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 Absorption Fine Structure (XAFS) measurements to determine Ni sorption processes on pyrophyllite, gibbsite, and montmorillonite over extended time periods (min-months). The kinetic investigations revealed that Ni sorption reactions (pH 5 7.5, [Ni]initial 5 3 mM, I 5 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 system Ni removal was almost complete after a reaction time of 24h while for the Ni/gibbsite and Ni/montmorillonite systems metal sorption continued up to '2 months. XAFS data revealed the presence of a mixed Ni/Al phase in the Ni/pyrophyllite and Ni/gibbsite systems after a reaction time of minutes. These results suggest that adsorption and nucleation processes (mixed Ni/Al phase formation) can occur simultaneously over time scales of only minutes. However, our finding of a fast 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 mixed Ni/Al phase in the Ni/mormorillonite system. As reaction time progressed, the number of second neighbor Ni atoms (NNi-Ni) at a distance of '3.05 Å increased in all sorption systems, suggesting further growth of a mixed Ni/Al phase with increasing reaction time. Our study suggests that three phenomena occur at the mineral/liquid interface: (1) nonspecific (i.e, outersphere complexation) and/or specific adsorption (i.e., inner-sphere complexation), (2) dissolution of Al, and (3) nucleation of a mixed Ni/Al phase. The rate-limiting 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 ratios in our sorption samples are within the range of Ni/Al ratios provided in the literature (1.3–5.6), one can estimate an average Al dissolution rate which seems to be enhanced compared to the Al dissolution rates of the minerals alone. This finding indicates that the dissolution of clay and aluminum oxide minerals can be promoted by metal ions such as Ni(II) through the formation of a mixed Ni/Al phase. Copyright © 1998 Elsevier Science Ltd