A fast and highly selective Congo red adsorption material based on a cadmium-phosphonate network

Abstract

In this work, a cadmium-phosphonate network was prepared based on a tetrahedral shaped tetraphosphonic acid linker. The resulting three-dimensional compound Cd₄(H₄L)₂(phen)₂(H₂O)₄ (1) exhibited a rapid and efficient adsorption of Congo red (CR) dye, the adsorption capacity of which reached 684 mg g⁻¹. Furthermore, this adsorbent showed excellent structural stability and adsorptive recyclability after three times of adsorption and desorption. The absorption kinetics fitted well with the pseudo-second-order kinetic model, revealing that the adsorption of the cadmium-phosphonate compound undergoes a chemical process, where hydrogen bonding between the amide groups (from CR) and the uncoordinated phosphonate oxygen atoms of the compound plays a pivotal role.