REMOVAL OF AQUATIC CHROMATE ION INVOLVING REHYDRATION REACTION OF CALCINED LAYERED DOUBLE HYDROXIDE (MG-AL-CO3)

A synthetic layered double hydroxide (LDH: Mg-Al-CO3) was heated to 56 0 degrees C for 3 h in air, and the resulting calcined LDH (1.00 g dm( -3)) was used in the sorption study of aquatic CrO42- (concentration r ange: 1.0 x 10(-3) - 5.0 x 10(-3) mol dm(-3)) under CO2 free atmospher e. Reconstructed LDH containing intercalated CrO42- between interlayer s was formed and this was confirmed by X-ray powder diffraction (XRD), Al-27 MAS NMR, FT-TR and UV/VIS spectroscopies. The sorption of CrO42 - by LDH resulted in increased hydroxide concentration, suggesting tha t ion-exchange based mechanism is involved: LDH + CrO42- reversible ar row LDH(CrO4) + 2OH(-). The equilibrium constant (K), for sorption of CrO42-, based on ion-exchange with two OH- per unit LDH (composition: Mg6Al2(OH)(18)), of 3.1 +/- 0.4 mol dm(-3) was evaluated by a non-line ar least squares fit procedure.