Aqueous TiO2 Suspensions Research Articles

EPR spin trapping evidence of radical intermediates in the photo-reduction of bicarbonate/CO2 in TiO2 aqueous suspensions.

Using the EPR spin trapping technique, we prove that simultaneous reactions take place in illuminated suspensions of TiO2 in aqueous carbonate solutions (pH ≈ 7). The adsorbed HCO3(-) is reduced to formate as directly made evident by the detection of formate radicals (˙CO2(-)). In addition, the amount of OH˙ radicals from the photo-oxidation of water shows a linear dependence on the concentration of bicarbonate, indicating that electron scavenging by HCO3(-) increases the lifetime of holes. In a weakly alkaline medium, photo-oxidation of HCO3(-)/CO3(2-) to ˙CO3(-) interferes with the oxidation of water. A comparative analysis of different TiO2 samples shows that formation of ˙CO2(-) is influenced by factors related to the nature of the surface, once expected surface area effects are accounted for. Modification of the TiO2 surface with noble metal nanoparticles does not have unequivocal benefits: the overall activity improves with Pd and Rh but not with Ru, which favours HCO3(-) photo-oxidation even at pH = 7. In general, identification of radical intermediates of oxidation and reduction reactions can provide useful mechanistic information that may be used in the development of photocatalytic systems for the reduction of CO2 also stored in the form of carbonates.

Improved Photocatalytic Activity of TiO2 on the Addition of CuWO4

Various methods that aim to improve the photocatalytic activity of TiO2 have been reported in the literature. Herein, we report that addition of CuWO4 into the aqueous suspension of TiO2 can result in significant enhancement in the rate of phenol degradation. As the amount of CuWO4 increased, the rate of phenol degradation increased and then decreased. A maximum rate of phenol degradation observed with 2 wt % CuWO4 was about 2.83 times that in the absence of CuWO4. A similar result was also observed with CuO. However, six consecutive tests showed that CuWO4/TiO2 was much more stable than CuO/TiO2, due to the very high stability of CuWO4 against photocorrosion. The improved activity of TiO2 is not due to CuWO4 and CuO themselves and also does not match their solubility in aqueous solution. Moreover, for the generation of OH radicals, and for the decomposition of H2O2 in aqueous solution, CuWO4/TiO2 was also more active than TiO2. Through a (photo) electrochemical measurement, a possible mechanism is proposed, involving electron transfer from the irradiated TiO2 to CuWO4 that facilitates the charge separation of TiO2 and consequently accelerates reactions at interfaces.

Retrieval of particle size and shape distributions of TiO2 and BaTiO3 aqueous suspensions using measurement data on the scattering matrix

Results of retrieval of size and shape distributions of particles of a dispersive medium using measured data on the scattering matrix of aqueous suspensions of titanium oxide and barium titanate are given. Retrieval of the distributions has been performed by solving the optimization problem for the sum of squared deviations of experimental values of matrix elements and their values calculated using the model of spheroidal scatterers. It has been shown that the particle size distribution is retrieved more accurately upon minimizing the sum of squared deviations for the sum of diagonal elements, while the more accurate shape distribution (ratio of dimensions) is obtained upon minimizing the sum of squared deviations only for the element F11.

ChemInform Abstract: Simultaneous Removal of Nitrite and Ammonia as Dinitrogen in Aqueous Suspensions of a Titanium(IV) Oxide Photocatalyst under Reagent‐Free and Metal‐Free Conditions at Room Temperature.

AbstractPhotocatalytic reduction of nitrite in an aqueous suspension of TiO2 in the presence of ammonia results in the formation of N2 under irradiation of UV light at room temperature.

Efficient removal of sulcotrione and its formulated compound Tangenta® in aqueous TiO2 suspension: Stability, photoproducts assessment and toxicity

The photocatalytic degradation of the herbicide sulcotrione (0.05mM) and its formulated compound Tangenta® in aqueous suspensions of TiO2 Degussa P25 was examined as a function of the different operational parameters. The optimum of the catalyst loading was found to be 2.0mgmL−1 under UVA light. In the first stage of the reaction, the photocatalytic degradation of sulcotrione alone and in Tangenta® followed the pseudo-first order kinetics, in which the heterogeneous catalysis proceeds via OH and holes. Further, it can be concluded that degradation rate of sulcotrione alone is about two times higher compared to formulated compound. The results showed that the disappearance of sulcotrione led to the formation of three organic intermediates and ionic byproducts (Cl−, SO42−, acetate and formate), whereas their mineralization was about 90% after 4h. Tentative photodegradation pathways were proposed and discussed. Also, there was no significant toxicity observed after the irradiation of sulcotrione solution and Tangenta® formulation using TiO2 catalyst on three mammalian cell lines.

Assessment of ζ-potential in TiO2 aqueous suspensions: A comparative study based on thermodynamic and rheological methods

It is widely demonstrated that the distinct electrokinetic techniques lead to different values of ζ-potential despite what should be expected from the classical theory. Electrophoresis and conductivity measurements were used to estimate the ζ-potential and the electrical double layer parameters of TiO2 aqueous suspensions. An assessment of the results was made following thermodynamic and rheological methods. In the first case, the Gibbs free energy increment was compared. In the second one, the yield stress/ζ-potential plots were used. In spite of the ζ-potentials obtained from conductivity, electrophoresis and electrophoresis-conductivity combined techniques are different, the Gibbs free energy increment values do not discriminate the best ζ-potential value. Moreover, the correlation between the yield stress and the square of the ζ-potential has shown that all the ζ-potentials can physically be acceptable. Therefore, the only reasonably conclusion from this study is that the use of combined techniques, like electrophoresis-conductivity, should always be taken as the more cautious approach of evaluating ζ-potentials.

Photocatalytic Oxidation of Dimethyl Methylphosphonate in Aqueous Suspensions of TiO2

Heterogeneous photocatalysis using simulated solar irradiation in the presence of titanium dioxide was studied using dimethyl methylphosphonate (DMMP) as a surrogate for organophosphorus compounds including pesticides and chemical warfare agents. A reduction in solution pH accompanied DMMP degradation with the pH approaching the pKa of DMMP. Addition of peroxide before reaction increased the amount of DMMP degraded. Nearly complete destruction of DMMP was only achieved with the largest addition of peroxide. Photoreaction of DMMP in a pH 10 solution and a pH 10 carbonate buffer degraded a similar percent of the solute, with a more gradual reaction observed for the buffered solution. Variations in reaction conditions were studied to evaluate the effect of potential spill conditions. The presence of fulvic acid in the solution slightly enhanced the oxidation of DMMP and the absence of dissolved oxygen inhibited DMMP oxidation. DMMP solutions prepared in simulated sea water reached similar degradation percentages as pure solutions, however, reacted more slowly.

Oxidation of Sulfonamides in Aqueous Solution by UV-TiO2-Fe(VI).

The photocatalytic degradation of sulfonamides in aqueous TiO2 suspension under UV irradiation has been investigated using potassium ferrate as electron acceptors. The results showed that the stability of Fe(VI) is dependent on pH significantly, and the stability reduces obviously in the presence of UV-TiO2. The experiments indicated that Fe(VI) could effectively scavenge the conduction band electrons from the surface of TiO2. The photocatalytic oxidation of sulfonamides with Fe(VI) was found to be much faster than that without Fe(VI). The SD, SM, and SMX concentration was greatly reduced by 89.2%, 83.4%, and 82.0%, respectively, after 10 min with UV-TiO2-Fe(VI), comparing to 65.2%, 66.0%, and 71.9%, respectively, with Fe(VI) only in the dark and 71.3%, 72.7%, and 76.0%, respectively, with UV-TiO2. The pH value of solution significantly influenced the sulfonamides degradation in UV-TiO2-Fe(VI) system. The degradation amount of sulfonamides after 10 min was a maximum at pH 7. The intermediate products of sulfonamides oxidation by UV-TiO2-Fe(VI) were analysed by LC-HESI-MS-MS and the results suggested that a majority of sulfonamides turned into large-molecule products without complete mineralization.

Adsorption and photocatalytic reduction of U(VI) in aqueous TiO2 suspensions enhanced with sodium formate

In this work, the effects of sodium formate on adsorption and photocatalytic reduction of U(VI) on the TiO2 surface were investigated. The results showed that sodium formate may enhance the adsorption of U(VI) on the TiO2 surface, the maximum adsorption capacity of U(VI) in the presence of sodium formate increased by 44 mg g−1. In addition, the photocatalytic reduction of U(VI) can be also enhanced, the reduction rate constant of U(VI) in the presence of sodium formate increased 17-fold. Results from XPS analysis clearly indicated the reduction of U(VI) to U(IV) on the TiO2 surface.

Deposition of nanostructures derived from electrostatically stabilised TiO2 aqueous suspension onto a biocomposite

A nanostructure derived from TiO2 particle deposition onto a biocomposite surface derived from coir dust (CD) was developed to control degradation using a spray dry technique. To stabilise and reduce the size of dispersed particles, the TiO2 powder was prepared in deionised water at pH 10 and sonicated at 20kHz and 400W. The coir dust was obtained from coconut kernel waste and underwent drying treatment before it was mixed with polypropylene (PP) as the substrate. The suspension consisted of particles with an average size and zeta value of 285nm and −19.2mV, respectively. The suspension was spray dried onto a hot-pressed substrate (biocomposite) with a surface roughness between 0.23 and 1.57μm at ambient temperature. Scanning electron microscopy image analysis and Fourier transform infrared spectroscopy analysis indicated that the TiO2 particles were successfully deposited onto the substrate, shown by the existence of a carboxylic acid group (COOH) in the CD matrix. Moreover, the weight of the deposited substrate increased exponentially with deposition time compared to pure PP substrate. However, the deposition rate of TiO2 nanoparticles was limited by the ratio of the substrate surface roughness to particle diameter, as predicted by a previous study.

Photocatalytic H₂ Evolution Using Different Commercial TiO₂ Catalysts Deposited with Finely Size-Tailored Au Nanoparticles: Critical Dependence on Au Particle Size.

One weight percent of differently sized Au nanoparticles were deposited on two commercially available TiO2 photocatalysts: Aeroxide P25 and Kronos Vlp7000. The primary objective was to investigate the influence of the noble metal particle size and the deposition method on the photocatalytic activity. The developed synthesis method involves a simple approach for the preparation of finely-tuned Au particles through variation of the concentration of the stabilizing agent. Au was deposited on the TiO2 surface by photo- or chemical reduction, using trisodium citrate as a size-tailoring agent. The Au-TiO2 composites were synthetized by in situ reduction or by mixing the titania suspension with a previously prepared gold sol. The H2 production activities of the samples were studied in aqueous TiO2 suspensions irradiated with near-UV light in the absence of dissolved O2, with oxalic acid or methanol as the sacrificial agent. The H2 evolution rates proved to be strongly dependent on Au particle size: the highest H2 production rate was achieved when the Au particles measured ~6 nm.

Photocatalytic oxidation mechanism of methanol and the other reactants in irradiated TiO2 aqueous suspension investigated by OH radical detection

The oxidation reaction mechanisms of organic additives, such as alcohols (MeOH and EtOH) and inorganic ions (I−, Br−and SCN−) in photocatalysis were elucidated by a fluorescence probe method by employing coumarin and coumarin-3-carboxylic acid as fluorescence probes. The general scheme of the oxidation and probing of OH radicals with coumarin was proposed. For the inorganic ions the photocatalytic oxidization was found to take place through OH radicals while for the organic molecules the oxidization should proceed by the exertion of the holes trapped on the TiO2 surface. This is supported by the fact that the quantum efficiency of the CO2 formation by MeOH oxidation was about ten times larger than that of the OH radical formation. The OH radicals in solution are most probably in equilibrium with the trapped holes, and the equilibrium constant could be estimated to be about 0.014 by assuming the diffusion controlled reaction of MeOH with trapped holes.

Radical intermediates in photoinduced reactions on TiO2 (an EPR spin trapping study).

The radical intermediates formed upon UVA irradiation of titanium dioxide suspensions in aqueous and non-aqueous environments were investigated applying the EPR spin trapping technique. The results showed that the generation of reactive species and their consecutive reactions are influenced by the solvent properties (e.g., polarity, solubility of molecular oxygen, rate constant for the reaction of hydroxyl radicals with the solvent). The formation of hydroxyl radicals, evidenced as the corresponding spin-adducts, dominated in the irradiated TiO2 aqueous suspensions. The addition of 17O-enriched water caused changes in the EPR spectra reflecting the interaction of an unpaired electron with the 17O nucleus. The photoexcitation of TiO2 in non-aqueous solvents (dimethylsulfoxide, acetonitrile, methanol and ethanol) in the presence of 5,5-dimethyl-1-pyrroline N-oxide spin trap displayed a stabilization of the superoxide radical anions generated via electron transfer reaction to molecular oxygen, and various oxygen- and carbon-centered radicals from the solvents were generated. The character and origin of the carbon-centered spin-adducts was confirmed using nitroso spin trapping agents.

A mechanism of the photocatalytic decomposition of 2,4-dinitrophenol on TiO2 immobilized on a glass surface

A reaction mechanism of the photocatalytic decomposition of 2,4-dinitrophenol (DNP) on TiO2 immobilized on a glass surface is investigated both theoretically and experimentally. The experimental results show that (1) the DNP concentration decreases linearly in the same manner; (2) the initial rate of decomposition increases with an increase in the initial DNP concentration, i.e., the reaction is not inhibited by DNP; and (3) product inhibition is present, but it is not as strong as that in the aqueous TiO2 suspension. To elucidate the mechanism of the photocatalytic reaction, a mathematical model is constructed by taking into consideration an electrostatic interaction between positively-charged TiO2 and negatively-charged DNP. The calculated results suggest that mixing a solution breaks down the DNP-aggregate layer formed in the neighborhood of TiO2 film and removes reactant inhibition by DNP. The results are also compared with our previous result for the DNP decomposition in an aqueous TiO2 suspension and the difference is identified.

Photocatalytic degradation kinetics and mechanisms of fungicide tebuconazole in aqueous TiO2 suspensions

The photocatalytic degradation of tebuconazole (TEB), a common fungicide with numerous agricultural and urban uses, has been investigated under simulated solar irradiation. Response surface methodology (RSM) was employed to assess individual and interactive effects of the three main independent parameters (initial TEB concentration, irradiation intensity and catalyst concentration) on the efficiency of the process. Optimization results showed that maximum TEB degradation and TOC removal was achieved at the optimum conditions: CTEB=1mgL−1, CTiO2=550 mg L−1, I=650Wm−2. By means of high resolution accurate mass liquid chromatography (HR-LC–MS) and gas chromatography–mass spectroscopy (GC–MS) nine transformation products were identified. The transformation of TEB pathways was found to proceed through tert-butyl chain cleavage, hydroxylation, oxidation and dechlorination pathways. Scavenging experiments showed that the degradation mechanism of TEB was mainly driven by HO• and h+. Finally, the application of an acute toxicity test (Vibrio fischeri) to selected water samples resulted in a non-inhibitory effect of the bacterial bioluminescence.

UV-induced photoactive adsorption mechanism of arsenite by anatase TiO2 with high surface hydroxyl group density

Adsorption and photo-oxidation using anatase TiO2 is a promising technique for arsenite removal from aqueous solution. However, adsorption and photo-oxidation at the solid/liquid interface of TiO2 in aqueous suspensions is a complex reaction process. In this study, we use a simple and inexpensive synthesis method for this type of TiO2 and assess its arsenite adsorption and photo-oxidation performance. The prepared anatase TiO2 is rich in hydroxyl groups, with a hydroxyl concentration of approximately 12.84nm−2. Under irradiation with UV light, the adsorption capacity increases and the residual concentration of arsenic in the solution is lower than 10μgL−1. According to zeta potential, FTIR, and XPS analyses, the hydroxyl groups on the anatase TiO2 surface are responsible for arsenic adsorption. The effect of hydroxyl radicals is investigated through scavenger experiments, the results of which indicate that hydroxyl radicals play a key role in the oxidation of arsenite on anatase TiO2 under UV irradiation. The number of hydroxyl groups on the anatase TiO2 increases upon UV illumination, indicating that these are the photoactive species for arsenic adsorption.

Photocatalytic degradation of p-chlorophenol by hybrid H2O2 and TiO2 in aqueous suspensions under UV irradiation

In this study, TiO2 particles were used as photocatalysts for the degradation of aqueous p-chlorophenol (p-CP) under UV irradiation. The effect of TiO2 dose (0–3 g/L), initial p-CP concentration, H2O2 concentration (2–45 mM), solution pH (4.6–9.5), and UV light intensity on the degradation of p-CP were examined. Four oxidative degradation processes, which utilized UV alone (direct photolysis), H2O2/UV, TiO2/UV, and H2O2/TiO2/UV, were compared in a batch photoreactor with a 100-W high-pressure mercury lamp. The photodegradation of p-CP could be described by the pseudo-first-order kinetics according to the Langmuir–Hinshelwood model. Moreover, the apparent degradation rate constants increased considerably from 3.5 × 10−3 min−1 (direct photolysis) to 19.9 × 10−3 min−1 (H2O2/TiO2/UV system).

Photocatalytic treatment of metribuzin herbicide over TiO2 aqueous suspensions: Removal efficiency, identification of transformation products, reaction pathways and ecotoxicity evaluation

Abstract The present study describes the detailed transformation and mineralization of metribuzin, a widely used herbicide, by TiO 2 mediated photocatalysis under simulated solar irradiation. Transformation of MET by heterogeneous photocatalysis with TiO 2 ([metribuzin] = 10 mg L −1 , [TiO 2 ] = 100 mg L −1 and I = 750 W m −2 ) was completed within 40 min irradiation, whereas 80% of mineralization was achieved within 300 min irradiation. The degradation for both metribuzin and total organic carbon (TOC) followed pseudo-first order kinetics. Rate constants of k app (MET) = 10.85 × 10 −1 min −1 and k app (TOC) = 5.2 × 10 −3 min −1 and half-lives ( t 1/2 ) of 6.4 min and 133.3 min were recorded for metribuzin and TOC respectively. High resolution accurate mass spectrometry coupled with liquid chromatography (HR-LC–MS) and gas chromatography–mass spectrometry (GC–MS) techniques were used to identify the degradation products. The main metribuzin phototransformation pathways were observed to be the cleavage of NH 2 and SCH 3 groups, hydroxylation and dealkylation of tert -butyl chain. The investigation of the role of the reactive species suggests that HO • , h + and O 2 •− play the principal role in the degradation mechanism of metribuzin. Substrate decomposition is accompanied by the formation of intermediates compounds more toxic than the parent molecule, as evidenced by the measurement of acute toxicity using the Vibrio fischeri bacteria test.

Titania-based photocatalytic degradation of two nucleotide bases, cytosine and uracil

The photocatalytic degradation of two components of DNA and RNA, cytosine (C4H5N3O) and uracil (C4H4N2O2) differing only by the presence of an amine or a carbonyl group was investigated in the presence of UV-irradiated TiO2 aqueous suspensions. The adsorption in the dark and under UV-A conditions, the photolysis, the kinetics of degradation, the fate of nitrogen and the identification of some intermediate products were investigated. The impact of pyrimidine cycles on the coverage of TiO2 under UV-A, the effect of NH2 substituent on the oxidation products and mineralization and the importance of carbonyl and amine groups on the fate of nitrogen atoms were evaluated. Electronic density was used to propose a possible chemical pathway. The comparison of the disappearance and mineralization rates in the photocatalytic process was discussed.

Photo-induced toxicity of titanium dioxide nanoparticles to Daphnia magna under natural sunlight

Titanium dioxide nanoparticles (TiO2 NP) are one of the most abundantly utilized nanoparticles in the world. Studies have demonstrated the ability of the anatase crystal of TiO2 NP to produce reactive oxygen species (ROS) in the presence of ultraviolet radiation (UVR), a co-exposure likely to occur in aquatic ecosystems. The goal of this study was to examine the photo-induced toxicity of anatase TiO2 NP under natural sunlight to Daphnia magna. D. magna were exposed to a range of UVR intensities and anatase TiO2 concentrations in an outdoor exposure system using the sun as the source of UVR. Different UVR intensities were achieved using UVR opaque and transparent plastics. AnataseTiO2-NP demonstrated the reciprocal relationship seen in other phototoxic compounds such as polycyclic aromatic hydrocarbons (PAHs) at higher UVR treatments. The calculated 8h LC50 of anatase TiO2 NP was 139ppb under full intensity ambient natural sunlight, 778ppb under 50% natural sunlight, and >500ppm under 10% natural sunlight. Mortality was also compared between animals allowed to accumulate a body burden of anatase TiO2 for 1h and organisms whose first exposure to anatase TiO2 aqueous suspensions occurred under UVR. A significantly greater toxic effect was observed in aqueous, low body burden suspensions than that of TiO2 1h body burdens, which is dissimilar from the model presented in PAHs. Anatase TiO2 presents a unique photo-induced toxic model that is different than that of established phototoxic compounds.