Implications for in-situ Groundwater Remediation
Open Access DepositedTransport and Remobilization of Colloidal Activated Carbon in Saturated Porous Media
Colloidal activated carbon (CAC) has emerged as a promising amendment for the in situ remediation of groundwater impacted with per- and polyfluoroalkyl substances (PFAS). The long-term effectiveness of CAC barriers depends on the successful emplacement, retention, and sustained stability of CAC particles within aquifer media under varying geochemical conditions. This dissertation systematically investigates how polyelectrolyte modifications, groundwater chemistry, and environmental aging processes influence the colloidal behavior of CAC, including aggregation, deposition, transport, and remobilization, in saturated porous media, thereby providing mechanistic insights into the physicochemical processes that govern barrier performance. Polyelectrolyte modification was found to be critical in regulating CAC stability, mobility and remobilization potential. Carboxymethyl cellulose (CMC) modified CAC maintained high mobility across a wide salinity range, enabling effective subsurface delivery. In contrast, poly(diallyldimethylammonium chloride) (PDM) modified CAC exhibited high mobility at low salinity (0.1 mM) but strong retention under high salinity (100 mM). Notably, CMC modified CAC deposited under high salinity became remobilized when salinity was lowered, whereas PDM modified CAC remained attached due to stronger attraction to quartz surfaces. These behaviors were further interpreted using extended DLVO (xDLVO) modeling, which revealed that polyelectrolyte coatings alter the energy barriers between CAC and collector surfaces, thereby controlling deposition and remobilization. Groundwater chemistry, particularly the presence of calcium ions (Ca²⁺) and natural organic matter (NOM), further influenced CAC deposition and remobilization. Elevated Ca²⁺ concentrations promoted CAC retention through cation bridging and electrostatic screening, whereas NOM introduced steric hindrance and competitive adsorption that reduced deposition and enhanced remobilization under decreasing ionic strength. Atomic force microscopy (AFM) measurements confirmed that Ca²⁺ increased, while NOM decreased, the adhesion energy between CAC particles and silica surface, underscoring the critical role of aquifer composition in determining CAC barrier stability. Long-term environmental aging also altered CAC surface properties and triggered particle remobilization, thereby compromising barrier integrity and potentially promoting PFAS loaded CAC migration. To assess these effects, CAC was subjected to physical, chemical (H₂O₂, Fenton, and acid), and biological aging to simulate subsurface aging processes. Electrokinetic characterization and column transport experiments under stepwise decreasing salinity revealed that most aging processes increased surface negativity and structural fragility, enhancing CAC release. In contrast, Fenton aging generated iron precipitation that neutralized surface charge and stabilized particle retention. xDLVO modulation linked these surface transformations to changes in remobilization energy barriers, demonstrating that aging processes critically governs CAC stability and long-term barrier performance. The long-term fate of polyelectrolyte modifications on CAC and their potential impact on colloidal behavior was further investigated. This study quantified the desorption of CMC (90k, 250k, 700k) and PDM from CAC under different salinity conditions (10 mM, 300 mM, and fluctuating 10–300 mM). The initial adsorbed mass was approximately 0.08 mg/m², and desorption remained minimal (<10%) over four months, indicating strong surface–polyelectrolyte interactions and physical confinement within CAC pores. Despite the limited desorption, post-desorption CAC exhibited enhanced aggregation and reduced mobility, and particle remobilization was further suppressed under fluctuating salinity. These results indicate that conformational rearrangements of the adsorbed polyelectrolytes, such as chain flattening, chain entanglement, and pore embedding, increase the aggregation potential and reduce CAC mobility, while at the same time contributing to barrier integrity and mitigating secondary impacts caused by downstream migration. This work offers mechanistic insight into the durability and retention of polyelectrolyte-modified CAC barriers for groundwater remediation. Together, these findings advance the mechanistic understanding of CAC colloidal behavior in complex subsurface environments and provide practical guidance for designing durable in situ CAC barriers that ensure long-term groundwater remediation.
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