Electrochemical oxidation of pesticide thiamethoxam on boron doped diamond anode: Role of operating parameters and matrix effect

The electrochemical oxidation of the neonicotinoid pesticide thiamethoxam (TMX) on a boron-doped diamond (BDD) anode was investigated. The effect of several operating parameters such as supporting electrolyte (0.025-0.1 M Na2SO4, 0.1 M NaCl), current density (4-40 mA cm−2), TMX concentration (1-10 mg L−1 of active ingredient in a commercial formulation) and initial solution pH (3-11) on degradation was evaluated. The apparent rate constant of a pseudo-first order kinetic model was found to increase with increasing (i) current density (from 0.036 to 1 min−1 at 4 and 40 mA cm−2, 2 mg L−1 TMX and 0.1 M Na2SO4) and (ii) Na2SO4 concentration (from 0.037 to 0.25 min−1 at 0.025 and 0.1 M Na2SO4, 2 mg L−1 TMX and 16 mA cm−2 current density) and decreasing (iii) TMX concentration (from 0.05 to 0.73 min−1 at 10 and 1 mg L−1 TMX, 0.1 M Na2SO4 and 16 mA cm−2 current density) and (iv) initial solution pH (from 0.017 to 2.13 at pH 11 and 3, 0.1 M Na2SO4, 2 mg L−1 TMX and 16 mA cm−2 current density). Experiments in actual matrices, such as secondary treated wastewater and bottled water, resulted in lower kinetics (0.02 and 0.04 min−1, respectively) than in pure water (0.22 min−1), signifying competitive interactions between TMX and the matrix constituents. Of the latter, organic matter (1-15 mg L−1 in the form of humic acid), bicarbonates (>100 mg L−1) and chlorides (>100 mg L−1) all had a detrimental effect on degradation. The rate constant of TMX degradation in pure water was reduced by 82%, 71% and 36% in the presence of 15 mg L−1 humic acid or 500 mg L−1 bicarbonates or 500 mg L−1 chlorides, respectively. Fast TMX degradation (i.e. complete removal of 2 mg L−1 TMX in 20 min at 16 mA cm−2, 0.1 M Na2SO4, in pure water) is accompanied by slower mineralization (i.e. 91% total organic carbon removal at 120 min), indicating the formation of more resistant transformation by-products.