Structure-induced selective adsorption of diverse dyes and Cr(vi) photoreduction using two new polyoxometalate-based metal–viologen complexes

Abstract

Organic dyes and heavy metals present in industrial wastewater pose a tremendous threat to human health and the aquatic ecosystem; therefore, it is vital to develop efficient and environmentally friendly materials for wastewater treatment. Hence, two bifunctional Keggin-type polyoxometalate (POM)-based metal–organic complexes (POMOCs), namely H[Cu^I(Hbcbpy)₄(PW₁₂O₄₀)₂]·7H₂O (BHU-3) and H[Co^II₂(bcbpy)₃(H₂O)₅(Co^IIIW₁₂O₄₀)]·6H₂O (BHU-4, bcbpy = 1-(4-carboxybenzyl)-4,4′-bipyridinium), in the presence of a viologen ligand were synthesized under hydrothermal conditions. BHU-3 exhibits a supramolecular structure with an interesting “windmill-like” [Cu(Hbcbpy)₄]⁵⁺ unit and two [PW₁₂O₄₀]³⁻ anions, whereas BHU-4 displays a one-dimensional (1-D) chain structure consisting of interesting “Y-like” metal–organic units [Co(bcbpy)₃]²⁺ and [Co^IIIW₁₂O₄₀]⁵⁻ anions. BHU-3 and BHU-4 represent rare bifunctional materials for the selective adsorption of organic dyes and photocatalytic reduction of Cr(VI). Interestingly, BHU-4 shows better selective adsorption for methylene blue (MB) and malachite green (MG) than BHU-3, and different charges in the polyoxoanions and potentials of BHU-3 and BHU-4 are the main factors affecting their adsorption abilities. On the contrary, BHU-3 performed significantly better than BHU-4 in the photoreduction of Cr(VI), which can be attributed to the fact that BHU-3 possesses higher efficiency of photogenerated carrier separation and migration. Furthermore, a series of characterization techniques explained the reasons for the differential performances of the complexes in the two applications.