Kinetics of Mixed Ni−Al Precipitate Formation on a Soil Clay Fraction

The kinetics of mixed Ni−Al layered double hydroxide (LDH) precipitate formation on a soil clay fraction was monitored using X-ray absorption fine structure (XAFS) spectroscopy. The kinetic behavior was monitored at pH 6.0, 6.8, and 7.5 in order to determine the effect of reaction pH on precipitate formation. XAFS analyses were performed on a Ni-reacted whole soil at pH 7.5 to determine the effect of metal oxides and organic matter on mixed Ni−Al LDH formation. The initial Ni concentration was 3 mM with a solid/solution ratio of 10 g L-1 in 0.1 M NaNO3. Initial Ni sorption kinetics on the soil clay were rapid at all pH values but differed at each pH for longer reaction times. The sorption kinetics at pH 7.5 were characterized by an extremely rapid initial step with nearly 75% of Ni sorbed within 20 h, followed by a slower step with nearly 100% of the Ni removed from solution within 150 h. XAFS analysis of the pH 7.5 sorption samples indicated the formation of a mixed Ni−Al LDH within 15 min. The sorption kinetics at pH 6.8 were initially rapid, followed by a slow step, and XAFS revealed the formation of a Ni−Al LDH within 2 h. At pH 6.0, Ni sorption did not exceed 20%, and XAFS analysis revealed no LDH formation within 72 h. XAFS analysis for the whole soil indicated a mixed Ni−Al phase formed at pH 7.5 after 24 h of reaction. These findings indicate that mixed metal precipitate formation occurs in heterogeneous clay systems and whole soils; therefore, they should be considered when predicting and modeling the fate of metals in subsurface environments.