Influence of Ca2+ on tetracycline adsorption on montmorillonite.

The adsorption of tetracycline (TC) on montmorillonite was studied as a function of pH and Ca(2+) concentration using a batch technique complemented with X-ray diffraction and transmission electron microscopy. In the absence of Ca(2+), TC adsorption was high at low pH and decreased as the pH increased. In the presence of Ca(2+), at least two different adsorption processes took place in the studied systems, i.e., cation exchange and Ca-bridging. Cation exchange was the prevailing process at pH<5, and thus, TC adsorption decreased by increasing total Ca(2+) concentration. On the contrary, Ca-bridging was the prevailing process at pH>5, and thus, TC adsorption increased by increasing Ca(2+) concentration. The pH 5 represents an isoadsorption pH where both adsorption processes compensate each other. TC adsorption became independent of Ca(2+) concentration at this pH. For TC adsorption on Ca(2+)-montmorillonite in 0.01 M NaCl experiments, the ratio adsorbed TC/retained Ca(2+) was close to 1 in the pH range of 5-9, indicating an important participation of Ca(2+) in the binding of TC to montmorillonite. X-ray diffraction and transmission electron microscopy showed that TC adsorption induced intercalation between montmorillonite layers forming a multiphase system with stacking of layers with and without intercalated TC.