Selective Sorption of Groundwater Contaminants using Novel Macrocyclic Materials
The increasing release of contaminants to the environment resulting from the consumption of chemicals in industrial products has highlighted the importance of exploring novel environmental remediation technologies. While individual contaminants can be managed by conventional treatment methods, removal of coexisting chemicals using conventional methods can be challenging. Adsorption has been known as a widely used treatment method over decades. Among different adsorbents activated carbon (AC) has been proven to effectively remove a wide range of environmental contaminants. However, there are challenges associated with AC when applied to complex mixtures of contaminants, such as per- and polyfluoroalkyl substances (PFAS) and other coexisting chemicals such as 1,4-dioxane and chlorinated volatile organic compounds (CVOCs). This dissertation explored the potential of macrocyclic adsorbents for removal of organic groundwater contaminants such PFAS, and CVOCs. Firstly, β-cyclodextrin and resorcinarene polymers crosslinked with tetrafuorophthalonitrile (β-CD-TFN, and Res-TFN) and were examined for the separation of 1,4-dioxane from CVOCs. The results highlighted the selectivity of the β-CD-TFN adsorbent for CVOCs, allowing for the effective separation of these compounds from 1,4-dioxane. Furthermore, the Res-TFN exhibited a remarkable affinity for CVOCs but very little adsorption of 1,4-dioxane. The study also demonstrated that Res-TFN outperformed AC in the selective adsorption of trichloroethylene (TCE) over 1,4-dioxane, indicating its potential as a selective adsorbent for separating CVOCs from 1,4-dioxane. This separation facilitates further bioremediation of both CVOCs and 1,4-dioxane since each group requires a specific type of bacteria with completely different environmental conditions. Moreover, the effectiveness of β-CD adsorbents in removing PFAS was investigated in this dissertation. The study revealed varying performance among three different β-CD adsorbents when exposed to different PFAS compounds. Benzyl chloride β-CD (β-CD-Cl) demonstrated significant adsorption of anionic PFAS compounds and also a non-ionic PFAS compound. However, this adsorbent did not remove any of the zwitterionic compounds in the mixture. The other two adsorbents β-CD crosslinked with hexamethylene diisocyanate and epichlorohydrin (β-CD-HDI and β-CD-EPI) displayed some removal. A positive correlation between log Kow of PFAS compounds and adsorption affinities was observed indicating the role of hydrophobic interactions in adsorption. This effect was stronger in β-CD-Cl compared to β-CD-HDI and β-CD-EPI. The performance of β-CD adsorbents in this study was compared to other adsorbents from previous studies. While β-CD-Cl’s performance was comparable to other adsorbents from literature, this comparison revealed varied performance of different adsorbents depending on the chemistry of target PFAS compound and also the adsorbent. This diversity highlighted the complex nature of adsorption processes when dealing with PFAS compounds. In summary, this dissertation highlighted the potential of macrocyclic adsorbents in the removal of groundwater organic contaminants in more complex settings. However, more investigations are required regarding their scalability, regeneration, and practical application on larger scales.