Fe(II)-Ca(II), Fe(II)-Na(I), and Fe(II)-Ca(II)-Na(I) exchange experiments on montmorillonite were performed in chloride background. These experiments show the possible sorption of Fe2+ and FeCl+ ion pairs in exchange site positions, a result confirmed with 77 K 57Fe Mössbauer experiments. The sorption data were modeled and the cation exchange selectivity for Fe(II) were found to be nearly equal to that of Ca(II). Vanselow selectivity coefficients, for Na-Fe2+ and Na-FeCl+ reactions, were found to be equal to 0.4 (0.5 for Ca2+) and 2.3 (2.5 for CaCl+) respectively. High affinity of montmorillonite for chloride ion pairs seems to be a common mechanism as first stated by Sposito et al., (Soil Sci. Soc. Am. J. 47, 51-56, 1983a), and should have implications e.g., on the chemistry of suspended particles in seawater. Exchange selectivity coefficients derived from this study and others were used to model experimental data on river water and seawater equilibrated particles. The agreement between simulations and experimental data is very good. The simulation shows the predominance of monovalent ion (Na+ and chloride ion pairs) sorption on clay particles in seawater. This sorption of monovalent ions leads to the dispersion of particles in seawater and to the extension of a ldquoplumerdquo of particles spreading away from river deltas, such as that of the River Amazon.