Carboxyl-Functionalized Covalent Organic Frameworks for the Adsorption and Removal of Triphenylmethane Dyes

The introduction of a functional moiety into a covalent organic framework (COF) is of great significance in promoting its wide applications. Here we show the one-pot synthesis of carboxyl-functionalized COF-TzDBd via the direct condensation of 1,3,5-tris(4-formyl-phenyl) triazine (Tz) and 4,4'-diamino-[1,1'-biphenyl]-2,2'-dicarboxylic acid (DBd). The prepared TzDBd showed good crystallinity, mesoporous pores, and high chemical stability. It was applied for ultrafast adsorption and the efficient elimination of triphenylmethane dyes. The adsorption isotherms, adsorption thermodynamics, kinetics, and reusability of the prepared carboxyl-COF were investigated in detail. The adsorption of crystal violet (CV) and brilliant green (BG) on TzDBd followed the pseudo-second-order adsorption kinetic model and reached equilibrium within 2 min at the initial concentration of 100 mg L-1. The adsorption of triphenylmethane dyes on TzDBd fit well with the Langmuir adsorption model and gave the maximum adsorption capacities of 307.7 and 276.1 mg g(-1) for CV and BG, respectively. The predesigned mesoporous pores, conjugated phenyl structure, and negatively charged carboxyl groups on TzDBd significantly facilitate the adsorption kinetics for large cationic triphenylmethane dyes. The ultrafast kinetics, high adsorption capacity, and good reusability endow the carboxyl-functionalized COF with great potential for removing triphenylmethane dyes from an aqueous environment.