Interaction of Synthetic Sulphate “Green Rust” with Phosphate and the Crystallization of Vivianite

Abstract

Abstract--Because layered Fe(II)Fe(III)-hydroxides (Green rusts, GRs) are anion exchangers, they represent potential orthophosphate sorbents in anoxic soils and sediments. To evaluate this possibility, two types of experiments with synthetic sulphate-interlayered GRs (GRso 4 = Fe2+4Fe3+a(OH)12SO 4 xH20 ) were studied. First, sorption of phosphate in GRso 4 was followed by reacting suspensions of pure GRso ~ synthesized by oxidation of Fe(II) with an excess of Na2HPO4 (pH 9.3). Second the possible incorporation of phosphate in GR during formation by Fe(II)-induced reductive dissolution of phosphate-containing ferrihydrites was examined in systems containing an excess of Fe(lI) (pH 7). With excess phosphate in solution, GRso ~ initially sorbed phosphate in the interlayer producing a basal layer spacing of 1.04 nm, but only --50% of the interlayer sulphate was exchanged with phosphate. This GR slowly transformed to vivianite within months. In the Fe(II)-rich systems, reaction with synthetic ferrihydrites produced GRso 4 similar to that produced by air oxidation. Reaction of Fe(II) with phosphate-containing ferrihydrites initially produced amorphous greenish phosphate containing precipitates which, after 3-4 h, crystallized to GRso 4 and vivianite. In these solutions, stable phosphate-free GRso 4 can form since precipitation of vivianite produced low phosphate activity. Consequently, in both systems GR or amorphous greenish precipitates act as reactive intermediates, but vivianite is the stable end-product limiting phosphate concentration in solution. It is also inferred that Fe(OH) 2 is an unlikely phosphate sorbent in mixed Fe(II)-Fe(III) systems because GR phases are more stable (less soluble) than Fe(OH)2.

Biotransformation of two-line silica-ferrihydrite by a dissimilatory Fe(III)-reducing bacterium: formation of carbonate green rust in the presence of phosphate

“…In contrast, green rust two compounds (GR2) intercalate tetrahedral anions (PO 4 3Ϫ , SO 4 2Ϫ ) for charge compensation. Phosphate GR exhibits a sharp peak (d 003 ) at 1.09 nm (8.14 o 2) (Hansen and Poulsen, 1999) that was not observed here. X-ray diffraction analysis alone could not identify whether our biogenic GR1 was of the CO 3 2Ϫ or Cl Ϫ form since both exhibit similar d-spacings.…”

“…At higher P concentrations, as noted with pure ferrihydrite , a GR-vivianite mineral association was generated by CN32. Vivianite appears to be a more stable phase than GR in the presence of P (Hansen and Poulsen, 1999), and, under the chemical conditions of our experiments, exhibits more rapid precipitation kinetics than GR. The GR to vivianite ratio in our experiments was controlled primarily by the total P concentration.…”

“…However, our results are not necessarily supportive of that point. Hansen and Poulsen (1999) studied P adsorption by synthetic GR(SO 4 2Ϫ ) and concluded that, while P may exchange as an interlayer anion, GR(PO 4 3Ϫ ) is metastable and transforms to vivianite, the stable end product. In the present study, we have observed the precipitation of GR-vivianite mixtures without any evidence for the intermediate formation of GR(PO 4 3Ϫ ).…”

Effect of Phosphate on the Formation of Nanophase Lepidocrocite from Fe(II) Sulfate

“…GRs are capable of adsorbing phosphate by exchanging a portion of interlayer anions (Hansen and Poulsen, 1999). Our experiments show that interlayer phosphate favors the transformation of GR into lepidocrocite and not goethite.…”

The significance of vivianite in archaeological settings

“…In the case of iron, the substrate used by these microorganisms is an amorphous Fe 2ϩ Fe 3ϩ hydroxide known as "green rust" (Hansen and Poulsen, 1999). In poorly oxygenated soils, some microorganisms degrade phosphate-containing minerals and organic matter, while in others the DMRB place themselves in direct contact with green rust deposits.…”