The adsorption of barium by Ca-exchanged clinoptilolite and montmorillonite is presented. The kinetics of adsorption of Ba2+ were evaluated contacting 1 g portion of each adsorber with 100 mL 0.1 N BaCl2 for 200 h. Adsorption by Ca-clinoptilolite is defined by second-order kinetics of rate constant K-v 8.232 x 10(-2) g mg(-1) h(-1) and maximum removal of 71.885 mg g(-1). It is a two-stage process initiated by a rapid uptake of Ba2+ followed by more moderate kinetics. The adsorption isotherms were determined contacting 0.2 g of each adsorber with 10 mL 0.1-0.005 N BaCl2 + CaCl2 solution, Ba2+/Ca2+ ratio 1, for periods of 7 days for the tuff and 2 days for the clay. The equilibrium adsorption is described by the Langmuir model, of equilibrium constant K 0.0151 L mg(-1) and maximum adsorption of 15.29 mg g(-1). The adsorption of Ba2+ by Ca-exchanged montmorillonite also follows a second-order reaction of rate constant K-p 3.179 x 10(-2) g mg(-1) h(-1), and calculated separation of 36.74 mg g(-1); the Langmuir isotherm is defined by the constant K 0.034 L mg(-1) and maximum adsorption of 15.29 mg g(-1). X-ray diffraction shows that the exchange of Ba2+ modifies the d(001) of Ca-montmorillonite from 15.4 to 12.4 angstrom. (C) 2009 Elsevier B.V. All rights reserved.