Enhanced degradation of 2,6-dimethylphenol by photocatalytic systems using TiO2 assisted with H2O2 and Fe(III).

In this study, several photocatalytic degradation systems were investigated using 2,6-dimethylphenol (2,6-DMP) as a model compound. Highly reactive species are formed in four systems, Fe(III), TiO2, TiO2/H2O2 and TiO2/Fe(III) where complete degradation of 2,6-DMP was achieved under UV radiation. Photodegradation of the 2,6-DMP has been described by pseudo-first order kinetic model in the presence of TiO2. In UV/TiO2-H2O2 system, the addition of H2O2 in the TiO2 suspension improves the degradation rate of 2,6-DMP from 70% to 100% for a H2O2 concentration of 10-2 M in 3 h. In homogeneous system, HO• and Fe2+ can be generated by the irradiation of Fe(III) solution. The speciation of Fe(III) obtained from Visual MINTEQ soft showed the formation of several species and Fe(OH)2+ were the most predominant and active species in a pH range of 2.5-3.5. At a low concentration of TiO2 (30 mg L-1), an important positive effect due to the iron addition has been shown in TiO2/Fe(III) system, the entrance of metallic ions at different concentrations enhanced the photocatalytic activity of TiO2. A degradation percentage of 90% was achieved in the UV/TiO2-Fe(III) system under optimal conditions against 57% in UV/TiO2 system. Strong synergistic effect was observed in the UV/TiO2-H2O2 binary system. On the basis of literature, a pathway for 2,6-DMP degradation was proposed. The mechanism of degradation of the 2,6-DMP did not involve only HO• radicals, an interaction of Fe(III) in the excited state with 2,6-DMP occurred giving rise to the formation of 2,6-dimethylphenoxyl radical.

Effect of Fe(III)-bicarboxylic complexes in removal pollutant under UV and sunlight in aqueous solutions.

In this study, we have applied Fe(III)-bi-carboxylic acid solutions containing citrate and oxalate ligands to degrade 3-méthylphénol (3MP) in aqueous solutions both under UV and sunlight. Under irradiation at 365 nm, the photodegradation of 3MP is markedly better in the presence of the Fe(III)Ox complex than in the Fe(III)Cit complex this fact is explained by an excess of H2O2 and Fe(II) generated by Fe(III)Ox photolysis creating the Fenton process. We mixtures were realized by varying the composition of the Fe(III)Cit and Fe(III)Ox in order to see the additives of the degradation efficiency of the pollutant. The results show that the addition of Fe(III)Ox to the Fe(III)Cit system evidently augmented the photodegradation rate at pH = 5.5. The Fe(III)Ox/Fe(III)Cit ratio is optimized at [Fe(III)Ox] (0.15/0.15)/[Fe(III)Cit] (0.15/0.6). Synergistic effect in the Fe(III)Ox/Fe(III)Cit binary system was confirmed. The addition of tertiobutanol (T-buOH) noticeably inhibited the photodegradation, indicating the involvement of •OH in the process. To verify the feasibility of photochemical processes in the environment, tests on the photodegradation of 3MP were performed under natural irradiation. The degradation was improved under excitation by sunlight in the presence of Fe(III)-bi-carboxylic acid solutions containing citrate and oxalate ligands. These results are very encouraging for the application of this system for the treatment of organic pollutants in aqueous solution.

Use of oxalic acid as inducer in photocatalytic oxidation of cresol red in aqueous solution under natural and artificial light.

This work was carried out in the field of water treatment using advanced oxidation processes (AOPs), especially photolysis of carboxylic acid that leads to the formation in situ of hydroxyl radical (·OH). Cresol red (CR) degradation induced by organic acids/UV system was investigated in aqueous solution. The preliminary study of CR-organic acid mixture in the dark and at room temperature allowed confirming the absence of interaction under our experimental conditions. However, upon irradiation at 365 nm, the proportion of elimination of CR was 89% after 5 h of irradiation. Indeed, the CR degradation efficiency depends on the acid concentration and the pH of the medium. The concentration of acid is optimized to the 5 × 10-3 M. pH 2.39 was the optimal one when C2HO- 4 was the most important species at this pH. The use of i-PrOH as ·OH confirmed the involvement of ·OH in photodegradation of CR induced by Ox. The addition of metal ions including Zn2+ and Cu2+ to the CR-organic acid mixture slowed the CR degradation unlike Fe2+, hence an improvement of its disappearance was observed. The results showed a faster degradation of the pollutant under excitation by sunlight. This environmentally friendly method appears to be very effective in the treatment of wastewater.

The effect of natural iron oxide and oxalic acid on the photocatalytic degradation of isoproturon: a kinetics and analytical study.

The photocatalytic degradation of isoproturon, a persistent toxic herbicide, was investigated in the presence of natural iron oxide and oxalic acid and under UV irradiation. The influence of the relevant parameters such as the pH and the iron oxide and oxalic acid concentrations has been studied. The presence of natural iron oxide and oxalic acid in the system effectively allow the degradation of isoproturon, whereas the presence of t-butyl alcohol adversely affects the phototransformation of the target pollutant, thus indicating that an OH radical initiated the degradation mechanism. The degradation mechanism of isoproturon was investigated by means of GC-MS analysis. Oxidation of both the terminal N-(CH3)2 and isopropyl groups is the initial process leading to N-monodemethylated (NHCH3), N-formyl (N(CH3)CHO), and CHCH3OH as the main intermediates. The substitution of the isopropyl group by an OH group is also observed as a side process.

Photocatalytic degradation of 2,4-dichlorophenoxyacetic acid and 4-chloro-2-methylphenoxyacetic acid in water by using TiO2.

The photocatalytic degradation of many chlorinated organic compounds by semiconductor particles, has been widely recognised as a promising method of water and wastewater treatment process. In the present work the photocatalytic transformation of 2,4-D and MCPA in aqueous solution with 2 g l(-1) suspended TiO2 is demonstrated. The formation of several intermediates has been observed. The photodegradation rate increases with increasing pH. The photocatalytic transformation of 2,4-D and MCPA over TiO2 in solution containing hydrogen peroxide has been also studied. H2O, accelerated significantly the reaction rate of 2, 4-D and MCPA. The partial inhibition by ethanol is attributed to the scavenging of OH radicals involved in the first step of the reaction. Finally, from these results, a mechanism is proposed. This photocatalytic method has good potential for application on a large scale.

Phototransformation of diuron in aqueous solution by UV irradiation in the absence and in the presence of H2O2.

The phototransformation of diuron has been studied by photolysis at 253.7 nm at 20 degrees C, in the absence and in the presence of H2O2. Experiments were conducted in batch and in continuous-flow reactors. In the absence of H2O2, the value of the quantum yield of photolysis of diuron at 253.7 nm was found to be equal to be 0.0125 +/- 0.0005 (using a molar absorption coefficient of 16500 +/- 500 M(-1) cm (-1) at 253.7 min) and insensitive to pH in the range 2-8.5. Oxidation rates of diuron by H2O2/UV could be predicted successfully by a kinetic model including photochemical and OH*-oxidation reactions using a value of 4.6 x 10(9) M(-1) s(-1) for the rate constant of the reaction of OH* with diuron. The model was verified for the various reactors used and under a wide range of conditions in pure water (pH: 2-8, [H2O2] : 0-0.1 M) and in the presence of hydrogenocarbonate ions (0-35 mM, pH = 8.3-8.4). The contribution of the carbonate radicals to the degradation rates of diuron was found to be insignificant under our experimental conditions.

Phototransformation of metoxuron [3-(3-chloro-4-methoxyphenyl)-1,1-dimethylurea] in aqueous solution.

UV irradiation of metoxuron in aerated aqueous solution at 254 nm or between 300 and 450 nm led initially to an almost specific photohydrolysis of the C-Cl bond, resulting in the formation of 3-(3-hydroxy-4-methoxyphenyl)-1,1-dimethylurea (MX3) and hydrogen chloride. The quantum yield was determined to be 0.020 (+/- 0.005) in solutions irradiated at 254 nm. Five minor photoproducts were also identified, in particular the dihydroxydimethoxybiphenyl derivatives resulting from the phototransformation of MX3. Irradiation increased the toxicity of an aqueous solution of metoxuron to the marine bacterium Vibrio fischeri.