Early diagenetic behaviour of selenium in freshwater sediments

Applied Geochemistry 15 (2000) 1439±1454

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Early diagenetic behaviour of selenium in freshwater sediments Nelson Belzile*, Yu.-Wei Chen, Rongrong Xu Department of Chemistry and Biochemistry, Laurentian University, Ramsey Lake Road, Sudbury, Ontario, Canada P3E 2C6 Received 9 December 1998; accepted 5 January 2000 Editorial handling by Y.K. Kharaka

Abstract The vertical distributions of dissolved Se species [Se(IV), Se(VI) and organic Se] and diagenetic constituents [Fe(II) and Mn(II)] were obtained in porewater samples of two Sudbury area lakes (Clearwater and McFarlane). The sedimentary concentration pro®les of total Se, Se species bound to Fe±Mn oxyhydroxides and to organic matter, and of elemental Se were also determined along with the concentrations of Fe, Mn and S in di€erent extractable fractions. Results indicated that the concentrations of total dissolved Se in porewater samples were very low, varying from around 2.0 nM to a maximum level of 6.5 nM, while the concentrations of total Se species in the solid phase varied between 2 and 150 nmol/g on a dry weight basis. The two lakes showed striking di€erences in the presence of Se(IV) and Se(VI) at the sediment±water interface (SWI). In Clearwater Lake, Se(VI) was present at this interface and Se(IV) was not detectable, whereas the opposite was found in McFarlane Lake. This suggests that reducing conditions might have existed near the SWI of McFarlane Lake at the sampling time; this hypothesis was con®rmed by several other measured chemical parameters. The pro®les of total dissolved Se of both lakes suggest upward and downward di€usion of dissolved Se species along the concentration gradients. Assuming that no precipitation occurred at the SWI, the ¯uxes of dissolved Se species across the SWI in Clearwater and McFarlane lakes were estimated to be 0.108 and 0.034 nmol cmÿ2 aÿ1, respectively. These values do not include the possible losses of volatile Se species due to microbial methylation. In the reducing sediments of both lakes, the formation of elemental Se and pyritic Se were found to be important mechanisms for controlling the solubility of Se in this environment. The main geochemical processes involving Se identi®ed in this study are: the adsorption of Se onto Fe±Mn oxyhydroxides at or near the SWI, the release of adsorbed Se by the reduction of the same oxyhydroxides and the mineralization of organic matter, and the removal of Se from porewaters to form elemental Se and a S mineral phase such as Se±pyrite or pure ferroselite. 7 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction Selenium is known for its dual essential and toxic

* Corresponding author. Tel.: +1-705-675-1151/2114; fax: +1-705-675-4844. E-mail address: [email protected] (N. Belzile).

character (Frankenberger and Benson, 1994). In most aquatic systems, Se can exist in four oxidation states, +VI, +IV, 0, and ÿII and in several organic forms (Cutter, 1982; Masscheleyn et al., 1989). The solubility and mobility of the Se species is largely dependent on pH and redox conditions (Masscheleyn et al., 1989; Weres et al., 1989). Thermodynamic calculations indicate that selenate, in the

0883-2927/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 8 8 3 - 2 9 2 7 ( 0 0 ) 0 0 0 1 1 - 1

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highly soluble form SeO42ÿ is the stable form of Se in oxic waters. Selenite SeO32ÿ is found in less oxic conditions and can be strongly adsorbed by Fe and Mn oxyhydroxides (Balistrieri and Chao, 1990). The microbial reduction of selenate and selenite to elemental Se (Garbisu et al., 1996; Oremland et al., 1990; Zhang and Moore, 1997) could be an important mechanism for the incorporation and retention of Se in soils and sediments since Se(0) occupies a large region of the Eh-pH stability ®eld. Similarly, the microbial oxidation of Se(0) to selenite and selenate can play a signi®cant role in oxic soils (Dowdle and Oremland, 1998). In sediments and their adjacent porewaters, Se is also subjected to chemically and/or microbiallymediated oxidation-reduction and methylation reactions often involving conversions between particulate and dissolved phases (Frankenberger and Engberg, 1998). Examples of such processes include the reduction of selenite Se(IV) and selenate Se(VI) to Se(0), the scavenging of Se(IV) by Fe and Mn oxides, the oxidation of organic selenide Se (ÿII), the precipitation of achavalite (FeSe) or ferroselite (FeSe2) or the incorporation of Se into solid phases such as pyrite (Belzile and Lebel, 1988; Myneni et al., 1997; Oremland et al., 1990; Tokunaga et al., 1997; Velinsky and Cutter, 1991). Under acidic and/or reducing conditions, ferric selenite may be reduced to elemental Se (Yao and Millero, 1995). Alkaline and oxidizing conditions favour the formation of selenate species, which are soluble and easily transported (Moore, 1991). The distribution of Se(IV) and Se(VI) in marine sediments and their porewaters appears to be related to changes in the redox environment of the sediments. Decrease of Se(IV) and Se(VI) in sediments could also be due to the formation of volatile (methylated) species (Frankenberger and Benson, 1994). Studies on freshwater Se geochemistry are scarce and it is crucial to understand the geochemical behaviour (diagenesis, solubility, mobility and transport) and biological availability of this potentially toxic element especially in an area that has been so strongly a€ected by Se pollution (Nriagu and Wong, 1983). The objectives of this study were to improve our knowledge on reactions that involve Se and modify its speciation in lake sediments. In this paper, the speciation of dissolved Se in porewater and the partitioning of solid Se species in sediments are presented for two Sudbury lakes di€ering in the acidity level and oxic status of their sediment±water interface. Other related parameters such as the dissolved and solid species of Fe, Mn and S were also determined. Based on these data, possible geochemical processes and di€usion mechanisms involving Se in oxic and anoxic freshwater environments are proposed.

2. Methodology

2.1. Sampling Sediment and their adjacent porewater samples were collected from two lakes, one acidic (Clearwater, 46822 ' N, 81803 ' W; pH=5.0) and one well bu€ered slightly alkaline (McFarlane, 46825 ' N, 80857 ' W; pH=7.5). The two lakes are located in the Sudbury area (Canada) about 15 km south of the metalliferous (Ni, Cu) Sudbury Nickel Irruptive. The two lakes are located only 5 km apart and therefore receive similar atmospheric loading of trace metals from the smelters nearby. Samples were collected at littoral sites in both lakes at depths of 5 and 7 m in Clearwater and McFarlane, respectively. For porewater samples, in situ di€usion samplers (peepers) were used. In this sampling approach, a volume of demineralized water contained with a membrane was allowed to equilibrate in situ with the interstitial waters (Carignan et al., 1985). The peeper consists essentially of a sheet of acrylic (plexiglass) into which two vertical rows of horizontal chambers (1 cm apart; total volume of one chamber 1 3.3 ml) were machined. The compartments were ®lled with demineralized water, and a sheet of ®ltration membrane (Gelman HT-200; 0.2 mm nominal pore size) was installed to cover all the compartments. A thin acrylic sheet with holes ®tting the compartment apertures was then screwed down tight to ®x the membrane and to isolate each cell. Before use, each sampler was immersed for at least 72 h in a chamber ®lled with demineralized water and bubbled with N2 to eliminate O2 from the chambers. At a given site, peepers were inserted vertically in the sediments. After a 10-day equilibration period, peepers were retrieved from the sediments by a diver, and porewater samples were collected immediately from individual chambers on the platform by piercing the ®ltration membrane with a micropipette. They were then transferred into pre-cleaned and preacidi®ed Te¯on bottles at low temperature and frozen in a refrigerator at a very low temperature (ÿ808C) to ensure that no bacterial or chemical alteration of Se speciation occurred during the storage. Two cores of sediments were collected by a diver with a lightweight plexiglass corer at two sites in both Clearwater Lake (June 1995) and in McFarlane Lake (July 1995). After retrieval, cores were immediately transported to the laboratory where they were extruded under inert atmosphere and cut into 0.5-cm sections for the top 2 and 1-cm sections thereafter. The subsamples were placed into polyethylene bottles, capped and frozen at ca. ÿ808C.

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2.2. Analyses To a 25 ml polyethylene bottle containing 4.0 ml of porewater, 2 ml of concentrated HCl (Analytical grade, Anachemia) was added and Se(IV) was analyzed by hydride generation atomic absorption spectrometry (HGAAS) coupled to two cooling traps to remove the water vapour and to preconcentrate the hydrogen selenide respectively (Cutter, 1986; Xu et al., 1997). For the determination of Se(VI), 6.0 ml of a porewater sample and 3.0 ml of concentrated HCl were added in a 30 ml round bottom glass ¯ask which was connected to a condensing system and heated in a thermostatically controlled glycerol bath at a temperature of 1008C for up to 40 min to ensure that all the Se(VI) was reduced into Se(IV) before the analysis (Cutter, 1978; Xu et al., 1997). The total dissolved inorganic Se [(IV)+(VI)] in porewater samples was analyzed by HGAAS analysis as mentioned above for Se(IV). The amount of Se(VI) could be obtained by subtracting the amount of Se(IV) from the total dissolved inorganic Se. For dissolved organic Se, Se(org), a 5.0-ml water sample was poured into a 20-ml quartz tube and a few drops of H2O2 30% (v/v) (Analytical grade, Fisher Scienti®c) were added to the tube. HCl was used to adjust the solution to pH 1.5. The sample was capped and then irradiated with UV light for 5 h utilizing a 1200 W Hg lamp with a main radiation wavelength of 253 nm. Under these conditions, over 95% of the dissolved organic C can be destroyed and almost all of the associated organic Se was oxidized into inorganic Se(IV) and Se(VI) (Takayanagi and Wong, 1985). The mixture was adjusted to a concentration of 4.0 M HCl with concentrated HCl. Se(VI) in the mixture was quantitatively reduced into Se(IV) using the HCl procedure mentioned above. The total dissolved Se [Se(IV)+Se(VI)+organic Se] in the porewater sample was then determined by HGAAS as mentioned above for Se(IV). The amount of organic Se was obtained by subtracting that of [Se(IV)+Se(VI)] from the total Se concentration. The total dissolved Fe and Mn were determined by ¯ame AAS. Total organic matter (TOM) refers to the total amount of organic substances present in dried sediment samples. Oven-dried samples (408C) of sediment (ca. 0.5 g) were weighed into clean pre-weighed crucibles. The samples were placed and heated in a mu‚e furnace at 7508C for 4 h then weighed after cooling down in a desiccator. The TOM present in the dried sediment samples was calculated by the di€erence of sample weight before and after heating. The water content of sediment samples was measured by the di€erence after drying them at 1058C for 4 h until constant weight. The sequential method used in the partitioning of

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Se for lake sediment samples in the experiments was a modi®ed version of the techniques developed by Tessier et al. (1979) and described in detail by Xu et al. (1997). Wet sediment samples were submitted to sequential extraction by (i) a 0.2 M oxalic acid solution bu€ered to pH 2 with ammonium oxalate for 8 h to remove trace Se mainly bound to Fe and Mn oxyhydroxides (identi®ed as Se(ox)), and (ii) an acidic solution (pH=2.0) of H2O2 (30% v/v) at 858C for 5 h to remove trace Se bound to organic matter in the sediments after the ®rst extraction (identi®ed as Se(red)). The Se species in both extracts were chemically converted into Se(IV) and analysed by HGAAS (Xu et al., 1997). The oxidized, Fe(ox), Mn(ox) and reduced, Fe(red), Mn(red) fractions of the sediments were obtained from the same extractions and determined by ¯ame AAS. A procedure described by Velinsky and Cutter (1990) was used to extract the elemental Se, Se(O) present from the sediment matrix. Separate dried (408C for ca. 48 h) sediment samples were extracted with a Na2SO3 solution adjusted to pH 7.0 for 8 h in an ultrasonic bath. The extract was then ®ltered and digested in 2.0 ml of concentrated HNO3. The extracted Se(0) was ®nally converted to Se(IV) before being analysed by HGAAS. For the determination of total Se, Fe and Mn, all sediment samples including two certi®ed reference standard marine sediments BCSS-1 and PACS-1 from the National & Research Council of Canada, were digested with a combination of concentrated acids, HCl, HNO3 and HF (Trace metal grade, Fisher Scienti®c), in a microwave oven (Elwaer and Belzile, 1995). The Se recovered from this digestion procedure was analysed after the required pre-reduction step, for total Se (Tot Se) by two methods, HGAAS and high performance liquid chromatography (HPLC) with ¯uorometric detection (Xu et al., 1997). Total reducible S (TRS) is de®ned as elemental S+pyrite S+acid volatile sul®de (AVS). Two multistep analytical schemes were used in this study for both TRS and AVS analyses on solid phase samples. The experimental apparatus and procedures for the determination can be found in Cornwell and Morse (1987), and Can®eld et al. (1986) for AVS and TRS respectively. Brie¯y, TRS was converted to H2S in a hot acidic Cr2+ solution in the presence of ethanol; AVS was converted to H2S in a cold acidic SnCl2 solution also in the presence of ethanol. In both cases, H2S gas was collected as a ZnS precipitate and ®nal concentrations were ultimately obtained by iodimetry. Elemental S was determined by gas chromatography-mass spectrometry after a CCl4 extraction (Chen et al., 1997). For all measured parameters, triplicate samples and

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standard addition were systematically run to control the quality of the analyses. 3. Results and discussion 3.1. Pro®les of dissolved species in porewaters 3.1.1. Clearwater Lake Vertical distributions of dissolved Se(IV), Se(VI), organic Se and total dissolved Se in porewater from the surface to a depth of 24 cm in sediments of Clearwater Lake are shown in Fig. 1. At the sediment±water interface (SWI), the average concentrations of Se(IV), Se(VI), organic Se and total Se were respectively non detectable, 1.2, 1.5 and 2.7 nM. This suggests that 55% of the total dissolved Se in porewaters at the sediment±water interface was associated with organic molecules. It is well known that aquatic organisms can incorporate inorganic Se and reduce it in a variety of selenide compounds such as the rather volatile methylated species and seleno-amino acids (selenocysteine and selenomethionine). Important proportions of dissolved organic Se have been measured in marine and

freshwater environments (Robberrecht and Van Grieken, 1982; Takayanagi and Wong, 1985). There is also evidence of the association between Se and humic substances in natural systems (Gustafsson and Johnsson, 1994; Wang et al., 1995). In Clearwater Lake sediments, the concentration of dissolved organic Se increased sharply with depth to a maximum of 4.7 nM at 4.0 cm indicating probably a release of this species from the decomposition of organic matter or desorption from oxidized forms of minerals. After this concentration peak, dissolved organic Se dropped to 1.8 nM between 4.2 and 10.0 cm and stayed constant between 10.0 and 20.0 cm before slightly increasing again below 20.0 cm. This decrease of the dissolved Se concentration with depth can be due to the formation of elemental Se by bacteria (Oremland et al., 1989) and to its precipitation/adsorption on a solid phase of the sediment. The possible association with Fe sul®des and particularly pyrite will be discussed further in the text. The concentration of Se(IV) at the SWI (Fig. 1) was below the detection limit (0.1 nM) suggesting either its oxidation into the oxic layer of the sediment or its precipitation/adsorption on Fe±Mn oxyhydroxides or organic matter present in this sediment layer.

Fig. 1. Depth distribution of porewater Se species in Clearwater Lake.

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The adsorption process is largely favoured for anionic species especially under acidic conditions (Balistrieri and Chao, 1990). Concentrations of selenite increased gradually with depth to a maximum of 2.0 nM at 10.0 cm and then stayed relatively constant at about 1.6 nM between 10.0 and 24.0 cm. A comparison of dissolved Se(IV) (and organic Se) pro®le with that of dissolved Fe (Fig. 2) shows similar trends for the two dissolved species, the possible association of these Se species with Fe oxyhydroxides and organic matter and the simultaneous release of dissolved Se(IV), organic Se and Fe(II) under reducing conditions can likely occur as Fe oxyhydroxides are used as oxidants during the mineralization of organic matter (Stumm and Morgan, 1996). The importance of Fe and Mn oxyhydroxides and the critical role they play in controlling the solubility of trace elements have been reported for As (Belzile and Tessier, 1990) and several other elements (Balistrieri et al., 1992; Davison, 1993; Tessier et al., 1996). In the water column and at the SWI, dissolved Se species can adsorb onto amorphous Fe oxyhydroxides. Selenite ions have more anity for Fe oxyhydroxides than for Mn oxides (Balistrieri and Chao, 1990) especially under the acidic conditions of Clearwater (pH=5.0) unfavourable for their formation and stab-

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ility. The adsorbed Se species could then be released when Fe oxyhydroxides are reduced at depths between 4.0 and 12.0 cm (Fig. 2). The concentration of Se(VI) (Fig. 1) stayed relatively constant at 1.0 nM from the surface of the sediment to a depth of 24.0 cm in the sediment. A laboratory study conducted by Balistrieri and Chao (1990) showed that Se(VI) does not adsorb on Mn dioxide and is less strongly adsorbed than Se(IV) on freshly prepared amorphous Fe oxyhydroxides. It is suggested in their study that negatively charged selenate oxyanions adsorb primarily by electrostatic interaction. Since the pH at the point of zero charge [pH(PZC)] of Mn dioxide is in the range of 1.5±2.5, selenate should not adsorb on Mn oxide. At the pH of Clearwater Lake, Mn oxides would not only be unstable because the pH conditions of this lake (5.0) do not favour the formation of Mn oxides but they would also be negatively charged and would not represent interesting sorption sites for the negatively charged selenate ions. Indeed, relatively high concentrations of Mn(II) in porewater at the SWI (Fig. 2) and relatively low concentrations of Mn oxides (mmol/g), expressed as Mn(ox) (vide infra ) were measured in the sediments of Clearwater Lake. Iron oxyhydroxides, on the other hand, have a

Fig. 2. Depth distribution of porewater Fe(II), Se(IV) and Mn(II) in Clearwater Lake.

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pH(PZC) in the range of 7.8±8.8, therefore their surfaces are expected to be positively charged at pH 5.0. They are likely to be much more ecient at interacting with selenate or selenite oxyanions, specially because Fe oxyhydroxides are present at much higher concentrations (mmol/g) at the pH of this lake. It is noticeable that dissolved Se(VI) remains detectable even at depths where reducing conditions exist, possibly due to the slow kinetics of reduction of Se(VI) to lower valence states or microbial action. The pro®le of total dissolved Se in porewater is similar to that of organic Se (Fig. 1) with the maximum concentration peak of 6.6 nM appearing at around 4 cm in depth. This maximum concentration peak generates a concentration gradient of total dissolved Se which can favour upward or downward di€usion with the corresponding precipitation/adsorption reactions in the upper or lower sediment layer or the di€usion of dissolved Se species into the water column of the lake. The vertical pro®les of dissolved Fe(II) and Mn(II) in porewaters of Clearwater Lake are presented in Fig. 2. The concentration of Fe was found to be undetectable at the surface of sediment due to the precipitation of Fe oxyhydroxides in the oxidized layer of

the sediment. Fe(II) increased sharply with depth to 350 mM at 4.0 cm, decreased between 4.0 and 6.0 cm to about 250 mM, and increased further down to a maximum of 430 mM at 10.0 cm to ®nally drop rapidly again between 10.0 and 12.0 cm to a rather constant value of 250 mM. These peaks have already been attributed to the dissolution of Fe oxyhydroxides under the reducing conditions that are induced by the microbial mineralization of organic matter. The pro®le of dissolved Mn exhibited a concentration of 4.5 mM at the SWI which indicates that the acidic conditions prevailing in this lake did not promote the formation of Mn oxyhydroxides thus maintaining relatively high concentrations of dissolved Mn(II) in the overlying water. Manganese(II) increased gradually with depth to 8.0 mM at 6.0 cm, decreased to 5.5 mM between 6.0 and 8.0 cm and then increased gradually to 12.5 mM between 8.0±24.0 cm. 3.1.2. McFarlane Lake At the time of sampling, the pH in McFarlane lake was 7.5. The vertical pro®les of dissolved Se(IV), Se(VI), organic Se and total Se in porewaters of McFarlane Lake are presented in Fig. 3. At this site,

Fig. 3. Depth distribution of porewater Se species in McFarlane Lake.

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dissolved selenite represented the most important fraction of the total dissolved Se. Contrary to the pro®le of dissolved Se(IV) in Clearwater Lake, the concentration in McFarlane was found to be about 1.3 nM at the surface of sediment. It increased sharply with depth to a maximum of 2.2 nM at 3.0 cm, dropped to about 1.3 nM between 3.0 and 6.0 cm and remained relatively constant below. The presence of measurable concentrations of Se(IV) at the SWI was a ®rst indication on the anoxic nature of sur®cial sediment of McFarlane Lake at that site. This was con®rmed by signi®cant concentrations (2.5 mM) of Fe(II) in porewaters (Fig. 4), the absence of an enriched layer of Fe oxyhydroxides, Fe (ox) and the presence of large concentrations of total reducible S (TRS) in the sur®cial sedimentary layer (vide infra ). The concentration peak of Se(IV) occurring at 3.0 cm could be attributed to the dissolution of Fe±Mn oxyhydroxides and the consequent release of adsorbed selenite since a peak was also observed in the dissolved Fe pro®le at the same depth (Fig. 4). The concentration of dissolved Se(VI) was found to be close to zero at the SWI (Fig. 3) reinforcing the hypothesis of a temporary anoxic interface. It increased with depth to a maximum of 1.0 nM at 3.0 cm, but it decreased gradually to 0.5 nM at 6.0 cm; below 6.0 cm it stayed constant at 0.5 nM.

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The pro®le of dissolved organic Se (Fig. 3) was slightly di€erent from those of Se(IV) and Se(VI). The concentration of organic Se was about 0.8 nM at the surface of sediment. It remained close to 0.5 nM between 0.0 and 3.0 cm, increased with depth to a maximum of 1.3 nM at 5.0 cm and then decreased gradually again with depth to 0.7 nM at 10.0 cm. This concentration peak appears to correspond more to the mineralization of organic matter and/or to the dissolution of Mn oxides than to the dissolution of Fe oxyhydroxides (Fig. 4). Between 10.0 and 20.0 cm, the concentration of dissolved organic Se stayed constant at 0.7 nM. The pro®le of total dissolved Se was similar to those of Se(IV) and (VI). The concentration of total Se (Fig. 3) was found to be 2.2 nM at the SWI which resulted mostly from the contribution of selenite and organic Se. It increased sharply with depth to 3.5 nM at 3.0 cm, but it decreased gradually between 3.0 and 5.0 cm, and then dropped rapidly with depth to 2.5 nM at 7.0 cm. Below 7.0 cm, it stayed constant at 2.5 nM. This pro®le is similar to that obtained in Clearwater Lake and suggests upwards and downwards diffusion. Due to the anoxic interface of McFarlane Lake, the di€usion of dissolved Se into the water column of this lake could more likely occur since it can-

Fig. 4. Depth distribution of porewater Fe(II), Se(IV) and Mn(II) in McFarlane Lake.

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not be e€ectively adsorbed by Fe and Mn oxyhydroxides normally present at the oxic SWI. A more intense microbial methylation of Se to volatile species can also be expected when reducing conditions are present at the SWI of McFarlane Lake (Flury et al., 1997). Vertical pro®les of dissolved Fe(II) and Mn(II) in porewater (Fig. 4) indicate the presence of both species at the surface of the sediment. At a depth of around 5.0 cm, a steep positive gradient could re¯ect the dissolution of buried Fe and Mn oxyhydroxides under the reducing conditions established during the anaerobic decomposition of organic matter. It can be speculated that these Fe and Mn oxyhydroxides were less reactive forms of oxides since they had resisted the reducing action of sediments to that depth. Porewater concentrations of Fe(II) were much smaller than in Clearwater Lake. 3.1.3. Di€usion of dissolved Se species The pro®les of total dissolved Se of both lakes suggest the possible di€usion of dissolved Se species along the concentration gradient generated by the maximum concentration peaks. As mentioned earlier, the downward di€usion can lead to the formation of elemental Se and to the precipitation of Se with Fe sul®des or to the formation of distinct mineral phases containing Se. The upward di€usion can lead to the reprecipitation/adsorption of dissolved Se with Fe±Mn oxyhydroxides present at the SWI or to the release of dissolved Se into the lake water. The latter process appears more likely to happen, at least locally, in the seasonally anoxic McFarlane Lake. Based on the assumption that all dissolved Se di€using upwards is transferred to the water column, we can estimate the porewater Se ¯ux across the SWI using Fick's ®rst law: J ˆ ÿfDO …@ C=@ Z †O

…1†

where J is the ¯ux of dissolved Se, f is the sediment porosity (0.96) calculated from the water content using the following formula (Berner, 1971): f=Vwater/ Vwater+Vsolid where V is the volume obtained from mass and density of water and sediment. DO is the free solution di€usion coecient for SeO42ÿ (4.7  10ÿ6 cm2 sÿ1 at 48C; Li and Gregory, 1974), and (@C/@Z )O is the concentration gradient calculated from the linear portion of total dissolved Se concentrations at the SWI (Figs. 1 and 3). The advective component (sedimentation) of the ¯ux was neglected but could be important in lakes characterized by large watersheds (Haygarth, 1994). For Clearwater Lake, the calculated ¯ux was estimated to be 0.108 nmol cmÿ2 aÿ1 which is comparable to the value of 0.112 nmol cmÿ2 aÿ1 reported by Takayanagi and Belzile (1988) for a marine environment. As for Clearwater Lake, the pro®le

of total dissolved Se of McFarlane Lake suggests downwards and upwards di€usion along the concentration gradient. The estimated ¯ux of dissolved Se across the SWI using the ®rst Fick's law was 0.034 nmol cmÿ2 aÿ1 in McFarlane Lake. These ¯ux values do not consider the contribution due to bioturbation which could be important in Clearwater Lake. However, it was assumed in the calculations that all dissolved Se could escape from the sediment before being trapped by Fe±Mn oxyhydroxides or organic matter present in sur®cial sediments. The authors have observed that it was not the case in Clearwater Lake where Fe and Mn oxyhydroxides were enriched at the oxic SWI (vide infra ). 3.2. Pro®les in solid sediments 3.2.1. Clearwater Lake The vertical pro®les of total solid Se and Se fractions extracted from the sediments of Clearwater Lake are shown in Fig. 5. The proportion of Se associated to pyrite or ferroselite was estimated from the four sets of data by subtracting the three extracted fractions (ox, red and 0) from the total. This estimated pyritic Se fraction is presented in Fig. 6. Fig. 5 indicates a clear sur®cial enrichment of Se associated to Fe and Mn oxyhydroxides, Se(ox) and of Se associated to organic matter, Se(red). Apart from the sur®cial sediments, Se(ox) represented only a minor fraction of the total as is expected under reducing conditions. Se(red) averaged 30% of the total at this location of Clearwater Lake. A recent study shows that selenate can be e€ectively removed from water to form organicallybound Se in wetland sediments (Zhang and Moore, 1997). Besides, it has been shown that the assimilation of Se by estuarine benthic organisms can be very ecient when it is present as particulate organo-Se (Luoma et al., 1992). The depth-distribution of Se(red) was similar to the pro®le of total organic matter (vide infra ), suggesting that the concentration of Se bound to organic matter was proportional to that of organic matter in sediments. It is important to mention that the extraction of sediment by the acidic H2O2 solution is not strictly selective and that, along with organic matter, some amorphous sul®de/selenide compounds and associated trace elements, can also be extracted (Tessier et al., 1979, 1985). It is therefore possible that the Se(red) pro®le also re¯ects the incorporation of Se into solid amorphous Fe sul®de, FeS, into a mixed phase such as FeSSe or the precipitation of achavalite, FeSe. Concentrations of Se(0) were very low near the oxidized surface of the sediment but increased sharply around 4 cm in depth to represent more than 60% of the total. Below this depth, Se(0) represented between 40 and 50% of total Se. Fig. 7 clearly indicates that

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Fig. 5. Depth distribution of total Se and extracted Se fractions (expressed as a percentage of the total) in sediments of Clearwater Lake.

Fig. 6. Depth distribution of total reducible (TRS), acid volatile S (AVS) and estimated pyritic Se in sediments of the Clearwater Lake.

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sur®cial sediments (®rst cm or so) of Clearwater Lake were enriched in total Fe and Fe oxyhydroxides identi®ed here as Fe(ox) and expressed in mmol/g, and to a lesser extent in total Mn and Mn oxides, Mn (ox), present at the mmol/g level. This con®rms once again the existence of oxidizing conditions at the SWI of Clearwater Lake. Below 2.5 cm, the concentration of total sedimentary Se kept increasing with depth and exceeded the sum of the three extracted Se species below 7.0 cm. This di€erence could be due to the formation of Se- and/or S-containing compounds such as FeS, FeSe, FeSeS, or more likely FeS2 FeSe2. Amorphous Fe sul®de (de®ned here as AVS) and pyrite (de®ned here as the di€erence between TRS and AVS+S(0)) were identi®ed in sediments of both lakes. The black deposits observed on Te¯on collectors (described in Belzile et al., 1989) below the SWI of both lakes con®rmed the presence of freshly formed amorphous Fe sul®des. The similar trends followed by TRS and concentrations of estimated pyritic Se (Fig. 6) support the hypothesis of Se being incorporated into FeS2 or forming the analog FeSe2. Previous calculations of the saturation index for common minerals of Fe and Mn (Carignan and Nriagu, 1985) have given values close to saturation for the mineral siderite, FeCO3 and amorphous Fe sul®de, FeS in both lakes.

Rhodocrosite, MnCO3 was close to saturation in McFarlane sediments but clearly undersaturated in Clearwater sediments. The large increase in total Se concentrations below 5 cm (Fig. 5) corresponds to the decrease in total dissolved Se observed in Fig. 1. It probably indicates that some dissolved Se was removed from the solution to form a solid phase. Under reducing conditions, Se could occur as elemental Se or form a distinct mineral phase such as ferroselite, FeSe2 or be more likely incorporated into a mixed phase such as FeSSe as suggested by Velinsky and Cutter (1990). Hypothetical mechanisms for the formation of Se mineral phases under reducing conditions are presented in Table 1 and discussed below. Fig. 8 clearly shows that the composition of the sedimentary layer between 1 and 4 cm signi®cantly dropped for both the water and organic matter content. This anomaly was re¯ected in the other vertical pro®les of solid species (Figs. 5, 6 and 7). X-ray ¯uorescence analysis performed on the solid sediments of this layer (results not shown) revealed that the chemical and mineralogical composition of this layer di€ered from the rest of the sediment core. Signi®cantly higher proportions of Si (with corresponding lower proportions of the other major elements and lower ashing

Fig. 7. Depth distribution of total Fe and Mn and of extracted fractions of both elements in sediments of Clearwater Lake.

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Table 1 Proposed mechanisms for the diagenetic formation of Fe sul®de (selenide) and pyrite (ferroselite) (1)

(2)

(3) (4)

2FeOOH+3HSÿ 4 2FeS+S0+H2O+3OHÿ 2FeOOH+3HSeÿ 4 2FeSe+Se0+H2O+3OHÿ FeS+S0 4 FeS2 (pyrite) FeSe+Se0 4 FeSe2 (ferroselite) 2FeOOH+3HSÿ 4 FeS+FeS2+H2O+3OHÿ 2FeOOH+3HSeÿ 4 FeSe+FeSe2+H2O+3OHÿ 3FeS+S0 4 Fe3S4 (greigite) 3FeSe+Se0 4 Fe3Se4 Fe3S4+S0 4 2FeS+FeS2 (pyrite) Fe3Se4+Se0 4 2FeSe+FeSe2 (ferroselite) Fe3S4+2 S0 4 3FeS2 Fe3Se4+2Se0 4 3FeSe2 Fe2++HSÿ+S5S2ÿ 4 FeS2+S4S2ÿ+H+ Fe2++HSeÿ+Se5Se2ÿ 4 FeSe2+Se4Se2ÿ+H+ FeS+H2S(aq) 4 FeS2+H2(g) FeSe+H2Se(aq) 4 FeSe2+H2(g)

loss) suggest the presence of a larger percentage of quartz and lower percentage of organic matter in the 1±5 cm layer. This can explain the lower water content of this layer since quartz particles are usually larger in size than clays which leads to lower porosity and lower water content (Berner, 1980). The presence of this distinct mineralogical layer can explain the lower TRS

Fig. 8. Variation with depth of water and organic matter content in sediments of Clearwater Lake.

Boesen and Postma, 1988

Sweeney and Kaplan, 1973 Rickard, 1975: Luther, 1991 Rickard, 1997; Rickard and Luther, 1997

values and estimated values of pyritic Se at these depths.

3.2.2. McFarlane Lake The concentration of total sedimentary Se reached a maximum value of 145 nmol/g at around 2 cm and decreased continuously down to values around 22 nmol/g in the sediments of McFarlane Lake (Fig. 9). The concentrations of Se species bound to Fe±Mn oxyhydroxides, Se(ox), and organic matter, Se(red), were constantly low at 0.05 and 0.53 nmol/g, respectively. Selenium bound to organic matter accounted for only <6.0% of the total sedimentary Se, while Se bound to Fe±Mn oxyhydroxides remained a very minor fraction. Elemental Se however, represented a signi®cant portion of the total (between 40 and 95%) all along the core. Se(0) was signi®cantly more abundant in sur®cial sediments in McFarlane when compared to Clearwater (Fig. 9). Once again, the pyritic Se fraction was estimated by subtracting the three extracted fractions from the total sedimentary Se (Fig. 10). In the pro®les of Fe and Mn extracted by reducing and oxidizing agents, the concentrations of Fe and Mn were relatively constant with depth from the SWI to 15 cm (Fig. 11), suggesting the existence of the anoxic conditions at the SWI discussed earlier. Some similarities existing between pro®les of this pyritic Se fraction and TRS (re¯ecting here the FeS2 content; Py=TRSÿ(AVS+S(0)) seem to support the incorporation of Se into pyrite, or the formation of FeSe2 (Fig. 10). The possible mechanisms leading to this incorporation or to the formation of a pure phase are presented and discussed in the next section (Table 1).

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Fig. 9. Depth distribution of total Se and of extracted Se fractions (expressed as a percentage of the total) in sediments of McFarlane Lake.

Fig. 10. Depth distribution of total reducible (TRS), acid volatile S (AVS) and estimated pyritic Se in sediments of McFarlane Lake.

N. Belzile et al. / Applied Geochemistry 15 (2000) 1439±1454

3.3. Possible formation of achavalite FeSe vs FeS and ferroselite FeSe2 vs FeS2 Several mechanisms have been proposed for the diagenetic formation of Fe sul®de and pyrite (Table 1). Mechanism 1 supposes the reduction of Fe oxyhydroxides by H2S to form Fe sul®de and elemental S; both species can further react to form pyrite as suggested by mechanism 2. Mechnism 2 can also include a step where the formation and transformation of greigite (Fe3S4) occur. In mechanism 3, the equilibrium existing between elemental S and polysul®de species is considered in the transformation of Fe sul®de into pyrite. More recently, the direct action of H2S on FeS has been proposed and described (mechanism 4). Because of the great similarity existing between the chemistries of Se and S, analogous mechanisms are proposed for the formation of achavalite (FeSe) and ferroselite (FeSe2). They are presented in Table 1. If those models of pyrite formation also apply to ferroselite, it would require the existence of elemental Se to react with achavalite in mechanisms 1 and 2. Elemental Se represented a signi®cant portion of the total sedimentary Se in both lakes which could reinforce the possible action of mechanisms 1 and 2. A comparison of the solubility products of both Fe sul®de and selenide

1451

( pKsp 1 18 and 26 for FeS and FeSe respectively) also suggests that selenide could substitute sul®de in the sediments and thermodynamically favour the formation of FeSe and FeSeS or FeSe2 according to Eq. (2) (Masscheleyn et al., 1989). FeS ‡ Se 2ÿ $ FeSe ‡ S 2ÿ

K1108

…2†

Dissolved Se species can then be removed from interstitial waters through the formation of ferroselite (FeSe2) or through the incorporation of Se into sul®de minerals such as amorphous Fe sul®de, FeS, FeSeS or pyrite FeS2. The presence of Fe sul®des has indeed been con®rmed by the extraction of acid volatile sul®de (AVS) and total reducible sul®de (TRS) from the same sediments. According to Figs. 6 and 10, pro®les of pyrite (representing the di€erence between TRS and AVS+SO) composed most of the sul®de minerals in reducing sediments and could control the solubility of Se through adsorption or incorporation of Se into crystal lattices of pyrite. This phenomenon has been observed to a lesser extent by Velinsky and Cutter (1991). Ferroselite could be formed in aquatic environments (Warren, 1968) and its stability ®eld in the S± Se±Fe system was between the ®elds of pyrite and Fe oxyhydroxides (Cutter and Velinsky, 1988). The other

Fig. 11. Depth distribution of total Fe and Mn and of extracted fractions of both elements in sediments of McFarlane Lake.

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removal mechanism of Se at the greater depth included precipitation as elemental Se, whose stability ®eld overlaps that of ferroselite (Cutter and Velinsky, 1988). 3.4. Comparison between the two environments Ð oxic vs anoxic interface The concentrations of dissolved Se species remained very low (less than 7 nM) in the sediments of both lakes. The lower concentration of dissolved Se(IV) at the SWI of Clearwater suggested that this species was more eciently adsorbed onto Fe oxyhydroxides at low pH values, while lower concentration of dissolved Se(VI) in the McFarlane SWI indicated possible anoxic conditions at the sampling time. Dissolved organic Se species were the dominant species in Clearwater Lake, while dissolved Se(IV) was dominant in McFarlane Lake. The pro®les of dissolved Se species in both lakes suggested downwards (formation of Se(0) and association with FeS and FeS2) and upwards (reprecipitation with Fe oxyhydroxides in Clearwater) di€usion. Sedimentary pro®les of solid Se species in both lakes con®rmed the incorporation of Se into Se(0), FeS or FeS2 or the formation of pure phases of FeSe and FeSe2. A noticeable di€erence in the absence of an enriched layer of Fe and Mn oxyhydroxides in McFarlane Lake (low relative proportions of Se(ox), Mn(ox) and Fe(ox)) was related to the possible anoxic conditions at the sampling site of this lake. The average atomic ratio Se:S was calculated to be 3.4 (21.0)  10ÿ4 (n = 17) in Clearwater Lake and 3.5

(20.8)  10ÿ4 (n = 17) in McFarlane Lake. This ratio was determined in Great Marsh, a coastal salt marsh in Delaware (Velinsky and Cutter, 1991) showing an average value of 2.0 (21.1)  10ÿ5. This suggests that Sudbury lake sediments are richer in Se when compared to Great Marsh considering that Sudbury lake sediments are already rich in S introduced through the mining and smelting activities. The pro®les of dissolved Fe and Mn in the porewaters of both lakes (Figs. 2 and 4 respectively) were similar, with much higher concentrations of Fe in Clearwater Lake and noticeably higher concentrations of Mn in McFarlane Lake. At the SWI, the concentration of Mn(II) was higher in Clearwater Lake (Fig. 2) due to the lower pH of this lake while the concentration of dissolved Fe was close to zero. Similarly, the concentrations of total and extractable Fe species in the sediment (Fig. 7) were also higher in Clearwater Lake compared with that of McFarlane Lake (Fig. 11). However, both total and extractable Mn species in the sediments are much higher in McFarlane Lake (Fig. 11) compared with those of Clearwater Lake (Fig. 7). Those di€erences in the background levels of Fe and Mn in the two lakes can probably be related to the watershed geochemistry. Higher concentrations of solid Mn in McFarlane Lake re¯ected the circumneutral pH of this lake favouring the precipitation of dissolved Mn(II) even though the reducing conditions existing at the sampling time did not favour the enrichment of sur®cial sediments in Mn oxides. The concentrations of TRS and AVS were larger by an order of magnitude

Fig. 12. Schematic model comparing the behaviour of Se in sur®cial sediments characterized by an oxic and anoxic interface with water.

N. Belzile et al. / Applied Geochemistry 15 (2000) 1439±1454

in McFarlane Lake compared to Clearwater Lake, suggesting more favourable conditions for pyritization in this lake.

4. Conclusion A schematic model is presented in Fig. 12 to summarize the geochemical behaviour of Se in lakes characterized by oxic and anoxic SWI. Based on porewater pro®les and solid phase data, it has been shown that the geochemistry of Se was controlled by several important processes such as adsorption/desorption, formation of Se(0), co-precipitation and di€usion. Chemical species such as Fe±Mn oxyhydroxides, organic matter, TRS and/or AVS appear to play an important role in the geochemistry of Se in sediments in both environments. Several chemical processes involving Se species can be microbially mediated. The ¯uxes across the SWI were estimated to be 0.108 and 0.034 nmol cmÿ2 aÿ1 in Clearwater and McFarlane, respectively, which is comparable to other similar environments. Pro®les of dissolved Se species in both lakes suggested downwards and upward di€usion. They indicated that selenite could escape to the water column under reducing conditions of the SWI whereas selenate could di€use more e€ectively at the oxic SWI of the acidic lake. Elemental Se and Se species existing as Se±pyrite (or as a pure Se phase such as ferroselite) in the solid phase could be considered as the major component of total Se in sediments of both lakes.

Acknowledgements We wish to express our gratitude to Greg Cutter, Andre Tessier, Evangelos Kakouros and an anonymous reviewer for their critical comments on the manuscript and Lynda S. Cutter for advice and help in the determination of elemental Se. Financial support from the Natural Sciences and Engineering Research Council of Canada and the Laurentian University Research Fund is acknowledged.

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