Glycine buffered synthesis of layered iron(II)-iron(III) hydroxides (green rusts)

Layered Fe^II-Fe^III hydroxides (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 (GR_SO4) by aerial oxidation of Fe^II or co-precipitation by adding Fe^III salt to an aqueous solution of Fe^II at constant pH. In both the oxidation and the co-precipitation methods pure crystalline GR_SO4 was 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; Fe^II-GLY complexes serve as a source of base (Fe^II-GLY+H₂O→Fe^II+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 GR_SO4 precipitation, 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 Fe^II measurements confirmed the purity, stoichiometry, and pyroaurite-type structure of the obtained GR_SO4. The formula of GR_SO4 was found to be Fe^II_4.08Fe^III_1.98(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 Fe^II concentration resulted in pure GR_SO4 products. Compared with the conventional unbuffered GR_SO4 synthesis method, the present method can provide pure products with a controllable, fast, and low-cost process.