The ease with which sorbed Cr(III) is released to solution via oxidation or cation exchange depends on its mode of complexation and the broader chemical environment in which it is held. This study was conducted to elucidate the chemical environment of hydrolysed Cr(III) sorbed onto montmorillonite, both alone and coprecipitated with Al. The 27 Al MAS NMR revealed that Cr in the coprecipitated hydroxy interlayer created a second environment for Al, slightly more shielded than that of Al in the montmorillonite 2:1 layer. This observation is evidence for the presence of numerous Al-O-Cr linkages, thus revealing a uniform distribution of Al and Cr within the hydroxy polymers. The 29 Si MAS NMR revealed a second environment for Si in the tetrahedral sheet of the Cr clays. This environment could be created only through a redistribution of electrons at the siloxane surface caused by inner-sphere complexation of Cr(III). Due to this inner-sphere complexation, sorbed Cr(III) is difficult to displace via simple exchange reactions and, consequently, its bioavailability is decreased.

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