Copper behaviour in a Podosol. 2. Sorption reversibility, geochemical partitioning, and column leaching

The sorption–desorption and leaching behaviour of Cu in a Podosol from south-east Queensland, Australia, was examined. Copper sorption was described by a linear distribution coefficient at low sorption levels (KDCa→0) of 481 L/kg and a sorption capacity (CS,Max) of 382 mg/kg. Selective removal of soil organic matter reduced these values by approximately 95%, indicating that Cu was sorbed predominantly to soil organic matter. The KDCa→0 and CS,Max values from Cu desorption experiments were 934 L/kg and 516 mg/kg, respectively, which indicates that sorption was not fully reversible. This irreversibility was related to aqueous Cu speciation (modelled with MINTEQA2), showing that aqueous complexes between Cu and dissolved organic carbon (DOC) comprised 28.3–72.8% and 21.3–45.4% of aqueous Cu in the sorption and desorption experiment, respectively. Sorption irreversibility was not evident when the corresponding data was presented as free Cu2+ isotherms. Both sorption and desorption experiments with free Cu2+ <0.2 mg/L were described by a KDCa→0 value of approximately 3000 L/kg. Sequential extraction of sorbed Cu indicated that at low concentrations, sorption occurred primarily via specific interactions, with non-specific sorption becoming increasing important at higher concentrations. Desorption of Cu in a column leaching experiment was attributable to exchange of sorbed Cu2+ with Na+. Leaching with a DOC solution of pH 7 and 135 mg/L greatly enhanced Cu mobility due to the formation of aqueous Cu–DOC complexes.