Fate and remediation of emerging organic contaminants in aquatic and soil environments

Abstract

The ubiquitous occurrence and growing abundance of many organic micropollutants in aquatic and soil environments poses significant risks to human health and ecosystems. While past research has improved our understanding of the processes controlling the fate and treatment of some classes of synthetic organic chemicals, the fate of many emerging and unregulated micropollutants of concern remain largely unknown. In this thesis research, advancement is made on the mechanistic understanding of the abiotic and biological processes that control the fate and treatment of emerging micropollutants. It first evaluates the influence of four prevailing terminal electron-accepting processes (TEAPs) and exposure to a mixture of nine contaminants of emerging concern (CEC) on both the microbial community structure and CEC degradation in agricultural soil. Results show soil microbial community composition is strongly impacted by different TEAPs but no significant change upon exposure to the mixture of CEC. Two of the CEC showed significant degradation in both bioactive and sterile conditions while six of the CEC only degraded in bioactive samples incubated under different TEAPs. Knowing that soil minerals can also play an important role in controlling the fate of some CEC, this thesis research further evaluates the influence of metal (hydr)oxide soil minerals on the abiotic degradation of nine organophosphate flame retardants (OPFRs). In the absence of minerals, base-catalyzed hydrolysis is confirmed for eight of the OPFRs in highly alkaline solutions (pH 9-12). In contrast, rapid degradation of all nine OPFRs is observed under circumneutral pH when metal (hydr)oxides are present, and their by-products confirmed ester hydrolysis as the active degradation pathway. Work from this thesis study also investigates the treatment of highly recalcitrant CEC structures (e.g., poly- and perfluoroalkyl substances (PFASs)) that have been increasingly detected in aquatic systems. The removal of 75 PFASs was assessed using 14 commercially available ion-exchange (IX)/non-ionic resins and granular activated carbon (GAC). Among the selected adsorbents, anion exchange resins (AERs) exhibited significant adsorption of PFASs compared to cation exchange resins, non-ionic resins, and GAC regardless of the PFAS’s predicted charge. Experimental results indicate that polarity of the AE functional group and polymer matrix plays a dominant role in dictating PFAS affinity for different resins. Structural characteristics of PFASs were also found significantly affect adsorption to AER. While increased PFAS carbon chain length is confirmed for higher selectivity, increased adsorption was also observed for anionic PFASs compared to similar size zwitterionic and cationic structures. In order to further assess the feasibility of IX technology for treating PFASs, regeneration of three PFAS-exhausted AERs were surveyed with 30 different regenerant schemes to evaluate regenerability for long-term applications. Findings from this work suggests regeneration of most resins loaded with long-chain PFASs (e.g., perfluorooctane sulfonate (PFOS)), including polystyrene- and polyacrylic-based AER, will require a combination of salt brine and co-solvent to desorb the PFOS and regenerate the AERs. This is consistent with the importance of both electrostatic and van der Waals interactions controlling PFAS binding to the AERs. The requirement of a co-solvent is expected to increase costs and safety concerns associated with regenerable AER treatment processes. Conclusions from this thesis improves our understanding on the importance of prevailing electron-accepting processes and soil mineral phases for the natural attenuation of important classes of emerging contaminants and provides insights for future decisions on selecting less persistent CECs. It also provides useful guidance for designing the most cost-effective and sustainable resin-based treatment systems for the remediation of PFASs and related contaminants that are recalcitrant to natural attenuation.