5.4.5: Sulfur disproportionation
- Page ID
- 132823
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)A disproportionation reaction is a type of redox reaction in which one compound of intermediate oxidation state is converted into two compounds, one that is more reduced and one that is more oxidized than the initial compound. Thus, fermentation is a type of disproportionation (Bak and Cypionka, 1987). Disproportionation reactions are also sometimes referred to as dismutations.
Some chemolithotrophic microorganisms can disproportionate sulfur compounds with intermediate oxidation states, as shown in the following example reactions with thiosulfate \(\left(\text{S}_{2} \text{O}_{3}^{2-}\right)\), sulfite \(\left(\text{SO}_{3}^{2-}\right)\), and elemental sulfur \(\left(\text{S}^{0}\right)\) (Bak and Cypionka, 1987; Finster, 2008; Thamdrup et al., 1993): \[\begin{align} & \text{S}_{2} \text{O}_{3}^{2-} + \text{H}_{2} \text{O} \longleftrightarrow \text{SO}_{4}^{2-} + \text{HS}^{-} + \text{H}^{+} \\ & 4 \ \text{SO}_{3}^{2-} + \text{H}^{+} \longleftrightarrow 3 \ \text{SO}_{4}^{2-} + \text{HS}^{-} \\ & 4 \ \text{S}^{0} + 4 \ \text{H}_{2}\text{O} \longleftrightarrow \text{SO}_{4}^{2-} + 3 \ \text{HS}^{-} + 5 \ \text{H}^{+} \end{align}\]
The reactions are essentially a combination of the microbial reactions in the two sections immediately above: intermediate sulfur species are both oxidized and reduced. Microorganisms that have found to be capable of using these reactions to support growth are primarily within bacterial class Deltaproteobacteria, including Desulfobacter, Desulfococcus, Desulfotomaculum, Desulfovibrio, Desulfomonile, Desulfobulbus, Desulfofustis, and Desulfocapsa (Finster, 2008).
The extent to which elemental sulfur disproportionation can occur appears to depend on the availability of elements that can scavenge sulfide. Thamdrup et al. (1993) found that elemental sulfur disproportionation only proceeded significantly if a sulfide sink such as iron or manganese were present, which can cause precipitation of metal sulfide phases. Similarly, Müller et al. (2020) found that cable Bacteria of family Desulfobulbaceae were only able to disproportionate elemental sulfur and thiosulfate if low sulfide concentrations were maintained by ferrous iron. Low sulfide concentrations help ensure that the reaction remains thermodynamically favorable.