SOLID-STATE AL-27 AND SI-29 NMR ANALYSIS OF HYDROXY-CR AND HYDROXY-ALINTERLAYERED MONTMORILLONITE

The ease with which sorbed Cr(III) is released to solution via oxidati on or cation exchange depends on its mode of complexation and the broa der chemical environment in which it is held. This study was conducted to elucidate the chemical environment of hydrolysed Cr(III) sorbed on to montmorillonite, both alone and coprecipitated with Al. The Al-27 M AS NMR revealed that Cr in the coprecipitated hydroxy interlayer creat ed a second environment for Al, slightly more shielded than that of Al in the montmorillonite 2:1 layer. This observation is evidence for th e presence of numerous Al-O-Cr linkages, thus revealing a uniform dist ribution of Al and Cr within the hydroxy polymers. The Si-29 MAS NMR r evealed a second environment for Si in the tetrahedral sheet of the Cr clays. This environment could be created only through a redistributio n of electrons at the siloxane surface caused by inner-sphere complexa tion of Cr(III). Due to this inner-sphere complexation, sorbed Cr(III) is difficult to displace via simple exchange reactions and, consequen tly, its bioavailability is decreased.