The adsorption of tetracycline (TC) on montmorillonite was studied as a function of pH and Ca2+ concentration using a batch technique complemented with X-ray diffraction and transmission electron microscopy. In the absence of Ca2+, TC adsorption was high at low pH and decreased as the pH increased. In the presence of Ca2+, 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 Ca2+ concentration. On the contrary. Ca-bridging was the prevailing process at pH > 5, and thus, TC adsorption increased by increasing Ca2+ concentration. The pH 5 represents an isoadsorption pH where both adsorption processes compensate each other. TC adsorption became independent of Ca2+ concentration at this pH. For TC adsorption on Ca2+-montmorillonite in 0.01 M NaCl experiments, the ratio adsorbed TC/retained Ca2+ was close to 1 in the pH range of 5-9, indicating an important participation of Ca2+ 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. (C) 2011 Elsevier Inc. All rights reserved.