Covalent organic framework with N-O dual-active sites for high-selectivity palladium recovery in metallurgical wastewater.

Selective recovery of Pd(II) from complex metallurgical wastewater is critical for mitigating metal pollution and realizing circular utilization of scarce precious-metal resources. Pristine covalent organic framework (COF) skeletons exhibit limited Pd(II) selectivity, while powdery COF adsorbents suffer from poor solid-liquid separability, restricting their practical application in wastewater treatment. Herein, an amidoxime-modified TpDB-AO with N-O diatomic synergistic chelating sites was fabricated via post-synthetic functionalization. This material can form stable five-membered chelate rings with Pd(II), greatly enhancing coordination affinity. Batch adsorption experiments demonstrate that TpDB-AO achieves an experimental uptake of 240.3 mg g-1 for Pd(II), substantially outperforming pristine TpDB (102.8 mg g-1). To address the inherent drawbacks of powdery COFs, TpDB-AO was further fabricated into a three-dimensional interconnected monolithic aerogel (TpDB-AO-AG). Dynamic breakthrough tests using real metallurgical wastewater reveal that 6 mg of TpDB-AO-AG achieves a cumulative treatment volume of 1720.8 mL at an initial Pd(II) concentration of 23.3 mg L-1, which is 35% higher than that of the unmodified aerogel. Spectral characterization combined with theoretical calculations verifies that N-O synergistic chelation dominates the highly selective capture of Pd(II), while electrostatic attraction serves as the auxiliary driving force. Unlike most studies that rely merely on simulated wastewater, this work validates the feasibility of TpDB-AO for practical industrial effluent treatment, providing reliable experimental and theoretical support for the design and preparation of high-performance adsorbents for precious metal recovery.