Effect of red mud on the mobility of heavy metals in mining-contaminated soils

The mobility of Cu2+, Zn2+, Ni2+, and Cd2+ in soils treated with red mud was experimentally studied to explore the feasibility of remediation of smelter-contaminated soils. Red mud samples were collected with the Bayer process (BRM) and confederate process (CRM) in the Aluminous Plant of Guizhou Province. Two farmed soil samples were collected from the Niujiaotang mining area, Guizhou Province, Southwest China. One sample was weakly polluted by fly ash; and the other was polluted severely by waste water from the smelter. For evaluating the potential of remediation, the concentrations of free metal ions and the distributions of metals in the soil were determined. The concentrations of free metal ions were measured by using the Donnan Membrane Technique, and the contributions of soil sorbents to the heavy metals adsorptions were calculated with Equilibrium Calculation of Speciation and Transport (ECOSAT). BRM reduced the concentrations of free metal ions in two kinds of soils, while CRM only favored the decrease of the concentrations of free metal ions in seriously contaminated soils. The experimental data also showed a tendency that the concentrations of free metal ions decreased proportionally with the amount of added red mud, which resulted from the increasing adsorption of heavy metal ions in the form of metal ion hydroxides.