9.02 - Arsenic and Selenium

The present study aims to investigate the hydrogeochemical evolution of heavy metals and assesses impacts of mining activities on the groundwater resources and potential human health risks in the coal mining areas of Ib valley coalfield. In this perspective, a total of one hundred and two mine water and groundwater samples were collected from different locations. The water samples were analysed for some selected heavy metals i.e. Mn, Cu, Pb, Zn, Ni, Co, As, Se, Al, Sr, Ba, Cd, Cr, V and Fe using ICP-MS. In addition, pH and SO₄²⁻ concentration were also measured following APHA procedure. The water pH in the Ib valley coalfields ranged from 3.26 to 8.18 for mine water and 5.23 to 8.52 for groundwater, indicating acidic to alkaline nature of water. Mn in mine water and Zn in groundwater environment were observed as the most dominant metals. The water hazard index (WHI) reflects that around 80% of mine water are non-toxic (WHI<5), 5% slightly toxic (5<WHI>10) and 15% extremely toxic (WHI>15). Relatively high pH and low concentration of dissolved metals and SO₄²⁻ in groundwater as compared to mine water indicate lesser impact of mining activities. The calculated drinking water quality index (DWQI) suggests that Mn, Al, Ni and Fe in mine water and Mn, Fe, Ni and Pb in groundwater were the major objectionable metals which caused the water quality deterioration for drinking uses. Further, the non-carcinogenic health risk assessment for adult male, female and child populations identifies Co, Mn, Ni as the key elements making the water hazardous for human health. Comparatively higher ratio of ingestion rate and body weight in child population might be causing higher health risks in child population as compared to adult male and adult female population.

Aqueous thiolated arsenic (As) species play an important role in the biogeochemical cycling of As in wetlands and hydrothermal systems. Although mine wastes such as tailings ponds and waste rock piles may harbor similarly sub-oxic and neutral to alkaline conditions that favor the formation and mobility of thio-As species, quantitative data on their existence in these systems is lacking. We conducted laboratory column experiments under contrasting redox conditions with waste rock from the Antamina mine, Peru, and processed tailings from Montague, Nova Scotia, Canada. Dissolved As concentrations between 1 and 7000 μg/L were recorded in drainages across these mine waste types, with up to 13 μg/L As present in thiolated form, predominantly monothioarsenate. Higher percentages of thio-As species (up to 5%) were observed in drainages from enargite-rich materials compared to arsenopyrite-bearing materials (<0.5%). The lower abundance of dissolved thio-As in the arsenopyrite-rich mine waste samples is attributed to their partially oxidized nature and reduced mineral reactivity under the experimental circumneutral drainage pH, the difference in S [-II/0]-to-As molar ratios compared to the enargite-rich mine waste samples, as well as the oxidation of di- and tri-thiolated As species by dissolved Fe. Overall, our results demonstrate that aqueous thiolated As species may occur in mine wastes with different As-bearing minerals and could play an important role in governing the mobility and fate of As in these systems.

Selenium (Se) is distributed into different environmental compartments by natural and anthropogenic activities, and generally discharged in the form of selenate [$Se{O}_4^{2-}$] and selenite [$Se{O}_3^{2-}$], which are both toxic. Physical-chemical and biological treatment processes have been reported to exhibit good treatment efficiencies for Se from aqueous streams, only a few demonstrated to achieve effluent concentrations <5 μg/L. Moreover, there are only a few numbers of studies that describe the progress in technological developments over the last decade. Therefore, to unify the state of knowledge, identify ongoing research trends, and determine the challenges associated with available technologies, this systematic review critically analyses the published research on Se treatment. Specific topics covered in this review include (1) Se chemistry, toxicity, sources and legislation, (2) types of Se treatment technologies, (3) development in Se treatment approaches, (4) Se recovery and circular economy and (5) future prospects. The current research has been found to majorly focused on Se removal via adsorption techniques. However, the key challenges facing Se treatment technologies are related to the presence of competing ions in the solution and the persistence of selenate compared to selenite during their reduction.

Handan lies in a geochemical anomaly belt characterized by enrichment of selenium (Se) in rock, soil, and water, contrasted with deficiency of Se in the North China Plain. In this study, 27 environmental soil samples and 96 rock samples in the Upper Paleozoic were analyzed by inductively coupled plasma optical emission spectrometer (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) for major and trace element concentrations to determine the source of Se in the environment.

Se concentrations in soil vary from 0.054 μg/g to 2.869 μg/g, with an average of 0.598 μg/g. Ignoring the anthropogenic contamination, Se concentrations in soils elevate with the decrease of distance from outcrops in Mountain Zishan (Mt. Zishan). There is a strong positive correlation between Se and Al₂O₃ (r = 0.679, P < 0.01, n = 27), and between Se and TiO₂ (r = 0.638, P < 0.05, n = 27). Furthermore, Se is significantly negatively correlated with MgO, Na₂O, K₂O, and Sr (r = −0.674, −0.755, −0.716, −0.739, respectively, P < 0.01 for all, n = 27). It indicates that Se in the soil is derived from weathering products.

Se is the most highly enriched trace element within Upper Paleozoic, which is composed of Benxi Formation (BX Fm.), Taiyuan Formation (TY Fm.), Shanxi Formation (SX Fm.), and Shihezi Formation (SHZ Fm.), in outcrops in Mt. Zishan area, western Handan. Se concentrations of rock samples vary from 0.021 μg/g to 4.971 μg/g, with an average of 0.401 μg/g. The maximum of Se concentration is 99.41 times higher than the Upper Continental Crust (UCC). The order of the rest trace elements enrichment degrees is S > Pb > As > Ga > Li > Bi > Th > Sc > In > U > Cd > Y > Rb > Ba > Cr > V > Be > Cs > Ni > Zn > Tl > Sr > Co > Mo > Cu. Se enrichment appears in two types of rocks, which are gray clay rocks and black mudstones and shales (including coal and gangue). Stratigraphically, Se is more highly enriched in TY Fm. and SX Fm., with average enrichment factor (EF) values of 14.64 and 5.59, respectively.

Chemical index of alteration (CIA) and several environmentally sensitive elements, such as Th, U, Sr, Ba, V, Cr, and Ce, were employed for sedimentary environment analysis. The results revealed that Se-enriched rocks were deposited mainly in a weak, reducing freshwater environment, and their sediments were subject to an intermediate chemical weathering process. The discrimination diagrams of SiO₂-TiO₂, F₁-F₂, ∑REE-La/Yb, and La/Sc-Co/Th indicate that the terrestrial clastic sediments were dominated by multiple types of original rocks. Se-laden clastic materials derived from mafic and ultramafic rocks in Yinshan ancient land provided main source of Se in Upper Paleozoic in western Handan, and adsorption of clayey and organic materials and biological accumulation induced Se enrichment ultimately.

Rivers throughout the world have been contaminated by arsenic dispersed from mining activities. The biogeochemical cycling of this arsenic has been shown to be due to factors such as pH, Eh, ionic strength and microbial activity, but few studies have examined the effects of both seasonal changes and microbial community structure on arsenic speciation and flux in mining-affected river systems. To address this research gap, a study was carried out in Huangshui Creek, Hunan province, China, which has been severely impacted by long-term historic realgar (α-As₄S₄) mining. Water and sediment sampling, and batch experiments at different temperatures using creek sediment, were used to determine the form, source and mobility of arsenic. Pentavalent (AsO₄³) and trivalent arsenic (AsO₃³⁻) were the dominant aqueous species (70–89% and 30–11%, respectively) in the creek, and the maximum concentration of inorganic arsenic in surface water was 10,400 μg/L. Dry season aqueous arsenic concentrations were lower than those in the wet season samples. The sediments contained both arsenate and arsenite, and relative proportions of these varied with season. 8.3 tons arsenic per annum were estimated to be exported from Huangshui Creek. Arsenic release from sediment increased by 3 to 5 times in high water temperature batch experiments (25 and 37 °C) compared to those carried out at low temperature (8 °C). Our data suggest that the arsenic-containing sediments were the main source of arsenic contamination in Huangshui Creek. Microbial community structured varied at the different sample sites along the creek. Redundancy analysis (RDA) showed that both temperature and arsenic concentrations were the main controlling factors on the structure of the microbial community. Protecbacteria, Bacteroidetes, Cyanobacteria, Firmicutes, Verrucomicrobia, and Planctomycetes were the stable dominant phyla in both dry and wet seasons. The genera Flavobacterium, Hydrogenophaga and Sphingomonas occurred in the most highly arsenic contaminated sites, which removed arsenic by related metabolism.Our findings indicate that seasonal variations profoundly control arsenic flux and species, microbial community structure and ultimately, the biogeochemical fate of arsenic.

Arsenic (As) is reputed to be a redox-sensitive element but has not been fully explored yet as a proxy in marine sediments and sedimentary rocks, compared to extensively examined elements such as molybdenum (Mo) and uranium (U). Here the behavior of As is compared to that of Mo and U in a number of paleo-environmental situations that have been studied before and are, therefore, rather well constrained. The results indicate that attention must be paid to the reference values used to normalize sedimentary As content in geochemical studies. The most-commonly used references (namely the Post-Archean Australian Shale and average upper crust of Earth, to be opposed to the Average Shale) tend to overestimate slightly As enrichments as shown by As enrichment factors being quasi systematically larger than 1, whatever the geological setting. The results also show that As enrichments are less straightforward compared to those of Mo and U, when using these elements as redox proxies. In addition, the depositional milieus that endured the most hydrologically-restricted and/or reducing (euxinic) conditions are not those with the highest As enrichments. Thus, arsenic shows a redox-sensitive behavior but is not adequate as a redox proxy. However As enrichments are correlated with Mo enrichments in two distinct types of situations: settings impacted 1) by particulate iron shuttling processes and, 2) by diagenetic fluid circulation (cold seeps). In both cases, the Mo-As correlation is induced by the strong relationship between these trace metals and reactive iron: Fe conveys both As and Mo to the sediment where they are trapped. Thus arsenic has a promising potential to allow recognition of past depositional environments associated with iron shuttling and (cold) seep circulations. Lastly, high-TOC, high-Mo, low-As situations point out possible occurrences of sulfurized organic matter in sediments. High Mo concentrations indicate reducing, sulfidic conditions prone to organic-matter accumulation (high TOC values), but low As concentrations indicate that reactive iron was a limiting factor, preventing As capture. Under such conditions, free H₂S was not associating with iron and could react with organic matter, causing its sulfurization.