Photocatalytic degradation of sulfamethoxazole in aqueous suspension of TiO2

The presence of drugs in the aquatic media has emerged in the last decade as a new environmental risk. The aim of this study is the evaluation of photocatalysis as a suitable process to degrade an antibiotic, the sulfamethoxazole. In this way, sulfamethoxazole in aqueous solution was treated by using titania in suspension as catalyst, and UV light. Sulfamethoxazole degradation and TOC reduction were improved when titania concentration was increased, until an optimum located between 0.5–1.0 g TiO₂/L. Under the studied conditions, 82% of sulfamethoxazole degradation and 23% of TOC reduction was achieved when working with 0.5 g TiO₂/L. The initial pH also seemed to influence the process in some extent, although the antibiotic degradation was not affected by this variable, TOC reduction was dramatically decreased when the initial pH was 2, probably due to interferences caused by the sulfate anion. The LC/MS study has been also carried out, and a mechanism has been proposed, through the identification of five intermediates. Sulfate and ammonium ions were also monitored in the solution finding that, as long as the sulfamethoxazole is degraded, the total amount of releasable ions was not reached. The SUVA parameter along the reaction shows a decrease on the aromatic content, but there is still a notable presence of the aromatic compounds after 15 h of reaction. Finally, the experimental data were fitted to different kinetic models. The best results were obtained for a model including the sulfamethoxazole and intermediates concentration.     

Effects of radiation absorption and catalyst concentration on the photocatalytic degradation of pollutants

This study is dealing with the reaction rate dependence on catalyst concentration and light absorption in photocatalytic processes. Models relating the reaction rate to the absorbed radiation by the catalyst (titania in suspension) are proposed. To apply these models, the system is divided into layers and each layer is divided into cells, assuming that there is only one particle (agglomerate) of catalyst in each cell. The extensive reaction rate can be calculated as the sum of the reaction rate in each cell, being this proportional to the light absorbed by each particle. Two different models are proposed for light propagation through the reaction medium (exponential and probabilistic model). The extinction coefficients have been estimated by using transmittance measurements related to sedimentation rates changing according to catalyst concentration and pH. The integration of these models, taking into account the expression of the reaction rate, allows to obtain equations that can explain the trends observed in the photocatalytic treatment of Cr(VI) and phenol, by using TiO₂ (Degussa-P25) in suspension.     

Photocatalytic degradation study of diclofenac over aqueous TiO2 suspensions

Diclofenac (2-[2′,6′-(dichlorophenyl)amino]phenylacetic acid) is a non-steroidal anti-inflammatory drug used to treat inflammatory and painful diseases of rheumatic and non-rheumatic origin. The present work deals with the photocatalytic transformation of diclofenac, under simulated solar irradiation using titania suspensions as catalyst, to assess the decomposition of the pharmaceutical compound, to identify intermediates, as well as to elucidate some mechanistic details of the degradation. The variation of TiO₂ amount and initial diclofenac concentration on the reaction rate, were systematically investigated. The use of the response surface methodology allowed to fit the optimal values of the parameters leading to the degradation of the pollutant. Also, a single polynomial expression modeling the reaction was obtained. Photomineralization of the substrate in terms of chlorine ions release was rather a quick process (within 1 h), while the amino moiety is mainly transformed into NH₄⁺ and in a lesser extend into NO₃⁻ ions. Evolution of CO₂ (loss of TOC) was found to occur within 2 h of irradiation. LC/MS was brought to bear in assessing the temporal course of the photocatalyzed process. Based on our findings a tentative degradation pathway is proposed for the photocatalytic degradation of diclofenac based on the formation of hydroxy-derivatives before the complete mineralization of the starting molecule. In addition Microtox bioassay (Vibrio fischeri) was employed in evaluating the ecotoxicity of solutions treated by photocatalysis. Results clearly demonstrate the efficiency of the photocatalytic process in the detoxification of the irradiated solution.     

Photocatalytic degradation of herbicide fluroxypyr in aqueous suspension of TiO2

The photocatalytic degradation of fluroxypyr (FLX) was studied using Degussa P-25 Titania as a catalyst. The disappearance of FLX was proved to follow a half-order kinetics. This implies that two active sites are involved in the adsorption of one molecule of fluroxypyr. In our conditions, complete mineralization of 40 ppm of pure fluroxypyr occurred within ca. 240 min of UV-radiation. Some reaction intermediates were identified by HPLC–MS (ESI+) and a tentative degradation pathway was proposed. The presence of HCl (pH 2) seemed to have an inhibiting effect on the initial rate, whereas degradation of FLX was accelerated at basic pH values (pH 8 or 10, achieved with CaCO₃ or NaOH, respectively). Photocatalysis proved to be an excellent new advanced oxidation technology (AOT) to eliminate fluroxypyr residues present in surface and ground waters.     

纳米TiO2-UV光催化降解乙酰甲胺磷影响因素的研究

考察了UV-纳米TiO2光催化降解乙酰甲胺磷的可行性,就高压汞灯照射时间及照射方式、催化剂种类及用量、pH值、反应温度、乙酰甲胺磷初始浓度等对光催化降解效率的影响进行了研究,并探索了催化剂的重复利用。结果表明,当添加0.1 g/L的纳米TiO2(德国P25),乙酰甲胺磷的起始浓度为20 mg/L,温度控制在25℃,反应体系的pH为11时,以高压汞灯持续照射80 min,即可实现对乙酰甲胺磷99.9%的光催化降解。     

TiO2, TiO2/Ag and TiO2/Au photocatalysts prepared by spray pyrolysis

TiO₂, TiO₂/Ag and TiO₂/Au photocatalysts exhibiting a hollow spherical morphology were prepared by spray pyrolysis of aqueous solutions of titanium citrate complex and titanium oxalate precursors in one-step. Effects of precursor concentration and spray pyrolysis temperature were investigated. By subsequent heat treatment, photocatalysts with phase compositions from 10 to 100% rutile and crystallite sizes from 12 to 120 nm were obtained. A correlation between precursor concentration and size of the hollow spherical agglomerates obtained during spray pyrolysis was established. The anatase to rutile transformation was enhanced with metal incorporations and increased precursor concentration. The photocatalytic activity was evaluated by oxidation of methylene blue under UV-irradiation. As-prepared TiO₂ particles with large amounts of amorphous phase and organic residuals showed similar photocatalytic activity as the commercial Degussa P25. The metal incorporated samples showed comparable photocatalytic activity to the pure TiO₂ photocatalysts.     

Photocatalytic degradation of Chromotrope 2R using nanocrystalline TiO2/activated-carbon composite catalysts

Composite catalysts made of nanocrystalline TiO₂ and carbon were prepared by a modified sol–gel method over activated carbon (AC). The composite catalysts were characterized by N₂ adsorption–desorption isotherm, TG, diffuse reflectance UV–vis spectroscopy, XRD and SEM. The photocatalytic activity was tested on the degradation of Chromotrope 2R (C2R) in aqueous medium under UV radiation. The composite catalysts exhibited higher activities than commercial Degussa P25 alone and the photocatalytic process was more efficient than the pure photolytic degradation. A modified Langmuir–Hinshelwood approach was used to study the kinetics and to determine the adsorption equilibrium constant and the reaction rate constant. Two different mechanisms are proposed and discussed in order to explain the observed synergy.     

Photocatalytic activity of Cu2O/TiO2, Bi2O3/TiO2 and ZnMn2O4/TiO2 heterojunctions

Cu₂O/TiO₂, Bi₂O₃/TiO₂ and ZnMn₂O₄/TiO₂ heterojunctions were studied for potential applications in water decontamination technology and their capacity to induce an oxidation process under VIS light. UV–vis spectroscopy analysis showed that the junctions-based Cu₂O, Bi₂O₃ and ZnMn₂O₄ are able to absorb a large part of visible light (respectively, up to 650, 460 and 1000 nm). This fact was confirmed in the case of Cu₂O/TiO₂ and Bi₂O₃/TiO₂ by photocatalytic experiments performed under visible light. A part of the charge recombination that can take place when both semiconductors are excited was observed when a photocatalytic experiment was performed under UV–vis illumination. Orange II, 4-hydroxybenzoic and benzamide were used as pollutants in the experiment. Photoactivity of the junctions was found to be strongly dependent on the substrate. The different phenomena that were observed in each case are discussed.     

Photocatalytic oxidation of ketones in the gas phase over TiO2 thin films: a kinetic study on the influence of water vapor

Photocatalytic oxidation of two ketones having different hydrophilicity, acetone and methyl isobutyl ketone (MIBK), over TiO₂ thin films supported on glass rings was studied under different relative humidity (RH) conditions. Adsorption isotherms of both ketones in the dark under different concentrations of water vapor were also measured and modeled considering a two-site Langmuir model. As expected, an increase in RH resulted in a decrease in the surface concentration of the organic molecules, but the photocatalytic oxidation rates did not show a parallel variation. In the case of acetone, almost total mineralization was achieved, and the kinetic constants obtained from fitting the data to the Langmuir–Hinselwood–Hougen–Watson (LHHW) model, were significantly larger in the presence of water vapor. Photocatalytic elimination of methyl isobutyl ketone (MIBK) is proposed to take place simultaneously by a direct oxidation route and by a sequential mechanism. Acetone is a stable intermediate in this last reaction pathway. Photodegradation of MIBK at different levels of RH was also modeled using LHHW kinetic expressions, and the rates obtained were slightly lower than those obtained for the photooxidation of acetone. However, a carbon balance was not achieved for MIBK, very likely due to the accumulation of partially oxidized products on the surface of the photocatalyst. The influence of water vapor on the photocatalytic oxidation rate of this larger ketone is more complex, and both inhibition and promotion effects can be envisaged. In addition, experimental results show that humidity exerts a significant influence in the mineralization efficiency of MIBK.     

Photocatalytic degradation of methylene blue with TiO2 nanoparticles prepared by a thermal decomposition process

The generation of TiO₂ nanoparticles by the thermal decomposition of titanium tetraisopropoxide (TTIP) was carried out experimentally using a tubular electric furnace at various synthesis temperatures (700-1300 °C) and TTIP heating temperatures (80-110 °C). The photocatalytic activity of the resulting TiO₂ nanoparticles was examined by measuring the rate of methylene blue decomposition. The TiO₂ nanoparticles were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) measurements and transmission electron microscopy (TEM). The crystallite size and crystallinity increased with increasing synthesis temperature and TTIP heating temperature. A TTIP heating temperature and synthesis temperature of 95 °C and 900 °C, respectively, were found to be the optimal synthesis conditions. The primary particle diameter obtained under optimum synthesis conditions was considerably smaller than the commercial photocatalyst (Degussa, P25). The specific surface areas were more than 134.4 m² g^− 1. Under the optimal conditions, the photocatalytic activity for methylene blue was higher than that of the commercial photocatalyst.     

Photocatalytic inactivation of Escherischia coli: Effect of concentration of TiO2 and microorganism, nature, and intensity of UV irradiation

The efficiency of photocatalytic disinfection, used to inactivate Escherischia coli K12 under different physico-chemical parameters, was examined. The photocatalyst chosen was the semiconductor TiO₂ degussa P25 and the irradiation was produced by an HPK 125 lamp. The effect of titania concentration was investigated using two E. coli concentrations. The photocatalyst concentration ranged from 0.1 to 2.5 g/L. The evolution of E. coli inactivation as function of time was discussed depending on the E. coli and TiO₂ concentrations. The optimal concentration of the photocatalyst, 0.25 g/L, is lower than that necessary to absorb all photons and to degrade the organic compounds. Some hypotheses are presented to explain this behaviour. The effect of the different domains of UV light (UVA, UVB, and UVC) was also studied and modification of the light irradiation intensity is discussed. No bacteria photolysis was obtained with UVA but the use UVC had, on the contrary, a detrimental effect on bacteria survival. The addition of titania at a low concentration, 0.25 g/L, improved the inactivation of E. coli in the presence of UVA and UVB, but a detrimental effect was observed under UVC. The disinfection efficiency increases as a function of light intensity, whatever the photocatalytic conditions (different TiO₂ concentrations and different UV domains). No bacterial growth was observed after disinfection, whether the system contained titania or not.     

Phorate degradation by TiO2 photocatalysis: Parameter and reaction pathway investigations

The photocatalytic degradation of phorate in aqueous suspensions was examined with the use of titanium dioxide (TiO₂) as a photocatalyst. About 99% of phorate was degraded after UV irradiation for 60 min. The photodegradation of phorate followed pseudo-first-order kinetics and parameters such as pH of the system, TiO₂ dosage, and presence of anions were found to influence the reaction rate. To obtain a better understanding of the mechanistic details of this TiO₂-assisted photodegradation of phorate with UV irradiation, the intermediates of the processes were separated, identified, and characterized by the solid-phase microextraction (SPME) and gas chromatography/mass spectrometry (GC/MS) techniques. The probable photodegradation pathways were proposed and discussed. To the best of our knowledge, this is the first study that reports the degradation pathways of phorate. The electrical energy consumption per order of magnitude for photocatalytic degradation of phorate was also calculated and showed that a moderated efficiency (E_EO = 96 kWh/(m³ order)) was obtained in TiO₂/UV process.     

Photocatalytic degradation kinetics of di- and tri-substituted phenolic compounds in aqueous solution by TiO2/UV

In this study, photocatalytic degradation of 2,4,6-trimethylphenol (TMP), 2,4,6-trichlorophenol (TCP), 2,4,6-tribromophenol (TBP), 2,4-dimethylphenol (DMP), 2,4-dichlorophenol (DCP) and 2,4-dibromophenol (DBP) has been studied by TiO₂/UV. Although degraded phenolic compound concentration increased by increasing initial concentration photocatalytic decomposition rates of di- and tri-substituted phenols at 0.1–0.5 mM initial concentrations decreased when the initial concentration increased. The fastest degradation observed for TCP and the slowest for TMP. Photodegradation kinetics of the compounds has been explained in terms of Langmuir–Hinshelwood kinetics model. Degradation rate constants have been observed to be extremely depended on electronegativity of the substituents on phenolic ring. Degradation rate constant and adsorption equilibrium constant of TCP were calculated as k 0.0083 mM min⁻¹ and K 9.03 mM⁻¹. For TBP and TMP the values of k and K were obtained as 0.0040 mM min⁻¹, 19.20 mM⁻¹, and 0.0017 mM min⁻¹, 51.68 mM⁻¹, respectively. Degradation rate constant of DBP was similar as DCP (0.0029 mM min⁻¹ for DBP and 0.0031 mM min⁻¹ for DCP) whereas adsorption equilibrium constants differed (48.40 mM⁻¹ for DBP and 30.52 mM⁻¹ for DCP). K and k of DMP found as 83.68 mM⁻¹ and 0.0019 mM min⁻¹, respectively. The adsorption equilibrium constants in the dark were ranged between 1.11 and 3.28 mM⁻¹ which are lower than those obtained in kinetics. Adsorption constants have inversely proportion with degradation rate constants for all phenolic compounds studied.     

钆掺杂二氧化钛纳米管光催化降解制药废水

采用阳极氧化法制备TiO2纳米管阵列,并在制备TiO2纳米管阵列的同时掺杂钆元素,形成钆掺杂TiO2纳米管阵列,研究其光催化降解制药废水的性能。结果表明,经过450℃煅烧的催化剂,在碱性条件下,钆掺杂浓度为0.007 5 mol/L时,催化效果最好,光催化5 h后,制药废水降解率达到97.41%,且该催化剂稳定性好,可重复使用。     

FTIR study of gas-phase alcohols photocatalytic degradation with TiO2 and AC-TiO2

In this paper the degradation of gaseous alcohols (methanol, ethanol, 1-propanol and 1-butanol) has been studied in a continuous reaction system. Methanol and ethanol, but not 1-propanol and 1-butanol, have caused the catalyst bare-TiO₂ to become deactivated. The catalyst containing activated carbon, AC-TiO₂, has not shown almost deactivation in any experiment.

To explain this behaviour variation, a thorough FTIR study concerning the interaction of gaseous alcohols with the catalyst surface and the effect of light on the process was performed. Thus, the interaction of the different catalysts with the selected alcohols, the formed species and the hydroxyl group evolution under irradiation was studied.     

Photocatalytic degradation of imidazolinone fungicide in TiO2-coated optical fiber reactor

The photocatalytic degradation of the fungicide fenamidone is studied in a TiO₂-coated optical fiber photoreactor. Fenamidone is slowly transformed with a kinetic order of 1 and a degradation rate of 0.02 h⁻¹. Intermediate products were isolated and identified by means of solid phase extraction (SPE) coupled to liquid chromatography-mass spectrometry techniques (LC-MS). A proposed degradation pathway of fenamidone is presented, involving mainly hydroxylation and oxidation reactions. Carboxylic acids and sulfate ions resulting from the same reaction in a powder reactor were also identified.     

Photocatalytic oxidation of methyl parathion over TiO2 and ZnO suspensions

The photocatalytic degradation of methyl parathion in aqueous solutions, using two different photocatalysts (TiO₂ and ZnO) has been investigated. The degradation of methyl parathion follows first order kinetics according to the Langmuir–Hinshelwood model. Complete degradation is achieved within 45 or 150 min when treated with illuminated TiO₂ or ZnO, respectively. It was observed that the initial rate increases linearly with an increase of the amount of catalyst up to a level where it reaches a plateau corresponding to the optimum of light absorption. The addition of an oxidant (K₂S₂O₈) leads to an increase in the rate of photooxidation. Moreover, illuminated TiO₂ suspensions were proved to be more effective in mineralizing the insecticide compared to ZnO suspensions. Measurements of phosphate, sulfate and nitrate ions gave valuable information about how this process is achieved. Addition of the oxidant enhances mineralization for both photocatalytic systems. Up to eight by-products were identified by GC–MS technique during the photocatalytic degradation of the insecticide that proceeds via oxidation, hydroxylation, dealkylation and hydrolysis of the ester group reaction pathways. Finally, the toxicity of the treated solution was reduced only in the presence of TiO₂, while ZnO suspensions appear to increase the toxicity due to photo-dissolution of ZnO releasing zinc in the treated solution.     

Photocatalytic conversion of CH4 and CO2 to oxygenated compounds over Cu/CdS–TiO2/SiO2 catalyst

Coupled semiconductor (CS) Cu/CdS–TiO₂/SiO₂ photocatalyst was prepared using a mutli-step impregnation method. Its optical property was characterized by UV–vis spectra. BET, XRD, Raman and IR were used to study the structure of the photocatalyst. Fine CdS was found dispersed over the surface of anatase TiO₂/SiO₂ substrate. Chemisorption and IR analysis showed methane absorbed in the molecular state interacted weakly with the surface of catalyst, and the interaction of CO₂ with CS produced various forms of absorbed CO₂ species that were primarily present in the form of formate, bidentate and linear absorption species. Photocatalytic direct conversion of CH₄ and CO₂ was performed under the operation conditions: 373 K, 1:1 of CO₂/CH₄, 1 atm, space velocity of 200 h⁻¹ and UV intensity of 20.0 mW/cm². The conversion was 1.47% for CH₄ and 0.74% for CO₂ with a selectivity of acetone up to 92.3%. The reaction mechanisms were proposed based on the experimental observations.