Properties of Anion Exchange Resins Processed by Three Grinding Methods

Rapid small-scale column tests (RSSCTs) are used to simulate full-scale treatment processes with granular activated carbon (GAC). This technique has recently been applied to ion exchange (IX) resins for adsorption of certain per- and polyfluoroalkyl substances (PFAS). However, RSSCTs rely on the preparation of size-reduced (ground) media, and there is currently a lack of consensus on resin grinding methods for IX resins and their associated effects on RSSCT correspondence. To evaluate impacts of resin size reduction and choice of grinding method, a polystyrene gel and macroporous strong base anion exchange resin were processed with a blender, a ball mill, and a mechanized mortar and pestle. Five mesh size fractions ranging from 40×60 to 200×325 were characterized along with the full-size (bulk) resin for surface morphology, water retention capacity, bed porosity (εb), anion exchange capacity (AEC) on a dry mass (Qm), filter capacity basis (Qb), and resin densities. Using a blender was the most time-efficient resin particle size reduction method. Grinding by blender, ball mill, and mortar and pestle each dramatically changed IX resin morphology from perfect spheres to rough, irregularly shaped particles with angular ridges and striations. Tukey’s test identified statistically significant, major (≥10%) grinding-induced changes in Qb and εb for both resins. Both εb and Qb are assumed to be constant in RSSCT scaling. Ion exchange column model simulations predict that grinding-induced changes resin properties could affect throughput to 5% breakthrough by as much as 18%. It is advisable that any study using ground resin should include a detailed description of grinding methodology and consider measuring Qb of ground and full-size resins.