Data on trace element concentrations in coal and host rock and leaching product in different pH values and open/closed environments

The data presented in this article are related to the research article entitled “Multivariate analysis of trace elements leaching from coal and host rock” [1]. During coal mining, coal and host rock undergo water-rock interaction, leading to release of trace elements to surface and ground water. The batch experiments were designed and implemented to investigate the leaching behavior and mechanisms during the process of water-rock interaction. In different experimental sets, various types of leaching water, open/closed environments, temperature, and initial pH values were used to evaluate their impact on leaching of trace elements. These data could be used to analyze leaching mechanisms of trace elements from coal and host rock, and understand, predict, control trace elements’ contamination to surrounding waters.


Data
Tables 1 and 2 show concentrations of major and trace elements in coal samples, and Tables 3 and 4 show concentrations in host rock samples after the first and second sampling and testing. Analysis of ICP-AES, ICP-MS were applied in the first the second testing, respectively. 26 and 16 elements were analyzed in the first and second testing, respectively.
Selected coal and host rock samples were used in the leaching experiments, to simulate leaching behavior of the trace elements. Tables 5 and 6 show leaching behavior of trace elements from coal and host rock, respectively, which were tested by ICP-MS. In the coal and host rock leaching experiments, different conditions, including type of leaching water, initial pH values, open/closed environments, and temperature, were set.

Description of the sampling area
Solid and water samples were collected at Xuzhou-Datun coal mine district, which is located at the north of Jiangsu province, eastern China (Fig. 1). Average temperature, relative humidity and air pressure in the region are 14 C, 73%, 101280 Pa, respectively. Average precipitation is 758 mm, ranging from 492 mm to 1178 mm, with an average evaporation 1623 mm. The geology in this area could be described as a series of sediment stratum cover the Archean system, from bottom to top these are Simian, Cambrian, middle-lower Ordovician, middle-upper Carboniferous, Permian, Jurassic, Cretaceous, Tertiary and Quaternary system.
The coal seams that are being mined are located in the Carboniferous and Permian systems, the former includes Benxi and Taiyuan formations, and the latter includes Shanxi and Lower-Shihezi formations, listed from bottom to top in both systems. The existing coal seams are located in the Shanxi formation (coal seam No. 7&9) and Lower-Shihezi (coal seam No. 2) Formation. In the lower formation, white feldspar, quartz granule-sandstone and silicon-mudstone cementation are the main Specifications table

Subject area
Geochemistry More specific subject area Environmental Geochemistry Type of data Tables  How data was acquired  ICP-AES (JY38S ICP-AES); ICP-MS (X-Series ICP-MS-Thermo Electron Co.) Data format (Raw data: important environmental concern elements were tested) Experimental factors Solid samples for testing (coal and host rock) were digested.

Experimental features
Host rock and coal samples were leaching using a batch mode, samples were collected using syringe. Different pH values and open/closed environments were applied in the experiments.

Data source location
Samples of coal, host rock for testing and leaching was collected in Xuzhou-Datun coal mine district, Jiangsu province, China.

Data accessibility
The data are available only in this article. Related research article Y. Shan, W. Wang, Y. Qin, L. Gao. Multivariate analysis of trace elements leaching from coal and host rock, Groundwater for Sustainable Development., 8, 2019, 402e412 [1].
Value of the data The data show major and trace element concentrations in a Chinese coal mine district, which can be used to compared with other coal mine districts. The data show leaching behavior of trace elements from coal and host rock, which can be used to investigate the environmental impact of coal mine water. The data show the impact of pH and open/closed conditions on leaching behavior, which can be used to analyze the leaching mechanisms of trace elements from coal and host rock. The data could be used to understand, predict, and control the contamination of trace elements on surrounding waters. minerals. Grey mudstone, sand-mudstone and sandstone are the main rocks in the middle Shanxi formation with some silicon-mudstone and siderite also present [2]. In the upper formation, greygreen middle-fine quartz sandstone, siltstone, mudstone are the main minerals. In this area, ground water was reported to be contaminated by coal mining and electricity plants [3,4].

Sampling
To investigate contamination of trace elements on surface and ground water by coal mining activities, 28 solid samples and 16 ground water and surface water samples were collected. The solid samples were collected from roof, floor, and coal from the working areas of the coal seam No. 2, 7, and 9 as shown in Tables 1e4 The ground water samples, including that from roof leaching, limestone aquifer and caved goaf, were collected in coal mines. Surface water samples were collected from lakes that located in both coal mine and non-coal mine district. 1000 mL Nalgene bottles were used to contain water samples, which were cleaned by acid in laboratory and rinsed twice before the sample were

Experiments
Some factors may control or impact the leaching process, leading to different migration behavior of major and trace elements. pH value is a key parameter in leaching behavior of trace elements from coal and host rock. Most of leaching study focus on the acid leaching behavior for the higher mobility of metal elements [5e9]. The group of Zhang have applied a series of experiments to investigate acid leaching of vanadium from coal [7e9]. However, metalloid elements may release more in an alkaline environment [6]. At the same time, high pH values are usually observed in coal mine water. Eh impact the leaching behavior in two aspects, higher kinetical ratio in one hand, and oxidation of some minerals that stable in an anaerobic environment in the other hand [5]. Temperature may promote the waterrock interaction according to the thermodynamics laws [10]. Some researchers have also discussed     "/" e under detection limit; "-" -missing value.  the impact of liquid/solid ratio [7], types of leaching water [5] on the trace elements' migration, as some major components, such as sulfate, may hider release of trace elements [5].
The leaching experiments were designed in batch mode to simulate water-rock interaction in a coal seam where the water moves slow and reaction tends to achieve equilibrium [7,11e13]. To avoid effect of content of solid samples on leaching behavior, most of the leaching experiments used the same coal/rock samples. All the glassware used in the experiments were soaked in 3.2 mol/L HNO 3 for two days so as to reduce cross-contamination. The selected solid samples were ground until 75 mm. 30 mg were weighted for each leaching experiment, for interacting with 1000 mL aliquot water. The water used in the experiments were ultra-pure water, surface water from coal mine district and non-coal mine district to simulate different environment. To investigate the immigration behavior of trace elements in both acid and alkaline environment, initial pH of ultra-pure water was set to 2, 5.6, 7 and 12. Temperature was controlled using a water bath at 38 C, or in raw temperature, which was about 15 C. To simulate a 'closed environment' with low pO 2 , (see Stumm and Morgan (1996) for details) [14], bottles were closed with a rubber stopper. Flasks were sealed and shaken every two hours. Leachate solutions were collected using syringes at 2, 6, 24 and 48 hours as the reaction may achieve equilibrium in hours in some conditions [7], some samples were collected even later to ten days to observe long time behavior [6]. 0.5 mol/L HNO 3 was added into each sample to reduce adsorption and hydration of trace elements. The pH and Eh of the solution during experiments was determined by a JENCO 6010 pH/ORP meter. The conditions in every experimental setting are illustrated in Tables 7 and 8 for coal leaching and host rock leaching, respectively.  While the solid sample were digested, 0.05 mg of samples were carefully weighted and put into tetrafluoroethylene crucibles, 3 mL of HNO 3 , 1mL of HF and 1 mL of HClO 4 were added and heated until all liquid consumed up. Then 3 mL of HCl were added and heated again until the consumption of the liquid. After that, the crucibles were rinsed using ultra-pure water, and then filter into a constant volume of 50 mL using volumetric flasks for analysis.

Testing
The data of ground water and surface water samples are shown in related article [1]. The data of solid samples and the water samples collected from the leaching experiments are shown in the DiB article, as supporting material for the related article. In the related article, selected attributes were used based on their concentration levels, proportion of under-detected data, and feature engineering.
Major ions and physical parameters of water samples were determined in Jiangsu Provincial Coal Geology Research Institute in line with Chinese standard protocols. pH was tested both in site and in the laboratory using Glass electrode method (GB/T 6920-86). Total dissolved solids and hardness were analyzed in term of the standard GB/T 8538 method. Mg and Ca were measured using atomic absorption spectrophotometric (GB 11905-89). K and Na were analyzed by flame atomic absorption spectrophotometry (GB 11904-89). Fe, Ammonia and Nitrate-Nitrogen were analyzed by phenanthroline spectrophotometry (HJ/T 345e 2007), phase molecular absorption spectrometry (HJ/T 195e2005) and gas-phase molecular absorption spectrometry (HJ/J 198e2005), respectively. Sulphate and chloride were determined using flame atomic absorption spectrophotometry (GB 13196-91) and silver nitrate titration (GB 11896-89), respectively.
Concentration of trace elements in solid and liquid samples were determined by ICP-AES and ICP-MS. The ICP-AES analysis was carried out in the Nanjing University using a JY38S ICP-AES model. Limit of detection and deviation for the analysis were 0.01 mg/mL and less than 2%, respectively. The ICP-MS analysis was carried out in the Analysis and Test Centre of China University of Mining and Technology using the X-Series ICP-MS-Thermo Electron Co., Rh was used as internal standard to determine limit of detection (0.5 pg/mL) and analytical deviation (less than 2%).