Interactions and transformations of chlorpyrifos in aqueous and colloidal systems
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Chlorpyrifos [o,o-diethyl o-(3,5,6-trichloro-2-pyridyl) phosphorothioate] is a broad-spectrum organophosphorus insecticide widely used in agricultural, industrial, and residential environments. Understanding interaction of chlorpyrifos with colloidal materials is fundamental for assessing fate and toxicity of chlorpyrifos in soil and aquatic environments. The objectives of this study were: (1) to quantify sorption and desorption of chlorpyrifos on colloidal materials in aqueous systems; (2) to investigate hydrolysis of chlorpyrifos in aqueous systems; (3) to evaluate the effects of colloidal materials on the fate of the sorbed chlorpyrifos in aqueous systems; and (4) to study abiotic transformation of chlorpyrifos in water;Six reference smectites, humic acid and suspended sediment collected from the Upper Cedar River near Janesville, Iowa were used for the present study. X-ray diffraction, Scanning Electron Microscopy, ICP-AES and chemical analysis were employed to characterize the colloidal materials. GC and HPLC were used to quantify chlorpyrifos and its degradates in aqueous systems. The results indicate a large difference in sorption affinities and variation in desorption among smectites. The sorption affinity for chlorpyrifos was not correlated to cation-exchange capacity, surface area or surface charge density, but may be related to the fabric of smectite quasicrystals. Substantial sorption-desorption hysteresis was found for all smectites and the suspended sediment. Chlorpyrifos was strongly sorbed by HA with negligible desorption in aqueous solution. Chlorpyrifos degraded in aqueous systems with half-lives ranged from 27 to 158 days, depending on the chemistry of aqueous solutions. The rate of chlorpyrifos hydrolysis decreased in the presence of suspended smectites in aqueous systems. The formation of chlorpyrifos-colloid complexes inhibited hydrolysis and influenced fate of chlorpyrifos in aqueous systems, suggesting that sorption of chlorpyrifos on suspended sediment may potentially prolong the exposure time of organisms such as fish to chlorpyrifos in aquatic environments. An abiotic transformation of chlorpyrifos to chlorpyrifos oxon in tap water is reported in this study. Active chlorine dispersed in tap water was responsible for the chemical transformation. The findings raise a new concern about the safety of domestic use of chlorpyrifos products.