Electrodegradation of 2,4-dichlorophenoxyacetic acid herbicide from aqueous solution using three-dimensional electrode reactor with G/β-PbO2 anode: Taguchi optimization and degradation mechanism determination

Abstract

This study aimed to investigate the electro-degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) from aqueous solution using two and three-dimensional electrode (2D and 3D) reactors with graphite(G)/β-PbO₂ anode. To increase the degradation efficiency, affecting parameters on the electro-degradation process were investigated and optimized by adopting the Taguchi design of experiments approach. The structure, morphology and electrochemical properties of the electrodes were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), linear sweep voltammetry and cyclic voltammograms. The controllable factors, i.e., electrolysis time, 2,4-D initial concentration, solution pH and current density (j) were optimized. Under optimum conditions, the 2,4-D degradation efficiency was 75.6% using 2D and 93.5% using 3D electrode processes. The percentage contribution of each controllable factor was also determined. The pH of the solution was identified as the most influential factor, and its percentage contribution value was up to 39.9% and 40.4% for 2D and 3D electrode processes, respectively. Considering the parameters of the kinetics, it was found that the degradation of 2,4-D and removal of COD using the G/β-PbO₂ electrode obey the pseudo-first order kinetics. In addition, the mineralization pathway of 2,4-D at G/β-PbO₂ electrode was proposed. The results also demonstrated that the 3D electrode process with G/β-PbO₂ anode can be considered as a useful method for degradation and mineralization of 2,4-D herbicides from aqueous solution.

Fig. 1. (a and b) The effect of electrolysis time, (c and d) the effect of pH, (e and f) the effect of 2,4-D concentration and (g and h) the effect of current density on the S/N ratio in the degradation of 2,4-D herbicide, a, c, e and g 2D and b, d, f and h 3D electrodegradation process. Circles on figures indicate optimum electrolysis time for electrochemical process.

Fig. 2. Degradation of 2,4-D herbicide solution before, during and after electrolysis at pH: 5 using G/β-PbO₂ electrode in constant current electrolysis processes, that monitoring by cyclic voltammetry technique at 100 mV s⁻¹ in 0.1 M Na₂SO₄ and 100 mg L⁻¹ of 2, 4-D on a glassy carbon electrode.

Fig. 3. LC/MS chromatographs after degradation of 2,4-D herbicide at the optimum conditions.

Scheme 1. Proposed pathway for electrocatalytic degradation of 2,4-D herbicide by anodic oxidation on G/β-PbO₂ electrode.

Fig. 4. Kinetics of (a) 2,4-D degradation, (b) COD removal at the optimum conditions (herbicide initial concentration = 100 mg L⁻¹, current density = 9 mA cm⁻², pH = 5, Na₂SO₄ dosage = 1 g/250 cc).