Sulfate-bridged sequential remediation of benzo[a]pyrene and cadmium co-contaminated soil

The remediation of soils co-contaminated with organic pollutants and heavy metals remains a persistent challenge, as most conventional remediation technologies target a single pollutant type and cannot simultaneously achieve efficient organic degradation and heavy metal immobilization. In this study, a sulfate-bridge-mediated sequential strategy was developed for benzo[a]pyrene (BaP) and cadmium (Cd) co-contaminated soil. In the first stage, a biochar-supported nano zero-valent iron (nZVI-BC) activated persulfate (PDS) system achieved 97.6% BaP removal while generating a high concentration of soil residual sulfate (1194.6 mg·kg^-1) as a by-product. This sulfate was deliberately repurposed as a terminal electron acceptor to drive the second stage, in which biochar-immobilized sulfate-reducing bacteria (BC-SRB) transformed bioavailable Cd into stable fractions, reducing the exchangeable Cd fraction by 44.1%. The efficient consumption of the residual sulfate in soil, together with the associated changes in Cd fractions during the biological stage, supports that sulfate serves as the key functional bridge coupling the two processes. Beyond pollutant removal, the sequential strategy mitigated the transient adverse impacts of oxidation on soil physicochemical properties and shifted the soil microbial community, with an increased relative abundance of Enterobacter and enrichment of indigenous sulfate-reducing taxa. This work demonstrates a strategy of converting an oxidative by-product into a microbial substrate to drive heavy metal biostabilization, offering a potential approach for the remediation of co-contaminated soils.