Geochemical modelling of arsenic and selenium leaching in alkaline water treatment sludge from the production of non-ferrous metals

Geochemical modelling of leaching of oxyanion forming elements such as arsenic (As) and selenium (Se) is frequently not successful. A consistent thermodynamic dataset of As and Se was therefore composed, not only including precipitation, but also adsorption and solid solution, and was applied to the pH-dependent leaching behaviour of As and Se in an alkaline residue with a pH 11.1 from the lime treatment of sulphuric acid wastewaters from the production of non-ferrous metals. The As and Se content ranged up to 6.7 wt% and 0.29 wt%, respectively and speciation analysis showed that 96.3% of As occured as arsenate whereas Se speciation comprised 79% selenate and 21.0% selenite. XRD and SEM/EDX analysis showed that arsenate occurred as rauenthalite (Ca3(AsO4)2·10H2O), associated with gypsum, the most important mineral. Arsenate and arsenite concentrations were only slightly below equilibrium with rauenthalite and calciumarsenite (CaHAsO3), respectively and consideration of adsorption and solid solution only marginally improved model predictions. Selenate (SeVI) and selenite (SeIV), on the other hand, were far from equilibrium with their corresponding calcium metalate. The application of solid solutions and adsorption of SeVI and SeIV oxyanions with gypsum, calcite and ettringite significantly improved model predictions but missing thermodynamic data and especially the lack of a comprehensive model for solid solution and surface exchange with calcite and ettringite still hampered efficient modelling.