Enzymatically induced phosphate precipitation (EIPP) for stabilization/solidification (S/S) treatment of heavy metal tailings

The accumulation of heavy metal tailings has caused serious environmental and geological concerns. Therefore, treatment and recycling of heavy metal tailings have become urgent and vital. To this end, this paper proposes a biological solidification and stabilization (S/S) method, which applies enzymatically induced phosphate precipitation (EIPP) to strengthen Pb-Zn tailings and reduce the hazard of the heavy metals. The effectiveness and mechanism of EIPP-S/S have been systematically explored, including the effect of enzyme and reactant concentration, and tailings density on the treatment efficiency. The results showed that MgHPO₄(H₂O)₃ (newberyite) and Pb₉(PO₄)₆ were the main products of EIPP. The newberyite strengthened the tailings through connecting tailing particles and filling the pores, and the solidification effect was positively related to the reactant content and tailing compactness. The strength of the treated tailing sample could reach 2.7 MPa at a relative density of 90% with a reactant concentration of 1.5 M. After treatment, about 40% of Pb in the tailings changed from an acid-soluble state to oxidation state and residual state, as a result of the precipitation of Pb₉(PO₄)₆. The concentration of leached Pb from the treated tailings under an acid rain environment was reduced from 0.86 mg/L before treatment to less than 0.02 mg/L, well below the limit of 0.1 mg/L in China's ground/surface water. This study provides new insights into the treatment and reuse of tailings by applying an innovative EIPP-S/S method, which can flexibly treat tailings in an environmentally friendly manner.