Treatment of acid rock drainage using a sulphate-reducing bioreactor with a limestone precolumn

High Efficacy Two-Stage Metal Treatment Incorporating Basic Oxygen Furnace Slag and Microbiological Sulfate Reduction

Lignocellulosic sulfate-reducing biochemical reactors (SRBRs) can be implemented as passive treatment for mining-influenced water (MIW) mitigating the potentially deleterious effects of MIW acidic pH, and high concentrations of metal(loid)s and SO42-. In this study, a novel two-stage treatment for MIW was designed, where basic oxygen furnace slag (slag stage) and microbial SO42- reduction (SRBR stage) were incorporated in series. The SRBRs contained spent brewing grains or sugarcane bagasse as sources of lignocellulose. The slag reactor removed >99% of the metal(loid) concentration present in the MIW (130 ± 40 mg L-1) and increased MIW pH from 2.6 ± 0.2 to 12 ± 0.3. The alkaline effluent pH of the slag reactor was mitigated by remixing slag effluent with acidic MIW before SRBR treatment. The SRBR stage removed the bulk of SO42- from MIW, additional metal(loid)s, and yielded a circumneutral effluent pH. Cadmium, copper, and zinc showed high removal rates in SRBRs (≥96%) and likely precipitated as sulfide minerals. The microbial communities developed in SRBRs were enriched in hydrolytic, fermentative, and sulfate-reducing taxa. However, the SRBRs developed distinct community compositions due to the different lignocellulose sources employed. Overall, this study underscores the potential of a two-stage treatment employing steel slag and SRBRs for full-scale implementation at mining sites.

Glycerol amendment enhances biosulfidogenesis in acid mine drainage-affected areas: An incubation column experiment

Microbial sulfate (SO₄²⁻) reduction in Acid Mine Drainage (AMD) environments can ameliorate the acidity and extreme metal concentrations by consumption of protons via the reduction of SO₄²⁻ to hydrogen sulfide (H₂S) and the concomitant precipitation of metals as metal sulfides. The activity of sulfate-reducing bacteria can be stimulated by the amendment of suitable organic carbon sources in these generally oligotrophic environments. Here, we used incubation columns (IC) as model systems to investigate the effect of glycerol amendment on the microbial community composition and its effect on the geochemistry of sediment and waters in AMD environments. The ICs were built with natural water and sediments from four distinct AMD-affected sites with different nutrient regimes: the oligotrophic Filón Centro and Guadiana acidic pit lakes, the Tintillo river (Huelva, Spain) and the eutrophic Brunita pit lake (Murcia, Spain). Physicochemical parameters were monitored during 18 months, and the microbial community composition was determined at the end of incubation through 16S rRNA gene amplicon sequencing. SEM-EDX analysis of sediments and suspended particulate matter was performed to investigate the microbially-induced mineral (neo)formation. Glycerol amendment strongly triggered biosulfidogenesis in all ICs, with pH increase and metal sulfide formation, but the effect was much more pronounced in the ICs from oligotrophic systems. Analysis of the microbial community composition at the end of the incubations showed that the SRB Desulfosporosinus was among the dominant taxa observed in all sulfidogenic columns, whereas the SRB Desulfurispora, Desulfovibrio and Acididesulfobacillus appeared to be more site-specific. Formation of Fe³⁺ and Al³⁺ (oxy)hydroxysulfates was observed during the initial phase of incubation together with increasing pH while formation of metal sulfides (predominantly, Zn, Fe and Cu sulfides) was observed after 1–5 months of incubation. Chemical analysis of the aqueous phase at the end of incubation showed almost complete removal of dissolved metals (Cu, Zn, Cd) in the amended ICs, while Fe and SO₄²⁻ increased towards the water-sediment interface, likely as a result of the reductive dissolution of Fe(III) minerals enhanced by Fe-reducing bacteria. The combined geochemical and microbiological analyses further establish the link between biosulfidogenesis and natural attenuation through metal sulfide formation and proton consumption.

Dynamics of Microbial Communities during the Removal of Copper and Zinc in a Sulfate-Reducing Bioreactor with a Limestone Pre-Column System

Biological treatment using sulfate-reducing bacteria (SRB) is a promising approach to remediate acid rock drainage (ARD). Our purpose was to assess the performance of a sequential system consisting of a limestone bed filter followed by a sulfate-reducing bioreactor treating synthetic ARD for 375 days and to evaluate changes in microbial composition. The treatment system was effective in increasing the pH of the ARD from 2.7 to 7.5 and removed total Cu(II) and Zn(II) concentrations by up to 99.8% and 99.9%, respectively. The presence of sulfate in ARD promoted sulfidogenesis and changed the diversity and structure of the microbial communities. Methansarcina spp. was the most abundant amplicon sequence variant (ASV); however, methane production was not detected. Biodiversity indexes decreased over time with the bioreactor operation, whereas SRB abundance remained stable. Desulfobacteraceae, Desulfocurvus, Desulfobulbaceae and Desulfovibrio became more abundant, while Desulfuromonadales, Desulfotomaculum and Desulfobacca decreased. Geobacter and Syntrophobacter were enriched with bioreactor operation time. At the beginning, ASVs with relative abundance <2% represented 65% of the microbial community and 21% at the end of the study period. Thus, the results show that the microbial community gradually lost diversity while the treatment system was highly efficient in remediating ARD.