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

The solute adsorption isotherm classification, examined theoretically in the previous paper, is illustrated by a number of further experiments which help to clarify some earlier obscurities and enable an isotherm to assist (a) in interpreting the adsorption mechanism and the solute orientation, (b) in identifying the presence of associated solute and of an impurity, and (c) in measuring the specific surface area of a solid. The production of the S-curve by either cooperative adsorption or by a competitive effect of an impurity in the solvent is illustrated and discussed. Thus p-nitrophenol is adsorbed from dry benzene by both silica and graphite. Traces of water in the solvent reduce the adsorption on silica by competition and change the curve from L- to S-shape, but have no effect on adsorption on graphite, where competition is negligible. The adsorption of aromatic hydrocarbons by alumina, which usually gives S-curves, is discussed. Cooperative adsorption can give the H-type of curve if the cooperating molecules move as a large unit from solvent to substrate, as in J-aggregate adsorption of cyanine dyes on silver halides. Examples of adsorption on inorganic and biological substrates giving C-type (linear) curves are quoted, illustrating the progressive penetration of micropores in the substrate suggested to be characteristic of this process. Data are given showing that maxima in isotherms for adsorption of high affinity solutes can be identified with completion of the monolayer. Surface tension measurements suggest that maxima may result from the presence of traces of a highly surface-active impurity.

Anthony P D'Silva

Papers

A general treatment and classification of the solute adsorption isotherm part. II. Experimental interpretation