Impact of TiO2 on the chemical and biological transformation of formulated chiral-metalaxyl in agricultural soils.

The impacts of TiO2 on the chemical and biological transformation of racemic metalaxyl wettable powder (rac-metalaxyl WP) in agricultural soils, and soil microorganisms were investigated. Under simulated solar irradiation, TiO2 highly promoted the transformation of rac-metalaxyl WP without changing the enantiomer fraction, with the promotion amplitude (60-1280%) being dependent on TiO2 characteristics. TiO2 characteristics showed different influence on the transformation of rac-metalaxyl WP in soils and aqueous solutions because their characteristics changed differently in soils. The impact of the mancozeb and other co-constituents on the transformation of rac-metalaxyl WP was smaller in soil media than in aqueous solution. Autoclave sterilization changed soil properties and subsequently weakened the promotion effects of TiO2 on the chemical transformations of rac-metalaxyl WP to 0-233%. Microorganism biomass and bacterial community were not statistically significant changed by TiO2 exposure regardless of rac-metalaxyl WP, suggesting that the promotional effects occurred mainly through chemical processes. The results also showed TiO2-soil interactions may be strengthened with TiO2 (Degussa P25) aging time in soils, which decreased its promotion amplitude from 1060% (without aging) to 880% (aging for 20 days). Intermediate formed in soil biological transformation process was different from that in TiO2 photocatalysis process.