Characteristics of Pores in Coals Samples Exposedto Acid Mine Drainage

: Acid mine drainage commonly occupies some pore space after underground 1 coal mining, and this severely depreciates coal pillars, thereby posing a significant risk 2 to mine stability. Considering that such depreciation is reflected in the microstructures 3 of these pillars, in the present study, we propose a static immersion method suitable for 4 coal seam samples immersion in the laboratory. We immersed the No. 2-2 coal seam 5 samples from the Ningtiao Tower Coal Mine in Yulin, Shaanxi Province, in different 6 acid mine drainage solutions and monitored the pH, oxidation–reduction potential 7 (ORP), electrical conductivity (EC), total dissolved solids (TDS) among other water 8 quality parameters for 300 h. After the immersion tests, samples were examined using 9 scanning electron microscopy (SEM). The pH, ORP, EC, and TDS of the prepared acid 10 mine drainage solutions increased significantly as the immersion time increased. 11 Changes in water quality parameters are attributed to the absorption of hydrogen ions 12 by insoluble clay minerals in the coal, which reduced the acidity, increased the pH value, 13 and enhanced the electrical conductivity of the acid mine drainage solutions. SEM 14 analysis reveals differences in the pore characteristics of (pore throat size and 15 orientation) of the coal samples, and these are caused by erosive effects of the acid mine 16 drainage solutions. Pores with throat sizes greater than 10 µm increased by 95% as the 17 pH of the acid mine drainage solutions decreased, while the dominant pore orientation 18 (60–90°) decreased to the 0–30° or increased to the 150–180° range, thereby increasing 19 their randomness. 20


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Acid mine drainage in a coal mine, which refers to the discharge of acidic wastewater 24 from current mining or mined coal seams, is a global concern 1 . It originates from 25 hydrochemical reactions between sulfide minerals contained in coal seams and 26 groundwater, during which hydrogen and heavy metal ions (e.g., manganese and iron) 27 are released, thereby elevating the acidity of the groundwater 2,3 . This mining byproduct 28 is a problem in countries, such as China 4,5 , the USA 4,5 , South Africa 8,9, and Brazil 10 , 29 where it is gradually impeding underground coal mining activities. 30 Acid mine drainage interacts with coal because of its complex composition 11 . In  Existing studies on the chemical effects of acidic water on coal involve soaking coal 42 gangue in aqueous solutions of nitric or sulfuric acid to simulate a static or dynamic 43 acidic water environment. Results show that metals precipitation depends on the host 44 environment, and thus, because an acidic environment affects the metals in coal, their 45 precipitation is also impacted . The ion precipitation potential increases as acidity of a 46 solution increases 3,20 . The ion leaching process, which represents the coal-water 47 interaction system, can be divided into the following stages: the calcite alkaline, silicate 48 acidic, sulfide acidic, and the continuous reaction 21 . According to previous studies, this 49 promotes enrichment in trace metals, thereby enhancing groundwater and soil pollution 50 over time 22 . 51 However, previous studies focus on the precipitation of metals in coal caused by the 52 water-rock interaction, the concentration of these metals in the environment, the 53 pollution methods, and the effects of coal mining on groundwater and soil. In addition, 54 these studies rely mostly on static immersion, and thus, the chemical erosion effects of 55 chemical degradation of coal by acid mine drainage has received little attention.

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Consequently, the effects of acidic water (sulfuric or hydrochloric acid and their 57 mixtures) on the precipitation of metals in coal and the alteration of its combustion 58 characteristics are commonly discussed without considering chemical degradation 59 caused by acid mine drainage. Further, coal microstructure is often investigated to 60 assess changes in minerals such as silica, pyrite, and clay minerals as well as trace 61 elements after acid immersion. Changes in the pore structure (pore throat size and pore 62 orientation) 23,24 remain poorly studied. 63 Therefore, in the present study, we conducted static immersion experiments on coal 64 samples immersed in simulated mine water of varying acidity levels (pH of 2-5), to 65 evaluate their impact on water quality parameters (pH, oxidation-reduction potential 66 (ORP), electrical conductivity (EC), and total dissolved solids (TDS)) over time. We 67 also examined the coal samples after the experiments using SEM to assess the pore size 68 and orientation characteristics associated with various acidic environments. The 69 findings of the present study improve understanding of the characteristics of pores in 70 coal samples exposed to acid mine drainage.  73 The coal used in the present study was obtained from a 2-2 coal seam in the Ningtiaota   Figure 2 Illustration of the coal sample preparation processs. 91 92 According to previous studies 25 , acid mine drainage in the coal seam is enriched in ions, 93 such as Fe 2+ , Fe 3+ , Al 3+ , Mg 2+ , Ca 2+ , Mn 2+ , Cl -, and SO4 2-, which promote acidification.

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Therefore, we used sodium sulfate, magnesium chloride, ferrous sulfate, ferric chloride, 95 and other salts to simulate acid mine drainage containing the main ions (initial pH = 5) 96 in the mine. The proportions of salts used are presented in Table 2 and the simulated 97 solutions were prepared using dilute hydrochloric acid (pH = 2).  100 Acid mine drainage immersion coal tests were performed using solutions with pH 101 values of 2, 3, 4, and 5 (Fig. 3). These solutions are henceforth denoted as pH-5, pH-4,   (Fig. 4). During the initial 24 h, sampling was performed hourly, 116 and every 12 h subsequently. Water quality parameters (pH, ORP, EC, and TDS) were 117 measured after each sampling using a high precision C-600 multi-functional pen-type 118 meter, until the end of each test. The precision and accuracy of the instrument for pH, 119 for example, were 0.01 and ± 0.05, respectively. changed significantly from 4 and 5 to 6.8 and 7.3, respectively, whereas that of the pH-147 2 solution barely increased from 2 to 2.08.

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The pH values of the immersion solutions increased by 4%, 30.67%, 70%, and 46% 149 relative to initial values of 2, 3, 4, and 5, respectively. pH changes variation after the        227 The EC and TDS are closely related because an increase in the EC value indicates an pH-2 is, however, approximately 6 orders of magnitude higher than those for the 233 solutions with initial pH values of 3, 4, and 5. This is because the acidity of the pH-2 234 solution is higher than those of the other solutions, and thus, precipitation and re-235 dissolution occurred in this strongly acidic environment. Therefore, although the solid 236 content of the solution was lower, the dissolved ions content increased. Therefore, the  as the acidity increases (Fig. 11a-d). The acidity of the solutions influenced the pore 256 throat size and geometry in the coals, with the pore throat size significantly increasing 257 from the highest to the lowest pH solution (marked by the red box in Fig. 11). The pores

b) pH-4, (c) pH-3, and (d) pH-2. 265
Coal pores in Fig. 12 reveal variable alterations in pore sizes and pore walls roughness 266 by the acidic solutions. As the pH decreases, the throat pore sizes obviously increase, 267 while the pore walls change from smooth and flat to rough. In Fig. 12a, the image 268 obtained at 20,000x magnification reveals smooth and intact pore walls with minor 269 accumulated amount particulate matter. However, as the acidity increased, the 270 roughness of pore walls increased, and cracks ( Fig. 12b-c)

Quantitative analysis of pore space in the immersed coals 280
To quantitatively characterize the influence of various solutions on coal pores, we  Table 3. According to the PCAS analysis and calculations, pore sizes produced the following   Pores with orientations between 60-90° are dominant (Fig. 15a), with approximately 320 110 to 180° pores. However, as the angle exceeds the 60-90° range, the number of pores gradually decrease. Therefore, pores mostly adopted the 60-90° range orientation, 322 with the asymmetric axis (the dashed line in red in Fig. 15a and b)     Illustration of the coal sample preparation processs.   Scanning electron microscope analysis process .      Morphology of pores in coal exposed to acidic solutions including (a) pH-5, (b) pH-4, (c) pH-3, and (d) pH-2, The magni cation from left to right are: 2,000x, 5,000x, and 20,000x .

Figure 13
Illustration of the identi cation and analysis process of coal pores using the PCAS software at 1,000x magni cation .  Orientation of coal pores associated with different acidic solutions including the (a) pore orientation distribution and (b) proportion of pores in different orientation ranges.

Figure 16
Effect of acidic solutions with different pH on structural parameters of coal pores including the relationship between (a) solution pH and pore size and (b) solution pH and pore direction .