Heterogeneous TiO2 Photocatalysis Research Articles

Heterogeneous photocatalytic degradation of ibuprofen in ultrapure water, municipal and pharmaceutical industry wastewaters using a TiO2/UV-LED system

Degradation and mineralization of ibuprofen (IBU) were investigated using Ultraviolet (UV) Light Emitting Diodes (LEDs) in TiO2 photocatalysis. Samples of ultrapure water (UP) and a secondary treated effluent of a municipal wastewater treatment plant (WWTP), both spiked with IBU, as well as a highly concentrated IBU (230 mg L−1) pharmaceutical industry wastewater (PIWW), were tested in the TiO2/UV-LED system. Three operating parameters, namely, pH, catalyst load and number of LEDs were optimized. The process efficiency was evaluated in terms of IBU removal using high performance liquid chromatography (HPLC) and ultra-high performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS). Additionally, the mineralization was investigated by determining the dissolved organic carbon (DOC) content. The chemical structures of transformation products were proposed based on the data obtained using liquid chromatography with a high resolution mass spectrometer ion trap/time-of-flight (LC-MS-IT-TOF). A possible pathway of IBU degradation was accordingly suggested. Bioassays were performed using the marine bacterium Vibrio fischeri to evaluate the potential acute toxicity of original and treated wastewaters. TiO2 heterogeneous photocatalysis was efficient to remove IBU from UP and from PIWW, and less efficient in treating the wastewater from the municipal WWTP. The acute toxicity decreased by ca. 40% after treatment, regardless of the studied matrix.

Pharmaceutical residues in aquatic environment and water remediation by TiO2 heterogeneous photocatalysis: a review

The occurrence of pharmaceutical compounds in the natural water sources has been reported as early as in the year 1980. Until now, the presence of pharmaceutical compounds in the aquatic environment has been frequently reported in the literature. Moreover, increasing evidence suggests that these contaminants have posed a threat to both humans and ecosystems. In this regard, the present review paper seeks to offer an overview of this environmental issue of pharmaceutical pollution where the subject matters to be reviewed include the effects, sources and mitigation strategies of pharmaceuticals in the aquatic environment. Besides, a review of the fundamentals and mechanisms of heterogeneous photocatalysis technology is also presented in this paper. Heterogeneous photocatalysis is a rapidly expanding technology which has been extensively investigated and applied in wastewater treatment for the remediation of persistent pollutants such as pharmaceutical compounds during the last decade. Furthermore, the ideal photocatalyst titanium dioxide (TiO2), which can collaborate and perform well in the photocatalysis treatment process, is also discussed. The advantages and limitations associated with the application of this treatment method are summarized and discussed in details. Finally, this review paper focuses on the future trend of the photocatalysis technology and identifies the barriers and lacking parts which need to be resolved in the near future.

Contaminants of emerging concern: a review of new approach in AOP technologies.

The presence of contaminants of emerging concern (CECs) such as pharmaceuticals and personal care products (PPCPs), endocrine-disrupting compounds (EDCs), flame retardants (FRs), pesticides, and artificial sweeteners (ASWs) in the aquatic environments remains a major challenge to the environment and human health. In this review, the classification and occurrence of emerging contaminants in aquatic environments were discussed in detail. It is well documented that CECs are susceptible to poor removal during the conventional wastewater treatment plants, which introduce them back to the environment ranging from nanogram per liter (e.g., carbamazepine) up to milligram per liter (e.g., acesulfame) concentration level. Meanwhile, a deep insight into the application of advanced oxidation processes (AOPs) on mitigation of the CECs from aquatic environment was presented. In this regard, the utilization of various treatment technologies based on AOPs including ozonation, Fenton processes, sonochemical, and TiO2 heterogeneous photocatalysis was reviewed. Additionally, some innovations (e.g., visible light heterogeneous photocatalysis, electro-Fenton) concerning the AOPs and the combined utilization of AOPs (e.g., sono-Fenton) were documented.

TiO2 Photocatalytic Cyclization Reactions for the Syntheses of Aryltetralones

This work focuses on a new strategy to overcome the overoxidation in heterogeneous TiO2 photocatalysis and to realize high-efficiency photosynthesis. We demonstrate that TiO2 photocatalysis can integrate C–C and C═O formation in a tandem manner to achieve efficient oxidative cyclization for the syntheses of aryltetralones. This protocol does not need any additive besides the inexpensive and/or recyclable TiO2, O2, and (solar) light. High yields with excellent diastereoselectivities are obtained for a wide scope of electron-rich substrates. Our findings demonstrate that in contrast to the conventional overoxidation, as long as the radical cations possess sufficient reactivity toward nucleophilic addition, single-electron transfer processes in TiO2 photocatalysis can be developed into a powerful tool to construct C–C bonds and even strained carbon rings.

Intensification of heterogeneous TiO2 photocatalysis using an innovative micro-meso-structured-photoreactor for n-decane oxidation at gas phase

The main goal of this work is to overcome barriers in heterogeneous TiO2 photocatalysis application towards indoor air decontamination by process intensification. The photocatalytic oxidation (PCO) of gas-phase n-decane was studied using a micro-meso-structured-photoreactor irradiated by simulated solar light, consisting of chambers and channels mechanically engraved in an acrylic slab. The network of chambers and channels is sealed with a borosilicate slab, allowing a good light penetration through the entire reactor depth. A sheet of cellulose acetate (CA) coated, in one side, with TiO2-P25 was assembled between the two slabs, allowing high spatial illumination homogeneity over the catalyst surface. The P25 thin film was in contact with the air stream flowing in the network. The reactor contains 1.95g of TiO2 in contact with the air stream per liter of reactor volume and the illuminated catalyst surface area, in contact with the gas phase, per reactor volume was calculated to be 349m2m−3, which is ∼1.4 times higher than that obtained for an annular photoreactor packed with CA monolithic structures coated with TiO2-P25. The PCO reaction rate, rdec, was assessed under different experimental conditions: amount of TiO2 supported in CA sheets, n-decane feed concentration, feed flow rate, relative humidity and UV irradiance. The highest rdec value (0.82μmolmin−1) was achieved when the following conditions were employed: mTiO2=75mg, Cdec,feed=1.31×10−2molm−3, Qfeed=220cm3min−1 and I=38.4Wm−2. The maximum reactivity of photocatalyst in combination with the photoreactor was 6.93×10−4molm−3reactor s−1, which is a 21.4 fold increase when compared with the annular photoreactor, highlighting the enhancement of mass and photons transfer.

Intensification of heterogeneous TiO2 photocatalysis using an innovative micro–meso-structured-reactor for Cr(VI) reduction under simulated solar light

Process intensification of heterogeneous TiO2 photocatalysis using a new type of static mixer micro–meso-structured-reactor, based on the NETmix reactor, was evaluated to promote the hexavalent chromium reduction to its trivalent state, in the presence of sacrificial agents. The photocatalytic micro–meso-structured reactor comprises a back acrylic slab where chambers and channels are imprinted and a frontal borosilicate slab with especially high UV transparency, irradiated by simulated solar light. An UV transparent cellulose acetate (CA) sheet coated with TiO2-P25 by a simple spray method was assembled between the two slabs of the photoreactor. The reactor geometry provides an efficient exposure of TiO2-P25 CA sheets to radiation, with an illuminated surface per unit of volume inside the reactor of 470m2m−3 and 1.36g TiO2 per liter of liquid inside the reactor. Beyond that, it provides a good degree of mixing using a laminar flow, maximizing heterogeneous and homogeneous Cr(VI) reduction reactions. Firstly, different organic species were tested as hole scavengers in order to avoid the Cr(III) re-oxidation. The effect of the degree of mixing inside the photoreactor, according to the Reynolds number, on the Cr(VI) photocatalytic reduction was then evaluated. Following, operating conditions such as TiO2 mass, tartaric acid concentration, pH value, temperature and initial Cr(VI) concentration were analyzed. Results indicate high reduction rates (1mmolCr(VI)m−3illuminated volumes−1) and a photonic efficiency of 3.96% for Re=830, 30mg TiO2 and a Cr(VI):tartaric acid molar ratio of 1:90 at pH 3.0 and 25°C. Finally, the CA sheets coated with thin films of TiO2-P25 showed to be suitable for continuous Cr(VI) reduction, without losing efficiency during 3 consecutive photocatalytic cycles.

Photocatalytic Disinfection and Removal of Emerging Pollutants from Effluents of Biological Wastewater Treatments, Using a Newly Developed Large-Scale Solar Simulator

The reuse of sewage water treatment is becoming a priority, especially in arid regions, where water is scarce. However, there is increasing concern about the presence of emerging micropollutants refractory to the conventional treatments in wastewater treatment plants. Solar TiO2 heterogeneous photocatalysis provides a valuable alternative for the simultaneous oxidation of chemical and inactivation of microorganisms. However, the variability of solar irradiance hinders the study of the process under controlled ultraviolet (UV) irradiance. This work reports the development of an innovative solar simulator and its validation against the use of natural sunlight in photocatalytic disinfection applications with simultaneous removal of emerging contaminants. The significant impact of osmotic and mechanical stress on the viability of E. coli bacteria was confirmed. UV irradiance and total-to-illuminated volume ratio showed no impact on the dependence of the bacteria inactivation kinetics on the energy accumulated...

Investigation of Mechanism Involved in TiO2 and Photo-Fenton Photocatalytic Degradation of Emerging Contaminant Sucralose in Aqueous Media

Sucralose is an artificial sweetener persistently present in wastewater treatment and aquatic environments. Two advanced oxidation processes (heterogeneous TiO2 and homogeneous Fenton photocatalysis) were employed for degradation of sucralose in aqueous phase. The aqueous samples were irradiated under a variety of experiment conditions (pH, light sources) and with different amount of TiO2, Fe (II) and H2O2. The disappearance of the sucralose followed pseudo-first-order kinetic according to the Langmuir- Hinshelwood model, where the rate constant increased with an increase in the initial sucralose concentration. Sucralose and intermediates were easily degraded and within 30minutes of irradiation complete mineralization was achieved in both advanced oxidation processes. During the oxidation process, the chloride ion was detected and accumulated in solution. Results demonstrated that both heterogeneous TiO2 and homogeneous photo-Fenton photocatalytic treatments are suitable for the elimination of sucralose from the aqueous solution.

ChemInform Abstract: Employing TiO2 Photocatalysis to Deal with Landfill Leachate: Current Status and Development

AbstractReview: 113 refs.

Abatement of toxicity of effluents containing Cr(VI) by heterogeneous photocatalysis. Toxicity assessment by AMPHITOX assay

Toxicity of a Cr(VI) solution before and after treatment by TiO2 heterogeneous photocatalysis (HP) was performed with AMPHITOX bioassay. Changes in toxicity on Rhinella arenarum larvae for 10-d were monitored after exposure to an untreated Cr(VI) solution and to the same solution after HP treatment. The HP treatment of a 41.60mgL−1 Cr(VI) solution reduced to 37.5% the concentration of the metal ion. A 10-fold reduction in toxicity at acute exposure (72h) and 150-fold reduction in toxicity after 240h was found. Further, the LOEC value increased from 0.001% for the untreated solution to 0.153% after HP treatment. Moreover, the safe concentration in untreated solution corresponded to 0.0001% sample, and it was 0.01% after the treatment, i.e., 100 times higher. A saving of water of about 100,000L per L of effluent would be possible through dilution to allow safer concentrations for discharge; the saving would reach the highest value (1,000,000L per L) at 240h. Sub-lethal effects were completely absent in larvae exposed to the treated solution. The AMPHITOX test allowed to detect chronic effects at low Cr concentrations, i.e. at environmentally relevant levels.

Photocatalytic degradation of thimerosal in human vaccine's residues and mercury speciation of degradation by-products

Thimerosal (sodium ethylmercury thiosalicylate) has been intensively used as a stabilizer of pharmaceuticals, mainly in human and veterinary vaccines. Yearly loss of thousands of vaccines occurs due to expiration, loss of cold chain and overstock; therefore safe disposal of expired vaccines containing thimerosal has become high priority. In this work, UVA light assisted TiO2 heterogeneous photocatalysis has been applied for the first time in order to degrade thimerosal contained in hepatitis B vaccine residues (HibTITER). The photocatalytic process was optimized using the Response Surface Methodology (RSM). Most favorable experimental conditions to achieve the maximum degradation of thimerosal were pH2 and 0.2gL−1 TiO2 under nitrogen bubbling. Under these conditions thimerosal contained in vaccine residues was completely degraded within 20min by UV-A assisted photocatalysis. The mercury in thimerosal was reduced to elemental mercury and removed from the solution in vapor form. The organic groups of thimerosal were oxidized to thiosalicylic acid, salicylic acid and dithiosalicylic acid, identified as by-products. The degradation pathway of thimerosal by photocatalysis and photolysis is proposed. This photocatalytic procedure is environmentally friendly and it could be considered as promising treatment approach for hazardous organomercurial compounds.

Heterogeneous photocatalytic removal of U(VI) in the presence of formic acid: U(III) formation

The efficiency of TiO2 heterogeneous photocatalysis under UV–visible light for removal of uranyl (UO22+) (0.25mM, pH 3) in the presence of formic acid (HCOOH) was investigated. The effect of the counterion of the uranium salt (perchlorate and acetate–nitrate) and the use of quartz and glass photoreactors were analyzed. The transformation of U(VI) in the presence of HCOOH was significantly high and the best removal conditions have been established. The highest efficiency was observed for uranyl perchlorate in the quartz photoreactor with 0.001M HCOOH, while uranyl acetate yielded a lower removal. HCOOH concentrations >0.01M provoked uranium reoxidation. U(VI) photocatalytic transformation was also studied by direct spectrophotometry using TiO2 nanoparticles in the presence of 1M HCOOH at pH<2. U(V), U(IV) and U(III) were detected. U(III) formation was also apparent in the absence of the photocatalyst. This is the first time that formation of U(III) in a photochemical U(VI) system is reported.

Mechanisms of removal of heavy metals and arsenic from water by TiO2-heterogeneous photocatalysis

Abstract This article is an overview of recent work performed in our laboratory on TiO2 heterogeneous photocatalysis of aqueous systems containing toxic forms of chromium, arsenic, lead, uranium and mercury. The cases of chromium and arsenic are treated in profundity. Photocatalytic treatments can convert the ionic species into their metallic solid form and deposit them over the semiconductor surface, or transform them into less toxic soluble species. When a transformation to the zerovalent state is possible, this allows the recovery of the metal from the waters, with an important economical return. Three types of mechanisms can be considered, all of them taking place through successive thermodynamically allowed monoelectronic electron transfer steps: (a) direct reduction by photogenerated electrons; (b) indirect reduction by intermediates generated by hole or hydroxyl radical oxidation of electron donors (reducing radicals); (c) oxidative removal by holes or hydroxyl radicals. Fundamentals of oxidative or reductive heterogeneous photocatalysis are described, with special emphasis on the role of organic donors present in the medium acting as synergists. This indicates the possibility of simultaneous treatment of pollutants of different chemical structure and properties present in the same medium.

Solar photo-Fenton using peroxymonosulfate for organic micropollutants removal from domestic wastewater: Comparison with heterogeneous TiO2 photocatalysis

This work aims at decontaminating biologically treated domestic wastewater effluents from organic micropollutants by sulfate radical based (SO4−) homogeneous photo-Fenton involving peroxymonosulfate as an oxidant, ferrous iron (Fe(II)) as a catalyst and simulated solar irradiation as a light source. This oxidative system was evaluated by using several probe compounds belonging to pesticides (bifenthrin, mesotrione and clothianidin) and pharmaceuticals (diclofenac, sulfamethoxazole and carbamazepine) classes and its kinetic efficiency was compared to that to the well known UV–Vis/TiO2 heterogeneous photocatalysis. Except for carbamazepine, apparent kinetic rate constants were always 10 times higher in PMS/Fe(II)/UV–Vis than in TiO2/UV–Vis system and more than 70% of total organic carbon abatement was reached in less than one hour treatment. Hydroxyl radical (OH) and SO4− reactivity was investigated using mesotrione as a probe compound through by-products identification by liquid chromatography-high resolution-mass spectrometry and transformation pathways elucidation. In addition to two OH based transformation pathways, a specific SO4− transformation pathway which first involved degradation through one electron transfer oxidation processes followed by decarboxylation were probably responsible for mesotrione degradation kinetic improvement upon UV–Vis/PMS/Fe(II) system in comparison to UVVis/TiO2 system.

Catalytic Role of Surface Oxygens in TiO2 Photooxidation Reactions: Aqueous Benzene Photooxidation with Ti(18)O2 under Anaerobic Conditions.

The important role played in TiO2 heterogeneous photocatalysis by the two-fold-coordinated TiO2 surface bridging oxygens (>Os2– or >OHs–) has been emphasized. Their ability to trap photogenerated h...

Effects of O2 and H2O2 on TiO2 Photocatalytic Efficiency Quantified by Formaldehyde Formation from Tris(hydroxymethyl)aminomethane

AbstractThe impact of O

Photocatalytic oxidation of treated municipal wastewaters for the removal of phenolic compounds: optimization and modeling using response surface methodology (RSM) and artificial neural networks (ANNs)

AbstractBACKGROUND: TiO2 heterogeneous photocatalysis should be optimized before application for the removal of pollutants in treated wastewaters. The response surface methodology (RSM) and artificial neural networks (ANNs) were applied to model and optimize the photocatalytic degradation of total phenolic (TPh) compounds in real secondary and tertiary treated municipal wastewaters.RESULTS: RSM was developed by considering a central composite design (CCD) with three input variables, i.e. TiO2 mass, initial concentration of TPh and irradiation intensity. At the same time a feed‐forward multilayered perceptron ANN trained using back propagation algorithms was used and compared with RSM. Under the optimum conditions established in experiments ([TPh]0 = 3 mg L−1; [TiO2] = 300 mg L−1; I = 600 W m−2) the degradation for both TPh and total organic carbon (TOC) followed pseudo‐first‐order kinetic model. Complete degradation of TPh took place in 180 min and reduction of TOC reached 80%. A significant abatement of the overall toxicity was accomplished as revealed by Microtox bioassay.CONCLUSIONS: It was found that the variables considered have important effects on TPh removal efficiency. The results demonstrated that the use of experimental design strategy is indispensable for successful investigation and adequate modeling of the process and that ANNs gave better modelling capability than RSM. Copyright © 2012 Society of Chemical Industry

Photolytic and photocatalytic degradation of fluoroquinolones in untreated river water under natural sunlight

The photodegradation of some among the most frequently prescribed fluoroquinolone antibacterials (FQs) was investigated in untreated river water under solar light as well as under the same conditions in the presence of suspended TiO2. The drugs considered included ciprofloxacin (CIP), danofloxacin (DAN), enrofloxacin (ENR), levofloxacin (LEV), marbofloxacin (MAR) and moxifloxacin (MOX), the last two belonging to the most recent FQ generation. The experiments were carried out in lab-scale batch reactor at concentrations (20–50μgL−1) comparable to those actually measured in surface waters, and the course of the reaction was monitored by high pressure liquid chromatography (HPLC) with fluorescence detector (FD). A first order kinetics was obeyed upon both direct photolysis and TiO2 heterogeneous photocatalysis. The photoproduced intermediates were identified by HPLC with electrospray ionization tandem mass spectrometry (ESI-MS/MS) and the degradation paths were identified. It was concluded that direct irradiation caused fluorine substitution and reductive elimination, while photocatalysis caused oxidative degradation of the amine side-chain (most efficient with tertiary amines and five-membered cyclic amines). The latter one was a minor process upon direct photolysis and involved hydrogen abstraction by excited states or photoproduced radicals. Photocatalytic decomposition occurred at a rate from two to five times faster than direct photolysis for all of the drugs, except for CIP, that is roughly proportional to the amine oxidation potential. The kinetic constants ranged from 0.061 to 0.66min−1 in direct photolysis, from 0.22 to 2.78min−1 in the presence of TiO2. In the latter process, a 90% abatement of the concentration of these otherwise highly persistent drugs was obtained in ca. 15min. This supports the contention that TiO2 photocatalysis under solar light is a convenient and efficient method for the remediation of pollutants at the μgL−1 levels despite the presence of other non-target matrix constituents. Noteworthy, TiO2 side-chain photo-oxidation was equally effective in the further degradation of the primary intermediates at a rate comparable to that of the parent compounds. The degradation proceeded further so that it could be expected that the antimicrobial activity, related to the FQ quinolone core more than to the substituent pattern, could be effectively eliminated.

Tris(hydroxymethyl)aminomethane as a Probe in Heterogeneous TiO2 Photocatalysis

AbstractIn this work we present a new route to probe the efficiency of aqueous TiO

TiO2-photocatalytic treatment coupled with biological systems for the elimination of benzalkonium chloride in water

TiO2 heterogeneous photocatalysis (HP) applied to the oxidation of benzalkonium chloride (BKC), a bactericide widely used in pharmaceutical and cosmetic formulations and risky for the biota, is presented. Although BKC is rapidly degraded by HP, mineralization degree is low, indicating the formation of intermediate compounds more resistant to the photocatalytic treatment.Combination of HP with a biological treatment improves the treatment. Both, use of TiO2-photocatalysis as a pretreatment to improve the biodegradability of wastewaters and as a polishing decontamination step after a biological treatment are analyzed. The biotreatment can be used when the effluent contains a relatively low BKC concentration and enough nutrients and carbon to sustain the life of the biomass. The bioreactor reduces both COD and BKC concentration. Once the level of COD is reduced, photocatalysis can be used for the total BKC elimination from the solution. On the other hand, HP can be used to reduce BKC concentration in effluents containing mostly this compound. Once the BKC concentration is low enough to be innocuous to the biomass, the effluent can be combined with a solution containing nutrients and carbon and treated by the biological method.