| In order to understand the pollution characteristics of As,Hg and Cd elements in atmospheric dust in Xi'an,a total of 13 sampling sites were set in this paper and a total of 156 dust samples were collected monthly from July 2017 to June 2018.The total amount and speciation characteristics of the three elements in the dust samples were analyzed,and the related pollution assessment and risk assessment were carried out.The following results have been drawn,which provide a scientific basis for the formulation of air environmental quality standards and the prevention and control of heavy metal pollution in the atmosphere of Xi'an.(1)The annual mean contents of As,Hg and Cd in atmospheric dust in Xi'an were 10.83,0.19 and 2.57 mg·kg-1,respectively.Among them,the mean value of As was lower than the background value of topsoil in Shaanxi Province,while the values of Hg and Cd were higher than the corresponding background values.In each month,the mean content of As was the highest in August,but compared with other seasons,the content of As was higher in winter,and the variation of As content was affected by seasonal changes to a certain extent.The mean content of Hg was the highest in January and the mean content of Cd in each month changed little.Among all functional areas,the mean content of As was the highest in cultural and educational areas,followed by leisure areas.The mean content of Hg was the highest in the commercial district.Cd content was highest in recreational areas.In different seasons,the mean content of As and Cd in cultural and educational areas and leisure areas was higher than that in other functional areas,while the mean content of Hg in commercial and cultural and educational areas was higher than that in other functional areas.In addition,the accumulation of Hg was mainly related to the daily household waste.Cd was mainly produced by industrial production emissions and traffic activities.On the one hand,As was affected by parent material;on the other hand,the pollution sources of Cd also had a positive effect on the accumulation of As.(2)The annual mean Igeo of As,Hg,and Cd in the atmospheric dust of Xi'an decreased in the order Cd>Hg>As.On the whole,Cd element presented strongly to extremely polluted,while the of Hg was moderately polluted and As was non-polluted.And As element in each season was non-polluted too.The pollution level of Hg was moderately to strongly polluted in winter,and moderately polluted in other three seasons.Cd was at strongly to extremely polluted levels in all four seasons.The annual mean PLI value of the three elements was 5.31,showing that it was very high contamination.In addition,the mean PLI value in spring,summer,autumn and winter showed an upward trend.(3)The BCR speciation analysis showed that the main occurrence form of As and Hg in the annual atmospheric dust in Xi'an was residual form,while the Cd mainly occurred in the form of acid extraction.In each season,the distribution of the As and Hg was residual>oxidizable form>reducible form>acid extraction,but the Cd for the acid extraction>reducible form>oxidizable form>residual form.The PBET simulation test results showed that the bioaccessibility of As in the annual dust was intestinal phase>gastric phase,while Hg and Cd showed gastric phase>intestine phase.The descending order of bioaccessibility of the three elements in gastric and intestine phase showed Cd>As>Hg.In each season,there were also the above characteristics.In addition,the mean bioaccessibility of the three elements in the gastric and intestine phase was different in each season.In the gastric phase,the bioaccessibility of the three elements in spring was greater than that in other seasons.As and Hg had the minimum bioaccessibility in autumn,while Cd had the minimum bioaccessibility in summer.In the intestine phase,As and C d had the same order of bioaccessibility,spring>summer>winter>autumn,Hg was spring>winter>summe>autumn.(4)Redundant analysis and Pearson correlation analysis showed that the bioaccessibility of Cd was greatly affected by its own total content,but the total content of As and Hg had little effect on their own bioaccessibility.And the bioaccessibility of As,Hg and Cd had little relationship with their respective reducible form.In addition,the acid extraction and reducible form of As had great influence on the dissolution process of As in intestinal phase.The reducible form of Hg also had a great influence on its bioaccessibility.The acid extraction,reducible form and oxidizable form of Cd all contributed more to the bioaccessibility of gastric and intestine phase.(5)The results of ecological risk showed that the potential ecological risk level of As element was low,Hg was at a high risk level,and Cd element had a very high ecological risk.The annual mean value of the combined potential ecological risk of the three elements was 1089.01,with a very high ecological risk level,which was mainly caused by Cd(75%).The potential ecological risk levels of each element in each season and the combined potential ecological risk levels of the three elements in each season were the same as those of the whole year,and the combined potential risk levels in winter(1107.83)were the highest.The mean annual RAC risk of the three elements in dust showed that Cd had a high risk,while As and Hg had a low risk.As and Hg were at low risk in each season,and the mean RAC values were spring>summer>autumn>winter and spring>autumn>winter>summer,respectively.While the mean RAC values of Cd was in order of summer>autumn>spring>winter,all of them were at high risk.(6)The health risk results showed that in four exposure scenarios,the noncarcinogenic risk of all kinds of people had the largest exposure concentration>95%UCL exposure concentration>available states of heavy metals>intestinal bioavailability,but all the HI was less than 1,indicating that the non-carcinogenic risk could be ignored.The total carcinogenic risk of As to both meals and females was the same in each exposure situation,between 10'6—10-4,within the acceptable range of humans.Since the carcinogenic risk of Cd was only caused by respiratory pathway,the total carcinogenic risk of Cd was shown male>female,which was far less than 10-6,and almost no carcinogenic risk was generated.In addition,the total carcinogenic risk in all exposure scenarios was almost caused by As element. |
PDF Full Text Request