Chemical speciation and risk assessment of cadmium in soils around atypical coal mining area of China

The distribution characteristics of Cadmium (Cd) fractions in soils around a coal mining area of Huaibei coalfield were investigated, with the aim to assess its ecological risk. The total Cd concentrations in soils ranged from 0.05 to 0.87 mg/kg. The high percentage of phyto-available Cd (58%) when redox or base-acid equilibria changed. Soil pH was found to be a crucial factor affecting soil Cd fraction, and carbonate-bound Cd can be significantly affected by both organic matter and pH of soils. The static ecological evaluation models, including potential ecological risk index (PERI), geo-accumulation index (Igeo) and risk assessment code (RAC), revealed a moderate soil Cd contamination and prensented high Cd exposure risk in studied soils. However, the dynamic evaluation of Cd risk, determined using a delayed geochemical hazard (DGH), suggested that our studied soils can be classified as median-risk with a mean probability of 24.79% for Cd DGH. These results provide a better assessment for the risk development of Cd contamination in coal mining areas

Investigation on Structural and Thermodynamic Characteristics of Perhydrous Bituminous Coal by Fourier Transform Infrared Spectroscopy and Thermogravimetry/Mass Spectrometry

Perhydrous bituminous coal has multi-purpose industrial applications determined mainly by its structural and thermodynamic parameters. The Huainan coalfield in China has a large reserve of perhydrous bituminous coal resource. However, little information is available concerning its physicochemical characteristics. In this study, perhydrous bituminous coal samples from number 13-1 and 8 coal seams were selected from 11 Huainan coal mines. The Fourier transform infrared spectroscopy (FTIR) showed that the chemical structures of perhydrous coal were characterized by relatively low aromaticity, implying that the process of hydrogen enrichment during coalification influenced the reactivity of aromatization and condensation of coal macromolecules. The thermogravimetry/mass spectrometry (TG/MS) experiments were conducted at different heating rates (10, 20, and 30 degrees C/min). At the heating rate of 20 degrees C/min, number 13-1 coal seam had similar pyrolysis characteristics as number 8 coal seam. Three pyrolysis stages could be divided at this heating rate, and the main pyrolysis temperature range is 300-600 degrees C. Variation in heating rates mainly affected the primary pyrolysis stage (450-550 degrees C) of perhydrous coals. With the increase of heating rates, the maximum weight loss rate of coal and the releasing rates of gaseous species increased. The second- and third-order reaction parameters fitted better representations of the non-isothermal pyrolysis processes of perhydrous coals from the Pan4 coal mine. The apparent activation energy was calculated with values ranging from 78 to 307 kJ/mol.</p

Thermal behavior and Raman spectral characteristics of step-heatingperhydrous coal: Implications for thermal maturity process

Perhydrous coal (PHC) had been widely investigated; however, a systematic research on the relationship between the structure of PHC and its thermal reactivity was rarely reported in the previous studies. The PHC was widely developed in the Carboniferous-Permian coal-bearing strata in the northern and eastern regions of the Huainan coalfield. In this study, we chose No. 13-1 PHC as the research object. Based on the macroscopic observation of No. 13-1 PHC of the borehole #9-3 and its logging response, a typical PHC sample was selected and pyrolyzed using a thermogravimetric analyzer (TGA) to acquire a set of coal samples after pyrolysis with wide range of random vitrinite reflectance (% VRo) from 0.86 to 7.79%. The thermal evolution process of PHC before and after pyrolysis was investigated by atomic force microscopy (AFM) and Raman spectroscopy. Research results show that: 1) in the experimental temperature range, the reflectance (VRo) range of PHC varied greatly; 2) with the increase of pyrolysis temperature, the Raman spectra of PHC changed significantly in the D band of disordered structure and the G band of ordered structure and the orderliness of its macromolecule particle arrangement was increasing gradually; 3) the relationship between the thermal reactivity of PHC and its Raman structure parameters was established, the FWHM of G-band can be used as indicators of the degree of thermal evolution of PHC; 4) compared with the G FWHM of normal coal with different rank, the thermal reaction temperature of PHC was advanced. Under the premise of comparable reaction temperature, the thermal maturity of PHC was gradually higher than that of bituminous coal, and slightly lower than anthracite, indicating that the actual PHC thermal maturity was suppressed by a higher amount of hydrogen

Comparative study on geochemical characterization of the Carboniferousaluminous argillites from the Huainan Coal Basin, China

Aluminous argillites were widely deposited in the Taiyuan Formation at the Huainan Coalfield at the southeast margin of the North China Plate. However, knowledge about their formation conditions and geochemical characterizations is not presently known. We recovered underground aluminous argillites at depths of 485-610 m from a borehole in the Zhangji Coal Mine and characterized their geochemical parameters, including major and trace elements, by X-ray fluorescence, inductively coupled plasma optical emission spectrometry, and inductively coupled plasma mass spectrometry. The provenance, climatic conditions during the weathering process of parent rocks, weathering extent, and depositional environments of Huainan aluminous argillites were investigated. Results show that Huainan aluminous argillites are depleted in alkalis and alkaline earth elements and enriched in Al, Fe, and Ti. The ratios of immobile trace elements such as Nb/Ta and Zr/Hf are similar in all the argillite samples. The NASC-normalized rare earth element (REE) patterns of the argillites show an enrichment of heavy REEs and depletion of light REEs, with positive Ce and negative Eu anomalies. The provenance analysis indicates that the studied aluminous argillites probably derived from the common parent rocks composed of felsic to intermediate igneous rocks. These argillites were presumably deposited under anoxic environments.</p

Mercury-binding forms in coals and their geological provenances in coals of different types

Mercury (Hg) is an element of environmental and geological significance. Quantification of different Hg-binding forms is crucial to understand geological Hg provenances and associated geochemical processes during coal formation. In this study, seven coal samples were selected, according to coal rank (i.e., middle volatile bituminous, C-3; low volatile bituminous, C-2; anthracite, C-1), chemical anomalies (high S coal, IBC-105; high Cl coal, C22650) and sampling environment (fresh coal, LH; weathered coal LHW), to determine their Hg-binding forms using well-established sequential extraction procedures coupled with sink-float experiment. In the thermally metamorphosed samples C-1 and C-2, a comparative enrichment of total Hg relative to C-3 is observed. Silicate- and organic-bound Hg are the dominant Hg-binding forms in C-1, suggesting possible Hg sources from magma silicate and secondary Hg enrichment by adsorption. Sulfide- and organic-bound Hg are the most abundant Hg-binding forms in IBC-105, whereas only organic-bound Hg dominates in CC22650. Weathering processes are suggested to transform the abundant sulfide-bound Hg in LH to silicate- and organic-bound Hg in its weathering product LHW.</p

Distribution of trace elements in feed coal and combustion residues from two coal-fired power plants at Huainan, Anhui, China

The rapid expansion of coal-fired power plants (CFPPs) in China has produced huge volume of toxic elements associated combustion residues, which pose great threat to local environment. In this study, feed coal, fly ash, bottom ash and FGD gypsum samples were collected from two different CFPPs at Huainan, Anhui, China. Feed coal and combustion residues were morphologically and mineralogically characterized by scanning electron microscopy equipped with energy-dispersive microanalyser and X-ray diffraction. Concentrations of thirteen major and trace elements in these samples were determined by inductively coupled plasma atomic emission spectrometry (B, Ti, Mn, Ni and Zn), inductively coupled plasma mass spectrometry (Cd, Co, Cr, Cu, Pb, Sn and V) and atomic fluorescence spectroscopy (As). The results show that most of the trace elements were concentrated in the fly ash, only Fe and Mn were enriched in the bottom ash. The diameters of ash particles removed by the electrostatic precipitator were in an inverse relationship with the enrichment factors of most trace elements. To address possible exposure of combustion residues in the environment, laboratory controlled leaching tests were carried out on fly ash and gypsum to understand the environmental behaviors of these elements. Both fly ash and gypsum were identified as not-hazardous wastes.</p

Distribution and fate of environmentally sensitive elements (arsenic, mercury, stibium and selenium) in coal-fired power plants at Huainan, Anhui, China

The present study has investigated the distribution of arsenic (As), mercury (Hg), stibium (Sb) and selenium (Se) in feed coal and its combustion by-products in two pulverized coal-fired power plants at Huainan city, Anhui province, China, both of which burned the bituminous coals. Experimental analysis of simultaneously sampled coal, bottom ash, fly ash and FGD products showed that the concentrations of As, Hg, Sb and Se in coal were in the ranges of typical Chinese power plants reported by others publications. Mercury was found to be the most volatile, whereas Sb was the least volatile. The overall material balances of As, Hg, Sb and Se were established based on the operation parameters of their respective boiler. The average removal efficiencies of As, Hg, Sb and Se by the ESP unit were 83%, 16%, 100%, and 72%, respectively, whereas those by the FGD process were 61%, 80%, 0% and 55%, respectively. The stack emission proportions of As, Hg and Se were 6%, 17% and 13%, respectively. And the total annual emissions of As, Hg and Se from two coal-fired power plants were estimated at 0.46 t, 0.04 t and 2.27 t, respectively.</p

Influences of magmatic intrusion on the macromolecular and pore structures of coal: Evidences from Raman spectroscopy and atomic force microscopy

Magmatic intrusion into coal-bearing sequences can significantly affect the rank of the intruded coal and change the macromolecular and pore structures. Here, we use Raman microscope and Atomic force microscope (AFM) to obtain quantitative information on these transformations. Six coal samples of different ranks taken from a magmatic intrusion zone of Huainan Coalfield, China, were studied. The Raman spectra were fitted with a combination of 8 Lorentzian bands and 1 Gaussian band. We found that the macromolecular structures in coals of different ranks have significant relationships with Raman spectral parameters, particularly the band area ratios (I-D1/I-G, I-D2/I-G, I-D3/I-G, I-D4/I-G and I-G/I-All). With the increase of coal rank, I-D1/I-G, I-D2/I-G, I-D3/I-G and I-D4/I-G show trends of decreasing intensity, suggesting an enhanced orientation of aromatic hydrocarbons. Observations by Atomic force microscopy indicate that the pore parameters (e. g. amount, size, shape) are quite different for different ranks of coals. The pore size of low-rank bituminous coal is much larger than high-rank anthracite and pore abundance is higher in the former. In addition, there is also good correspondence between the surface topography of coal grains and coal rank.</p

Variation of Hg content in low sulfur coals in relation to the coal-forming environment: a case study from Zhuji Coal Mine, Huainan Coalfield, North China

Twenty-nine low sulfur coal samples were selected to determine the magnitude and variability of mercury (Hg) content in a well-documented stratigraphy system including ten continuous coal seams in Zhuji Coal Mine, Huainan Coalfield, Anhui Province, North China. Mercury content of samples was measured on a direct mercury analyzer and confident results were obtained as evaluated by standard references, sample replicates and procedural blanks. The calculated overall mine average Hg content is 71.19&thinsp;&plusmn;&thinsp;9.28&nbsp;ng/g based on seam averages and weighting by the estimated reserve of each coal seam. The estimated Hg emission potential for Huainan coalfield is obviously lower than that calculated from coal emission factor in industrial use. An increasing trend of Hg content with the evolution of depositional environment was observed from Nos. 3 to 11-2 coal seams. Combining the evidence of sedimentology and paleontology, a better understanding was gained of the mechanism of Hg sequestration in specific coal benches. A large portion of Hg residing in the low sulfur coals presumably integrated to the functional groups of organic constitution, whereas pyrite was generally abundant in the high sulfur coals.</p

Distribution of environmentally sensitive elements in residential soils near a coal-fired power plant: Potential risks to ecology and children's health

One hundred and twelve soil samples were collected from residential areas surrounding a coal-fired power plant at Huainan City, Anhui Province, China. The concentrations of environmentally sensitive elements (ESEs As, Cd, Cr, Cu, Hg, Mn, Ni, Pb, V and Zn) in soil samples were determined, and their potential ecological and health risks were assessed. Mean concentrations of ESEs in the downwind soils of the power plant are relatively higher than those in the upwind soils, pointing to a potential ESEs input from coal combustion. The calculated ecological risk of ESEs in soils indicates a relatively low ecological risk. Hazard quotient (HQ) of ESEs in downwind soils is 1.5, suggesting a potential health risk for children. However, the carcinogenic risk values of ESEs in soils are within the acceptable non-hazardous range of 1E-06-1E-04.</p