Chromium leaching mechanism of coal mine water——a modeling study based on Xuzhou-Datun coal mine dis

藏等9lineatwww.sclencedirect.comMININGScience DirectSCIENCE ANDTECHNOLOGYELSEVIERMining Science and Technology 20(2010)0097-0102www.elsevier.com/locate/jcumtChromium leaching mechanism of coal mine watera modeling study based on Xuzhou-Datun coal mine districtSHAN Yao, QIN Yong, WANG WenferSchool of Resource and Earth Science, China University of Mining Technology Xuzhou 221116, ChinaAbstract: In order to investigate chromium contamination of coal mine water, to analyze chromium leaching mechanism and toevaluate environment pollution potential of coal mine water, we perform site investigations, physical and computer modeling in theXuzhou- Datun coal mine district. The result of our test samples shows that chromium concentration was 9 ug/L in roof leachate and3 ug/L in coal leachate. The host rock has a higher pollution potential than that of coal seams. Leaching experiments and XRD testresults indicate that chromium is released from the process of transforming illite to kaolinite. The pH, pe and temperature of coalmine water affect the chromium leaching behavior. Modeling results suggest that the adsorption of adsorbents controls chromiumoncentration in coal mine water. The chromium adsorption ratio is quite low in both an acid and in an alkaline environmentTherefore, coal mine water has a high pollution potential. Under other conditions, chromium adsorption is stronger in a neutralwater environment, so that chromium concentrations may be very lowKeywords: chromium; coal mine water; leaching1 Introductioning and found that 40% of its chromium wasleached. That led to the discovery of the associationChromium is an essential element in the human of chromium and clay. Huggins et al. studied chro-body and can be used in treating resistance to insu- mium in American coal and found two major in-lin.An appropriate supply of chromium is therefore stances of its occurrence, i. e, in clay and in associa-helpful to treat diabetes. On the other hand, an exces- tion with organic matter!. The chromium, largelysive intake of chromium may hurt human organs. associated with clay minerals, is found in illite,ac-Kidneys may be damaged when people are in contact counting for between 10% and 50% of all chromium.with or overdose on chromium compounds, leading The percentage of chromium found in organic asso-to a change in renal functions, in enzymes and protein ciation in coal ranges from about 50% to 90% of allconcentration. Kidney necrosis can occur in cases of chromium amounts, difficult to leach. Song in a studyheavy overdoses". In a wide range of industrial op- of coal geochemistry in the north of Shanxi Province,erations, such as electroplating, stainless steel pro- argued that chromium is found mainly in illite!l .Liduction and catalytic processes, large amounts of et al. and Dai et al. thought the main chromium oc-toxic chromium compounds are generated and then currence in coal was in association with clay and finedischarged into the environment 5-ol. Although chro- particle minerals2-131. Both teams used sequentialmium is less toxic than lead and mercury, the extraction and statistical analyseshexavalent form of chromium has been shown to beIn coal mine water, minerals may be transformedcarcinogenic.So, understanding the chromiumand release chromium under suitable conditions oftransformation behavior and mechanism is of real pH, pe and temperature. Mineral-water interaction isimportance in environmental protection.an important process that releases toxic metals intoThe occurrence of chromium in coal seams has the environment and endangers human health andreceived worldwide attention for its environment im- environmental health alike. This interaction processpact. The chromium in coal seams may occur in or- includes a series of equilibrium and kinetic reactionsganic substances, clay minerals and some detrital between water-rock and water-coal, i. e, in oxidation-minerals. Finkelman et al. investigated coal leach- reduction, dissolution-precipitation, adsorption-desorpchamao the dinsolution-precipitaeceived 23 June 2009: 12 September 2009中国煤化工important roles inCorresponding author. Tel: 8651683590091the bCNMHGAE-mail address: yaoshan@ gmaiL.cordoi:10.10l6S16745264(09)60168Xas clay, iron and manganese oxidized minerals mayScience and TechnologyVol 20 No. 1be the controlling factor that affect the geochemical concentrations in coal mine water and to study poten-behavior of elements4!. In industrial processes, a tial controlling geochemical mechanisms empiricallyvariety of materials such as red mud, activated carbon, and through modeling analysisalumina silica, lignite, low-grade manganese ore,crushed coconut shell, rice husks and sawdust2 Study areaused as adsorbents for the removal of chromium'This study was conducted in the Datun coal mineOf this material, iron oxidized minerals are usually district of Xuzhou, in northern Jiangsu province. Itthe strongest adsorbents 6covers an area of 400 km"( Fig. 1)The aim of this project was to assess chromiumnFig. 1 Location of study areaThis area is covered by a quaternary layer, 67 to aquifers, one in the Lower-Shihezi formation and the305 m thick. A series of sediment strata, shown in Fig. other above the coal seam in the Shanxi formation2, cover the Archeozoic system. From the bottom to The fifth aquifer is a limestone aquifer in the Taiyuanthe top stratum, these are layers of the Sinian, Cam- formation of the Carboniferous layer and the last aqbrian, middle-lower Ordovician, middle-upper Car- uifer is also a limestone aquifer but in the Ordovicianboniferous, Permian, Jurassic, Cretaceous, Tertiary layer. According to a coal mine report, the aquifers inand Quaternary system.both the Ordovician layer(600 m thick) and the Car-There are six aquifers in the sediment strata(Fig. boniferous Taiyuan formation(180-200 m thick)2). a grit aquifer in the quaternary layer, a conglom- the main water sources of the coal seam.erate rock aquifer in the Jurassic layer, two sandstoneJurassic laverTertiary layerCretaceous layoal seamJurassic laOrdovician layerSinian layerCoal seamCarboniferous layer/ Ordovician layer-Archeozoic layer6. Limestonea Coal mines1000mFig 2 Study area with detailed geology and location of aquifers3 MethodfromA number nf them are listed in中国煤化工 ected in1000mL3.1 Sampling and testsNalgeCNMHd-cleaned by 3%HNollecting the sam-Over the period from March 2006 to June 2007, wples. The pe and pH of water samples were measuredcollected 48 water samples and 30 rock/coal samples by a JENCO 6010 pH/ORP meter in the field. CoalSHAN Yao et alChromium leaching mechanism of coand rock samples were collected from the working lected using syringes after 2, 6, 24 and 48h. Somearea and put into plastic bags which were immedi solutions were collected also after 5 and 10 days. Aately closed.solution of 3% HNO, was added to all the samples toTable 1 List of the collected sampleskeep the elements dissolvedThe solid samples used in the experiments are coalLocationample type Coal mine layerand host rock from coal mine roofs and floors. theleaching water is surface water and deionised water.Surface water SurfaceThe surface water was collected from Zhaoyang LakeSanthejian coal mine, Xuzhou Carbonate water aquifers in Ordovician with ion strength of 24.6 mmol/L. The deionised wa-Sanhejian coal mine, Xuzhou Carbonate water (aquifers 4)0a]yuan formatioter had a resistance of 18.2 mQ2. We set different PH,pe and temperature levels in the leaching experimentshejian coal mine, Xuzhou Coal mine water Shanxi formation(No 9)in order to analyze the leaching behavior of the ele-Sanhejian coal mine, Xuzhou Coal mine water Shanxi formation(No 9) ments. HCl and NaOH were added to the deionisedunhejian coal mine, xuzhou Leaching water Shanxi formation (No 9) water to titrate the pH of 2, 5.6 and 12 in order toYaoqiao coal mine, Datun Leaching water Shanxi formation(No. 7 compare the leaching behavior of elements in an acid,Sanhejian coal mine, Xuzhou CoalShanxi formation (No 9) neutral and alkali environment. The pe of the experiLongdong coal mine, DatunShanxi formation (No 9) ments was controlled by sealing the flask with a corkYaoqiao coal mine, DatunShanxi formation(No.7) to stop oxygen exchanges. Some experiments wereSanhejian coal mine, XuzhouShanxi formation(No9) carried out when the flasks were sealed with a cork toLongdong coal mine, Datun Roofhanxi formation(No9) simulate the experimental system in a closed envi-Yaoqiao coal mine, Datun RoofShanxi formation (No. 7) ronment, while other experiments were conducted inSanhejian coal mine, Xuzhou FloorShanxi formation(No 9)the open. The temperature was controlled by a waterYaoqiao coal mine, Datun Floorbath to keep it at 40C or air temperature, i.e., 15C.The pH and pe of the experimental system wereWe tested for the major ions of the water samples tested by the JENCO 6010 pH/ORP meter which wasat the Jiangsu Provincial Coal Research Institute, fol- also used in the field study.lowing Chinese standards (GB/T 6920-86, GBT8358GB/T8358,GB190489,GB11905-8,Hr33 Modeling345-2007,HT195-2005,HJ198-2005,GBAs a supporting analytical tool, geochemistry13196-91, GB 11896-89) The mineral content of solid modeling is widely used in the area of environmentalsamples were tested by XRD(Philips Pw 1830 dif- geochemistry and environmental engineering(zhaofractomoter system using Cu Ka radiation) in the et al. and Sun et al. ) In order to analyze theImperial College London. Before the test, I g samples mechanism of chromium leaching from coal minewere crushed in an agate ball mill, and then crushed water, we modeled the adsorption behavior of chro-in a rod(Micronising Mill) with water or ethanol to mium using the software of MIntEQ (version 2. 53),form a fine suspension; this was then mounted on a si and Geochemist workbench(version 7.0)wafer and driedThere are two main aspects which control chro-Trace elements of water/coal/rock samples were mium behavior, i.e., dissolution-precipitation andtested by ICP-MS at the China University of Mining adsorption-desorption equilibrium. We carried outand Technology. The type of ICP-MS was an X-series two types of modeling: the first one was to calculatemade by the Thermo Electron Co. USA. Forward the element speciation in the solutions, while thepower was 1200 w. Nebuliser, auxiliary and coolant other one was to simulate chromium adsorption be-argon speeds are 0.80 L/min, 0.75 L/min and 13. 50 havior. The leaching mechanism is discussed in sec-L/min, respectively. The internal Rh standard was tion 3, combining the leaching experiments and modused to determine the detection limit(0.5 pg/mL) and eling resultserror range(less than 2%).The chromium speciation was calculated byGWB. We set some parameters in the modeling, in-3.2 Leaching experimentscluding a 1000 mL solution with ion strength of 0.1Leaching experiments were conducted using batch mol/L, a chromium concentration of 1.7x10/Lmode to simulate coal mine water conditionsand some other major elements, based on the coaldifferent values of pH, open/closed environment andmine water test results. The temperature of the envi-temperature in a coal seam where water movement is ronment was set at 40Cslow and dissolution and adsorption reactions werechromium adsorption behaviorset to be in equilibrium.中国煤化工 d in the modelingt the beginning of the experiments, deionizedum concentrationwater was added to 1000 mL water, including surfacewasCaMH GHfo amount at 0.1water,in 1500 mL-bottles and mixed with 30 g of g. The adsorption ratio on the Hfo surface was calcurock or coal powder(120 mesh). Solutions were col-lated for different ph and pe levels. InMining Science and TechnologyVoL20 No. 1we used the Diffuse Layer/Surface Complexation boniferous and Permian coal of northern China has 15model(DU/SC) as suggested by Dzombak et al. and ug/g of chromium. Dai et al. concluded theet alwe used 600 m/g as a specific surface of the Hfo, a coal should be 15.35 wgg and reported that themmog strong adsorption sitelloy ite and a 0.056chromium concentration in Late-Paleozoic layer ofnorthern China area is 14.98 Hg/g 3). In our test(Ta-ble 2), the chromium content in the study area ranged4 Results and discussionbetween 6 and 30 Hgg and in the coal seam roof be-tween 60 and 110 ug/g. We conclude that the chro-4.1 Chromium concentrations in the study areamium content in the study area is slightly higher thanThe average amount of crust in Chinese coal is 100 the average level in China and roofs and floors haugg and that of chromium 12 ug/g, while the Car-igher concentrations than coal seamsTable 2 Cr concentration in samplesSample No. Cr content(ug/mL) Sample No. Cr content(ug/mL)I Sample No. Cr content(ug/mL) Sample No. Cr content(ug/mL)1.7567836o126.78Sample tests in the field showed that the chromium. chromium concentration and lesser amounts in alka-concentrations in surface water were 3.0 and 1.7 line and neutral leachates. In surface water, the chro-ug/mL (samples I and 2), in carbonated water 2.3 and mium concentration decreased slightly at first and2.6 Hg/mL (samples 3 and 4), in coal leaching water then remained relatively stable3.2 and 3.6 ug/mL(samples 5 and 6) and in waterOur results are comparable with those of theleaching from the roof 8.8 and 9.3 ug/mL(samples 7 ies by Wang et al. and Song 44. They studiedand 8). The result shows that the chromium concen- leaching behavior of coal, bottom ash and fly ash fortrations are slightly different between surface water, 60 h and found that chromium is easier released incarbonated water and in coal leaching water. The water in an acid environment. The results explainconcentrations are much higher in roof leaching water, why acid coal mine water has a high level of chro-which suggests that the leaching occurs mainly in the mium. On the other hand, the results also indicatedhost rock of the coal seamsthat more chromium could be released in an alkaline4.2 Leaching behavior of chromiumenvironment, which needs further analysisIn considering the solid samples, the experimentsFig 3 shows the chromium leaching behavior of showed that the chromium concentrations in the hostthe host rock under different pH levels in the leaching rock before and after the experiments were 104 ug/g,experiment. Four types of leaching water have been 63 Hg/g(pH=2) and 89 ug/g(pH=12) respectivelyused,i.e,, surface water(with ion strength of 24.6 This shows that chromium leached from the solidmmol/L and an initial pH of 8.5)and three levels of samples.deionized water with initial pH of 2, 5.6 and 12. The Fig 4 shows the XRD results from the roof sam-temperature during the experiment was set at 40C. ples before and after leaching for deionised water atoh levels of 2 and 12. where curve a shows theInitial pH=2original mineral composition in the samples, whilecurves B and C show the samples after leaching usingacid and alkaline deionised water. As the XRd testresults show(Fig. 4), the original sample has 17%illite and 28% kaolinite. The samples leached by acidhave average amounts of 12%o illite and 35% kaolinite---.Initial pH5.6The samples leached by alkaline have 15% illite and29% kaolinic. The result shows that the amount ofTime(hour)illite declined and that of kaolinite increased duringFig 3 Results of experiments of chromium leachingxperiment.Combing previous studies for arguments that theWhen using deionised water, the chromium con- main occurrence of chromium in coal seams is ancentration in leachate increased during the first fourto six hours and then decreased. The chromium con-中国煤化工 found that illite-in coal mine water.centrations in the 48 h samples were 4.528 Hg/L WeCNMHGing process,illite(acid), 0.118 Hg/L(neutral)and 2.038 Hg/L(alkaline). transforms to kaolinite and that chromium is releasedThe result shows that acid leachate has the highestin the process of mineral transformation from illite toChromium leaching mechanism of coal mine waterkaolinite and the reason why chromium concentration charges positive electricity at low pH levels by pro-is higher in roof leaching water than that in coal tonation, so adsorbents adsorb Cro4- and desorb Cr+.In an alkaline environment the adsorbent surface is adis-protonation and charges negative electricity, sothe adsorption ability of Cris enhanced and that ofCrO4 reducedWe modeled the CrOA adsorption behavior on theHfo surface using MINTEQ, as shown in Fig. 6.shown, CrO4 has weak adsorption ability in an acid减人入environment but this becomes stronger at higher pHlevels. When the pH is less than 7, CrO4 can be fornearly 100% adsorbed. As the ph increases, its adsorption ability decreases sharply. The curve shows以uthat adsorption accounted for 13. 4% at a pH level of8.6 and almost 0% at pH 12. These results are quite0203040506070similar to the results obtained by Sujata Mallick et12. They found that CrO42 adsorption achieved aFig 4 Chromium concentrations in roof samples before andmaximum value on manganese nodule surfaces whenfter leaching experiment-XRD resultsthe pH of the solution was 3 and declined with anincreasing pH. The adsorption of CrO4 can be4.3 Adsorptionglected if the pH is higher than 9.The leaching experiments indicate that adsorptionffects or controls the chromium occurrence and migration behavior 4. Therefore, the study of adsorp-tion modeling is used to analyze the chromium adsorption behavior and its mechanisnChromium has two main levels of speciation incoal mine water, i.e., Cr and CrO4. Fig. 5 showsthe chromium speciation in coal mine water calcu4006007.008008.20840860lated by the GWB. The main speciation is Cr+in theacid environment. CrCI is a type of complex speciFig. 6 Modeling result of the ratio of chromiumation if (cI] is high in coal mine water. CrO4 be-adsorption vs PH using MINTEQcomes the main speciation in an alkaline environ-The modeling results explain the chromium leaching behavior, i.e. i) in an acid environment, CrO42Acid environmenthas a strong adsorption ability while that of Cr+isweak: Cr" is the main speciation. Therefore, thechromium adsorption is weak at low pH and thechromium concentration in the leaching water is highii)in a neutral environment, both speciations havestrong adsorption ability. So, most of the chromiumcan be adsorbed, which leads to low chromium concentrations in the leachate. Hence, the concentration10in surface water leachate decreased duing experiments; ii) in an alkaline environment, the68101214Cr adsorption ability is enhanced and that of CrO4decreases; in our case, the Cro4 became the mainFig 5 pH-pe diagram of chromium species inspeciation in the solution. Therefore, the chromiumconcentration in alkaline solutions can be high. In ourstudy, the ph of coal mine water was around 8.5,Cr*and CrO4- express different adsorption be- hence the adsorption of CrO42-was about 20%achavior. The adsorption ability of Cr*is stronger in an cording to our modeling results, which lead to a highalkaline solution than in an acid solution for charging chrorntration in the water of coal minespositive electricity; CrO4 shows the opposite beIn中国煤化工 ainage may havehavior. Gulsin Arslan et al. studied the CrO4 adsorp- relatition and found that CrO4 is easily adsorpted in an willCNMHGhown in our studyacid environment. This phenomenon can be explained the chiconcentration increases with increasingby electrostatic theorhe adsorbent surface ph in alkaline coal mine water, which may be lessScience and Technologyvol20 No. 1than that of the acid mine drainage. However, thepacts of coal. Intenational Journal of Coal Geologymain speciation in an alkaline environment is Cro1999(40):91-10The toxicity of +6 valance chromium can be 1000 [10] Huggins FE, Shah N, Huffman G P, Kolker A, crowleytimes greater than that of +3 valance chromium 2S Palmer C A. Finkelman R B. Mode of occurrence ofHence. chromium contamination in alkaline coalchromium in four US coals. Fuel Processing Technology,000,63(2):79-92.mine water should attract more attention[11] Song D Y. Buring-Leaching Migration Behavior ofHarmful Trace Elements in Coal and Its Environment5 ConclusionsEfect [Ph D dissertation], 2003. (In Chinese)I)Chromium concentration in roof leaching water [12] LiD H, Tang Y G Geological genesis of coal geocal anomalies of the late Permian coals of the Qiis higher than that in coal leaching water.2)Chromium leaching is controlled by the ph of2005,51(2):163-168.( in Chinese)coal mine water. Chromium is easiest to leach in an [13] Dai S F, Ren d Y, liu jR, Li SS Occurrence and dis-acid environment, followed by alkaline and neutralbution of minor toxic elements in coals of the ferfeng coalfield, Hebei province, North China. Journal of3)Chromium in coal mine water is released from358-362. (In Chinese)the transformation process of illite to kaolinite.[14]Li D, He X W, Wang CR, Li Y, Shao L N. Study of the4) The leaching behavior is related to chromiumreclamation of coal mine waste water with high turbidity,adsorption on adsorbent surfaces. Cr+tends to behigh iron and high manganese concentration. Journal ofsouped in an acid environment and adsorped in anChina University of Mining Technology, 2008, 37(1)alkaline environemt; the behavior of CrO4 is just the125-128. (In Chinese)opposite. The main chromium speciation in an acid [15] Mallick S, Dash S S, Parida K M. Adsorption ofenvironment is Crand CrO4 2-in the alkaline, whichhexavalent chromium on manganese nodule leachedresidue obtained from NH3-SO2 leaching Journal ofleads to high chromium concentrations in acid andColloid and interface Science, 2006(297):419-425.alkaline coal mine water[16] Comelis g Johnson C A, Van g T Leaching mechanismof oxyanionic metalloid and metal species in alkalineAcknowledgements23(5):955-976.Science Foundation of China (Nos 40572095, [17] Zhao F H, Sun H F, Li w S Migration of hazardouselements in acid coal mine drainage. 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