Dissolution Of Sulfide Minerals Research Articles

Water-Rock Interaction and Freeze-Thaw Cycles as Drivers of Acid Rock Drainage Generation by a Rock Glacier in the European Alps.

Little is known to date about the processes governing natural acid rock drainage (NARD) generated by rock glaciers. We used paragneiss samples from a catchment with NARD generated by a rock glacier in the Italian Alps for long-term leaching experiments under conditions that are possible within rock glaciers. The findings clearly suggest that at a low acid neutralization capacity of the rock, the dissolution of sulfide minerals, even if they are present in trace amounts, may be the most important process that controls the groundwater acidity at 1 °C, a typical temperature of groundwater discharge from rock glaciers. The acidic conditions increase the solubility and mobility of aquifer lithology-specific trace elements, and concentrations of some heavy metals of geogenic origin (e.g., Mn and Ni) may greatly exceed health standards after a six month interaction of water with paragneisses. Diurnal freeze-thaw cycles were found to be 6-7 times more effective in transformation of coarse rock fragments to fine-grained debris with fresh, reactive mineral surfaces, compared with temperatures above freezing. Cyclic freezing favors an enhanced formation of amorphous silica, a highly effective adsorbent for metal ions, and its redissolution within unfrozen layers of rock glaciers may represent an additional source of trace elements.

Geochemical Modeling and Quality Assessment of the Surface Waters from the Mouhoun River System, Burkina Faso: Implications of Water–Rock Interactions and Anthropogenic Activities

Major ion and heavy metal geochemistry was used to investigate the factors that control the Mouhoun River water quality in western Burkina Faso. Major cation concentrations was in the following decreasing order: Ca2+> Na+> K+> Mg2+, reflecting silicate weathering. However, relatively high HCO3- and SO42- concentrations could be related to dissolution of calcite and sulfide minerals, respectively. The average heavy metal concentrations in the riverine system were in the following order: FeT>Ag>Mn>Zn>Pb>Cu>Ni>AsT>Cr>Hg>Co>Cd with FeT and Ag exceeding the World Health Organization permissible limits. Concentrations of all heavy metals, except Cr, were higher in the Mouhoun system compared to the average world river concentrations. Adsorption and speciation models showed that Zn, Ag, Cd, Co, Cr and Ni are likely to remain in their free ionic forms, and thus posing serious threats to aquatic life and human health. Ag-Hg-Ni-Cd association around Poura and Tenado on the principal component plot suggested that the widespread artisanal gold mining in these areas may have contributed to these metals’ loading. In contrast, NH4+, Pb, Cu, Cr, AsT and Zn were clustered around a densely populated and industrialized Kou watershed, pointing to agricultural and industrial sources of these pollutants. Furthermore, the Kou River had the highest metal index followed by Tenado, Samendeni, Poura and Boromo. Hence, the chemistry of the Mouhoun River and its tributary appears to be mainly controlled by population density and various anthropogenic activities taking place in their drainage basins. The findings could provide avenues for sound and sustainable water resource management in a water scarce semi-arid environment.

New insights into the controversy of reactive mineral-controlled arsenopyrite dissolution and arsenic release

Serious arsenic (As) contaminations could commonly result from the oxidative dissolution of As-containing sulfide minerals, such as arsenopyrite (FeAsS). Pyrite (Py) and calcite (Cal) are two typically co-existing reactive minerals and represent different geological scenarios. Previous studies have shown that a high proportion of Py can generate a stronger galvanic effect and acid dissolution, thereby significantly promoting the release of arsenic. However, this conclusion overlooks calcite's antagonistic effect on the release of As in the natural environment. That antagonistic effect could remodel the linear relationship of pyrite on the oxidative dissolution of arsenopyrite, thus altering the environmental risk of As. We examined As release from arsenopyrite along a gradient of Py to Cal molar ratios (Py:Cal). The results showed that the lowest As release from arsenopyrite was surprisingly found in co-existing Py and Cal systems than in the singular Cal system, let alone in the singular Py system. This phenomenon indicated an interesting possibility of Py assistance to Cal inhibition of As release, though Py has always been regarded as a booster, also evidenced in this research, for As release from arsenopyrite. In singular systems of Py and Cal, As continued to be released for 60 days. However, in co-existing Py and Cal systems, As was released non-linearly in three stages over time: initial release (0–1 Day), immobilization (1–15 Days), and subsequent re-release (>15 Days). This is a new short-term natural attenuation stage for As, but over time, this stage gradually collapses. During the re-release stage (> 15 Days), a higher molar ratio of Py:Cal (increasing from 1:9 to 9:1) results in a lower rate constant k (mg·L−1·h−1) of As release (range from 0.0011 to 0.0002), and a higher abundance of secondary minerals formed (up to 26 mg/g goethite and hematite at Py: Cal=9:1). This demonstrates that increasing the Py:Cal molar ratio results in the formation of more secondary minerals which compensate for the higher potential antagonistic mechanisms generated by pyrites, such as acid dissolution and galvanic effect. These results explain the mechanisms of the high-risk characteristics of As both in acidic mine drainage and karst aquifers and discover the lowest risk in pyrite and calcite co-existing regions. Moreover, we emphasize that reactive minerals are important variables that can't be ignored in predicting As pollution in the future.

Microbial community structure in an uranium-rich acid mine drainage site: implication for the biogeochemical release of uranium.

The generation of acid mine drainage (AMD) characterized by high acidity and elevated levels of toxic metals primarily results from the oxidation and dissolution of sulfide minerals facilitated by microbial catalysis. Although there has been significant research on microbial diversity and community composition in AMD, as well as the relationship between microbes and heavy metals, there remains a gap in understanding the microbial community structure in uranium-enriched AMD sites. In this paper, water samples with varying levels of uranium pollution were collected from an abandoned stone coal mine in Jiangxi Province, China during summer and winter, respectively. Geochemical and high-throughput sequencing analyses were conducted to characterize spatiotemporal variations in bacterial diversity and community composition along pollution groups. The results indicated that uranium was predominantly concentrated in the AMD of new pits with strong acid production capacity, reaching a peak concentration of 9,370 μg/L. This was accompanied by elevated acidity and concentrations of iron and total phosphorus, which were identified as significant drivers shaping the composition of bacterial communities, rather than fluctuations in seasonal conditions. In an extremely polluted environment (pH < 3), bacterial diversity was lowest, with a predominant presence of acidophilic iron-oxidizing bacteria (such as Ferrovum), and a portion of acidophilic heterotrophic bacteria synergistically coexisting. As pollution levels decreased, the microbial community gradually evolved to cohabitation of various pH-neutral heterotrophic species, ultimately reverting back to background level. The pH was the dominant factor determining biogeochemical release of uranium in AMD. Acidophilic and uranium-tolerant bacteria, including Ferrovum, Leptospirillum, Acidiphilium, and Metallibacterium, were identified as playing key roles in this process through mechanisms such as enhancing acid production rate and facilitating organic matter biodegradation.

Component separation and origin estimation of mining-influenced water based on fluoride ions and water isotopes in underground legacy mine, Central Japan

Study regionA legacy underground tungsten mine in a mountainous area, central Japan. Study focusWe analyzed mining-influenced water (MIW) from mine voids and surface water from rivers to determine the dissolved ion concentrations and water isotopes (δ18O and δ2H). The results were interpreted using principal components and cluster analyses, as well as Eh-pH (Pourbaix) diagram. By integrating the obtained analytical results with mine-related data, we conducted component separation of MIW and estimation of their origins. New hydrological insights for the regionThe MIW and part of surface water were characterized by the SO42− and F− generated via the dissolution of sulfide minerals and fluorite (CaF2) from the ore deposit. MIW components were successfully separated using these indicators and water isotopes. The results of component separation indicated that the MIW consisted of two components, namely infiltrating water that rapidly passes through upper mine voids to reach the mine void at ground level, and groundwater that undergoes some degree of residence time before flowing into the ground level void from shaft I.

Effects of water-table changes following rainfall events on arsenic fate and transport in groundwater–surface water mixing zones

This study investigated the effects of groundwater–surface water (GW–SW) interactions on the fate and transport of arsenic (As) following rainfall events and subsequent water-table changes in GW–SW mixing zones, comprising the riparian and hyporheic zones, near an abandoned gold mine. During the dry and wet periods, stream conditions changed from flow-through to gaining, respectively. Water-table changes caused by rainfall events controlled flow paths between riparian zones and the stream, affecting spatiotemporal variation in the redox and pH conditions of the aquatic environment. Subsequently, the fate and transport of As in GW–SW mixing zones was responsive to variations in redox and pH conditions. Through the oxidative dissolution of As-bearing sulfide minerals and the reductive dissolution of iron (Fe) oxides with adsorbed As, As was released into the groundwater in the riparian zones and transported to the stream and streambed along the baseflow discharge. However, As was also immobilized in the sediment through adsorption onto Fe-oxides and coprecipitation with calcium (Ca) and zinc (Zn), suggesting that the sediment acts as a sink-and-source of As in aquatic environments. Therefore, water-table changes and GW–SW interactions could play an important role in the fate and transport of As in aquatic environments, specifically groundwater–riparian–streambed–stream systems. The findings of this study will provide scientific insights into the mechanisms of As in aquatic environments, aiding in improved decision–making to ensure safe and sustainable water management in response to future climate change.

Integrating Tide‐Driven Wetland Soil Redox and Biogeochemical Interactions Into a Land Surface Model

AbstractRedox processes, aqueous and solid‐phase chemistry, and pH dynamics are key drivers of subsurface biogeochemical cycling and methanogenesis in terrestrial and wetland ecosystems but are typically not included in terrestrial carbon cycle models. These omissions may introduce errors when simulating systems where redox interactions and pH fluctuations are important, such as wetlands where saturation of soils can produce anoxic conditions and coastal systems where sulfate inputs from seawater can influence biogeochemistry. Integrating cycling of redox‐sensitive elements could therefore allow models to better represent key elements of carbon cycling and greenhouse gas production. We describe a model framework that couples the Energy Exascale Earth System Model (E3SM) Land Model (ELM) with PFLOTRAN biogeochemistry, allowing geochemical processes and redox interactions to be integrated with land surface model simulations. We implemented a reaction network including aerobic decomposition, fermentation, sulfate reduction, sulfide oxidation, methanogenesis, and methanotrophy as well as pH dynamics along with iron oxide and iron sulfide mineral precipitation and dissolution. We simulated biogeochemical cycling in tidal wetlands subject to either saltwater or freshwater inputs driven by tidal hydrological dynamics. In simulations with saltwater tidal inputs, sulfate reduction led to accumulation of sulfide, higher dissolved inorganic carbon concentrations, lower dissolved organic carbon concentrations, and lower methane emissions than simulations with freshwater tidal inputs. Model simulations compared well with measured porewater concentrations and surface gas emissions from coastal wetlands in the Northeastern United States. These results demonstrate how simulating geochemical reaction networks can improve land surface model simulations of subsurface biogeochemistry and carbon cycling.

Identifying sources of acid mine drainage and major hydrogeochemical processes in abandoned mine adits (Southeast Shaanxi, China).

Acid mine drainage (AMD) has resulted in significant risks to both human health and the environment of the Han River watershed. In this study, water and sediment samples from typical mine adits were selected to investigate the hydrogeochemical characteristics and assess the environmental impacts of AMD. The interactions between coexisting chemical factors, geochemical processes in the mine adit, and the causes of AMD formation are discussed based on statistical analysis, mineralogical analysis, and geochemical modeling. The results showed that the hydrochemical types of AMD consisted of SO4-Ca-Mg, SO4-Ca, and SO4-Mg, with low pH and extremely high concentrations of Fe and SO42-. The release behaviors of most heavy metals are controlled by the oxidation of sulfide minerals (mainly pyrite) and the dissolution/precipitation of secondary minerals. Along the AMD pathway in the adit, the species of Fe-hydroxy secondary minerals tend to initially increase and later decrease. The inverse model results indicated that (1) oxidative dissolution of sulfide minerals, (2) interconversion of Fe-hydroxy secondary minerals, (3) precipitation of gypsum, and (4) neutralization by calcite are the main geochemical reactions in the adit, and chlorite might be the major neutralizing mineral of AMD with calcite. Furthermore, there were two sources of AMD in abandoned mine adits: oxidation of pyrite within the adits and infiltration of AMD from the overlying waste rock dumps. The findings can provide deeper insight into hydrogeochemical processes and the formation of AMD contamination produced in abandoned mine adits under similar mining and hydrogeological conditions.

Incomplete sulfide oxidation under sub-oxic conditions: Rates and aqueous sulfur speciation

The oxidative dissolution of sulfide minerals consumes oxygen and can generate redox gradients in mine wastes that affect the mobility of redox-sensitive solutes. However, the controls of oxygen depletion on sulfide reactivity and sulfur mobilization remain poorly constrained, complicating the long-term assessment of drainage quality dynamics. We investigated pyrite and Fe-rich sphalerite weathering using aqueous S speciation and quantitative mineralogical analyses under controlled, ambient versus sub-oxic, conditions at 6 < pH < 9. Titration assays revealed that oxidation rates for pyrite (up to 10−10 mol m−2 s−1) were higher than for sphalerite (up to 10−11 mol m−2 s−1) and, for both minerals, decreased by two orders-of-magnitude under sub-oxic (<50 ppm O2) conditions. Precipitation of secondary S-bearing minerals was restricted to elemental S, with no sulfate- or thiosulfate-salts detectable >0.01 wt%. Sulfate was the dominant aqueous S species produced in all experiments, but significant amounts of thiosulfate, tetrathionate, and sulfite were also found. These dissolved intermediate S species were abundant under both sub-oxic and oxic conditions, and broadly in line with known sulfide oxidation mechanisms, even though the weathering of pyrite produced several factors less incompletely oxidized S than that of sphalerite, under otherwise comparable conditions. This work shows that a range of intermediate S species may be produced even in low-reactivity conditions and may help optimize the management of such mine waste materials.

Comparison of sphalerite, djurleite, and chalcopyrite leaching by chemically and biologically generated ferric sulfate solutions

Ferric leaching, including the oxidation of sulfide raw materials of nonferrous metals in a ferric sulfate solution, is a well-known hydrometallurgical process that is widely applied in practice. Recent studies have indicated differences in the leaching kinetics of sphalerite concentrates when using solutions of ferric sulfate obtained by dissolving a commercial reagent or after bio-oxidation of iron-containing substrates by acidophilic chemolithotrophic microorganisms. In this research, the leaching of powder samples of natural zinc and copper sulfide minerals (sphalerite, djurleite, and chalcopyrite) was compared using two different solutions. The first one was an aqueous solution of a chemically pure ferric sulfate reagent, and the second solution contained ferric sulfate obtained via biooxidation of the chemically pure reagent of ferrous sulfate by an acidophilic bacterium Leptospirillum ferriphilum. The leaching solutions contained 10.1 g/L of ferric iron, pH 1.3. The experiments were carried out in batch mode in a mechanically stirred reactor. When using a solution of the chemical reagent of ferric sulfate, the level of dissolution of sulfide minerals was higher than in the case of the biological reagent: by 21.3% after 5 h (80 °C; 1% pulp density) for sphalerite, by 11.8% after 3 h (80 °C; 1% pulp density) for djurleite, and by 9.1% after 5 h (80 °C; 0.5% pulp density) for chalcopyrite. A comparison of the results of experiments with the biological reagent throughout from which microbial cells were preliminarily removed and the chemical reagent to which microbial cells were added confirmed the pivotal role of microbial cell lysis products in decreased efficiency of leaching with the biological reagent. Metabolomics analysis revealed several potential metabolites of L. ferriphilum involved in the observed negative effect on the leaching effectiveness, including nucleosides, alkaloids, and benzaldehydes. By contrast, the efficiency of zinc leaching from sphalerite was improved in the presence of metabolism products of L. ferriphilum in the first hours of the process.

Assessment of seasonal changes in groundwater quality of waste rock dump in temperate continental climate, northern Japan

Understanding seasonal groundwater quality changes in temperate continental climate waste rock dumps (WRDs) is necessary for sustainable environmental risk prevention and legacy mine contamination management. Therefore, we conducted a field investigation of a WRD to determine the mechanisms controlling its groundwater quality dynamics. The research aimed to understand the impact of seasonal changes on heavy metals released from the WRD. Three monitoring wells were installed in the WRD to investigate the pH, electrical conductivity (EC), and groundwater level (GL). The mineral composition of the waste rock was determined. Groundwater and river water samples from the monitoring wells and rivers surrounding the WRD were collected for chemical analysis. The sphalerite and galena concentrated in the WRD were assumed to be the main sources of Zn, Pb, and Cd contamination. Summer rainfall was the dominant recharge source of river water, which rapidly infiltrated to the WRD, altering the pH, EC, and GL of the groundwater. The pH, EC, and GL were stable in winter because snowpack covering the surface soil prevented groundwater recharge to the WRD. However, snow melting affected the pH, EC, and GL in the WRD. The sources of groundwater recharge (rainfall, river water, and snowmelt) altered the behaviour of the heavy metals in the WRD through two main mechanisms: the dissolution of sulphide minerals and efflorescent salts upon contact with the recharge water, and the dilution effect of the recharge water, which mixes with the groundwater in the WRD, reducing the heavy metal concentration. Sulphide mineral and efflorescent salt dissolution were significant in the deepest monitoring well and rainfall was the dominant recharge source which increased sulphide mineral and efflorescent salt dissolution in the WRD.

Copper leaching from the chalcopyrite-bearing MoS2 concentrate by mixed chlorides solution

In this study, the dissolution of copper sulfide minerals by the ferric (FeCl3) and ferrous (FeCl2) chloride leaching for upgrading the content of molybdenum disulfide (MoS2) in a molybdenite concentrate was investigated. The effect of various parameters was studied on the copper dissolution behaviour from the concentrate. In this matter, the copper dissolution was reached 94.84 % under the optimized leaching conditions. The kinetics of copper dissolution from the concentrate was established using a shrinking core model (SCM), and the process was controlled by diffusion, with a corresponding activation energy of 18.63 kJ mol-1 at the temperature range of 343?373 K. The amount of cop-per in the leachate was tested by the inductively coupled plasma-optical emission spectrometer (ICP-OES) and the solid phase was studied by X-ray diffraction (XRD), and scanning electron microscope (SEM). Results of the experiments show that the content of MoS2 in the solid residue was increased up to 88.59 % after the leaching.

Extracellular polymeric substances enhance dissolution and microbial methylation of mercury sulfide minerals.

Due to the extremely low solubility, mercury sulfide minerals, as the major environmental mercury sinks, are generally considered to be inert mercury species with minimal bioavailability. Here, we demonstrate that extracellular polymeric substances (EPS), continuously secreted and released by anaerobic methylating bacteria, enhance the dissolution processes of cinnabar (α-HgS) minerals. The enhancing effects of EPS occur to a greater extent in the dissolution of nanoparticulate α-HgS compared to the bulk-scale counterpart. The released EPS-Hg(II) species are available for microbial methylation to produce bioaccumulative neurotoxin, methylmercury. This is probably due to the abundant aromatic proteins in EPS that strongly interact with surface Hg(II) via inner-sphere complexation as well as cation-π interaction. Our study discovers the potential environmental risks of "inert" mercury sulfide minerals in natural microbial habitats, particularly benthic biofilms with abundant microbial EPS, transformed to the severely toxic methylmercury. The mechanistic findings will facilitate an accurate understanding of the interactions between soft and transition metals and microorganism-derived organics, which may dictate the environmental fate and impact of these elements.

Metagenomic Features Characterized with Microbial Iron Oxidoreduction and Mineral Interaction in Southwest Indian Ridge.

The Southwest Indian Ridge (SWIR) is one of the typical representatives of deep-sea ultraslow-spreading ridges, and has increasingly become a hot spot of studying subsurface geological activities and deep-sea mining management. However, the understanding of microbial activities is still limited on active hydrothermal vent chimneys in SWIR. In this study, samples from an active black smoker and a diffuse vent located in the Longqi hydrothermal region were collected for deep metagenomic sequencing, which yielded approximately 290 GB clean data and 295 mid-to-high-quality metagenome-assembled genomes (MAGs). Sulfur oxidation conducted by a variety of Gammaproteobacteria, Alphaproteobacteria, and Campylobacterota was presumed to be the major energy source for chemosynthesis in Longqi hydrothermal vents. Diverse iron-related microorganisms were recovered, including iron-oxidizing Zetaproteobacteria, iron-reducing Deferrisoma, and magnetotactic bacterium. Twenty-two bacterial MAGs from 12 uncultured phyla harbored iron oxidase Cyc2 homologs and enzymes for organic carbon degradation, indicated novel chemolithoheterotrophic iron-oxidizing bacteria that affected iron biogeochemistry in hydrothermal vents. Meanwhile, potential interactions between microbial communities and chimney minerals were emphasized as enriched metabolic potential of siderophore transportation, and extracellular electron transfer functioned by multi-heme proteins was discovered. Composition of chimney minerals probably affected microbial iron metabolic potential, as pyrrhotite might provide more available iron for microbial communities. Collectively, this study provides novel insights into microbial activities and potential mineral-microorganism interactions in hydrothermal vents. IMPORTANCE Microbial activities and interactions with minerals and venting fluid in active hydrothermal vents remain unclear in the ultraslow-spreading SWIR (Southwest Indian Ridge). Understanding about how minerals influence microbial metabolism is currently limited given the obstacles in cultivating microorganisms with sulfur or iron oxidoreduction functions. Here, comprehensive descriptions on microbial composition and metabolic profile on 2 hydrothermal vents in SWIR were obtained based on cultivation-free metagenome sequencing. In particular, autotrophic sulfur oxidation supported by minerals was presumed, emphasizing the role of chimney minerals in supporting chemosynthesis. Presence of novel heterotrophic iron-oxidizing bacteria was also indicated, suggesting overlooked biogeochemical pathways directed by microorganisms that connected sulfide mineral dissolution and organic carbon degradation in hydrothermal vents. Our findings offer novel insights into microbial function and biotic interactions on minerals in ultraslow-spreading ridges.

Hydrochemical and Isotopic Characteristics in the Surface Water of the Fenhe River Basin and Influence Factors

The Fenhe River Basin is the mother river of Shanxi Province. Due to the over-exploitation of water resources and the impact of social and economic development, the ecological environment has deteriorated. After a series of treatment and protection measures, the water quality has since been improved. Mathematical statistics, Piper diagrams, Gibbs model, hydrogen and oxygen isotopes, and other methods were used to analyze the characteristics and sources of hydrochemistry in the surface water of the Fenhe River basin, which revealed the evolution process of surface water quality of the Fenhe River basin. The results showed that the content of the main hydrochemical components in the main stream surface water of Fenhe River basin increased gradually along the runoff path. The hydrochemical types of surface water of Fenhe River basin were mainly HCO3·SO4·Cl-Ca·Na·Mg and SO4·HCO3·Cl-Ca·Na·Mg. There were great differences in hydrochemical components of tributaries and karst water in the basin. There were also great differences in hydrochemical components of tributaries in the basin. The hydrochemical types of surface water of karst water were mainly SO4·HCO3-Ca·Mg. The hydrochemical composition of surface water in Fenhe River basin was mainly affected by rock weathering and evaporation crystallization, whereas rainfall had little effect. Na+ and K+ mainly came from the dissolution of evaporated salt rocks with Na in the surrounding loess. Ca2+, Mg2+, and HCO3- mainly came from the dissolution of carbonate rocks. SO42- may have also come from the dissolution of sulfide minerals in the loess layer around Fenhe River in addition to the dissolution of gypsum. The values of δD and δ18O of Fenhe River surface water were gradually enriched from upstream to downstream. The characteristics of hydrogen and oxygen isotopes further showed that the surface water was mainly affected by evaporation. The results of this study can provide evidence for ecological restoration and protection and ecological civilization construction in the Fenhe River basin.

Copper mobilisation from Cu sulphide minerals by methanobactin: Effect of pH, oxygen and natural organic matter.

Aerobic methane oxidation (MOx) depends critically on the availability of copper (Cu) as a crucial component of the metal centre of particulate methane monooxygenase, one of the main enzymes involved in MOx. Some methanotrophs have developed Cu acquisition strategies, in which they exude Cu‐binding ligands termed chalkophores under conditions of low Cu availability. A well‐characterised chalkophore is methanobactin (mb), exuded by the microaerophilic methanotroph Methylosinus trichosporium OB3b. Aerobic methanotrophs generally reside close to environmental oxic–anoxic interfaces, where the formation of Cu sulphide phases can aggravate the limitation of bioavailable Cu due to their low solubility. The reactivity of chalkophores towards such Cu sulphide mineral phases has not yet been investigated. In this study, a combination of dissolution experiments and equilibrium modelling was used to examine the dissolution and solubility of bulk and nanoparticulate Cu sulphide minerals in the presence of mb as influenced by pH, oxygen and natural organic matter. In general, we show that mb is effective at increasing the dissolved Cu concentrations in the presence of a variety of Cu sulphide phases that may potentially limit Cu bioavailability. More Cu was mobilised per mole of mb from Cu sulphide nanoparticles compared with well‐crystalline bulk covellite (CuS). In general, the efficacy of mb at mobilising Cu from Cu sulphides is pH‐dependent. At lower pH, e.g. pH 5, mb was ineffective at solubilizing Cu. The presence of mb increased dissolved Cu concentrations between pH 7 and 8.5, where the solubility of all Cu sulphides is generally low, both in the presence and absence of oxygen. These results suggest that chalkophore‐promoted Cu mobilisation from sulphide phases is an effective extracellular mechanism for increasing dissolved Cu concentrations at oxic–anoxic interfaces, particularly in the neutral to slightly alkaline pH range. This suggests that aerobic methanotrophs may be able to fulfil their Cu requirements via the exudation of mb in natural environments where the bioavailability of Cu is constrained by very stable Cu sulphide phases.

Selenium Migration Mode in Coal Seams: Insights from Multivariate Analysis, Leaching Investigation, and Modelling

Processes controlling selenium concentrations ([Se]) in mine waters were studied at an operating coalmine district in Xuzhou city, China. The geochemistry and mobility of selenium was studied through leaching experiments, multivariate analysis, and numerical modeling. Results showed that selenium leaching was influenced by selenium occurrence in minerals, pH, electron activity (pe), and sulfur concentration in the water. Selenium occurrence in host rock was mainly sulfide minerals, and clay minerals in coal, respectively. Therefore, the oxidation and dissolution of sulfide minerals and transformation of clays may control the release of selenium. Experimental leaching experiments suggested selenium tends to leach more when the solution has more sulfur dissolved. A positive relationship is established between pH and the amount of Se released into solution with four times more Se released at pH 12 compared to pH 2 when leached with high-purity water. This release behavior is higher in O2-rich environments. The numerical modeling results showed that pH, pe, and sulfur presence in the solution play important roles in selenium adsorption. Selenium was desorbed from adsorbing surfaces under alkaline conditions, specifically when the solution pH was higher than 8. Higher pe values in the solution caused reduced selenium adsorption. In addition, dissolved sulfur competed with selenate for surfaces of adsorption, thus, selenium adsorption decreases as the sulfur concentration increased.

Oxidative Dissolution of Arsenic-Bearing Sulfide Minerals in Groundwater: Impact of Hydrochemical and Hydrodynamic Conditions on Arsenic Release and Surface Evolution.

The dissolution of sulfide minerals can lead to hazardous arsenic levels in groundwater. This study investigates the oxidative dissolution of natural As-bearing sulfide minerals and the related release of arsenic under flow-through conditions. Column experiments were performed using reactive As-bearing sulfide minerals (arsenopyrite and löllingite) embedded in a sandy matrix and injecting oxic solutions into the initially anoxic porous media to trigger the mineral dissolution. Noninvasive oxygen measurements, analyses of ionic species at the outlet, and scanning electron microscopy allowed tracking the propagation of the oxidative dissolution fronts, the mineral dissolution progress, and the change in mineral surface composition. Process-based reactive transport simulations were performed to quantitatively interpret the geochemical processes. The experimental and modeling outcomes show that pore-water acidity exerts a key control on the dissolution of sulfide minerals and arsenic release since it determines the precipitation of secondary mineral phases causing the sequestration of arsenic and the passivation of the reactive mineral surfaces. The impact of surface passivation strongly depends on the flow velocity and on the spatial distribution of the reactive minerals. These results highlight the fundamental interplay of reactive mineral distribution and hydrochemical and hydrodynamic conditions on the mobilization of arsenic from sulfide minerals in flow-through systems.

Hydrogeochemical mineral exploration in deeply weathered terrains: An example from Mumbwa, Zambia

Locating mineral deposits in areas of thick or transported overburden is notoriously difficult. Post-mineral cover is prevalent in many parts of the globe and has led to prospective geological sequences being missed by traditional methods of exploration. Hydrogeochemistry is particularly applicable for the exploration of Iron Oxide Copper Gold (IOCG) deposits because, when compared to larger porphyry or sediment-hosted systems, IOCG deposits tend to be smaller and high-grade with a limited lateral footprint to intersect with grid-drilling; groundwater interactions and ion dispersion tend to produce a much larger anomaly target than regolith geochemistry alone and require fewer samples. As a case study, we examine the hydrogeochemistry of the Kitumba IOCG deposit, located in the Mumbwa district of west-central Zambia. We present physicochemical data (Eh, pH, TDS, conductivity), major and trace element concentrations, and isotopic compositions (δ98Mo, 87Sr/86Sr, and δ65Cu) from groundwaters interacting with the Kitumba deposit and surrounding prospects. A hydrogeochemical footprint of As, Mo, Fe, Mn, and Zn is dispersed from the deposit. Groundwater 87Sr/86Sr values (0.708832 to 0.731807) reflect the mixing in varying proportions of waters that have interacted with distinct lithological endmembers in the Mumbwa area, corresponding to a complicated tectonic and metamorphic history. We report fractionation of 1.34 to 1.60‰ (∆65Cugroundwater – chalcopyrite) between proximal groundwater and primary chalcopyrite, which we postulate may be related to the oxidative dissolution of primary sulfide minerals. The δ98Mo3134 values of groundwaters proximal to known ore bodies are isotopically distinct (−1.08 ± 0.18‰ 2SE to 0.64 ± 0.08‰ 2 SE) from background aquifers (2.08 ± 0.12‰ 2SE). The trace element and isotopic hydrogeochemical patterns described in this study document water-rock and water-deposit interactions and demonstrate the potential of non-traditional stable isotopes to be employed in district-scale reduction of exploration ground and vectoring towards undisturbed ore deposits similar to Kitumba.

Dissolution kinetics and solubilities of copper sulfides in cyanide and hydrogen peroxide leaching: Applications to increase selective extractions

Accurate quantification of secondary and primary sulfide minerals is fundamental for resource evaluation, ore processing, and long-term sustainability of mining operations. In addition to visual mapping and automated mineral quantification, chemical analysis can also be harnessed to characterize the mineralogy of ore deposits. By evaluating the conditions in which certain minerals can be selectively dissolved from others, a chemical evaluation could provide geochemical speciation data of low-abundance minerals, such as copper/iron sulfides present in low-grade copper ores. The selective dissolution of copper sulfide minerals is, however, understudied. Here, we evaluate the use of potential selective dissolution conditions to differentiate supergene copper sulfides from hypogene copper sulfides. By characterizing the dissolution kinetics of chalcocite, covellite, bornite, enargite, chalcopyrite, and pyrite concentrates, we found that alkaline cyanidation (and not hydrogen peroxide or acid leaching in the presence of oxidizing agents) selectively dissolves supergene copper sulfides, which can be applied in a sequential extraction scheme to estimate the sulfide mineralogy of tailings samples. Cyanide completely dissolved chalcocite and covellite within 5–15 min, whereas dissolution in acid oxidative media only partially dissolved copper sulfides. Pyrite, chalcopyrite, enargite, and bornite under 0.5% KCN leaching (1 mg/mL) for 10 min showed approximately 1, 10, 30, and 40% of copper recovery, respectively. Cyanide leaching applied in sequential extractions of porphyry copper tailings samples from the Piuquenes impoundment, La Andina, Chile, improved the selective dissolution of secondary sulfides compared to a previously proposed hydrogen peroxide dissolution method, thus allowing their differentiation from primary sulfide minerals. The selective leaching of supergene sulfides by cyanidation provides a cheap and efficient method to estimate the copper sulfide mineralogy in copper ores, facilitating the sustainability and resource evaluation of mining operations.