The effects of inherent minerals in biochars and low molecular weight organic acids (LMWOAs) on chlorpyrifos and chlorpyrifos-methyl adsorption by biochars are unclear. We examined the sorption of chlorpyrifos and chlorpyrifos-methyl onto giant reed-derived biochars before and after deashing or LMWOA aging. The effect of citric acid (CA) as a co-solute on the sorption of chlorpyrifos and chlorpyrifos-methyl was also investigated. With increasing temperature (300-600 °C), the adsorption capacity of biochars increased from 4.32 to 14.8 mg/g for chlorpyrifos and from 15.0 to 50.5 mg/g for chlorpyrifos-methyl. This can be explained by the fact that higher temperature biochar had more aromatic units and pores for capturing more sorbates. The deashing and LMWOA aging treatments exposed more carbon surfaces and improved the porosity of biochar, thus favoring sorption. Further, the deashing treatment resulted in greater sorption enhancement, when compared with the LMWOA aging treatment. At pH 6.5, CA2- and CA3- chelated Ca2+ via bridging at CA concentration below 10 mmol/L, thus reducing the competition of Ca2+ for aromatic surfaces and COO-/OH groups. When the CA concentration was above 20 mmol/L, CA2-, CA3-, and [Ca(CA)2]x- inhibited the sorption of chlorpyrifos and chlorpyrifos-methyl by competing for carbon sites and pores of biochar. These findings will help guide the practical application of biochar in pesticide-contaminated water and soil, and to better understand the role of biochar in the transport, fate, and bioavailability of organophosphorus pesticides in the rhizosphere.
Keywords: Co-solute; Deashing; Low molecular weight organic acid; Organophosphate pesticide; Pyrolytic temperature; Rhizosphere.
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