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Electroanalytical characterization of polysulfides (SX2-) in model solution and euxinic seawater conditions

Sulfur (S), an ubiquitous element in the environment, occurs in different oxidation states from -2 (sulfide, S2-) to +6 (sulfate, SO42−). Between the thermodynamically stable end members (S2-, SO42−), a variety of intermediate S species exist, examples of which are sulfite, polythionates, thiosulfate, elemental sulfur (S0), and polysulfides (SX2-). S species with intermediate oxidation states are produced during a variety of biotic and abiotic processes including the oxidation of hydrogen sulfide (HS-) and sulfide minerals, SO42− reduction, and the transformation of organic S compounds. Reduced forms of S, reduced sulfur species (RSS: S2-, HS-, S0, SX2-, RSH), are most important forms for processes related to S biogeochemistry and environmental quality [1, 5]. Polysulfides are important RSS which play a role in numerous environmental processes, and so far there is not methodology for their direct determination in natural water environments, and conditions with presence of other RSS. In this work for the first time differential pulse voltammetry (DPV) at the Hg electrode [1], was used for determination of SX2- in natural euxinic seawater environment (Rogoznica Lake, pH=7.0 - 7.5 ; oxygen concentration = 0 – 0.4 mg L-1), i.e. conditions where total RSS (mainly in form of HS-) could varied between 10-8 M to 10-3 M [2, 5]. In this approach an adsorption phenomenon associated with SX2- or a product of it’s cathodic reaction with the Hg electrode created a characteristic current minimum at -1.0 V (vs.Ag/AgCL), which magnitude is proportional to the concentration of S(0) existing in the form of polysulfidic sulfur [1]. In our case, well defined DPV peaks were recorded for model K2SX NaCl/NaHCO3 carbonate buffer solution (pH~9.5) in the concentration range between 10 mg L-1 and 500 mg L-1 (Figure 1a). The same approach, and obtained calibration, demonstrated presence of SX2- in the chemocline and anoxic euxinic layer of Rogoznica Lake, in the concentration range between 18 mg L-1 to 75 mg L-1 (Figure 1b). Additionally, sampled DC polarography (SDC) at the Hg electrode revealed in the studied samples a ratio between anodic and cathodic currents, i.e. the ratio between sulfide and S(0) presence in the SX2- [1, 2]. Established electrochemical approach for RSS determination [2-5], based on accidification and purging step, determined total concentration of RSS and presence of its volatile and non-volatile fractions. Concentration of SX2- as well as total RSS varied with environmental conditions (depth, oxygen concentration, light intensity) that are seasonaly dependent (Figure 1b). [1] S. Kariuki et al., Anal. Chim. Acta 442 (2001) 277-285. [2] I. Ciglenečki et al., J. Electrochem. Sci. Eng. 4(4) (2014) 155-163. [3] T.F. Rozan et al., Anal. Chim. Acta 415 (2000) 175–184. [4] I. Ciglenečki and B. Ćosović, Electroanal. 9 (1997) 775-780. [5] E. Bura-Nakić et al., Geochim Cosmochim Acta 73 (2009) 3738-375.

polysulfides ; electroanalytical methods ; differential pulse voltammetry (DPV) ; euxinic seawater conditions

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