Acidic Microenvironments in Waste Rock Characterized by Neutral Drainage: Bacteria–Mineral Interactions at Sulfide Surfaces

Abstract

Microbial populations and microbe-mineral interactions were examined in waste rock characterized by neutral rock drainage (NRD). Samples of three primary sulfide-bearing waste rock types (i.e., marble-hornfels, intrusive, exoskarn) were collected from field-scale experiments at the Antamina Cu-Zn-Mo mine, Peru. Microbial communities within all samples were dominated by neutrophilic thiosulfate oxidizing bacteria. However, acidophilic iron and sulfur oxidizers were present within intrusive waste rock characterized by bulk circumneutral pH drainage. The extensive development of microbially colonized porous Fe(III) (oxy)hydroxide and Fe(III) (oxy)hydroxysulfate precipitates was observed at OPEN ACCESSMinerals 2014, 4 171 sulfide-mineral surfaces during examination by field emission-scanning electron microscopy-energy dispersive X-ray spectroscopy (FE-SEM-EDS). Linear combination fitting of bulk extended X-ray absorption fine structure (EXAFS) spectra for these precipitates indicated they were composed of schwertmannite [Fe 8 . The presence of schwertmannite and K-jarosite is indicative of the development of localized acidic microenvironments at sulfide-mineral surfaces. Extensive bacterial colonization of this porous layer and pitting of underlying sulfide-mineral surfaces suggests that acidic microenvironments can play an important role in sulfide-mineral oxidation under bulk circumneutral pH conditions. These findings have important implications for water quality management in NRD settings.

Evolution of Acid Mine Drainage Formation in Sulphidic Mine Tailings

“…These nano-environments can develop thin layers of acidic water that do not affect the bulk pH of the water chemistry. With progressive oxidation, the nano-environments may change to microenvironments [30]. Evidence of acidic microenvironments in the presence of near neutral pH for the bulk water can be inferred from the presence of jarosite (this mineral forms at pH around 2) in certain soil horizons where the current water pH is neutral [31].…”

Surface transformations of platinum grains from Fifield, New South Wales, Australia

“…More importantly, the presence of acicular Fe-hydroxides provides evidence that acidic conditions occur at the grain surface, which presents a possible mechanism for chemical dissolution of platiniferous materials (Guilbert 1986;Sillitoe 2005). The oxidation of Fe promotes the formation of acid through subsequent hydrolysis of water, which can alter the local chemical conditions (see Dockrey et al 2014), forming acid, which can enhance Pt solubilization (Sillitoe 2005); in particular, the precipitation of acicular Fe-hydroxides immediately on grain surfaces (Fig. 4) may produce an acidic nano-environment (interface) contributing to grain dissolution and the release of additional Fe from the Pt-Fe alloy surface (Guilbert 1986;Hattori et al 2010).…”

Gold nugget morphology and geochemical environments of nugget formation, southern New Zealand

“…All of the above processes occur via inorganic reactions that have been verified under laboratory conditions and rates (Webster, 1986;Rimstidt and Vaughan, 2003;Melashvili et al, 2015). In addition, it is possible that these or similar reactions may have been mediated, or rate-enhanced, by microbiological processes (Chen et al, 2014;Dockrey et al, 2014;Corkhill and Vaughan, 2009). Most such microbiological activity occurs under acidic conditions (Chen et al, 2014), but Dockrey et al (2014) suggest that acidic microenvironments on oxidising sulphide mineral surfaces can host appropriate bacteria in an overall circumneutral pH environment, such as the nugget-forming sites described herein.…”