Carboxyl groups on g-C3N4 for boosting the photocatalytic U(VI) reduction in the presence of carbonates

The photocatalytic U(VI) reduction provides a promising strategy for extracting uranium from seawater. However, seawater contains ~2.75 mM carbonates, which would seriously inhibit the photocatalytic reduction of U(VI), restricting the practical application of this method. This study is devoted to realizing the photo-reduction of U(VI) in carbonates-containing system by developing a highly efficient photocatalyst. Carboxylated carbon nitride (CCN) catalysts with different amounts of carboxyl groups were prepared (CCN-5 and CCN-24) by oxidating bulk carbon nitride (BCN). Compared to BCN, the adsorption ability of CCN-24 for U(VI) increased by 30% and 9% in the presence of 2.0 mM and 10.0 mM NaHCO3, respectively. Moreover, the modification of electron-withdrawing carboxyl groups reduced the conduction band position and promoted the separation efficiency of electrons and holes, facilitating the photocatalytic performance for the reduction of U(VI). Under the illumination of visible-light, CCN exhibited high reactivity for the photocatalytic U(VI) reduction in carbonates-containing system. In the presence of 10.0 mM HCO3−, CCN-24 showed a U(VI) reduction rate of 0.0867 min−1, being ~33 times of that for BCN (0.0026 min−1). Electron spin resonance (ESR) and radical trapping investigation proved that •O2− was the main active reduction species for U(VI), and more •O2− and fewer oxidizing •OH radicals were generated on CCN than BCN. It was also verified that CCN could remain stable and keep high photocatalytic reactivity after at least 5 recycles. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) confirmed that U(VI) was finally reduced to U(IV) and UO2+x was finally formed on the photocatalyst surface. This work opens up the possibility of extracting uranium from carbonates-containing solutions via photocatalysis method, expending the application of this method in natural systems.