The interfacial reaction between calcium-bentonite zinc ion was studied by a radioisotopic tracer method. Both the quantity of zinc ions incorporated into the solid and the quantity of calcium ions leaving the solid were determined. A comparison of the two quantities and the Langmuir representation obtained suggest that other processes can occur besides ion exchange. These processes were separated; ion exchange and adsorption isotherms were constructed, and the isotherm parameters determined. The real ion exchange process was evaluated on the basis of the law of mass action as well. The equilibrium constant of the process and the surface activity coefficients of calcium and zinc ions were determined. The variation of the surface activity coefficient as a function of surface concentration is explained by supposing exchange sites of different binding energy.
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