V-goethite

Fig. 3.8 Fraction of metals dissolved vs. fraction of Fe dissolved from synthetic metal substituted goethites. Left Ni-, Co- and Mn-goethites in 0.5 M HCI at 75 °C (Giovanoli Cornell, 1992, with permission). Right Four synthetic V-goethites in 6 M HCI at 25 °C (Schwertmann, Pfab, 1994, with permission).

Pyrite, high Pb cj Pyrite, low Pb V Goethite, low Pb y Goethite, high Pb <0> Aerosols Arsenates/MSMA... 

Sorption of methyl arsenic onto metal oxides has received scant attention. Lafferty and Loeppert (2005) found that MMAs(III) and DMAs(III) were not appreciably sorbed onto goethite or ferrihydrite within the pH range of 3 to 11, while arsenite was strongly sorbed to both the oxides. In contrast, MMAs(V) and arsenate were sorbed from pH 3 to 10 in great... 

Desorption of arsenate, MMAs(V) and DMAs(V) from goethite and ferrihydrite by phosphate and sulfate was studied by Lafferty and Loeppert (2005). These arsenic compounds were desorbed more efficiently by phosphate than sulfate. In desorption envelopes, the amount of arsenate desorbed generally increased as the number of methyl groups increased... 

A series of laboratory experiments by Leuz et al. (2006), showed that the Fe oxyhydroxide, goethite, (FeOOH) is an important sorbent for Sb(lll) and Sb(V) species in the near-neutral pH range. 

Spectroscopic techniques may provide the least ambiguous methods for verification of actual sorption mechanisms. Zeltner et al. (Chapter 8) have applied FTIR (Fourier Transform Infrared) spectroscopy and microcalorimetric titrations in a study of the adsorption of salicylic acid by goethite these techniques provide new information on the structure of organic acid complexes formed at the goethite-water interface. Ambe et al. (Chapter 19) present the results of an emission Mossbauer spectroscopic study of sorbed Co(II) and Sb(V). Although Mossbauer spectroscopy can only be used for a few chemical elements, the technique provides detailed information about the molecular bonding of sorbed species and may be used to differentiate between adsorption and surface precipitation. 

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).

The band positions of Fe oxides are also influenced by the substitution for Fe by other cations in the structure, as indicated partly by their colour. Scheinost et al. (1999) noticed a linear shift in the position of the Ai " Ti transition from 943 to 985 nm and that of the Ai " T2 transition from 653 to 671 nm for 47 synthetic goethites whose Al-substitution (Al/(Al-i-Fe) ranged between 0 and 0.33 mol mol (R = 0.92 for both). Mn "-substituted goethites showed bands arising from Mn " near 454 and 596 nm. The overall reflectivity in the visible range decreased as structural Mn increased from 0 to 0.20 mol mol (Vempati et al., 1995). The same effect has been observed for V "-substituted goethites (Schwertmann Pfab, 1994). The position of the EPT band of Mn "-substituted hematite shifted to 545 nm and that of the Ai " T2 transition to 700 nm (Vempati et al., 1995). The position of the same transition shifted from ca. 600 to 592 nm as the Al-substitution in hematite rose from 0 to 0.125 mol mol (Kosmas et al., 1986). Crystal size and crystal shape also have an effect on diffuse reflectance, as shown for hematite (see Fig. 6.12). As the crystals become smaller, reflectance increases and needles also reflect more than cubes, i. e. the colour becomes more vivid. 

It is to be expected that reductive dissolution of Fe oxides becomes faster as the electron activity increases, i.e. the lower the redox potential (Eh) of the aqueous system, the faster the dissolution. Fischer (1987) dissolved goethite at pH 3 and RT in an Eh range of between -0.3 and -rO.l V and found that the dissolution rate. Ink, decreased linearly from about 5 to 1 mg Ee " L min (r = 0.96). Organic and inorganic additives that shift the redox potential in a negative direction, accelerate dissolution of iron oxides (Frenier Growcock, 1984). 

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. 

Ainsworth, C.C. Sumner, M.E. Hurst,V.J. (1985) Effect of aluminum substitution in goethite on phosphorus adsorption I. Adsorption and isotopic exchange. Soil Sci. Soc. Am. J. 49 1142-1149... 

Barron,V. Torrent, J. (1996) Surface hydroxyl configuration of different crystal faces of hematite and goethite. J. Colloid Interface Sci. 177 407-411... 

Barron,V Galvez, N. Hochella Jr., M.E. Tor-rent, J. (1997) Epitaxial overgrowth of goethite on hematite synthesized in phosphate media A scanning force and transmission electron microscopy study. Am. Min. 82 1091-1100... 

Barron,V Rendon, J.L. Torrent, J. Serna, C.J, (1984) Relation of infrared, crystallochemical, and morphological properties of Al-substi-tuted hematites. Clays Clay Min. 32 475-479 Barrow, J.J. Cox,V.C. (1992) The effects of pH and chloride concentration on mercury sorption. I. Goethite. J. Soil Sci. 43 437-450 Barrow, N. Madrid, L. Posner, A.M. (1981) A partial model for the rate of adsorption and desorption of phosphate by goethite. J. Soil Sci. 32 399-407... 

Cerpa, A. Garcia-Gonzalez, M.T. Tartaj, P. Re-quena, J. Garcell, L. Serna, C.J. (1999) Mineral-content and particle-size effects on the colloidal properties of concentrated lateri-tic suspensions. Clays Clay Min. 47 515-521 Cervini-Silva, J. Sposito, G. (2002) Steady-state dissolution kinetics of aluminum-goethite in the presence of desferrioxamine-B and oxalate. Environ. Sci. Technol. 36 337-342 Cesco, S. Rdmheld.V. Varanini, Z. Pinton,... 

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