Schwertmannite arsenate

In addition to sulphate, selenate (J. M. Bigham, unpubl.) and chromate (S. Regen-spurg unpubl.) can also be incorporated in the tunnels of synthetic schwertmannite. Whether or not two different Se-O distances (based on EXAFS) attributable to surface and tunnel selenate, respectively, exist in the Se-form is still under discussion (Waychunas et al., 1995, 1995 a). The Cr form has the bulk composition Fei6Oi6(OH)i0.23(CrO4)2.gg. In fact, synthetic schwertmannite formed in the sul-phate/arsenate system tolerates arsenate only up to a As/(As-rS) mole ratio of ca. 0.5, and it is likely that most of this arsenate is surface-bound. Above this ratio, a new, very poorly ordered Fe-hydroxy arsenate with two broad XRD peaks at ca. 0.31 and 0.16 nm and BhfS at 4.2K and ca. 1.5 K of 41.6 and 47.3T, respectively, forms (Carlson et al. 2002). From this one may conclude that, whereas the tetrahedral oxyanions with hexavalent central cations (S Se Cr) can be accomodated in the tunnel positions, the pentavalent cations can not, or not as easily. Schwertmannite from acid mine water contained between 6 and 70 g kg As (Carlson et al. 2002). 

The acidophilic Fe reducer Addiphilium cryptum JF-5 reductively disssolved pure schwertmannite and sulphate was released whereas arsenated and chromated schwertmannite was not reduced, and so remained inactivated, probably because of their biotoxicity (Regenspurg et al. 2002). A method for Fe removal as magnetite, recently proposed by Morgan et al. (2003), consists of oxidizing it at pH 10.5 in the presence of magnetite seeds. 

K. (2002) Potential remobdization of toxic anions during reduction of arsenated and chro-mated schwertmannite by the dissimilatory Fe(III)-reducing bacterium AcidiphUium cryp-tum JF-5 Water.Air, Soil pollution Focus 2 ... 

Waychunas, G.A. fuller, C.C. Davis, J.A. Big-ham, J.M. Schwertmann, U. (1995a) EX-AFS and simulation study of selenate and arsenate anion positions in coprecipitated schwertmannites. Abstract Amer. Geophys. Union... 

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