We examined the sorption of nickel and zinc in three soils that were unamended, freshly amended, or preconditioned with gamma-irradiated sewage sludge. Metal sorption was also studied in the same soils treated with a 0.01 N CaCl2-extract of the sludge. Adsorption of both metals was greatest in the unamended soils, less in soils preconditioned with sludge, and least in the freshly amended soils and sludge-extract-treated soils. We attempted to explain treatment effects on the basis of (1) reduced metal activity as a result of increased soil solution ionic strength, (2) reduced metal activity as a result of inorganic-metal complexes, (3) increased competition for sorbing sites, and (4) reduced metal activity as a result of organic-metal complexation. None of the hypotheses completely explained the observed treatment effects, although the data suggested that organic-metal complexes play a significant role in reduced free metal activity, particularly for Ni. Despite the reduction in metal retention effected by various sludge treatments, little Ni or Zn leaching is expected in these sludge-amended soils, for sorption of both metals is very high.
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