Enhanced ozonation of polystyrene nanoplastics in water with CeOx@MnOx catalyst.

The nanoplastics released into the environment pose a critical threat to creatures, and thus it is necessary to remove them. However, their natural decomposition usually takes years or even decades, which raises an imminent demand for an efficient removal technology. Herein we report a core-shell CeOx@MnOx catalyst for enhancing ozonation of polystyrene nanoplastics in water. Ozonation achieves 31.67% molecular weight removal of polystyrene nanoplastics in the first 10 min reaction, which is increased to 51.67% in catalytic ozonation by MnOx and further improved to 73.33% in catalytic ozonation via CeOx@MnOx. The remarkable thing is the CeOx@MnOx could achieve almost 96.70% molecular weight removal after 50 min reaction. The specific catalytic mechanism is ozone decomposes into reactive oxygen radicals (•OH, •O2- and 1O2) after capturing electrons from MnOx, improving the polystyrene nanoplastics removal. Meanwhile, the Mn averaged valence state increases, making it harder to donate electrons to ozone. This can be alleviated by encapsulating the CeOx core in the MnOx, enabling electrons replenishment from the CeOx core to the MnOx shell, which is verified by the experiment and density functional theory calculations. The repeated experiment demonstrates the CeOx@MnOx possesses excellent stability, maintaining 95.25-96.70% removal efficiency of polystyrene nanoplastics. This research provides a possibility for the efficient removal of nanoplastics in water.

Separation and identification of nanoplastics in tap water.

Microplastics pollution in freshwater has attracted global attentions, but when microplastics are broken into nanoplastics, they may present higher toxicity mainly due to their greater potential to cross biological membranes. So far almost no work has been done on the separation and identification of nanoplastics in tap water. Herein we removed large particles from tap water by 0.45 µm filter and then sequentially screened nanoparticles in filtrate by Anopore with pore size of 200, 100, and 20 nm, the most frequent particle sizes of which concentrate at 255 nm, 148 nm, and 58 nm, respectively. Based on characterization of FTIR, AFM-IR and Pyr-GC/MS, the polymers were identified to be polyolefins, polystyrene, polyvinyl chloride, polyamide, and some plastic additives. The abundance of nanoplastics with the most frequent particle sizes in range of 58-255 nm was 1.67-2.08 µg/L in tap water. This work provides a feasible method for separation and identification of nanoplastics in tap water, and manifests the existence of nanoplastics, which poses a potential threat to the health of residents.

α-Calcium Sulfate Hemihydrate Nanorods Synthesis: A Method for Nanoparticle Preparation by Mesocrystallization.

The past decades have witnessed great advances in nanotechnology since tremendous efforts have been devoted for the design, synthesis, and application of nanoparticles. However, for most mineral materials such as calcium sulfate, it is still a challenge to prepare their nanoparticles, especially with uniform size and high monodispersity. In this work, we report a route to regulate the morphology and structure of α-calcium sulfate hemihydrate (α-HH) and successfully synthesize and stabilize its mesocrystals for the first time. The ellipsoidal mesocrystals in length of 300-500 nm are composed by α-HH nanoparticles arranged in the same crystallographic fashion and interspaced with EDTA. The time-dependent experiments indicate the α-HH aggregates evolve from irregular structure to mesocrystal structure with the subsequent growth of subunits and then partially fuse into single crystals. Disorganizing the mesocrystal structure before the emergence of fusion reaps α-HH nanorods in a length of 30-80 nm and a width of 10-20 nm with high monodispersion. This ingenious concept paves an alternative way for nanoparticle preparation and is readily extended to other inorganic systems.

Immobilizing Water into Crystal Lattice of Calcium Sulfate for its Separation from Water-in-Oil Emulsion.

This work report a facile approach to efficiently separate surfactant-stabilized water (droplet diameter of around 2.0 µm) from water-in-oil emulsion via converting liquid water into solid crystal water followed by removal with centrifugation. The liquid-solid conversion is achieved through the solid-to-solid phase transition of calcium sulfate hemihydrate (CaSO4. 0.5H2O, HH) to dihydrate (CaSO4·2H2O, DH), which could immobilize the water into crystal lattice of DH. For emulsion of 10 mg mL(-1) water, the immobilization-separation process using polycrystalline HH nanoellipsoids could remove 95.87 wt % water at room temperature. The separation efficiency can be further improved to 99.85 wt % by optimizing the HH dosage, temperature, HH size and crystalline structure. Property examination of the recycled oil confirms that our method has neglectable side-effect on oil quality. The byproduct DH was recycled to alpha-HH (a valuable cemetitious material widely used in construction and binding field), which minimizes the risk of secondary pollution and promotes the practicality of our method. With the high separation efficiency, the "green" feature and the recyclability of DH byproduct, the HH-based immobilization-separation approach is highly promising in purifying oil with undesired water contamination.

Controlled synthesis of monodisperse α-calcium sulfate hemihydrate nanoellipsoids with a porous structure.

We report a facile and green chemical solution approach to synthesize monodisperse α-calcium sulfate hemihydrate (α-HH) nanoellipsoids with a length of 600 nm and a width of 300 nm by simply mixing Ca(2+) and SO4(2-) glycerol-water precursor solutions in the presence of Na2EDTA. The α-HH nanoellipsoid is formed through a Na2EDTA-mediated self-assembly of small primary building blocks (α-HH domains: ∼14 nm). The study on the morphological evolution of α-HH reveals that the controlled synergy of supersaturation (precursor concentration) and Na2EDTA is crucial for the development of α-HH into nanoellipsoids. Further thermal annealing of the nanoellipsoid could make the α-HH domains transit into calcium sulfate anhydrites and grow up, generating the gaps between them and resulting in a porous structure. This work paves a new way for preparing high-quality α-HH nanoellipsoids with a monodisperse nanosize and a porous structure, promising their future application in many fields such as biomedicine.

Switching reactive oxygen species reactions derived from Mn-Pt anchored zeolite for selective catalytic ozonation.

Rationally switching reactive oxygen species (ROS) reactions in advanced oxidation processes (AOPs) is urgently needed to improve the adaptability and efficiency for the engineering application. Herein we synthesized bimetallic Mn-Pt catalysts based on zeolite to realize the switching of ROS reactions in catalytic ozonation for sustainable degradation of organic pollutants from water. The ROS reactions switched from singlet oxygen (1O2, 71.01%) to radical-dominated (93.79%) pathway by simply introducing defects and changing Pt/Mn ratios. The oxygen vacancy induced by anchoring Mn-Pt species from zeolite external surface (MnPt/H-Beta) to internal framework (MnPt@Si-Beta) exposes more electron-rich Pt2+/Pt4+ redox sites, accelerating the decomposition of O3 to generate •OH via electron transfer and switching ROS reactions. The Mn site acted as a bridge plays a critical role in conducting electrons from organic pollutants to Pt sites, which solidly solves the electron loss of catalysts, facilitating the efficient degradation of pollutants. A 34.7-fold increase in phenol degradation compared with the non-catalytic ozonation and an excellent catalytic stability are achieved by MnPt@Si-Beta/O3. The 1O2-dominated ROS reaction originated from MnPt/H-Beta/O3 exhibits superior performances in anti-interference for Cl-, HCO3-, NO3-, and SO4-. This work establishes a novel strategy for switching ROS reactions to expand the targeted applications of O3 based AOPs for environmental remediation.

Identification and quantification of nanoplastics (20-1000 nm) in a drinking water treatment plant using AFM-IR and Pyr-GC/MS.

Nanoplastics, owing to their small particle size, pose a significant threat to creatures, deserving heightened attention. Numerous studies have investigated microplastics pollution and their removal efficiency in drinking water treatment plants, none of which have involved nanoplastics due to lacking a suitable analytical method. This study introduced a feasible method of combing AFM-IR and Pyr-GC/MS to identify and quantify nanoplastics (20-1000 nm) for a preliminary understanding of their fate during drinking water treatment processes. Resolving of chemical functional groups and pyrolysis products from AFM-IR and Pyr-GC/MS data demonstrated the presence of PE and PVC nanoplastics in this drinking water treatment plant. The initial influent abundances of PE and PVC nanoplastics were 0.86 µg/L and 137.31 µg/L, with subsequent increase to 4.49 µg/L and 208.64 µg/L in ozonation contact tank unit. Then a gradual decreasing was observed along water process, achieving 98.4% removal of PE nanoplastics and 44.0% removal of PVC nanoplastics, respectively. Although this drinking water treatment plant has exhibited a certain level of nanoplastics removal efficiency, particular attention should be directed to the oxidation unit, which appears to be a significant source of nanoplastics. This study will lay a foundation for revealing nanoplastics pollution in the environment.

Assessment of microorganisms in drinking water disinfected by catalytic ozonation with fluorinated ceramic honeycomb and NaClO disinfectants under laboratory and pilot conditions.

While sodium hypochlorite (NaClO) has long been used to disinfect drinking water, concerns have risen over its use due to causing potentially hazardous byproducts. Catalytic ozonation with metal-free catalysts has attracted increasing attention to eliminate the risk of secondary pollution of byproducts in water treatment. Here, we compared the disinfection efficiency and microbial community of catalytic ozone with a type of metal-free catalyst fluorinated ceramic honeycomb (FCH) and NaClO disinfectants under laboratory- and pilot-scale conditions. Under laboratory conditions, the disinfection rate of catalytic ozonation was 3∼6-fold that of ozone when the concentration of Escherichia coli was 1 × 106 CFU/ml, and all E. coli were killed within 15 s. However, 0.65 mg/L NaClO retained E. coli after 30 min using the traditional culturable approach. The microorganism inactivation results of raw reservoir water disinfected by catalytic ozonation and ozonation within 15 s were incomparable based on the cultural method. In pilot-scale testing, catalytic ozonation inactivated all environmental bacteria within 4 min, while 0.65 mg/L NaClO could not achieve this success. Both catalytic ozonation and NaClO-disinfected methods significantly reduced the number of microorganisms but did not change the relative abundances of different species, i.e., bacteria, viruses, eukaryotes, and archaea, based on metagenomic analyses. The abundance of virulence factors (VFs) and antimicrobial resistance genes (ARGs) was detected few in catalytic ozonation, as determined by metagenomic sequencing. Some VFs or ARGs, such as virulence gene 'FAS-II' which was hosted by Mycobacterium_tuberculosis, were detected solely by the NaClO-disinfected method. The enriched genes and pathways of cataO3-disinfected methods exhibited an opposite trend, especially in human disease, compared with NaClO disinfection. These results indicated that the disinfection effect of catalytic ozone is superior to NaClO, this finding contributed to the large-scale application of catalytic ozonation with FCH in practical water treatment.

Mn anchored zeolite molecular nest for enhanced catalytic ozonation of cephalexin.

The molecular nest structured catalysts have demonstrated better performance than the traditional supported catalysts. However, they have not been tried in antibiotics or other organic pollutants removal from water by advanced oxidation processes (AOPs). Here we synthesized Mn anchored zeolite molecular nest (Mn@ZN) for the catalytic ozonation of cephalexin (CLX), which is the widely used antibiotic and also a refractory pollutant in water. The ozonation catalyzed by Mn@ZN achieves 97% of CLX degradation in only 2 min and a reaction rate constant of 0.2454 L·mg-1·s-1, which is 79.2 times higher than that of the non-catalytic ozonation. Even after ten cycles, the 0.46Mn@ZN/O3 still achieves a CLX degradation efficiency higher than 88% in 2 min, presenting an excellent stability. Mn ions stabilized by the molecular nests facilitate Lewis acid sites and oxygen vacancies, providing active sites for O3 sorption and decomposition into ·O2- and 1O2 through electrons transfer for the radical reaction with CLX. DFT calculation indicates that both the oxygen vacancy formation energy and the O3 adsorption energy of Mn@ZN are reduced by the Mn species introduction. This study finds a fascinating catalyst of Mn@ZN for the catalytic ozonation of antibiotics, and also a smart design strategy for zeolite confined metals catalysts for water treatment.

Toxicity of polystyrene nanoplastics to human embryonic kidney cells and human normal liver cells: Effect of particle size and Pb2+ enrichment.

Nanoplastics pollution in drinking water has aroused wide concern, but their effects on human health are still poorly understood. Herein we explore the responses of human embryonic kidney 293T cells and human normal liver LO2 cells to polystyrene nanoplastics, mainly focusing on the effects of particle sizes and enrichment of Pb2+. When the exposed particle size is higher than 100 nm, there is no obvious death for these two different cell lines. As the particle size decreases from 100 nm, cell mortality goes up. Although the internalization of polystyrene nanoplastics in LO2 cells is at least 5 times higher than that in 293T cells, the mortality of LO2 cells is lower than that of 293T cells, illustrating that LO2 cells are more resistant to polystyrene nanoplastics than 293T cells. Additionally, the Pb2+ enrichment on polystyrene nanoplastics in water can further enhance their toxicity, which should be taken seriously. The cytotoxicity of polystyrene nanoplastics to cell lines works through a molecular mechanism involving oxidative stress-induced damage of mitochondria and cell membranes, resulting in a decrease in ATP production and an increase in membrane permeability. Referenced to nanoplastics pollution in drinking water, there is no necessary to panic about the adverse effects of plastic itself on human health, but the enrichment of contaminants should get more attention. This work provides a reference for the risk assessment of nanoplastics in drinking water to human health.

Control of α-calcium sulfate hemihydrate morphology using reverse microemulsions.

Alpha calcium sulfate hemihydrate (α-HH) is an important class of cementitious material and exhibits considerable morphology-dependent properties. In the reverse microemulsions of water/n-hexanol/cetyltrimethylammonium bromide (CTAB)/sodium dodecyl sulfonate (SDS), the morphology and aspect ratio of α-HH are successfully controlled by adjusting the mass ratio of CTAB/H(2)O and the concentration of SDS. As the ratio of CTAB/H(2)O is increased from 1.3 to 4.5, the crystal length decreases from 120 to 150 µm to 0.5-1.2 µm with the corresponding aspect ratio reduced sharply from 180 to 250 to 2-7. With increasing SDS concentration, the crystal morphology gradually changes from submicrometer-sized long column to rod, hexagonal plate, and even nanogranule. The preferential adsorption of CTAB on the side facets and SDS on the top facets contributes to the morphology control. This work presents a simple, versatile, highly efficient approach to controlling the morphology of α-HH on a large scale and will offer more opportunities for α-HH multiple applications.

Synergistic catalysis of ozonation and photooxidation by sandwich structured MnO2-NH2/GO/p-C3N4 on cephalexin degradation.

Antibiotics such as cephalexin (CLX) are often detected in water and sewage, and advanced oxidation processes (AOPs) are usually the most effective method to degrade them. Currently, the synergy of AOPs has raised lively interest in water and wastewater treatment. Here the sandwiched catalyst of MnO2-NH2/GO/p-C3N4 (MN/GO/CN) is synthesized, in which graphene oxide (GO) acts as "core layer" connecting aminated MnO2 (MnO2-NH2) for catalytic ozonation and proton-functionalized g-C3N4 (p-C3N4) for photocatalysis. The MN/GO/CN combines the AOPs of catalytic ozonation and photocatalysis, initiates hydroxyl radicals 4.2 times the sum of catalytic ozonation and photocatalysis, and achieves the first order kinetics constant of 2.4 × 10-2/s, which is 2.7, 8.1 and 20.1 times that of catalytic ozonation, photo ozonation, and photocatalysis, respectively, and consequently reduces CLX from 1.0 mg/L to below the detection limit within 2.5 min, demonstrating the strong synergism between the AOPs. The sandwich structure enables GO to mediate the electron transfer between p-C3N4 and MnO2-NH2, which not only hinders electron-hole recombination on p-C3N4, but also speeds redox electron cycle on MnO2 to promote the catalytic activity. The simultaneous catalytic ozonation and photocatalysis by sandwiched bifunctional catalyst to obtain synergistic effect will find its broad prospect in water and wastewater treatment.

Catalytic ozonation of aqueous 4-methylquinoline by fluorinated ceramic honeycomb.

Metal-free catalysts for catalytic ozonation have attracted more and more attentions to eliminate the risk of secondary pollution of heavy metals in water or wastewater treatment. Herein we prepared fluorinated ceramic honeycomb (FCH) with the dip-calcination method using NH4F as the modifier over ceramic honeycomb (CH) to catalyze the ozonation of 4-methylquinoline (4-Meq), a typical harmful quinoline derivate discharged from coal or petroleum industries. The ozonation degraded 54.9% of 4-Meq and removed 14.4% of chemical oxygen demand (COD) in 30 min, while the FCH catalytic ozonation degraded 77.8% of 4-Meq and removed 29.2% of COD. In addition, FCH has a stable catalytic performance and can effectively remove 4-Meq as well as COD in real coal gasification wastewater. The fluorination endows the surface of the FCH with abundant Si-F groups as active acid sites and aluminum-attached hydroxyl groups, and then enhance the ozone decomposition to generate free reactive oxygen species (ROS). Those ROS includes free hydroxyl radicals, free superoxide radicals as well as singlet oxygen, and the free hydroxyl radical plays a major role in the degradation and COD removal of 4-Meq. The degradation of 4-Meq follows two pathways of the demethylation, benzene ring opening and the pyridine ring-opening. This work demonstrates an efficient catalyst for ozonation to root out the risk of the heavy metals pollution from catalysts, and provides an insightful understanding of the FCH catalytic ozonation.

Distribution behavior of arsenate into α-calcium sulfate hemihydrate transformed from gypsum in solution.

Transforming gypsum into α-calcium sulfate hemihydrate (α-HH) provides a promising utilization pathway for the abundant amount of chemical gypsum. The transformation follows the route of "dissolution-recrystallization", during which the arsenic pollutant in gypsum is released into the solution, and hence raises the possibility of being distributed into the product of α-HH, a potential harm that has always been neglected. Investigation of the transformation process at neutral pH revealed that the arsenate ions in solution were distributed into α-HH and generated an enrichment of arsenic by 4-6 times. Arsenate ions distributed into α-HH by substitution for lattice sulfate, adsorption on α-HH facets and occupation for surface sulfate sites. While at higher concentrations, calcium arsenate coprecipitated with α-HH or even crystallized independently. Increasing temperature accelerated the phase transformation and restrained arsenate migration into α-HH due to the lag of distribution balance. The pH of solution modulated the dominant arsenate species and decreasing pH weakened arsenate substitution capacity for sulfate in α-HH. This work uncovers arsenate distribution mechanism during the transformation of gypsum into α-HH and provides a feasible method to restrain arsenate distribution into product, which helps to understand arsenate behavior in hydrothermal solution with high concentration of sulfate minerals and provides a guidance for controlling pollutants distribution into product.

The fabrication and characterization of novel carbon doped TiO2 nanotubes, nanowires and nanorods with high visible light photocatalytic activity.

Novel carbon doped TiO(2) nanotubes, nanowires and nanorods were fabricated by utilizing the nanoconfinement of hollow titanate nanotubes (TNTs). The fabrication process included adsorption of ethanol molecules in the inner space of TNTs and thermal treatment of the complex in inert N(2) atmosphere. The structural morphology of carbon doped TiO(2) nanostructures can be tuned using the calcination temperature. X-ray diffraction, Raman and Brunauer-Emmett-Teller studies proved that the doped carbon promoted the crystallization and phase transition by acting as nucleation seeds. X-ray photoelectron spectroscopy (XPS) showed that O-Ti-C and Ti-O-C bonds were formed in the nanostructures. Additional electronic states from the XPS valence band due to carbon doping were observed. This evidence indicated the electronic origin of the band gap narrowing and visible light absorption. The differences in chemical and electronic states between the surface and bulk of as-prepared samples confirmed that carbon was doped into the lattice of TiO(2) nanostructure through an inner doping process. The as-prepared catalysts exhibited enhanced photocatalytic activity for degradation of toluene in gas phase under both visible and simulated solar light irradiation compared with that of commercial Degussa P25. This novel fabrication approach can valuably contribute to designing nanostructured photocatalytic materials and modifying various nanotube materials.

Near-Infrared-Triggered Release of Ca2+ Ions for Potential Application in Combination Cancer Therapy.

Calcium ion (Ca2+ ), an abundant species in the body, is a potential therapeutic ion with manageable side effects. However, the delivery of such a highly charged species represents a great challenge. Here, a nanosystem based on Au nanocages (AuNCs) and a phase-change material (PCM) for delivering calcium chloride (CaCl2 ) into cancer cells and thereby triggering cell death upon near-infrared (NIR) irradiation is demonstrated. In the absence of NIR irradiation, the nanosystem, denoted CaCl2 -PCM-AuNC, shows negligible cytotoxicity because the Ca2+ ions are fully encapsulated in a solid matrix. Upon NIR irradiation, the Ca2+ ions are swiftly released due to the melting of PCM matrix in response to photothermal heating. The sudden increase in intracellular Ca2+ causes disruption to the mitochondrial Ca2+ homeostasis and thus the loss of mitochondrial membrane potential, subsequently resulting in cell apoptosis. This nanosystem provides a new method for cancer treatment by tightly managing the intracellular concentration of a physiologically essential element.

The role of surface hydroxyl concentration on calcinated alumina in catalytic ozonation.

Alumina has been used as a catalyst for ozonation, surface hydroxyl on which is regarded as the active center for ozone attack, but the influences of hydroxyl generation are still vague. Here, we prepared alumina with different hydroxyl concentrations by adjusting calcination temperatures, of which the catalytic activity was evaluated with the mineralization degree of phenol, and then revealed the active sites of hydroxyl generation with characterization of XRD, Py-IR, and NH3-TPD. The results show that the greater the hydroxyl concentration, the higher the catalytic activity, demonstrating that surface hydroxyl contributes to its catalytic activity. The effect of calcination temperatures on hydroxyl concentration and catalytic activity is in accordance with the amount of weak Lewis acid sites on the surface of alumina, illustrating the surface hydroxyl derived from the decomposition of water adsorbed on weak Lewis acid sites. However, the catalytic performance of the alumina decreases slowly in a long-term reaction owing to the active center reduction resulted from the coverage by organic acids from phenol degradation. The present work reveals the influences of hydroxyl generation which are beneficial for adjusting surface hydroxyl regarded as active site for ozone attack and the reason of catalyst deactivation, which provides guideline for the rational design of catalyst.

Application of chitosan as flocculant for coprecipitation of Mn(II) and suspended solids from dual-alkali FGD regenerating process.

Heavy metals and suspended solid (SS) needed to be removed from the recirculation of dual-alkali flue gas desulfurization (FGD) system. The feasibility of coprecipitation of heavy metal and SS by water-soluble chitosan was studied in a lab scale experiment. The association between chitosan and metal ions was verified through DSC and FT-IR. The pH investigation revealed that at the pH ranged from 5 to 9, there were three stages for different actions: adsorption of chitosan for Mn(II), precipitation of manganese hydroxide and coprecipitation of manganese hydroxide and chitosan-Mn(II) complex. The ion selectivity experiments showed that the occurrence of Ca(II) in the solution had little influence on the adsorption of chitosan for Mn(II). The decrease rate of adsorption capacity was about 0.0023 mmol g(-1) per 1 mg L(-1) Ca(II). When adsorption and flocculation of chitosan occurred at the same time and at the sufficient addition of chitosan, chitosan not only made solids flocculate but also enhanced sorption capacity of chitosan. Application of chitosan for coprecipitation of Mn(II) and SS could remove Mn(II) efficiently and improve the settling characteristics of SS from dual-alkali FGD regenerating process.

Continuous processing of phase-change materials into uniform nanoparticles for near-infrared-triggered drug release.

We report a method based on interfacial, anti-solvent-induced precipitation in a fluidic device for the continuous and scalable processing of phase-change materials (PCMs) into uniform nanoparticles with controlled diameters in the range of 10-100 nm. A eutectic mixture of lauric acid and stearic acid, with a well-defined melting point at 39 °C, serves as an example to demonstrate the concept. In the fluidic device, a coaxial flow is created by introducing a PCM solution in ethanol and a lipid solution in water (the anti-solvent) as the focused and focusing phases, respectively. The formation of lipid-capped PCM nanoparticles is governed by diffusion-controlled mixing of ethanol and water. During the production, both doxorubicin (DOX, an anticancer drug) and indocyanine green (ICG, a near-infrared dye) can be readily loaded into the PCM nanoparticles to give a smart drug release system. Upon irradiation with near-infrared light, the photothermal heating caused by ICG can melt the PCM and thereby trigger the release of DOX. This work not only provides a new technique for the continuous processing of PCMs and other soft materials into uniform nanoparticles with controlled sizes but also demonstrates a biocompatible system for controlled release and related applications.

Photocatalytic oxidation of nitrogen oxides using TiO2 loading on woven glass fabric.

TiO2 loading on woven glass fabric is applied to treat nitrogen oxides (NOx) by photocatalytic oxidation (PCO). In this paper, the PCO behavior of NO at high concentrations was studied by PCO of NOx at source levels (20-168 ppm). The PCO efficiency reached 27% in this experiment, while the inlet NOx concentration was 168 ppm (147 ppm NO). The dependency of the reaction rate on several key influencing factors (relative humidity, space time, inlet concentration, oxygen percentage) was also studied. The results illustrate that the resulting hydroxyl radical and active oxide play an important role in the oxidation of NOx. The reactions are limited by the thermodynamic equilibrium after ca. 15s space time. A possible explanation for the catalyst deactivation is the accumulation of nitric acid and nitrous acid on the TiO2 surface during the PCO of NOx. However, the photocatalytic activity can be recovered with a simple heat treatment. The results from the study of the effect of the inlet concentration were described with the Langmuir-Hinshelwood model.