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

Experimental evidence and published results show that when dyes are faded they are usually oxidised, probably by atmospheric oxygen. In certain systems reduction may occur, e.g. when the substrate is more readily oxidised than the dye or under anaerobic conditions in the presence of a hydrogen donor.

Oxidation and Reduction in Light Fading of Dyes

Tests have been made with over 300 dyed or pigmented materials exposed to mercury-tungsten or mercury lamps. The colorations were of a wide range of hue and light-fastness properties (1 to 8 and above) and in several depths on different substrates. The mercury-tungsten fluorescent lamp (500 W) gave results identical with those obtained with daylight or xenon arclight for 75% of the very varied selection of 174 patterns examined, and 25 % were within one-half of a grade, except for five which differed by one grade. When the blue standards are faded in this lamp (at 45% r.h.), they have an average interval factor of about 2.1. The required times of exposure are similar to those for the xenon arclight. The equipment required is extremely simple, and its initial and operating costs are considerably lower than those of methods hitherto used for testing in artificial light. The equipment can readily be used on the laboratory bench and is suitable for routine testing of the light fastness of any coloured material. In addition to the control of humidity, temperature control is possible, though this is not usually necessary. The mercury-vapour lamp (400 W) has also been similarly examined, using 138 patterns. It is not recommended for general use but is valuable for routine sorting tests of materials of very high fastness (BS grading above 7). The blue standard patterns fade, under exposure to these lamps, in the same sequence and with similar interval spacings, as in daylight or xenon light.

Economical and Efficient Fading Lamps

All lightfading rates of dyed or pigmented materials can be divided into five main classes and some subgroups, according to the shape of the arithmetic plot of rate of loss of dye. The physical condition and environment of the coloring matter appear to be principal factors determinipg the shape; a change in concentration does not usually change the class of fade.Class I fades are approximately first-order curves and are indicative of dye dispersed as single molecules or very small associated units. Class II fades are initially class I but later become linear (zero order), probably because a high proportion of coloring matter either is firmly embedded in the substrate molecules or is present as relatively large associated units. Class III fades resemble the later portion of Class II for similar reasons.Class IV fades have a point of inflection, with a more rapid fade following an initial slow change or even an apparent negative fade, i.e., an increase in depth of color; class V fades accelerate continuousl...

Some Observations on the Kinetics of Dye Fading

Giles, Charles H. (The University of Strathclyde, Glasgow, Scotland), and Robert B. McKay. Adsorption of cationic (basic) dyes by fixed yeast cells. J. Bacteriol. 89:390–397. 1965.—The adsorption of 10 typical cationic dyes on formalin-fixed yeast cells has been studied by determining isotherms, and the results are consistent with an ion-exchange mechanism. The adsorption on this complex substrate is similar to that on the simpler substrate, alumina. The dyes are probably aggregated when adsorbed, and the size of the aggregates increases with increase in the molecular weight of the dye ion. After considering the possible adsorption sites, and comparing the data with adsorption on simpler substrates, we suggest that the most important adsorption sites may be phosphate or other strongly acidic groups.

Adsorption of cationic (basic) dyes by fixed yeast cells.

The techniques used in textile dyeing are introduced, and the factors important in commercial dyeing emphasized. The contents are organized into three sections: 1) general principles; 2) practical experiments; and 3) background theory. Part 1 considers the basic facts about textile materials and the practices of laboratory dyeing. Part 2 offers experimental instructions. Most of the experiments are designed to be carried out using the simplest equipment, but some advanced experiments use pH meters, spectrophotometers, and color measuring equipment. Part 3 presents the background theory to the experiments.

Charles H. Giles

Papers

Oxidation and Reduction in Light Fading of Dyes

Economical and Efficient Fading Lamps

Some Observations on the Kinetics of Dye Fading

Adsorption of cationic (basic) dyes by fixed yeast cells.

Giles's laboratory course in dyeing