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Application of the electrochemical method for monitoring polysulfides (SX2-) in marine euxinic environment (Rogoznica Lake, Croatia)

Rogoznica Lake (RL) is small and shallow naturally eutrophicated marine lake, situated on the eastern Adriatic coast in Croatia (43°32’ N, 15° 58’ E). The main characteristic of the RL is seasonally stratified water column into the upper oxic and lower anoxic layers.[1] Due to intense sulfate reduction, the anoxic layer is rich in dissolved HS- (up to 5 mM).[1] At the oxia-anoxia boundary, a pinkish colored chemocline develops seasonally, and it is characterized by a dense population of purple phototrophic sulfur bacteria (PSB).[2] PSB are known to store sulfur S(0) intracellularly, which by cell lysis would be released in water, and under euxinic conditions solubilized by HS- in nucleophilic polysulfides, SX 2-. SX 2- play important role in trace metal, C and S biogeochemical cycles. Due to large range of reduced sulfur species (RSS: S2-, HS- , S0 , SX 2-, RSH) concentrations (nM – mM) in the RL water column, this environment is an ideal site for testing and development of methodology for RSS characterization and speciation.[1, 3, 4 and references therein] The direct determination of SX 2- in samples containing other RSS remains a challenge ; partly because many of the S species are thermodynamically unstable. In this work, for the first time in natural samples, differential pulse voltammetry (DPV) was used for direct determination of SX 2- in the euxinic aqueous environment (RL). In this approach an adsorption phenomenon associated with SX 2- 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.[5] In our case, well defined DPV peaks were recorded for model K2SX solution. The same approach, and obtained calibration, demonstrated presence of SX 2- in the RL water column, bellow the chemocline (up to 0.1 mM polysulfide S (0)). Additionally, in the studied samples, total RSS concentration was measured by cyclic voltammetry (CV), [1–4] while sampled DC polarography (SDC) at the Hg electrode revealed a ratio between anodic and cathodic currents, i.e. the ratio between sulfide and S(0) presence in the molecule of SX 2- . [2, 4] Concentrations of SX 2-, as well astotal RSS, varied seasonally depending on redox conditions in the lake. However, position of oxia-anoxia boundary (chemocline position), light intensity, and HS- concentration greatly influence on SX 2- distribution and concentration within the RL water column. Acknowledgements. This work was funded by Croatian Science Foundation, IP-2018-01-1717- MARRES REFERENCES [1] I. Ciglenečki, Z. Ljubešić, I. Janeković, M. Batistić, in R.D. Gulati, E.S. Zadereev, A.G. Degermendzhi (eds) “Ecology of meromictic lakes”. Springer 2017, Cham, p 125−154. [2] M. Čanković, J. Žučko, I. Dupčić Radić, I. Janeković, I.Petrić , I. Ciglenečki, G.Collins, Syst Appl Microbiol 2019, 42, 126016, 13 [3] I. Ciglenečki, M. Marguš, E. Bura – Nakić, I. Milanović, J. Electrochem. Sci. Eng. 2014, 4(4), 155-163. [4] E. Bura-Nakić, G.R. Helz, I. Ciglenečki, B. Ćosović, Geochim Cosmochim Acta 2009, 73, 3738– 3751. [5] S. Kariuki, M. J. Morra, K. J. Umiker, I. Francis Cheng, Anal. Chim. Acta 2001, 442, 277- 285.

polysulfides ; electrochemical methods ; Rogoznica Lake

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