Two birds with one stone: Simultaneous adsorption and reduction of Cr(VI) in water by an engineered thiol group-perylene diimide covalent organic polymer

Abstract

Hexavalent chromium (Cr(VI)) poses severe environmental risks due to its high toxicity, mobility, and carcinogenicity. Herein, a thiol-functionalized perylene diimide-based covalent organic polymer (PDI-DBD-SH) was prepared through one-step thermal polymerization for integrated Cr(VI) adsorption, reduction, and immobilization. The material combines redox-active –SH sites with a rigid π-conjugated PDI-based polyimide backbone, enabling thiol-mediated Cr(VI) reduction and PDI-assisted charge migration. PDI-DBD-SH exhibited pH-dependent Cr(VI) removal, reaching a maximum adsorption capacity of 53.42 mg g-1 at pH 3.0. The XPS analysis and alkaline desorption test demonstrated that Cr(VI) was electrostatically enriched on the material surface and reduced to Cr(III), which was subsequently immobilized on PDI-DBD-SH, accompanied by the oxidation of –SH groups to S–S and –SO3H species. Density functional theory and frontier orbital analyses revealed favorable Cr(VI) adsorption, thiol-to-Cr(VI) electron transfer, and charge delocalization through the PDI backbone. PDI-DBD-SH also maintained effective Cr(VI) removal in real water matrices. This work highlights a conjugated-backbone-assisted strategy for constructing covalent organic polymers capable of integrated Cr(VI) capture, reduction, and immobilization.