Sorption reactions at mineral/surface interfaces play a dominant role in determining trace metal mobility, toxicity, and bioavailability in soils. The formation of surface precipitate phases may result in permanent sequestration of trace metals into relatively bio-unavailable phases, thereby significantly reducing their impact on the surrounding environment. Understanding the true impact of metal soil contamination on the surrounding environment requires a better knowledge of the processes controlling surface precipitate formation and dissolution than is currently available, and the development of models for metal behavior that incorporate this knowledge. Recent advances in techniques, such as x-ray absorption spectroscopy (XAS), that allow us to obtain molecular scale information on metal speciation in soil samples with minimal disturbance of the surrounding matrix, coupled with observations of precipitate formation kinetics and thermodynamic stability, allow us to gain a more accurate picture of the factors influencing metal precipitation under natural conditions. In this work, we have used a suite of macroscopic and molecular scale techniques to investigate the speciation of Ni in several soils under a range of soil conditions and time scales. The results have been coupled with desorption and bioavailability studies to determine the effect of precipitate formation on the fate of Ni in contaminated soils.

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.