Fundamental aspects of lignocellulosic material as slow-release electron donor for biological sulfate reduction

Biological sulfate reduction combined with chemical precipitation of metal sulfides is an good alternative to mitigate environmental impacts of acid mine drainage (AMD) and to recover base metals. This process requires a consistent supply of electron donors, and this study investigated the use of Brewery Spent Grain (BSG) as a slow-release electron donor (SRED) to sustain sulfate reduction and enhance metal sulfide precipitation as settling particles. A microbial consortium was enriched in batch reactors using poultry manure as the primary inoculum to evaluate its performance, metabolic pathways, and resilience to sulfide toxicity. Consortium enrichment successfully promoted a synergistic consortium where fermentative bacteria, notably Macellibacteroides and Enterococcus, hydrolyzed the lignocellulosic BSG to release lactate, hydrogen, propionate, and butyrate. These substrates were subsequently utilized by SRB groups, including Bacteroides graminisolvens, Desulforhabdus sp., and Desulfomicrobium, to drive sulfate reduction. The enrichment process increased the sulfide concentration and the sulfate removal rate. Optimization assays revealed that a ratio of 8 g BSG/g SO₄^2- provided the best conditions for sulfate removal. Conversely, excess BSG led to reactor acidification, slowing performance and causing excessive gas production. Sulfide toxicity tests demonstrated that while high sulfide accumulation may increase the lag phase, it does not significantly interfere with the ultimate sulfate consumption rate. The sum of those results provides optimized parameters for biogenic sulfide generation using BSG, giving a new perspective on the integration of industrial by-products into circular AMD remediation, establishing a foundation for more sustainable mining practices aligned with future global demands.