**1. Introduction**

Cereal–legume mixtures are usually cultivated for grain or green fodder, sometimes as a green manure. Compared to their pure sowing, cereal and legume mixtures are characterized by a higher total protein yield, more stable yielding, especially in unfavorable habitats, a better legume health, and higher nutritional value [1,2]. An additional advantage of the mixture is soil enrichment by legumes with symbiotically fixed nitrogen [3–5]. In the research mixtures of oats with common vetch were tested.

Oat (*Avena sativa* L.) is a cereal with phytosanitary properties in the crop rotation because it is rarely infested by fungal pathogens of stem base and roots [6]. The tolerance of oats to soil acidification, poor soil conditions, low temperature, and higher soil humidity make them a frequent component of many crop rotations, especially in mountainous regions, with a higher share of rainfall [7]. Oats' grain is an excellent feed for horses and dairy cattle because of its chemical composition. Depending on the cultivar, grains of oats contain ca. 100 g kg−1 dry matter (d.m.) of total protein, 40–50 g kg−1 d.m. of crude fat, 100 g kg−1 d.m. of crude fiber, 60 g kg−1 d.m. of nitrogen-free extract [8–10]. The biological value of oat protein is not high, but it contains many valuable amino acids, such as lysine and arginine [10]. Of all cereals, oats have the most fiber, mainly in their husks, which

**Citation:** Puzy ´ ˙ nska, K.; Puzy ´ ˙ nski, S.; Synowiec, A.; Bocianowski, J.; Lepiarczyk, A. Grain Yield and Total Protein Content of Organically Grown Oats–Vetch Mixtures Depending on Soil Type and Oats' Cultivar. *Agriculture* **2021**, *11*, 79. https://doi.org/10.3390/ agriculture11010079

Received: 15 December 2020 Accepted: 15 January 2021 Published: 18 January 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

reduces their digestibility and energy value [11,12]. Oat products and grain quality can be improved by mixing with legumes [13,14].

Common vetch (*Vicia sativa* L.) contains high amounts of protein in seeds (approx. 33% of dry matter) and vegetative parts, i.e., in straw (approx. 60–120 g kg−1 d.m.) and green fodder (150–250 g kg−1 d.m.) [15]. Vetch seeds can be used as a supplement for animals' feed in the absence or limited access to soybean or cornmeal [16]. Ceglarek et al. [17] underline the high content of thiamine acids and methionine in its protein, in comparison to other legume species. Common vetch is ideal for green forage as it has thin stems rich in fine leaves. The slender shoots of vetch can reach a length of up to 150 cm, so it can easily lodge [18]. Common vetch, like oats, is a good forecrop [19]. However, unlike oats, it has high soil demands. It is also characterized by high water requirements, especially during flowering due to the pile root system and a high transpiration rate [20].

The oat–vetch mixture for grain or green forage production combines the advantages of two different species, e.g., reduced fertilization needs due to symbiotic nitrogen fixation. When mixed with oats, vetch plants are less prone to lodging so that harvesting can be done in one step with a combine harvester. The oat and vetch mixture improves soil structure and growth of succeeding crops. In the mixture, the oat protein complements the vetch's sulfur amino acids, and the vetch protein has a positive effect on the quality of the feed [19,20].

The share of vetch seeds in the mixture with oat is variable [20,21], and for that reason, it is not very popular in cultivation. Moreover, with low rainfall, vetch cannot withstand competition for water with oat, and its share in the mixture yield is small [22]. Another important factor influencing the yield of the mixture are different soil requirements of its components. A proper selection of cultivars for the mixture is essential, especially cereal cultivars characterized by lower competitiveness toward the legume component [21]. To date, there are very few reports in the literature on the effect of cultivar choice on the yield of the cereal and legume mixtures in conditions of organic farming. For this reason, this study aimed to analyze the yield, its components and protein content of grain of four oat cultivars grown organically in a mixture with Hanka's vetch on two different soils.

#### **2. Materials and Methods**

#### *2.1. Field Site and Experiment Descriptions*

The research was carried out in 2012–2014 in the Experimental Station Mydlniki-Krakow (50◦05 N 19◦51 E) in the southern Poland. The experiment was set up in a randomized block design, with four replications on two types of soils: Stagnic Luvisol (S.L.) and Haplic Cambisol (H.C.) [23], located about 1 km apart. The area of the experiment was under organic farming management since 2009. The description of the soils is given in Table 1. The preceding crop was winter spelt (*Triticum spelta* cv. 'Frankenkorn').

**Table 1.** Characteristic of the soils.

The mixtures of oat with common vetch (*Vicia sativa*, cv. 'Hanka'; breeder: FN Granum, Wodzierady, Poland) were cultivated for grain. The common vetch was mixed with one of the four oats' cultivars, namely 'Celer', 'Grajcar', 'Kasztan', or 'Furman'. A characteristic of the oats' cultivars is presented in Table 2. The mixtures were sown on 23 March 2012; 16 April 2013; and 20 March 2014, on plots of 18 m2 (3 × 6 m) area, using plot drill (Hege 80) at row space 13.0 cm. A total of 32 plots were established each year. The planned density

of crops was 500 plants m−2 of oat and 75 plants m−2 of vetch. Crops were cultivated organically.

#### **Table 2.** Characteristics of oats' cultivars.


**3.7 Age**

Sleep difficulties are closely correlated with older age in patients with chronic uremia (Iliescu et al., 2003). Yoshioka et al.(1993) found that advanced age and long-term dialysis therapy directly affected patients experiencing sleep problems. The disorders are similar to those described in the general population, where the prevalence and severity of sleep disorders are also associated with old age. Each decade of age increases the risk of insomnia by 239%, and the risk of overt clinical insomnia by 51% (De Santo et al., 2005). Mollaoğlu (2004) reported a negative correlation between age and sleep quality, with sleep quality decreasing with advanced age in their study of 105 HD patients. In addition, communitybased studies have shown that sleep quality could be deteriorated in elderly patients due to increased frequency of physical diseases, multiple drug use, primary sleep disturbances, or lifestyle modifications (Brandenberger et al., 2003; Kamel & Gammack 2006).

### **3.8 Depression**

The psychiatric condition most commonly causing sleep disorders is depression that may affect up to 50% of this patient population (Covic et al., 2006). The relationship between depression and sleep disorders is well known both in the general population and in patients undergoing hemodialysis (İliescu et al.,2003). Depression can be a cause, as well as a result, of insomnia. Dialysis patients with a Pittsburg Quality Index Sleep score of >5 (patients with a "difficult sleep") have a prevalence of overt depression of 20%, while among ESRD patients reporting a normal sleep**,** the prevalence of depression is almost nil (İliescu et al.,2003).


**1. Introduction**

218 Electrochemical Cells – New Advances in Fundamental Researches and Applications

Massart, D.L., Vandegiste, B.M.G., Buydens, L.M.C., de Jong, S., Lewi, P.J., Smeyers-

Melamed, D. (2005) Monitoring arsenic in the environment: a review of science and

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Terlecka, E. (2005). Arsenic speciation analysis in water samples: a review of the hyphenated techniques. *Environmental Monitoring and Assessment*. Vol. 107, pp. 259-284 Trojanowicz, M. (2009). Recent developments in electrochemical flow detections-A review.

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Hydrochemistry of groundwater from the Tierra de Pinares region (Douro basin, Spain) affected by high levels of arsenic. *Proceedings of Arsenic 2008, 2nd International Congress on arsenic in the environment: Arsenic from nature to humans.* pp. 83-84, Pollution of top and subsurface soil, as well as underneath groundwater has been one of the consequences of industrial activities; actually environmental professionals are facing a time consuming issue when they look for potential solutions for heavy metals and organic compounds removal. Although many soil remediation technologies are available, electrokinetic processing has been an emerging technology offering advantages for a wide variety of pollutants being either organic or inorganic; as well as its versatility of being applied in soil wetting conditions ranging from unsaturated to saturated; one of the main advantages of this technology is the fact that this process can be applied to low permeability soils, like clays.

Initially electrokinetics was applied for soil consolidation, in this process water flux is forced by an electrical field action, an approach to explain how it works is based on setting up a soil structural change analysis based on modification of soil matrix, plasticity index and crystalline state (Gray, 1970). For clayey soils it has been accepted that they behave like an osmotic membrane, therefore it is important to understand how physicochemical factors affect its response in regulating osmotic pressure into the soil matrix (Fritz, 1986). Another report (Darmawan, 2002) reinforce the necessity of knowing how the solid matrix response to the electric field, since obtained electrical current is function of electrolyte concentration, buffering capacity, and chemical form of involved metals, these can be in either soluble, electrostatically adsorbed, or surface complexed forms; also, metal migration is favored when soil is dominated by clay minerals, otherwise migration is lowered when soil has a high buffering capacity and/or high humic content, the last one acts like an additional resistance to the current transference throughout the soil.

Electrokinetics as a remediation experimental procedure requires having a wetted soil in which electrodes are inserted and terminals are connected to a power source. As soon as an electric field is generated, electrode reactions take place producing protons (H+) at the anode

Electrode Materials a Key Factor to Improve Soil Electroremediation 221

reduce its activity as experimental conditions favor metal deposition and so far electrode passivation. Then, in order to increase active sites number, reduce passivation and increase useful lifetime; some efforts had addressed electrode modification using oxide materials such as CarbonTiO2, TiSnO2-Sb, TiIrO2-Ta2O5, TiRuO2. Electrodes prepared in this way have been named Dimensionally Stable Anodes (DSA) and they have proven be effective in organic degradation (Comminellis, 1994), because in addition to their high capacity to generate hydroxyl radicals, they also are mechanically resistant to abrupt pH changes.

Titanium dioxide exists in three crystalline forms namely anatase, rutile, and brookite, from which the first is the one exhibiting higher catalytic activity; which is highly dependent on the surface specific area of the particulate form, a phenomena which is exemplified by Baiju et al (2009), in their work they used anatase-TiO2 in dye removal; these authors provide evidence on how switching from a particulate form to nanotubes it results in a concurrent change in the mechanism of dye removal from an aqueous solution. Although, other authors (Yigit & Inan, 2009) have provided evidence on how a mixture anatase-rutile provides a

In wastewater treatment use of Dimensionally Stable Anodes (DSA) made of a Ti mesh covered with a film of iridium or tin has probed being an effective tool for organics degradation while keeping its mechanical resistance (León et al, 2009); also, there is a theory proposed by Comminellis (1991) in which it is shown that the iridium DSA posses a highly reactive surface which directly oxidizes the substrate, at this respect an improvement in oxidant activity is obtained when titanium mesh is covered with a film including a mixture of oxides like Ir-Ta (Hu et al, 2002). Use of modified electrodes favors electrocatalytic processes by supporting higher current densities and so far reducing the corresponding oxidation potentials, because inner sphere mechanisms allow for hydroxyl radicals generation at an interfacial level, by which there it is possible to increase organic molecules decomposition reaching total mineralization levels. About the electrode matrix it can be affirmed that DSA usually exhibit higher mechanical resistance because they are prepared onto a rigid matrix, while the ones based on Reticulated Vitreous Carbon (RVC)

Considering that few attention have been dedicated to the role that electrode material plays on soil electroremediation, since a higher electrode activity would allow developing conditions which could enhance pollutant removal, specially when the pollutant is an hydrocarbon product. In this sense, this contribution presents results from two research approaches in which it is tested how inclusion of a catalytic specie, like anatase (TiO2) in a reticulated vitreous carbon (RVC) or a titanium anode covered with IrO2-Ta2O5 film will help to improve occurrence of water electrolysis reactions; also, each experimental approach allowed to elucidate the role played by materials used as anode and cathode, as well as the influence exerted by additional resistance factors like electrode position in respect to the soil interphase. Finally, it is an opportunity to test the theory by which it is expected an enhancement of electrokinetic hydrocarbon removal as consequence of the higher electrode

Experimental work is organized as follows: 1) Electrodes made of Reticulated Vitreous Carbon (RVC), in which anode was modified by inclusion of an anatase deposit (TiO2), and keeping constant a RVC cathode, this provides two combinations bare RVC electrodes and RVC-TiO2 anode with bare RVC cathode; 2) Anode made of a titanium plate covered with

higher efficiency in humic acid mineralization.

have higher reactive surface but a lower mechanical resistance.

activity.

and hydroxide (-OH) at the cathode; concentration of these ions increases exponentially creating and acid front which moves from anode to cathode, and a basic front moving from cathode to anode; during its passage through the soil protons and hydroxides interact with sorbed pollutants releasing them into solution. Soluble ion transport occurs by three mechanisms: 1) Diffusion due to concentration gradients, 2) Convection due to fluid movement and 3) Migration due to the electric field.

A sample of initial published results (Acar et al, 1994; Hamed et al, 1991; Khan & Alam, 1994; Kim et al, 2002; Pamucku et al 1990; Pamucku & White, 1992; Reddy et al, 1999) is enough to claim that this method is highly efficient on restoration actions for clayey soils having very low heavy metal concentrations, for which regular mining procedures would result very expensive; although, for this method one of the minuses is the time required to get metal removals above 90%. Most of these studies report soil characterization providing information about: sand, clay, silt content; organic matter as well as hydraulic permeability. Although, few reports have covered soil electrical resistance, which according to Vázquez et al (2004), it could be used as a method for analyzing soil behavior in presence or absence of an electrical field.

In order to improve the process and get shortening of experimental times, applied efforts have covered a wide set of conditions. Some examples of reported research have addressed for modifying pH and current density (Hamed& Bhadra, 1997), chemical conditioning of electrode wells (Reed et al, 1995; Murillo-Rivera et al, 2009), cation inclusion (Colleta et al, 1997), as well as addition of complexing (Yeung et al, 1996) and lixiviant agents (Cox et al 1996); another approach has been the inclusion of reactive barriers into the soil matrix (Cundy & Hopkinson, 2005; Ruiz et al, 2011).

When treating PAHs (Polyaromatic Hydrocarbons) soil pollution it is important to care about lateral effects like lowering the electroosmotic flow rate (EOFR), which can be consequence of controlling pH at the electrode wells, by doing this an affectation of soil and/or solution chemistry can be induced producing an accumulation at the neutral or alkaline soil regions (Saicheck & Reddy, 2003). Another factor affecting EOFR derives from surfactant inclusion, then it becomes necessary to evaluate if it is worthy lowering EOFR for increasing PAHs removal (Kolosov et al, 2001). Also, pollutant mobilization should be evaluated as a function of pH control and surfactant addition, since flow can occurs in anodic direction (Ribeiro et al, 2005), or be enhanced in the cathodic direction.

Finally, organics and metal removal can be affected by geometry cell and flow direction, in this sense a report about an upward electrokinetic soil remediation (Wang et al, 2007) points out that removal efficiency is increased for organics when electroremediation cell is smaller in diameter, or larger in height; otherwise for metals, removal is improved when the cell is smaller in diameter or shorter in height.

Electrode materials are a key parameter to assure that electron transference takes place at fast rates. Selection of materials should be based on thermodynamic and kinetic response, so interface molecular interactions are fast enough to release the oxidant species. Also, it must be considered aspects like mechanical, thermic and corrosion resistance; as well as the procedures and solutions used in surface cleaning, pretreatment, and surface activation. In this sense, materials which satisfy these requirements are: vitreous carbon, titanium, stainless steel, platinum, gold and silver; but it must not be discarded that all materials can

and hydroxide (-OH) at the cathode; concentration of these ions increases exponentially creating and acid front which moves from anode to cathode, and a basic front moving from cathode to anode; during its passage through the soil protons and hydroxides interact with sorbed pollutants releasing them into solution. Soluble ion transport occurs by three mechanisms: 1) Diffusion due to concentration gradients, 2) Convection due to fluid

A sample of initial published results (Acar et al, 1994; Hamed et al, 1991; Khan & Alam, 1994; Kim et al, 2002; Pamucku et al 1990; Pamucku & White, 1992; Reddy et al, 1999) is enough to claim that this method is highly efficient on restoration actions for clayey soils having very low heavy metal concentrations, for which regular mining procedures would result very expensive; although, for this method one of the minuses is the time required to get metal removals above 90%. Most of these studies report soil characterization providing information about: sand, clay, silt content; organic matter as well as hydraulic permeability. Although, few reports have covered soil electrical resistance, which according to Vázquez et al (2004), it could be used as a method for analyzing soil behavior in presence or absence of

In order to improve the process and get shortening of experimental times, applied efforts have covered a wide set of conditions. Some examples of reported research have addressed for modifying pH and current density (Hamed& Bhadra, 1997), chemical conditioning of electrode wells (Reed et al, 1995; Murillo-Rivera et al, 2009), cation inclusion (Colleta et al, 1997), as well as addition of complexing (Yeung et al, 1996) and lixiviant agents (Cox et al 1996); another approach has been the inclusion of reactive barriers into the soil matrix

When treating PAHs (Polyaromatic Hydrocarbons) soil pollution it is important to care about lateral effects like lowering the electroosmotic flow rate (EOFR), which can be consequence of controlling pH at the electrode wells, by doing this an affectation of soil and/or solution chemistry can be induced producing an accumulation at the neutral or alkaline soil regions (Saicheck & Reddy, 2003). Another factor affecting EOFR derives from surfactant inclusion, then it becomes necessary to evaluate if it is worthy lowering EOFR for increasing PAHs removal (Kolosov et al, 2001). Also, pollutant mobilization should be evaluated as a function of pH control and surfactant addition, since flow can occurs in

Finally, organics and metal removal can be affected by geometry cell and flow direction, in this sense a report about an upward electrokinetic soil remediation (Wang et al, 2007) points out that removal efficiency is increased for organics when electroremediation cell is smaller in diameter, or larger in height; otherwise for metals, removal is improved when the cell is

Electrode materials are a key parameter to assure that electron transference takes place at fast rates. Selection of materials should be based on thermodynamic and kinetic response, so interface molecular interactions are fast enough to release the oxidant species. Also, it must be considered aspects like mechanical, thermic and corrosion resistance; as well as the procedures and solutions used in surface cleaning, pretreatment, and surface activation. In this sense, materials which satisfy these requirements are: vitreous carbon, titanium, stainless steel, platinum, gold and silver; but it must not be discarded that all materials can

anodic direction (Ribeiro et al, 2005), or be enhanced in the cathodic direction.

movement and 3) Migration due to the electric field.

(Cundy & Hopkinson, 2005; Ruiz et al, 2011).

smaller in diameter or shorter in height.

an electrical field.

reduce its activity as experimental conditions favor metal deposition and so far electrode passivation. Then, in order to increase active sites number, reduce passivation and increase useful lifetime; some efforts had addressed electrode modification using oxide materials such as CarbonTiO2, TiSnO2-Sb, TiIrO2-Ta2O5, TiRuO2. Electrodes prepared in this way have been named Dimensionally Stable Anodes (DSA) and they have proven be effective in organic degradation (Comminellis, 1994), because in addition to their high capacity to generate hydroxyl radicals, they also are mechanically resistant to abrupt pH changes.

Titanium dioxide exists in three crystalline forms namely anatase, rutile, and brookite, from which the first is the one exhibiting higher catalytic activity; which is highly dependent on the surface specific area of the particulate form, a phenomena which is exemplified by Baiju et al (2009), in their work they used anatase-TiO2 in dye removal; these authors provide evidence on how switching from a particulate form to nanotubes it results in a concurrent change in the mechanism of dye removal from an aqueous solution. Although, other authors (Yigit & Inan, 2009) have provided evidence on how a mixture anatase-rutile provides a higher efficiency in humic acid mineralization.

In wastewater treatment use of Dimensionally Stable Anodes (DSA) made of a Ti mesh covered with a film of iridium or tin has probed being an effective tool for organics degradation while keeping its mechanical resistance (León et al, 2009); also, there is a theory proposed by Comminellis (1991) in which it is shown that the iridium DSA posses a highly reactive surface which directly oxidizes the substrate, at this respect an improvement in oxidant activity is obtained when titanium mesh is covered with a film including a mixture of oxides like Ir-Ta (Hu et al, 2002). Use of modified electrodes favors electrocatalytic processes by supporting higher current densities and so far reducing the corresponding oxidation potentials, because inner sphere mechanisms allow for hydroxyl radicals generation at an interfacial level, by which there it is possible to increase organic molecules decomposition reaching total mineralization levels. About the electrode matrix it can be affirmed that DSA usually exhibit higher mechanical resistance because they are prepared onto a rigid matrix, while the ones based on Reticulated Vitreous Carbon (RVC) have higher reactive surface but a lower mechanical resistance.

Considering that few attention have been dedicated to the role that electrode material plays on soil electroremediation, since a higher electrode activity would allow developing conditions which could enhance pollutant removal, specially when the pollutant is an hydrocarbon product. In this sense, this contribution presents results from two research approaches in which it is tested how inclusion of a catalytic specie, like anatase (TiO2) in a reticulated vitreous carbon (RVC) or a titanium anode covered with IrO2-Ta2O5 film will help to improve occurrence of water electrolysis reactions; also, each experimental approach allowed to elucidate the role played by materials used as anode and cathode, as well as the influence exerted by additional resistance factors like electrode position in respect to the soil interphase. Finally, it is an opportunity to test the theory by which it is expected an enhancement of electrokinetic hydrocarbon removal as consequence of the higher electrode activity.

Experimental work is organized as follows: 1) Electrodes made of Reticulated Vitreous Carbon (RVC), in which anode was modified by inclusion of an anatase deposit (TiO2), and keeping constant a RVC cathode, this provides two combinations bare RVC electrodes and RVC-TiO2 anode with bare RVC cathode; 2) Anode made of a titanium plate covered with

Electrode Materials a Key Factor to Improve Soil Electroremediation 223

Soil sample corresponds to a hydrocarbon polluted weathered soil, for which a physical characterization is done by using the ASTM D4318 as well as the USCS-P13-B-2 procedure, by which it was established soil type and textural composition. Since this soil has hydrocarbon pollutants, its concentration was determined as oil and grease by the Soxhlet technique. According to published results (Murillo-Rivera et al, 2009), 0.1M NaOH solution is an electrolyte that works fine for hydrocarbon polluted soils, so this one was chosen as the

Experimental cell was a rectangular one (10 cm length, 2 cm width, 4 cm high), a current density of 20 mA cm-2 was imposed with a PDC-GP 4303DU Power Source, in a

Considering that a DSA anode provides oxidant species at higher rates, then for this hydrocarbon polluted soil electroremediation it was chosen an electrode arrangement, considering a modified DSA made of a titanium plate covered with an iridium-tantalum film (TiIrO2-Ta2O5), and two types of cathode: carbon felt (CF) and a titanium plate (Ti). Also, in these set of experiments it was considered two electrode positions: in the first one, a physical barrier of filter paper was included between soil and electrode, while in the second

Registered experimental parameters were: applied electrical current, developed electrical potential, with these it was possible to calculate cell resistance and energy consumption. At the end of each experiment soil was cut in 3 sections (anode, middle, cathode), hydrocarbon removal was estimated from soil residual concentrations, which were extracted by the Soxhlet technique, and later analyzed by UV-Vis (XLS, Perkin-Elmer) gas chromatography

Soil characterization results are reported in Table 1. As it can be observed this soil is classified as a Low plasticity Clay (CL), then it will no exhibit a great volume change during experimentation; also, clay and silt content indicate that this is a low permeability soil.

> plasticity clay (CL)

Plasticity Chart (Helwany,

2007, page 13)

Parameter Value Methodology Liquid Limit (LL) % 36 ASTM D4318-10 Plastic Limit (PL) % 24 ASTM D4318-10 Plasticity Index (PI) % 12 ASTM D4318-10

Sand % 34 USCS-P13-B-2 Silt % 53 USCS-P13-B-2 Clay % 13 USCS-P13-B-2

**2.2 Cathode modification**

galvanostatic mode during 4 hours.

**3. Results**

**3.1 Anode modification**

electrolyte for soil wetting, and wells replenishment.

one the electrode was set in direct contact with soil sample.

coupled to mass spectroscopy (CG-EM, Agilent GC 19091-413).

Classification Low

Table 1. Physical and textural properties of Guanajuato soil.

IrO2-Ta2O5, and two cathode materials carbon felt (CF) and titanium plate (Ti), plus a variation of its position respect to the soil matrix. These experiments are described in the following paragraphs.