## 3 Results and Discussion
### Adsorption Isotherms
#### 3.4.3 Adsorption Mechanism  
The anion exchangers used for DY50 sorption have functional groups of the weak base nature, i.e., -N(CH3)2. Their sorption capacities declared by the manufacturer correspond to the functional group contents and are about 1.3, 1.6 and 1.8 eq/L for A21, A24 and A23, respectively. If the retention mechanism of DY50 occurred solely by anion exchange reaction, then A23 resin should exhibit the highest sorption capacity for DY50. However, the determined sorption capacity based on the isotherm models used is 290.3 mg/g (which is close to the experimentally determined value of 284.3 mg/g) and is the lowest among the studied resins. The mechanism of dye retention in the anion exchanger phase is a result of the formation of an ionic pair between the functional groups of the anion exchangers and the sulphonic groups of the dye to a considerable extent (Figure 9) and can be described by the following reaction:  
\[nRCH_{2}NH(CH_{3})_{2}^{+}Cl^{-}+R^{\prime}(SO_{3}^{-})_{n}\rightarrow[RCH_{2} NH(CH_{3})_{2}^{+}]_{n}(SO_{3}^{-})_{n}R^{\prime}+nCl^{-}\]  
where \(RCH_{2}NH(CH_{3})_{2}^{+}Cl^{-}\)--weakly basic anion exchanger in the chloride form, \(R^{\prime}(SO_{3}^{-})_{n}\) --anionic form of dye of \(n\) sulphonic groups. However, the interactions between the delocalized electrons of benzene rings in the anion exchanger matrix and the delocalized electrons of dye aromatic rings (\(\pi\)-\(\pi\) type interactions), van der Waals and hydrogen bonds known as non-specific interactions can also be considered. Figure 9: Proposed mechanism of DY50 interaction with the weakly basic anion exchangers of the polystyrene, phenol-formaldehyde and polyacrylic matrices. This suggests that the adsorption of dyes is dependent on the composition of the anion exchanger matrix. The \(q_{e}\) value of A24 possessing the polyacrylic matrix was found to be 666.5 mg/g. The polystyrene resin A21 adsorbed 610 mg of DY50 per gram, while the phenol- formaldehyde anion exchanger adsorbed dye in the amount of two times less. Resin of an acrylic matrix demonstrates beneficial exchange equilibria and kinetics towards large organic ions such as dyes compared with the styrene ones [75]. A simplistic explanation for this lies with the greater hydrophilic nature of the aliphatic skeletal structure of the acrylic matrix, which in turn means van der Waals type interactions between the resin matrix and the hydrocarbon structure of an organic counter-ion (i.e., dye anion). The larger the organic anion the greater seems the affinity for an anion exchanger. Another parameter that may influence the sorption capacity values is the matrix structure. Two of the studied anion exchangers, i.e., A21 and A23 have macroporous structure while A24 has gel structure. There are no actual internal macroscopic structural pores (holes or channels) in gel type resin and it can be likened to a dense electrolyte-gel within which the dissociated counter-ions are able to diffuse. The non-uniform distribution of crosslinking and functional groups in gel type resin implies the rise of regions showing different structural characteristics within a given resin bead, and consequently non-symmetrical strains within the structure. Additionally, during aqueous conditioning and rinses, some of the stresses and strains within a resin are induced which enhanced the hydrophilic nature. Macroporous resins, being highly crosslinked, possess quite a heterogeneous distribution of structurally dense and tortuous regions of high charge density therefore the rate of exchange can be discernibly slower when compared with gel equivalents [75]. Since macroporous resins possess real pores, the number of functional groups per unit dry weight of matrix is usually less than that for an equivalent gel product as reflected by their slightly lower dry weight capacities. This, in turn, can affect the dye retention capability.