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Electron transfer on a titanium dioxide/electrolyte solution interface has been studied. As observed by other researchers on similar interfaces (TiO2- and ZnO- electrolyte solution), slow consumption of OH- ions was found. A theoretical model has been developed for calculating the change in Fermi energy levels of both electrolyte solution and semiconductor, showing that ion consumption from the solution is favoured by a decreased difference in their Fermi energies. A kinetic constant (υ) is found to characterise the consumption process, its value increasing with electrolyte and semiconductor mass concentrations. Furthermore, this process may be used to estimate the point of zero charge of a titanium dioxide colloidal dispersion.En este trabajo se ha estudiado un proceso de transferencia de electrones en la interfase dióxido de titanio/electrolito acuoso. Tal y como han observado otros investigadores en interfases similares (TiO2- y ZnO- electrolito), se ha detectado un consumo lento de iones OH-. Para dar explicación a este proceso, se ha desarrollado un modelo teórico basado en el cálculo de las energías de Fermi en el semiconductor y en el electrolito. De este modo, se demuestra que dicho consumo de iones está favorecido por una disminución de la diferencia entre ambos niveles de Fermi. Para caracterizar el proceso de consumo lento de OH- se define una constante cinética (υ), cuyo valor aumenta a medida que crece la concentración másica de semiconductor y de electrolito en la suspensión. Adicionalmente, este fenómeno proporciona una herramienta para determinar experimentalmente el punto de carga nula de la suspensión de dióxido de titanio en el medio acuoso.

Abstract: In recent years, there has been a tremendous amount of research and development in the area of photocatalysis (heterogeneous and homogeneous), a process included in a special class of oxidation techniques defined as Advanced Oxidation Processes (AOPs), all characterized by the same chemical feature, production of ls. This paper reviews the use of sunlight to produce the ls by TiO2 photocatalysis and photo-Fenton process. The reacting systems necessary for performing solar photocatalysis are described. The paper also summarizes most of the research carried out related to solar photocatalytic degradation of water contaminants, and how it could significantly contribute to the treatment of persistent toxic compounds. It outlines how to enhance the process efficiency by integration with biotreatment. Various solar reactors for photocatalytic water treatment mainly based on non-concentrating collectors built during the last few years are also described in detail in this review, as well as the use of the solar photocatalytic processes to inactivate microorganisms present in water, placing special emphasis on experimental systems made to optimize this disinfection technique. [Copyright &y& Elsevier]

A new granulated version of the well-known P-25 titanium dioxide (VP AEROPERL® P-25/20 (Aeroperl)) has been tested to determine whether its photocatalytic efficiency is good enough for use in photocatalytic water purification and to find out if it can be separated from water more easily than its well-known homologue, powdered Degussa P-25, a significant technical improvement that might eliminate the tedious final filtration necessary with a slurry. Furthermore, a new commercial catalyst (PC-100 from Millennium Inorganic Chemicals), having a surface area and structure that are both different from Degussa P-25, has also been studied. All the experiments were carried out in sunlight in the pilot compound parabolic collector (CPC) plant at the Plataforma Solar de Almerı´a. Three different substrates were chosen as model molecules for this study: dichloracetic acid, phenol and the pesticide imidacloprid. Results show that Degussa Aeroperl is not a good alternative to powdered Degussa P-25 because of its spontaneous sedimentation during photocatalysis. Millennium PC-100 efficiency seems to be in the same range as that of Degussa P-25. In this work we also attempt to demonstrate that the comparison of efficiencies of different photocatalysts is not a trivial matter. Many factors are involved and interfere in the testing of photocatalyst behaviour during the degradation of a contaminant. A thorough comparison of photocatalyst activity should include reactions with several different substrates under varied experimental conditions. [Copyright &y& Elsevier]

The technical feasibility and performance of photocatalytic degradation of four water-soluble pesticides (diuron, imidacloprid, formetanate and methomyl) have been studied at pilot scale in two well-defined systems of special interest because natural-solar UV light can be used: heterogeneous photocatalysis with titanium dioxide and homogeneous photocatalysis by photo-Fenton. The pilot plant is made up of compound parabolic collectors (CPCs) specially designed for solar photocatalytic applications. Experimental conditions allowed disappearance of pesticide and degree of mineralisation achieved in the two photocatalytic systems to be compared. In order to assure that the photocatalytic results are consistent, hydrolysis and photolysis tests have been performed with the four pesticides. The initial concentration tested with imidacloprid, formetanate and methomyl was 50 and 30 mg/l with diuron, and the catalyst concentrations were 200 mg/l and 0.05 mM with TiO2 and iron, respectively. Total disappearance of the parent compounds and 90% mineralisation have been attained with all pesticides tested, methomyl being the most difficult to be degraded with both treatments. First-order rate constants, initial rate, time necessary for mineralising 90% of the initial TOC and hydrogen peroxide consumption were calculated in all cases, enabling comparison both of treatments and of the selected pesticide reactivity. [Copyright &y& Elsevier]

Interest has grown in the modification of titanium dioxide with graphene to improve the photocatalytic behaviour. In this work, titanium dioxide–reduced graphene oxide (TiO 2 –RGO) composites were synthesised by the photocatalytic reduction of exfoliated graphene oxide (GO) by TiO 2 (Evonik P25) under UV irradiation in the presence of methanol as a hole acceptor. The composite materials were characterised using high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Raman and XPS analysis provided evidence that GO was converted to RGO by photocatalytic reduction. The TiO 2 –RGO composites were compared to TiO 2 in suspension reactors for the disinfection of water contaminated with Escherichia coli and Fusarium solani spores under real sunlight. Very rapid water disinfection was observed with both E. coli and F. solani spores. An enhancement in the rate of inactivation of E. coli was observed with the TiO 2 –RGO composite compared to P25 alone. The rate of inactivation of F. solani spores was similar for both the TiO 2 –RGO and P25. When the major part of the solar UVA was cut-off ( λ > 380 nm) using a methacrylate screen, there was a marked increase in the time required for inactivation of E. coli with P25 but no change in the inactivation rate for the TiO 2 –RGO. There is evidence of singlet oxygen production with visible light excitation of the TiO 2 –RGO composites which would lead to E. coli inactivation. [ABSTRACT FROM AUTHOR]

[Display omitted] • TiO 2 -rGO at the solar assisted photodegradation of complex mixing of pesticides. • Best photocatalytic performances with oxygen, regardless of the water matrix. • H 2 O 2 has not improved the final photocatalytic efficiency in any case. • P25-rGO promoted the photo-oxidation of the complex pesticides mix studied. • HBK-rGO presented excellent solar photoefficiency with 200 μg L-1 of pesticides. New titania-reduced graphene oxide (TiO 2 -rGO) nanocomposites have been studied to assess their application in the solar assisted photodegradation of a complex mixing of pesticides (pyrimethanil, isoproturon, alachlor and methomyl) at solar pilot plant, opening the opportunity to extend the use of these hybrid photocatalysts to complex wastewater effluents. TiO 2 -rGO nanocomposites were prepared by hydrothermal method from two commercial TiO 2 , Hombikat UV-100 (HBK) and Aeroxide® P25 (P25). The effect of two different water matrices (simulated and natural) on TiO 2 -rGO solar photoefficiency with two pesticides concentrations (5 mg·L−1 and 200 μg·L−1), and the additional use of an electron scavenging as oxidant agent, such as hydrogen peroxide in a comparison with oxygen from air, has been studied in compound parabolic collectors (CPC) photoreactors. Best photocatalytic performances were always found with oxygen over both TiO 2 -rGO nanocomposites, regardless of the water matrix employed. The use of H 2 O 2 as oxidant agent has not improved the final photocatalytic efficiency in any case. The generation of secondary highly reactive species in the reaction media from reaction of SO 4 ●- /HO radicals, and the more negatively surface charge with low S BET of P25-rGO nanocomposite, besides its lower hydrodynamic size with lower particle aggregation in the reaction conditions, have been responsible to promote the photo-oxidation of the complex pesticides mix studied. Finally, HBK TiO 2 -reduced graphene oxide (HBK-rGO) nanocomposite has presented excellent solar photoefficiency in the photodegradation of 200 μg·L−1 of each pesticide with the oxygen form air, especially under natural water matrix, because of its large negatively charge surface area where probably recombination of electrons and holes is prevented. [ABSTRACT FROM AUTHOR]

Abstract: The purpose of this study was to propose a municipal wastewater treatment method based on solar Advanced Oxidation Processes (AOPs) permitting reuse of the treated wastewater. Experiments were performed in a pilot compound parabolic collector (CPC) solar plant at the Plataforma Solar de Almería. Mineralisation was monitored by measuring the dissolved organic carbon (DOC), and the concentration profile of each compound during degradation was determined by HPLC–UV. Two different approaches, photo-Fenton (pH=2.8) and TiO2 were tested with 9 different emerging contaminants at 100μgL−1 each (acetaminophen, antipyrine, atrazine, caffeine, diclofenac, isoproturon, progesterone, sulfamethoxazole, and triclosan) at low iron and TiO2 concentrations. Photo-Fenton was by far more effective than TiO2 for degrading these contaminants, and was therefore selected for further study. The 9 contaminants were tested under the following conditions without pH adjustment: (i) Fe=5mgL−1, D.I. water; (ii) Fe=5, 15 and 55mgL−1, standard fresh water; (iii) Fe=5mgL−1, standard fresh water without NaHCO3. Initial amount of hydrogen peroxide was 50mgL−1, frequently analysed and added to maintain this concentration. It was demonstrated that low efficiency in some cases is mainly due to bicarbonates, and it is therefore proposed that the process be improved, either by increasing the iron concentration, or eliminating bicarbonates. [Copyright &y& Elsevier]

Agricultural wastewater is characterized by high organic matter and traces of organic pollutants. Effluents from the agro-food industry in particular are a potentially high environmental risk which requires appropriate, comprehensive treatment. This study evaluated the treatment of real wastewater from an agro-food processing plant by an Immobilized Biomass Reactor working in batch and continuous operating modes. Organic matter oxidation and the removal of reference organic pollutants were analyzed. Finally, the efficiency of a photocatalytic tertiary treatment with supported TiO 2 for the IBR effluent containing persistent microcontaminants was also studied. After the combined treatment, imazalil and thiabendazole were completely removed and over 90% of acetamiprid had been eliminated, but only with the additional contribution of hydrogen peroxide as an electron acceptor. [ABSTRACT FROM AUTHOR]

Highlights: [•] Immobilized photocatalyst based on TiO2 on glass spheres was successfully applied for ECs removal. [•] Regeneration with H2O2/UV or calcination are the most efficient methods to recover photoactivity. [•] F-, S- and Cl-containing compounds are preferentially adsorbed and easily removed from titania acid sites. [•] Compounds containing amine and amide groups are resistant to the photodegradation. [ABSTRACT FROM AUTHOR]

Highlights: [•] Structure/activity relationships over nanostructured iron-doped titania photocatalysts for solar degradation of phenol. [•] Iron leaching shown to be most relevant for the photoactivity. [•] Results compared in terms of the oxidizing agent, either H2O2 or oxygen from air. [ABSTRACT FROM AUTHOR]

Highlights: [•] Photocatalysis with TiO2 in the laboratory and in a pilot plant scale is compared. [•] The effect of clouds in the solar pilot plant is checked. [•] Initial pollutant removal is very similar in the pilot plant and in the laboratory. [•] TOC removal is faster at the pilot plant than at the laboratory. [•] Differences found in both installations are attributed to the light sources. [ABSTRACT FROM AUTHOR]

Abstract: The appropriate catalyst concentration for industrial wastewater treatment is several hundred milligrams per liter in solar photoreactors. Nonetheless, when the purpose of eliminating emerging contaminants in municipal wastewater treatment plant effluents is for reuse of the water, and they are present at extremely low concentrations, a tertiary treatment with a much lower photocatalyst concentration might be possible. TiO2 concentrations of only tens of mgL−1 were selected to evaluate the influence of catalyst load, initial hydrogen peroxide dose and radiation intensity on the degradation rate of five emerging contaminants (100μgL−1 of ofloxacin, sulfamethoxazole, flumequine, carbamazepine, and 2-hydroxy-biphenyl) spiked in a real municipal wastewater treatment plant effluent. Response surface methodology based on a spherical central composite design was used to optimize the parameters to find the maximal degradation rate. The experiments were carried out using an Evonik P-25 titanium dioxide suspension in a Suntest solar simulator. It has been demonstrated that the use of hydrogen peroxide is highly recommendable for working with TiO2 at low concentrations and high photon flux must be avoided. It has also been demonstrated that too low (less than 40mgL−1) TiO2 concentration is not recommendable. [Copyright &y& Elsevier]

Abstract: The synergism produced between ozone and TiO2 black light photocatalytic oxidation of methanol has been studied following the rate of formaldehyde formation during photocatalytic oxidation, ozonation and photocatalytic ozonation experiments. Methanol was selected as a model compound due to its low reaction rate with molecular ozone and its scavenging character for both, free hydroxyl radicals and trapped holes. TiO2-P25 was used as photocatalyst and black light blue lamps (emitting with a maximum at 365nm) as radiation source. The effect of ozone concentration and pH was evaluated. Absorbed light intensity by the photocatalyst was also determined to calculate the quantum yields of photocatalytic reactions. Three main processes need to be considered during photocatalytic ozonation: direct ozone-methanol reaction, indirect ozone reactions and photocatalytic reactions, which allow calculating the quantum yield of photo-generated oxidizing species. The presence of ozone exerts a positive effect in the reaction rate of oxidizing species formation due to light induced reactions also enhancing the quantum yield from 0.34 to 0.80moleinstein−1 at pH=3 (where indirect ozone reactions are negligible). This parameter increased from 0.29 to 3.27moleinstein−1 at pH=7 likely due to indirect ozone reactions that cannot be disregarded. The positive effect of ozone in the photocatalytic induced reactions has been attributed to the reaction of dissolved ozone and hydrogen peroxide (formed upon methanol direct ozonation) as electron acceptors, thus reducing the recombination process on the catalyst surface to some extent. A simplified economic study is also presented. [Copyright &y& Elsevier]

Abstract: Nanostructured iron-doped TiO2 catalysts with anatase structure and two different iron contents (0.7 and 3.5wt.%) have been prepared by a combined sol–gel/microemulsion method and examined with respect to their behaviour for photocatalytic degradation of aqueous phenol with H2O2 under solar light. The activity results are compared with those obtained for similarly prepared undoped systems as well as Degussa P25 reference catalyst and have been complemented with structural/morphological and electronic characterization analyses achieved by XRD, TEM, Raman, S BET, UV–vis DRS and XPS techniques. In contrast to typically observed results, the highest photoactivity is obtained for the catalyst with highest iron loading while that with 0.7% Fe displays lower photoactivity than undoped samples. Such unusual behaviour has been attributed to the contribution of new photoactive iron complexes generated on the surface of the catalyst as a consequence of iron leaching and readsorption processes taking place during the course of phenol photodegradation process. [Copyright &y& Elsevier]

Abstract: This article reports on degradation using TiO2 immobilized on glass spheres of 15 emerging contaminants (ECs) at low concentrations in simulated and real Municipal Wastewater Treatment Plant (MWTP) effluents. A sol containing titanium isopropoxide, commercial P25 and polyethylene glycol (PEG) was prepared, and a layer of the photoactive precursor was immobilized on glass spheres by dip-coating. The raw materials and the xerogels were characterized by N2 adsorption/desorption, XRD and TGA-DTA. The xerogel showed the TiO2 anatase, rutile and brookite crystal phases characteristic of the reference materials. The diffraction pattern showed no modification of the crystal size from incorporation of the PEG. Two different TiO2 anatase crystal sizes were identified, which was attributed to the different TiO2 precursors used in the synthesis route. Degradation of the emerging contaminants (acetaminophen, antipyrine, atrazine, carbamazepine, diclofenac, flumequine, hydroxybiphenyl, ibuprofen, isoproturon, ketorolac, ofloxacin, progesterone, sulfamethoxazole and triclosan), at an initial concentration of 100μgL−1 each was determined by ultra-performance liquid chromatography (UPLC-UV) and mineralization was monitored by measuring the dissolved organic carbon (DOC). The experiments were performed in a pilot compound parabolic collector (CPC) solar plant at the Plataforma Solar of Almeria (Spain). 85% of the compounds were degraded within 120min of illumination time depending on the water. The results show the potential application of this technology as a good alternative to suspension systems for the treatment of polluted water. [Copyright &y& Elsevier]

Abstract: The solar light photocatalytic degradation of ethidium bromide (EtBr) in aqueous suspensions has been analyzed over titania-based catalysts. A nanosized titania and an iron-doped TiO2 (with ≈2wt.% of iron content), synthesized within reverse microemulsions, were compared with a commercial Degussa P-25 titania. Structural and/or light handling properties of a different nature arise upon doping the titania with iron. These include favoring a titania crystal phase transformation from anatase to rutile and lowering the band gap energy. Almost total degradation of EtBr upon solar light assisted photodestruction and with significant total organic carbon (TOC) removal can be achieved over the three studied catalysts although the highest mineralization activity is achieved over the P25 titania sample. The results evidence that heterogeneous photocatalysis can be a promising advanced oxidation process to treat diluted EtBr residues typically generated in biomedical and biomolecular laboratories. [Copyright &y& Elsevier]

Abstract: Titanium dioxide photocatalysis (200mg/L) and photo-Fenton were applied to the treatment of several different pesticides considered priority substances (PS) by the European Commission dissolved in water at 50mg/L or at maximum water solubility (alachlor, atrazine, chlorfenvinphos, diuron, isoproturon and pentachlorophenol) alone (20mg/L of iron) and as a mixture (10mg/L of iron). All tests were performed in new twin 75-L compound parabolic collector (CPC) pilot plants driven by solar energy. Total organic carbon (TOC) mineralisation, disappearance of the parent compound and inorganic ion release are discussed as a function of treatment time. Photo-Fenton treatment was found to be shorter than TiO2 and more appropriate for these compounds and mixtures of them. [Copyright &y& Elsevier]

Abstract: TiO2 solar photocatalysis has been proven to be a degradation process for aqueous organic contaminant leading to total mineralisation of a large number of compounds. Furthermore, the interest in using this technique for water disinfection has grown in the last decade. Recent publications have reported photokilling of bacteria and viruses by TiO2 photocatalysis. Therefore, solar photocatalysis disinfection seems to be a very promising process, which could help to improve public health in rural areas of developing countries. The objective of this work was to assess the feasibility of using TiO2 solar photocatalysis to disinfect water supplies for future applications in developing countries. This article reviews the viability of solar photocatalysis for disinfection in low cost compound parabolic collectors, using sunlight and titanium dioxide semiconductor, both applied as slurry and supported. We report on the bactericidal action of TiO2 on a pure culture of Escherichia coli with a low cost photoreactor based on compound parabolic collectors. The influence of different experimental set-ups and parameters are also analysed. The results and potential application of the solar photocatalysis technology to water disinfection are studied within the frame of two research EU projects whose objective consist on the development of a fully autonomous solar reactor system to purify drinking water in remote locations of developing countries. [Copyright &y& Elsevier]