Nickel sorption mechanisms in a pyrophyllite-montmorillonite mixture

Nickel sorption on pyrophyllite, montmorillonite and a 1:1 pyrophyllite-mon tmorillonite mixture was studied at pH 7.5 and a reaction time of 40 min. T he main modes of Ni uptake under these reaction conditions are adsorption o n montmorillonite and surface precipitation on pyrophyllite. For the clay m ixture, where adsorption on the montmorillonite component and surface preci pitation on the pyrophyllite component compete for Ni uptake, X-ray absorpt ion fine structure spectroscopy (XAFS) was used to estimate the distributio n of Ni over the mixture components. This was done by comparison to pyrophy llite-montmorillonite mixtures with known Ni distributions over the mixture components. Nickel uptake on singly reacted pyrophyllite was slightly high er than on singly reacted montmorillonite. This was consistent with the XAF S results for the clay mixture, which suggested that the pyrophyllite compo nent sorbed slightly more Ni than the montmorillonite component. Our findin gs suggested that both adsorption and surface precipitation were important mechanisms in the overall Ni uptake in the clay mixture, and that neither s orption mechanism truly out-competed the other in the reaction time of 40 m in employed. Therefore, both mechanisms should be considered when modeling Ni sorption in similar systems. (C) 1999 Academic Press.