Formation of Green Rust Sulfate: A Combined in Situ Time-Resolved X-ray Scattering and Electrochemical Study

Abstract

The mechanism of green rust sulfate (GR-SO(4)) formation was determined using a novel in situ approach combining time-resolved synchrotron-based wide-angle X-ray scattering (WAXS) with highly controlled chemical synthesis and electrochemical (i.e., Eh and pH) monitoring of the reaction. Using this approach,GR-SO(4) was synthesized under strictly anaerobic conditions by coprecipitation from solutions with known Fe(II)/Fe(III) ratios (i.e., 1.28 and 2) via the controlled increase of pH. The reaction in both systems proceeded via a three-stage precipitation and transformation reaction. During the first stage,schwertmannite (Fe(8)O(8)(OH)(4.5)(SO(4))(1.75)) precipitated directly from solution at pH 2.8-4.5. With increasing pH (>5), Fe(2+) ions adsorb to the surface of schwertmannite and catalyze its transformation to goethite (alpha-FeOOH) during the second stage of the reaction. In the third stage, the hydrolysis of the adsorbed Fe(2+) ions on goethite initiates its transformation to GR-SO(4) at pH >7. The GR-SO(4) then continues to crystallize up to pH approximately 8.5. These results suggest that with an Fe(II)/Fe(III) ratio of < or = 2 in the initial solution the structural Fe(II)/Fe(III) of the GR-SO(4) will be close to that of the starting composition.

Kinetic study of cis-dichloroethylene (cis-DCE) and vinyl chloride (VC) dechlorination using green rusts formed under varying conditions

“…1 shows the inverse relationship between the amount of NaOH added and the dissolved Fe(II). A dramatic loss of dissolved Fe(II) was previously reported between pH 7 and 8 when dissolved Fe(II) was adsorbed or precipitated during GR precipitation (Ahmed et al, 2010). Although this report showed that its sample had various dissolved Fe(II) concentrations within narrow pH ranges, the role of dissolved Fe(II) in the surface activation of GRs and the consequent effect on the resulting solid phases' reactivity were not the focus.…”

Formation of iron (hydr)oxides during the abiotic oxidation of Fe(II) in the presence of arsenate

“…The peak at 11.4 • is assigned to the (0 0 3) plane of green rust [42] that comes from the products of both dissolved Fe(II) and Fe (hydr)oxides such as lepidocrocite, goethite and ferrihydrite, and it can be further transformed into lepidocrocite by air oxidation [43,44]. High pH is more suitable for the formation of green rust which makes it more significant in XRD patterns at pH 8.0 than those at pH 6.0 and 7.0 [19]. Previous studies suggested the accelerated transformation of lepidocrocite to green rust in the presence of phosphate [43,45].…”

“…The oxidative hydrolysis of Fe(II) is ready to form schwertmannite at pH 2.5-4.0 with the combination of a small amount of goethite, while ferrihydrite and goethite are the dominant species in the pH range of 4.4-8.4 [18]. Under anaerobic conditions, the hydrolysis of the mixed Fe(II) and Fe(III) solution results in the formation of schwertmannite at pH 2.8-4.5, following by the formation of goethite at pH > 5.0, while green rust at pH > 7.0 [19].…”

Free energy of formation for green rust sodium sulphate (NaFeII6FeIII3(OH)18(SO4)2(s))

“…Unfortunately, the mechanisms by which GR SO 4 forms during the various procedures are poorly understood; it not clear if precursors are required for GR SO 4 formation, or if the phase can precipitate directly from solution. Neither has the role of potential precursors been thoroughly established, although it has been suggested that electron transfer from adsorbed Fe II to precursor goethite leads to brucitelike Fe II /Fe III hydroxide layers (Ahmed et al, 2010). Thus, we cannot unambiguously state that GR SO 4 formation mechanisms differ for the various synthesis methods, but should this be the case, variation in structure, composition and charge of the first formed green rust-like solid could affect the degree of Na + incorporation.…”