Municipal sludge-assisted microbial passivation of pyritic waste rock for sustainable prevention of acid mine drainage: Pilot-scale evidence

Acid mine drainage (AMD) generated from pyritic waste-rock piles is a persistent environmental challenge. Although sulfate-reducing bacteria (SRB) have shown potential for AMD remediation, the lack of cost-effective carbon sources remains an important constraint on their large-scale application. In this study, an integrated system consisting primarily of municipal sludge, forest soil and SRB-containing microbial inoculum was further evaluated at pilot scale for the passivation of pyritic waste rock. In the amended cell, leachate pH increased from approximately 3 to ≥6.8, while dissolved Fe decreased to ∼0.01 mg L⁻¹ and sulfate concentrations stabilized at 15–100 mg L⁻¹. Mineralogical and surface analyses showed a significant reduction in sulfate-bearing secondary minerals, enrichment of reduced Fe and S species, and the development of carbon-rich surface coatings on pyrite-bearing materials. Microbial community analysis further revealed increased microbial diversity and a marked shift from communities enriched in Fe/S-oxidizing taxa, including Acidiferrobacter, Leptospirillum , and Sulfobacillus , toward communities containing higher abundances of taxa previously associated with anaerobic Fe-S transformations, including Desulfitobacterium, Desulfosporosinus , and Proteiniclasticum , together with increased microbial network connectivity. These coupled changes were consistent with the establishment of a more reducing and passivating geochemical environment, which effectively suppressed acid generation and Fe and sulfate release. Follow-up observations at 90 and 180 days further showed that circumneutral pH and relatively low Fe and sulfate concentrations were maintained during the monitoring period. Overall, this approach provides promising pilot-scale evidence for the source-oriented management of pyrite-rich mine wastes and the beneficial reuse of municipal sludge as a carbon-rich amendment.