Hydroxylation and dealumination of a metakaolinite-rich brick under acid conditions, and their influences on metal adsorption: One- and two-dimensional (1H, 27Al, 23Na, 29Si) MAS NMR, and FTIR studies

Brick was treated at 90 degrees C with a 6 M solution of HCl at various reaction times (1-26 h), leading to hydroxylation and dealumination of brick metakaolinite. Alkali treatment of HCl-treated brick improved the adsorption performance of this material in the removal of divalent metals (Fe2+ and Pb2+). The effects of chemical and thermal treatments on the structure of the brick were examined using one- (1D) and two-dimensional (2D) NMR and FTIR spectroscopies. The applied techniques included: (1D)H-1, Na-23, Si-29 and Al-27 MAS NMR; and Al-27/H-1 D-HMQC and H-1/Si-29 CP-HETCOR NMR. Correlation NMR methods provided new insights into the complicated issue relative to the nature of generated H-1 atoms and their connectivity/proximity with neighboring Al-27 and Si-29 atoms in hydroxylated metakaolinite. The contribution of Bronsted acid sites before and after their neutralization by NaOH on the metal-adsorption process was interpreted in terms of "active" negatively charged sites at the material surface. The metal adsorption capacity of neutralized brick was found to be intimately related to its Na amount and/or number of ESiO-Na+ and EAlO-Na+ functions. (C) 2016 Elsevier Inc. All rights reserved.