Selenium Speciation in Kerogenfrom Two Chinese SeleniumDeposits: EnvironmentalImplicationsH A N J I E W E N , * , † , ‡ J E A N C A R I G N A N , ‡

Y U Z H U O Q I U , † A N D S H I R O N G L I U †

Institute of Geochemistry, Chinese Academy of Sciences,Guiyang, 550002 China, and Centre de RecherchesPetrographique et Geochimiques, CNRS, 15,Rue Notre-Dame-Pauvrves, B. P. 20, 54501,Vandoeuvre-les-Nancy Cedex, France

Selenium is an essential trace element for humans,animals, and vegetation. Its occurrence in the environmentis characterized by specific chemical and biochemicalproperties that control its elemental solubility, toxicity, andenvironmental behavior. The Laerma Se-Au deposit andYutangba Se deposit are two important Se-bearing depositsfound recently in China. In one of these areas (Yutangba),a serious environmental impact happened involving Sepoisoning. Previous studies have shown that Se in bothdeposits is closely related to organic matter, especiallykerogen fractions, but detailed relationships between Seand kerogen and Se chemical forms were not reported. Inthis study, the different speciation of Se is identified bytransmission electron microscopy (TEM) and othergeochemical techniques (infrared spectra (IS) and X-raydiffraction (XRD)) from kerogen samples extracted from orerocks of both deposits. The occurrence of organicallybound Se in the Laerma deposit and elemental Se nanograinsin the Yutangba deposit is observed, indicating thediversity of formation mechanisms and possible chemicalforms of Se in Se-rich rocks. The formation of elementalSe associated with organic matter is likely related to redoxconditions, whereas organic species are related to thehigher sulfur content of kerogen and possibly result fromS-Se substitutions. This discovery provides new evidencewith which to assess potential Se mobility duringweathering of ore-bearing rocks. In an altered rock, theelemental Se in kerogen is more steadily mobilized and ispotentially accumulated by vegetation, which mayexplain the sudden prevalence of Se poisoning in theYutangba area. In contrast, organically bound Se seemsmore resistant to chemical alteration compared to other Sespecies so that its bioavailability may be very restricted.

IntroductionSelenium is an essential trace element for human and animalhealth and vegetation. However, its overabundance ordepletion may cause serious biological and ecologicalproblems, such as Se toxicosis (Se excess) and Chronic Keshandisease (because of Se depletion). Indeed, in 1963, a major

incident related to Se poisoning happened in the Yutangbaregion of China, where villagers were forced to evacuate theirhomes (1, 2).

The main industrial source of Se is mainly restricted toore deposits that are genetically associated with volcanism,where it occurs as an accessory element. Volcanogenic goldand base-metal massive sulfide deposits are potentially themost viable Se sources for industrial purposes (3-7).However, the recent discovery of Se-rich deposits (the LaermaSe-Au deposit and the Yutangba Se deposit) and Se-bearingformations in central China, such as those in the Laermaregion near the boundary between Sichuan and GansuProvinces, the Ziyang-langao region of southern ShaanxiProvince, and the northwestern Hunan and western HubeiProvinces in China, have shown the potential for Se to be amajor element in some ore deposits (8, 9). These Se-bearingformations in China may have been responsible for somemajor environmental hazards in these areas. However,hazards such as poisoning of the population have not beenfound in all high Se regions. Even in an isolated high Seregion, the biological and ecological effects of Se were clearlyvariable (2, 10).

Most Se-bearing formations in China comprise carbon-aceous cherts and shales containing abundant organic matterwith which Se is either bounded or adsorbed (9, 11). However,the mechanisms of Se enrichment associated with organicmatter and Se speciation in solid organic matter (kerogen)remain unclear, although the chemical form of Se is of primaryimportance in estimations of its potential mobility andtoxicity in the environment.

Here, we report transmission electron microscope (TEM)images, X-ray diffraction, and infrared spectra of Se-bearingkerogen from two major Se ores in China, the Laerma Se-Audeposit and the Yutangba Se deposit. We found that, in thefresh rocks from the ore bodies, Se may either occur as (1)elemental nanograins or as (2) organically bound or adsorbedspecies according to redox conditions prevailing during Semineralization and to the sulfur content in kerogen, respec-tively. Selenium mobility in the environment highly dependson its speciation, the organically bound species probablybeing very resistant to dissolution.

Geological SettingThe Laerma Se-Au deposit and Yutangba Se deposit arehosted in the selenium-bearing formations (SBFs) which weredefined by Wen and Qiu in 2002 (9) (Se-bearing formation-SBF- is a suite of rocks with Se and other multielementanomalies, the Se concentration generally being larger than5 ppm, and is characterized by specific temporal-spatialdistribution characteristics). Detailed petrology, mineralogy,geochemistry, and organic biomarker studies have shownthat several selenium minerals occur in the Laerma deposit(12, 13), and a few of these have been identified in theYutangba Se deposit (14, 15). Our preliminary studies alsoindicated that about 75% and 66% of total Se in these oredeposits, respectively, are closely associated with or occurwithin the solid organic matter (11). The geological settingand geochemical data are listed in Table S1 (see Table S1,Supporting Information).

The Laerma Se-Au Deposit. This deposit is located atthe plunging end in the western part of the Baiyigou Anticlinein the Qingling region along Sichuan-Guansu boundary (SeeFigure S1, Supporting Information). It is hosted in lowerCambrian Taiyangding Group which comprises a series ofcarbonaceous cherts and slates. It is characterized byabundant organic matter and anomalies in Au, Se, U, Cu,

* Corresponding author phone: (33) 3 83 59 42 11; fax: (33) 3 8351 17 98; e-mail: hjwen@crpg.cnrs-nancy.fr.

† Chinese Academy of Sciences.‡ Centre de Recherches Petrographique et Geochimiques.

Environ. Sci. Technol. 2006, 40, 1126-1132

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Mo, Sb, and PGE. This siliceous formation occurs not onlythroughout West Qinling but also extends eastward to EastQinling, such as in the Ziyang, Langao, and Ankang regionsin the Shaanxi Province. Being apparently lithologicallycontrolled, the Se ore bodies are mostly distributed in chertsand transitional facies to slates, both vertically and hori-zontally. Morphologically, the ore bodies are generally lensesof varying scales, ranging from tens to hundreds of meters,with minor veins. Se is present in relatively high concentra-tions in both host rocks and ores, averaging 8.7 ppm in chert,3.1 ppm in slate, 89 ppm in chert-type ores, and 55 ppm inslate-type ores. Locally, Se may reach extreme concentrationsof up to 500 ppm. The Laerma deposit was formed atintermediate to low temperatures (142-269 °C) and at lowpressures (9-30 Mpa) and gold and Se may have beentransported as S-Se-Au complex (12), which is typical ofhydrothermal deposits associated with submarine exhala-tions.

A large number of selenium minerals and Se-bearingminerals have been identified, such as tiemannite (HgSe),clausthalite (PbSe), antimonselite (Sb2Se3), kullerudite (NiSe2),an unknown Ni-As-S-Se mineral phase, and Se-stibnite(13). In addition, Se was detected in nearly all sulfides,particularly in pyrite and stibnite. Although many differentinorganic species of Se were identified, Wen and Qiu (11)showed experimentally that about 75% of total Se was closelyrelated to organic matter for both chert and slate formations.

The Yutangba Se Deposit. The Yutangba Se deposit is theonly sedimentary-type Se deposit known in the world to date.It is located in the northern wing of Suanhe syncline in thenortheastern part of upper Yangtze platform fold belt (seeFigure S1, Supporting Information). The ore-bearing layers,extending thousands of meters, are found between thecarbonaceous chert and carbonaceous shale of the LowerPermian Maokou formation. The nine orebodies, so fardiscovered, are distributed along the abovementioned litho-logical interfaces and are mainly lenticular, ranging from 30m to 150 m in length, from 0.7 m to 5.2 m in thickness, andfrom 14 m to 35 m in depth. The ores show typical syngeneticsedimentary characters, common with aphanitic and bio-genic textures, and laminated and massive structures. Se ispresent in relatively high abundance in both host rocks andores, with up to 1.3% on average in the enriched zones.

A few Se minerals, including klockmannite (CuSe), es-kebornite (CuFeSe2), chalcomenite (CuSeO3‚2H2O), andnative Se, have been identified and a small amount of Se wasincorporated into the lattice of pyrite because of isomorphoussubstitution. Therefore, Song (8) suggested that Se occurredmainly in the form of adsorbed Se to organic matter,accounting for 66% of the total Se. On the other hand, Zhenget al. (16) proposed that Se occurred mainly in the form ofmicroparticulates of elemental Se in association with organicmatter. Zhu et al. (15) provided further evidence regardingthe occurrence of native selenium, but their work was mainlyconcerned with altered rocks and burned coal stones. Seoccurrences have been divided into three categories: (1) theprimary native Se occurring in carbonaceous-siliceous rocks,(2) micro-Se crystals formed during the weathering of Se-rich rocks, and (3) larger Se crystals related to combustionof stone coal (15). However, relationships between primarynative Se and carbonaceaous rocks were not fully demon-strated and need further investigation.

Yao et al. (14) proposed that Se was derived from thevolcanic matter related to the Emeishan basalt eruption.These distal sources of magmas and hydrothermal activitymust have supplied large amounts of Si and Se. Under suchenvironmental conditions, the high productivity of siliceousplankton may have consumed the excess Se and Si, diedrapidly, and fallen to the sea floor to form Se-rich carbon-aceous cherts.

Materials and MethodsPreparation of Samples. The Laerma Se-Au deposit wassampled at two mining pits that have been mined as majorore bodies. Samples were also collected from engineeringdrill cores. The Yutangba Se deposit was sampled at onemining pit and two outcrops that represent major ore veins.The samples are relatively with less weathered surfaces. Majorand trace elements, organic carbon, and total sulfur wereanalyzed for 48 samples from the Laerma deposit and for 20samples from the Yutangba deposit. Major results have beendescribed by Wen and Qiu (9, 11) and are reported in TableS1 (see Table S1, Supporting Information). Subsequently, onthe basis of the chemical data and sample distribution, 12samples from the Laerma deposit and 10 samples from theYutangba deposit were selected for kerogen extraction andwere prepared for TEM analysis. We believe that these rocksreflect the mineralogy and chemistry of the original orebodies.

Samples collected from the abovementioned depositswere treated to obtain over 98% pure kerogen. The detailsof the experimental procedure are as follows.

After removing the weathered surface, samples werecleaned with distilled water and were dried and then werecrushed to -200 meshes. The powdered samples were treatedwith CHCl3 in a Soxhlet’s extractor to extract soluble organicmatter and then were treated with HF-HCl to remove silicateand carbonate minerals. Kerogen and sulfides (mainly pyrite)were obtained from the solid residue and were separatedfrom each other using heavy liquid (CHBr3). To get high-purity kerogen samples, this procedure was repeated at leastthree times until no sulfide could be observed under abinocular microscope. Chemical purification involving oxi-dation or reduction, as described by Fu and Qin (17), wasavoided here in part because of the low sulfide concentrationin ores but mostly because of the high volatility of Se undereither oxidizing or reducing conditions. Finally, the kerogenseparated from collected samples was frozen and dried.

Transmission Electron Microscopy (TEM). A smallaliquot of purified kerogen was pulverized in alcohol andwas dispersed ultrasonically. Afterward, a drop of suspendedliquid was dripped on carbon-Formvar-coated 200-meshcopper or nickel grids and was dried by airing. To eliminatebackground noise, copper and nickel grids were usedalternately for all samples. Images were obtained with a JEM-200FX TEM at 140 V under standard operating conditionswith the liquid nitrogen anticontaminator in place. Theenergy-dispersive X-ray spectroscopy (EDS) was performedat 35 kV by using a current of 10 mA and a counting time of100 s (EDS model: JF-1). The magnification observed wasadjusted in accordance with the grain size of samples. Allexperiments were conducted in the TEM laboratory of theInstitute of Geochemistry, Chinese Academy of Sciences.

X-ray Diffraction (XRD) and Infrared Spectra (IS).Powder X-ray diffraction (XRD) measurements were madeon a Rigaku D/MaxγA X-ray diffractometer with Cu KRradiation (λ ) 1.54178Å). The infrared spectra used a Nicolet750-type Fourier transform infrared spectrometer purgedwith nitrogen gas that incorporated a DTGS detector.

ResultsTEM observations showed that there was no other impurityfound in kerogen samples except for very minor sulfide andsilicate minerals. The TEM images of kerogen from the twodeposits were similar, showing cellular noncrystal grainedtexture and amorphous fibrous aggregation structure. Car-bon, sulfur contents, and maturity degree in kerogen fromboth deposits slightly differed from one another (Table 1);however, detailed observations have shown significantdifferences in Se occurrences in kerogen from the twodeposits.

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The Laerma Se-Au Deposit. With the exception of a fewSe-bearing minerals, such as Se-bearing pyrite, Se-bearingarsenopyrite, and tiemannite (relic minerals after samplestreatments), only one elemental Se particle was found insamples from the Laerma Se-Au deposit. The Se grain wasembedded in the kerogen and was approximately 300 nm insize (Figure 1A). The result of EDS analysis showed almostpure elemental Se with low peaks of S, C, O, and Ca (Figure1B).

The color of the kerogen from the Laerma deposit wasusually dark and gray as observed under TEM, suggesting arelatively high degree of aromatization (18), which isconsistent with the results of organic geochemistry andbiomarker studies (9). This kerogen contained significantmineral elements as observed by the EDS analysis. Indeed,in addition to C, O, and S, signal peaks corresponding to Si,Ca, V, As, and Fe were measured (Figure 1C). Amorphoussilicon may have been incorporated into kerogen, since chertwas hosting the organic matter. This would readily explainthe presence of Si peaks in the EDS spectrum. The presenceof As and Fe is more enigmatic, but these elements do notoccur as nanometer-size minerals because of their homo-geneous distribution in all kerogen samples, and this fordifferent sizes of the electronic beam spot during the EDSanalysis. Sulfur was present in kerogen from both depositsbut, using the C peak in EDS spectra as a reference, thecontent of S in kerogen from the Laerma deposit wasobviously higher than that in kerogen from the Yutangbadeposit as suggested by elemental analysis (Table 1). Finally,EDS analyses on more than 30 points revealed that Se wascontained in kerogen from the Laerma deposit and that itsdistribution was also homogeneous.

The analysis of infrared spectra from the kerogen samplesof Laerma Se-Au deposit was performed to evaluate thetype of organic bonds Se was involved with. Characteristic

peaks were observed at 3392, 2914, 1718, and 1575 cm-1 ininfrared spectra (17) (Figure 2). The broad peak in the vicinityof 3392 cm-1 resulted from -OH oscillation in hydroxyl,phenol-hydroxy, and H2O. The small peaks at 2914 and 2853cm-1 may indicate the presence of alkyl and alkene, whilethe minor peak at 1718 cm-1 may be attributed to CdO incarbonyl and carboxyl. The prominent peak at 1575 cm-1

was a reflection of CdO oscillation in acid hydroxy andquinone and of the CdC oscillation in alkene, aromatic ring,and polycyclic aromatic hydrocarbon. The strong peak at1081 cm-1 was clearly caused by quartz impurity. The infraredspectra of samples from the Laerma deposit suggest that thefunctional groups in kerogen are mainly oxygen-bearingradicals.

The Yutangba Se Deposit. In contrast to kerogen fromthe Laerma deposit, a large amount of elemental Se grainswere identified in kerogen from this Se deposit (Figure 3A,D, E). The elemental Se particles were attached to orembedded in kerogen, sometimes with a concentric mor-phology. Most of the elemental Se particles were of nanograinsize with the largest grains observed being approximately500 nm in diameter. The EDS spectra obtained from theseSe grains indicated that they were made of pure Se (Figure3B). Cl peaks might reflect relics of HCl from samplepreparation. C and O peaks were most likely associated withkerogen, which is mainly comprised of C, H, O, S, N, and P.The lack of any other metal peaks in the spectrum indicatesthat Se is in its elemental state [Se0] rather than as a metalselenide [Se-2]. In contrast to Laerma kerogen, no Se wasdetected in the pure kerogen fraction, for which only

FIGURE 1. (A) TEM image of an elemental Se grain formed in kerogen from the Laerma Se-Au deposit. Originally, there was only a singlegrain but the Se particle, heated by the electron beam, volatilized and divided into two parts; (B) EDS of the native Se grain in plate Aindicated by arrow 1; (C) EDS of kerogen collected from the area in plate A indicated by arrow 2.

TABLE 1. Organic Chemical Parameters of the CollectedSamples

LaermaSe-Au deposit

YutangbaSe deposit

Corg(%) 1.46 (48)a 20.4% (20)a

R0(maturity degree) 2.62-3.52 (7)a 1.67-3.23 (8)a

(3.14)b (2.53)b

S in kerogen (%) 1.8 (3)a 0.6 (3)a

a The number of samples determined, in brackets. b The mean value,in brackets.

FIGURE 2. Infrared spectra of kerogen (sample L-4) from the LaermaSe-Au deposit.

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characteristic peaks of C, O, and S, and a few Ca and V, wereidentified by EDS analyses (Figure 3C). In addition, a fewnative copper grains were also observed in some kerogensamples, indicative of the strong reductive role of organicmatter (see Figure S2, Supporting Information).

Generally, elemental Se occurs as amorphous and crys-talline phases. The crystalline forms can further be dividedinto the monoclinic Se (informally called the “red” Se) andhexagonal Se (“gray” Se). To identify the possible crystallinestructure of native Se grains, samples of kerogen with veryhigh Se contents (up to 31 000 ppm, which can providereliable peak intensities) were analyzed by powder X-raydiffraction (19) (Figure 4). In addition to a small amount ofbarium fluorosilicate (BaSiF6) and graphite, the X-ray dif-fraction spectra revealed an amorphous structure of el-emental Se because Se peaks did not appear in d spacingmeasurements. Barium fluorosilicate minerals may haveformed after chert was decomposed by HF and the appear-

ance of graphite suggests that a small part of the kerogen hasbeen degraded to the graphite phase.

At the Yutangba deposit, Se occurs mainly as primaryelemental Se nanograins with amorphous structures andembedded in kerogen. This kind of Se occurrence is fairlyspecific in organic matter, and a few related studies havebeen reported in the literature.

DiscussionCombined TEM, XRD, and infrared spectra studies revealeddifferent chemical forms of Se in kerogen from the Laermaand Yutangba deposits, even though these two deposits arecomparable with each other by having the same specific rockassemblage (carbonaceous cherts and carbonaceous shales)and showing the same type of organic matter (sapropel-type).

Selenium Speciation in Kerogen. Two main physicalforms for Se were observed: (1) nanograins of elemental Se(Se0) embedded in kerogen and (2) homogeneously distrib-uted Se within the kerogen matrix. For the latter form, twotypes of Se occurrence are possible: (1) extremely smallelemental Se grains distributed in kerogen homogeneously,which were difficult to be detected and identified underbackscattered electron images, and (2) organically boundSe. Indeed, Se has a strong affinity with organic matter, andabundant biochemical evidence shows its strong tendencyto be enriched in an organism (20). Common Se-organicbonds include Se-H, O-Se-O, Se-C, Se-N, and so forth(21). The infrared spectra results suggested that the functionalgroups in kerogen are mainly oxygen-bearing radicals, whichfavor the formation of covalent bonds with Se as mentionedabove. Therefore, it is likely that Se might be stably attachedto kerogen via bonding with radicals such as -COOH, -OH,and -NH2. Theoretically, low-maturity organic matter (kero-gen, Table 1) containing more oxygen-bearing radicals, suchas that from the Yutangba deposit, is more favorable for Seto form covalent bonds with oxygen-bearing radicals in

FIGURE 3. (A, D, E) TEM image of an elemental Se grain in kerogen from the Yutangba Se deposit; (B) EDS of the elemental Se grainin plate A indicated by arrow 1; (C) EDS of kerogen collected from the area in plate A indicated by arrow 2.

FIGURE 4. XRD pattern of kerogen with a high Se concentration,collected from the Yutangba Se deposit. The shaded peaks representthe d spacing of graphite (d1/ln ) 3.35, d2/ln ) 1.67, d3/ln ) 1.54);other peaks represent the d spacing of barium fluosilicate (d1/ln) 3.58, d2/ln ) 1.95, d3/ln ) 2.23); peaks of the crystalline seleniumshould appear at d1/ln ) 3.00, d2/ln ) 3.78, d3/ln ) 2.07 indicatedby the dashed line in the figure.

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kerogen than that with higher-maturity organic matter fromthe Laerma deposit. Therefore, Yutangba ore rocks shouldpreferentially display organically bound Se. However, theinverse situation was revealed by EDS spectra of kerogenfrom both deposits.

Sulfur elemental analyses of kerogen from the Laermadeposit indicated higher organic sulfur contents than thatfrom Yutangba Se deposit (Table 1), which was confirmedby the EDS spectra. In addition, many sulfur-bearing organiccompounds have been identified by GC-MS measurementof the Laerma deposit ore rocks, such as alkylthiophene series,alkyl-tetrahydro-thiophene series, dibenzothiophene series,methyldibenzothiophene series, methyl-benzothiopheneseries, and so forth (22). Se may combine with organic matterby substitution to sulfur (23) as it is a common constituentin most organisms and because of the great similarity inchemical properties between these two elements. At themolecular level, Se occurs in analogues of sulfur-containingamino acids (e.g., selenomethionine, selenocysteine) and isfound in diverse enzymes (24). Previous experiments havesuggested the uptake of Se by sulfur-containing biologicalspecies such as algae, bacteria, as well as viruses (25). Nelsonet al. (22) showed that between 1.1% and ∼96.0% (averaging13.8%) of sulfur in microorganisms can be replaced by Se toform species such as R-SxSe1-x with substitutions beingpossibly enhanced in dilute solution. Thus, organically boundSe is expected in organic matter, and our results suggest thatthis Se eventually becomes enriched in the kerogen fraction,presumably by substituting for S, and now represents up to75% of the total Se found in the ore body of the Laermadeposit.

Redox Conditions. In the Yutangba Se deposit, explorationwork, carried out by the third geological team of the HubeiProvince, China Geological Survey, indicated that all the orebodies were distributed just above or in the close vicinity ofthe actual water table (8), which is favorable to redoxreactions. The mineralized environment is similar to the fieldof “common soil conditions” in an Eh-pH diagram for thesystem Se-H2O (Figure 5). In this redox field (oxidizingconditions area), Se-2 in rocks and ores is readily oxidizedto 0, +4, or +6 valences. The oxidizing environment doesnot favor the bonding of S into kerogen (29) and similarly Sebecause of the limitation and instability of bonding withoxygen-bearing radicals. On the other hand, Se+4 and Se+6

may be readily reduced again as the water migrates deeperand eventually encounters lower redox potential. In Yutangba,this may be the case as high Se+4 or Se+6 solutions enteredfractures or cavities in rocks rich in organic matter. In Figure5, the shaded area (dash-dotted lines) shows the possible

predominance of elemental Se and Se minerals under thepH values (ranging from 4 to 10) found in the common soilconditions. In the field of water stability, elemental Se0

predominates over H2Se and HSe- species with -2 anionvalence, although the relative proportions may change slightlyby changing dissolved Se concentration or by adding othermetals in solution (30). Elemental Se grains found in fracturesand cavities of organic matter were preserved with relativestability because the organic matter acted as a reducingbarrier. Indeed, the dominance of elemental Se0 in theYutangba kerogen implies that further reduction into anionSe-2 was limited. This is probably because that under thecommon conditions (hatching lines), the Eh of the infiltratingsolutions did not get low enough to produce Se-2, which isin accordance with the mineral associations as shown inTable S1 (see Table S1, Supporting Information), or that,once formed, the Se0 is kinetically resistant to furtherreduction. As a result, selenides may be absent or may bemarginally formed under local stronger redox conditions.

Compared to the Yutangba Se deposit, the mineralizingenvironment of the Laerma Se-Au deposit was relativelysimple and was probably a closed reducing system, whereSe-2 was the dominant form (11). This environment isfavorable to both the preservation of high sulfur kerogenand the formation of organic Se compounds by substitutionto sulfur.

Environmental Implications. Our results, which showthat 66% of total Se at the Yutangba deposit is amorphouselemental Se in kerogen, are of great importance for bothfurther exploration work and ore-melting processes. Theseobservations also provide new information regarding thesudden prevalence of Se poisoning in the Yutangba region.Indeed, although native Se has a large stability field in theEh-pH diagram of Figure 5, Zhu et al. (15) suggested thatnative Se in altered carbonaceous cherts and shales may bedissolved, remobilized, and transported to produce newnative Se. However, while the Se-rich formations extendedseveral tens of kilometers in the Yutangba region, restrictedareas and specific groups of local residents were affected bySe poisoning (10). Besides natural factors (heavy rain) andhuman land use, exploitation and utilization of stone coalby local people seemed to be significant in the extent ofpoisoning of the population (15).

Vegetation Se concentration depends significantly on thesoluble and bioavailable fraction of soil Se and not necessarilyon the total Se in soils (31). Furthermore, the mobility of soilSe is mainly controlled by Fe(III), Mn, and Al oxides in moistand semimoist acidic soils and by Ca, Mg, and K oxides insemidry alkaline soils (32). Se is usually not toxic in areasdominated by sulfide-rich rocks where Se is present at theppm level. Weathered sulfides form iron oxide such as Fe-rich gossans where specific adsorption of Se oxyanion takesplace by ion exchange with surface groups of hydrous ironoxides and of hematite (Fe2O3). In this case, a hydrous surfacefilm having the adsorption properties of goethite (FeOOH)forms rapidly (33, 34). Experimental work indicates thatadsorption on hydrous ferric oxides removes between 95%and 99% of the Se oxyanion from solutions having a pH valueof 8 and lower (33, cited therein). The formation of metalselenites, or sometimes observed selenides (like ferroselite,FeSe2), with heavy metals (Fe, Cu, Pb, Cd, Hg, etc.) preventsdissolution and thereby decreases the toxicity of these metalsin soils (32). Sulfur-related Se in Yutangba and Laermadeposits might form stable metal selenites after beingweathered so that its bioavailability remains restricted.

Very few studies exist on the mobility of organically boundSe in kerogen in contrast to other Se occurrences. Thechemical structure of kerogen will probably control itschemical stability and degradation kinetics. Kulp and Pratt(34) studied Se speciation in samples of upper Cretaceous

FIGURE 5. Eh-pH diagram for the system Se-H2O under conditionsof total Se ) 10-6 molar/L, Po2 ) 1 bar, and T ) 25 °C. The shadowarea represents the possible predominance field to form theelemental Se and other Se minerals. Properties for Se species arefrom refs 5, 7, 26-28 and from SUPCRT92 software.

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chalk and shale from South Dakota and Wyoming by thesequential extraction method. They found that the majorityof Se was associated with the residual fraction (mainlykerogen), averaging 41.9% in chalk and 35.2% in shales. Seremained in the residual fraction after six steps of pre-extraction by water, phosphate, NaOH, sodium sulfite, aceticacid, and Cr(II) reduction/volatilization, respectively. Thissuggests that elemental Se and metal selenides may formdissolved Se-2 under alkaline and reductive conditions. Whenredox conditions become more oxidant, Se-2 transformseasily into oxidized Se+4 or Se+6. In the experimentalconditions, organically bound Se in kerogen was moreresistant to dissolution, suggesting that this Se form is morestable over a given range of redox conditions. It is assumedthat very high oxidizing conditions are needed to oxidizekerogen and release the organically bound Se as an aqueousand bioavailable ion.

These observations and interpretations have strongimplications for the mobility of Se occurring at the YutangbaSe deposit and the Laerma Se-Au deposit. We suggest thatelemental Se is the chemical form most readily mobilized byalteration. Indeed, although the kerogen coating certainlymay act as a barrier for weathering agents to oxidize Se0,naturally altered rocks and anthropogenic treatments mayresult in kerogen degradation and may expose elemental Seto oxidation. The fact that 66% of total Se occurred aselemental Se at the Yutangba deposit provided new evidencefor its potential mobility during weathering, thus explainingthe sudden prevalence of Se poisoning in this area. In contrast,organically bound Se, which represents up to 75% of totalSe found at the Laerma deposit, seems more resistant tochemical alteration. The combination of Se organic com-pounds in kerogen and abundant sulfides limits considerablythe formation of soluble Se and its subsequent bioavailability.In addition to Se speciation, the Laerma deposit is locatedin a high altitude region with a cold climate, where physicalalteration dominates chemical weathering. The oppositesituation is found in the Yutangba region. Accordingly,combined with the high metal contents and the cold and dryclimate, sulfur-related and organically bound Se from theLaerma Au-Se deposit is not likely to be mineralized bynatural chemical weathering agents so that its bioavailabilityis very restricted.

AcknowledgmentsThis project was financially supported by the China NSF(No. 40003008 and No. 60633110). The paper benefited fromthe suggestions made by two anonymous reviewers and isimproved with careful English editing by Dr. Alice Williamsof CRPG-CNRS.

Supporting Information AvailableDetails of site locations for two Se deposit and its geologyand geochemistry; additional TEM image and EDS spectrafor elemental Cu grain in kerogen. This material is availablefree of charge via the Internet at http://pubs.acs.org.

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Received for review August 25, 2005. Revised manuscriptreceived December 13, 2005. Accepted December 16, 2005.

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