Adsorption of PFAS from Subsurface using Montmorillonite Grafted Chitosan Beads

Abstract

Per- and polyfluoroalkyl substances (PFAS) are highly persistent pollutants with widespread contamination in the environment, posing significant risks to human health. To address the urgent need for effective remediation strategies, adsorption techniques have gained prominence as a promising approach for PFAS removal. However, the currently available commercial adsorbents have limitations, such as the lack of studies on their performance in soil, and the use of non-renewable materials. To address these issues, a novel adsorbent based on montmorillonite grafted chitosan beads (MTGCB) is developed that combines the advantages of both natural materials and efficient against PFAS removal. This thesis focuses on the comprehensive analysis of MTGCB for the efficient removal of PFAS from water, stabilization in PFAS contaminated soil, and their impact on the mechanical properties of the soil and compare them with commercial adsorbent granular activated carbon (GAC). The introduction highlights the detrimental effects of PFAS on human health, emphasizing the critical need for effective remediation strategies. Adsorption techniques, such as those employed in this study, are instrumental in removing PFAS from water, stabilizing contaminated soil, and enhancing the mechanical properties of the soil. The research comprises three main components: (1) adsorption tests to evaluate the efficiency of MTGCB and GAC for PFAS removal from water, (2) leaching tests to assess the stabilization properties of these adsorbents in soil, and (3) shear parameter analysis to investigate the impact of the adsorbents on the mechanical properties of the soil. The results demonstrate that both GAC and MTGCB exhibit effective removal of PFAS compounds, with higher removal rates observed for long chain PFAS compounds. GAC shows excellent adsorption capacity, reaching equilibrium within 24h for all PFAS compounds, while MTGCB exhibits 48h except PFBA which is 24h due to its molecular size. Both adsorbents conform well to the Langmuir and Freundlich model, and MTGCB demonstrates higher capacity for PFOS, PFBS and PFBA due to hydrophobicity and electrostatic interactions, while for PFOA it was similar to GAC. Leaching tests indicate that GAC provides excellent stabilization with minimal leaching, while MTGCB shows a decreasing trend in leaching with increasing adsorbent percentage. Longer carbon chains and sulfonic groups in PFAS contribute to better stabilization. Optimal addition percentages of 2% for GAC and 10% for MTGCB are recommended for achieving soil stabilization. Furthermore, compaction tests reveal a decrease in maximum dry unit weight of soil with the addition of both adsorbents due to their low specific gravity and increased water content in GAC due to presence of its water holding capacity while in MTGCB the variation was negligible. However, the friction angle and cohesion increase, indicating improved mechanical properties, with GAC's angular shape and MTGCB's presence of montmorillonite aiding in binding soil particles together. An optimal addition percentage of 5% is suggested for achieving enhanced mechanical properties. Considering the stabilization and mechanical properties of the soil, the optimal addition percentage of MTGCB is found to be 10%, while for GAC, 2% addition is determined to be effective. Overall, this research highlights the potential of MTGCB as a promising alternative to GAC for the stabilization of PFAS in soil. MTGCB demonstrates comparable adsorption performance, effective leaching control, and the ability to improve soil mechanical properties. The findings provide valuable insights for the application of these adsorbents in PFAS removal and soil stabilization, contributing to the development of efficient soil remediation strategies and environmental risk assessments.