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Effect of sterilization and experimental conditions on the isotopic exchange of nickel in two contrasting soils

A detailed study of the isotopic exchangeability of native nickel is presented for two contrasting soils. Non‐sterile soils were compared with γ‐sterilized soil to assess the role of microbial activity in modifying nickel speciation. The distribution coefficient, Kd, was measured in a range of suspensions and in moist soil. Solution and soil solids were separated by double centrifuging. Soils were wetted with either water or 0.01 m CaCl2 solution. The change in Kd was also recorded for the soils incubated moist over a 6‐week period. Kd values decreased markedly as suspension was more concentrated, and data obtained in moist soil followed the same trend. Data obtained at a soil:solution ratio of 1:1 are closer to those obtained for soil incubated at field capacity (or drier) than those obtained at the more usual 1:10 ratio. Sterilization caused a marked increase in nickel solubility (about a threefold decrease in Kd for one soil and a 25% decrease for the other). Soil analysis showed that sterilization modified only water‐extractable organic carbon and EDTA‐extractable Mn. The effect of solution composition on Kd was less marked. These effects are attributed largely to dissolved organic carbon and easily extractable manganese oxides. There was relatively little change in Kd over the 6‐week period, suggesting that except for strongly anoxic soils, the distribution of nickel measured after 24 hours is a good estimation of Kdin situ.