Competitive Ion Exchange Behavior on Oxides

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Abstract
Cation adsorption data using MnCl2 and the proton isotherm of a Si oxide were compared with pH as the variable. The cation adsorption data quantitatively correlated with the proton isotherm analysis; the H+/Mn2+ exchange ratio was 6.0. The presence of adsorption plateaus are due to pH‐dependent (step) reactions; this was suggested as the only reasonable explanation for adsorption isotherms covering several pH units. These plateaus are very important to surface adsorption modeling, and may be significant in determining experimental conditions for other soil and colloidal research. The proton isotherm obtained using the backtitration technique had some pH‐ and salt‐dependent shifts that may involve competitive reaction mechanisms between the ions in solution and the oxide surface. Cation adsorption studies were also used to determine the total number of sites available; this resulted in an initial estimate of 13.6 µmol m−2. The authors review several topics related to ion exchange modeling and data interpretation; emphasis is placed on avoiding an oversimplified view of the chemical nature of the solidaqueous phase.