Photocatalytic Detoxification of Polluted Waters. In: Boule P, editor. Environmental Photochemistry. Berlin: Springer Berlin Heidelberg; 1999. pp. 285-351. [DOI: 10.1007/978-3-540-69044-3_11]

Current Developments in the Effective Removal of Environmental Pollutants through Photocatalytic Degradation Using Nanomaterials

Photocatalysis plays a prominent role in the protection of the environment from recalcitrant pollutants by reducing hazardous wastes. Among the different methods of choice, photocatalysis mediated through nanomaterials is the most widely used and economical method for removing pollutants from wastewater. Recently, worldwide researchers focused their research on eco-friendly and sustainable environmental aspects. Wastewater contamination is one of the major threats coming from industrial processes, compared to other environmental issues. Much research is concerned with the advanced development of technology for treating wastewater discharged from various industries. Water treatment using photocatalysis is prominent because of its degradation capacity to convert pollutants into non-toxic biodegradable products. Photocatalysts are cheap, and are now emerging slowly in the research field. This review paper elaborates in detail on the metal oxides used as a nano photocatalysts in the various type of pollutant degradation. The progress of research into metal oxide nanoparticles, and their application as photocatalysts in organic pollutant degradation, were highlighted. As a final consideration, the challenges and future perspectives of photocatalysts were analyzed. The application of nano-based materials can be a new horizon in the use of photocatalysts in the near future for organic pollutant degradation.

Electrically Induced Alignment of Semiconductor Nanosheets in Niobate-Clay Binary Nanosheet Colloids toward Significantly Enhanced Photocatalysis

Aqueous binary colloids of niobate and clay nanosheets, prepared by the exfoliation of their mother layered crystals, are unique colloidal systems characterized by the separation of niobate and clay nanosheet phases, where niobate nanosheets form liquid crystalline domains with the size of several tens of micrometers among isotropically dispersed clay nanosheets. The binary colloids show unusual photocatalytic reactions because of the spatial separation of photocatalytically active niobate and photochemically inert clay nanosheets. The present study shows structural conversion of the binary colloids with an external electric field, resulting in the onsite alignment of colloidal nanosheets to improve the photocatalytic performance of the system. The colloidal structure is reshaped by the growth of liquid crystalline domains of photocatalytic niobate nanosheets and by their electric alignment. Niobate nanosheets are assembled by the domain growth process and then aligned by AC voltage, although clay nanosheets do not respond to the electric field. Photocatalytic decomposition of the cationic rhodamine 6G dye, which is selectively adsorbed on clay nanosheets, is examined for the niobate-clay binary nanosheet colloids with or without domain growth and electric field. The fastest decomposition is observed for the electrically aligned colloid without the domain growth, whereas the sample with the domain growth and without the electric alignment shows the slowest decomposition. The results demonstrate the improvement of the photocatalytic performance by changing the colloidal structure, even though the sample composition is the same.

Design of Metal@Titanium Oxide Nano-heterodimers by Laser-Driven Photodeposition: Growth Mechanism and Modeling

In order to circumvent the usual nucleation of randomly distributed tiny metallic dots photodeposited on TiO₂ nanoparticles (NPs) induced by conventional UV lamps, we propose to synthesize well-controlled nanoheterodimers (NHDs) using lasers focused inside microfluidic reactors to strongly photoactivate redox reactions of active ions flowing along with nanoparticles in water solution. Since the flux of photons issued from a focused laser may be orders of magnitude higher than that reachable with classical lamps, the production of electron-hole pairs is tremendously increased, ensuring a large availability of carriers for the deposition and favoring the growth of a single metallic dot as compared to secondary nucleation events. We show that the growth of single silver or gold nanodots can be controlled by varying the beam intensity, the concentration of the metallic salt, and the flow velocity inside the microreactor. The confrontation to a build-in model of the metallic nanodot light-induced growth onto the surface of TiO₂ NPs shows the emergence of a predictable "master behavior" on which individual growths obtained from various tested conditions do collapse. We also characterized the associated quantum yield. Eventually, we successfully confronted our model to growth data from the literature in the case of silver on TiO₂ and gold on II-VI semiconducting NPs triggered by UV lamps. It shows that for the photosynthesis of NHDs the efficiency of the electron-hole pair production rate matters much more than the number of pairs produced and that the use of laser light can provide a photodeposition-based synthesis at the nanoscale.

Direct and indirect photodegradation of estriol in the presence of humic acid, nitrate and iron complexes in water solutions

The photochemical behavior of a natural estrogen estriol (E3) was investigated in the presence of the natural photoreactive constituents including nitrate, iron(III), and humic acid (HA). The direct photodegradation of E3 increased with increasing incident light intensity, decreasing initial concentration of E3 and increasing pH in the range of 6.0 to 10.0. The direct photodegradation of the deprotonated speciation of E3 was much faster than that of its protonated form. The presence of NO3(-) and iron(III) promoted the photochemical loss of E3 in the aqueous solutions. The quenching experiments verified that hydroxyl radicals were predominantly responsible for the indirect photodegradation of E3. HA could act as photosensitizer, light screening agent and free radical quencher. For the first time, the enhancement or inhibition effect of HA on photodegradation was found to depend on the irradiation light intensity. HA enhanced the photodegradation of E3 under sunlight or weak irradiation of simulated sunlight. In contrast, under high irradiation light intensity, HA inhibited the photodegradation. The hydroxylation photoproducts were identified using GC-MS and the photodegradation pathway of E3 was proposed.

TiO2/β-SiC foam-structured photoreactor for continuous wastewater treatment

INTRODUCTION

This study of photocatalytic degradation of wastewater was carried out in alveolar cell β-SiC foam-structured photocatalytic reactors working in a recirculation mode. The immobilization of TiO2 on β-SiC foams was efficiently obtained through a sol-gel technique in acidic conditions.

DISCUSSION

In order to optimize degradation yields obtained by the foam-structured prototype reactor for the photocatalytic water treatment, the operating conditions of the photoreactor have been investigated and the efficiency of the process was evaluated by measuring the photocatalytic degradation of Diuron (3-(3,4-dichlorophenyl)-1,1-dimethyl-urea)) under UV irradiation. Kinetic studies were carried out by investigating the influence of different parameters controlling the reaction (TiO2 loading and β-SiC foam cell size). The ageing of TiO2/β-SiC foam photocatalytic materials and the mineralization (TOC, Cl-, NO3- and NH4+) of Diuron were investigated.

Photocatalytic degradation of phenanthrene on soil surfaces in the presence of nanometer anatase TiO2 under UV-light

The effect of nanometer anatase TiO2 was investigated on the photocatalytic degradation of phenanthrene on soil surfaces under a variety of conditions. After being spiked with phenanthrene, soil samples loaded with different amounts of TiO2 (0 wt.%, 1 wt.%, 2 wt.%, 3 wt.%, and 4 wt.%) were exposed to UV-light irradiation for 25 hr. The results indicated that the photocatalytic degradation of phenanthrene followed the pseudo first-order kinetics. TiO2 significantly accelerated the degradation of phenanthrene with the half-life reduced from 45.90 to 31.36 hr for TiO2 loading of 0 wt.% and 4 wt.%, respectively. In addition, the effects of H2O2, light intensity and humic acid on the degradation of phenanthrene were investigated. The degradation of phenanthrene increased with the concentration of H2O2, light intensity and the concentration of humic acids. It has been demonstrated that the photocatalytic method in the presence of nanometer anatase TiO2 was a very promising technology for the treatments of soil polluted with organic substances in the future.

Investigation on the photocatalytic degradation of pyrene on soil surfaces using nanometer anatase TiO2 under UV irradiation

Photocatalytic degradation of pyrene on soil surfaces was investigated in the presence of nanometer anatase TiO(2) under a variety of conditions. After being spiked with pyrene, soil samples loaded with different amounts of TiO(2) (0%, 1%, 2%, 3%, and 4%, w/w) were exposed to UV irradiation for 25h. The results indicated that the photocatalytic degradation of pyrene followed pseudo-first-order kinetics. TiO(2) accelerated the degradation of pyrene generally as indicated by the half-life reduction from 45.90 to 31.36h, corresponding to the TiO(2) amounts from 0% to 4%, respectively. The effects of H(2)O(2), light intensity and humic acids on the degradation of pyrene were also investigated. The degradation of pyrene increased along with increasing the concentration of H(2)O(2), light intensity and the concentration of humic acids. All results indicated that the photocatalytic method in the presence of nanometer anatase TiO(2) was an advisable choice for the treatments of PAHs polluted soil in the future.

Indirect photolysis of perfluorochemicals: hydroxyl radical-initiated oxidation of N-ethyl perfluorooctane sulfonamido acetate (N-EtFOSAA) and other perfluoroalkanesulfonamides

Selected perfluorinated surfactants were irradiated in aqueous hydrogen peroxide solutions using artificial sunlight to study transformation under aquatic environmental conditions. Indirect photolysis mediated by hydroxyl radical was observed for N-ethyl perfluorooctane sulfonamidoethanol (N-EtFOSE), N-ethyl perfluorooctane sulfonamido acetate (N-EtFOSAA), N-ethyl perfluorooctane sulfonamide (N-EtFOSA), and perfluorooctane sulfonamide acetate (FOSAA). An upper limitforthe bimolecular reaction rate constant for reaction of *OH and N-EtFOSAA was determined to be (1.7 +/- 0.7) x 10(9) M(-1)s(-1). A proposed reaction pathwayfor degradation of the parent perfluorochemical, N-EtFOSE, to the other perfluoroalkanesulfonamides and perfluorooctanoate (PFOA) was developed and includes oxidation and N-dealkylation steps. As they did not undergo additional degradation, perfluorooctane sulfonamide (FOSA) and PFOA were the final degradation products of hydroxyl radical-initiated oxidation. UV-visible absorption spectra for the perfluorochemicals, showing absorbance in the UV region below the range of natural sunlight are also reported. In sunlit environments, indirect photolysis of perfluorochemicals is likely to be important in the determination of their environmental fate given the slow rates expected for biotransformation and weak sorption. Photolytic conversion of perfluorochemicals into refractory perfluorinated acids, mainly PFOA, could mean that a significant fraction of these compounds will accumulate in the world's oceans.

Mechanistic aspects regarding the direct aqueous environmental photochemistry of phenol and its simple halogenated derivatives. A review

We have reviewed the mechanistic aspects regarding the direct aqueous phase environmental photochemistry of phenol and its simple halogenated derivatives. These compounds are important industrial and natural products, are ubiquitous in aquatic systems, and their acute and chronic toxicity makes their environmental fate of interest. Work over the past two decades has unified the photochemistry of phenol and its simple halogenated derivatives. In general, three photochemical pathways dominate in aqueous solution depending on the nature of the substrate: (1) photoionization, (2) photochemical aryl-halogen bond homolysis, and (3) photochemical aryl-halogen bond heterolysis. Photoionization typically results in an array of biaryl radical coupling products which are only relevant for highly concentrated waste streams. Photolytic aryl-halogen bond homolysis will primarily give photoreduction products where reducing agents such as dissolved organic matter or reduced metal cations are present, and radical coupling products in highly concentrated waste streams. The 2- and 4-substituted halophenols may undergo photochemical aryl-halogen bond heterolysis upon irradiation to give an aryl cation. The aryl cation can be attacked by water to give the corresponding hydroxylated derivative, or may deprotonate to generate alpha- and gamma-ketocarbenes, respectively. Following their formation, the singlet alpha-ketocarbenes may undergo Wolff rearrangements to cyclopentadiene-ketenes that are subsequently hydrolyzed to cyclopentadiene carboxylic acids. The triplet alpha- and gamma-ketocarbenes are attacked by oxygen and hydrolyzed to give benzoquinones, directly hydrolyzed to yield hydroquinones, reduced to give phenols, or could take part in coupling reactions in highly concentrated waste streams to give dimers and hydroxybiaryl complexes. Additional studies in natural water samples are required to assess the relative importance of these direct irradiation mechanisms relative to indirect photolysis and other abiotic and biotic degradation and environmental partitioning pathways across the continuum of marine, freshwater, and wastewater biogeochemical signatures.

Universal behavior of photochemical deposition in liquid solutions driven by a one-photon transition

Even if photochemical deposition of nearly all types of materials has been used for decades to pattern almost any kind of substrate for various applications (catalysis, chemical sensing, magnetic data storage, optoelectronics, spin-dependent electron transport, and solar cells), a rationalized description is still missing. This paper aims at fulfilling this lack by presenting a unified approach of the photodeposit growth initiated by a one-photon photochemical reaction. We experimentally investigate the robustness of growth scalings predicted for photochemical deposition driven by a continuous laser wave. Three types of one-photon photochemical reactions (photoexcitation of chromates, photodissociation of permanganates, and photocondensation of colloidal selenium) and three parameters (solvent p H variations, concentration in photoactive reagent, and influence of the exciting optical wavelength) were cross analyzed. In all the cases, including data taken from the literature, the same dynamic master behavior emerges from the data rescaling of measured deposit growth laws. The nice agreement observed between system-independent predictions and the whole data set strongly supports a universal description of the photodeposit growth whatever the photosensitive medium and the involved one-photon chemical reaction. Such an approach also points out the quantitative sorting of photochemical reactions in terms of deposition efficiency. This rationalization of the kinetics of photodeposition anticipates new methodologies to predict, design, and control substrate micropatterning for chemical, lithographic, and optoelectronic applications.

Different net effect of TiO2 sintering temperature on the photocatalytic removal rates of 4-chlorophenol, 4-chlorobenzoic acid and dichloroacetic acid in water

Our purpose was to show that the sintering temperature of TiO(2) can have a different net effect (thought to arise from a decrease in surface area against a decrease in recombination rate of charge carriers) on the photocatalytic removal rate of various organic pollutants in water. For that, we have chosen four chlorinated pollutants, viz. 4-chlorophenol (4-CP), 2,5-dichlorophenol (2,5-DCP), 4-chlorobenzoic acid (4-CBA) and dichloroacetic acid (DCAA). Their photocatalytic removal was studied over four TiO(2) samples (from Millennium Chemicals or affiliate) all obtained identically by TiOSO4 thermohydrolysis with subsequent calcination at various temperatures, TiO(2) Degussa P25 was used for comparison. At equal TiO(2) mass in the slurry photoreactor, the pseudo-first-order removal rate constant k increased with the calcination temperatures for the three aromatic pollutants, whereas it was the opposite for the aliphatic acid. Results obtained with P25 were consistent with the reasoning based on the combined effects of surface area and charge recombination rate. Similar k values for 4-CP and 2,5-DCP, irrespective of the TiO(2), further illustrate the importance of the molecular structure. For 4-CBA, the possibility of decarboxylation in addition to an attack on the ring, as well as a much higher extent of adsorption, can explain a higher k with respect to the chlorophenols. The implication of these results is that the hole attack mechanism for carboxylic acids is much more sensitive to surface area variation than would be the (diffusible) OH radical mechanism for aromatics which could react in the near-surface solution-phase.

Kinetic study of photocatalytic degradation of carbamazepine, clofibric acid, iomeprol and iopromide assisted by different TiO2 materials--determination of intermediates and reaction pathways

The light-induced degradation of clofibric acid, carbamazepine, iomeprol and iopromide under simulated solar irradiation has been investigated in aqueous solutions suspended with different TiO2 materials (P25 and Hombikat UV100). Kinetic studies showed that P25 had a better photocatalytic activity for clofibric acid and carbamazepine than Hombikat UV100. For photocatalytic degradation of iomeprol Hombikat UV100 was more suitable than P25. The results can be explained by the higher adsorption capacity of Hombikat UV100 for iomeprol. The study also focuses on the identification and quantification of possible degradation products. The degradation process was monitored by determination of sum parameters and inorganic ions. In case of clofibric acid various aromatic and aliphatic degradation products have been identified and quantified. A possible multi-step degradation scheme for clofibric acid is proposed. This study proves the high potential of the photocatalytic oxidation process to transform and mineralize environmentally relevant pharmaceuticals and contrast media in water.

Photocatalytic transformation of dyes and by-products in the presence of hydrogen peroxide

The efficiency of the photocatalytic degradation of dyes and dyeing industry pollutants on immobilized photocatalysts can be improved by addition of hydrogen peroxide, due to its photocatalytic decomposition on TiO2. Experiments were carried out with two azodyes, Acid Orange-7 (AO-7) and Tartrazine (Tart), with 3-nitrobenzenesulfonic add (3-NBSA) which is a chemical intermediate in the dye industry and with real industrial wastewaters, using a thin-film fixed bed reactor. The effect of hydrogen peroxide is only significant for concentrations higher than 5 x 10(-3) M (170 mg l(-1)).