Trends in soil sorption coefficients within common antimicrobial families

Sorption coefficients (Kd) of fluoroquinolone, tetracycline, and sulfonamide antimicrobial compounds were measured for seven soils between pH 4.5 and 8.5 using batch sorption techniques. Soils were chosen to encompass a range of cation exchange capacity (CEC), iron and aluminum oxide and organic matter contents. For most soil-pH pairs, enrofloxacin, norfloxacin, and ciprofloxacin Kd values were within a factor of 2 (0.3 log units) of each other. Lower enrofloxacin sorption than norfloxacin or ciprofloxacin sorption was observed at pH 8.5 for the two most aluminosilicate clay-rich soils, likely due to compound differences in acid dissociation constants, yielding greater anion species abundance for enrofloxacin. Tetracycline, oxytetracycline and chlortetracycline Kd values also were within a factor of 2 for each soil-pH pair. Measured tetracycline and fluoroquinolone compound Kd values could be estimated within a factor of 2 using published empirical multi-linear regression models. In contrast, sulfonamide Kd values varied among compounds, as expected for sorbates that interact primarily with soil organic matter. Results of this research indicate that substituent groups have little effect on sorption interactions of compounds from the tetracycline and fluoroquinolone family that interact with soils primarily through cation exchange, surface complexation and cation bridging sorption mechanisms.