Effect of hydroxide polymenrs on cation exchange of montmorillonite

Al hydroxide polymers (AlHO) can significantly influence the cation exchange behaviour of clays. We have determined the effect of synthesized AlHO on Ca¿Na, Zn¿Na and Pb¿Na exchange for a series of exchanger compositions and two Al loadings at pH 6.0 and an ionic strength of 0.01 m. The preference for Ca on the siloxane surface of the clay¿AlHO system (CAlHO) was greater than for the pure clay, and the average KV (Vanselow selectivity coefficient) was determined to be 2.16 and 1.24, respectively. The selectivity coefficients for the exchange reactions Zn¿Na and Pb¿Na were not directly determined in CAlHO systems, because heavy-metal ions bind as well to the clay surface as to the AlHO over a wide range of pH. We have estimated the effect of the presence of AlHO on the selectivity coefficients of Zn¿Na and Pb¿Na exchange by extrapolation of the experimental results of Ca¿Na, Zn¿Na and Pb¿Na exchange for pure clay and Ca¿Na exchange for CAlHO. The average KV was increased by the presence of the AlHO from 1.23 to 2.16 for Zn¿Na exchange and from 1.59 to 2.77 for Pb¿Na exchange. The increase in the preference for the divalent cations is probably caused by parallel alignment of clay platelets by sorption of AlHO. Increasing the amount of AlHO did not change the selectivity for Ca¿Na exchange, and probably the structure of the system or the arrangement of the clay platelets and AlHO particles was not substantially changed. This was supported by the linear reduction of the cation exchange capacity with amount of AlHO present at pH 6.6. It seems likely that the selectivity coefficients for Ca¿Na, Zn¿Na and Pb¿Na exchange that we found apply in naturally occurring montmorillonite¿AlHO systems.