Purification of acidic wastewater containing Cd(II) using a red mud-loess mixture: Column test, breakthrough curve, and speciation of Cd.

In this study, the safety of a red mud-loess mixture (RM-L) for the remediation of groundwater polluted by acid mine drainage (AMD) containing Cd(II) in mining areas was systematically analyzed and clarified. The effects of the initial concentration, flow rate, and packing height on the breakthrough performance and longevity of RM-L as a permeable reactive barrier (PRB) packing material were explored by column tests. The results show that the breakthrough time, saturation time, and adsorption capacity of Cd(II) in RM-L increased with decreasing initial concentration and flow rate, as well as increasing packing height. Moreover, RM-L had a long-term effective acid buffering capacity for acidic wastewater containing Cd(II). An increase in the packing height led to a longer longevity of the PRB than the theoretical value. In addition, the speciation of Cd on RM-L was dominated by carbonate form and iron-manganese oxide form. The surface of the RM-L particles evolved from a dense lamellar structure to small globular clusters after purifying the acidic wastewater containing Cd(II), due to the corrosion of H and the reoccupation and coverage by increasingly enriched adsorbates and precipitates of heavy metal ions.