International Journal of Scientific Research in
Environmental Science and Toxicology

This study investigated the glyphosate adsorption by water treatment residual using the method of indirect Chemical Oxygen Demand (COD) in aqueous solution using the electrocoagulation process. The optimum glyphosate removal was obtained with typical operating conditions: initial pH of 5.8, initial herbicide concentration of 100 mg/L, current density 25 mA/cm², type of electrolyte, and salt concentration of 1 g/L and temperature of 20^oC. The results showed that the COD of glyphosate removal were 89.8% by using iron (Fe) electrodes at 40 min, 89.8% by using stainless steel S-S at 60 min, while were 84.8 % by using aluminum Al electrodes at the 80 min. It was found that the data fitted to Freundlich (R² = 0.964) model. In glyphosate the electrocoagulation process can be described by a pseudo first order with rate constant (0.0059, 0.0052 and 0.005 min- 1) by using Fe, S-S and Al electrodes respectively. The consumption of electrical energy for glyphosate at optimum conditions as follows: (9.999, 13.9 and 19.0619 KWh/m3) using Fe, S-S and Al electrodes respectively.

Keywords: Electrocoagulation; Electrodes; Herbicide; Glyphosate; Adsorption; Kinetics

Nowadays herbicides production and disposition considerably increase in the environment, and many of them become contaminants [1] Glyphosate (C3H8NO5P) is an active ingredient of the most widely used herbicide and it is supposed to be specific on plant metabolism and less toxic than other pesticides. However, glyphosate was found to be toxic to human embryonic and placental cells and can disrupt the animal cell cycle in urchin eggs [2] In addition, it has been found that glyphosate can disrupt the digestive, reproductive, and respiratory system and increase health risks [3-4]. Glyphosate is strong polar, water soluble, and low volatile. It can inhibit the plant enzyme from being involved in the synthesis of the aromatic amino acids [5]. Due to the different manufacturing processes and application methods, a significant amount of glyphosate reaches the surface water, groundwater, and soil. Glyphosate can cause widespread and persistent contamination because of its complexation with heavy metals [6]. In drinking water, the presence of glyphosate should be lower than 0.7 mg/L according to the USA National Primary Drinking Water Regulations (NPDWRs) [7]. Therefore, it is necessary to develop high efficient ways to remove the glyphosate from water. Different methods used to removal of glyphosate such as advanced oxidation [8], chemical precipitation [9], and membrane separation [10], biodegradation [11] and adsorption [12]. Electrocoagulation (EC) is one of the most feasible and environmentally friendly methods due to the advantages of high adsorption efficiency, economy, and no secondary pollution. Literature reports the ability of EC to treat wastes, e.g., textile wastewater [13-14], liquid from the food [15], textile industries [16], aquacultural wastewater [17], herbicide [18] and polymer [19].

This work focuses on the enhancement of electrocoagulation process for Chemical Oxygen Demand COD removal of glyphosate herbicide from aqueous solution using iron (Fe), stainless steel (SS) and aluminum (Al) electrodes and to investigate the kinetic and adsorption isotherm studies on the removal efficiency.

For all electrocoagulation experiments, a commercial formulation of the herbicide glyphosate from Monsanto Agro Sciences, USA, was used. The properties of the glyphosate are given in Table (1). Mineral salts were routinely used for all electrocoagulation experiments. Sodium chloride, potassium chloride, calcium chloride, sodium sulfate, sodium carbonate, sodium hydroxide, sulfuric acid, potassium dichromate, was of analytical grade and purchased from Merck. Distilled water was used for the preparation of solutions. Standard solutions of potassium dichromate (K2Cr2O7), sulfuric acid (H2SO4) reagent with silver sulfate (Ag2SO4), Mercury sulfate (HgSO4) and sulfuric acid (H2SO4) were prepared to measure the COD. A stock solution of herbicide (1000 mg/L) was prepared by dissolving an accurate quantity of the herbicide in distilled water and suitably diluted to the required initial concentrations. Different standard solutions of herbicide with concentration from 50–200 mg/L were prepared to measure its removal at different conditions.

The solution volume of 100 mL was used for each experiment as the electrolyte. The pH of the electrolyte was adjusted, if required, with HCl or NaOH (Merck, Darmstadt, Germany) solutions and measured by a pH meter (AC28-TOA electronics Ltd, Japan) at the beginning and during the experiment. After adjusting the initial solution pH to the desired value, the current density was set. Magnetic stirring at 250 rpm provided a homogeneous solution in the batch reactor intervals, samples were taken for analysis. Temperature studies were carried at varying temperature (10- 40 oC) to determine the type of reaction. The chemical oxygen demand (COD) was analyzed using UV–Vis Spectrophotometer (model UV 1601 is from Shimadzu, Japan).

Chemical oxygen demand (COD) is a measure of the amount of the oxygen used in the chemical oxidation of inorganic and organic matter present in water. Although COD is not a specific compound, it is considered a conventional pollutant under the federal Clean Water Act, and it has been widely used by regulatory agencies worldwide to gauge overall treatment plant efficiencies. It is also an indicator of the degree of pollution in the effluent and of the potential environmental impact of the discharge of wastewater in bodies of water. COD is determined using a strong chemical oxidant under standard conditions. Compounds that contribute to COD are: biodegradable organic compounds, nonbiodegradable compounds and inorganic oxidizable compounds [20].

The calculation of COD removal efficiencies after electrocoagulation treatment was performed using the following formula [21]. $$C_R\left(\%\right)=\left[\left(C_0–C\right)/C_0\right]\times 100 \left(1\right)$$
Where C0 and C are concentrations of wastewater before and after electrocoagulation [22].

In an EC process the coagulating ions are produced ‘in situ’ and it involves three successive stages:

* formation of coagulants by electrolytic oxidation of the ‘sacrificial electrode

* destabilization of the contaminants, particulate suspension, and breaking of emulsions and

* Aggregation of the destabilized phases to form flocs. The destabilization mechanism of the contaminants, particulate suspension, and breaking of emulsions have been described in broad steps and may be summarized as follows :

a. Compression of the diffuse double layer around the charged species by the interactions of ions generated by oxidation of the sacrificial anode.

b. Charge neutralization of the ionic species in the wastewater by counter ions produced by the electrochemical dissolution of the sacrificial anode. The counter ions reduce the electrostatic inter-particle repulsion to the extent that van der waals attraction predominates, thus causing coagulation. In this process a zero net charge results.

c. Floc formation: a sludge blanket is created from the floc that formed as a result of the coagulation process. That entraps and bridge colloidal particles that are still remaining in the aqueous medium [23].

Figure (1) represents the mechanism of the removal of herbicides by EC; this mechanism will be explained with two specific examples involving iron and aluminum since these two metals have been extensively used to clarify pollutant water [24]. Electrocoagulation of herbicide solution using iron (Fe), stainless steel (S-S) and aluminum (Al) electrodes takes place according to the following mechanisms [25].

The primary reactions taking place in the Electrocoagulation cell with different anode materials are as follows:

Anode: $4F{e}_{(s)}\to 4F{e}^{2+}{}_{(aq)}+8e^{-} \left(2\right)$

4Fe²⁺_(aq) +10H₂O(l) + O₂(aq) → 4Fe(OH)_3(s)+8H+_(aq)

Cathode:
8H⁺_(aq)+8e− → 4H₂(g)
Overall:
4Fe²⁺aq+10H₂O_(l)+O_2(aq) → 4Fe(OH)_3(s)+4H_2(g)
Cathode:
8H⁺_(aq)+8e⁻ → 4H_2(g)
Overall: 4Fe(s)+10H₂O_(l)+O_2(aq) → 4Fe(OH)_3(s)+4H_2(g)
Mechanism 2
Anode:
Fe (s)→ Fe²⁺ (aq) + 2e⁻ (6) Fe²⁺ (aq)+2OH−(aq) → Fe(OH)_2(s)
Cathode :
2H₂O_(l)+2e⁻ → H_2(g) + 2OH⁻_(aq)
Overall:
Fe_(s) + 2H₂O_(l) → Fe(OH)₂(s)+H_2(g)
Aluminum electrodes
Anode: Al(s) → Al³ _+(aq)+3e⁻
Cathode: 3H₂O(l)+3e− → 3/2H₂+3OH−
Overall: Al_(s)+3H₂O_(l) → Al(OH)_3(s)

Effect of initial pH
The pH is one of the most important factors influencing the performance of electrocoagulation process. Figure (2) shows the effect of pH on COD removal efficiency at a period time using Fe, S-S and Al electrodes at initial concentration of 100 mg/L, a current density of 25 mA/cm2, interelectrode distance of 1 cm, temperature of 20oC and NaCl concentration of 1 g/L. in neutral medium the removal efficiency is much higher for all electrodes. From fig 2 the results showed that the COD of glyphosate removal were 89.8% by using iron (Fe) electrodes at 40 min, 89.8% by using stainless steel S-S at 60 min, while were 84.8 % by using aluminum Al electrodes at the 80 min.

Effect of initial glyphosate concentration (mg/L)
The effect of initial herbicides concentration using COD removal efficiency was treated at 40 min, 60 min and 80 min using Fe, S-S and Al electrodes respectively at different initial concentrations in the range of ( 50-200 mg/L) in the optimum conditions: a current density of 25 mA/cm2, NaCl concentration of 1 g/L, temperature of 20oC, pH of 5.8 and inter-electrode distance of 1 cm. Fig (3) show that the removal efficiencies of COD falls from 89.8 to 42.8%, 89.8 to 49.4% and 84.8 to 58.4% as initial glyphosate concentration increase from 50 – 200 mg/L using Fe, S-S and Al electrodes respectively. Further increasing of the initial concentration leads to decrease in the herbicides removal efficiency. According to Faraday‘s law, a constant amount of metal hydroxides is dissolved from the Fe, S-S and Al anode and passes to the solution for the same current density and electrolysis time for herbicides concentrations. Consequently, the same amount of metal hydroxides is produced in the aqueous solution. This is probably why the amount of hydroxyl and metal ions produced on the electrodes was not sufficient to adsorb at high glyphosate concentrations at a constant current density.

Effect of current density (mA/cm²)
In EC process, the most important parameter for controlling the coagulant dosing rate and reaction rate into the medium sample is current density. The current density applied in this study between (12.5 - 50 mA/cm2). The reactions were carried out at 20 min, 40 min, 60 min, 80 min and 100 min using Fe, S-S and Al electrodes under the following conditions: the initial concentration of 100 mg/L, pH of 5.8, and inter-electrode distance of 1 cm, temperature of 20 oC and NaCl concentration of 1 g /L. Figure (4) shows the effect of current density for the removal of glyphosate COD from aqueous solutions in a period time (20-100 min) using Fe, S-S and Al electrodes. The removal efficiency of COD was 89.8%, 89.8% and 84.8 using Fe, S-S and Al electrodes. The results show the optimum condition of current density was 25 mA/cm2 for all electrodes. The removal efficiency of COD increased up by increasing the current density [26]. The increase of coagulant and bubbles generation rate lead to the increase number of H2 bubbles and decrease their size with increasing current density resulting in a faster removal of herbicides[23-29].

Effect of type of electrolyte Figure (5) displays the effect of electrolyte types on the COD removal efficiency of glyphosate at 40 min, 60 min and 80 min using Fe, S-S and Al electrodes respectively in the presence of different supporting electrolytes including NaCl, KCl, CaCl₂, Na₂SO₄ and Na₂CO₃ were studied at initial concentration of 100 mg/L, a current density of 25 mA/cm², inter-electrode distance of 1 cm temperature of 20oC and pH of 5.8. The best removal efficiencies of COD were in the presence of chloride ions such as NaCl, KCl and NaCl, KCl, CaCl₂ using Fe, Al and S-S electrodes at time of 40, 60 and 80 min respectively. But other electrolytes which not contain chloride ions as Na₂SO₄ and Na₂CO₃, the removal efficiencies reached to lower value at the same time. In the presence of chloride ions of NaCl, KCl, and NaCl, KCl, CaCl₂ (which provide the effective Cl- ion) electrolytes the removal efficiency of COD were high due to formation of hypochlorite (OCl-) and hypochlorous acid (HOCl). This behavior may be due to the small ionic size of K+ and Na+ which increases the ion mobilities and the loss ability of Clion. In case of CaCl₂ electrolyte the removal efficiency decreases due to the presence of calcium hypochlorite which is insoluble in water. But in other electrolytes which not contain chloride ions, the removal efficiency of COD dropped. [30].

Effect of electrolyte concentration (g/L) From figure (6) shows that, as the electrolyte concentration increased, the removal efficiency increased due to the increment of the electrical conductivity reaching the maximum value at 1 g/L NaCl. However, a further increase of electrolyte concentration did not improve. As the electrolyte concentration increased the COD removal efficiency increased due to the increment of the electrical conductivity. It can be attributed that the increase of the conductivity by the addition of sodium chloride [31] and the produced amount of metallic hydroxide this leading to a reduction of the oxide layer and an enhancement of the anodic dissolution of the electrode material and herbicide removal increases[32]. However, with the increase in NaCl concentration ˃ 1 g/L for glyphosate respectively the removal efficiency decreased using all electrodes.

Effect of temperature (oC)
The result from figure (7) indicates that increasing temperature has a negative effect on removal efficiencies of COD. Where at 20oC the glyphosate COD removal reached to 89.8%, 89.8% and 84.8% using Fe, S-S and Al electrodes respectively. While at higher temperature (40oC) the COD removal dropped to 67%, 25.3% and 65.8% using Fe, S-S and Al electrodes respectively. Indicated that increasing temperature has a negative effect on both removal efficiency of herbicides and COD values, it may be concluded that at low temperature the dissolution of anode occurs at a lower rate. When the temperature was over 40oC, the removal efficiency began to decrease. In this case, the volume of colloid M (OH)n will decrease and pore production on the metal anode well be closed [33].

Effect of anode materials
The effect of different anode materials on the removal efficiency of glyphosate was evaluated using iron, stainless steel and aluminum anodes. Due to the effective adsorption nature, the contaminants in the water will be removed by the adsorption with metal hydroxides produced from the chemical coagulants like Fe , S-S and Al salts. By this method coagulants are introduced

without corresponding sulfate or chloride and more removal efficiencies are proved [34]. By eliminating competing anions and using a highly pure coagulant source, lower metals residuals are obtained and less sludge is produced than when metal salts are utilized.

A contaminant free ion source allows maximum adsorptive removal of the various dissolved forms of metals that could be present and require treatment. The main advantage in the case of Fe and S-S electrode is that the residual iron if any present in treated water will not cause any health problem like aluminum. Finally table (4) represents the Comparison between the Electrocoagulation methods for removal of glyphosate with other methods.

Kinetic studies of glyphosate
Rate of reaction describes the rates of change in concentration of reactant per unit time [35]. Figure (8) represents the glyphosate removal using EC method of which exhibited pseudo first order with good correlation coefficient 0.978, 0.942 and 0.948 using Fe, S-S and Al electrodes according to following equation:
$${\text{Ln At/A}}_0\text{ =-Kt}$$
Where Ao, At, t and k are the glyphosate absorbance at initial time reaction, glyphosate absorbance after reaction, time of reaction (min) and reaction rate constant (min-1) respectively. The straight lines in plots show a good agreement of experimental data with the kinetic models for different removal rate. The calculated k values from the Figure (8) were 0.0059, 0.0052 and 0.005 min-1 using Fe, S-S and Al electrodes.

Isotherm modeling
In this study, two adsorption isotherms viz., Freundlich and Langmuir models were applied to establish the relationship between the amount of COD for glyphosate adsorbed onto the iron hydroxides and its equilibrium concentration in the electrolyte containing contaminant ions.

The Langmuir equation
The Langmuir isotherm is valid for monolayer adsorption onto a surface containing a finite number of identical sites. The model assumes uniform energies of adsorption onto the surface and no transmigration of adsorbate in the plane of the surface [36]. Based upon these assumptions, Langmuir represented the following equation:
$$q_e=1/q_m+1/\left(q_m{K}_L{C}_e\right)$$
Where qe (mg/g) is amount adsorbed at equilibrium, Ce (mg/L) equilibrium concentration, qm is the Langmuir constant representing maximum monolayer adsorption capacity and KL (L/mg) is the Langmuir constant related to energy of adsorption. The essential characteristics of the Langmuir isotherm can be expressed as the dimensionless constant R L [37].
$$R_L=1/1+\left(K_L+C_o\right)$$
Where Co is the initial concentration, KL the constant related to the energy of adsorption (Langmuir Constant). The RL values between 0 and 1 indicate the favorable adsorption.

Freundlich adsorption isotherm
This is commonly used to describe the adsorption characteristics for the heterogeneous surface [38]. The Freundlich adsorption isotherm typically fits the experimental data over a wide range of concentrations. These data often fit the empirical equation proposed by Freundlich:
$$Logqe=Log{K}_f+1/nLogCe$$
Where kf (mg/g) and n (dimensionless) are constants that account for all factors affecting the adsorption process, such as the adsorption capacity and intensity. The Freundlich constants Kf and n are determined from the intercept and slope, respectively, of the linear plot of log qe versus log Ce. According the equation (14 and 16) the contaminants are usually adsorbed at the surface of the metal hydroxides generated during the electrocoagulation process. Figures (9-10) and Table 2 represent the Langmuir and Freundlich Isotherms studies of equilibrium.

Operating Cost One of the most important parameters that must be determined to evaluate a method of wastewater treatment is the cost. The operating cost includes material (mainly electrodes) cost, operating cost (mainly electrical energy) and other cost items such as labor, maintenance, sludge dewatering and disposal. In this study, energy and electrode material costs have been taken into account as major cost items.

Electrical Energy Consumption
In an electrochemical process, the most important economical parameter is energy consumption Ec (KWh/m³) [39]. This parameter is calculated from the following expression: [40]
$$Ec =\frac{V.I.t}{volume.1000}$$
Where V, I, t and Volume stand for average voltage of the EC system (V), electrical current intensity (A), reaction time (h) and treated solution volume (m3) respectively. According the equation (17), The electrical energy consumption for glyphosate by electrocoagulation was (9.999, 13.9 and 19.0619 KWh/m³) at 40, 60 and 80 min electrolysis time using Fe, S-S and Al electrodes respectively at the applied current of 0.1A and cell voltage (15, 13.9 and 14.3 V).

Mass of loss from anode electrode
The maximum possible mass of Fe and Al electrochemically generated from sacrificial anodes for a particular electrical current was calculated using Faraday‘s law of electrolysis [41]:
$$\text{m = I}\text{.A}\text{.M}\text{.t / V}\text{.Z}\text{.F}$$
where m is the mass of the anode material dissolved (g), I the current density (A/ m²), A the active electrode area (m²), M the molar mass of the anode material (g /mol), t electrolysis time (s), V volume of the reactor (m3), z the number of electrons transferred, and F the Faraday‘s constant (96,485 C/mol1). The cathode dissolution was not considered according to equation (32). The maximum possible mass of Fe and Al electrochemically generated from sacrificial anodes, was (0.0007, 0.0005 and 0.00045 Kg/m3) ions Fe+2, Fe+3, Al+3 respectively.

Determine the residual concentration of iron In glyphosate samples
It appears to be more of a nuisance than a potential health hazard. Iron in water 0.1 mg/L for ferrous iron and 0.2 mg/L ferric Iron. Water used in industrial processes usually contain less than 0.2 mg/L iron [42]. According to following equation Beer-lambert:
$${\rm A}=\epsilon \text{ b C}$$
Where A is absorbance, ε is the molar absorptivity, b is the path length of the sample and C is the concentration of the compound in solution. The concentration measurements of iron (II) were found 0.054 ppm for glyphosate.

1. COD removal for glyphosate were 89.8%, 89.8% and 84.8% using Fe electrodes at 40 min, S-S electrodes at 60 min and Al electrodes at 80 min respectively.

2. Electrical energy consumption (9.999, 13.9 and 19.0619 KWh/ m3) using Fe, S-S and Al electrodes respectively for glyphosate, with typical operating conditions: a current density 25 mA/ cm2, an initial pH of 5.8, NaCl concentration of 1 g/L, an initial glyphosate concentration of 100 mg/L, inter-electrode distance of 1 cm and temperature of 20^oC.

3. The results were concluded that the electrode material play an important role in electrocoagulation method for treatment of herbicides in aqueous solution.

4. The removal rate of glyphosate followed first order reactions using Fe, S-S and Al electrodes with rate constant 0.0059 min- 1, 0.0052 min-1 and 0.005 min-1 respectively.

5. The glyphosate adsorption was best fitted by the Freundlich adsorption isotherm and the results were in good agreement with the experimental data.

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