This proof-of-concept project aims to advance the development and application of a new generation of layered double hydroxide (LDH) adsorbents modified with covalently bonded organic functional groups for the treatment of per- and polyfluoroalkyl substances (PFAS) under various environmental systems. The organically modified LDHs combine the favorable features of (1) LDH minerals that show strong positive surface charge and electrostatic interactions with PFAS under environmentally relevant pH conditions and (2) organic modifiers that exhibit enhanced hydrophobic interactions with PFAS. More importantly, the pH-dependent surface charges of LDH minerals, together with the use of zwitterionic organic modifiers, can enable regeneration of the spent adsorbents under alkaline conditions without the use of organic solvents. As the first step of adsorbent development, this project aims to (1) identify compositions of organically modified LDH adsorbents for efficient PFAS treatment, (2) demonstrate the proof-of-concept for spent adsorbent regeneration with solutions free of organic solvents, and (3) identify optimum formulations of regeneration solutions with reduced impacts.

Three research tasks are outlined in this project. A suite of model PFAS with various chain lengths and functional groups will be examined based on their environmental prevalence and structures. In Task 1, the project team will prepare and characterize a series of multifunctional LDH adsorbents consisting of (1) long-chain alkyl groups to enhance hydrophobic interactions with PFAS and (2) zwitterionic groups to facilitate PFAS desorption under alkaline conditions. In Task 2, the adsorption performance of the organically modified LDHs will be determined in terms of PFAS adsorption kinetics, capacity, and removal efficiency under complex water matrices. Task 3 will investigate regeneration strategies of spent adsorbents and provide an initial assessment of adsorbent reusability. The most promising adsorbent candidates and regenerant formulations will be identified.

This project will provide a crucial first step and strong basis for the development of efficient and regenerable adsorbents for ex situ treatment of various PFAS-impacted aqueous media. Compared to existing adsorbents, employment of adsorbents with enhanced PFAS adsorption kinetics and capacity can reduce the material usage rate and thus enhance the adsorbent service life in PFAS treatment. Furthermore, regeneration of spent adsorbents with alkaline aqueous solutions free of organic solvents can substantially reduce the socioeconomic and safety burdens related to handling and treatment of the associated waste streams. The adsorbents can be used as a stand-alone technology for rapid PFAS removal, or be combined with destructive technologies to develop a treatment train for complete PFAS treatment. Findings of this research will lay the foundation for the establishment of more cost-effective and efficient PFAS treatment processes for the remediation of Department of Defense sites, thereby fortifying the Department of Defense's operational capabilities and securing warfighter preparedness. (Anticipated Project Completion - 2027)