Adsorption of Hg2+, Cr3+, Pb2+, Cu2+, Zn2+, Ba2+, Ni2+, Mn2+, Cd2+, Ba2+ and Ag+ by Ca-montmorillonite from monocationic solutions followed second-order kinetics, attaining removals q(e) of Pb2+ 6.187 x 10(2), Hg2+ 4.369 x 10(2), Cu2+ 2.045 x 10(2), Zn2+ 1.576 x 10(2), Ag+ 1.375 x 10(2), Cr3+ 7.917 x 10(1), Ni2+ 7.300 x 10(1), Ba2+ 6.815 x 10(1), Cd2+ 2.003 x 10(1) and Mn2+ 1.632 x 10(1) mg g(-1), greater than those by montmorillonite. Equilibrium adsorption by montmorillonite from bimodal metal +Ca2+ solutions followed the Langmuir model with adsorptions of Zn2+ 3.477 x 10(1), Ba2+ 6.984, Hg2+ 3.681 x 10(1), Ni2+ 4.849, Mn2+ 3.820 and Cd2+ 6.978 mg g(-1), and by Ca-montmorillonite of Zn2+ 3.523 x 10(1). Ba2+ 1.073 x 10(1) and Ni2+ 5.969 mg g(-1). Within the pH range established by acid cationic solutions and near neutral montmorillonite and Ca-montmorillonite, at pHs below 5 precipitate oxides and hydroxides of Cr3+. Pb2+ and Cu2+ while at higher pHs Hg-2(2+), Zn2+, Ba2+, Cd2+, Ni2+, Mn2+ and Ag+ remain in solution and are adsorbed as simple and hydroxyl cations by the clay, keeping the selectivity sequence Hg-2(2+) > Zn2+ > Ba2+ > Cd2+ > Ni2+ > Mn2+. (C) 2011 Elsevier B.V. All rights reserved.