Adsorption of amphoteric aromatic compounds by hyper-cross-linked resins with amino groups and sulfonic groups

For efficient enrichment of amphoteric aromatic compounds p-aminophenol (PAP), p-aminobenzoic acid (PABA) and sulfanilic acid (SA) from water, two types of hyper-cross-linked resins NJ-6 with sulfonic groups and NJ-99 with tertiary amino groups were compared in this study. The kinetics plots show that the adsorption of PAP on NJ-6 and NJ-99 are both favorable processes with over 80% and 67% of the equilibrium capacity reached within 30 min and all of the kinetics data follows the pseudo-second-order rate equation well. Intra-particle diffusion is not the only rate-limiting step except in the first 6 min. In FT-IR spectra, the peaks concerned with the functional groups of the resins were weakened or shifted after adsorption. The ratio of the equilibrium adsorption capacities (Qe) between NJ-99 and NJ-6 increase with the decreasing pKa1 values of adsorbates and the character of adsorption on NJ-99 will be changed from reversible to irreversible with the decreasing pKa1 values of adsorbates, too. Therefore, we may deduce that the interaction between acidic groups of the adsorbates and amino groups of NJ-99 would play a more dominant role with the decreasing pKa values of the adsorbates. The Koble–Corrigan model, a combination of the Langmuir and Freundlich isotherm models give the best fit to the equilibrium adsorption isotherms of the adsorbates on NJ-99 at different temperatures. Molecular form of the adsorbate is favorable for the adsorption.

► Functional groups of the resins play a dominant role in adsorption.
► Koble–Corrigan model gives the best fit to the equilibrium data.
► Adsorption kinetics agrees with the pseudo second-order model.
► Comparison of isotherms at 303 K fallen from 318 K or risen from 288 K to verify reversibility.
► Molecular state favored adsorption on hyper-cross-linked resins.