Mechanism and process optimization of a newly proposed carbonate and phosphate co-biomineralization approach for...

Solidification/stabilization treatment by biomineralization methods can mitigate the heavy metals (HMs) leaching from lead-zinc tailings through simple spray construction. The solidification-efficient enzyme-induced carbonate precipitation (EICP) process suffers from low HM stabilization efficiency and ammonia-nitrogen pollution, whereas the stabilization-effective enzyme-induced phosphate precipitation (EIPP) process is costly. To overcome these limitations, this study proposes a two-step EIPP+EICP combined mineralization method in which EIPP is conducted first and the EICP reaction is subsequently introduced. The method exhibited mineralization rates of approximately 80 % and ammonia removal rates of > 50 %. Small MgHPO₄·3H₂O crystals formed during the first-step EIPP captured NH₄⁺ and converted into MgNH₄PO₄·6H₂O. While larger crystals formed multilayer particles with an MgHPO₄·3H₂O core coated by MgNH₄PO₄·6H₂O, MgCO₃·3H₂O, and MgHPO₄·3H₂O microcrystals. The optimal concentration ratios of EICP and EIPP reaction solutions for high, medium, and low combined mineralization targets were 1:0.2, 0.5:0.2, and 0.2:0.1, respectively. The co-mineralization method significantly captured free Zn²⁺, Cd²⁺, and Pb²⁺ by 92 %, 93 %, and 99 %, respectively into phosphate and carbonate precipitates in solutions, much more than that of the single EICP (22 %, 2 %, and 91 %). Furthermore, it stabilized bioavailable Zn, Cd, and Pb in lead-zinc tailings by 44 %, 53 %, and 23 %, respectively. In summary, the two-step EIPP+EICP method can stabilize HMs, offering the advantages of low cost and eco-friendliness, and providing a new strategy for the in situ solidification/stabilization of loose piles, such as lead-zinc tailings.