Experimental and DFT study of adsorption-reduction mechanism of Au(III) and Cr(VI) by β-cyclodextrin/polydopamine coated UiO-66-NH2 magnetic composites

Herein, a magnetic metal–organic framework composite was constructed by loading CoFe2O4 nano-particles on β-CD-TEPA/PDA functionalized UiO-66-NH2 for Au(III)/Cr(VI) elimination. The effective synthesis and excellent structural performances of the target materials were proved by advanced characterization technology, and the introduction of β-CD-TEPA/PDA significantly enhanced adsorption-reduction efficiency of Au(III)/Cr(VI). Batch adsorption experiments showed that the water environmental conditions remarkably affected the removal process, while isotherm and kinetic adsorption demonstrated that the elimination process belonged to a uniform chemical adsorption. The maximum adsorption amounts of Au(III) (445.5 mg/g, pH = 2.0) were greater than that of Cr(VI) (96.2 mg/g, pH = 2.0), which was probably caused by the differences in the morphology and size of the two target ions. The synthesized adsorbents exhibited good adsorption selectivity for Au(III)/Cr(VI) in a mixed system, and it could easily be removed from the solution by a magnet. The spectroscopic evidence and DFT calculation revealed that Au(III)/Cr(VI) initially was bound with -NH2 and β-CD-TEPA via electrostatic attraction and chelate action. Subsequently, Au(III) was readily reduced to Au(0) and Cr(VI) was converted into Cr(III) without additional reducing agents. About Au(III) (19.20%) and Au(0) (80.80%) species co-existed onto the spent adsorbent, while it was 56.44% and 43.56% for Cr(VI) and Cr(III). Hence, this study opened up the possibility to expand the applications of MOFs materials for wastewater remediation.