Structural Incorporation of Trace Metals in Goethite (a-FeOOH) in Di-Metal Systems.
Navdeep Kaur, Balwant Singh, Markus Gräfe, and Brendan J. Kennedy. The University of Sydney, The Faculty of Agriculture, Food and Natural Resources, A03 - Ross Street, Sydney, 2006, Australia
Knowledge of the mechanisms by which trace metals associate with goethite in multi-element systems is essential to assess their bioavailability and to manipulate the role of goethite in controlling these processes under natural conditions. To mimic the formation of goethite and its association with trace elements in nature, an attempt was made to synthesize goethite in the presence of two foreign metal cations. Thus, the ability of goethite to incorporate simultaneously two foreign metal cations in its structure was investigated. Goethites were synthesized using the Fe (III) hydrolysis pathway via the formation of ferrihydrite. The di-metals systems investigated were Cd-Zn; Cd-Cr; Cd-Pb; Pb-Cr, and Pb-Cu. For comparison, a pure (non-substituted) goethite was also prepared. All goethites were prepared following a modified method of Cornell and Schwertmann (1991). The nominal foreign metal concentration was 10 mol% (5 mol% for each metal) in all goethites, except the Cd-Zn substituted sample, which was 13 mol% (6.5 mol% for each metal). The transformation periods were 90 days for Cd-Zn, 35 days for Cd-Cr, and 25 days for Cd-Pb; Pb-Cr and Pb-Cu systems as compared to 60 h for the pure system i.e. goethite with no metal substitution. Zinc retarded the rate of transformation of ferrihydrite to goethite the most and Pb the least. Following their transformation, the samples were washed with DDI-water, dried at 55°C, and gently crushed. Any amorphous materials were removed by extracting the solids five times with 0.3 M ammonium oxalate + 0.3 M oxalic acid buffer (pH = 3) in solid-solution ratios of 1:100 (g/ml) for 2 h in the dark. An additional extraction with 0.01 M HCl was given to remove surface adsorbed oxalate and to dissolve any metal-oxalate precipitated phases. Finally an extraction with DDI-water and acetone was applied before drying the purified samples in an oven at 55°C. Goethites containing Pb were purified using 0.2 M HClO4 to avoid the formation of insoluble Pb-oxalate (s) or PbCl2 (s) phases. Analyses of extracts showed that two to three extractions were sufficient to remove amorphous materials. X-ray powder diffraction (XRD) indicated that only goethite had formed in all cases except for some additional hematite in the Pb-Cu substituted sample. A small amount of Cd(OH)2 (s) was observed in unextracted and extracted samples of Zn-Cd substituted goethite, which was removed by further extractions with 0.2M HClO4. A purified subsample (1:1000, g/ml) was dissolved in 6M HCl to assess the metal incorporation into the structure. The chemical analyses indicated substitutions of both metals upto 10.3, 9.0, 6.5 and 6.1 mol% for Cd-Cr; Pb-Cr; Cd-Pb; and, Cd-Zn substituted samples, respectively. Chromium appeared to enhance the incorporation of Cd and Pb into goethite structure, whereas Zn appeared to suppress Cd incorporation. In the Cd-Cr system, the substitutions of Cd and Cr for Fe were 5.1 and 5.2 mol%, respectively, while in the Zn-Cd system, the substitutions of Cd and Zn for Fe were 0.7 and 5.5 mol%, respectively. In the Pb-Cr system, the substitution Pb and Cr for Fe were 2.8 and 6.1 mol%, respectively, while in the Pb-Cd system, the substitution of Pb and Cd for Fe were 1.1 and 5.4 mol%, respectively. Both, Pb and Cr appeared to enhance the substitution of Fe by Cd. At similar levels of nominal metal concentrations, the extent of substitution for the studied metals was Cr> Zn> Cd> Pb. This order could be related to the similarity in ionic radii and valence of these cations in comparison with Fe3+ (0.0645 nm) and their effect on the rate of transformation. It is hypothesized that the substitution of Cr3+ (0.0615 nm) did not cause significant structural strain and thus permitted the substitution of other metals, viz., Pb4+ and Cd2+. In contrast, Zn2+ (0.074 nm) caused structural strains and a charge imbalance and thus restricted the incorporation of the even larger Cd2+ (0.095 nm) ions. Lead (Pb4+) ions, (0.0775 nm) appeared to enhance the incorporation of Cd2+, which may be related to a charge balancing factor in the structure rather than a cation radius factor. To ascertain the actual mechanisms of substitution in di-metal systems, further studies employing synchrotron-based X-ray diffraction and extended X-ray absorption fine structure spectroscopy will be conducted to determine the unit cell parameters and the local coordination environments of the metals in the goethite lattice structure, respectively. The effect of metal cation substitution on the crystal morphology and thermal stability will also be evaluated using transmission electron microscopy and thermogravimetric analyses, respectively.