Dual-S-scheme 3D Ag2CO3/BiOI/BiOCl microsphere heterojunction for improving photocatalytic mercury removal

A novel 3D microspherical Ag2CO3/BiOI/BiOCl heterojunction photocatalyst was successfully prepared via a one-pot coprecipitation method. The pore characteristics, morphologies and optical properties of the composite photocatalysts were characterized in detail by means of XRD, FTIR, SEM, TEM, UV–Vis DRS, N2 adsorption-desorption, XPS, ESR and PL. The effects of the CO32−/Bi3+ molar ratio, pH value, fluorescent lamp irradiation, SO2, NO and scavengers on the wet removal of gaseous Hg0 by the composites were studied through a series of experiments. The results showed that more than 98% of Hg0 from simulated flue gas was removed by the optimized 3% Ag2CO3–BiOI/BiOCl system within 60 min. The photocatalyst had good acid, alkali and NO resistance, but SO2 significantly inhibited it. The specific surface area of the Ag2CO3/BiOI/BiOCl photocatalyst was much higher than that of a single component, which was 6.80 and 1.56 times that of Ag2CO3 and BiOI/BiOCl, respectively. For 3% Ag2CO3–BiOI/BiOCl, the Hg0 removal efficiency remained at approximately 94% in the sixth cycle. Density functional theory (DFT) revealed that the formation of heterojunctions could enhance the electron transition and efficient separation of photoinduced electron-hole pairs. Furthermore, the free radical capture experiment showed that anionic superoxide radical (•O2−) was the main active substance for the removal of Hg0. A reasonable dual-S-scheme charge transfer mechanism for an enhanced photocatalytic removal of Hg0 is proposed. [Display omitted] •The optimal photocatalytic Hg0 removal efficiency can reach up to 98%.•Ag2CO3/BiOI/BiOCl microsphere maintained superior activity and stability.•A dual S-scheme heterojunction among Ag2CO3, BiOI and BiOCl was formed.•The enhanced activity was attributed to an effective separation of carriers.