EFFECTS OF AGING PROCESSES AND CO-EXISTING SUBSTANCES ON THE PERFORMANCE OF COLLOIDAL ACTIVATED CARBON FOR IN SITU REMEDIATION OF PERFLUOROALKYL ACIDS

Abstract

Per- and polyfluoroalkyl substances (PFAS) are emerging organic contaminants that are ubiquitous and persistent in environments, bioaccumulative in ecosystems, and pose potential risks to human health and wildlife. PFAS have been detected in human tissues, raising concerns about their potential impacts on public health. Among PFAS, perfluoroalkyl acids (PFAAs) are particularly challenging to remediate due to their high detection rates, greater water solubility, and widespread mobility in the environment, especially in soil and groundwater. Colloidal activated carbon (CAC), characterized by its small size (< 1 µm) and high surface aera (> 1000 m2/g), has gained attention as an injectable adsorbent for in-situ remediation technology for PFAA-contaminated groundwater. However, the long-term effectiveness of CAC for in-situ PFAA removal is challenged by the complexity of the subsurface environment. Environmental factors, such as fluctuations in humidity, chemical reactions, and biological activity (e.g., biofilm formation) in the subsurface area, can lead to aging effects that alter the surface properties of CAC. Additionally, the presence of co-existing substances can influence the adsorption behavior of PFAAs, collectively diminishing the performance of CAC. Limited information is available on the extent and mechanisms by which these factors influence CAC’s effectiveness, posing a barrier to its widespread implementation for remediating PFAA-impacted sites. However, over time, the complex and variable environmental conditions can induce aging (mechanical, chemical, and/or biochemical) processes in CAC, altering its surface properties and potentially affecting its PFAA removal efficiency during the long-term in-situ application. These aging processes include physical aging (i.e., driven by mechanical stresses such as wet-dry cycles), chemical aging (i.e., through oxidative reactions involving oxidants or mineral catalysts), and biological aging (i.e., caused by microbial colonization and biofilm formation). Those processes might affect the adsorption efficiency of CAC for PFAA by altering the availability of effective surface sites and accessibility of pore structure, and changing surface elementary composition and charge. Currently, there is little information on how aging processes affect the removal of PFAA by CAC. In this project, controlled laboratory physical (i.e., wet-dry cycling, W/D), chemical (i.e., H2O2 (H), Fenton reaction (F), and acid treatment (A)), and biological (i.e., bacterial inoculation on CAC surface (B)) aging processes were applied to CAC at the laboratory scale to simulate the aging processes in the subsurface environments. The impact of controlled laboratory aging treatments, designed to stimulate the aging processes of CAC, on the effectiveness of CAC in removing PFAA compounds were evaluated by characterizing aged CACs for surface properties, including specific surface area, pore structure, surface morphology, elemental composition, surface functional groups, point of zero charge, and anion exchange capacity). Adsorption parameters (including the Freundlich exponent (n), the Freundlich constant (KF), and the partition coefficient (Log Kd)) were determined from adsorption isotherm experiments using a mixed PFAA solution containing perfluorobutanoic acid (PFBA), perfluorobutanesulfonic acid (PFBS), perfluoro-n-pentanoic acid (PFPeA), perfluorohexanoic acid (PFHxA), perfluorooctanoic acid (PFOA), and perfluorooctanesulfonic acid (PFOS). These results were compared with unaged CAC to elucidate mechanisms by which aging affect CAC performance in mitigating PFAA contamination. Physical and chemical aging treatments induced varying changes in CAC surface properties, including surface area, pore structure, surface oxygen content, and surface charge. H₂O₂ and physical aging treatments caused minor changes on CAC compared to unaged CAC, whereas Fenton-reagent and acid aging treatments markedly increased surface oxygen content, reduced surface area, and enhanced surface negativity. These changes collectively suggest a reduction in the adsorption capacity of CAC for PFAA removal. Consistent with the observed aging effects on surface properties, the results from adsorption isotherm experiments indicated the tendency of reduction in the adsorption of all tested PFAAs, except for PFOS, on CAC following both physical and chemical aging. Especially for short-chain PFAAs, such as PFPeA and PFBA, Fenton-aging treatment led to decreases in KF and Log Kd by up to 63%-76% and 45%-53%, respectively. Surface characterization analyses of CAC after biological aging treatment revealed a significant reduction in the effective surface area of CAC, primarily due to biofilm or biological residue occupying adsorption sites and blocking pores. Additionally, biological aging treatment increased the contents of oxygen and hydrophilic functional groups on CAC surface, such as carboxyl/carboxylate and amino groups, leading to decreased surface hydrophobicity and reduced sorption capacity for most PFAAs, with the exception of PFOS. Similar to physical and chemical aging treatments, biological aging posed a greater challenge to the adsorption capacity of CAC for shorter-chain PFAA, such as PFPeA and PFBA, with reductions in KF and Log Kd reaching 56%-74% and 37%-47%, respectively. These results suggest that the aging process is an important, yet often overlooked, factor in determining the long-term effectiveness of the CAC sorptive barrier for PFAS removal, especially for shorter-chain, hydrophilic PFAA. The presence of co-existing substances in the subsurface environments, such as dissolved organic matters (DOM), inorganic ions, and other co-contaminants (e.g., diesel range organics (DRO)), may exacerbate the impacts of aging processes on CAC’s performance for PFAS removal. In this study, the combination effects of aging and various environmental solutes (i.e., DOM, DRO, or inorganic ions like Cl- and Ca2+) on CAC performance were assessed in a multi-PFAA solution containing PFOS, PFOA, PFHxA, PFPeA, PFBS, and PFBA. Batch and column adsorption experiments were utilized to evaluate the adsorption coefficients (i.e., n, KF, and Log Kd), adsorption capacity (Qm) and accumulative desorption ratio). The results indicated that adsorption affinities of each tested PFAA generally aligned with their hydrophobicity, with the observed trend: PFOS > PFOA >> PFHxA ~ PFBS > PFPeA > PFBA, suggesting that hydrophobic interactions play a key role in PFAA adsorption. This ranking remained invariant across fresh or aged CAC and different aqueous environmental conditions, suggesting that the relative affinities among PFAA are primarily governed by their physiochemical properties, particularly their fluorinated carbon chain length and functional groups. Meanwhile, batch isotherm experiments demonstrated that high ionic strength (101 mM adjusted by NaCl), DOM (1 or 10 ppm), and DRO (1.73 ppm) inhibit PFAA adsorption by altering electrostatic interactions and suppressing effective adsorption sites on both fresh and aged CAC. The adverse effects of aging processes, such as Fenton reagent and biological aging, compounded these inhibitory mechanisms of co-existing substances by altering CAC surface properties, particularly reducing its effectiveness in removing short-chain PFAA adsorption. Notably, the presence of 10 ppm DOM or 1.73 ppm DRO completely suppressed the adsorption capacities of Fenton reagent and biological aged CAC for PFPeA and PFBA, leading to non-detectable adsorption occurred. Column experiments revealed that aged CAC exhibited reduced adsorption capacities and enhanced desorption. Particularly, in the column filled with Fenton’s reagent aged and biologically aged CAC, short-chain PFAAs (C<7) that had initially adsorbed were fully desorbed and remobilized. These findings suggest that complex environmental solutes, particularly with the presence of DOM and DRO, can diminish the long-term effectiveness of CAC barriers. Moreover, aging processes such as Fenton-induced reaction and biological aging might intensify these inhibitory effects, further reducing the long-term effectiveness of CAC for in-situ PFAA removal and accelerating the remobilization of PFAAs from adsorbed CAC sorptive barrier. While previous studies have investigated the aging effects on other carbon-based adsorbents (e.g., granular and powdered activated carbon, and biochar) in various contexts, this study is the first to evaluate the potential effects of in-situ aging on the performance of CAC for PFAA removal under controlled laboratory conditions. By examining combined impacts of aging and environmental solutes on PFAA adsorption by CAC particles, and how aging processes influence PFAA adsorption-desorption behaviors, this work provides valuable insights into the long-term operation of in-situ CAC barriers for remediating PFAA in subsurface environment. The findings underscore the critical need for comprehensive characterization of aging effects on CAC, refinement of predictive models of CAC barrier for considering aging effects, and the development of improved guidance for site-specific CAC barrier applications.