Acid Mine Drainage Sources Research Articles

Oxidative Dissolution Mechanisms of Pyrite {100} and {210} Faces: An Atomic Study

ABSTRACT The oxidative dissolution of pyrite discharging into tailing pond as gangue minerals attracts intensive interesting as this forms the main source of acid mine drainage. However, the related dissolution mechanisms on the individual crystal faces of pyrite particles remain unclear. In this study, the oxidative dissolution mechanisms of pyrite {100} and {210} faces under acidic and neutral conditions were investigated. The results indicated that the dissolution rate of pyrite correlates positively to the reactivity of pyrite face. The reactivity of {210} was higher than that of {100}, mainly due to the geometric arrangements of the atoms. In addition, the oxidative dissolution on pyrite faces follows the chemical reaction model. Further density functional theoretical (DFT) calculation indicated that {210} face showed obvious Fe-S breakage and S-S formation while {100} face only showed slight atoms movement during relaxation process. Moreover, a higher conductivity and electron transfer ability of {210} faces than that of {100} face was found base on scanning electrochemical microscopy. This study therefore provides new insights into oxidative dissolution of pyrite crystal faces at an atomic level.

Recovery of Rare Earth Elements from Acid Mine Drainage with Supported Liquid Membranes: Impacts of Feedstock Composition for Extraction Performance.

Acid mine drainage (AMD) from inactive coal mines can be enriched in rare earth elements (REEs) and has gained much attention as an alternative source for these technology-critical metals. However, AMD is a relatively low-grade REE resource in which the abundance of impurities and the composition variability of the feedstock create major uncertainties for the performance of REE extraction technologies. This study sought to identify AMD feedstock variables that influence the extraction efficiency of REEs by supported liquid membranes (SLMs). SLM separation is a process involving a hydrophobic membrane embedded with an extracting solvent that facilitates the selective extraction of REE ions. The major aims were to (1) assess the effectiveness of SLM-based REE separation from several AMD samples representing a spectrum of aqueous composition, (2) determine the effects of AMD storage and holding time on extraction performance, and (3) assess the impact of AMD pretreatment (e.g., filtration and pH adjustment) on REE recovery. The results showed that relative extraction fluxes of REE correlated with AMD characteristics such as pH and major ions such as Fe, Ca, and Mn. The purity of the acid strippant product, expressed as the REE dry weight content, depended on the initial REE concentrations in the AMD source rather than the flux of individual REEs across the membrane. For AMD samples stored for 3 months prior to extraction, REE recovery by SLM separations was substantially decreased if oxidation of Fe(II) to Fe(III) was observed during sample storage. Pretreatment of AMD feedstocks by pH adjustment did not substantially improve the separation performance. Overall, this study establishes primary water quality parameters of AMD that influence the SLM separation flux and product purity. Such insights contribute to a mechanistic understanding of critical metals extractions by SLM for complex and nontraditional feedstocks such as AMD wastes.

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.

Microbiological impacts of acid mine drainage on urban agriculture in Soweto, South Africa

The late 17th century discovery of gold in Johannesburg was accompanied by skewed income distribution in the world's most unequal society and more prosaic curses like acid mine drainage (AMD). This acid water contaminates ecosystems such as patches of urban agriculture that communities rely on to supplement income and nutritional needs. We investigated the extent to which AMD contaminates streams, irrigation water and subsequently, soil and vegetation on small-scale farms. We targeted microbial communities as the first line of biotic casualties in AMD infiltrated substrates. Sites were all in Soweto, Johannesburg, the first was just adjacent to a mine dump, the second was in a midstream section and the last was downstream where locals cultivate fields. We tested contamination of river sediment, river water, cultivated and uncultivated soil, borehole water used for irrigation and the crops themselves. The Biolog EcoPlate™ microbial community profiling kit was used, it measures the ability of microbes to use various carbon sources. The MicrotoxⓇ bio-assay was also used, it exploits a luminescent bacterium to indicate the level of biological toxicity in a medium. Both methods suggest severe contamination of environmental substrates close to the AMD source. The contamination attenuates downstream but was detected in soil, water and crops ~20 km away from the mine. Therefore, the impacts of mining are systemic in the catchment and authorities must appreciate the seriousness of the situation so that mitigation strategies can devised and implemented immediately.

Pilot Scale Testing of Lignite Adsorption Capability and the Benefits for the Recovery of Rare Earth Elements from Dilute Leach Solutions

Naturally occurring organic materials containing humic acids show a strong affinity towards rare earth elements (REE) and other critical elements. Leaching experiments on lignite coal waste produced from construction sand production revealed that the contained REEs were associated with the organic matter. Furthermore, adsorption studies revealed that the lignite waste was capable of extracting REEs from a model solution and increased the REE content of the lignite waste by more than 100%. As such, this study aimed to utilize the lignite waste to adsorb REEs from pregnant leach solutions and acid mine drainage sources having low REE concentrations and subsequently leach the lignite material to produce pregnant leach solutions containing relatively high amounts of REEs, which benefits the performance and economic viability of downstream separation and purification processes. An integrated flowsheet was developed based on this concept and tested at a pilot scale. The pregnant leachate solution (PLS) was generated from a heap leach pad containing 2000 tons of Baker seam coarse refuse. The pilot scale circuit was comprised of aluminum precipitation, adsorption using the waste lignite, and rare earth-critical metal (RE-CM) precipitation stages in succession. The results indicated that the aluminum precipitation stage removed over 88% and 99% of the Al and Fe, respectively. The adsorption stage increased the REE content associated with the waste lignite from 457 ppm to 1065 ppm on a whole mass basis. Furthermore, the heavy REE (HREE) content in the feedstock increased by approximately 250%, which raised the percentage of HREE in the REE distribution by 19 absolute percentage points. In addition to the REEs, concentrations of other critical elements such as Mn, Ni, and Zn also improved by 75%, 37%, and 250%, respectively. Bench-scale tests revealed that increasing the solids concentration in the waste lignite and PLS mix from 1% to 20% by weight enhanced the adsorption efficiency from 32.0% to 99.5%, respectively. As such, a new flowsheet was proposed which provides significantly higher REE concentrations in the PLS that can be fed directly to solvent extraction and/or oxalic acid precipitation and, thereby, enhancing process efficiency and economics.

Modelling of Acid Mine Drainage in Open Pit Lakes Using Sentinel-2 Time-Series: A Case Study from Lusatia, Germany

Strong acid mine drainage (AMD) processes in the flooded, formerly open pits in the Lusatia area present an enormous environmental challenge for the rehabilitation of the post-mining landscape. Extensive and costly monitoring is required for optimal AMD management and remediation planning and control. Because of the large size of the area and the dimension of the problem, the regular sampling can only provide limited point data, which needs to be extrapolated to the entire area. Consequently, the search for effective approaches for extrapolating the point data to the area of all water bodies is essential for rehabilitation success monitoring and for understanding the dependencies between AMD and environmental factors such as land use, weather conditions, geology, and hydrogeology. The main aim of this study was to investigate the suitability of Sentinel-2 multispectral imagery and artificial neural networks (ANNs) for the quantitative mapping of acid mine drainage (AMD) constituents, such as dissolved iron, pH value, and sulfate in large water bodies, for an area of approximately 7220 km2 (the area of the pit lakes is about 185 km2). Correlations between different chemical water parameters were also investigated. An extensive water monitoring dataset was used to train artificial neural networks for the identification of dependencies between the multispectral remote sensing data and the water quality ground measurements. Respective relationships have been identified, especially for dissolved iron and pH. These trained ANNs have been used to produce water quality maps with high spatial (10 × 10 m) and temporal (any cloud-free period) resolution, which show the wide variability of water quality in the different parts of the mining region. Concrete sources of AMD can be identified using the water quality maps of single lakes, and the success of sanitation measures such as liming was visualized. The approach opens many doors for the optimization of both the monitoring program and sanitation technology.

Rare earth element recovery in hard-rock acid mine drainage and mine waste: A case study in Idaho Springs, Colorado

While the geochemistry of hard-rock generated acid mine drainage (AMD) and waste rock has been extensively studied for environmental and toxicological impacts, their critical metal contents have only recently begun to be characterized. We investigated rare earth element (REE) geochemistry in AMD and waste rock in the Idaho Springs gold district, Colorado. Specifically, we found that two AMD inputs to the Argo Tunnel Wastewater Treatment Plant (ATWTP) contain high dissolved REE concentrations (0.7 and 1.3 mg*L−1), and that REEs could be concentrated using a two-stage pH adjustment. The first stage precipitated iron and removed thorium by adsorption, while the second stage precipitated aluminum, which removed REEs by adsorption. Solids generated were 2670 and 2800 mg*kg−1 for each of the REE-bearing AMD sources, and at 90% REE recovery, the ATWTP could produce nearly 1000 kg of REEs each year with an economically favorable MREE enrichment. REE leaching in waste rock was also investigated, but results indicate that it is unlikely to be a significant source of REEs. More work is needed to characterize both the critical metal content (including other metals besides REEs) and economic feasibility of extraction for mine waste and AMD in hard-rock mining environments.

Iron Isotopes in Acid Mine Drainage: Extreme and Divergent Fractionation between Solid (Schwertmannite, Jarosite, and Ferric Arsenate) and Aqueous Species.

Examination of stable Fe isotopes is a powerful tool to explore Fe cycling in a range of environments. However, the isotopic fractionation of Fe in acid mine drainage (AMD) has received little attention and is poorly understood. Here, we analyze Fe isotopes in waters and Fe(III)-rich solids along an AMD flow-path. Aqueous Fe spanned a concentration and δ56Fe range of ∼420 mg L-1 and + 0.04‰ at the AMD source to ∼100 mg L-1 and -0.81‰ at ∼450 m downstream. Aqueous As (up to ∼33 mg L-1) and SO42- (up to ∼2000 mg L-1), like aqueous Fe, decreased in concentration down the flow-path. X-ray absorption spectroscopy indicated that downstream attenuation in aqueous Fe, As, and SO42- was due to the precipitation of amorphous ferric arsenate (AFA), schwertmannite, and jarosite. The Fe(III) in these solids displayed extreme variability in δ56Fe, spanning +3.95‰ in AFA near the AMD source to -1.34‰ in schwertmannite at ∼450 m downstream. Similarly, the isotopic contrast between solid Fe(III) precipitates and aqueous Fe (Δ56Feppt-aq) dropped along the flow-path from about +4.1 to -1.1‰. The shift from positive to negative Δ56Feppt-aq reflects divergence between competing equilibrium versus kinetic fractionation processes.

Development of a chemical-free process utilizing ozone oxidative precipitation for the recovery of cobalt and manganese from acid mine drainage

Acid mine drainage (AMD) has been identified as a viable secondary resource of Co, and Mn. However, recovery of these elements while treating AMD is challenging due to the unfavorable pH and Eh conditions for Co-Mn precipitation. This study investigated the effects of various ligands (hydroxide, carbonate, ammonium, sulfate, and phosphate) and oxidizers (sodium persulfate, potassium permanganate, and ozone) on the precipitation of Co-Mn from AMD through experimental and solution chemistry studies. Ozone was found to be the most effective agent for the recovery of these elements. Based on the results, an AMD treatment process was formulated for selective recovery of multiple critical elements, including Al, REEs, Co, and Mn. More than 95 % of Co and Mn were recovered through the proposed process utilizing chemical-less ozone oxidative precipitation. A precipitate containing 0.9 % Co and 54.6 % Mn was obtained from an AMD source containing 0.9 ppm Co and 41.8 ppm Mn.

Effects of abandoned coal mine on the water quality

ABSTRACT Improving effective strategies to control acid mine drainage (AMD) in abandoned mine areas is a significant challenge because of water quality issues. AMD is a serious water pollution problem. The water in the area where the mine is located acquires an acidic character, the pH level of the environment decreases, the concentration of heavy metals increases with their dissolution in the acidic environment, and can cause serious environmental pollution by creating a toxic effect for all living things. However, acidic characteristics are not observed in the waters at high pH levels; a basic environment is formed; the heavy metal dissolution slows down; and the precipitation of minerals begins. In such an environment, it is not possible to observe AMD formation and heavy metal pollution. If AMD source is characterized, the most effective strategies can be developed to control the acidic effluents. The main aim of this work was to investigate the potential impact of Ovacik abandoned, coal mine (Cankiri, Turkey) on the water quality in the areas around the mine. Water samples were collected along the open pit area (abandoned), dump sites and coal storage area. Effluent characterizations were investigated for the better evaluation of the water contamination level to develop the successful treatment strategies. In the studies conducted in the same field in 2010–2011, it was determined that the water pH changes between 6.64 and 8.13. Similarly, in this study, the results showed that the pH values of water streams in the abandoned mine were measured between 7.28 and 8.94 with no heavy metal contamination. Although the coal deposit exhibits AMD source minerals, the rock formations which have alkaline content prevented AMD in the considered site.

Assessment of the Transfer of Trace Metals to Spontaneous Plants on Abandoned Pyrrhotite Mine: Potential Application for Phytostabilization of Phosphate Wastes.

The abandoned Kettara pyrrhotite mine (Marrakech region, Morocco) is a real source of acid mine drainage (AMD) and heavy metal pollution from previous mining operations—which has spread, particularly because of wind erosion. A store-and-release cover system made of phosphate wastes was built on the site for preventing AMD. To ensure the integrity of this cover and its durability, it is desirable to revegetate it (phytostabilization) with plants adapted to the edaphoclimatic conditions of the region. In this paper, a study was carried out on the spontaneous vegetation around the phosphate cover in order to consider the selection of plants to promote the stabilization of the Kettara mine tailings pond. Nine species of native plants with their rhizospheric soils growing in agricultural soils and tailings from the Kettara mine were collected, and metals (As, Cd, Co, Cu, Pb, Zn, Ni, Cr) were analyzed. The soil analysis showed that the tailings contained high concentrations of Cu (177.64 mg/kg) and Pb (116.80 mg/kg) and that the agricultural soil contained high concentrations of As (25.07 mg/kg) and Cu (251.96 mg/kg) exceeding the toxicity level (Cu > 100 mg/kg, Pb > 100 mg/kg, As > 20 mg/kg). The plant analysis showed low trace metal accumulation in Scolymus hispanicus, Festuca ovina, Cleome brachycarpa, Carlina involucrata and Peganum harmala. These species had a bioconcentration factor (BCF) greater than 1 and a translocation factor (TF) less than 1, demonstrating a high tolerance to trace metals. Therefore, they are good candidates for use in the phytoremediation of the Kettara mine tailings. These species could also potentially be used for the phytostabilization of the phosphate waste cover of the Kettara mine, thus completing the rehabilitation process of this area.

Temporal evolution of acid mine drainage (AMD) leachates from the abandoned tharsis mine (Iberian Pyrite Belt, Spain)

Acid mine drainage (AMD) due to the mining of sulfide deposits is one of the most important causes of water pollution worldwide. Remediation measures, especially in historical abandoned mines, require a deep knowledge of the geochemical characteristics of AMD effluents and metal fluxes, considering their high spatial and temporal evolution, and the existence of point and diffuse sources with a different response to rainfall events. This study investigates the temporal variations and hydrogeochemical processes affecting the composition of main AMD sources from the Tharsis mines (SW Spain), one of most important historical metal mining districts in the world. To address this, a fortnightly-monthly sampling was performed during two years in the main AMD sources and streams within the mine site covering different hydrological conditions. A seasonal pattern was observed linked to hydrological variations; higher pollutant concentrations were observed during the dry season (maximum values of 4,6 g/L of Al, 11,8 g/L of Fe, and 67 g/L of sulfate) and lower ones were observed during the rainy periods. Stream samples exhibited a negative correlation between electrical conductivity (EC) and flow, while positive values were observed in AMD sources, where groundwater fluxes were predominant. High flow also seems to be the main driver of Pb fluxes from AMD sources, as the concentration of Pb in waters increased notably during these events. The precipitation of secondary Fe minerals may limit the mobility of As and V, being retained in the proximity of mine sites. The concentration of Zn in waters seems to be controlled by the original grade in the metal deposit from which the waste is generated, together with the age of these wastes. The pollutant load delivered by the Tharsis mines to the surrounding water courses is very high; e.g., mean of 733 ton/yr of Al or 2757 ton/yr of Fe, deteriorating the streams and reservoirs downstream.

Community-Based Monitoring Detects Sources and Risks of Mining-Related Water Pollution in Zimbabwe

Although mining and mineral processing are vital for many economies in the Global South, they are associated with enormous challenges of managing potentially devastating environmental impacts. In contexts where environmental oversight agencies often lack financial and personal capacities to fulfill their role, community-based monitoring might be a valid alternative to monitor potential environmental impacts. In this study, we present the setup and the implementation of a citizen science project to monitor water quality parameters in a river downstream of a coal mining area in Hwange, Western Zimbabwe. In a joint effort over 1.5 years, community monitors and scientists took close to 800 water samples in the Deka River and effluent channels. The data allowed identifying sources of pollution and relating these to past and present mining activities. The primary source of acid mine drainage came from abandoned underground mine sites. Illegal mine water dumping from active mine sites accentuated the problem and resulted in fish kills and food risks for the local population. Concentrations of manganese, nickel and arsenic were exceeding national fresh water guidelines and international drinking water standards. Manganese concentrations exceeded guidelines by a factor of 70 resulting in a public health risk. In this study, we showed that community-based monitoring offers a promising approach to establish a high-quality dataset for assessing mining-related risks if the implementation of sampling protocols is followed tightly. The monitoring scheme significantly improves the collection and interpretation of water quality data in challenging contexts where governmental institutions and industrial players are not enforcing environmental standards.

Formation and characterization of acid mine drainage in the Madzharovo ore field, Southeastern Bulgaria

The formation of acid mine drainage (AMD) is a serious environmental problem in areas with mining and processing industries worldwide. Their generation is associated with chemical and biological processes of oxidation of sulfide minerals, mainly pyrite. Sources of AMD can be deposits of sulfide minerals and coal with a high content of pyrite sulfur, mining waste and some tailings. The impact of AMD on surface and groundwater in mining areas continues for decades after the cessation of extraction. An example of the negative impact of generated acid mine drainage on the state of surface waters is in the region of Madzharovo. Years after the cessation of mining, the waters at the discharge points "Momina Skala", "Harman Kaya" and "Pandak Dere" are characterized by low pH values and high concentrations of iron, copper, zinc, cadmium, lead and manganese.

Acid mine drainage (AMD) contamination in coal mines and the need for extensive prediction and remediation: a review

Globally, the major source of environmental pollution as a result of mineral exploitation and processing is acid mine drainage (AMD). AMD refers to outflowing streams of acidic constituents from pyrite-bearing ore mines. The exposure of pyrite (FeS2) in coal waste dumps to atmospheric oxygen and water in the presence of microbial communities promotes the formation of sulphuric acid which leaches out the inherent heavy metals into the mine discharge, a phenomenon called pyrite oxidation. AMDs are usually characterized by a convoy of toxic heavy metals, most of which are transition elements (copper, nickel, zinc, etc.) and arsenic at concentrations higher than the limits permitted by environmental regulations. The impact of this acidic discharge from coal mines on downstream/underground waters and farm lands within the corresponding mining zones have been severally reported by previous researchers, but not so much have been discussed on extensive prediction and remediation. It is in view of this that the current paper reviews the need for extensive prediction and remediation approach for coal mines under the following subheadings; General introduction, AMD sources identification, representative sampling, adoption of a prediction model, determination of AMD potential and quality via static and kinetic tests and the development of an economically sustainable remediation strategy. It is thought that this article would be useful to academia as well as policy makers that are responsible for the development and implementation of environmental regulations in coal mines.

Acid Mine Drainage Sources and Impact on Groundwater at the Osarizawa Mine, Japan

Understanding the origin of acid mine drainage (AMD) in a closed mine and groundwater flow system around the mine aids in developing strategies for environmental protection and management. AMD has been continuously collected and neutralized at Osarizawa Mine, Akita Prefecture, Japan, since the mine was closed in the 1970s, to protect surrounding river water and groundwater quality. Thus, water samples were taken at the mine and surrounding groundwaters and rivers to characterize the chemical properties and environmental isotopes (δ2H and δ18O). The results showed that the quality and stable isotope ratios of AMD differed from those of groundwater/river water, indicating that the recharge areas of AMD. The recharge area of AMD was evaluated as the mountain slope at an elevation of 400–500 m while that of the surrounding groundwater was evaluated at an elevation of 350–450 m, by considering the stable isotopes ratios. This indicates that the groundwater affected by AMD is limited to the vicinity of the mine and distributed around nearby rivers.

Mine waters as a secondary source of rare earth elements worldwide: The case of the Iberian Pyrite Belt

Acid Mine Drainage (AMD) generates a great concern worldwide due to its severe impact to water resources during hundreds and even thousands of years after the cessation of mining activity if control measures are not implemented. AMD treatment is an environmental necessity, but also constitute a tremendous opportunity for the valorization of potential secondary sources of elements of economic interest. The knowledge of the hydrogeochemistry of REE in AMDs and their distribution using normalized patterns would help discrimination of the most potentially marketable AMD sources. To achieve this goal and to estimate the total economic potential of a severely AMD-affected region, chemistry and flow data were determined in spatially and temporally-distributed samples of numerous AMD sources collected throughout the Iberian Pyrite Belt (IPB). Due to high anual metal loads of elements such as Al (6600 ton), Zn (1600 ton), Cu (600 ton), Co (26 ton), Ni (10 ton), LREE (10.7 ton/yr), MREE (2.1 ton/yr), HREE (1 ton/yr), Y (3.7 ton) or Sc (0.7 ton), AMDs of the IPB would have an economic potential of 24.1 M$/yr (being REE 22.6% of this potential). Although the technical and economic limitations would impose a more realistic value of 4.2–10.3 M$/yr. The magnitude of this economic potential cannot be compared with active mines, however the longevity of the AMD generation processes and the need to achieve an environmental improvement make valorization of these leachates an interesting option to recover metals, which would help to treatment plants costs, improving notably the quality of water bodies in abandoned mining sites.

ASSESSING THE ACID MINE DRAINAGE POTENTIAL OF A STOCKPILED GOLD BEARING SULPHIDE ORE

This work seeks to assess the acid mine drainage (AMD) potential of stockpiled sulphidic gold bearing ore that was left untreated for a period of four (4) years. The mineralogical composition of the stockpiled sulphidic gold ore was determined using X-Ray Diffractometry (XRD) and Reflected Light Microscope. The AMD potential and kinetic studies on the ore sample were determined using Acid Base Accounting (ABA) and Humidity Cell (ASTMD 5744). The XRD and microscope analyses showed that the gold ore contain arsenopyrite and pyrite as the major sulphide minerals. The ABA confirmed that the gold sulphide ore could be a potential source of AMD with respect to AMD marginal of safety if the ore is left untreated for a very long period. Humidity Cell kinetic analysis showed a possibility of mobilizing Arsenic (As) into the geo-environment when there is a long period of oxidation of abandoned sulphide run-of-mine (ROM) ore.

Kinetics of Pyrite Oxidation in Simulated Mine Tailings Pond Conditions

Pyrite oxidation is the predominant source of acid mine drainage (AMD), which is one of the severe environmental impacts of mining. AMD destroys flora and fauna in the nearby ecosystem, and it causes grave health problems to the people. To further the understanding and help in the mitigation of AMD, this electrochemical study investigates the effect of the type and concentration of oxidant, and the presence of impurity (chalcopyrite) to the rate of pyrite oxidation, while simulating the conditions in tailings ponds. Open Cell Potential (OCP) and Electrochemical Impedance Spectroscopy (EIS) analysis were done using a three-electrode cell set-up, which is composed of a pyrite rotating disc electrode (RDE), mercury/mercurous sulfate reference electrode in saturated potassium sulfate, and a platinum counter electrode, with a sulfuric acid working electrolyte. Polarization resistance obtained after fitting an electrical circuit to the EIS data was related to the rate of pyrite oxidation, while OCP was used to describe its oxidation mechanism. It was found out that the oxidation of pyrite by ferric ions preferentially occurred rather than by dissolved oxygen. The parameter setting with the lowest relative oxidation rate were: electrolyte with no ferric addition, and with nitrogen purging; and RDE with 10 % chalcopyrite addition. Reaction orders calculated for the effect of ferric ions and dissolved oxygen were equal to 1; but ferric has a higher slope value in the plot. Presence of chalcopyrite in the RDE caused the preferential oxidation of chalcopyrite instead of the pyrite due to their galvanic interaction. To minimize pyrite oxidation, ferric and dissolved oxygen concentration should be minimized, and chalcopyrite impurities should be increased. Keywords: Electrochemical Impedance Spectroscopy, Acid Mine Drainage, Kinetics, Rotating-Disc Electrode, Pyrite

Understanding the inorganic carbon transport and carbon dioxide evasion in groundwater with multiple sulfate sources during different seasons using isotope records

The geochemical cycle of carbon under anthropogenic activities controls the transport of carbon between surface sediments, hydrosphere and atmosphere, especially for atmospheric and marine carbon dioxide. Currently, the anthropogenically-induced carbon cycle in groundwater is an important part of the cycle in inland waters and cannot be underestimated as a geochemical process affecting the global carbon cycle. Therefore, there is a need to conduct deep research using isotopes in which the isotopic mixing and fractionation response to the carbon cycle can be assessed. The results showed that the δ34S-SO42− and δ18O-SO42− values in groundwater inherited the isotopic characteristics of four sulfate sources and shifted from acid mine drainage (AMD) derived from pyrite oxidation to atmospheric sulfates and agricultural fertilizers as well as domestic sewage along the groundwater flow path. In particular, the evolution of sulfate sources was accompanied by changes in the δ13C-DIC values in different seasons. During the dry season, the Rayleigh function described a possibility of enriched δ13C-DIC values in an AMD-affected aquifer, where a dropping water table or decreasing water content in unsaturated zones provided favorable acid buffering conditions. Bicarbonate neutralized protons to CO2 that was in turn spilled over the water surface. During the wet season, the depleted δ13C-DIC values in the AMD-affected aquifer reflected another possibility of the carbon cycle in which a rising water table caused a similar closed system to some extent and slowed down the replenishment of bicarbonate so that δ13C-DIC almost recorded the δ13C of soil CO2. In spite of the same AMD source, their influences on the carbon cycle in the dry and wet seasons were CO2 evasion and deficient CO2 uptake, respectively. By contrast, regardless of the season, the carbon cycle gradually shifted towards a net consumption of CO2 in the aquifer with atmospheric and agricultural sulfates as well as domestic sewage, and δ13C-DIC inherited the characteristic of HCO3−, indicating that these sources in anthropogenically-induced carbon processes hardly involved in isotopic fractionation of δ13C-DIC.