Photocatalytic degradation of organic micropollutants: Inhibition mechanisms by different fractions of natural organic matter

Photo-transformation of isoproturon under UV-A irradiation: The synergy of nitrite and natural organic matter

Isoproturon (IPU), a widely utilized phenylurea herbicide, is recognized as an emerging contaminant. Previous studies have predominantly attributed the degradation of IPU in natural waters to indirect photolysis by natural organic matter (NOM). Here, we demonstrate that nitrite (NO2-) also serves as an important photosensitizer that induces the photo-degradation of IPU. Through radical quenching tests, we identify hydroxyl radicals (•OH) and nitrogen dioxide radicals (NO2•) originating from NO2- photolysis as key players in IPU degradation, resulting in the generation of a series of hydroxylated and nitrated byproducts. Moreover, we demonstrate a synergistic effect on the photo-transformation of IPU when both NOM and NO2- are present in the reaction mixture. The observed rate constant (kobs) for IPU removal increases to 0.0179 ± 0.0002 min-1 in the co-presence of NO2- (50 μM) and NOM (2.5 mgC/L), surpassing the sum of those in the presence of each alone (0.0135 ± 0.0004 min-1). NOM exhibits multifaceted roles in the indirect photolysis of IPU. It can be excited by UV and transformed to excited triplet states (3NOM*) which oxidize IPU to IPU•+ that undergoes further degradation. Simultaneously, NOM can mitigate the reaction by reducing the IPU•+ intermediate back to the parent IPU. However, the presence of NO2- alters this dynamic, as IPU•+ rapidly couples with NO2•, accelerating IPU degradation and augmenting the formation of mono-nitrated IPU. These findings provide in-depth understandings on the photochemical transformation of environmental contaminants, especially phenylurea herbicides, in natural waters where NOM and NO2- coexist.

Fate of sulfamerazine by synchronous adsorption and photocatalysis dependent on natural organic matter properties

Natural organic matter (NOM) can impede the removal of organic micro-pollutants (OMPs) through several mechanisms, including inner filter effect, competition with the target OMP, and radical scavenging, during synchronous adsorption/photocatalysis of multi-functional composites. In this study, the fate and inhibitory mechanisms of sulfamerazine (SMZ, a model OMP) that occurred in presence of seven different NOM samples (i.e. three standard NOM surrogates, a river water sample, a carbon filter effluent and two different sand filter effluents) during the adsorption/photocatalysis by a composite of Bi2O3-TiO2 supported on powdered activated carbon (Bi2O3-TiO2/PAC, abbreviated as BTP) when exposed to visible light irradiation were revealed. The results indicated that adsorption played a greater attribution than photocatalysis on SMZ removal. The primary impediment to the adsorption and photocatalytic degradation of SMZ was attributed to the presence of terrestrial-derived, humic-like NOM fractions with high aromaticity. The adsorption efficacy of SMZ was weakened by the absorption of NOM and its degradation products onto the BTP surface. The inner filter effect, competition between NOM and SMZ, and radical scavenging were responsible for the reduced photocatalysis of SMZ. In the cases of real water matrices, the presence of inorganic anion and co-existed NOM reduced the removal of SMZ. In summary, the findings of this work offer a comprehensive comprehension of the impact of NOM fractions on photocatalysis, emphasizing the necessity to examine the interplay between NOM and background inorganic constituents in the degradation of OMP via adsorption/photocatalysis.

Construction of BiOCl/bismuth-based halide perovskite heterojunctions derived from the metal-organic framework CAU-17 for effective photocatalytic degradation

The designed synthesis of an S-scheme heterojunction has possessed a great potential for improving photocatalytic wastewater treatment by demonstrating increased the photoredox capacity and improved the charge separation efficiency. Here, we introduce the fabrication of a heterojunction-based photocatalyst comprising bismuth oxychloride (BiOCl) and bismuth-based halide perovskite (BHP) nanosheets, derived from metal-organic frameworks (MOFs). Our composite photocatalyst is synthesized through a one-pot solvothermal strategy, where a halogenation process is applied to a bismuth-based metal-organic framework (CAU-17) as the precursor for bismuth sourcing. As a result, the rod-like structure of CAU-17 transforms into well-defined plate and nanosheet architectures after 4 and 8 h of solvothermal treatment, respectively. The modulation of the solvothermal reaction time facilitates the establishment of an S-scheme heterojunction, resulting in an increase in the photocatalytic degradation efficiency of rhodamine B (RhB) and sulfamethoxazole (SMX). The optimized BiOCl/BHP composite exhibits superior RhB and SMX degradation rates, achieving 99.8% degradation of RhB in 60 min and 75.1% degradation of SMX in 300 min. Also, the optimized BiOCl/BHP composite (CAU-17-st-8h sample) exhibited the highest rate constant (k = 3.48 × 10-3 min-1), nearly 6 times higher than that of the bare BHP in the photocatalytic degradation process of SMX. The enhanced photocatalytic efficiency can be endorsed to various factors: (i) the in-situ formation of two-components BiOCl/BHP photocatalyst, derived from CAU-17, effectively suppresses the aggregation of pristine BHP and BiOCl particles; (ii) the S-scheme heterostructure establishes a closely-knit interfacial connection, thereby facilitating efficient pathways for charge separation/transfer; and (iii) the BiOCl/BHP heterostructure enhances its capacity to absorb visible light. Our investigation establishes an effective strategy for constructing heterostructured photocatalysts, offering significant potential for application in photocatalytic wastewater treatment.

Prediction of Nanoparticle Photoreactivity in Mixtures of Surface Foulants Requires Kinetic (Non-equilibrium) Adsorption Considerations

The adsorption of foulants on photocatalytic nanoparticles can suppress their reactivity in water treatment applications by scavenging reactive species at the photocatalyst surface, screening light, or competing for surface sites. These inhibitory effects are commonly modeled using the Langmuir-Hinshelwood model, assuming that adsorbed layer compositions follow Langmuirian (equilibrium) competitive adsorption. However, this assumption has not been evaluated in complex mixtures of foulants. This study evaluates the photoreactivity of titanium dioxide (TiO2) nanoparticles toward a target compound, phenol, in the presence of two classes of foulants ─ natural organic matter (NOM) and a protein, bovine serum albumin (BSA) ─ and mixtures of the two. Langmuir adsorption models predict that BSA should strongly influence the nanoparticle photoreactivity because of its higher adsorption affinity relative to phenol and NOM. However, model evaluation of the experimental phenol decay rates suggested that neither the phenol nor foulant surface coverages are governed by Langmuirian competitive adsorption. Rather, a reactivity model incorporating kinetic predictions of adsorbed layer compositions (favoring NOM adsorption) outperformed Langmuirian models in providing accurate, unbiased predictions of phenol degradation rates. This research emphasizes the importance of using first-principles models that account for adsorption kinetics when assumptions of equilibrium adsorption do not apply.

Recent advances in visible light driven inactivation of bloom forming blue-green algae using novel nano-composites: Mechanism, efficiency and fabrication approaches

Over the years, algae have proved to be a water pollutant due to global warming, climate change, and the unregulated addition of organic compounds in water bodies from diffused resources. Harmful algal blooms (HABs) are severely affecting the health of humans and aquatic ecosystems. Among available anti-blooming technologies, semiconductor photocatalysis has come forth as an effective alternative. In the recent past, literature has been modified extensively with a decisive knowledge regarding algal invasion, desired preparation of nanomaterials with enhanced visible light absorption capacity and mechanisms for algal cell denaturation. The motivation behind this review article was to gather algal inactivation data in a systematic way based on various research studies, including the construction of nanoparticles and purposely to test their anti-algal activities under visible irradiation. Additionally, this article mentions variety of starting materials employed for preparation of various nano-powders with focus on their synthesis routes, analytical techniques as well as proposed mechanisms for lost cellular integrity in context of reduced chlorophyll' a' level, cell rapture, cell leakage and damages to other physiological constituents; credited to oxidative damage initiated by reactive oxidation species (ROS). Various floating and recyclable composited catalysts Ag2CO3-N: GO, Ag/AgCl@ZIF-8, Ag2CrO4-g-C3N4-TiO2/mEP proved to be game-changers owing to their enhanced VL absorption, adsorption, stability, separation and reusability. An outlook for the generalized limitations of published reports, cost estimations for practical implementation, issues and challenges faced by nano-photocatalysts and possible opportunities for future studies are also proposed. This review will be able to provide vast insights for coherent fabrication of catalysts, breakthroughs in experimental methodologies and help in elaboration of damage mechanisms.

Photo-oxidation of Micro- and Nanoplastics: Physical, Chemical, and Biological Effects in Environments

Micro- and nanoplastics (MNPs) are attracting increasing attention due to their persistence and potential ecological risks. This review critically summarizes the effects of photo-oxidation on the physical, chemical, and biological behaviors of MNPs in aquatic and terrestrial environments. The core of this paper explores how photo-oxidation-induced surface property changes in MNPs affect their adsorption toward contaminants, the stability and mobility of MNPs in water and porous media, as well as the transport of pollutants such as organic pollutants (OPs) and heavy metals (HMs). It then reviews the photochemical processes of MNPs with coexisting constituents, highlighting critical factors affecting the photo-oxidation of MNPs, and the contribution of MNPs to the phototransformation of other contaminants. The distinct biological effects and mechanism of aged MNPs are pointed out, in terms of the toxicity to aquatic organisms, biofilm formation, planktonic microbial growth, and soil and sediment microbial community and function. Furthermore, the research gaps and perspectives are put forward, regarding the underlying interaction mechanisms of MNPs with coexisting natural constituents and pollutants under photo-oxidation conditions, the combined effects of photo-oxidation and natural constituents on the fate of MNPs, and the microbiological effect of photoaged MNPs, especially the biotransformation of pollutants.

Enhanced Fenton-like oxidation (Vis/Fe(III)/Peroxydisulfate): The role of iron species and the Fe(III)-LVF complex in levofloxacin degradation

Traditional Fenton and Fenton-like processes are affected by the sluggish kinetics of Fe(II) regeneration and Fe(III) accumulation. This research revealed that the degradation efficiency of pollutants was significantly increased by adding Fe(III) to the Vis/PS system. A mechanism is proposed in which photosensitivity pollutants can boost Fe(III) to produce Fe(II) under visible light irradiation. Intriguingly, Fe(III) rapidly combines with LVF in aqueous environments to form Fe(III)-LVF complexes. This research confirms that Fe(III)-pollutant complexes are generated. The proportion of complexes are calculated using mathematical models. Furthermore, the production of Fe(IV) is verified in the Vis/PS/Fe(III) system, which also plays a vital role in boosting LVF degradation. Overall, this study provides comprehensive insights into the degradation mechanism of micropollutants, involving hydroxyl radical (OH∙), Fe(IV), and Fe(III)-LVF complexes, providing an efficient and green strategy for contaminant removal during wastewater treatment.

Catalytic degradation of crystal violet and methyl orange in heterogeneous Fenton-like processes

Metals-loaded (Fe3+, Cu2+ and Zn2+) activated carbons (M@AC) with different loading ratios (0.1%, 0.5%, 1%, 5% and 10%) were prepared and employed for catalytic degradation of dye model compounds (crystal violet (CV) and methyl orange (MO)) in wastewater by heterogeneous Fenton-like technique. Compared with Cu@AC and Zn@AC, 0.5% Fe3+ loaded AC (0.5Fe@AC) had better catalytic activity for dyes degradation. The effects of dyes initial concentration, catalyst dosage, pH and hydrogen peroxide (H2O2) volume on the catalytic degradation process were investigated. Cyclic performance, stability of 0.5Fe@AC and iron leaching were explored. Degradation kinetics were well fitted to the pseudo-second-order model (Langmuir-Hinshelwood). Almost complete decolorization (99.7%) of 400 mg L-1 CV was achieved after 30 min reaction under the conditions of CV volume (30 mL), catalyst dosage (0.05 g), H2O2 volume (1 mL) and pH (7.7). Decolorization of MO reached 98.2% under the same conditions. The abilities of pyrolysis char (PC) of dyeing sludge (DS) and metal loaded carbon to remove dye pollutants were compared. The intermediate products were analyzed and the possible degradation pathway was proposed. This study provided an insight into catalytic degradation of triphenylmethane- and aromatic azo-based substances, and utilization of sludge char.

Broad-bandgap porous graphitic carbon nitride with nitrogen vacancies and oxygen doping for efficient visible-light photocatalytic degradation of antibiotics

Effective degradation methods are required to address the issue of antibiotics as organic pollutants in water resources. Herein, a two-stage thermal treatment method was used to prepare porous graphitic carbon nitride (g-C3N4) modified with nitrogen vacancies and oxygen doping at the N-(C)3 position and deep in the g-C3N4 framework. Compared with bulk g-C3N4 (BCN) (7 ± 1 m2/g), the modified sample (RCN-2h) possesses a larger specific surface area (224 ± 1 m2/g), a larger bandgap (by 0.19 eV), and a mid-gap state. In addition, RCN-2h shows 15.4, 11.2, and 9.5 times higher photodegradation rates than BCN for the degradation of 100% ofloxacin (OFX) (within 15 min), tetracycline (within 15 min), and sulfadiazine (within 35 min), respectively. The RCN-2h catalyst also exhibits superior stability and reusability. Systematic characterization and density functional theory calculations demonstrate that the synergistic effect of the porous structure, nitrogen vacancies, and oxygen doping in RCN-2h provides additional reaction sites, improved charge separation efficiency, and shorter diffusion paths for reactants and photogenerated charge carriers. Trapping experiments reveal that •O2- is the main active species in OFX photodegradation, and a possible photodegradation pathway is identified using liquid chromatography-mass spectrometry. Benefiting from the simplicity of synthesis methods and the superiority of elemental doping, carbon nitride materials with functional synergy have great potential for environmental applications.

Recent advances of semiconductor photocatalysis for water pollutant treatment: mechanisms, materials and applications

Semiconductor photocatalysis has become an increasing area of interest for use in water treatment methods. This review systematically presents the recent developments of emerging semiconductor photocatalysis system and their application in the removal of water pollutants. A brief overview of the semiconductor photocatalysis mechanism involved with the generation of reactive oxygen species (ROS) is provided first. Then a detailed explanation of the development of TiO2-based, g-C3N4-based, and bismuth-based semiconductor materials and their applications in the degradation of water pollutants are highlighted with recent illustrative examples. Furthermore, the future prospects of semiconductor photocatalysis for water treatment are critically analyzed.

Effective sequestration of tetracycline from aqueous streams using metal-free chemically functionalized porous g-C3N4

The facile preparation of visible-light-driven low-cost photocatalysts with extraordinary catalytic activity is highly beneficial in treating emerging pharmaceutical contaminants. Herein, oxalic acid-induced chemically functionalized graphitic carbon nitride (OCN) was prepared using a one-pot calcination method for the degradation of tetracycline. The estimated structural, morphological, and optical properties proved the formation of highly porous oxalic acid functionalized g-C3N4 (OCN) with enhanced surface area and abundant amino groups. The photocatalytic degradation studies reported a maximum tetracycline removal of 92% within 90 min of visible light illumination and followed pseudo-first-order kinetics (k = 0.03068min-1). The phenomenal photocatalytic efficacy of the functionalized OCN is ascribed to the increased presence of amino groups, strengthening visible light absorption. The enriched surface area also generated many active sites for the reclamation of tetracycline. The radicals trapping studies show that holes and superoxides are mainly responsible for the redemption of tetracycline. The degradation pathways of the tetracycline using OCN were predicted using HRMS. This study provides more insights into the reclamation of tetracycline using a highly efficient metal-free photocatalyst.

Dissolved organic matter promotes photocatalytic degradation of refractory organic pollutants in water by forming hydrogen bonding with photocatalyst

Removing refractory organic pollutants in real water using photocatalysis is a great challenge because coexisting dissolved organic matter (DOM) can quench photogenerated holes and thus prevent generation of reactive oxygen species (ROS). Herein, for the first time, we develop a hydrogen bonding strategy to avoid the scavenging of photoexcited holes, by which DOM even promotes photocatalytic degradation of refractory organic pollutants. Theoretical calculations combined with experimental studies reveal the formation of hydrogen bonding between DOM and a hydroxylated S-scheme heterojunction photocatalyst (Mo-Se/OHNT) consisting of hydroxylated nitrogen doped TiO2 (OHNT) and molybdenum doped selenium (Mo-Se). The hydrogen bonding is demonstrated to change the interaction between DOM and Mo-Se/OHNT from DOM-Ti (IV) to a hydrogen bonded complexation through the hydroxyl/amine groups of DOM and the OHNT in Mo-Se/OHNT. The formed hydrogen network can stabilize excited-state of DOM and inject its electron to the conduction band rather than the valence band of the OHNT upon light irradiation, realizing the key to preventing hole quenching. The electron-hole separation in Mo-Se/OHNT is consequently improved for generating more ROS to be involved in removing refractory organic pollutants. Moreover, this hydrogen bonding strategy is generalized to nitrogen doped zinc oxide and graphitic carbon nitride and applies to real water. Our findings provide a new insight into handling the DOM problem for photocatalytic technology towards water and wastewater treatment.

Effective degradation of tetracycline via persulfate activation using silica-supported zero-valent iron: process optimization, mechanism, degradation pathways and water matrices

Pure zero-valent iron (ZVI) was supported on silica and starch to enhance the activation of persulfate (PS) for tetracycline degradation. The synthesized catalysts were characterized by microscopic and spectroscopic methods to assess their physical and chemical properties. High tetracycline removal (67.55%) occurred using silica modified ZVI (ZVI-Si)/PS system due to the improved hydrophilicity and colloidal stability of ZVI-Si. Incorporating light into the ZVI-Si/PS system improved the degradation performance by 9.45%. Efficient degradation efficiencies were recorded at pH 3-7. The optimum operating parameters determined by the response surface methodology were PS concentration of 0.22 mM, initial tetracycline concentration of 10 mg/L, and ZVI-Si dose of 0.46 g/L, respectively. The rate of tetracycline degradation declined with increasing tetracycline concentration. The degradation efficiencies of tetracycline were 77%, 76.4%, 75.7%, 74.5%, and 73.75% in five repetitive runs at pH 7, 20 mg/L tetracycline concentration, 0.5 g/L ZVI-Si dose, and 0.1 mM PS concentration. The degradation mechanism was explained, and sulfate radicals were the principal reactive oxygen species. The degradation pathway was proposed based on liquid chromatography-mass spectroscopy. Tetracycline degradation was favorable in distilled and tap water. The ubiquitous presence of inorganic ions and dissolved organic matter in the lake, drain, and seawater matrices interfered with the tetracycline degradation. The high reactivity, degradation performance, stability, and reusability of ZVI-Si substantiate the potential practical application of this material for the degradation of real industrial effluents.

Antibiotics oxytetracycline removal by photocatalyst titanium dioxide and graphitic carbon nitride in aquaculture wastewater

The surge in the use of antibiotics, especially in aquaculture, has led to development of antibiotic resistance genes, which will harm environmental and public health. One of the most commonly used antibiotics in aquaculture is oxytetracycline (OTC). Employing photocatalysis, this study compared OTC degradation efficiency of two different types of common photocatalysts, TiO₂ and graphitic carbon nitride (GCN) in terms of their photochemical properties and underlying photocatalytic mechanism. For reference purpose, self-synthesized GCN from urea precursor (GCN-Urea) and commercial GCN (GCN-Commercial) were both examined. OTC adsorption-photocatalysis removal rates in pure OTC solution by TiO₂, GCN-Urea and GCN-Commercial were attained at 95%, 60% and 40% respectively. Photochemical properties evaluated included light absorption, band gap, valence and conduction band positions, photoluminescence, cyclic voltammetry, BET surface area and adsorption capability of the photocatalysts. Through the evaluations, this study provides novel insights towards current state-of-the-art heterogeneous photocatalytic processes. The electron-hole recombination examined by photoluminescence is not the key factor influencing the photocatalytic efficacies as commonly discussed. On the contrary, the dominating factors governing the higher OTC degradation efficiency of TiO₂ compared to GCN are the high mobility of electrons that leads to high redox capability and the high pollutant-photocatalyst affinity. These claims are proven by 86% and 40% more intense anodic and cathodic cyclic voltammetry curve peaks of TiO₂ as compared to both GCNs. OTC also demonstrated 1.7 and 2.3 times higher affinity towards TiO₂ than GCN-Urea and GCN-Commercial. OTC removal by TiO₂ in real aquaculture wastewater only achieved 50%, due to significant inhibition effect by dissolved solids, dissolved organic matters and high ionic contents in the wastewater.

Fe3O4 quantum dots mediated P-g-C3N4/BiOI as an efficient and recyclable Z-scheme photo-Fenton catalyst for tetracycline degradation and bacterial inactivation

Photo-Fenton technology integrated by photocatalysis and Fenton reaction is a favorable strategy for water remediation. Nevertheless, the development of visible-light-assisted efficient and recyclable photo-Fenton catalysts still faces challenges. This study successfully constructed a novel separable Z-scheme P-g-C₃N₄/Fe₃O₄QDs/BiOI (PCN/FOQDs/BOI) heterojunction via in-situ deposition method. The results showed that the photo-Fenton degradation efficiency for tetracycline over optimal ternary catalyst reached 96.5% within 40 min at visible illumination, which was 7.1 and 9.6 times higher than its single photocatalysis and Fenton system, respectively. Moreover, PCN/FOQDs/BOI possessed excellent photo-Fenton antibacterial activity, which could completely inactivate 10⁸ CFU·mL^-1 of E. coli and S. aureus within 20 and 40 min, respectively. Theoretical calculation and in-situ characterization revealed that the enhanced catalysis behavior resulted from the FOQDs mediated Z-scheme electronic system, which not only facilitated photocreated carrier separation of PCN and BOI while maintaining maximum redox capacity, but also accelerated H₂O₂ activation and Fe³⁺/Fe²⁺ cycle, thus synergistically forming more active species in system. Additionally, PCN/FOQDs/BOI/Vis/H₂O₂ system displayed extensive adaptability at pH range of 3-11, removal universality for various organic pollutants and attractive magnetic separation property. This work would provide an inspiration for design of efficient and multifunctional Z-scheme photo-Fenton catalyst in water purification.

Organophosphate Esters (OPEs) Flame Retardants in Water: A Review of Photocatalysis, Adsorption, and Biological Degradation

As a substitute for banned brominated flame retardants (BFRs), the use of organophosphate esters (OPEs) increased year by year with the increase in industrial production and living demand. It was inevitable that OPEs would be discharged into wastewater in excess, which posed a great threat to the health of human beings and aquatic organisms. In the past few decades, people used various methods to remove refractory OPEs. This paper reviewed the photocatalysis method, the adsorption method with wide applicability, and the biological method mainly relying on enzymolysis and hydrolysis to degrade OPEs in water. All three of these methods had the advantages of high removal efficiency and environmental protection for various organic pollutants. The degradation efficiency of OPEs, degradation mechanisms, and conversion products of OPEs by three methods were discussed and summarized. Finally, the development prospects and challenges of OPEs’ degradation technology were discussed.

Inactivation of algae by visible-light-driven modified photocatalysts: A review

Harmful algal blooms have raised great concerns due to their adverse effects on aquatic ecosystems and human health. Recently, visible light-driven (VLD) photocatalysis has attracted attention for algae inactivation owing to its unique characteristics of low cost, mechanical stability, and excellent removal efficiency. However, the low utilization of visible light and the high complexation rate of electron-hole (e^--h⁺) pairs are essential drawbacks of conventional photocatalysts. Scientific efforts have been devoted to modifying VLD photocatalysts to enhance their antialgal activity. This review concisely summarizes the anti-algae performance of the latest modified VLD photocatalysts. The summary of the mechanisms in VLD photocatalytic inactivation demonstrates that reactive oxygen species (ROS) can induce oxidative damage to algal cells and photocatalytic degradation of released organic matter. In addition, the factors, such as photocatalyst dosage, algal concentration and species, and the physicochemical properties of different water matrices, such as pH, natural organic matter, and inorganic ions, affecting the efficacy of VLD catalytic oxidation for algae removal are briefly outlined. Thereafter, this review compiles perspectives on the emerging field of VLD photocatalytic inactivation.

Cobalt-Phosphate (Co-Pi)-Modified WO3 Photoanodes for Performance-Enhanced Photoelectrochemical Wastewater Degradation

Photocatalytic technology, with features of wide applicability, mild reaction conditions and sunlight availability, satisfies the requirements of "green chemistry". As the star photoanode material for photoelectrochemical catalysis, WO₃ has a suitable band gap of 2.8 eV and a strong oxidation capacity, as well as displaying great potential in organic wastewater degradation. However, its performance is usually hindered by competition with water oxidation to generate peroxides, rapid charge complexation caused by surface defect sites, and so on. Herein, WO₃ films modified with cobalt-phosphate (Co-Pi/WO₃) film were prepared and involved in photocatalytic organic wastewater degradation. A degradation rate constant of 0.63311 h^-1 was obtained for Co-Pi/WO₃, which was much higher than that of WO₃, 10.23 times that of direct photocatalysis (DP) and 23.99 times that of electrocatalysis (EC). After three cycles of degradation, the film can maintain a relatively good level of stability and a degradation efficiency of 93.79%.

Insight into the performance of UV/chlorine/TiO2 on carbamazepine degradation: The crucial role of chlorine oxide radical (ClO•)

The UV/chlorine (UC) system is a homogeneous advanced oxidation process with increasing attention in water decontamination. The addition of TiO₂ is a newly found strategy to enhance the generation of hydroxyl radical (HO^•) and chlorine radical (Cl^•) in the UC system. However, the crucial role of chlorine oxide radical (ClO^•, generated by the reactions of HO^• and Cl^• with chlorine) on pollutant degradation, has not been noticed in UV/chlorine/TiO₂ (UCT), the heterogeneous photocatalytic system for chlorine activation. Herein, the role of ClO^• in UCT was clarified through quenching experiments combined with model simulations during carbamazepine degradation. Tert-butyl alcohol completely inhibited while bicarbonate only partly suppressed carbamazepine degradation in UCT, indicating the important role of ClO^•. The second-order reaction rate constant between ClO^• and carbamazepine (k_{ClO•,carbamazepine}) was fitted to be (1.21 ± 0.08) × 10⁷ M^-1 s^-1 by the kinetic model, which avoided the influence of carbonate radical (CO₃^•-), whose contribution couldn't be excluded during k_{ClO•,carbamazepine} determination in commonly used competitive kinetic methods with bicarbonate. With the obtained k_{ClO•,carbamazepine}, model simulation suggested that ClO^• contributed about 50 % to carbamazepine degradation in UCT, and its concentration was less affected under varied conditions (solution pH, chlorine, bicarbonate, and chloride concentration) to keep an efficient carbamazepine degradation. On the contrary, pollutant degradation dominated by HO^• in UCT was largely inhibited with the increase of pH, chlorine, and bicarbonate concentration. In addition to the promotion of degradation efficiency, less disinfection byproducts and lower energy requirement were found in UCT compared with UC. Furthermore, UCT could maintain satisfactory degradation efficiency and energy saving in ground water and surface water samples. Results of this study unraveled the crucial role of ClO^• for pollutant degradation in UCT, and showed bright prospects and great potentials of the system in water treatment.

Contribution of filtration and photocatalysis to DOM removal and fouling mechanism during in-situ UV-LED photocatalytic ceramic membrane process

The use of ceramic membranes and ultraviolet light-emitting diodes (UV-LEDs) has advanced the application of photocatalytic membrane for water treatment. We systematically evaluated the contribution of filtration and photocatalysis to dissolved organic matter (DOM) removal and fouling mechanism during in-situ UV-LED photocatalytic ceramic membrane filtration. The results showed that physical rejection primarily led to removal of 4-15 kDa molecules and photocatalysis further increased the removal of 1-4 kDa molecules, causing small sized microbial humic-like or protein-like materials in the permeate. In-situ UV-LED photocatalysis had an excellent effect on membrane fouling mitigation regardless of DOM sources. The dominant fouling mechanism changed from partial blockage to gel layer formation with increasing Ca²⁺ concentration but did not change with UV treatment. Correlation analysis revealed that the removal of 1-4 kDa molecules contributed to the mitigation of both reversible and irreversible fouling resistance, and the small molecules were the major cause of irreversible fouling resistance. Removal of 1-4 kDa terrestrial humic acid-like contributed to the pore blockage mechanism for synthetic water. Removal of 4-15 kDa protein-like materials was closely correlated to the pore blockage mechanism for real water. Trihalomethanes (THMs) and haloacetic acids (HAAs) formation potential (FP) were both significantly reduced after photocatalytic ceramic membrane process, but precursors of nitrogenous disinfection by-products (N-DBPs) with high toxicity were not removed by filtration or by photocatalysis, which deserves attention. Membrane rejection made higher contribution to better DBPFP control than photocatalysis. This study provides novel insights into the impact of UV-LED on DOM removal, DBPFP control and fouling mitigation, promoting the development of photocatalytic ceramic membrane filtration.

Impact of water matrices on oxidation effects and mechanisms of pharmaceuticals by ultraviolet-based advanced oxidation technologies: A review

The binding between water components (dissolved organic matters, anions and cations) and pharmaceuticals influences the migration and transformation of pollutants. Herein, the impact of water matrices on drug degradation, as well as the electrical energy demands during UV, UV/catalysts, UV/O₃, UV/H₂O₂-based, UV/persulfate and UV/chlorine processes were systemically evaluated. The enhancement effects of water constituents are due to the powerful reactive species formation, the recombination reduction of electrons and holes of catalyst and the catalyst regeneration; the inhibition results from the light attenuation, quenching effects of the excited states of target pollutants and reactive species, the stable complexations generation and the catalyst deactivation. The transformation pathways of the same pollutant in various AOPs have high similarities. At the same time, each oxidant also can act as a special nucleophile or electrophile, depending on the functional groups of the target compound. The electrical energy per order (EEO) of drugs degradation may follow the order of EEO_UV > EEO_UV/catalyst > EEO_UV/H2O2 > EEO_UV/PS > EEO_UV/chlorine or EEO_UV/O3. Meanwhile, it is crucial to balance the cost-benefit assessment and toxic by-products formation, and the comparison of the contaminant degradation pathways and productions in the presence of different water matrices is still lacking.

Tetracycline photolysis revisited: Overlooked day-night succession of the parent compound and metabolites in natural surface waters and associated ecotoxicity

Despite the extensive study of tetracycline photolysis in aquatic environments, the phototransformation of tetracycline and its metabolites under natural day-night succession has not been examined. In this study, we investigated tetracycline photolysis and associated ecotoxicity in two natural surface waters and one artificial ultrapure water under simulated day/night cycling over two days. Previously unrecognized and highly pH- and temperature-dependent dark interconversions of tetracycline metabolites were observed. The liquid chromatography-mass spectrometry/mass spectrometry analysis identified a range of isomerized, hydroxylated, demethylated, deaminated, and open-ring photoproducts. The hydrolysis of tetracycline, isotetracycline, and several intermediate products was proposed as the major mechanism for the observed dark transformations. Exposure studies employing Escherichia coli indicated that although the tetracycline degradation products had lower bacterial toxicities than the parent compound, increasing toxicity with irradiation time after the near-complete degradation of the parent compound in natural waters implied that product mixtures retain ecotoxicity. The dark transformations also affected the bacterial toxicity and fluorescence properties of irradiated tetracycline solutions. Overall, this study provides new insights into the photochemical behavior of tetracycline and its associated ecological risk in aquatic environments.

Dissolved organic matters with low molecular weight fractions exhibit high photochemical potential for reactive oxygen formation

The photochemical properties of dissolved organic matter (DOM) were highly related to the molecular weight (MW) and organic compositions. In this study, the bulk algae- and macrophyte-derived DOM (ADOM and MDOM, respectively) and Suwannee River humic acid (SRHA) were applied and fractionated into low MW- (LMW, <1 kDa) and high MW-(HMW-, 1 kDã0.45 μm) fractions. The formation and mechanisms of photochemically produced reactive intermediates (e.g., HO•, ¹O₂, and ³CDOM*) for these bulk and MW-fractionated samples were compared via the irradiation experiment, fluorescence and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Results showed that humic-/fulvic-like substances were mainly distributed in the LMW fraction which occupied about 44-60% of total organic carbon for ADOM and MDOM and 13% for SRHA. Photochemical experiments showed that the autochthonous DOMs (e.g., ADOM and MDOM) were characterized with comparable formation rates and quantum yields of reactive oxygens with the allochthonous SRHA, suggesting the high photochemical formation potential. Further analysis showed obvious MW-dependent heterogeneities that, irrespective of DOM types, the LMW-fraction exhibited higher formation rates and quantum yields, followed by the bulk- and then the HMW-fractions. The fluorescence and FT-ICR-MS results indicated that the unique biochemical classes, i.e., humic-/fulvic-like moieties and protein-/lipid-derived compounds in the LMW fractions may be responsible for the high apparent quantum yields. This study highlighted the importance of simultaneous characterization of MW and organic compositions for evaluating the photochemical potential and other behaviors and effects of aquatic DOMs.

Insights into the Crucial Role of Electron and Spin Structures in Heteroatom-Doped Covalent Triazine Frameworks for Removing Organic Micropollutants

The water shortage crisis, characterized by organic micropollutants (OMPs), urgently requires new materials and methods to deal with it. Although heteroatom doping has been developed into an effective method to modify carbon nanomaterials for various heterogeneous adsorption and catalytic oxidation systems, the active source regulated by intrinsic electron and spin structures is still obscure. Here, a series of nonmetallic element-doped (such as P, S, and Se) covalent triazine frameworks (CTFs) were constructed and applied to remove organic pollutants using the adsorption-photocatalysis process. The external mass transfer model (EMTM) and the homogeneous surface diffusion model (HSDM) were employed to describe the adsorption process. It was found that sulfur-doped CTF (S-CTF-1) showed a 25.6-fold increase in saturated adsorption capacity (554.7 μmol/g) and a 169.0-fold surge in photocatalytic kinetics (5.07 h^-1), respectively, compared with the pristine CTF-1. A positive correlation between electron accumulation at the active site (N1 atom) and adsorption energy was further demonstrated with experimental results and theoretical calculations. Meanwhile, the photocatalytic degradation rates were greatly enhanced by forming a built-in electric field driven by spin polarization. In addition, S-CTF-1 still maintained a 98.3% removal of 2,2',4,4'-tetrahydroxybenzophenone (BP-2) micropollutants and 97.6% regeneration after six-cycle sequencing batch treatment in real water matrices. This work established a relation between electron and spin structures for adsorption and photocatalysis, paving a new way to design modified carbon nanomaterials to control OMPs.

The transformation of Benzophenone-3 in natural waters and AOPs: The roles of reactive oxygen species and potential environmental risks of products

Benzophenone-3 (BP-3) is a widespread emerging organic pollutant. However, little is known about the synergistic effect of various reactive oxygen species (ROS) in natural waters and wastewater treatment plants on its transformation. In this study, the indirect photochemical behavior of BP-3 in the natural aquatic environments and the degradation process in the AOPs system were investigated by theoretical chemistry calculations. Besides the potential eco-toxicity effects, health effects, and bioaccumulation of the transformation products were assessed by computational toxicology. Results of transformation mechanism and kinetics showed that OH· and ¹O₂ are the keys to the transformation of BP-3, whereas the role of HO₂· and O₃ can be ignored. AOPs based on OH· and ¹O₂ could lead to the rapid transformation of BP-3, while the transformation of BP-3 in natural waters is slow, and even environmental persistence can be observed. However, dissolved organic matter (DOM) promotes the indirect phototransformation of BP-3 in natural waters. A variety of transformation products are generated under the synergistic effects of ROS, H₂O, and ³O₂. Assessments of environmental risks indicated that the potential eco-toxicity and health effects of the main products are significantly lower than that of the parent BP-3. More importantly, low bioaccumulation of transformation products would not enlarge their eco-toxicity and health effects. This study not only gives valuable insights into the indirect phototransformation of BP-3 in natural waters but also provides theoretical support for the feasibility of BP-3 degradation in industrial wastewater by AOPs based on OH· and ¹O₂.

Accurate Removal of Toxic Organic Pollutants from Complex Water Matrices

Characteristic emerging pollutants at low concentration have raised much attention for causing a bottleneck in water remediation, especially in complex water matrices where high concentration of interferents coexist. In the future, tailored treatment methods are therefore of increasing significance for accurate removal of target pollutants in different water matrices. This critical review focuses on the overall strategies for accurately removing highly toxic emerging pollutants in the presence of typical interferents. The main difficulties hindering the improvement of selectivity in complex matrices are analyzed, implying that it is difficult to adopt a universal approach for multiple targets and water substrates. Selective methods based on assorted principles are proposed aiming to improve the anti-interference ability. Thus, typical approaches and fundamentals to achieve selectivity are subsequently summarized including their mechanism, superiority and inferior position, application scope, improvement method and the bottlenecks. The results show that different methods may be applicable to certain conditions and target pollutants. To better understand the mechanism of each selective method and further select the appropriate method, advanced methods for qualitative and quantitative characterization of selectivity are presented. The processes of adsorption, interaction, electron transfer, and bond breaking are discussed. Some comparable selective quantitative methods are helpful for promoting the development of related fields. The research framework of selectivity removal and its fundamentals are established. Presently, although continuous advances and remarkable achievements have been attained in the selective removal of characteristic organic pollutants, there are still various substantial challenges and opportunities. It is hopeful to inspire the researches on the new generation of water and wastewater treatment technology, which can selectively and preferentially treat characteristic pollutants, and establish a reliable research framework to lead the direction of environmental science.

Adsorption and catalytic degradation of preservative parabens by graphene-family nanomaterials

Parabens pose increasing threats to human health due to endocrine disruption activity. Adsorption and degradation of parabens by three types of graphene-family nanomaterials (GFNs) were therefore investigated. For a given paraben, the maximum adsorption capacities (Q⁰) followed the order of reduced graphene oxide (RGO) > multilayered graphene (MG) > graphene oxide (GO); for a given GFN, Q⁰ followed the order of butylparaben (BuP) > propylparaben (PrP) > ethylparaben (EtP) > methylparaben (MeP), dominated by hydrophobic interaction. MeP removal by all the three GFNs was highly enhanced (0.55-4.37 times) with the assistance of H₂O₂ due to additional catalytic degradation process, and MG showed the highest removal enhancement. ∙OH was confirmed as the dominant radicals responsible for parabens degradation. For MG and RGO, the metal impurities (Fe, Cu, Mn, and Co) initiated Fenton-like reaction with H₂O₂ to generate ∙OH. GO contained oxygen-centered free radicals, which were responsible for ∙OH formation via transferring electron to H₂O₂. Four degradation byproducts of MeP were identified, including oxalic, propanedioic, fumaric, and 2,5-dihydroxybenzoic acids. Combined with density function theory calculations, the degradation sites and pathways were identified and confirmed. These findings provide useful information on mechanistic understanding towards the adsorption and degradation of parabens by GFNs.

Visible-light-driven Z-scheme protonated g-C3N4/wood flour biochar/BiVO4 photocatalyst with biochar as charge-transfer channel for enhanced RhB degradation and Cr(VI) reduction

For the simultaneous photocatalytic reduction of hexavalent chromium (Cr(VI)) and the degradation of rhodamine B (RhB), directional charge-transfer channels and efficient separation of photogenerated holes and electrons are important. Herein, a Z-scheme heterojunction photocatalyst, protonated g-C₃N₄/BiVO₄ decorated with wood flour biochar (pCN/WFB/BiVO₄), was prepared through a hydrothermal reaction and electrostatic self-assembly for Cr(VI) photoreduction and RhB photodegradation. The morphological features, crystalline structure, chemical composition, optical properties, specific surface area, and photoelectrochemical properties of the prepared samples were investigated. The pCN/WFB/BiVO₄ photocatalyst exhibited superior removal performance when used to remove Cr(VI) and RhB separately or RhB-Cr(VI) system. The biochar bridge served as a charge-transfer channel between two semiconductors, and the electrons in protonated g-C₃N₄ (pCN) and BiVO₄ achieved a charge balance. This led to the formation of a Z-scheme heterojunction, fast photogenerated charge separation, and a powerful redox ability. The pCN/WFB/BiVO₄ photocatalyst provides new insight into the mechanisms responsible for boosting multicomponent photocatalytic reactions, while constituting a promising candidate for wastewater treatment.

Performance of TiO2/UV-LED-Based Processes for Degradation of Pharmaceuticals: Effect of Matrix Composition and Process Variables

Ultra-violet light-emitting diode (UV-LED)-based processes for water treatment have shown the potential to surpass the hurdles that prevent the adoption of photocatalysis at a large scale due to UV-LEDs' unique features and design flexibility. In this work, the degradation of five EU Watch List 2020/1161 pharmaceutical compounds was comprehensively investigated. Initially, the UV-A and UV-C photolytic and photocatalytic degradation of individual compounds and their mixtures were explored. A design of experiments (DoE) approach was used to quantify the effects of numerous variables on the compounds' degradation rate constant, total organic carbon abatement, and toxicity. The reaction mechanisms of UV-A photocatalysis were investigated by adding different radical scavengers to the mix. The influence of the initial pH was tested and a second DoE helped evaluate the impact of matrix constituents on degradation rates during UV-A photocatalysis. The results showed that each compound had widely different responses to each treatment/scenario, meaning that the optimized design will depend on matrix composition, target pollutant reactivity, and required effluent standards. Each situation should be analyzed individually with care. The levels of the electrical energy per order are still unfeasible for practical applications, but LEDs of lower wavelengths (UV-C) are now approaching UV-A performance levels.

Photodegradation of three antidepressants in natural waters: Important roles of dissolved organic matter and nitrate

Antidepressants have become ubiquitous emerging organic pollutants. Therefore, it is essential to investigate photodegradation of the antidepressants in environment waters for their ecological risk assessment. However, photodegradation behavior of antidepressants varied from different structures and photodegradation mechanism was rarely known for most antidepressants. Herein, citalopram (CIT), paroxetine (PAR) and fluvoxamine (FLUVO) were employed to study the photodegradation behavior of antidepressants in lake water. Results show that direct photolysis of CIT decreased when pH increased from 6 to 9 while the pH effect was not obvious on direct photolysis of FLUVO and PAR. Photodegradation of CIT occurred from its triplet-state and can undergo self-photosensitization through reaction with the generated hydroxyl radical (·OH). In lake water, PAR and FLUVO are degraded mainly via direct photolysis, while CIT is transformed mainly via indirect photolysis. Dissolved organic matter (DOM) and NO₃^- was proved to be the main factors affecting antidepressants photodegradation in lake water. DOM and NO₃^- showed inhibition effect on photodegradation of FLUVO and PAR, while promoted CIT degradation. The promotion effect of CIT was mainly through reaction with ·OH and excited triplet-state of DOM while singlet oxygen played a minor role. Based on the photodegradation products identified by MS/MS, the photodegradation pathways were proposed for CIT and PAR, respectively. For FLUVO, one (Z)-isomer degradation product was also found. Understanding the photodegradation behavior of antidepressants is vital for providing data to do ecological risk assessment.

Highly efficient AgBr/h-MoO3 with charge separation tuning for photocatalytic degradation of trimethoprim: Mechanism insight and toxicity assessment

A highly solar active AgBr/h-MoO₃ composite was constructed by a facile precipitation method, and the charge separation tuning was achieved by photoreduction of AgBr. The photoreduced Ag⁰ on AgBr/h-MoO₃ acted as charge transfer bridge to form Z-scheme heterostructure, while the high degree of Ag reduction converted the material into type-II heterostructure. The synthesized optimal material promoted charge separation and visible light activity due to the incorporation of highly solar active AgBr, which showed ca. 2 times activity on trimethoprim (TMP) degradation than h-MoO₃. The contribution of reactive species on TMP degradation followed the order of O₂^- >¹O₂ > h⁺, which agree well with the proposed charge separation mechanism. The photocatalytic degradation mechanism of TMP was proposed based on the radical quenching, intermediate analysis and DFT calculation. The toxicity analysis based on QSAR calculation showed that part of the degradation intermediates are more toxic than TMP, thus sufficient mineralization are required to eliminate the potential risks of treated water. Moreover, the material showed high stability and activity after four reusing cycles, and it is applicable to treat contaminants in various water matrix. This work is expected to provide new insight into the charge separation tuning mechanism for the AgX based heterojunction, and rational design of highly efficient photocatalysts for organic contaminants degradation by solar irradiation.

A critical review on bismuth oxyhalide based photocatalysis for pharmaceutical active compounds degradation: Modifications, reactive sites, and challenges

Pharmaceutical active compounds (PhACs), as a kind of widely used pharmaceutical drugs, has attracted much attention. The bismuth oxyhalides (BiOX)-based photocatalysis can remove PhACs efficiently due to its unique layered structure, optical and electronic properties. Nevertheless, the rapid recombination of photogenerated electron-hole pairs, and the inherent instability of structure have limited its practical application. In order to solve these problems, recent modification studies tend to focus on facet control, elemental doping, bismuth-rich strategies, defect engineering and heterojunction. Therefore, the objective of this review is to summarize the recent developments in multiply modified strategies for PhACs degradation. The synthesis methods, photocatalytic properties and the enhancement mechanism are elaborated. Besides, based on theoretical calculation, the reactive sites of typical PhACs attacked by different reactive oxygen species were also proposed. Subsequently, challenges and opportunities in applications are also featured which include factors, viz., dissolution of halogen ions, instability under visible light, applications of real water/wastewater, intermediates and byproducts toxicity analysis of BiOX-based photocatalysis. Finally, the perspectives of BiOX-based photocatalysis for PhACs photodegradation in actual water applications are highlighted.

Toward Scaling-Up Photocatalytic Process for Multiphase Environmental Applications

Recently, we have witnessed a booming development of composites and multi-dopant metal oxides to be employed as novel photocatalysts. Yet the practical application of photocatalysis for environmental purposes is still elusive. Concerns about the unknown fate and toxicity of nanoparticles, unsatisfactory performance in real conditions, mass transfer limitations and durability issues have so far discouraged investments in full-scale applications of photocatalysis. Herein, we provide a critical overview of the main challenges that are limiting large-scale application of photocatalysis in air and water/wastewater purification. We then discuss the main approaches reported in the literature to tackle these shortcomings, such as the design of photocatalytic reactors that retain the photocatalyst, the study of degradation of micropollutants in different water matrices, and the development of gas-phase reactors with optimized contact time and irradiation. Furthermore, we provide a critical analysis of research–practice gaps such as treatment of real water and air samples, degradation of pollutants with actual environmental concentrations, photocatalyst deactivation, and cost and environmental life-cycle assessment.

Do Gas Nanobubbles Enhance Aqueous Photocatalysis? Experiment and Analysis of Mechanism

The performance of photocatalytic advanced oxidation must be improved in order for the technology to make the jump from academic research to widespread use. Research is needed on the factors that cause photocatalysis to become self-limiting. In this study, we introduced, for the first time, nanobubbles continuously into a running photocatalytic reactor. Synthetic air, O2, and N2 bubbles in the size range of 40 to 700 nm were added to a reaction system comprising P25 TiO2 photocatalyst in stirred aqueous solution excited by UV-A lamps, with methyl orange as a target contaminant. The removal of methyl orange was tested under conditions of changing pH and with the addition of different radical scavengers. Results indicated that the oxygen and air nanobubbles improved the photocatalytic degradation of methyl orange—the removal efficiency of methyl orange increased from 58.2 ± 3.5% (N2 aeration) to 71.9 ± 0.6% (O2 aeration). Dissolved oxygen (DO) of 14.93 ± 0.13 mg/L was achieved using O2 nanobubbles in comparison to 8.43 ± 0.34 mg/L without aeration. The photodegradation of methyl orange decreased from 70.8 ± 0.4% to 53.9 ± 0.5% as pH increased from 2 to 10. Experiments using the scavengers showed that O2− was the main reactive species in photocatalytic degradation under highly dissolved oxygen conditions, which also accounted for the observation that the removal efficiency for methyl orange decreased at higher pH. However, without photocatalyst, nanobubbles alone did not improve the removal of methyl orange, and nanobubbles also did not increase the degradation of methyl orange by only photolysis. These experiments show that oxygen and air nanobubbles can act as environmentally friendly catalysts for boosting the performance of photocatalytic water treatment systems.

Recent advances on preparation and environmental applications of MOF-derived carbons in catalysis

Carbon materials derived from metal organic frameworks (MOFs) have excellent properties of high surface area, high porosity, adjustable pore size, high conductivity and stability, and their applications in catalysis have become a rapidly expanding research field. In this review, we have summarized the synthesis strategies of MOF-derived carbons with different physical and chemical properties, obtained through direct carbonization, co-pyrolysis and post-treatment. The potential applications of derived carbons, especially monometal-, bimetal-, nonmetal-doped and metal-free carbons in organo-catalysis, photocatalysis and electrocatalysis are analyzed in detail from the environmental perspective. In addition, the improvement of catalytic efficiency is also considered from the aspects of increasing active sites, enhancing the activity of reactants and promoting free electron transfer. The function and synergy of various species of the composites in the catalytic reaction are summarized. The reaction paths and mechanisms are analyzed, and research ideas or trends are proposed for further development.

Selective removal of pharmaceuticals and personal care products from water by titanium incorporated hierarchical diatoms in the presence of natural organic matter

Natural organic matter (NOM), such as humic acids, fulvic acids, and tannic acids, is ubiquitous in water bodies and hinders the photodegradation of pharmaceuticals and personal care products (PPCPs). We prepared titanium incorporated hierarchical diatoms as a novel photocatalyst to selectively remove PPCPs (triclosan, bisphenol A or BPA, and N, N-Diethyl-meta-toluamide or DEET) in the presence of NOM (humic acid). Diatom (Stephanodiscus hantzschii) grown in a titanium(IV) bis(ammonium lactato) dihydroxide solution integrated 7.2% ± 1.4% (mass fraction) of titanium in their cell wall and formed silica-titania frustules. The photodegradation of triclosan, BPA, and DEET by both silica-titania frustules and titania nanopowder (a control photocatalyst) follows pseudo-first-order kinetics. Under ultraviolent light irradiation, the titanium-content-normalized pseudo-first-order removal rate constants of triclosan, BPA, and DEET by silica-titania frustules were 3, 4, and 4-times those by titania nanopowder, respectively, at a humic acid concentration of 10 mg•L^-1. Incorporation of titanium did not alter the morphology and hierarchical nano/microstructures of the diatom. The silica-titania frustules were rich in nanopores with a diameter of 20 ± 4 nm (mean ± standard deviation), allowing PPCPs with a small molecular weight (typically < 600 g•mol^-1) to pass through while efficiently rejecting NOM with high molecular weights. The silica-titania frustules with hierarchical nano/microstructures served as a prefiltration unit by selectively allowing PPCPs to pass through the nanopores and are therefore promising for photodegradation and environmental remediation applications.

Visible-light-driven photocatalytic disinfection of raw surface waters (300-5000 CFU/mL) using reusable coated Ru/WO3/ZrO2

We selected ruthenium (Ru) to improve the photocatalytic activity of a WO₃/ZrO₂ composite. The synthesized Ru/WO₃/ZrO₂ was then compared to a benchmark photocatalyst (S-TiO₂) in terms of photocatalytic disinfection of raw surface waters collected from the Nile Delta region, Egypt. The photocatalysts were immobilized on aluminum plates with polysiloxane to test them in repetitive cycles under the irradiation of a metal-halide lamp. Bacterial concentrations in the raw waters ranged from 300 to 5000 CFU/mL (CFU: colony-forming units) and different species and genus were detected including gram-negative (e.g., shigella, salmonella, vibrio parahaemolyticus, and vibrio cholera) and gram-positive bacteria (e.g., enterococcus). Ru/WO₃/ZrO₂ deactivated over 90 % of the bacterial content within 120 min for most sources, whereas S-TiO₂ did not perform as highly. The bacterial count after 240 min of irradiation was below the detection limit for all different water sources. Moreover, the inhabitation of photocatalytic disinfection by natural organic matter (NOM) was investigated. Ru/WO₃/ZrO₂ was stable for four continuous cycles (960 min in total), suggesting the viability for practical application.

Portable point-of-use photoelectrocatalytic device provides rapid water disinfection

Portable water purification devices are needed to provide safe drinking water in rural communities, developing communities with low quality centralized water distribution, and military or recreational applications. Filtration, ultraviolet light, or chemical additives provide a spectrum of alternatives to remove pathogens from water. For the first time, we design, fabricate and demonstrate the performance of a small portable photoelectric point-of-use device, and document its performance on pathogen inactivation. The device utilizes a commercial teacup from which TiO₂ nanotube photoanodes were produced in-situ and, with a small rechargeable battery powered 365 nm light emitting diode, was able to achieve 5-log inactivation of Escherichia coli in 10 s and 2.6-log of Legionella in 60 s of treatment in model water samples. Treatment of natural water achieved a 1-log bacteria inactivation after 30 s due to matrix effects. The electro-photocatalytic disinfection reactor in a kup (e-DRINK) can provide a feasible and affordable solution to ensure access to clean water. More broadly, this work demonstrates the potential for illumination to improve the efficiency of electrocatalytic surfaces.

Photocatalytic Nanofiltration Membrane Using Zr-MOF/GO Nanocomposite with High-Flux and Anti-Fouling Properties

Photocatalytic nanofiltration (NF) membranes with enhanced flux and anti-fouling properties were prepared from a layered in situ nanocomposite of metal organic framework (i.e., UiO-66) and graphene oxide (UiO-66_GO) on a polyamide NF membrane using a pressure-assisted self-assembly method. For filtering pure water and humic acid, the composite membrane with a 10% UiO-66_GO loading (UiO-66_GO/NF-10%) showed a higher water flux (up to 63 kg/m2 h bar), flux recovery (80%), and total fouling resistance (33%) than the pristine NF membrane. Physical and chemical characterization revealed that this performance was attributed to improvements in hydrophilicity, porosity, surface smoothness, and charge repulsion. The UiO-66_GO/NF-10% composite membrane exhibited better physical stability with a relatively low mass loss (8.64%) after five washes than the membranes with mass loadings of 5 and 15 wt%. Furthermore, the UiO-66_GO/NF-10% composite membrane exhibited considerable photocatalytic activity under ultraviolet (UV) irradiation (bandgap: 3.45 eV), which reduced irreversible fouling from 20.7% to 2.4% and increased flux recovery to 98%. This study demonstrated that surface modification with the UiO-66_GO nanocomposite produced a high-flux anti-fouling photocatalytic NF membrane, which is promising for water purification.

Photocatalytic Degradation of Organic Micropollutants in Water by Zr-MOF/GO Composites

Nanocomposites of UiO-66 and graphene oxide (UiO-66_GO) were prepared with different GO contents by a one-step hydrothermal method, and their photocatalytic activities for the degradation of carbamazepine (CBZ) were investigated under ranges of GO loading, catalyst dose, initial pollutant concentration, and solution pH. The UiO-66_GO nanocomposites showed photocatalytic rate constant up to 0.0136 min−1 for CBZ degradation and its high overall removal efficiency (>90%) in 2 h. The photocatalytic rate constant over the UiO-66_GO nanocomposite was about 2.8 and 1.7 times higher than those over pristine GO and UiO-66, respectively. The enhancement of photocatalytic activity by GO was attributed to increased surface area and porosity, improved light absorption, and narrowed band gap. The composite also showed substantial recyclability and stability over five consecutive cycles of photocatalytic degradation. The experimental results indicated that O2●− and OH● are the responsible radicals for photocatalytic degradation, which helped us propose a photocatalytic mechanism for the enhanced CBZ photodegradation. This work provides a reference for the development of GO-based composite photocatalysts and expands the application of UiO-66 as a photocatalyst for the degradation of persistent micropollutants in water.