Thermodynamics and kinetics of 1,8-dihydroxy-2-(imidazol-5-ylazo)-naphthalene-3,6-disulphonic acid immobilization on Dowex resin

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The sorption of 1,8-dihydroxy-2-(imidazol-5-ylazo)-naphthalene-3,6-disulphonic acid (IACA) onto the anion exchange resins Dowex 1-X8 and Dowex 2-X8 was studied by following the absorption spectra of solid phase, as the function of acidity, dye concentration, temperature and time. The aim of this work was to elucidate the thermodynamics and kinetics of the sorption process and to investigate the influence of the sorption on the ionic forms of the modified dye. The Langmuir model was applied to describe equilibrium isotherms of dye sorption onto Dowex 1-X8, 50–100 mesh at pH 4.5. The values of Gibbs free energy Δ G were between −26.0 kJ/mol and −30.5 kJ/mol, in the temperature range from 0 to 25 °C. The results of the kinetic experiments, performed in batch conditions, showed that sorption was pseudo-first order process. IACA diffusion coefficients from 5.2 × 10 −9 to 8.2 × 10 −9 cm 2/s for Dowex 1-X8 and from 4.5 × 10 −9 to 7.1 × 10 −9 cm 2/s for Dowex 2-X8 were obtained. Activation energy values calculated by using Arrhenius equation, were 25.8 kJ/mol, for Dowex 1-X8, and 25.4 kJ/mol for Dowex 2-X8. The apparent equilibrium constants of protonation of the imidazole moiety (log K p = 2.4) and dissociation of OH group from naphthalene nucleus (p K d = 6.4) of the modified dye were determined from the spectrophotometric data using Donnan model, and compared to the values obtained in the solution.