Copper behaviour in a Podosol. 1. pH-dependent sorption - desorption, sorption isotherm analysis, and aqueous speciation modelling

Abstract

The effects of pH and Cu loading on the solid/solution partitioning of Cu in a Podosol from south-east Queensland, Australia was examined. Sorption–desorption of Cu exhibited maximum linear distribution coefficients (KD) at approximately pH 5. Observed decrease in KD values at pH >5 was attributed to increased solubility of native dissolved organic carbon (DOC) at higher pH and subsequent formation of non-sorbing Cu–DOC complexes. Speciation modelling with the MINTEQA2 code indicated that >90% of aqueous Cu was present as Cu–DOC complexes at pH >5.5. The effect of Cu loading was examined with sorption isotherm analysis at pH 5 using solid : solution ratio approaches that were both constant (1 : 2 and 1 : 10) and variable. As the solid : solution ratio increased, the proportion of Cu sorbed decreased due to the formation of Cu-DOC complexes. However, this effect was negligible once these Cu–DOC complexes were accounted for via free Cu2+ sorption isotherms. This indicated that Cu2+ sorption at concentrations <0.08 mg/L was described by a KD value of approximately 3000 L/kg. Despite this relatively high KD value for Cu2+ sorption, the results indicate that Cu–DOC complexes significantly enhance Cu solubility in soils high in DOC.