Abstract
The co-occurrence of trichloroethylene (TCE) and perfluorooctanoic acid (PFOA) in groundwater presents a significant remediation challenge due to their disparate physicochemical properties and the potential for inhibitory co-contaminant interactions. This thesis develops and critically evaluates an integrated remediation framework combining tailored biostimulation of indigenous microbial consortia with a novel nanomaterial, sulfidated zero-valent iron supported on layered double oxides (S-nZVI@LDO), to address this complex contamination scenario. The research systematically investigates the performance of each system component before examining their combined effects, revealing profound and often antagonistic biogeochemical interactions.This work first establishes that targeted biostimulation with a lactate-methanol mixture can effectively overcome PFOA-induced inhibition of microbial reductive dechlorination. In PFOA-impacted systems, biostimulation suppressed competing methanogenesis by up to 84 %, redirecting electrons to achieve near-complete TCE dechlorination to ethene (32.1 µM) and increasing the pseudo-first-order rate constant (Kobs) by 70 % (from 0.0138 to 0.0234 day⁻¹). This enhancement was underpinned by the enrichment of key functional bacteria, including Dehalococcoides (Dhc) spp. and syntrophic partners like Pseudomonas. However, a comparative analysis of four distinct microbial consortia revealed a critical trade-off: while lactate alone yielded the fastest initial TCE degradation (Kobs = 0.226 day⁻¹), it consistently stalled at the intermediate cis-1,2-dichloroethene (cis-1,2-DCE). Methanol co-amendment was essential for driving the reaction to completion while at the cost of significantly reduced kinetics (a 27–80 % decrease in Kobs), underscoring that optimal biostimulation strategies are dictated by the intrinsic metabolic potential of the site-specific microorganisms.
In parallel, the engineered S-nZVI@LDO was demonstrated to be an efficient and dual-function abiotic composite. It achieved simultaneous removal of TCE (73.3 %) and PFOA (37.8 %) from co-contaminant solutions, with negligible competitive interference (≤ 4 % efficiency loss). Characterization confirmed distinct removal mechanisms: TCE removal occurred via reductive dechlorination and hydrophobic sorption, whereas PFOA removal was primarily governed by electrostatic attraction and hydrogen bonding. The material proved effective in real world groundwater (58.2 % TCE and 33.7 % PFOA removal) and exhibited a high affinity for a range of per- and polyfluoroalkyl substances (PFASs), including 100 % removal of the alternative GenX.
The integration of these two promising strategies, however, uncovered a "remediation paradox" characterized by dose-dependent antagonism. While S nZVI@LDO dosages ≥ 2 g/L achieved complete abiotic removal of the parent TCE, they induced a significant collapse in the microbial population and completely suppressed the essential terminal dechlorinator, Dhc. This cytotoxicity halted the biological pathway, leading to the accumulation of vinyl chloride (VC) at concentrations up to 47.5 µM, which is a more hazardous daughter product than in the unamended biological system. At a lower, less toxic dose of 0.5 g/L, the nanoparticles misdirected microbial activity, stimulating methanogens and partial dechlorinators (Desulfitobacterium) at the expense of Dhc, resulting in the accumulation of cis-1,2-DCE and VC.
Furthermore, this study reveals that S-nZVI@LDO is not a permanent sink for PFOA in a biologically active environment. While the material initially sequestered PFOA, aqueous concentrations rebounded to near-initial levels over the 90-day incubation. This phenomenon was driven by microbial colonization of the nanoparticle surface, which potentially created a hydrophilic, negatively charged biofilm. This biological coating effectively masked the sorbent's intrinsic hydrophobic and electrostatic properties, neutralizing its primary sorption mechanisms and causing the release of previously captured PFOA.
In conclusion, this thesis provides a mechanistic framework for remediating complex co-contaminants, demonstrating that the efficacy of combined abiotic biotic systems is governed by the intricate and often antagonistic interactions at the nanoparticle-microbe interface. The findings challenge the paradigm of simple co application, revealing that the conditions required for effective chemical reduction can be profoundly detrimental to the biological processes needed for complete dechlorination. This work underscores the critical need to move beyond parent compound-based metrics of success and to develop advanced remedial strategies, to manage these complex interactions and achieve sustainable environmental outcomes.
| Date of Award | 15 Nov 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Jun He (Supervisor), George Zheng Chen (Supervisor) & Tengwen Long (Supervisor) |