Oxide nanolayer formation on surface of modified blast furnace sludge particles during voltammetric cycling in alkaline media

Abstract

Knowledge of the properties of metallurgical waste is essential for the assessment of their recycling. In this work, the formation of iron oxide nanolayers during voltammetric cycling in 1 M NaOH on the particle surface of blast furnace sludge after acid leaching (BFSL) was studied. Most importantly, the effect of hydrogen on these processes was of particular interest. For these purposes, the study combines electrochemical methods, cyclic voltammetry on solid and carbon paste electrodes, with analytical optical methods (TEM). On the solid iron electrode surface as a model system, nanostructured magnetite (Fe 3 O 4 ) was identified as the main oxidation product, and, to a lesser extent, also maghemite (γ-Fe 2 0 3 ). It was found that the charges corresponding to Fe 3 O 4 formation and its reduction together with the hydrogen evolution reaction (HER) occurring at E = − 1500 mV depend on the number of cycles and have a similar course. Additionally, in the first phase of the cycling, the accumulation of maghemite on the solid Fe-electrode surface during cycling affects the growth of the oxide layer and catalytically increases the yield of the HER. Concerning the measurement with BFSL-modified CPE, on the BFSL surface, haematite is transformed into magnetite during cycling, resulting in the same Fe 3 O 4 nanolayer as on the solid iron electrode. In this layer, the same redox processes take place, including the influence of hydrogen in the initial stage of cycling.

Freezing Enhances Leaching of Ferrous Ions but Hinders Reductive Dissolution of Ferric Ions from Iron Oxides

“…Also, if dissolved ion species are supplied from the electrode by its reduction, the redox reaction of Fe 2+ /Fe 3+ ( E 0 = 0.77 vs SHE) is observable in this potential window. In actuality, former studies using the magnetite electrode observed the Fe 2+ /Fe 3+ redox couple on voltammograms even when ferrous or ferric ions were not added to solutions, suggesting that dissolved iron species are supplied from magnetite. − …”

“…Figure a shows CVs recorded using the magnetite electrode at 25 °C, where the potential ( E ) was scanned in the potential range, E = 0.3 → −0.5 → 0.8 → 0.3 V (vs Ag/AgCl). CV is well characterized by one redox couple (anodic peak A 1 and cathodic peak C 1 ) at E ∼ 0.4 V and one cathodic peak (C 2 ) at E ∼ −0.1 V. Former studies assigned the C 1 /A 1 couple to the redox of dissolved Fe 2+ /Fe 3+ and C 2 to the reduction of ferric ions on the surface of the magnetite crystal (Fe magn III ). − Thus, dissolved Fe 2+ , which is supplied at C 2 , is oxidized to Fe 3+ at A 1 , and then, Fe 3+ is reduced at C 1 in the subsequent cathodic scan to give the C 1 /A 1 redox couple. Because dissolved iron species does not exist in the original solution, the A 1 /C 1 redox couple did not appear when the potential was initially scanned in the anodic direction, that is, E = 0.3 → 0.8 V, as shown in Figure b.…”

“…The dissolution of iron from minerals is a critical process governing biological activity particularly in the hydrosphere. ,, The present work has indicated that freezing possibly plays a significant role in this process of environmental importance. Iron undergoes complex equilibria in neutral or basic solutions; for example, oxyhydroxides rather than oxides become stable, , and complexation is facilitated . Therefore, the system is more complicated in high pH media because more factors should be taken into consideration.…”

Electrochemical characterization of leached steel-making sludge

“…Once Fe(0) is formed due to hydrogen evolution at a potential of approximately − 1.5 V, the anodic peak can be observed. The presence of metallic iron was proved in 43 . When starting in the cathodic direction from 0 to − 1.5 V, a sufficient amount of Fe(II) is formed and anodic peaks also occur.…”