Tetracycline sorption to coal and soil humic acids: An examination of humic structural heterogeneity

The sorption properties of tetracycline were compared between a coal humic acid (CHA) and a soil humic acid (SHA) under various solution chemistry conditions. Structural characteristics of the two humic acids were quantified in detail by elemental analysis, solid-state (13)C nuclear magnetic resonance (NMR), acid-base titration, and Zeta potential measurement. The CHA consists primarily of poly(methylene)-rich aliphatics with more aromatic C--O and higher negative surface charge, while SHA is mainly composed of young material residues of lignin, carbohydrates and peptides, and oxidized charcoal. At pH 5 the sorption affinities of tetracycline to SHA and CHA were very close, but remarkably stronger than that to functionality-free model polymeric sorbents (polyethylene and polystyrene). Meanwhile, despite the much lower hydrophobicity, tetracycline displayed stronger sorption to the humic acids than nonpolar, nonionic 1,3,5-trichlorobezene. It is thus concluded that specific complexation (H--bonding and cation exchange) with the humic functionality overwhelmed hydrophobic effect in sorption when tetracycline was dominated by the zwitterion. Furthermore, modifying solution chemistry conditions (pH, ionic strength of NaCl and CaCl2, and the presence of Zn2+) generally caused more prominent effects on tetracycline sorption to CHA than to SHA, which was attributed to the higher surface charge of CHA. Results of this research demonstrate the importance of the structural nature of humic acids in antibiotic sorption.