Photoinduced reactivity of titanium dioxide

The photocatalytic degradation of azo dyes containing different functionalities has been reviewed using TiO2 as photocatalyst in aqueous solution under solar and UV irradiation. The mechanism of the photodegradation depends on the radiation used. Charge injection mechanism takes place under visible radiation whereas charge separation occurred under UV light radiation. The process is monitored by following either the decolorization rate and the formation of its end-products. Kinetic analyses indicate that the photodegradation rates of azo dyes can usually be approximated as pseudo-first-order kinetics for both degradation mechanisms, according to the Langmuir–Hinshelwood model. The degradation of dyes depend on several parameters such as pH, catalyst concentration, substrate concentration and the presence of electron acceptors such as hydrogen peroxide and ammonium persulphate besides molecular oxygen. The presence of other substances such as inorganic ions, humic acids and solvents commonly found in textile effluents is also discussed. The photocatalyzed degradation of pesticides does not occur instantaneously to form carbon dioxide, but through the formation of long-lived intermediate species. Thus, the study focuses also on the determination of the nature of the principal organic intermediates and the evolution of the mineralization as well as on the degradation pathways followed during the process. Major identified intermediates are hydroxylated derivatives, aromatic amines, naphthoquinone, phenolic compounds and several organic acids. By-products evaluation and toxicity measurements are the key-actions in order to assess the overall process.

TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations A review

Fenton chemistry encompasses reactions of hydrogen peroxide in the presence of iron to generate highly reactive species such as the hydroxyl radical and possibly others. In this review, the complex mechanisms of Fenton and Fenton-like reactions and the important factors influencing these reactions, from both a fundamental and practical perspective, in applications to water and soil treatment, are discussed. The review covers modified versions including the photoassisted Fenton reaction, use of chelated iron, electro-Fenton reactions, and Fenton reactions using heterogeneous catalysts. Sections are devoted to nonclassical pathways, by-products, kinetics and process modeling, experimental design methodology, soil and aquifer treatment, use of Fenton in combination with other advanced oxidation processes or biodegradation, economic comparison with other advanced oxidation processes, and case studies.

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Advanced Oxidation Processes for Organic Contaminant Destruction Based on the Fenton Reaction and Related Chemistry

Removal of synthetic dyes from wastewaters: a review.

A review of classic Fenton's peroxidation as an advanced oxidation technique.

UV/TiO2 photocatalytic oxidation of commercial dyes in aqueous solutions

Oxidation kinetics and mechanisms of 2,4-dichlorophenol (2,4-DCP) by Fenton's reagent were studied. The effect of pH, concentration of H2O2, and Fe2+, and 2,4-DCP on both oxidation and dechlorination kinetics was investigated. A mathematical model was developed to describe the kinetics of 2,4-DCP oxidation and chloride ion production at constant concentrations of H2O2 and Fe2+. The optimal ratio of H2O2 to Fe2+ for the oxidation of 2,4-DCP was determined to be 11 which is the same as predicted by the previously developed kinetic model.

2,4-Dichlorophenol Oxidation Kinetics by Fenton's Reagent

Stability of 5,5-dimethyl-1-pyrroline-N-oxide as a spin-trap for quantification of hydroxyl radicals in processes based on Fenton reaction

Decolorization kinetics and mechanisms of commercial dyes by H2O2/iron powder system

Photocatalytic degradation kinetics of eleven azo dyes by TiO2/UV was studied using a Rayonet photoreactor. The degradation kinetics of azo dyes seems to be significantly influenced by their electrical nature, the number of azo bonds present in a dye molecule, and type of auxiliary groups attached to the azo bond. For example, trisazo dyes were the most difficult to degrade, and monoazo dyes were the most readily degradable, while the disazo dyes were in the middle range. The experimental results indicate that positive hole oxidation is the dominant mechanism contributing to the degradation of neutral or negatively charged dyes at pH 3. The degradation kinetics of positively charged basic dyes may be through reduction by electrons in the conduction bands of the excited TiO2 particles. On the other hand, hydroxyl radicals should be the major oxidation species at pH greater than 7. The pH effects on the degradation kinetics of different dyes may be due to the intrinsic reactivities of the neutral and ionize...

Walter Z. Tang

Papers

UV/TiO2 photocatalytic oxidation of commercial dyes in aqueous solutions

2,4-Dichlorophenol Oxidation Kinetics by Fenton's Reagent

Stability of 5,5-dimethyl-1-pyrroline-N-oxide as a spin-trap for quantification of hydroxyl radicals in processes based on Fenton reaction

Decolorization kinetics and mechanisms of commercial dyes by H2O2/iron powder system

TiO2/UV Photodegradation of Azo Dyes in Aqueous Solutions