Study of Fe II/Fe III ratio in thin films of carbonate or sulphate green rusts obtained by potentiostatic electrosynthesis

Abstract

Thin films of carbonate or sulphate green rusts were synthesised from potentiostatic oxidation of solutions containing ferrous species and bicarbonate or sulphate ions at slightly alkaline pHs and ambient temperature. The thin films were characterised by means of electrochemical quartz crystal microbalance, scanning electron microscopy, X-ray diffraction and infrared reflection–absorption spectroscopy. The composition of carbonate or sulphate green rusts was studied through chemical titration, inductively coupled plasma-optical emission spectroscopy (ICP-OES) and gravimetry and is as follows: [Fe II (2 R) Fe III 2(OH) (4 R−2 R′+6) (H 2O) (2 R′−2) ] 2 R′+ ·[ R′CO 3,(2 R-{3 or 4} R′ + 2)·H 2O] 2 R′− and [Fe II (2 R) Fe III 2(OH) (4 R−2 R″+6) (H 2O) (2 R″−2) ] 2 R″+ ·[ R″SO 4,(4 R − 4 R′ + 4)·H 2O] 2 R″− Fe II/Fe III ratio R depends on the concentration of soluble ferrous species that are located at electrode surface during the potentiostatic synthesis and increases as the potential is made more negative. R-values are in the ∼2–2.5 range for carbonate green rust, and in the ∼2–2.2 range for sulphate green rust. The obtained R′- and R″-values are 1–1.3 and 1.1–1.4 for carbonate and sulphate green rusts, respectively. The present study excludes the existence of green rusts with Fe II/Fe III ratio, R, lower than 2. The R-values lower than 2 that are found in some studies might relate to mixtures of the green rust and its solid-state oxidation product exGR-Fe(III) (ferric oxyhydroxycarbonate or ferric oxyhydroxysulphate) rather than to pure green rust.