Influence of Mg2+ Ions on the Formation of Green Rust Compounds in Simulated Marine Environments

Abstract

Green rust compounds (GR), i.e., Fe(II-III) layered double hydroxides, are important transient compounds resulting from the corrosion of steel in seawater. The sulfated variety, GR(SO42−), was reported as one of the main components of the corrosion product layer, while the chloride variety, GR(Cl−), was more rarely observed. The carbonate variety, GR(CO32−), is favored by an increase in pH and forms preferentially in the cathodic areas of the metal surface. Since Mg(II) is abundant in seawater, it may have a strong influence on the formation of GR compounds, in particular as it can be incorporated in the hydroxide sheets of the GR crystal structure. In the present work, the influence of Mg2+ on the precipitation reaction of GR(SO42−) was investigated. For that purpose, Mg2+ was substituted, partially or entirely, for Fe2+. The GR was then prepared by mixing a solution of FeCl3·6H2O, Na2SO4·10H2O, NaCl, FeCl2·4H2O and/or MgCl2·4H2O with a solution of NaOH. The precipitation of the GR was followed or not by a 1-week aging period. The obtained precipitate was characterized by X-ray diffraction. It was observed that Mg(II) favored the formation of chloride green rust GR(Cl−) and magnetite Fe3O4 at the detriment of GR(SO42−). The proportion of GR(Cl−) and Fe3O4 increased with the Mg(II):Fe(II) substitution ratio. Without Fe(II), the precipitation reaction led to iowaite, i.e., the Mg(II)-Fe(III) compound structurally similar to GR(Cl−). It is forwarded that the presence of Mg2+ cations in the hydroxide sheets of the GR crystal structure is detrimental for the stability of the crystal structure of GR(SO42−) and favors the formation of other mixed valence Fe(II,III) compounds.

Effect of Ni2+, Zn2+, and Co2+ on green rust transformation to magnetite

“…The incorporation of metal impurities is expected to weaken the GR structure, making the mineral less stable and more susceptible to transformation relative to the pure phase. This explanation also agrees with previous observations of decreased GR mineral stability and increased susceptibility towards mineral transformation due to divalent cation sorption [ 3 , 24 , 26 , 27 ].…”

“…The kinetics of GR transformation to magnetite is known to dependent on pH, Eh, and temperature [ 19 ]. However, little effort has been directed into elucidating the effect of trace metals on anoxic GR transformation even though metal sorption is known to modify GR surface charge and mineral reactivity [ 2 , 3 , 20 – 24 ]. So far, the effect of Ni 2+ , Zn 2+ , and Co 2+ sorption on GR transformation to magnetite has not yet been studied.…”

Influence of Iron Substitution and Solution Composition on Brucite Carbonation

“…Additionally, reaction of 44 mol % Fe(II)-brucite in SO 4 2− solutions produced minor amounts of GR(SO 4 ) after 2 h, with a final abundance of 5.9 wt %. It is well known that green rust (i.e., fougerite) can contain a significant amount of Mg via substitution for Fe(II), 42 and recently Refait et al 57 found that Mg substitution favors uptake of carbonate and chloride ions over sulfate into the green rust interlayers. The carbonation of 44 mol % Fe(II)-brucite also produced dypingite in Cl-(12.4 wt %) and SO 4 -rich solutions (42.5 wt %).…”

Influence of Organic Matter/Bacteria on the Formation and Transformation of Sulfate Green Rust

“…Green rust compounds can be precipitated by mixing a solution of Fe(II) and Fe(III) salts with NaOH solution [19]. Based on this, a method was developed to prepare GR(SO 4 2− ) under conditions simulating seawater, i.e., using Fe(II) and Fe(III) chlorides and adding sodium chloride and sodium sulfate to obtain a suspension with overall chloride and sulfate concentrations similar to those typical of seawater [23,29]. In the present study, this method was used once again.…”