Acidification and solubilisation of heavy metals from single and dual-component model solids

The solubilisation of Ni, Cd and Pb from 3 model components of soils and sediments is investigated as a function of pH. In presence of increasing acidity, Ni, Cd and Pb are strongly solubilised from kaolinite and amorphous ferric hydroxide. Metal solubility tends to decrease, on the other hand, when humic acid is present. The quantity of metal solubilised from the dual-component, amorphous ferric hydroxide-humic acid system is smallest in the pH range 3.5 to 5.5. In the system kaolinite-humic acid, the solubility is highest at neutral pH. In these solid mixtures, metal solubility is always minimal (close to nil) in the pH range between the pHzpt of the humic acid and the pHzpt of the mineral solid. The solubilisation of heavy metals from soils and sediments is highly pH-dependent but does not always increase with decreasing pH. The solubility of Cd, Ni and Pb depends not only on pH but also on interactions between soil components.