Stability of hectorite in weakly acidic solutions. III. Adsorption of heavy metal cations and hectorite solubility

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Abstract
The heavy metal cations studied were adsorbed by hectorite suspended in 0.05 M calcium chloride as a pH-dependent reaction in the order, Cu > Zn > (Co, Mn) > Ni. The adsorption of heavy metal cations was accompanied by the removal from solution of silicic acid released by clay dissolution. It is proposed that the reaction takes place on edge surfaces such that the clay lattice is effectively extended. This result complements other work in which it has been shown that the adsorption of silicic acid by other silicate clays results in an increased adsorption of zinc, cobalt and nickel. The present study did not provide evidence of the exchange of zinc and other cations for lattice magnesium.The solubility of hectorite was decreased in the presence of heavy metal cations in the same order that favoured their adsorption. It is proposed that they limit the dissolution of hectorite in weakly acidic solutions by adsorption on or near the sites most prone to proton attack, that is, the magnesium ions of the octahedral layer exposed at edges and defects.