Chantal Guillard

Photocatalytic degradation pathway of methylene blue in water

The TiO2/UV photocatalytic degradation of methylene blue (MB) has been investigated in aqueous heterogeneous suspensions. In addition to a prompt removal of the color, TiO2/UV-based photocatalysis was simultaneously able to oxidize the dye, with an almost complete mineralization of carbon and of nitrogen and sulfur heteroatoms into CO2 ,N H4 + ,N O3 − and SO4 2− , respectively. A detailed degradation pathway has been determined by a careful identification of intermediate products, in particular aromatics, whose successive hydroxylations lead to the aromatic ring opening. These results suggest that TiO2/UV photocatalysis may be envisaged as a method for treatment of diluted waste waters in textile industries.

PHOTOCATALYTIC DEGRADATION OF VARIOUS TYPES OF DYES (ALIZARIN S, CROCEIN ORANGE G, METHYL RED, CONGO RED, METHYLENE BLUE) IN WATER BY UV-IRRADIATED TITANIA

Abstract The photocatalytic degradation of five various dyes has been investigated in TiO 2 /UV aqueous suspensions. It was attempted to determine the feasibility of such a degradation by varying the chemical structures, either anthraquinonic (Alizarin S (AS)), or azoic (Crocein Orange G (OG), Methyl Red (MR), Congo Red (CR)) or heteropolyaromatic (Methylene Blue (MB)). In addition to a prompt removal of the colors, TiO 2 /UV-based photocatalysis was simultaneously able to fully oxidize the dyes, with a complete mineralization of carbon into CO 2 . Sulfur heteroatoms were converted into innocuous SO 4 2− ions. The mineralization of nitrogen was more complex. Nitrogen atoms in the −3 oxidation state, such as in amino-groups, remain at this reduction degree and produced NH 4 + cations, subsequently and very slowly converted into NO 3 − ions. For azo-dye (OG, MR, CR) degradation, the complete mass balance in nitrogen indicated that the central NN azo-group was converted in gaseous dinitrogen, which is the ideal issue for the elimination of nitrogen-containing pollutants, not only for environmental photocatalysis but also for any physicochemical method. The aromatic rings were submitted to successive attacks by photogenerated OH radicals leading to hydroxylated metabolites before the ring opening and the final evolution of CO 2 induced by repeated subsequent “photo-Kolbe” reactions with carboxylic intermediates. These results suggest that TiO 2 /UV photocatalysis may be envisaged as a method for treatment of diluted colored waste waters not only for decolorization, but also for detoxification, in particular in textile industries in semi-arid countries.

Influence of chemical structure of dyes, of pH and of inorganic salts on their photocatalytic degradation by TiO2 comparison of the efficiency of powder and supported TiO2

Abstract Anionic (Alizarin S (AS), azo-Methyl Red (MR), Congo Red (CR), Orange G (OG)) and cationic (Methylene Blue (MB)) dyes were degraded, either individually or in mixtures, by using UV-irradiated TiO 2 in suspension or supported on glass and on paper. The influence of the chemical structure of different dyes as well as that of pH and of the presence of inorganic salts on the photocatalytic properties of TiO 2 has been discussed. The role of adsorption is suggested, indicating that the reaction occurs at the TiO 2 surface and not in the solution. S and N hetero-atoms are respectively mineralized into SO 4 2− , NO 3 − and NH 4 + , except azo-groups which mainly formed N 2 which represents an ideal case for a decontamination reaction. The fate of nitrogen strongly depends on its initial oxidation degree. High photocatalytic activities have been found for TiO 2 coated on glass by the sol–gel method. Its efficiency was intermediate between those of PC-500 and P-25 powders. The efficiency of PC-500 TiO 2 sample, fixed on paper by using a binder, is slightly less important than that of the powder. The presence of a silica-binder with an acidic pzc is suggested to be at the origin of the decrease in efficiency.

Heterogeneous photocatalysis : an emerging technology for water treatment

Abstract Heterogeneous photocatalysis is capable of destroying many organic micropollutants in aqueous medium. The principle of the method and a brief survey of the relevant literature are presented. On the basis of studies on the degradation of various substituted aromatics in UV-irradiated TiO2 aqueous suspensions carried out principally in our laboratory, we indicate the kinetic characteristics of the method, the nature of the intermediates and the degradation pathways, as well as the influence of pH or common ions and other parameters. The photocatalytic recovery of noble metals and the detoxification of water containing certain inorganic ions are also described. The advantages and disadvantages of the photocatalytic method, which begins to be commercialized, are briefly discussed.

Comparison of various titania samples of industrial origin in the solar photocatalytic detoxification of water containing 4-chlorophenol

The degradation of 4-chlorophenol (4-CP) was used as a model reaction to investigate the photocatalytic properties of different industrial TiO2 catalysts and to compare their efficiencies in the treatment of contaminated waters. Parallel experiments have been performed using either artificial UV-light in a batch photoreactor in laboratory experiments or solar energy in the flow reactor of the pilot plant at Plataforma Solar de Almeria (PSA) in Spain. Depending on the kinetic criteria chosen for comparison ((i) initial rate of pollutant disappearance; (ii) amounts of intermediate products present in solution at a given time; (iii) time necessary to obtain total mineralization), different classifications of photocatalyst activities were observed. Among various physical characteristics such as particle size, structure or active site density, which may intervene on the photocatalytic activity, the influence of the surface area appeared of prime importance. For lower surface area catalysts, there is a decrease in the readsorption rate of intermediate products and consequently in the overall photodegradation rate. By contrast, for higher surface area catalysts, it was observed a lower final rate of total organic carbon (TOC) disappearance because of a very low coverage in pollutant which favors electron–hole recombination. The comparison of the kinetic results for decontaminating water at PSA and in laboratory experiments indicated the same kinetic order (apparent first order) for 4-CP disappearance and the same apparent quantum yield. However, fewer intermediate products and a faster TOC disappearance were observed in the solar pilot reactor at PSA, which, in addition, worked with a smaller optimum concentration of suspended titania. This was ascribed to the design of the photoreactor.

Why inorganic salts decrease the TiO2 photocatalytic efficiency

Methylene Blue (MB) has been chosen as a model molecule to evaluate the impact of inorganic salts, present in textile waste waters, on the adsorption properties and on the photocatalytic efficiency of TiO2. No OH∘ radical scavenging by anions such as NO3−, Cl−, SO42−, PO43−, and CO32− was observed at neutral and basic pH, while this phenomenon can be suggested at acidic pH for some anions except carbonate anions which are totally neutralized and/or eliminated as CO2 in these conditions. The decrease in the rate MB photocatalytic degradation in the presence of inorganic salts was shown to be due to the formation of an inorganic salt layer at the surface of TiO2, which inhibits the approach of MB molecules. The correlation between the amount of MB adsorbed and the rate of its photocatalytic degradation, whatever the nature of the salt, its concentration and the pH of the solution, indicates (i) that photocatalysis occurs at the surface and not in the solution and (ii) that OH− ions added at basic pH do not participate to the increase in the photocatalytic efficiency by inducing an increase in OH∘ formation. They increase the surface density in adsorption sites TiO−. The effect of various salts is similar on various titania samples of industrial origin (Millennium TiO2 PC 500, PC 50, and Degussa P 25). It is however more important on Millennium PC 10 probably because of its smaller surface area.

Photocatalytic pollutant removal in water at room temperature: case study of the total degradation of the insecticide fenitrothion (phosphorothioic acid O,O-dimethyl-O-(3-methyl-4-nitro-phenyl) ester)

The photocatalytic purification of water containing fenitrothion has been performed in TiO2 aqueous suspensions. The mineralization into CO2, H2PO4−, SO42−, NO3− was achieved. Several intermediates have been identified by HPLC and GC/MS. They illustrate the transformation of PS into PO, the splitting between the phosphorous part and the aromatic moiety and the transfer of methyl or methoxy groups. Formate ions, principally produced from the CH3O and CH3, groups and acetate ions were also detected as intermediate products.

Photocatalytic degradation of sulfonylurea herbicides in aqueous TiO2

The degradation of two sulfonylurea (Sus) herbicides, cinosulfuron and triasulfuron, has been investigated in aqueous solutions containing TiO2 suspensions as photocatalyst. Cinosulfuron and triasulfuron have been rapidly transformed with different kinetic orders (1 and 1/2, respectively). The calculated quantum efficiencies (φ) of both Sus have been found similar (0.73%). Intermediate products were identified using HPLC–UV or HPLC–MS analyses. A photocatalytic degradation pathway has been proposed, leading to cyanuric acid as a final product. This molecule is known as resistant to all oxidation technologies.

Effect of operating parameters on the testing of new industrial titania catalysts at solar pilot plant scale

Abstract 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 Almeria. 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.

Factors influencing the photocatalytic degradation of sulfonylurea herbicides by TiO2 aqueous suspension

The photocatalytic degradations of six sulfonylurea herbicides have been investigated in aqueous solutions containing TiO2 suspensions as photocatalyst, in order to assess influence of various parameters, such as adsorption, initial concentration and photon flux on the photocatalytic process. Results show that adsorption is an important parameter controlling the apparent kinetic order of the degradation (either one in the case of associative adsorption or a half in the case of dissociative adsorption). In respect to the Langmuir–Hinshelwood model, the photocatalytic reaction is favoured by a high concentration. A higher efficiency is observed under the lower photon fluxes, the limiting factor being the electron–hole pair recombinations.