The combined effect of thiols and solar radiation on the redox chemistry of iron in seawater

Abstract

[eng] Objectives. In this work we have designed a set of experiments to define the mechanism how organic compounds with reduced Sulphur groups (RSS), commonly exudated by phytoplankton, have a modulating effect on the redox chemistry of iron in seawater. The studied organic compounds included several thiols, such as cysteine, cystine, glutathione, oxidized glutathione, 3-mercaptopropionic acid, thyoglicolic acid, and N-acetylcysteine and 2- aminoethanesulfonic acid (taurine). Methods. The first method evaluates the potential Fe(II) oxidation to Fe(III) after the complexation with organic compounds. Reaction is carried out in a flow system monitored using chemiluminescence detection with luminol. The second experimental setup involves Cathodic stripping voltammetry for the quantitative determination of thiol species to assess thiol stability when exposed to solar radiation. The third experiment evaluates whether solar radiation can promote the reduction of Fe(III) to Fe(II) in the presence of thiols by chemiluminescence detection. Results. Our data shows that some RSS have a strong effect stabilizing Fe(II) in seawater delaying its oxidation. The Cathodic Stripping Voltammetry results show a thiol photodegradation over time. Final results demonstrates a fast reduction from Fe(III) to Fe(II) when exposed to solar radiation, in presence of organic complexes containing reduced sulphurs. Conclusion. Based on our findings we propose possible mechanisms that explain the rapid reduction of Fe(III) to Fe(II) under simulated marine conditions, in which the solar radiation effect in presence of reduced sulphur compounds induces an electron shuttle that triggers the reduction of a functional group inside the natural Fe-Ligand complex.