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 560 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 range: 1.0 x 10(-3) - 5.0 x 10(-3) mol dm(-3)) under CO2 free atmosphere. Reconstructed LDH containing intercalated CrO42- between interlayers 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 that ion-exchange based mechanism is involved: LDH + CrO42- reversible arrow 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-linear least squares fit procedure.