Estimating Ion Exchange Resin Intraparticle Diffusion Coefficients: Method Development and Nitrate Application

Gel‐type anion exchange (AEX) resin performance is impacted by internal mass transfer, which is represented by the solid‐phase intraparticle diffusion coefficient ( D s ). Importantly, D s values are required inputs to ion exchange treatment process models but are rarely reported. Previous experiments using gel‐type AEX resins exhibited external mass transfer control, preventing D s estimation for several anions, including perfluoroalkyl substances. To reduce external mass transfer resistance and allow D s estimation, a stainless‐steel centrifugal stirrer device was standardized and validated with nitrate (model anion). Initially, experiments were conducted across mixing speeds (750–1750 rpm) to balance (1) maximizing external mass transfer and (2) preventing flow disturbances. Subsequently, experiments used an optimized speed to estimate nitrate D s at multiple initial concentrations on three gel‐type AEX resins. The developed methodology was useful for determining nitrate D s and should enable improved D s estimates for other contaminants, including perfluoroalkyl substances, needed for ion exchange treatment process models.