Study of some parameters responsible for glyphosate herbicide mineralization by Electro - Fenton process

Vietnam Journal of Science and Technology 55 (4C) (2017) 238-244 STUDY OF SOME PARAMETERS RESPONSIBLE FOR GLYPHOSATE HERBICIDE MINERALIZATION BY ELECTRO - FENTON PROCESS Thanh Son Le 1, * , Tuan Duong Luu 1, 2 , Tuan Linh Doan 1 , Manh Hai Tran 1 1 Insitute of Environmental Technology, VAST, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam. 2 Graduate University of Science and Technology, VAST, 18 Hoang Quoc Viet, Cau Giay, Ha Noi, Viet Nam * Email: thanhson96.le@gmail.com Received: 11 August 2017; Accepted for publication: 16 October 2017 ABSTRACT Glyphosate (C3H8NO3P) is a highly effective broad-spectrum, post-emergence, non- selective organophosphate herbicide and commonly applied in Viet Nam. The removal of glyphosate in aqueous solution has been investigated by electro - fenton process which is based on the continuous production of ● OH radicals by reaction between Fe 2+ catalyst and H2O2 electrochemical generated on cathode. The carbon felt (60 cm 2 ) and Pt gauze (45 cm 2 ) were used as cathode and anode of the electro-fenton system, successively. Monitoring the total organic carbon (TOC) during the electrolysis proved that pH, current intensity, electrolysis time and catalyst concentration influenced the glyphosate mineralization efficiency. At the optimal conditions: [Fe 2+ ] = 0.1 mM; pH = 3; [Na2SO4] = 0.05M ; I = 0.5A and the compressed air was bubbled through the solutions, the experiment results showed that 84.4 % Glyphosate was mineralized to CO2, H2O and inorganic acid after 50 min. Keywords: electro-fenton, glyphosate, felt carbon, mineralization. 1. INTRODUCTION As an agriculture-based country, Vietnam’s demand for pesticides is very high and this demand increased every year according to the agricultural growth. However, their overuse, incorrect use and storage have posed a serious threat to health and aquatic ecosystems in agricultural areas. Moreover, most of storehouses were seriously downgraded, the drainage system at the warehouse was almost poor, so pesticides may enter and contaminate surface and ground water. They are toxic and carcinogenic in nature even at low concentration [1], so usually have direct adverse effect on the living organisms [2]. Thus, the treatment of pesticide residues in general and the water polluted by the pesticide in particular are very imperative. Many methods for removing pesticides have been recently developed. Among them, physical methods such as adsorption, membrane filtration, coagulation/flocculation and biological methods generate sludge which still presents a serious pollution problem for the environment at Study of some parameters responsible for glyphosate herbicide mineralization 239 the end of the treatment. A more efficient and non-selective process, advanced oxidation processes (AOPs), generating powerful oxidant of hydroxyl radical (2.8 V oxidation potential) which promote oxidation of pesticides until mineralization seem to be more promising [3]. As a novel AOP, electro-Fenton bases on the continuous electrochemical generation of H2O2 by reaction (1) to a contaminated acid solution containing Fe 2+ or Fe 3+ as catalyst [4]. ● OH is then produced in the medium by the Fenton’s reaction between Fe2+ and H2O2 (Eq. (2)). This catalytic reaction is propagated from Fe 2+ regeneration mainly occurring by the cathodic reduction of Fe 3+ (Eq. (3)). O2 + 2H + + 2e  H2O2 (1) H2O2 + Fe 2+  Fe3+ + OH- + ●OH (2) Fe 3+ +e -  Fe2+ (3) In this paper, we represent a detailed discussion on the effects of some operating parameters such as pH, applied current, catalyst concentration on the degradation of Glyphosate, a highly effective broad-spectrum, post-emergence, non-selective and commonly applied organophosphate herbicide [5], by electro – fenton process. 2. MATERIALS AND METHODS 2.1. Electrochemical system Electrochemical system included a digital DC generator VSP4030 (B&K Precision, CA, US) and two electrodes placed a cylindrical glass cell of 7 cm diameter. The cathode was a 60 cm 2 piece of carbon felt, placed on the inner wall of the cell covering the totality of the internal perimeter. The anode was cylindrical Pt gauze (45 cm 2 area) placed on the centre of the cell and surrounded by the cathode (Fig.1). The distance between the electrodes was 1 cm. Compressed air was bubbled through the solutions at about 1 L.min -1 to supply O2 for producing H2O2 from reaction (1). A catalytic quantity of ferric ion was introduced into the solution before the beginning of electrolysis. All solutions were vigorously stirred with a magnetic bar to allow mass transfer. The pH of solutions was adjusted by sulphuric acid. 2.2. Materials The carbon felt was purchased from A Johnson Matthey Co., Germany. Analytic grade glyphosate (C3H8NO3P, Sigma Aldrich NY, USA) was used without further purification. Iron (II) sulphate heptahydrate (99.5 %, Merck) and sodium sulphate (99 %, Merck) were used as catalyst and supporting electrolyte, respectively. Sulphuric acid (98 %, Merck) was used to adjust the pH of solution. All solutions were prepared with ultra-pure water obtained from a Millipore Milli-Q system with resistivity >18 MΩ.cm at room temperature. Figure 1. Scheme of the experimental set-up used for the electro-Fenton treatments: (1) Electrolytic cell, (2) carbon-felt cathode, (3) platinum anode, (4) magnetic stir bar, (5) digital DC generator. Thanh Son Le, Tuan Duong Luu, Tuan Linh Doan, Manh Hai Tran 240 2.3. Analytical procedures The pH was monitored using a Hanna HI 991001 pH-meter (Hanna instruments Canada Inc.). The mineralization (conversion to CO2, H2O and inorganic ions) of glyphosate solutions was monitored from the decay of their total organic carbon (TOC), determined on a Shimadzu TOC-VCPH analyzer (Shimadzu Scientific Instruments, Kyoto, Japan). The percentage of TOC removal was then calculated from Eq. (4): (4) where TOCt and TOCo are the experimental TOC values at time t and initial time, respectively. 3. RESULTS AND DISCUSSION 3.1.The effect of initial pH To study the effect of pH on the degradation efficiency of glyphosate by using Fenton process, a series of experiments was carried out under acidic conditions at different pH values in the range of 2 and 6 (Figure 2). It was found that the optimum pH for degradation of glyphosate by electro-Fenton was about 3, where the mineralization percentage reached near 60 % in 50 min. Indeed, increase in pH from 3 to 6 caused a decrease in mineralization efficiency from 58 % to 30 % in 50 min and it can be explained as follows. At pH above 3, Fe 3+ could start to be precipitated in the form of amorphous Fe(OH)3 which are less reactive than the dissolved ions and thus the iron concentration is decreased in the solution reducing the degradation efficiency [6]. Also, this hydroxide could partially coat the electrode surface inhibiting Fe 2+ regeneration at the cathode (Eq. (3)). Moreover, at high pH, H2O2 could be catalytically decomposed to oxygen, that reduced its concentration in the solution, potentially creating a hazardous situation [7]. In contrast, decreasing the pH from 3 to 2 reduced the mineralization efficiency from 58 % to 43 % in 50 min. This could be explained by the formation of oxonium ion (H3O2 + ) at pH below 3 (Eq .(5)), which enhanced the stability of and probably to reduce substantially the reactivity with Fe 2+ [8]. A low pH also promotes H evolution at the cathode (Eq. (6)) and then reduces the number of active sites for generating H2O2 [9]. Also, the in situ H2O2 decomposition reaction (Eq. (7)) was possibly carried out and promoted to drop the yields of H2O2 in the medium. It was another ignorable reason for the competition of electrons with H2O2 generation reaction [10]. 0 10 20 30 40 50 0 10 20 30 40 50 60 T O C r em o v al ( % ) Time (min) pH=2 pH=3 pH=4 pH=5 pH=6 Figure 2. Effect of pH and medium on TOC removal for the degradation of 200 ml of 0.1 mM glyphosate aqueous solution with [Fe 2+ ] = 0.1 mM during electro-Fenton treatment at I = 0.1 A. Study of some parameters responsible for glyphosate herbicide mineralization 241 H2O2 + H + → H3O2 + (5) 2 H + + 2e  H2 (6) H2O2 + 2H + + 2e − → 2H2O (7) Based on the observed pH effect, all subsequent experiments were performed at pH 3.This result is consistent with those results in the literature [11 - 12]. 3.2. The effect of electrolysis time and current applied In order to investigate the effect of electrolysis time and current applied on the glyphosate mineralization, several experiments were performed at room temperature and different current values in the range of 0.1 A – 0.50 A, in the presence of 0.1 mM of Fe2+ as catalyst and pH = 3. According to Figure 3, the TOC gradually decayed with electrolysis time and this TOC decay rate has increased by raising the current from 0.1 A to 0.5 A, i.e the degradation efficiency of glyphosate was improved by increasing applied current value. This effect could be related to the amount of •OH radicals produced by the Fenton’s reaction (2), where H2O2 is generated in situ by O2 reduction on the cathode (Eq. (1)). Indeed, at higher current, H2O2 formation rate according to reaction (1) and regeneration rate of Fe 2+ catalyst from reaction (3) could be accelerated, leading to the generation of higher amount of hydroxyl radicals from Fenton’s reaction (Eq. (2)). These results were quite similar to those recorded by Ting et al. [9] and Dirany et al. [13]. However, the use of high current in electro- oxidation process in case of carbon felt electrodes will cause surface corrosion, which reduces their service life [14] and thus we chose 500 mA for the following experiments. 3.3. Influence of Fe 2+ concentration To determine the effect of Fe 2+ concentration on the degradation of glyphosate, several experiments were performed at room temperature and pH = 3, under current controlled electrolysis 500 mA with the concentration of ferrous ions varying from 0.05 – 0.5 mM. The results are reported in Figure. 4. As can be seen, the TOC removal rate increases with increasing Fe 2+ concentration from 0.05 to 0.1 mM and after this value, the degradation rate decreases by raising Fe 2+ concentration. This result can be explained as follows. With the increase of Fe 2+ concentration from 0.05 mM to 0.1 mM, more ●OH was produced by the Fenton’s reaction (Eq. (2)), and consequently an enhanced TOC removal efficiency was observed [15]. In addition, at low concentration of Fe 2+ below 0.1 mM, H2O2 was electrogenerated excessively compared with Fe 2+ concentration. Hence, the excess amount of H2O2 could react with the ● OH to form HO2 ● Figure 3. Effect of current on TOC removal for the degradation of 200 ml of 0.1 mM glyphosate aqueous solution with [Fe 2+ ] = 0.1 mM during electro-Fenton treatment at pH 3. Thanh Son Le, Tuan Duong Luu, Tuan Linh Doan, Manh Hai Tran 242 radical which is less powerful and destructive oxidant rather than ● OH radical (Eq. (8)) [16], so the glyphosate removal efficiency was reduced. H2O2 + ● OH  H2O + HO2 ● (8) In the opposite, the decrease of TOC removal rate by increasing Fe 2+ concentration from 0.1 to 0.5 mM could be related to the progressively fall of ● OH concentration due to consumption of ● OH via the reaction with the excess of ferrous ions (Eq. (9)) [17]. In addition, Fe 3+ formed also could react with H2O2 (Eq. (9) and (10)) resulting in decrease of the TOC removal [18]. Furthermore, use over high concentration of Fe 2+ can generate a large quantity of ferric oxide sludge, resulting in much more requirement of separation and disposal of the sludge. So, 0.1 mM is optimal concentration of ferrous catalyst. Fe 2+ + ● OH  Fe3+ + HO- (9) Fe 3+ + H2O2  Fe−OOH 2+ + H + (10) Consequently, under optimal conditions: pH = 3, I = 0.5 A, [Fe 2+ ] = 0.1 mM, glyphosate initial concentration = 0.1mM, after 50 min, 84.4 % glyphosate was mineralized (Figure 4). This result is in the same trend with that obtained by Ozcan et al. [19] for the degradation of Acid Orange 7 and Hammami et al. [20] for the degradation of direct orange 61. 4. CONCLUSION The decontamination of aqueous glyphosate solutions was firstly carried out by electro- Fenton process using graphite-felt cathode and Fe 2+ as the catalyst. The obtained results demonstrated that pH, electrolysis time, current intensity and Fe 2+ catalyst concentration influenced the glyphosate mineralization efficiency. Under the optimal conditions, pH = 3, [Fe 2+ ] = 0.1mM, I = 0.5 A, [Na2SO4] = 0.05 M, the initial glyphosate concentration of 0.1 mM could be mineralized 84.4 % within 50 min. 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