Flexible Thiourea-Based Covalent Organic Frameworks for Ultrahigh Mercury Removal from Aqueous Solutions

Covalent organic frameworks (COFs) have gained significant interest because of their exceptional performance in wastewater remediation. However, most COF adsorbents are synthesized using symmetrical rigid aromatic linkers, which limit the COF molecular design. Herein, we introduced a highly desirable thiourea moiety into a flexible crystalline framework through the condensation of the aliphatic thiocarbohydrazide linker with 1,3,5-triformylphloroglucinol knot. The synthesized flexible thiourea-based COFs, Thio-COF-90 and Thio-COF-120, existed in both keto and enol tautomeric forms simultaneously. Adsorption mechanism studies demonstrated that mercury adsorption favored keto and thiol tautomeric forms over their respective enol and thione tautomeric forms. Thio-COF-120 showed higher adsorption kinetics as it exhibited a high keto form as compared with that of Thio-COF-90. Furthermore, Thio-CFO-120 possessed a very high Hg2+ adsorption capacity (4277 mg/g) owing to its abundant available chelating sites and its adsorption isotherm was fitted well by the Freundlich model. Thio-COF-120 was stable under harsh conditions and showed high removal efficiencies (>97%) under strong acidic conditions (1-3 M HCl) with an exceptional Kd value of 8.5 × 104. Moreover, Thio-COF-120 adsorbed Hg2+ selectively in the existence of competing metal ions attributable to the presence of soft–soft interaction between sulfur in Thio-COF-120 and mercury ions.