Simultaneous incorporation of Cr, Zn, Cd, and Pb in the goethite structure

Abstract

In order to improve our understanding of how the goethite crystal structure is affected by the incorporation of metals (and by variations in the amount of the incorporation), and to review any possible synergistic and antagonistic effects of co-metals, the present investigation focused on the incorporation of multiple (di-, tri-, and tetra-) metals, i.e. Cr, Zn, Cd, and Pb, in the goethite crystallographic structure. A series of single- and multi-metal M-Cr/Zn/Cd/Pb-substituted goethites with M/(M+Fe) molar ratios = 0.10 were prepared. The general sequence of metal entry in single-metal substituted goethites was Zn = Cr > Cd > Pb and in multi-metal-substituted goethites was Zn > Cr ⩾ Cd > Pb. Simultaneous incorporation of Cr, Zn, Cd, and Pb up to 10.5 mole % was achieved in goethite. Synchrotron X-ray diffraction and extended X-ray absorption fine structure (EXAFS) techniques were employed to assess the structural characteristics of the synthesis products. Rietveld refinement of XRD data showed small changes in unit-cell parameters and Fe/M-Fe/M distances due to M substitution(s). A typical goethite-like crystalline structure remained intact, however. The unit-cell parameters were mutually, linearly correlated, though Fe/M-Fe/M distances were not, indicating that complex changes occurred at the local scale. In single-metal substituted goethites, incorporation of Cr reduced the unit-cell volume by 0.13% while that of Zn, Cd, and Pb increased it by 1.09, 3.58, and 0.56%, respectively. The changes in multi-metal-substituted goethites appeared to be the complex combination of that of the individually incorporated metals. The X-ray absorption near edge structure study of Pb-substituted goethites showed that the majority of associated Pb was Pb2+, while Pb4+ was preferred over Pb2+ in the bulk structure. Measurements by EXAFS at the Fe K-edge indicated that the Fe polyhedra contracted in the presence of Cd2+ and Pb2+, providing room for the substitution of larger cations. Measurements by EXAFS at the Cr and Cd K-edges indicated symmetric Cr/Cd polyhedra with single Cr/Cd-O distances and, at Fe and Zn K-edges and the Pb LIII-edge, indicated asymmetric polyhedra with two sets of Fe/Zn/Pb-O distances. The Zn octahedra were possibly Zn(OH)4O2, which enlarged the metal-metal corner-sharing distance to 3.86 Å. This configuration of ligands around the Zn2+ cation might occur to balance local charges. Symmetric polyhedra appeared to reduce steric strains in the structure, compared to the asymmetric polyhedra. The result was that Cr enhanced the incorporation of Zn, Cd, and Pb, while the converse was true for Zn.

Association of trace elements and dissolution rates of soil iron oxides

“…Because of the variability of the chemical environment in soil during the precipitation of Fe oxides, and their ability to incorporate other trace elements into their structures, Fe oxides are an important source and sink of environmentally significant trace elements. Incorporation of several elements such as cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), manganese (Mn), nickel (Ni), titanium (Ti), and zinc (Zn) occurs in the structure of synthetic goethites (Fitzpatrick et al 1978;Lim-Nunez and Gilkes 1987;Singh et al 2002;Cornell and Schwertmann 2003;Huynh et al 2003;Kaur et al 2009aKaur et al , 2009b, which lends further support to the association of various trace elements with Fe oxides. Trace elements are commonly associated with Fe oxides in soils; however, it is not known whether these elements occur within a separate phase or substitute for Fe 3+ in the structure of Fe oxides (Singh and Gilkes 1992;Trolard et al 1995).…”

“…The initial greater release of these elements relative to Fe also suggests that some fraction of the element was present in a discrete solid or soluble phase or adsorbed on to Fe oxides. There are several reports indicating that these elements can be adsorbed by Fe oxides and/or incorporated into Fe-oxide structures (Quin et al 1988;Angove et al 1999;Christophi and Axe 2000;Manceau et al 2000;Wells et al 2006;Kaur et al 2009a;Marcussen et al 2009). The dissolution pattern for Ti was varied (Fig.…”

MINERALOGY OF NEUTRAL MINE DRAINAGE IN THE TAILINGS OF SIDERITE-Cu ORES IN EASTERN SLOVAKIA

“…Substitution of different cations has a diverse effect on the goethite structure. In single-metal substituted goethite, incorporation of Cr reduces the unit-cell volume while that of Zn, Cd, and Pb causes its expansion (Kaur et al 2009). Moreover, incorporation of cations such as Cd, Zn, Cu, Ni, and Co has a strong impact on the lattice parameter a (Gerth 1989).…”

Probing the local environment of substitutional Al $$^{3+}$$ 3 + in goethite using X-ray absorption spectroscopy and first-principles calculations

“…Goethite is also one of the most stable Fe(III) oxide along with hematite. The high specific surface area of this mineral gives it the ability to control the mobility of metals and nutrients through adsorption and incorporation mechanisms (Trivedi et al, 2001;Kaur et al, 2009). Even if many elements can be incorporated through the replacement of iron atoms into the goethite structure, aluminium is the most common chemical impurity.…”