Nickel sequestration in a kaolinite-humic acid complex.

Incorporation of first row transition metals into stable surface precipitates can play an important role in reducing the bioavailability of these metals in neutral and alkaline soils. Organic coatings may interfere with this sorption mechanism by changing the surface characteristics and by masking the mineral surface from metal sorptives. In this study, kinetic sorption and desorption experiments were combined with extended X-ray absorption fine structure (EXAFS) spectroscopy to elucidate the effect of humic acid (HA) coatings on the formation and stabilization of nickel precipitates at the kaolinite-water interface. Initial Ni uptake (pH 7.5, [Ni]i = 3 mM, and I = 0.02 M NaNO3) increased with greater amounts of HA coated onto the kaolinite surface. Ni uptake continued over an extended period of time without reaching an apparent equilibrium. EXAFS analysis of the Ni sorption complex structures formed over time (up to 7 months) revealed the formation of a Ni-Al layered double hydroxide (LDH) precipitate at the kaolinite surface in the absence of HA. HA alone formed an inner-sphere complex with Ni (with 2 carbon atoms at an average radial distance of 2.85 A). A Ni-Al LDH precipitate phase was formed at the kaolinite surface in the presence of a 1 wt % HA coating. However, with 5 wt % HA coated at the kaolinite surface, the formation of a surface precipitate was slowed significantly, and the precipitate formed was similar in structure to Ni(OH)2(s). The Ni(OH)2 precipitate was not resistant to proton dissolution, while the Ni-Al LDH precipitate was. These results augment earlier findings that the incorporation of Ni and other first row transition metals into stable surface precipitates is an important sequestration pathway for toxic metals in the environment, despite the presence of ubiquitous coating materials such as humic acids.