Nickel sorption mechanisms in a pyrophyllite-montmorillonite mixture

Nickel sorption on pyrophyllite, montmorillonite and a 1:1 pyrophyllite-montmorillonite mixture was studied at pH 7.5 and a reaction time of 40 min. The main modes of Ni uptake under these reaction conditions are adsorption on montmorillonite and surface precipitation on pyrophyllite. For the clay mixture, where adsorption on the montmorillonite component and surface precipitation on the pyrophyllite component compete for Ni uptake, X-ray absorption fine structure spectroscopy (XAFS) was used to estimate the distribution of Ni over the mixture components. This was done by comparison to pyrophyllite-montmorillonite mixtures with known Ni distributions over the mixture components. Nickel uptake on singly reacted pyrophyllite was slightly higher than on singly reacted montmorillonite. This was consistent with the XAFS results for the clay mixture, which suggested that the pyrophyllite component sorbed slightly more Ni than the montmorillonite component. Our findings suggested that both adsorption and surface precipitation were important mechanisms in the overall Ni uptake in the clay mixture, and that neither sorption mechanism truly out-competed the other in the reaction time of 40 min employed. Therefore, both mechanisms should be considered when modeling Ni sorption in similar systems. (C) 1999 Academic Press.