THE KINETICS OF MIXED NI-AL HYDROXIDE FORMATION ON CLAY AND ALUMINUM-OXIDE MINERALS - A TIME-RESOLVED XAFS STUDY

In this study kinetic investigations were combined with X-ray Absorpti on Fine Structure (XAFS) measurements to determine Ni sorption process es on pyrophyllite, gibbsite, and montmorillonite over extended time p eriods (min-months). The kinetic investigations revealed that Ni sorpt ion reactions (pH = 7.5, [Ni](initial) = 3 mM, I = 0.1 M (NaNO3)) were initially fast (8-35% of the initial Ni was removed within the first 40 min). Thereafter, the rate of sorption decreased significantly and depended on the type of mineral surface. For the Ni/pyrophyllite syste m Ni removal was almost complete after a reaction time of 24h while fo r the Ni/gibbsite and Ni/montmorillonite systems metal sorption contin ued up to approximate to 2 months. XAFS data revealed the presence of a mixed Ni/Al phase in the Ni/pyrophyllite and Ni/gibbsite systems aft er a reaction time of minutes. These results suggest that adsorption a nd nucleation processes (mixed Ni/Al phase formation) can occur simult aneously over time scales of only minutes. However, our finding of a f ast growing mixed Ni/Al phase cannot be extrapolated to other sorption systems. A reaction time of 48 h was required for the presence of a m ixed Ni/Al phase in the Ni/mormorillonite system. As reaction time pro gressed, the number of second neighbor Ni atoms (NNi-Ni) at a distance of approximate to 3.05 Angstrom increased in all sorption systems, su ggesting further growth of a mixed Ni/Al phase with increasing reactio n time. Our study suggests that three phenomena occur at the mineral/l iquid interface: (1) nonspecific (i.e, outer-sphere complexation) and/ or specific adsorption (i.e., inner-sphere complexation), (2) dissolut ion of Al, and (3) nucleation of a mixed Ni/Al phase. The rate-limitin g step is the dissolution of Al from the surface, which depends on the mineral substrate. Using the Ni linear sorption rates observed in the Ni/gibbsite and Ni/montmorillonite systems and assuming the Ni/Al rat ios in our sorption samples are within the range of Ni/Al ratios provi ded in the literature (1.3-5.6), one can estimate an average Al dissol ution rate which seems to be enhanced compared to the Al dissolution r ates of the minerals alone. This finding indicates that the dissolutio n of clay and aluminum oxide minerals can be promoted by metal ions su ch as Ni(II) through the formation of a mixed Ni/Al phase. Copyright ( C) 1998 Elsevier Science Ltd.