Adsorption of tetracycline on kaolinite with pH-dependent surface charges

Kaolinite is a major type of clay minerals in soils of warm and humid climate. Although it has a much lower cation exchange capacity (CEC) and specific surface area compared to swelling clays, its ubiquitous existence as well as its pH-dependent surface charge makes it an important component to study the interactions between contaminants and soils. Tetracycline (TC) is a group of broad spectrum antibiotics used extensively in human and veterinary medicine. It has a high aqueous solubility and a long environmental half-life. In this study, the interactions between TC and kaolinite in aqueous solution were investigated in batch tests and supplemented by FTIR analyses. The adsorption of TC on kaolinite was mainly on the external surfaces via cation exchange as confirmed by stoichiometric desorption of exchangeable cations and simultaneous adsorption of H+ rather than due to complexation. Under acidic conditions, a reduction in surface charge, thus the CEC, resulted in more desorption of exchangeable cations compared to TC adsorption. Fitting of the experimental data to the adsorption of different species revealed that TC+ accounted for 4/5 of the total TC adsorbed with the remaining by zwitterion TC0, possible via hydrogen bonding. At higher temperature, the pKa2 and pKa3 values seem to shift a pH unit lower. Due to its pH-dependent charge of kaolinite, TC adsorption is more pH dependent. The TC adsorption capacity on kaolinite was much lower compared to that on swelling clays. However, the adsorption rate constant was faster than that on swelling clays owing to surface adsorption instead of intercalation. Despite its low TC adsorption capacity, the ubiquitous existence of kaolinite in soils of warm climate may play a vital role in the fate and transport of TC in these soils.

The fitted values are: KdTC+ = 232 L/kg, KdTC0 = 49 L/kg, KdTC- = −2 L/kg for adsorption of cationic, zwitterionic, and anionic TC.Figure optionsDownload high-quality image (104 K)Download as PowerPoint slideResearch highlights
► Kaolinite is a good adsorbent for tetracycline (TC) in aqueous solution.
► Cation exchange is the dominant mechanism for TC adsorption.
► TC adsorption is on the external surfaces of kaolinite.
► Solution pH had strong influence on TC adsorption.