Indigenous Sulfate-Reducing Bacteria for Metal Immobilization in Acid-Generating Mine Waste: Laboratory Proof-of-Concept and Arsenic Mobilization Risk at the Recsk Cu–Zn Deposit, Hungary

Abstract Mine waste from the Recsk Cu–Zn deposit (Hungary) generates strongly acidic, metal-rich leachates. This laboratory microcosm study tested whether indigenous sulfate-reducing bacteria (SRB) in two contrasting waste types (flotation tailings, H2; waste rock, H7) could be reactivated after neutralization to immobilize dissolved metals. A representative subset of 28 field samples was enriched in modified Postgate E medium at pH 6–7. Conventional PCR targeting the dsrAB and apsA genes yielded positive or tentative-positive results in 17 of 28 samples, with no significant difference between waste types (Fisher’s exact test, p = 0.71). A pH-gradient experiment identified pH 6.0–6.6 as the practical window for rapid sulfide formation (fitted optimum ≈ 6.3; R 2 = 0.87). Neutralization required 2.9 ± 1.0 mg Ca(OH)₂ g⁻ 1 for H2 and 5.8 ± 3.3 mg g⁻ 1 for H7 (Welch’s t-test, p = 0.012). In the five selected endpoint systems (four SRB-positive systems plus one PCR-negative non-sulfidogenic reference), Cu, Zn, and Mn were below site-specific discharge limits in all cases and Fe in four of five. One As-rich H2 reduced sample showed dissolved arsenic of ≈ 13 mg L⁻ 1 after treatment, exceeding the site-specific limit (0.5 mg L⁻ 1 ) by a factor of ≈ 26. Because arsenic speciation was not measured and the mineralogical evidence is trace-level, the mobilization mechanism remains unresolved. The data support SRB treatment as a laboratory proof-of-concept, but indicate that As-bearing sulfosalt mineralogy should be screened before pilot application.