Evaluation of arsenic mineralogy and geochemistry in gold mine-impacted matrices: Speciation, transformation, and potential associated risks.

The risk of arsenic (As) contamination from gold mining is a long-term environmental concern for mines worldwide. Researchers have mainly focused on As contamination induced by tailings, however, less attention has been paid mineralogically to differentiate the fate of As among different As-bearing matrices. This paper presents a detailed study of the mineralogical and morphological features of three typical As-bearing matrices (waste rock, ores, and tailings) using bulk chemical, microscopic and spectroscopic analyses, and reveals the geochemical behavior of As in those matrices. Results from mineral composition identified by RoqSCAN revealed that the matrices were dominated by quartz, k-feldspar, albite, muscovite, and clay minerals, with subordinate ankerite, chlorite, smectite, hematite, arsenopyrite, pyrrhotite, apatite, pyrite, halite, and calcite. The sequential extraction scheme indicated that As in waste rock, ores and tailings was mainly hosted in arsenopyrite. Microscopic analysis observed that waste rock was significantly different from the ores and tailings in terms of mineralogical and morphological characteristics. For waste rock, from arsenopyrite to hematite, As content decreased from 46.12 wt% to 3.54 wt%. However, arsenopyrite presented as unweathered euhedral crystals or slight fragmentation in ores and tailings and a narrower oxidation rims than that of waste rock. The leaching test of SPLP showed that the highest As leaching was found in waste rock (0.246 mg/L) which was significantly higher than those in ores (0.080 mg/L) and tailings (0.148 mg/L). The As in waste rock displayed weaker geochemical stability than in ores and tailings, as supported by mineralogy analysis. Health risk assessment suggested waste rock had a higher health risk for both adults and children compared with ores and tailings. These findings reaffirm that understanding of As fate among different source materials is paramount for securing humans from As hazards. More must be done to decelerate the continuous oxidation of waste rock, thus mitigating As release into nature.

The geochemical stability of typical arsenic-bearing sinter in the Tibetan plateau: Implications from quantitative mineralogy.

High­arsenic (As) sinter deposited from geothermal water is a potentially overlooked hazardous matrix and there remain substantial gaps in our comprehension of the stability of As sequestered within it. In this study, qualitative and quantitative analysis of the mineralogy of As-bearing sinter was conducted by Mineral Liberation Analyzer (MLA) in geothermal areas of the Tibetan Plateau to reveal the geochemical stability of As. Our results indicated that the contents of As in sinter were 3 orders of magnitude higher than the local soil. The dominant host minerals of As were calcite (40.9 %), thenardite (22.5 %), calcium silicate (13.0 %), and halite (8.1 %). Additionally, it was found that a relatively higher As bioavailability was extracted by ethylene diamine tetraacetic acid (EDTA), with a leaching rate of 41.2 %. Notably, the X-ray diffraction (XRD) showed that the thenardite and halite were decomposed after the leaching. The combination of mineralogy and geochemistry data suggested that calcite and calcium silicate were a crucial mechanism for As retention in sinter, while the dissolution of saline minerals (e.g., thenardite, halite, and calcium chloride) served as the primary sources for As release. This finding unveils the potential risks and mechanisms associated with high-As sinter, providing scientific guidance for risk management of sinter.

Monthly variations of groundwater arsenic risk under future climate scenarios in 2081-2100.

The seasonal variations of shallow groundwater arsenic have been widely documented. To gain insight into the monthly variations and mechanisms behind high groundwater arsenic and arsenic exposure risk in different climate scenarios, the monthly probability of high groundwater arsenic in Hetao Basin was simulated through random forest model. The model was based on arsenic concentrations obtained from 566 groundwater sample sites, and the variables considered included soil properties, climate, topography, and landform parameters. The results revealed that spatial patterns of high groundwater arsenic showed some fluctuations among months under different future climate scenarios. The probability of high total arsenic and trivalent arsenic was found to be elevated at the start of the rainy season, only to rapidly decrease with increasing precipitation and temperature. The probability then increased again after the rainy season. The areas with an increased probability of high total arsenic and trivalent arsenic and arsenic exposure risk under SSP126 were typically found in the high-arsenic areas of 2019, while those with decreased probabilities were observed in low-arsenic areas. Under SSP585, which involves a significant increase in precipitation and temperature, the probability of high total arsenic and trivalent arsenic and arsenic exposure risk was widely reduced. However, the probability of high total arsenic and trivalent arsenic and arsenic exposure risk was mainly observed in low-arsenic areas from SSP126 to SSP585. In conclusion, the consumption of groundwater for human and livestock drinking remains a threat to human health due to high arsenic exposure under future climate scenarios.

Characteristics, sources, and risk assessment of thallium and associated with metal(loid)s in the Yarlung Tsangpo River Basin, southern Tibetan Plateau.

The Tibetan Plateau (TP) is known as the water tower of Asia, and the water quality has long been a focus of public concern, especially in the Yarlung Tsangpo River Basin (YTRB), a unique area that is climate-sensitive, geologically complex, eco-fragile, and densely populated. Thallium (Tl) is a typical metal that is more toxic than Pb, Cd, and As and often occurs in sulfide minerals. Although large-scale polymetallic sulfide mineralization developed in the YTRB, the geochemical dispersion and potential risk of Tl in aquatic environments of the YTRB remain poorly understood. In this study, the concentration, distribution, source, and health risk of Tl and associated metal(loid)s in the hot springs and surface water in the YTRB were systematically analyzed. The results showed that the trace elements (Cd, Cr, Zn, Cu, Al, Sr, Ni, Co, Mn, Pb) in water environments are within the recommended limits, except for Tl and As. Principal component analysis (PCA) and correlation analysis (CA) showed that the elements of Tl and As were positively related to each other in either both hot spring water and surface water, indicating their common origin. Spatial variations suggested that high levels of Tl and As observed in the north YTRB, which may be relevant to the reduction-dissolution of Tl (As)-bearing minerals and the magmatic hydrothermal system formed in the shallow part of the northern YTRB. Furthermore, source apportionment identified natural sources of Cu, Ni, Cr, Co, Mn, Zn, and Cd and anthropogenic inputs of Al and Pb. Exposure assessment studies have found that ingestion is the primary route of As and Tl exposure to local population, and balneological and bathing purposes do not constitute a human health concern. This study offers valuable insights into the risk of naturally occurring Tl enrichment being hidden in As-rich hydrosphere in the YTRB and other regions with similar geoenvironmental contexts.

Stabilization of soil arsenic by natural limonite after mechanical activation and the associated mechanisms.

Arsenic (As) is an environmentally hazardous contaminant which have a serious threat to human health. In recent years, sustainability has drawn increasing attention in the environmental remediation field. Application of natural minerals as a class of iron-containing materials for soil As remediation is meaningful and challenging. In this paper the As sorption ability and soil stabilization of mechanical activated limonite has been studied. Mechanical activation can effectively enhance the adsorption performance of natural limonite. The positive effect of mechanical activation on limonite mainly include: (1) particle size reduction and specific surface area increase; (2) reduction of limonite crystallinity and increase of surface active sites; (3) mineral phase transformation to amorphous iron oxides substances. The average grain size of limonite reduces from 16.8 µm to 0.214 µm after activation while the specific surface area increases from 10.26 m2/g to 56.74 m2/g. The maximum adsorption capacities of mechanically activated limonite (Lm) for As (III) and As (V) were 9.14 mg/g and 8.26 mg/g, respectively at pH 7.0, higher than untreated limonite (L0). Mechanically activated limonite can effectively stabilize As in soils. When Lm dosage was 10%, the stabilization effects could reach about 78%. Limonite could transform the soil As from non-specifically and specifically sorbed fraction to amorphous iron hydrous oxides bounded fractions. Mechanically activated limonite (Lm) exhibited good adsorption and stabilization performance advantages for As in soils.

[Mineral Characteristics of Arsenic in the Active Area of the Banbishan Gold Mine and Its Effect on Arsenic Accumulation in Farmland Soil].

To explore the source and pollution characteristics of soil arsenic, mineralogy and chemical analysis methods were used to analyze the ore, waste rock, sediment, and river and soil samples around the mining area. Under a polarized light microscope, As-bearing mineral-arsenopyrite was found in the soil, ore, and waste rock around the Banbishan gold mine. Moreover, arsenopyrite in the waste rock has already experienced weathering and oxidation, and the oxidized arsenopyrite easily migrates and is released in the soil, which is potentially harmful. Because of the effect of mining transportation activities and indigenous smelting, arsenic was mainly distributed in the topsoil, at a depth of 0-20 cm, in the farmland on both sides of the road and in the places where villagers were gathered. The soil arsenic content in Xiaowulan Village and Gaozhangzi Village ranged from 7.2 to 196.2 mg·kg-1 and exceeded the rate of arsenic by 45.9% and 82.1%. According to the assessment by the RAC method, the farmland soil in Xiaowulan Village and Gaozhangzi Village were mainly at low to medium risk, although some soil points in Xiaowulan Village were at high risk. In general, the effects of the mining activities of the surrounding environment were not optimistic. As-bearing minerals in the oxidation of long-term weathering can cause much arsenic to be activated, which in turn, affects the local crops and long-term residents living around the mining area. It is suggested to carry out risk assessments for arsenic in the soil-crop-atmospheric-human system, and further study the conversion rules and mechanisms of arsenopyrite during weathering, to provide scientific guidance for the environmental protection of cultivated land.

[Adsorption and Mechanism of Arsenic by Natural Iron-containing Minerals].

Natural iron-containing minerals present in the geosphere in the form of crystalline minerals can be used as adsorption material for removal of arsenic from wastewater and remediation of arsenic-contaminated soils. In this paper, the adsorption and desorption of arsenic onto different iron-containing materials including hematite, limonite, siderite, ilmenite, magnetite, Fe2O3, Fe3O4, and Fe-Mn binary oxide (FMBO) were studied in laboratory experiments. The mechanism of arsenic adsorption was analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and fourier transform infrared spectroscopy (FTIR). The results showed that arsenic adsorption is fitted by pseudo-second-order kinetics and the Langmuir isotherm model for almost all adsorbents, suggesting monolayer adsorption of arsenic onto the minerals. The sorption efficiency and capacity of arsenic by FMBO are much higher than those of other materials. Furthermore, limonite has high sorption efficiencies for both As(Ⅲ) and As(Ⅴ) among the natural iron-containing minerals, and their adsorption capacities are 3.96 mg·g-1 and 2.99 mg·g-1, respectively. The XRD results showed that natural limonite contains a large number of weak crystalline mineral components such as goethite, which can provide relatively abundant arsenic adsorption sites. Thus, limonite appears to be the most suitable natural mineral for arsenic adsorption.