Iron-modified biochar facilitates simultaneous stabilization of perfluorooctanoic acid and heavy metals during vermicomposting of sludge.

The ubiquitous coexistence of perfluorooctanoic acid (PFOA) and heavy metals (HMs) in sludge restricts its resource utilization through vermicomposting. Biochar-based adsorbents are mixed with sludge to improve its vermicomposting effectiveness, yet their potential for PFOA and HMs stabilization remains unexplored. In this study, iron-modified biochar (Fe-BC) was used to enhance simultaneous stabilization of PFOA and HMs during vermicomposting of sludge. The sludge mixed with 0% (VC), 1% (V1), 3% (V3), and 5% (V5) of Fe-BC was separately vermicomposted by Eisenia fetida. The results showed higher organic matter loss in Fe-BC treatments, with the highest being 43.84% in V5, while VC had 27.69%. Moreover, earthworm biomass increased by 61.92% in V5 and only by 26.33% in VC. Microbial community analysis revealed that the addition of Fe-BC significantly enriched the phylum Pseudomonadota, with potential for organic matter degradation. Under V5, the relative abundance of Pseudomonadota reached 49.71% in vermicompost and 71.41% in earthworm gut. The PFOA content was reduced by 17.40% in V5, contrary to its increase of 15.15% in VC. Meanwhile, V5 had higher reduction efficiency of Pb and Zn at over 85% and 78%, respectively. PFOA and HMs stabilization was mainly governed by adsorption, complexation, and bioaccumulation processes. The study suggests the use of Fe-BC to improve vermicomposting of sludge by minimizing toxicological effects associated with PFOA and HMs.