Arsenic in Chinese coals- Distribution, modes of occurrence, and environmental effects

ReviewArsenic in Chinese coals:Distribution,modes of occurrence,and environmental effectsYu Kang a ,b ,Guijian Liu a ,b ,⁎,Chen-Lin Chou c ,Ming H.Wong d ,Liugen Zheng a ,Rui Ding aaCAS Key Laboratory of Crust-Mantle Materials and Environment,School of Earth and Space Sciences,University of Science and Technology of China,Hefei,Anhui 230026,China bState Key Laboratory of Loess and Quaternary Geology,Institute of Earth Environment,The Chinese Academy of Sciences,Xi'an,Shaanxi 710075,China cIllinois State Geological Survey (Emeritus),Champaign,IL 61820,USA dCroucher Institute for Environmental Sciences,and Department of Biology,Hong Kong Baptist University,Kowloon Tong,Hong Kong,Chinaa b s t r a c ta r t i c l e i n f o Article history:Received 4May 2011Received in revised form 12October 2011Accepted 13October 2011Available online 10November 2011Keywords:ArsenicChinese coalModes of occurrence Geologic factorsEnvironmental impactArsenic,one of the most hazardous elements occurring in coals,can be released to the environment during coal processing and combustion.Based on the available literature and published results obtained in our lab-oratory,the content,distribution and the modes of occurrence of As in Chinese coals,and its environmental and impacts are reviewed in this article.With the 4763sets of data (from the literature)rearranged,the ar-ithmetic mean As concentration of each province and weighted mean As concentration of the entire country (using the expected coal reserves as the weighting factor)were calculated.The weighted mean As concentra-tion in Chinese coals is 3.18mg/kg,with As concentration increasing from northern China to southern China.The As concentration in coal varies with coal-forming ages and coal ranks.Arsenic has several modes of oc-currence in coals.According to results obtained by other studies and our own experiments,As is mainly as-sociated with mineral matter (such as pyrite and other sul fide minerals)in coals,although a signi ficant amount of arsenic is associated with organic matter.The accumulation of As in coal is controlled by many geological factors during coal-forming processes,including plant decomposition,sedimentary environments,and epigenetic hydrothermal activity.During the combustion of coal,As is released to the air,water,and soil,causing serious environmental pollution.More than 45%of the coal consumed in China is utilized by power plants,and it is estimated that nearly 522tonnes,21tonnes and 252tonnes of As are emitted into the atmo-sphere by industries,residential buildings and coal-fired power plants,respectively,every year.©2011Elsevier B.V.All rights reserved.Contents 1.Introduction ...............................................................22.Abundance and distribution of As in Chinese coals .............................................32.1.Abundance and distribution of As in different provinces .......................................32.2.Abundance of As in coals of different coal-forming ages .......................................52.3.Distribution of As in coals of different ranks ............................................63.Modes of occurrence of As in coals ....................................................63.1.Inorganic occurrence anic occurrence ........................................................74.Geologic factors that in fluence the distribution of As in coals ........................................74.1.Different plant species associated with coal formation ........................................74.2.Coal-forming environment .....................................................74.3.Coali fication ...........................................................84.4.Intrusion of hydrothermal fluids through fractures .........................................85.Environmental effects ..........................................................85.1.Behavior of As in coal combustion .................................................85.2.Primary estimation of As emission from industries and residential coaluse..............................9Science of the Total Environment 412-413(2011)1–13⁎Corresponding author at:CAS Key Laboratory of Crust-Mantle Materials and Environment,School of Earth and Space Sciences,University of Science and Technology of China,Hefei,Anhui 230026,China.Tel.:+865513603714.E-mail address:lgj@ (G.Liu).0048-9697/$–see front matter ©2011Elsevier B.V.All rights reserved.doi:10.1016/j.scitotenv.2011.10.026Contents lists available at SciVerse ScienceDirectScience of the Total Environmentj o u r n a l h o me p a g e :w ww.e l s e v i e r.c om /l o c a t e /s c i t o t e n v5.3.Annual atmospheric As emissions from coal-fired power plants (9)5.4.As contamination in the vicinity of power plants (9)5.5.Acid mine drainage(AMD)and leaching experiments (10)6.Conclusions (10)Acknowledgments (10)References (10)1.IntroductionArsenic is ubiquitous in the environment.It is distributed through-out the rock,soil and aqueous environments,and is also present in trace amounts in living organisms,including human bones and soft tissues(Beyer and Cromartie,1987;Gault et al.,2008;Ng et al., 2003;Wang and Mulligan,2006;Yudovich and Ketris,2005).Arsenic is widely distributed in the earth's crust,with a crustal abundance (CLARKE value)of about2mg/kg(Wedepohl,1995).Arsenic and ar-senide are widely used in industrial foundries,and manufacturing of agriculture and medical products.For instance,As is used as an addi-tive in the alloy for manufacturing high-strength structural iron, abrasion-resistant steel,and storage batteries,as well as pesticides and preservatives.Typical background levels of As in the atmosphere are0.2–1.5ng/m3in rural areas,0.5–3ng/m3in urban areas and b50ng/m3at industrial sites(DG Environment,2000).Arsenic and As compounds have been classified as Group I carcin-ogens(IARC,1987).They can enter the human body through respira-tion,ingestion,and skin absorption(Liu et al.,2007a).Once ingested, As may distribute to various parts of the human body and accumulates in hair and nails,leading to chronic As poisoning(arseniasis)(Belkin et al.,1997a;Finkelman et al.,1999,2002;Huang et al.,2002;Zhang et al., 2000a,b;Zheng et al.,1996,1999;Zhou et al.,2002).Arsenic poisoning may cause non-cancer effects such as hypertension,cardiovascular and cerebrovascular diseases,diabetes mellitus,keratosis and neurotoxicity as well as cancers of skin,lung,bladder and liver.Cancers involving other organs have also been implicated(IPCS,2001).Because of the high toxicity and mobility of As,and the direct health impacts in epi-demic areas,As contamination in air,water,and soil from both geolog-ical and anthropogenic sources and the occurrence,distribution,and mobility of As have received significant attention in recent years.It has become a public environmental health concern(Ng et al.,2003).One of the major sources of As contamination arises from the com-bustion of As-containing coal(Belkin et al.,1997b;Finkelman et al., 1999,2002;Wang and Mulligan,2006;Zheng et al.,1999).Coal is the predominant energy resource in China,with a consumption of 2.59billion tonnes in2007,which accounts for68.7%of the total en-ergy consumption of the whole country(China Statistical Yearbook, 2009),and the consumption has increased to about3.02billion-tonnes in2009(NBS,2009).The annual coal consumption in China during the most recent three decades is shown in Fig1.In addition to China,it is envisaged that coal will continue to be a very important energy source,over the next several decades,in many other develop-ing countries(Finkelman et al.,2002;Wang and Mulligan,2006).Coal contains more than80elements,among which more than20 trace elements are hazardous(Finkelman,1994;Swaine,1990,2000; Swaine and Goodarzi,1995).The hazardous trace elements may be released into the environment during the processes of mining,trans-porting,processing,and usage(Finkelman et al.,2002),and therebyTable1Arsenic concentrations in Chinese coal of different provinces and the whole country, including autonomous regions and the municipality directly under the Central Government.City,province,or autonomous region Mean(mg/kg)Predicted resource(100million tonnes)Number ofsamplesBeijing 1.7586.725Inner Mongolia 2.1412250.4795 Ningxia 2.171721.1134 Xinjiang 2.2118037.371 Qinghai 2.82380.425 Shandong 2.9405.13143 Jiangsu 2.9850.4925 Shanxi 3.663899.18268 Shaanxi 3.692031.119 Hebei 4.9601.3967 Anhui 4.91611.59112 Gansu 5.151428.8720 Henan 5.19919.7156 Heilongjiang 5.67176.1341 Hunan 6.1445.3536 Sichuan a7.38303.791157 Jilin7.5730.0365 Guangdong8.039.114 Hainan8.070.011 Jiangxi8.4740.84383 Guizhou9.071896.9201 Hubei10.03 2.048 Liaoning10.0559.2735 Fujian1225.576 Zhejiang130.448 Guangxi17.6217.6422 Yunnan21.78437.871172 Taiwan25.38 1.84 China9.65b45470.247633.18ca Including Chongqing.b Mean.c Weighted average.Table2Average As concentrations in coal of different countries and the whole world,as well as of the continental crust(units:mg/kg).WeightedaverageArithmeticmeanGeometricmeanNumber ofsamplesReferenceChinese coal 3.189.654763This study Chinese coal51915Chen et al.(2002)Chinese coal 4.71018Cui and Chen,1998Chinese coal276.6 4.2132Ren et al.,1999a,bChinese coal 6.4 4.0297Wang et al.,2006a,bChinese coal 5.0Zheng et al.,2006Americancoal24 6.57676Finkelman,1993Americancoal6.18823Coleman andBragg,1990U.K coal18.1Spears andZheng,1999 Australiancoal2Swaine andGoodarzi,1995 World coal5Valkovic,1983 Uppercontinentalcrust2Wedepohl,1995continentalcrust1.7Wedepohl,19952Y.Kang et al./Science of the Total Environment412-413(2011)1–13pollute the environment.Arsenic compounds(the highly toxic oxides such as As2O3and As2O5)are introduced into the air,water bodies, and soils during coal combustion and pose serious health threats to human beings(Belkin et al.,1997b;Finkelman et al.,1999,2002;Zheng et al.,1999).For example,As poisoning has affected at least3000people in Guizhou Province(Zheng et al.,1996).In Guizhou,chili peppers that are dried over coal-burning stoves in-doors could be a principal source of As poisoning.Fresh chili peppers contain less than1mg/kg As and the chili peppers dried over the oven contain much higher As concentrations.Significant As exposure may also come from other tainted foods,ingestion of dusts,and from inhala-tion of indoor air polluted by As derived from coal combustion.Jin et al. (2003)reported that the population of As-contaminated areas caused by burning coal in Guizhou and Shaanxi province is estimated to be 333,905,of which48,438people are exposed to As poisoning by pollut-ed air and food crops.Guizhou is the most typical area of As contamina-tion in China as well as in the world,while As poisoning is also found in the Qingba area,Shaanxi Province(Jin et al.,2003).Arsenic concentrations in Chinese coals vary greatly from0to 35,037mg/kg(Belkin et al.,1997a,b;Ding et al.,1999).Most previous assessments of As in Chinese coals(Chen et al.,2002;Chen and Tang, 2002;Dai et al.,2004,2005,2010;Ren et al.,1999a,b,2006;Wang et al.,2006a;Zheng et al.,2006)calculated and used the arithmetic mean to evaluate the As levels in China.Ren et al.(1999a,b)estimated that the arithmetic mean of As in Chinese coals was as high as 277mg/kg because he took a super-high As value(32,000mg/kg) into account,although the reserves of such superhigh-As coal are ex-traordinarily small(Dai et al.,2011).The coal in southwest China usu-ally contains abnormally high concentrations of arsenic because of many geological factors(Belkin et al.,1997a,b;Dai et al.,2011;Li et al.,2002;Ren et al.,1999a,b).In this situation,a simple arithmetic mean is inappropriate.A weighting factor should be adopted to esti-mate the mean As concentration.In this study,we took coal reserves in different provinces as the weighting factor.The major aim of this article was to provide a comprehensive re-view of the concentrations of As in coal and associated environmental impacts in China.This involved examination of the available litera-ture,featuring screening and selection of data.Data derived from on-going experiments conducted by the authors of this article were also used as a supplement.More specifically,the major objectives of this review were to investigate(a)As concentrations in Chinese coals;(b)the modes of As occurrence;(c)factors governing the abundance of As in coals;and(d)associated environmental impacts.2.Abundance and distribution of As in Chinese coalsResearch related to As in coal started as early as the middle of the nineteenth century(Yudovich and Ketris,2005).Chinese scientists have studied the abundance and distribution of As in Chinese coals since the1960s(Liu et al.,2002a;Tang and Huang,2000).To deter-mine the As content in coals,many analytical methods were adopted. Liu et al.(2004b)and Wang et al.(2006a,b)used the molybdenum blue spectrophotometric method.Instrumental neutron activation analysis(INAA)was also employed to determine the As concentration (Ren et al.,1999a;Song et al.,2007;Zeng et al.,1998).However,the most commonly used method now is to subject powdered coal sam-ples to wet digestion followed by inductively coupled plasma atomic emission spectrometry(ICP-AES),atomicfluorescence spectrometry (AFS)or high-resolution-/inductively coupled plasma mass spec-trometry(HR-/ICP-MS)because of the high sensitivity of these tech-niques(Dai et al.,2005;Shao et al.,2003;Zeng et al.,2005;Zhao et al.,2008;Zhuang et al.,1998,1999,2003).With the advancement of analytical techniques,data for trace elements,including arsenic, in coal have been updated,especially data for deep-mine coals. Based on the literature data and some data generated from our labo-ratories,4763sets of data related to As in coal from different prov-inces were used to calculate the arithmetic mean and weighted mean As concentrations of the entire country(using the expected coal reserves as the weighting factor).The weighted mean was used to estimate the amount of As emitted by coal combustion in China.2.1.Abundance and distribution of As in different provincesA total of4763sets of data related to As analyses(including34gen-erated from the authors'laboratories)were collected from28provinces in China(except Hong Kong,Macau,Tibet,Shanghai and Tianjin),with the data from the Sichuan and Chongqing provinces combined together (Chen and Tang,2002;Cui and Chen,1998;Dai et al.,2005;Dou et al., 1998;Fan and Fan,2000;Feng et al.,1998;He et al.,2002;Li et al.,1993, 1994;Liu et al.,2003b,2004b;Lu et al.,1995a;Qian and Yang,2003;Ren et al.,1999a,2006;Shao et al.,2003;Wang et al.,2003;Wang et al., 2006a,b;Wu et al.,2004;Xiao,1989;Yang et al.,1983;Yuan,1999; Zeng et al.,1998,2000,2005;Zhang et al.,1992;Zhang et al.,2004a; Zhou,1991;Zhou,1998;Zhuang et al.,1998,1999,2001,2003).FullTable3Average As concentrations in coal of different coal-forming ages.(units:mg/kg).Coal-forming ages Wang et al.,2006a,b Cui and Chen,1998This studyAverage mg/kg Number of samples Average mg/kg Number of samples Average mg/kg Number of samplesEarly Carboniferous 5.832 4.233 4.29 3.4735Middle Carboniferous1 5.681Late Carboniferous 5.3957 1.365 3.21122Early Permian 4.285 2.1249 2.63 3.04334Late Permian 6.558 2.5117 3.83175Early Triassic12.54212.509.4642Late Triassic8.6118.41128.42123Early Jurassic1113 1.358 3.08 6.3071Middle Jurassic 6.934 2.8386 3.98120Late Jurassic8.6228.22408.23262Tertiary1314 2.2167.247.2430Table4Arsenic concentrations in Permian coals of11provinces(units:mg/kg).Data obtainedfrom Huang et al.(2001),Chen et al.(2002),Zhang et al.(2002),Luo(2005)and Luo etal.(2008).Provinces Arithmetic mean Range Number of samplesShanxi 1.500.1–3.260Shaanxi 1.190.60–2.2123Shandong 1.530.40–3.509Henan0.990.34–1.9411Anhui 3.410.80–19.4045Jiangsu 1.000.40–5.206Hunan7.770.8–47.928Guangxi 3.36–1Sichuan20.60810–25.006Chongqing 3.070.77–12.258Guizhou326.090.15–35,037.002733Y.Kang et al./Science of the Total Environment412-413(2011)1–13particulars of the sources of samples were carefully checked in the cited reference to avoid data duplication.The mean As concentra-tions,the expected coal resources,and the number of samples are listed in Table1.The spatial distribution of As concentrations in coals among differ-ent provinces of China is compared.According to the coal industry standard of China(MT/T803,1999),the As concentration in coal is classified into four categories:0–4,4–8,8–25,>25mg/kg.Provinces with different As concentrations in coals can be divided into four groups(Fig.2).The concentration of As in Taiwan is greatest of all 34provinces,and then followed by provinces in South China.North-west China,together with inner Mongolia,Shanxi,Shandong and Jiangsu have low As concentrations.The results show that(1)The concentration of As in Chinese coals increases gradually from north-ern China to southern China,with the value of the north(mostly less than4mg/kg)distinctively lower than that of the south(mostly ranged from4to25mg/kg).(2)Since the amount of coal resources in southern China is much smaller than that in northern China,the weighted mean reflects a low As concentration in coal,e.g.the As con-centration in coals of the two provinces with the largest coal resources, Xinjiang and Inner Mongolia,is2.21and2.14mg/kg,respectively.(3) The coalfields with high As concentrations(up to several hundred mg/ kg)are located in southern China,especially southwestern China.It has been reported that the As concentration in coals in southwestern China is18.2mg/kg(Chen et al.,1985;Chen et al.,1989;Cui and Chen,1998).Ding et al.(2001)analyzed32coal samples from this area,and found that12samples had As values in excess of1000mg/ kg and an anomalous sample contained up to35,000mg/kg As.The geographical distribution of coal resources in China is ex-tremely unbalanced(Tian et al.,2010)and the concentration of As is also different due to variations in the coal-forming plants and the coal-forming geological environments(Zheng et al.,2006).Therefore, merely to calculate the arithmetic mean value is too simplistic,and then leads to an inaccurate description of the As concentration in Chi-nese coals(Dai et al.,2011).Two weighting factors could be adopted here,i.e.the coal reserves and coal production/consumption.A coal production/consumption weighting could reflect the mean concen-tration of Chinese coal in one year,but differs from year to year.The coal reserves weighting is a better approach to the evaluation of a mean As concentration of all exploited and untapped coal resources. Furthermore,the unevenness of the sample distribution in different provinces has to be taken into account because it also will cause data inaccuracies.So,it is necessary to adopt the coal reserves of each province as weighting factor to calculate mean As concentration in Chinese coals.According to Table1,the As concentrations in most Chinese coal-fields ranged from0to10mg/kg.The arithmetic mean and weighted mean of As concentration are9.65mg/kg and3.18mg/kg,respective-ly.The rather high arithmetic mean of9.65mg/kg is caused by the uneven distribution of samples,because the existing literature fo-cused on the high and super-high As regions that greatly increased the proportion of high values.However,the arithmetic mean value obtained is much lower than the arithmetic mean As concentration in American coals(24mg/kg)and Russian coals(25mg/kg)(Finkelman, 1993;Swaine,1990)(Table2).Cui and Chen(1998)and Chen and Tang(2002)calculated the ar-ithmetic mean As concentrations in Chinese coals at4.7mg/kg and 5mg/kg respectively.Wang et al.(2006a)calculated on the basis of 297coal samples and indicated the arithmetic mean As concentration in Chinese coals to be6.4mg/kg,with a geometric mean of4.0±8.5mg/kg.Ren et al.(1999b)took samples with superhigh-As coals (32,000mg/kg)into account and estimated that the arithmeticTable5The worldwide As emission ratio a from coal-fired power plants in ten countries.Country Emitted viafly ash Total emission(solid+gas)Nationwide emission ReferenceRussia16.10%Kizilstein,2002.Bulgaria30%–40%Vassilev and Vassileva,1997.US 3.90%Helble,2000.Spain 1.20%Otero-Rey et al.,2003.China 6.28%195t Luo et al.,2004.China 2.16%Guo et al.,2004.10–20%at900–1000°C and15–30%at>1400°C Yudovich and Ketris,2005.Japan0.56%0.251t Ito et al.,2006.Thailand51mg/MWh Kuprianov and Tanetsakunvatana,2006. Canada0.85%Goodarzi,2006.India 1.60%Reddy et al.,2005.China0.011g/t coal Yi et al.,2008.Estonia0.68g/ton coal Häsänen et al.,1997.Worldwide15–100μg/MJ232–1550t Nriagu and Pacyna,1988. Mathematical model14%Helble,2000.a As emission ratio:a ratio of the total As in coals could be released to the atmosphere through combustion and dust remover.Table6Range and average ash content in different Chinese coals.(Ash%).Location Ranges(%)Arithmetic mean(%)Geometric mean(%)Number of samples ReferenceChina 1.51–82.923.90737This study Huainan,Anhui a13.03–55.9626.6823.2615Authors'work Northern China 2.72–82.2421.2614.5231Song et al.,2007 Southern China11.70–40.0023.3522.115Zhuang et al.,2007 West Shandong 2.80–47.517.3613.6329Querol et al.,1999 Leping,Jiangxi 4.87–82.9728.7423.7446Querol et al.,2001 Huaibei,Anhui 1.51–24.9813.4612.229Zheng et al.,2008 Huainan,Anhui10.70–39.925.425.2508Sun et al.,2010 Huainan,Anhui10.70–20.5014.6814.2111Huang et al.,2001 Yanzhou,Shandong a9.26–24.8913.4612.647Liu et al.,2003d Yanzhou,Shandong a 4.20–60.8118.0215.3246Liu et al.,2004ba From feed coal in power plants.4Y.Kang et al./Science of the Total Environment412-413(2011)1–13mean of As in Chinese coals was as high as277mg/kg(geometric mean4.2mg/kg),based on132coal samples.It is obvious that the weighted mean As concentration in Chinese coals is lower than the arithmetic mean values calculated by other authors(Table2).It is caused by the unevenness of the coal reserves in each province. Inner Mongolia has the largest reserves of coal,but the mean coal As concentration is only2.14mg/kg(Table1).With respect to the coal reserves,85.47%of the total reserves in nine provinces(Beijing, Inner Mongolia,Ningxia,Xinjiang,Qinghai,Shandong,Jiangsu,Shan-xi,Shaanxi)have As concentrations less than4.0mg/kg.Likewise, about72.42%of the total reserves have As concentrations less than 3.0mg/kg(Beijing,Inner Mongolia,Ningxia,Xinjiang,Qinghai,Shan-dong,Jiangsu).So,it is clear that the weighted mean would be more representative than the arithmetic mean,since the inaccuracy brought about by uneven geographical and geological distribution of the As concentration data has been partly removed.2.2.Abundance of As in coals of different coal-forming agesThe As concentration in Chinese coals varies significantly among different coal-forming ages(Table3),with Triassic coals containing the highest(mean:9.46mg/kg),followed by Tertiary coals(mean: 7.24mg/kg),and then Jurassic coals(mean:6.30mg/kg)while Car-boniferous and Permian coals contain rather low concentrations(less than5mg/kg)(Cui and Chen,1998;Wang et al.,2006a).Zheng et al. (2006)found a similar geological order in As concentration of Chinese coals,that is Triassic coals(11.1mg/kg)>Tertiary coals(10.5mg/kg) >Jurassic,Carboniferous,Permian coals(b5mg/kg).The variation of As concentrations in coals of different ages in southwestern China is shown in Fig3.Coals of all geologic ages from southwestern China(Gui-zhou,Sichuan,and Yunnan Provinces)have higher As concentrations than those from other regions in China.The reducing order of As concen-trations was Sinian>Tertiary>Cambrian>Silurian>Late Triassic>Late Permian>Early Permian>Early Jurassic>Early Carbonic,similar to,but not exactly the same as the order of the whole of China.Arsenic concentrations in Permian coals of11provinces were also calculated(Table4).The As concentrations in southern provinces (Hunan,Guangxi,Sichuan,Chongqing and Guizhou)were much higher than those in northern provinces(Shanxi,Shaanxi,Shandong and Henan,mostly b4mg/kg).The enrichment of trace elements in coal is a lengthy and complicated process affected by several geolog-ical factors.Ren et al.(1999a,b)and Dai et al.(2011)pointed out that the Southwest Coal Basin in southwestern China is a fault-controlled coal basin.Hydrothermalfluids(deep circulating and igneous differ-entiated hydrothermalfluids)might lead to a significant increase in As concentration.The enrichment of As in coals of southwest China in different coal-forming ages was controlled by tectonic setting and hydrothermalfluids(Dai et al.,2011;Li et al.,2002;Ren et al., 1999a,b).In addition,although the As concentration is high in south-western China,it varies widely in different coal-mining areas.Chen and Tang(2002)indicated that the As concentrations were signifi-cantly controlled by large geofaults.High-As coals mainly spread in large faults parallel to the axis of anticline and were correlated closely to the mineralization of Au,Hg and Sb(Chen and Tang,2002;Ding et al.,2001;Zheng et al.,1999).In summary,the high-As concentra-tions in coals of southwestern China were formed by several geolog-ical processes,including initial tectonic setting of the coal basin and then the evolution of the geofault and effects of hydrothermalfluids.Zhang et al.(2004b)indicated that Triassic coals contain higher As concentrations than Late Permian coals in southwestern Guizhou.It has been observed that Mesozoic and Cenozoic coals in China gener-ally contain higher As than Late Paleozoic coals,with Early Triassic coals the highest at a mean concentration of12.5mg/kg(Cui and Chen,1998;Wang,2000;Zhang et al.,1999).Li et al.(2002)found that As concentrations in Early Palaeozoic stone coals are far higher than in Late Permian and Late Triassic coals.Zhang et al.(1999)also reported that Permian coals in southern China contain higher As con-centrations(mean:8.3mg/kg)than Carboniferous–Permian coals in northern China(mean:3.0mg/kg).In addition to the geological fac-tors mentioned above,a most important reason is that the main coal-forming plants were different.For example,the coal-forming plants of Early Palaeozoic were algae and other lower sea plants(ma-rine-environment),while the Late Permian and Late Triassic coal-forming plants were higher land plants.The capability of sea plants (algae)to accumulate trace elements was much higher than land plants (Li et al.,2002).The marine-environment itself could also lead to accu-mulation of As in coals,not only because the As concentrations were high in ocean water,but also because the marine coal-formingTable7Assessment of atmospheric As emissions from industrial and residential coal use.Industrial and residential consumption was obtained from China Statistical Yearbook(2004,2005, 2006,2007).Years Industrialconsumption(milliontonnes)Industrial As emission(tonnes)Residentialconsumption(million tonnes)Residential As emission(tonnes)Emission factor a Coal-fired power plant ratio b Emission factor a Coal-fired power plant ratio b20031505.68602.80348.5790.3512.9220.9220041801.35721.17417.0290.4512.9320.9420052024.44810.49468.6796.1313.7522.2520062255.39902.95522.1392.7813.2721.48a Emission factor obtained from Wang et al.,2005.When calculate the industrial emission,we take the washing process into account.According to Wang et al.,2008,about25%of industrial coal would be washed,leaving only40%(1–60%)of total As concentrations before combustion.b Emission ratio from coal-fired power plant data(Table5).Table8Annual coal consumption and assessment of atmospheric As emissions from coal-fired power plants in China during2003to2006.Years Coal production(million tonnes)Coal consumption(million tonnes)Utilized by power plants(million tonnes)Emission of As a(tonnes)Soots released from coal-fired powerplants b(tonnes)Solid-phase Asemissions c(tonnes)20031667.01637.3779.8180.53,128,735204.0 20041992.31936.0919.6212.93,202,496208.8 20052204.72165.61031.0238.73,808,701248.3 20062373.02392.21187.6274.93,864,315252.0a Calculations based on data from China Statistical Yearbook,2004,2005,2006,2007.b Data from:Environmental Statistics Annals(2004,2005,2006).c Calculated basing on solid phase method.5Y.Kang et al./Science of the Total Environment412-413(2011)1–13。