Hydrogen sulfide occurrence states in China's coal seams

The occurrence states of hydrogen sulfide in coal seams are crucial in preventing and controlling hydrogen sulfide emission in coal mines and the safe development of coal bed methane. In this study, the research status of the occurrence states of free-state, adsorbed-state, and water-soluble hydrogen sulfide in coal seams was systematically analyzed. H2S anomaly areas in China's coal seams are mainly located in the Carboniferous-Permian and Jurassic series of northern, eastern, central, and northwest regions of China. Bacterial sulfate reduction accounts for most of the hydrogen sulfide anomalies of low-rank coal, while thermochemical decomposition thermal desorption spectroscopy and thermochemical sulfate reduction may also result in hydrogen sulfide anomaly in medium- and high-rank coal. In contrast, magmatism-induced hydrogen sulfide anomalies are rarely found. Absorbed-state hydrogen sulfide anomalies are prevailing, while water-soluble and free-state hydrogen sulfide anomalies are relatively scarce. Coal seam's porosity mainly controls the hydrogen sulfide adsorption, pressure, coalification degree, pore volume, and specific area, while water-soluble hydrogen sulfide is influenced by pressure, sulfate-reducing bacteria, burn, porosity, fractures, water temperature, and hydrodynamic conditions. The fractures in coal seams, their burial depth, coal quality, coal rank, roof, and floor lithology are the main factors controlling the free-state hydrogen sulfide preservation. The absorbed-state hydrogen sulfide in coal seams is mainly mitigated by varying the ventilation mode, increasing the ventilation capacity, spraying alkali fog into the air, and injecting alkali liquid into coal seams for governance.


Geneses of H 2 S anomaly in coal seams
Overall, anomalous enrichment of H 2 S in coal seams can be classified into three states: free-state, adsorbed-state, and water-soluble ones. The formation mechanisms mainly include biogenic formation, thermochemical formation, and magmatism (Berner, 1984;Chambers and Trudinger, 1979;Dai et al., 2002;Zhang et al., 2005). To be specific, the biogenic formation refers to bacterial sulfate reduction (BSR), while the thermochemical formation mainly refers to thermochemical decomposition (thermal desorption spectroscopy (TDS)) and thermochemical sulfate reduction (TSR) .
BSR can be regarded as the main biogenic origin of H 2 S in coal seams. The occurrence of BSR activity should satisfy the following three basic conditions: availability of organic matter, sulfate, and sulfate-reducing bacteria (SRB) (Huang et al., 2016;Simonton and King, 2013). The anaerobic environment for the occurrence of the reduction is favorable for the storage and aggregation of H 2 S. Accordingly, BSR or BSR-including mixed causes can mainly account for H 2 S anomalies in China's coal seams. The abundance of BSR-induced H 2 S is generally <3%. Additionally, the formation medium condition should be suitable for the growth and reproduction of SRB Asaoka et al., 2018;Machel, 2001;You et al., 2009). Given this, BSR usually occurs in shallow coal seams.
Thermochemical formation mainly includes TSR and TDS. The former one is the main factor controlling the formation of anomalous H 2 S enrichment. High temperatures (exceeding 150°C), sufficient organic matter, and sulfate are three basic conditions required for TSR. Spontaneous combustion of coal rock can directly affect the production of H 2 S in the TSR process. After being heated or baked to high temperatures, sulfur in the coal rock is partly oxidized to SO 2 and partly dissolved in water to form sulfate, providing conditions for TDS and TSR to form H 2 S. The concentration of TDS-generated H 2 S is generally <2% (Sosńicka and Lüders, 2020;Vengosh et al., 2014;Zhang, 2007;Zhang et al., 2008).
Magmatic activities melt the rocks in the deep crust, and the generated volatile components, including H 2 S, enter into the coal seams after degassing separation. Therefore, the content of H 2 S under magmatism mainly depends on the magma constitutes and gas migration conditions, being very unstable. Furthermore, the formation's H 2 S can be preserved only under certain reservoir conditions (Wu et al., 2013).
The factors in coal seams that can affect the concentration of H 2 S include the total sulfur content in the coal, SRB, reservoir pressure, coalification degree, hydrodynamic condition, and the spontaneous combustion of coal rocks. Further identification of genetic types of H 2 S in coal seams should consider various factors, including coal-forming environment, thermal evolution history of coal rocks, the constituting characteristics of C and S isotopes, and gas components.
According to the National Coal Board coal classification standard (Spears et al., 1999), coal with a total sulfur content exceeding 2.5% is considered a high-sulfur coal. Besides, coals with H 2 S concentrations exceeding 1000 ppm referred to as high-concentration H 2 S anomalies. While most coal seams in East China fall into high-sulfur coal classification, high-sulfur Fenghuangshan and Tiexin Coal Mines in North China contain mostly medium-and low-concentration H 2 S anomalies. In contrast, among the Longtan Coal Mine, Binlang Coal Mine, and Guang'an Coal Mine in Southwest China, the first two mines have high-sulfur coals and high-concentration H 2 S anomalies, while the latter has medium-concentration one. In general, the H 2 S concentration in high-sulfur coals significantly exceeds those in medium-and low-sulfur coals, reaching as high as 57.14%.
Noteworthy is that pyrite is one of the common metal minerals in coal seams, and H 2 S is the basic condition for generating coal seam pyrite. Due to the aggressive chemical properties of H 2 S, iron ions are susceptible to its action, forming a relatively stable sulfide-pyrite, consuming a large amount of H 2 S in the gas reservoir. Because the valence of sulfur in pyrite is higher than that of H 2 S, it shows the sulfur isotope of pyrite. The composition of the sulfur isotope is higher than that of H 2 S (Zhao et al., 2021a). Deng (2015) described the two main forms of pyrite produced by consuming H 2 S in coal as grain-shaped and raspberry-shaped ones: 1. A grain-shaped crystal pyrite is formed by the direct precipitation of H 2 S into raw pyrite. This occurs in a reducing environment with pH < 6.5, when the coal seam water contains sulfate-reduced saturated water-soluble H 2 S and Fe 2+ ions, which concentrations are less than that of FeS. 2. A raspberry-shaped pyrite is formed by evolution of pyrite of complex origin. In the coal seam water environment with pH > 6.5, when the dissolved S 2− in the coal-forming environment is relatively abundant, the organic matter can react with SO 4 2− to form H 2 S, and Fe 3+ is reduced to Fe 2+ . At this time, Fe 2+ reacts with H 2 S. The reaction produces FeS, which may continue to undergo several sulfide stages and eventually form a raspberry-shaped pyrite. Deng (2015) studied the Zhunnan Coalfield and found that the measured sulfur isotope values in the study area were quite low, ranging from −14.5‰ to 11.6‰. Among them, the δ 34 S value range of pyrite in coal was 8.7‰ to 11.6‰, with an average of 10.2‰. The δ 34 S values of H 2 S gas in coal seams were negative and ranged from −14.5‰ to −9.4‰, with an average of −12.3‰. The δ 34 S value in the underground water body of the coal mine was −0.6‰, while the δ 34 S value measured in the crude oil of the regional boundary of Houxia was 14.17‰. Using the above-mentioned average values of 10.2‰ and −12.3‰, the total δ 34 S value could be assessed as δ 34 S Pyriteδ 34 S H 2 S = 10.2 -(−12.3‰) = 22.5‰ > 22‰. Thus, the regional H 2 S gas generally exhibited the characteristics of BSR genesis. Wen (2018) conducted an in-depth study of the Huayingshan mining area and reported that most coal fields had H 2 S content above 2%, being high H 2 S reservoirs. The sulfur isotope value distribution range was mostly between +10‰ and +15‰. According to the relationship between the content of H 2 S of different genetic types and the δ 34 S value, H 2 S in this region has the characteristics of TSR origin.

Distribution characteristics of H 2 S anomaly areas in the coal seams
From China's coal mines with proven H 2 S geneses, a comprehensive analysis of the formation of H 2 S anomaly coal mines revealed that BSR was the main factor controlling the anomalous enrichment of H 2 S, among H 2 S abnormal coal mines, the share of pure BSR-induced ones was 42.31%, while shares of BSR/TSR and BSR/TDS/TSR ones in mixed BSR-related cases were 19.23% and 7.69%, respectively, as shown in Figure 1. It can be observed that the anomalies of free-state H 2 S and adsorbed-state H 2 S have coincident formation geneses, namely, BSR, magmatism, TSR, and the mixed BSR/ TSR and BSR/ TDS/ TSR causes. Except for magmatism, the anomaly of watersoluble H 2 S shows almost the same formation geneses with free-state and adsorbed-state H 2 S anomalies.  Table 1 and Figure 2, H 2 S anomaly areas in China's coal seams are mainly distributed in Inner Mongolia, Hebei, and Shanxi in North China, Shandong in East China, Sichuan and Chongqing in Southwest China, Xinjiang, Shaanxi, and Ningxia in Northwest China, and Henan and Hunan in Central China. H 2 S anomaly areas in coal seams are mainly found in Carboniferous-Permian and Jurassic coal series. In terms of anomaly genesis, BSR can primarily account for H 2 S anomaly in China, followed by the mixed genesis, while magmatism is rarely found (see Figure 1). As listed in Table 1, BSR genesis is mainly responsible for H 2 S anomalies in North and Northwest China. In contrast, H 2 S anomalies in East and Southwest China were formed mainly under magmatism and TDS (see Table 1).

As shown in
In terms of occurrence state, as shown in Table 1, H 2 S in China's anomalous coal seams exists in the adsorbed state; free-state H 2 S is relatively common in East and North China, while watersoluble H 2 S is scarce. H 2 S in free and water-soluble states is rarely found in Southwest China. In contrast, in Northwest China, H 2 S anomaly coal seams contain H 2 S in the adsorbed, watersoluble, and free states.
H 2 S anomalies in China are present in low-, medium-, and high-rank coal seams. However, these coal seams differ in coal type among different regions. Specifically, H 2 S anomaly coal seams in East China are mainly composed of gas coal, as shown in Table 1. H 2 S anomaly coal seams in Southwest China are mainly composed of fat coal, coking coal, and anthracite, while those in Northwest China are mainly composed of lignite, long flame coal, non-caking coal, gas coal, and gas-fat coal.   (1) Based on the concentration of H 2 S in coal gas, H 2 S anomalies can be subdivided into highconcentration (>1000 ppm), medium-concentration (100-1000 ppm), and low-concentration (6.6-100 ppm) ones. Most of China's H 2 S abnormal coal seams are medium-and lowconcentration ones, while high-concentration H 2 S anomalies are scarce. The H 2 S anomaly concentration scale is related to H 2 S genesis. As shown in Table 1, the H 2 S anomaly concentration scale in coal seams nationwide is dominated by medium-and low-concentration anomalies, while highconcentration ones are relatively rare. The abnormal concentration scale is related to the origin of H 2 S. The total sulfur content and abnormal concentration scale of H 2 S related to BSR and TSR are lower, while those related to magmatism are generally higher. In general, the total sulfur content and abnormal concentration scales in high-rank coals are generally higher than those in middle-and low-rank ones, and the total sulfur content and H 2 S concentration exhibit a pronounced positive correlation (Figure 3). For example, in North China, anthracite coals generally have higher total sulfur content and abnormal concentration scale coking coals, with several exceptions. The first one is the Fenghuangshan Coal Mine: although its total sulfur content is as high as 3.21%, its abnormal concentration scale is only moderately abnormal, not reaching a highconcentration anomaly level. The second one is the Xinwei Coal Mine, although its coal rank is anthracite, its total sulfur content is not high. The medium-rank coal (including fat coal and coking coal), with high total sulfur content and anomaly concentration, prevails in Southwest China (Zhao et al., 2021b). Coal seams in Northwest China mainly contain low-and medium-rank coals, including lignite, long flame coal, non-caking coal, gas coal, and gas-fat coal, with low and medium total sulfur content and anomaly concentration.

Free-state H 2 S occurrence
Free-state H 2 S is generally found in coal seams with well-developed fractures and weak hydrodynamic conditions. Besides, coal seam roofs and floors, as the overlying rocks, are compact in lithology. Numerous researchers have revealed dependencies between tectonic activities, burial depth, coal quality, and free-state H 2 S via field measurements and numerical simulations. Gas-collecting bags are commonly used to collect coal bed methane (CBM) samples with free-state H 2 S. After being separated by the chromatographic column, H 2 S is combusted in the reaction kettle. The combustion products react with ozone, and the amplification reaction produces chemoluminescence, which is detected by a photomultiplier. Accordingly, the content of H 2 S in a CBM sample can be analyzed in the data analysis module. This method is quite expensive and involves potentially harmful oxidizing agents, such as ozone (Cheng et al., 2013).
By performing in-situ measurements and numerical simulations on the content of H 2 S in coal seams, Fu et al. (2015) revealed the effects of burial depth and coal quality on H 2 S in coal seams of the Xishan Coal Mine, Xinjiang, China. They reported that the H 2 S content was in negative correlation with the contents of CBM, CH 4 , CO 2 , and N 2 . Besides, the content of H 2 S was also in negative correlation with moisture (Figure 4(a)) and ash yield (Figure 4(b)), and in positive correlation with the content of volatile yield (Figure 4(c)) and total sulfur (Figure 4(d)). However, no obvious correlation between the H 2 S content and the coal seam burial depth was detected (Figure 4(e)). Besides, they analyzed the genesis of the H 2 S anomaly and reported that the coal seam partly absorbed the generated H 2 S under magmatism of diabase at the late Yanshan Orogeny in the No. 3 Coal Seam of the Bayi Coal Mine in Zaozhuang, China. The remaining free-state H 2 S was distributed in pores and fractures of coal seams. Thus, anomaly enrichment areas of free-state H 2 S were formed on the west side of the dry rock wall with no water or faults (Song et al., 2016). Free-state H 2 S diffusion to the coal mining tunnels is quite high; therefore, minimizing the concentration of free-state H 2 S in tunnels is vital to ensure miners' safety. Safe and convenient passive protective ways of reducing H 2 S hazards in mining tunnels imply more effective ventilation methods, increased exhaust air rate, spraying alkaline fog into the air for neutralization, and wearing anti-H 2 S masks. For example, in the +469 m B 3+6 fully mechanized caving face on the east wing in the Wudong Coal Mine's western region (Gao, 2020), the mined-out area and the coal seam were above and below the working face, respectively. The measured concentration of H 2 S in the advanced detection hole reached 14,300 ppm. H 2 S in a free state would inevitably enter the tunnel's return flow during the working face's recovery process. The concentration of H 2 S was abnormally high (∼60 ppm) at the back of the working face. H 2 S in the adsorbed state could be further desorbed into a free-state one during the coal drawing process, increasing the latter's concentration and jeopardizing underground workers' safety. The tunnel was ventilated with a 754.8 m 3 /min ventilation capacity to mitigate this problem, which dropped the concentration of H 2 S in the working face below 28 ppm. Besides, workers were obliged to wear anti-H 2 S masks for more effective personal protection. Another example is the Baozigou Mine in Gansu Jingchuan County, which is a low-concentration H 2 S anomaly coal mine (Jia et al., 2018). Before the treatment, the concentration of H 2 S in the working face was 90 ppm, which far exceeded the safety limit of 6.6 ppm. Some protective measures, such as spraying the alkaline liquid and improving the ventilation condition, were recommended to reduce the concentration of H 2 S to below 6.6 ppm. On the one hand, to avoid the appearance of large-vortex core regions in the working face, the air cylinder could be shifted by 3 m from the working face to ensure the migration of H 2 S toward the sidewall of the tunnel with the airflow rather than being taken to the vortex region. On the other hand, three high-pressure nozzles that were originally installed for absorbing free-state H 2 S and dust gushed from the drum during the coal cutting process could be removed, and six single high-pressure alkaline-liquid-sprayers should be reinstalled around the heading machine. As an optimal solution, two nozzles were set on the bottom of the cutting drum of the heading machine 5°toward the inside of the connecting rod, while two more nozzles were set on both sides of the front part of the heading machine at a spraying angle of 30°and 45°, respectively. The front nozzles absorbed the free-state H 2 S desorbed from the crushed coal near the absorbing air cylinder and H 2 S and dust in the convolutional airflow on the air return side, while the rear nozzles further purified the escaped H 2 S and dust. After adopting the above measures, the concentration of free-state H 2 S at a distance of 5 m from the heading machine's driving on the air intake side was reduced below 6.6 ppm. In particular, the concentration of free-state H 2 S on the air return side was reduced below 6.1 ppm, which satisfied the above safety requirements.

H 2 S in the adsorbed state
The adsorbed-state H 2 S is the main occurrence form of H 2 S in coal seams. At present, the H 2 S accumulation process is extensively explored via isothermal adsorption parallel tests, contrastive analysis, generalized gray-scale correlation analysis, and quantum chemical analysis. Fu et al. (2011Fu et al. ( , 2015 conducted the H 2 S isothermal adsorption parallel tests under equilibrium water conditions, which revealed that the Langmuir curve could describe the H 2 S adsorption pattern by coal rock, i.e., the isothermal curve consisted of (i) rapid adsorption, (ii) relatively slow adsorption, and (iii) equilibrium adsorption stages, as shown in Figure 5. Moreover, the distribution of coal mines with abnormally high H 2 S concentration was controlled by tectonic structures (Meng and Li, 2018;Shen et al., 2018;Wang et al., 2018). For instance, the Fukang Coal Mine, located in the Mesozoic folded belt of the Bogeda Mountain between the Fukang and Yaomoshan Fractures, had H 2 S anomaly coal seams, mainly distributed in the protruded cambered part of the thrust nappe and well-developed tectonic coal regions. Similarly, the Choumeigou Mine, the Xinlong Mine, the Jinlong Mine, and the Kanglong Mine with H 2 S anomaly were located in the protruded cambered part in the middle thrust nappe, which were closed inverted anticline structures. The coal seams with high-concentration H 2 S anomaly were crushed, and gas escaping during the coal seam lifting process provided space for later adsorption of H 2 S. The coal seams were confined by the Nanchi steel reservoir fault, the Choumeigou reverse fault, the Ganhezi reverse fault, the Wugonggou fault, and the Xiaolongkou reverse fault. Accordingly, the late-adsorbed H 2 S was confined, which led to anomalous enrichment of absorbed-state H 2 S in the coal seams.
Some scholars also carried out mercury injection tests and isothermal adsorption tests under equilibrium water conditions and analyzed the adsorption rules of H 2 S by coal at different ranks Xue et al., 2016Xue et al., , 2017. It was found that pressure and coal rank were the main factors influencing the H 2 S-absorbing capacity of coal. As shown in Figure 6, coal's H 2 S-adsorbing capacity increased with pressure and the degree of coal metamorphism. Besides, H 2 S adsorption by coal also depended on the pore distribution characteristics of the coal sample. A larger number of micro-and transition pores contributed to the adsorption of H 2 S by coal, while medium and large pores were unfavorable for the adsorption (Cheng et al., 2017;Guo et al., 2007;Luo et al., 2014). Lin et al. (2017) and Zhang (2018) analyzed the effects of coal seam's thermal evolution temperature, adsorption characteristics, pore characteristics, total sulfur content, and the reducibility index on anomalous H 2 S enrichment. They elaborated a method of quantitative determination of Figure 5. Isothermal adsorption curve of hydrogen sulfide (H 2 S) by coal at 30°C: rapid adsorption, slow adsorption, and equilibrium stages .
the generalized relation degree of various factors. The effect of the studied factors on anomalous H 2 S enrichment was ranked in the decreasing order as follows: the reducibility index, the content of total sulfur, the adsorption constant, evolutional thermal temperature, the Brunauer-Emmett-Teller (BET) specific surface area, and the burial depth. Further analysis revealed that large fractional dimensions, more complex pores, uneven surface, high looseness degree, large specific area, and large adsorption constant promoted the adsorption of H 2 S in coal seams, causing high-concentration anomaly of adsorbed-state H 2 S. Besides, it was found that the adsorption of H 2 S positively correlated with the content of total sulfur, so that the latter could be used for roughly evaluating the adsorbing capability of H 2 S in the research area. Liang et al. (2016) analyzed the adsorbed H 2 S characteristics of coal surfaces via quantum chemical analysis. They established the molecular model of H 2 S-containing coal surface in Tiexin, Shanxi, to assess the adsorption energy of H 2 S and CH 4 by coal surface molecules (Bertoncini et al., 2000;Yang et al., 2002). It was found that under the co-existence condition of H 2 S and CH 4 , the adsorption energy values of H 2 S and CH 4 by coal were 2.230 and 94.861 kJ/mol, respectively. Therefore, the coal seam's adsorption of CH 4 exceeded that of H 2 S, so that the former process inhibited the latter one. Through calculation, the adsorption energy of the mixed gas exceeded the sum of the individual adsorption energy values of the single gas with the same numbers and kinds, suggesting that coal's adsorption capability of the H 2 S/CH 4 mixed gas far exceeded the adsorption of a single gas. Therefore, H 2 S promoted the adsorption of CH 4 by the coal seam.
Based on the occurrence characteristics of adsorbed-stated H 2 S in the coal seam, some active measures such as advanced detection, coal seam pressure-difference pre-drainage H 2 S, and spraying alkaline liquid for the neutralization of H 2 S can be applied. Since active prevention and treatment should always consider the coal seams' occurrence condition and the mining technologies, they are expensive and problematic. The Gaojiabao Coal Mine was used as a case study by several researchers (Dai et al., 2002;Wu et al., 2016;Xu, 2020); the concentration of H 2 S in the No. 4 coal seam was ∼50 ppm. The adsorbed-state H 2 S in coal seams could be easily disturbed and spread into the air. The recommended measures for controlling the adsorbed-state H 2 S in the coal seams were reduced to drilling the coal seam surface for advanced detection of H 2 S before mining, followed by the injection of alkaline liquid in the H 2 S anomaly regions of the coal seam for the neutralization of H 2 S. Moreover, to enhance the treatment performance, the corresponding alkaline injection amount could be assessed by the distribution characteristics of H 2 S content in the coal seams . Given the neutralization capability, treatment efficiency, cost, and equipment structure, NaHCO 3 was a lucrative alkali-injection solute. The analysis of physical properties of coal seams in the Gaojiabao Coal Mine, operation safety, the requirement on drilling equipment, and the final hole sealing difficulty and degree revealed that a drilling hole with a 65 mm diameter provided the optimal solution. When the overall drilling length exceeded the coal seam width, the overflow induced by excessive alkaline liquid amount could be effectively avoided. Based on the working face length, the drilling depth was set at 80 m. Before injecting the alkaline liquid, the airflow's H 2 S concentrations in the working face and the air return ways reached 30 and 40 ppm, respectively. After the injection of alkaline liquid, they were reduced to 11 and 22 ppm, i.e., by 60% and 50%, respectively, which implied a good governance effect.
Water-soluble H 2 S Liu (2014) pointed out that the burned areas in the coal mines at the southeast margin of the Junggar Basin included well-developed fractures: the surface water seeped through burnt rocks forming underground water storage units. Under the spontaneous combustion of coal seams, the sulfur in the coal was partly oxidized to SO 2 ; the latter was dissolved in water, forming sulfate ions, which were then decomposed to produce H 2 S under TDS and TSR mechanisms. The generated H 2 S gas could be dissolved in the coal seams of burnt rocks or driven away by the underground water, thereby forming water-soluble anomalous H 2 S enrichment areas in the regions with poorly developed burnt rocks or favorable water-resisting layers (Cai et al., 2009;Su et al., 2017). Deng et al. (2017Deng et al. ( , 2018Deng et al. ( , 2020 analyzed mineral (underground) water characteristics at the southeast margin of the Junggar Basin and H 2 S genesis. They revealed the co-existence of large content of H 2 S with water in the coal seams. Moreover, the H 2 S content positively correlated with the CO content and pressure value. No correlation between the H 2 S content and the coal seam burial depth was detected, as shown in Figures 7 and 8.
Generally, CH 4 in water has a form of a water-soluble gas with quite stable properties. When water is abundant in SRB, the latter can use CH 4 in water as a sulfate for reductive dissolution of a hydrogen donor, producing H 2 S under dissimilation and promoting the reaction between CO 2 in water and soluble Ca 2+ ions to form calcium carbonate crystals. BSR reaction is a kind of exothermic reaction. High temperature can suppress the forward reaction; therefore, the decline in water temperature is favorable for the production of H 2 S, and simultaneously more CH 4 can be consumed (Song et al., 2017). Consider a particular case: the southern margin of the Junggar Basin contained coal with a great amount of carbonate, sulfate, and abundant organic matter. The No. 4 spring showed declining temperature year by year, leading to constant consumption of CH 4 and increasing H 2 S and CO 2 contents. Moreover, the content of positive Ca 2+ ions in confined water at deep coal mines dropped, and burn-in regional coal rocks mostly occurred in the shallow part. A limited amount of H 2 S was produced under TDS and TSR. These hydrochemical characteristics strongly indicated that active BSR action was the principal cause of the anomaly concentration of water-soluble H 2 S in this region (Deng et al., 2018).
Water-soluble H 2 S can generally be governed via sealing, dredging drainage, and spraying lime powder into the water gushing port. The following analysis was performed by taking the Huayingshan Mine and the Xishan Coal Mine in Urumqi as examples. On account of the karst landscape and complexly developed underground rivers, the maximum water inflow in the Huayingshan Mine reached up to 200,000 m 3 /h (Lei et al., 2011). Moreover, H 2 S escaped from fractures and could be dissolved in water. The aquifer in coal seams was close to the coal seam, triggering water inrush accidents and bringing a great threat to safety production. Some sealing and dredging drainage measures were recommended to prevent the potential hazard caused by the prevention of water-soluble H 2 S. Multi-component composite grouts (including barite powder, bentonite, and sodium carboxymethyl cellulose binder) were the most lucrative for sealing pores and fractures in the coal seams, isolating water, and achieve the goal of sealing water-soluble H 2 S. Next, watersoluble H 2 S could be dredged and discharged to the specified positions, and lime or alkaline liquid would be sprayed for neutralization. Insofar as the Xishan Coal Mine had a moderate water inrush of about 924.43 m 3 /h and high content of H 2 S in the water of 38 ppm , it was recommended that lime or alkaline liquid should be periodically sprayed into water gushing port for prevention. The tunnel should also maintain regular water discharge to prevent gas dissolution in water and reduce the hazard risks.

Prospects
Due to the limitations of available experimental and calculation techniques, the main research efforts have been focused on the occurrence state of adsorbed-state H 2 S in coal seams. However, the occurrence states of free-state and water-soluble H 2 S are also topical, insofar as freestate and water-soluble H 2 S is more inclined to rush and leak, causing enormous potential safety hazards to safety production. Scholars can improve the sampling techniques and the current methods and explore innovative theories in the future to reduce the potential risks. Firstly, the sampling schemes of free-state and water-soluble H 2 S can be enhanced in terms of occurrence to explore more targeted sampling techniques. Secondly, the existing dissolved quantity measurement methods should be refined. For example, physical extraction or reactive precipitation method can be considered for enhancing measurement accuracy. Thirdly, some portable devices for non-contact rapid on-site measurement and the analysis of H 2 S content of collected water and gas samples should be developed to acquire first-hand information at the site under the premise of ensuring personnel safety. Fourthly, the physical properties of coal seams on the storage and migration of H 2 S-containing coal-water should be investigated in depth. Fifthly, the effects of hydrodynamic and fire-burning conditions on acidic gas generation and storage mechanism should be clarified. Finally, the difference in sulfur utilization capability of different ranks of coals by SRB under the same hydrodynamic conditions should be examined in detail. However, most available active or passive H 2 S prevention techniques can only be regarded as local prevention measures. There is still a large gap between safety in a coal mine and high-efficiency mining. In future studies aiming to establish comprehensive and efficient H 2 S prevention techniques, more fundamental research efforts should be made to develop economical, safe, and high-efficiency H 2 S absorbents for particular site conditions. Conclusions 1. High-concentration H 2 S coal seams in China's coal seams are mainly found in the Carboniferous, Permian, and Jurassic series, located in Xinjiang, Shandon, Hebei, Henan, Shanxi, Hunan, Sichuan, Shannxi, Ningxia, and Inner Mongolia. BSR genesis plays a dominant role in H 2 S anomalies (mainly medium-and low-concentration H 2 S anomalies). H 2 S anomaly mainly exists in adsorbed states, while H 2 S anomalies in water-soluble and free states are relatively scarce. 2. Free-state H 2 S generally exists in the coal seams with developed fractures and weak hydrodynamic conditions. Both roofs and floors, as the overlying layers, show compact lithological characteristics. The burial depth and coal quality can significantly affect the enrichment of freestate H 2 S in coal seams. 3. The Langmuir curves adequately describe the adsorption characteristics of adsorbed-state H 2 S. The maximum adsorption capacity of H 2 S by the coal seams positively correlates with the number of transition pores and micropores, being negatively correlated with the number of medium and large pores. In terms of the effects on anomaly enrichment of adsorbed-state H 2 S, the reducibility index ranks the first, followed by total sulfur content, the adsorption constant, the thermal evolution temperature, the BET specific area, and the burial depth. 4. In coal seams, the reduction of water temperature can promote BSR reaction for the production of H 2 S, and the coal fire area in the shallow seam is conductive to TDS or TSR reaction. Anomaly enrichment areas of water-soluble H 2 S are easily formed in the regions with poorly developed burnt rocks or favorable water-resisting layers. 5. Some measures can be adopted for the prevention of H 2 S in coal mines. In terms of free-state H 2 S, such measures include: changing the ventilation mode to a more effective one, increasing the exhaust air rate, spraying alkaline fog into the air for neutralization, and wearing anti-H 2 S masks to reduce the hazard. In terms of adsorbed-state H 2 S, the most effective would be the advanced detection, pressure-difference pre-pumping H 2 S, and spraying alkaline liquid for neutralization. In terms of water-soluble H 2 S, the above measures can be reduced to sealing, dredging, and spraying lime powder at the water gushing port for governance. 6. More research efforts should be focused on free-state and water-soluble H 2 S, the improvement of sampling techniques, experimental methods, and the development of relevant innovative theories to reduce the H 2 S-related safety risks.