Near-Ultraviolet Light-Driven Photocatalytic Chlorine Activation Process with Novel Chlorine Activation Mechanisms

Abstract

In this study, the near-ultraviolet (near-UV) light-driven TiO2 photocatalytic chlorine activation process, termed as the UV365/TiO2/chlorine process, degrades a representative recalcitrant micropollutant, carbamazepine (CBZ), at an apparent first-order rate constant (k CBZ ′) that is 34.2 and 3.9 times higher than those without TiO2 and chlorine, respectively. In this process, chlorine serves a more important role as a catalyst to enhance the yield of hydroxyl radicals (HO·) without being consumed, in addition to its role as a radical precursor to produce HO· and reactive chlorine species. k CBZ ′ increased with increasing TiO2 dosages from 1.0 to 20.0 mg/L and light intensities from 0.1 to 0.33 mW/cm2 and with decreasing chlorine dosages from 5.0 to 1.0 mgCl2/L. Increasing pH increased the overall radical concentrations but transformed HO· and Cl· to less reactive ClO·, leading to a decreasing k CBZ ′ from pH 6.0 to 9.0. The dual roles of chlorine in the process enabled the rapid CBZ degradation at a chlorine dosage about 1/20 to that of the conventional UVC/chlorine process, at a TiO2 dosage about 1/200 to that of the UVC/TiO2 process and at an electrical energy per order of CBZ degradation at least 2-order of magnitude lower than those of the existing UVC-based processes.

Roles of Activated Carbon in UV/Chlorine/Activated Carbon–TiO2 Process for Micropollutant Abatement and DBP Control

“…The inhibition in k′ MTZ can be ascribed to the scavenging of HO • and chlorine by NOM. 9,18 The negative impact of NOM was reduced for UV/chlorine/AC-TiO 2 , which can be ascribed to the promoted concentrations of HO • and h + in UV/chlorine/AC-TiO 2 .…”

“…9,14−16 Moreover, O 2 •− generated in photocatalytic processes can react with HOCl to produce HO • at 7.5 ± 0.38 × 10 6 M −1 s −1 (eq 7). 17 Further, HOCl/OCl − can facilitate the separation of h + -e − pairs via eqs 3−6, 9,10,18 which in turn enhances the photocatalytic efficiency. Regarding DBP control, the faster decay of chlorine in UV/chlorine/TiO 2 results in a considerably lower chlorine exposure than that in UV/chlorine, which causes reduced DBP formation.…”

“…The combination of UV/chlorine and TiO 2 photocatalysis has proved to be effective for enhancing micropollutant abatement and for reducing DBP formation. , The underlying mechanism is shown in eqs –. TiO 2 , which has a valence band (VB) position of 2.7 V and conduction band (CB) position of −0.4 V, is capable of oxidizing H 2 O to HO • and reducing dissolved oxygen (DO) to superoxide radicals (O 2 •– ). − Hole–electron (h + -e – ) pairs generated during photocatalytic processes can activate HOCl/OCl – (p K a = 7.5) to enhance the production of reactive radicals, e.g., HO • and RCS, as shown in eqs –. ,− Moreover, O 2 •– generated in photocatalytic processes can react with HOCl to produce HO • at 7.5 ± 0.38 × 10 6 M –1 s –1 (eq ). Further, HOCl/OCl – can facilitate the separation of h + -e – pairs via eqs –, ,, which in turn enhances the photocatalytic efficiency.…”

“…TiO 2 , which has a valence band (VB) position of 2.7 V and conduction band (CB) position of −0.4 V, is capable of oxidizing H 2 O to HO • and reducing dissolved oxygen (DO) to superoxide radicals (O 2 •– ). − Hole–electron (h + -e – ) pairs generated during photocatalytic processes can activate HOCl/OCl – (p K a = 7.5) to enhance the production of reactive radicals, e.g., HO • and RCS, as shown in eqs –. ,− Moreover, O 2 •– generated in photocatalytic processes can react with HOCl to produce HO • at 7.5 ± 0.38 × 10 6 M –1 s –1 (eq ). Further, HOCl/OCl – can facilitate the separation of h + -e – pairs via eqs –, ,, which in turn enhances the photocatalytic efficiency. Regarding DBP control, the faster decay of chlorine in UV/chlorine/TiO 2 results in a considerably lower chlorine exposure than that in UV/chlorine, which causes reduced DBP formation …”

Illuminating Human Norovirus: A Perspective on Disinfection of Water and Surfaces Using UVC, Norovirus Model Organisms, and Radiation Safety Considerations

“…Additionally, water supplies can have varying concentrations of chlorine or other disinfectant residuals. Some of these disinfectants are typically activated by UVC for various reasons but often through the formation of chlorine or oxygen radicals [123]. Therefore, it is critical to know the concentration of disinfectants in a test and recommended to design the study around them.…”