Effect of disinfection on the photoreactivity of effluent organic matter and photodegradation of organic contaminants.

Chlorine, UV254, and ozone are three typical processes commonly used for wastewater disinfection, which could change the photoreactivity of dissolved organic matter (DOM) in effluents of wastewater treatment plants (WWTPs). The photoinduced reactive species (RS) from DOM, primarily including the excited triplet state of DOM (3DOM*), singlet oxygen (1O2), and hydroxyl radical (•OH), play important roles in the attenuation of contaminants. However, the effect of disinfection processes on the photosensitized degradation of contaminants is poorly understood. This paper presents the first evidence that 3DOM*, 1O2, and •OH interaction with three typical contaminants (diphenhydramine, cimetidine, and N,N-diethyl-m-toluamide (DEET)) was largely impacted by DOM after disinfection. The results of electron spin resonance (ESR) spectrometry and laser flash photolysis (LFP) experiments demonstrated that the chlorination increased the formation rate of 3DOM* and 1O2, while UV254 irradiation and ozonation decreased the formation rate of these RS. All these three disinfection processes promoted the photoproduction of •OH and increased the photodegradation rate constants (kobs) of DEET by 26-361%. The kobs of diphenhydramine, cimetidine, and DEET correlated positively with the formation rate of 3DOM*, 1O2, and •OH, respectively. The bimolecular reaction rate constant of 3DOM* with diphenhydramine increased by ∼41% after chlorination. These findings suggest that disinfection processes altered the photogeneration of RS from DOM, which significantly impacts the fate of trace pollutants in aquatic environments.

Effect of permanganate oxidation on the photoreactivity of dissolved organic matter for photodegradation of typical pharmaceuticals.

Permanganate has been widely used in the remediation of contaminated water due to its relatively strong oxidation properties and ease of use. The ubiquitous dissolved organic matter (DOM) in natural waters causes a significant sink of permanganate in treatments, which further impacts the photoformation of reactive species and the removal of trace pollutants by DOM. Significantly, the effect of permanganate oxidation on the photoreactivity of DOM remains unknown. The present paper investigated for the first time the photophysical and photochemical properties variation of DOM from different sources after permanganate oxidation. Results showed that the permanganate oxidation caused a decrease in UV absorbance, fluorescence intensity, aromaticity, and molecular weight for all tested DOM samples, as well as photoformation rate of DOM triplet states (3DOM⁎), singlet oxygen (1O2), and hydroxyl radical (OH) under simulated sunlight. Quantum yield of 1O2 showed positively linear correlations with both triplet quantum yield coefficient (fTMP) and E2/E3 (ratio of absorbance at 254 and 365 nm) for all the DOM samples before and after permanganate oxidation. The quantum yield of OH exhibited no significant correlation with fTMP or E2/E3. Permanganate oxidation inhibited the DOM-photosensitized indirect photodegradation of pollutants that do not absorb sunlight (e.g., decreased by 15-29%). For the tested pollutants that undergo direct photolysis under sunlight, their photodegradation was promoted (e.g., increased by 1-19%) in the permanganate oxidized DOM solutions due to the decrease of light-screening effect by DOM. These findings suggest that permanganate oxidation affects the photoreactivity of DOM and the corresponding photochemical fate of organic pollutants in natural waters.

Mechanistic Investigation of Enhanced Photoreactivity of Dissolved Organic Matter after Chlorination.

Chlorine is commonly used in disinfection processes in wastewater treatment plants prior to discharge of the effluents into receiving waters. Effluent organic matter and humic substances constitute up to 90% of dissolved organic matter (DOM) in receiving water, which induces photogeneration of reactive species (RS) such as excited triplet state of DOM (3DOM*), singlet oxygen (1O2), and hydroxyl radical (•OH). The RS plays an important role in the attenuation of trace pollutants. However, the effect of chlorine disinfection on the photoreactivity of the DOM has remained unclear. Here, we investigated the physicochemical properties and subsequent RS variation after chlorination of DOM. Solid-state 13C cross-polarization/magic angle-spinning NMR and Fourier transform ion cyclotron resonance mass spectrometry verified that the aromaticity, electron-donating capacity (EDC), and average molecular weight of DOM decreased markedly after chlorination. It was found for the first time that the photoproduction of 3DOM*, 1O2, and •OH increased markedly after chlorination of DOM upon irradiation of simulated sunlight. The quantum yields of 3DOM*, 1O2, and •OH were positively correlated with E2/E3 (ratio of the absorbance at 254 to 365 nm) while negatively correlated with EDC before and after chlorination. These findings highlight the synergetic effect of chlorine disinfection on the photosensitization of DOM under irradiation of sunlight, which will promote the removal of trace pollutants in surface waters.

Photogeneration of Reactive Species from Biochar-Derived Dissolved Black Carbon for the Degradation of Amine and Phenolic Pollutants.

Due to agricultural waste combustion and large-scale biochar application, biochar-derived dissolved black carbon (DBC) is largely released into surface waters. The photogeneration of reactive species (RS) from DBC plays an important role in organic pollutant degradation. However, the mechanistic interactions between RS and pollutants are poorly understood. Here, we investigated the formation of DBC triplet states (3DBC*), singlet oxygen (1O2), and hydroxyl radical (•OH) in straw biochar-derived DBC solutions and photodegradation of typical pharmaceuticals and personal care products (PPCPs). Laser flash photolysis and electron spin resonance spectrometry showed that DBC exhibited higher RS quantum yields than some well-studied dissolved organic matter. The RS caused rapid degradation of atenolol, diphenhydramine, and propylparaben, selected as target PPCPs in this study. The 3DBC* contributed primarily to the oxidation of selected PPCPs via one-electron-transfer interaction, with average reaction rate constants of 1.15 × 109, 1.41 × 109, and 0.51 × 109 M-1 s-1, respectively. •OH also participated in the degradation and accounted for approximately 2.7, 2.5, and 18.0% of the total removal of atenolol, diphenhydramine, and propylparaben, respectively. Moreover, the photodegradation products were identified using high-resolution mass spectrometry, which further confirmed the electron transfer and •OH oxidation mechanisms. These findings suggest that DBC from the combustion process of agricultural biomass can efficiently induce the photodegradation of organic pollutants under sunlight in aquatic environments.

Grape Seed Extract Assisted Synthesis of Dual-Functional Anatase TiO2 Decorated Reduced Graphene Oxide Composite for Supercapacitor Electrode Material and Visible Light Photocatalytic Degradation of Bromophenol Blue Dye.

The grape extract is a potential natural reducing agent because of its high phenolic content. The extracts of seeds, skin, and pulp of grape were prepared by digestion, grinding, and soxhlet methods and used for reducing graphene oxide (GO). The reduced GO made using the soxhlet extract of grape seed (GRGO) was hydrothermally treated with titanium dioxide (TiO2) for the synthesis of GRGO-TiO2 nanocomposite. The X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), UV-vis, photoluminescence, and Raman spectra studies further confirmed the formation of GRGO and the GRGO-TiO2 hybrid. Scanning electron microscope and transmission electron microscope studies showed the decoration of spherical TiO2 particles (<100 nm) on the few-layered GRGO sheets. The GRGO-TiO2 hybrid was explored as a working electrode for supercapacitors and visible light photocatalyst for water decontamination. GRGO-TiO2 showed higher specific capacitance (175 F g-1) than GRGO (150 F g-1) and TiO2 (125 F g-1) in an aqueous electrolyte. GRGO-TiO2 exhibited 83.6% capacitance retention even after 2000 cycles, indicating the good stability of the material. Further, under visible light irradiation (λ > 400 nm), GRGO-TiO2 showed ∼30% higher photo-oxidation of the bromophenol blue (BPB) dye than TiO2. Also, GRGO-TiO2 decreased the total organic carbon content of BPB from 92 to 18 ppm. Overall, the soxhlet extract of grape seed was found to be a cost-effective reducing agent for the preparation of GRGO, which is a suitable material to be used in supercapacitors and photocatalysis.

Synergetic effect of Sn doped ZnO nanoparticles synthesized via ultrasonication technique and its photocatalytic and antibacterial activity.

The current work reports the photocatalytic and antibacterial performance of tin (Sn) doped zinc oxide (ZnO) nanoparticles synthesized via ultrasonic aided co-precipitation technique. The increase of Sn concentration decreased the lattice parameter and increased the crystallite size without changing the ZnO structure. The hexagonal shaped particles and sheets obtained for 3% and 5% Sn substituted ZnO, respectively. The increase of dopant concentration reduced the reflectance and optical band gap energy of Sn doped ZnO. The vibrational band present at 1443 cm-1 confirmed the successful bond formation of Sn-O-Zn. The 5% Sn doped ZnO nanoparticles exhibited greater dye elimination rate of methylene blue compared to 3% Sn. The antibacterial activity of Sn doped ZnO showed the higher zone of inhibition about 14 mm against different pathogens. The 5% Sn doped ZnO photocatalyst improve the transfer rate of photo excite carrier and decrease the rate of recombination which greatly influence on the photocatalytic and antibacterial performance.

Photocatalytic activity of SnO2/Fe3O4 nanocomposites and the toxicity assessment of Vigna radiata, Artemia salina and Danio rerio in the photodegraded solution.

The study was undertaken to design SnO2/Fe3O4 nanocomposite by sonochemical method and to assess the photodegradation of organic dye. Textural, composition and structural features of the bare SnO2 and SnO2/Fe3O4 samples were characterized using scanning electron microscope (SEM), energy dispersive X-ray (EDX), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The X-ray diffraction of as-synthesized SnO2/Fe3O4 nanocomposites confirms the presence of tetragonal and cubic structure. The results disclose that the incorporation of Fe3O4 in SnO2 decrease the crystallite size and increase the surface area compared with bare SnO2 nanoparticle. The as-prepared photocatalyst shows higher efficiency than the bare SnO2 under sunlight irradiation. Vigna radiata seeds (VR), Artemia salina (AS) and Zebra fish (Danio rerio (DR) were used to check the toxicity level of the treated and untreated Rhodamine B (RhB) dye solution. These models displayed good consistency for examining the harmfulness of the solutions. The results suggests SnO2/Fe3O4 nanocomposite exhibited a good efficacy in the dye wastewater treatment. Further, the degradation efficiency was confirmed by the toxicity examination.

Effect of UV254 disinfection on the photoformation of reactive species from effluent organic matter of wastewater treatment plant.

UV254 is one of the main disinfection methods used in wastewater treatment plants (WWTPs) for the inactivation of pathogens in the effluents before being discharged into the receiving waters. The effluent organic matters (EfOM) are well-known photosensitizers for the generation of reactive species, mainly including the triplet states of EfOM (3EfOM*), singlet oxygen (1O2) and hydroxyl radical (•OH), which contribute to the removal of trace pollutants in water. However, the effect of UV254 disinfection on the photoreactivity of EfOM remains unclear. Here we investigated the photophysical and photochemical properties variation of EfOM after UV254 disinfection, along with humic substances (HS) as comparison. The UV254 disinfection caused a decrease of aromaticity, fluorescence intensity and molecular weight for all samples, while a reduction formation of triplet state of these dissolved organic matters (3DOM*), 1O2, hydrogen peroxide (H2O2), and superoxide anions (O2•-) under simulated sunlight was observed. In contrast, the generation of •OH was increased after UV254 disinfection. The quantum yield of 1O2 was positively correlated with triplet quantum yield coefficient (fTMP) in all cases. However, the quantum yield of •OH exhibited positive and negative correlations with fTMP for EfOM and HS, respectively. The quantum yields showed positive correlations with E2/E3 (ratio of the absorbance at 254 to 365 nm) for untreated DOM samples, while for the first time we found the trends differ distinctly after UV254 disinfection. These findings indicate that UV254 disinfection in WWTPs significantly increases the potential of •OH photoproduction from effluents and the cost-effective solar irradiation after UV254 disinfection is expected to be a novel technique for further removal of pathogen and trace organic pollutants in wastewater effluents and receiving waters.