| In recent years,selenium(Se)rich agriculture has developed rapidly in China.Se in the supergene environment of Se-rich areas in Hubei,Hunan,Shaanxi,Zhejiang and other provinces mainly comes from the weathering of black shale.The black shale was rich in Se and had high content of heavy metal elements,resulting in high background value of relevant heavy metal elements in the weathered soil,which has become a bottleneck restricting the development of Se rich agriculture in the black shale areas in China.In this paper,Enshi City,Hubei Province,the?world capital of Se?was selected as the study area.Under the guidance of multi-disciplinary theories of ecological geochemistry,sedimentology and sedimentary geochemistry,and based on the massive geochemical survey data of Se and heavy metals in rocks,soil,water(streams and precipitation)and crops in the supergene environment of the area.According to the geotectonic and lithofacies paleogeographic background,combined with the combination characteristics and speciation analysis of Se and heavy metal elements,the distribution characteristics and controlling factors of Se and heavy metal elements in rocks,soil,water,plants in the study area were systematically clarified for the first time,and the element migration model under the supergene environment and Se-enrichment model of Late Permian black rock series in the study area were established.In addition,the intake of Se and heavy metals of residents in different Se environmental effect areas in the study area was quantitatively estimated for the first time,and the annual flux of Se and heavy metals in precipitation and the leaching of that in rock and soil by stream water were quantitatively estimated,which provided scientific basis for the development of characteristic Se rich industry,ecological environment protection and improvement of residents'health level in the study area.It also provides a reference for the ecological evaluation of Se and heavy metals in the exposed areas of black rock series in China.The main understandings are as follows:(1)The geochemical characteristics of Se and heavy metals in sedimentary rocks and the controlling factors of Se content in rocks were found out.The Paleozoic and Mesozoic sedimentary rocks are mainly exposed in the study area.From the lithological point of view,the enrichment degree of Se was arranged from high to low as follows:black rock series,coal measures,carbonate rocks,fine clastic rocks and coarse clastic rocks.Geotectonic setting was the main controlling factor of Se content in sedimentary rocks.In the extensional tectonic environment,mantle Se comes to the surface with volcanic activity,and was absorbed and assimilated by marine plankton,and then stored in a closed basin with carbon.This was the source and sink of Se in Se-rich strata.Organic carbon,lithology and redox conditions were also important influencing factors of Se content in sedimentary rocks.The main occurrence state of Se in the rocks of Gufeng Formation of Permian was organic state,and the residual state was very low.However,the proportion of residual state in the rocks of Dalong Formation deposited during the mass extinction period had increased greatly,which indicates that the assimilation and absorption of Se by plankton was the original Se enrichment mechanism in Gufeng period,while the marine ecology was seriously damaged in Dalong period,and a large amount of Se entered into the lattice of clay minerals.There was a positive correlation between Se and Ni,as,Cd,Hg,Cu,Cr,Zn(P<0.01),but no significant correlation with Pb;among the eight heavy metal elements of Ni,as,Cd,Hg,Cu,Cr,Zn,Pb,only Pb has no significant correlation with Cd,and other heavy metal elements have a positive correlation with each other(P<0.01).The weathering and leaching of black rock series bring not only se but also some harmful heavy metals to the supergene environment.(2)The geochemical distribution characteristics and controlling factors of Se,heavy metals in the soil of the study area were found out,and the geochemical grade of land quality in the study area was evaluated.Compared with the national topsoil,Se,Cd,Co,Mn,F,Na2O and K2O in the topsoil of the study area were extremely enriched,while Ca O and Sr were extremely deficient.Cluster analysis showed that:N,P,S,SOC and Cr,V,Mo,Se,Cd,Ni,Zn,Cu as a group,reflecting the symbiotic association of biological elements and Se and related heavy metal elements;Fe2O3,K2O,Al2O3,Mn,as,I,Pb,Ge,Co as a group,reflecting the binding effect of soil oxides on related trace elements;Ca O was closely related to p H,soil acidification was related to soil Ca deficiency;The F elements were strongly adsorbed by Mg O.Soil physical and chemical properties such as parent rock,iron and manganese oxides,SOC,particle size and chemical weathering intensity were important factors controlling the distribution of elements in soil.According to?Specification of land Quality Geochemical Evaluation?(DZ/T0295-2016),the distribution areas of excessive,high,moderate,marginal and deficient total Se in the topsoil of the study area accounted for 5.06%,82.10%,12.59%,0.14%and0.11%of the total area respectively.According to the soil environmental quality standard.According to?Soil environment quality risk control standard for soilcontamination of agriculture land?(GB 15618-2018),the safety risk of As,Hg,Pb,Zn,Cr,Cu,Ni in agricultural surface soil was relatively low,and the priority protection area accounts for more than 90%;the risk of Cd element in soil was higher,and the priority protection,risk management,control and safe utilization areas accounted for 25.46%,66.48%and 8.06%respectively.(3)The biogeochemical characteristics of Se and heavy metals in rice,corn,potato,sweet potato,cabbage,radish,pepper,tea were found out,and their Se enrichment and safety were evaluated.From high to low,the Se enrichment rates were as follows:rice(n=189,64.55%),cabbage(n=155,58.06%),radish(n=140,49.29%),pepper(n=125,32.80%),corn(n=360,29.72%),tea(n=236,25.42%),sweet potato(n=186,11.29%),potato(n=309,4.85%).The main heavy metal element exceeding the standard in crop samples was Cd,and the exceeding rate from high to low was as follows:rice(40.70%),pepper(36%),corn(7.78%),potato(5.18%),sweet potato(4.84%),cabbage(3.87%),radish(3.57%).In addition,11.11%of the rice samples exceeded the standard.The over standard rates of As,Hg and Pb in eight crops were very low.As,Cr,Pb,Hg and Cd in all tea samples did not exceed the standard.The enrichment ability of crops to Zn,Cu and Cd was relatively strong,but to As,Cr,Ni,Hg and Pb was weak.The contents of Se,Ni,Mo and Cd in the edible parts of crops were significantly positively correlated with the corresponding elements in the root soil(P<0.01),which was mainly controlled by the se rich parent rock of Permian system;the contents of Zn in tea and as in radish were negatively correlated with the corresponding elements in the root soil(P<0.01),which may be caused by the antagonistic effect of Se,Zn and As in crops.(4)The geochemical characteristics of elements in atmospheric precipitation and stream water in the study area were found out,and the contribution of atmospheric precipitation to Se and heavy metals in soil and the leaching of elements in rock and soil by stream water were estimated.Cluster analysis showed that As,Hg and Se in precipitation samples were closely related to the most volatile trace elements in coal combustion,and Pb,Zn and Mn were related to vehicle exhaust and coal combustion.The correlation between Fe and Mn in stream water samples was the highest,which was related to the redox potential of water body;Se,Mo and Cd belong to the same group in stream water,which had excellent element geochemical inheritance with their eluviated rock and soil.The precipitation and stream water in the study area were generally clean,but the stream water in the Se excess area may cause excessive Se exposure.It is estimated that the amount of Se removed from rock and soil by stream water was about 6323kg per year,which was higher than the annual contribution of precipitation of643kg.The ratios of annual leaching of As,Cu,Zn,Cd and Pb in rock and soil to annual contribution of precipitation were 1.18,1.28,0.88,4.33 and 0.15 respectively.The relative balance of Cu,Zn and as was maintained,and the loss of Cd was serious;the leaching of Pb was limited,which may be related to the poor migration of Pb in weakly alkaline stream water.(5)The migration model of Se and heavy metals in rock soil(water body)-plant in the study area was preliminarily established,and the prevention and control suggestions of soil heavy metal pollution were put forward.After the rocks were exposed to the earth's surface due to tectonic movement,the water-soluble,ion-exchange of Se and the heavy metal elements such as Cd were leached by precipitation and can directly enter the soil and water body of supergene environment,while the proportion of strongly organic bound,residual and other inert states in weathering residues was relatively increased.Plants can directly absorb water-soluble and ion-exchange elements in the soil.The inert state of Cu,Zn,Pb,As and Hg in the surface soil was the main form of occurrence,while the main form of Cd in the soil was ion-exchange state,which has high bioavailability,leading to relatively serious over standard of Cd in crops.In addition,coal combustion and automobile exhaust contribute a lot to As,Hg,Se,Pb,Mn and Zn in the atmosphere.These elements enter the soil and surface water environment with precipitation,and can also participate in the biosphere chemical cycle.The content of heavy metals in the soil of the study area was closely related to the total amount,p H and SOC.The heavy metals in soil mainly come from the geological background,so the operability of reducing the total amount of heavy metals was not strong.Appropriate adjustment of soil p H and application of organic fertilizer are effective measures to reduce the risk of soil heavy metal pollution.(6)According to the dietary structure of the study area,the intake of Se and related elements in different Se environmental effect areas was estimated by MOS method,and the ecological effect was evaluated.The intake of essential trace elements for human body in the study area was estimated.The Se intake of adult residents in the areas with excessive,high and moderate Se was 427.4?g/d,75.6?g/d and 25.9?g/d respectively.The Se intake of residents in the areas with excessive Se might be too high.The Se intake of residents in the areas with high Se and the Cu and Zn intake of residents in the areas with adequate Se were close to RNI.The rice had the largest contribution to Se and related heavy metal intake in crops.Reducing rice planting area in Se-excess area or planting rice varieties with low concentration of heavy metals is an effective way to reduce the ecological risk of heavy metals in the study area.The intake of As,Ni,Pb and Hg was far lower than TDI,there was no risk of dietary exposure,and the intake of Cd exceeded the limit.However,urinary NAG of residents in the high Se soil area showed that Cd,Cr and Mo did not cause significant harm to human health,which may be caused by the antagonistic effect of Se on Cd,Mo and Cr.Therefore,the exposure dose limits of Cd,Mo and Cr can be appropriately increased in the high Se soil area of the study area. |
PDF Full Text Request