Fabrication of recoverable Bi2O2S/Bi5O7I/ZA hydrogel beads for enhanced photocatalytic Hg0 removal in the presence of H2O2

Abstract

The effective removal of elemental mercury (Hg⁰) is a global challenge due to its toxicity and bioaccumulation threat to public health and ecosystems. Photocatalytic technology by visible light-driven photocatalysts is promising for Hg⁰ removal. However, effective separation of photocatalyst powders from reaction solution limits its widespread use. To solve the problem, we report for the first time the successful fabrication of recoverable Bi₂O₂S/Bi₅O₇I/ZA hydrogel beads for enhanced photocatalytic Hg⁰ removal in the presence of H₂O₂. Characterization techniques such as XRD, TEM, EDS, XPS, UV–vis DRS, PL, etc. are employed to understand the physicochemical properties and photoelectric performance of the photocatalysts. The serial BOSI photocatalysts all outperform the single component, which is attributed to the formation of a heterojunction between Bi₅O₇I and Bi₂O₂S. The coupling of 2-BOSI-ZA beads with H₂O₂ shows favorable synergistic effect, with Hg⁰ removal efficiency in the following order: H₂O₂ + 2-BOSI-ZA < 2-BOSI-ZA + FSL < H₂O₂ + 2-BOSI-ZA + FSL. TPC, EIS, and PL tests confirm that the introduction of Bi₂O₂S effectively suppresses charge carrier recombination. ESR and free radicals capture experiments demonstrate that the main species responsible for removal of Hg⁰ are ^•O₂^– and ^•OH. Density functional theory calculations exhibit that the internal electric field (IEF) between Bi₅O₇I and Bi₂O₂S contributes to the spatial charge separation of the heterojunction. The IEF leads to an S-scheme carrier transfer mechanism at the Bi₂O₂S/Bi₅O₇I interface that benefits the carrier separation on Bi₅O₇I, resulting in an enhanced photocatalytic performance. This work can provide further inspiration for designing hydrogel photocatalysts with an excellent activity in conjunction with oxidants in the field of mercury pollution control.