EDTA-Functionalized Covalent Organic Framework for the Removal of Heavy-Metal Ions

The removal of heavy-metal ions from wastewater has drawn intense attention, because of their toxicity, bioaccumulation tendency, and persistency in nature. Adsorption is regarded as one of the most promising methods, because of its simplicity and efficiency. In the present work, we report the preparation of a novel EDTA-functionalized covalent organic framework (COF) for the removal of heavy-metal ions. First, a COF named TpPa-NO2 was reduced to TpPa-NH2 by using Na2S2O4 as a reductant, and then EDTA dianhydride was grafted onto TpPa-NH2 to obtain TpPa-NH2@EDTA through post-modification. Both the COF morphology and structure remained unchanged after post-modification. The TpPa-NH2@EDTA showed excellent performance in adsorbing different types of heavy-metal ions, such as soft Lewis acid (Ag+, Pd2+), hard Lewis acid (Fe3+, Cr3+), and borderline Lewis acid (Cu2+, Ni2+), and the removal efficiencies are all >85% within 5 min, because of the strong chelation effect of EDTA. The TpPa-NH2@EDTA also showed high adsorption ability in a pH ≥3 environment and have an adsorption capacity of >50 mg/g for the six representative heavy-metal ions. This work provides a new idea for the application of COF materials in the removal of heavy-metal ions from wastewater.