THE EFFECT OF HYDROXY-ALUMINUM PRECIP ITAT ION ON THE EXCHANGE PROPERTIES OF MONTMORILLONITE 1 Key Words--Cation Exchange, Hydroxy-aluminum Precipitate, Interlayer Precipitate,

The exchange isotherms of Na-Ca on montmorillonite w re determined by Banin (1968). It was found that the selectivity coefficient was not constant but changed with the relative frac-tion of the adsorbed ions. Banin associated this variation with the structural properties of the mineral, namely, tactoid for-mation in the calcium state, and the presence of different ex-change sites affecting the energy of exchange. Shainberg and Kemper (1966) calculated that the affinity of the internal sur-face of calcium is 2-4 times greater than that of the external surface. As long as the platelets were completely separated, the two ions were randomly distributed in the exchange com-plex. When tactoids formed, demixing took place, and the so-dium ions concentrated onthe external surfaces of the packets (Mrring and Glaeser, 1953; Shainberg and Otoh, 1968)+ When suspensions of montmorillonite were titrated with Al-salt and base, A1 precipitated as hydroxy-aluminum com-plexes (WeismiUer et al., 1967). Results of X-ray powder dif-fraction showed that at least some of these precipitates occurred as interlayers between the clay platelets (Rich, 1960; Barnhisel and Rich, 1963; Keren et al., 1977). A number of workers have reported a reduction in cation-exchange capac-ity (CEC) of clays following the formation of Al-interlayer