**5. Conclusions**

dyes onto chitosan is based on the strong electrostatic interactions between dissociated sul‐ fonate groups of the dye and protonated amino groups of the chitosan. Many studies con‐ firm that the above process is favored under acidic conditions, where the total "charge" of chitosan adsorbent is more positive (due to the stronger protonation of amino groups at

Proposing our diffusion concept, it is obvious from the experimental data that: (i) the tem‐ perature's dependence of the coefficients Dp suggests that the transport mechanism is not simply a pure diffusion through the pores, and (ii) the small values of Dp versus Dw set ques‐ tions about the existence of surface diffusivity. The above observations are compatible to the nature of interactions between the reactive dye and the adsorbent, which is described above. The strong electrostatic interaction in the adsorption sites inhibits the surface diffusion. Moreover, the electrostatic forces have a relatively large region of action. Using as reference the non-grafted chitosan (Ch), the existence of charges of opposite sign at the pore walls cre‐ ates a surface charge gradient in addition to the adsorbate gradient in the adsorbent particle. This charge gradient drags the oppositely charged dye molecules inside the particle and leads to enhanced effective pore diffusivity. This fact could completely explain the increase of diffusivity related with the grafting groups (the more positively charged grafted groups, the stronger attraction of negatively charged dye molecule). As the density of the adsorption sites increases, the charge density in the particle increases, leading to higher effective pore diffusivity values. The temperature's dependence of this electrostatically facilitated diffu‐ sion process is weaker than that of the pure diffusion process. However, the opposite phe‐ nomenon is occurred in the case of sulfonate-chitosan derivative (Ch-g-Sulf), where the surface charge is of the same sign as that of the dye molecule, inhibiting the diffusion proc‐ ess. The above concept of the adsorption process of reactive dyes on chitosan derivatives suggests the development of models that take into account explicitly the electrostatic inter‐ action between dye and adsorbent, instead of considering them only by the modification, which they create to the effective pore diffusivity Dp. Conclusively, by employing the phe‐ nomenological model based on the pair pore-surface diffusion for the transport of the dye in the adsorbent particle information about the actual mechanism of adsorption and on interac‐

Chitosan adsorbents present considerable advantages such as their high adsorption capaci‐ ty, selectivity and also the facility of regeneration. The regeneration of the adsorbent may be crucially important for keeping the process costs down and to open the possibility of recov‐ ering the pollutant extracted from the solution. For this purpose, it is desirable to desorb the adsorbed dyes and to regenerate the chitosan derivative for another cycle of application. Generally, the regeneration of saturated chitosan for non-covalent adsorption can be easily achieved by using an acid solution as the desorbing agent. Researchers proposed to desorb the dye from the beads by changing the pH of the solution [52,53] and they showed that the beads could be reused five times without any loss of mechanical or chemical efficacy. The

acidic pH values) [78,79].

198 Eco-Friendly Textile Dyeing and Finishing

tion between adsorbent and adsorbate can be extracted.

**4. Desorption conditions and Reuse**

The treatment of industrial dyeing effluent that contains the large number of organic dyes by adsorption process, using easily available low-cost adsorbents, is an interesting alterna‐ tive to the traditionally available aqueous waste processing techniques (chemical coagula‐ tion/flocculation, ozonation, oxidation, photodegradation, etc.). Undoubtedly, low-cost adsorbents offer a lot of promising benefits for commercial purposes in the future. The dis‐ tribution of size, shape, and volume of voids species in the porous materials is directly relat‐ ed to the ability to perform the adsorption application. The comparison of adsorption performance of different adsorbents depend not only on the experimental conditions and analytical methods (column, reactor, and batch techniques) but also the surface morphology of the adsorbent, surface area, particle size and shape, micropore and mesopore volume, etc. Many researchers have made comparison between the adsorption capacities of the adsorb‐ ents, but they have nowhere discussed anything about the role of morphology of the adsorb‐ ent, even in case of the inorganic material where it plays a major role in the adsorption process. The pH value of the solution is an important factor which must be considered dur‐ ing the designing of the adsorption process. The pH has two kinds of influences on dye: (i) an effect on the solubility, and (ii) speciation of dye in the solution (it depends on dye class). It is well known that surface charge of adsorbent can be modified by changing the pH of the solution. The high adsorption of cationic or acidic dyes at higher pH may be due to the sur‐ face of adsorbent becomes negative, which enhances the positively charged dyes through electrostatic force of attraction and vice versa in case of anionic or basic dyes. In case of chi‐ tosan-based materials, the literature reveals that maximum removal of dyes from aqueous waste can be achieved in the pH range of 2-4. However, physical and chemical processes such as drying, autoclaving, cross-linking reactions, or contacting with organic or inorganic chemicals proposed for improving the sorption capacity and the selectivity. The production of chitosan also involves a chemical deacetylation process. Chitosan is characterized by its easy dissolution in many dilute mineral acids, with the remarkable exception of sulfuric acid. It is thus necessary to stabilize it chemically for the recovery of dyes in acidic solutions. Several methods have been developed to reinforce chitosan stability. The advantage of chi‐ tosan over other polysaccharides is that its polymeric structure allows specific modifications without too many difficulties. The chemical derivatization of the polymer by grafting new functional groups onto the chitosan backbone may be used to increase the adsorption effi‐ ciency, to improve adsorption selectivity, and also to decrease the sensitivity of adsorption environmental conditions. It is interesting to note the relationships between physicochemi‐ cal properties and/or sources of chitosan and the dye-binding properties. Most of the prop‐ erties and potential of chitosan as adsorbent can be related to its cationic nature, which is unique among abundant polysaccharides and natural polymers, and its high charge density in solution.

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The common adsorbent, commercially available activated carbon has good capacity for the removal of pollutants. But its main disadvantages are the high price of treatment and diffi‐ cult regeneration, which increases the cost of wastewater treatment. Thus, there is a demand for other adsorbents, which are of inexpensive material and do not require any expensive additional pretreatment step. However,there is no direct answer to the question which adsorb‐ ent is better: chitosan (raw material, preconditioned chitosan, grafted or cross-linked chito‐ sans) or activated carbons? The best choice depends on the dye and it is impossible to determine a correlation between the chemical structure of the dye and its affinity for either carbon or chitosan. Each product has advantages and drawbacks. In addition, comparisons are diffi‐ cult because of the scarcity of information and also inconsistencies in data presentation.
