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 70 mM GLY (pH 8.0), whereas in the absence of GLY, synthesis failed under similar conditions. Glycine functions as both a pH buffer and a ligand; Fe$^{II}$-GLY complexes serve as a source of base (Fe$^{II}$-GLY + H$_2$O -> 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$^{II}_{4.08}$Fe$^{III}_{1.98}$(OH)$_{11.6}$(SO$_4$)$_{1.00}$, and the tabular GR crystals had a lateral size of 100–500 nm and a thickness of about 40 nm. Upscaling of the synthesis by either 25 times in volume or 20 times in FeII 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.

In Journal of Colloid and Interface Science