Spectroscopic Evidence for the Formation of Mixed-Cation Hydroxide Phases upon Metal Sorption on Clays and Aluminum Oxides

Retention of heavy metal ions on soil mineral surfaces is an important process for maintaining environmental quality. A thorough understanding of the kinetics and mechanisms of heavy metal sorption on soil mineral surfaces is therefore of fundamental importance. The present study examines the kinetics and mechanisms of Ni(II) sorption onto pyrophyllite, kaolinite, gibbsite, and montmorillonite. Ni sorption reactions were initially fast (15-40% of the initial Ni was removed within the first hour). Thereafter, the rate of sorption decreased significantly. X-ray absorption fine structure (XAFS) spectroscopy was used to determine the local structural environment of Ni(II). Data analysis reveals the presence of polynuclear Ni surface complexes. Ni-Ni bond distances (3.00-3.03 A) were distinctly shorter than in Ni(OH)2(s) (3.09 A). We propose that the reduction of the Ni-Ni distances is caused by the formation of mixed Ni/Al hydroxide phases. The XAFS spectra and derived structural parameters are similar to those in takovite (Ni6Al2(OH)16CO3.H2O), thus suggesting the presence of a Ni phase of similar structure. Even though dissolved Al could not be detected in our samples, Al could have been released into solution and incorporated into mixed Ni/Al hydroxide-like phases. The formation of such phases can explain the finding that the dissolution rates (Si-release) are strongly enhanced (relative to the dissolution rates of the clays alone) as long as Ni sorption is pronounced. We suspect that the release of Al into solution is the rate-determining step for the formation of mixed Ni/Al hydroxide-like phases in our study. Our study demonstrates that mixed Ni/Al hydroxide-like compounds can form when Ni is introduced into a suitable environment in which there is a source of hydrolyzed species of Al. One can speculate that the formation of mixed-cation hydroxide compounds also represents a plausible "sorption mode" for other divalent metal ions when silicates or oxides are present. It has been shown that similar mixed-cation hydroxide compounds can be synthesized when Mg(II), Ni(II), Co(II), Zn(II), or Mn(II) is added to suspensions containing Al(III), Fe(III), and Cr(III). Thus, the formation of mixed-cation hydroxide compounds should be considered when conducting metal sorption experiments, modeling metal surface complexation, determining speciation, and assessing the risk of the migration of contaminants in polluted sites.