Preparation and characteristics of high performance paper containing titanium dioxide photocatalyst supported on inorganic fiber matrix

A novel paper-based material containing titanium dioxide (TiO₂) photocatalyst was successfully prepared by a papermaking technique with the internal addition of inorganic fibers on which TiO₂ particles were supported. Photodegradation performance of acetaldehyde gas, an indoor pollutant, and the durability of the TiO₂-containing papers were investigated under UV irradiation. Ceramic fiber suspension and polydiallyldimethylammonium chloride as a cationic flocculant were mixed, followed by the addition of TiO₂ suspension and anionic polyacrylamide. Subsequently, the inorganic mixture was poured into a pulp suspension, and TiO₂ handsheets then prepared by a papermaking method. The tensile strength of TiO₂-containing paper without a ceramic carrier decreased by more than 30% after 240-h UV irradiation (2 mW/cm²), although the strength of the TiO₂ sheet with ceramic fibers remained reasonably stable. The efficiency of acetaldehyde decomposition by the TiO₂ paper containing an inorganic carrier was nearly equal to that of the carrier-free TiO₂ paper. Scanning electron microscopic observation suggested that most TiO₂ particles were predominantly supported on the inorganic fiber matrix, and were mostly out of contact with organic pulp fibers. The TiO₂ paper with an inorganic carrier demonstrated both excellent photocatalytic performance and durability, which before had been mutually incompatible for organic materials containing TiO₂ photocatalyst. The two-stage mixing procedure for TiO₂ sheet-making is promising for the simple manufacture of high performance paper with photocatalytic ability.     

Heterogeneous photocatalytic decomposition of benzene on lanthanum-doped TiO2 film at ambient temperature

Lanthanum-doped anatase TiO₂ thin films on glass prepared via a sol–gel process have been shown to have much higher photocatalytic activity for the degradation of gaseous benzene than pure anatase TiO₂ thin film. The photodecomposition of benzene on both types of TiO₂ films follows the first-order kinetics while the CO₂ and CO formation followed the zero-order kinetics. GC/MS identification of the intermediates produced during the photodegradation of benzene revealed that doping lanthanum into TiO₂ thin film favors a cleavage of benzene ring. An optimal lanthanum amount with respect to photocatalytic activity was about 2.5 wt% (La₂O₃/TiO₂).     

Study on photocatalytic degradation of gaseous dichloromethane using pure and iron ion-doped TiO2 prepared by the sol-gel method

The synthesis of TiO₂ and Fe–TiO₂ by sol–gel method is demonstrated and characterized. The characterization of TiO₂ and Fe–TiO₂ is performed with instruments, including TGA/DTA, FTIR, UV–Vis, N₂ adsorption and SEM. Dichloromethane is used for the photocatalytic activity test. From the results of dichloromethane photocatalyitc degradation, the calcined temperature of TiO₂ and the presence of water vapor influence the photocatalytic activity. The optimum doping amount of iron ions is 0.005 mol%, and this can enhance the photocatalytic activity, while too great an amount will make the iron ions become recombination centers for the electron–hole pairs and reduce the photocatalytic activity. UV–Vis diffuse reflectance spectra of Fe–TiO₂ show an increase in absorbency in the visible light region with the increase in iron ions doping concentration The intermediate of dichloromethane photodegradation includes CHCl₃, CCl₄, CH₂Cl₂ and COCl₂. The presence of iron ions may reduce the adsorption of Cl element on the surface of the photocatalyst.     

Photocatalytic activity of the calcined H-titanate nanowires for photocatalytic oxidation of acetone in air

Hydrogen titanate (H-titanate) nanowires were prepared via a hydrothermal reaction of TiO₂ powders (P25) in KOH solutions and then calcined at various temperatures. The phase structure, crystallite size, morphology, specific surface area, and pore structures of the calcined H-titanate nanowires at various temperatures were characterized with field emission scanning electron microscope, X-ray diffraction, transmission electron microscopy and nitrogen adsorption–desorption isotherms, and their photocatalytic activities were evaluated by photocatalytic oxidation of acetone in air. With increasing calcination temperature, the specific surface area and porosity of the calcined samples steadily decreased. At a calcination temperature range of 400–600 °C, the calcined H-titanate nanowires showed higher photocatalytic activity than P25 powders for photocatalytic oxidation of acetone. Especially, at 500 °C, the calcined H-titanate nanowires showed the highest photocatalytic activity, which exceeded that of P25 by a factor of about 1.8 times. This can be attributed to the synergetic effect of larger specific surface area, higher pore volume and the presence of brookite TiO₂. With further increase in the calcination temperature (700–900 °C), the photocatalytic activity of the samples decreased obviously owing to the growth of TiO₂ crystallites.     

Photocatalytic degradation of phenol by visible light-responsive iron-doped TiO2 and spontaneous sedimentation of the TiO2 particles

Fe-doped TiO₂ was prepared by the calcination of Fe_xTiS₂ (x = 0, 0.002, 0.005, 0.008, 0.01) and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV–visible diffuse reflectance spectra. All the Fe-doped TiO₂ were composed of an anatase crystal form and showed red shifts to a longer wavelength. The activity of the Fe-doped TiO₂ for the degradation of phenol was investigated by varying the iron content during UV (365 nm) and visible light (405 nm and 436 nm) irradiation. The degradation rate depended on the Fe content and the Fe-doped TiO₂ was responsive to the visible light as well as the elevated activity toward UV light. The molar ratio of 0.005 was the optimum for both the UV and visible light irradiations. The result was discussed on the basis of the balance of the excited electron–hole trap by the doped Fe³⁺ and their charge recombination on the doped Fe³⁺ level. The Fe-doped TiO₂ (x = 0.005) was more active than P25 TiO₂ under solar light irradiation. The suspended Fe-doped TiO₂ spontaneously precipitated once the stirring of the reaction mixture was terminated.     

Photocatalytic bacterial inactivation by polyoxometalates

The photocatalytic inactivation (PCI) of Escherichia coli (Gram-negative) and Bacillus subtilis (Gram-positive) was performed using polyoxometalate (POM) as a homogeneous photocatalyst and compared with that of heterogeneous TiO₂ photocatalyst. Aqueous suspensions of the microorganisms (10⁷–10⁸ cfu ml⁻¹) and POM (or TiO₂) were irradiated with black light lamps. The POM-PCI was faster than (or comparable to) TiO₂-PCI under the experimental conditions employed in this study. The relative efficiency of POM-PCI was species-dependent. Among three POMs (H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀, and H₄SiW₁₂O₄₀) tested in this study, the inactivation of E. coli was fastest with H₄SiW₁₂O₄₀ while that of B. subtilis was the most efficient with H₃PW₁₂O₄₀. Although the biocidal action of TiO₂ photocatalyst has been commonly ascribed to the role of photogenerated reactive oxygen species such as hydroxyl radicals and superoxides, the cell death mechanism with POM seems to be different from TiO₂-PCI. While TiO₂ caused the cell membrane disruption, POM did not induce the cell lysis. When methanol was added to the POM solution, not only the PCI of E. coli was enhanced (contrary to the case of TiO₂-PCI) but also the dark inactivation was observed. This was ascribed to the in situ production of formaldehyde from the oxidation of methanol. The interesting biocidal property of POM photocatalyst might be utilized as a potential disinfectant technology.     

Reaction pathways of dimethyl phthalate degradation in TiO2-UV-O2 and TiO2-UV-Fe(VI) systems

The photocatalytic degradation of dimethyl phthalate (DMP) in aqueous TiO₂ suspension under UV illumination has been investigated using oxygen (O₂) and ferrate (Fe(VI)) as electron acceptors. The experiments demonstrated that Fe(VI) was a more effective electron acceptor than O₂ for scavenging the conduction band electrons from the surface of the catalyst. Some major intermediate products from DMP degradation were identified by HPLC and GC/MS analyses. The analytical results identified dimethyl 3-hydroxyphthalate and dimethyl 2-hydroxyphthalate as the two main intermediate products from the DMP degradation in the TiO₂–UV–O₂ system, while in contrast phthalic acid was found to be the main intermediate product in the TiO₂–UV–Fe(VI) system. These findings indicate that DMP degradation in the TiO₂–UV–O₂ and TiO₂–UV–Fe(VI) systems followed different reaction pathways. An electron spin resonance analysis confirmed that hydroxyl radicals existed in the TiO₂–UV–O₂ reaction system and an unknown radical species (most likely an iron–oxo species) is suspected to exist in the TiO₂–UV–Fe(VI) reaction system. Two pathway schemes of DMP degradation in the TiO₂–UV–O₂ and TiO₂–UV–Fe(VI) reaction systems are proposed. It is believed that the radicals formed in the TiO₂–UV–O₂ reaction system preferably attack the aromatic ring of the DMP, while in contrast the radicals formed in the TiO₂–UV–Fe(VI) reaction systems attack the alkyl chain of DMP.     

TiO2-assisted degradation of a perfluorinated surfactant in aqueous solutions treated by gliding arc discharge

The plasma–chemical degradation of Forafac 1110, a perfluorinated non-ionic surfactant, in aqueous solutions was investigated using TiO₂ catalysts. The considered plasma was the gliding arc in humid air, which results from an electric discharge at atmospheric pressure and quasi-ambient temperature. Two titanium dioxide powders were used and their synergistic effects on the Forafac degradation were compared. The results were discussed through the evolution of the pH, the conductivity, the fluoride ions concentration released in solutions, the surfactant concentration remaining after treatment and the chemical oxygen demand (COD) measurement.

The combination of the plasma–chemical treatment with heterogeneous catalysis through the use of TiO₂ accelerated the Forafac degradation, since only 60 min was sufficient to remove 96% instead of 360 min needed in the absence of TiO₂. The use of anatase and rutile under the trade-name of Rhodia TiO₂ and Merck TiO₂, respectively, led to different results, because Rhodia TiO₂ has proven to be more efficient. It would seem that the crystalline phase as well as the crystallite size, explain the efficiency of anatase. The advantage of the plasma-catalysis is due to the fact that there is a significant production of the OH^• radicals not only generated by the gliding arc discharge but also by TiO₂.     

Photodegradation of surfactants. XV: Formation of SO4 ions in the photooxidation of sulfur-containing surfactants

Three types of surfactants and related reference compounds containing sulfonate (-SO₃Na), sulfate (-OSO₃Na) or thioether carboxylate (-S-Cn-COOK) group were photodecomposed in an aqueous heterogeneous dispersion system. The photomineralization to SO₄²⁻ions was examined for the surfactants with different chemical structures. The photocatalytic activities of TiO₂ and ZnO were compared for the sulfonates of dodecylbenzene sulfonate (DBS) and polystyrene sulfonate (PSS), the sulfates of sodium dodecyl sulfate (SDS), and the potassium salts of S-dodecylthioglycol acid (TGA), S-dodecylthiopropionic acid (TPA) and S-dodecylthiomalic acid (TMA). ZnO catalyst exhibited higher activity in the formation of SO₄²⁻ion than TiO₂ catalyst.     

Removal of humic acid using TiO2 photocatalytic process--fractionation and molecular weight characterisation studies

The photocatalytic removal of humic acid (HA) using TiO₂ under UVA irradiation was examined by monitoring changes in the UV₂₅₄ absorbance, dissolved organic carbon (DOC) concentration, apparent molecular weight distribution, and trihalomethane formation potentials (THMFPs) over treatment time. A resin fractionation technique in which the samples were fractionated into four components: very hydrophobic acids (VHA), slightly hydrophobic acids, hydrophilic charged (CHA) and hydrophilic neutral (NEU) was also employed to elucidate the changes in the chemical nature of the HA components during treatment. The UVA/TiO₂ process was found to be effective in removing more than 80% DOC and 90% UV₂₅₄ absorbance. The THMFPs of samples were decreased to below 20 μg l⁻¹ after treatments, which demonstrate the potential to meet increasingly stringent regulatory level of trihalomethanes in water. Resin fractionation analysis showed that the VHA fraction was decreased considerably as a result of photocatalytic treatments, forming CHA intermediates which were further degraded with increased irradiation time. The NEU fraction, which comprised of non-UV-absorbing low molecular weight compounds, was found to be the most persistent component.     

Applicability of micellar electrokinetic chromatography to kinetic studies of photocatalytic oxidation of dibenzo-p-dioxin

Micellar electrokinetic chromatography (MEKC) using sodium dodecyl sulfate was tested for its ability to analyze mixtures containing three dioxin derivatives. Dioxin-containing samples, subjected to photocatalytic oxidation using TiO₂ and/or SnO₂ as the semiconductor catalyst were analyzed by MEKC to obtain the rate constant for each dioxin when it was degraded individually or in the presence of the other two dioxins. A significant degree of analyze competition was observed and the mixed catalyst offered important synergism.     

Sequential photochemical-biological degradation of chlorophenols

UV/TiO₂/H₂O₂, UV/TiO₂ and UV/H₂O₂ were compared as pre-treatment processes for the detoxification of mixtures of 4-chlorophenol (4CP), 2,4-dichlorophenol (DCP), 2,4,6-trichlorophenol (TCP) and pentachlorophenol (PCP) prior to their biological treatment. When each chlorophenol was initially supplied at 50 mg l⁻¹, UV/TiO₂/H₂O₂ treatment supported the highest pollutant removal, COD removal, and dechlorination efficiencies followed by UV/TiO₂ and UV/H₂O₂. The remaining toxicity to Lipedium sativum was similar after all pre-treatments. Chlorophenol photodegradation was always well described by a first order model kinetic (r² > 0.94) and the shortest 4CP, DCP, TCP and PCP half-lives of 8.7, 7.1, 4.5 and 3.3 h, respectively, were achieved during UV/TiO₂/H₂O₂ treatment. No pollutant removal was observed in the controls conducted with H₂O₂ or TiO₂ only. Inoculation of all the photochemically pre-treated mixtures with activated sludge microflora was followed by complete removal of the remaining pollutants. Combined UV/TiO₂/H₂O₂-biological supported the highest detoxification, dechlorination (99%) and COD removal (88%) efficiencies. Similar results were achieved when each chlorophenol was supplied at 100 mg l⁻¹. COD and Cl mass balances indicated UV, UV/H₂O₂, and UV/TiO₂ treatments lead to the formation of recalcitrant photoproducts, some of which were chlorinated.     

Compound-specific chlorine isotope analysis of polychlorinated biphenyls isolated from Aroclor and Clophen technical mixtures

The combined electrochemical oxidation-solar-light/immobilized TiO₂ film process was conducted to degrade an azo dye, Reactive Black 5 (RB5). The toxicity was also monitored by the Vibrio fischeri light inhibition test. The electrochemical oxidation rapidly decolorized RB5 (55, 110 μM) with a supporting electrolyte of 2 g l⁻¹ NaCl at current density 277 A m⁻² and pH 4. However, TOC mineralization and A₃₁₀ removal were low. Additionally, the treated solution showed high biotoxicity. RB5 at 110 μM significantly retarded the de-colorization efficiency by using the solar-light/immobilized TiO₂ film process. The combined electrochemical oxidation-solar-light/immobilized TiO₂ process effectively increased the removal of color, A₃₁₀, and TOC. The toxicity was also significantly reduced after 3 h of solar irradiation. The results indicated that the low-cost combined process is a potential technique for rapid treatment of RB5.     

Radiolysis of aqueous 4-nitrophenol solution with Al2O3 or TiO2 nanoparticles

Aqueous 4-nitrophenol solutions containing TiO₂ or Al₂O₃ nanoparticles were irradiated with electron beam. 4-nitrophenol was decomposed by the ionizing radiation process in the absence of the nanoparticles. The addition of TiO₂ or Al₂O₃ (2 g l⁻¹) before irradiation improved the removal of 4-nitrophenol, total organic carbon (TOC) but also nitrogen (TN). To identify the origin of the loss (catalysis or simply adsorption), TiO₂ or Al₂O₃ nanoparticles were added after irradiation. Experiments show that the effect of the presence of TiO₂ or Al₂O₃ during irradiation is just due to adsorption.     

Photocatalytic degradation of trinitrotoluene and other nitroaromatic compounds

The photocatalytic degradation of 2,4,6-trinitrotoluene and ten other nitroaromatic compounds in aerated TiO₂ suspensions has been studied. The following order of reactivity was observed: nitrotoluenes > nitrobenzene > dinitrotoluenes . dinitrobenzenes > 2,4,6-trinitrotoluene > 1,3,5-trinitrobenzene, which reflects the known influence of nitro groups towards the attack of electrophilic reagents on the aromatic molecule.     

Pulsed discharge plasma induced Fenton-like reactions for the enhancement of the degradation of 4-chlorophenol in water

To sufficiently utilize chemically active species and enhance the degradation rate and removal efficiency of toxic and biorefractory organic pollutant para-chlorophenol (para-CP), the introductions of iron metal ions (Fe²⁺/Fe³⁺) into either pulsed discharge plasma (PDP) process or the PDP process with TiO₂ photo-catalyst were tentatively performed. The experimental results showed that under the same experimental condition, the degradation rate and removal efficiency of para-CP were greatly enhanced by the introduction of iron ions (Fe²⁺/Fe³⁺) into the PDP process. Moreover, when iron ions and TiO₂ were added together in the PDP process, the degradation rate and removal energy of para-CP further improved. The possible mechanism was discussed that the obvious promoting effects were attributed to ferrous ions via plasma induced Fenton-like reactions by UV light irradiation excited and hydrogen peroxide formed in pulsed electrical discharge, resulting in a larger amount of hydroxyl radicals produced from the residual hydrogen peroxide. In addition, the regeneration of ferric ions to ferrous ions facilitates the progress of plasma induced Fenton-like reactions by photo-catalytic reduction of UV light, photo-catalytic reduction on TiO₂ surface and electron transfer of quinone intermediates, i.e. 1,4-hydroquinone and 1,4-benzoquinone.     

Capillary electrophoresis applied to kinetic studies of photocatalytic oxidation of substituted anilines

Cyclodextrin modified capillary electrophoresis (CMCE) and micellar electrokinetic chromatography (MEKC) were tested for their ability to analyze mixtures containing substituted anilines. CMCE separated analytes by their partitioning preference between the two substituted cyclodextrins while MEKC exploited the analyte partitioning between the micelles and the bulk aqueous phase. MEKC was selected to analyze samples that were subjected to photocatalytic oxidation using TiO₂ and/or SnO₂ as the semiconductor catalyst. The rate constant for each compound was determined when it was degraded individually or in the presence of other derivatives in the same family, indicating a significant degree of analyte competition and synergism offered by the mixed catalyst. The results demonstrated that CE, with its minimal sample preparation and requirement, is a valuable tool, particularly suitable for studying highly toxic pollutants, without generating a high volume of waste.     

Arsenic removal from water employing heterogeneous photocatalysis with TiO2 immobilized in PET bottles

Arsenic oxidation (As(III) to As(V)) and As(V) removal from water were assessed by using TiO₂ immobilized in PET (polyethylene terephthalate) bottles in the presence of natural sunlight and iron salts. The effect of many parameters was sequentially studied: TiO₂ concentration of the coating solution, Fe(II) concentration, pH, solar irradiation time; dissolved organic carbon concentration. The final conditions (TiO₂ concentration of the coating solution: 10%; Fe(II): 7.0 mg l⁻¹; solar exposure time: 120 min) were applied to natural water samples spiked with 500 μg l⁻¹ As(III) in order to verify the influence of natural water matrix. After treatment, As(III) and total As concentrations were lower than the limit of quantitation (2 μg l⁻¹) of the voltammetric method used, showing a removal over 99%, and giving evidence that As(III) was effectively oxidized to As(V). The results obtained demonstrated that TiO₂ can be easily immobilized on a PET surface in order to perform As(III) oxidation in water and that this TiO₂ immobilization, combined with coprecipitation of arsenic on Fe(III) hydroxides(oxides) could be an efficient way for inorganic arsenic removal from groundwaters.     

Effects of Fe2O3, organic matter and carbonate on photocatalytic degradation of lindane in the sediment from the Liao River,China

Fourteen sediment samples with different content of Fe₂O₃ were collected from the lower reaches of the Liao River in China. The photodegradation of lindane on the surfaces of these sediments was investigated to observe the effects of Fe₂O₃ and other photoinducable substances, such as TiO₂ and organic substances, on photodegradation of lindane. A partial least-squares (PLS) analysis model was developed to find out the statistical relationship between the photodegradation and the contents of these photoinducable substances. It was concluded from the PLS analysis that inorganic carbon and organic carbon have negative effects, whereas Fe₂O₃ and TiO₂ accelerate the photodegradation of lindane in the sediment samples when 365 nm UV light was used as light source. In all cases of the experiments, the photodegradation of lindane in the sediment samples were fitted for pseudo-first-order kinetics.     

Comparative studies of the Fe3+/2+-UV, H2O2-UV, TiO2-UV/vis systems for the decolorization of a textile dye X-3B in water

The degradation of a common textile dye, Reactive-brilliant red X-3B, by several advanced oxidation technologies was studied in an air-saturated aqueous solution. The dye was resistant to the UV illumination (wavelength λ  320 nm), but was decolorized when one of Fe³⁺, H₂O₂ and TiO₂ components was present. The decolorization rate was observed to be quite different for each system, and the relative order evaluated under comparable conditions followed the order of Fe²⁺–H₂O₂–UV  Fe²⁺–H₂O₂ > Fe³⁺–H₂O₂–UV > Fe³⁺–H₂O₂ > Fe³⁺–TiO₂–UV > TiO₂–UV > Fe³⁺–UV > TiO₂–visible light (λ  450 nm) > H₂O₂–UV > Fe²⁺–UV. The mechanism for each process is discussed, and linked together for understanding the observed differences in reactivity.