Insights into tetracycline adsorption onto kaolinite and montmorillonite: experiments and modeling

Adsorption of tetracycline (TC) on kaolinite and montmorillonite was investigated using batch adsorption experiments with different pH, ionic strength, and surface coverage. As a result, pH and ionic strength-dependent adsorption of TC was observed for the two clay minerals. The adsorption of TC decreased with the increase of pH and ionic strength, and high initial TC concentration had high adsorption. In addition, a triple-layer model was used to predict the adsorption and surface speciation of TC on the two minerals. As a result, four complex species on kaolinite (a parts per thousand X(-)a (TM) H3TC+, a parts per thousand X(-)a (TM) H2TC +/-, a parts per thousand SOH(0)a (TM) H2TC +/-, and a parts per thousand SOH(0)a (TM) HTC-) and three species on montmorillonite (a parts per thousand X(-)a (TM) H3TC+, a parts per thousand X(-)a (TM) H2TC +/-, and a parts per thousand SOH(0)a (TM) HTC-) were structurally constrained by spectroscopy, and these species were also successfully fitted to the adsorption edges of TC. Three functional groups of TC were involved in these adsorption reactions, including the positively charged dimethylamino group, the C=O amide I group, and the C=O group at the C ring. Combining adsorption experiments and model in this study, the adsorption of TC on kaolinite and montmorillonite was mainly attributed to cation exchange on the surface sites (a parts per thousand X-) compared to surface complexation on the edge sites (a parts per thousand SOH) at natural soil pH condition. Moreover, the surface adsorption species, the corresponding adsorption modes, and the binding constants for the surface reactions were also estimated.