Complexation of the antibiotic tetracycline with humic acid

The effect of solution chemistry and sorbate-to-sorbent ratio on the interaction of the antibiotic tetracycline with Elliott soil humic acid (ESHA) was investigated using equilibrium dialysis and FITEQL modeling. Tetracycline speciation strongly influenced its sorption to ESHA. Sorption was strongly pH-dependent with a maximum around pH 4.3, and competition with H+ and electrolyte cation (Na+) was evident. The pH-dependent trend was consistent with complexation between the cationic/zwitterionic tetracycline species and deprotonated sites in ESHA (mainly carboxylic functional groups). Modification of ESHA by Ca2+ addition increased tetracycline sorption suggesting that ternary complex formation (ESHA–metal–tetracycline) may be important at higher concentrations of multivalent metal cations. The macroscopic data (pH-envelope and sorption isotherms) were successfully modeled using a discrete log K function with the FITEQL 4.0 chemical equilibrium program indicating that ESHA–tetracycline interaction could be reasonably represented as complex formation of a monoacid with discrete sites in humic acid. Sorption–desorption hysteresis was observed; both sorption and desorption isotherms were well described by the Freundlich equation.