Enhanced in situ Pb(II) passivation by biotransformation into chloropyromorphite during sludge composting

Composting is an effective technology for the disposal and utilization of solid biowastes. However, conventional composting is inefficient for the passivation of heavy metals in solid biowastes, thus limiting the applications of compost derived from solid biowaste. Here, a thermophilic biomineralization strategy was proposed and demonstrated during sludge composting for in situ heavy metals passivation via thermophiles inoculation. It was found that Thermus thermophilus could promote the transformation of Pb(II) into the most stable chloropyromorphite [Pb₅(PO₄)₃Cl, K_sp = 10^-84.4] during composting. After 40 days of composting with T. thermophilus FAFU013, the most insoluble residual fractions of Pb increased by 16.0% (from 76.5% to 92.5%), which was approximately 3 times higher than that of the uninoculated control. The DTPA-extractable Pb decreased to 11.5%, which was 14.4% less compared with the uninoculated control, indicating a significant Pb passivation by inoculation of T. thermophilus FAFU013. A series of batch experiments revealed that Pb(II) could be rapidly accumulated by selective biosorption and gradually transformed into chloropyromorphite through the biomineralization of T. thermophilus FAFU013. This study provides new insight into the mechanism of heavy metal passivation during composting and the problem associated with the disposal of Pb-contaminated solid biowastes through the biomineralization of thermophiles.