Efficient and highly selective photocatalytic reduction of Cr(VI) by an active two-dimensional covalent organic framework

In this paper, a two-dimensional covalent organic framework material (named HDU-27) with thiazole structure and π-π conjugation has been successfully synthesized using the ligands tri acetaldehyde-based methoxyphenyl and benzo [1,2-d:4,5-d]thiazole-2,6-diamine in an ammonia-formaldehyde condensation reaction and applied to the photocatalytic reduction study of chromium (Cr). The photocatalytic experiments showed that HDU-27 had an excellent photocatalytic reduction of Cr(VI) with a high Cr(VI) reduction efficiency of 99.5 ​% at pH ​= ​2 for 60 ​min. In the presence of interfering ions, HDU-27 exhibited excellent selectivity for Cr(VI). Mechanistic experiments and electron spin resonance characterization identified e⁻ and ·O₂⁻ radicals as the main active species in Cr(VI) photocatalytic reduction. In addition, the photocatalytic rate of HDU-27 was significantly accelerated in the Cr(VI)/organic pollutant (RhB, MB, MO) mixed system due to the oxidation of the organic matter and the increase in the separation of photogenerated e⁻−h⁺. The cycling experiments demonstrated the excellent reduction stability and recyclability of HDU-27 in the photocatalytic reduction of Cr(VI). HDU-27 is an excellent photocatalyst with potential application for removing organic pollutants and heavy metal ions from contaminated wastewater.