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Anion exchange remediation of per- and polyfluoroalkyl substances from groundwater impacted by aqueous film-forming foams
Ellis, Anderson Clayton
Ellis, Anderson Clayton
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2023
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Abstract
Ubiquitous detection of per- and polyfluoroalkyl substances (PFASs) in groundwater has occurred on a global scale resulting from these compounds’ use as surfactants in aqueous-film forming foams (AFFF). The high mobility, solubility, and recalcitrance of these contaminants renders most conventional water treatment processes ineffective at removing PFASs, highlighting the need for advanced separation technologies. While PFAS plumes are typically remediated using granular activated carbon (GAC) at present, anion exchange resins (AERs) have emerged as a promising technology due to their enhanced affinity for PFASs, high capacity, and rapid adsorption kinetics. The emergence of novel ‘PFAS-selective’ AERs has received increased attention due to their specialized functional groups that lead to extremely high PFAS affinities, though little information is available regarding these adsorbents’ performance in field settings, their environmental sustainability, or their practical regenerability despite being marketed as single-use products. Studies detailed in this thesis investigate the performance of commercially available AERs compared to conventional adsorbents in remediating a diverse suite of PFASs in complex groundwater matrices, furthering the understanding of PFAS uptake, adsorption mechanisms, treatment system sustainability, estimated costs, resin regenerability, and resource recovery associated with ex-situ PFAS remediation schemes.
Resin products were parameterized in batch screening studies, enabling down-selection of promising products for use in a field-scale pilot system installed at a PFAS source zone near Philadelphia, PA and operated for eight months. Breakthrough of PFASs followed a trend of chromatographic elution, with perfluorocarboxylic acids (PFCAs) eluting prior to perfluorosulfonic acids (PFSAs) and short-chain structures eluting prior to longer-chain analogues. Resins marketed as ‘PFAS-selective’ were found to have greater adsorption capacity and affinity for PFASs compared to regenerable AERs, with longer empty-bed contact times (EBCTs) being found to prevent premature PFAS elution. Polystyrene-based resins treated most PFAS structures >10-fold longer prior to breakthrough than polyacrylic resins, as all analytes eluted through the regenerable polyacrylic resin bed within two weeks of operation. PFSA structures were fully removed by PFAS-selective resins for the duration of the pilot study. Post-mortem analysis of spent resin beds revealed competitive adsorption among PFAS structures, with evidence supporting competitive displacement mechanisms whereby higher-affinity structures cause lower-affinity compounds to desorb from exchange sites.
Field pilot breakthrough data was used to inform sustainability and cost models using a life cycle assessment (LCA) framework. Models found both regenerable and PFAS-selective AERs to be more sustainable than GAC, and PFAS-selective resins were identified as the lowest-cost system among the three options. Primary drivers of system sustainability and costs were found to be (1) PFAS treatment goals, (2) frequency of sorbent changeout/regeneration, (3) resource recovery, particularly for regenerable AERs, and (4) disposal of PFAS-laden wastes. PFAS-loaded AERs from the field pilot were used to assess the regenerability of ‘PFAS-selective’ resins and evaluate regeneration parameters crucial to induce PFAS desorption. Batch screening of regenerant solution constituents identified the combination of methanol and sodium chloride to be ideal for promoting desorption of PFAS from resins, with poor regeneration being observed when using solvent-only or brine-only solutions. Continuous-flow column work found regenerable AERs to experience complete regeneration using 70% organic cosolvent fractions, with PFAS-selective resins requiring 90% cosolvent to achieve complete desorption. These selective resins were found to require additional bed volumes of regenerant to achieve complete desorption, though EBCT was found to have minimal effect on regeneration efficacy.
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