Experimental and theoretical investigation into the elimination of organic pollutants from solution by layered double hydroxides

This work aims at revealing the role of pristine layered double hydroxide (LDH) materials in the elimination of organic pollutants from solution. Typical LDH samples, ZnCr- and MgAl-LDHs (with Zn2+/Cr3+ or Mg2+/Al3+ molar ratio 2), are prepared and used for the removal of methylene blue (MB), methyl orange (MO), and formaldehyde. The systematic investigations of structural characterization and periodic density functional theory (DFT) calculation of the LDH samples demonstrate that: (1) no electron-hole pairs could be generated for MgAl-LDHs under the irradiation of visible light due to the large calculated gap energy above 5.0 eV. (2) ZnCr-LDHs are sensitive to the irradiation of visible light with the calculated gap energy between 2.0 and 3.0 eV, but the rapid charge recombination and low efficiency in electron/hole separation would suggest that photocatalytic activity of ZnCr-LDHs would be greatly limited. In the experimental work, ZnCr- and MgAl-LDHs show no photocatalytic activity for the removal of formaldehyde under the visible light. The disposal of the organic dyes molecules in the solution would be caused by the photoassisted degradation and surface adsorption effect rather than the photocatalysis impact for both LDH samples. This is confirmed by the elimination tests that carry out in the dark condition with the similar procedure under visible light irradiation. Moreover, the two types of LDH samples exhibit the different adsorption capability for the MB and MO molecules due to the different colloidal properties of the LDH samples, which is revealed by Zeta potential measurement. The above finding that elimination of organic dyes from solution by the pristine LDH samples through photoassisted degradation and adsorption processes would be important for the rational design and use of clay-like materials for the treatment of sewage containing toxic compounds. (C) 2013 Elsevier B.V. All rights reserved.