Acidic Pit Lakes Research Articles

Contaminant generation and transport from mine pit lake to perennial stream system: Multidisciplinary investigations at the Big Ledge Mine, Nevada, USA

The Big Ledge mine located in northeastern Nevada, USA is a former barite mine site that now hosts an acidic pit lake and highly concentrated waste rock seepage, which flows into an intermittent stream. In an effort to identify the source(s) of the seepage, stable isotopic analyses were conducted (δ2H, δ18OH2O, δ18OSO4, and δ34S) in the pit lake, seepage, and groundwater. Additionally, inverse geochemical modeling was utilized to evaluate mixing relationships and mineral equilibria. Mass-balance calculations were also performed on the seepage/stream system to quantify discrete and diffuse solute inputs to surface waters. The stable isotopic composition of the pit lake and seepage display evaporative enrichment with the seeps being intermediate between the pit lake and groundwater, indicating that water in the seeps is partially sourced in the upgradient pit lake. Inverse modeling corroborates the isotopic mixing relationships, suggests that sulfide minerals could be dissolving along the flow path in the waste rock, and predicts the precipitation of sulfate salts from the concentrated seepage waters. Mineralogical analysis of efflorescent salts at the site, including epsomite (MgSO4·7H2O), pickeringite (MgAl2[SO4]4·22H2O), and melanterite (FeSO4·7H2O), corroborates the inverse modeling. Mass-balance calculations indicate the presence of diffuse acid mine drainage inflows along the creek, which, along with discrete inflows from waste rock seepage, contribute to sustained dissolved-metal transport in the perennial Tabor Creek downgradient from the mine.

Microbial Communities of Polymetallic Deposits' Acidic Ecosystems of Continental Climatic Zone With High Temperature Contrasts.

Acid mine drainage (AMD) systems are globally widespread and are an important source of metal pollution in riverine and coastal systems. Microbial AMD communities have been extensively studied for their ability to thrive under extremely acidic conditions and for their immense contribution to the dissolution of metal ores. However, little is known on microbial inhabitants of AMD systems subjected to extremely contrasting continental seasonal temperature patterns as opposed to maritime climate zones, experiencing much weaker annual temperature variations. Here, we investigated three types of AMD sites in Eastern Transbaikalia (Russia). In this region, all surface water bodies undergo a deep and long (up to 6 months) freezing, with seasonal temperatures varying between −33 and +24°C, which starkly contrasts the common well-studied AMD environments. We sampled acidic pit lake (Sherlovaya Gora site) located in the area of a polymetallic deposit, acidic drainage water from Bugdaya gold-molybdenum-tungsten deposit and Ulan-Bulak natural acidic spring. These systems showed the abundance of bacteria-derived reads mostly affiliated with Actinobacteria, Acidobacteria, Alpha- and Gammaproteobacteria, chloroplasts, Chloroflexi, Bacteroidetes, and Firmicutes. Furthermore, candidate taxa “Ca. Saccharibacteria” (previously known as TM7), “Ca. Parcubacteria” (OD1) and WPS-2 were represented in substantial quantities (10–20%). Heterotrophy and iron redox cycling can be considered as central processes of carbon and energy flow for majority of detected bacterial taxa. Archaea were detected in low numbers, with Terrestrial Miscellaneous Euryarchaeal Group (TMEG), to be most abundant (3%) in acidic spring Ulan-Bulak. Composition of these communities was found to be typical in comparison to other AMD sites; however, certain groups (as Ignavibacteriae) could be specifically associated with this area. This study provides insight into the microbial diversity patterns in acidic ecosystems formed in areas of polymetallic deposits in extreme continental climate zone with contrasting temperature parameters.

Coprecipitation of Co2+, Ni2+ and Zn2+ with Mn(III/IV) Oxides Formed in Metal-Rich Mine Waters

Manganese oxides are widespread in soils and natural waters, and their capacity to adsorb different trace metals such as Co, Ni, or Zn is well known. In this study, we aimed to compare the extent of trace metal coprecipitation in different Mn oxides formed during Mn(II) oxidation in highly concentrated, metal-rich mine waters. For this purpose, mine water samples collected from the deepest part of several acidic pit lakes in Spain (pH 2.7–4.2), with very high concentration of manganese (358–892 mg/L Mn) and trace metals (e.g., 795–10,394 µg/L Ni, 678–11,081 µg/L Co, 259–624 mg/L Zn), were neutralized to pH 8.0 in the laboratory and later used for Mn(II) oxidation experiments. These waters were subsequently allowed to oxidize at room temperature and pH = 8.5–9.0 over several weeks until Mn(II) was totally oxidized and a dense layer of manganese precipitates had been formed. These solids were characterized by different techniques for investigating the mineral phases formed and the amount of coprecipitated trace metals. All Mn oxides were fine-grained and poorly crystalline. Evidence from X-Ray Diffraction (XRD) and Scanning Electron Microscopy coupled to Energy Dispersive X-Ray Spectroscopy (SEM–EDX) suggests the formation of different manganese oxides with varying oxidation state ranging from Mn(III) (e.g., manganite) and Mn(III/IV) (e.g., birnessite, todorokite) to Mn(IV) (e.g., asbolane). Whole-precipitate analyses by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES), and/or Atomic Absorption Spectrometry (AAS), provided important concentrations of trace metals in birnessite (e.g., up to 1424 ppm Co, 814 ppm Ni, and 2713 ppm Zn), while Co and Ni concentrations at weight percent units were detected in asbolane by SEM-EDX. This trace metal retention capacity is lower than that observed in natural Mn oxides (e.g., birnessite) formed in the water column in a circum-neutral pit lake (pH 7.0–8.0), or in desautelsite obtained in previous neutralization experiments (pH 9.0–10.0). However, given the very high amount of Mn sorbent material formed in the solutions (2.8–4.6 g/L Mn oxide), the formation of these Mn(III/IV) oxides invariably led to the virtually total removal of Co, Ni, and Zn from the aqueous phase. We evaluate these data in the context of mine water pollution treatment and recovery of critical metals.

Causes and impacts of a mine water spill from an acidic pit lake (Iberian Pyrite Belt)

In May 2017, a spill from La Zarza pit lake (SW Spain) resulted in the release of approximately 270,000 m3 of extremely acidic waters to the Odiel River. Around 780 × 103 kg of Fe, 170 × 103 kg of Al, 2.15 × 103 kg of As and high amounts of other trace metals and metalloids were spilled. The purpose of this study is to explain the causes, consequences and impacts of the mine spill on the receiving water bodies. To this end, an extensive sampling along the mine site, river and estuary as well as a hydrological model of the pit lake was performed. Around 53 km of the Odiel River's main course, which was already contaminated by acid mine drainage (AMD), were affected. The mine spill resulted in an incremental impact on the Odiel River water quality. Thus, dissolved concentrations of some elements increased in the river up to 450 times; e.g. 435 mg/L of Fe and 0.41 mg/L of As. Due to low pH values (around 2.5), most metals (e.g., Cu, Zn, Mn, Cd) were transported in the dissolved phase to the estuary, exhibiting a conservative behavior and decreasing their concentration only due to dilution. However, dissolved concentrations of Fe, Cr, Pb, Se, Sb, Ti, V and especially As decreased significantly along the river due to Fe precipitation and sorption/coprecipitation processes. At the upper zone of the estuary, a noticeable increment of metal concentrations (up to 77 times) was also recorded. The water balance illustrates the existence of groundwater inputs (at least 16% of total) to the pit lake, due probably to local infiltration of rainwater at the mining zone. The probable existence of an ancient adit connected to the pit lake indicates that potential releases could occur again if adequate prevention measures are not adopted.

Watanabea acidotolerans: A new trebouxiophyte lineage (Chlorophyta) inhabiting low pH environments from Europe to South America

SUMMARYHere we studied a new Watanabea strain CAUP H 8901‐CRYO isolated from an extremely acidic (pH 2.6) pit lake in Czechia. We used morphological, ultrastructural and molecular data to ascertain its taxonomic position and phylogenetic relationships within the genus Watanabea. The morphology of the studied strain matched the formal Watanabea description, including its most characteristic feature, the production of two types of autospores. The new isolate morphologically and ultrastructurally resembled the Watanabea type species, W. reniformis. However, it differed from W. borysthenica by having a pyrenoid not surrounded by starch grains. Molecular phylogenetic analyses of nuclear 18S rDNA and plastid rbcL showed that the strain CAUP H 8901‐CRYO formed an independent lineage within the genus Watanabea. Furthermore, analyses of nuclear ITS2 secondary structure demonstrated an additional side loop coiling from the helix III, the unique feature of CAUP H 8901‐CRYO, not present in the authentic strain of W. reniformis SAG 211‐9b. The ITS2 secondary structures of the two strains also differed by one compensatory base change (CBC) and a number of additional nucleotide substitutions. The new acidotolerant strain and another Watanabea isolate from a naturally acidic lake in Argentina were identical in their plastid rbcL sequence. Consequently, here we proposed a new species, Watanabea acidotolerans, to accommodate a lineage of microalgae with a possible worldwide distribution and tolerance to low pH habitats.

Extreme enrichment of arsenic and rare earth elements in acid mine drainage: Case study of Wiśniówka mining area (south-central Poland)

The Wiśniówka rock strip mining area (south-central Poland) with quartzite quarries, acid water bodies and tailings piles is one of the most unique acid mine drainage (AMD) sites throughout the world. This is due to the occurrence of enormous amounts of pyrite unknown in sedimentary formations worldwide. Of the two mineralization zones, one that is the most abundant in arsenical pyrite occurs in the lowermost Upper Cambrian formation of the Podwiśniówka quarry. The As-rich pyritiferous clastic rocks are exposed as a result of deep quartzite extraction during 2013–2014. In addition, the clayey-silty shale interbeds are enriched in rare earth element (REE) minerals. The mining operation left an acidic lake with a pH of about 2.4–2.6 and increased contents of sulfates, metal(loid)s and REE. The Podwiśniówka pyrite-rich waste material was stacked up in many places of the mining area giving rise to strongly acidic spills that jeopardized the neighboring environment. One of these unexplored tailings piles was a source of extremely sulfate- and metal(loid)-rich pools with unusual enrichments in As (up to 1548 mg L−1) and REE (up to 24.84 mg L−1). These distinctly exceeded those previously reported in the Wiśniówka area. A broad scope of geochemical, mineralogical and petrographic methods was used to document these specific textural and mineralogical properties of pyrite facilitating its rapid oxidation. The pyrite oxidation products reacted with REE-bearing minerals releasing these elements into acid water bodies. Statistical methods were employed to connect the obtained tailings pool hydrogeochemical data with those derived from this and the previous studies of the Podwiśniówka and Wiśniówka Duża acid pit lakes. In contrast to metal(loid) profiles, the characteristic shale-normalized REE concentration patterns turned out to be more suitable for solving different AMD issues including provenance of mine waste material in the tailings pile examined.

Hydrological characterization and prediction of flood levels of acidic pit lakes in the Tharsis mines, Iberian Pyrite Belt

Opencast mining operations frequently lead to the creation of large voids that become anthropogenic lakes when the water table recovers. In the case of sulfide mining the stored water is of an acidic nature with significant concentrations of toxic metals and, therefore, a high pollutant potential. The main goal of the present work is to characterize the hydrological functioning and evolution of four acidic mine pit lakes in the abandoned mines of Tharsis, which is the second most important mine district in the Iberian Pyrite Belt (IPB). We present a simple methodology based on the use of the available orthophotographs and a Digital Terrain Model (DTM) together with the water balance of the pit lakes, which could be applied to other abandoned mining sites, where there is often a lack of hydrogeological information that prevents the application of more complex models. The accumulation of large volumes (5.2 × 106 m3) of acidic and metal-rich waters in these pit lakes poses a serious environmental concern, with dissolved concentrations up to 2000 mg/L of Fe, 223 mg/L of Al, etc. Sierra Bullones and Filón Norte are connected underground and present the same evolution, with water transfers from Sierra Bullones to Filón Norte. The water level in both pit lakes is increasing, with an average rise of 2.8 m/yr since the beginning of flooding. However, the increase in the evaporation rate, as a result of the larger flooded area as the water level rise, would induce a hydrological equilibrium before reaching the overflow level, leading to the formation of a terminal lake. On the other hand, the water level in Filón Centro and Filón Sur pit lakes remain approximately stable. The first behaves as a flow-through or terminal lake, depending on the annual rainfall, while the second acts permanently as a flow-through lake.

Metal mobility and mineral transformations driven by bacterial activity in acidic pit lake sediments: evidence from column experiments and sequential extraction

Biogeochemical processes occurring in lake sediments are crucial for element cycling. In acidic pit lakes, they play a major role in acidity neutralization and toxic metal immobilization. However, the relationship between bacterial activity (e.g., iron and sulfate reduction), mineral neoformation, and metal mobility is often difficult to establish and has been rarely reported. The aim of this study was to identify mineral products formed by bacterial activity and their influence on trace metal dynamics during early diagenesis. The study focuses on several acidic pit lakes in SW Spain. We combined chemical analyses of intact sediment cores taken from shallow and deep zones with experimental sediment core incubations aimed at visualizing geochemical and mineralogical transformations occurring in the sediments during early diagenesis. The sediment incubations commenced with a total resuspension and homogenization and lasted for periods of several months. During this time, pore water and overlying water was extracted and analyzed to record geochemical changes. The sediments were also sectioned and chemically analyzed after the experiments to investigate carbon, nutrient, sulfur, iron, and trace metal mobility. Intact sediment cores were analyzed by sequential extraction and differential X-ray diffraction to examine metal speciation and the presence of secondary minerals not detectable by conventional techniques. Shallow sediments are more biogeochemically reactive due to their higher organic carbon content. In columns prepared with these littoral sediments, a sharp pH gradient appeared in the upper 4 cm soon after the beginning of the incubations, suggesting alkalinity generation and concentration of microbial metabolisms (iron and sulfate reduction). This pH gradient provokes the precipitation of Al and leads to the formation of a milky, gelatinous layer in the water/sediment interface. This layer quickly dehydrates and recrystallizes to more stable phases. At the same time, bacterially produced H2S reacts with different metals (Cu, Zn) which are thus immobilized in the sediments as secondary sulfides. However, these two metals are also removed from solution by purely abiotic mechanisms like sorption or coprecipitation. Carbon seems to be the major factor limiting bacterially mediated metal and acidity attenuation in sediments of these acidic pit lakes. Metal immobilization is not a simple process and takes place through a combination of direct precipitation (Al oxyhydroxysulfates, Cu and Zn sulfides) and indirect mechanisms (sorption, coprecipitation, cation exchange). The precipitation of Al is of prime importance for pH buffering and immobilization of other metals (Cu, Zn). This process is not evident in routine chemical analyses of sediment cores, but it can be inferred by a combination of experimental and laboratory techniques such as the ones used here. P and N fertilization of these pit lakes would greatly increase the natural rates of acidity neutralization.

Arsenic in the Wiśniówka acid mine drainage area (south-central Poland) – Mineralogy, hydrogeochemistry, remediation

This paper summarizes different aspects of arsenic mineralogy and geochemistry in the Wiśniówka acid mine drainage (AMD) area, south-central Poland. The basic source of this metalloid in AMD water bodies is micro-grained pyrite that forms two different mineralized zones within the Upper Cambrian rock formation. Due to specific textural and geochemical characteristics, this mineral undergoes rapid oxidation on exposed quarry walls and unvegetated tailings piles, thus contributing to elevated arsenic concentrations in acid pit lakes and ponds, and locally intermittent pools. The latter were highlighted by the highest As levels reaching even 370 mg/L. Of the three AMD large reservoirs, the Podwiśniówka (eastern) lake revealed distinctly raised As concentrations varying from 7.02 to 22.2 mg/L. Another interesting water body is the Marczakowe Doły acid pond showing diverse seasonal As concentration patterns in colloids and a water column. Results of geochemical modeling showed that the saturation indices (SI) of hematite and goethite exhibited the highest values (range of 6.96 to 12.84 and 2.47 to 5.39, respectively) tending to precipitate in all the water bodies examined. In addition, the positive SI values of nanosized ε-Fe2O3 (1.64 to 4.13) and schwertmannite (0.90 to 2.10) were found only in the Wiśniówka Duża and Marczakowe Doły acid waters. Laboratory experiments revealed that the best chemical and cost-effective method (out of the limited number of options) of arsenic removal from the AMD waters examined was the use of high-grade powdered limestone (fraction 0.0–0.09 mm) or more effective although more expensive limewater (calcium hydroxide). The positive SI values of hematite, goethite, ε-Fe2O3 and schwertmannite of neutralized solutions suggest that these minerals should precipitate. Due to diverse concentrations in the examined rock series, arsenic has been used for fingerprinting “hot” places within the mining and neighboring areas, as well as for predicting the directions and effects of strip mining for quartzites. The As signature can also be applied to monitor a detrimental AMD impact on local surface and underground waters.

Autumnella lusatica gen. nov. and sp. nov. (Chlorophyta, Trebouxiophyceae), a new phytoplankton species in acidic lignite pit lakes

:We describe a novel green algal species that has been recorded for a decade but not specified during phytoplankton monitoring of acidic pit lakes in Saxony, Germany. Based on sequences of 18S rDNA, the ITS1-5.8S rDNA-ITS2 region and 16S rDNA, the novel alga is described as Autumnella lusatica gen. nov. & sp. nov. within the class Trebouxiophyceae. A close phylogenetic relationship led to the assignment of the morphologically similar Koliella corcontica Hindák to the new genus. Autumnella lusatica grows well in a neutral culture medium for green algae. In the type locality, the new species is characterized by (1) tolerance against acidity down to pH 3.0; (2) ability to propagate and establish enormous biovolumes at conditions of acidic pH, low total phosphorus (TP) and total inorganic carbon (TIC) concentrations; and (3) unusual seasonal growth dynamics with maximum and minimum biovolumes in October and June, respectively. As in other lakes, A. lusatica was abundant in the oligotrophic lake, Haberdorfer See, and caused chlorophyll a concentrations up to 30 μg l−1 at pH values between 3.0 and 3.5, TIC concentrations of usually < 1 mg l−1 and TP of < 10 μg l−1 in the euphotic zone. During its main growth phase, A. lusatica increased the total carbon in the euphotic zone on average by 2 mg l−1 or a factor of eight by C accumulation. Its growth rate could be even greater than that of some typical mixotrophic competitors. During lake stratification, the species preferred the epilimnion, supported by the formation of oil droplets in the cytoplasm.

Microbially mediated aluminosilicate formation in acidic anaerobic environments: A cell-scale chemical perspective.

Through the use of scanning transmission electron microscopy (STEM) combined with other complementary techniques (SEM, cryo-TEM, HRTEM, and EELS), we have studied the interaction of microorganisms inhabiting deep anoxic waters of acidic pit lakes with dissolved aluminum, silica, sulfate, and ferrous iron. These elements were close to saturation (Al, SiO2 ) or present at very high concentrations (0.12m Fe(II), 0.12-0.22m SO42- ) in the studied systems. The anaerobic conditions of these environments allowed investigation of geomicrobial interactions that are difficult to see in oxidized, Fe(III)-rich environments. Detailed chemical maps and through-cell line scans suggest both extra- and intracellular accumulation of Al, Si, S, and Fe(II) in rod-like cells and other structures (e.g., spherical particles and bacteriomorphs) of probable microbial origin. The bacterial rods showed external nanometric coatings of adsorbed Fe(II) and Al on the cell surface and cell interiors with significant presence of Al, Si, and S. These microbial cells coexist with spherical particles showing similar configuration (Fe(II) external coatings and [Al, Si, S]-rich cores). The Al:Si and Al:S ratios and the good Al-Si correlation in the cell interiors suggest the concurrent formation of two amorphous phases, namely a proto-aluminosilicate with imogolite-like composition and proto-hydrobasaluminite. In both cases, the mineralization appears to comprise two stages: a first stage of aluminosilicate and Al-hydroxysulfate precipitation within the cell or around cellular exudates, and a second stage of SO42- and Fe(II) adsorption on surface sites existing on the mineral phases in the case of (SO42- ) or on presumed organic molecules [in the case of Fe(II)]. These microbially related solids could have been formed by permineralization and mineral replacement of senescent microbial cells. However, these features could also denote biomineralization by active bacterial cells as a detoxification mechanism, a possibility which should be further explored. We discuss the significance of the observed Al/microbe and Si/microbe interactions and the implications for clay mineral formation at low pH.

Quantifying Lake Water Quality Evolution: Coupled Geochemistry, Hydrodynamics, and Aquatic Ecology in an Acidic Pit Lake.

Assessment of water quality evolution in the thousands of existing and future mine pit lakes worldwide requires new numerical tools that integrate geochemical, hydrological, and biological processes. A coupled model was used to test alternative hypothesized controls on water quality in a pit lake over ∼8 years. The evolution of pH, Al, and Fe were closely linked; field observations were reproduced with generic solubility equilibrium controls on Fe(III) and Al and a commonly reported acceleration of the abiotic Fe(II) oxidation rate by 2-3 orders of magnitude. Simulations indicated an ongoing acidity loading at the site, and the depletion of Al mineral buffering capacity after ∼5 years. Simulations also supported the existence of pH limitation on nitrification, and a limitation on phytoplankton growth other than the commonly postulated P and DIC limitations. Furthermore, the model reproduced the general patterns of salinity, pH, Al, and Fe during an uncontrolled river breach in 2011, however, incorporating sediment biogeochemical feedbacks is required to reproduce the observed postbreach internal alkalinity generation in the lake. The modeling approach is applicable to the study of hydrological, geochemical, and biological interactions for a range of lake and reservoir management challenges.

GEOCHEMICAL INVESTIGATION AND MODELLING OF AN ACID PIT LAKE FROM A HIGH SULFIDATION ORE DEPOSIT: KIRKI, NE GREECE

Open pit mining of a high sulfidation epithermal type deposit at Kirki (Thrace, NE Greece) resulted in the formation of an acid pit lake by infilling of the open cast by rain and drainage waters after mine closure. The acidic and oxidative pit lake waters show high concentrations of trace metals largely due to the high toxic metals content of the ore, the limited buffering capacity of host rocks and the direct exposure of the ore zone to weathering. The floor of the pit lake is covered by a finegrained mineral precipitate that comprises mainly detrital minerals, originating from erosion of the rocks exposed on the walls of the open pit. Secondary anglesite, several species of the jarosite-group, rozenite, melanterite, gypsum, bukovskyite, beaverite, scorodite and minor goethite are also detected. The mineral precipitate presents significant heavy metal content indicating effective removal of metals from the acidic waters. The speciation/mass transfer computer code PHREEQC-2 and the MINTEQ database were employed for geochemical modelling of the equilibrium between the acidic pit lake waters and the secondary phases of the mineral precipitate.

Rare earth and trace element signatures for assessing an impact of rock mining and processing on the environment: Wiśniówka case study, south-central Poland.

A detailed hydrogeochemical study was performed in the Wiśniówka mining area (south-central Poland). This covered three acid pit bodies, historic tailings acid ponds, acid pools, and additionally two neighboring rivers. All these acid mine drainage (AMD) waters are characterized by the pH in the range of 1.7 (pools) to 3.5 (tailings ponds). The most interesting is the Podwiśniówka acid pit lake that shows a very low pH (2.2–2.5) and very high concentrations of SO4 2− (2720–5460 mg/L), Fe (545–1140 mg/L), Al (86.2 mg/L), As (9603–24,883 μg/L), Co (1317–3458 μg/L), Cr (753–2047 μg/L), Cu (6307–18,879 μg/L), Ni (1168–3127 μg/L), and rare earth element (REE) (589–1341 μg/L). In addition, seeps that drain the Podwiśniówka mine tailings and partly aggregate piles form strong acid pools in the mining area. Along with these pools, in which As and REE contents reach 369,726 and 6288 μg/L, respectively, these waters are among the most distinctive As- and REE-rich AMD surface waters across the world. It is noteworthy that the Podwiśniówka acid pit lake and Wiśniówka Duża acid pit sump exhibit different element signatures and REE concentration patterns normalized to North American Composite Shale (NASC): the Podwiśniówka acid pit lake always shows a characteristic roof-shaped medium REE (MREE) profile with distinct enrichments in Gd, Eu, and Tb whereas the other one displays a step-shaped heavy REE (HREE) profile with positive Tb and Gd anomalies. The REE undergo fractionation during weathering and the subsequent leaching of dissolved and suspended fractions from rocks to acid water bodies where these and other elements are further fractionated by geochemical processes. This study shows that the individual REE have greater affinities for Mn, HREE for Fe and SO4 2−, and only La and Ce for Al. This specific water geochemistry has enabled us to (i) pinpoint the location of AMD “hot spots” originated from quartzite mining and processing operations conducted by current and previous mining companies, (ii) predict the directions and effects of future strip mining for quartzites in the Wiśniówka Duża and Podwiśniówka open pits, and (iii) evaluate the potential impact of mining and processing effluents on the quality of rivers.

Reaction time scales for sulphate reduction in sediments of acidic pit lakes and its relation to in-lake acidity neutralisation

Sulphate reduction is a key reaction to remove acidity from water bodies affected by acid mine drainage. In this study, 35SSO42− reduction rates determined in sediments from a variety of acidic lignite pit lakes have been compiled. The rates decreased with pH and are strongly dependent on carbon substrate. The rates were fitted to a Monod model adapted to the specific conditions of acidic pit lakes (APL) sediments: i) sulphate reduction rate is independent from sulphate concentration due to the high concentration typically observed in APL systems (10–30 mM), ii) the observed pH dependency of sulphate reduction was accounted for by an inhibition function Finihibt which considers the occurrence of low cell numbers of sulphate reducing bacteria at pH values < 4.75. Simulated steady-state sulphate reduction rates are predicting measured rates at carbon substrate concentrations of <10 μM. Estimated steady-state reaction time scales range between 2.4 h at pH 7 and 41 h at pH 3 at a carbon half-saturation constant of KC−S = 100 μM and are increasing with increasing KC−S values. Time scales at low pH are too long to allow for significant generation of alkalinity during the time of residence of groundwater passing through the top and hence most reactive zone of APL sediments which has important implications for the remediation of acidic pit lakes.

Geochemistry of dissolved aluminum at low pH: Extent and significance of Al–Fe(III) coprecipitation below pH 4.0

This work examines the geochemical behavior of dissolved aluminum in sulfate-rich acidic waters. Our observations were obtained during several years of geochemical and mineralogical research in the San Telmo acidic pit lake and other pit lakes of SW Spain. The work includes scanning and transmission electron microscopy (SEM, TEM) of suspended mineral colloids found in deep lake waters. Energy dispersive spectroscopy (EDS) coupled to scanning and high resolution transmission electron microscopy (STEM, HRTEM) revealed not only the presence and formation of discrete, sub-micron Al solids like alunite, but also the abundance and distribution of Al into Fe(III) phases typical of acid mine drainage, such as schwertmannite and jarosite, at a nanometric resolution. The main conclusion emerging from our work is that the fate and transport of Al at low pH (<4.0) can be largely influenced by adsorption on and/or coprecipitation with both schwertmannite and jarosite. Under the geochemical conditions studied (SO42−=10−2M, Fe(III)∼Al=10−3M), alunite formation may occur at pH>3.3, as suggested by mineralogical observations and geochemical modelling. Below this pH, and contrary to the extended assumption, Al is not truly conservative, and in the presence of ferric iron, both metals may co-precipitate at a substantial extent to form either particles of Al-rich schwertmannite (containing up to ca. 8at.% Al with [Fe/(Fe+Al)]=0.77) and/or crystals of H3O+- to K+-jarosite (containing up to ca. 10at.% Al with [Fe/(Fe+Al)]=0.54). This Al incorporation seems to take place by adsorption on particle surfaces in schwertmannite and by atomic substitution for Fe3+ in jarosite. Alunite is also unstable at this low pH range with respect to jarosite, which may lead either to isomorphic transformation and/or to chemically zoned crystals with jarositic rims around previously formed alunite cores. As a whole, the compositional spectrum of the analyzed jarosites and alunites describes a discontinuous, coupled (Al3+–Fe3+, H3O+–K+) solid solution series with an apparent gap at intermediate compositions. However, this gap seems to follow geochemical aspects more than crystallographic factors (i.e., immiscibility). The combination of this macroscopically invisible Al incorporated into Fe(III) solids along with subordinate alunite formation may cause significant Al removal even at very low pH (e.g., 20% decrease in Al concentration in San Telmo at pH<3.1). Furthermore, this Fe(III)–Al co-precipitation may also affect the fate of toxic trace elements like As and Pb.

Assessment of factors limiting algal growth in acidic pit lakes--a case study from Western Australia, Australia.

Open-cut mining operations can form pit lakes on mine closure. These new water bodies typically have low nutrient concentrations and may have acidic and metal-contaminated waters from acid mine drainage (AMD) causing low algal biomass and algal biodiversity. A preliminary study was carried out on an acidic coal pit lake, Lake Kepwari, in Western Australia to determine which factors limited algal biomass. Water quality was monitored to obtain baseline data. pH ranged between 3.7 and 4.1, and solute concentrations were slightly elevated to levels of brackish water. Concentrations of N were highly relative to natural lakes, although concentrations of FRP (<0.01 mg/L) and C (total C 0.7-3.7 and DOC 0.7-3.5 mg/L) were very low, and as a result, algal growth was also extremely low. Microcosm experiment was conducted to test the hypothesis that nutrient enrichment will be able to stimulate algal growth regardless of water quality. Microcosms of Lake Kepwari water were amended with N, P and C nutrients with and without sediment. Nutrient amendments under microcosm conditions could not show any significant phytoplankton growth but was able to promote benthic algal growth. P amendments without sediment showed a statistically higher mean algal biomass concentration than controls or microcosms amended with phosphorus but with sediment did. Results indicated that algal biomass in acidic pit lake (Lake Kepwari) may be limited primarily by low nutrient concentrations (especially phosphorus) and not by low pH or elevated metal concentrations. Furthermore, sediment processes may also reduce the nutrient availability.

High-Field FTICR-MS Data Evaluation of Natural Organic Matter: Are CHON5S2 Molecular Class Formulas Assigned to 13C Isotopic m/z and in Reality CHO Components?

The analysis of dissolved organic matter (DOM) using high-field Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) poses challenges in molecular formula assignment. The consideration of (13)C isotopes provides new insights into the consistent elemental formula solutions. Modern software helps to overcome misinterpretation, but false assignments of molecular classes to mass peaks have rarely been elucidated until now. It will be demonstrated that this can be important with formula assignments comprising exactly five nitrogen and two sulfur atoms in DOM data sets: the molecular class CHON5S2. The existence of such components in DOM under tripeptide Met-His-Cys formed with the formula C14H23O4N5S2 cannot be excluded; however, components containing 5 N and 2 S should be suspected to not be highly abundant. The true elemental compositions of such unusual "N5S2 moieties" were calculated using Suwannee River fulvic acid (SRFA) data from the literature and one data set from acidic pit lake pore water. The replacement of a H3N5S2 moiety with a (13)C1(12)C5O4 moiety explained more than 95% of the questionable "N5S2 moieties". This finding was proved by calculation of δ(13)C‰ values from relative peak intensities.

Low-crystallinity products of trace-metal precipitation in neutralized pit-lake waters without ferric and aluminous adsorbent: Geochemical modelling and mineralogical analysis

AbstractThe removal of dissolved trace metals during neutralization of acid mine drainage has usually been described and modelled as a progressive, pH-dependent sorption onto standard ferric or aluminous adsorbent. In the absence of adsorbent mineral surfaces, trace metals tend to form amorphous to low-crystallinity compounds which are often difficult to characterize. Here, we study the behaviour of the more soluble metals (Cu2+, Zn2+, Mn2+, Co2+, Ni2+, Cd2+) in the absence of ferric and aluminous adsorbent by neutralization experiments with waters from two acidic pit lakes. The objectives of our study were to identify the mineral products formed by trace-metal precipitation and the pH ranges at which these metals are removed from the solutions. Both geochemical modelling and detailed mineralogical and chemical analyses (XRD, SEM, TEM, XRF, ICP-AES) were undertaken to characterize the products. The schwertmannite and hydrobasaluminite colloids formed in the initial neutralization stages were removed from the waters at pH 3.5 and 5.1, respectively. These two minerals had previously adsorbed the Cr3+and Pb2+initially present in the solutions. The Cu precipitates were amorphous to X-rays, though chemical and modelling data suggest that Cu probably precipitated as a precursor of brochantite (Cu4(SO4)(OH)6·2H2O) at pH &gt;6.0, together with minor quantities of other Cu hydroxysulfates (langite, antlerite) and Cu(OH)2. At higher pH, other divalent metals (Zn2+, Mn2+) precipitated as silicates, carbonates and/or (possibly) minor oxides and (oxy)hydroxides. The high concentration of aqueous SiO2in the solutions allowed Zn to precipitate as willemite (Zn2SiO4) at pH &gt;7.0. Similarly, the presence of inorganic carbon (originally as CO2(aq.)) greatly influenced the nature of the corresponding precipitate of Mn. This metal was initially present as Mn2+and experienced a partly oxidative precipitation forming, in combination with Mg2+, the hydroxyl carbonate desautelsite (Mg6Mn2(CO3)(OH)16·4H2O) at pH 9.0–10.0. The formation of Mn3+/Mn4+oxides and hydroxides (hausmannite, manganite, birnessite) could not be demonstrated, although geochemical calculations support their subordinate formation. Other metallic cations such as Co2+, Ni2+and Cd2+did not form discrete mineral phases but were totally removed by sorption onto and/or incorporation into the cited Zn and Mn compounds. The discrepancies between theoretical and demonstrated mineralogy and the significance of these minerals for future pit-lake remediation initiatives are discussed.

Selective precipitation of schwertmannite in a stratified acidic pit lake of Iberian Pyrite Belt

AbstractSelective precipitation of schwertmannite was identified in the water column of a chemically stratified acidic lake (pH 2.0 2.5). A set of analytical techniques was used to identify accurately the mineral phase (X-ray diffraction, X-ray fluorescence spectrometry, scanning electron microscopy coupled with energy dispersive spectroscopy). The lake has two clearly differentiated layers: a thin superficial layer that extends to ∼2 m in depth with less dissolved solid content and, therefore, lower density, which contrasts with a thicker lower layer that extends to 34 m in depth. The upper layer is strongly influenced by climatic factors, triggering dilution (rainfall), concentration (dry season) and intense seasonal thermal fluctuations, while the lower layer is more compositionally and thermally stable.Schwertmannite precipitate was initially observed only in the upper layer, adhered to a plastic buoy and the anchor line. The latter was confirmed through a test involving a precipitation device that consisted of maintaining a series of plastic plates at different depths over a period of several months. Only the plates located in the upper layer became covered with precipitate, whereas the plates that were submerged in the lower layer remained clean. These observations clearly differed from the saturation indices of schwertmannite (SIsch) calculated usingPHREEQCand using Bighamet al.(1996) or Kawano and Tomita (2001) solubility products. Schwertmannite would not precipitate in the lake with the former but would precipitate in both layers with the latter. It is the constant provided by Yuet al.(1999) that makes SIschin both layers approach the observed behaviour: SIsch&gt;0 displays oversaturation in the upper layer while SIsch&lt;0 displays undersaturation in the lower layer. The value of the product of solubility that better adjusts to this situation is log Ksch= 9.5 10. Using the method by Yuet al.(1999) to establish the apparent solubility, a range of log Kschvalues was defined between 10.5 and 11.5 for the system studied.