Kinetics of the formation and dissolution of Ni precipitates in a gibbsite/amorphous silica mixture

There have been a number of studies that have examined metal precipitation reactions on an array of natural soil materials. While many of these invest igations have focused on model single-component systems, recent research ha s appeared on metal precipitation on soils and clay fractions of soils. How ever, few studies have explored mixed model component systems, which may le ad to a better understanding of metal reactions on soils and clay fractions . Furthermore, only a few studies have appeared on the stability of the met al surface precipitates. In light of this, we investigated Ni sorption and dissolution kinetics and mechanisms on a mixture of gibbsite and amorphous silica by combining macroscopic studies with X-ray absorption fine structur e (XAFS) and diffuse reflectance spectroscopies (DRS), and high-resolution thermogravimetric analysis (HRTGA). Batch sorption experiments were conduct ed at pH 7.5 and at different reaction times to elucidate the sorption proc ess and to study the role of residence time on metal precipitate stability. Spectroscopic and HRTGA investigations revealed alpha-Ni(OH)(2) precipitat es formed on the gibbsite/silica mixture initially and over time evolved to a Ni phyllosilicate. The available Si source was derived from partial diss olution of the sorbent during Ni sorption. With increasing residence time, the precipitate phases drastically increased in stability, as shown by decr easing amounts of Ni release as effected by nitric acid (HNO3) and ethylene diaminetetraacetic acid (EDTA) treatments. This aging effect may be explain ed by the silicate-for-nitrate exchange during the first days of reaction a nd subsequently by silicate polymerization and partial grafting onto the hy droxide layers to form a phyllosilicate precursor phase (R.G. Ford, A.C. Sc heinost, K.G. Scheckel, and D.L. Sparks, Environ. Sci. Technol. 33(18), 314 0-3144, 1999). (C) 2000 Academic Press.