Bioaugmentation with Acidithiobacillus species accelerates mineral weathering and formation of secondary mineral cements for hardpan development in sulfidic Pb-Zn tailings
Introduction
Sulfidic and metallic tailings contain abundant reactive minerals such as pyrite (FeS2), pyrrhotite (Fe1−xS), sphalerite (Zn[Fe]S) and galena (PbS), which pose high risks of acidic (metallic) drainage (AMD) due to their oxidation potentials (Rosso and Vaughan, 2006, Moncur et al., 2009). In the weathering processes, potentially toxic elements including As, Cd, Pb, and Zn would be released from the sulfide minerals into aqueous phase, leading to the formation of AMD and secondary pollution risks in seasonal rainfall events (Lindsay et al., 2015). Successful closure of sulfidic tailings storage facilities (TSF) requires hydrogeochemical stabilization in the mineral phase to avoid secondary risks of metal(loid) dissolution (Huang et al., 2012). Our recent investigation of a long-term field trial (>15 years) discovered a thick and continuous layer of hardpan cap (30–>50 cm thick) naturally formed at the surface of sulfidic Cu‒Pb‒Zn tailings for supporting native vegetation under semi-arid climatic conditions (Gravina et al., 2004, Liu et al., 2018). The hardpan exhibited a high degree of hydrogeochemical stability with heavy metals largely sequestered within the hardpan layer and encapsulated in the cemented structure of the hardpans (Liu et al., 2019). Using cement-based technology (such as magnesium phosphate cement) for Pb-Zn contaminated wastes stabilization indicates great benefits for its time- and cost-efficiency (Wang et al., 2019, Wang et al., 2020). These observations have prompted the present study to investigate whether it is possible to accelerate the weathering of tailings minerals towards the formation of hardpan and immobilization of heavy metals.
The weathering of sulfides, co-dissolution of carbonates, and Si-rich minerals to form large amounts of secondary mineral gels are fundamental processes to cement tailing mineral particles, which are in fact a typical geological indicator of hardpan formation (Gilbert et al., 2003, DeSisto et al., 2011, Liu et al., 2018). The cement layers of tailing crusts are physically dense and mechanically hard under semi-arid climatic conditions (Graupner et al., 2007). A previous study identified the microstructural roles of Ca-sulfate/gypsum evaporites and Fe‒Si gels as cements in maintaining structural integrity of the massive hardpan caps naturally formed at sulfidic Cu‒Pb‒Zn tailings (Liu et al., 2018). The secondary mineral gels (such as amorphous Fe‒Si minerals) within the hardpan profile exhibit strong capacities for sequestration of heavy metals in sulfidic tailings (Hayes et al., 2012, Liu et al., 2019, Shi et al., 2020, Yu et al., 2020). Although these findings suggest the potential role of hardpan in managing and rehabilitating the sulfidic tailings, the natural formation process may take several decades for achieving structural integrity, mineralogical polymerization, and geochemical inertness (Huang et al., 2014). As a result, it is necessary to investigate how to accelerate the hardpan development before realizing the hardpan potential.
Tolerant sulphur (S) and iron (Fe)-oxidizing bacteria (such as the genus Acidithiobacillus: A. thiooxidans and A. ferrooxidans) are reported to be able to survive in sulfidic tailings and can catalyze the oxidation of Fe2+ and S2- in pyrite and other metal sulfides at a much higher rate than chemical oxidation (Fortin et al., 1995, Gadd, 2010, Schippers et al., 2010). It is known that A. thiooxidans is more capable of S oxidation than A. ferrooxidans at acid pH conditions (Bosecker, 1997). The combined functions of those two microbial species would significantly enhance the oxidation rates and continual oxidation of sulfide minerals by restricting the formation of intermediate precipitates of S0 (Dopson and Lindström, 1999). These combined microbial processes would result in the formation of large amounts of secondary minerals (Johnson and Hallberg, 2003), even under the initial circumneutral pH conditions (Southam and Beveridge, 1992, Mielke et al., 2003). However, under semi-arid climatic conditions, the abundance and activities of indigenous Fe/S-oxidizing bacteria in the tailings are very limited due to unfavorable conditions for microbial proliferation (Li et al., 2014, Li and Huang, 2015, Lindsay et al., 2015). In the meantime, the water migration into the capping layer via capillary suction and leaching into the tailings profile are infrequent, thus slowing down the overall weathering process (Banwart and Malmström, 2001, Huang et al., 2014).
The present study aims to investigate if the weathering of primary minerals and formation of secondary mineral gels could be stimulated by bioaugmentation with exogenous Fe/S-oxidizing bacteria (A. ferrooxidans and A. thiooxidans) in sulfidic Pb-Zn tailings maintained under moist conditions over a short timeframe. It was hypothesized that exogenous Fe and S oxidizers added to the sulfidic tailings could accelerate the weathering of primary Fe-bearing minerals (e.g., pyrite and biotite) and the formation of secondary mineral gels (e.g., secondary Fe (oxyhydr)oxides or hydroxysulfate) in the tailings. These secondary mineral gels would sequester the toxic heavy metals such as Zn and Pb due to their strong metal sorption capacity. A suite of microspectroscopic technologies including synchrotron-based X-ray absorption fine structure spectroscopy (XAFS) and X-ray fluorescence microscopy (XFM) coupled with X-ray absorption near edge fine structure spectroscopy (XANES) were employed to examine the microstructure changes, secondary Fe-mineral formation, and mechanisms of heavy metal sequestration in the tailings.
Section snippets
Materials
The sulfidic tailings were bulk-sampled at an operating TSF of Cannington mine in Northwest Queensland, Australia (21°85 'S, 140°90 'E). Tailings geochemistry background information is provided in the Supplementary Information (SI) (Table S1). The bulk tailings were air-dried, ground manually, and sieved through a 1 mm stainless steel sieve. Approximately 120 g tailings were loaded into an autoclaved Buchner Funnel (70 mm in diameter, 40 mm in height), which was bottom-lined with a 60 µm
Leachate geochemistry in response to microbial inoculation
The deliberate leachate collection was solely for the purpose of non-destructive monitoring of geochemical dynamics of the soluble elements partially indicative of mineral weathering in the tailings. During the five month of column incubation period, the pH in the “T + B”, “T-B”, and “T-B+A” ranged from 2.5 to 4.0 at each sampling cycle (Fig. S4), while that of “T” remained relatively stable at approximately 6.0–7.0 over the same period. Similarly, the leachate EC in the “T + B”, “T-B”, and
Fe/S-oxidizers accelerated mineral weathering and formation of secondary jarosite-like minerals
The findings in the present study evidently demonstrated that bioaugmentation with exogenous Fe/S-oxidizers was necessary for accelerating weathering of sulfidic minerals and formation of large amounts of secondary Fe-bearing minerals. The Fe oxidizing bacteria (A. ferrooxidans) may have gained energy via Fe(II) oxidation to Fe(III) under acidic pH condition, while the chemical oxidation of Fe(II) is not favored in extreme acid conditions (Hallberg and Johnson, 2001, Blowes et al., 2003). The
Conclusions
Building on the wealth of knowledge about the characterization of acidic/neutral and metallic drainages in sulfidic tailings, the present study has advocated a bioaugmenting approach with combined Fe/S-oxidizing bacteria to accelerate mineral weathering and secondary mineral formation for potential hardpan development. This may attenuate the AMD potential in the surface layer of sulfidic tailings. It is concluded that: (1) the weathering of pyrite and biotite-like minerals was rapidly
CRediT authorship contributions statement
Yunjia Liu: Conceptualization, Investigation, Methodology, Formal analysis, Data curation, Writing - original draft preparation. Songlin Wu: Methodology, Investigation, Formal analysis, Writing - review & editing. Gordon Southam: Methodology, Investigation, Writing - review & editing. Ting-Shan Chan: Methodology. Ying-Rui Lu: Methodology. David J. Paterson: Methodology. Longbin Huang: Supervision, Conceptualization, Project administration, Funding acquisition, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was undertaken on the XAS beamline 01C1 at National Synchrotron Radiation Research Centre, Taiwan and XFM beamline at the Australian Synchrotron (AS182/XFM/13331), part of ANSTO. The authors also acknowledge the facilities, and the scientific and technical assistance, of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy and Microanalysis, The University of Queensland (UQ). The study was supported by UQ research higher degree grant (
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