Sorption of cupric, dichromate and arsenate ions in some New Zealand soils

Concentrated solutions of copper (Cu2+), dichromate (Cr2O72-) and arsenate (AsO43-) ions (CCA solutions) are used extensively in the New Zealand timber preservation industry. These ions are therefore, potential soil pollutants at timber treatment sites. Sorption of these three ions was examined by the surface and sub-surface horizons of two free-draining New Zealand soils over a range of soil solution pH values. Copper sorption by both soils increased substantially with increasing pH and was greater in the surface compared with the sub-surface horizons. Less dichromate was sorbed than the other two ions and was similar in both surface and sub-surface horizons for each soil. Dichromate sorption increased with decreasing pH. Arsenate sorption was intermediate between copper and dichromate and was less influenced by changes in soil solution pH. Arsenate sorption was generally greater in the sub-surface horizons of both soils. Sorption from solutions containing all three ions was not greatly different to sorption from solutions containing the single metal ions. Sorption behaviour for each ion is related to its chemistry and the soil chemical properties of each horizon. Results suggest that in the event of soil contamination by CCA solution, the immediate leaching potential of the initial ions species present would increase in the following order: Cu2+ < HAsO42-/H2AsO4- << Cr2O72-.