Photocatalytic water treatment has been the subject of extensive research over the last three decades. However, at striking odds with the abundance of literature reports, there is extremely low number of systems currently using photocatalysis for water treatment in field, and no single municipal utility uses photocatalytic treatment to date. This dissertation contributes to identifying technological roadblocks in this field preventing transfer of research to practice, and provides potential approaches to address these challenges.
Micron-sized titanium dioxide hierarchical spheres (TiO2-HS) were assembled from nanosheets to address two common limitations of photocatalytic water treatment: (1) inefficiency associated with scavenging of oxidation capacity by non-target water constituents, and (2) energy-intensive separation and recovery of the photocatalyst slurry. These micrometer-sized spheres are amenable to low-energy separation, and over 99 % were recaptured from both batch and continuous flow reactors using microfiltration. Using nanosheets as building blocks resulted in a large specific surface area—three times larger than that of commercially available TiO2 powder (Evonik P25). Anchoring food-grade cyclodextrin onto TiO2-HS (i.e., CD-TiO2-HS) provided hydrophobic cavities to
entrap organic contaminants for more effective utilization of photo-generated reactive oxygen species. CD-TiO2-HS removed over 99% of various contaminants with dissimilar hydrophobicity (i.e., bisphenol A, bisphenol S, 2-naphthol and 2,4-dichlorophenol) within 2 h under a low intensity UVA input (3.64×10-6 Einstein/L/s). As with other catalyst (including TiO2 slurry), periodic replacement or replenishment would be needed to maintain high treatment efficiency (e.g., we demonstrate full reactivation through simple re-anchoring of CD). Alternatively, the durability of the coating can be improved as follows.
Considering that CD-TiO2-HS particles had limited durability as the CD coatings are susceptible to ROS attack. We developed an ROS-resistant fluorinated CD polymer (CDP) that can both adsorb contaminants and resist degradation by ROS, yielding a more efficient material for “trap and zap” water treatment. We produced the CDP through crosslinking β-cyclodextrin with tetrafluoroterephthalonitrile and optimized the thickness of the coating on TiO2 microspheres to improve the efficiency of contaminant degradation. Specifically, increasing the CDP content can enhance BPA adsorption but can also occlude photocatalytic sites and hinder photocatalytic degradation. CDP-TiO2 exhibited minimal photoactivity loss after 1,000 h of repeated use in DI water under UVA irradiation, and no release of organic carbon from the coating was detected. Photocatalytic treatment using CDP-TiO2 only showed a small decrease in BPA removal efficiency in secondary effluent after four 3-h cycles, from 80.2% to 71.7%. In contrast, CD-TiO2 and P25 removed only 29.8% and 6.2% of BPA after 4 cycles, respectively. Altogether, the CDP-TiO2 microspheres presented represent promising materials for potential use in photocatalytic water treatment.
Besides emerging contaminants, there is also a growing need to mitigate the discharge of antibiotic resistance genes (ARGs) from municipal wastewater treatment systems. Here, molecular-imprinted graphitic carbon nitride (MIP-C3N4) were synthesized for selective photocatalytic degradation of a plasmid-encoded ARG (blaNDM-1, coding for multidrug resistance New Delhi metallo-beta-lactamase) in secondary effluent. Molecular imprinting with guanine enhanced ARG adsorption, which improved utilization of photogenerated oxidizing species to degrade blaNDM-1 rather than being scavenged by background non-target constituents. Consequently, photocatalytic removal of blaNDM-1 in secondary effluent with MIP-C3N4 (k = 0.111 ± 0.028 min-1) was 37 times faster than with bare C3N4 (k = 0.003 ± 0.001 min-1) under UVA irradiation. MIP-C3N4 can efficiently catalyzed the fragmentation of blaNDM-1, which decreased the potential for ARG repair by transformed bacteria.
Overall, micron-sized photocatalysts assembled from nanosheets preserve the photoactivity, and enable low-energy separation of photocatalysts after treatment for reuse. This curtails the energy requirement in slurry reactors using nano-sized photocatalysts. The “trap and zap” strategy can mitigate the scavenging effect from background constituents in realistic water matrices and secondary effluent, enhance the photocatalysis efficiency, and endow photocatalyst with selective treatment capacity towards target contaminants. Polymerization and crosslinking with fluorinated monomers can significantly improve the catalyst durability, which allows for resilient applications in a repeated use scenario.