Pollution,Risk And Source Of Heavy Metal In Indoor And Outdoor Re-Suspended Deposited Dust Of Primary And Secondary School Campuses

In the rapid development of urbanization and industrialization,a large number of heavy metal pollutants are released into the urban environment with human activities such as industrial production,traffic operation and residents’life,and most of the pollutants accumulate in the dust deposited on the impervious interface of the city.Among them,re-suspended deposited dust（RDD）with particle size less than 100μm is repeatedly raised under the action of external forces such as traffic flow,air flow and people flow,and has become an important pollution"source"in the urban environment.As an important urban functional area,urban campus is the main activity place for students in their growth and development period.Heavy metals in RDD deposited on the impermeable interface of campus have potential health hazards to school students,which has attracted extensive attention of scholars at home and abroad in recent years.Usually,school students spend most of their school time indoor,so the heavy metals in campus indoor dust have a more direct impact on their health.However,the current research on heavy metals in urban campus dust is mainly focused on the outdoor dust,while less attention is paid to the heavy metals in indoor dust,and mainly focuses on the pollution of all heavy metals in outdoor bulk dust.The basic scientific problems such as the pollution,risk,source and optimal control of pollution factors of heavy metals in RDD on the impermeable interface between indoor and outdoor campus are still unclear,systematic qualitative and quantitative research is still needed.In view of the sensitivity of primary and secondary school students to pollutants and the research status of heavy metal pollution in urban surface dust,this study takes Xi’an primary and secondary school campuses（specifically divided into four types:primary school,junior high school,senior high school and junior high school,of which junior high school and senior high school share campuses）as the research object,The magnetic susceptibility,particle size,morphology,chemical properties,components and other physical and chemical properties of RDD on the indoor and outdoor impermeable interface of campus and the content of heavy metals in RDD were determined and analyzed.On this basis,the campus heavy metal pollution and potential ecological risk analysis are carried out from indoor and outdoor aspects,focusing on the systematic study of health risk assessment.Finally,combining the experimental results of RDD physical and chemical properties,heavy metal content and content spatial distribution characteristics with the results of multivariate statistics and positive definite matrix receptor model,the sources of heavy metals in campus indoor and outdoor RDD are identified and quantified,and the key pollution sources and pollution elements are determined respectively.The main results and conclusions are as follows:1.The physical and chemical characteristics,heavy metal content distribution characteristics and correlation of RDD on the indoor and outdoor impermeable interface of primary and secondary schools in Xi’an are clarified.In the indoor and outdoor RDD samples on the impermeable interface of primary and secondary schools in Xi’an,the average contents of V,Mn and Ba are close to the background values of corresponding elements in Shaanxi Province,and the average contents of Ni,Co,Cr,Cu,Zn and Pb are 1.3-7.8 times of the background values respectively;The content level of most heavy metals measured in outdoor RDD is lower than that in indoor RDD,but the dispersion of outdoor heavy metal content is greater than that in indoor RDD.Magnetic susceptibility of RDD samples(χ_lf),the proportion of dust with small particle size and the content of heavy metals are high in the traditional industrial areas in the east and west of the city,and low in the cultural and religious areas and development zones in the South and north of the city.The indoor and outdoor RDD of the campus is mainly composed of altered minerals,soil debris,metal pellets,a small amount of tire debris and unknown particles.The morphology of RDD single particles is mainly flake,strip,spherical and irregular.The main chemical components of RDD single particles are Si,AI,CA,O,S,Mg,K,C and Na,but the content in particles with different morphology is different.The proportion of spherical particles and hematite particles in indoor RDD is higher than that in outdoor RDD.2.The pollution levels and differences of indoor and outdoor RDD heavy metals in four types of campus were clarified.In primary school,junior middle school,senior high school and junior-high school,the comprehensive pollution degree of indoor and outdoor RDD heavy metals in primary school campus is the lowest,which is moderate pollution;The comprehensive pollution degrees of indoor and outdoor RDD heavy metals in junior high school,senior high school and junior-high school campuses is high,and they all reach severe pollution.From the perspective of spatial distribution,the comprehensive pollution level of heavy metals in indoor and outdoor RDD from primary and secondary school campuses located in the east and west industrial zones of the city is higher than that in the south and north cultural,educational and development zones of the city.The pollution of Cu,Zn and Pb in the indoor and outdoor RDD of the four types of campus is the most serious,reaching moderate to severe pollution.3.The potential ecological risks of heavy metals in indoor,outdoor and overall RDD of four types of primary and secondary school campuses were systematically evaluated.The comprehensive potential ecological risk of heavy metals in indoor and outdoor RDD of primary and secondary schools in Xi’an is small,but there are some differences between indoor and outdoor potential ecological risks of different types of campuses.The indoor and outdoor comprehensive potential ecological risk of primary school is lower than that of junior middle school,senior high school and junior-high school;the comprehensive potential ecological risks outside the campus of primary school,junior middle school,senior high school and junior high school are lower than those inside.The ecological risks of Pb,Cu and Co in the indoor and outdoor RDD of four types of primary and secondary school campuses are high,especially Pb,which has serious ecological hazards,and the ecological risks of other heavy metals are small.4.The health risks of heavy metals in RDD of primary and secondary schools to different types of students in different scenes were accurately evaluated.Based on the health risk evaluation model of US EPA,the health risk assessment of primary and secondary students in three different scenarios on campus（specifically divided into indoor,outdoor and indoor-outdoor scenarios,in which indoor-outdoor is the combination of indoor and outdoor scenarios）was carried out by using some modified exposure parameters of urban children in Shaanxi province.The evaluation results show that there is no non-carcinogenic and carcinogenic risk of heavy metals in indoor outdoor RDD of primary and secondary schools for all kinds of students,and the total non-carcinogenic risk index and total carcinogenic risk index of heavy metals in indoor RDD are significantly higher than those in outdoor RDD.Among the four types of students,the total non-carcinogenic risk and total carcinogenic risk of male students were higher than those of female students;the total non-carcinogenic risk and total carcinogenic risk index of outdoor RDD heavy metals on primary school boys/girls in grades 1-3 are the highest,and the total non-carcinogenic risk index and total carcinogenic risk index of indoor RDD heavy metals on high school boys/girls are the highest.Respiratory inhalation and hand-mouth intake are the main ways for students to be exposed to RDD heavy metals outside the campus;hand-mouth exposure pathway is the main exposure pathway for students to RDD heavy metals in campus indoor.Cr,Mn and Pb are the key heavy metal elements causing health risks to students.5.The source and contribution rate of heavy metals in indoor and outdoor RDD of primary and secondary schools were diagnosed and quantified.There are differences in the main sources of heavy metals in indoor and outdoor RDD dust in primary and secondary schools.In outdoor RDD of campus,Co-Ba mainly comes from building source,Cu-Pb-Zn from traffic source,Cr-Ni from coal-fired industrial source and V-Mn from natural source;in indoor RDD,Cu-Zn-Ba comes from traffic source,Cr-Ni-Pb from coal-fired steel smelting source,V-Mn from natural source and Co from building chalk dust source.The contribution rate of anthropogenic sources to the contents of 9 heavy metals in indoor RDD of primary and secondary schools in Xi’an was 69.96%,and that of natural sources was 30.04%;the contribution rate of anthropogenic sources to the content of 9 heavy metals in campus outdoor RDD was57.63%,and that of natural sources was 42.37%.The contribution rate of anthropogenic sources to the content of heavy metals in indoor RDD is about 12.33%higher than that in outdoor.6.The optimal control pollution sources of heavy metals in indoor and outdoor RDD of primary and secondary school campuses were determined.The contribution rate of coal-fired steel smelting source to the content of 9 heavy metals in campus indoor RDD is the highest.In addition,building chalk dust source is another important contribution source of heavy metals in campus indoor RDD;building-traffic source is the anthropogenic source with the highest contribution rate to 9 kinds of heavy metals in outdoor RDD.The contribution rate of traffic sources to 9 kinds of heavy metals in campus indoor suspended dust is lower than that in outdoor suspended dust,but the contribution rate of coal-burning related sources to 9 kinds of heavy metals in indoor suspended dust is higher than that in outdoor suspended dust.It shows that the heavy metals in indoor suspended dust in primary and secondary schools are more significantly affected by anthropogenic sources.Based on the systematic study of heavy metals in RDD on the indoor and outdoor impermeable interface of primary and secondary schools in Xi’an,aiming at the existing problems,this paper puts forward some suggestions to improve the campus environmental quality and reduce the health risk of students.1.It is suggested that relevant urban departments should effectively control the heavy metal pollution caused by industry and traffic and reduce the number and intensity of urban heavy metal pollution sources;2.to improve the cleanliness of the indoor and outdoor environment of the campus and reduce the accumulation of dust on the campus.Professional air particulate matter filtration equipment shall be installed indoors to prevent the RDD accumulated indoors from rising again after being disturbed by students’activities,resulting in secondary health hazards to students;3.to strengthen the education of students’awareness of environmental protection and living habits.Finally,it can effectively reduce the risk and harm of heavy metals in urban dust to campus and students.