Carbonate and sulphate green rusts—Mechanisms of oxidation and reduction

The oxidation and reduction of carbonate, GR(CO₃), and sulphate, GR(SO₄), green rusts (GR) have been studied through electrochemical techniques, electrochemical quartz crystal microbalance (EQCM), FTIR, XRD and SEM. The used samples were made of thin films electrodeposited on gold substrate. The results from the present work, from our previous studies and from literature were compiled in order to establish a general scheme for the formation and transformation pathways involving carbonate or sulphate green rusts. Depending on experimental conditions, two routes of redox transformations occur. The first one corresponds to reaction via solution and leads to the formation of ferric products such as goethite or lepidocrocite (oxidation) or to the release of Fe^II ions into the solution (reduction) with soluble Fe^II-Fe^III complexes acting as intermediate species. The second way is solid-state reaction that involve conversion of lattice Fe²⁺ into Fe³⁺ and deprotonation of OH groups in octahedra sheets (solid-state oxidation) or conversion of lattice Fe³⁺ into Fe²⁺ and protonation of OH groups (solid-state reduction). The solid-state oxidation implies the complete transformation of GR(CO₃) or GR(SO₄) to ferric oxyhydroxycarbonate exGRc-Fe(III) or ferric oxyhydroxysulphate exGRs-Fe(III), for which the following formulas can be proposed, Fe^III₆(OH)_(12−2y)(O)_(2+y)(H₂O)_(y)(CO₃) or Fe^III₆(OH)_(12−2z)(O)_(2+z)(H₂O)_(6+z)(SO₄) with 0 ≤ y or z ≤ 2. The solid-state reduction gives ferrous hydroxycarbonate exGRc-Fe(II) or ferrous hydroxysulphate exGRs-Fe(II), which may have the following chemical formulas, [Fe^II₆(OH)₁₀(H₂O)₂]·[CO₃, 2H₂O] or [Fe^II₆(OH)₁₀(H₂O)₂]·[SO₄, 8H₂O].