An evaluation on S-type adsorption isotherm in the model of crosslinked polyhydroxamates/oxazine dyes/water interactions

The effects of crosslinker and dye type on swelling and S-type adsorption properties of crosslinked polyhydroxamates (CHP) were investigated. CHPs containing N,N?-methylenebisacrylamide (N), or ethylene glycol dimethacrylate (E) were used in the swelling, diffusion, and adsorption experiments in solutions of oxazine dyes such as Brilliant Cresyl Blue, Nile Blue, and Cresyl Violet. Swelling and diffusion parameters of CHPs in dye solutions (such as equilibrium swelling, half time of swelling, swelling value at half time, network parameter, diffusion exponent, and diffusion constant) were calculated. It is understood from the time of swelling to reach equilibrium that CHPs swell very fast. CHP-E in all dyes solutions swelled considerably more than CHP-N. Dye solution diffusion into CHPs was determined to be of non-Fickian character. It has been observed that the swelling properties of hydrogels are highly influenced by the crosslinker type. The adsorption of oxazine dyes onto CHPs is similar to the S-type adsorption in the Giles classification system. When it was seen that the experimental data fit the Sigmoidal 4 parameter equation with a high correlation (r²?>?0.995), the use of this equation determined the adsorption parameters such as the highest bonding rate or monolayer coverage, the transition point of the isotherm, the magnitude of the absorbent's absorbability and the slope parameter. Site-size, maximum fractional occupancy, the binding ratio at the transition point, binding constant, the initial binding constant, partition coefficient, and adsorption free energy values were also calculated by using the found adsorption values. Dye adsorption from all dyes solutions to CHP-E is considerably higher than CHP-N. An increasing linear relationship was found between swelling and adsorption. In conclusion, the sigmoidal equation approach can be a useful tool for chemists, chemical, agricultural and environmental engineers, polymer scientists to find the adsorption parameters of polymer adsorbents, and at the same time, it can be said that CHP can be used as a good sorbent in the removal of some chemical agents (such as dye molecules, organic molecules, biologically active molecules).