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Electrochemical determination of metal sulfide species: Discrimination between colloidal and "trully dissolved" phases

In this work, the electrochemical behaviour of different metal-sulfide particles and nanoparticles (CuS, HgS, PbS, FeS, MnS and ZnS) at the HMDE (hanging mercury drop electrode) and PIGE (paraffin-impregnated graphite electrode) in 0.55 M NaCl + 0.03 M NaHCO3 electrolyte, pH = 8.5. was studied. The aim was to investigate the possibility of using voltammetry in the detection of metal-sulfide nanoparticles in natural waters. The proposed electrochemical approach was applied to the detection of metal sulfide particles in natural samples. The results of this work surprisingly demonstrate that cathodic curves obtained in suspensions of different metal-sulfide species at the HMDE were characterized by well pronounced reduction peaks situated between -0.9 and -1.8 V (vs. Ag/AgCl electrode) (1, 2). Our results also demonstrate that positions and charges of observed peaks will depend on experimental conditions, i.e. ageing of the particles, deposition potential, accumulation time as well as mineral phases formed in the solution (Fig. 1A) (1, 3). In some metal-sulfide suspensions (FeS, HgS) more than one reduction wave can be observed (situated at -1.1 and -1.65 V vs. Ag/AgCl, (Fig 1B) ; these can be assigned to the reduction of different metal-sulfide phases or to some more complex voltammetric processes. Similar reduction peaks to those observed in the model solutions, were observed in natural samples from the anoxic, sulfidic environment (crater Pavin Lake, France) which can be assigned to the reduction of FeS dissolved and/or colloidal species (Fig. 1B). Experiments done with abrasively stripping voltammetry showed that anodic stripping peaks recorded in abrasively transferred solid state phases of synthetic zinc, ferrous, manganese and copper sulfides at the PIGE after a prior electrogeneration step are essentially almost identical for all investigated phases (4). After a preelectrolysis at potentials ranging from -1.0 to -1.5 V (vs. SCE) a well-defined anodic stripping peak corresponding to the oxidation of metal deposits generated at deposition potentials is obtained around -0.74 V (vs. SCE). Consistent with our work on CuS, FeS, PbS and HgS nanoparticles and powders at the Hg electrode (1-3), observed peaks can be assigned to the oxidation of metal deposits (Cu, Fe, Zn, Mn) formed during the reductive accumulation which with the presence of traces of sulfide in the diffusion layer form metal sulfide species retained to the electrode surface according to the following reaction: a) reduction: MS + 2e- + H+ → Mº ; ; ; ; + HS- b) oxidation (stripping scan): Mo(PIGE) + HS- → MS↓ + H++ 2e- Preliminary results obtained by the electrochemical quartz crystal microbalance (EQCM) technique, which was performed to follow the mass changes during the electrochemical transformations of different metal sulfide phases in electrolyte solutions, will be presented and discussed.

metal-sulfide ; particles ; nanoparticles ; electrochemical determination

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