Indigenous sulfate-reducing bacteria for metal immobilization in acid-generating mine waste: laboratory assessment and arsenic mobilization risk at the Recsk Cu–Zn deposit, Hungary

Abandoned mine waste from high-sulfidation epithermal deposits generates acid mine drainage (AMD) that contaminates surrounding water bodies with potentially toxic elements. This study investigates the use of indigenous sulfate-reducing bacteria (SRB) to immobilize metals in waste from the Recsk Cu–Zn mining complex, Hungary. Twenty-eight samples from two waste heaps—H ₂ flotation tailings and H7 waste rock—were collected from oxidized and reduced layers. Initial pH ranged from 1.9 to 3.9; neutralization to pH 6–7 with Ca(OH) _₂ required 2.9 ± 1.0 mg g ^− 1 for tailings and 5.8 ± 3.3 mg g ^− 1 for waste rock (p ≈ 0.01). PCR targeting the dsrAB and apsA genes confirmed SRB presence in 17 of 28 enriched samples, with no significant difference between heaps (p ≈ 0.71). The high intrinsic sulfate content of both materials (H2: 2.4 ± 1.9 g kg ^− 1 ; H7: 14 ± 6 g kg ^− 1 ) eliminated the need for external sulfate supplementation, simplifying the nutrient requirements for field application. Biogenic sulfide precipitation at pH 6.0–6.6 reduced leachable Fe, Cu, Zn, and Mn below Hungarian regulatory thresholds for waste disposal. However, sulfidogenic treatment of tennantite-bearing waste rock mobilized arsenic (> 250 µg L ^− 1 ), likely attributed to reductive dissolution of Fe(III) oxyhydroxides. A step-by-step treatment framework is proposed that integrates pre-neutralization, SRB bioreactors, and an arsenic polishing step for field-scale implementation.