Heterogeneous TiO2 Photocatalysis Research Articles

Selective conversion of furfural to furfuryl alcohol by heterogeneous TiO2 photocatalysis

Selective synthesis of furfuryl alcohol from furfural conversion via semiconductor photocatalytic route has appeared as a promising solution for transforming biomass into high‐value‐added products under mild temperature and pressure conditions. Titanium dioxide (TiO2) photocatalysts were prepared by a simple sol‐gel method followed by a calcination treatment of 500 – 1000 ºC, resulting in materials with distinct physicochemical properties. The photocatalytic efficiency of the TiO2 samples was examined in the selective production of furfuryl alcohol from furfural under UV‐LED irradiation. The influence of various organic solvents, including ethanol, methanol, 2‐propanol, and acetonitrile, was evaluated to optimise the selectivity towards furfuryl alcohol production. Photocatalysts with larger anatase to rutile ratio and increased density of oxygen vacancies (defects) exhibited superior performance for furfuryl alcohol production. The presence of these defects on the catalyst surface leads to a significant enhancement in the photocatalytic efficiency by acting as crucial active sites. Among the TiO2 samples, the highest conversion of furfural into furfuryl alcohol was observed with the TiO2 sample calcined under an air atmosphere at 600 ºC (TiO2‐600), achieving 85% yield and 100% selectivity for furfural after 30 min reaction using ethanol as solvent.

Nitrate radicals generated by TiO2 heterogeneous photocatalysis: Application to the cleavage of C=C double bond to carbonyl compounds

The oxidative cleavage of alkenes represents a relevant chemical process with immense potential for the synthesis of partially oxidized compounds. Several methodologies have been proposed for achieving oxidative cleavage of alkenes, aiming to obtain carbonyl products. However, none of these approaches have been successfully scaled up for industrial applications beyond the laboratory scale. In this study, we report the selective photocatalytic oxidative cleavage of limonene to limononaldehyde using irradiated TiO2 suspensions in the presence of silver nitrate under ambient conditions. Results demonstrate that within one hour of irradiation, a notable conversion and selectivity of 60% towards limononaldehyde can be achieved. The investigation reveals that the oxidation process is primarily driven by nitrate radicals, generated through hole-induced oxidation of nitrate ions. Notably, this selective photocatalytic approach shows potentials that bode well for its application to other olefinic substrates in addition to limonene. These results present a promising pathway for the scalable industrial synthesis of partially oxidized compounds through selective photocatalytic oxidation of alkenes.

Environmental assessment of a single-family photocatalytic greywater treatment system based on the design and operating conditions

A life cycle assessment was carried out to compare the environmental impacts of an alternative water supply for toilet flushing in households with those of water currently supplied for this purpose through the municipal potable water network. In the proposed system, greywater from showers, baths and wash-hand basins is collected and treated using heterogeneous TiO2 photocatalysis. Several scenarios comprising different design and operating conditions were studied. Using the ReCiPe (H) 2016 method, the following environmental impact categories were assessed: global warming, stratospheric ozone depletion, terrestrial acidification, freshwater eutrophication, marine eutrophication, human carcinogenic toxicity, human non-carcinogenic toxicity, terrestrial ecotoxicity, freshwater ecotoxicity, marine ecotoxicity, and water consumption. The results show that for the water reuse device to be less impactful than the municipal water system in at least 7 of the selected impact categories by more than 20 %, the product between the number of reactors and the operating time should not exceed 8 reactors·h·day−1, also taking the influence of the UVA-LED's power into account. Of the 18 alternative scenarios analyzed, 14 present a potential for reducing the environmental impacts compared to the municipal water supply system in at least 7 of the 11 evaluated environmental impact categories. For instance, the municipal supply of one cubic meter of water and its subsequent wastewater treatment in the baseline scenario produces 4.21 kg CO2 eq·m−3, whereas only 1.62 kg CO2 eq·m−3 are produced in the optimum scenario of the greywater reuse system. Moreover, the use of solar photovoltaic panels as a source of electricity for the systems is found to be an effective measure to further improve the environmental performance, mainly for the impact categories of global warming, stratospheric ozone depletion, and terrestrial acidification. The study also highlights the benefits of recycling components and materials in the end-of-life stage of the proposed system.

Electron and Energy Transfer Mechanisms: The Double Nature of TiO2 Heterogeneous Photocatalysis.

Photocatalytic chemical transformations in the presence of irradiated TiO2 are generally considered in terms of interfacial electron transfer. However, more elusive energy-transfer-driven reactions have been also hypothesized to occur, mainly on the basis of the indirect evidence of detected reaction products whose existence could not be justified simply by electron transfer. Unlike in homogeneous and colloidal systems, where energy transfer mechanisms have been investigated deeply for several organic syntheses, understanding of similar processes in heterogeneous systems is at only a nascent level. However, this gap of knowledge can be filled by considering the important achievements of synthetic heterogeneous photocatalysis, which bring the field closer to industrial exploitation. The present manuscript summarizes the main findings of previous literature reports and, also on the basis of some novel experimental evidences, tentatively proposes that the energy transfer in TiO2 photocatalysis could possess a Förster-like nature.

Challenges of TiO2 heterogeneous photocatalysis on cytostatic compounds degradation: state of the art.

The following work provides a perspective on the degradation of cytostatic pollutants through TiO2 heterogeneous photocatalysis. Cytostatic drugs are emerging pollutants used for cancer treatment found in hospital and domestic wastewater. Small amounts of cytostatic pollutants may pose severe health problems in human beings, animals, and plants after prolonged contact. This research presents a general review of some water treatment methods, such as aerobic activated sludge, enzymatic degradation, nanofiltration and chlorination, that have been used for the degradation or elimination of cytostatic drugs in wastewater. In recent years, photocatalysis has become important to solve this problem; these advanced oxidation process uses pure and modified TiO2 to degrade cytostatic contaminants and convert them into non-harmful substances or to eliminate them completely. This work contains a comprehensive review of the heterogeneous photocatalysis process and mechanism, and its application on the removal of cytostatic pollutants. Even if research on the topic is still scarce, this literature review provides interesting highlights on the scope of the research field, and the path such research could follow.

Degradation of hormones in tap water by heterogeneous solar TiO2-photocatalysis: Optimization, degradation products identification, and estrogenic activity removal

Hormones are one of the most hazardous classes of Contaminants of Emerging Concern (CEC) found in tap water, being significantly recalcitrant regarding traditional water treatment processes. In this work, the hormones estrone, 17β-estradiol, 17α-ethinylestradiol, and estriol, present in tap water, were degraded by heterogeneous solar photocatalysis in a flat-plate photochemical reactor (recycle mode) using TiO2 (P25, Evonik) as the photocatalyst. A fully automated analytical method based on online SPE and LC-UV was developed and validated for monitoring those molecules. For a fixed amount of sample (125 μL), the limit of quantification was 10 μg L−1 (for all four hormones) with coefficients of variation smaller than 20%, and accuracy between 80% and 120%, all of which are acceptable figures for this kind of determination. The reactional system was optimized with the aid of a 22 factorial design. At optimized conditions (pH ≈ 6.8, flow rate 250 mL min−1), the removal of the four hormones (250 μg L−1 each) was approximately 85% after 3 h of treatment. However, even after 9 h of treatment, it was not possible to completely remove the estrogenic activity (MCF-7 cells) of the treated tap water. The structures of some degradation products were proposed.

Bromate removal from water intended for human consumption by heterogeneous photocatalysis: Effect of major dissolved water constituents

This study focuses on the influence of major dissolved constituents naturally found in waters intended for human consumption on bromate (BrO3−) reduction by heterogeneous photocatalysis. The individual and combined effect of chloride (Cl−), bicarbonate/carbonic acid (HCO3−/H2CO3), nitrate (NO3−), sulphate (SO42−) and humic acids (HAs) on BrO3− reduction was evaluated in synthetic waters (SWs). Additionally, freshwaters (FWs) from a drinking water treatment plant (DWTP) were tested and directly compared to SWs. Cl− was beneficial for contents in the range 0.47–1.4 mM, with negligible influence for lower and higher contents. NO3− had a null effect regardless of its content (0.024–0.81 mM). HCO3−/H2CO3 (0.061/0.45 mM), SO42− (0.12–2.6 mM) and HAs (0.11–1.0 mM C) had a negative effect in the tested contents. The BrO3− reduction rate was 2.8 times lower in SW with a mixture of water constituents compared to SW without constituents addition. This decline on BrO3− reduction rate corresponded to the sum of the individual species contribution and so there was no evidence of synergetic effects. By contrast, the use of FWs provided BrO3− reduction rates only slightly lower than that found for SW without constituents addition (∼1.2-fold), which can be attributed to: (i) the distinct characteristics of the organic matter of FWs (HAs, fulvic acids and humins with distinct molecular weights and functional groups) compared to that of SW (pure HAs), and/or (ii) the presence in FWs of other inorganics in addition to those here addressed. The heterogeneous TiO2 photocatalysis proved to be a promising process for BrO3− reduction in DWTPs.

A step forward on NETmix reactor for heterogeneous photocatalysis: Kinetic modeling of As(III) oxidation

This study focuses on the kinetic modeling of As(III) oxidation by heterogeneous TiO2 photocatalysis, assisted by ultraviolet light emitting diodes, using a static mixer – NETmix reactor – in two different configurations: back-side illumination and front-side illumination. First, preliminary experimental assays were performed varying the load of TiO2-P25 and the incident radiation flux for each configuration, and the observed pseudo-first-order kinetic constants were obtained. A model was built based on a mechanistic reaction pathway for As(III) oxidation, taking into account the radiation absorption by the catalytic surface. Three kinetic parameters were estimated considering the solution of the material balance for the existing chemical species and the evaluation of the radiation field inside the reactor by solving the Radiative Transfer Equation. The incident radiation flux was determined at every point on the catalyst surface. It was concluded that the initial reaction rate per unit area is similar in both reactor configurations. However, when compared per unit volume, front-side illumination exhibits twice the reaction rate as the back-side illumination. Moreover, according to the obtained parameters, the front-side configuration showed higher As(V) adsorptivity when compared to the back-side. In addition, the dependence of the reaction rate upon the incident radiation level proved to be the same regardless of the illumination mechanism. Nevertheless, considering that the radiation field is not uniform, the complete expression of the dependence with the local incident radiation flux in the reaction rate equations must be used. The results obtained with the model predictions were contrasted with the experimental data regarding As(III) concentrations and showed a good agreement for both reactor configurations.

Solar-Induced Removal of Benzophenones Using TiO2 Heterogeneous Photocatalysis at Lab and Pilot Scale

This work presents the main results regarding the use of solar heterogeneous photocatalysis with TiO2 on the removal of Benzophenone-1 and Benzophenone-2 (BP1 and BP2) in aqueous samples at lab and pilot scale. BP1 and BP2 are two UV-filters that have been classified as potential endocrine disruptors, and their presence in water has been reported by different researches. Experiments were conducted at lab scale using simulated sunlight radiation, while at pilot scale a solar compound cylinder-parabolic (CCP) collector and direct sunlight were employed. Effects of catalyst concentration and solution pH on contaminants elimination were assessed. Pollutants were completely removed at lab conditions in just 60 min of reaction, while under pilot scale tests elimination was around 90.0% in 360 min. Mineralization of treated samples was around 60%. In general, it was found that the photo-generation of hydroxyl (HO·) radicals contributes to BP1 and BP2 removal, and that higher doses of TiO2 could have an inhibitory effect. Additionally, presence of some inorganic ions on the aqueous matrix (drinking water) used in the pilot-scale tests would be associated to a reduction on the extent of pollutants degradation, which together with aspects such as the type of reactor and the atmospheric and climate conditions would limit the process efficiency in comparison with results at lab-scale experiments.

Investigation of mechanism and kinetics in the TiO2 photocatalytic degradation of Indigo Carmine dye using radical scavengers

This study is aimed at investigating the influence of different scavenger species of radicals that might possibly be involved in the TiO2/UVA photocatalytic degradation of Indigo Carmine dye. The effect caused by the presence of hydroxyl radicals (1-butanol and 2-propanol), positive holes (h+) (potassium iodide) and singlet oxygen (azide) was studied. Kinetics and optimal degradation conditions were evaluated using a factorial experiment design. The highest pseudo-first-order kinetics (k = 5.22 × 10−2 ± 0.002 and t1/2 = 13.25 ± 0.49 min) was achieved at pH 4.0, 6 mg L−1 of Indigo Carmine dye and 12 mg L−1 of TiO2. Mineralization was not achieved, and direct photolysis was not observed under the studied conditions. Indigo Carmine degradation occurs mainly due to oxidation in the positive hole (h+) followed by singlet oxygen action and on a smaller scale by hydroxyl radical. The use of the aforementioned radical scavengers made it possible to verify the mechanism and kinetics of Indigo Carmine dye through TiO2 heterogeneous photocatalysis.

Effect of TiO2 phase on the photocatalytic degradation of methylene blue dye

Several industrial processes make use of dyes in order to impart colour into their products. These processes release large amounts of dye-containing wastewater which can be hazardous to human health and the environment. This has necessitated the development of cheap, reliable and sustainable technologies that can remediate the effluent, heterogeneous TiO2 photocatalysis is one such technology that has proven successful. Different polymorphs of TiO2 exist and literature suggests that the photocatalytic efficiency of various phases depends on the target pollutant. This study investigated the photocatalytic degradation of methylene blue (MB) dye using of three commercial TiO2 powders with different crystal phases as catalysts. The materials properties for each catalyst were first determined using x-ray diffraction and microscopy. Degradation studies were then conducted on 10 ppm MB solutions using the three powders. Degussa P25 TiO2, a mixture of anatase and rutile was the most efficient compared to the neat anatase and rutile powders, degrading 81.4% of the MB. This catalyst was chosen for optimisation studies, where 95% degradation was achieved at a 0.5 g/L catalyst loading and pH of 10. The degree of mineralisation decreased with increasing MB concentration. The effect of metal doping on the photocatalytic performance of the catalyst was also investigated. Copper doping increased degradation by 2% whilst zinc reduced it to 90%. The complete decolourisation of the MB coupled with the significant decrease in TOC upon completion of the degradation process, indicate that Degussa P25 catalysed photocatalysis is a viable pre-treatment technology in dye contaminated wastewater treatment.

Tube-in-tube membrane reactor for heterogeneous TiO2 photocatalysis with radial addition of H2O2

A tube-in-tube membrane reactor, operating with radial addition of H2O2, was employed for the intensification of heterogeneous H2O2/TiO2 photocatalytic processes. Polluted water is continuously fed into the annular space between the shell side of an ultrafiltration membrane and an outer quartz tube. The membrane shell-side surface is coated with a thin-film of TiO2-P25. The oxidant (H2O2) is internally fed to the membrane, which permeates through the membrane pores, being dosed and uniformly delivered to the active catalyst sites. Radiation is externally supplied via four UVC or UVA lamps. The helical movement of water around the membrane shell side improves the radial dispersion of the oxidant, promoting efficiently its UVC photolytic cleavage into OH within the annular reaction zone. Tests were performed using pure water and a secondary effluent from a wastewater treatment plant, both spiked with 100 µg L−1 of 17β-estradiol (E2) and 17α-ethinylestradiol (EE2). The efficiency of the photochemical (UVC and UVC/H2O2) processes was first assessed as a function of H2O2 dose. Then, E2 and EE2 photocatalytic oxidation efficiency by UV/TiO2 and UV/H2O2/TiO2 was evaluated according to the amount of TiO2-P25 deposited on the outer surface of the membrane, H2O2 dose, H2O2 addition mode, light source and aqueous solution matrix. The UVC/H2O2/TiO2 system with radial H2O2 addition showed the highest oxidation ability, resulting in E2 and EE2 removal percentages of 51/32% and 48/30%, respectively, for the synthetic/real matrices, using a low UVC fluence (45 mJ cm−2).

Removal of bromate from drinking water using a heterogeneous photocatalytic mili-reactor: impact of the reactor material and water matrix.

The main goal of this study was to evaluate the removal of bromate from drinking water using a heterogeneous photocatalytic mili-photoreactor, based on NETmix technology. The NETmix mili-reactor consists of a network of channels and chambers imprinted in a back slab made of acrylic (AS) or stainless steel (SSS) sealed, through mechanical compression and o-rings, with an UVA-transparent front borosilicate glass slab (BGS). A plate of UVA-LEDs was placed above the BGS window. TiO2-P25 thin films were immobilized on the BGS (back-side illumination, BSI) or SSS (front-side illumination, FSI) by using a spray deposition method. The photoreduction rate of a 200 μg L-1 (1.56 μM) BrO3- solution was assessed taking into account the following: (i) catalyst film thickness, (ii) catalyst coated surface and illumination mechanism (BSI or FSI), (iii) solution pH, (iv) type and dose of sacrificial agent (SA), (v) reactor material, and (vi) water matrix. In acidic conditions (pH 3.0) and in the absence of light/catalyst/SA, 28% and 36% of BrO3- was reduced into Br- only by contacting with AS and SSS during 2-h, respectively. This effect prevailed during BSI experiments, but not for FSI ones since back SSS was coated with the photocatalyst. The results obtained have demonstrated that (i) the molar rate of disappearance of bromates was similar to the molar rate of formation of bromides; (ii) higher BrO3- reduction efficiencies were reached in the presence of an SA using the FSI at pH 3.0; (iii) formic acid ([BrO3-]:[CH2O2] molar ratio of 1:3) presented higher performance than humic acids (HA = 1 mg C L-1) as SA; (iv) high amounts of HA impaired the BrO3- photoreduction reaction; (v) SSS coated catalyst surface revealed to be stable for at least 4 consecutive cycles, keeping its photonic efficiency. Under the best operating conditions (FSI, 18 mL of 2% wt. TiO2-P25 suspension, pH 3.0), the use of freshwater matrices led to (i) equal or higher reaction rates, when compared with a synthetic water in the absence of SA, and (ii) lower reaction rates, when compared with a synthetic water containing formic acid with a [BrO3-]:[CH2O2] molar ratio of 1:3. Notwithstanding, heterogeneous TiO2 photocatalysis, using the NETmix mili-reactor can be used to promote the reduction of BrO3- into Br-, attaining concentrations below 10 μg L-1 (guideline value) after 2-h reaction. Graphical Abstract .

Comparison of the removal of 21 micropollutants at actual concentration from river water using photocatalysis and photo-Fenton

The development of technologies capable of eliminating a wide spectrum of micropollutants of diverse nature from waters becomes essential. Advanced oxidation processes (AOPs) seem to be the most promising technologies but the most appropriate AOP to remove the micropollutants is still under debate. In this study, for the first time, the comparison of the efficiency of TiO2 heterogeneous photocatalysis and iron-impregnated faujasite heterogeneous photo-Fenton process is conducted towards the removal of a mixture of 21 micropollutants from a real river at low concentrations. The sampling area is situated on the Meurthe River, downstream the wastewater treatment plant of the city of Nancy in the northeast of France. To assess the impact of the micropollutants concentration and the matrix composition, two sampling campaigns have been performed at different periods of the year. Among the targeted micropollutants, 14 pharmaceuticals, 1 personal care product, 4 endocrine disruptors and 2 perfluorinated molecules are specifically followed. Their concentrations range from few ng L−1 to hundreds ng L−1. The adsorption of all the micropollutants is higher onto the iron-impregnated faujasite compared to that onto TiO2 due to the larger surface area of the iron-impregnated faujasite. The iron-impregnated faujasite offers superior photo-Fenton oxidation efficiency towards the degradation of the micropollutants over the TiO2 photocatalysis system. For almost all the micropollutants, the contaminants are totally removed after 30 min by photo-Fenton while a longer period is necessary by photocatalysis. A relationship between the removal efficiencies and the octanol–water partition coefficient (log Kow) of each micropollutant has been extracted from the data. A further economic evaluation confirms that the photo-Fenton process is also the better in terms of cost.

Intensification of heterogeneous TiO2 photocatalysis using the NETmix mili-photoreactor under microscale illumination for oxytetracycline oxidation

This study focuses on the intensification of heterogeneous TiO2 photocatalysis for the removal of a contaminant of emerging concern (CEC), oxytetracycline (OTC), as a polishing step of urban wastewaters, using an innovative NETmix mili-photoreactor under UVA-LEDs illumination. The effect of catalyst coated surface per reactor volume and the illumination mechanism, back-side (BSI) or front-side (FSI) irradiation, on OTC oxidation were evaluated. For that, a thin film of photocatalyst was uniformly deposited on the front borosilicate slab (BS) (BSI mechanism; 333 m2catalyst m−3reactor) or on the network of channels and chambers imprinted in the back stainless-steel slab (SSS) (FSI mechanism; 989 m2catalyst m−3reactor) using a spray system. OTC removal was also assessed as a function of TiO2 film thickness immobilized on both slabs. The photocatalyst reactivity in combination with photoreactor was significantly enhanced (3.4 times) from 0.64 to 2.19 mmolOTC m−3illuminated reactor volume s−1, when considering the BSI and FSI mechanisms, respectively. In addition, the influence of UVA-LEDs intensity on OTC oxidation rate was investigated. UVA-LEDs plates were placed on the top of the NETmix borosilicate window. Moreover, the effect of water matrix was assessed using a secondary effluent from an urban wastewater treatment plant fortified with OTC. OTC oxidation rate was only inhibited in about 1.3 times in the presence of the real matrix, showing the ability of the NETmix to overcome matrix effects due to its unique characteristics. Catalyst film stability over four consecutive reaction cycles was evaluated using synthetic and real matrices fortified with OTC.

Effect of catalyst coated surface, illumination mechanism and light source in heterogeneous TiO2 photocatalysis using a mili-photoreactor for n-decane oxidation at gas phase

This work evaluates the effect of the catalyst coated surface per reactor volume, illumination mechanism (back side (BSI) or front side (FSI) illumination) and light source on heterogeneous TiO2 photocatalysis using a mili-photoreactor, for the oxidation of a volatile organic compound in a gas stream. A thin film of TiO2-P25 was uniformly deposited on the front borosilicate window of the mili-photoreactor (BSI mechanism) or on the network of channels and chambers imprinted in the back stainless steel slab (FSI mechanism) using a spray system. BSI and FSI mechanisms were assessed using two different UVA LED systems (9 LEDs – LED9 and 18 LEDs – LED18), UVA lamps and simulated solar light as radiation sources. Under BSI and solar light the maximum reaction rate (rdec) obtained was 1.0 µmol min−1, whereas a 1.7-fold increase was attained using BSI-LED9. Although under FSI mechanism the catalyst coated surface per reactor volume is three times higher than under BSI, the maximum rdec for FSI showed only a 1.4 and 1.1-fold increase using simulated solar light and LED9, respectively, when compared with BSI. In order to provide a better light distribution over the catalyst coated surface, a new LED plate with 18 LEDs was designed and tested. Under FSI or BSI mechanisms and using the LED18 system, a ∼3.6 fold increase on the maximum rdec was observed when compared to the LED9 system. However, even for the LED18 system, similar maximum reaction rates are observed for BSI and FSI mechanisms. This means, that regardless of the light source configuration used, ∼60% of the catalyst coated surface is not illuminated.

Intensifying heterogeneous TiO2 photocatalysis for bromate reduction using the NETmix photoreactor

This work focuses on the intensification of BrO3− (200 μg L−1) reduction by TiO2-assisted heterogeneous photocatalysis, using the NETmix mili-photoreactor illuminated by UVA light-emitting diodes (UVA-LEDs). The mili-photoreactor was assembled in two configurations: i) catalyst deposition on the channels and chambers of a back stainless steel slab (SSS) and ii) catalyst deposition on the front borosilicate glass slab (BGS), allowing the study of front-side (FSI) and back-side (BSI) illumination mechanisms, respectively. The BrO3− reduction rate in aqueous solution was assessed as a function of: i) pH; ii) dissolved oxygen (DO); iii) addition of formic acid (CH2O2) as a sacrificial agent (SA); iv) photocatalyst film thickness; v) illumination mechanism; vi) irradiation intensity; vii) temperature; and viii) water matrix. Higher BrO3− reduction rates were observed using the FSI mechanism and lower pH values. Nitrogen injection (to eliminate DO) did not significantly improve the reaction rate and the addition of CH2O2 had a negative effect at pH 6.5. Neither temperature nor irradiance increase showed a considerable improvement on the reduction rate. Moreover, TiO2 film remains stable for at least 13 consecutive reactions without significant catalyst leaching. The chemically pre-treated fresh water (FW) matrix negatively affected the reaction rate when compared with the synthetic water (SW), under the best operational conditions (SSS: pH = 5.5, 287 mg of TiO2, 25 °C, SA absence, [DO] = 232–263 μM). This was associated with the presence of both inorganic and organic matter at much higher concentrations than BrO3−. Notwithstanding, heterogeneous TiO2 photocatalysis, using the NETmix mili-photoreactor, was successfully applied to fresh water, achieving [BrO3−] < 10 μg L−1 (guideline value) after 2-hour reaction.

Green synthesis of bromine by TiO2 heterogeneous photocatalysis and/or ozone: A kinetic study

Elemental bromine is an industrially relevant compound traditionally produced from bromide ions by using chlorine as the oxidizing agent. Problems related to transportation and handling of the corrosive, expensive and toxic chlorine make green synthetic alternatives highly desirable. In this paper the green synthesis of bromine from bromide in aqueous solutions under mild conditions by means of TiO2 photocatalysis and/or ozonation has been investigated from a kinetic point of view. The ozonation in the absence of the photocatalyst follows a first order kinetic with respect to both ozone and bromide. The kinetics of the reactions in the presence of the photocatalyst has been described by means of a Langmuir-Hinshelwood type model and the values of the kinetic constants and of the apparent adsorption constants have been determined. The results obtained constitute the basis for practical applications of this green and novel bromine synthesis and can be used for future reactor engineering and scale up of the process.

Selective endoperoxide formation by heterogeneous TiO2 photocatalysis with dioxygen

We report the selective formation of endoperoxides by aerobic TiO2 photocatalysis through the cyclic addition of dioxygen and a non-conjugated diene, the first heterogeneous catalytic system for endoperoxide synthesis. This green protocol does not require any additive and the photocatalyst is abundant and recyclable, providing a yield up to 64% and >20:1 diastereoselectivity. Mechanistic investigations were carried out by using product analysis, kinetic studies, O-18 labelling experiments, electron-spin resonance and a set of quenching experiments. Superoxide (but not singlet oxygen, triplet oxygen or peroxide) is directly involved in the reaction cascade to form the endoperoxide product. The new findings may be helpful for future for designing eco-friendly and energy sustainable strategies for selective oxygenation reactions using semiconductors, O2 and sunlight.

Antituberculosis drugs degradation by UV-based advanced oxidation processes

Several advanced oxidation processes (AOPs) were investigated for the degradation of isoniazid (INH) and rifampicin (RIF) in aqueous solution: TiO2 and ZnO heterogeneous photocatalysis and homogeneous and heterogeneous UV-A photo-Fenton processes. Under optimized conditions (pH 6.0 and 500mgL−1 of photocatalyst) the UV-A TiO2-photocatalysis removed approximately 90% of INH and 60% of RIF at 60min. Under similar experimental conditions, the degradation efficiency of ZnO was significantly lower to INH. The low-cost UV-A homogeneous photo-Fenton process removed 70% of INH and 80% of RIF at 60min. Photo-Fenton process using iron-immobilized in chitosan beads showed lower degradation efficiency (7% to INH and 50% to RIF) probably due to low iron availability in the catalyst surface. A radical scavenging assay was performed to investigate the effect of free radicals (hydroxyl radical (OH), electron vacancy (hole, h+), superoxide radical anions (O2−), and singlet oxygen (1O2)) on INH and RIF degradation during TiO2/UV-A photocatalysis. The two drugs have distinct degradation mechanisms, while RIF is degraded by h+, INH suffers influence of various active species. The TiO2/UV-A photocatalysis was employed to degrade INH/RIF residue.