Enhanced uranium self-photocatalytic extraction via the adsorption effect of acid-modified SiO2†

Uranium photocatalytic extraction has been regarded as a promising strategy for nuclear wastewater management. Many studies show that the enhanced adsorption capacity of the photocatalyst can improve the uranium extraction capacity. However, the mechanism of the synergistic effect between adsorption and photocatalysis is still unclear. Herein, we proposed a novel adsorption trigger for the uranium self-photocatalytic extraction strategy that can enhance the transition from soluble uranyl ions to the insoluble metastudtite. Silica (SiO₂) containing different amounts of surface silicon hydroxyl groups were utilized as both adsorbent and photocatalyst, and they were modified with H₂SO₄, HCl and HNO₃, respectively. The SiO₂ materials possessed varying specific surface areas, ordered as SiO₂(H₂SO₄) (350.12 m² g⁻¹) > SiO₂(HNO₃) (286.80 m² g⁻¹) > SiO₂(HCl) (264.85 m² g⁻¹) > SiO₂ (9.70 m² g⁻¹). The adsorption capacity of U(VI) on SiO₂(H₂SO₄) was the highest as 137.95 mg g⁻¹, which was seven times higher than that of the unmodified SiO₂. The improvement in adsorption performance with acid-modified SiO₂ enhanced the aggregation of uranyl ions, thereby accelerating the self-photocatalytic extraction of uranyl ions. Additionally, the modification of SiO₂ with H₃PO₄ introduced phosphate groups that significantly diminished the photocatalytic activity of uranyl ions, sequentially halting the self-photocatalytic degradation of uranium. Furthermore, SiO₂(H₂SO₄) showed good reusability and stability after being recycled and reused five times as well as good selectivity for U(VI). This work delivers further understanding of the effect of catalyst adsorption on photocatalytic reactions, serving as a guide for the development and synthesis of advanced photocatalysts in future studies.