A consecutive chlorination and alkaline hydrolysis process for rapid degradation and detoxication of malathion in aqueous solution

Although oxidation and advanced oxidation can degrade malathion from water, unfortunately it is generally transformed into more toxic products, such as malaoxon. In the present study, a new approach has been proposed for the degradation and detoxication of malathion in aqueous solution using a combined process of chlorination followed by alkaline hydrolysis. The possible transformation pathway was proposed after identification of the main intermediate products using ultra-performance liquid chromatography - electrospray tandem mass spectrometry. The results showed that on average 90.3% of malathion was transformed into malaoxon during chlorination via a desulfurization reaction. After that, in the subsequent hydrolysis reaction, malaoxon was decomposed to malaoxon monoacid and diethyl 2-mercaptosuccinate via a carboxyl ester hydrolysis and a competing elimination reaction, respectively. At the same time, toxicity assessment of the treated solutions during the whole treatment process was performed based on the inhibition of acetylcholinesterase. It was found that during chlorination, the toxicity index H-AchE of the malathion aqueous solution increased sharply to a maximum within 5 min due to the formation of malaoxon in solution. In the subsequent hydrolysis process, the toxicity index of the treated solution was almost unchanged during the first 10 min, but then quickly decreased to about zero within the next 10 min. This indicated a complete detoxication of the newly formed malaoxon and the whole malathion solution.