Thio-groups decorated covalent triazine frameworks for selective mercury removal

Covalent triazine frameworks (CTFs) as a kind of covalent organic framework (COF) materials show great potential for practical application by virtue of their high stability and facile large-scale synthesis. In this work, we developed three CTFs (MSCTF-1, MSCTF-2, and xSCTF-2) of different pore size decorated with S-groups using different functionalization methods for achieving selective Hg²⁺ removal from aqueous solutions. The material structures were comprehensively studied by gas adsorption, IR and XPS, etc. Among them, the MSCTF-2 with 24.45% S content showed the highest Hg²⁺ adsorption capacity of 840.5 mg g^‒1, while MSCTF-1 exhibiting much larger distribution coefficient of 1.67 × 10⁸ mL g^‒1 renders an exceptionally high efficiency for reducing the concentration of Hg²⁺ contaminated water to less than 0.03 μg L^‒1. Moreover, the MSCTFs show distinct features of: (i) high selectivity toward Hg²⁺ over various transition metal ions; (ii) high stability over a wide pH range from pH 1 to 12; and (iii) good recyclability with 94% of Hg²⁺ removal over five consecutive cycles. The Hg²⁺ adsorption on functionalized CTFs followed pseudo-second-order kinetics and Langmuir isotherm. Our results revealed the material structure-performance relationship that the adsorption capacities depend on the binding site density whereas the distribution coefficient is essential to the removal efficiency.