A novel covalent organic framework for efficient photocatalytic reduction of Cr(vi) and synergistic removal of organic pollutants under visible light irradiation

Abstract

Polluted wastewater containing organic pollutants and heavy metals is increasing and harms environmental ecosystems and human health. Designing and producing an efficient photocatalyst is therefore crucial for treating polluted wastewater. In this study, electron-deficient benzothiadiazole was used as the electron acceptor, and electron-rich thiophene as the electron donor using a solvothermal method. The successful preparation of a novel visible-light responsive donor–acceptor type covalent organic framework (HDU-26) has been demonstrated using various characterization techniques. The conjugated structure of the electron-donating and electron-accepting units promoted the charge transfer of the HDU-26 photocatalyst, effectively reduced the band gap, and suppressed the electron–hole (e⁻–h⁺) complexation. The photocatalytic performance was investigated under the influence of different parameters. Under optimal conditions, HDU-26 achieved a photocatalytic reduction efficiency of up to 99.5% for Cr(VI). Over five reaction cycles, HDU-26 was still able to maintain a high level of photocatalytic activity, with the Cr(VI) removal rate reaching 97.4%. The major active species for the photocatalytic reduction of Cr(VI) were e⁻ and ·O₂⁻ radicals. In addition, the oxidation of organic matter results in an increase in photogenerated e⁻–h⁺ separation. Moreover, the photocatalytic rate of HDU-26 was significantly accelerated due to the synergistic effect in the mixed system of Cr(VI)/organic pollutants (RhB, MB, MO). In conclusion, the high availability of active sites, excellent stability, and recyclability of HDU-26 indicate its potential applications for removing organic pollutants and heavy metal ions from contaminated wastewater.