Chemical Speciation Analysis Of Heavy Metals And Risk Assessment In PM_2.5 In Guiyang City

ln recent years,with the increasing number of environmental pollution sources in Guiyang City,such as the rapidly development of industry,the increasing of automobile ownership,and the rapid increase of coal and oil consumption,there are more toxic and harmful substances have been released into the air.The pollutants in the air are very unfavorable to be diluted and diffused due to the environment of Guiyang City,which locates at the intermountain basin,with typical karst landform characteristics,and the breeze and the static windy climate condition.Heavy metals of PM_2.5 can enter the human lungs directly via respiration,serious impact on human health.Heavy metals can exist in different chemical speciation,and its bioavailability in the environment is also different.To study the bioavailability and the potential ecological risk of heavy metals for the environment and the human more accurately,PM_2.5 samples were collected at three selected representative sampling sites with different seasons in Guiyang City.The chemical speciation of Cu,Zn,Mn,Cd,and Pb in PM_2.5samples were investigated using the Fernández and modified BCR sequential extraction procedure.The chemical speciation distribution characteristics,seasonal variation characteristics,and spatial variation characteristic were discussed.The bioavailability were also estimated based on the results of chemical speciation analysis.The potential ecological risk of those five heavy metals in PM_2.5 were assessed by potential ecological risk indexes of H?kanson.And the human health risks of the heavy metals were also evaluated with the USEPA’s human exposure assessment model.In addition,the release process of Cu,Zn,Mn,Cd,and Pb in PM_2.5 were simulated by simulated acid rain experiment,the influence of different p H value of acid rain on the release rate of heavy metals,and the transformation of chemical speciation of heavy metals under the effect of simulated acid rain were investigated.The main conclusions are as follow:1、The chemical speciation distribution of Cu,Mn,Zn,Cd,and Pb in PM_2.5 samples in Guiyang City were different.The chemical speciation results were investigated using the Fernández sequential extraction procedure showed that the Zn,Cd and Pb were found mainly in various forms:the soluble and exchangeable（F₁）,carbonate-bound,oxides-bound and reducible-bound（F₂）forms.The Mn primarily existed in the soluble and exchangeable（F₁）,carbonate-bound,oxides-bound and reducible-bound（F₂）,and residual forms such as silicates（F₄）.The Cu was mainly distributed in the soluble and exchangeable（F₁）,carbonate-bound,oxides-bound and reducible-bound（F₂）,organic-bound,oxidisable-bound and sulphidic-bound（F₃）forms.The chemical speciation results were investigated using the modified BCR sequential extraction procedure showed that more than 80percent of Zn,Cd,and Pb were presented in the acid-soluble fraction.The Mn primarily existed in the acid-soluble fraction and the residual fraction.The Cu was mainly distributed in the acid-soluble fraction and the oxidizable fraction.2、The chemical speciation distribution proportion of heavy metal s were quite different due to the different sampling districts.The chara cteristics of spatial variation were that the easily mobile fractions of he avy metals in Baiyun were the highest,followed by that in Yunyan,an d the fractions in Huaxi were the lowest.Those five heavy metals sho wed the same order of concentration in various chemical fractions wit h a totalamount in all seasons.However,the chemical speciation distri bution proportion of heavy metals was quite different with different sa mpling seasons.The characteristics of seasonal variation were that the ea sily mobile fractions of heavy metals in the heating quarter was higher than that in the non-heating quarter.3、The bioavailability of the heavy metals were investigated using the Fernández and modified BCR sequential extraction procedure.The evaluation results are the same for the two methods.The order of bio availability of the heavy metals was determined as Cd>Zn≈Pb>Mn>Cu.The Cd,Zn and Pb were classified as bioavailable element s due to K>0.8.While the Cu,Mn,were remarked as potential bioa vailable elements because the values of K ranged from 0.2 to 0.8.Th e bioavailability of heavy metals is different in different areas from th e point of spatial variation characteristic.The samples collected at Bai yun show the highest bioavailability of the heavy metals which reveal ed that the serious pollution originated from human activities,and the sa mples collected at Huaxi exhibit the lowest bioavailability of the heav y metals.Meanwhile,the samples collected in the heating quarter displa y higher bioavailability of the heavy metals than that in the non-heati ng quarter.4、Simulated acid rain experimental results showed that the release process of Cu,Pb,and Mn in PM_2.5 samples can be divided into the early rapidly dissolution stage,and the later slowly dissolution stage.The Cd and Zn were released faster than Mn due to the Cd and Zn are more sensitive to hydrogen ions.The release rates of heavy metals are increase with the decrease of the p H value of the acid rain.For the treatment of simulated acid rain,the order of release rates of the heavy metals was Cd>Zn>Mn>Cu>Pb.Most of the released heavy metals are transform from bioavailable fractions.5、The risk assessment of the heavy metals with bioavailability were investigated using the Fernández and modified BCR sequential extraction procedure.The estimation results are the same for the two methods.The results of potential ecological risk assessment indicated that the Mn in PM_2.5of Guiyang City present a low potential ecological hazard level,the Zn display medium ecological hazard level,and the Pb exhibit strong ecological hazard level,while the Cu and Cd reach a serious pollution level.The potential ecological risk of heavy metals in the district of Baiyun were the highest,followed by the district of Yunyan,and the district of Huaxi were the lowest.However,the potential ecological risk are all reach the serious pollution level for all the three sampling sites.The results of health risk assessment indicated that the order of non-carcinogenic risk of the five heavy metals was Mn>Pb>Cu>Cd>Zn.The assessment results showed that there were no non-carcinogenic risk for the Cu,Pb,Cd,and Zn metals via respiration for both children and adults.Mn may pose potential non-carcinogenic risk to human health,especially for children.Cd may have potential carcinogenic risk for adult,especially for adult male.