title: Glycine buffered synthesis of layered iron(II)-iron(III) hydroxides (green rusts).
abstract: Layered FeII-FeIIIhydroxides (green rusts, GRs) are efficient reducing agents against oxidizing contaminants such as chromate, nitrate, selenite, and nitroaromatic compounds and chlorinated solvents. In this study, we adopted a buffered precipitation approach where glycine (GLY) was used in the synthesis of sulfate-interlayered GR (GRSO4) by aerial oxidation of FeIIor co-precipitation by adding FeIIIsalt to an aqueous solution of FeIIat constant pH. In both the oxidation and the co-precipitation methods pure crystalline GRSO4was precipitated in the presence of 70mM GLY (pH 8.0), whereas in the absence of GLY, synthesis failed under similar conditions. Gycine functions as both a pH buffer and a ligand; FeII-GLY complexes serve as a source of base (FeII-GLY+H2O→FeII+H-GLY+OH-) during GR formation, supplying about 45% of the total base required for the synthesis. The GLY buffer decreases pH fluctuations during base addition and hence allows for fast GRSO4precipitation, minimizing byproduct formation. The use of other pH buffers [4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid and 2-amino-2-(hydroxymethyl)-1,3-propanediol] was also tested but failed. Mössbauer spectroscopy, X-ray diffraction, Fourier transform infrared, transmission electron microscopy, and FeIImeasurements confirmed the purity, stoichiometry, and pyroaurite-type structure of the obtained GRSO4. The formula of GRSO4was found to be FeII4.08FeIII1.98(OH)11.6(SO4)1.00, and the tabular GR crystals had a lateral size of 100-500nm and a thickness of about 40nm. Upscaling of the synthesis by either 25 times in volume or 20 times in FeIIconcentration resulted in pure GRSO4products. Compared with the conventional unbuffered GRSO4synthesis method, the present method can provide pure products with a controllable, fast, and low-cost process.
authors.affiliation: Department of Plant and Environmental Sciences, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark. Electronic address: wey@plen.ku.dk.
authors.forename: Weizhao
authors.initials: W
authors.lastname: Yin
authors.affiliation: Department of Plant and Environmental Sciences, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark; Department of Chemistry, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark.
authors.forename: Lizhi
authors.initials: L
authors.lastname: Huang
authors.affiliation: Department of Chemistry, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark.
authors.forename: Emil Bjerglund
authors.initials: EB
authors.lastname: Pedersen
authors.affiliation: Department of Physics, Technical University of Denmark, Fysikvej, 2800 Kongens Lyngby, Denmark.
authors.forename: Cathrine
authors.initials: C
authors.lastname: Frandsen
authors.affiliation: Department of Plant and Environmental Sciences, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark.
authors.forename: Hans Christian B
authors.initials: HCB
authors.lastname: Hansen
keywords: Glycine ligand; Green rust sulfate; Homogenous precipitation; Layered double hydroxides (LDHs)