pH-dependent ciprofloxacin sorption to soils: Interaction mechanisms and soil factors influencing sorption

A study of ciprofloxacin sorption to 30 soils from the eastern United States revealed a statistically significant effect of pH on the solid–water distribution coefficient (Kd). Cation exchange capacity was the key soil factor influencing the extent of sorption at all pH values (3–8), with soil metal oxide content playing a smaller role at higher pH. Although, cation exchange, cation bridging and surface complexation mechanisms all appeared to contribute to the net extent of ciprofloxacin sorption to soils, the ciprofloxacin molecule appeared to be best suited for cation exchange. Notably, the ciprofloxacin cation (dominant at pH ≤ 5) exhibited a greater potential for cation exchange than the net neutral zwitterion (relevant at pH > 6). In addition to ciprofloxacin speciation in aqueous solution, the availability of surface sites for cation exchange (cation exchange capacity) and surface complexation (metal oxide content) played a key role in determining the shape of the pH edge and the potential contributions from individual sorption mechanisms to the net extent of sorption. Soils not limited by the availability of cation exchange sites and possessing a high effective cation exchange capacity (ECEC > 20 cmolc/kg), exhibited distinct pH edges (general decrease in sorption with pH) and the highest extent of sorption at all pH values. In these soils, cation exchange was found to be a dominant mechanism of sorption, with potential contributions from cation bridging and surface complexation being either unimportant or not discernable. In soils of lower cation exchange capacity (ECEC < 20 cmolc/kg), limited with respect to the availability of cation exchange sites, sorption initially increased with increasing pH, peaked at pH 5.5 and decreased at pH 7–8. In these soils, a specific contribution from surface complexation to the net extent of sorption was surmised at high pH values in metal oxide-rich soils, possessing low cation exchange capacities.