Parameter optimization and degradation mechanism for electrocatalytic degradation of 2,4-diclorophenoxyacetic acid (2,4-D) herbicide by lead dioxide electrodes

Abstract

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).

Fig. 1. Normal probability distribution of residuals for 2,4-D removal efficiency: (a₁, b₁ and c₁), and Residuals versus predicted for 2,4-D removal efficiency (a₂, b₂ and c₂); (a) SS316/β-PbO₂, (b) Pb/β-PbO₂ and (c) G/β-PbO₂.

Fig. 2. Response surface plot for electrochemical process with different anode electrodes; SS316/β-PbO₂ anode (Part I), Pb/β-PbO₂ anode (Part II) and G/β-PbO₂ anode (Part III): (a) j: 1 mA cm⁻², (b) j: 3 mA cm⁻², (c) j: 5 mA cm⁻² and (d) overlay plot for optimal region (pH:7, j: 3 mA cm⁻², time: 50 min).

Fig. 3. LSV grame of β-PbO₂ electrodes in Na₂SO₄ (0.25 M) at pH = 6.0. Scan rate = 100 mV s⁻¹.

Fig. 4. (Part I) Degradation of 2,4-D solution before, during and after electrolysis using modified β-PbO2 electrodes, (Part II) degradation of 2,4-D solution during different electrolysis times using modified β-PbO₂ electrodes (Part III) (a) degradation of 2,4-D solution before and (b) after electrolysis using unmodified electrodes (at pH = 3 in constant current electrolysis processes monitored by cyclic voltammetry technique at 100 mV s⁻¹ in 0.1 M Na₂SO₄ on a glassy carbon electrode).

Scheme 1. Proposed pathway for electrocatalytic degradation of 2,4-D herbicide by anodic oxidation on unmodified graphite, Pb and stainless still electrodes.

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.