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

Application of chitosan, a natural aminopolysaccharide, for dye removal from aqueous solutions by adsorption processes using batch studies: A review of recent literature

A theoretical basis is developed for the classification of adsorption isotherms for solutes in dilute solution, which relates their characteristic shapes to parameters of the solvent and any second solute. Three of the four main classes (S, L, H) are accounted for by differences in relative magnitude of the activation energies of desorption of solutes and solvent. The S-shaped isotherm is also accounted for by an additional concentration-dependence of this parameter, implying cooperative adsorption. The subgroups of each class are also explained. The linear (C) isotherm is explained by penetration of substrate micropores by solute, with or without solvent, whereby new adsorption sites are opened up; the theory predicts the experimentally found sharp inflection to a plateau in the C-curves. Curves of the other classes can also in some cases show a linear branch above the turning point, representing conditions like those for C-curves. The theoretical treatment is valid whether the adsorbing surface is energetically uniform, or, as in most systems, nonuniform.

A general treatment and classification of the solute adsorption isotherm. I. Theoretical

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Sorption isotherms: A review on physical bases, modeling and measurement

Adsorption of phenolic compounds by activated carbon—a critical review

858. Mechanism of adsorption of cationic dyes by alumina, and a note on heat changes in solution adsorption

Use of dyes for specific surface measurement

Eight non–ionic (disperse) dyes adsorbed in films of cellulose acetates, nylon and poly(ethylene terephthalate) (PET) have been faded by visible and near–ultraviolet radiation at temperatures in the range 15 to 100°C, and in atmospheres of relative humidities (r. h.) from zero to saturation. Fading is accelerated by rise in temperature, and, at r. h. above 5%, by rise in r. h. Activation energies of fading (Ef) (r. h. > 5%) range between about 3 and 18 kcal/mole, and depend on the nature of the substrate rather than on the structure of the dye. Values rise with increasing crystallinity of the polymer structure, except for PET which gives anomalously low values. The extent of dye–fibre interaction is at a minimum with PET and the rate–controlling step is a purely photochemical one. With the other substrates, the rate of fading is controlled by the dye–substrate interaction. The accelerating action of atmospheric humidity over normal ranges is partly due to its influence on the dye–substrate bonding and partly to its effect in swelling the substrate and thus increasing the diffusion coefficient of oxygen.

Activation Energies of Light Fading of Non–ionic Disperse Dyes in Hydrophobic Polymers

Surface-tension (γ) values of solutions of several anionic and cationic dyes in water have been measured by the ring method over a range of concentrations. All of these dyes reduce the surface tension of water, i.e. they are surface-active, the most effective being monosulphonated and cationic dyes. Polysulphonated dyes are less effective, but even a tetrasulphonated naphthaleneazonaphthalene dye reduces γ noticeably. Addition of sodium chloride causes a further reduction of γ with a polysulphonated dye. The results suggest that all ionised dyes are surface-active, and thus will tend to concentrate at a water interface. This may partly explain their adsorption by fibres.

Surface Tension of Ionised Dye Solutions

Published evidence relating the rate of fading of a colorant to its physical state in the substrate, and to the photochemical layer effect, is discussed, and a new theoretical Model of light fading is proposed. In this the rate of oxygen diffusion from the atmosphere is considered, first in traversing the polymeric substrate, and then in entering an embedded colorant particle, under illumination. Expressions are derived which show that the rate of fading should vary with I/a2 (a = radius of particle), for large particles. As the particles become smaller, the rate tends towards a 1/a dependence, and eventually, with very small particles, it is independent of their size. Recently reported rates of fading of pigments of measured particle size gave results in agreement with the present treatment; the smallest particles thus considered are of the same order of magnitude as those of high light fastness direct dyes in cellulose, as observed by electron microscopy. Thus the model may be applicable to at least some water-soluble dyes as well as pigments.

Charles H. Giles

Papers

858. Mechanism of adsorption of cationic dyes by alumina, and a note on heat changes in solution adsorption

Use of dyes for specific surface measurement

Activation Energies of Light Fading of Non–ionic Disperse Dyes in Hydrophobic Polymers

Surface Tension of Ionised Dye Solutions

The Relation between Light Fastness of Colorants and their Particle Size