Degradation of diuron in water by electrochemical advanced oxidation in a microreactor: effects of anion contamination on degradation and toxicity

Abstract

Diuron is a toxic herbicide that has contaminated underground and surface water and caused serious problems in many countries. Diuron residues in water can be treated using electrochemical advanced oxidation processes (EAOPs), and microreactor EAOPs can be developed into portable water treatment units for use in contaminated areas. Up to 91 % of diuron in 10 ppm solution could be degraded within 100 s of residence time in the microreactor. Nitrates and sulfates, which are anions commonly found in natural water, retard EAOP degradation of diuron by scavenging hydroxyl radicals and suppressing hydroxyl radical generation at the anode by adsorbing to the anode surface. The apparent first-order rate constant for diuron degradation is decreased from 2.69 × 10−2 s−1 in deionized water to 1.81 × 10−2 and 1.64 × 10−2 s−1, in water containing 50 mM nitrate and 50 mM sulfate, respectively. Sulfate does not alter the diuron degradation pathway, but nitrate forms reactive species that interact with diuron and its degradation intermediates. The concentrations of intermediates increase when the EAOP is made less effective by the presence of nitrate or sulfate, but the intermediates are less toxic than diuron. At a long residence time (e.g., 100 s), EAOP treatment decreases diuron toxicity more when the water is contaminated with nitrate or sulfate than when the water is pure, under the same conditions.