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

Alumina inhibits pyrite oxidative dissolution by regulating solid film passivation layer and S, Fe, and Al speciation transformation

The oxidation of pyrite results in the formation of a solid film passivation layer on its surface. This layer effectively hinders the direct interaction between H2O, O2, and the pyrite surface, thereby impeding the oxidation dissolution of pyrite. There are few studies on whether alumina (Al2O3), a common aluminum-containing oxide, affects the formation of a solid film passivation layer on the surface of pyrite and inhibits the oxidation dissolution of pyrite. This research investigates the impact of Al2O3 incorporation on the speciation transformation of S, Fe, and Al on the surface of pyrite during oxygen pyrite process. The oxidation of pyrite followed the "polysulfide-thiosulfate" complex oxidation pathway. When <1.5 g/L Al2O3 was introduced, it increase pyrite oxidation, whereas ≥1.5 g/L Al2O3 prevented pyrite oxidation. The process of Al2O3 dissolution results in the consumption of H+ and the subsequent release of Al3+. This, in turn, facilitates the hydrolysis of Fe3+ and Al3+ to generate a secondary mineral layer on the pyrite surface. As a result of the accumulation of S promotes the formation of polysulfide chemical (FeSn) or iron deficiency sulfide (Fe1-xS), resulting in the formation of a solid film passivation layer composed of sulfur film and secondary mineral layer. The results demonstrated that Al2O3 can promote the formation of a solid film passivation layer on the surface of pyrite, which has significant implications for controlling the oxidation dissolution process of pyrite and offers a new perspective for the source control of acid mine drainage.

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.

High-resolution temporal monitoring of rare earth elements in acidic drainages from an abandoned sulphide mine (iberian pyrite belt, Spain)

Rare earth elements (REE) are strategic elements due to their economic importance. However, the studies dedicated to the distribution and behaviour of REE in aquatic systems have been scarce until a few decades ago. This work studies the seasonal variations of REE concentrations in acid mine drainage (AMD) affected water courses and the factors controlling their mobility under different hydrological conditions. To address this issue, a high-resolution sampling was performed for two years in selected sampling sites. REE concentrations were very high (median values of 2.7-3.4 mg/L, maximum of 7.0 mg/L). These values are several orders of magnitude higher than those found in natural waters, highlighting the importance of AMD processes on the release of REE to the hydrosphere. No good correlations were found between pH and REE concentration, while REE correlated positively (r Spearman coefficient of 0.78-0.94) with EC and negatively (r -0.88 to -0.90) with discharge in AMD-affected streams. A conservative behaviour of REE was observed due to the strongly acidic conditions observed in the study area. The waters also showed an enrichment in MREEs over LREEs and HREEs (mean values of GdN/LaN>1.8 and YbN/GdN < 0.7), typical of AMD waters. An asymmetry in the content of LREE and HREE was observed in AMD samples studied, which could be explained by the preferential dissolution of LREE or HREE-enriched minerals within each waste heaps. Multivariate analysis suggests the influence of Mn-rich minerals existent in the study area as a potential source of LREE.

Acid mine drainage and smelter-derived sources affecting water geochemistry in the upper Nakdong River, South Korea

Both the smelter and acid mine drainage (AMD) in uppermost streams impact water geochemistry and deteriorate water quality. Efficient water quality management requires identifying the contribution of each source to stream water geochemistry. In this study, we aimed to determine the natural and anthropogenic sources (AMD and smelting) affecting water geochemistry by considering seasonality. Water samples were collected, from May 2020 to April 2021, in a main channel (Nakdong River) and tributaries in a small watershed including mines and smelters. The watershed is characterized by a carbonate-rich area in the upper-middle reaches and silicate-rich area in the middle-lower reaches. On the plots of Ca/Na vs. Mg/Na and 2(Ca + Mg) vs. HCO₃ + 2SO₄, the water geochemistry was predominantly explained by the carbonate and silicate weathering associated with sulfuric and carbonic acids. According to typical δ¹⁵N values for sources, nitrate contribution from soil-N mainly impacted water geochemistry, regardless of seasonality; the contribution from agricultural activity and sewage was negligible. Water geochemistry in the main channel samples was discriminated before and after passing through the smelter. The effects of the smelter were evident in elevated SO₄, Zn, and Tl concentrations and in δ⁶⁶Zn values; this was further supported by the relationships between Cl/HCO₃ and SO₄/HCO₃ and between δ⁶⁶Zn and Zn. These results were pronounced during winter, when the flush-out effect was absent. Our results suggest that multi-isotopes and chemical composition analyses can trace multiple sources influencing the water geochemistry in watersheds containing AMD and smelters.

Anomalous concentrations of rare earth elements in acid mine drainage and implications for rare earth resources from late Permian coal seams in northern Guizhou

Rare earth elements (REEs) have attracted much attention in recent decades due to their growing applications in high-tech industries. Coal and acid mine drainage (AMD) are considered promising alternative sources due to their high concentrations of REEs. Here, AMD with anomalous REEs concentrations was reported in a coal-mine area in northern Guizhou, China. The AMD had a total concentration as high as 22.3 mg/l, suggesting that regional coal seams may be enriched with REEs. Five segments from borehole samples, which contained coal, rocks from the roof and floor of the coal seam were collected from the coal mine site to investigate the abundance, enrichment, and occurrence of REE-bearing minerals. Elemental analysis showed that the REE contents in the coal, mudstone and limestone from the coal seam roof, and claystone from the floor (all dating to the late Permian) varied greatly, with averages of 388, 549, 60.1 mg/kg and 2030 mg/kg, respectively. Encouragingly, the REEs content in the claystone is over an order of magnitude higher than the average content reported in most other coal-based materials. The enrichment of REEs resources in regional coal seams is particularly associated with the contribution of REEs in the claystone that comprises the coal seam floor, rather than just the coal, as considered in previous studies. The minerals in these claystone samples were dominated by kaolinite, pyrite, quartz and anatase. Two types of REE-bearing minerals, bastnaesite and monazite, were detected in the claystone samples by SEM-EDS analysis, and they were found to be adsorbed by a large amount of clay minerals, mainly kaolinite. Additionally, the results of chemical sequential extraction also confirmed that the majority of the REEs in the claystone samples are mainly in their ion-exchangeable, metal oxide and acid-soluble forms, which are viable prospects for REE extraction. Therefore, the anomalous concentrations of REEs and most of them are in extractable phases, which demonstrates that the claystone from the floor of the late Permian coal seam should be a potential secondary source of REEs. Future studies will further consider the extraction model and the economic benefits of REEs from the floor claystone samples.

Suppression of pyrite oxidation by co-depositing bio-inspired PropS-SH-tannic acid coatings for the source control acid mine drainage

In previous works, both tannic acid (TA) and organosilane-based passivators have been proven to possess good inhibition effects on pyrite oxidation, which could effectively prevent acid mine drainage (AMD) generation at the source. However, the hydrophilicity of TA passivation film and the complex coating process of organosilane-based passivators (high temperature conditions were required during the process carried out) may limit their further practical use. Therefore, to achieve the purpose of better coating treatment of pyrite under mild conditions, TA and γ-mercaptopropyltrimethoxysilane (PropS-SH) were introduced to synergistically passivate pyrite in this work. Electrochemistry tests and chemical leaching experiments both confirmed that PropS-SH-TA coated pyrite had better oxidation resistance than raw pyrite and single PropS-SH or TA coated pyrite. Additionally, the analyses of scanning electron microscopy (SEM) measurements and static water contact angle tests demonstrated that a scaly coating was formed on PropS-SH-TA coated pyrite surface, which may be the reason for the significant improvement of its surface hydrophobicity. Finally, the study on the film-forming mechanism of PropS-SH-TA composite passivator displayed that the benzoquinone derivatives formed by TA could copolymerize with PropS-SH through Michael addition or Schiff base reaction, which constructed a dense hydrophobic film on pyrite surface. The newly formed composite film could provide a better oxidation barrier for pyrite based on TA passivation film.

Rare earth elements - Source and evolution in an aquatic system dominated by mine-Influenced waters

Acid mine drainage (AMD), formed by the instability of sulfides, typically generates acidity and releases potentially toxic elements and sulfate to the environment, among other pollutants. An example is the group of rare earth elements (REE) that may have high toxic behavior. This toxicity leads to degradation of soils, water reservoirs and rivers, promoting serious risks for the ecosystems. So, the main goal of the present work is to study the hydrochemical properties of a system with mine-influenced waters during the rainy season, focusing on the origin, evolution/behavior, and concentration of REE. The study area is the São Domingos mining complex, located in one of the largest metallogenetic provinces in the world (Iberian Pyrite Belt), known by the evidences of AMD contamination. The obtained results reveal extraordinarily low pH (0.4), high electrical conductivity, reaching 26,200 μS/cm, and high values of sulfate and acidity. Regarding the REE, the determined concentration exceeded that observed in normal pH of neutral freshwaters by 2-3 times the order of magnitude. The results revealed that Y and Ce are distinguished in practically all sampled sites, due to its higher concentrations, with maximum values of 221.8 and 166.9 μg/L. In general, the concentrations increase as the water pH decreases. The statistical analysis indicates that REE elements may have a common origin, mutual dependence, and similar behavior during transport with typical AMD elements and composition of host rocks. Most samples show enrichment in middle REE (MREE) (Gdn/Lun), like the classic signature of AMD. In turn, colloids and AMD-precipitates may be participating in the incorporation of these elements. Therefore, due to potential risk of impacts on ecosystems, REE are a topic of relevant interest for future studies in order to assist monitoring processes and help government decisions related to water quality management.

Construction of a hydrogeochemical conceptual model and identification of the groundwater pollution contribution rate in a pyrite mining area

To effectively restore the groundwater environment of the Shiping Mine (SPM) area, which is contaminated by acid mine drainage (AMD), a hydrogeochemical conceptual model was constructed based on groundwater chemistry and environmental stable isotopes. The contribution rate of various pollution sources in the groundwater environment was quantitatively analyzed using an optimized stable isotope mass balance model. A total of 68 groups of water samples were collected. The sampling period covered the dry, intermediate, and wet periods of a complete hydrological year. Samples were taken from rain, springs, mine drainage, tailings leachate, and surface water; and the detection and analysis indicators included 24 parameters, such as inorganic salts, heavy metals, and isotopes. A hydrogeochemical and statistical data analysis was performed. The main source of groundwater replenishment was found to be atmospheric precipitation, with the water-rock interaction of calcite and pyrite, and mining activities being the main controlling factors of hydrogeochemical processes. Acid mine drainage significantly enhanced the dissolution of various minerals, and the detection rate of Zn, Cu, As, Cd, and Pb increased from 0-30%-100% when compared with groundwater in the area upstream of the mines. The optimized mass balance model results revealed that the contribution rates of upstream groundwater, mine water and leachate were 0.78-0.86, 0.08-0.18, and 0.04-0.06 for Heidong underground river, respectively; were 0.27-0.36, 0.62-0.68, and 0.03-0.05 for Tiantang underground river, respectively. Furthermore, based on the water balance analysis, 34-70% of the mine water was found to infiltrate directly through karst fissures and karst pipes and could not be collected at the mine entrance. Acid mine drainage that directly infiltrated through runoff could easily be ignored due to the hidden migration path, which may cause the groundwater environment to be remediated less effectively than expected.