Mercury forms and their transformation in pyrite under weathering

Effects of water-coal interactions on coal mine water quality in China: a lixiviation experiment and actual water quality investigation

Water-coal interactions are dominant factors that affect water quality in coal mines. Using lixiviation, the effects of water-coal interactions on pH, salinity, and hazardous elemental enrichment in coal mine water and associated trends were simulated and analyzed. The salinity and hazardous element contents were low in the alkaline solution filtrate. However, the salinity and contents of hazardous elements (As, Cr, Zn, Cu, Mn, Co, Ni, Cd, Pb, U, and Be) in acid solution filtrate increased significantly with a decrease in pH. The pH of the solution filtrate was affected by the mineral composition of the coal, wherein the pyrite content could generally determine the pH. In addition, the spatial distribution and utilization potential of coal mine water quality in China was determined based on water quality data surveys. For water-deficient regions in northern China, particularly in the northwest, the local mine water had high salinity, a high pH, and a low content of hazardous elements; therefore, the reuse of mine water for water consumption is feasible. Conversely, the mine water in the southwest region had high salinity and a low pH and was enriched in harmful elements with potential ecological and health risks. The actual water quality characteristics of the coal mine water matched the results of the laboratory simulation analysis, confirming the effect from water-coal interactions. This work provides a reference for understanding the determinants of coal mine water quality and the potential for water environment protection.

Dissolution of harmful trace elements from coal and the environmental risk to mine water utilization

Under the pressure of water shortages, coal mine water has been allocated as a national water resource in China. However, the existence of harmful trace elements (HTEs) in coal mine water causes environmental risks and health concerns over its reuse. Through a lixiviation experiment, the dominant factors affecting the dissolution of HTEs in coal were simulated and analyzed, and the environmental risks of HTEs in coal mine water in China were evaluated for the first time. The average dissolved content levels of HTEs from coal were Mn > Cu > Zn > Ni > Ba > Cr > Co > V > Mo > Se > U > Pb > Cd, and the average maximum dissolution rates were Ni > Co > Mo > Zn > Cu > Cd > Mn > Se > Ba > Cr > U > Pb > V. Oxidation-reduction potential (Eh) and pH are the dominant factors controlling HTE dissolution. Higher oxygen exposure levels induce Eh and pH development, resulting in more HTE dissolution. This study constructed the dissolution potential index (F_C) of HTEs from coal. Based on the results of the F_C model, the areas with the highest migration potential and environmental risk of HTEs from coal seams to mine water are located in southern China, especially in the southwest, followed by areas of eastern Inner Mongolia and Shanxi and Shaanxi provinces. The corresponding risks in other regions are relatively low; thus, mine water utilization remains an effective option. This study provides an effective reference for the analysis of HTE enrichment in coal mine water and an evaluation of its safe utilization.

Concentration and speciation of mercury in atmospheric particulates in the Wuda coal fire area, Inner Mongolia, China

Coal-seam fire is a source of atmospheric mercury that is difficult to control. The Wuda Coalfield in Inner Mongolia, China, is one of the most severe coal fire disaster areas worldwide and has been burning for more than 50 years. To investigate atmospheric mercury pollution from the Wuda coal fire, gaseous elemental mercury (GEM) concentrations and atmospheric particulate mercury (PHg) speciation were measured using a RA-915+ mercury analyzer and the temperature-programmed desorption method. Near-surface GEM concentrations in the Wuda Coalfield and adjacent urban area were 80 ng m^-3 (65-90 ng m^-3) and 52 ng m^-3 (25-95 ng m^-3), respectively, which are far higher than the local background value (22 ng m^-3). PHg concentrations in the coalfield and urban area also reached significantly high levels, 33 ng m^-3 (25-45 ng m^-3) and 22 ng m^-3 (14-29 ng m^-3), respectively (p < 0.05). There is no clear evidence that PHg combines with organic carbon or elemental carbon, but PHg concentration appears to be controlled by air acidity. PHg mainly exists in inorganic forms, such as HgCl₂, HgS, HgO, and Hg(NO₃)₂·H₂O. This work can provide references for the speciation analysis of atmospheric PHg and the safety assessment of environmental mercury.

Environmental geochemical maps of harmful trace elements in Chinese coalfields

Coal resource utilization and environmental protection is a critical global issue. This study aims to address the need for geochemical maps of harmful trace elements (HTEs) in Chinese coalfields and to extract scientific information from these maps. Based on data extracted from the Trace Elements in Coal of China database, geochemical maps of As, Cd, Cr, F, Hg, Ni, Pb, and Se in Chinese coalfields were generated, for the first time, using the ArcGIS platform. Differences in regional HTE concentrations were attributed to multiple factors, including the type of coal-forming environment, terrigenous debris, and groundwater effect. However, on a national scale, the spatial distribution pattern of HTEs in coal is affected by the abundance of elements in the earth's crust. Herein, the enrichment anomaly of HTEs in coal were found to be significantly correlated with fault locations, and hydrothermal fluid action was characterized as the primary causal factor. HTE abundance in coal is the result of geochemical cycles in the earth's crust. Additionally, stratum fracture zones may serve as conduits and material sources for the migration of HTEs from deep layers to shallow layers, including coal seams. This study provides an essential reference for extensive map applications and coal environmental management while advancing our understanding of the spatial distribution patterns of chemical elements in coal.

Role of H+, HF, SO42- and kaolin in fixing Hg of coal fire sponge

Coal fire sponges (CFS) are common in coal-fire areas. Due to the enrichment of Hg in CFS, large amounts of Hg are released by CFS into the atmosphere via natural weathering or solar radiation. Therefore, CFS should be of concern in Hg pollution management and control globally. In addition, CFS changes the Hg cycle path by capturing Hg from coal fires that would have entered the atmosphere. In this study, the concentration, distribution, species, and enrichment mechanism of CFS Hg were investigated. The results showed that the Hg concentration in CFS ranged from 1008 to 35,310 ng/g, with an average of 8932 ng/g (CFS number, n = 153). The Hg concentration of CFS in different types of land was found to be significantly inhomogeneous. To determine the status of subterranean spontaneous combustion, the Hg concentration was added, which can improve the effect of coal-fire monitoring. Compared to the background area topsoil, CFS was enriched in Hg, acid, SO₄^2-, and total fluoride. The Hg species in CFS was primarily HgSO₄, followed by HgO. However, the primary Hg species in the surrounding topsoil were HgCl₂ and HgO. By the simulation experiment, it was determined that hydrofluoric acid (HF) was beneficial to activate the stable species in the coal-fire areas. HgCl₂, HgO, or Hg⁰ were ionized by acid liquor or HF, which can promote Hg migration and increase the adsorbed ratio; in the presence of SO₄^2-, the primary Hg species was HgSO₄. Ultimately, Hg was absorbed by clay minerals and organic matter. The high-efficiency activation of steady Hg species by the coal-fire HF should be studied further.

Spatial distribution of harmful trace elements in Chinese coalfields: An application of WebGIS technology

Harmful trace elements in coal have caused serious damage to the environment and human health. Understanding their spatial distribution is helpful for environmental health assessment and for their effective control and utilization. To further explore the geospatial distribution of harmful trace elements found in Chinese coals, this work constructed the Trace Elements in Chinese Coals Database Management System (TECC), and analysed the spatial distribution of harmful trace elements by applying spatial data algorithms and visual technology of WebGIS. The main results are as follows: (1) The mean concentrations of 25 harmful trace elements (Ag, As, B, Ba, Be, Cd, Cl, Co, Cr, Cu, F, Hg, Mn, Mo, Ni, P, Pb, Sb, Se, Sn, Th, Tl, U, V, Zn) in Chinese coals are provided, using the "reserve-concentration" weighted calculation method; (2) Using As, Hg, F, and U as examples, the spatial distribution of harmful trace elements in Chinese coalfields is visually displayed; (3) Harmful trace elements are extremely unevenly distributed in Chinese coalfields; they are mainly concentrated in south China, especially in the southwest region, and some elements may also be concentrated in coals from northwest, northeast, and north China. The enrichment of harmful trace elements in Chinese coals is the result of a combination of multiple factors, such as the nature of the region the coal is sourced from, sedimentary facies, coal-forming plants, and magmatic hydrothermal processes. This work can serve as a reference for the study of harmful trace elements in coal, including assessment of their environmental and health impacts.

Study on Mercury Species in Coal and Pyrolysis-Based Mercury Removal before Utilization

Coal-fired mercury (Hg) pollution control is an important global environmental context. Eight coal samples from different coal fields in China were used to investigate Hg species and the Hg removal effects under different pyrolysis conditions in the presence of nitrogen. These conditions included temperature, particle size, and residence time. The study concludes that the temperature is the most important factor affecting Hg removal from coal, and the mercury release activity at specific temperatures depends on the species and content of Hg present. Large particle size limits the removal rate of Hg, and coal particles smaller than 40 mesh are more favorable for the rapid removal of Hg. For most coal types, pyrolysis of 10-15 min can achieve the ultimate Hg removal effect. Rapid pyrolysis at 600 °C in nitrogen is feasible to remove Hg from coal. Consequently, the Hg removal rate reaches 88-100%, the loss rate of coal calorific value is 2-12%, and approximately 17-58% of S is removed synergistically. HgS, HgSe, HgSO₄, organic matter Hg, and HgO are the main types of Hg species detected in coal, whose thermal decomposition characteristics are the essential criteria for determining the type of Hg removal process. This research will facilitate the improvement of pollution control methods for coal-source Hg pollution.