Comparison of photocatalysis and photolysis of malathion, isomalathion, malaoxon, and commercial malathion--products and toxicity studies.

Malathion, one of the most widely applied insecticides, is still used in agriculture. There are many studies regarding its degradation under different experimental conditions, but few deal with its transformation products, i.e. malaoxon and isomalathion. Thus, malathion, malaoxon, isomalathion, and Radotion (one of its over 6000 commercial forms) were studied in terms of their degradation kinetics, identification of their transformation products, their toxicity, and their degree of mineralization, during UV photolysis (lambda = 254 nm) and TiO(2) photocatalysis (lambda = 355 nm). The degradation kinetics was similar for all four starting materials. More than 75% of theoretically expected sulfur in PS and P-S groups was oxidized after 240 min of photolysis and photocatalysis. On the other hand, less than 30% of stoichiometrically predicted amounts of phosphate was detected in the photolytic experiments, but more than 80% of expected phosphate was detected after photocatalytic treatment of all four organophosphorous materials. Several transformation products were identified by mass spectra of representative gas chromatographic peaks. Oxidation and isomerization were found as the main reactions of butenedioc acid diethyl esters and their analogs. The formation of malaoxon, isomalathion or trimethyl phosphate esters correlated well with the induced toxicity (inhibition of acetylcholinesterase), which was observed in photocatalysis of malathion and Radotion, and in photolysis of malaoxon and Radotion.