Abdollah Dargahia,
Amin Ansarib,
Davood Nematollahib,
Ghorban Asgaria,
Reza Shokoohia and
Mohammad Reza Samarghandi*c
aDepartment of Environmental Health Engineering, School of Health, Hamadan University of Medical Sciences, Hamadan, Iran
bDepartment of Chemistry, Faculty of Chemistry, Bu-Ali-Sina University, Hamadan, Iran
cDepartment of Environmental Engineering School of Public Health, Hamadan University of Medical Sciences, Hamadan, Iran. E-mail: samarghandi@umsha.ac.ir
First published on 12th February 2019
2,4-Dichlorophenoxyacetic acid (2,4-D) is one of the most commonly used herbicides in the world. In this work, the electro-catalytic degradation of 2,4-D herbicide from aqueous solutions was evaluated using three anode electrodes, i.e., lead dioxide coated on stainless steel 316 (SS316/β-PbO2), lead dioxide coated on a lead bed (Pb/β-PbO2), and lead dioxide coated on graphite (G/β-PbO2). The structure and morphology of the prepared electrodes were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The process of herbicide degradation was monitored during constant current electrolysis using cyclic voltammetry (CV). In this study, the experiments were designed based on the central composite design (CCD) and were analyzed and modeled by response surface methodology (RSM) to demonstrate the operational variables and the interactive effect of three independent variables on 3 responses. The effects of parameters including pH (3–11), current density (j = 1–5 mA cm−2) and electrolysis time (20–80 min) were studied. The results showed that, at j = 5 mA cm−2, by increasing the reaction time from 20 to 80 min and decreasing the pH from 11 to 3, the 2,4-D herbicide degradation efficiency using SS316/β-PbO2, Pb/β-PbO2 and G/β-PbO2 anode electrodes was observed to be 60.4, 75.9 and 89.8%, respectively. Moreover, the results showed that the highest COD and TOC removal efficiencies using the G/β-PbO2 electrode were 83.7 and 78.5%, under the conditions pH = 3, electrolysis time = 80 min and j = 5 mA cm−2, respectively. It was also found that G/β-PbO2 has lower energy consumption (EC) (5.67 kW h m−3) compared to the two other studied electrodes (SS316/β-PbO2 and Pb/β-PbO2). The results showed a good correlation between the experimental values and the predicted values of the quadratic model (P < 0.05). Results revealed that the electrochemical process using the G/β-PbO2 anode electrode has an acceptable efficiency in the degradation of 2,4-D herbicide and can be used as a proper pretreatment technique to treat wastewater containing resistant pollutants, e.g., phenoxy group herbicides (2,4-D).
2,4-Dichlorophenoxyacetic acid (2,4-D) is a herbicide, which is extensively used around the world. The excellent properties including the low cost and the high efficiency to control the broadleaf weeds in a variety of settings such as crops, rights-of-way, lawns and forests have introduced the 2,4-D as the most commonly used herbicides.5–7
Degradation of 2,4-D is very slow in water; its half-life in water range from one to several weeks under aerobic conditions, while it can be more than 120 days under anaerobic conditions.8 The International Agency for Research on Cancer of the World Health Organization categorized this herbicide as “possibly carcinogenic to humans, according to insufficient evidence in humans and limited evidence in experimental animals”.9 The issues associated with the exposure to this herbicide have been studied in different applicators, but the risk of 2,4-D to human health has not thoroughly assessed.10 The results of the previous studies clarified that the consumption of 2,4-D by laboratory animals has resulted in the objectionable effects on eyes, thyroid, kidneys, adrenal glands, ovaries and testicles.11 It is also observed that the utilization of this herbicide is associated with teratogenic, genotoxic, neurotoxic, immune suppressant and cytotoxic effects.11,12 The extensive use and poor biodegradability of 2,4-D are led to its ubiquitous presence in the environment, which is led to the contamination of surface and ground waters.13 Hereupon, the attempts have vastly implemented to eliminate this herbicide through the different techniques such as adsorption,14,15 biological decomposition8,16,17 and plasma-ozonation.9 However, these techniques have serious disadvantages and issues, e.g., high cost, incomplete pollutant removal, production of toxic by-products, need to add the chemical compounds, sludge production and need for more treatment.18
Over the last two decades, the electrochemical technologies, due to their high efficiency, environmentally friendly and versatility have provided great developments in wastewater treatment, especially for the elimination of the bio-refractory substances.19 Since the electrochemical advanced oxidation processes (EAOPs) are not involved in the toxic reagents and the electron is considered as their main reagents, they account for as the eco-friendly emergent techniques for water remediation.20 The degradation process is performed by the direct reaction of organics at the anode surface by charge transfer, whereas the degradation of pollutants, at high current, is preferentially carried out due to the oxidation of the pollutants by the oxidizing agents produced at the anode.21,22 The anodic oxidation or electrochemical oxidation (EO) is the most convenient and common EAOP, in which the physisorbed M (·OH) radical produced by water oxidation degrades the organics in this technique.21 The anodes, which have incredible potential for the oxygen evolution reaction (OER) and are capable to generate the weakly adsorbed hydroxyl radicals, are considered as the brilliant anodes for electro-oxidation of organic compounds.23 Hence, the anodes such as boron-doped diamond (BDD), SnO2–Sb, IrO2, and PbO2 have emerged as the most talented anodes for organic degradation.24,25 However, although the BDD electrode is chemically stable and shows the excellent ability in the degradation of the organics, it is still very expensive.26 The SnO2–Sb electrode is much less expensive, but it is deactivated after a short period of use.27 On the other hand, although PbO2 anodes show the lower electrochemical activity in the degradation of organics, compared to BDD and Sb-doped SnO2 electrodes, they offer the advantages including the easier production by electrodeposition, low electrical resistivity, and good electrochemical activity. Moreover, their stability was observed to be remarkable at high potentials and at different pH values.28,29
In the present study, three anode electrodes, i.e., lead dioxide coated on stainless steel 316 (SS316/β-PbO2), lead dioxide coated on the lead bed (Pb/β-PbO2), lead dioxide coated on graphite (G/β-PbO2) were employed to study the degradation of 2-4-D herbicide from aqueous solutions. The experiments were designed based on central composite design (CCD) and were analyzed using the response surface methodology (RSM), which it provides proper statistical tools to design and to optimize the studied process. Morphology of prepared electrodes is characterized by scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). The process of herbicide degradation was monitored during constant current electrolysis using cyclic voltammograms (CV) techniques. After the electrolysis process and complete degradation of 2,4-D herbicide, the leaching of lead ions and the mechanism of 2,4-D degradation were evaluated.
Elemental analysis of the β-PbO2 was performed using Energy-dispersive X-rays (EDX) technique and results were presented in Fig. S3 (Part II).† As shown in Fig. S3 (Part II),† the weight percentage of oxygen (O) and lead (Pb), the main elements existed in the β-PbO2, were 20.9%, 79.1% for graphite electrode and 19.5%, 80.5% for Pb electrode and 20.1%, 79.9% for SS316, respectively.
X-ray diffraction patterns (XRD) measurements were used to estimate the phases and crystallinity of PbO2 and the deposited film. Fig. S3 (Part III†) is related to the XRD of the PbO2 layer deposited on the graphite, lead and SS316 interlayers in which the diffraction peaks of the β form of PbO2 has represented. All XRD results show the tetragonal structure of β-PbO2. Unlike the orthorhombic α-PbO2, the tetragonal β-PbO2 is characterized by favorable conductivity,38,42 which can exceptionally assist the anode employed in electro-oxidation of contaminants in aqueous solution. The main peaks, which it is observed at 2θ of 25.4°, 32.0°, 36.2°, 49.1°, 53.5°, 58.7°, 62.5° and 75.4° for these three electrodes (graphite, lead and SS316), are corresponded to the (110), (101), (200), (211), (220), (310), (301) and (321) plane of β-PbO2, respectively.32,39 It is important to note that the existence of β-PbO2 in all samples was proven. In order to calculate the average size of β-PbO2, Debye–Scherrer formula was applied;32 based on the results obtained from this formula, the size of β-PbO2 crystals in the G/β-PbO2, SS316/β-PbO2 and Pb/β-PbO2 electrodes was obtained to be 30.2, 44.7, and 36.0 nm, respectively. These results are consistent with the SEM results.
Run | A: pH | B: j (mA cm−2) | C: time (min) | 2,4-D herbicide removal efficiency (%) | ||
---|---|---|---|---|---|---|
SS316/β-PbO2 (E1) | Pb/β-PbO2 (E2) | G/β-PbO2 (E3) | ||||
1 | 11 | 3 | 50 | 28.08 | 44.81 | 57.91 |
2 | 7 | 3 | 50 | 33.94 | 50.89 | 65.34 |
3 | 7 | 3 | 50 | 34.5 | 55.1 | 68.3 |
4 | 3 | 3 | 50 | 39.15 | 59.87 | 74.99 |
5 | 3 | 1 | 80 | 36.03 | 52.62 | 68.16 |
6 | 7 | 3 | 20 | 16.79 | 31.95 | 45.61 |
7 | 7 | 1 | 50 | 28.72 | 48.25 | 60.56 |
8 | 11 | 5 | 20 | 18.53 | 28.74 | 44.5 |
9 | 7 | 3 | 50 | 30.86 | 48.5 | 61.5 |
10 | 3 | 5 | 80 | 60.46 | 75.93 | 89.88 |
11 | 7 | 5 | 50 | 47.27 | 64.03 | 78.29 |
12 | 3 | 1 | 20 | 14.84 | 28.71 | 44.71 |
13 | 7 | 3 | 80 | 42.89 | 61.13 | 75.34 |
14 | 11 | 1 | 80 | 18.84 | 27.36 | 43.78 |
15 | 11 | 5 | 80 | 33.88 | 50.22 | 62.17 |
16 | 7 | 3 | 50 | 36.67 | 50.4 | 64.62 |
17 | 7 | 3 | 50 | 29.9 | 46.83 | 66.9 |
18 | 3 | 5 | 20 | 23.23 | 33.91 | 47.66 |
19 | 7 | 3 | 50 | 34.21 | 52.4 | 62.99 |
20 | 11 | 1 | 20 | 10.68 | 18.75 | 33.94 |
Response | Modified equations with significant terms | Model | R2 | Adj. R2 | Adeq. precision | S.D | CV | Press |
---|---|---|---|---|---|---|---|---|
Y1 | +34.33 − 6.33A + 7.47B + 10.76C − 4.41AC + 2.85BC − 6.67C2 | Quadratic | 0.954 | 0.933 | 27.09 | 3.06 | 9.89 | 369.83 |
Y2 | +52.11 − 8.12A + 7.71B + 12.52C − 4.48AC + 3.87BC − 11.18C2 | Quadratic | 0.953 | 0.931 | 25.54 | 3.75 | 8.07 | 425.49 |
Y3 | +66.14–8.31A + 7.13B + 12.29C − 4.77AC + 3.33BC − 10.56C2 | Quadratic | 0.956 | 0.935 | 26.49 | 3.54 | 5.82 | 466.52 |
MOx + H2O → MOx(HO˙) + H+ + e− | (1) |
Afterward, the electrochemically generated MOx(HO˙), as one of the strongest oxidant, mineralize the organic matter (eqn (2)).32,43 Furthermore, the MOx(HO˙) can generate the O2 gas (eqn (3)); this reaction acts as a competitor for the reaction shown in eqn (2).
R + MOx(HO˙) → mCO2 + nH2O + xH+ + ye− | (2) |
2MOx(HO˙) → 2MOx + O2 + 2H+ + 2e− | (3) |
According to reports, the oxygen evolution reaction is considerably dependent on the value of oxygen evolution over-potential; so that, the oxygen evolution reaction is identified as the dominant reaction for the electrodes with the low oxygen evolution over-potential. However, this reaction is difficult for the electrodes that have higher oxygen evolution over-potential and; thus, for these kinds of electrodes, the reaction (2) occurs earlier than the reaction (3), which it leads to progress the efficiency of the mineralization reaction.32,43
In order to determine the performance of the electrochemical process, the 2,4-D herbicide degradation was calculated as a response. A quadratic model describes the changes in 2,4-D degradation efficiency as a function of variables. According to Table 2, A (pH), B (j) and C (time) are the most important functions of the model for the electrochemical process using the SS/β-PbO2, Pb/β-PbO2 and G/β-PbO2 anode electrodes. The 2,4-D degradation efficiency using the coated electrodes was investigated at various pH values (3–11), electrolysis time (20–80 min) and current density (1–5 mA cm−2). The surface plot for 2,4-D degradation by the electrochemical process using SS316/β-PbO2, Pb/β-PbO2 and G/β-PbO2 anode electrodes is depicted in (Fig. 2 (Part I–III)) and is representative of the interaction effect of time and pH. As can be seen, increasing the electrolysis time and reducing the pH enhanced the 2,4-D degradation efficiency. As the results showed, by decreasing the pH from 11 to 3 under the same conditions (j = 5 mA cm−2, time = 80 min), the efficiency of the electrochemical process in degradation of 2,4-D using the SS316/β-PbO2, Pb/β-PbO2 and G/β-PbO2 electrodes was increased by 26.5, 25.7 and 27.7%, respectively. In addition, the degradation efficiency of this herbicide using SS316/β-PbO2, Pb/β-PbO2 and G/β-PbO2 was increased by 37.2, 42.0 and 42.2% by an increase in the electrolysis time from 20 to 80 under identical conditions (j: 5 mA cm−2, pH = 3) (Fig. 2 (Part I–III)), respectively. Also, the results showed that 2,4-D degradation efficiency using the SS316/β-PbO2, Pb/β-PbO2 and G/β-PbO2 electrodes varied between 10.6–60.4%, 18.7–75.9% and 33.9–89.8%, respectively. The highest 2,4-D degradation efficiency was observed at pH = 3, time = 80 min and j = 5 mA cm−2. It was found that the lowest efficiency is achieved at the maximum pH value and the lowest electrolysis time and current density. It was evident that the pH was more effective on the 2,4-D degradation efficiency compared to electrolysis time. The obtained results are consistent with the results of Aquino Neto et al. (2009);44 in their study, it was observed that increasing the pH from 2 to 11 is led to decrease the degradation efficiency of glyphosate herbicide (GH); so that, the highest GH degradation was achieved at pH = 2 (38%). In their study, the lower pH values diminished the oxygen evolution reaction in favor of organic compound oxidation,44 which is agreed with the results of the present study. Jaafarzadeh et al. (2018) observed that decreasing the pH and increasing the electrolysis time are led to improve the 2,4-D removal efficiency; it is in line with the present study.41 In the present study, increasing the 2.4-D degradation in acidic pH can be attributed to the higher efficiency of the hydroxyl radical production in acidic pH values and production of some ions such as Cl, Cl2 and HOCl that have higher efficiency at acidic pH values.45,46 The effect of current density on the degradation of 2,4-D is presented in Fig. 2 (Part I–III (a–c)). Considering the figuers from (Fig. 2 Part I (a–c)) to (Fig. 2 Part III (a–c)), it can be concluded that increasing the current density leads to reducing the electrolysis time and increasing the efficiency of 2,4-D herbicide degradation. The greater hydroxyl radical generation at the higher current density may be the reason for this event; so that, the herbicide degradation efficiency using SS316/β-PbO2 electrodes (from 36.0 to 60.4%), Pb/β-PbO2 (from 52.6 to 75.6%) and G/β-PbO2 (68.1 to 89.8%) is developed by increasing the current density from 1 mA cm−2 to 5 mA cm−2 at the same conditions (pH = 3 and time = 80 min), which is consistent with the study conducted by Jaafarzadeh et al. (2018).41 In their study, the results showed that increasing the current density increases the removal efficiency of herbicide; so that, increasing the current from 10 mA cm−2 to 40 mA cm−2 is led to enhance the removal efficiency from 52% to 82%.41 In the study conducted by Yahiaoui et al. (2013), it was observed that the tetracycline (TC) removal efficiency is developed by increasing the current density; so that, the removal efficiency increased from 33.5 to 77.7% by increasing the current density from 2.5 to 25 mA cm−2,47 which is in accordance with the present study. As discussed before, this effect should be related to increasing the production of electrogenerated OH˙ radicals from the water discharge by increasing current density.
The results showed that all three types of electrodes have suitable performance in degradation of the 2,4-D herbicide; but among them, the G/β-PbO2 anode electrode was able to provide the highest herbicide degradation efficiency compared to other studied electrodes. The high potential of the G/β-PbO2 can be considered as the reason for its superior performance in the electrochemical generation of HO˙ radicals compared to other studied electrodes. The properties of G/β-PbO2 electrode include the high oxygen evolution potential, good electric conductivity, good electrodeposition and high specific area, which lead to the exceptional performance of this electrode.32,48–50
To justify the performance of β-PbO2 electrodes in the process of electro-catalytic degradation of the 2,4-D herbicide, the experiment was carried out to determine the anodic potential of the three electrodes prepared (SS/β-PbO2, Pb/β-PbO2 and G/β-PbO2) for water oxidation in 0.25 M Na2SO4 electrolyte solution at pH of 6 and scan speed of 100 mV s−1. Fig. 3 shows that the oxygen evolution over-potential is high for all three electrodes, and these electrodes have well expanded the potential window of the water oxidation. According to Fischbacher et al. (2013), the high oxygen evolution over-potential on the surface of these electrodes produces a large amount of HO˙, which increases the oxidation efficiency and improves the performance of these electrodes for degradation of organic pollutants.51
According to this data, our β-PbO2 electrodes clearly have high oxygen overpotential and produces effectively H2O2 and O3. According to the reaction shown below, the reaction of generated H2O2 with O3 produces hydroxyl radicals.
H2O2 + 2O3 → 2HO˙ + 3O2 |
Optimum condition | 2,4-D herbicide degradation (%) | ||
---|---|---|---|
SS316/β-PbO2 | Pb/β-PbO2 | G/β-PbO2 | |
Experimental results | 33.94% | 50.89% | 65.34% |
Model response | CI low: 32.24, CI high: 36.42 | CI low: 49.54, CI high: 54.67 | CI low: 63.72, CI high: 68.56 |
Error | 0.97 | 1.18 | 1.12 |
Standard deviation | ± 3.06 | ± 3.75 | ± 3.54 |
In optimum conditions, the energy consumption (EC) for three SS316/β-PbO2, Pb/β-PbO2 and G/β-PbO2 electrodes was investigated (Fig. S4†). The results showed that the EC for the three electrodes of SS316/β-PbO2, Pb/β-PbO2 and G/β-PbO2 was 6.46, 6.15 and 5.67 kW h m−3, respectively. According to the results, the G/β-PbO2 electrode has a lower EC compared to two other electrodes due to high efficiency in the 2,4-D herbicide degradation, which it was even lower than the EC reported in studies conducted by Pipi et al. (2014),52 Souza et al. (2015),53 Hashim et al. (2017 (ref. 54)) and Kobya et al. (2016);55 in the mentioned studies, the energy consumption was observed to be 16.9, 455.5, 6.21 and 11.17 kW h m−3, respectively.
As shown in Fig. 4 (Part I), during the electrolysis process, the peak current of A1 was reduced and new peaks of C0 and A0 emerged. Reducing the peak current of A1 indicates the consumption of the raw material of the herbicide and the appearance and increase of the peaks of the C0 and A0 represents the formation of the intermediates, which are produced during the electrolysis process. As seen in Fig. 4 (Part II), at the beginning of the electrolysis, the peak current of A1 diminished and the peaks of A0/C0 begin to emerge. This process continues until the peak of A1 disappears completely. The complete removal of A1 peak shows the complete degradation of the 2,4-D herbicide. According to the Scheme 1 and Fig. 4 (Part III), it is observed that the 5-benzoquinone and 5-parabensucinone are the byproducts and intermediates resulted from the oxidation of the 2,4-D herbicides. Also, the appearance of the A0/C0 peak during electrolysis specifies the presence of these intermediate in this electrolysis (Fig. 4 (Part III)).
Scheme 1 Proposed pathway for electrocatalytic degradation of 2,4-D herbicide by anodic oxidation on unmodified graphite, Pb and stainless still electrodes. |
By the continuation of the electrolysis process, the peaks of A0/C0 started to decrease and, finally, these peaks also disappeared completely, which it is indicative of the complete degradation of the intermediate formed during the electrolysis process (Scheme 2). The empirical observations in the Souza et al. (2015)53 and the empirical data validates the mechanism presented in Scheme 2.
Scheme 2 Proposed pathway for electrocatalytic degradation of 2,4-D herbicide by anodic oxidation on modified G/β-PbO2, SS316/β-PbO2 and Pb/β-PbO2 electrodes. |
The comparison of the 2,4-D degradation mechanism using the PbO2 coated electrode and the un-coated electrodes used in previous studies disclosed that, using the β-PbO2, the 2,4-D herbicide as well as the intermediates generated during the electrolysis process are completely degraded and the final products of degradation are the water (H2O) and carbon dioxide (CO2) (Fig. 4 (Part I and II) and Scheme 2); however, the uncoated graphite and SS316 can only degrade the herbicide and are not able to degrade the intermediates during the electrolysis and, as a result, these intermediates remain as the by-products in final solution after electrolysis (Fig. 4 (Part III) and Scheme 1). PbO2, due to having the high overvoltage for production of the oxygen and high potential window for oxidation of water, is capable to electrochemically production of the strong oxidizing species such as ozone (O3) and hydrogen peroxide (H2O2) during the electrolysis process, which the reactions between these two oxidants are led to produce the hydroxyl radical. The electrochemically produced hydroxyl radical through the reaction between the O3 and H2O2 and through the decomposition of the water on the surface of the Pb electrode are the driving force and the main factor leading to fulfill the degradation process and to convert the raw material and the intermediate formed during the electrolysis process to water (H2O) and carbon dioxide (CO2).8,16,32,53,56 It should be noted that the proposed mechanism for degradation the 2,4-D herbicide in this study was reported based on the results of LC/MS, GC/MS and HPLC in the studies conducted by Souza et al. (2015),53 Sanchis et al. (2013),8 Jaafarzadeh et al. (2018),41 Fontmorin et al. (2012)16 and according to results of LC/MS spectrum obtained in our previous study34 and the electrochemical data obtained in our current study. Furthermore, in order to prove the proposed mechanism for degradation of 2,4-D herbicide and identification of intermediates, after 80 min, electrolysis was stopped and the LC-MS spectrum of the 2,4-D herbicide solution was provided. Table 4 and Fig. S5† shows the presence of molar mass for each of the intermediates in the corresponding spectrum, which it is accounted for as the reason for the correctness of pathway for the degradation of the 2,4-D herbicide and the proposed mechanism (Schemes 1 and 2).
According to deactivation of the electrode by increasing the cell potential to 10 V, the life-time of electrodes studied are observed to be as follows: G/PbO2(80 h) > SS/PbO2 electrode (52 h) > Pb/PbO2 electrode (40 h).46 The remarkable stability of the graphite electrode may be due to several factors. One of these factors is the penetration of lead dioxide particles into the inner layers of the graphite bed, which provides more interaction and adhesion of the PbO2 film to the graphite substrate. Another factor, as shown in SEM (Fig. S3, Part I†) and XRD (Fig. S3, Part III†) images, is the fact that decreasing the size of β-PbO2 particles on the graphite surface can reduce the defect density at the electrode surface and increase the electrochemical stability of G/β-PbO2.32 Therefore, the modification of the surface of the electrodes not only decreases or eliminates the possibility of the penetration of electrolyte through the cracks and pores but also leads to an increase in internal pressure caused by the production of oxygen gas inside the electrode. Furthermore, the greater life-time of the SS316/β-PbO2 electrode, compared to the Pb/β-PbO2 electrode, can be due to the smooth and dense morphology of coated lead dioxide particles, which inhibits the penetration of the electrolyte into the coating and the formation of a passive layer on the surface. In addition, due to the improvement of the oxygen evolution potential (OEP) for G/β-PbO2 and SS316/β-PbO2 electrodes, the penetration of reactive oxygen species into the stainless steel and graphite bed is also inhibited.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra10105a |
This journal is © The Royal Society of Chemistry 2019 |