Excellent photoreduction performance of U(VI) on metal organic framework/covalent organic framework heterojunction by solar-driven

• MOF-Ti, MOF-In and MOF@COF were synthesized and applied for U(VI) photoreduction. • NH 2 -MIL-125(Ti)@TpPa-1 heterojunction was constructed broadened the range of visible light response. • The intimate link (π-π interaction) between NH 2 -MIL-125(Ti) and TpPa-1 promoted charge-carrier transfer and the efficiency of separation. • The presence of active Ti 3+ , O V • and •O 2 − played the dominant roles in converting U(VI) into U(IV). The conversion of uranium(U) from soluble U(VI) into insoluble U(IV) by photocatalytic reduction is an implementable removal technology. In this investigation, two common MOFs photocatalysts (MOF-In, MOF-Ti) and the assembled MOF@COF (2D Schiff base, NH 2 -MIL-125(Ti)@TpPa-1) were applied for photocatalytic reduction of U(VI). These as-synthesized photocatalysts were characterized by XRD, SEM, TEM, XPS, DRS, EIS, photocurrent and Mott-Schottky plots. The results shown that the constructed NH 2 -MIL-125(Ti)@TpPa-1 heterojunction not only broadened the scope of visible light response to orange light (600 nm), but also expedited the separation of photogenerated carriers and facilitated the U(VI) photoreduction. Specially, the photoreduction removal rates of U(VI) were NH 2 -MIL-68(In) (55.6%), NH 2 -MIL-125(Ti) (57.7%), NH 2 -MIL-125(Ti)@TpPa-1 (81.6%), and the heterojunction of NH 2 -MIL-125(Ti)@TpPa-1 was about 1.5-fold higher than that of NH 2 -MIL-125(Ti). Meanwhile, during the photoreduction process, photogenerated electrons and superoxide radicals played the dominant roles in converting U(VI) into U(IV). Moreover, the presence of active Ti 3+ and oxygen vacancy could effectively promote superoxide radical generation and inhibit recombination of photogenerated carriers. Combined with the U(VI) adsorption by NH 2 -MIL-125(Ti)@TpPa-1 in previously studied, altogether, MOF@COF hybridization can play a synergistic role of adsorption-photocatalytic reduction in the practical application of U(VI) elimination. Therefore, it can be predicted that 2D COF-based hybridization materials have a brilliant application prospect in pollutant purification by solar energy.