Electrochemical and Colorimetric Measurements Show the Dominant Role of FeS in a Permanently Anoxic Lake

Abstract

Recent publications have shown that the anodic reaction between FeS and Hg can be used for electrochemical detection of colloidal and particulate FeS in natural waters. Anodic waves that were recorded around -0.45 V (vs Ag/AgCl) in model solutions correspond to the electrochemical transformation of nanoparticulate FeS to HgS. Here, as a further step, the proposed approach is tested on anoxic, sulfidic, and iron-rich samples of a meromictic freshwater lake (Lake Pavin, France). Based on new and more comprehensive work on FeS electrochemistry in model and anoxic Lake Pavin samples, a new interpretation is given for previously recorded voltammetric signals in sulfide and iron rich environment, usually designated FeS(aq), and its role in controlling solubility of different FeS phases. A comparison of the depth profiles of S(-II) measured by voltammetry and the methylene blue method showed that the majority of S(-II) is in the form of FeS. In the monimolimnion layer, thermodynamic calculations based on total Fe(II) and S(-II) concentration, measured by ferrozine and the methylene blue method, predict precipitation of FeS with log K(s) values between -3.6 and -3.8, very close to mackinawite's K(s) value. In the upper part of the same layer, precipitation of greigite is predicted. It is shown that modification of a Hg electrode by surface-formed FeS has a significant influence on voltammetric Fe(II) determination, since reduction of Fe(II) under such conditions occurs both on bare (-1.4 V) and on FeS modified Hg surfaces (-1.1 V); Fe(II) may be underdetermined when only the -1.4 V peak is measured.

Mineralogical Diversity in Lake Pavin: Connections with Water Column Chemistry and Biomineralization Processes

“…This maar lake is ferruginous (total Fe(II) concentrations reaching more than 1 mM) and displays very low free sulfide concentrations [33,45,46]. The redoxcline located at around a 55-m depth constitutes the upper limit of the monimolimnion, which extends down to a 92-m depth.…”

“…Sulfides were stabilized by precipitation with zinc (+100 µL Zn acetate 0.01 M for a 10-mL sample); this tube was used for both sulfate and sulfide determinations. Sulfide species determined by this method comprise free H 2 S and HS´, as well as metal-bound sulfide nanoparticles (in particular, nanoparticles of FeS, as discussed later [33]). A glass tube (Exetainer Labco, Lampeter, Wales) was filled without air bubbles for alkalinity measurement and a second one for nitrogen species.…”

“…Whereas all previously-described Gallionella stalks were composed of Fe(III)-oxyhydroxides, such as ferrihydrite, goethite [72,76] or lepidocrocite [36] in association with organic matter, here, they consisted of Fe phosphates (most probably associated with organics) (Figure 7d). This suggests a strong control of the chemical composition of the solution over the composition of these biominerals, whereas organics would act as preferential nucleation sites for the deposition of these phases [33,76]. Whereas Fe phosphates in the mixolimnion of Lake Pavin have been shown to exhibit a mixed Fe redox state [20], it would be of interest to investigate the local Fe redox state of these stalks.…”

“…(4) Finally, multiple sulfur-cycling microorganisms are present in these lakes [17,32]. Chemical and Fe isotope analyses of sedimentary pyrite (FeS 2 ) suggest that these microbes may contribute to Fe sulfide precipitation in the water column [5,33,34].…”

Mercury distribution in sediments of a shallow tropical reservoir in Brazil

“…Once inside the epilimnion of the reservoir, Hg can either be eliminated towards the atmosphere after photochemical reduction (Xiao et al 1995, Peretyazhko et al 2006, transferred to the hypolimnion after adsorption on particulate matter (like other heavy metals), or undergo further transformations such as methylation (Guimarães et al 2006) or (co)precipitation (e.g., with Fe species; Bura-Nakić et al 2013).…”