Sorption of three tetracyclines by several soils: Assessing the role of pH and cation exchange

Tetracyclines (TCs) are widely used in veterinary medicine for treatment and prevention of disease and are present in animal waste products. Detection of TCs in soil, sediments, and water, and the growing concern of their potentially adverse effect on natural ecosystems have resulted in a need to understand their behavior in aqueous soil systems. TCs have multiple ionizable functional groups such that at environmentally relevant pH values, they may exist as a cation (+ 0 0), zwitterion (+ - 0), or a net negatively charged ion (+ - -), which complicates predicting their sorption, availability, and transport. We investigated the sorption of oxytetracycline (OTC), tetracycline (TC), and chlortetracycline (CTC) by several soils varying in pH, clay amount and type, cation exchange capacity (CEC), anion exchange capacity (AEC), and soil organic carbon in 0.01 N CaCl2, 0.001 N CaCl2, and 0.01 N KCl. All three TCs are highly sorbed, especially in acidic and high clay soils. When normalized to CEC, sorption tends to decrease with increasing pH. A sorption model in which species-specific sorption coefficients normalized to pH-dependent CEC (K-d(+00) ,K-d(+-0) and K-d(+--) and weighted by the pH- dependent fraction of each species fit the data well across all soils except for a soil rich in gibbsite and high in AEC. Resulting K-d(+00) values were more than an order of magnitude larger than values for either K-d(+-0) and K-d(+--) values such that K-d(+00) alone described most of the sorption observed as a function of pH for eight soils that varied in their mineralogy and pH (pH ranged from 4 to 8).