Copper behaviour in a podosol. I: pH-dependent sorption-desorption, sorption isotherm analysis and aqueous speciation modelling

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 \u3e5 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 \u3e90% of aqueous Cu was present as Cu–DOC complexes at pH \u3e5.5. 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 \u3c0.08 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 DO