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Arsenic Removal from Drinking Water

Parvathy V Kumar, M.Tech Sem1 (Environmental Engg.) 1. Introduction Arsenic (atomic number 33) is ubiquitous and ranks 20th in natural abundance, comprising about 0.00005% of the earths crust, 14th in the seawater, and 12th in the human body. Its concentration in most rocks ranges from 0.5 to 2.5 mg/kg, though higher concentrations are found in finer grained argillaceous sediments and phosphorites. It is a silver-grey brittle crystalline solid with atomic weight 74.9; specific gravity 5.73, melting point 817 OC (at 28 atm), boiling point 613 OC and vapor pressure 1mm Hg at 372 OC [1]. Since its isolation in 1250 a.d. by Albertus Magnus, this element has been a continuous center of controversy. Arsenics history in science, medicine and technology has been overshadowed by its notoriety as a poison in homicides. Arsenic is viewed as being synonymous with toxicity. Arsenic oxide, termed white lace, was once a poisoners tool of choice because it was effective, ubiquitous, and had no taste. Napoleon was once thought to have been poisoned with arsenic while in exile at Elba in 1821. In turn, Rasputin routinely took arsenic to protect himself from poisoning. After 1836, when an English chemist named James Marsh developed a reliable technique for detecting arsenic in the bodily fluids of the deceased, arsenic poisoning began to wane. Not long after, agricultural applications of arsenic in pesticides and herbicides, as well as its use in pigment, became widespread. Arsenic was combined with lead to make lead arsenate, an early insecticide. Arsenic was an important component of Salvarsan, an early treatment for syphilis, and in embalming. CCA a compound of copper, chromate, and arsenate is widely used as a wood preservative. As2O3 was used as an insecticide, in rat poison, and as a weed killer. The introduction of organic pesticide alternatives (for example, DDT), coupled with environmental and human health concerns, led to the gradual discontinuation of many arsenic-based pesticides by the 1960s, but many areas contaminated by the practice remain. More than 245 minerals contain arsenic and although the ultimate source of arsenic is geological, human activities such as mining, the burning of fossil fuels, and pesticide application, also cause arsenic pollution. Arsenic exists in four oxidation states, +V (arsenate), +III (arsenite), 0 (arsenic), and III (arsine). In addition to arsenite, arsenate, and their methylated derivatives, there are fish arsenic (arsenobetaine, AB and arsenocholine, AC) and arsenosugar compounds of environmental interest [2]. The molecular formulae of organic arsenic compounds are given in Table 1.

Table 1: Some arsenic compounds of interest [2] Arsenic is mobilized by natural weathering reactions, biological activity, geochemical reactions, volcanic emissions and other anthropogenic activities. Soil erosion and leaching contribute to 612108 and 2380108 g/year of arsenic, respectively, in dissolved and suspended forms in the oceans. Most environmental arsenic problems are the result of mobilization under natural conditions. However, mining activities, combustion of fossil fuels, use of arsenic pesticides, herbicides, and crop desiccants and use of arsenic additives to livestock feed create additional impacts. 2. Arsenic Toxicity Arsenic is toxic to both plants and animals and inorganic arsenicals are proven carcinogens in humans. The toxicity of arsenic to human health ranges from skin lesions to cancer of the brain, liver, kidney, and stomach. A wide range of arsenic toxicity has been determined that depends on arsenic speciation. Generally inorganic arsenic species are more toxic than organic forms to living organisms, including humans and other animals. The oral LD50 for inorganic arsenic ranges from 15293 mg (As) kg1 and 11150 mg (As) kg1 bodyweight in rats and other laboratory animals respectively. Exposure to arsenic trioxide by ingestion of 7080 mg has been reported to be fatal for humans. Arsenite (As III) is usually more toxic than arsenate (As V). The toxicity of trivalent arsenic is related to its high affinity for the sulfhydryl groups of biomolecules such as glutathione (GSH) and lipoic acid and the cysteinyl residues of many enzymes. The formation of As(III)sulfur bonds results in various harmful effects by inhibiting the activities of enzymes such as glutathione reductase, glutathione peroxidases, thioredoxin reductase, and thioredoxin peroxidase [2]. It is generally accepted that pentavalent arsenicals do not directly bind to sulfhydryl groups to cause toxic effects. However, a recent study reported that sulfide-activated pentavalent arsenic could bind to the sulfhydryl group of GSH and thereby show distinct behavior and toxicity. The toxicity of DMMTAV is comparable to that of trivalent arsenicals. The toxicity of DMMTAV may be caused by the

production of reactive oxygen species (ROS) during its exposure, which may cause mutagenesis and DNA damage, initiating cancer [1]. Arsenic (III) is a hard acid and preferentially complexes with oxides and nitrogen. Conversely, arsenic (V) behaves like a soft acid, forming complexes with sulfides. Inorganic forms of arsenic most often exist in water supplies. Arsenic is uniquely sensitive to mobilization (pH 6.58.5) and under both oxidizing and reducing conditions among heavy metalloids. Two forms are common in natural waters: arsenite (AsO33) and arsenate (AsO43), referred to as arsenic(III) and arsenic(V). Pentavalent (+5) or arsenate species are AsO43, HAsO4 2, H2AsO4 while trivalent (+3) arsenates include As(OH)3, As(OH)4 , AsO2OH2 and AsO3 anaerobic environments such as groundwater [1]. Elemental arsenic is ordinarily a steel gray metal-like material that sometimes occurs naturally. However, arsenic is usually found in the environment combined with other elements such as oxygen, chlorine, and sulfur. Arsenic combined with these elements is called inorganic arsenic. Arsenic combined with carbon and hydrogen is referred to as organic arsenic. The organic forms are usually less toxic than the inorganic forms. Most inorganic and organic arsenic compounds are white or colorless powders that do not evaporate. They have no smell, and most have no special taste. Most cases of arsenic-induced toxicity in humans are due to exposure to inorganic arsenic, and there may be some differences in the potency of different chemical forms, e.g., arsenites tend to be more toxic than arsenates. Ingested arsenic is well absorbed across the gastrointestinal tract into the bloodstream and distributed to many organs. Health effects from ingesting arsenic are stomachache, nausea, vomiting and diarrhea, fatigue, abnormal heart rhythm, blood-vessel damage resulting in bruising, and impaired nerve function causing a pins and needles sensation in the hands and feet. The most characteristic effect of long-term oral exposure to inorganic arsenic is a pattern of skin changes. These include hyperpigmentation and hypopigmentation of the skin and the appearance of small corns or warts on the palms, soles, and torso. A small number of the corns may develop into skin cancer. Ingestion of arsenic also causes peripheral vascular diseases and is associated with carotid atherosclerosis. Ingesting inorganic arsenic has also been reported to increase the risk of cancer in the bladder, lungs, liver, and kidneys [3].

. Pentavalent species predominate and are

stable in oxygen rich aerobic environments. Trivalent arsenites predominate in moderately reducing

Table 2: Arsenic concentrations in different parts of the world [2]. 3. Arsenic Contamination Inorganic arsenic, the more toxic form, is found in groundwater, surface water, and many foods, such as rice, grains, and fish (although only a very small percentage of arsenic in fish is inorganic). On average, there is more exposure to inorganic arsenic from drinking water than from food [2]. Studies in countries where the population has had long-term exposure to arsenic in groundwater indicate that 1 in 10 people who drink water containing 500 mg of arsenic per litre may ultimately die from cancers caused by arsenic, including lung, bladder and skin cancers [4]. There are many reports of chronic arsenism resulting from drinking well water containing highly elevated levels of arsenic in endemic areas worldwide. The worst cases were located in Asia: Bangladesh, India, China, and Taiwan. Estimated number of affected population totaled more than 100 million. In these areas, the soil naturally contains high levels of arsenic which leaches into groundwater that is used for drinking water. Areas of Bangladesh have shallow aquifers composed of arsenic-containing sediment. Arsenic is released into groundwater when the oxygen levels in the aquifer become low and iron and manganese oxyhydroxides that bind arsenic dissolve and release it into the surrounding water. Arsenic concentrations in drinking water from four villages in the endemic area in Bangladesh ranged from 10 to 2,040 g/l. In West Bengal, India, it is estimated that more than one million persons are drinking arsenic-laced water. Analysis of 20,000 tube-well waters revealed that 62% have

arsenic at levels as high as 3,700 g/l [3]. Similarly high arsenic concentrations have been reported recently from the USA, China, Chile, Taiwan, Mexico, Argentina, Poland, Canada, Hungary, and Japan. Among 21 countries in different parts of the world affected by groundwater arsenic contamination, the largest population at risk is in Bangladesh followed by West Bengal in India [1]. Table 2 summarizes the arsenic concentration at different parts of the world. Incidentally, the contamination of groundwater by arsenic in Bangladesh is the largest poisoning of a population in history, with millions of people exposed [4]. 4. Standards and Guidelines Until recently, Arsenic was often not on the list of constituents in drinking water routinely analysed by national laboratories, water utilities and non-governmental organizations (NGOs) and so the body of information about the distribution of Arsenic in drinking water is not as well known as for many other drinking-water constituents. In recent years, it has become apparent that both the WHO guideline value and current national standards are quite frequently exceeded in drinking water sources, and often unexpectedly so. Indeed, arsenic is now recognised as the most serious inorganic contaminants in drinking water on a worldwide basis. The WHO provisional guideline of 10 ppb (0.01 mg/L) arsenic has been adopted as the drinking water standard. However, many countries have retained the earlier WHO guideline of 50 ppb (0.05 mg/L) as their standard or as an interim target including Bangladesh and China. Table 3 lists the drinking water standards followed in different countries.

Table 3: Maximum permissible arsenic content in drinking water in different countries.

5. Arsenic removal Most common methods for removing arsenic from drinking water are based on the following processes: 1. Precipitation processes, including coagulation/filtration, direct filtration, coagulation assisted microfiltration, enhanced coagulation, lime softening, and enhanced lime softening 2. Adsorptive processes, including adsorption onto activated alumina, activated carbon and iron/manganese oxide based or coated filter media 3. Ion exchange processes, specifically anion exchange 4. Membrane processes, including nano-filtration, reverse osmosis and electrodialysis. Arsenic removal involves far more than removing arsenic from water. It also requires disposal of waste solids (e.g. spent filter material, coagulant sludges) and residual fluids likely to contain high levels of arsenic and possibly other hazardous or radioactive (e.g. uranium, radon) constituents. If the water requires pretreatment (e.g. lowering of pH for adsorption processes, or raising it for limesoftening) the waste situation becomes more complicated. At the same time, most treatments tend to affect effluent levels of dissolved components other than arsenic - sometimes in a deleterious fashion. For example, removal of arsenic through ion exchange also tends to lower bicarbonate levels increasing the corrosivity of the effluent. Coagulation using Al(III) or Fe(III) salts can result in increased concentrations of these metals in the treated water, as well as the anions (e.g. sulfate) from the salts used in the process. Lastly, most arsenic treatment technologies work better for arsenate than arsenite, so a preoxidation step is often used.

5.1 Precipitation processes

Adsorption co-precipitation with hydrolysing metals such as Al3+ and Fe3+ is the most common treatment technique for removing arsenic from water. Sedimentation followed by rapid sand filtration or direct filtration or microfiltration is used to remove precipitate. Coagulation with iron and aluminium salts and lime softening is the most effective treatment process. To improve efficiency of this method, a priory oxidation of As (III) to As (V) is advisable. Hypochlorite and permanganate are commonly used for the oxidation. Atmospheric oxygen can also be used, but the reaction is very slow.

Water treatment with coagulants such as aluminium alum, Al2(SO4)3.18H2O, ferric chloride , FeCl3 and ferric sulfate Fe2(SO4)3.7H2O are effective in removing arsenic from water. In the coagulationflocculation process aluminium sulfate, or ferric chloride, or ferric sulfate is added and dissolved in water under efficient stirring for one to few minutes. Aluminium or ferric hydroxide micro-flocs are formed rapidly. The water is then gently stirred for few minutes for agglomeration of micro-flocs into larger easily settable flocs. During this flocculation process all kinds of microparticles and negatively charged ions are attached to the flocs by electrostatic attachment. Arsenic is also adsorbed onto coagualted flocs. As trivalent arsenic occurs in non-ionized form, it is not subject to significant removal. Oxidation of arsenic(III) to arsenic(V) is thus required as a pretreatment for efficient removal. This can be achieved by addition of bleaching powder (chlorine) or potassium permanganate. Arsenic adsorbed on aluminium hydroxide flocs as Al-As complex is removed by sedimentation. Filtration may be required to ensure complete removal of all flocs. Similar reactions take place in case of ferric chloride and ferric sulfate with the formation of Fe-As complex as end product which is removed by the process of sedimentation and filtration. 5.1.1 Oxidation Arsenic is present in groundwater in As(III) and As(V) forms in different proportions. Most treatment methods are effective in removing arsenic in pentavalent form and hence include an oxidation step as preteatment to convert arsenite to arsenate. Arsenite can be oxidized by oxygen, ozone, free chlorine, hypochlorite, permanganate, hydrogen peroxide and fulton's reagent but Atmospheric oxygen, hypochloride and permanganate are commonly used for oxidation in developing countries: H3AsO3 + O2 = H2AsO4 - + 2 H + H3AsO3 + HClO = HAsO4-- + Cl - + 3H+ 3H3AsO3 + 2KMnO4 = 3HAsO4- - + 2MnO2 + + 2K+ + 4H+ + H2O Air oxidation of arsenic is very slow and can take weeks for oxidation but chemicals like chlorine and permanganate can rapidly oxidize arsenite to arsenate under wide range of conditions [5]. 5.1.2 Passive Sedimentation Passive sedimentation received considerable attention because of rural people's habit of drinking stored water from pitchers. Oxidation of water during collection and subsequent storage in houses

may cause a reduction in arsenic concentration in stored water. Experiments conducted in Bangladesh showed zero to high reduction in arsenic content by passive sedimentation. Arsenic reduction by plain sedimentation appears to be dependent on water quality particularly the presence of precipitating iron in water. It is also reported that more than 50% reduction in arsenic content is possible by sedimentation of tubewell water containing 380-480 mg/L of alkalinity as CaCO3 and 812 mg/L of iron but this technique cannot be relied to reduce arsenic to desired level. Most studies showed a reduction of zero to 25% of the initial concentration of arsenic in groundwater. In rapid assessment of technologies passive sedimentation failed to reduce arsenic to the desired level of 50 g/L in any well. 5.1.3 In-situ Oxidation In in-situ oxidation of arsenic and iron in the aquifer, the aerated tubewell water is stored in a tank and released back into the aquifers through the tubewell by opening a valve in a pipe connecting the water tank to the tubewell pipe under the pump head. The dissolved oxygen in water oxidizes arsenite to less mobile arsenate and also the ferrous iron in the aquifer to ferric iron, resulting a reduction in arsenic content in tubewell water. Experimental results show that arsenic in the tubewell water following in-situ oxidation is reduced to about half due to underground precipitation and adsorption on ferric iron. 5.1.4 Solar Oxidation This is a simple method of solar oxidation of arsenic in transparent bottles to reduce arsenic content of drinking water. Ultraviolet radiation can catalyze the process of oxidation of arsenite in presence of other oxidants like oxygen. Experiments show that the process on average can reduce arsenic content of water to about one-third.

5.2 Adsorptive processes

Adsorptive processes involve the passage of water through a contact bed where arsenic is removed by surface chemical reactions. Activated alumina, activated carbon, iron oxide coated or based filter media are used for these processes. A critical review of adsorption processes for arsenic removal can be found in [1]. Use of activates carbon is a well established technique. Activated carbon is a crude form of graphite with a random or amorphous highly porpus structure with a broad range of pore sizes,from visible cracks and crevices, to crevices of molecular dimensions. Active carbons have been prepared from

coconut shells, wood char, lignin, petroleum coke, bone-char, peat, sawdust, carbon black, rice hulls, sugar, peach pits, fish, fertilizer waste, waste rubber tire, etc. Wood (130,000 tonnes/year), coal (100,000 tonnes/year), lignite (50,000 tonnes/year), coconut shell (35,000 tonnes/year), and peat (35,000 tonnes/year) are most commonly used [1]. Adsorption capacity depends on activated carbon properties, adsorbate chemical properties, temperature, pH, ionic strength, etc. Many activated carbons are available commercially but few are selective for heavy metals. They are also expensive. Despite carbons prolific use to treat wastewater, it remains expensive, requiring vast quantities of activated carbon. Hence alternate adsorption media have been sought for, and use of iron oxide coated sand, yttrium carbonate, activated alumina, iron oxide impregnated active alumina, manganese dioxide, zirconium resins, titanium dioxide, granular ferric hydroxide, Portland cement etc. is reported. Exhaustive listing of all these basic and alternate adsorbents is provided in [1].

5.3 Ion exchange processes

In these processes, ions held electrostatically on the surface of a solid phase are exchanged for ions of similar charge dissolved in water. Usually, a synthetic anion exchange resin is used as a solid. Ion exchange removes only negatively charged As(V) species. If As(III) is present, it is necessary to oxidise it.

5.4 Membrane processes

Microfiltration (MF), utrafiltration (UF), nano-filtration (NF), reverse osmosis (RO) and electrodialysis reversal (EDR) can remove arsenic through filtration, electric repulsion, and adsorption of arsenic-bearing compounds. The use of MF and UF membranes is dependent on the size distribution of arsenic bearing particles in water. To increase removal efficiency with a low percentage of particulate arsenic content, MF can be combined with coagulation processes. Nanofiltration membranes are capable of removing significant portions of the dissolved arsenic compounds in natural waters. Reverse Osmosis (RO) is very effective in removing dissolved arsenic. Electrodialysis reversal (EDR) can also be used for removal of arsenic. A water recovery of 85% is achievable. Reported arsenic removal varies from 28% to 86%. In general, membrane filtration is more effective for removal As (V) than for As (III). 6. Emerging methods for arsenic removal Besides the methods mentioned in Sec. 5, several new methods have been reported recently. Some interesting methods are shortly described below [7].

1. Fe-Mn-Oxidation 2. Green sand filtration 3. Coagulation assisted Microfiltration 4. In situ (sub-surface) arsenic immobilization 5. Enhanced coagulation (aka electrocoagulation, electroflotation) 6. Biological arsenic removal

6.1 Fe-Mn Oxidation

Conventional iron and manganese removal can result in significant arsenic removal, through coprecipitation and sorption onto ferric or manganic hydroxides. Most low-cost methods for arsenic and manganese removal rely on aeration and filtration through porous media such as sand and gravel. Any method that effectively removes iron and manganese could be evaluated to see if arsenic is also removed effectively.

6.2 Manganese Greensand

Greensand is a granular material composed of the mineral glaucite, which has been coated with manganese oxide. It is a natural zeolite (microporous mineral), and has strong ion exchange properties, and will remove iron, manganese, arsenic, sulphide, and many other anions. Like manganese dioxide coated sand, greensand surface is strongly oxidizing, and is thus able to remove both arsenite and arsenate. This method is especially interesting to utilities where Fe and Mn are already being removed using a manganese greensand filter. It is possible that a small pH adjustment from 8+ to 6.5 may be all that is required to bring the facility into compliance.

6.3 Coagulation assisted Microfiltration

In coagulation assisted microfiltration technology, microfiltration is used in a manner similar to a conventional gravity filter. The advantages of MF over conventional filtration are a more effective microorganism barrier, removal of smaller floc sizes and an increased plant capacity. The microfiltration membrane system works to remove arsenic from water by the addition of an ironbased coagulant, such as ferric chloride, to the water. The arsenic is adsorbed onto positively charged ferric hydroxide particles, which are then removed by microfiltration. In pilot studies, the removal of arsenic to below 2 g/l is reported in waters with a pH between 6 and 7.

6.4 In situ (sub-surface) arsenic immobilization

When arsenic is mobilized in groundwater under reducing conditions, it is also possible to immobilize the arsenic by creating oxidized conditions in the subsurface. In Germany, in order to remediate an aquifer containing high arsenite, high ferrous iron, low-pH groundwater, potassium permanganate was injected directly into contaminated wells, oxidizing arsenite, which coprecipated with ferric oxides as ferric arsenate. Arsenic concentrations were reduced by over 99%, from 13,600 tot 60 g/l. In another project atmospheric oxygen was used to reduce arsenic concentrations in situ from approximately 20 to 5 g/l, while iron and manganese levels were also lowered. However, this oxidation is a slow process. Under reducing conditions, and in the presence of sulphur, arsenic can precipitate out of solution and form relatively insoluble arsenic sulphides. However, arsenic sulphides like all metal sulphides are not stable when in contact with acidic conditions (low pH values). In situ immobilization has the great advantage of not producing any wastes that must be disposed of. However, experience is limited, and the technique should be considered with caution.

6.5 Enhanced coagulation (or electrocoagulation, electroflotation)

With enhanced coagulation, or electrocoagulation or electroflotation, soluble anodes made from iron or aluminium are used. Reported advantages mentioned in laboratory studies are the in-situ oxidation of As(III) tot As(V), and better removal efficiencies than with classical coagulation. Also organic arsenic, fluoride and dissolved metals are removed by this method. Final As concentrations in groundwater are below 10 g/l, even with high initial concentrations. An additional advantage of enhanced coagulation is the reported removal of natural organic matter (NOM). Recent studies showed two to three times better effects on the removal of humic substance than required by US EPA. The method has many advantages for purification of highly humic waters. It simply and efficiently neutralizes molecules to form flocs and coagulates. The coagulation evolves at a high specific reduction of the coagulant, which makes the method suitable from a techno-economic viewpoint and with respect to environmental aspects. The efficient floc separation allows a long safe operation of sand filter without washing.

6.6 Biological arsenic removal

Arsenic in water can be removed by microbiological processes. Two main types of metal-microbe interactions can be potentially used for the removal of arsenic from ground water. They are (a) microbial oxidation of arsenic (III) to arsenic (V) to facilitate its removal by conventional arsenic

removal processes, and (b) bioaccumulation of arsenic by microbial biomass. The Biological Activated Carbon (BAC) system, developed by the Mainstream BMS Ltd.,Vanscoy, Saskatchewan and Davnor Water Treatment Technologies Ltd., Calgary, Alberta, is a biologically active filtration unit. In this system a granular activated carbon (GAC) filter is used, which are continuously aerated to enhance the growth of biological activity within the filter media. This system has been used in rural Saskatchewan on experimental basis for seven years with consistent arsenic removal exceeding 90% (Pokhrel et al., 2005). This system could also remove more than 99% of iron and also dissolved organic material. Solar oxidation and Removal of Arsenic (SORAS) is a simple method that uses irradiation of water with sunlight in PET or other UV transparent bottles to reduce arsenic level from drinking water. The process is developed by Swiss Federal Institute of Environmental Science and Technology, Switzerland (EAWAG) and Swiss Agency for Development and Cooperation (SDC). The method is based on photochemical oxidation of As (III) followed by precipitation or filtration of As (V) adsorbed on Fe (III) oxides. Field tests in Bangladesh show removal efficiency between 45-78% with an average of 67%. Concerning the Bangladesh guideline value of 50 g/l, SORAS can treat raw water having an arsenic concentration below 100 150 g/l. 7. Comparison of the Arsenic Removal Methods

Table 4: Applicability of different techniques [6] Green : Suitable; Yellow : Somewhat suitable; Red : Not suitable

Table 4 compares some of the arsenic removal techniques in terms of their applicability [6], from which it can be inferred that enhanced coagulation is the only technique which can be tailored for low cost or high efficiency use in domestic as well as community applications, as well as in brackish water treatment. Further, some of these techniques are compared in Table 5 in terms of their advantages and disadvantages [1].

Table 5: Comparison of some of the arsenic removal methods [1].

To summarize, each of the techniques is advantageous in certain aspects as well as disadvantageous in certain other aspects. Hence, suitability of a technique will obviously be application specific.

[1] Dinesh Mohana, Charles U. Pittman Jr., Arsenic removal from water / wastewater using adsorbents a critical review, Journal of Hazardous Materials 142, pp: 1-53 (2007). [2] Virender K Sharma, Mary Sohn, Aquatic arsenic: toxicity, transformations and remediation, Environment International 35, pp: 743-759 (2009). [3] C. H. Selene, J. Chou, Christopher T. De Rosa, Case studies Arsenic, International Journal of Hygene and Environmental Health 206, pp: 381-386 (2003). [4] Allan H. Smith, Elona O. Lingas, Mahfuzar Rahman, Contamination of drinking water by arsenic in Bangladesh: a public health emergency, Bulletin of the World Health Organization 78(9), pp: 1093 -1103 (2000). [5] P. Ratna Kumar, Sanjeev Chaudhari, Kartic C Khilar, S. P. Mahajan, Removal of arsenic from water by electrocoagulation, Chemosphere 55, pp: 1245-1252 (2004). [6] L. Feenstra, J. van Erkel, L. Vasak, Arsenic in groundwater: overview and evaluation of removal methods, Report No. SP 2007-2, International Groundwater Resources Assessment Centre, World Health Organization (2007). [7] T. B. Mlilo, L. R. Brunson, D. A. Sabatini, Arsenic and fluoride removal using simple materials, Journal of Environmental Engineering 136, pp: 391-398 (2010).