[33] McGee AB, Schmierbach MR, Bazzaz FA. Photosynthesis and growth of populations of *Populus deltoids* from contrasting habitats. The American Midland Naturalist.

[34] Yoon J, Cao X, Zhou Q, Ma LQ. Accumulation of Pb, Cu, and Zn in naïve plants growing on contaminated Florida site. The Science of the Total Environment. 2006;**368**(2):456-464

[35] Cui S, Zhou Q, Chao L. Potential hyperaccumulation of Pb, Zn, Cu and Cd in endurant plants distributed in an old smeltery, Northeast China. Environmental Geology.

plants in the vicinity of a Korean Cu-W mine. Sensors. 2008;**8**(4):2413-2423

it? And what makes them so interesting? Plant Science. 2011;**180**:169-181

mium in hydroponics. Water Air and Soil Pollution. 2009;**197**:23-34

review. Journal of Biotechnology Research. 2016;**2**(11):80-91

Nuclear Engineering and Design. 2017;**315**:194-199

**242**:425-432

198 Heavy Metals

Phytologist. 1994;**127**:61-68

remediation. 1999;**1**:115-123

1981;**105**:305-311

2007;**51**(6):1043-1048

[49] Hattab S, Dridi B, Chouba L, Kheder MB, Bousetta H. Photosynthesis and growth responses of pea *Pisum sativum* L. under heavy metals stress. Journal of Environmental Sciences. 2009;**21**(11):1552-1556

**Chapter 12**

Provisional chapter

**Phytoremediation: Halophytes as Promising Heavy**

DOI: 10.5772/intechopen.73879

The continued accumulation of trace and heavy metals in the environment presents a significant danger to biota health, including humans, which is undoubtedly undermining global environmental sustainability initiatives. Consequently, the need for efficient remediation technologies becomes imperative. Phytoremediation is one of the most viable options in this regard. Hundreds of plants in laboratory experiments demonstrate the potential to remediate varying concentrations of heavy metals; however, the remediation capacity of most of these plants proved unsatisfactory under field conditions. The identification and selection of plants with higher metal uptake capacity or hyperaccumulators are one of the limitations of this technology. Additionally, the mechanism of heavy metal uptake by plants remains to be sufficiently documented. The halophyte plants are famous for their adaptation to harsh environmental conditions, and hence could be the most suitable candidates for heavy metal hyperaccumulation. The state of Qatar in the Gulf region encompasses rich resources of halophytes that have the potential for future investment toward human and environmental health. This chapter, therefore, gives an overview of phytoremediation, with emphasis on halophytes as suitable heavy metal hyperaccu-

mulators for improved remediation of heavy metal–contaminated areas.

Keywords: halophytes, phytoremediation, heavy metals, hyperaccumulation

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Phytoremediation: Halophytes as Promising Heavy

**Metal Hyperaccumulators**

Metal Hyperaccumulators

Mohammed H. Abu-Dieyeh

Mohammed H. Abu-Dieyeh

Abstract

http://dx.doi.org/10.5772/intechopen.73879

Kamal Usman, Mohammad A. Al-Ghouti and

Kamal Usman, Mohammad A. Al-Ghouti and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

#### **Phytoremediation: Halophytes as Promising Heavy Metal Hyperaccumulators** Phytoremediation: Halophytes as Promising Heavy Metal Hyperaccumulators

DOI: 10.5772/intechopen.73879

Kamal Usman, Mohammad A. Al-Ghouti and Mohammed H. Abu-Dieyeh Kamal Usman, Mohammad A. Al-Ghouti and Mohammed H. Abu-Dieyeh

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73879

#### Abstract

[49] Hattab S, Dridi B, Chouba L, Kheder MB, Bousetta H. Photosynthesis and growth responses of pea *Pisum sativum* L. under heavy metals stress. Journal of Environmental

[50] Turan M, Esringu A. Phytoremediation based on canola (*Brassica napus* L.) and Indian mustard (*Brassica juncea* L.) planted on spiked soil by aliquot amount of Cd, Cu, Pb, and

[51] Pulford ID, Watson C. Phytoremediation of heavy metal-contaminated land by trees—A

[52] Pulford ID, Dickinson NM. 90 Phytoremediation technologies using trees. Trace Elements in the Environment: Biogeochemistry, Biotechnology, and Bioremediation.

[53] Gerhardt KE, Huang XD, Glick BR, Greenberg BM. Phytoremediation and rhizoremediation of organic soil contaminants: Potential and challenges. Plant Science. 2009;

[54] Pulford ID, Watson C, McGregor SD. Uptake of chromium by trees: Prospects for phytoremediation. Environmental Geochemistry and Health. 2001;**23**(3):307-311

[55] Cluis C. Junk-greedy greens: Phytoremediation as a new option for soil decontamina-

[56] Raskin I, Ensley BD. Phytoremediation of Toxic Metals. New Jersey, USA: John Wiley

Sciences. 2009;**21**(11):1552-1556

2005;**14**:383

200 Heavy Metals

**176**(1):20-30

and Sons; 2000

Zn. Plant, Soil and Environment. 2007;**53**(1):7

tion. BioTechology Journal. 2004;**2**:61-67

review. Environment International. 2003;**29**:529-540

The continued accumulation of trace and heavy metals in the environment presents a significant danger to biota health, including humans, which is undoubtedly undermining global environmental sustainability initiatives. Consequently, the need for efficient remediation technologies becomes imperative. Phytoremediation is one of the most viable options in this regard. Hundreds of plants in laboratory experiments demonstrate the potential to remediate varying concentrations of heavy metals; however, the remediation capacity of most of these plants proved unsatisfactory under field conditions. The identification and selection of plants with higher metal uptake capacity or hyperaccumulators are one of the limitations of this technology. Additionally, the mechanism of heavy metal uptake by plants remains to be sufficiently documented. The halophyte plants are famous for their adaptation to harsh environmental conditions, and hence could be the most suitable candidates for heavy metal hyperaccumulation. The state of Qatar in the Gulf region encompasses rich resources of halophytes that have the potential for future investment toward human and environmental health. This chapter, therefore, gives an overview of phytoremediation, with emphasis on halophytes as suitable heavy metal hyperaccumulators for improved remediation of heavy metal–contaminated areas.

Keywords: halophytes, phytoremediation, heavy metals, hyperaccumulation

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### 1. Introduction

Heavy metals and other organic compounds constitutes the major environmental contaminants, and the trials of phytoremediation to free pollutants from waste water and contaminated soil dates back to hundreds of years ago in plants such as the Thlaspi caerulescens and Viola calaminaria, which were reported to remediate high concentration of heavy metals [1]. Anthropogenic activities arising from industrialization largely contribute to the proliferation of these contaminants, either by direct leakage or accidents during transport of solid and liquid wastes from storage and industrial facilities [2, 3]. Strategies to clean up environmental contaminants, both organic and inorganic are either by physical, chemical and or biological treatments [4, 5]. However, physical and chemical methods are recognized for a number of disadvantages or limitations such as high cost and labor intensiveness. Additionally, chemical processes create another pollution and are especially costly since they generate heaps of sludge [6]. In view of this context, new and better approaches to clean up of metal contamination were thought up and became imperative, hence the exploration of various bio-based techniques. The use of biological agents is considered cheap, safer and has limited or no negative impact to the environment [7]. Bio-based remediation methods include bio-augmentation, bioremediation, bioventing, composting and phytoremediation. However, phytoremediation proves the most viable and useful alternative and has gain an increasing attention in recent times [8, 9]. The adverse and negative effects associated with these elements make them targets for phytoremediation [10]. Phytoremediation offer several advantages. It is cheap, promotes biodiversity, reduces erosion, less destructive and decreased energy consumption leading to reduced carbon dioxide emission [11]. To date, about 400 plant species were suggested to be metal hyper-accumulators [12]. However, few studies reported the toxicity of several metals combined [13], and while hyperaccumulation of nickel (Ni), cadmium (Cd), manganese (Mn), zinc (Zn) and selenium (Se) have been well established, the same is yet be available or demonstrated beyond doubt in plant species for copper (Cu), chromium (Cr), lead (Pb), thallium (Th) and cobalt (Co) metals. For instance, Cu is an important element for growth and general plant physiology, owing to its role as a cofactor to various types of enzymes involved in the transfer of electrons during metabolic processes, such as the tricarboxylic acid (TCA) cycle [14, 15]. However, at high concentrations, it is toxic to plants signaled by stunted growth, and although there is some physiological insight to Cu stress in plants, the responses are still vague at the functional level [16]. The accumulation of heavy metals in plant tissues results in a wide range of negative effects on growth. Although it affects seed germination, growth of seedlings and photosynthetic processes, which generally leads to the inhibition of the plants important enzymatic activity [17, 18], however, plants responds differently [19]. In dealing with the heavy metal stress, the root tissue is the first to be exposed to the associated toxins, and its cell wall has a mechanism of exchange that fixes the heavy metal ions, thereby limiting the transmission of the toxins to other plant tissues [20, 21]. Several studies reported many plants, including desert species as good phytoremediation agents, however, few are metal hyperaccumulators and their selection for efficient phytoremediation is still a challenge. This is demonstrated by slow growth, above ground biomass, root system and harvest [22]. Accordingly, successful heavy metal phytoremediation requirement of hyperaccumulation capacity in candidate plants position halophytes as suitable phytoremediators. This is due to their extensive stress tolerance mechanism, which enables them thrive in saline soil and in

Phytoremediation: Halophytes as Promising Heavy Metal Hyperaccumulators

http://dx.doi.org/10.5772/intechopen.73879

203

In simple terms, phytoremediation refers to a process where plants are employed to reduce or free up organic and inorganic contaminants from the environment [13] with the aid of associated microbes. The process by which contaminants are remediated differs; these may be in the form of removal, transfer, degradation and immobilization from either soil or water [23]. It is a unique approach capitalizing on plants roots ability for the initial uptake of pollutants, and eventually accumulating them onto the shoot tissue by translocation across the stem. Compared to other conventional treatment techniques, phytoremediation is new, with a great potential to providing the much-needed green technology solution to our deteriorating environment. To date, hun-

During phytoremediation, plants growing on soil or water contaminated with trace or heavy metals could absorb or tolerate these elements differently, depending on the physiological means involved and the kinds of metals present [25]. According to Halder and Ghosh [26] phytoremediation techniques are categorized into five; phytoextraction, phytofiltration,

Phytoextraction is a technique of phytoremediation where plants take up metals by translocation, and accumulate them in a form that can be extracted on its tissue [27]. It is one of the most common types of phytoremediation and the names; phytoabsorption, phytoaccumulation and phytosequestration are often used interchangeably to refer to phytoextraction [28]. It is considered as the major phytoremediation technique among all others for the removal of metals from contaminated water, sediment and soil. The efficiency of this remediation process depends on a number of factors from soil properties, metal bioavailability and speciation to the type of plant species. However, high concentration of absorbed metals usually ends up in the shoot biomass of the plant in harvestable form [12]. A number of recent studies reported various plant species that

Plants able to exhibit phytoextraction strategy in metal sequestration may potentially be hyper accumulators, referring to plants that consistently accumulate certain threshold of metal concentration in their shoot tissue, which varies according to the metals [22]. Generally, all hyper accumulators should possess characteristics such as high growth rate, widely branched shoot, high bioaccumulation and translocation capacity, high above ground biomass, easily cultivated and harvested [22, 33]. However, Ali, Khan [28] demonstrated two methods or approaches for metal phytoextraction in different plants, one producing less above ground biomass but

dreds of plant species were suggested as potential phytoremediation agents [24].

phytovolatilization, phytostabilization and phytotransformation.

demonstrate phytoextraction strategy from both water and soil media [29–32].

other desert conditions.

2. Phytoremediation

2.1. Phytoremediation techniques

2.1.1. Phytoextraction

due to their extensive stress tolerance mechanism, which enables them thrive in saline soil and in other desert conditions.

### 2. Phytoremediation

1. Introduction

202 Heavy Metals

Heavy metals and other organic compounds constitutes the major environmental contaminants, and the trials of phytoremediation to free pollutants from waste water and contaminated soil dates back to hundreds of years ago in plants such as the Thlaspi caerulescens and Viola calaminaria, which were reported to remediate high concentration of heavy metals [1]. Anthropogenic activities arising from industrialization largely contribute to the proliferation of these contaminants, either by direct leakage or accidents during transport of solid and liquid wastes from storage and industrial facilities [2, 3]. Strategies to clean up environmental contaminants, both organic and inorganic are either by physical, chemical and or biological treatments [4, 5]. However, physical and chemical methods are recognized for a number of disadvantages or limitations such as high cost and labor intensiveness. Additionally, chemical processes create another pollution and are especially costly since they generate heaps of sludge [6]. In view of this context, new and better approaches to clean up of metal contamination were thought up and became imperative, hence the exploration of various bio-based techniques. The use of biological agents is considered cheap, safer and has limited or no negative impact to the environment [7]. Bio-based remediation methods include bio-augmentation, bioremediation, bioventing, composting and phytoremediation. However, phytoremediation proves the most viable and useful alternative and has gain an increasing attention in recent times [8, 9]. The adverse and negative effects associated with these elements make them targets for phytoremediation [10]. Phytoremediation offer several advantages. It is cheap, promotes biodiversity, reduces erosion, less destructive and decreased energy consumption leading to reduced carbon dioxide emission [11]. To date, about 400 plant species were suggested to be metal hyper-accumulators [12]. However, few studies reported the toxicity of several metals combined [13], and while hyperaccumulation of nickel (Ni), cadmium (Cd), manganese (Mn), zinc (Zn) and selenium (Se) have been well established, the same is yet be available or demonstrated beyond doubt in plant species for copper (Cu), chromium (Cr), lead (Pb), thallium (Th) and cobalt (Co) metals. For instance, Cu is an important element for growth and general plant physiology, owing to its role as a cofactor to various types of enzymes involved in the transfer of electrons during metabolic processes, such as the tricarboxylic acid (TCA) cycle [14, 15]. However, at high concentrations, it is toxic to plants signaled by stunted growth, and although there is some physiological insight to Cu stress in plants, the responses are still vague at the functional level [16]. The accumulation of heavy metals in plant tissues results in a wide range of negative effects on growth. Although it affects seed germination, growth of seedlings and photosynthetic processes, which generally leads to the inhibition of the plants important enzymatic activity [17, 18], however, plants responds differently [19]. In dealing with the heavy metal stress, the root tissue is the first to be exposed to the associated toxins, and its cell wall has a mechanism of exchange that fixes the heavy metal ions, thereby limiting the transmission of the toxins to other plant tissues [20, 21]. Several studies reported many plants, including desert species as good phytoremediation agents, however, few are metal hyperaccumulators and their selection for efficient phytoremediation is still a challenge. This is demonstrated by slow growth, above ground biomass, root system and harvest [22]. Accordingly, successful heavy metal phytoremediation requirement of hyperaccumulation capacity in candidate plants position halophytes as suitable phytoremediators. This is

In simple terms, phytoremediation refers to a process where plants are employed to reduce or free up organic and inorganic contaminants from the environment [13] with the aid of associated microbes. The process by which contaminants are remediated differs; these may be in the form of removal, transfer, degradation and immobilization from either soil or water [23]. It is a unique approach capitalizing on plants roots ability for the initial uptake of pollutants, and eventually accumulating them onto the shoot tissue by translocation across the stem. Compared to other conventional treatment techniques, phytoremediation is new, with a great potential to providing the much-needed green technology solution to our deteriorating environment. To date, hundreds of plant species were suggested as potential phytoremediation agents [24].

### 2.1. Phytoremediation techniques

During phytoremediation, plants growing on soil or water contaminated with trace or heavy metals could absorb or tolerate these elements differently, depending on the physiological means involved and the kinds of metals present [25]. According to Halder and Ghosh [26] phytoremediation techniques are categorized into five; phytoextraction, phytofiltration, phytovolatilization, phytostabilization and phytotransformation.

### 2.1.1. Phytoextraction

Phytoextraction is a technique of phytoremediation where plants take up metals by translocation, and accumulate them in a form that can be extracted on its tissue [27]. It is one of the most common types of phytoremediation and the names; phytoabsorption, phytoaccumulation and phytosequestration are often used interchangeably to refer to phytoextraction [28]. It is considered as the major phytoremediation technique among all others for the removal of metals from contaminated water, sediment and soil. The efficiency of this remediation process depends on a number of factors from soil properties, metal bioavailability and speciation to the type of plant species. However, high concentration of absorbed metals usually ends up in the shoot biomass of the plant in harvestable form [12]. A number of recent studies reported various plant species that demonstrate phytoextraction strategy from both water and soil media [29–32].

Plants able to exhibit phytoextraction strategy in metal sequestration may potentially be hyper accumulators, referring to plants that consistently accumulate certain threshold of metal concentration in their shoot tissue, which varies according to the metals [22]. Generally, all hyper accumulators should possess characteristics such as high growth rate, widely branched shoot, high bioaccumulation and translocation capacity, high above ground biomass, easily cultivated and harvested [22, 33]. However, Ali, Khan [28] demonstrated two methods or approaches for metal phytoextraction in different plants, one producing less above ground biomass but significantly accumulate metals in high concentration and vice versa in the other plant species, with final metal accumulation in agreement with those of hyper accumulators. Consequently, hyper accumulation is more important in phytoremediation than volume of biomass produced, and this suggest the use of hyper accumulators as more acceptable since it has advantages such as safe disposal, cheap process and easy handling [28].

erosion or leaching [27]. It is applicable in the stabilization of metals in contaminated soil, sediment or water environments, which ensures they are not transferred to the food chain from the soil by translocating to other parts of food crops or to the underground water. This is possible by sorption via the root, precipitation and subsequent metal reduction around the plant rhizosphere, for instance the toxic Cr6+ is converted to Cr3+, which is less toxic [42, 43]. Variation exists as to how prone a metal is to phytostabilization and is subject to its chemical character. This is evidenced in a comparative study to evaluate metal accumulation capacity of two aquatic macrophytes Phragmites australis and Typha domingensis, where both are found to stabilize As

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205

Although phytostabilization offer some advantages over other phytoremediation techniques, it is however limited to temporary measure to deal with pollutants contamination owing to the fact that metals are only inactivated and their movement restricted, but still remains in the contaminated environmental compartment [45]. It is useful in emergencies, since it can rapidly immobilize pollutants from soil, water or sediment. Equally important, it ensures that contaminants are not translocated to other plant tissues by trapping most of it in the plant root [46]. Considering the strategies employed in phytostabilization, plants that can appropriately fall under this mechanism is their ability to tolerate and immobilize metals and other contaminants, low translocation capacity from root to plant aerial parts and of course extensive and fibrous tap root system [7]. Among several studies that reported plants species with these characteristics [47–49] demonstrating the phytostabilization of Zn, Pb, Cu and Cd by different

Phytotransformation or phytodegradation is another technique of phytoremediation where contaminants and other nutrients are chemically modified through plant metabolism and render associated contaminants inactive in both plant root and shoot tissues [6]. Plant metabolic enzymes act on the surrounding contaminants, thereby transforming them to a less toxic form, plants rhizosphere microbes also aid in the transformation process of the compounds [50]. Although this mechanism is mostly against organic contaminants, inorganic compounds such as metals were also suggested, in which case a strategy akin to phytostabilization is employed to convert toxic metals to less toxic form [51]. However, this technique seem less efficient and reliable compared to others in that it requires longer period of time, strict soil characteristic such

In phytovolatilization, contaminants are converted in to a volatile form and released to the air via plants leaves stomata [27, 34]. However, this mechanism merely transfers contaminants from one environmental compartment to another, which may somehow return back to the original source (soil) by precipitation and hence could be less popular to other phytoremediation techniques especially phytoextraction and phytofiltration [34, 52]. It is commonly employed when treating groups of highly volatile metals like Hg and As. Phytovolatilization of As involves the conversion of elemental As to selenoaminoacids, such as selenomethione, which is modified by

as depth and underground water availability and often require soil amendments.

methylation to a volatile and less toxic form, dimethylselenide [53].

and Hg but inefficient in the phytostabilization of other metals [44].

plants in soil and sediment polluted environments.

2.1.4. Phytotransformation/phytodegradation

2.1.5. Phytovolatilization

### 2.1.2. Phytofiltration

Phytofiltration or rhizofiltration, as used interchangeably, refers to the absorption or adsorption of contaminants from surface wastewater by plant roots thereby preventing them from leaching to the underground water [34]. It is a type of phytoremediation technique that can be demonstrated in situ by directly growing plants in the polluted water body [24]. Although it is commonly applicable using aquatic plant species [35], there are suggestions that the process may be applied to terrestrial plants, which remediate metals to precipitate with the aid of microbes root bio filter [36]. Indeed, root exudates cause metal precipitation which alters the rhizosphere pH level [37]. Many terrestrial plants including grasses grown in a hydroponic culture were shown to effectively remove metals such via phytofiltration [38]. In the same study, Indian mustard was especially reported to accumulate higher fold of metal concentration far beyond the initial concentration, and the removal is by tissue specific adsorption mediated by root metal concentration.

Quite a number of studies have shown many species of aquatic macrophytes that demonstrate phytofiltration potential. While experimenting for phytoremediation under different water conditions polluted with heavy metals, Liao and Chang [39] found that Eichhonia crassipes absorb and accumulates metal contaminants, it has also exhibit high growth rate and increased biomass production and thus considered a good phytofiltration agent. This plant species absorb high concentrations of Pb, Ni, Zn and Cu which accumulates much higher in the root tissue than the shoot, suggesting the important role of fibrous and tap root system found in the plant, which is one of the key characteristics of potential phytofiltration agent. In a similar study, other aquatic plant species including Salvinia herzogii, E. crassipes, Pistia stratiotes and Hydromistia stolonifera were shown to absorb high concentration of Cd with P. stratiotes accumulating higher Cd concentration and exhibiting faster growth rate, a feature attributed to possible complimentary mechanism for the enhanced metal uptake [40]. Absorption of Cd in the root of all the plants relates to the added concentration. In another study by Thayaparan, Iqbal [41] also reported that Azolla pinnata have shown a great potential in the removal of high Pb concentration by phytofiltration from polluted water. As in phytoextraction, potential phytofiltration agents should tolerate high metal concentration, exhibit fast and high growth rate as well as above ground biomass, however, in contrast to phytoextraction, they are expected to show limited translocation capacity of absorbed metals from root to shoot tissues [24]. For efficient phytofiltration, this is an advantage over phytoextraction, since low translocation of contaminants means reduced contamination of other parts of the plant.

#### 2.1.3. Phytostabilization

In this technique, pollutants are converted into a less toxic or bioavailable form by the continuous precipitation of the plant rhizosphere. This is achieved either by surface run off prevention, erosion or leaching [27]. It is applicable in the stabilization of metals in contaminated soil, sediment or water environments, which ensures they are not transferred to the food chain from the soil by translocating to other parts of food crops or to the underground water. This is possible by sorption via the root, precipitation and subsequent metal reduction around the plant rhizosphere, for instance the toxic Cr6+ is converted to Cr3+, which is less toxic [42, 43]. Variation exists as to how prone a metal is to phytostabilization and is subject to its chemical character. This is evidenced in a comparative study to evaluate metal accumulation capacity of two aquatic macrophytes Phragmites australis and Typha domingensis, where both are found to stabilize As and Hg but inefficient in the phytostabilization of other metals [44].

Although phytostabilization offer some advantages over other phytoremediation techniques, it is however limited to temporary measure to deal with pollutants contamination owing to the fact that metals are only inactivated and their movement restricted, but still remains in the contaminated environmental compartment [45]. It is useful in emergencies, since it can rapidly immobilize pollutants from soil, water or sediment. Equally important, it ensures that contaminants are not translocated to other plant tissues by trapping most of it in the plant root [46]. Considering the strategies employed in phytostabilization, plants that can appropriately fall under this mechanism is their ability to tolerate and immobilize metals and other contaminants, low translocation capacity from root to plant aerial parts and of course extensive and fibrous tap root system [7]. Among several studies that reported plants species with these characteristics [47–49] demonstrating the phytostabilization of Zn, Pb, Cu and Cd by different plants in soil and sediment polluted environments.

#### 2.1.4. Phytotransformation/phytodegradation

Phytotransformation or phytodegradation is another technique of phytoremediation where contaminants and other nutrients are chemically modified through plant metabolism and render associated contaminants inactive in both plant root and shoot tissues [6]. Plant metabolic enzymes act on the surrounding contaminants, thereby transforming them to a less toxic form, plants rhizosphere microbes also aid in the transformation process of the compounds [50]. Although this mechanism is mostly against organic contaminants, inorganic compounds such as metals were also suggested, in which case a strategy akin to phytostabilization is employed to convert toxic metals to less toxic form [51]. However, this technique seem less efficient and reliable compared to others in that it requires longer period of time, strict soil characteristic such as depth and underground water availability and often require soil amendments.

### 2.1.5. Phytovolatilization

significantly accumulate metals in high concentration and vice versa in the other plant species, with final metal accumulation in agreement with those of hyper accumulators. Consequently, hyper accumulation is more important in phytoremediation than volume of biomass produced, and this suggest the use of hyper accumulators as more acceptable since it has advantages such

Phytofiltration or rhizofiltration, as used interchangeably, refers to the absorption or adsorption of contaminants from surface wastewater by plant roots thereby preventing them from leaching to the underground water [34]. It is a type of phytoremediation technique that can be demonstrated in situ by directly growing plants in the polluted water body [24]. Although it is commonly applicable using aquatic plant species [35], there are suggestions that the process may be applied to terrestrial plants, which remediate metals to precipitate with the aid of microbes root bio filter [36]. Indeed, root exudates cause metal precipitation which alters the rhizosphere pH level [37]. Many terrestrial plants including grasses grown in a hydroponic culture were shown to effectively remove metals such via phytofiltration [38]. In the same study, Indian mustard was especially reported to accumulate higher fold of metal concentration far beyond the initial concentration, and the removal is by tissue specific adsorption

Quite a number of studies have shown many species of aquatic macrophytes that demonstrate phytofiltration potential. While experimenting for phytoremediation under different water conditions polluted with heavy metals, Liao and Chang [39] found that Eichhonia crassipes absorb and accumulates metal contaminants, it has also exhibit high growth rate and increased biomass production and thus considered a good phytofiltration agent. This plant species absorb high concentrations of Pb, Ni, Zn and Cu which accumulates much higher in the root tissue than the shoot, suggesting the important role of fibrous and tap root system found in the plant, which is one of the key characteristics of potential phytofiltration agent. In a similar study, other aquatic plant species including Salvinia herzogii, E. crassipes, Pistia stratiotes and Hydromistia stolonifera were shown to absorb high concentration of Cd with P. stratiotes accumulating higher Cd concentration and exhibiting faster growth rate, a feature attributed to possible complimentary mechanism for the enhanced metal uptake [40]. Absorption of Cd in the root of all the plants relates to the added concentration. In another study by Thayaparan, Iqbal [41] also reported that Azolla pinnata have shown a great potential in the removal of high Pb concentration by phytofiltration from polluted water. As in phytoextraction, potential phytofiltration agents should tolerate high metal concentration, exhibit fast and high growth rate as well as above ground biomass, however, in contrast to phytoextraction, they are expected to show limited translocation capacity of absorbed metals from root to shoot tissues [24]. For efficient phytofiltration, this is an advantage over phytoextraction, since low translo-

cation of contaminants means reduced contamination of other parts of the plant.

In this technique, pollutants are converted into a less toxic or bioavailable form by the continuous precipitation of the plant rhizosphere. This is achieved either by surface run off prevention,

as safe disposal, cheap process and easy handling [28].

mediated by root metal concentration.

2.1.3. Phytostabilization

2.1.2. Phytofiltration

204 Heavy Metals

In phytovolatilization, contaminants are converted in to a volatile form and released to the air via plants leaves stomata [27, 34]. However, this mechanism merely transfers contaminants from one environmental compartment to another, which may somehow return back to the original source (soil) by precipitation and hence could be less popular to other phytoremediation techniques especially phytoextraction and phytofiltration [34, 52]. It is commonly employed when treating groups of highly volatile metals like Hg and As. Phytovolatilization of As involves the conversion of elemental As to selenoaminoacids, such as selenomethione, which is modified by methylation to a volatile and less toxic form, dimethylselenide [53].

### 3. Metal hyperaccumulator plants

Several plants species are known to tolerate high concentration of toxic metals. Tolerant species are best described as excluders, where metal uptake and translocation to different tissue parts are limited. While others that are capable of accumulating higher concentrations with improved translocation from the root to shoot part of the plant, thereby significantly reducing its availability in the soil, and they do so with no visible sign of toxicity effects. To date, heavy metals have no standard definition by recognized bodies in the area. Various researchers use different characteristics and levels in their description such as atomic mass and number, density, chemical character as well as their toxicity; however, there appears no connection between such properties [54]. According to Wang and Chen [55], three categories of heavy metals arising from both natural and artificial sources are of interest, these includes valuable metals e.g. Ag, Au, Pd, Pt, harmful metals e.g., As, Cu, Co, Cd, Cr, Hg, Ni, Pb, and radionuclides such as Am, Th, Ra and so on. The non-biodegradability and stable nature of heavy metals suggests increased exposure to living species including humans [54], periodic reviews of toxic metals effects are documented by many research groups [56–58].

in the protection of plants against the toxicity of heavy metals in soil, sediment and water [65, 69, 70]. The expression of MTs and PCs, alongside organic acid synthesis, together functions in heavy metal uptake by plants and also their translocation to other tissue parts [42]. The expression of these natural chelators could be enhanced to increase the efficiency of heavy metal accumulation and translocation. Currently, there are many ongoing studies aimed at characterizing and identifying biomolecules involved in the transport and detoxification of heavy metals. This will aid in understanding the whole detoxification process involved in plants [28, 71], and to achieve this, the importance of comparative proteomic studies cannot

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Plant shoot is an important tissue in phytoremediation process; it is especially responsible for accumulating the highest metals concentration when the subject plant employs phytoextraction technique, which is subject to the type of metal elements and bioavailability. In recent times, there has been an increased interest in the proteomics study of plant hyperaccumulators with the aim of characterizing and identifying proteins acting in metal sequestration and detoxification [72]. These are possible with the advancement in modern mass spectrometry techniques such as twodimensional liquid chromatography matrix-assisted laser desorption/ionization time-of-flight (2D-LC/TOFMS), time of flight/mass spectrometry (TOF/MS), two-dimensional polyacrylamide gel electrophoresis (2D PAGE), and liquid chromatography- tandem mass spectrometry (LC–MS/ MS). For instance, the proteome of many plants species including Thlaspi caerulescens, Pteris vittata, Helianthus annuus and Agrostis tenuis were recently searched for heavy metals detoxifying proteins; several key functional proteins were found that protect plants against oxidative stress, as well as those responsive to biotic and abiotic stress condition among others [12, 73, 74].

In the proteomics study of plant metal hyperaccumulators, comparison could be made, even when these studies are from different plants and metals. In 2005, [75] found that prolonged exposure of Alyssum lesbiacum to Ni in an optimized experimental condition induced only three proteins, and one of these proteins, iron superoxide dismutase (Fe-SOD), was demonstrated to have antioxidant activity [76], while the other two proteins were identified as chloroplast phosphoglycerate kinase and a transketolase both having a role in the carbohydrates metabolism. In Anemone halleri, photosynthetic protein (chlorophyll a/b binding) and membrane protein (photosystem II) were constantly translated and upregulated when treated with Zn and Cd, which is linked to the improved metabolic energy demand in this metal

At high metal concentrations, increased proteins induction are involved in the defense against antioxidants and energy metabolism has been consistently observed; examples includes Renal Epithelial Protein (APX), Superoxide dismutase (SOD), cytochrome P450 and Glutathione S-transferase (GST). These suggest that, for the uptake, translocation and accumulation of heavy metals concentration on the shoot tissue, plants require the functional photosynthetic process as well as the activity of proteins that scavenge oxygen radical species [13]. Metabolic energy active proteins were also suggested to have important roles in metal tolerance by plants. The proteomes of T. caerulescens with variable tolerance to Cd and Zn metals were

be over-emphasized.

3.1.1. The shoot proteome

hyperaccumulating plant [77].

When determining hyperaccumulators of toxic metal, the most important factor is the concentration of the metal ion threshold. Therefore, plants can be regarded as hyperaccumulators, when capable of accumulating toxic metals concentration to about 50 to 100 times more than nonhyperaccumulator plants [13, 59]. For instance, the threshold for Zn and Mn hyperaccumulation in plant shoot is pegged at 1% of dry biomass, 0.01% for Cd and 0.1% respectively for Ti, Se, Sb, Pb, Ni, Cu, Cr, Co, and As [13, 60]. To date, few plant species are classified as hyperaccumulators, the majority of them (3/4) are tolerant to Ni and belongs to the Brassicaceae family native to Western Asia and Southern Europe, with up to 48 species implicated in Ni accumulation of around 3% dry shoot mass [60–62]. There is increasing interest in plant hyperaccumulators in recent times, owing to their potential use in metal contaminated soil and water detoxification [25, 63].

#### 3.1. The role of metal chelators in hyperaccumulation

The phytoremediation of heavy metals involve many physiological, biochemical and molecular activities. In this process, especially phytoextraction involves the accumulation and translocation of heavy metals to plant tissues. Plant metal chelators or phytochelatins (PCs) and metallothioneins (MTs) are the most common transporter proteins for heavy metal phytoremediation. MTs are cysteine rich proteins that are famous for metal binding and greatly assist in the process of sequestration of metals in ionic form [64]. PCs are glutathione synthase products and they binds to heavy metals thereby forming central part of the phytodetoxification mechanism [65, 66]. The induction of phytochelatins is induced by the activity of an enzyme, phytochelatins synthase, which is triggered by the activity of metal ions present [34, 67]. In an experiment to demonstrate the role of synthases, mutants in model plant Arabidopsis thaliana were shown to be hypersensitive to Hg and Cd, which is attributed to their inability to produce PCs [68]. On the other hand, MTs are genetically encoded metal binding peptides and usually bear low molecular weight. A number of studies demonstrate MTs role in the protection of plants against the toxicity of heavy metals in soil, sediment and water [65, 69, 70]. The expression of MTs and PCs, alongside organic acid synthesis, together functions in heavy metal uptake by plants and also their translocation to other tissue parts [42]. The expression of these natural chelators could be enhanced to increase the efficiency of heavy metal accumulation and translocation. Currently, there are many ongoing studies aimed at characterizing and identifying biomolecules involved in the transport and detoxification of heavy metals. This will aid in understanding the whole detoxification process involved in plants [28, 71], and to achieve this, the importance of comparative proteomic studies cannot be over-emphasized.

### 3.1.1. The shoot proteome

3. Metal hyperaccumulator plants

206 Heavy Metals

detoxification [25, 63].

3.1. The role of metal chelators in hyperaccumulation

Several plants species are known to tolerate high concentration of toxic metals. Tolerant species are best described as excluders, where metal uptake and translocation to different tissue parts are limited. While others that are capable of accumulating higher concentrations with improved translocation from the root to shoot part of the plant, thereby significantly reducing its availability in the soil, and they do so with no visible sign of toxicity effects. To date, heavy metals have no standard definition by recognized bodies in the area. Various researchers use different characteristics and levels in their description such as atomic mass and number, density, chemical character as well as their toxicity; however, there appears no connection between such properties [54]. According to Wang and Chen [55], three categories of heavy metals arising from both natural and artificial sources are of interest, these includes valuable metals e.g. Ag, Au, Pd, Pt, harmful metals e.g., As, Cu, Co, Cd, Cr, Hg, Ni, Pb, and radionuclides such as Am, Th, Ra and so on. The non-biodegradability and stable nature of heavy metals suggests increased exposure to living species including humans [54], periodic

reviews of toxic metals effects are documented by many research groups [56–58].

When determining hyperaccumulators of toxic metal, the most important factor is the concentration of the metal ion threshold. Therefore, plants can be regarded as hyperaccumulators, when capable of accumulating toxic metals concentration to about 50 to 100 times more than nonhyperaccumulator plants [13, 59]. For instance, the threshold for Zn and Mn hyperaccumulation in plant shoot is pegged at 1% of dry biomass, 0.01% for Cd and 0.1% respectively for Ti, Se, Sb, Pb, Ni, Cu, Cr, Co, and As [13, 60]. To date, few plant species are classified as hyperaccumulators, the majority of them (3/4) are tolerant to Ni and belongs to the Brassicaceae family native to Western Asia and Southern Europe, with up to 48 species implicated in Ni accumulation of around 3% dry shoot mass [60–62]. There is increasing interest in plant hyperaccumulators in recent times, owing to their potential use in metal contaminated soil and water

The phytoremediation of heavy metals involve many physiological, biochemical and molecular activities. In this process, especially phytoextraction involves the accumulation and translocation of heavy metals to plant tissues. Plant metal chelators or phytochelatins (PCs) and metallothioneins (MTs) are the most common transporter proteins for heavy metal phytoremediation. MTs are cysteine rich proteins that are famous for metal binding and greatly assist in the process of sequestration of metals in ionic form [64]. PCs are glutathione synthase products and they binds to heavy metals thereby forming central part of the phytodetoxification mechanism [65, 66]. The induction of phytochelatins is induced by the activity of an enzyme, phytochelatins synthase, which is triggered by the activity of metal ions present [34, 67]. In an experiment to demonstrate the role of synthases, mutants in model plant Arabidopsis thaliana were shown to be hypersensitive to Hg and Cd, which is attributed to their inability to produce PCs [68]. On the other hand, MTs are genetically encoded metal binding peptides and usually bear low molecular weight. A number of studies demonstrate MTs role Plant shoot is an important tissue in phytoremediation process; it is especially responsible for accumulating the highest metals concentration when the subject plant employs phytoextraction technique, which is subject to the type of metal elements and bioavailability. In recent times, there has been an increased interest in the proteomics study of plant hyperaccumulators with the aim of characterizing and identifying proteins acting in metal sequestration and detoxification [72]. These are possible with the advancement in modern mass spectrometry techniques such as twodimensional liquid chromatography matrix-assisted laser desorption/ionization time-of-flight (2D-LC/TOFMS), time of flight/mass spectrometry (TOF/MS), two-dimensional polyacrylamide gel electrophoresis (2D PAGE), and liquid chromatography- tandem mass spectrometry (LC–MS/ MS). For instance, the proteome of many plants species including Thlaspi caerulescens, Pteris vittata, Helianthus annuus and Agrostis tenuis were recently searched for heavy metals detoxifying proteins; several key functional proteins were found that protect plants against oxidative stress, as well as those responsive to biotic and abiotic stress condition among others [12, 73, 74].

In the proteomics study of plant metal hyperaccumulators, comparison could be made, even when these studies are from different plants and metals. In 2005, [75] found that prolonged exposure of Alyssum lesbiacum to Ni in an optimized experimental condition induced only three proteins, and one of these proteins, iron superoxide dismutase (Fe-SOD), was demonstrated to have antioxidant activity [76], while the other two proteins were identified as chloroplast phosphoglycerate kinase and a transketolase both having a role in the carbohydrates metabolism. In Anemone halleri, photosynthetic protein (chlorophyll a/b binding) and membrane protein (photosystem II) were constantly translated and upregulated when treated with Zn and Cd, which is linked to the improved metabolic energy demand in this metal hyperaccumulating plant [77].

At high metal concentrations, increased proteins induction are involved in the defense against antioxidants and energy metabolism has been consistently observed; examples includes Renal Epithelial Protein (APX), Superoxide dismutase (SOD), cytochrome P450 and Glutathione S-transferase (GST). These suggest that, for the uptake, translocation and accumulation of heavy metals concentration on the shoot tissue, plants require the functional photosynthetic process as well as the activity of proteins that scavenge oxygen radical species [13]. Metabolic energy active proteins were also suggested to have important roles in metal tolerance by plants. The proteomes of T. caerulescens with variable tolerance to Cd and Zn metals were compared and there was a higher accumulation of the extrinsic subunit of photosystem II protein, which led to its stabilization in the more metal tolerant variant as against the less tolerant accession. In addition to GST and cytochrome P450 earlier mentioned, other proteins such as aspartate aminotransferase and thioredoxin are commonly found, and linked to the sequestration of xenobiotics including metals. GSTs have particularly been demonstrated to be up regulated in many other living species including bacteria and fungi treated with metals like Zn, Cu and Cd [78]; hence GST were suggested to confer resistance to toxic genes in these cells.

### 3.1.2. The root proteome

In plants, the root tissue is the first to be exposed to all potential toxicants whether in the soil or surface water and hence serve as the gateway route through which they can subsequently be translocated to other tissue parts. Plants diversity as to hyperaccumulators and nonhyperaccumulators exist, this is due to the fact that, while some species bear the complete mechanism of enhanced metal uptake and eventual translocation, others have limited sequestration capacity in their root vacuoles [79]. In non-hyperaccumulators plant roots, Zn transporters are only detectable in the absence of Zn, whereas in hyperaccumulators, there is constitutive expression of these proteins such as ZT1 even in Zn deficient condition [80, 81]. In T. caerulescens, the iron transporter protein IRT1 was found to be involved in Zn and Cd uptake [82], similarly, root proteome study of this hyperaccumulator and A. lesbiacum were conducted by Tuomainen, Tervahauta [83] to evaluate peptides involved in Zn and Cd hyperaccumulation. In these studies, various classes of proteins were identified, their availability and or abundance varies relative to metal exposure and accessions. As in the case with similar studies on shoot proteome of hyperaccumulators, ROS scavenging proteins were more abundant in the more metal tolerant accessions compared to the less metal tolerant species. It was concluded that the changes in the enzyme, superoxide dismutase (SOD) availability upon which Zn depends in the different accessions may be linked to ROS increase.

domingensis remediated heavy metals from industrial waste water and solution cultures; similarly, members of the halophytes plant family Brassicaceae were reported to be important phytoremediation agents [3, 59, 91] and the tree plant Prosopis juliflora exhibited phytoremediation of heavy metals potential [92, 93]. Additionally, such as other plants, such as Phragmites australis that were previously shown to clean petroleum-polluted soils may be good candidates for the phytoremediation of typical oil and gas produced wastewater [94], others with similar potentials includes Medicago such as Medicago sativa and Glycine max which also demonstrated strong petroleum polluted soil phytoremediation activity [95, 96]. Some examples of other species tested for phytoremediation studies and their metal uptake capacity are summarized in

S/No Plant species Metal (s) Metal accumulation (mg/kg) References 1 Atriplex halimus subsp. schweinfurthii Cadmium 606.51 [97]

 Arthrocnemum macrostachyum Lead 620 [99] Crucianella maritima Zinc 390 [99] Dittrichia viscosa Lead 270 [99] Tamarix smyrnensis Bunge Lead 800 [100]

6 Typha domingensis Selenium 30 [90]

7 T. lotifolia L Cadmium 210 [102]

Table 1. Examples of phytoremediation studies using species belong to Qatari flora and/or their relatives.

A. halimus L. Cadmium 830 [98] Zinc 44

Cadmium 800

Lead 59.13 [101]

Phytoremediation: Halophytes as Promising Heavy Metal Hyperaccumulators

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209

Lead 150 [103]

The accumulation of trace and heavy metals in the environment present a great risk to biota health. These contaminants are implicated in a wide range of human diseases and various long-term negative environmental consequences, thereby endangering overall sustainable development initiatives worldwide. Many conventional treatment strategies are widely practiced for the remediation of these contaminants. However, traditional remediation processes have many disadvantages, from complicating environmental pollution to high operational cost among others. Phytoremediation is one of the most promising alternatives in this regard, and laboratory experiments have demonstrated the capacity of hundreds of plants species to remediate different heavy metal contaminants. However, there still exist limitations in the application of this emerging technology. This may be linked to exposure to other stress factors in field conditions, and especially in extreme environments, which could significantly affect

Table 1 above.

8 Paspalum conjugatum L. Prosopis laevigata

4. Conclusion

An important organelle, cell wall, in the plant root is also affected by its exposure to heavy metal stress. The putative protein, glycosyl hydrolase family 18, involved in the formation of cell wall structure was shown to be regulated in accordance to treatment conditions and accession. These proteins, which are particularly known to be involved in cell wall expansion, differ in terms of abundance between the root proteome of two accessions, which in turn also affect the capacity of metals uptake; higher Ni and Cd accumulation was observed in the variant with more protein abundance [13]. Despite the recent advancement in proteomics technology, root protein transporters are yet to be differentially identified. Indeed, this is in agreement with transcriptomics studies, with analyzed data suggesting the constitutive expression of metal genes transporters in plant metal hyperaccumulators [13, 81, 84].

#### 3.2. The halophytes of Qatar are promising heavy metal hyperaccumulators

Some studies demonstrated the potential of several Qatari plants as good phytoremediation candidates, many among which are heavy metal hyperaccumulators. Examples includes species belonging to the genus Zygophyllum, which are as either metal tolerant or accumulators when tested on both polluted soil and wastewater media [85–89]. Others include Typha domingensis and Phragmites australis [2]. According to Carvalho and Martin [90], Typha

Table 1. Examples of phytoremediation studies using species belong to Qatari flora and/or their relatives.

domingensis remediated heavy metals from industrial waste water and solution cultures; similarly, members of the halophytes plant family Brassicaceae were reported to be important phytoremediation agents [3, 59, 91] and the tree plant Prosopis juliflora exhibited phytoremediation of heavy metals potential [92, 93]. Additionally, such as other plants, such as Phragmites australis that were previously shown to clean petroleum-polluted soils may be good candidates for the phytoremediation of typical oil and gas produced wastewater [94], others with similar potentials includes Medicago such as Medicago sativa and Glycine max which also demonstrated strong petroleum polluted soil phytoremediation activity [95, 96]. Some examples of other species tested for phytoremediation studies and their metal uptake capacity are summarized in Table 1 above.

### 4. Conclusion

compared and there was a higher accumulation of the extrinsic subunit of photosystem II protein, which led to its stabilization in the more metal tolerant variant as against the less tolerant accession. In addition to GST and cytochrome P450 earlier mentioned, other proteins such as aspartate aminotransferase and thioredoxin are commonly found, and linked to the sequestration of xenobiotics including metals. GSTs have particularly been demonstrated to be up regulated in many other living species including bacteria and fungi treated with metals like Zn, Cu and Cd [78]; hence GST were suggested to confer resistance to toxic genes in these cells.

In plants, the root tissue is the first to be exposed to all potential toxicants whether in the soil or surface water and hence serve as the gateway route through which they can subsequently be translocated to other tissue parts. Plants diversity as to hyperaccumulators and nonhyperaccumulators exist, this is due to the fact that, while some species bear the complete mechanism of enhanced metal uptake and eventual translocation, others have limited sequestration capacity in their root vacuoles [79]. In non-hyperaccumulators plant roots, Zn transporters are only detectable in the absence of Zn, whereas in hyperaccumulators, there is constitutive expression of these proteins such as ZT1 even in Zn deficient condition [80, 81]. In T. caerulescens, the iron transporter protein IRT1 was found to be involved in Zn and Cd uptake [82], similarly, root proteome study of this hyperaccumulator and A. lesbiacum were conducted by Tuomainen, Tervahauta [83] to evaluate peptides involved in Zn and Cd hyperaccumulation. In these studies, various classes of proteins were identified, their availability and or abundance varies relative to metal exposure and accessions. As in the case with similar studies on shoot proteome of hyperaccumulators, ROS scavenging proteins were more abundant in the more metal tolerant accessions compared to the less metal tolerant species. It was concluded that the changes in the enzyme, superoxide dismutase (SOD) availability upon

which Zn depends in the different accessions may be linked to ROS increase.

An important organelle, cell wall, in the plant root is also affected by its exposure to heavy metal stress. The putative protein, glycosyl hydrolase family 18, involved in the formation of cell wall structure was shown to be regulated in accordance to treatment conditions and accession. These proteins, which are particularly known to be involved in cell wall expansion, differ in terms of abundance between the root proteome of two accessions, which in turn also affect the capacity of metals uptake; higher Ni and Cd accumulation was observed in the variant with more protein abundance [13]. Despite the recent advancement in proteomics technology, root protein transporters are yet to be differentially identified. Indeed, this is in agreement with transcriptomics studies, with analyzed data suggesting the constitutive

expression of metal genes transporters in plant metal hyperaccumulators [13, 81, 84].

Some studies demonstrated the potential of several Qatari plants as good phytoremediation candidates, many among which are heavy metal hyperaccumulators. Examples includes species belonging to the genus Zygophyllum, which are as either metal tolerant or accumulators when tested on both polluted soil and wastewater media [85–89]. Others include Typha domingensis and Phragmites australis [2]. According to Carvalho and Martin [90], Typha

3.2. The halophytes of Qatar are promising heavy metal hyperaccumulators

3.1.2. The root proteome

208 Heavy Metals

The accumulation of trace and heavy metals in the environment present a great risk to biota health. These contaminants are implicated in a wide range of human diseases and various long-term negative environmental consequences, thereby endangering overall sustainable development initiatives worldwide. Many conventional treatment strategies are widely practiced for the remediation of these contaminants. However, traditional remediation processes have many disadvantages, from complicating environmental pollution to high operational cost among others. Phytoremediation is one of the most promising alternatives in this regard, and laboratory experiments have demonstrated the capacity of hundreds of plants species to remediate different heavy metal contaminants. However, there still exist limitations in the application of this emerging technology. This may be linked to exposure to other stress factors in field conditions, and especially in extreme environments, which could significantly affect physiological function and general growth. An example is above the ground biomass accumulation, a key requirement for plants that is critical to phytoremediation success. The identification and selection of plants with higher metal uptake capacity or hyperaccumulators, even in the presence of other stress condition is therefore the objective of many phytoremediation studies in recent times. Additionally, our limited understanding on the molecular mechanism of heavy metal remediation, such as the exact role of transporter proteins is compounding progress in this area. However, it is obvious that several stress response molecules are key to the tolerance and or accumulation of heavy metal contaminants by potential phytoremediators. The halophytes are famous for their adaptation to stress environmental conditions, and hence could be the most suitable candidates in the search for appropriate heavy metal hyperaccumulators and consequent elucidation of mechanism of uptake. Indeed, these are significant steps essential to improving the efficiency of phytoremediation for large scale, field and industrial applications.

[3] Lone MI et al. Phytoremediation of heavy metal polluted soils and water: Progresses and

Phytoremediation: Halophytes as Promising Heavy Metal Hyperaccumulators

http://dx.doi.org/10.5772/intechopen.73879

211

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[13] Visioli G, Marmiroli N. The proteomics of heavy metal hyperaccumulation by plants.

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### Acknowledgements

This book chapter was financially supported for publication by Qatar National Library (a member of The Qatar Foundation). The statements made herein are solely the responsibility of the authors. The authors wish to acknowledge the funding support of Academic Research Office at Qatar University for the grant (QUST-CAS-SPR-2017-2033).

### Conflict of interest

The authors wish to declare no conflict of interest.

### Author details

Kamal Usman, Mohammad A. Al-Ghouti and Mohammed H. Abu-Dieyeh\*

\*Address all correspondence to<EMAIL_ADDRESS>
Department of Biological and Environmental Sciences, College of Arts and Sciences, Qatar University, Doha, Qatar

### References

[3] Lone MI et al. Phytoremediation of heavy metal polluted soils and water: Progresses and perspectives. Journal of Zhejiang University. Science. B. 2008;9(3):210-220

physiological function and general growth. An example is above the ground biomass accumulation, a key requirement for plants that is critical to phytoremediation success. The identification and selection of plants with higher metal uptake capacity or hyperaccumulators, even in the presence of other stress condition is therefore the objective of many phytoremediation studies in recent times. Additionally, our limited understanding on the molecular mechanism of heavy metal remediation, such as the exact role of transporter proteins is compounding progress in this area. However, it is obvious that several stress response molecules are key to the tolerance and or accumulation of heavy metal contaminants by potential phytoremediators. The halophytes are famous for their adaptation to stress environmental conditions, and hence could be the most suitable candidates in the search for appropriate heavy metal hyperaccumulators and consequent elucidation of mechanism of uptake. Indeed, these are significant steps essential to improving the efficiency of phytoremediation for large scale, field and industrial applications.

This book chapter was financially supported for publication by Qatar National Library (a member of The Qatar Foundation). The statements made herein are solely the responsibility of the authors. The authors wish to acknowledge the funding support of Academic Research

Office at Qatar University for the grant (QUST-CAS-SPR-2017-2033).

Kamal Usman, Mohammad A. Al-Ghouti and Mohammed H. Abu-Dieyeh\*

Department of Biological and Environmental Sciences, College of Arts and Sciences, Qatar

[1] Lasat MM. Phytoextraction of toxic metals. Journal of Environmental Quality. 2002;

[2] Yasseen BT, Al-Thani RF. Ecophysiology of wild plants and conservation perspectives in the State of Qatar. In: Stoytcheva M, Zlatev R, editor. Agricultural Chemistry. Croatia:

The authors wish to declare no conflict of interest.

\*Address all correspondence to<EMAIL_ADDRESS>
InTech; 2013. pp. 210. ISBN: 978-953-51-1026-2

Acknowledgements

210 Heavy Metals

Conflict of interest

Author details

University, Doha, Qatar

31(1):109-120

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**Chapter 13**

Provisional chapter

**Assessment of Heavy Metals in Landfill Leachate: A**

DOI: 10.5772/intechopen.74009

Landfilling of solid wastes has gained increasing acceptance due to the ease of disposal. However, such activity has consequences if the landfill site is not designed according to specification or does not have a leachate liner and collection system. Leachate possesses potential risk to surface and groundwater aquifer within the area surrounding the landfill site. The aim of this chapter is to assess the physicochemical parameters and heavy metal levels in leachate generated from a periurban landfill site situated in Thohoyandou, Limpopo Province, South Africa. Physicochemical parameters were measured onsite using standard methods, while heavy metals were analyzed with flame atomic absorption spectrometer (FAAS) after nitric acid digestion. pH, conductivity and turbidity values ranged from 6.97 to 7.68, 426 to 2288 μS/cm and 12.78 to 295.5 NTU, respectively. Most levels of the determined heavy metals exceeded the effluent discharge guideline limit of South African Department of Water Affairs. This could potentially spike their levels in surface and groundwater. Adequate measures should be put in place to manage the leachate generated from

Keywords: groundwater, heavy metals, landfill, leachate, potential risk, surface water

Solid waste management is of global concern in both developing and developed countries. Despite much awareness aimed at reducing the waste generated due to anthropogenic activities,

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Assessment of Heavy Metals in Landfill Leachate:

**Case Study of Thohoyandou Landfill, Limpopo**

A Case Study of Thohoyandou Landfill, Limpopo

**Province, South Africa**

Province, South Africa

http://dx.doi.org/10.5772/intechopen.74009

John O. Odiyo

John O. Odiyo

Abstract

landfill sites.

1.1. General background

1. Introduction

Joshua N. Edokpayi, Olatunde S. Durowoju and

Joshua N. Edokpayi, Olatunde S. Durowoju and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

#### **Assessment of Heavy Metals in Landfill Leachate: A Case Study of Thohoyandou Landfill, Limpopo Province, South Africa** Assessment of Heavy Metals in Landfill Leachate: A Case Study of Thohoyandou Landfill, Limpopo Province, South Africa

DOI: 10.5772/intechopen.74009

Joshua N. Edokpayi, Olatunde S. Durowoju and John O. Odiyo Joshua N. Edokpayi, Olatunde S. Durowoju and John O. Odiyo

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.74009

#### Abstract

Landfilling of solid wastes has gained increasing acceptance due to the ease of disposal. However, such activity has consequences if the landfill site is not designed according to specification or does not have a leachate liner and collection system. Leachate possesses potential risk to surface and groundwater aquifer within the area surrounding the landfill site. The aim of this chapter is to assess the physicochemical parameters and heavy metal levels in leachate generated from a periurban landfill site situated in Thohoyandou, Limpopo Province, South Africa. Physicochemical parameters were measured onsite using standard methods, while heavy metals were analyzed with flame atomic absorption spectrometer (FAAS) after nitric acid digestion. pH, conductivity and turbidity values ranged from 6.97 to 7.68, 426 to 2288 μS/cm and 12.78 to 295.5 NTU, respectively. Most levels of the determined heavy metals exceeded the effluent discharge guideline limit of South African Department of Water Affairs. This could potentially spike their levels in surface and groundwater. Adequate measures should be put in place to manage the leachate generated from landfill sites.

Keywords: groundwater, heavy metals, landfill, leachate, potential risk, surface water

### 1. Introduction

#### 1.1. General background

Solid waste management is of global concern in both developing and developed countries. Despite much awareness aimed at reducing the waste generated due to anthropogenic activities,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

there has been an increase in solid waste generation throughout the world [1–3]. This could partly be due to increase in population, industrialization and urbanization. Different efforts geared toward effective management of solid waste due to the perceived adverse health and environmental impacts have been reported [4–6].

1.2. Principles of leachate formation

and health impacts.

Figure 1. Generalized pattern of leachate generation [21].

Leachate is formed due to the interaction between the waste in the landfill and water from soil moisture, precipitation, snow and other liquid waste disposed of in the landfill site. Leachate can also be formed as a result of chemical and biochemical processes within the landfill. There is nonuniform and intermittent percolation of moisture through the solid waste in a landfill as a result of leachate generation [5, 12, 13]. This consequently leads to the removal of soluble fractions from the waste into the leachate. A wide variety of contaminants such as heavy metals, some other persistent organic pollutants like flame retardants, polycyclic aromatic hydrocarbons (PAHs), phenols, polychlorinated biphenyls (PCBs), chlorinated alkyl phosphate, polyfluoroalkyl and other fluorinated chemicals, pharmaceuticals and personal care products, polybrominated diphenyl ethers (PBDEs), polychlorinated dibenzofurans (PCDFs), polychlorinated dibenzodioxins (PCDDs) and polychlorinated diphenyl ethers (PCDEs) are also collected into the leachate as it drains through the pile of waste in the landfill [14–20]. This makes leachate a wastewater of high strength

Assessment of Heavy Metals in Landfill Leachate: A Case Study of Thohoyandou Landfill, Limpopo Province…

http://dx.doi.org/10.5772/intechopen.74009

221

Leachate control should, therefore, be included in the design of solid waste management systems. The volume and flow rate of the leachates are dependent upon the percolation of water through the waste layers [5]. The transfer of contaminants into the leachate and biodegradation processes are affected by the flow patterns and velocity of the leachate. The general pattern of the generation of leachate in landfills was reported by Farquhar [21] as presented in Figure 1.

Percolation from the unsaturated zone into the saturated zone is thus possible, and this gives a pathway for groundwater contamination. Surface runoff through the landfill also provides a possible route to surface water contamination majorly during precipitation events. The generation of leachate from landfills if not properly managed can lead to several adverse environmental

because of the high levels of various classes of contaminants it contains.

Landfilling remains one of the most commonly used methods for solid waste management in most parts of the world. Its efficiency and safety coupled with cost make it the preferred method [7–9]. Several advances in landfill technology have been reported to enhance its suitability for solid waste management. Leachate is often generated from landfill processes due to the increasing presence of soil moisture and other favorable environmental factors [1, 5].

In most developing countries, the facility for leachate collection and treatment is often not part of the design of landfill sites. One of the adverse effects caused by solid waste disposal onto landfills is the contamination of surface and groundwater by leachate. The extent of such contamination depends on the quality of leachate generated from the landfill [4, 7]. The composition of a landfill waste varies from place to place and depends on a number of factors. Some of them include the location of the site, the socioeconomic status of the people the landfill is serving, the technology used in the landfill and the age of the waste generated including prevailing environmental factors such as weather conditions of the landfill site, all of which influences the composition of leachate [1, 4, 7].

Blight and Fourie [10] reported the variation of the solid waste component disposed in landfill sites (Table 1). This ranges from simple organic and inorganic materials to complex ones.

Traditionally, in most developing countries, the segregation of hazardous waste from nonhazardous waste before disposal into a landfill site is uncommon. Edokpayi et al. [11] reported the disposal of several household hazardous waste into sanitary landfill sites. When both kinds of wastes are thus disposed of, the leachate quality will be poorer than if the hazardous waste was pretreated before landfilling or treated using other available methods.

\*Building rubble landfilled, in which sand cover is included.

\*\*Building rubble not landfilled, in which cover soil is not included.

+ Scavenged before landfilling.

Table 1. Percentage of solid waste composition from different parts of the world [10].

### 1.2. Principles of leachate formation

there has been an increase in solid waste generation throughout the world [1–3]. This could partly be due to increase in population, industrialization and urbanization. Different efforts geared toward effective management of solid waste due to the perceived adverse health and

Landfilling remains one of the most commonly used methods for solid waste management in most parts of the world. Its efficiency and safety coupled with cost make it the preferred method [7–9]. Several advances in landfill technology have been reported to enhance its suitability for solid waste management. Leachate is often generated from landfill processes due to the increasing presence of soil moisture and other favorable environmental factors [1, 5]. In most developing countries, the facility for leachate collection and treatment is often not part of the design of landfill sites. One of the adverse effects caused by solid waste disposal onto landfills is the contamination of surface and groundwater by leachate. The extent of such contamination depends on the quality of leachate generated from the landfill [4, 7]. The composition of a landfill waste varies from place to place and depends on a number of factors. Some of them include the location of the site, the socioeconomic status of the people the landfill is serving, the technology used in the landfill and the age of the waste generated including prevailing environmental factors such as weather conditions of the landfill site, all of which

Blight and Fourie [10] reported the variation of the solid waste component disposed in landfill sites (Table 1). This ranges from simple organic and inorganic materials to complex ones.

Traditionally, in most developing countries, the segregation of hazardous waste from nonhazardous waste before disposal into a landfill site is uncommon. Edokpayi et al. [11] reported the disposal of several household hazardous waste into sanitary landfill sites. When both kinds of wastes are thus disposed of, the leachate quality will be poorer than if the hazardous waste

Ash, soil, building rubble, other 8–14 18 39\* 1\*\* 3 16 41 16 Paper and cardboard 34–44 29 19 23 12 14 9 6 Plastic 8–10 7 7\* 13 5 7 1+ 12 Metals 11–13 8 2+ 10 6 4 1+ 8 Glass 4–9 10 1+ 8 6 3 1+ 9 Organic, putrescible 12–22 25 32 45 68 56 47 49

USA UK Cape Town Johannesburg Gaborone Lima Delhi Abu Dhabi

was pretreated before landfilling or treated using other available methods.

environmental impacts have been reported [4–6].

220 Heavy Metals

influences the composition of leachate [1, 4, 7].

Components % Composition by undried mass

\*Building rubble landfilled, in which sand cover is included. \*\*Building rubble not landfilled, in which cover soil is not included.

Table 1. Percentage of solid waste composition from different parts of the world [10].

+

Scavenged before landfilling.

Leachate is formed due to the interaction between the waste in the landfill and water from soil moisture, precipitation, snow and other liquid waste disposed of in the landfill site. Leachate can also be formed as a result of chemical and biochemical processes within the landfill. There is nonuniform and intermittent percolation of moisture through the solid waste in a landfill as a result of leachate generation [5, 12, 13]. This consequently leads to the removal of soluble fractions from the waste into the leachate. A wide variety of contaminants such as heavy metals, some other persistent organic pollutants like flame retardants, polycyclic aromatic hydrocarbons (PAHs), phenols, polychlorinated biphenyls (PCBs), chlorinated alkyl phosphate, polyfluoroalkyl and other fluorinated chemicals, pharmaceuticals and personal care products, polybrominated diphenyl ethers (PBDEs), polychlorinated dibenzofurans (PCDFs), polychlorinated dibenzodioxins (PCDDs) and polychlorinated diphenyl ethers (PCDEs) are also collected into the leachate as it drains through the pile of waste in the landfill [14–20]. This makes leachate a wastewater of high strength because of the high levels of various classes of contaminants it contains.

Leachate control should, therefore, be included in the design of solid waste management systems. The volume and flow rate of the leachates are dependent upon the percolation of water through the waste layers [5]. The transfer of contaminants into the leachate and biodegradation processes are affected by the flow patterns and velocity of the leachate. The general pattern of the generation of leachate in landfills was reported by Farquhar [21] as presented in Figure 1.

Percolation from the unsaturated zone into the saturated zone is thus possible, and this gives a pathway for groundwater contamination. Surface runoff through the landfill also provides a possible route to surface water contamination majorly during precipitation events. The generation of leachate from landfills if not properly managed can lead to several adverse environmental and health impacts.

Figure 1. Generalized pattern of leachate generation [21].

#### 1.3. Management and treatment of leachates

Leachate management is possible if the design of the landfill permits adequate lining, collection and storage systems. Landfill liners should be a part of the initial design of the landfill site. These liners are crucial as they act as a barrier preventing the leachate generated from the landfill to drain into the environment or groundwater aquifer [13, 22]. Various materials used as leachate liners include natural clays, geomembranes (constructed from various plastic materials such as polyvinyl chloride (PVC) and high-density polyethene (HDPE)), geotextiles, geonet and geosynthetic clay liners [13, 22–25]. The lining system can be made from a single material such as clay or a combination of clay and other materials. Basically, there are three types of lining systems which are single, composite or double lining systems. Various factors should be considered when selecting a lining system for a landfill as some lining materials can be easily degraded by chemicals in the leachate.

In developing countries, the lining of landfills and systems for collection and treatment of leachate are often found in some major cities, while same facilities in other cities, periurban or rural areas do not have a provision for leachate management. Collection of leachate in landfills is important to prevent it from accumulating up to the quantity that it can spill over, causing pollution in waterways and water courses [5, 12]. Poverty, lack of required expertise, corruption and misappropriation of funds could be some of the reasons that have limited adequate leachate management in the developing world.

Different methods have been reported for the management of leachate in the different parts of the world. The simpler method is to incorporate the leachate as an influent in a wastewater treatment facility [26]. This method is cheap since there will be no need for the construction of a new facility for the treatment. Conveyance pipe from the landfill to the wastewater treatment plant is sufficient, provided that the distance between both facilities is in close proximity. Depending on the leachate quality, pretreatment may be required before combining it with the influent of the municipal wastewater treatment works. However, in some cases, when a large quantity of leachate is generated on a daily basis, or the distance between both facilities is far apart, then onsite treatment of leachate will be the preferred option or the reticulation of the leachate back into the landfill. Several physical (evaporation, floatation, air stripping, filtration and membrane processes), chemical (coagulation and precipitation, adsorption, ion exchange, chemical oxidation) and biological methods (the use of ponds in various aerobic and anaerobic modes) have been reported for leachate treatment [26–30]. Cost, expertise, the age of the landfill, estimated volume of leachate generated, climatic condition and efficiency of the method often determine the treatment method of choice. Treatment of leachate using biological methods is more common in South Africa [26].

#### 1.4. Study area

The landfill site with geographical coordinates 23� 0'12.3" S and 30� 28'0.16<sup>00</sup> E is located in Thulamela Municipality of Limpopo Province, South Africa (Figure 2a).

Figure 2. (a) Map of the study area. (b) Some pictures from Thohoyandou landfill site.

Assessment of Heavy Metals in Landfill Leachate: A Case Study of Thohoyandou Landfill, Limpopo Province…

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Assessment of Heavy Metals in Landfill Leachate: A Case Study of Thohoyandou Landfill, Limpopo Province… http://dx.doi.org/10.5772/intechopen.74009 223

1.3. Management and treatment of leachates

222 Heavy Metals

be easily degraded by chemicals in the leachate.

leachate management in the developing world.

methods is more common in South Africa [26].

The landfill site with geographical coordinates 23�

Thulamela Municipality of Limpopo Province, South Africa (Figure 2a).

1.4. Study area

Leachate management is possible if the design of the landfill permits adequate lining, collection and storage systems. Landfill liners should be a part of the initial design of the landfill site. These liners are crucial as they act as a barrier preventing the leachate generated from the landfill to drain into the environment or groundwater aquifer [13, 22]. Various materials used as leachate liners include natural clays, geomembranes (constructed from various plastic materials such as polyvinyl chloride (PVC) and high-density polyethene (HDPE)), geotextiles, geonet and geosynthetic clay liners [13, 22–25]. The lining system can be made from a single material such as clay or a combination of clay and other materials. Basically, there are three types of lining systems which are single, composite or double lining systems. Various factors should be considered when selecting a lining system for a landfill as some lining materials can

In developing countries, the lining of landfills and systems for collection and treatment of leachate are often found in some major cities, while same facilities in other cities, periurban or rural areas do not have a provision for leachate management. Collection of leachate in landfills is important to prevent it from accumulating up to the quantity that it can spill over, causing pollution in waterways and water courses [5, 12]. Poverty, lack of required expertise, corruption and misappropriation of funds could be some of the reasons that have limited adequate

Different methods have been reported for the management of leachate in the different parts of the world. The simpler method is to incorporate the leachate as an influent in a wastewater treatment facility [26]. This method is cheap since there will be no need for the construction of a new facility for the treatment. Conveyance pipe from the landfill to the wastewater treatment plant is sufficient, provided that the distance between both facilities is in close proximity. Depending on the leachate quality, pretreatment may be required before combining it with the influent of the municipal wastewater treatment works. However, in some cases, when a large quantity of leachate is generated on a daily basis, or the distance between both facilities is far apart, then onsite treatment of leachate will be the preferred option or the reticulation of the leachate back into the landfill. Several physical (evaporation, floatation, air stripping, filtration and membrane processes), chemical (coagulation and precipitation, adsorption, ion exchange, chemical oxidation) and biological methods (the use of ponds in various aerobic and anaerobic modes) have been reported for leachate treatment [26–30]. Cost, expertise, the age of the landfill, estimated volume of leachate generated, climatic condition and efficiency of the method often determine the treatment method of choice. Treatment of leachate using biological

0'12.3" S and 30�

28'0.16<sup>00</sup> E is located in

Figure 2. (a) Map of the study area. (b) Some pictures from Thohoyandou landfill site.

The site is located 180 km from the provincial city of Polokwane. In 2014, the facility received 210,000 tons of waste. Thohoyandou is a semiarid region with variations in temperature between 12 and 22C in the dry season (winter) and between 20 and 40C in the wet season (summer) [31]. Precipitation also varies between 340 and 2000 mm with an average rainfall of 800 mm. The region is classified as a humid subtropical dry forest biozone [32]. The landfill site is very close to the municipal wastewater treatment plant. Common surface water sources in the region include the Dzindi, Luvhuvhu and Mvudi Rivers. Thohoyandou landfill currently receives all kinds of sanitary wastes from the periurban center, which is Thohoyandou and other villages in the suburb. At the time of this study, there was no leachate collection and treatment system on the site. Residential areas are located around the landfill. Some other major land use around the landfill includes subsistence agriculture, schools and wastewater treatment plant. Some pictures taken from the landfill are presented in Figure 2b.

3. Results and discussion

Table 2. Physicochemical parameters of leachate samples.

(South Africa)

Parameters Thohoyandou landfill

NR indicates values that were not reported.

alkaline pH values.

The pHs of the leachate from all sites of sampling (Table 2) were found within the acceptable range of wastewater discharge onto the water resource by the South African Department of Water Affairs [33]. An average leachate pH of 7.39 was determined in this study which is suitable for methanogenic bacteria [34]. Various average pH values below and above this value have been reported for leachate in different parts of the developing world (Table 3). pH usually affects the speciation of metals in solutions with most metals easily speciated to their toxic forms in acidic pH below the value of 4, while some precipitate out of the solution at

Assessment of Heavy Metals in Landfill Leachate: A Case Study of Thohoyandou Landfill, Limpopo Province…

Samples pH Conductivity (μS/m) Turbidity (NTU)

Alexandria landfill (Egypt) [34]

Semur landfill (Indian) [35]

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225

El-Jadida landfill (Morocco) [36]

L1 7.60 486 22.56 L2 7.47 485 28.83 L3 7.68 2288 295.5 L4 7.42 604 25.43 L5 7.20 721 33.15 L6 6.97 426 12.78

pH 7.39 8.9 6.9 8.8 Conductivity (μS/cm) 835 30,725 NR 26,000 Turbidity (NTU) 69.71 NR NR NR Na (mg/L) 33.88 364 462 NR K(mg/L) 22.94 5400 1241 NR Ca (mg/L) 46.27 8.4 NR 190 Mg (mg/L) 16.58 302.5 NR 235

Generally, the presence of inorganic charged particles such as carbonates, chlorides, sulfates and bicarbonate ions in water is often estimated in terms of the conductivity of the water. Conductivity oftentimes is also used as a surrogate for total dissolved solids [37]. Table 2 shows the levels of conductivity determined from all the sampling sites. This level complied

Table 3. Comparison of the average physicochemical leachate characteristics in some developing countries.

### 2. Materials and methods

### 2.1. Sampling

Six leachate samples were collected during the wet season of 2014 from various places within the premises of the landfill site. The samples were collected in a precleaned 1-L polyethylene sample container. The containers were washed with livid detergent and soaked with 1% nitric acid. Subsequently, the sample containers were rinsed with deionized water. Physicochemical parameters such as pH, conductivity and turbidity were measured in situ, while samples for the determination of heavy metals were preserved using concentrated nitric acid. The samples were transported on ice to Hydrology and Water Resources Department of the University of Venda. Four major metals (Calcium (Ca), Potassium (K), Magnesium (Mg) and Sodium (Na)) and 10 heavy metals (Aluminum (Al), Cadmium (Cd), Chromium (Cr), Cobalt (Co), Copper (Cu), Iron (Fe), Lead (Pb), Manganese (Mn), Nickel (Ni) and Zinc (Zn)) were selected and analyzed due to their availability in landfill leachate. The samples were preserved at 4C until further analysis.

#### 2.2. Instrumentation

pH and conductivity multimeter supplied by Fischer Thermoscientific was used for field measurements of pH and conductivity, respectively, while a turbidimeter supplied by HANA was employed to determine the turbidity of the samples. The measuring devices were calibrated following the protocol approved by the manufacturer. Flame atomic absorption spectrometer (FAAS) supplied by Perkin Elmer was used for the analysis of major and heavy metal content in the leachate samples. Calibration standards were prepared from AAS grade reagent for all the metals of interest.

### 3. Results and discussion

The site is located 180 km from the provincial city of Polokwane. In 2014, the facility received 210,000 tons of waste. Thohoyandou is a semiarid region with variations in temperature between 12 and 22C in the dry season (winter) and between 20 and 40C in the wet season (summer) [31]. Precipitation also varies between 340 and 2000 mm with an average rainfall of 800 mm. The region is classified as a humid subtropical dry forest biozone [32]. The landfill site is very close to the municipal wastewater treatment plant. Common surface water sources in the region include the Dzindi, Luvhuvhu and Mvudi Rivers. Thohoyandou landfill currently receives all kinds of sanitary wastes from the periurban center, which is Thohoyandou and other villages in the suburb. At the time of this study, there was no leachate collection and treatment system on the site. Residential areas are located around the landfill. Some other major land use around the landfill includes subsistence agriculture, schools and wastewater treatment plant. Some pictures taken from the landfill are presented

Six leachate samples were collected during the wet season of 2014 from various places within the premises of the landfill site. The samples were collected in a precleaned 1-L polyethylene sample container. The containers were washed with livid detergent and soaked with 1% nitric acid. Subsequently, the sample containers were rinsed with deionized water. Physicochemical parameters such as pH, conductivity and turbidity were measured in situ, while samples for the determination of heavy metals were preserved using concentrated nitric acid. The samples were transported on ice to Hydrology and Water Resources Department of the University of Venda. Four major metals (Calcium (Ca), Potassium (K), Magnesium (Mg) and Sodium (Na)) and 10 heavy metals (Aluminum (Al), Cadmium (Cd), Chromium (Cr), Cobalt (Co), Copper (Cu), Iron (Fe), Lead (Pb), Manganese (Mn), Nickel (Ni) and Zinc (Zn)) were selected and analyzed due to their availability in landfill leachate. The samples were preserved at 4C

pH and conductivity multimeter supplied by Fischer Thermoscientific was used for field measurements of pH and conductivity, respectively, while a turbidimeter supplied by HANA was employed to determine the turbidity of the samples. The measuring devices were calibrated following the protocol approved by the manufacturer. Flame atomic absorption spectrometer (FAAS) supplied by Perkin Elmer was used for the analysis of major and heavy metal content in the leachate samples. Calibration standards were prepared from AAS grade reagent

in Figure 2b.

224 Heavy Metals

2.1. Sampling

until further analysis.

2.2. Instrumentation

for all the metals of interest.

2. Materials and methods

The pHs of the leachate from all sites of sampling (Table 2) were found within the acceptable range of wastewater discharge onto the water resource by the South African Department of Water Affairs [33]. An average leachate pH of 7.39 was determined in this study which is suitable for methanogenic bacteria [34]. Various average pH values below and above this value have been reported for leachate in different parts of the developing world (Table 3). pH usually affects the speciation of metals in solutions with most metals easily speciated to their toxic forms in acidic pH below the value of 4, while some precipitate out of the solution at alkaline pH values.

Table 2. Physicochemical parameters of leachate samples.

Table 3. Comparison of the average physicochemical leachate characteristics in some developing countries.

Generally, the presence of inorganic charged particles such as carbonates, chlorides, sulfates and bicarbonate ions in water is often estimated in terms of the conductivity of the water. Conductivity oftentimes is also used as a surrogate for total dissolved solids [37]. Table 2 shows the levels of conductivity determined from all the sampling sites. This level complied with the DWA wastewater discharge limit [33] and was much lower than the levels reported in Alexandria landfill leachate in Egypt (Table 3), Columbia (22,000 μS/cm) and Morocco (26,000 μS/cm) [34, 36, 38]. However, Rahim et al. [39] reported a lower conductivity level of 31.68 μS/cm for landfill leachate in Malaysia.

The turbidity of the leachate samples obtained was very high. Liquids with high turbidity often indicate the likelihood of silt, clay, fine organic matter and microscopic organisms that are suspended in the water. Site three recorded a very high turbidity level, much higher than the levels from other sites in the landfill.

Figure 3 shows the levels of major metals (Na, K, Mg and Ca) in the leachate of Thohoyandou landfill. The levels found are not higher than what can be disposed of into water bodies. The only concern is the continuous migration of salt-rich leachate into surface and groundwater sources which could increase their levels in the river and groundwater aquifers. Various sequences of major metals' occurrence in leachate varied greatly from one location to the other. The sequence for major metals determined in this study follows the trend: Ca > Na > K > Mg (Table 3), while at Alexandria landfill in Egypt, the sequence is K > Na > Mg > Ca [34]. Level of Mg is higher than Ca in the leachate of all the landfills reported except Thohoyandou landfill. Although rock type in groundwater aquifers contributes significantly to its salinity level, continuous percolation of metal-rich leachate may also contribute to increased groundwater salinity.

The levels of heavy metals determined in this study are presented in Figures 4 and 5. Variable levels of heavy metals were determined in the landfill site. On average, Fe, Mn and Al are present in higher concentrations than the other metals. A possible reason for the high level of these metals could be due to their relative abundance on Earth. Previous studies have shown that Fe and Al are present in higher concentrations in river water, soils and sediments around Thohoyandou when compared to other metals [32, 37, 40]. The relatively low levels of the

more hazardous heavy metals could suggest that the waste generated from this region is not largely from a highly industrialized area. It is, however, noteworthy to mention that most of the heavy metals were present in concentrations higher than the normal wastewater discharge guideline values except for Zn and Cr. Ni, Al and Co does not have guideline values for their discharge. Table 4 shows the comparison of average heavy metal concentration in leachate from landfills in some developing countries. Fe recorded the highest level from all the countries except in Jebel Chakir, Landfill of Tunisia, where Fe was not part of the metals determined [34, 36, 37, 41, 42]. Mn and Zn were also part of the metals that generally existed in high

Figure 4. Levels of Co, Cr, Pb, Cu, Ni and Zn in different sampling sites of Thohoyandou landfill.

Assessment of Heavy Metals in Landfill Leachate: A Case Study of Thohoyandou Landfill, Limpopo Province…

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Figure 5. Levels of Fe, Mn and Al in different sampling sites of Thohoyandou landfill.

concentration.

Figure 3. Concentrations of major metals in leachate samples.

Assessment of Heavy Metals in Landfill Leachate: A Case Study of Thohoyandou Landfill, Limpopo Province… http://dx.doi.org/10.5772/intechopen.74009 227

Figure 4. Levels of Co, Cr, Pb, Cu, Ni and Zn in different sampling sites of Thohoyandou landfill.

with the DWA wastewater discharge limit [33] and was much lower than the levels reported in Alexandria landfill leachate in Egypt (Table 3), Columbia (22,000 μS/cm) and Morocco (26,000 μS/cm) [34, 36, 38]. However, Rahim et al. [39] reported a lower conductivity level of

The turbidity of the leachate samples obtained was very high. Liquids with high turbidity often indicate the likelihood of silt, clay, fine organic matter and microscopic organisms that are suspended in the water. Site three recorded a very high turbidity level, much higher than

Figure 3 shows the levels of major metals (Na, K, Mg and Ca) in the leachate of Thohoyandou landfill. The levels found are not higher than what can be disposed of into water bodies. The only concern is the continuous migration of salt-rich leachate into surface and groundwater sources which could increase their levels in the river and groundwater aquifers. Various sequences of major metals' occurrence in leachate varied greatly from one location to the other. The sequence for major metals determined in this study follows the trend: Ca > Na > K > Mg (Table 3), while at Alexandria landfill in Egypt, the sequence is K > Na > Mg > Ca [34]. Level of Mg is higher than Ca in the leachate of all the landfills reported except Thohoyandou landfill. Although rock type in groundwater aquifers contributes significantly to its salinity level, continuous percolation of metal-rich leachate may also contribute to increased groundwater

The levels of heavy metals determined in this study are presented in Figures 4 and 5. Variable levels of heavy metals were determined in the landfill site. On average, Fe, Mn and Al are present in higher concentrations than the other metals. A possible reason for the high level of these metals could be due to their relative abundance on Earth. Previous studies have shown that Fe and Al are present in higher concentrations in river water, soils and sediments around Thohoyandou when compared to other metals [32, 37, 40]. The relatively low levels of the

31.68 μS/cm for landfill leachate in Malaysia.

the levels from other sites in the landfill.

Figure 3. Concentrations of major metals in leachate samples.

salinity.

226 Heavy Metals

Figure 5. Levels of Fe, Mn and Al in different sampling sites of Thohoyandou landfill.

more hazardous heavy metals could suggest that the waste generated from this region is not largely from a highly industrialized area. It is, however, noteworthy to mention that most of the heavy metals were present in concentrations higher than the normal wastewater discharge guideline values except for Zn and Cr. Ni, Al and Co does not have guideline values for their discharge. Table 4 shows the comparison of average heavy metal concentration in leachate from landfills in some developing countries. Fe recorded the highest level from all the countries except in Jebel Chakir, Landfill of Tunisia, where Fe was not part of the metals determined [34, 36, 37, 41, 42]. Mn and Zn were also part of the metals that generally existed in high concentration.

References

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Table 4. Comparison of the average heavy metal characteristics in some developing countries.

### 4. Conclusion

This chapter has provided the much-needed baseline data of leachate for Thohoyandou landfill. The levels of Fe and Mn determined in the leachate of this study were very high as they exceeded the guideline value for wastewater discharge. Al concentration was also high. The most interesting result indicated that leachate from Thohoyandou landfill is of comparable quality to leachate from other developing countries.

### Acknowledgements

The University of Venda is appreciated for providing the facilities to conduct this study.

### Author details

Joshua N. Edokpayi\*, Olatunde S. Durowoju and John O. Odiyo

\*Address all correspondence to<EMAIL_ADDRESS>
Department of Hydrology and Water Resources, University of Venda, South Africa

## References

4. Conclusion

NR indicates values that were not reported.

Heavy metals (mg/L)

228 Heavy Metals

Thohoyandou landfill (South Africa)

Alexandria landfill (Egypt) [34] Semur landfill (Indian) [35]

Table 4. Comparison of the average heavy metal characteristics in some developing countries.