Environ. Sci. Technol. 2004, 38, 4263-4268
Molybdenum Scavenging by Iron Monosulfide GEORGE R. HELZ,* TRENT P. VORLICEK,† AND MANI D. KAHN Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
Molybdenum profiles in dated sediment cores provide useful historical information about anoxia in anthropogenically impacted natural waters but would be of greater service if Mo fixation mechanisms were better understood. Here, we explore Mo scavenging by precipitated FeS in a model system consisting of an FeIII-bearing kaolinite (KGa-1B) dispersed in NaHS solutions. Test solutions contain 18 µM thiomolybdates (mainly MoOS32-). Optically measuring dissolved polysulfides monitors the rate of FeS production from FeIII minerals. Even though the exposed clay surface area is large (450 m2/L), the clay itself sorbs little Mo at pH 8.6. As FeS forms, Mo is taken up in initial Mo/Fe mole ratios of 0.04-0.06, irrespective of HS- concentration (4-40 mM range). After about a day, Mo expulsion from the solids begins, accompanied by net polysulfide consumption. These changes reflect recrystallization of amorphous FeS to more ordered products such as greigite. FeS captures some MoO42- but captures thiomolybdates more effectively. Kaolinite accelerates conversion of MoOS32to MoS42-, as predicted previously, and thiomolybdates facilitate reduction of FeIII minerals in the clay compared to Mo-free solutions. FeS is a potentially effective, transient scavenging agent for Mo in sulfidic environments, although FeS2 and organic matter appear to be the ultimate sedimentary hosts.
Introduction Along coasts of industrialized countries, zones of O2-depleted bottom water (“dead zones”) appear to be increasing in size and number (1-6). Both human and natural factors regulate this phenomenon. On one hand, anthropogenic plant nutrients, especially fixed N, promote microbial O2 consumption by stimulating the downward rain of biodegradable organic particles. On the other, natural factors such as weather and river basin discharges govern O2 renewal in bottom waters. Methods for reconstructing the history of anoxia can help scientists disentangle the influences of these human and natural factors. This is a necessary step in learning to predict and ultimately to manage anoxia. Profiles of Mo concentrations in dated sediments provide one such method at both freshwater and marine sites (e.g. refs 7-9). A drawback of Mo as an indicator of past anoxia is that its sedimentary geochemistry remains to be understood and is probably complex. As expected from its long mean residence time in the ocean (0.8 My), MoO42- is not readily * Corresponding author phone: (301)405-1797; fax: (301)314-9121; e-mail:
[email protected]. † Current address: Department of Chemistry and Geology, Minnesota State University, Mankato, MN 56001. 10.1021/es034969+ CCC: $27.50 Published on Web 07/10/2004
2004 American Chemical Society
scavenged by common silicate, carbonate, and oxide minerals at the pH of seawater (10); Mn oxides are an exception (11). Presence of sulfide seems to be one requirement for Mo scavenging and deposition in reducing environments (12-15). Sulfide converts MoO42- to particle-reactive thiomolybdates, MoOxS4-x2- (16, 17). Zerovalent sulfur (S0) donors (for example, polysulfide ions) react with MoOS32- to form MoIV or MoV species, some of which are strongly scavenged by pyrite, FeS2 (18). Pyrite is thought to be the most important host-phase of Mo in euxinic sediments and black shales (14, 19-23). Nevertheless, classic work by Sugawara (24) and Bertine (25) suggests that pyrite precursors, iron monosulfides, initially capture Mo from solution. We explore this process here. Compared to the earlier studies, which relied on HSgeneration by sulfate-reducing bacteria, our approach better documents the chemical processes. In the earlier studies, no information was obtained on Mo speciation, and neither total sulfide (ΣS2-) nor total zerovalent sulfur (ΣS0) was quantified. The possibility that microorganisms were the scavenging agents could not be excluded. Following common usage, we employ FeS and iron monosulfide as generic terms here for HCl-soluble iron sulfides. In field studies, these often are quantified collectively as acid-volatile sulfide (AVS). Specific materials include amorphous precipitates (Fe∼1S), mackinawite (Fe0.87-1.1S), and greigite (Fe0.75S). They form a class distinct from HCl-insoluble iron sulfides such as pyrite (FeS2).
Experimental Section Materials. Scavenging by FeS under controlled conditions employed the Clay Mineral Society’s standard kaolin, KGa1B (available from the University of Missouri-Columbia). Previously we have studied catalysis of MoS42- hydrolysis by this clay at very low HS- concentrations (17). Measurements of this clay’s specific surface area yield 11.3, 11.7, and 12.5 m2/g (26-28). Roughly one-third by weight consists of colloidal-sized particles ( 7 (10). We do not expect intermediate members of the MoOxS4-x2- series to behave differently. The most likely explanation for Mothio loss is coprecipitation with FeS. Persuasive evidence for this is provided by Figure 4, which presents excellent correlations between ΣMothio loss and ΣS0 production. The slopes given in the key to Figure 4 represent initial molar ratios of Mo lost per S0 produced. Because FeS is produced in a 2:1 ratio to S0 (reaction 1), dividing the slopes by 2 gives the molar ratio of Mo lost per FeS formula unit precipitated. The implication of these correlations is that FeS takes up 4-6 mol % Mo. In support of the idea that Mo loss is linked to production of 4266
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FIGURE 4. Correlations of ΣMothio vs ΣS0 during the first 6 h of the experiments in Figure 3. Linear fits are used for the 3.89 and 9.76 mM HS- data and a quadratic fit for the 41.1 mM HS- data. For each data set, m represents the initial slope, which is equal to the change in moles of ΣMothio per mole of ΣS0. FeS and ΣS0, note that ΣMothio reaches minima in Figure 3 at ∼20 h, just when ΣS0 reaches maxima. In an early study of Mo scavenging by FeIII-bearing kaolinite, illite, and montmorillonite in sulfidic solutions, Bertine (25) reached a similar conclusion about the involvement of FeS but did not quantify molar ratios. After ∼20 h, the character of the reactions in Figure 3 changes. Total zerovalent S begins to decline and ΣMothio begins to increase, implying that Mo is now being released from the FeS reservoir. Both processes suggest that initially formed, amorphous FeS is recrystallizing to a slightly oxidized product, most likely greigite (i.e. S0 + 3FeS f Fe3S4). It is interesting that this process does not occur in Figure 2, where no Mo is present. One possible explanation is that dissolved
FIGURE 5. Sulfidation of Mo as a function of the concentration of H2S (aq) (data from ref 16); rMo is the amount of each Mo species as a fraction of ΣMo. Arrows at top show H2S (aq) concentrations necessary to sulfidize FeIII oxyhydroxides (stable to the left of an arrow) to FeII sulfides (stable to the right of an arrow): F ferrihydrite, G - goethite, M - mackinawite, A - amorphous FeS. Note: at the pH of marine anoxic basins, total dissolved sulfide (ΣS2-) is about 10-fold larger than H2S (aq). Mo species promote greigite formation by facilitating S-atom transfer to FeS surfaces. Another possibility is that 4-6 mol % Mo in amorphous FeS simply increases its instability, providing an extra driving force for recrystallization. An alternate hypothesis for the decline in ΣS0 after ∼20 h might invoke formation of optically invisible Mo polysulfides, as described in ref 18. However, formation of Mo polysulfides would entail further MoOxS4-x2- loss, whereas regeneration is observed. Furthermore, the degree of Mo polysulfide formation ought to depend on S8 concentration, and it would be difficult to account for similar amounts of ΣS0 decline in the 3.89 and 41.1 mM HS- experiments, which differ by 2 orders of magnitude in [S8]. It appears that significant Mo polysulfide formation requires higher S8 concentrations than occur here (18).
Implications for Mo Enrichment in Sediments The success of trace element scavenging in natural waters depends both on the trace element’s chemical form and on the chemical nature of available particles. Sulfide can promote Mo scavenging by influencing both participants in this process. Figure 5 shows the effect of the [H2S] concentration on the equilibrium speciation of Mo (data from ref 16). A terrigenous, FeIII-bearing particle that falls into euxinic water will encounter rising [H2S] and thus increasingly more sulfidized MoOxS4-x2-. Results in this paper suggest that more sulfidized MoOxS4-x2- anions are more susceptible to scavenging by FeS, even though FeS appears capable of binding some unsulfidized MoO42-. At the same time, rising H2S generates FeS from FeIII minerals. The key implication of Figure 4 is that sulfidation of the FeIII minerals in KGa-1B, not the clay minerals themselves, controls capture of dissolved Mo. Previously (17), we observed very little capture of thiomolybdates by KGa-1B when [H2S] was too low to sulfidize much of the FeIII component in this clay. Along the top of Figure 5, we indicate some [H2S] thresholds for conversion of FeIII oxyhydroxides to FeS (see Supporting Information for the supporting calculations). For H2S concentrations near the left side of the diagram in Figure 5, Mo scavenging by the sulfide-mediated processes discussed in this paper is expected to be very limited. At low [H2S], virtually all Mo is in the MoO42- form and thermodynamics permits FeS production from only the least stable fraction of FeIII (ferrihydrite example in Figure 5). Conditions
for scavenging become much more auspicious as [H2S] rises toward 10-5 M (the action point of the geochemical switch (16)). At this [H2S] level, the average degree of sulfidation of MoOxS4-x2- rises greatly, and relatively more stable and abundant forms of FeIII, like goethite, become capable of producing FeS (Figure 5). The latter effect would be expected to increase greatly the Mo-reactive particle surface area. In nature, the Mo/Fe mole ratios achievable in FeS precipitates ought to be much lower than the ratios of 0.040.06 in our experiments because dissolved Mo is far lower. Yet, some remarkably high Mo/Fe ratios have been observed on suspended particles in sulfidic waters. In Lake Pavin (39, 40), Mo/Fe reaches 0.007 on particles just below a chemocline, where [H2S] ≈ 17 µM; this compares with Mo/Fe of 0.00008 just above the chemocline and 0.00003 in average continental crust. Possibly pH, which we did not vary in our work, helps explain high Mo/Fe ratios on particles in Lake Pavin. Bertine (25) hypothesized that FeS takes up Mo as a MoS3 component in solid solution. A reaction representing this process would be
2H+ + MoOS32- f MoS3 (ss) + H2O
(3)
This reaction suggests that higher [H+] in nature compared to our experiments might compensate for lower [MoOS32-]. Below the chemocline in Lake Pavin, [H+] is ∼102 greater than in our experiments, whereas dissolved Mo is ∼104 lower. In the mass action law for reaction 3, these differences would offset one another. Measurements of dissolved Mo in sediment pore waters in the deepest part of the Santa Barbara Basin have identified two ΣS2- thresholds for Mo fixation (8). At depths in the sediment where ΣS2- first exceeds 10-7 M, binding of some dissolved Mo to solids ensues. Deeper in the sediment, where ΣS2- exceeds 10-4 M (i.e. [H2S] exceeds 10-5 M), dissolved Mo disappears entirely from pore water. The latter threshold can be understood in terms of Figure 5. This threshold lies near the [H2S] concentration needed for production of significant amounts of MoOS32- + MoS42- and for sulfidation of comparatively stable FeIII oxyhydroxides, like goethite. In the presence of polysulfides, MoOS32- can go on to form MoIV or MoV species that could be strongly scavenged by pyrite (18), as this mineral is formed from FeS during diagenesis. Furthermore, evidence from bioinorganic chemists about the reactivity of such MoIV or MoV species (41) suggests that they could be actively involved in reactions that bind Mo to sedimentary organic matter. On the other hand, the 10-7 M ΣS2- threshold occurs at a [H2S] concentration where we expect Mo fixation to be very inefficient or negligible. Possibly 10-7 M is not a true chemical threshold but instead represents the average [H2S] at a horizon where highly sulfidic microenvironments have become abundant in the Santa Barbara Basin core. Reduced sediments are spatially quite heterogeneous (42). Diffusion of Mo into highly sulfidic microenvironments and its fixation there could be misconstrued as fixation at low [H2S] in the bulk sediment.
Acknowledgments This research supported by NSF Grants EAR 9980532 and EAR 0229387 and by Rollinson and Howard Hughes Undergraduate Research Fellowships to M.D.K. We appreciate the constructive criticism of an anonymous reviewer.
Supporting Information Available Data plotted in Figure 3 and calculation of H2S (aq) thresholds for FeIII oxyhydroxides f FeII sulfides in Figure 5. This VOL. 38, NO. 16, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review September 4, 2003. Revised manuscript received May 28, 2004. Accepted June 1, 2004. ES034969+
