Anion exchange resin mass transfer: method development and parameter determination for inorganic anions & PFAS
Citation:
Smith, S., D. Wahman, L. Haupert, B. Gray, E. Hughes, E. Kleiner, S. Pedigo, E. Stebel, C. Gastaldo, G. Abulikemu, J. Quinn, T. Sanan, J. Pressman, AND G. Sorial. Anion exchange resin mass transfer: method development and parameter determination for inorganic anions & PFAS. Presented at WQTC, Dallas, TX, November 05 - 09, 2023.
Impact/Purpose:
This presentation will cover USEPA research on per- and polyfluoroalkyl substances (PFAS) treatment in drinking waters in support of a multi-solute anion exchange column model. Column and batch experiments were conducted to validate the model for PFAS with experimental data, assess impacts of natural organic matter (NOM) on PFAS removal with anion exchange, and validate methodology for obtaining intraparticle mass transfer coefficient (Ds) estimates.
Description:
Per- and polyfluoroalkyl substances (PFAS) are ubiquitous in the environment and some are known to have deleterious health effects. Recently the United States Environmental Protection Agency published a proposed drinking water regulation that included six PFAS (PFOA, PFOS, PFNA, PFHxS, PFBS, and HFPO-DA or "GenX chemicals"). Single-use strong-base anion exchange (SBA) resins are a promising technology for PFAS treatment and have demonstrated some advantages over GAC. Predictive models can help understand the design, engineering, and costs of treatment; however, no publicly available model exists for PFAS on SBA resins and kinetic parameters tend to be scarce in the literature. Furthermore, more data is needed on effects of natural organic matter (NOM), which is present in source waters, affects SBA removal efficiency, and should be incorporated into treatment models. The presented research aims to (1) validate a multi-solute SBA column model for PFAS with experimental data, (2) assess impacts of NOM on PFAS removal, (3) validate a batch stirrer apparatus for use in kinetic studies, and (4) obtain intraparticle mass transfer coefficient (Ds) estimates for nitrate and sulfate as a first step toward estimates for PFAS. Four pilot-scale SBA systems with paired columns are supplied with water containing nine PFAS (including those in the proposed rule) and inorganic anions – two with and two without NOM. A completed experiment included 0.5- and 1.0-min EBCTs and a current experiment is operating at 1.5- and 2.5-min EBCTs to observe the fastest eluting PFAS (PFBA, GenX, and PFHxA) in greater detail. Column results validate and provide context to the SBA model, and the impact of NOM on PFAS removal will be explored. Batch stirrers are developed to estimate kinetic mass transfer parameters for PFAS on SBA resins. Although intraparticle mass transfer coefficients (Ds) can be fitted from column effluent data, the batch stirrer can provide more direct experimentally determined estimates. The batch stirrer assembly, adapted from literature, creates high flow over a small mass of resin, minimizing film layer thickness to isolate the effect of intraparticle diffusion. Kinetic parameter estimates (Ds and film transfer coefficient, or kL) are obtained from the Homogeneous Surface Diffusion Model (HSDM) using fractional uptake curves of the experimental data. The stirrer method is being systematically tested with nitrate, and Ds estimation will be demonstrated for nitrate and sulfate prior to use with PFAS. The presentation will cover both column and stirrer experiments, providing essential information on PFAS anion exchange breakthrough behavior, NOM impacts, and kinetic parameter determination.