Priority organic pollutants such as phthalates, which are plasticizers associated with plastic production, represent a significant threat to human health. These compounds, which can be released to the environment during plastic synthesis, from storage sites, and from landfills, exert adverse effects associated with the immune, reproductive, and cognitive systems, and are known to be carcinogens and endocrine disruptors.
Phthalates demonstrate a significant degree of aqueous solubility, compared to other toxic organic pollutants, presenting abundant pathways for human exposure. An effective method to reduce the environmental burden of such chemicals is to design efficient catalytic systems for the breakdown of these toxic species. Most methods employ catalysts that become deactivated over time. As such, a new generation of materials is required for long-term environmental remediation of priority organic pollutants that is reusable with minimal to no loss of reactivity. We hypothesize that photoactivatable semiconductor structures with integrated catalytic nanodomains can be sustainable, highly efficient phthalate degradation systems. To that end, semiconductor materials can be used for both reductive and oxidative degradation processes, where the optimal photocatalyst for specific pollutants, as well as the mechanism by which degradation proceeds, remain unestablished. For instance, different inorganic materials, heterostructure compositions (more than one material integrated together), and their spatial arrangement can have dramatic implications on the photocatalytic reactivity and reaction rates for different priority organic pollutants. Once photodegradation reactivity and breakdown pathways are confirmed for phthalates, the effect of the reactivity will be correlated to sample toxicity using mixtures of chemicals and distributions observed at the Homestead Air Force Base Superfund site, as identified through the studies performed by the Research Support Core. It is likely that complete degradation may not be required to abate toxicity, all of which remains unknown for these selected toxicants.
To address these capabilities and abate the environmental toxicity associate with phthalates, the following specific aims have been developed:
Nanostructural rational design of semiconductor materials with selectable band gaps for phthalate degradation
Semiconductor materials can drive redox processes via their band gap structure, where integration of noble metal nanoparticles at the material surface facilitates chemical degradation. Unlike commonly studied nanospheres and thin films, materials with a higher surface-to-volume ratio are advantageous for catalysis. To optimize the reactivity for organic pollutant degradation, we propose to create a library of materials with unique architectures, compositions, and arrangements to systematically evaluate and compare reactivity for H2 and radical generation for reductive and oxidative degradation processes, respectively, in control solvents, real sample environments, and simulated systems that mimic conditions that are commonplace at Superfund sites.
Elucidating phthalate breakdown reaction rates and pathways using heterostructure photo-catalysts
By employing the H2 and radicals generated in situ (Specific Aim 1), sustainable photodriven redox pollutant degradation is envisioned for multiple degradation cycles. This will be enhanced via the incorporation of metallic nanoparticles on the photocatalyst surface to facilitate exciton lifetimes and electron transfer processes. To probe the catalytic abilities of the materials, the degradation of phthalates with different substituents will be examined under both reductive and oxidative conditions. Both the overall reaction rate and breakdown pathways, identifying the major intermediate products generated, will be elucidated to determine optimal reaction processes. Monitoring of the reaction pathways will be achieved using GC-MS and LC-MS with the Research Support Core and the sensors of Project 3. Changes in reactivity are envisioned based upon the selected material composition, redox conditions (reductive vs. oxidative), and phthalate substrate employed, where new knowledge will be achieved leading to the generation of new photocatalysts with optimized reactivity.
Pollutant breakdown reaction toxicity and effects of complex pollutant mixtures
Armed with knowledge concerning breakdown pathways for the selected phthalates, changes in the reaction toxicity will be fully examined. The different intermediates that are likely to be generated are anticipated to elicit different toxicity effects, which will be studied in collaboration with Projects 1 and 2. In addition, using the knowledge gained from the Research Support Core, phthalate mixtures will also be studied for photocatalytic breakdown that mirror compositions identified at the Homestead Air Force Base Superfund site. These complex mixtures will result in the production of multiple different intermediates simultaneously, presenting new opportunities for understanding the toxicity of such complex systems. Together, this strongly integrates Project 4 with the overall theme and research activities of the University of Miami Superfund program.