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

Thirty-five dyed fabrics, representing eleven different dye-fibre combinations, have been tested for light fastness in the Microscal Light Fastness Tester, under illumination from the 400-W mercury vapour lamp, and to a 1500-Wxenon arc in a standard commercial machine. Evaluation of the results was made by a panel of nine observers, and mean gradings on the BSI scale were calculated. Alt but three of the dyeings gave ratings by the mercury lamp that were within three-quarters of a grade of those by xenon illumination, 17 of them being within one-quarter of a grade. The mercury lamp is therefore considered to be a satisfactory illuminant for commercial light-fastness evaluation; it would offer very considerable economies in operation.

The three above-mentioned (all vat) dyeings were rated lower by between 1.2 and 1.4 mean grades with mercury compared to xenon illumination. Reasons are given for a suggestion that traces of an oxidant remaining from the dyeing operations may have been responsible for the more rapid fading in the mercury lamp. There is a possibility therefore that the mercury lamp may be useful in detecting colorations that might show anomalously rapid fading outdoors where there is a large proportion of violet and near-ultraviolet radiation.

Comparison of Light Fastness Assessments with the Microscal (Mercury Vapour Illuminant) Light Fastness Tester and Xenon Arc Illumination

Apparent deviations from Beer's law that exist with many dyes in aqueous solution have been measured over an extended concentration range. Three types of ‘curve’ can be distinguished: A, linear, generally found in solutions <10-1 M; B, concave to the concentration axis; C, a new and composite type having a lower portion of type B and an upper of type A (positive slope). C is observed with some surface-active dyes and appears to be caused by micellisation. Where the size distribution of the absorbing species does not change over the concentration range under study, e.g. as for aqueous dispersions of pigments, no deviations from Beer's law are observed. Dyes that show marked deviations from the law in aqueous solution due to aggregation phenomena may exhibit ideal behaviour over a thousand-fold concentration range in organic solvents.

The Effects of Association on the Beer's Law Behaviour of Dye Solutions

Cellulose monolayers have been spread on dilute solutions of Janus Red B (C. I. 26115) a cationic substantive dye (I); a monochlorotriazinyl (hot–dyeing) reactive dye (II) and a dichlorotriazinyl (cold–dyeing) reactive dye (III), under various conditions. I forms mixed films, in which the dye molecules (which are surface–active) are probably oriented between the cellulose chains. The substantivity of this dye for cellulose fibres has no apparent influence on its behaviour towards the film, thus re–inforcing previous conclusions that substantivity is largely a result of fibre morphology. II, at high pH, reacts with the film, making it relatively incompressible, but not causing much expansion. The dye, after reaction, is believed to be oriented beneath the film. III shows much more marked effects than II. With rise in pH and on standing for 24 h or more, it causes a large expansion of the film, which becomes incompressible with an immeasurably high viscosity. It is suggested that the dye molecules are in the surface, interposed between, and crosslinking, the cellulose chains, and appear to have reacted on each side of each glucose residue.

Study of Cellulose Substantivity and of Cellulose–Reactive Dye Reactions by the Monolayer Method

Surface-tension measurements of three wool-reactive dyes in water-benzyl alcohol mixture show that, contrary to previous hypotheses, the presence of the organic solvent depresses the concentration of dye at the interface with air, and most probably with wool. It is suggested that this effect accelerates dye diffusion into the fiber by reducing “drag” of diffusing dye molecules in the intermolecular channels of the fiber.Determination of surface pressure/area relationships and surface-film viscosity measurements at measured times after spreading protein monolayers on acid solutions of the same dyes in pure water and solutions of dimethyl formamide, methanol, isopropanol, n-butanol, and benzyl alcohol, respectively, shows that the presence of solvent accelerates covalent bond formation between dye and protein. It is suggested that the solvent shields hydrophobic regions of the protein and allows easier access of the dye to the reactive polar groups in the protein; also, it may retard the side reaction of dy...

Charles H. Giles

Papers

Comparison of Light Fastness Assessments with the Microscal (Mercury Vapour Illuminant) Light Fastness Tester and Xenon Arc Illumination

The Effects of Association on the Beer's Law Behaviour of Dye Solutions

Study of Cellulose Substantivity and of Cellulose–Reactive Dye Reactions by the Monolayer Method

An Examination of the Mechanism of Solvent-Assisted Dyeing by Surface-Tension and Monolayer Measurements

British Light‐fastness Standards