Study On Geochemical Behavior Of Heavy Metals And Rare Earth Element In The Black Shale-Soil-Moss System From The Lower Cambrian Black Shale Area In Guizhou Province,China

Black shale is widely distributed in China.For example,the South China black shale horizon,one of the largest black shale areas on earth,extends discontinuously in an east-west direction for more than thousands of kilometers,and often produces large deposits such as Ni-Mo,V,and U deposits.Due to the accumulation of a large amount of organic matter and sulfide in the diagenetic process,black shale is rich in a variety of trace metal elements（such as Cd,Cu,Cr,Ni,Mo,U,Zn,and rare earth element（REE）,etc.）,which is known as typical"polymetallic layer".The exposed black shale is susceptible to chemical weathering easily and produce acid rock drainage because of their abundance of sulfide and organic matter,and then various heavy metals are likely to be activated and released into the surrounding environment.Therefore,heavy metal pollution in black shale area has become a prominent problem restricting the healthy development of ecological environment.Mosses are the most primitive species of higher plants,some moss taxa can grow and flourish in the harsh environment where vascular plants can not survive,so they are the pioneer of ecological restoration in mining areas.To date,studies on moss tolerance to heavy metals and ecological restoration mainly focus on lead-zinc mining area,gold-mercury mining area,manganese mining area,etc.However,the geochemical behaviors and isotopic fractionation of heavy metals and rare earth element in the rock-soil-moss system in the black shale area are still unclear.In this study,the typical black shale area of the Lower Cambrian which located in southeast Guizhou（Yacha and Gaoqiao,Sansui County）was selected as the research area.Various analytical techniques were used in this study,including High resolution transmission electron microscopy（TEM-EDS）,TESCAN Integrated Mineral Analyzer（TIMA）,micro-X-ray fluorescence spectroscopy（Micro-XRF）,and copper isotopes.We systematically studied the enrichment and transfer characteristics of heavy metals and rare earth element in black shale-soil-moss system.It was also investigated that the physiological and biochemical response mechanism of moss under polymetallic stress,and the detoxification and tolerance mechanisms of polymetallic in moss cells were also discussed.After that,the magnitude and direction of Cu isotopic fractionation in the black shale-soil-moss system were studied.The aim of this study is to reveal heavy metals and rare earth element biogeochemical process in the black shale-soil-moss system and the influence mechanism.It will provide scientific basis for moss as the pioneering plant of vegetation ecological restoration in the black shale area.The results obtained in this study are as follows:（1）Black shale is characterized by high concentration of heavy metals（including REE）and various sulfide minerals in the study areas.Black shale is susceptible to chemical weathering easily under oxidation conditions,and will release heavy metals and rare earth element to varying degrees,and then pose a threat to surrounding environment.Specifically,both Yacha and Gaoqiao black shale are rich in Cd,Pb,Cu,Cr,Mo,Zn,U,and rare earth element,and also show obvious heterogeneity.The highest value of Cd,Cu,Cr,Zn and U in the black shale is 598 mg/kg,1035 mg/kg,4630 mg/kg,4290 mg/kg and 524 mg/kg,respectively.Compared with world shale and black shale,The enrichment coefficient of Cd in black shale is the highest（94.7）among all heavy metal elements in Yacha,while the enrichment coefficient of U is the highest（46.2）in Gaoqiao area.In addition,the average concertration of rare earth element in Yacha（217.00 mg/kg）and Gaoqiao（289.04 mg/kg）is significantly higher than that of the Upper Continental Crust（146.40 mg/kg）and North American shale（173.21 mg/kg）,and is characterized by HREE enrichment.Because the black shale is rich in sulfide minerals such as pyrite,chalcopyrite and sphalerite,it is easy weathered under oxidation conditions and releasing heavy metals and rare earth element in different degrees.The release level and rate of Mo,Cd,Ni,Zn and As are significantly higher than other heavy metals and rare earth elements..（2）The soil developed on top of black shale is characterized by low pH value,high organic matter content,and enriched in a variety of heavy metals（such As Cd,Mo,U,Cu,Cr,and As）and rare earth element.Clay minerals,secondary sulfate minerals and high content of organic matter formed during the weathering of black shale are responsible for the enrichment of heavy metals and rare earth element in soil.The black shale soils both in the two study areas are contaminated with polymetallic,and present a very high ecological risk（YC:RI=5792.22;GQ:RI=2424.46）,Cd is the major pollutant in soil because it has the highest ecological risk factor（YC:E_i=4566.92;GQ:E_i=1474.96）.The lithological signature of REE in black shale is readily inherited by overlying soil,and characterized by strong enrichment in HREE.Rare earth element in soil of Yacha presents moderate ecological risk（RI=293.73）,while presents high ecological risk in Gaoqiao（RI=450.29）,Lu is the major pollutant in soil because it has the highest ecological risk factor（YC:E_i=71.40;GQ:E_i=117）.（3）A total of 8 moss species belonging to 5 families and 5 genera are collected from the black shale areas,among which Pohlia flexuosa Harv.in Hook（P.flexuosa）is the dominant moss species.The average concertration of Cu in P.flexuosa is 568mg/kg and 689 mg/kg,respectively,with an extreme value of 2180 mg/kg in Yacha and Gaoqiao regions.Therefore,it can be considered as a potential Cu hyperaccumulator.Also,P.flexuosa have been shown to be effective bioaccumulators of REE,with the highest content up to 198.69 mg/kg.Correlation analysis showed that heavy metals and rare earth element are mostly originating from soil substrate that P.flexuosa lives on and less from the atmospheric deposition,and there is a synergistic absorption effect of various heavy metals in P.flexuosa.In addition,the concentration of heavy metals and rare earth element in the senescent tissues of P.flexuosa are higher than that in the new growing tissues,which may be the result of proximal pollution.The shale-normalized results of REEs concentrations in the parent rock-soil-moss system followed a remarkably identical pattern and were characterized by strong enrichment in HREEs,indicating moss P.flexuosa could be used as a powerful tool to reflect the content and composition of REEs in black shale areas.（4）Moss P.flexuosa has evolved a wide spectrum of strategies to counteract the toxic effects of heavy metals,including cell-wall immobilization,vacuolar sequestration,and activating a series of antioxidant non-enzymatic and enzymatic responses.Specifically,Cd、Cu、Pb、Ni、Zn、U,Th,and rare earth element are mainly absorbed in the extracellular（heavy metals 75.29%and rare earth elements 70.71%,respectively）to alleviate their toxicity,while Cr,Mo and As are mainly located in the intracellular.In addition,moss P.flexuosa enhanced the fixation of heavy metals by increasing the contents of cellulose,pectin and the thickness of cell wall.Among them,Ni,Cu,Zn,Cd,Pb,La,and Ce are mainly trapped in the cell wall to achieve detoxification,while Cr is mainly stored in vacuoles.Moreover,moss P.flexuosa can reduce the damage of heavy metals by increasing the activity of antioxidant enzymes（SOD and CAT）and the content of metallothionein（GSH and MTs）.（5）Cu isotopic fractionation occurs in the black shale-soil-moss system.At the black shale-soil interface,the adsorbing and fixation of Cu by clay minerals（illite,montmorillonite,kaolinite）in the soil results in the secondary enrichment of Cu in the soil（τ>0）,and then leading to heavy Cu isotopes in the soil relative to the parent rock(Δ⁶⁵Cu_{soil-black shale}<0).Meanwhile,in the soil-moss system,moss P.flexuosa preferently absorbed heavy Cu isotopes in the process of Cu absorption and enrichment(Δ⁶⁵Cu_{soil-black shale}>0).