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Photocatalytic hydrogen evolution has attracted tremendous interest as it offers a process for generation of green hydrogen based on solar radiation. But the process is extremely complex as many factors influence photocatalytic hydrogen evolution. While novel photocatalysts such as MOFs, perovskites, conjugated polymers, carbon-based materials, quantum dots, and others face challenges such as stability and scalability, semiconductor photocatalysts and cocatalysts such as TiO 2 /Pt, Ni–CdS, CdS/ZnO, CdS/ZnS have the advantages of simplicity in synthesis, stability, and scalability. Despite decades of research, challenges such as charge separation, spectrum usage, low hydrogen generation efficiency, photocatalyst stability and scalability persist. This review discusses what has been achieved so far in the domain of photocatalytic hydrogen evolution and presents a critical analysis of challenges to be addressed in developing a robust and practical photocatalytic hydrogen generation system based on efficiency of photocatalysts and their stability and the extreme importance of efficient reactor designs. [Display omitted] • Challenges associated with photocatalytic hydrogen generation systems. • Photocatalysts, cocatalysts and their stability challenges in hydrogen evolution. • Potential of semiconductor nanostructures, plasmonic nanoparticles and QDs. • Importance of reactor designs in enhancing photocatalytic hydrogen evolution. [ABSTRACT FROM AUTHOR]

Methyl orange (MO) is a poisonous, carcinogenic, and genotoxic azo dye with high chemical stability, high solubility in water, and very low biodegradability. The removal is difficult by conventional water purification methods. The degradation of MO with photocatalytic membrane reactors loaded with TiO 2 nanoparticles(NPs) is reported in some publications, and the degradation efficiency is less than 50%. The present study presents the use of titanium dioxide nanoparticles deposited on the surface of nanofiltration membranes to assess the photocatalytic degradation of reactive dye. The morphological analyses showed the presence of a thin titanium dioxide layer on the surface of the membranes. In addition, the photocatalytic membranes with 0.25 wt% of TiO 2 showed an interesting photocatalytic activity over four hours, with 82.3% of the MO degraded. The degradation followed a pseudo-second-order kinetics model, and the reusability of the prepared membranes was also verified. • Development of photocatalytic membranes for recalcitrant dye removal. • The photocatalytic performance of the membranes depends on the TiO 2 amounts. • An interesting MO degradation (82.3% After 210 min) has been obtained. • Low efficiency loss was detected during the membrane reusability. [ABSTRACT FROM AUTHOR]

Environmental pollution has become a worldwide issue. Rapid industrial and agricultural practices have increased organic contaminants in water supplies. Hence, many strategies have been developed to address this concern. In order to supply clean water for various applications, high-performance treatment technology is required to effectively remove organic and inorganic contaminants. Utilizing photocatalytic membrane reactors (PMRs) has shown promise as a viable alternative process in the water and wastewater industry due to its efficiency, low cost, simplicity, and low environmental impact. PMRs are commonly categorized into two main categories: those with the photocatalyst suspended in solution and those with the photocatalyst immobilized in/on a membrane. Herein, the working and fouling mechanisms in PMRs membranes are investigated; the interplay of fouling and photocatalytic activity and the development of fouling prevention strategies are elucidated; and the significance of photocatalysis in membrane fouling mechanisms such as pore plugging and cake layering is thoroughly explored. [ABSTRACT FROM AUTHOR]

Trials on ciprofloxacin removal from real aqueous systems were performed using a photocatalytic membrane reactor with catalyst in suspension. Four sets of four treatment cycles were carried out and experimental results were processed from the statistical point of view in order to assess results reproducibility. Global ciprofloxacin removal efficiencies of more than 99.85% were obtained for all treatment cycles. Coupling of photocatalytic degradation with membrane process allowed catalyst separation and reuse within further treatment cycles. Statistical analyse of experimental data recommend the use of photocatalytic membrane reactors as a feasible option for the advanced removal of pharmaceutical products like ciprofloxacin from real wastewater systems.

Dimethyl phthalate is low molecular weight phthalate used on a wide scale within industry for manufacture of plastics, solvents, adhesives, lubricants, coatings and due to its poor biodegradability is not removed by classic wastewater treatment processes. It was proved that it affects the endocrine system, presenting carcinogenic, teratogenic and mutagenic effects on upon living organisms. Therefore, there is a need for more advanced treatment methods such as advanced oxidation processes for dimethyl phthalate removal from aqueous systems. A hybrid process consisting from photo catalysis and membrane based processes was investigated and the optimum parameters for dimethyl phthalate removal were established. In respect to photocatalytic process the influence of photo catalyst dose, pH, irradiation time were studied and process kinetics were set up using a synthetic dimethyl phthalate solution. In order to recover and reuse the photocatalyst a membrane process was used and optimum working pressure was determined. In the case of real wastewater matrix the addition of hydrogen peroxide was needed, in the photocatalytic stage, in order to improve process efficiency. Four treatment cycles, using real wastewater spiked with dimethyl phthalate, were performed with the reuse of photo catalyst proving that the use of photocatalytic membrane reactor with suspended photo catalyst represents a promising method for phthalates removal from aqueous systems.

Environmental pollution has become a worldwide issue. Rapid industrial and agricultural practices have increased organic contaminants in water supplies. Hence, many strategies have been developed to address this concern. In order to supply clean water for various applications, high-performance treatment technology is required to effectively remove organic and inorganic contaminants. Utilizing photocatalytic membrane reactors (PMRs) has shown promise as a viable alternative process in the water and wastewater industry due to its efficiency, low cost, simplicity, and low environmental impact. PMRs are commonly categorized into two main categories: those with the photocatalyst suspended in solution and those with the photocatalyst immobilized in/on a membrane. Herein, the working and fouling mechanisms in PMRs membranes are investigated; the interplay of fouling and photocatalytic activity and the development of fouling prevention strategies are elucidated; and the significance of photocatalysis in membrane fouling mechanisms such as pore plugging and cake layering is thoroughly explored.

The development of effective disinfection treatment processes is crucial to help the water industry cope with the inevitable challenges resulting from the increase in human population and climate change. Climate change leads to heavy rainfall, flooding and hot weather events that are associated with waterborne diseases. Developing effective treatment technologies will improve our resilience to cope with these events and our capacity to safeguard public health. A submerged hybrid reactor was used to test the efficiency of membrane filtration, direct photolysis (using ultraviolet-C low-pressure mercury lamps, as well as ultraviolet-C and ultraviolet-A light-emitting diodes panels) and the combination of both treatment processes (membrane filtration and photolysis) to retain and inactivate water quality indicator bacteria. The developed photocatalytic membranes effectively retained the target microorganisms that were then successfully inactivated by photolysis and advanced oxidation processes. The new hybrid reactor could be a promising approach to treat drinking water, recreational water and wastewater produced by different industries in small-scale systems. Furthermore, the results obtained with membranes coated with titanium dioxide and copper combined with ultraviolet-A light sources show that the process may be a promising approach to guarantee water disinfection using natural sunlight. [ABSTRACT FROM AUTHOR]

According to the World Health Organization, antibiotic resistance is one of the main threats to global health. The excessive use of several antibiotics has led to the widespread distribution of antibiotic-resistant bacteria and antibiotic resistance genes in various environment matrices, including surface water. In this study, total coliforms, Escherichia coli and enterococci, as well as total coliforms and Escherichia coli resistant to ciprofloxacin, levofloxacin, ampicillin, streptomycin, and imipenem, were monitored in several surface water sampling events. A hybrid reactor was used to test the efficiency of membrane filtration, direct photolysis (using UV-C light emitting diodes that emit light at 265 nm and UV-C low pressure mercury lamps that emit light at 254 nm), and the combination of both processes to ensure the retention and inactivation of total coliforms and Escherichia coli as well as antibiotic-resistant bacteria (total coliforms and Escherichia coli) present in river water at occurrence levels. The membranes used (unmodified silicon carbide membranes and the same membrane modified with a photocatalytic layer) effectively retained the target bacteria. Direct photolysis using low-pressure mercury lamps and light-emitting diode panels (emitting at 265 nm) achieved extremely high levels of inactivation of the target bacteria. The combined treatment (unmodified and modified photocatalytic surfaces in combination with UV-C and UV-A light sources) successfully retained the bacteria and treated the feed after 1 h of treatment. The hybrid treatment proposed is a promising approach to use as point-of-use treatment by isolated populations or when conventional systems and electricity fail due to natural disasters or war. Furthermore, the effective treatment obtained when the combined system was used with UV-A light sources indicates that the process may be a promising approach to guarantee water disinfection using natural sunlight.

A porous support with sufficient strength and high UV transmittance is of key importance for potential applications of the light conducting photocatalytic membrane reactors. In this work, we report a simple method for the preparation of amorphous fused silica support with boric acid as sintering aid. The effect of experimental conditions on sintering was investigated by orthogonal test. The effect of porosity and refractive index of pore medium on the UV transmittance of the supports was also studied. It is demonstrated that boric acid not only assists the sintering of the fused silica supports with short sintering time, but also inhibits the crystallization of cristobalite. Fused silica supports prepared at 1150 °C for 1 h show average porosity of 42.3%, bending strength of 14.9 MPa and UV transmittance as high as 64.2% in water. These properties make fused silica a good candidate as support for light conducting photocatalytic membrane reactors. [ABSTRACT FROM AUTHOR]

This study successfully prepared and tested sulfur- and nitrogen-co-doped TiO2-coated α-Al2O3 (S,N-doped TiO2/Al2O3) hollow fiber (HF) membranes for efficient photocatalytic degradation of gaseous ammonia (NH3). Thiourea was used as a sulfur- and nitrogen-doping source to produce a S,N-doped TiO2 photocatalyst powder. For comparative purposes, undoped TiO2 powder was also synthesized. Through the application of a phase-inversion technique combined with high-temperature sintering, hollow fibers composed of α-Al2O3 were developed. Undoped TiO2 and S,N-doped TiO2 photocatalyst powders were coated on the α-Al2O3 HF surface to obtain undoped TiO2/Al2O3 and S,N-doped TiO2/Al2O3 HF membranes, respectively. All prepared samples were characterized using XRD, TEM, XPS, UV-Vis, SEM, BET, FT-IR, and EDS. S and N dopants were confirmed using XPS and UV-Vis spectra. The crystal phase of the undoped TiO2 and S,N-doped TiO2 photocatalysts was a pure anatase phase. A portable air purifier photocatalytic filter device was developed and tested for the first time to decrease the amount of indoor NH3 pollution under the limits of the lachrymatory threshold. The device, which was made up of 36 S,N-doped TiO2/Al2O3 HF membranes, took only 15–20 min to reduce the level of NH3 in a test chamber from 50 ppm to around 5 ppm, confirming the remarkable performance regarding the photocatalytic degradation of gaseous NH3.

This study aims to investigate the effect of binder used in dip-coating techniques on ceramic photocatalytic membrane reactor (PMR) and to evaluate the performance of the PMR in the treatment of secondary effluent from two hospitals. The catalytic membrane was dipped and coated with TiO2, then either sodium lauryl sulfate (SLS) or polyethylene glycol (PEG) was used as binders. The photocatalytic membranes were characterized using scanning electron microscopy and X-ray diffraction. Organic matter in the wastewater effluent was measured and characterized using chemical oxygen demand, dissolved organic carbon (DOC), and fluorescent excitation-emission matrix (EEM). DOC removal efficiencies of the PMR experiments using the TiO2-PEG and the TiO2-SLS membranes were 20.0% and 28.5%, respectively. EEM characterization revealed four fractions of organic matter were presented in the system, namely tyrosine-like, tryptophan-like, humic-like and fulvic-like organic matter. The TiO2-assisted PMR had preferential removal of protein-like dissolved organic matter (DOM) over humic-like substances, as protein-like DOM was preferably oxidized by hydroxyl radicals in the reactor. [ABSTRACT FROM AUTHOR]

The presence of contaminants of emerging concern (CEC), such as pharmaceuticals, in water sources is one of the main concerns nowadays due to their hazardous properties causing severe effects on human health and ecosystem biodiversity. Niflumic acid (NFA) is a widely used anti-inflammatory drug, and it is known for its non-biodegradability and resistance to chemical and biological degradation processes. In this work, a 10 wt.% TiO2/PVDF–TrFE nanocomposite membrane (NCM) was prepared by the solvent casting technique, fully characterized, and implemented on an up-scaled photocatalytic membrane reactor (PMR). The photocatalytic activity of the NCM was evaluated on NFA degradation under different experimental conditions, including NFA concentration, pH of the media, irradiation time and intensity. The NCM demonstrated a remarkable photocatalytic efficiency on NFA degradation, as efficiency of 91% was achieved after 6 h under solar irradiation at neutral pH. The NCM proved effective in long-term use, with maximum efficiency losses of 7%. An artificial neural network (ANN) model was designed to model NFA’s photocatalytic degradation behavior, demonstrating a good agreement between experimental and predicted data, with an R2 of 0.98. The relative significance of each experimental condition was evaluated, and the irradiation time proved to be the most significant parameter affecting the NFA degradation efficiency. The designed ANN model provides a reliable framework l for modeling the photocatalytic activity of TiO2/PVDF-TrFE and related NCM.

Reducing irreversible fouling in polymer membranes by integrating photocatalytic and membrane processes as the self-cleaning photocatalytic membrane is a promising candidate for improving membrane filtration performance. In this study, mixed matrix photocatalytic membranes were prepared from the combination of different morphologies ZnO- g -C 3 N 4 heterostructure in the polymer matrix by the phase-separation method. To investigate the self-cleaning and performance properties of mixed matrix photocatalytic membranes prepared from different morphologies heterostructures, the photocatalytic membrane reactor with a visible-light source was applied. Nanoflower/nanosheet (NF/NS) ZnO- g -C 3 N 4 photocatalytic membrane showed good self-cleaning performance owing to the high photocatalytic performance of NF/NS ZnO- g -C 3 N 4 heterostructure by the reduction of irreversible membrane fouling, thus improving the antifouling and filtration performance of the membrane. Also, the morphology and the uniform distribution of the NF/NS ZnO- g -C 3 N 4 heterostructure in the membrane matrix caused good hydrophilic properties, high porosity, and a more symmetrical structure in the (NF/NS) ZnO- g -C 3 N 4 photocatalytic membrane (F 4). For the F 4 membrane, the permeability and rejection values increased from 40.35 L m−2 h−1 and 90.9% in the dark environment to 84.37 L m−2 h−1 and 97.4% under visible-light for dye pollutants. Accordingly, F 4 had the best filtration and self-cleaning performance, which can be used as a promising visible-light photocatalytic membrane in wastewater treatment processes. [Display omitted] • Cross-flow photocatalytic membrane reactor (PMR) in visible light was utilized. • High-performance membrane by reducing the reversible resistance (cake-like layer). • NF/NS photocatalytic membrane showed the best self-cleaning performance. • MB rejection for NF/NS membrane in dark and light is 90.9% and 97.4%, respectively. [ABSTRACT FROM AUTHOR]

In recent years, reactive ammonia (NH3) has emerged as a major source of indoor air pollution. In this study, Al2O3-based hollow fiber membranes functionalized with nitrogen-doped titanium dioxide were produced and successfully applied for efficient heterogeneous photocatalytic NH3 gas degradation. Al2O3 hollow fiber membranes were prepared using the phase inversion process. A dip-coating technique was used to deposit titanium dioxide (TiO2) and nitrogen-doped titanium dioxide (N-TiO2) thin films on well-cleaned Al2O3-based hollow fiber membranes. All heterogeneous photocatalytic degradation tests of NH3 gas were performed with both UV and visible light irradiation at room temperature. The nitrogen doping effects on the NH3 heterogeneous photocatalytic degradation capacity of TiO2 were investigated, and the effect of the number of membranes (30, 36, 42, and 48 membranes) of the prototype lab-scale photocatalytic membrane reactor, with a modular design, on the performances in different light conditions was also elucidated. Moreover, under ultraviolet and visible light, the initial concentration of gaseous NH3 was reduced to zero after only fifteen minutes in a prototype lab-scale stage with a photocatalytic membrane reactor based on an N-TiO2 photocatalyst. The number of Al2O3-based hollow fiber membranes functionalized with N-TiO2 photocatalysts increases the capacity for NH3 heterogeneous photocatalytic degradation.

The development of effective disinfection treatment processes is crucial to help the water industry cope with the inevitable challenges resulting from the increase in human population and climate change. Climate change leads to heavy rainfall, flooding and hot weather events that are associated with waterborne diseases. Developing effective treatment technologies will improve our resilience to cope with these events and our capacity to safeguard public health. A submerged hybrid reactor was used to test the efficiency of membrane filtration, direct photolysis (using ultraviolet-C low-pressure mercury lamps, as well as ultraviolet-C and ultraviolet-A light-emitting diodes panels) and the combination of both treatment processes (membrane filtration and photolysis) to retain and inactivate water quality indicator bacteria. The developed photocatalytic membranes effectively retained the target microorganisms that were then successfully inactivated by photolysis and advanced oxidation processes. The new hybrid reactor could be a promising approach to treat drinking water, recreational water and wastewater produced by different industries in small-scale systems. Furthermore, the results obtained with membranes coated with titanium dioxide and copper combined with ultraviolet-A light sources show that the process may be a promising approach to guarantee water disinfection using natural sunlight.

BACKGROUND In the recent years, photocatalytic membrane process has gained interest in wastewater treatment applications. In this study, the ability of advanced oxidation technology coupled membrane process was evaluated during chloramphenicol (CAP) filtration. Titanium dioxide doped hydroxyapatite (TiO2‐HAP) based photocatalytic membrane for chloramphenicol (CAP) degradation was investigated. The TiO2‐HAP photocatalyst was synthesized by a facile hydrothermal technique and characterized by the transmission electron microscope, X‐ray diffraction, and FTIR spectroscopy. Varying concentrations of TiO2‐HAP photocatalyst incorporated polysulfone (PSf) membranes were fabricated to enhance the photocatalytic activity and antifouling propensity. The photocatalytic activity of TiO2‐HAP incorporated PSf membranes was evaluated by a low‐pressure cross‐flow lab‐scale photocatalytic membrane reactor (PMR) for degradation of chloramphenicol in water. These results were compared with pristine PSf membrane. RESULTS: The degradation of chloramphenicol was measured by liquid chromatography‐mass spectrometry (LC–MS). The photocatalytic degradation experiments revealed that the highest degradation of 61.59% was observed for the PSf/4 wt% TiO2‐HAP nanocomposite membrane. CONCLUSION: This study highlights the advantages of applying the photocatalytic TiO2‐HAP incorporated PSf composite membranes for pharmaceutical wastewater treatment applications. © 2020 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

Dye-containing wastewater should be treated effectively using eco-friendly technologies in order to prevent the adverse impacts on the environment and natural water resources. This review summarizes the recent technologies used commonly for dye removal from wastewater, such as biological methods, advanced oxidation processes (AOP), electrocoagulation, adsorption, membrane technology, and photocatalytic reactors using novel nanomaterials. On the other hand, this review also addresses the performances, operating conditions, important process parameters, and the advantages and disadvantages of different treatment systems. Besides, in order to achieve efficient color removal, several previous studies have also focused on hybrid treatment technologies. Among the different hybrid treatments, the MBR (membrane bioreactor) and the PMR (photocatalytic membrane reactor) were reviewed in detail as they have shown to be practically promising for color removal from textile wastewater. Regarding the factors that influence the performance of PMR systems, the role of nanoparticles were discussed by considering their mechanisms of color removal. [ABSTRACT FROM AUTHOR]

• Monolayer TMPyP@SPSf/PES membrane was prepared by electrostatic assembly strategy. • The visible light response region of TMPyP was expanded after it was immobilized. • Photocatalytic membrane reactor (PMR) exhibited excellent performance for dye degradation. • The PMR can be applied for degradation of different dyes and can be reused after regeneration. Porphyrin-based catalytic oxidation of hydrocarbon bonds is considered to be the most representative biomimetic catalysis. To mimic the biomimetic catalytic oxidation of nature under illumination of visible light, a monolayer of meso -tetrakis (1-methylpyridinium-4-yl) porphyrin immobilized sulfonated polysulfone/ polyethersulfone blend membrane (TMPyP@SPSf/PES) was prepared via a facile electrostatic assembly method. The visible light response region of TMPyP was expanded after it was immobilized on the SPSf/PES membrane surface, which resulted in the increase of the photocatalytic performance, and the Rhodamine B (RhB) degradation was increased from 30.0% to 93.4%. A photocatalytic membrane reactor (PMR) equipped with TMPyP@SPSf/PES membrane was employed for RhB treatment under continuous flow filtration coupling with photocatalysis, and the optimized degradation was up to 98.3%. Moreover, the TMPyP@SPSf/PES membrane can be reused for photocatalytic degradation of RhB after regeneration in TMPyP solution. More importantly, the TMPyP@SPSf/PES membrane can also efficiently degrade other cationic and anionic dyes (such as degradation of methylene blue 99.1%, acid fuchsin 96.8%). Finally, the oxidation mechanism and degradation pathway of RhB were further investigated by electron spin resonance (ESR), ultra-performance liquid chromatography and high-definition mass spectrometry (UPLC HDMS) and gas chromatography-Mass spectrometry (GC–MS). It was revealed that the photogenerated hole (h+), superoxide radical (·O 2 −), and singlet oxygen (1O 2) controlled the oxidation process. The degradation of RhB includes N -de-ethylate, wrecked of chromophore structures and opening-ring of benzene. [ABSTRACT FROM AUTHOR]

In this work, synergistic effect of solar photocatalysis integrated with adsorption process towards the degradation of Congo red (CR) was investigated via two different approaches using a photocatalytic membrane reactor. In the first approach, sequential treatments were conducted through the adsorption by graphene oxide (GO) and then followed by photocatalytic oxidation using Fe-doped ZnO nanocomposites (NCs). In the second approach, however, CR solution was treated by photocatalytic oxidation using Fe-doped ZnO/rGO NCs. These nanocomposites were synthesized by a sol-gel method. The NCs were characterized by X-ray diffraction (XRD), photoluminescence (PL), Fourier transmission infrared (FTIR), ultraviolet–visible (UV-vis) spectroscopy, and field emission scanning electron microscopy (FESEM). It was observed that Fe-doped ZnO could enhance the photoactivity of ZnO under solar light. When Fe-doped ZnO were decorated on GO sheets, however, this provided a surface enhancement for adsorption of organic pollutants. The photocatalytic performances using both approaches were evaluated based on the degradation of CR molecules in aqueous solution under solar irradiation. Nanofiltration (NF) performance in terms of CR residual removal from water and their fouling behavior during post-separation of photocatalysts was studied. Serious flux declined and thicker fouling layer on membrane were found in photocatalytic membrane reactor using Fe-doped ZnO/rGO NCs which could be attributed to the stronger π–π interaction between rGO and CR solution. [ABSTRACT FROM AUTHOR]

Photocatalytic membrane reactor (PMR), coupling photocatalysis and membrane separation, has shown a considerable potential in foulant removal. However, the serious membrane fouling and fragile membrane stability still limit the application of current PMR systems. Here, we report a strategy that PdCu alloy deposited on TiO 2 , which is incorporated with polyvinylidene fluoride membrane, achieves high humic acid (HA) rejection efficiency (100%) with enhanced membrane permeation and anti-fouling performance. Detailed characterizations demonstrate that alloying effect of PdCu-TiO 2 plays multiple roles in improving PMR performance: (1) providing the paired Pd-Cu sites for enhancing HA adsorption and transferring electrons from Pd-Cu sites to HA to improve photocatalytic HA removal; (2) inducing the photothermal effect due to the carrier damping, which directly enhances membrane permeation. The remarkable properties of PdCu-TiO 2 membrane in PMR confirm the significance of alloying effect for treating aqueous organic pollutants. [Display omitted] • Photocatalytic membrane reactor with enhanced membrane permeation and anti-fouling performance. • Alloying effect of PdCu-TiO2 for enhancing the adsorption and removal of humic acid. • Photothermal effect induced by alloying effect for enhancing membrane permeation. • Orbitals hybridizations of Cu 3d - and Pd 3d - O 2p for facilitating electrons transfer [ABSTRACT FROM AUTHOR]

The research on photocatalytic membrane reactors (PMRs) started around the year 2000 with the study of wastewater treatment by degradation reactions of recalcitrant organic pollutants, and since then the evolution of our scientific knowledge has increased significantly, broadening interest in reactions such as the synthesis of organic chemicals. In this paper, we focus on some initial problems and how they have been solved/reduced over time to improve the performance of processes in PMRs. Some know-how gained during these last two decades of research concerns decreasing/avoiding the degradation of the polymeric membranes, improving photocatalyst reuse, decreasing membrane fouling, enhancing visible light photocatalysts, and improving selectivity towards the reaction product(s) in synthesis reactions (partial oxidation and reduction). All these aspects are discussed in detail in this review. This technology seems quite mature in the case of water and wastewater treatment using submerged photocatalytic membrane reactors (SPMRs), while for applications concerning synthesis reactions, additional knowledge is required.

Photocatalytic membrane reactors (PMRs) have been widely used in wastewater treatment over the past few years. In this study, P-doped g-C 3 N 4 (PCN), a metal-free, visible light (Vis)-driven photocatalyst, was prepared and coated on an Al 2 O 3 substrate followed by integration with an inorganic Al 2 O 3 hollow fiber membrane module for use as a PMR. The 10 wt % of PCN exhibited the highest degradation activity for methyl blue (MB) removal under Vis irradiation because the C sites and vacancies within the heptazine rings of the CN units were substituted with P to improve charge separation and reduce the number of unpaired electrons. The PMR exhibits higher efficiency and stability in the removal of MB, methyl orange, phenol solution, and a mixture of the three organic compounds than do individual hollow fiber membranes or photocatalysis systems. The TOC (total organic carbon) analysis revealed that more than 92% of the phenol was decomposed and mineralized in the PMR, which also had a MB removal efficiency of greater than 90% when repeatedly used for four times. These results indicate that the PMR developed in this study is highly active and stable, and can serve as a promising system for effective removal of organic pollutants in wastewater. Photocatalytic membrane reactor integrated with P-doped g-C 3 N 4 with Al 2 O 3 hollow fiber membrane module exhibits excellent photocatalytic activity. Image 1 • P replaces C in g-C 3 N 4 structure to improve charge separation and photocatalysis. • P-doped g-C 3 N 4 was fabricated and integrated with photocatalytic membrane reactor. • PMR can be used for wastewater treatment under irradiation of simulated sunlight. • PMR degrades MB under irradiation with light of wavelength larger than 400 nm. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • AOP-UF hybrid systems are effective for OTC removal from waste and surface waters. • Adsorption on TiO 2 particles and on membrane contributed for OTC and DOC removal. • The permeate flux proved to be affected by photocatalyst loading and oxidant dosage. • The membrane did not influence the DOC removal by UVC/H 2 O 2 with 120 mg H 2 O 2 L−1. • Effluent matrix affects the mineralization and OTC removal efficiency. Abstract In this study, the efficiency of three different hybrid systems coupling ultrafiltration (UF) with (i) UVC/H 2 O 2 , (ii) UVC/TiO 2 , and (iii) UVC was evaluated for the treatment of a secondary effluent (SE) from a municipal wastewater treatment plant and a surface water (SW) from Miedwie Lake, both spiked with 5 mg L−1 of oxytetracycline (OTC). A ceramic membrane made of TiO 2 was tested. The effect of H 2 O 2 concentration (30 to 120 mg L-1) on the UVC/H 2 O 2 -UF system and of P25-TiO 2 loading (0.5 to 1.5 g L-1) in suspension on the photocatalytic UVC/TiO 2 -UF system were investigated. A photonic flux of 5.1 J s-1 was provided in all systems. The maximum pure water flux (PWF) was 111 L m-2 h-1. Adsorption on the photocatalyst particles and/or on the membrane surface was found to be an important contribution for the removal of OTC and dissolved organic carbon (DOC). The UF membrane contributed significantly to photocatalyst and pollutants rejection in the photocatalytic membrane reactor (PMR) with the UVC/TiO 2 system; whereas when using the UVC/H 2 O 2 process, with the highest H 2 O 2 dose, the membrane effect was negligible. Using SE as reaction matrix in the UVC/TiO 2 -UF system with 1.0 g L-1 of TiO 2 , the complete OTC removal was achieved in 5 h with a mineralization of 49%. For the same reaction period, a DOC removal of 52% was achieved with the UVC/H 2 O 2 -UF system (120 mg H 2 O 2 L-1). A similar permeate flux decrease (ca. 40%) was observed in both cases. Furthermore, the highest reduction of permeate flux (60%) was observed when using the UVC-UF system. Using SW as reaction matrix, higher OTC degradation rates and percentage of mineralization were reached for the same reaction period, when compared with SE, due to the lower COD and inorganic salts concentration present in the surface water. [ABSTRACT FROM AUTHOR]

Photocatalysis combined with membrane technology could offer an enormous potential for power generation in a renewable and sustainable way. Herein, we describe the one-step hydrogen and oxygen evolution through a photocatalytic membrane reactor. Experimental tests were carried out by means of a two-compartment cell in which a modified Nafion membrane separated the oxygen and hydrogen evolution semi-cells, while iron ions permeating through the membrane acted as a redox mediator. Nanosized Au/TiO2 and Au/CeO2 were employed as suspended photocatalysts for hydrogen and oxygen generation, respectively. The influence of initial Fe3+ ion concentration, ranging from 5 to 20 mM, was investigated, and the best results in terms of hydrogen and oxygen evolution were registered by working with 5 mM Fe3+. The positive effect of gold on the overall water splitting was confirmed by comparing the photocatalytic results obtained with the modified/unmodified titania and ceria. Au-loading played a key role for controlling the photocatalytic activity, and the optimal percentage for hydrogen and oxygen generation was 0.25 wt%. Under irradiation with visible light, hydrogen and oxygen were produced in stoichiometric amounts. The crucial role of the couple Fe3+/Fe2+ and of the membrane on the performance of the overall photocatalytic system was found. [ABSTRACT FROM AUTHOR]

Though having been studied for more than four decades, photocatalysis is still hardly applied in large-scale water and wastewater treatment. This is mainly due to the low energy efficiency of conventional photocatalytic reactors. A recently introduced reactor concept, the fixed-bed photocatalytic membrane reactor (FPMR), can potentially overcome this shortcoming. Its performance for the degradation of organic compounds is examined for different modes of operation. For this purpose, FPMRs based on commercial submerged ceramic membranes were used, which are covered by a photocatalytic layer of pyrogenic titania. The FPMRs were performed in two operational modes: closed-loop reactor and continuous-flow reactor. Oxalic acid was used as a model organic compound to conduct photocatalysis. The influence of light intensity and catalyst layer thickness as well as mass transfer inside the layer were thoroughly studied. The evaluation of the reactors was based on the apparent quantum yield, the photocatalytic space–time yield and specific energy consumption. The results demonstrate that the FPMRs are considerably more efficient than other reported reactor concepts. Its apparent quantum yield can reach up to 11.1%. Furthermore, its specific light energy consumption is merely around 0.1 kWh/g (TOC). The results also reveal that closed-loop FPMRs achieve higher efficiencies than continuous-flow FPMRs. Last but not least, a quantitative model for calculating the reaction rate constant of a photocatalytic membrane from observed reaction rate constant of a photocatalytic membrane reactor was developed and experimentally verified. [ABSTRACT FROM AUTHOR]

Photocatalytic organic synthesis/conversions and water treatment under visible light are a challenging task to use renewable energy in chemical transformations. In this review a brief overview on the mainly employed visible light photocatalysts and a discussion on the problems and advantages of Vis-light versus UV-light irradiation is reported. Visible light photocatalysts in the photocatalytic conversion of CO2, conversion of acetophenone to phenylethanol, hydrogenation of nitro compounds, oxidation of cyclohexane, synthesis of vanillin and phenol, as well as hydrogen production and water treatment are discussed. Some applications of these photocatalysts in photocatalytic membrane reactors (PMRs) for carrying out organic synthesis, conversion and/or degradation of organic pollutants are reported. The described cases show that PMRs represent a promising green technology that could shift on applications of industrial interest using visible light (from Sun) active photocatalysts.

• Steroid hormone (SH) removal in a TiO 2 coated PES membrane was quantified • Organic matter can assume role of enhancer or inhibitor to SH degradation • As a photosensitizer, humic acid at 5 mgC/L enhanced SH removal from 47 to 72% • Superoxide radical was identified as a predominant species for SH degradation • Tannic acid inhibited the SH removal by screening light and scavenging reactions Water treatment in photocatalytic membrane reactors (PMR) holds great promise for removing micropollutants from aquatic environments. Organic matter (OM) that is present in any water matrix may significantly interfere with the degradation of steroid hormone (SH) micropollutants in PMRs. In this study, the interference of various OM types, humic acid (HA), Australian natural organic matter (AUS), worm farm extract (WF), tannic acid (TA), and gallic acid (GA) with the SH degradation at its environmentally relevant concentration (100 ng/L) in a flow-through PMR equipped with a polyethersulphone-titanium dioxide (PES-TiO 2) membrane operated under UV light (365 nm) was investigated. Results of this study showed that OM effects are complex and depend on OM type and concentration. The removal of β-estradiol (E2) was enhanced by HA at its levels below 5 mgC/L while the enhancement was abated at higher HA concentrations. The E2 removal was inhibited by TA, and GA, while no significant interference observed for AUS, and WF. The data demonstrated diverse roles of OM that acts in PMRs as a light screening agent, a photoreactive species scavenger, an adsorption alteration trigger, and a photosensitizer. The time-resolved fluorescence measurement showed that HA, acting as a photosensitizer, promoted the sensitization of TiO 2 by absorbing light energy and transferring energy/electron to the TiO 2 substrate. This pathway dominated the mechanism of the enhanced E2 degradation by HA. The favorable effect of HA was augmented as increasing the light intensity from 0.5 to 10 mW/cm2 and was weakened at higher light intensities due to the increased scavenging reactions and the limited amount of HA. This work clarifies the underlying mechanism of the OM interference on photocatalytic degradation of E2 by the PES-TiO 2 PMR. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Three flat-sheet immobilized photocatalytic membrane reactors (PMRs) based on Bi 2 WO 6 -g-C 3 N 4 /PVDF were constructed and compared. • A novel vertical PMR exhibited excellent photocatalytic removal performance. • Anti-fouling property of the PMR was analyzed by filtering bovine serum albumin (BSA), humic acid (HA) and sodium alginate (SA) and XDLVO theory. • PMR possessed superior long-term stability even after 6-cycles in 18 h continuous flow mode. • The atrazine's degradation pathway and its intermediates' toxicity were investigated. The high efficiency, anti-fouling performance and long-term stability of photocatalytic membrane reactors (PMRs) are critical in practical applications. Here, three flat-sheet immobilized PMRs based on Bi 2 WO 6 -g-C 3 N 4 /polyvinylidene fluoride (PVDF) photocatalytic membrane were constructed and compared for atrazine (ATZ) degradation under simulated sunlight. The photocatalytic removal rate of ATZ by the vertical PMRs was remarkable, at 78.98% in 6 h, and had a stable removal rate (about 65%) under continuous flow mode (18 h). Additionally, the reactor presented superior reusability and long-term stability with an ATZ removal rate of 65% even after at least 6 cycles in continuous flow mode. By filtering bovine serum albumin (BSA), humic acid (HA) and sodium alginate (SA) under simulated solar irradiation, it demonstrated good anti-fouling ability, which was further proved by the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. The degradation pathway and evolution of the intermediates were proposed by HPLC-QTOF-MS/MS and DFT calculations. The PMR in this work has great potential in the water treatment industry. [ABSTRACT FROM AUTHOR]

In this study, to overcome the problems of photo corrosion and structural instability, an attempt was made to construct a semiconductor heterojunction with MOF that can resist photo corrosion and reduction to adjust the electron (e−) transport path. The crystal stability and photo corrosion resistance are improved by creating Z-type heterojunctions to change the electron transfer path of Ag photocatalysts. MOFs have unique optical, electrical, and catalytic properties and thus play a key role in optimizing electron transfer pathways. Even if a semiconductor-catalyst heterojunction is constructed, migration-matched catalysts induce a cascade of crystal reduction and photo corrosion in a built-in electric field. At this time, PENS provides an electron acceptor platform, which can temporarily store photoelectrons, prevent the decrease of photoelectrons to the crystal, and improve electron transfer efficiency. The degradation of antibiotics and dyes assessed the resulting impact on photocatalytic efficiency under simulated sunlight, and photocatalytic activity was measured with an eye toward real outdoor applications. Due to the synergistic effect of the double self-cleaning structure, the ultrafiltration membrane exhibits superhydrophilic/underwater superoleophobic properties. It shows excellent separation efficiency and recyclability in the separation of oil-in-water emulsion experiments. [Display omitted] • Z-scheme facilitates the separation and transfer of charge carriers. • Flexibility, temperature resistance and acid and alkali resistance were achieved. • Membrane reactor exhibits efficient photocatalytic mineralization of antibiotics. • Superhydrophilic/underwater superoleophobic to achieve efficient oil-water separation. • Very robust performance over re-use cycles (×8 times) was achieved. This study fabricated PENS/TA/ZIF-67@Ag 2 S (PZA), a double self-cleaning network antifouling polymer membrane reactor, by the non-solvent-induced phase separation (NIPS) method. Due to the synergistic effect of the double self-cleaning structure, it showed an excellent separation efficiency of 99.65% and a high flux of 1198.011 L·m−2·h−1 in the separation of oil-in-water emulsion experiments. After eight cycles, Oil-water separation efficiency remains >99% recyclable. The Z-type heterogeneous conjunctival reactor we constructed has a high-efficiency degradation rate of 99.94% and a mineralization rate of 80.315% for sulfamethoxazole (SMX) within 120 min. From the results of mechanical properties, TGA and DSC, it can be seen that PZA has good strength, toughness and high temperature resistance. According to the experimental analysis of active substances, the active ingredients produced by PZA are •OH., h+, •O 2 –. Based on Density functional theory (DFT) calculations and intermediate analysis, sulfamethoxazole's active sites and degradation pathways (SMX) were explored. According to the Quantitative Structure-Activity Relationship (QSAR) of the Toxicity Estimation Software Tool (TEST), the toxicity of the intermediates produced during the degradation of SMX was predicted. This study provides a feasible strategy for fabricating Z-scheme metal-organic framework (MOF)-based polymer membrane reactors for efficient green antibiotic wastewater treatment and oil-water self-cleaning. [ABSTRACT FROM AUTHOR]

In this study, cobalt-doped MIL-88B(Fe) (x CoMIL-88B(Fe)) was successfully synthesized and grown on an Al 2 O 3 support. The cobalt doping of MIL-88(Fe) not only enhanced its visible-light absorption but also improved free electron–hole pair separation. On adding peroxymonosulfate (PMS), 5CoMIL-88B(Fe) powder demonstrated superior photocatalytic phenol removal than that without the PMS or catalyst. The reactive seeding growth of cobalt-doped MIL-88B(Fe) was employed to prepare a 5CoMIL-88B(Fe) membrane coupled with a filtration module to produce a photocatalytic membrane reactor (PMR). The permeate flux of the photocatalytic membrane reactor (PMR) was approximately 2900–4000 L m−2 h−1 bar−1 (LMH), which demonstrated over 90% phenol removal efficiency and exhibited high stability, reusability, recyclability, and high photocatalytic activity for phenol degradation over 10 cycles utilizing 5CoMIL-88B(Fe)@Al 2 O 3 membrane with PMS. This is the first study to demonstrate a cobalt-doped MIL-88B(Fe) membrane through the reactive seeding method for a PMR system, which is an effective and sustainable process for wastewater treatment. [Display omitted] • 5CoMIL-88B(Fe)@Al 2 O 3 was used for photocatalytic membrane reactors. • 90% phenol removal efficiency were acheived with 6 mM PMS in photocatalytic membrane reactors. • Stable permeance flux (2900–4000 LMH) can be achieved in the PMR system. [ABSTRACT FROM AUTHOR]

The objectives of the current study are to assess and compare the performance of a developed photocatalytic membrane reactor (PMR) in treating industrial waste (e.g., organic dye waste) against membrane distillation. The current PMR is composed of a feed tank, which is a continuous stirred photocatalytic reactor containing slurry Titanium dioxide (TiO2) particles that are activated by using ultraviolet lamp irradiation at a wavelength of 365 nm, and a poly-vinylidene flouride (PVDF) membrane cell. The experimental setup was designed in a flexible way to enable both separate and integrated investigations of the photocatalytic reactor and the membrane, separately and simultaneously. The experimental work was divided into two phases. Firstly, the PVDF membrane was fabricated and characterized to examine its morphology, surface charge, and hydrophobicity by using a scanning electron microscope, surface zeta potential, and contact angle tests, respectively. Secondly, the effects of using different concentrations of the TiO2 photocatalyst and feed (e.g., dye concentration) were examined. It is found that the PMR can achieve almost 100% dye removal and pure permeate is obtained at certain conditions. Additionally, a kinetic analysis was performed and revealed that the photocatalytic degradation of dye follows a pseudo-first-order reaction.

After the dyeing process, part of the dyes used to color textile materials are not fixed into the substrate and are discharged into wastewater as residual dyes. In this study, a heterogeneous photocatalytic process combined with microfiltration has been investigated for the removal of C.I. Disperse Red 73 from synthetic textile effluents. The titanium dioxide (TiO2) Aeroxide P25 was selected as photocatalyst. The photocatalytic treatment achieved between 60% and 90% of dye degradation and up to 98% chemical oxygen demand (COD) removal. The influence of different parameters on photocatalytic degradation was studied: pH, initial photocatalyst loading, and dye concentration. The best conditions for dye degradation were pH 4, an initial dye concentration of 50 mg·L−1, and a TiO2 loading of 2 g·L−1. The photocatalytic membrane treatment provided a high quality permeate, which can be reused.

In this study, a slurry photocatalytic membrane reactor (PMR) was developed and evaluated for the degradation of aqueous phenanthrene (PHE). During continuous process with a hydraulic retention time (HRT) of 140 min, the maximum PHE degradation and total organic carbon (TOC) removal efficiencies were found to be 97% and 79%, respectively. The reuse and recovery potential of TiO2 was studied with continuous recycling. The major intermediates during photodegradation of PHE were found to be phenanthrenequinone, phenanthenol and fluorine. This study also includes an investigation of membrane fouling caused by hydrophilic nano TiO2. The cake layer observed on the membrane surface was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and energy dispersive spectroscopy (EDS). In addition, the effect of operating parameters such as pH and permeate flux on membrane fouling were also investigated. Low permeate flux and alkaline conditions reduced membrane fouling. [ABSTRACT FROM AUTHOR]

Within the Adenoviridae family, human adenovirus 40 (HAdV40) is the most UV resistant serotype. This study explores photocatalytic inactivation of the pathogenic HAdV40 by UV light with TiO 2 catalyst in the presence of dissolved organic matter. Scavenging of reactive oxygen species (ROS) by dissolved organics is significant: 9.6 mg(DOC)/L decreases [OH ] by the factor of ∼17.4 with respect to that in deionized water. Regardless of the pre-filter choice ( d pore  = 0.03 μm, 0.45 μm, or 0.8 μm), the removal of the total HAdV40 by photocatalytic UV depends linearly on UV 254 fluence, Φ 254 , indicating that water quality impacts the DNA damage via fluence attenuation only. In contrast, the removal of infectious HAdV40 depends on the nature of dissolved constituents in the sample; at low Φ 254 , for the same value of fluence photocatalytic UV disinfection is more effective when using the smallest pore size (0.03 μm) pre-filter. The differences in concentration, molecular weight, and type of organic molecules that pass through different prefilters should translate into variations in the extent of quenching. Apparently, filtration through a 0.03 μm membrane produces sufficiently pure permeate for the adenovirus to become an ROS’s target competitive with remaining organic scavengers. Combining a photocatalyst-coated microfiltration membrane ( d pore  = 0.8 μm) with UV light into a photocatalytic membrane reactor removes ∼3.0 log of infectious HAdV40 in the high throughput continuous flow process with the initial specific permeate flux of 3322 ± 266 (L m −2  h −1  bar −1 ). The results point to the potential feasibility of photocatalytic disinfection as a tertiary treatment applied to a relatively high quality effluent typical for membrane filtration plants. [ABSTRACT FROM AUTHOR]

A comparison of effectiveness of two advanced oxidation processes, photocatalysis and ultraviolet (UV) irradiation enhanced with H 2 O 2 addition, coupled with ultrafiltration (UF) for treatment of secondary effluent (SE) from a municipal wastewater treatment plant (WWTP) is presented. The results were compared with photolysis – UF system. The concentration of H 2 O 2 in the UV/H 2 O 2 – UF mode ranged from 0.03 to 0.3 g/L, the photocatalyst TiO 2 P25 loading was changed from 0.5 to 2 g/L. A ceramic membrane with ZrO 2 separation layer was used. No significant influence of photocatalyst or H 2 O 2 concentration on permeate flux was observed, however, both PMR (photocatalytic membrane reactor) and UV/H 2 O 2 – UF systems were less prone to fouling than the photolysis – UF one. Adsorption was found to be an important stage of total organic carbon (TOC) removal in PMR, while UF contributed mainly to photocatalyst rejection with minor effect on the overall treatment efficiency. The highest TOC removal of organic contaminants after 5 h of operation was found at 1.5 gTiO 2 /L in PMR (61%) and 0.15 gH 2 O 2 /L in UV/H 2 O 2 – UF process (65%). The quality of permeate obtained in both systems was comparable, the product of the latter one exhibited lower ecotoxicity towards crustaceans Thamnocephalus platyurus than the PMR permeate. [ABSTRACT FROM AUTHOR]

Photocatalytic membrane reactors have been recently considered as promising reactor types for photodegradation of organic compounds. In this work, a novel reactor concept named fixed-bed photocatalytic membrane reactor (FPMR), which relies on dead-end microfiltration of the catalyst particles, is introduced and studied. A quantitative model for evaluating the influence of mass transfer rate and intrinsic reaction rate on the overall photocatalytic degradation rate is developed and experimentally validated. The results show that the mass transfer rate contributes significantly to the overall reaction rate constant of the FPMR. They further reveal that the overall mass transfer coefficient and overall reaction rate constant of the reactor are greater than 4 s −1 . Those remarkably high rates are comparable to those of new photocatalytic microreactors which are two to three orders of magnitude higher than traditional photocatalytic reactors. Hence, the new reactor concept forms a powerful approach for designing photocatalytic microreactors. [ABSTRACT FROM AUTHOR]

The potential of photocatalytic membrane reactors (PMR) to degrade cytostatic drugs is presented in this work as an emerging technology for wastewater treatment. Cytostatic drugs are pharmaceutical compounds (PhCs) commonly used in cancer treatment. Such compounds and their metabolites, as well as their degraded by-products have genotoxic and mutagenic effects. A major challenge of cytostatic removal stands in the fact that most drugs are delivered to ambulant patients leading to diluted concentration in the municipal waste. Therefore safe strategies should be developed in order to collect and degrade the micro-pollutants using appropriate treatment technologies. Degradation of cytostatic compounds can be achieved with different conventional processes such as chemical oxidation, photolysis or photocatalysis but the treatment performances obtained are lower than the ones observed with slurry PMRs. Therefore the reasons why slurry PMRs may be considered as the next generation technology will be discussed in this work together with the limitations related to the mechanical abrasion of polymeric and ceramic membranes, catalyst suspension and interferences with the water matrix. Furthermore key recommendations are presented in order to develop a renewable energy powered water treatment based on long lifetime materials. [ABSTRACT FROM AUTHOR]

The utilization of membranes in the water industry has been growing rapidly; however, the technology still experiences problems with fouling. A potential solution is to immobilize photocatalyst particles on the surface of the membranes to encourage in situ degradation of the organic contaminants contributing to the fouling. In this study, we developed a photocatalytic membrane (PM) by coating a silicon carbide membrane with Zr/TiO 2 sol. The performance of the PM in degrading different concentrations of humic acid was evaluated comparatively under UV irradiation of two wavelengths, 275 and 365 nm. The results indicated that (i) the PM achieved high levels of humic acid degradation, (ii) the photocatalytic activity of the PM reduced the formation of fouling and hence the loss of permeability, (iii) the formation of fouling was reversible; no trace of fouling was observed after cleaning, and (iv) the PM showed high durability during multiple rounds of operation. [Display omitted] • A photocatalytic membrane (PM) was developed by coating Zr/TiO 2 on a SiC membrane. • The PM showed high removal efficiency (86%) in removing HA representing NOMs. • The PM showed higher long-term permeability compared with the uncoated membrane. • The PM tended to form reversible fouling through photocatalytic reactions. [ABSTRACT FROM AUTHOR]

This paper presents an overview of recent reports on photocatalytic membrane reactors (PMRs) in organic synthesis as well as water and wastewater treatment. A brief introduction to slurry PMRs and the systems equipped with photocatalytic membranes (PMs) is given. The methods of PM production are also presented. Moreover, the process parameters affecting the performance of PMRs are characterized. The applications of PMRs in organic synthesis are discussed, including photocatalytic conversion of CO2, synthesis of KA oil by photocatalytic oxidation, conversion of acetophenone to phenylethanol, synthesis of vanillin and phenol, as well as hydrogen production. Furthermore, the configurations and applications of PMRs for removal of organic contaminants from model solutions, natural water and municipal or industrial wastewater are described. It was concluded that PMRs represent a promising green technology; however, before the application in industry, additional studies are still required. These should be aimed at improvement of process efficiency, mainly by development and application of visible light active photocatalysts and novel membranes resistant to the harsh conditions prevailing in these systems.

Photocatalysis combined with membrane technology could offer an enormous potential for power generation in a renewable and sustainable way. Herein, we describe the one-step hydrogen and oxygen evolution through a photocatalytic membrane reactor. Experimental tests were carried out by means of a two-compartment cell in which a modified Nafion membrane separated the oxygen and hydrogen evolution semi-cells, while iron ions permeating through the membrane acted as a redox mediator. Nanosized Au/TiO2 and Au/CeO2 were employed as suspended photocatalysts for hydrogen and oxygen generation, respectively. The influence of initial Fe3+ ion concentration, ranging from 5 to 20 mM, was investigated, and the best results in terms of hydrogen and oxygen evolution were registered by working with 5 mM Fe3+. The positive effect of gold on the overall water splitting was confirmed by comparing the photocatalytic results obtained with the modified/unmodified titania and ceria. Au-loading played a key role for controlling the photocatalytic activity, and the optimal percentage for hydrogen and oxygen generation was 0.25 wt%. Under irradiation with visible light, hydrogen and oxygen were produced in stoichiometric amounts. The crucial role of the couple Fe3+/Fe2+ and of the membrane on the performance of the overall photocatalytic system was found.

Hybrid processes combining membrane separation and heterogeneous photocatalysis represent an exciting technology because each technique complements the advantages and overcomes the challenges of the other. This combination gives plants set-up named Photocatalytic Membrane Reactors (PMRs). PMRs can be designed in two configurations: reactors in which the catalyst is suspended in the reaction mixture and reactors that utilize the photocatalyst immobilized/deposited on membrane surface, giving a photocatalytic membrane. PMRs allow to perform a variety of chemical reactions (e.g. degradation or synthesis of organic compounds) and separation in one step thus minimizing environmental and economic impacts in agreement with the Green Chemistry principles. Different PMR configurations (pressurized, submerged, with photocatalytic membrane, photocatalytic reactors with membrane distillation, with membrane dialysis, with a pervaporation membrane) have been applied in water treatment for degradation of different organic pollutants (such as dyes, pharmaceuticals and other pollutants) and in the synthesis of organic compounds (such as phenol, vanillin and phenylethanol). The main experimental results of the different applications will be described and critically examined in this review. [ABSTRACT FROM AUTHOR]

The investigations on the influence of humic acids (HAs) being representatives of Natural Organic Matter (NOM) and inorganic salts (NaHCO 3 , Na 2 HPO 4 and Na 2 SO 4 ) representing inorganic ions in natural waters, on fouling and stability of a polyethersulfone ultrafiltration membrane in a photocatalytic membrane reactor (PMR) are reported. The influence of inorganic ions on the permeate flux was dependent on their concentration and was more significant at higher salts content. The HCO 3 − ions contributed to the permeate flux decline to the highest extent, which was attributed to both a higher feed pH and a formation of a dense fouling layer. In the presence of SO 4 2 − the permeate flux was higher than in case of other ions which was explained in terms of lower thickness of the fouling cake caused by repulsion of TiO 2 particles by the membrane under such conditions. The most severe decrease of the permeate flux observed in the presence of a mixture of HAs and inorganic salts was attributed to the formation of a thick fouling cake caused by bridging effect between HAs molecules and TiO 2 particles under high ionic strength conditions. The inorganic salts contributed also to a decrease of HAs decomposition rate due to the hole and hydroxyl radicals scavenging effect of the ions. It was found that the conditions prevailing in the PMR contributed to the loss of the membrane separation properties towards dextrans due to the abrasive action of TiO 2 particles. However, the extent of the observed decrease in the dextrans rejection was dependent on the feed composition. [ABSTRACT FROM AUTHOR]

A comparison of the efficiency of treatment of surface water from Miedwie Lake (Poland) in hybrid systems coupling various advanced oxidation processes (TiO2 photocatalysis, UVC photolysis and UV/ H2 O2 ) and ultrafiltration (UF) is presented. A ceramic membrane with ZrO2 separation layer (Inside Céram, 5,000 g mol–1) was used. Application of photocatalysis contributed to the improvement of the permeate flux compared with photolysis by ca. 16%–35%. Similar effect was observed in case of UV/ H2 O2 process with addition of 0.15 gH2 O2 L–1. Adsorption of organic contaminants on TiO2 influenced significantly on the treatment efficiency in the photocatalytic membrane reactor (PMR). The highest removal rate of total organic carbon was observed in case of 1 g TiO2 L–1. Similar efficiency of mineralization was found in UV/H2 O2 –UF process with application of 0.15 gH2 O2 L–1. The UV/H2 O2 –UF system was found to be a promising alternative for the PMR during treatment of surface water. [ABSTRACT FROM AUTHOR]

The investigations on the influence of TiO 2 photocatalyst (Aeroxide® P25) loading, feed cross-flow velocity (CFV) and feed water composition on the fouling of ceramic TiO 2 ultrafiltration membrane and removal of humic acids (HA) in a photocatalytic membrane reactor (PMR) are presented. Moreover, the stability of the ceramic membrane after 400 h of operation in the PMR is discussed. In case of model solution of HA in ultrapure water it was revealed that by an increase of TiO 2 loading from 0.5 to 1.5 g/dm 3 the membrane fouling can be avoided. Alkaline conditions (pH 9) had a negative influence on the permeate flux and humic acids removal, whereas at pH 3 and pH 6.5 no membrane fouling was noticed. In the presence of HCO 3 − , SO 4 2− and HPO 4 2− at both low and high concentrations the membrane fouling was more severe than in the absence of these species, what was attributed to a lower efficiency of HA removal in the feed. Addition of Ca 2+ and Mg 2+ to the feed containing the inorganic anions allowed to improve the efficiency of HA removal and reduce membrane fouling. After 400 h of PMR operation a deterioration of membrane separation properties was observed what was attributed to the abrasive action of photocatalyst particles. [ABSTRACT FROM AUTHOR]