Adsorption of Low-Molecular-Weight Carboxylic Acids to Anion Exchange Resins with Various Properties and the Mechanisms of Interaction

Anion exchange resin (AER) is often used to produce ultrapure water by removing low-molecular-weight (LMW) carboxylic acids (CAs). However, the adsorption performance of resins requires improvement, and the adsorption mechanisms are not fully clear. In this study, eight AERs with different polymer matrices, porosity types, and ammonium group properties were used to investigate the adsorption of common LMW CAs. The adsorption efficiency of strongly basic AERs (80–100%) was much higher than that of weekly basic AERs (2–56%). The adsorption kinetics were limited by intraparticle diffusion. In particular, for CAs with a molecular volume of > 83.5 Å, the gellular AERs showed an obvious size exclusion effect that was not as apparent for the macroporous AERs. The adsorption capacity and affinity were highly related to the polymer matrix. The polystyrene AERs showed 1.60–3.00 times higher maximum adsorption and 2.00–3.80 times greater adsorption affinity than their polyacrylic counterparts. Adsorption also depended on the CA structure. The carbon and carboxyl number of the CA affect the total negative atomic charge density, which in turn affects the CA–AER electrostatic interaction. Besides, hydrogen bonds between the carboxyl group and the AER matrix increased CA adsorption; this effect was especially prominent for diprotic CAs.