Passive treatment of percolate from complex heavy-metal mining waste

Solid waste of the abandoned arsenic mine of Baccu Locci (Sardinia) is complex and difficult to treat. Concentrations of heavy-metals like Cd, Cu, Ni, Pb and Zn are above limits in acid (pH=4) percolate. When the pH of the waste is raised for treatment of the heavy-metal-cation contamination, then oxyanionic arsenate desorbs and its concentration in the percolate may increase well beyond limit for particular waste types. In order to avoid arsenate oxyanion mobilization during heavy-metal-cation immobilization, Baccu Locci mining waste was placed on a layer of porous transformed alkaline red mud (TRM) pellets, instead of mixed with alkaline reagents. The TRM pellets are composed of alumina refinery residue that mainly consists of micron-sized hematite and that has been consolidated by ordinary Portland cement. The TRM pellets have an acid neutralizing capacity (ANC) of app. 4.5 moles H+/kg and can raise the pH of percolate well above 8, causing immobilization of heavy-metal cations. Upon exhaustion and decrease of pH, the hematite provides an excellent sorbent for arsenate. Comparing pH and heavy-metal concentrations in 600 L of percolate from an 80 kg waste column without and an 80 kg waste column with a bottom layer of TRM pellets (7 wt%), during a test of half a year; shows that the TRM pellets effectively neutralize the percolate (average pH = 7.4), immobilize the heavy-metal cations (81 - 99%) and immobilize the arsenate oxyanion (78%).