Ionic Nanocovalent Organic Frameworks for Enhanced Gold Recovery from Electronic Wastewater

Although many covalent organic frameworks (COFs) can reduce Au³⁺ to Au⁰, the reduction mechanism is unclear. These COFs are formed by reversible imine bonds, and their adaptation to acidic environments takes long periods of time, which is difficult for chemical synthesis. As a result, it is great to be able to synthesize a new class of amide-linked (CONH) nano-COFs with fewer active sites. Although amide-based COFs have been reported for gold recovery, the COFs are prepared using postmodification techniques (e.g., dynamic nucleophilic exchange and oxidation). In this study, electrically neutral nano-COF (Nano-COFA) and ionic nano-COF (Ionic-nano-COFA) linked by amide bonds are synthesized. The recovery of gold by Nano-COFA (Ionic-nano-COFA) is primarily a fast chemisorption process. Nano-COFA (Ionic-nano-COFA) has a maximum adsorption capacity of 1334 (1736) (25 °C), 2044 (2541) (35 °C), and 2933 (3200) (45 °C) mg g^-1, respectively. Selective adsorption and cyclic regeneration experiments demonstrate that the two types of nano-COFs exhibit high selectivity for gold and reusability. Additionally, the CN bonds act as reducing agents, changing Au³⁺ to Au⁰ without affecting the crystallinity of the COFs. For the treatment of real electronic wastewater, both types of nano-COFs show high selectivity and adsorption efficiency.