Environmental Geochemistry of Trace Elements in Two Huai Coal Mining Districts and Environmental Application of Compounds of Trace Elements in Photocatalysis

兩淮礦區微量元素的環境地球化學及其化合物在光催化中的應用研究

Student thesis: Doctoral Thesis

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Author(s)

  • Zhicao YAN

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date20 Jun 2014

Abstract

Because of widespread uses of coals many toxic elements including Hg have been released into the environment. Although many trace elements in coals are present in very small amounts, their environmental impacts and potential human health effects are of great concern because of the enormous amount of coal consumption. In this dissertation, some new findings related to the environmental geochemistry of trace elements from Double Huai coal mining districts were reported, including the abundance, distribution, modes of occurrence, enrichment factors, human health problems, and applications of trace elements in the environment.

With the aim of the better understanding of the geochemistry of trace elements in coal, 416 borehole samples of coal, one parting, two floor and two roof mudstones were collected from 9 minable coal seams in 24 boreholes drilled during exploration in the Huainan Coalfield, Anhui, China, and 47 elements were determined by various analytical techniques. The depositional environment, abundance, distribution, and affinity of elements were investigated. Results showed that the boron concentration in the coals indicates a brackish water depositional environment in this study region and the marine influence decreased from coal seam 1 to 13-1. Some potentially toxic elements (e.g., Se, Cd) are higher in concentration than their averages for Chinese coals. The roof, floor and parting samples have higher contents of some elements than do the coal seams. The minerals in the coals from the Huainan Coalfield were found to consist mainly of clays, carbonates and sulfides. The elements can be classified into two groups based on their stratigraphic distribution from coal seam 1 to 13-1, and the characteristics of each group are discussed in detail. The elements may be classified into four groups (i.e., Group 1 to 4) according to their correlation coefficients with ash yield. The elements in Group 1, Group 2, and Group 3 are strongly correlated with ash yield, while the elements in Group 4 have weak or negative correlation coefficients.

Mercury (Hg) and its compounds are among the most toxic organic compounds in the environment. The combustion of some local coals has caused severe health problems which are linked to high Hg contents in coals. The Hg concentrations in 108 samples, comprising 81 coal samples, 1 igneous rock, 2 parting rock samples and 24 water samples from the Huaibei Coal Mining District, China, were determined by cold-vapor atomic fluorescence spectrometry. The abundance and distribution of Hg in different coal mines and coal seams have been studied. The weighted average Hg concentration for all coal samples in the Huaibei Coalfield is 0.42 mg/kg, which is about twice that of Chinese average coals. From the southwestern to the northeastern coalfield in the Huaibei Coal Mining District, the Hg concentration shows a decreasing trend, which is presumably related to the magmatic activity and fault structures. Relatively high Hg levels are observed in coal seams Nos. 6, 7, and 10 in southwestern coal mines. Correlation analysis indicates that Hg in southwestern and southernmost coals with high Hg concentrations is associated with pyrite. Additionally, thirty-nine low-sulfur Permian coal samples were selected for the discussion of the geochemistry of Hg in Permian low-sulfur coals in the Zhuji Coal Mine, Huainan Coalfield, Anhui province, China. Mercury contents of coal samples were determined by cold-vapor atomic fluorescence spectrometry (CV-AFS). The average Hg content of the overall mine is 71.2 ng/g weighted by the estimated reserve of each coal seam. An increasing trend of Hg content was observed from bottom to top stratigraphically. As for Hg enrichment in some bench coal samples from No. 3 coal seam, magmatic activity is responsible for this phenomenon; the pyrite formed during seawater invasion possibly in return accumulated Hg from seawater and this mechanism is possibly responsible for the high Hg content in some coal samples from Nos. 4 to 11-1 coal seam. The mechanism that the organic matter can capture Hg from the leachable fractions of Hg for partings is possibly responsible for the Hg enrichment in some coal samples from No. 11-2 coal seam. Additionally, the anomalously high Hg levels coincide with high sulfur contents in this study, and the main contributor to the high Hg in these coals is presumably related to the sulfides such as pyrite.

The surface waters from ponds and stream creeks in the coal mining subsidence areas in some coal mining districts have been used as drinking water and for breeding fish by the residents, so it is very important to assess the Hg concentration in these surface waters in the coal mining districts. The surface water samples were collected from the Huaibei Coal Mining District. The Hg concentrations in surface waters in the Huaibei Coal Mining District ranged from 10 to 60 ng/L, and displayed a decreasing trend with distance from a coal waste pile. However, the concentrations are lower than the regulated levels for Hg in drinking water. Although many publications have reported the geochemistry of trace elements in surface waters, few have reported about the geochemistry of trace elements in groundwater, partly due to the difficulty in sampling. In fact, the geochemistry of trace elements (e.g., rare earth elements) has been widely used to discriminate the source of groundwater. In this study, we especially analyzed the geochemistry of rare earth elements in groundwater. To investigate the distribution of trace elementsin groundwater, the concentrations of rare earth elements (REEs) were determined by ICP-MS for groundwater and aquifer limestone samples collected from the Taiyuan Formation limestone aquifer in the Wolonghu Coal Mine, Anhui province, China. The results showed that the groundwater is characterized by a warm temperature (ranging from 35.8 to 39.1 ℃), roughly neutral pH (7.34-8.27) and high total dissolved solids (TDS, 1380-2850 mg/L). The REEs in groundwater are characterized by light rare earth elements (LREEs) depletion relative to heavy rare earth elements (HREEs), positive Eu anomalies and negative Ce anomalies. The fractionation of REEs in groundwater can be effected by the inorganic species of REEs. The negative Ce anomalies probably reflect the signature of the aquifer rocks (limestone rocks), whereas positive Eu anomalies are probably attributed to the redox conditions. The similarities of REEs patterns between groundwater and aquifer rocks suggest that aquifer rocks play important roles in controlling the REE characteristics of groundwater. The characteristics of REEs and major ions in groundwater from the Taiyuan Formation aquifer are different from other aquifers in the Wolonghu Coal Mine. Although the groundwater from Taiyuan Formation limestone aquifer is not well discriminated from other aquifers by REEs in this study, the obtained results will provide important information for building the foundation of water source discrimination by using REEs in the Wolonghu Coal Mine in the future.

Compounds of trace elements have been widely used as photocatalysts for the degradation of organic compounds in water. In this study, bismuth tantalum oxide is reported as a photocatalyst for the degradation of organic organic compounds. Bismuth tantalum oxide is a potentially intelligent photocatalyst, which is able to perform a relatively good photocatalytic activity under visible light. Silver loaded bismuth tantalum oxide photocatalyst possess an orderly structure and uniform distribution in thickness, and shows a good photocatalytic activity for degradation of organic organic compounds under visible light. Additionally, La doped bismuth tantalum oxide can also increase the photoactivity significantly in comparison with bismuth tantalum oxide.