Effect of bioprecipitation of secondary minerals mediated by sulphate reducing bacteria (SRB) on metal mobility in mine impacted environment: preliminary data

Mine and related activities are sources of huge volumes of mine wastes, tailings and residues of metallurgy, often characterized by high contents of metals and semimetals such as Zn, Pb, Fe, As, etc. These materials, when exposed to surface/near surface conditions, can be subjected to oxidation processes leading to the mobilization and dispersion of contaminants in soils and waters. Centuries of intensive mine exploitation, mainly addressed to zinc (Zn) and lead (Pb) extraction from sulphide and calamine deposits, left a seriously impacted environment in the Iglesiente and Arburese mine districts (SW Sardinia). Studies performed at the watershed scale, showed high sulphate (SO4 2-) and metal contents (mainly Zn and Fe) in rivers flowing in the area, however significant differences up to three orders of magnitude have been observed among them: from 6 kg/day of Zn load in Rio San Giorgio to 2000 kg/day in Rio Irvi waters. Microscale investigations (X-Ray Powder Diffraction (XRPD), Scanning Electron Microscopy energy dispersive spectroscopy (SEM-EDS), etc.) carried out on streambed sediments, allowed to recognize the presence of biogeochemical barriers, in well-developed hyporheic zone, that significantly affects metals mobility. Of particular interest is the presence of secondary sulphide minerals (e.g., framboidal FeSX and ZnS) the precipitation of which is mediated by sulphate reducing bacteria (SRB), under reducing conditions. With the aim to better understand the bioprecipitation processes, anaerobic batch experiments have been performed at the laboratory scale. Specifically, highly polluted Rio Irvi water (Zn = 550 mg/l) was inoculated with selected native SRB isolated from core sediments sampled along Rio San Giorgio and Rio Naracauli riverbanks. Different SRB microbial communities and novel metal-tolerant sulphidogenic microorganisms have been identified by the next-generation sequencing (NGS) approach. SEM-EDS analysis carried out on solids recovered at the end of experiments, showed the presence of (bio)precipitates having Zn and S composition and tubular morphology. At the same time, up to 100% of Zn removal has been determined by chemical analysis, performed by inductively coupled plasma optical emission spectroscopy (ICP-OES), of recovered solution (Paganin et al., 2021). These findings indicate the effectiveness of SRB in limiting metal mobility, suggest their potential in metal recovery, and highlight the importance of selecting native microbial communities already adapted to extreme environments.