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Experimental Estimation of the Bisulfite Isomer Quotient As a Function of Temperature: Implications for Sulfur Isotope Fractionations in Aqueous Sulfite Solutions

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Experimental Estimation of the Bisulfite Isomer Quotient As a Function of Temperature: Implications for Sulfur Isotope Fractionations in Aqueous Sulfite Solutions Lawrence Berkeley National Laboratory Recent Work Title Experimental estimation of the bisulfite isomer quotient as a function of temperature: Implications for sulfur isotope fractionations in aqueous sulfite solutions Permalink https://escholarship.org/uc/item/7bb6q2wt Authors Eldridge, DL Mysen, BO Cody, GD Publication Date 2018 DOI 10.1016/j.gca.2017.10.005 Peer reviewed eScholarship.org Powered by the California Digital Library University of California Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta 220 (2018) 309–328 www.elsevier.com/locate/gca Experimental estimation of the bisulfite isomer quotient as a function of temperature: Implications for sulfur isotope fractionations in aqueous sulfite solutions Daniel L. Eldridge ⇑, Bjorn O. Mysen, George D. Cody Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, DC 20015, United States Received 1 June 2017; accepted in revised form 6 October 2017; available online 16 October 2017 Abstract À 2À Bisulfite (HSO3 ) and sulfite (SO3 ) compounds play key roles in numerous geochemical and biochemical processes extend- ing from the atmosphere to the subseafloor biosphere. Despite decades of spectroscopic investigations, the molecular compo- À sition of HSO3 in solution remains uncertain and, thus, the role of bisulfite in (bio)chemical and isotope fractionation À À processes is unclear. We report new experimental estimates for the bisulfite isomer quotient (Qi = [(HO)SO2 ]/[(HS)O3 ]; [] = concentration) as a function of temperature from the interpretation of Raman spectra collected from aqueous NaHSO3 + À solutions contained in fused silica capsules. In pure NaHSO3 solutions (1Na :1HSO3 , stoichiometric) over [NaHSO3]= 0.2–0.4 m (moles/kg H2O), the following relationship is obtained: : ð : Þ ðQ Þ¼878 59 32 98 À : ð : Þ; ln i T 1 9642 0 1081 where T = 278–358 K (5–85 °C) (based on 50 determinations; additional significant figures are provided to avoid rounding À À errors). This relationship suggests that the minor isomer, (HS)O3 , may comprise 23–38% (±3%) of the mole fraction of HSO3 over 5–85 °C, respectively, which is higher in relative abundance than has generally been understood previously. We addition- ally provide estimates for Qi in NaHSO3 solutions containing a total ionic strength of m = 1.0 m (0.2 m NaHSO3 + 0.8 m NaCl) and different pH (3.3–4.5), but do not appear to resolve any significant differences in Qi as a function of these additional variables. These new values of Qi are employed to re-assess the bulk sulfur isotope fractionations among bisulfite and other S À (IV) compounds as a function of temperature, which appear to be highly dependent on the amount of (HS)O3 present. Our new constraints on the bisulfite isomer quotient may allow for a detailed assessment of the molecular composition and isotope À mass balance of S(IV) solutions containing HSO3 , and may be useful in the further investigation of the mechanisms and iso- tope fractionations associated with a number of processes that involve bisulfite compounds. These may include the intracel- lular enzymatic transformations of sulfite and bisulfite compounds that occur as part of dissimilatory sulfate reduction, which is a major and geologically important form of anaerobic respiration. Ó 2017 Elsevier Ltd. All rights reserved. Keywords: Sulfite; Bisulfite; Raman spectroscopy; Isotope effects; Dissimilatory sulfate reduction 1. INTRODUCTION Aqueous sulfite compounds (overall oxidation state: S4+, or S(IV)) are produced and consumed by a variety of ⇑ Corresponding author. chemical and biological processes in natural environments E-mail address: [email protected] (D.L. Eldridge). https://doi.org/10.1016/j.gca.2017.10.005 0016-7037/Ó 2017 Elsevier Ltd. All rights reserved. 310 D.L. Eldridge et al. / Geochimica et Cosmochimica Acta 220 (2018) 309–328 that span conditions from those of the atmosphere to the and consumption of sulfite compounds (Parey et al., 2013; subseafloor biosphere. Sulfite compounds can be produced Wing and Halevy, 2014; Leavitt et al., 2015). Our under- in the atmosphere by the hydrolysis of volcanogenic or standing of the precise mechanisms and isotope effects asso- anthropogenic SO2(g) emissions during the overall genera- ciated with the enzyme-mediated transformation of sulfite tion of acid rain and sulfate aerosols (e.g., Brandt and compounds within sulfate reducing microorganisms will van Eldik, 1995). In low-temperature aqueous environ- require a detailed understanding of the molecular composi- À ments where aqueous sulfide (H2S/HS ) is continuously tion of sulfite compounds in aqueous solution. generated by anaerobic respiration (dissimilatory sulfate Sulfite compounds exhibit a complex speciation in aque- reduction), redox interfaces are often established based on ous solution that is often ignored in many models and con- opposing sulfide and oxygen concentration gradients where siderations of the general DSR metabolism. This speciation sulfite compounds can persist at relatively low concentra- alone may result in complex chemical and isotope fraction- tions due to their continuous production and consumption ation behavior (Eldridge et al., 2016). A basic understand- by chemical and biological oxidation processes (e.g., eux- ing of the molecular composition of S(IV) solutions is inic basins and organic-rich marine sediments: Millero, based on the relative stabilities of sulfite compounds via 1991; Zhang and Millero, 1993; Zopfi et al., 2001, 2004; the following equilibrium reactions: Li et al., 2010). As part of this cycling, sulfite compounds SO ð Þ $ SO ð Þ ð1Þ can be additionally utilized by some microorganisms for 2 g 2 aq þ $ À þ þ ð Þ sulfite compound disproportionation (e.g., Bak and SO2ðaqÞ H2O HSO3 H 2 Pfennig, 1987), an intriguing metabolism whose influence HSOÀ $ SO2À þ Hþ ð3Þ on global and environmental-scale sulfur cycling is still 3 3 uncertain (Johnston, 2011). At the intracellular level, sulfite The relative distribution of S(IV) species therefore compounds are key intermediate substrates produced and depends principally on the pH of the aqueous solution, consumed by enzyme-mediated reactions within microor- and also on temperature and other variables such as ionic ganisms (bacteria and archaea) that carry out dissimilatory strength (m). The equilibrium constants and other thermody- sulfate reduction (e.g., Ishimoto and Fujimoto, 1961; Peck, namic data (e.g., DH°rxn) for these overall reactions are gen- 1962; Kobayashi et al., 1974; Oliveira et al., 2008; Parey erally well-constrained (Goldberg and Parker, 1985; Millero et al., 2010, 2013), and assimilatory sulfate reduction in et al., 1989). For example, at 25 °C and infinite dilution (low bacteria, archaea, and plants that yields reduced sulfur ionic strength), SO2(aq) dominates in solutions where pH < À 2À for building principle amino acids (cysteine and methion- 1.9, HSO3 dominates over pH = 1.9–7.2, and SO3 at pH ine) and other functions (e.g., Canfield, 2001). > 7.2 (‘‘dominates” means comprises 50% relative molar À Of the many natural processes involving S(IV) com- fraction). This indicates that HSO3 is an abundant if not pounds, dissimilatory sulfate reduction (DSR) is worthy dominant form of S(IV) over a wide range of solution con- of special consideration because it serves as a ubiquitous ditions relevant to many natural systems including the intra- and influential pathway of anaerobic respiration that oper- cellular environment of sulfate reducing organisms. ates globally in organic-rich seafloor sediments (e.g., This simple picture is complicated by the more nuanced Jørgensen, 1982; Bowles et al., 2014), and has had a pro- and somewhat uncertain molecular composition of bisulfite À found impact in the surficial geochemistry of the Earth (HSO3 ), which may be comprised of two distinct isomers throughout geologic history as inferred from the rock (Golding, 1960; Connick et al., 1982; Horner and record (e.g., Canfield, 2001, 2004; Farquhar et al., 2010; Connick, 1986; Littlejohn et al., 1992; Risberg et al., Rickard, 2014). In addition to stimulating a global oxida- 2007). Bisulfite compounds can also undergo reactions with 2À À tive sulfur cycle (Jørgensen, 1977, 1982; Jørgensen et al., each other to form a dimer (S2O5 ,HS2O5 ) when the over- À 1990; Canfield and Teske, 1996; Jørgensen and Nelson, all HSO3 concentration is relatively high (Golding, 1960; À 2004), aqueous sulfide (H2S/HS ) produced by DSR pro- Connick et al., 1982; Beyad et al., 2014). The two isomers À vides a primary source of reduced sulfur for authigenic pyr- of HSO3 differ in the structural site of protonation. One ite (FeS2) formation in marine sedimentary environments isomer contains a proton bound to an oxygen atom that À (e.g., Rickard, 2014). DSR has long been known to produce can be represented as (HO)SO2 , and the other isomer con- relatively large and variable sulfur isotope fractionations tains a proton bound to the sulfur atom represented as (HS) À between extracellular sulfate and sulfide (e.g., Kaplan and O3 . An overall equilibrium for isomerization can be written Rittenberg, 1964; Sim et al., 2011a, 2011b; Leavitt et al., as: 2013, 2015) that can exert primary controls on the sulfur ðHSÞOÀ $ðHOÞSOÀ ð4Þ isotopic composition of geologically-preserved sulfur min- 3 2 erals such as authigenic pyrite and sulfate minerals, which A corresponding equilibrium quotient (Qi) can be writ- in turn have been used to search for the earliest signs of ten to represent their relative concentrations in solution the metabolism in the ancient rock record (e.g., Shen ([] = concentration): et al., 2001, 2009; Shen and Buick, 2004; Ueno et al., ½ðHOÞSOÀ Q ¼ 2 ð5Þ 2008; Wacey et al., 2011; see also Johnston, 2011). Such i ½ðHSÞOÀ variability is generally understood to arise from factors that 3 affect the overall expression of step-wise isotope fractiona- Experimental studies have investigated values for Qi by tions associated with a series of intracellular enzyme- means of 17O-NMR, Raman, and sulfur K-edge XANES mediated reactions that centrally involve the production spectroscopies (Horner and Connick, 1986; Littlejohn D.L.
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