Electrochemical oxidation of benzoic acid in water over boron-doped diamond electrodes: Statistical analysis of key operating parameters, kinetic modeling, reaction by-products and ecotoxicity

The electrochemical oxidation of benzoic acid over boron-doped diamond electrodes was studied. Experiments were conducted in a flow-through electrolytic cell at current intensities ranging from 11 to 24 A, an electrolyte concentration of 0.05 M and initial substrate concentrations ranging from 16 to 185 mg  L−1. Liquid chromatography (LC) coupled to diode array detector was employed to follow benzoic acid concentration profiles, while chemical oxygen demand and dissolved organic carbon (DOC) analyses were carried out to assess the extent of mineralization. In preliminary experiments, the effect of different electrolytes (NaNO3, NaCl or Na2SO4) and the initial pH of the solution (10 or 3.8) was evaluated. The effects of operating parameters such as applied current intensity, electrolysis time and initial benzoic acid concentration on the degradation and mineralization efficiency were investigated with the application of factorial design methodology and simple linear models describing and predicting adequately the removal of the substrate and DOC were developed. The initial substrate concentration and the treatment time constitute important parameters with regard to the efficiency of the process. Benzoic acid conversion proceeds through the hydroxylation of the aromatic ring as evidenced by the formation of several hydroxylated derivatives identified by LC coupled to mass spectroscopy (LC/MS-MS). Of these, monohydroxybenzoic acids appear to be quite stable to electrochemical oxidation. Toxicity tests with marine bacteria V. fischeri showed that, at the conditions in question, degradation by-products are consistently more toxic than the parent compound even after deep oxidation.