Modulating conjugated microporous polymers via molecular isomerism engineering for photoenhanced extraction of uranium from rare earth wastewater

Summary: When designing new conjugated microporous polymers (CMPs), major efforts are typically focused on selecting specific linkages and variations of building blocks to control their terminal structure and function. However, the constitutional isomerism of CMPs has seldom been considered so far as a crucial aspect. Herein, we successfully synthesize a pair of isomeric CMPs, named DBP-TEP (with phenanthrenequinone units) and DBQ-TEP (with anthraquinone units). Benefiting from the construction of the extended D-π-A conjugated skeleton, our isomeric CMPs are characterized by low band gaps and high charge transport efficiency, thus remarkably enhancing uranium photoreduction performance. Interestingly, the experiment results show that the adsorption capacity of DBP-TEP is 1.5 times that of DBQ-TEP under light irradiation. Additionally, both experiments in real-life and theoretical calculations prove that the phenanthrenonequinone unit does have a profound effect on the photocatalytic activity, which is predominantly attributed to the stronger electronic push-pull system and higher π-conjugated system in DBP-TEP. Science for society: Rare earth elements are a key component of modern technology. However, the mining of rare earth elements can lead to polluted waters entering the environment. Rare earth mine tailwater discharged from mining and leaching wastewater from improper mine disposal can result in a large amount of residual radionuclide uranium mainly in the form of UVI. To combat this, the reduction of highly environmentally mobile UVI to relatively immobile and low-solubility UIV has recently been recognized as one of the most promising technologies. Our study presents a green and efficient photocatalyst—a material that can absorb light influencing the rate of a chemical reaction—named 2,7-dibromo-9,10-phenanthrenedione (DBP) tetrakis(4-ethynylphenyl)methane (TEP), to extract uranium from rare earth wastewater. This study demonstrates that isomer pairs are key to efficiency: DBP-TEP (with phenanthrenequinone units) has much better photocatalytic performance in extracting uranium than 2,6-dibromoanthraquinone DBQ-TEP (with anthraquinone units).