In this thesis, mechanisms of the photocatalytic chlorine activation are investigated by using bismuth vanadate (BiVO₄) as the model photocatalyst to produce photogenerated species, including holes (h_VB⁺), electrons (e_CB⁻), and superoxide radicals (O₂^•−), under the visible light irradiation. This strategy is termed as the vis/BiVO₄/chlorine process. Carbamazepine (CBZ), one of the most frequently detected pharmaceuticals in the aquatic environment, can only be degraded by HO• and ClO• produced from the chlorine activation by the photogenerated h_VB⁺, e_CB⁻, and O₂^•− in the vis/BiVO₄/chlorine process, while it cannot be degraded by the vis/BiVO₄ (without chlorine) or vis/chlorine process (without BiVO₄). The reactions between h_VB⁺ and chlorine depend on the valance band potentials of the p...[ Read more ]

In this thesis, mechanisms of the photocatalytic chlorine activation are investigated by using bismuth vanadate (BiVO₄) as the model photocatalyst to produce photogenerated species, including holes (h_VB⁺), electrons (e_CB⁻), and superoxide radicals (O₂^•−), under the visible light irradiation. This strategy is termed as the vis/BiVO₄/chlorine process. Carbamazepine (CBZ), one of the most frequently detected pharmaceuticals in the aquatic environment, can only be degraded by HO• and ClO• produced from the chlorine activation by the photogenerated h_VB⁺, e_CB⁻, and O₂^•− in the vis/BiVO₄/chlorine process, while it cannot be degraded by the vis/BiVO₄ (without chlorine) or vis/chlorine process (without BiVO₄). The reactions between h_VB⁺ and chlorine depend on the valance band potentials of the photocatalysts and the chlorine species. At pH 5.0, the valence band potential of BiVO₄ at +2.25 eV is more positive than the E°(Cl⁺/HOCl). Thus, h_VB⁺ can oxidize HOCl to produce HO• and Cl⁺, which reacts rapidly with H₂O to regenerate chlorine. At pH 7.0 and pH 9.0, with increasing E°(Cl⁺/HOCl), the valence band potential of BiVO₄ is more negative than the E°(Cl⁺/HOCl) while it is still more positive than the E°(ClO•/ClO⁻). Thus, h_VB⁺ can only oxidize HOCl/ClO⁻ to produce ClO•. The reactions between chlorine and e_CB⁻ are chlorine species-dependent. HOCl can be activated by e_CB⁻ to generate HO• via the direct one-electron transfer pathway or the indirect one-electron transfer pathway using O₂ as an electron shuttle to form O₂^•−and transfer the electron to HOCl. However, ClO⁻ cannot be activated via the one-electron transfer pathways to produce radicals, and it only undergoes the two-electron transfer pathways by reacting with e_CB⁻/O₂^•− to produce Cl⁻ and HO⁻. The contributions of photogenerated h_VB⁺ and e_CB⁻/O₂^•− to the CBZ degradation in the vis/BiVO₄/chlorine process were further calculated to be 66% and 34% at pH 7.0, respectively. While h_VB⁺ can always activate chlorine to produce radicals, e_CB⁻ and O₂^•− may be wasted when they undergo the two-electron transfer pathways to reduce chlorine to Cl⁻ and HO⁻. The chlorine activation efficiencies by e_CB⁻ via the one-electron transfer pathways, defined as the ratios of the amount of e_CB⁻ consumed in the one-electron transfer pathways to that generated in the vis/BiVO₄/chlorine process, were further quantified in a photo-assisted electrochemical reactor. Under the low DO concentration, the chlorine activation efficiencies by e_CB⁻ at pH 5.0 and pH 7.0 were 0.2998 and 0.1614, respectively, and the chlorine activation via the direct one-electron transfer pathway achieved higher efficiency and played a more important role. While under the normal DO concentration, the chlorine activation efficiencies by e_CB⁻ at pH 5.0 and pH 7.0 were 0.1333 and 0.1070, respectively, and the chlorine activation via the indirect one-electron transfer pathway became more important. Nonetheless, the ratios of the amount of CBZ degraded (Q_CBZ) over the amount of e_CB⁻ generated (Q_e) were comparable under both the low and normal DO concentrations at pH 7.0, because the stronger HO• scavenging effects under the low DO concentration offset the higher HO• yield in the direct one-electron transfer pathway. The findings of this study identify the roles of photogenerated species and quantify the chlorine activation efficiencies by e_CB⁻ in the photocatalytic chlorine activation process and thus bring insights to guide the design of photocatalysts for the chlorine activation with higher radical yields and light utilization efficiency.