Leaching behaviour and solubility — Controlling solid phases of heavy metals in municipal solid waste incinerator ash

This paper highlights the uses and limitations of thermodynamic calculations in the planning of leach tests in the laboratory or for research in the field. Heavy metal solubility has been studied in leachate from Landfill Lostorf, AG, Switzerland. Also, the influence of pH on the solubility of Cu, Pb, Cd and Zn has been determined in the laboratory. The results have been compared with the maximum allowable heavy metal concentrations in equilibrium with the appropriate (hydr)oxides and carbonates. Copper is supersaturated with respect to Cu(OH)2 in both laboratory and field studies. Complexation with organic ligands is a probable explanation for this observation. Both Zn and Pb are undersaturated with respect to pure (hydr)oxides and carbonates, though agreement between calculations and measurements are close enough, that PbCO3 could be controlling the solubility in the laboratory experiments. The markedly lower concentrations of Pb in the field in comparison with the laboratory data could be explained by the affinity for Pb to bind to solids and the higher solid:solution ratio in field conditions. The solubility of Cd could be controlled by the formation of CdCO3. The relatively high concentrations of Mo (up to 10−5 ) in the landfill leachate could be limited by the precipitation of CaMoO4.