## 2 Hybrid Treatment Methods
### Adsorption/Oxidation Treatment  
[PERSON] et al. [43] showcased the efficacy of coupling ion-exchange resin with electrochemical (EO) oxidation in augmenting PFAS treatment within groundwater through a pilot plant field trial. The treatment sequence encompassed two pivotal steps:  
* Ion exchange: This initial phase facilitated the separation and concentration of PFASs from groundwater. The ion exchange resin exhibited the capability to eliminate 18 distinct PFASs from the groundwater, encompassing PFHxS, PFOS, and 6:2 FTS. The resin effluent displayed PFAS concentrations below detectable limits, except for PFBA, PFPeA, and PFHxA. The resin was regenerated by leaching with methanoland NaI solution, and the methanol in the leachate was removed via distillation. The resulting solution was treated using EO. * Electrochemical oxidation (EO): subsequent to ion exchange, this stage annihilated the concentrated PFASs. The EO process manifested a notable reduction in PFAS precursor concentrations by 74.1-96.8% and four extended-chain PFAAs (namely PFHxS, PFHpS, PFOA, and PFOS) concentrations by 59.2-94.0%. The amalgamated ion exchange/EO treatment train stands out compared to isolated EO treatment due to its energy efficiency. Notably, the heightened ionic strength of ion exchange effluent, a result of increased salt concentrations, obviates the necessity for electrolyte dosing to enhance electrical conductivity, a requirement in a standalone EO system. Recently, [PERSON] et al. [44] conducted a study on the extraction of four structurally different perfluoroalkyl ether carboxylic acids (PFECAs) from tap water (0.2 mg C/L TOC) and groundwater (7.3 mg C/L TOC). The PFECAs were removed using an ion exchange (column-based)/EO hybrid method. Their findings revealed that the ion exchange material A592E (a microporous resin) successfully adsorbed >92% of the PFECAs. Moreover, a solution comprising ammonium sulphate + methanol effectively desorbed 60-100% of the adsorbed PFECs. Subsequently, EO treatment resulted in the removal of >99.9% of the PFECAs after distilling off >99.9% of methanol. [PERSON] et al. [45] similarly deployed an adsorption/oxidation treatment train to address PFAS removal from simulated groundwater. Their approach entailed two sequential stages:  
* Adsorption: Employing granular activated carbon (GAC), this phase concentrated and adsorbed PFASs from groundwater. * Oxidation: GAC underwent activated persulfate oxidation to dismantle the adsorbed PFASs. PFOS was not transformed even with high oxidant doses, but PFOA degraded into shorter-chain compounds. The GAC/oxidation treatment train's appeal stems from its relatively lower cost. However, a noteworthy consideration lies in the post-treatment disposal of GAC, potentially constituting a significant expense. In summary, both the ion exchange/EO and GAC/oxidation treatment trains exhibit efficacy in addressing PFASs within groundwater. The ion exchange/EO treatment train holds an advantage in energy efficiency and electrolyte-free operation, albeit at a higher cost. Conversely, the GAC/oxidation treatment train presents a more economical solution, albeit with the added cost of GAC disposal. The optimal treatment train selection hinges on specific site conditions and budgetary considerations. Nanoscale materials exhibit significant potential as adsorbents for PFAS treatment due to their large surface area and corresponding reactivity, as demonstrated by [PERSON] et al. [46]. Their adaptability allows for facile modifications, such as the incorporation of surface-bound metal catalysts and fluorine, effectively enhancing their PFAS removal capacities. [PERSON] et al. [46] showcased multiple recent studies focusing on PFAS removal from aqueous samples utilizing various nano-enhanced adsorbents, along with strategies for subsequent targeted destruction of PFASs at or near the material's surface. Furthermore, the authors presented integrated methods that involved adsorption on nanoscale materials followed by directed PFAS destruction.