Modeling the degradation and disinfection of water pollutants by photocatalysts and composites: A critical review

Application of artificial intelligence in (waste)water disinfection: Emphasizing the regulation of disinfection by-products formation and residues prediction

Water/wastewater ((waste)water) disinfection, as a critical process during drinking water or wastewater treatment, can simultaneously inactivate pathogens and remove emerging organic contaminants. Due to fluctuations of (waste)water quantity and quality during the disinfection process, conventional disinfection models cannot handle intricate nonlinear situations and provide immediate responses. Artificial intelligence (AI) techniques, which can capture complex variations and accurately predict/adjust outputs on time, exhibit excellent performance for (waste)water disinfection. In this review, AI application data within the disinfection domain were searched and analyzed using CiteSpace. Then, the application of AI in the (waste)water disinfection process was comprehensively reviewed, and in addition to conventional disinfection processes, novel disinfection processes were also examined. Then, the application of AI in disinfection by-products (DBPs) formation control and disinfection residues prediction was discussed, and unregulated DBPs were also examined. Current studies have suggested that among AI techniques, fuzzy logic-based neuro systems exhibit superior control performance in (waste)water disinfection, while single AI technology is insufficient to support their applications in full-scale (waste)water treatment plants. Thus, attention should be paid to the development of hybrid AI technologies, which can give full play to the characteristics of different AI technologies and achieve a more refined effectiveness. This review provides comprehensive information for an in-depth understanding of AI application in (waste)water disinfection and reducing undesirable risks caused by disinfection processes.

Preparation and Application of Single-Atom Cobalt Catalysts in Organic Synthesis and Environmental Remediation

The development of high-performance catalysts plays a crucial role in facilitating chemical production and reducing environmental contamination. Single-atom catalysts (SACs), a class of catalysts that bridge the gap between homogeneous and heterogeneous catalysis, have garnered increasing attention because of their unique activity, selectivity, and stability in many pivotal reactions. Meanwhile, the scarcity of precious metal SACs calls for the arrival of cost-effective SACs. Cobalt, as a common non-noble metal, possesses tremendous potential in the field of single-atom catalysis. Despite their potential, reviews about single-atom Co catalysts (Co-SACs) are lacking. Accordingly, this review thoroughly summarized various preparation methodologies of Co-SACs, particularly pyrolysis; its application in the specific domain of organic synthesis and environmental remediation is discussed as well. The structure-activity relationship and potential catalytic mechanism of Co-SACs are elucidated through some representative reactions. The imminent challenges and development prospects of Co-SACs are discussed in detail. The findings and insights provided herein can guide further exploration and development in this charming area of catalyst design, leading to the realization of efficient and sustainable catalytic processes.

g-C3N4 modified with non-precious metal Al with LSPR as an efficient visible light catalyst

The issue of water pollution has emerged as a formidable challenge, prompting a pressing need for solutions. The utilization of metal nanoparticles with surface plasmon resonance and semiconductor composite photocatalysts is regarded as a highly effective approach to solve this problem. g-C3N4 is an effective catalyst for the degradation of organic pollutants. Its photocatalytic performance is usually enhanced by the use of the noble metal Au Ag. However, the high cost of these materials limits their application. In this study, we present the synthesis of Al NPs/g-C3N4 nanocomposites using the bridging effect of ligands. The characterized of transmission electron microscopy (TEM), X-ray diffractometer (XRD) and ultraviolet-visible spectroscopy (UV-Vis) proved that Al NPs/g-C3N4 with a wider light absorption range were successfully synthesized. The effects of ligands, (glutathione (GSH), glutamic acid (GAG), and cysteine (CYS)), Al diameter (40 to 200 nm) and the ratio of Al to g-C3N4 (1:1 to 5:1) on the photocatalytic degradation of methylene blue (MB) by Al NPs/g-C3N4 were also evaluated. The results showed that the optimum degradation efficiency of Al NPs/g-C3N4 for MB at 5 mg/L reached 100% within 60 min, which was 11 times higher than that of pure g-C3N4. The principal analysis of Al enhancing the photocatalytic performance of g-C3N4 was studied through transient photocurrent spectroscopy (TPC), electrochemical impedance spectroscopy (EIS), and steady-state transient fluorescence spectroscopy (PL). The results confirmed that hot carriers generated by localized surface plasmon resonance (LSPR) of Al nanoparticles increase the carrier concentration. In addition, the Schottky barrier generated by Al and g-C3N4 could also improve the carrier separation rate and increase the carrier lifetime. This work is expected to solve the problem of organic wastewater treatment and lay the foundation for subsequent research on photocatalysis.

Solar photocatalysis application in UWWTP outlets - simulations based on predictive models in flat-plate reactors and pollutant degradation studies with in silico toxicity assessment

The application of the solar photocatalysis for the degradation of residual pollutants found in surface water was demonstrated. Semi-pilot scale flat-plate cascade reactor (FPCR) was used to study the degradation of model organic pollutants: enrofloxacin (ENRO), 17β-estradiol (E2) and 1H-benzotriazole (1H-BT) over TiO2 thin-film supported on glass fibers. A modular panel with full-spectra solar lamps with appropriate UVB and UVA irradiation levels was used as a simulation of sunlight. Pollutant degradation in FPCR was estimated using predictive models; intrinsic reaction rate constants (ki) for ENRO, E2 and 1H-BT independent of the reactor size, flow rate and irradiation conditions were determined: 9.60, 3.35 and 0.37 109 s-1 W-0.5 m1.5, respectively. Main degradation products (DPs), formed upon hydroxylation, ring opening and oxidation, were identified using LC-QTOF-MS. The ecotoxicological impact was assessed via T.E.S.T. and ECOSAR open-source tools showing the formation of less harmful DPs after sufficient reaction time. Pollutant degradation was simulated at four locations of interest, i.e. exhausts from urban wastewater treatment plants (UWWTPs) in Zagreb, Croatia (45°N), Krakow, Poland (50°N), Sevilla, Spain (37°N) and Ioannina, Greece (39.6°N). Results have proved that a simple flat-plate system with supported photocatalysts can be easily scaled up and incorporated at the outlet of UWWTP for the reduction of pollutant load and related toxicity. The exhaust canal in Zagreb with the estimated length of a photocatalytic layer of 122 m for the > 90% degradation of all target pollutants was discussed as the best installation site among studied locations. ENVIRONMENTAL IMPLICATION: A multi-disciplinary approach to the tentative application of TiO2 solar photocatalysis outdoors to reduce pollutant loads and toxicity in surface waters was demonstrated. Possible application at four selected locations in Europe, as an additional step in water treatment after urban wastewater treatment plants (UWWTPs) was discussed. Target pollutants were studied under environmentally relevant conditions (sunlight levels, water matrix, simulation of process on a real scale at selected geographical location), at both higher and low concentrations.

Optimized photodegradation of palm oil agroindustry waste effluent using multivalent manganese-modified black titanium dioxide

This article presents a methodological approach to use manganese (Mn³⁺Mn⁷⁺)-modified black titanium dioxide (Mn/BTiO₂) as a photocatalyst to optimize and improve visible-light-driven photodegradation of treated agro-industrial effluent (TPOME). A modified wet chemical process was used to prepare BTiO₂. The BTiO₂ was then wet impregnated with Mn and calcined at 300 °C for 1 h to produce Mn/BTiO₂. The activity of Mn/BTiO₂ was investigated in terms of photo-assisted elimination of chemical oxygen demand (COD), phenolic compounds (PCs), color, and total organic carbon (TOC). Using the design of experiments (DOE), the conditions of the photocatalytic process, including photocatalyst loading, Mn concentration, hydrogen peroxide (H₂O₂) dose, and irradiation time, were optimized. Under the optimum conditions (0.85 g/L photocatalyst loading, 0.048 mol/L H₂O₂ dose, 0.301 wt.% Mn concentration, and 204 min irradiation time) COD, PCs, color, and TOC removal efficiencies of 88.87%, 86.04%, 62.8%, and 84.66%, respectively, were obtained. Statistical analysis showed that the response variable's removal from TPOME estimation had high R² and low RMSE, MSE, MAD, MAE, and MAPE values, indicating high reliability. This study demonstrated the significant potential of the developed photocatalytic system for the treatment of waste effluent generated by the palm oil industry and other agro-industries, with the ability to simultaneously reduce a number of organic pollution indicators (OPIs).

Visible Light-Mediated Inactivation of H1N1 Virus UsingPolymer-Based Heterojunction Photocatalyst

It is well known that viruses cannot replicate on their own but only inside the cells of target tissues in the organism, resulting in the destruction of the cells or, in some cases, their transformation into cancer cells. While viruses have relatively low resistance in the environment, their ability to survive longer is based on environmental conditions and the type of substrate on which they are deposited. Recently, the potential for safe and efficient viral inactivation by photocatalysis has garnered increasing attention. In this study, the Phenyl carbon nitride/TiO₂ heterojunction system, a hybrid organic-inorganic photocatalyst, was utilized to investigate its effectiveness in degrading the flu virus (H1N1). The system was activated by a white-LED lamp, and the process was tested on MDCK cells infected with the flu virus. The results of the study demonstrate the hybrid photocatalyst's ability to cause the virus to degrade, highlighting its effectiveness for safe and efficient viral inactivation in the visible light range. Additionally, the study underscores the advantages of using this hybrid photocatalyst over traditional inorganic photocatalysts, which typically only work in the ultraviolet range.

Removing microplastics from aquatic environments: A critical review

As one of the typical emerging contaminants, microplastics exist widely in the environment because of their small size and recalcitrance, which has caused various ecological problems. This paper summarizes current adsorption and removal technologies of microplastics in typical aquatic environments, including natural freshwater, marine, drinking water treatment plants (DWTPs), and wastewater treatment plants (WWTPs), and includes abiotic and biotic degradation technologies as one of the removal technologies. Recently, numerous studies have shown that enrichment technologies have been widely used to remove microplastics in natural freshwater environments, DWTPs, and WWTPs. Efficient removal of microplastics via WWTPs is critical to reduce the release to the natural environment as a key connection point to prevent the transfer of microplastics from society to natural water systems. Photocatalytic technology has outstanding pre-degradation effects on microplastics, and the isolated microbial strains or enriched communities can degrade up to 50% or more of pre-processed microplastics. Thus, more research focusing on microplastic degradation could be carried out by combining physical and chemical pretreatment with subsequent microbial biodegradation. In addition, the current recovery technologies of microplastics are introduced in this review. This is incredibly challenging because of the small size and dispersibility of microplastics, and the related technologies still need further development. This paper will provide theoretical support and advice for preventing and controlling the ecological risks mediated by microplastics in the aquatic environment and share recommendations for future research on the removal and recovery of microplastics in various aquatic environments, including natural aquatic environments, DWTPs, and WWTPs.

Novel Indium Vanadium Oxide Nanosheet-Supported Nickel Iron Oxide Nanoplate Heterostructure for Synergistically Enhanced Photocatalytic Degradation of Tetracycline

Semiconductor-based heterogeneous photocatalytic oxidation processes have received considerable attention for the remediation of toxic pollutants. Herein, InVO4/NiFe2O4 nanocomposites were synthesized using a facile hydrothermal technique. Furthermore, various characterization results revealed the successful loading of NiFe2O4 nanoplates over InVO4 nanosheets, thereby signifying the formation of a heterostructure. The performance of the synthesized photocatalyst was tested for tetracycline (TC) antibiotic removal. The optimized InVO4/NiFe2O4 nanocomposite exhibits maximum photodegradation of TC molecules (96.68%) in 96 min; this is approximately 6.47 and 4.93 times higher than that observed when using NiFe2O4 and InVO4, respectively. The strong interaction between the InVO4 nanosheets and NiFe2O4 nanoplates can improve the visible-light absorption and hinder the recombination of charge carriers, further enhancing the photocatalytic performance. Moreover, hydroxyl radicals play a crucial role in the photodegradation of TC antibiotics.

Inactivation of a pathogenic NDM-1-positive Escherichia coli strain and the resistance gene blaNDM-1 by TiO2/UVA photocatalysis

Proliferation of bla_NDM-1 in water and wastewater is particularly concerning because of multidrug-resistance and horizontal transfer of the gene. In the present study, a pathogenic NDM-1-positive Escherichia coli strain (named E. coli NDM-1) and the bla_NDM-1 gene were treated with titanium dioxide (TiO₂)/ultraviolet A (UVA) photocatalysis. Effects of catalyst dose, UVA intensity, and phosphate on bacteria and intracellular and extracellular bla_NDM-1 genes were determined. With increases in TiO₂ dose and UVA intensity, the inactivation rate of E. coli NDM-1 increased greatly in saline solution. However, phosphate in water hindered adsorption of bacteria to TiO₂ and partly changed the TiO₂ photocatalytic pathway, resulting in low degradation efficiency. Although inactivation of E. coli NDM-1 was highly efficient, TiO₂/UVA photocatalysis had little effect on removal of the bla_NDM-1 gene. During the 2-h photocatalytic experiments, E. coli cells decreased by 4.7-log, while the bla_NDM-1 gene decreased by 0.7- ~ 1.5-log. Moreover, the degradation rate of extracellular bla_NDM-1 was ~2.7 times higher than that of intracellular genes. Abundance and transformation frequency of residual bla_NDM-1 genes remained high, even when bacteria were completely inactivated, indicating potential health risks. Increases in treatment time and UVA irradiation intensity are needed to remove the bla_NDM-1 gene to sufficiently low levels.

Covalent and Non-covalent Functionalized Nanomaterials for Environmental Restoration

Nanotechnology has emerged as an extraordinary and rapidly developing discipline of science. It has remolded the fate of the whole world by providing diverse horizons in different fields. Nanomaterials are appealing because of their incredibly small size and large surface area. Apart from the naturally occurring nanomaterials, synthetic nanomaterials are being prepared on large scales with different sizes and properties. Such nanomaterials are being utilized as an innovative and green approach in multiple fields. To expand the applications and enhance the properties of the nanomaterials, their functionalization and engineering are being performed on a massive scale. The functionalization helps to add to the existing useful properties of the nanomaterials, hence broadening the scope of their utilization. A large class of covalent and non-covalent functionalized nanomaterials (FNMs) including carbons, metal oxides, quantum dots, and composites of these materials with other organic or inorganic materials are being synthesized and used for environmental remediation applications including wastewater treatment. This review summarizes recent advances in the synthesis, reporting techniques, and applications of FNMs in adsorptive and photocatalytic removal of pollutants from wastewater. Future prospects are also examined, along with suggestions for attaining massive benefits in the areas of FNMs.

TiO2/Karaya Composite for Photoinactivation of Bacteria

TiO₂/Karaya composite was synthesized by the sol-gel method for the photoinactivation of pathogens. This is the first time that we have reported this composite for an antimicrobial approach. The structure, morphology, and optical properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-rays (EDS), Fourier transform infrared spectroscopy (FTIR), and diffuse reflectance, and the surface area was characterized by the BET method. The XRD and EDS results showed that the TiO₂/Karaya composite was successfully stabilized by the crystal structure and pore diameter distribution, indicating a composite of mesoporous nature. Furthermore, antibacterial experiments showed that the TiO₂/Karaya composite under light was able to photoinactivate bacteria. Therefore, the composite is a promising candidate for inhibiting the growth of bacteria.

Photocatalytic degradation of (micro)plastics using TiO2-based and other catalysts: Properties, influencing factor, and mechanism

(Micro)plastics pollution has raised global concern because of its potential threat to the biota. The review on recent developments of photocatalytic degradation of (micro)plastics is still insufficient. In this study, we have discussed various bare and composites photocatalysts involved in the photocatalytic degradation of (micro)plastics. The photocatalytic mechanisms and factors affecting the degradation were also discussed. To improve the performance of photocatalysts, their surface is modified with metal or non-metal dopants. These doped photocatalysts are then compounded with a variety of environmentally friendly and nontoxic polymers to prepare multifunctional composites. The generation of reactive oxygen species (ROS) plays an important role in the photocatalytic degradation of (micro)plastics, and superoxide ions (O₂^-) and hydroxyl radicals (OH) participate in the photocatalytic degradation, leading to the breaking of the polymer chain and the production of some intermediates. Although satisfactory progress has been achieved in the photodegradation of (micro)plastics, most photocatalytic degradation technologies investigated to date cannot realize the complete mineralization of (micro)plastics. Furthermore, based on the current challenges of the existing photocatalytic degradation technologies, perspectives for future research directions have been proposed. This review presents a systematic summary of the progress made in the photocatalytic degradation of (micro)plastics and offers a comprehensive reference for future research on improving the (micro)plastics photocatalytic degradation efficiency.

Regrowth of Escherichia coli in environmental waters after chlorine disinfection: shifts in viability and culturability

Bacterial regrowth after water/wastewater disinfection poses severe risks to public health. However, regrowth studies under realistic water conditions that might critically affect bacterial regrowth are scarce. This study aimed to assess for the first time the regrowth of Escherichia coli (E. coli) in terms of its viability and culturability in environmental waters after chlorine disinfection, which is the most widely used disinfection method. Post-chlorination regrowth tests were conducted in 1) standard 0.85% NaCl solution, 2) river water receiving domestic wastewater effluents, and 3) river water that is fully recharged by domestic wastewater effluents. The multiplex detection of plate count and fluorescence-based viability test was adopted to quantify the culturable and viable E. coli to monitor the regrowth process. The results confirmed that chlorine treatment (0.2, 0.5 and 1.0 mg L-1 initial free chlorine) induced more than 99.95% of E. coli to enter a viable but non-culturable (VBNC) state and the reactivation of VBNC E. coli is presumably the major process of the regrowth. A second-order regrowth model well described the temporal shift of the survival ratio of culturable E. coli after the chlorination (R2: 0.73-1.00). The model application also revealed that the increase in initial chlorine concentration and chlorine dose limited the maximum regrowth rate and the maximum survival ratio, and the regrowth rate and percentage also changed with the water type. This study gives a better understanding of the potential regrowth after chlorine disinfection and highlights the need for investigating the detailed relation of the regrowth to environmental conditions such as major components of water matrices.

Silica Particles Derived from Natural Kaolinite for the Removal of Rhodamine B from Polluted Water

This manuscript deals with the thermal and chemical modification of a natural kaolinite that shows excellent performance in the capture of a cationic organic pollutant from the aqueous phase. Kaolinite was calcined at 700 °C and treated with HCl to remove aluminium and to obtain a siliceous material. The structural changes and the physico-chemical properties of the materials at different stages of thermal and chemical modification were investigated with several techniques, including XRPD, MAS-NMR, SEM-EDX, FT-IR, and N2 physisorption at 77 K. The ability of the parent kaolinite and siliceous material to capture the organic dye, Rhodamine B, from the aqueous phase was investigated by means of UV-Vis spectroscopy. The siliceous material exhibited better adsorption capacity with respect to the parent kaolinite. Finally, the functional stability of the siliceous material was tested over three cycles of regeneration and adsorption.

Phyto-mediated photocatalysis: a critical review of in-depth base to reactive radical generation for erythromycin degradation

Erythromycin (ERY), designated as a risk-prioritized macrolide antibiotic on the 2015 European Union watch list, is the third most commonly used antibiotic, most likely due to its ability to inhibit the protein. ERY has revealed record-high aquatic concentrations threatening the entire ecosystem and hence demands priority remedial measures. The inefficiency of various conventional ERY degradation methodologies opened up a gateway to advanced technologies. The conventional approach comprising of a chemically formulated, single photocatalyst has a major drawback of creating multiple environmental stresses. In this context, photocatalysis is grabbing tremendous attention as an efficient and cost-effective antibiotic treatment approach. Several studies have ascertained that ZnO, TiO₂, Fe₃O₄, and rGO nanoparticles possess remarkable pollution minimizing operational capabilities. Additionally, composites are found much more effective in antibiotic removal than single nanoparticles. In this review, an attempt has been made to provide a comprehensive baseline for efficient reactive radical production by a phyto-mediated composite kept under a certain source of irradiation. Considerable efforts have been directed towards the in-depth investigation of rGO-embedded, phyto-mediated ZnO/TiO₂/Fe₃O₄ photocatalyst fabrication for efficient ERY degradation, undergoing green photocatalysis. This detailed review provides photocatalytic nanocomposite individualities along with a hypothetical ERY degradation mechanism. It is assumed that derived information presented here will provoke innovative ideas for water purification incorporating green photocatalysis, initiating the construction of high-performance biogenic hierarchical nanocatalysts.

Review on inactivation of airborne viruses using non-thermal plasma technologies: from MS2 to coronavirus

Although several non-thermal plasmas (NTPs) technologies have been widely investigated in air treatment, very few studies have focused on the inactivation mechanism of viruses by NTPs. Due to its efficiency and environmental compatibility, non-thermal plasma could be considered a promising virus-inactivation technology. Plasma is a partly or fully ionized gas including some species (i.e., electrons, free radicals, ions, and neutral molecules) to oxidize pollutants or inactivate harmful organisms. Non-thermal plasmas are made using less energy and have an active electron at a much higher temperature than bulk gas molecules. This review describes NTPs for virus inactivation in indoor air. The different application processes of plasma for microorganism inactivation at both laboratory and pilot-scale was also reviewed This paper reports on recent advances in this exciting area of viral inactivation identifying applications and mechanisms of inactivation, and summarizing the results of the latest experiments in the literature. Moreover, special attention was paid to the mechanism of virus inactivation. Finally, the paper suggests research directions in the field of airborne virus inactivation using non-thermal plasma.

Optimization of electrochemical activation of persulfate by BDD electrodes for rapid removal of sulfamethazine

Boron-doped diamond electrodes have been employed for the removal of sulfamethazine (SMZ) from water by electrochemical activation of persulfate (EO/BDD-PS). A set of experiments with a central composite design (CCD) was conducted to optimize the operating parameters such as persulfate dose, solution pH, and current density by response surface methodology (RSM). The experimental results indicated a rapid degradation of SMZ even at high initial concentrations. For instance, complete degradation of 50 mg L^-1 of SMZ was attained after 15 min at the optimum operating conditions (persulfate loading = 0.40 g L^-1, pH = 4, and current density = 21 mA cm^-2). The oxidation mechanism of EO/BDD-PS process was studied based on the reactive oxidant species (ROS) revealing that both (OH) and contributed to the degradation of SMZ in the EO/BDD-PS system. Furthermore, the oxidation pathway has been proposed by the suspect screening and tandem mass spectrometry analysis. The performance of EO/BDD-PS showed faster SMZ degradation than electro-Fenton and anodic oxidation processes using the same BDD electrochemical reactor under the same conditions. Furthermore, we provided a cost estimation study revealing that a full-scale application of the EO/BDD-PS system for the treatment of similar contaminated water costs about $2.23 m^-3.

Exfoliation of 2D materials by saponin in water: Aerogel adsorption / photodegradation organic dye

The biggest challenge for the paint industry is to clean the contaminated waste dye solution before it released into the water or to reuse it to create new paint and to protect the water from environmental pollution. Here in this work, exfoliating layered transition metal dichalcogenide materials prepare to the exfoliated 2D materials thin sheets in water with the assistance of natural saponin. Then, the three-dimensional (3D) MoS₂-aerogel composite was synthesized by using greenway exfoliated two-dimensional (2D) MoS₂ thin sheets to form MoS₂-aerogel composite. The prepared 3D MoS₂-aerogel composite demonstrates excellent 94% methylene blue (MB) dye adsorption ability over 5 min. Moreover, the adsorbed MB of the MoS₂-aerogel shows ∼80% dye degradation activity in the presence of visible light. Therefore, these synthesized 3D MoS₂-aerogel composite could be an excellent candidate for photocatalytic applications in the future.

Central-collapsed structure of CoFeAl layered double hydroxides and its photocatalytic performance

Layered double hydroxides (LDHs) has been regarded as one of the most potential photocatalysts for degradation of the pollutants, due to the tunable elements in the laminates, high surface area and exposed active sites. Developing a photocatalyst with a visible light activity and fast charge separation efficiency is a main research focus. In this work, a central-collapsed CoFeAl-LDHs was formed via the selective etching Al³⁺ in the laminates, which relied on the function of OH^- produced by urea hydrolysis. The Central-collapsed structure of CoFeAl-LDHs exhibited enhanced adsorption activity and photocatalytic efficiency. The results show that the pseudo-second-order kinetic model and the Langmuir model are suitable for adsorption behavior. This etching cavity is beneficial to the adsorption of MB and provides a better platform for the direct interaction between MB and CoFeAl-LDHs. The morphology and photoelectrochemical properties of the central-collapsed structure of LDHs were characterized and used to explore the relationship between the etching degree and photocatalytic activity. The photocatalytic properties of all the samples under visible light irradiation were evaluated, and LDH-6 has the best photocatalytic activity. This work provides a novel approach for the fabrication of central-collapsed structure of layered double hydroxides photocatalysts to meet environmental and energy requirements.

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.

Role of nanocatalyst in the treatment of organochlorine compounds - A review

Since a few centuries ago, organochlorine compounds (OCs) become one of the threatened contaminants in the world. Due to the lipophilic and hydrophobic properties, OCs always discover in fat or lipid layers through bioaccumulation and biomagnification. The OCs are able to retain in soil, sediment and water for long time as it is volatile, OCs will evaporate from soil and condense in water easily and frequently, which pollute the shelter of aquatic life and it affects the function of organs and damage system in human body. Photocatalysis that employs the usage of semiconductor nanophotocatalyst and solar energy can be the possible alternative for current conventional water remediation technologies. With the benefits of utilizing renewable energy, no production of harmful by-products and easy operation, degradation of organic pollutants in rural water bodies can be established. Besides, nanophotocatalyst that is synthesized with nanotechnology outnumbered conventional catalyst with larger surface area to volume ratio, thus higher photocatalytic activity is observed. In contrast, disadvantages particularly no residual effect in water distribution network, requirement of post-treatment and easily affected by various factors accompanied with photocatalysis method cannot be ignored. These various factors constrained the photocatalytic efficiency via nanocatalysts which causes the full capacity of solar photocatalysis has yet to be put into practice. Therefore, further modifications and research are still required in nanophotocatalysts' synthesis to overcome limitations such as large band gaps and photodecontamination.

Modeling the removal of methylene blue dye using a graphene oxide/TiO2/SiO2 nanocomposite under sunlight irradiation by intelligent system

In this study, a model to improve the degradability of methylene blue (MB) dye using graphene oxide/TiO2/SiO2 nanocomposite under sunlight irradiation is investigated. The effect of operative parameters comprising catalyst concentration, initial dye concentration, and pH on the photocatalytic batch during removal of MB is studied. Fractional factorial design (FFD) and response surface methodology (RSM) are used to design the experiment layout. Graphene oxide (GO)/TiO2/SiO2 nanoparticles are synthesized through sonication and sol–gel methodologies. In the experiments, three levels of catalyst varied in the percentage of TiO2 pointed as (I) TiO2:GO (100%), (II) TiO2:GO:SiO2 (50%), and (III) TiO2:GO:SiO2 (25%) are used. The irradiation interval was 7 h at solar radiation energy 6.35–5.00 kW h/m2/day. In the experiments, three levels of catalyst varied in the percentage of TiO2 pointed as (I) TiO2:GO (100%), (II) TiO2:GO:SiO2 (50%), and (III) TiO2:GO:SiO2 (25%) are used. The synthesized catalysts are characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscope, and X-ray photoelectron spectroscopy. ANOVA under 23 FFD is conducted to evaluate the effect of independent factors depending on the value of F as pH of solution, weight of catalyst, and concentration of MB. The adsorption kinetics, experimental design with FFD, and RSM are investigated in this study. The Surface Adsorption kinetics were statistically analyzed, the model that best described the results of each experiment was determined out of the two evaluated kinetics (pseudo-first order, pseudo-second order), for the three photocatalyst composites I, II, and III with the parameters; weight of the catalyst, pH, and initial MB concentration, also percentage degradation is evaluated. RSM results are consistent with the kinetic model; first, the pH is considered as the most significant parameter affecting the removal of the organic pollutant, and second, catalyst II gives the highest percentage removal efficiency of MB. FFD results are consistent with both models where the effect of the independent factor depending on the value of F was pH of solution > weight of catalyst > initial concentration of MB. The percentage removal was in the range from 30 to 99%.

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.

Metal–organic framework micromotors: perspectives for environmental applications

Metal–organic framework micromotors possessing a self-propulsion system have been proposed as a new generation of advanced materials for various environmental applications.

Review on heterogeneous photocatalytic disinfection of waterborne, airborne, and foodborne viruses: Can we win against pathogenic viruses?

Microbial pathogenic contaminations have world widely represented a serious health hazard to humans. Viruses, as a member of microbial contaminants, seriously threaten human health due to their high environmental resistance, having small sizes, and causing an extensive range of diseases. Therefore, selecting an appropriate technology to remove viral contaminants from the air, water, and foods is of prominent significance. Traditional methods for viral disinfection have not proven to be highly practical and effective because they need high energy resources and operational expenses. In recent years, semiconductor-based photocatalysis has attracted more attention in the field of microorganism inactivation due to its outstanding performance and mild reaction conditions. Therefore, this review primarily concentrates on the recent development in viral inactivation/disinfection by heterogeneous photocatalysts. Moreover, the photocatalytic viral inactivation of waterborne, airborne, and foodborne viruses is discussed. Given the appealing merits of heterogeneous photocatalytic disinfection of viruses, there is no doubt that this technology will be an impressively active research field and a source of comfort and confidence to humans in battling against viruses.

A novel photocatalytic reactor for the extended reuse of W-TiO2 in the degradation of sulfamethazine

In this study, a photocatalytic reactor with a novel engineering design has been used for the extended degradation of sulfamethazine (SMZ). The reactor employed four consecutive stainless-steel plates immobilized by tungsten-dope TiO₂ (W-TiO₂) using polysiloxane. The characterization of W-TiO₂ by X-ray diffraction (XRD), Raman spectroscopy, and energy-dispersive X-ray (EDX) denoted successful doping of tungsten in the lattice of anatase crystals of TiO₂ suggesting a high photocatalytic activity under UV and visible light. A Box-Behnken experimental design was employed for the optimization of the operating parameters such as solution pH, flow rate, and the initial SMZ concentration. The residual SMZ concentration was below the detection limit after 30 min of the photocatalytic reaction under the optimum operating conditions. A highly remarkable degradation of SMZ was observed in five consecutive cycles, which reveals an extended stable photocatalytic activity offered by the reactor design. The transformation products were identified by tandem mass spectrometry, and they were employed to propose the degradation pathway. These results highlight the importance of using the photocatalysts in retained forms and open additional avenues for the practical application of photocatalysis in wastewater treatment.

Unconventional electro-Fenton process operating at a wide pH range with Ni foam cathode and tripolyphosphate electrolyte

We propose an unconventional electro-Fenton (EF) system with a nickel-foam (Ni-F) cathode and tripolyphosphate (3-PP) electrolyte at near-neutral pH (EF/Ni-F-3-PP) to overcome pH restrictions in EF while preventing Ni-F corrosion. Response surface modelling was used to optimize the main operating parameters with a model prediction analysis (R² = 0.99): pH = 5.8, Fe²⁺ = 3.0 mM and applied current = 349.6 mA. Among the three variables, the pH exerted the highest influence on the process. Under optimal conditions, 100 % of phenol removal was achieved in 25 min with a pseudo-first-order apparent rate constant (k_app) of 0.2 min^-1, 3.2-fold higher than the k_app of EF/Ni-F with SO₄^2- electrolyte at pH 3. A mineralization yield of 81.5 % was attained after 2 h; furthermore, it was found that 3-PP enhanced H₂O₂ accumulation by preventing bulk H₂O₂ decomposition. Finally, toxicity evaluation revealed the formation of toxic by-products at the early stages of treatment, which were totally depleted after 2 h, demonstrating the detoxifying capacity of the system. In conclusion, this study shows for the first time the potential of Ni-F as a cathode for EF under near-neutral conditions, rendered possible by the 3PP electrolyte. Under these conditions, the Ni-F corrosion issue could be alleviated.

A Sharp Jump in Photocatalytic Activity of Elemental Sulfur for Dye Degradation in Alkaline Solution

Elemental sulfur is a low-cost and abundant substance as one of the largest by-products of the oil industry which was widely used in many industrial activities. Cyclo-octasulfur (S₈ ) is one of the sulfur allotropes that is a very stable substance in standard conditions. In this study, we report a low-cost and fast method for the degradation of methyl violet in water under visible light and also sunlight by using elemental sulfur (S₈ ). The results show that sulfur is a good photocatalyst which operates under visible light and can be utilized for degradation of methyl violet. The photocatalytic degradation of methyl violet in acidic, neutral, and alkaline media was investigated, and it was found that the photocatalytic efficiency increases dramatically in alkaline solution. The effects of the initial concentration of the dye, photocatalyst dosage, solution pH, and photocatalyst reusability were investigated. The kinetics of the reaction were studied in detail, and the photocatalytic rate equation was presented.

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.

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

Natural organic matter (NOM) can inhibit the photocatalytic degradation of organic micropollutants (OMPs) through inner filter effect, reactive oxygen species (ROS) scavenging, and competitive adsorption. However, previous studies have focused solely on the bulk properties of NOM and our understanding of the inhibition mechanism by NOM fractions during photocatalytic degradation of OMP is still fragmentary. In this study, five well-characterized different NOM samples (i.e., secondary treated wastewater, river water, and three standard NOM surrogates) were used to elucidate the inhibition mechanisms during photocatalytic degradation of carbamazepine (a model OMP) using TiO₂ and its composites with carbon nanotubes (CNT-TiO₂) under UVC and solar-light irradiation. The results indicated that terrestrially derived NOM with high aromaticity, a low oxygen/carbon atom ratio, and large molecular weight is the major fraction that participates in ROS scavenging, competitive adsorption, and inner filter effect. Furthermore, the modeling analysis suggested that inner filter effect due to NOM and ROS scavenging was the most influential inhibitory mechanism. In the case of secondary treated wastewater, the presence of high concentrations of inorganic species (e.g., PO₄^3-, Cl^-, and NO₃^-) together with NOM significantly reduced the photocatalytic degradation of carbamazepine. Overall, the methods and the results of this study provide a comprehensive understanding of the effects of NOM fractions on photocatalysis and highlight the need to further consider the interplay between NOM and background inorganic constituents in photocatalytic degradation of OMP.