Kinetic studies of Ni sorption onto three soils with different particle sizes, clay mineralogy and soil organic matter content were conducted at several pH values between 6 and 7.5 for times ranging from days up to one year. Measurements of Ni loss from solution were coupled with quick x-ray absorption spectroscopy studies to resolve the onset of precipitate formation during the first 24-72 hours after Ni addition. Ni speciation in the soils at 24 hours, 30 days, 6 months and 12 months was determined using both bulk and µ-XAS to determine the extent of precipitate formation compared to Ni sorption and the identity of the precipitate formed in each case. Desorption studies were conducted after 1, 6 and 12 months to assess the impact of soil aging on the stability of the initially formed precipitates and the overall solubility of the surface bound Ni. These experimental results were then compared to models developed from previously determined thermodynamic solubility constants for a variety of model Ni precipitate phases.
Nickel speciation results from this series of experiments show that precipitate formation is dependent on a number of factors. Generally speaking, no precipitates were found in any of the soils below pH 6.5, a result consistent with predictions from equilibrium modeling. In kinetic experiments at pH ³ 7, formation of mixed nickel-aluminum hydroxide surface precipitates occurred within 12-24 hours of Ni addition to the soil, but the final Ni speciation was highly dependent on the clay mineralogy of the specific soils. The availability of substrate cations, particularly aluminum, strongly influenced the type of Ni precipitate formed, with Ni-Al hydroxides dominating in the two kaolinite-containing soils and Ni phyllosilicate phases forming in the third, montmorillonite-dominated soil. The mixed Ni-Al hydroxide phases were preferred over more thermodynamically favorable phyllosilicate phases due to rapid formation kinetics that act to bind up available Ni soon after Ni addition to the system. Increased presence of soil organic matter had a strong influence on Ni speciation within the first 72 hours, resulting in less surface precipitate formation, but the effect decreased at longer times.
The formation of surface precipitates had a significant effect on Ni desorption and bioavailability. The bioavailable Ni fraction, as measured by a Ni sensitive bacterial biosensor strain of R. metallidurans, decreased from 70-90% of total Ni in the sorption dominated systems (pH 6) to approximately 25% in soils containing either Ni-Al hydroxide or Ni phyllosilicate phases. Nickel desorption in 0.1 mM HNO3 (pH 4) also decreased substantially when surface precipitates were present. Aging the soils for up to one year in contact with the Ni solution had no effect on Ni desorption percentages, suggesting that the initial precipitates formed are relatively stable over the long term. Overall, these results show that the formation of surface precipitate phases will have a substantial effect on Ni mobility and bioavailability in contaminated soils, and should be incorporated into models and predictions of both short and long term metal behavior in natural systems.