Modelling of chromatographic carboxylic acid fractionation with a strong anion exchange resin in sulfate form

Abstract

Chromatographic fractionation of complex mixtures of carboxylic acids with a strong anion exchange resin in sulfate form as separation material was modelled. The studied system contained lactones of two isosaccharinic acids, four dissociating monocarboxylic acids (lactic acid, glycolic acid, acetic acid, and formic acid), and two strong electrolytes (Na2SO4 and H2SO4). The sorption equilibria in the studied system are complex because of three distinct sorption mechanisms: affinity-based interactions, site sharing, and ion exchange. Modelling of the sorption equilibria was attempted using simple sorption isotherms models: the Sheindorf–Rebuhn–Sheintuch (SRS), the Langmuir, and the Sips models. For simplicity, the electrolytes were treated as molecular components (without dissociation). In the case of pure carboxylic acids, both the SRS (residuals 10–5–10–4) and Sips models (residuals ∼ 10–7–10–5) correlated well with the experimental data. However, the Langmuir model was found to be inadequate. In the case of competitive sorption, the SRS model (residuals 10–6–10–4) correlated well with the data, but the Sips model’s (residuals ∼ 10–2) predicted sorption was too strong. Additionally, the sorption of Na2SO4 was well predicted by the SRS model. For each component pair, only a single interaction parameter was required with the SRS model. The sorption equilibria (SRS) model and the column mass balance model were validated against data from batch column experiments done with carboxylic acid-rich feed solutions. Pure water or aqueous carboxylic acid and salt solutions were used as eluents. The model accurately predicted the elution profiles in each case. The characteristic shapes of the carboxylic acid elution profiles with the effects resulting from competitive sorption were well accounted for. Strong sensitivity of the studied system towards changes in pH and H2SO4 concentration was observed.