PII S0016-7037(98)00136-7 The kinetics of mixed Ni-Al hydroxide formation on clay and aluminum oxide minerals: A time-resolved XAFS study

Abstract—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, outer