Significance of anion exchange in pentachlorophenol sorption by variable-charge soils

Sorption data and subsequent predictive models for evaluating acidic pesticide behavior on variable-charge soils are needed to improve pesticide management and environmental stewardship. Previous work demonstrated that sorption of pentachlorophenol (PCP), a model organic acid, was adequately modeled by accounting for pH-and pKa-dependent chemical speciation and using two organic carbon-normalized sorption coefficients; one each for the neutral and anionic species. Such models do not account for organic anion interaction to positively charged surface sites, which can be significant for variable-charge minerals present in weathered soils typical of tropical and subtropical regions. The role of anion exchange in sorption of ionizable chemicals by variable-charge soils was assessed by measuring sorption of PCP by several variable-charge soils from aqueous solutions of CaCl2, CaSO4, Ca(H2PO4)2 as a function of pH. Differences in sorption from phosphate and chloride electrolyte solutions were attributed to pentachlorophenolate interactions with anion exchange sites. Suppression of PCP sorption by phosphate ranged from negligible in a soil with essentially no positively charge sites, as measured by negligible anion exchange capacity, to as much as 69% for variable-charge soils. Pentachlorophenolate exchange correlated well with the ratio of pH-dependent anion exchange capacity to net surface charge. Sorption reversibility of PCP by both CaCl2 and Ca(H2PO4)2 solutions was also demonstrated. Results for PCP clearly demonstrate that sorption to anion exchange sites in variable-charge soils should be considered in assessing pesticide mobility and that phosphate fertilizer application may increase the mobility of acidic pesticides.