Distribution And Leaching Characteristic Of Environmentally Sensitive Trace Elements In Coal Ash

Coal ash is the combustion product of coal,coal gangue and slime in coal-fired power plant,it is one of the main sources of industrial solid waste in China.Trace elements in coal ash leach out easily with natural weathering and leaching,which have certain impacts on surrounding ecological environment.Samples of non-weathering ash from coal-fired power plant and weathering ash from waste landfill in Huaibei city were collected.The characteristics,content and occurrence state,and dynamic leaching rates of trace elements in coal ash were determined by using sequential chemical experiments and precise instrumental tests.The results showed as follows:(1)The granularities of fly ash ranged from 1μm to 100μm,the granularities of bottom ash were above 250μm.The contents of moisture in coal ash ranged from 0.14%to 3.46%,and the contents of fixed carbon were low,which ranged from 0.04%to 0.46%.The total contents of sulfur ranged from 0.03%to 1.42%,so the samples belonged to low-sulfur ash.The major components of sulfur in coal ash were sulfate and organic state.The main minerals of coal ash were quartz,silinaite,moganite,berlinite,seamanite,borcarite,amesite,callaghanite,chlorite,callaghanite,goethite,srebrodolskite,and so on.Among them,the content of quartz was the highest,accounting for more than 60%.(2)The contents of Mn and V in coal ash were the highest,and the content of Hg was the lowest.The contents of trace elements were related to the type of samples,and the contents of As,Co,Cr,Cu,Mn,Ni,Pb,V,Zn decreased with the increase of weathering time,but the content of Hg increased instead.Compared with feed coal,trace elements in coal ash were relatively enriched,the average content of Hg was 3.73 times of feed coal in coal ash.As,Co,Cr,Cu,Hg,Ni,Pb,V and Zn were more likely to gather in fly ash,but Mn was more easily gathered in bottom ash.Compared with the backgrounds of soil environment in Huaibei city,three elements such as Hg,Cu and V were seriously enriched in coal ash,which were 138,3.53 and 2.61 times of the background values,respectively.(3)Trace elements in coal ash had various occurrence states,Co,Cr,Cu,Ni,Pb and Zn in residual state were the highest,and the other forms were significantly different,indicating most of these elements were in stable form,but a part of active form(exchangeable and carbonate fractions)of trace elements would migrate and transform in surrounding environment.Trace elements in coal ash were closely related to sulfide minerals.As,Co,Cr,Cu,Mn,Ni,Pb,V and Zn released gradually due to the decomposition of low-sulfur minerals with low lattice energy.(4)Leaching rates of trace elements reduced with the extend of time and decrease of pH value,but As,Pb and Zn increased in alkaline leaching liquid(pH=8.36).In different conditions,the leaching rates of Cr and As were the highest.The weathering degree of coal ash had great influences on the precipitation of trace elements,so the leaching rates of Pb and V increased in weathering ash.Leaching behavior of trace elements was closely related to their contents and occurrence states in coal ash.(5)Based on the results of biological toxicity of coal ash,all samples were less likely to cause the toxicity effect.RAC risk assessment showed that Co,Cr,Mn,Pb and V had slight ecological risk,Ni and Zn would have moderate ecological risk,and active forms of As would cause serious ecological risk.Maximum leaching amount of As,Cr and Pb obviously exceeded the corresponding environmental quality standard for groundwater.Therefore,the pollution caused by As,Cr and Pb in low-sulfur coal ash deserved more attention.