Pathways of solar light-induced photocatalytic degradation of azo dyes in aqueous TiO2 suspensions

The photocatalytic degradation of aqueous solutions of Acid Orange 7 in TiO₂ suspensions has been investigated with the use of a solar light simulating source. The photoreaction was followed by monitoring the degradation of the dye and the formation of intermediates and final products, as functions of time of irradiation, both in solution and on the photocatalyst surface. It has been found that the dye adsorbs on TiO₂ and undergoes a series of oxidation steps, which lead to decolorization and formation of a number of intermediates, mainly aromatic and aliphatic acids. These molecules are further oxidized toward compounds of progressively lower molecular weight and, eventually, to CO₂ and inorganic ions, such as sulfate, nitrate and ammonium ions. A TiO₂-mediated photodegradation mechanism for Acid Orange 7 is proposed on the basis of quantitative and qualitative detection of intermediate compounds.     

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.     

Preparation and characterization of surface bond-conjugated TiO2/SiO2 and photocatalysis for azo dyes

Surface bond-conjugated TiO₂/SiO₂ was prepared by means of the impregnation method. Based on the results of XRD, FTIR, XPS and BET measurements, the growth of titania (predominantly anatase) on the silica substrate seems to occur by anchoring of the TiO₂ phase through Ti–O–Si cross-linking bonds. The structure model of TiO₂/SiO₂ was proposed. Compared to B–TiO₂, the most efficient catalyst is 30 wt.% TiO₂/SiO₂ (Ims30), which showed three times higher photoactivity for the degradation of reactive 15 (R15). In addition, the catalyst had a higher photoactivity on a silica of smaller particle size than on the silica of larger particles. Silica gel plays the basic role of dispersion and support for power TiO₂. The isoelectric point of the catalyst was 3.0 pH units by the measurement of zeta-potential, indicating the presence of the surface acidity of the catalyst. The photodegradation and the adsorption of R15 and cationic blue X-GRL (CBX) were investigated with the change of initial aqueous pH.     

Enhanced adsorption of two azo dyes produced by carbon modification of TiO2

The process of removal of two azo dyes (Reactive Red 198 and Direct Green 99) from water was investigated. The adsorption of azo dyes onto surfaces of pristine TiO₂, P25 and carbon-modified TiO₂ (at 120 °C for 24 h) was presented. The Freundlich model of adsorption isotherm was found for pristine TiO₂ and TiO₂-P25. Modification of TiO₂ by carbon lead to the change from the Freundlich model to the Langmuir model of adsorption isotherm. For the TiO₂-C photocatalyst the adsorption capacity was determined, which was almost two times higher for Direct Green 99 than Reactive Red 198 dyes. As a result we observed the increase of photocatalytic activity of carbon-modified TiO₂ photocatalyst.     

Hydroxyapatite photocatalytic degradation of calmagite (an azo dye) in aqueous suspension

The hydroxyapatite (HAP) is prepared by precipitation method and examined for the photocatalytic degradation of calmagite, a toxic and non-biodegradable azo-dye compound. The physicochemical properties of hydroxyapatite material were characterized using BET surface area, XRD, FT-IR, and SEM analysis. The FT-IR analysis of the hydroxyapatite revealed that the peak intensity due to absorbance of surface PO₄³⁻ group centered at wave number 1030 cm⁻¹ is drastically decreased upon exposure to UV for 1 h. The study includes dark adsorption experiments at different pH conditions, influence of the amount of catalyst, and effect of pH on photocatalytic degradation of dye, chemical oxygen demand (COD) removal, biological oxygen demand (BOD₅) increase and SO₄²⁻ and NO₃⁻ ions evolution during the degradation. At optimum photocatalytic experimental conditions the same is compared with commercial degussa P-25 TiO₂. The photocatalytic treatment significantly reduced the COD (92% removal) and increased the BOD₅/COD ratio to 0.78. Considerable evolution of SO₄²⁻ (8.5 mg L⁻¹) and NO₃⁻ (12.2 mg L⁻¹) ions are achieved during the degradation process, thus reflecting the usefulness of the hydroxyapatite photocatalytic treatment in calmagite removal in wastewater.     

Conversion of benzoic acid during TiO2-mediated photocatalytic degradation in water

The photocatalytic degradation of benzoic acid in water over Degussa P-25 TiO₂ suspensions was studied. UVA irradiation at 365 nm was supplied by a medium pressure mercury lamp providing 25 mW cm⁻² light intensity. Experiments were conducted at benzoic acid initial concentrations between 25 and 150 mg L⁻¹, catalyst loadings between 0.2 and 1 g L⁻¹ and initial solution pH values between 2 and 10.6. Conversion increased with increasing catalyst loading up to about 0.6 g L⁻¹ and it was favored at alkaline or neutral conditions but impeded at extremely acidic conditions. Increasing initial substrate concentration led to decreased benzoic acid conversion, which was found to follow a Langmuir–Hinshelwood kinetic expression. High performance liquid chromatography (HPLC) was employed to follow benzoic acid concentration profiles as well as to identify reaction by-products, while chemical oxygen demand (COD) and total organic carbon (TOC) analyses were carried out to assess the extent of mineralization. Benzoic acid hydroxylation by-products, namely 2-, 3- and 4-hydroxybenzoic acids as well as phenol were identified as reaction intermediates, although these contributed only a small fraction of the residual organic content. Although benzoic acid at 50 mg L⁻¹ was not ecotoxic to marine photobacteria Vibrio fischeri, its photodegraded solution exhibited substantial toxicity, which, however, proved not to be due to the identified intermediates.     

Performance evaluation of a continuous flow immobilized rotating tube photocatalytic reactor (IRTPR) immobilized with TiO2 catalyst for azo dye degradation

Photocatalytic oxidation is becoming an attractive technique for the degradation of hazardous organic contaminants. Reactor design plays an important role in the treatment efficiency. A novel immobilized photocatalytic reactor presented in this paper, consists of TiO₂ coated rotating PVC tubes in a continuous flow reactor; irradiated with UV lamps. Using reactors in series approach, the effects of key parameters—initial dye concentration, rotational speed, pH and flow rate, on color removal were evaluated for reactive red dye. Low initial concentration and acidic pH favored the dye removal. Rate of color removal increased with speed initially but remained constant at higher speed. Though the effect of flow rate was complex, in combination with initial concentration it had a significant effect on the energy consumption. Langmuir–Hinshelwood type kinetic model fitted the decolorization kinetic well and the rate constants were evaluated. 90–99.99% color removal and 55–70% TOC removal were obtained depending on operating conditions.     

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.     

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 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.     

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.     

Kinetic study for photocatalytic degradation of volatile organic compounds in air using thin film TiO2 photocatalyst

The present paper examined the kinetics of photocatalytic degradation of volatile organic compounds (VOCs) including gaseous trichloroethylene (TCE), acetone, methanol and toluene. Variable parameters were initial concentration of VOCs, water vapor content and photon flux of ultra-violet (UV) light. A batch photo-reactor was specifically designed for this work. The photocatalytic degradation rate increased with increasing the initial concentration of VOCs, but maintained almost constant beyond a certain concentration. It matched well with the Langmuir–Hinshelwood (L–H) kinetic model. For the influence of water vapor in a gas phase photocatalytic degradation rate, there was an optimum concentration of water vapor in the degradation of TCE and methanol. And, water vapor enhanced the photocatalytic degradation rate of toluene, whereas it inhibited that of acetone. As for the effect of photon flux, it was found that photocatalytic degradation occurs in two regimes with respect to photon flux.     

Photocatalytic degradation of 2,4-dichlorophenol by TiO2/UV: Kinetics, actinometries and models

Heterogeneous photocatalysis has proved to be a useful tool for the degradation of water pollutants as it can be observed by the high number of references appeared the last 30 years. Despite this strong research, some points, as more satisfactory and accurate kinetic models, still have to be more developed. In this work, as reaction model, the photocatalytic treatment of 2,4-dichlorophenol has been studied by using TiO₂ in suspension as catalyst. The influence of different variables (TiO₂ concentration, pH effect, radiation) on the reaction rate has been tested. Afterwards, two new kinetic models are proposed. The first one takes into account the effect of intermediates in the degradation rate of the parent pollutant, which allows a better agreement for long time reactions. The second one attempts to include the radiation effect into the kinetic expression by means of a new term which refers to the absorbed light by the photocatalyst. Both models fit quite well with the experimental data and seem to be interesting future tools for the comparison and scaling-up of different photocatalytic systems.     

TiO2/H2O2 mediated photocatalytic transformation of UV filter 4-methylbenzylidene camphor (4-MBC) in aqueous phase: Statistical optimization and photoproduct analysis

Central composite design (CCD) based on response surface methodology (RSM) was employed in the present study in combination with “profiling and desirability function” to evaluate the effects of main variables (TiO₂, H₂O₂, and light intensity) affecting 4-methylbenzylidene camphor (4-MBC) photocatalytic degradation and for optimization of the process.Besides statistical optimization of the UV filter decomposition, the investigation involved a study of the identification of intermediate compounds, mineralization, as well as toxicity evaluation. The transformation of 4-MBC proceeds through: (i) demethylation of the bridged structure; (ii) hydroxylation of the methylbenzylidene moiety; (iii) bihydroxylation/demethylation reaction. Even if all the identified compounds are degraded themselves within 30 min, the complete carbon mineralization is only achieved after 24 h of irradiation. Toxicity assays using Vibrio fischeri, have shown that 4-MBC intermediates exhibit acute toxicity.     

Photoelectro-Fenton combined with photocatalytic process for degradation of an azo dye using supported TiO2 nanoparticles and carbon nanotube cathode: Neural network modeling

In this work, treatment of an azo dye solution containing C.I. Basic Red 46 (BR46) by photoelectro-Fenton (PEF) combined with photocatalytic process was studied. Carbon nanotube-polytetrafluoroethylene (CNT-PTFE) electrode was used as cathode. The investigated photocatalyst was TiO₂ nanoparticles (Degussa P25) having 80% anatase and 20% rutile, specific surface area (BET) 50 m²/g, and particle size 21 nm immobilized on glass plates. A comparison of electro-Fenton (EF), UV/TiO₂, PEF and PEF/TiO₂ processes for decolorization of BR46 solution was performed. Results showed that color removal follows the decreasing order: PEF/TiO₂ > PEF > EF > UV/TiO₂. The influence of the basic operational parameters such as initial pH of the solution, initial dye concentration, the size of anode, applied current, kind of ultraviolet (UV) light and initial Fe³⁺ concentration on the degradation efficiency of BR46 was studied. The mineralization of the dye was investigated by total organic carbon (TOC) measurements that showed 98.8% mineralization of 20 mg/l dye at 6 h using PEF/TiO₂ process. An artificial neural network (ANN) model was developed to predict the decolorization of BR46 solution. The findings indicated that artificial neural network provided reasonable predictive performance (R² = 0.986).     

Kinetic study of the photocatalytic recovery of Pt from aqueous solution by TiO2, in a closed-loop reactor

The photocatalytic recovery of platinum

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ions from aqueous solution by TiO₂ (anatase) at conditions met in hydrometallurgical practice (low platinum concentration, low pH values and high chloride concentration) was studied. A continuous flow system with recirculation of the TiO₂ suspension through an illuminated column was applied. The addition of methanol as a “hole scavenger” was proved essential for the reaction to proceed. The influence of various parameters, such as initial platinum and methanol concentration, TiO₂ concentration, pH and flow rate, on the recovery rate was studied. The rate determining stage was found to be the adsorption of methanol molecules on the particles surface. The process seems to be effective and recovery yields up to 95% were achieved.     

TiO2 deactivation during gas-phase photocatalytic oxidation of ethanol

The deactivation of TiO₂ Degussa P25 during the gas-phase photocatalytic oxidation of ethanol has been studied. Water vapor plays a clear competitive role for surface sites adsorption, thus hampering the ethanol photo-oxidation. Dark adsorption of ethanol on a fresh catalyst shows a Langmuirian behavior with the formation of a monolayer of adsorbate. Dark adsorption in a TiO₂ surface that has been used in consecutive photocatalytic experiments of ethanol degradation gives non-Langmuirian isotherms, indicating the existence of noticeable changes of the catalyst surface structure. After several irradiations the catalyst activity decreases. Such deactivation has been investigated, observing that the rate constant of ethanol and acetaldehyde (its main degradation product) oxidation decreases with irradiation time. Several surface treatments have been studied in order to find suitable procedures for catalytic activity recovery, but regular decay of activity is always observed after every treatment.     

The kinetics of photocatalytic degradation of trichloroethylene in gas phase over TiO2 supported on glass bead

In this investigation, a packed bed filled with coated titanium dioxide glass beads to study the kinetics of photocatalytic degradation of trichloroethylene under irradiation of 365 nm UV light. In the range of 100–500 ml/min of flowrate, the reaction rate for 6 μM TCE increased with an increasing flowrate upto 300 ml/min, while was not affected by the flowrate at the values higher than 300 ml/min. For moisture in the range of 9.4–1222.2 μM, the reaction rate of TCE was decreased with an increasing humidity. The adsorbed water on the catalyst surface could compete with the adsorption of TCE on the sites. The reaction rate of 6 μM TCE increased as the light intensity increased, and was proportional to the 0.61 order of the light intensity. Among the three L–H bimolecular form models, the experimental data had the best fit for one of models:

     

Synthesis of Bi2O3/TiO2 nanostructured films for photocatalytic applications

Dendritic growth of bismuth oxide nanostructured films was accomplished by reactive magnetron sputtering. The deposition of the Bi₂O₃ template layers was adapted to abide a vapour-liquid-solid mechanism in order to develop a 3D growth morphology with high surface area templates for photocatalytic applications. TiO₂ photocatalytic thin films were deposited at a later stage onto Bi₂O₃ layers. The obtained heterostructured films were characterized by scanning electron microscopy, X-ray diffraction and atomic force microscopy. Additionally, the photocatalytic efficiency was assessed by conducting an assay using methylene blue dye as testing pollutant under a UV-A illumination. The photocatalytic tests revealed that the Bi₂O₃ layers functionalized with TiO₂ thin films are more efficient at degrading the pollutant, by a factor of 6, when compared with the individual layered films.