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

摘要

Recent publications have shown that the anodic reaction between FeS and Hg can be used for electrochemical detection of colloidal and partic-ulate 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, thermo-dynamic 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.