Degradation of thiophosphate fungicide, tolclofos-methyl by calcium hydroxide and zerovalent iron in soil

Abstract

Degradation of thiophosphate fungicide, tolclofos-methyl in the aquatic environment is mainly biotic, whereas both hydrolytic and photolytic breakdowns are slow and of minor importance. Thus, hydrolysis of tolclofos-methyl during Ca(OH)2 treatments, degradation of fungicide tolclofos-methyl by zerovalent iron, and effect of ferric salts during zerovalent iron treatment were investigated. Unannealed zerovalent iron was used as zerovalent iron source. Tolclofos-methyl was found to undergo hydrolysis only at pH level as high as 12 in aqueous solutions, and its degradation in the soil appeared to be achieved only by biotic degradation. Ferric sulfate and ferric chloride were the most effective for degrading tolclofos-methyl by unannealed zerovalent iron in aqueous solution. Order of effectiveness of salts is FeCl3>Fe2(SO4)3>FeSO4>Al2(SO4)3; only ferric chloride salts were effective for enhancing tolclofos-methyl degradation in the soil slurry. Degradation rate of tolclofos-methyl in the soil slurry was observed up to 25 and 37% by adding 1 and 3% FeCl3 respectively. It was 2–3 orders of magnitude higher than the degradation by unannealed zerovalent iron alone. Enhanced effect of ferric chloride on tolclofos-methyl degradation by unannealed zerovalent iron was retarded when 5% salts was added into soil slurry. Addition of FeCl3 into soil, in the absence of unannealed zerovalent iron, was found to degrade tolclofos-methyl to some extent.