Copper geochemistry in an acidic, sandy soil: sorption-desorption, aqueous speciation and mobility
Burton, ED, Phillips, IR, Hawker, DW & Lamb, DT 2006, 'Copper geochemistry in an acidic, sandy soil: sorption-desorption, aqueous speciation and mobility', Proceedings 18th World Congress of Soil Science, Philadelphia, PA, 9-15 July, International Union of Soil Sciences.
The geochemical behavior of Cu in an acidic, sandy Podosol was examined. Sorption-desorption of Cu exhibited maximum linear distribution coefficients (Kd) at approx. pH 5. Observed depression of Kd values at pH > 5, were attributed to increased solubility of native Dissolved Organic Carbon (DOC) at higher pH and subsequent formation of poorly-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 both constant (1:2 and 1:10) and variable solid:solution ratio approaches. 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. At a 1:10 solid:solution ratio, Cu sorption was described by a linear distribution coefficient at low sorption levels (Kd[low]) of 481 L/kg and a sorption capacity (CS,Max) of 382 mg/kg. Selective removal of soil organic matter reduced these values by approx. 95 %, indicating that Cu was sorbed predominantly to soil organic matter. The Kd[low] 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, showing that aqueous complexes between Cu and DOC comprised 28.3 to 72.8 % and 21.3 to 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 Kd[low] value of approx. 3000 L/kg. The results show that many aspects of Cu behavior (including sorption-desorption hysteresis) can be explained by considering aqueous speciation.