Removal of Per- and Poly-Fluoroalkyl Substances (PFAS) from Landfill Leachate using Modified Coal Fly-Ash (CFA)

Introduction
On April 10, 2024, the Environmental Protection Agency (EPA) established Maximum Contamination Levels (MCLs) for six PFAS compounds, emphasizing the urgency of effective remediation methods1. Landfills, a key source of PFAS contamination, show concentrations ranging from 2 to 47,000 ng/L in leachates 2-4. Current techniques such as reverse osmosis (RO) and granular activated carbon (GAC) achieve >90% PFAS removal but generate secondary waste, increasing energy and costs5-7. Coal fly ash (CFA), a byproduct of power generation, offers a sustainable alternative for PFAS remediation. Acid-thermal modification enhances CFA's surface area and functional groups, enabling efficient PFAS removal through electrostatic interactions and hydrophobic effects as shown in Figure 110. Studies have demonstrated significant PFAS removal efficiencies with acid-modified CFA due to its altered point of zero charge (pHpzc) and protonated surface8-10. Additionally, thermal treatment improves CFA's structural stability, reactivity, and adsorption capacity10. This dual modification process creates a robust and cost-effective adsorbent for PFAS in landfill leachates, offering environmental and economic advantages over RO and GAC. Acid-thermally modified CFA represents a promising solution for addressing the global challenge of PFAS contamination in complex matrices.

Materials and Methods
Materials
Three types of CFA (A, B, C) were collected from Chandler Concrete Co. (NC, USA). CFA-A was the original CFA from a coal-powered plant, CFA-B was reburned buried ash, and CFA-C was from another coal-powered plant. Their properties are listed in Table 1. Landfill leachate was collected from a municipal landfill (VA, USA) and stored at 4°C. Water quality parameters are shown in Table 2. PFAS chemicals for isotherm and kinetic study (PFBA, PFHxA, PFOA, PFDA, PFDoDA) were obtained from Sigma-Aldrich, and a mix of 25 PFAS analytes was sourced from Wellington Laboratories.

CFA Modification & Charaterization CFA was modified using 1—3 M HCl (Fisher Scientific) at a 1:2 ratio (1 g CFA to 2 mL acid). The mixture was baked at 120°C for 24 h, ground into powder, and analyzed pre- and post-modification using SEM. BET analysis determined the CFA surface area. pHpzc was evaluated using the pH drift method, and elemental composition was analyzed via XRF.

Leachate Adsorption Test Modified CFA (0—1000 g/L) was mixed with 50 mL landfill leachate at 150 rpm for 24 h at 20°C. Filtered samples were analyzed for PFAS and water quality parameters. QA measures included duplicate and triplicate analyses. Water quality was measured using standard methods (APHA). PFAS concentrations were analyzed via EPA Method 533 and CIC (EPA Method 1621). All analyses and curve fitting were performed using OriginPro 2024.

Results & Discussion
The performance evaluation of CFA adsorbents determined that 2.5 M HCl-modified CFA at 400 g/L was optimal for PFAS removal from leachate. Among the PFAS, PFBS exhibited the highest adsorption across all CFA types, with CFA-B achieving the best performance. CFA-B removed 41.3% of total PFAS, comprising 20.8% sulfonic-PFAS and 20.5% carboxylic-PFAS. CFA-A removed 38.2%, with 17.0% sulfonic- and 21.2% carboxylic-PFAS. CFA-C was the least effective, removing 26.2% of PFAS, split as 11.6% sulfonic- and 14.6% carboxylic-PFAS. Despite differences, all CFAs showed similar adsorption between sulfonic and carboxylic PFAS. CFA-B's superior performance is attributed to its enhanced surface area, increasing from 1.9 to 6.4 m2/g following 2.5 M HCl modification, compared to 1.7 to 5.4 m2/g for CFA-A and 1.1 to 2.8 m2/g for CFA-C. The increase in surface area and pore volume (2-3x) significantly improved adsorption capacity. Additionally, the higher CaO and iron oxide content in CFA-B facilitated adsorption through pore formation, ionic interactions, and ligand-binding sites. Iron oxides promoted surface complexation with PFAS, while CaO enhanced ionic interactions, particularly under acidic conditions. Silica, however, had limited impact due to its negative charge repelling anionic PFAS.

CFA-B was especially effective in adsorbing short-chain PFAS (C < 6), with 59.9 ng/g compared to 54.5 ng/g (CFA-A) and 39.1 ng/g (CFA-C). Long-chain PFAS (C > 6) adsorption was lower, with CFA-B at 9.2 ng/g, CFA-A at 9.4 ng/g, and CFA-C at 4.7 ng/g. This trend is likely due to either CFA's higher affinity for short-chain PFAS or the higher initial concentrations of short-chain compounds in the leachate. Overall, CFA-B's modified physicochemical properties provided multiple adsorption pathways, including hydrophobic interactions, electrostatic attractions, and chemical bonding, making it the most effective adsorbent for PFAS removal during leachate treatment.

Conclusion
2.5 M HCl-modified CFA, especially CFA-B, effectively removed PFAS from landfill leachate, with a 41.3% removal rate driven by CFA adsorbents exhibited higher adsorption for short-chain PFAS due to their higher initial concentrations and greater affinity, highlighting the material's suitability for diverse PFAS compounds. Acid-modified CFA provides a cost-effective, scalable alternative to conventional PFAS treatment methods, leveraging waste material for treatment.