Visible-Light Activation of a Dissolved Organic Matter–TiO₂ Complex Mediated via Ligand-to-Metal Charge Transfer

Abstract

Given the widespread use of TiO2, its release into aquatic systems and complexation with dissolved organic matter (DOM) are highly possible, making it important to understand how such interactions affect photocatalytic activity under visible light. Here, we show that humic acid/TiO2 complexes (HA/TiO2) exhibit photoactivity (without significant electron–hole activation) under visible light through ligand-to-metal charge transfer (LMCT). The observed visible-light activities for pollutant removal and bacterial inactivation are primarily linked to the generation of H2O2 via the conduction band. By systematically considering molecular-scale interactions between TiO2 and organic functional groups in HA, we find a key role of phenolic groups in visible-light absorption and H2O2 photogeneration. The photochemical formation of H2O2 in river waters spiked with TiO2 is notably elevated above naturally occurring H2O2 generated from background organic constituents due to LMCT contribution. Our findings suggest that H2O2 generation by HA/TiO2 is related to the quantity and functional group chemistry of DOM, which provides chemical insights into photocatalytic activity and potential ecotoxicity of TiO2 in environmental and engineered systems.

Multiple roles of humic acid in the photolysis of sulfamethoxazole: kinetics and mechanism

“…42 Besides, HA could induce ROS under UV irradiation, promoting the direct photolysis of organic contaminants. 43 In this study, HA exhibited complex influences on SMX photolysis: promotion with low-concentration and inhibition with high-concentration, which might be attributed to the effects of HA on two types of photolysis. To prove these, we designed batch experiments to investigate the roles of HA in direct and indirect photolysis of SMX.…”

Multiple roles of humic acid in the photolysis of sulfamethoxazole: kinetics and mechanism

“…42 Besides, HA could induce ROS under UV irradiation, promoting the direct photolysis of organic contaminants. 43 In this study, HA exhibited complex influences on SMX photolysis: promotion with low-concentration and inhibition with high-concentration, which might be attributed to the effects of HA on two types of photolysis. To prove these, we designed batch experiments to investigate the roles of HA in direct and indirect photolysis of SMX.…”

Photoswitch Mediated Electron Highway Driving Direct Pollutant‐to‐Oxidant Electron Transfer in Ultrafast Fenton‐Like Reactions

“…In a parallel process, water was oxidized by the holes in the VB to produce • OH, which also interacted with the oxidant to generate secondary active species. The final mechanism was photosensitized activation, including dye sensitization and ligand-to-metal charge transfer (LMCT) sensitization (Figure 1h ) [ 23,24 ]. In dye sensitization, electrons in the dye molecule were excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), subsequently transferring to the CB, facilitating the activation of the oxidant.…”

Overlooked Complexation and Competition Effects of Phenolic Contaminants in a Mn(II)/Nitrilotriacetic Acid/Peroxymonosulfate System: Inhibited Generation of Primary and Secondary High-Valent Manganese Species

“…We subsequently monitored the ultraviolet–visible (UV–vis) spectrum of each system, as the formation of a strong chemical bond is usually accompanied by the emergence of a new absorption band or the shift of the peak position. ,, A new broad peak emerged at around 270 nm in the Mn(II)/NTA/PMS system, indicating the generation of Mn(III)NTA . However, no significant change in peak intensity or position was observed in the spectra of Mn(II)/NTA/4-OCH 3 and Mn(II)/NTA/PMS/4-OCH 3 systems (compared to Mn(II)/NTA and 4-OCH 3 alone) (Figure S14).…”

Multiple roles of humic acid in the photolysis of sulfamethoxazole: kinetics and mechanism

“…42 Besides, HA could induce ROS under UV irradiation, promoting the direct photolysis of organic contaminants. 43 In this study, HA exhibited complex influences on SMX photolysis: promotion with low-concentration and inhibition with high-concentration, which might be attributed to the effects of HA on two types of photolysis. To prove these, we designed batch experiments to investigate the roles of HA in direct and indirect photolysis of SMX.…”

Photoswitch Mediated Electron Highway Driving Direct Pollutant‐to‐Oxidant Electron Transfer in Ultrafast Fenton‐Like Reactions

“…In a parallel process, water was oxidized by the holes in the VB to produce • OH, which also interacted with the oxidant to generate secondary active species. The final mechanism was photosensitized activation, including dye sensitization and ligand-to-metal charge transfer (LMCT) sensitization (Figure 1h ) [ 23,24 ]. In dye sensitization, electrons in the dye molecule were excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), subsequently transferring to the CB, facilitating the activation of the oxidant.…”

Overlooked Complexation and Competition Effects of Phenolic Contaminants in a Mn(II)/Nitrilotriacetic Acid/Peroxymonosulfate System: Inhibited Generation of Primary and Secondary High-Valent Manganese Species

“…We subsequently monitored the ultraviolet–visible (UV–vis) spectrum of each system, as the formation of a strong chemical bond is usually accompanied by the emergence of a new absorption band or the shift of the peak position. ,, A new broad peak emerged at around 270 nm in the Mn(II)/NTA/PMS system, indicating the generation of Mn(III)NTA . However, no significant change in peak intensity or position was observed in the spectra of Mn(II)/NTA/4-OCH 3 and Mn(II)/NTA/PMS/4-OCH 3 systems (compared to Mn(II)/NTA and 4-OCH 3 alone) (Figure S14).…”

Multiple roles of humic acid in the photolysis of sulfamethoxazole: kinetics and mechanism

“…42 Besides, HA could induce ROS under UV irradiation, promoting the direct photolysis of organic contaminants. 43 In this study, HA exhibited complex influences on SMX photolysis: promotion with low-concentration and inhibition with high-concentration, which might be attributed to the effects of HA on two types of photolysis. To prove these, we designed batch experiments to investigate the roles of HA in direct and indirect photolysis of SMX.…”

Photoswitch Mediated Electron Highway Driving Direct Pollutant‐to‐Oxidant Electron Transfer in Ultrafast Fenton‐Like Reactions

“…In a parallel process, water was oxidized by the holes in the VB to produce • OH, which also interacted with the oxidant to generate secondary active species. The final mechanism was photosensitized activation, including dye sensitization and ligand-to-metal charge transfer (LMCT) sensitization (Figure 1h ) [ 23,24 ]. In dye sensitization, electrons in the dye molecule were excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), subsequently transferring to the CB, facilitating the activation of the oxidant.…”

Overlooked Complexation and Competition Effects of Phenolic Contaminants in a Mn(II)/Nitrilotriacetic Acid/Peroxymonosulfate System: Inhibited Generation of Primary and Secondary High-Valent Manganese Species

“…We subsequently monitored the ultraviolet–visible (UV–vis) spectrum of each system, as the formation of a strong chemical bond is usually accompanied by the emergence of a new absorption band or the shift of the peak position. ,, A new broad peak emerged at around 270 nm in the Mn(II)/NTA/PMS system, indicating the generation of Mn(III)NTA . However, no significant change in peak intensity or position was observed in the spectra of Mn(II)/NTA/4-OCH 3 and Mn(II)/NTA/PMS/4-OCH 3 systems (compared to Mn(II)/NTA and 4-OCH 3 alone) (Figure S14).…”