期刊
CHEMICAL ENGINEERING JOURNAL
卷 405, 期 -, 页码 -出版社
ELSEVIER SCIENCE SA
DOI: 10.1016/j.cej.2020.126681
关键词
Phosphate; UiO-66; Linker exchange; Defect; Adsorption mechanism; Stability
资金
- Chinese Postdoctoral Science Foundation [2019 M650798]
- National Major Scientific and Technological Projects for Water Pollution Control and Management [2017ZX07402003]
- Chinese Research Academy of Environmental Sciences Central Public Welfare Scientific Research Project [2016YSKY-027]
Research into the adsorption mechanism of phosphate in different types of MOFs revealed that at lower concentrations, phosphate preferentially adsorbed on defect sites, while above a certain threshold, linker exchange became the major mechanism.
Recently, UiO-66 type metal organic frameworks (MOFs) have been studied as adsorbents for phosphate removal from wastewater; however, the adsorption mechanism with different concentrations of phosphate, especially the roles of defects and linker exchange, are not entirely clear. In this work, the phosphate adsorption mechanism in HCl-UiO-66 and HCl-NH2-UiO-66 were examined by testing the relationship between the adsorbed phosphate and released linker in the adsorption process. Adsorption kinetics studies (3.0 mM phosphate solution) and adsorption isotherm analyses (0.3-6.0 mM phosphate solution) revealed highly linear relationships (R-2 = 0.8949-0.9978) between the adsorbed phosphate and released linker for both adsorbents. Further analysis showed that when the adsorbed phosphate was lower than 0.56 mmol/g for HCl-UiO-66 and 0.51 mmol/g for HCl-NH2-UiO-66, phosphate was preferentially adsorbed on the defect sites in the MOFs with no linker release. However, when the adsorbed phosphate exceeded these levels, the defects were saturated by phosphate and linker exchange became the major adsorption mechanism. Based on density functional theory calculations, the obtained strengths of the linkers (H2BDC: Terephthalic acid; NH2-H2BDC: 2-Aminoterephthalic Acid; NO2H2BDC: 2-Nitroterephthalic Acid) and phosphate species as Lewis bases fall in the order, PO43- > HPO42> NH2BDC2 > BDC2> H2PO4- > NO2-BDC2> H3PO4, which clearly evidences why linkers can be exchanged by phosphate. Given the propensity for ligand exchange and the resulting structural destabilization, UiO-66 type MOFs is not suggested as ideal adsorbents for phosphate removal from wastewater with high phosphate concentrations.
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