Geochemical Characteristics And Potential Risks Of Heavy Metals In Desulfurization Gypsum From Coal-fired Power Plants

China has rich coal reserves,coal-fired power generation will be the main way of power generation for a long time.With the development of society and economy,the output of FGD(flue gas desulphurization)gypsum has increased every year.By 2014,the annual output of FGD gypsum has reached over 70million tons.Due to the low utilization rate,a large amount of FGD gypsum is in storage,the accumulated amount of FGD gypsum is huge.The release of heavy metals contained in FGD gypsum during the disposal,storage and recycling is a potential threat to the environment.During the disposal,storage and recycling of FGD gypsum,it will encounter different conditions in the natural environment,such as rain leaching and sunlight,high temperature heating during calcination,and therefore it is necessary to understand the total level,mobility and stability of the pollutants in FGD gypsum.China has a vast territory and also many power plants,regional differences in resources and technology may lead to differnence of the total level and mobility of the pollution in FGD gspsum from different regions.We collected the FGD gypsum samples from 70 power plants in 20 provinces and systematically analyzed the pollution level of Hg,As,Pb,Cd,Cr,Mn,Zn and Ni,as well as their mobility and stability in the FGD gypsum samples,used groundwater migragtion,particle dispersion simulation modules to estimate the health risks of heavy metals in FGD gypsum to the recepotrs,the main results and conclusions are as follows:The total concentration of mercury Hg in FGD gypsum ranged from ND to4330μg/kg,the average concentration was 868μg/kg,the median 642μg/kg.The total Hg content in FGD gypsum from Shanxi province and central region is higher than that from other areas.The total As content in the FGD gypsum ranged from 8.81 to 102.77 mg/kg,the median value was 13.98 mg/kg and the average concentration was 17.10 mg/kg.Most of the samples(78%)had As content of 10-20 mg/kg.The content of As in the samples from western and northern part of China was higher,there were no big difference of As content from different part of China.The concentrations of Zn,Cr and Mn in the desulphurized gypsum from Shanxi Province were found to be between 40.10and 96.05 mg/kg,12.92-61.13 mg/kg and 6.50-56.14 mg/kg,respectively,while the concentrations of Ni,Pb and Cd in the range of 0.70-31.72 mg/kg,0.011-13.27 mg/kg and ND-1.62 mg/kg,respectively.The average concentration of heavy metals followed the order of Zn>Cr>Mn>Ni>Pb>Cd.The chemical extraction tests revealed that the mercury in the FGD gypsum was mainly in the strong complex phase,and the regional difference was not significant.Samples from west part of China had higher percentage of F1,F2and F3,which were potentially available for organisms in the environment.Sequential extraction tests discovered that that arsenic was mainly present in the FGD gypsum as F5(residue phase)and Shanxi province and central regions of China has higher percentration of F5 in FGD gypsum,while FGD gypsum from western part has relatively higher F1-F4 percentages which will more available by the environment.Sequential extraction tests revealed that heavy metals stayed as different phase in different FGD gypsum samples,the bioavailability of heavy metals in FGD gypsum was in the order of Mn>Zn>Cd>Cr>Pb>Ni,while the mobility potential followed the order of Cd>Mn>Ni>Pb>Zn>Cr.RAC asessement found that all samples showed environmental risks according to the chemical speciation distribution of heavy metals in FGD gypsum.The estimation of annual release Hg and As from FGD gypsum during leaching process were 1.45±0.36 tons and 1.58±0.57 tons respectively.And Zn,Cr,Cd,Pb,Ni and Mn from FGD gypsum in Shanxi province were 131.8±30.9,50.6±12.6,0.7±0.4,4.7±1.1,6.9±1.6 and 74.4±14.4 tons,respectively.The leaching amount of Hg in the TCLP experiment were in the range of ND-19μg/L,and SPLP ND-16.02μg/L.MEP test results revealed that Hg concentration in extracted solutions descreased as a function of time and up to9.7%of Hg was released.The leaching amount of As in the TCLP experiment were in the range of ND-205.20μg/L,and SPLP ND-16.02μg/L,there were only 9 samples which had dectable As in the SPLP extracted solutions.Long-term leaching test found that up to 11.1%of the total As was released.Leaching toxicity tests for heavy metals in FGD gypsum from Shanxi province revealed that higher Mn and Zn concentration in extracted solutions,which suggested high environmental risks.The difference between concentrations of Pb,Cr and Ni were small.Zn and Mn in SPLP extracted solution were higher than other elements,and Cd and Cr concentration from SPLP tests were higher than those from TCLP test,which meant that Cr and Cd was more affected by acid rain.The concentration of six element in extracted solutions from TCLP and SPLP tests exceeded different classes of environmental quality standard for surface water.The Hg evaporation from FGD gypsum at the room temperature and the calcinling temperatures showed that at room temperature,the evaporation of mercury from FGD gsypum was up to 50%of total amount,while F3(organic chelated phase)and F5(residue phase)were the main conctributors during the evaporation.,At the high temperature conditions,large amount of mercury release was happened at 150 to 300℃.In this study,health risk assessment was applied with groundwater migration module and wind erosion module,non-cancer and cancer risks were calculated.Health risk assessment for heavy metals in FGD gypsum under scenarios of ground water migration showed that noncancer risks mainly from As,Pb,Hg,Mn in extracted solution.Specifcally Hg was the main contributor.When the Hg concentration in extracted sltion below 0.005mg/L,there was no noncaner risk from both single element and the sum of total heavy metals for all the simulation scenario.Under the land treatment scenario cancer risk from intake of contaminated drinking water come from As in the extracted solution,when the As concentration was higher than 0.0014 mg/L,there was no cancer risk for all the simuation scenarios.While 60%of FGD gypsum samples studied posed no cancer and noncaner risk to the receptors under the groundwater migration simulation scenario.Noncancer risk resulted from heavy metals tranfered with FGD gypsum particles dispersion showed that As was the main contributor for both noncancer and cancer risks.When As concentration in FGD gypsum below 20 mg/kg noncancer risk and cancer risk were all below the treshhold values.While 87%of FGD gypsum samples posed no cancer and noncaner risk from heavy metals in FGD gypsum to the receptors due to the FGD gypsum particle dispersion.