Reduction of SeO₄^2- Anions and Anoxic Formation of Iron(II)−Iron(III) Hydroxy-Selenate Green Rust

Abstract

Iron(II)-iron(III) hydroxy-salts known as green rusts were recently discovered as minerals present in hydromorphic soils and sediments. Due to their high reactivity, they are envisioned as potential reducing agents of a number of pollutants such as nitrate, chromate, or selenate. The interaction of selenate ions with such iron(II)-containing hydroxy compounds was studied by monitoring the oxidation processes of the iron phases with transmission Mo ¨ssbauer spectroscopy measured at 14 K and by following the evolution of Se(VI) in solution by capillary electrophoresis. This interaction involved the hydroxy-selenate green rust, a compound isomorphous to the hydroxy-sulfate GR(SO 4 2-). Its chemical composition, Fe II y Fe III 2 (OH) 2(y+2) SeO 4 ‚ 8H 2 O, varied with time since y starts at 5.5 and ends at 4. GR(SeO 4 2-) was obtained from Fe(OH) 2 precipitates by simultaneous accumulation of SeO 4 2anions inside the solid phase and reduction of an equal amount of SeO 4 2anions to Se(IV) species. These species were found to be less mobile, partially bound to iron compounds and/or forming iron salts. Finally, the hydroxy-selenate GR2(SeO 4 2-) can form without any other oxidizing agent than selenate itself.

Ordering in FeII–III Hydroxysalt green rusts from XRD and Mössbauer analysis (chloride, carbonate, sulphate, oxalate...); about the structure of hydrotalcite-like compounds

“…In contrast studies concerning the other type of stacking for GR, i.e. GR2 as obtained for SO 4 2j or SeO 4 2j , led to one unique value of x at 0.33 [2,8].…”

“…Hydroxychloride GR1(Cl j ) was prepared this way by adjusting the excess of Cl j with respect to the stoichiometry of Fe(OH) 2 when mixing FeCl 2 and NaOH. A large range of composition was obtained from {[Cl j ]/[OH j ]} = 1.14 to 1.20, which led to compounds Fe II 3 Fe III (OH) 8 Cl [3] and Fe II 2 Fe III (OH) 6 Cl [4], respectively. XRD patterns were those of a GR1 with parameters a 0 = 0.319 nm and c = 2.385 nm.…”

Microbial studies of a selenium-contaminated mine site and potential for on-site remediation

“…The data also indicate that microbial populations are very important in this transformation as demonstrated in comparing the iron amendment column results. There have been reports of green rust independently reducing selenate [42,51]. If this were the only factor in reduction and binding of the Se, then the extracted Se would be consistent in all of the columns containing iron.…”

Biotite dissolution and Cr(VI) reduction at elevated pH and ionic strength

“…Thus, the differences in Cr(VI) removal in these experiments suggests an additional reduction mechanism, namely heterogeneous reduction of Cr(VI) by secondary Fe(II) precipitates such as Fe(OH) 2 or green rust. Refait et al (2000) found green rust formation through reduction of Se(VI) by Fe(OH) 2 as indicated by the reaction: …”