Cr-goethite

Other substituents also influence the dissolution rate. Of these, and, in particular, Cr substantially stabilized goethite against proton attack, whereas the opposite effect was found for Mn (Fig. 12.23) (Schwertmann, 1991). Similar results were obtained by Lim-Nunez and Gilkes (1987). Unsubstituted and Cr-goethites had... 

Recent experiments have determined that the isotopic fractionation between adsorbed and dissolved Cr(VI) is very small (Ellis et al. submitted). In batch experiments, dissolved Cr(VI) was equilibrated with finely ground alumina or goethite. The results of these experiments should be appficable to natural settings, as oxides chemically similar to alumina and goethite provide the main adsorption sites for Cr(VI) anions. Because the analytical precision of the measurements was 0.2%o, it was necessary to amplify any isotopic shifts so that effects smaller than 0.1 %o could be detected. This was accompfished by repeating the sorption step ten or more times, so that a fractionation between dissolved and adsorbed Cr(VI) of 0.04%o was detectable at the 95% confidence level. 

Wielinga et al. (2001) demonstrated this process by incubating goethite anaerobically at pH 7 with lactate and an iron-reducing bacterium, and introducing Cr(VI) after commencement of Fe(III) reduction (Figure 7.7). In treatments without Cr(VI), accumulation of Fe(II) in solution continued, but in the treatments with Cr(VI) it was reversed in abiotic controls there was no accumulation of Fe(II). Chromate can also be reduced abiotically by sulfide. 

Fig. 3.6 Relationship between the unit cell edge length b of synthetic goethites and the level of structurally incorporated Al ", Mn ", Cr ", Co " and V " (data from Schulze Schwertmann, 1984 Stiers Schwertmann, 1985 Schwertmann et al., 1989 Gerth, 1990 and Schwertmann Pfab, 1994, respectively).

As in the case of Al, other cations which replace Fe " also modify goethite properties. The yellow colour of pure goethite turns reddish upon incorporation of Cr ",... 

Fig. n.9 a) Retention of Cr " by goethite Tcr (unitless) vs. Cr " equilibrium concentration showing the transition from adsorption to surface precipitation, b) Structural model of incorporated and adsorbed Cr " by goethite (Charlet. Manceau, 1992, with permission). 

Fig. 12.23 Dissolution-time curves for metal (M)-substituted goethites (M/(Fe-i-M)) as compared to pure goethite without substitution Mn (0.057) (0.060) (0.079) Cr -" (0.078)...

Rose and Elliott, 2000) whereas toxic elements, such as As and Cr, will probably be readsorbed by the goethite depending on pH (S. Regenspurg, pers. comm.). 

Upon adsorption of Fe " at a solid surface, the standard redox potential of the Fe /Fe pair is reduced substantially from 0.77 V to 0.35-0.45 V (Wehrli, 1990) thereby facilitating the electron transfer. Buerge and Hug (1999) have demonstrated that this higher reactivity may be responsible for the fact that solid phases (Fe oxides, Si02, and clay minerals) in natural systems accelerate Cr reduction and that goethite and lepidocrocite are by far more active in this respect than the rest of the solid phases, because these two FeOOH forms adsorb much more Fe ". The authors attribute this to better overlap and charge delocalization at the surface of the Fe oxides. 

Hoins, U. Charlet, L. Sticher, H. (1993) Ligand effect on the adsorption of heavy metals. The sulphate-cadmium goethite case. Water, Air, Soil Pollution 68 241-255 Holm, G. (1985) Substitution selectivity of some transition elements (Cr, Mn, Co, Ni, Cu, Zn) during formation of P-FeOOH. Geologiska Foreningsi Stockholm Forhandlingar 107 297-300... 

Goethite EXAFS study of As(V) and Cr(VI) adsorption on goethite. Monodentate binding favored at low surface coverage of As(V), bidentate at high surface coverage Fendorf et al. (1997)... 

For Fe oxides, FTIR spectroscopy provides a rapid means of identification. It can detect traces (1-2%) of goethite in a sample of hematite. In addition, low levels of impurities arising from insufficient washing, for example nitrate in ferrihydrite (band at 1384 cm ) and carbonate in goethite (ca. 1300 and 1500 cm ) can be detected. Broadening of the absorption bands reflects a decrease in crystal perfection (Cambier, 1986). Structural substitution of Fe by A1 (Schulze Schwertmann, 1984), Mn (Stiers Schwertmann 1985), and Cr (Schwertmann et al. 1989) causes a shift in the positions of the bands. 

The Feo/Fet ratio of the product increases from 0.002 to 0.057 as the Cr/ Cr+Fe in the system increases indicating nearly complete transformation to goethite in all cases. The Cr-for-Fe substitution ranges from between 0 and 12 mol%, i.e. not all the Cr added to the system is incorporated into goethite. The goethites becomes considerably darker as substitution increases (see Plate VI). The size of the acicular crystals decreases with increasing substitution and resistance to dissolution in 6 M HCl at RT in-... 

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