Elsevier

Applied Geochemistry

Volume 113, February 2020, 104511
Applied Geochemistry

Bioprecipitation of As4S4 polymorphs in an abandoned mine adit

https://doi.org/10.1016/j.apgeochem.2019.104511Get rights and content

Highlights

  • Investigation of a freshly dewatered ore mine adit, which was flooded for 75 years.

  • Almost pure bonazziite (β-As4S4) was found in a submerged mine timber.

  • Bioprecipitation of various minerals is confined to single wood tracheids.

  • Mineral assemblies show a complex succession of redox conditions within the timber.

  • Intensive iron and sulfur redox cycling kept suboxic conditions in the flooded adit.

Abstract

Opportunities to study the anthropogenic cementation zones within permanently flooded mine works, where toxic elements accumulate by processes not fully described, are generally very rare. We examined macroscopic arsenic sulfide deposits inside a decomposed timbering of an abandoned mine adit, which had been flooded for 75 years. Bonazziite (β-As4S4) was always the dominant mineral in the deposits (up to 96.5%), while realgar and the ZnS phase were present in minor amounts. These findings differ from near-surface environments, where the most common biogenic As sulfide is realgar and other polymorphs are usually present in trace amounts only. Bonazziite was chemically highly pure with an average As/S molar ratio 0.9. It was 34S depleted compared to both aqueous sulfate and primary sulfides, which is consistent with biogenic sulfur reduction. Sulfide encrustation was confined to individual wood tracheids, indicating the extent of microenvironments that were oversaturated with respect to these minerals. The sulfides were accompanied by older gypsum and younger arsenian schwertmannite indicating complex succession of the redox conditions within the timbering. Authigenic electrum was found in one sample. Analysis of microbial communities revealed the dominance of sulfate reducers and fermenting microorganisms within the timber, similar to realgar-precipitating spots in wetland soils as previously documented. An anoxic environment in the mine water was maintained by autotrophic iron oxidation. The results suggest that a highly reduced microenvironment embedded in anoxic, but relatively oxidized surroundings is the key feature of the realgar and bonazziite-precipitating systems.

Introduction

Microbial sulfate reduction commonly occurs in anoxic environments, where organic matter and sulfate is present. In the presence of dissolved metals and metalloids, hydrogen sulfide produced by microorganisms causes precipitation of biogenic sulfide minerals. These minerals are generally characterized by their physical association with microbial habitats and depletion of the 34S isotope, which is a signature of enzymatic sulfate reduction (Canfield, 2001). Bioprecipitation of arsenic sulfides is well established in both laboratory and field conditions (Alam and McPhedran, 2019; Langner et al., 2013; Lee et al., 2007; Le Pape et al., 2017; McFarlane et al., 2015). Recent studies revealed increasing evidence of macroscopic As-sulfide (AsS) accumulation in various environments with natural or anthropogenic As enrichment (Demergasso et al., 2007; Drahota et al., 2013, 2017; Kerr et al., 2018; Knappová et al., 2019; Langner et al., 2013).

Besides the geochemical anomalies on the earth's surface, arsenic represents a common pollutant in active and abandoned underground mines. The most usual source of arsenic is oxidation of arsenopyrite and other As-bearing sulfides present in different types of ore mineralizations. Concentrations of As can reach tens or even hundreds of mg/L in highly mineralized acid mine drainage (Falteisek et al., 2016; Nordstrom et al., 2000; Xiao et al., 2009). Unlike most heavy metals, As is also relatively mobile in neutral mine waters (Lindsay et al., 2015) where it can reach several mg/L (Cidu et al., 2018; Drewniak et al., 2010). The aqueous concentration and mobility of As is usually affected by the formation of secondary minerals in the empty or backfilled mine works. Oxidation, neutralization, and evaporation of the solution after their discharge from rock pores into the underground mine works are the main mechanisms of secondary mineral formation (Majzlan et al., 2014). These minerals can be observed in historical mines in copious amounts (Culka et al., 2016; Filippi et al., 2015; Siuda and Macioch, 2018). Their formation contributes to the natural attenuation of toxic elements including As, but the immobilized elements can easily be released following a change in pH, redox conditions, or hydrologic regime. The processes that take place in naturally drained parts of abandoned mines are relatively well described (Filippi et al., 2015; Majzlan et al., 2014; Siuda and Macioch, 2018).

The deep, permanently flooded parts of old mines represent a different type of environment that is generally inaccessible for systematic investigation. They can represent a significant reservoir of potentially hazardous elements since most of the water slowly passes through permanently flooded levels before it is released to the environment from many abandoned mines (Kay et al., 2012; von Gunten et al., 2018; Roesler et al., 2007). The physicochemical parameters of water discharging from the flooded mines show that sulfate reduction and methanogenesis are common processes at these sites (Alam and McPhedran, 2019; Drewniak et al., 2010; Roesler et al., 2007). The precipitation of insoluble sulfides thus may potentially affect mobility of toxic elements in the flooded mines. However, our understanding of these processes is based on scarce examples of sulfide precipitation (Labrenz et al., 2000) and mostly on assumptions that have been inferred from bulk water parameters measured ex situ or at a discharge site. Spatial heterogeneity and the highly localized processes in the flooded mine environment are usually not considered.

Understanding of the bioprecipitation of As sulfides at low-temperature conditions is based mostly on surface environments. In complex soil and mine waste systems, a suboxic, but not strongly reduced, shell around the sulfide-precipitating microenvironment is usually present (Drahota et al., 2013, 2017; Kerr et al., 2018; Knappová et al., 2019; Langner et al., 2013). The main process maintaining the suboxic zone is oxidation of organic carbon. Unlike the surface environments, organic carbon is frequently the limiting nutrient for microbial growth in underground mines. It is present mostly in the form of partially decomposed timber, which is insoluble and represents a relatively recalcitrant substrate for most prokaryotes. Microbial necromass represented by decomposing biofilms of autotrophic iron and sulfur oxidizers may constitute an additional carbon source at sites, where extensive chemolithotrophic primary production takes place (Denef et al., 2010). Redox conditions are driven mainly by the balance between aerobic chemolithotrophs and heterotrophic microorganisms capable of anaerobic respiration in abandoned ore mines (Kimura et al., 2011; Ziegler et al., 2009). Taken together, the environment of mine adits is represented by the water column and underlying chemogenic sediments, whose composition and physicochemical conditions result from the internal biogeochemical processes. This system is thus less complex than the soil profiles or mine wastes, where random alternation of various layers and materials can play a significant role. Bioprecipitation of AsS in flooded mines is thus worthy of study not only because it can generate significant reservoirs of potentially hazardous elements, but also because the biogeochemical processes are better tractable in this environment compared to the complex soil profiles.

Mineralogy of the biogenic AsS polymorphs generally varies from amorphous As2S3 to crystalline orpiment (As2S3) and realgar (α-As4S4) while small amounts of other polymorphs have been detected (Demergasso et al., 2007; Langner et al., 2013; Lee et al., 2007). Traces of uzonite (As4S5) have been found in mixture with other As-sulfides in naturally produced AsS accumulations (Demergasso et al., 2007). Another AsS polymorph, bonazziite (β-As4S4) is well known as a synthetic high-temperature polymorph of As4S4. Native bonazziite has been described from a hydrothermal deposit (Bindi et al., 2015). It has been recently found in relatively low amounts as a constituent of biogenic realgar accumulations in wetland soil, which did not experienced elevated temperatures (Knappová et al., 2019). Low amounts of bonazziite were also found to precipitate together with realgar in microcosm experiments derived from wetland soils (Falteisek et al., 2019). Filaments composed of monomineral bonazziite have been described only from a thermophilic pure culture (Ledbetter et al., 2007). McFarlane et al. (2015) recognized β-As4S4 as a product of spontaneous recrystalization of extracellular AsS nanowires. These findings indicate the role of extracellular biopolymers in determination crystallinity of the mineral. Despite research efforts, factors determining the biogenic AsS mineralogy remain largely unclear.

Here, we report the finding of biogenic As-sulfides dominated by bonazziite in an abandoned mine adit that had been flooded for approximately 75 years. Mineralogy, water geochemistry and microbiology are described in relatively unique condition shortly after draining of the adit. This enabled a unique opportunity to study the sulfidation processes under almost preserved conditions of an abandoned flooded mine.

Section snippets

Site description

The Lehnschafter mine (50°41′27.42″N, 13°43′16.66″E) is located in the town of Mikulov, in the eastern part of the Krušné Hory Mts. (northern Czech Republic). The mine is opened by several adits located at various altitudes (top-down: Lehnschafter, Allerheiligen, Liebenfrauen, and Kreuzstollen). Mining operations were conducted from 16th century and ceased in 1858. After partial restoration, the mine is operated as an open-air museum since 2012.

The As–Ag–Pb hydrothermal vein-type deposit is

Field observations

During examination of the Liebenfrauen adit, which have been conducted 6 days after the progress of draining enabled entering the flooded area, yellow powdery coatings on timbers and even on some spots of the rocky walls were found throughout the gallery (Fig. 2). They were located mostly, but not exclusively, in the lower half of the adit's cross section. Few local accumulations reached the thickness up to ca 5–7 mm on the adit walls. Timbers exposed to the mine water in lower part of the adit

Arsenic mineralogy

Drainage of the Liebenfrauen adit enabled examination of the products of biogeochemical processes taking place in suboxic, near-neutral, As- and Fe-rich mine water during the past 75 years. In addition to the common minerals associated with weathering of sulfidic ore deposits (Fe (oxyhydr)oxides, schwertmannite, gypsum) were found the secondary AsS polymorphs: bonazziite and realgar. Localization of the sulfide minerals on the surface and inside the partially decomposed timbering together with

Conclusions

We described extensive precipitation of arsenic sulfide polymorphs bonazziite and realgar together with unidentified ZnS phase in the timbering of a recently dewatered historical Liebenfrauen mine adit. It is the first described site, where bonazziite exceeded other As sulfides, representing up to 96.5% of all minerals in the sample. No secondary Fe sulfides were observed, while arsenian schwertmannite and gypsum intimately associated with As sulfides. Microbiological and S isotope data

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 study was supported by the Czech Science Foundation (GACR 16-09352S) and the Center for Geosphere Dynamics (UNCE/SCI/006). Part of the laboratory equipment for this study was purchased from and the Operational Programme Prague – Competitiveness (CZ.2.16/3.1.00/21516). We thank to Pavel Chaloupka for allowing us to visit the Liebenfrauen adit, which is part of an old mine that he has been restoring and has opened to the public. A number of colleagues helped with sample processing and

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