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NICKEL SOLUBILITY AND PRECIPITATION IN SOILS: A THERMODYNAMIC STUDY

Edward Peltier^1,*,, Ramakumar Allada^2,, Alexandra Navrotsky² and Donald L. Sparks¹

¹ Environmental Soil Chemistry Research Group, Department of Plant and Soil Sciences, 152 Townsend Hall, University of Delaware, Newark, DE 19717-1303, USA
² Thermochemistry Facility, Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA

^* E-mail address of corresponding author: epeltier{at}udel.edu

The formation of mixed-metal-Al layered double hydroxide (LDH) phases similar to hydrotalcite has been identified as a significant mechanism for immobilization of trace metals in some environmental systems. These precipitate phases become increasingly stable as they age, and their formation may therefore be an important pathway for sequestration of toxic metals in contaminated soils. However, the lack of thermodynamic data for LDH phases makes it difficult to model their behavior in natural systems. In this work, enthalpies of formation for Ni LDH phases with nitrate and sulfate interlayers were determined and compared to recently published data on carbonate interlayer LDHs. Differences in the identity of the anion interlayer resulted in substantial changes in the enthalpies of formation of the LDH phases, in the order of increasing enthalpy carbonate<sulfate<nitrate. Substitution of silica for carbonate resulted in an even more exothermic enthalpy of formation, confirming that silica substitution increases the stability of LDH precipitates. Both mechanical mixture and solid-solution models could be used to predict the thermodynamic properties of the LDH phases. Modeling results based on these thermodynamic data indicated that the formation of LDH phases on soil mineral substrates decreased Ni solubility compared to Ni(OH)₂ over pH 5–9 when soluble Al is present in the soil substrate. Over time, both of these precipitate phases will transform to more stable Ni phyllosilicates.

Key Words: Aqueous Solubilities • Calorimetry • Enthalpy of Formation • Layered Double Hydroxides • Ni Precipitation

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