Research On Ccurrence And Migration Mechanism And Risk Of Heavy Metals By Biomass Combustion From Metal Mine Area

Forest fire is one of the most common disturbance factors in forest ecosystem.For forest areas with high heavy metal background concentration in soil(mining sites,industrial polluted areas,etc.),fire may lead to the re-release and migration of fixed heavy metals due to the accumulation of more metal elements in soil or vegetation,which brings risks to forest plan burning and ecological restoration in mining area.This study took area of Huangshi National Mining Park,Hubei Province,China,as the research object.Vegetation and soil from the park was collected for laboratory simulation combustion.The calorimeter platform was used to carry out mine-park-biomass combustion,only soil heating and co-combustion of biomass and soil experiment.The coupling effect of heat radiation and biomass ash on the physical and chemical properties of soil and the migration and transformation mechanism of heavy metals were identified.In addition,the comprehensive pollution of heavy metals in ash after fire were evaluated,and the potential environmental leaching risk of heavy metals from soil after forest fire were evaluated by toxic leaching experiment(TCLP)and pot experiment.The results are as follows:(1)The combustion products contained a variety of heavy metal,such as As,Cr,Cu,Zn,etc.The contents of As,Cr,Cu and Pb in smoke particles from camphor tree combustion were higher than those of other combustible.BCR’ results showed that the acid soluble and reducible states of As were higher in ash,while Cr mainly existed in oxidizable and residual states.In addition,the proportion of available state of As decreased gradually from 74%(400°C)to 41%(800°C),indicating that the increase of temperature significantly reduced the bioavailability of As.Besides,with the increase of temperature,the concentration for most of heavy metals in smoke particles decreased,while the concentration of most heavy metals in ash increased to varying degrees.Risk assessment results indicated that the presence of heavy metals in ash may cause moderate or higher levels of pollution.The total pollution level of heavy metals in ash of camphor tree branch was higher than that of other combustibles.(2)The coupling effect of ash and thermal radiation significantly changed the physical and chemical properties of mine soil.Secondly,the coupling effect increased the total amount of Cr,As,Cu and Zn in soil,indicating that the thermal radiation and ash generated by biomass combustion were important factors affecting the concentration of heavy metals in mine soil.In addition,the proportion of available state of Cr decreased from 50% to 40%,and that of available state of As increased from 14% to 22% under the coupling effect.While the available state of Cu and Zn was not obvious changed,indicating that the coupling effect inhibited and promoted the migration of Cr and As,respectively.(3)TCLP evaluation showed that the leaching capacity of Cr,As,Cu and Zn in soil increased both only soil heating and co-combustion of soil and biomass.The highest leaching mass fractions of Cr,As,Cu and Zn were 0.88 mg/kg,0.81 mg/kg,654.13 mg/kg and 35.58 mg/kg,respectively,and Cu and Zn exceeded the environmental supervision value.Pot experiment results showed that the addition of biomass ash significantly affected the absorption levels of heavy metals Cr,Cu and Zn in aboveground and underground parts of maize seedlings.Heavy metals were mainly concentrated in the underground part of the plant when ash was not added.With the increase of ash content,the content of Cr and Cu in each part of maize seedlings increased first and then decreased,while the absorption level of Zn decreased in the underground part and increased in the aboverground part of maize seedlings.