Sequential Process Combination of Photocatalytic Oxidation and Dark Reduction for the Removal of Organic Pollutants and Cr(VI) using Ag/TiO₂

Abstract

We investigated a sequential photocatalysis-dark reaction, wherein organic pollutants were degraded on Ag/TiO under UV irradiation and the dark reduction of hexavalent chromium (Cr(VI)) was subsequently followed. The photocatalytic oxidation of 4-chlorophenol (4-CP), a test organic substrate, induced the generation of degradation intermediates and the storage of electrons in Ag/TiO which were then utilized for reducing Cr(VI) in the postirradiation period. The dark reduction efficiency of Cr(VI) was much higher with Ag/TiO (87%), compared with bare TiO (27%) and Pt/TiO (22%). The Cr(VI) removal by Ag/TiO (87%) was contributed by adsorption (31%), chemical reduction by intermediates of 4-CP degradation (26%), and reduction by electrons stored in Ag (30%). When formic acid, humic acid or ethanol was used as an alternative organic substrate, the electron storage effect was also observed. The postirradiation removal of Cr(VI) on Ag/TiO continued for hours, which is consistent with the observation that a residual potential persisted on the Ag/TiO electrode in the dark whereas little residual potential was observed on bare TiO and Pt/TiO electrodes. The stored electrons in Ag/TiO and their transfer to Cr(VI) were also indicated by the UV-visible absorption spectral change. Moreover, the electrons stored in the preirradiated Ag/TiO reacted with O with showing a sign of low-level OH radical generation in the dark period.

Materials and Mechanisms of Photo‐Assisted Chemical Reactions under Light and Dark Conditions: Can Day–Night Photocatalysis Be Achieved?

“…[68] This study essentially showedt hat stored electrons were capable of reducingh eavymetal ions in the dark as well. [70] It was reportedt hat the photocatalytic oxidationo f4 -CP produced intermediate compounds and stored electrons in the Ag/TiO 2 system forf urtheru tilizationt or educe Cr VI in the dark. In practice, the presence of O 2 inhibits the storageo fe lectrons by scavenging them.…”

“…In as ingle reaction, organic pollutants, such as 4-CP,f ormic acid, humic acid, or ethanol,w ere degraded with Ag/TiO 2 under UV light, followed by the dark reduction of hexavalent Cr VI . [70] It was reportedt hat the photocatalytic oxidationo f4 -CP produced intermediate compounds and stored electrons in the Ag/TiO 2 system forf urtheru tilizationt or educe Cr VI in the dark. Accordingly,t he dark reduction of Cr VI was much higherf or Ag/TiO 2 (87 %) and continued for few hours.T he observed dark-reduction efficiency of the Ag/TiO 2 system was proposed to be the collective effects of 1) adsorption,2 )chemical reduction by intermediates of 4-CP degradation,a nd 3) reduction by electrons stored in Ag (Figure 13).…”

Ag Nanoparticles/AgX (X=Cl, Br and I) Composites with Enhanced Photocatalytic Activity and Low Toxicological Effects

“…Recent reports have shown that highly efficient plasmonic photocatalysts such as Ag/g‐C 3 N 4 , Ag/TiO 2 , Ag/AgVO 3 , Ag/Ag 3 PO 4 , Ag/Ag 2 MoO 4 , and Ag/Ag 2 WO 4 can be prepared and their results indicate that Ag decoration can dramatically enhance the photocatalytic performance of the pure photocatalyst via the SPR effect. Recently, considerable efforts have been placed on the design and fabrication of heterojunctions to improve the applications of AgX‐based materials .…”

Efficient adsorption/reduction of aqueous hexavalent chromium using oligoaniline hollow microspheres fabricated by a template‐free method

“…Besides metal cations, the real wastewater contains several kinds of anions and organic matter. The coexistence of high‐affinity ligands such as PO 4 3− and organic pollutants can hinder the removal efficiency of Cr(VI) and affect regeneration of the adsorbents , …”

“…The coexistence of high-affinity ligands such as PO 4 3− and organic pollutants can hinder the removal efficiency of Cr(VI) and affect regeneration of the adsorbents. 57,58…”