Influence of Silicon and Phosphorus on Structural and Magnetic Properties of Synthetic Goethite and Related Oxides

Abstract

Abstract--A series of synthetic goethites containing varying amounts of Si and P dopants were characterized by X-ray powder diffraction, electron diffraction, microbeam electron diffraction, and M6ssbauer spectroscopy. Very low level incorporation produced materials having structural and spectral properties similar to those of poorly crystalline synthetic or natural goethite. At higher incorporation levels, mixtures of noncrystalline materials were obtained which exhibited M6ssbauer spectra typical of noncrystalline materials mixed with a superparamagnetic component. Microbeam electron diffraction indicated that these mixtures contained poorly crystalline goethite, poorly crystalline ferrihydrite, and a noncrystalline component. If the material was prepared with no aging of the alkaline Fe 3+ solution before the addition of Na2HPO4 or Na2SiO3, materials were obtained containing little if any superparamagnetic component. If the alkaline Fe 3+ solution was aged for 48 hr before the addition, goethite nuclei formed and apparently promoted the precipitation of a superparamagnetic phase. The M6ssbauer-effect hyperfine parameters and the saturation internal-hyperfine field obtained at 4.2 K were typical of those of goethite; however, the M6ssbauer spectra indicated that the ordering temperature, as reflected in the relaxation rate and/or the blocking temperature, decreased with increasing incorporation of Si and P. The complete loss of crystallinity indicates that Si and P did not substitute for Fe, but rather adsorbed on crystal-growth sites, thereby preventing uniform crystal growth.

Retention of Cobalt by Pure and Foreign-Element Associated Goethites

“…Also, the relative position of hydroxyl groups on the surface is presumably altered, possibly leading to different sorption properties as compared to pure goethite. The incorporation of Si did not cause any change of the unit-cell dimensions as also shown by Cornell et al (1987) and Quin et al (1988). This indicates that the close association of this element with goethite occurs by occlusion and not by isomorphous substitution.…”

“…In the presence of Si relatively large crystals are formed (sample GSi: length ~600 nm, width ~ 200 nm) which are multi-domainic (Figure 2). This effect could be due to the presence of strongly sorbing silicate anions linking together goethite particles (Cornell et al 1987, Quin et al 1988. Furthermore, this sample exhibited the smallest X R D line-broadening of all samples.…”

Ferrihydrite: Surface Structure and Its Effects on Phase Transformation

“…As a result, A1 is much less effective in retarding the phase transformation than Si. It is known that only 2% of Si can be incorporated into the goethite lattice (Quin et al 1988), while the substitution orAl for Fe in hematite and goethite can reach 15% and 30%, respectively (Schwertmann and Cornell 1991). Quin et al (1988) speculated that "the Si... must have been adsorbed strongly on crystal growth sites.…”

“…The phase transformation rate depends on the surface water content as well as temperature (Schwertmann and Cornell 1991;Johnston and Lewis 1983). It is retarded by chemisorbed species such as organics, phosphate, and silicate (Schwertmann and Thalmann 1979;Karim 1984;Quin et al 1988).…”

Association of trace elements and dissolution rates of soil iron oxides

“…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.…”