Effects of sewage sludge and barley straw treatment on the sorption and retention of Cu, Cd and Pb by coppermine Anthropic Regosols.

To evaluate the involvement of cation exchange in the competitive and separate sorption and retention of Cu(2+), Cd(2+) and Pb(2+) by soils developing from a copper mine spoil, and to determine the effects of sludge and barley straw treatment on the intensity and reversibility of sorption and retention, isotherms were constructed by means of batch sorption/desorption experiments in which displaced Ca(2+), Mg(2+), K(+) and Al(3+) were also determined. Amendment with sludge and barley straw was associated with an increase in pH of about 4 units; approximately 75-, 1900- and 55-fold increases in CEC(e), organic matter content and Mn oxides content, respectively; and greatly increased capacity for the sorption and retention of Pb, Cu and Cd. Most heavy metal sorption came about through displacement of the predominant cation in the exchange complex (Al(3+) in unamended soils, and Ca(2+) in amended soils), but the greater total sorption from multi-metal solutions also involved the displacement of other exchangeable cations. The parameter K(r) clearly reflected the lower sorption and retention capacities of unamended minesoils (K(r)<0.2 for all three metals, as against K(r) approximately 0.54 (Cd) or K(r)>0.97 (Pb and Cu) for amended minesoils); the competition for sorption sites in multi-metal solutions (for any given metal, the K(r) for single-metal solutions was invariably greater than the corresponding K(r) for multi-metal solutions); and the order of preference among metals for sorption and retention (Pb>Cd>or=Cu for sorption on unamended soils, which had virtually no organic matter, an important Cu-binding component; Pb>Cu>or=Cd otherwise). The values of the hysteresis index HI were likewise in agreement with previous results on the reversibility of the sorption of these metals, identifying Pb and Cd as the most and the least irreversibly sorbed metals, respectively. The amendment combination investigated successfully increased the immobilization of Pb, Cu and Cd by this minesoil, but a change in the amendment dosage is necessary in order to achieve near-neutral pH and minimize the predominance of Ca(2+) in the exchangeable cation complex.