The Occurrence State And Speciation Of Selenium And Its Environmental Behaviors In Rock-Soil-Plant From Typical High-Se Areas

Selenium(Se)is an essential trace element for humans and animals,Se deficient or excess are both harm to health.At the same time,nearly 72% of areas in China are lack of Se in soils,so the low Se level in the geological environment severely affects the health of the residents in our country.In addition,there are two famous Se-enriched regions in China because the sudden incidence of Se poisoning,one is Enshi City of Hubei Province;the other is Ziyang County of Shaanxi Province.Thus,the issue faced by Chinese people is how to improve the daily intake of Se for Se-deficient residents and then the healthy condition.As we all know,the safest and most effective way to obtain Se for humans is dietary supplement,so the Se concentration of plants are the direct controlling factor for the Se level in human.However,the bioavailability Se in environment are controlled by its concentration,speciation and physicochemical properties in soils and rocks,which then influence the Se content of plants.Apart from the total Se content,the Se fractions in plants could also affect the absorption efficiency of Se for human bodies.Therefore,studying the Se fractions and its rules of release,migration and transformation in the rock-soil-plant system is not only important to understand the eco-geochemical cycle of Se,but also benefit for the scientific development of Se mineral resources,the healthy development of Se related industries and the final national war of alleviation of poverty.In this study,the research areas are two typical Se-enriched regions(Enshi and Ziyang),with the fund of China Geological Survey Project,we have conducted the eco-geochemical evaluation of Se for the study area by investigating the distribution,speciation and transformation of Se in rock-soil-plant system.Based on the Se fraction and speciation of Se-enriched rocks(including pyrite),the calcination and activation of Se-enriched slate and the cultivation of plants were also carried out;in addition,we have also discussed the mechanism of the activation process and the Se absorption rules by plants.As a consequence,these data could provide theoretic reference and technical support for Se resource exploiting,Se-enriched mineral fertilizer developing and Se-enriched agricultural products cultivating and evaluating.The main conclusions are as follows:(1)Environmental geochemical investigation of selenium in two famous Se-enriched areas,Enshi and Ziyang,of ChinaThe Se concentration of Se-enriched rocks and soils from Ziyang,Shaanxi Province is,on average,25.74?g/g and 21.49?g/g,which are both lower than those of Enshi,Hubei Province(420.0?g/g and 30.81?g/g);in both of the two areas,the enrichment of trace element in rocks is similar,Mo,Cd,Sb,V,Cr,As,Ni,Cu and Bi are enriched,while the element Pb is normal or depleted.The plant Se contents not only vary from species,but also affected by the total Se and bioavailable Se in cultivated soils.The highest Se content(275?g/g)was found in elancai(Thlaspi arvense L)from Enshi City,and followed by: pepper > corn > sweet potato > eggplant > tea > potato > cucumber.(2)Discussion on the key factors of Se levels in Se-enriched rocks and soilsThe SPSS analysis showed,the rock Se concentrations are not only affected by the contents of major elements SiO2,Al2O3,MgO,Fe2O3+MnO,P2O5 and S,but also present positive correlations with element S and C,which are related to the present of sulfides and organic matter.Also,Se concentration in Se-enriched rock is positively correlated to many concomitant trace elements,reflecting the common features of black rock series.Additionally,Ziyang rock Se concentrations could also have certain relationships with mineral contents,for example,a positive correlation between Se vs illite and Se vs pyrite,while negative between Se vs quartz and Se vs Albite.In addition,it is also found that the soil Se concentrations are controlled by the parent material and the organic matter,however,no obvious relationship was found between soil Se and the pH value.(3)The Se fractions and distribution in rocks/soils from two typical Se-enriched regionsThe Se fractions in rock are dependent on its age and lithology.In Ziyang Se-enriched carbonaceous slate,the main Se fractions are sulfide/selenide and elemental Se,base-soluble orgainc Se and residual Se,and the bioavailable Se is 5.12±3.60%;and in Ziyang Se-enriched chert,the main Se fractions are Se fractions are base-soluble orgainc Se,sulfide/selenide and elemental Se,and residual Se,and the bioavailable Se is 6.54±0.86%.In Enshi Se-enriched carbonaceous shale,the main Se fractions are base-soluble organic Se,sulfide/selenide and elemental Se,and ligand-exchangeable Se,and the bioavailable Se is as high as 17.81±6.80%.In addition,the characteristics of soil Se fractions present a high succession of the underlying strata.Also the bioavailable Se in Ziyang soils(4.17 ± 3.35%)is lower than that of Enshi soils(14.14±8.43%).(4)The distribution and transformation of Se in Se-enriched pyrite from Ziyang and its possible originThe Se concentration is relatively high in Lower Cambrian pyrite-rich rock from Ziyang,Shaanxi Province,and the highest total Se value reaches 73.1?g/g.EMPA data show the average Se and As concentration in pyrite are 281±337?g/g(n=280)and 666±533?g/g(n=154),respectively,which are very close to the results of LA-ICPMS;the EMPA results also indicate that 2 types of Se and As distribution are found in pyrite,the type I pyrite is characterized by a Se-bear and As-poor core,but no similar core is found in the type II pyrite.In addition,the Se-enriched strata are considered as hydrothermal sedimentary origin,and the pyrite should be formed under an anoxic semi-deep sea environment.Hydrothermal fluid might be the main source of Se,at the same time,there could also be some magma activations or metamorphic hydrothermal fluids.It is also found that Se mainly substitutes for S in the pyrite structure,also few elemental Se may exist in some high Se “veins”.With the alteration of the pyrite,the produced limonite would play a key role in the surface migration cycle of Se for its strong adsorption ability.Furthermore,most of the As/Se values are less than 2.25 in the altered pyrites,therefore,it is thought to be a indicator for evaluating the pollution degree of pyrite.(5)A calcination and activation procedure for Se-enriched slate was developedThe procedure can significantly improve the bioavailability of Se in Se-enriched slate,the optimal sorbent is CaO and the best temperature is 600?.Under the best condition,the proportion of Se(VI)is the highest in the total bioavailable Se,of which the bioavailability is regarded as the highest.Meanwhile,in the Se-enriched slate suitable for activation,its Se fractions should be associated with organic matter and/or sulfide minerals.The mechanism of Se capture by Ca-based materials is related to physical adsorption,chemical adsorption,chemical reactions or a combination of these three.Thus,the calcination procedure proposed can support the preparation of Se fertilizer by Se-enriched slate resource.(6)The effectiveness of the activated rock powder were verified by cultivation experimentsAs indicated by the cultivation of potato and garlic,no matter fertilizing soil with Na2SeO3 or activated rock powder,both of them can significantly enhance the Se enrichment of plant,but the plant Se concentrations are different under different Se sources.For example,during the cultivation of potato,the Se fertilizers' efficiency is in order: activated rock powder > Na2SeO3 > original rock powder;however,in the cultivation of garlic,the absorption efficient of Se by garlic from activated rock powder is higher than from Na2SeO3 only when the dose is lower than 70mg/m2,which demonstrates further studies and discussions are still needed for obtaining the suitable plants and the best method to fertilize soil with activated rock powder.In spite of this situation,these cultivation experiments can still prove the fact that the activation procedure can improve the Se bioavailability of Se-enriched slate,which is of great meaningful to accelerate the Se fertilizer development and obtain high stability,controllability and qualification rate.(7)The establishment and application of organic Se speciations' quantitative analysis methodAn efficient and rapid analytical method for 5 Se species,including Se(VI),Se(IV),SeMet,SeCys2 and MeSeCys,is developed by using the HPLC-ICPMS equipped with a Hamilton PRP X-100 chromatographic column and a 50?L quantitative loop,which costs only 10 min with 82 Se as a signal monitor.The optimum mobile phase is 5 mM citric acid added with 2% methanol at pH 5.5,and the flow rate is 1.0 ml/min.And the results show,the dominant Se speciation is SeMet in the Se-enriched products when fertilized with Na2SeO3.However,it is different in garlic and elancai,Se(VI)is the main Se speciation in the garlic,while MeSeCys,SeCys2 and SeCys2 are the main speciation in the leaf,root and stem of elancai,respectively.Therefore,the adsorption and conversion rates of plants are different,and investing the organic Se speciation in plants could not only understand the transformation and conversion way of Se,but also effectively promote the development of high-end Se-enriched agricultural products and their safe and normative production.