Treatment of textile dyehouse wastewater by TiO2 photocatalysis

The oxidative degradation of an actual textile dyehouse wastewater was investigated by means of photocatalysis in the presence of TiO2. The UV-A-induced photocatalytic oxidation over TiO2 suspensions was capable of decolorizing the effluent completely, as well as reducing chemical oxygen demand (COD) sufficiently (COD reduction generally varied between about 40% and 90% depending on the operating conditions) after 4 h of treatment. Two crystalline forms of TiO2, viz. anatase and rutile, were tested for their photocatalytic activity and anatase was found to be more active than rutile. The extent of photocatalytic degradation was found to increase with increasing TiO2 concentration up to 0.5 g/L TiO2, above which degradation remained practically constant, reaching a plateau. Furthermore, textile effluent degradation was enhanced at acidic conditions (i.e. pH = 3) and in the presence of hydrogen peroxide. To assess catalyst activity on repeated use, experiments were performed where the catalyst was recovered and reused; after three successive uses, TiO2 had sufficiently retained its photocatalytic activity. Finally, the luminescent marine bacteria Vibrio fischeri was used to assess the acute ecotoxicity of samples prior to and after the photocatalytic treatment and it was found that ecotoxicity was fully eliminated following photocatalytic oxidation.     

Photocatalytic degradation of aqueous pollutants using silica-modified TiO(2)

Photocatalytic degradation (PCD) of several aqueous pollutants was investigated using a porous silica-coated titanium dioxide (SiO(2)-TiO(2)) photocatalyst. Several cationic, neutral and anionic pollutants were tested. The results indicate that modifying the surface properties of TiO(2) using silica significantly enhances the PCD rate of the cationic pollutants. The rate enhancement decreased with an increase in substrate concentration, especially for the quaternary amines, and was attributed to the decrease in initial adsorption. However, no significant rate-increase resulted for acetate and phenol. Results suggest that the increased presence of cationic pollutants at the catalyst surface caused the rate enhancement.     

Optical density and photonic efficiency of silica-supported TiO2 photocatalysts

Over the last years, many research groups have developed supported TiO2-based materials in order to improve the engineering applications of photocatalytic technologies. However, not many attempts have been made to evaluate the optical behavior of these materials. This work focuses on the study of the photonic efficiencies of silica-supported TiO2 photocatalysts following the photodegradation of dichloroacetic acid (DCA) as model compound. Catalysts with different types of silica support and titania loadings were tested and their activity was found to be in correlation with the results of the clusters size distribution of the TiO2 nanocrystals. The photonic efficiency of the supported photocatalysts depends extremely on the optical density of the solid suspensions. Influence of the textural properties of the support and the titania loading on the optical density as well as on the photonic efficiency of the materials are discussed. The dependence of the absorption of radiation by the suspension on the catalyst concentration is also analyzed.     

A photoelectrocatalytic process that disinfects water contaminated with Mycobacterium kansasii and Mycobacterium avium

Nontuberculous mycobacteria are resistant to conventional water treatment; indeed, they have been recovered from a wide variety of environmental sources. Here, we applied the photoelectrocatalytic technique using a Ti/TiO₂–Ag photoanode to inactivate mycobacteria. For a mycobacteria population of 5 × 10⁸ CFU mL⁻¹, we achieved 99.9 and 99.8% inactivation of Mycobacterium kansasii and Mycobacterium avium with rate constant of 6.2 × 10⁻³ and 4.2 × 10⁻³ min⁻¹, respectively, after 240 min. We compared the proposed method with the photolytic and photocatalytic methods. Using a mycobacteria population of 7.5 × 10⁴ CFU mL⁻¹, the proposed Ti/TiO₂–Ag photoanode elicited total mycobacteria inactivation within 3 min of treatment; the presence of Ag nanoparticles in the electrode provided 1.5 larger degradation rate constant as compared with the Ti/TiO₂ anode (1.75 × 10⁻² for M. kansassi and 1.98 × 10⁻² for M. avium). We monitored the degradation of the metabolites released during cellular lysis by TOC removal, sugar release, chromatography, and mass spectrometry measurements; photoelectrocatalysis and Ti/TiO₂–Ag photoanodes furnished the best results.     

Application of nano TiO(2) towards polluted water treatment combined with electro-photochemical method

A novel composite reactor was prepared and studied towards the degradation of organic pollutants in this work. In the reactor, a UV lamp was installed to provide energy to excite nano TiO(2), which served as photocatalyst, leading to the production of hole-electron pairs, and a three-electrode electrolysis system was used to accumulate H(2)O(2) which played an important role in the degradation process. The reactor was evaluated by the degradation process of rhodamine 6G (R-6G), and the data obtained in the experiments showed that the combination of the photochemical and electrochemical system raised the degradation rate of R-6G greatly; the working mechanism of the reactor was also discussed in the article. The prepared reactor was also utilized to treat polluted water from dyeing and printing process. After continuous treatment for 0.5h, chemical oxygen demand biochemical oxygen demand, quantity of bacteria and ammonia nitrogen of the polluted water were reduced by 93.9%, 87.6%, 99.9% and 67.5%, respectively, which indicated that the method used here could be used for effective organic dyes degradation.     

Enhancement of photocatalytic activity by metal deposition: characterisation and photonic efficiency of Pt, Au and Pd deposited on TiO2 catalyst

Pt, Au and Pd deposited TiO2 have been prepared and characterised by surface analytical methods such as surface area, XRD, and scanning electron micrograph and photophysical characterisation by diffuse reflectance spectroscopy. The photocatalytic activity of the doped catalysts was ascertained by the photo-oxidation of leather dye, acid green 16 in aqueous solution illuminated with low-pressure mercury lamp ( approximately 254 nm). The effect of metal contents on the photocatalytic activity was investigated. The highest photonic efficiency was observed with metal deposition level of less than 1 wt%.     

Electrochemically assisted photocatalytic degradation of Acid Orange 7 with beta-PbO2 electrodes modified by TiO2

Modification of beta-PbO(2) electrodes was carried out by TiO(2) co-deposition and characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The 2.0 g TiO(2) (the amount of TiO(2) used in 200 mL electrodeposition solution) modified beta-PbO(2) electrode was more compact and more uniform in comparison with the unmodified beta-PbO(2) electrode. TiO(2) particles were tightly attached on and between beta-PbO(2) crystals on modified beta-PbO(2) electrode. It was also used in electrochemically assisted photocatalytic degradation (EAPD) of Acid Orange 7. Compared with the total efficiency by a single application of ultraviolet irradiation and electrochemical procedure, application of a 1.5 V potential in EAPD improved the apparent first-order rate constant by 44.2% for 2.0 g TiO(2) modified beta-PbO(2) electrode even if it was not freshly used. A synergetic effect was significant. Within the amount of TiO(2) investigated, the more TiO(2) used in electro-deposition solution, the higher the degradation efficiencies were. Effects of initial dye concentration, initial pH values and applied potentials across the electrodes were investigated. Acidic condition and high potentials applied across the electrodes favored color or TOC removal of the dye. Decolorization rate decreased with an increase in the dye concentration in the range of 5-50mg/L. Experiments above demonstrate that TiO(2) modified beta-PbO(2) electrode, which realized TiO(2) immobilization successfully, performed well in EAPD of Acid Orange 7.     

Development of De-NOx selective catalytic reduction catalyst using V2O5 –TiO2 supported on a ceramic filter candle

This study reports the development of a V₂O₅-TiO₂-based selective catalytic reduction (SCR) catalyst supported on alumina-silicate water filter candles for NO_x abatement. The synthesised catalysts have been characterised by thermogravimetry/differential scanning calorimetry, X-ray diffraction, scanning electron microscope and measurement of NO_x removal performance. The catalyst synthesised under optimal conditions shows NO_x reduction efficiency of >97%. SCR performance has been augmented using O₂ as a carrier gas. This simple, reproducible procedure can have wide applications in NOx abatement.     

Development of a TiO2 modified optical fiber electrode and its incorporation into a photoelectrochemical reactor for wastewater treatment

Electrochemical advanced oxidation processes (EAOPs) are used to chemically burn non biodegradable complex organic compounds that are present in polluted effluents. A common approach involves the use of TiO₂ semiconductor substrates as either photocatalytic or photoelectrocatalytic materials in reactors that produce a powerful oxidant (hydroxyl radical) that reacts with pollutant species. In this context, the purpose of this work is to develop a new TiO₂ based photoanode using an optic fiber support. The novel arrangement of a TiO₂ layer positioned on top of a surface modified optical fiber substrate, allowed the construction of a photoelectrochemical reactor that works on the basis of an internally illuminated approach. In this way, a semi-conductive optical fiber modified surface was prepared using 30 μm thickness SnO₂:Sb films on which the photoactive TiO₂ layer was electrophoretically deposited. UV light transmission experiments were conducted to evaluate the transmittance along the optical fiber covered with SnO₂:Sb and TiO₂ showing that 43% of UV light reached the optical fiber tip. With different illumination configurations (external or internal), it was possible to get an increase in the amount of photo-generated H₂O₂ close to 50% as compared to different types of TiO₂ films. Finally, the electro-Fenton photoelectrocatalytic Oxidation process studied in this work was able to achieve total color removal of Azo orange II dye (15 mg L⁻¹) and a 57% removal of total organic carbon (TOC) within 60 min of degradation time.     

Photocatalytic degradation of non-steroidal anti-inflammatory drugs with TiO2 and simulated solar irradiation

The aim of this work is to evaluate and compare the degradation achieved for three non-steroidal anti-inflammatory drugs (NSAIDs) by heterogeneous TiO2 photocatalytic means in aqueous solution at laboratory scale. The selected pharmaceutical compounds were diclofenac (DCF), naproxen (NPX) and ibuprofen (IBP). These compounds were used in their sodium salt chemical form. Previous experiments (adsorption, photolysis and thermodegradation) were developed to evaluate non-catalytic degradation for each NSAID. Photocatalytic experiments were carried out in a Xe-lamp reactor in order to study the influences of different operational conditions (catalyst load, temperature and dissolved oxygen concentration). These results showed that the optimum amount of TiO2, to achieve maximum degradation, of IBP was 1g/L. In contrast, the maximum degradation for DCF or NPX was observed at a TiO2 loading of 0.1g/L. Temperature had a significant effect only for NPX degradation, achieving almost 99% phototransformation. No significant differences were observed for DCF and IBP at 20, 30 and 40 degrees C. Dissolved oxygen concentration was an important parameter to increase the degradation for NPX and IBP. However, it was observed that its rate of mineralization did not increase. Intermediate metabolites were detected in all cases. Hydroxyl metabolites were the most important residual compounds after the photocatalytic treatment of IBP. The inhibition percentage of bioluminescence from Vibro fischeri--as a toxicity parameter--increased during the irradiation time due to the residual concentration of the hydroxyl metabolites generated. However, after 120 min, in experiments with 40 mg/L of dissolved oxygen, a decrease of the % inhibition was observed. Only photocatalytic treatment of IBP drives to a satisfactory biodegradability index BOD5/COD (between 0.16 and 0.42) and, only in this case, a post-biological treatment could be suggested.     

Optimization of capacity and kinetics for a novel bio-based arsenic sorbent, TiO2-impregnated chitosan bead

The optimization of TiO₂-impregnated chitosan beads (TICB) as an arsenic adsorbent is investigated to maximize the capacity and kinetics of arsenic removal. It has been previously reported that TICB can 1) remove arsenite, 2) remove arsenate, and 3) oxidize arsenite to arsenate in the presence of UV light and oxygen. Herein, it is reported that adsorption capacity for TICB is controlled by solution pH and TiO₂ loading within the bead and enhanced with exposure to UV light. Solution pH is found to be a critical parameter, whereby arsenate is effectively removed below pH 7.25 and arsenite is effectively removed below pH 9.2. A model to predict TICB capacity, based on TiO₂ loading and solution pH, is presented for arsenite, arsenate, and total arsenic in the presence of UV light. The rate of removal is increased with reductions in bead size and with exposure to UV light. Phosphate is found to be a direct competitor with arsenate for adsorption sites on TICB, but other relevant common background groundwater ions do not compete with arsenate for adsorption sites. TICB can be regenerated with weak NaOH and maintain full adsorption capacity for at least three adsorption/desorption cycles.     

A practical demonstration of water disinfection using TiO2 films and sunlight

The scope of this study is the assessment of the efficiency of solar disinfection by heterogeneous photocatalysis with sol-gel immobilized (titanium dioxide) TiO2 films over glass cylinders. The solar disinfection process known as SODIS was considered as a reference. Spring water naturally polluted with coliform bacteria was exposed to sunlight in plastic bottles with and without TiO2 over simple solar collectors and the disinfection effectiveness was measured. Total and fecal coliforms quantification was performed by means of the chromogenic substrate method in order to obtain the efficiency of each disinfection treatment. The disinfection with TiO2 was more efficient than the SODIS process, inactivating total coliforms as well as fecal coliforms. On a sunny day (more than 1000 W m(-2) irradiance), it took the disinfection with immobilized TiO2 15 min of irradiation to inactivate the fecal coliforms to make them undetectable. For inactivation of total coliforms, 30 min was required, so that in less than half the time it takes SODIS, the treated water complies with the microbial standards for drinking water in Mexico. Another important part of this study has been to determine the bacterial regrowth in water after the disinfection processes were tested. After SODIS, bacterial regrowth of coliforms was observed. In contrast, when using the TiO2 catalyst, coliforms regrowth was not detected, neither for total nor for fecal coliforms. The disinfection process using TiO2 kept treated water free of coliforms at least for seven days after sun irradiation. This demonstration opens the possibility of application of this simple method in rural areas of developing countries.     

Solar Fenton and solar TiO2 catalytic treatment of ofloxacin in secondary treated effluents: Evaluation of operational and kinetic parameters

Two different technical approaches based on advanced oxidation processes (AOPs), solar Fenton homogeneous photocatalysis (hv/Fe²⁺/H₂O₂) and heterogeneous photocatalysis with titanium dioxide (TiO₂) suspensions were studied for the chemical degradation of the fluoroquinolone ofloxacin in secondary treated effluents. A bench-scale solar simulator in combination with an appropriate photochemical batch reactor was used to evaluate and select the optimal oxidation conditions of ofloxacin spiked in secondary treated domestic effluents. The concentration profile of the examined substrate during degradation was determined by UV/Vis spectrophotometry. Mineralization was monitored by measuring the dissolved organic carbon (DOC). The concentrations of Fe²⁺ and H₂O₂ were the key factors for the solar Fenton process, while the most important parameter of the heterogeneous photocatalysis was proved to be the catalyst loading. Kinetic analyses indicated that the photodegradation of ofloxacin can be described by a pseudo-first-order reaction. The rate constant (k) for the solar Fenton process was determined at different Fe²⁺ and H₂O₂ concentrations whereas the Langmuir-Hinshelwood (LH) kinetic expression was used to assess the kinetics of the heterogeneous photocatalytic process. The conversion of ofloxacin depends on several parameters based on the various experimental conditions, which were investigated. A Daphnia magna bioassay was used to evaluate the potential toxicity of the parent compound and its photo-oxidation by-products in different stages of oxidation. In the present study solar Fenton has been demonstrated to be more effective than the solar TiO₂ process, yielding complete degradation of the examined substrate and DOC reduction of about 50% in 30 min of the photocatalytic treatment.     

Investigating synergism during sequential inactivation of MS-2 phage and Bacillus subtilis spores with UV/H2O2 followed by free chlorine

A sequential application of UV as a primary disinfectant with and without H₂O₂ addition followed by free chlorine as secondary, residual disinfectant was performed to evaluate the synergistic inactivation of selected indicator microorganisms, MS-2 bacteriophage and Bacillus subtilis spores. No synergism was observed when the UV irradiation treatment was followed by free chlorine, i.e., the overall level of inactivation was the same as the sum of the inactivation levels achieved by each disinfection step. With the addition of H₂O₂ in the primary UV disinfection step, however, enhanced microbial inactivation was observed. The synergism was observed in two folds manners: (1) additional inactivation achieved by hydroxyl radicals generated from the photolysis of H₂O₂ in the primary UV disinfection step, and (2) damage to microorganisms in the primary step which facilitated the subsequent chlorine inactivation. Addition of H₂O₂ in the primary disinfection step was also found to be beneficial for the degradation of selected model organic pollutants including bisphenol-A (endocrine disruptor), geosmin (taste and odor causing compound) and 2,4-D (herbicide). The results suggest that the efficiency of UV/free chlorine sequential disinfection processes, which are widely employed in drinking water treatment, could be significantly enhanced by adding H₂O₂ in the primary step and hence converting the UV process to an advanced oxidation process.     

Surface modification of TiO2 by a ruthenium(II) polypyridyl complex via silyl-linkage for the sensitized photocatalytic degradation of carbon tetrachloride by visible irradiation

A new ruthenium(II) photosensitizer, [Ru(II)(py-pzH)(3)](2+) (where py-pzH=3-(2'-pyridyl)pyrazole), has been synthesized. The complex displayed outstanding excited state redox properties (estimated Ru(III)/Ru(II)* approximately -1.24 V vs. NHE) and was expected to sensitize the injection of electrons into the conduction band of anatase TiO(2) upon visible irradiation. The photosensitizer was anchored onto the surface of anatase TiO(2) particles via in situ silylation. The silyl-linkage displayed excellent stability in both aqueous media, over a wide pH range, and in common organic solvents. The resultant material, TiO(2)-[Ru(II)(py-pz-Si identical with )(3)], was found to be able to mediate degradation of CCl(4) in neutral aqueous medium under broad band visible irradiation (lambda>450 nm). The relation between the rate of degradation and concentration of substrate was explored and the mechanism of the photodegradation of the perhalogenated organic was discussed.     

The effect of engine emission on canola biodiesel blends with TiO2

The role of nanoparticles and nanofluid additives for biodiesel has gained consistent position in the current trend as they contribute to increase the performance of the engine with lower emission. In addition, additives also help to increase the engine reliability and lifespan. In this work, the effects of canola biodiesel blends of 20% proportions with diesel were investigated at 100% of engine load. The fuel is tested in a multi-cylinder water-cooled direct ignition (DI) engine. There are numerous notable works on nanofluid; however, the addition of TiO₂ nanoparticle as additive to produce canola biodiesel fuel is very limited. With the addition of the TiO₂ nanoparticle on Canola biodiesel blend in the DI engine, the exhaust property of gases such as CO, HC and NO_X is reduced. Furthermore, the combustion characteristics of the engine are improved. The canola biodiesel blends also resulted in lower NO_x emission as well as low smoke.     

Photocatalytic oxidation of DBP precursors using UV with suspended and fixed TiO2

French River water (Nova Scotia, Canada) was separated into six different natural organic matter (NOM) fractions, including hydrophobic acids, bases and neutrals and hydrophilic acids, bases and neutrals. The raw water, as well as each of the NOM fractions were analysed for disinfection by-product (DBP) formation potential before and after advanced oxidation with UV/TiO₂ to determine the efficacy of this treatment for the removal of DBP precursors. The UV/TiO₂ treatment was carried out with a nanostructured thin film (NSTF), coated with TiO₂ which is compared with the use of a TiO₂ suspension. For the raw river water, removals of total trihalomethane formation potential (TTHMFP) and total haloacetic acid formation potential (THAA₉FP) were found to be approximately 20% and 90%, respectively, with 50 mJ/cm² UV exposure and 1 mg/L TiO₂. For the fractionated samples, approximately 75% of both trihalomethane (THM) and haloacetic acid (HAA) precursors were found to be associated with the hydrophobic acid fraction. For this individual fraction the same UV/TiO₂ treatments exhibited approximately 20-25% removal of both TTHMFP and THAA₉FP, suggesting that the fractionation process may have affected the treatability of HAA precursors or may have altered the results of the oxidation processes.     

Solar photocatalytic thin film cascade reactor for treatment of benzoic acid containing wastewater

A solar photocatalytic cascade reactor was constructed to study the photocatalytic oxidation of benzoic acid in water under various experimental and weather conditions at HKUST. Nine stainless steel plates coated with TiO(2) catalyst were arranged in a cascade configuration in the reactor. Photolytic degradation and adsorption were confirmed to be insignificant total organic carbon (TOC) removal mechanisms. A turbulent flow pattern and, hence, improved mixing in the liquid film were achieved due to the unique cascade design of the reactor. The photoinduced consumption of oxygen during reactions was demonstrated in a sample experiment. The proposed rate equations provided good fits to 90 data points from 17 experiments. The regression results showed that the TOC removal rates averaged over 30 min intervals did not illustrate significant dependence on TOC(0) and that I(mean) was more important in affecting the photocatalytic process within the ranges of the data examined. The percentage removal of TOC in 7 l of 100 mg/l (or 100 ppm) benzoic acid solutions increased from 30% to 83% by adding 10 ml of hydrogen peroxide solution (30 wt%). Hydrogen peroxide was also shown to enhance the efficiency of the degradation process at elevated temperatures. Ortho-, meta- and para-hydroxybenzoic acids were identified by HPLC analysis as the intermediates of benzoic acid during reactions without the addition of hydrogen peroxide solutions.     

Photocatalytic degradation of reactive black 5 in aqueous solutions: Effect of operating conditions and coupling with ultrasound irradiation

The degradation of reactive black 5 (RB 5), a representative diazo dye found in textile effluents, by means of ultraviolet irradiation (9W UVA) over TiO(2) suspensions, ultrasound irradiation (80kHz, 135W) and their combined application was investigated. Several commercial TiO(2) catalysts were screened and an anatase Hombicat UV 100 sample exhibited considerable activity in terms of solution decoloration, COD and ecotoxicity reduction. Photocatalytic degradation increased with increasing TiO(2) loading (in the range 0.05-1g/L) and decreasing dye concentration (in the range 120-20mg/L) and solution pH (in the range 9-2.6). At the typical conditions employed in this study (60mg/L dye, 0.25mg/L catalyst, ambient pH=5.8, oxygen sparging), complete decoloration was achieved after 60min of reaction. Addition of H(2)O(2) up to 0.01M hindered degradation, scavenging the photogenerated holes and hydroxyl radicals. Ultrasound irradiation resulted in low decoloration, e.g. less than 10% after 60min at 60mg/L dye and oxygen sparging and slightly improved under an argon atmosphere. The simultaneous application of ultraviolet and ultrasound irradiation resulted in increased decoloration compared to that achieved by photocatalysis and sonolysis operating separately; moreover, the overall sonophotocatalytic effect was greater than the additive effect of the two processes, implying possible synergy.     

Effect of ferric ion added on photodegradation of alachlor in the presence of TiO2 and UV radiation

The effect of in situ photodeposited ferric ion onto TiO2 surface on the degradation of alachlor was investigated in the presence of the UV radiation. The photodegradation rate of alachlor could be described as an apparent first order. The rate constant (K(a)) of alachlor increased from 0.021 to 0.060 h(-1) as the number of coating times increased from 1- to 5-times in the absence of ferric ion, where the corresponding thicknesses of the TiO2 film were 67 and 174 nm. The rate constant (K(a)) increased from 0.030 to 0.060 h(-1) as pH value decreasd from pH 9 to 5 in the presence of only TiO2 immobilised with 5-times of coating. The rate constant increased slightly from 0.031 to 0.050 h(-1) as the concentration of ferric ion increased from 0.75 to 7.5 mg Fe3+ l(-1) in the absence of TiO2. However, those increased from 0.051 to 0.110 h(-1) in the presence of both TiO2 and ferric ion. In situ photodeposition of ferric ion onto the TiO2 surface enhanced the rate constant of photodegradation of alachlor by about 80% with an adding 7.5 mgFe3+ l(-1). During the alachlor photodegradation, three kinds of non-toxic organic compounds derived from alachlor were detected in 1 h.