Geochemical Characteristics Of Heavy Metals And Selenium In Selenium-Rich Areas

Selenium-rich soils are important selenium resources and important basis for the development of selenium-rich agricultural products.However,selenium-rich soils are often accompanied by heavy metals such as cadmium and chromium.Heavy metals in soils not only directly affect the quality of soil environment,but also affect headwaters,animals,plants and human health.Heavy metal pollution and hazards are called"chemical time bombs"in soils,which bring serious safety risks to the soil ecological environment in selenium-rich areas.Enshi,Located Hubei province,is known as the"Selenium Capital of the World".It relies on Selenium-rich resources and the Selenium-rich industry has developed well.Lichuan City,which is close to Enshi,is rich in selenium resources.In order to better exploit and utilize selenium resources,it is not only required that the soils are rich in selenium,but also that there is no serious pollution of heavy metals in soils.In this paper,the geochemical characteristics of heavy metals and their coexistence with selenium in the soils of Wangying Town,Lichuan City are studied,and the environmental quality and ecological risk of the soils are evaluated,which is of great significance to the development of selenium-rich soil resources and selenium-rich agricultural productsThe area of selenium-enriched area in this study is 105.45km~2.Through collecting and testing 45 rock samples,521 soil samples and 41 crop samples,the content characteristics of heavy metals such as arsenic(As),cadmium(Cd),chromium(Cr),lead(Pb),mercury(Hg),zinc(Zn),nickel(Ni),copper(Cu)and selenium(Se)in rocks,soils and crops in the study area were identified,the coexistence relationship between heavy metals and selenium was discussed,and the environmental quality and ecological risk of soil were evaluated.The main conclusions are as follows:(1)The distribution of heavy metals in the rocks in the study area is uneven.The Permian black carbonaceous mudstones are especially rich in arsenic,cadmium and mercury,which are 3.75,2.95 and 4.28 times of the element abundance values of the continental crust,respectively.The Triassic limestone is relatively rich in arsenic.The regional distribution of heavy metals in soils is uneven,and the spatial distribution characteristics are similar.The background values of arsenic,cadmium,chromium,copper,mercury,nickel,lead and zinc are 16.42mg/kg,0.373mg/kg,91.71mg/kg,32.21mg/kg,0.107mg/kg,36.46mg/kg,35.06mg/kg and 96.69mg/kg,respectively.Cadmium is obviously enriched,which is 3.85 times of the average value of national soils.It inherits the characteristics of cadmium in rocks.The ratio of the other seven elements to the average value of national soils is between 1.2 and 1.75,showing relative enrichment.Therefore,the regional geological background should be taken into account in the exploitation and utilization of selenium-rich resources.(2)Se content in Permian black carbonaceous mudstone is obviously higher than that in limestone,up to 14.12 mg/kg,which is 282 times of the clark value of selenium in the crust.The background value of Se in soils is 0.5363mg/kg,which is1.85 times of the average value of national soils,and the average content is 0.6023mg/kg.Geochemical grade of selenium in agricultural soils in the evaluation area is mainly secondary.Selenium content of Coptis chinensis is the highest in 7 crops,reaching 0.4016 mg/kg.(3)Heavy metal elements differ significantly in different soils of parent materials,soil types and land using,and are affected by pH values and organic matters.The average contents of arsenic,cadmium,chromium,copper,mercury,nickel,lead and zinc in Permian parent material area are higher than those in Triassic parent material area;The contents of arsenic,cadmium,chromium,copper,lead and zinc are the highest in argillaceous mountain brown soil,while the contents of mercury and nickel are the highest in carbonate mountain yellow brown soil and carbonate mountain brown soil,respectively.Cadmium and mercury are more abundant in farmland.At the same time,it is found that cadmium and copper are positively correlated with pH values,arsenic is negatively correlated with organic matters,and cadmium is positively correlated with organic matters.(4)Geochemical assessment of soil environment in the study area shows that the comprehensive quality of soil environment is excellent,and there is a moderate potential ecological risk.The area of clean to slightly polluted area is 90.02 km~2,accounting for 85.37%of the total area of the study area;the area of slightly polluted to moderately polluted area is 14.40 km~2,accounting for 13.66%,and the area of heavily polluted area is 1.03km~2,accounting for 0.98%.Among the eight heavy metals,cadmium is the main pollution factor,which is greatly influenced by human activities.The light to heavy pollution area is 14.75 km~2.(5)There is a significant correlation between chromium,cadmium,nickel and selenium in rocks and soils,which has a certain safety risk for the development of selenium-rich agricultural resources.Rice,Coptis chinensis and Lentinus edodes in crop samples all meet the selenium-enriched standard,but cadmium exceeds the standard in varying degrees.Therefore,the contradiction between selenium and cadmium in the development of selenium-enriched agriculture must be solved and effective measures should be taken to strengthen the prevention and control of cadmium pollution.