Characterization of polysulfides (Sx2−) in seawater euxinic conditions by electroanalytical methods

Abstract

Polysulfides (S_x²⁻) are important reduced sulfur species (RSS) that play a role in numerous environmental processes. A sound analytical method for the measurement of S_x²⁻ in euxinic waters is lacking. In this work, differential pulse voltammetry (DPV) at the Hg electrode was used to measure the presence of S_x²⁻ in a model seawater solution and an euxinic marine lake (Rogoznica Lake - RL Croatia), in which the concentration of RSS, mainly HS⁻, varies between 100 and 4000 μM. In the DPV, an adsorption phenomenon associated with S_x²⁻ reduction on Hg produces a characteristic current minimum at −1.0 V (vs. Ag/AgCl), the magnitude of which is proportional to the concentration of polysulfidic sulfur.

The DPV current minima were recorded in the model solution K₂S_x NaCl/NaHCO₃ (pH ∼ 8.2) in a concentration range from 10 to 100 μM of polysulfidic sulfur. Total RSS was measured by cyclic voltammetry, and sampled DC voltammetry showed the ratio between HS⁻ and S⁰ within the S_x²⁻. Using the same methodology, the presence of S_x²⁻ below the chemocline enriched by photoptrophic sulfur bacteria was measured in the euxinic layer of RL at concentrations of up to 70 μM of polysulfidic sulfur. The results suggest that euxinic RL samples can be considered as polydisperse solutions of S⁰ or of S-rich compounds that can change their physicochemical properties and speciation during sample manipulation. These changes can be detected by electrochemistry. Atomic force microscopy proved the release of bacterial cellular S⁰ during the acidification and purging step in the electrochemical measurements, which contributed to the voltammetric RSS signal.