Photocatalytic Oxidation Process Research Articles

Sequential adsorption - photocatalytic oxidation process for wastewater treatment using a composite material TiO2/activated carbon

A composite material was tested to eliminate phenol in aqueous solution combining adsorption on activated carbon and photocatalysis with TiO2 in two different ways. A first implementation involved a sequential process with a loop reactor. The aim was to reuse this material as adsorbent several times with in situ photocatalytic regeneration. This process alternated a step of adsorption in the dark and a step of photocatalytic oxidation under UV irradiation with or without H2O2. Without H2O2, the composite material was poorly regenerated due to the accumulation of phenol and intermediates in the solution and on TiO2 particles. In presence of H2O2, the regeneration of the composite material was clearly enhanced. After five consecutive adsorption runs, the amount of eliminated phenol was twice the maximum adsorption capacity. The phenol degradation could be described by a pseudo first-order kinetic model where constants were much higher with H2O2 (about tenfold) due to additional OH radicals. The second implementation was in a continuous process as with a fixed bed reactor where adsorption and photocatalysis occurred simultaneously. The results were promising as a steady state was reached indicating stabilized behavior for both adsorption and photocatalysis.

Fabrication of α-Fe2O3/In2O3 composite hollow microspheres: A novel hybrid photocatalyst for toluene degradation under visible light

The α-Fe2O3/In2O3 composite hollow microspheres were first synthesized through a well-designed two-step hydrothermal approach with an aim to promote the photocatalytic activity of the pure In2O3. The morphologies, phase structures, and optical properties of the resultant samples were systematically characterized by field-emission scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, UV–vis diffuse-reflectance, and photoluminescence spectroscopy. The α-Fe2O3 nanoparticles acted as visible-light sensitizer, which were well-decorated on the surface of the In2O3 hollow microspheres. Meanwhile, the investigation of photocatalytic performance confirmed that the visible-light induced photocatalytic degradation rate of gaseous toluene was improved after the introduction of α-Fe2O3 component, which was about 1.6 times higher than that of pure In2O3 sample under identical conditions. Furthermore, some intermediates formed during the photocatalytic oxidation process were also indentified by in␣situ FTIR spectroscopy. The enhanced photocatalytic performance of the α-Fe2O3/In2O3 composites mainly stemmed from the strong visible-light-harvesting ability and the efficient spatial separation of photo-generated electron–hole pairs.

2D double-layer-tube-shaped structure Bi2S3/ZnS heterojunction with enhanced photocatalytic activities

Bi2S3/ZnS heterojunction with 2D double-layer-tube-shaped structures was prepared by the facile synthesis method. The corresponding relationship was obtained among loaded content to phase, morphology, and optical absorption property of Bi2S3/ZnS composite. The results shown that Bi2S3 loaded could evidently change the crystallinity of ZnS, enhance the optical absorption ability for visible light of ZnS, and improve the morphologies and microstructure of ZnS. The photocatalytic activities of the Bi2S3/ZnS sample were evaluated for the photodegradation of phenol and desulfurization of thiophene under visible light irradiation. The results showed that Bi2S3 loaded greatly improved the photocatalytic activity of ZnS, and the content of loaded Bi2S3 had an impact on the catalytic activity of ZnS. Moreover, the mechanism of enhanced photocatalytic activity was also investigated by analysis of relative band positions of Bi2S3 and ZnS, and photo-generated hole was main active radicals during photocatalytic oxidation process.

Evaluation of a solar/UV annular pilot scale reactor for 24 h continuous photocatalytic oxidation of n-decane

A pilot scale single-pass continuous-flow annular photoreactor (rannulus=21.3mm) was designed and manufactured featuring a compound parabolic collector (CPC) to capture both direct and diffuse solar radiation and, simultaneously, artificial UVA lamps, in order to examine the possibility of working continuously day and night. Cheap, lightweight and easily shaped cellulose acetate materials, which are transparent in the TiO2 activation range, were used as supports, as an alternative to classic porous solid monolithic structures. Such configuration allows that only 1% of the reactor volume is filled with photoactive material.The photocatalytic oxidation (PCO) of n-decane (Cdec, feed=10ppm, Qfeed=2Lmin−1, τ=44s) over cellulose acetate monolithic structures coated with different TiO2-based photocatalytic films (Lcatalytic bed=144cm) was studied under solar and artificial UVA radiation. Gas-phase n-decane conversions up to 100% were attained using P25 or PC500 photocatalytic films under solar irradiances starting from 16WUVm−2 in the morning (sunrise, increasing temperature) and down to 3WUVm−2 in the afternoon (sunset, decreasing temperature). An increase of the UV irradiance at total n-decane conversion promotes the formation of CO2. The excess of photons reaching the photocatalytic bed favours the direct reaction pathway of CO2 generation. The PCO of n-decane enhanced 29% resulting in 100% of conversion using PC500 film instead of P25 film, under artificial UVA radiation (I=29Wm−2). Results indicate that combining both radiation sources, a 24h continuous PCO process towards the removal of n-decane can be accomplished.

Modeling of photodegradation process to remove the higher concentration of environmental pollution

Environmental organic pollutants are mineralized to harmless final-products such H2O and CO2 by photocatalytic advanced oxidation processes (AOPs). In photocatalytic-AOPs, an appropriate concentration of p-Cresol was mixed with certain amount of ZnO in 500 mL deionized water according to an experimental-design. Then the mixture was irradiated by UV-A lamp at different pH for 6 h. At specific time intervals, the sampling was carried out to calculate the efficiency of the photodegradation. Therefore, the photodegradation as a system consists of four input variables such irradiation time, pH, amount of ZnO and p-Cresol’s concentration while the only output was the efficiency. In this work, the system was modeled and optimized by semi-empirical response surface methodology. To obtain the empirical responses, the design was performed in laboratory. Then observed responses were fitted with several well-known models by regression process to suggest a provisional model. The suggested model which was validated by several statistical evidence, predicted the desirable condition with higher efficiency. The predicted condition consisted of irradiation time (280 min), pH (7.9), photocatalyst (1.5g L−1), p-Cresol (95 mg L−1) and efficiency (95%) which confirmed by further experiments. The closed confirmation results has presented the removal (efficiency = 94.7%) of higher p-Cresol concentration (95 mg L−1) at shorter irradiation time in comparison with the normal photodegradation efficiency (97%) which included irradiation time (300 min), pH (7.5), photocatalyst amount (1.5g L−1) and p-Cresol (75 mg L−1). As a conclusion, the modeling which is able to industrial scale up succeeded to remove higher concentration of environmental organic pollutants with ignorable reduction of efficiency.

Iron-based catalysts for photocatalytic ozonation of some emerging pollutants of wastewater

A synthetic secondary effluent containing an aqueous mixture of emerging contaminants (ECs) has been treated by photocatalytic ozonation using Fe3+ or Fe3O4 as catalysts and black light lamps as the radiation source. For comparative purposes, ECs have also been treated by ultraviolet radiation (UVA radiation, black light) and ozonation (pH 3 and 7). With the exception of UVA radiation, O3-based processes lead to the total removal of ECs in the mixture. The time taken to achieve complete degradation depends on the oxidation process applied. Ozonation at pH 3 is the most effective technique. The addition of iron based catalysts results in a slight inhibition of the parent compounds degradation rate. However, a positive effect is experienced when measuring the total organic carbon (TOC) and the chemical oxygen demand (COD) removals. Photocatalytic oxidation in the presence of Fe3+ leads to 81% and 88% of TOC and COD elimination, respectively, compared to only 23% and 29% of TOC and COD removals achieved by single ozonation. The RCT concept has been used to predict the theoretical ECs profiles in the homogeneous photocatalytic oxidation process studied. Treated wastewater effluent was toxic to Daphnia magna when Fe3+ was used in photocatalytic ozonation. In this case, toxicity was likely due to the ferryoxalate formed in the process. Single ozonation significantly reduced the toxicity of the treated wastewater.

Unexpected toxicity to aquatic organisms of some aqueous bisphenol A samples treated by advanced oxidation processes.

In this study, photocatalytic and catalytic wet-air oxidation (CWAO) processes were used to examine removal efficiency of bisphenol A from aqueous samples over several titanate nanotube-based catalysts. Unexpected toxicity of bisphenol A (BPA) samples treated by means of the CWAO process to some tested species was determined. In addition, the CWAO effluent was recycled five- or 10-fold in order to increase the number of interactions between the liquid phase and catalyst. Consequently, the inductively coupled plasma mass spectrometry (ICP-MS) analysis indicated higher concentrations of some toxic metals like chromium, nickel, molybdenum, silver, and zinc in the recycled samples in comparison to both the single-pass sample and the photocatalytically treated solution. The highest toxicity of five- and 10-fold recycled solutions in the CWAO process was observed in water fleas, which could be correlated to high concentrations of chromium, nickel, and silver detected in tested samples. The obtained results clearly demonstrated that aqueous samples treated by means of advanced oxidation processes should always be analyzed using (i) chemical analyses to assess removal of BPA and total organic carbon from treated aqueous samples, as well as (ii) a battery of aquatic organisms from different taxonomic groups to determine possible toxicity.

Experimental study on coupling photocatalytic oxidation process and membrane separation for the reuse of dye wastewater

The coupling of photocatalytic oxidation process and membrane separation has been studied for the reuse of dye wastewater. The operating conditions of the reactor such as irradiation time, solution pH, and work pressure were investigated. Water quality and extent of fouling were measured, respectively, in terms of removal rate of water quality index and membrane flux decline. The results showed that the highest activity for congo red degradation was reached under pH 4 in about 90min of irradiation time. In membrane separation, when pH 10, the decline of the UF membrane flux was slower. However, the decline of the RO membrane flux was slower when pH 4. Taking into accounts of the power savings and efficiency, 0.5MPa was used as the work pressure of RO. Photocatalysis as a pre-treatment can reduce the membrane flux decline of UF and RO membrane by about 12% and 8%, respectively. After being treated by photocatalysis, UF and RO, the experimental results of chemical oxygen demand (COD), conductivity, hardness and concentration of congo red in the effluent are about 25mg/L, 88μs/cm, 28mg/L and 0.32mg/L, respectively. All the results exceed the quality standard of reuse water for textile industry.

Graphene/TiO2 based photo-catalysts on nanostructured membranes as a potential active filter media for methanol gas-phase degradation

Photocatalytic oxidation processes (PCO) represent more and more promising technologies for air purification and the development of new solutions in pollution sensing and prevention by using adequate nanostructures with unique properties has gained more interest in the scientific community. The present work is meant to show the production, characterization and the photocatalytic performance of nanostructured membranes based on electrospun polyacrylonirile (PAN) scaffolds and graphene/titania based catalysts. Three different systems of photocatalyst were chosen in order to compare their photocatalytic properties: pristine TiO2, TiO2 plus a few-layers graphene and TiO2/reduced-graphene composite obtained by hydrothermal method from graphene oxide. Results of the photocatalytic performance on methanol gas-phase degradation, revealed a higher reaction rate of the graphene based photocatalysts wherein an effective charge transfer, enhanced by graphene, has been supposed to reduce the charge recombination increasing the photocatalytic activity of TiO2 nanoparticles. Moreover, it has been found that the performance of the nanostructured membranes can be restored by stripping with an inert gas several times and this property makes them a good candidate as active filter media.

자외선 LED를 이용한 자유 시안의 광촉매 산화

This study was initiated to remove free cyanide from wastewater using the process of photocatalytic oxidation. UV lamp has been extensively used as a light source in conventional photocatalytic oxidation, but numerous drawbacks of UV lamp have been raised so far. Thus, this study focused on evaluating the applicability of UV LED as an alternative light source to overcome the drawbacks of UV lamp. Furthermore, the effects of diverse operational parameters on the performance of process were investigated. The results demonstrated the applicability of UV LED as a substitute of UV lamp. Also, the results show that the performance of process was improved by the increase in the number of UV LEDs used. To acquire economic feasibility as well as high efficacy, however, it is required to determine the optimum number of UV LED prior to practical implementation of the process. Among the three types of photocatalysts (anatase, rutile, and Degussa P25) tested, the Degussa P25 showed the greatest performance, and it was proven that the process was not improved as much as the Degussa P25 through simple mixing of anatase and rutile without any pretreatment. In addition, the removal efficiency of free cyanide appeared to be increased with the decrease in the particle size of TiO2 photocatalyst. Besides, the process was enhanced with injection of oxygen which is considered as a major electron acceptor in the photocatalytic oxidation.

Textural and electronic structure engineering of carbon nitride via doping with π-deficient aromatic pyridine ring for improving photocatalytic activity

Molecular doping of conjugated carbon nitride (CN) with π-deficient pyridine ring was applied for the modification of CN photocatalysts. According to the density functional theory (DFT) calculations, the incorporation of π-deficient pyridine ring entities in the conjugated CN matrix can effectively modulate the intrinsic electronic and band structure of CN by relocating its π-electrons. And then, a series of pyridine-doped CN photocatalysts were synthesized via thermal copolymerizing dicyandiamide (DCDA) with 2, 6-diaminopyridine (DPY). Integrating π-deficient pyridine ring into the CN network by modification with DPY does not alter the crystal structures or the core chemical skeleton of CN. A significant alteration in the texture and morphology was also observed for the modified CN samples. Moreover, integrating π-deficient pyridine ring into the conjugated CN network can actually engineer the electronic structure with tunable band-gap and promotes the migration and separation of photo-generating electron–hole pairs, which are in well agreement with the theoretical calculation results. The combined benefits of the molecular doping in terms of electronic, optical, surface and texture properties lead to a significant improvement in the photocatalytic activity for methyl orange (MO) degradation under visible light irradiation. The O2−/OOH radical is the major oxidation species in the photocatalytic oxidation process. The modulated CN incorporating π-deficient aromatic systems possess a higher reduction potential because the extension of optical absorption of CN mostly results from the up-shift of HOMO, which is favorable for the photocatalytic degradation of organic pollutant.

Photodegradation of odorous 2-mercaptobenzoxazole through zinc oxide/hydroxyapatite nanocomposite

In this study, ZnO/HAP nanocomposite with excellent photocatalytic activities was successfully synthesized by sol–gel method and used for degradation of 2-mercaptobenzoxazole (MBO) as model of odorous mercaptan compound in water. To optimize the performance of ZnO/HAP photocatalytic capabilities, ZnO/HAP loading (0.05–0.3 g/L), irradiation time (15–180 min), pH (3–11) and initial concentration of MBO (10–100 ppm) were investigated. At neutral pH of 7, the highest amount of the MBO (99.45 %) was degraded by ZnO/HAP nanocomposite through photocatalytic oxidation process within 2 h of irradiation time. A maximum adsorption capacity of 197.64 mg g−1 was obtained for ZnO/HAP under optimized conditions. BET results indicated that ZnO/HAP had a surface area of 182.36 mg2g−1 which was much greater than pure ZnO nanoparticles (31.2 mg2g−1). TEM image demonstrated a spherical shape structure of ZnO/HAP with average particle size of 25 nm in diameter. The XRD patterns revealed the principal components of ZnO/HAP including HAP and ZnO. FTIR spectrum results supported formation ZnO and HAP by their stretching mode in composite. Comparison of photocatalytic activity of ZnO/HAP with pure ZnO and HAP nanoparticles had clearly recognized that latter is the most active photocatalyst in the degradation of MBO using UV light as source energy. The reason for greater activity of ZnO/HAP was due to its larger specific surface area (182.36 m2g−1) and high generation of active $${\text{HO}}^{\cdot}$$ and $${\text{O}}_{2}^{- 2 \cdot}$$ species.

Enhanced photocatalytic activity of single-phase, nanocomposite and physically mixed TiO2 polymorphs

In this study, testing of TiO2 polymorphs (anatase, rutile, brookite) and their mixtures (anatase/rutile, anatase/TiO2-B) in heterogeneous photocatalytic oxidation process was conducted at ambient conditions in a batch slurry reactor. The efficiency of bare TiO2 catalysts was evaluated based on the degree of bisphenol A (BPA) removal, which is a well-known endocrine disrupting compound (EDC). The obtained results indisputably show that BPA removal is strongly affected by catalyst morphology, crystallite size, structure and specific surface area. Detailed interpretation of catalyst properties combined with BPA removal rates leads to the conclusion that photocatalytic oxidation is the most prominent either by using pure anatase particles or high surface area anatase/TiO2-B nanocomposite. However, the highest extent of mineralization was observed in the presence of high specific surface area nanotubular anatase/TiO2-B nanocomposite. Interestingly, when anatase and rutile particles were physically mixed, an additional beneficial effect on BPA degradation was observed. Interpretation of the obtained results shows that a synergistic effect between the respective phases takes place, and consequently enhances the overall activity. This phenomenon was explained by the proposed mechanism of overall hydroxyl radicals concentration increment due to transfer of OH formed on the surface of anatase particles (via H2O oxidation with photogenerated holes in the valence band) to rutile particles.

Sequential‐based process modelling of VOCs photodegradation in fluidized beds

AbstractSequential modular simulation (SMS), as a new modelling approach, was developed to simulate the photocatalytic oxidation (PCO) of gaseous pollutants in fluidized bed reactors. In the experimental part, the PCO of gaseous acetone was studied in a fluidized bed photo‐reactor (FBPR) and the influence of operating conditions (inlet concentration, relative humidity (RH) and superficial gas velocity) on both acetone conversion and mineralization was investigated. It was found that the RH, as a key factor in PCO reactions, had a contradictory effect on the conversion and mineralization of acetone. In the modelling part, the bed was divided into several sections in which the bubble and emulsion phases were considered as a plug flow and a completely mixed flow reactor, respectively. Dynamic two‐phase model was adopted as the hydrodynamic sub‐model and the Langmuir–Hinshelwood (LH) mechanism as the kinetic sub‐model. Kinetic constants of the latter sub‐model were estimated using experimental data from the literature. A new dimensionless number (HA number) was introduced to determine the optimum number of sections, as the most important factor in model predictions. The performance of the proposed model was compared with the experimental data obtained in this study and several sets of experimental data from the literature. The results showed that the simple and easy‐to‐achieve approach, which has the capability of integrating into the industrial process simulators such as Aspen Plus© and Aspen HYSYS©, can be used to simulate the behaviour of non‐ideal FBPRs in PCO processes.

Photocatalytic oxidation of aqueous ammonia using atomic single layer graphitic-C3N4.

Direct utilization of solar energy for photocatalytic removal of ammonia from water is a topic of strong interest. However, most of the photocatalysts with effective performance are solely metal-based semiconductors. Here, we report for the first time that a new type of atomic single layer graphitic-C(3)N(4) (SL g-C(3)N(4)), a metal-free photocatalyst, has an excellent photocatalytic activity for total ammonia nitrogen (TAN) removal from water. The results demonstrated that over 80% of TAN (initial concentration 1.50 mg · L(-1)) could be removed in 6 h under Xe lamp irradiation (195 mW · cm(-2)). Furthermore, the SL g-C(3)N(4) exhibited a higher photocatalytic activity in alkaline solution than that in neutral or acidic solutions. The investigation suggested that both photogenerated holes and hydroxyl radicals were involved the TAN photocatalytic oxidation process and that the major oxidation product was NO3(-)-N. In addition, SL g-C(3)N(4) exhibited good photocatalytic stability in aqueous solution. This work highlights the appealing application of an inexpensive metal-free photocatalyst in aqueous ammonia treatment.

High adsorption and efficient visible-light-photodegradation for cationic Rhodamine B with microspheric BiOI photocatalyst

BiOI microspheres were synthesized using an EG-assisted one-pot solvothermal method. The morphology, structure, surface area, pore-size distribution and surface electrical property of the BiOI photocatalyst were characterized using XRD, SEM, TEM, BET and zeta potential measurements. The results showed that the microspheric structure consisting of numerous nanosheets manifested a large BET area 59.3 m2 g−1 with a mesoporous structure of 14.1 nm. Besides, its surfaces were negatively charged in general. Adsorption kinetics and isotherm analysis indicated that the adsorption of cationic Rhodamine B (RhB) onto BiOI followed the pseudo-second-order model, and the Freundlich equation sufficiently described the sorption isotherm. As the pH increased, the equilibrium adsorption concentration exhibited a downward trend. The adsorption mechanism was tentatively proposed using FTIR and XPS spectral analysis. The three main functional mechanisms involved in the adsorption process were aperture adsorption, electrostatic interaction and chemical coordination effect. BiOI exhibited high photodegradation efficiency for highly concentrated RhB solution and adsorption enhanced photocatalytic efficiency by improving the utilization of reactive species in the photocatalytic oxidation process.

Effects of pH and temperature on photocatalytic activity of PbTiO3 synthesized by hydrothermal method

PbTiO3 photocatalyst was synthesized successfully by facile hydrothermal method. The effects of the hydrothermal reaction temperatures and the pH values of the systems on the photocatalytic activities of PbTiO3 were investigated in detail. The photocatalytic activities of samples were evaluated by the degradation of methyl orange (MO) aqueous solution under simulated solar irradiation. The as-obtained PbTiO3 sample exhibits anisotropical growth along the (0 0 1) plane, and its photocatalytic activity is about 3 times higher than that of PbTiO3 prepared by precipitation method. Moreover, the as-prepared PbTiO3 has high stability during photocatalytic oxidation process, and does not cause secondary pollution.

In-situ anion exchange synthesis of AgBr/Ag2CO3 hybrids with enhanced visible light photocatalytic activity and improved stability

AgBr/Ag2CO3 hybrids were synthesized via an in situ anion-exchange reaction between Ag2CO3 and NaBr. The obtained hybrids were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), UV–vis diffuse reflectance spectroscopy (DRS) and terepthalic acid-photo luminescence (TA-PL) technique. The as prepared AgBr/Ag2CO3 hybrids exhibited wide absorption in the visible light region and displayed efficient and higher photocatalytic activities towards the degradation of dye molecules (Ponceau BS) as compared to pure AgBr and Ag2CO3 samples under visible light irradiation (λ>400nm). The enhanced photocatalytic activity of AgBr/Ag2CO3 was related to the efficient separation of electron–hole pairs derived from matching band potentials between AgBr and Ag2CO3, as well as the good electron trapping role of Ag° nanoparticles in situ formed on the surface of AgBr and Ag2CO3 particles during photocatalytic oxidation process. The quenching effect of different scavengers of reactive species suggested that OH and h+ play major role in the degradation of PBS. The enhanced stability of the hybrid was attributed to the trapping of photogenerated electrons from the surface of catalyst by Ag° nanoparticles which suppress the photocorrosion.

The Combination Sewage Treatment Process of Nanofiltration and Photocatalytic Oxidation for the Removal of Endocrine Disruptors

The potential hazard effects of Endocrine Disruptors (EDCs) are drawing growing attention from the public,and the traditional sewage water treat process works in an unstable way in the removal of EDCs and can be influenced by many factors.The membrane technology shows good interception efficiency of EDCs,and the photocatalytic oxidation,with its wide application range of different types of polluted water,can thoroughly convert EDCs to inorganic materials.The combination process of nanofiltration and photocatalytic oxidation can easily extract EDCs from the water at first step,and decompose the EDCs in the concentrated polluted water and the back fluch water collected at the second step.The combination process is a reliable,practical and effective methods for EDCs removal.

UV/TiO₂ photocatalytic oxidation of recalcitrant organic matter: effect of salinity and pH.

Photocatalytic oxidation processes have interest for water treatment since these processes can remove recalcitrant organic compounds and operate at mild conditions of temperature and pressure. However, performance under saline conditions present in many water resources is not well known. This study aims to explore the basic effects of photocatalysis on the removal of organic matter in the presence of salt. A laboratory-scale photocatalytic reactor system, employing ultraviolet (UV)/titanium dioxide (TiO₂) photocatalysis was evaluated for its ability to remove the humic acid (HA) from saline water. The particle size and zeta potential of TiO₂ under different conditions including solution pH and sodium chloride (NaCl) concentrations were characterized. The overall degradation of organics over the NaCl concentration range of 500-2,000 mg/L was found to be 80% of the non-saline equivalent after 180 min of the treatment. The results demonstrated that the adsorption of HA onto the TiO₂ particles was dependent on both the pH and salinity due to electrostatic interaction and highly unstable agglomerated dispersion. This result supports UV/TiO₂ as a viable means to remove organic compounds, but the presence of salt in waters to be treated will influence the performance of the photocatalytic oxidation process.