Probe Compounds to Quantify Cation Exchange and Complexation Interactions of Ciprofloxacin with Soils

Ciprofloxacin (CIP)-soil sorption interactions by surface complexation (via -COOH group) and cation exchange (-NH3+ group) were estimated by use of the structurally related probe compounds flumequine (M) (-COOH) and phenylpiperazine (PP) (-NH3+). Comparison of CIP and FQ sorption by surface complexation on goethite indicated a 0.7 times lower driving force for sorption, K-xs, for CIP than for FQ, with a model that assumed functional group interactions were enhanced by the hydrophobicity of the rest of the molecule. Similarly, K-xs was 9.5 times greater for CIP than for PP for sorption by cation exchange on kaolinite and montmorillonite. Use of the pure phase sorbent K-xs scaling factors between PP, FQ, and CIP for eight soils with a range of clay and oxide contents yielded total sorbed CIP concentrations that were within a factor of 2 (at pH 7.2) or less (at pH 5) of observed values. The estimated relative contributions of CIP cation-exchange versus complexation interactions increased for soils with increasing cation-exchange capacity. The agreement between independently estimated and observed CIP sorption indicates that the use of probe compounds has the potential to provide quantitative measures of sorption contributions for other complex sorbates with multiple functional groups, including other emerging contaminants and pesticides.