Selenium Speciation in Kerogen from Two Chinese Selenium

Jan 19, 2006 - In one of these areas (Yutangba), a serious environmental impact happened involving Se poisoning. Previous studies ... Citation data is...
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Environ. Sci. Technol. 2006, 40, 1126-1132

Selenium Speciation in Kerogen from Two Chinese Selenium Deposits: Environmental Implications H A N J I E W E N , * ,†,‡ J E A N C A R I G N A N , ‡ YUZHUO QIU,† AND SHIRONG LIU† Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550002 China, and Centre de Recherches Petrographique 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 environment is characterized by specific chemical and biochemical properties that control its elemental solubility, toxicity, and environmental behavior. The Laerma Se-Au deposit and Yutangba Se deposit are two important Se-bearing deposits found recently in China. In one of these areas (Yutangba), a serious environmental impact happened involving Se poisoning. Previous studies have shown that Se in both deposits is closely related to organic matter, especially kerogen fractions, but detailed relationships between Se and kerogen and Se chemical forms were not reported. In this study, the different speciation of Se is identified by transmission electron microscopy (TEM) and other geochemical techniques (infrared spectra (IS) and X-ray diffraction (XRD)) from kerogen samples extracted from ore rocks of both deposits. The occurrence of organically bound Se in the Laerma deposit and elemental Se nanograins in the Yutangba deposit is observed, indicating the diversity of formation mechanisms and possible chemical forms of Se in Se-rich rocks. The formation of elemental Se associated with organic matter is likely related to redox conditions, whereas organic species are related to the higher sulfur content of kerogen and possibly result from S-Se substitutions. This discovery provides new evidence with which to assess potential Se mobility during weathering of ore-bearing rocks. In an altered rock, the elemental Se in kerogen is more steadily mobilized and is potentially accumulated by vegetation, which may explain the sudden prevalence of Se poisoning in the Yutangba area. In contrast, organically bound Se seems more resistant to chemical alteration compared to other Se species so that its bioavailability may be very restricted.

Introduction Selenium is an essential trace element for human and animal health and vegetation. However, its overabundance or depletion may cause serious biological and ecological problems, such as Se toxicosis (Se excess) and Chronic Keshan disease (because of Se depletion). Indeed, in 1963, a major * Corresponding author phone: (33) 3 83 59 42 11; fax: (33) 3 83 51 17 98; e-mail: [email protected]. † Chinese Academy of Sciences. ‡ Centre de Recherches Petrographique et Geochimiques. 1126

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incident related to Se poisoning happened in the Yutangba region of China, where villagers were forced to evacuate their homes (1, 2). The main industrial source of Se is mainly restricted to ore deposits that are genetically associated with volcanism, where it occurs as an accessory element. Volcanogenic gold and base-metal massive sulfide deposits are potentially the most viable Se sources for industrial purposes (3-7). However, the recent discovery of Se-rich deposits (the Laerma Se-Au deposit and the Yutangba Se deposit) and Se-bearing formations in central China, such as those in the Laerma region near the boundary between Sichuan and Gansu Provinces, the Ziyang-langao region of southern Shaanxi Province, and the northwestern Hunan and western Hubei Provinces in China, have shown the potential for Se to be a major element in some ore deposits (8, 9). These Se-bearing formations in China may have been responsible for some major environmental hazards in these areas. However, hazards such as poisoning of the population have not been found in all high Se regions. Even in an isolated high Se region, the biological and ecological effects of Se were clearly variable (2, 10). Most Se-bearing formations in China comprise carbonaceous cherts and shales containing abundant organic matter with which Se is either bounded or adsorbed (9, 11). However, the mechanisms of Se enrichment associated with organic matter and Se speciation in solid organic matter (kerogen) remain unclear, although the chemical form of Se is of primary importance in estimations of its potential mobility and toxicity in the environment. Here, we report transmission electron microscope (TEM) images, X-ray diffraction, and infrared spectra of Se-bearing kerogen from two major Se ores in China, the Laerma Se-Au deposit and the Yutangba Se deposit. We found that, in the fresh rocks from the ore bodies, Se may either occur as (1) elemental nanograins or as (2) organically bound or adsorbed species according to redox conditions prevailing during Se mineralization and to the sulfur content in kerogen, respectively. Selenium mobility in the environment highly depends on its speciation, the organically bound species probably being very resistant to dissolution.

Geological Setting The Laerma Se-Au deposit and Yutangba Se deposit are hosted in the selenium-bearing formations (SBFs) which were defined by Wen and Qiu in 2002 (9) (Se-bearing formation -SBF- is a suite of rocks with Se and other multielement anomalies, the Se concentration generally being larger than 5 ppm, and is characterized by specific temporal-spatial distribution characteristics). Detailed petrology, mineralogy, geochemistry, and organic biomarker studies have shown that several selenium minerals occur in the Laerma deposit (12, 13), and a few of these have been identified in the Yutangba Se deposit (14, 15). Our preliminary studies also indicated that about 75% and 66% of total Se in these ore deposits, respectively, are closely associated with or occur within the solid organic matter (11). The geological setting and geochemical data are listed in Table S1 (see Table S1, Supporting Information). The Laerma Se-Au Deposit. This deposit is located at the plunging end in the western part of the Baiyigou Anticline in the Qingling region along Sichuan-Guansu boundary (See Figure S1, Supporting Information). It is hosted in lower Cambrian Taiyangding Group which comprises a series of carbonaceous cherts and slates. It is characterized by abundant organic matter and anomalies in Au, Se, U, Cu, 10.1021/es051688o CCC: $33.50

 2006 American Chemical Society Published on Web 01/19/2006

Mo, Sb, and PGE. This siliceous formation occurs not only throughout West Qinling but also extends eastward to East Qinling, such as in the Ziyang, Langao, and Ankang regions in the Shaanxi Province. Being apparently lithologically controlled, the Se ore bodies are mostly distributed in cherts and transitional facies to slates, both vertically and horizontally. Morphologically, the ore bodies are generally lenses of varying scales, ranging from tens to hundreds of meters, with minor veins. Se is present in relatively high concentrations 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 in slate-type ores. Locally, Se may reach extreme concentrations of up to 500 ppm. The Laerma deposit was formed at intermediate to low temperatures (142-269 °C) and at low pressures (9-30 Mpa) and gold and Se may have been transported as S-Se-Au complex (12), which is typical of hydrothermal deposits associated with submarine exhalations. A large number of selenium minerals and Se-bearing minerals 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 different inorganic species of Se were identified, Wen and Qiu (11) showed experimentally that about 75% of total Se was closely related to organic matter for both chert and slate formations. The Yutangba Se Deposit. The Yutangba Se deposit is the only sedimentary-type Se deposit known in the world to date. It is located in the northern wing of Suanhe syncline in the northeastern part of upper Yangtze platform fold belt (see Figure S1, Supporting Information). The ore-bearing layers, extending thousands of meters, are found between the carbonaceous chert and carbonaceous shale of the Lower Permian Maokou formation. The nine orebodies, so far discovered, are distributed along the abovementioned lithological interfaces and are mainly lenticular, ranging from 30 m to 150 m in length, from 0.7 m to 5.2 m in thickness, and from 14 m to 35 m in depth. The ores show typical syngenetic sedimentary characters, common with aphanitic and biogenic textures, and laminated and massive structures. Se is present in relatively high abundance in both host rocks and ores, with up to 1.3% on average in the enriched zones. A few Se minerals, including klockmannite (CuSe), eskebornite (CuFeSe2), chalcomenite (CuSeO3‚2H2O), and native Se, have been identified and a small amount of Se was incorporated into the lattice of pyrite because of isomorphous substitution. Therefore, Song (8) suggested that Se occurred mainly in the form of adsorbed Se to organic matter, accounting for 66% of the total Se. On the other hand, Zheng et al. (16) proposed that Se occurred mainly in the form of microparticulates of elemental Se in association with organic matter. Zhu et al. (15) provided further evidence regarding the occurrence of native selenium, but their work was mainly concerned with altered rocks and burned coal stones. Se occurrences have been divided into three categories: (1) the primary native Se occurring in carbonaceous-siliceous rocks, (2) micro-Se crystals formed during the weathering of Serich rocks, and (3) larger Se crystals related to combustion of stone coal (15). However, relationships between primary native Se and carbonaceaous rocks were not fully demonstrated and need further investigation. Yao et al. (14) proposed that Se was derived from the volcanic matter related to the Emeishan basalt eruption. These distal sources of magmas and hydrothermal activity must have supplied large amounts of Si and Se. Under such environmental conditions, the high productivity of siliceous plankton may have consumed the excess Se and Si, died rapidly, and fallen to the sea floor to form Se-rich carbonaceous cherts.

Materials and Methods Preparation of Samples. The Laerma Se-Au deposit was sampled at two mining pits that have been mined as major ore bodies. Samples were also collected from engineering drill cores. The Yutangba Se deposit was sampled at one mining pit and two outcrops that represent major ore veins. The samples are relatively with less weathered surfaces. Major and trace elements, organic carbon, and total sulfur were analyzed for 48 samples from the Laerma deposit and for 20 samples from the Yutangba deposit. Major results have been described by Wen and Qiu (9, 11) and are reported in Table S1 (see Table S1, Supporting Information). Subsequently, on the basis of the chemical data and sample distribution, 12 samples from the Laerma deposit and 10 samples from the Yutangba deposit were selected for kerogen extraction and were prepared for TEM analysis. We believe that these rocks reflect the mineralogy and chemistry of the original ore bodies. Samples collected from the abovementioned deposits were treated to obtain over 98% pure kerogen. The details of the experimental procedure are as follows. After removing the weathered surface, samples were cleaned with distilled water and were dried and then were crushed to -200 meshes. The powdered samples were treated with CHCl3 in a Soxhlet’s extractor to extract soluble organic matter and then were treated with HF-HCl to remove silicate and carbonate minerals. Kerogen and sulfides (mainly pyrite) were obtained from the solid residue and were separated from each other using heavy liquid (CHBr3). To get highpurity kerogen samples, this procedure was repeated at least three times until no sulfide could be observed under a binocular microscope. Chemical purification involving oxidation or reduction, as described by Fu and Qin (17), was avoided here in part because of the low sulfide concentration in ores but mostly because of the high volatility of Se under either oxidizing or reducing conditions. Finally, the kerogen separated from collected samples was frozen and dried. Transmission Electron Microscopy (TEM). A small aliquot of purified kerogen was pulverized in alcohol and was dispersed ultrasonically. Afterward, a drop of suspended liquid was dripped on carbon-Formvar-coated 200-mesh copper or nickel grids and was dried by airing. To eliminate background noise, copper and nickel grids were used alternately for all samples. Images were obtained with a JEM200FX TEM at 140 V under standard operating conditions with the liquid nitrogen anticontaminator in place. The energy-dispersive X-ray spectroscopy (EDS) was performed at 35 kV by using a current of 10 mA and a counting time of 100 s (EDS model: JF-1). The magnification observed was adjusted in accordance with the grain size of samples. All experiments were conducted in the TEM laboratory of the Institute of Geochemistry, Chinese Academy of Sciences. X-ray Diffraction (XRD) and Infrared Spectra (IS). Powder X-ray diffraction (XRD) measurements were made on a Rigaku D/MaxγA X-ray diffractometer with Cu KR radiation (λ ) 1.54178Å). The infrared spectra used a Nicolet 750-type Fourier transform infrared spectrometer purged with nitrogen gas that incorporated a DTGS detector.

Results TEM observations showed that there was no other impurity found in kerogen samples except for very minor sulfide and silicate minerals. The TEM images of kerogen from the two deposits were similar, showing cellular noncrystal grained texture and amorphous fibrous aggregation structure. Carbon, sulfur contents, and maturity degree in kerogen from both deposits slightly differed from one another (Table 1); however, detailed observations have shown significant differences in Se occurrences in kerogen from the two deposits. VOL. 40, NO. 4, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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

Corg(%) R0(maturity degree) S in kerogen (%)

Laerma Se-Au deposit

Yutangba Se deposit

1.46 (48)a 2.62-3.52 (7)a (3.14)b 1.8 (3)a

20.4% (20)a 1.67-3.23 (8)a (2.53)b 0.6 (3)a

a The number of samples determined, in brackets. in brackets.

b

The mean value,

The Laerma Se-Au Deposit. With the exception of a few Se-bearing minerals, such as Se-bearing pyrite, Se-bearing arsenopyrite, and tiemannite (relic minerals after samples treatments), only one elemental Se particle was found in samples from the Laerma Se-Au deposit. The Se grain was embedded in the kerogen and was approximately 300 nm in size (Figure 1A). The result of EDS analysis showed almost pure elemental Se with low peaks of S, C, O, and Ca (Figure 1B). The color of the kerogen from the Laerma deposit was usually dark and gray as observed under TEM, suggesting a relatively high degree of aromatization (18), which is consistent with the results of organic geochemistry and biomarker studies (9). This kerogen contained significant mineral 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). Amorphous silicon may have been incorporated into kerogen, since chert was hosting the organic matter. This would readily explain the presence of Si peaks in the EDS spectrum. The presence of As and Fe is more enigmatic, but these elements do not occur as nanometer-size minerals because of their homogeneous distribution in all kerogen samples, and this for different sizes of the electronic beam spot during the EDS analysis. Sulfur was present in kerogen from both deposits but, using the C peak in EDS spectra as a reference, the content of S in kerogen from the Laerma deposit was obviously higher than that in kerogen from the Yutangba deposit as suggested by elemental analysis (Table 1). Finally, EDS analyses on more than 30 points revealed that Se was contained in kerogen from the Laerma deposit and that its distribution was also homogeneous. The analysis of infrared spectra from the kerogen samples of Laerma Se-Au deposit was performed to evaluate the type of organic bonds Se was involved with. Characteristic 1128

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FIGURE 2. Infrared spectra of kerogen (sample L-4) from the Laerma Se-Au deposit. peaks were observed at 3392, 2914, 1718, and 1575 cm-1 in infrared spectra (17) (Figure 2). The broad peak in the vicinity of 3392 cm-1 resulted from -OH oscillation in hydroxyl, phenol-hydroxy, and H2O. The small peaks at 2914 and 2853 cm-1 may indicate the presence of alkyl and alkene, while the minor peak at 1718 cm-1 may be attributed to CdO in carbonyl and carboxyl. The prominent peak at 1575 cm-1 was a reflection of CdO oscillation in acid hydroxy and quinone and of the CdC oscillation in alkene, aromatic ring, and polycyclic aromatic hydrocarbon. The strong peak at 1081 cm-1 was clearly caused by quartz impurity. The infrared spectra of samples from the Laerma deposit suggest that the functional groups in kerogen are mainly oxygen-bearing radicals. The Yutangba Se Deposit. In contrast to kerogen from the Laerma deposit, a large amount of elemental Se grains were identified in kerogen from this Se deposit (Figure 3A, D, E). The elemental Se particles were attached to or embedded in kerogen, sometimes with a concentric morphology. Most of the elemental Se particles were of nanograin size with the largest grains observed being approximately 500 nm in diameter. The EDS spectra obtained from these Se grains indicated that they were made of pure Se (Figure 3B). Cl peaks might reflect relics of HCl from sample preparation. C and O peaks were most likely associated with kerogen, which is mainly comprised of C, H, O, S, N, and P. The lack of any other metal peaks in the spectrum indicates that Se is in its elemental state [Se0] rather than as a metal selenide [Se-2]. In contrast to Laerma kerogen, no Se was detected in the pure kerogen fraction, for which only

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 grain in plate A indicated by arrow 1; (C) EDS of kerogen collected from the area in plate A indicated by arrow 2. ance of graphite suggests that a small part of the kerogen has been degraded to the graphite phase. At the Yutangba deposit, Se occurs mainly as primary elemental Se nanograins with amorphous structures and embedded in kerogen. This kind of Se occurrence is fairly specific in organic matter, and a few related studies have been reported in the literature.

Discussion FIGURE 4. XRD pattern of kerogen with a high Se concentration, collected from the Yutangba Se deposit. The shaded peaks represent the 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 selenium should appear at d1/ln ) 3.00, d2/ln ) 3.78, d3/ln ) 2.07 indicated by the dashed line in the figure.

characteristic peaks of C, O, and S, and a few Ca and V, were identified by EDS analyses (Figure 3C). In addition, a few native copper grains were also observed in some kerogen samples, indicative of the strong reductive role of organic matter (see Figure S2, Supporting Information). Generally, elemental Se occurs as amorphous and crystalline phases. The crystalline forms can further be divided into the monoclinic Se (informally called the “red” Se) and hexagonal Se (“gray” Se). To identify the possible crystalline structure of native Se grains, samples of kerogen with very high Se contents (up to 31 000 ppm, which can provide reliable peak intensities) were analyzed by powder X-ray diffraction (19) (Figure 4). In addition to a small amount of barium fluorosilicate (BaSiF6) and graphite, the X-ray diffraction spectra revealed an amorphous structure of elemental Se because Se peaks did not appear in d spacing measurements. Barium fluorosilicate minerals may have formed after chert was decomposed by HF and the appear-

Combined TEM, XRD, and infrared spectra studies revealed different chemical forms of Se in kerogen from the Laerma and Yutangba deposits, even though these two deposits are comparable with each other by having the same specific rock assemblage (carbonaceous cherts and carbonaceous shales) and showing the same type of organic matter (sapropeltype). Selenium Speciation in Kerogen. Two main physical forms for Se were observed: (1) nanograins of elemental Se (Se0) embedded in kerogen and (2) homogeneously distributed Se within the kerogen matrix. For the latter form, two types of Se occurrence are possible: (1) extremely small elemental Se grains distributed in kerogen homogeneously, which were difficult to be detected and identified under backscattered electron images, and (2) organically bound Se. Indeed, Se has a strong affinity with organic matter, and abundant biochemical evidence shows its strong tendency to be enriched in an organism (20). Common Se-organic bonds include Se-H, O-Se-O, Se-C, Se-N, and so forth (21). The infrared spectra results suggested that the functional groups in kerogen are mainly oxygen-bearing radicals, which favor the formation of covalent bonds with Se as mentioned above. Therefore, it is likely that Se might be stably attached to kerogen via bonding with radicals such as -COOH, -OH, and -NH2. Theoretically, low-maturity organic matter (kerogen, Table 1) containing more oxygen-bearing radicals, such as that from the Yutangba deposit, is more favorable for Se to form covalent bonds with oxygen-bearing radicals in VOL. 40, NO. 4, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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

kerogen than that with higher-maturity organic matter from the Laerma deposit. Therefore, Yutangba ore rocks should preferentially display organically bound Se. However, the inverse situation was revealed by EDS spectra of kerogen from both deposits. Sulfur elemental analyses of kerogen from the Laerma deposit indicated higher organic sulfur contents than that from Yutangba Se deposit (Table 1), which was confirmed by the EDS spectra. In addition, many sulfur-bearing organic compounds have been identified by GC-MS measurement of the Laerma deposit ore rocks, such as alkylthiophene series, alkyl-tetrahydro-thiophene series, dibenzothiophene series, methyldibenzothiophene series, methyl-benzothiophene series, and so forth (22). Se may combine with organic matter by substitution to sulfur (23) as it is a common constituent in most organisms and because of the great similarity in chemical properties between these two elements. At the molecular level, Se occurs in analogues of sulfur-containing amino acids (e.g., selenomethionine, selenocysteine) and is found in diverse enzymes (24). Previous experiments have suggested the uptake of Se by sulfur-containing biological species such as algae, bacteria, as well as viruses (25). Nelson et al. (22) showed that between 1.1% and ∼96.0% (averaging 13.8%) of sulfur in microorganisms can be replaced by Se to form species such as R-SxSe1-x with substitutions being possibly enhanced in dilute solution. Thus, organically bound Se is expected in organic matter, and our results suggest that this Se eventually becomes enriched in the kerogen fraction, presumably by substituting for S, and now represents up to 75% of the total Se found in the ore body of the Laerma deposit. Redox Conditions. In the Yutangba Se deposit, exploration work, carried out by the third geological team of the Hubei Province, China Geological Survey, indicated that all the ore bodies were distributed just above or in the close vicinity of the actual water table (8), which is favorable to redox reactions. The mineralized environment is similar to the field of “common soil conditions” in an Eh-pH diagram for the system Se-H2O (Figure 5). In this redox field (oxidizing conditions area), Se-2 in rocks and ores is readily oxidized to 0, +4, or +6 valences. The oxidizing environment does not favor the bonding of S into kerogen (29) and similarly Se because of the limitation and instability of bonding with oxygen-bearing radicals. On the other hand, Se+4 and Se+6 may be readily reduced again as the water migrates deeper and eventually encounters lower redox potential. In Yutangba, this may be the case as high Se+4 or Se+6 solutions entered fractures or cavities in rocks rich in organic matter. In Figure 5, the shaded area (dash-dotted lines) shows the possible 1130

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predominance of elemental Se and Se minerals under the pH values (ranging from 4 to 10) found in the common soil conditions. In the field of water stability, elemental Se0 predominates over H2Se and HSe- species with -2 anion valence, although the relative proportions may change slightly by changing dissolved Se concentration or by adding other metals in solution (30). Elemental Se grains found in fractures and cavities of organic matter were preserved with relative stability because the organic matter acted as a reducing barrier. Indeed, the dominance of elemental Se0 in the Yutangba kerogen implies that further reduction into anion Se-2 was limited. This is probably because that under the common conditions (hatching lines), the Eh of the infiltrating solutions did not get low enough to produce Se-2, which is in accordance with the mineral associations as shown in Table S1 (see Table S1, Supporting Information), or that, once formed, the Se0 is kinetically resistant to further reduction. As a result, selenides may be absent or may be marginally formed under local stronger redox conditions. Compared to the Yutangba Se deposit, the mineralizing environment of the Laerma Se-Au deposit was relatively simple and was probably a closed reducing system, where Se-2 was the dominant form (11). This environment is favorable to both the preservation of high sulfur kerogen and the formation of organic Se compounds by substitution to sulfur. Environmental Implications. Our results, which show that 66% of total Se at the Yutangba deposit is amorphous elemental Se in kerogen, are of great importance for both further exploration work and ore-melting processes. These observations also provide new information regarding the sudden prevalence of Se poisoning in the Yutangba region. Indeed, although native Se has a large stability field in the Eh-pH diagram of Figure 5, Zhu et al. (15) suggested that native Se in altered carbonaceous cherts and shales may be dissolved, remobilized, and transported to produce new native Se. However, while the Se-rich formations extended several tens of kilometers in the Yutangba region, restricted areas and specific groups of local residents were affected by Se poisoning (10). Besides natural factors (heavy rain) and human land use, exploitation and utilization of stone coal by local people seemed to be significant in the extent of poisoning of the population (15). Vegetation Se concentration depends significantly on the soluble and bioavailable fraction of soil Se and not necessarily on the total Se in soils (31). Furthermore, the mobility of soil Se is mainly controlled by Fe(III), Mn, and Al oxides in moist and semimoist acidic soils and by Ca, Mg, and K oxides in semidry alkaline soils (32). Se is usually not toxic in areas dominated by sulfide-rich rocks where Se is present at the ppm level. Weathered sulfides form iron oxide such as Ferich gossans where specific adsorption of Se oxyanion takes place by ion exchange with surface groups of hydrous iron oxides and of hematite (Fe2O3). In this case, a hydrous surface film having the adsorption properties of goethite (FeOOH) forms rapidly (33, 34). Experimental work indicates that adsorption on hydrous ferric oxides removes between 95% and 99% of the Se oxyanion from solutions having a pH value of 8 and lower (33, cited therein). The formation of metal selenites, or sometimes observed selenides (like ferroselite, FeSe2), with heavy metals (Fe, Cu, Pb, Cd, Hg, etc.) prevents dissolution and thereby decreases the toxicity of these metals in soils (32). Sulfur-related Se in Yutangba and Laerma deposits might form stable metal selenites after being weathered so that its bioavailability remains restricted. Very few studies exist on the mobility of organically bound Se in kerogen in contrast to other Se occurrences. The chemical structure of kerogen will probably control its chemical stability and degradation kinetics. Kulp and Pratt (34) studied Se speciation in samples of upper Cretaceous

chalk and shale from South Dakota and Wyoming by the sequential extraction method. They found that the majority of Se was associated with the residual fraction (mainly kerogen), averaging 41.9% in chalk and 35.2% in shales. Se remained in the residual fraction after six steps of preextraction by water, phosphate, NaOH, sodium sulfite, acetic acid, and Cr(II) reduction/volatilization, respectively. This suggests that elemental Se and metal selenides may form dissolved Se-2 under alkaline and reductive conditions. When redox conditions become more oxidant, Se-2 transforms easily into oxidized Se+4 or Se+6. In the experimental conditions, organically bound Se in kerogen was more resistant to dissolution, suggesting that this Se form is more stable over a given range of redox conditions. It is assumed that very high oxidizing conditions are needed to oxidize kerogen and release the organically bound Se as an aqueous and bioavailable ion. These observations and interpretations have strong implications for the mobility of Se occurring at the Yutangba Se deposit and the Laerma Se-Au deposit. We suggest that elemental Se is the chemical form most readily mobilized by alteration. Indeed, although the kerogen coating certainly may act as a barrier for weathering agents to oxidize Se0, naturally altered rocks and anthropogenic treatments may result in kerogen degradation and may expose elemental Se to oxidation. The fact that 66% of total Se occurred as elemental Se at the Yutangba deposit provided new evidence for its potential mobility during weathering, thus explaining the sudden prevalence of Se poisoning in this area. In contrast, organically bound Se, which represents up to 75% of total Se found at the Laerma deposit, seems more resistant to chemical alteration. The combination of Se organic compounds in kerogen and abundant sulfides limits considerably the formation of soluble Se and its subsequent bioavailability. In addition to Se speciation, the Laerma deposit is located in a high altitude region with a cold climate, where physical alteration dominates chemical weathering. The opposite situation is found in the Yutangba region. Accordingly, combined with the high metal contents and the cold and dry climate, sulfur-related and organically bound Se from the Laerma Au-Se deposit is not likely to be mineralized by natural chemical weathering agents so that its bioavailability is very restricted.

Acknowledgments This project was financially supported by the China NSF (No. 40003008 and No. 60633110). The paper benefited from the suggestions made by two anonymous reviewers and is improved with careful English editing by Dr. Alice Williams of CRPG-CNRS.

Supporting Information Available Details of site locations for two Se deposit and its geology and geochemistry; additional TEM image and EDS spectra for elemental Cu grain in kerogen. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review August 25, 2005. Revised manuscript received December 13, 2005. Accepted December 16, 2005. ES051688O