New technologies are needed to address the removal of contaminants to increase the safety and realization of wastewater reuse practices for urban water sustainability. The AIMS Lab will employ surface chemistry analysis, advanced analytical tools and water chemistry techniques in the conceptualization, design, characterization and application of novel materials to examine creative solutions to water security challenges. Specifically, the AIMS Lab is currently investigating:
1. Low-cost media for stormwater treatment
One of the challenges facing urban water sustainability is reduced replenishment of groundwater. In rural cities and regions with open or forested land cover, rainfall is able to infiltrate into soils to recharge groundwater aquifers and replenish surface water reservoirs (e.g., rivers and lakes). The opposite is true for densely populated, urbanized regions (like Seattle, WA) where surfaces are covered with engineered infrastructure such as buildings and roadways. Rainfall in these cities comes into contact with these impervious surfaces to generate large volumes of runoff that travels over these urban landscapes.
“Urban” stormwater runoff is a major component of the urban water cycle, yet it is a highly under-utilized resource. In fact, it is often viewed as a waste or a nuisance causing street flooding, back-up into homes and overflow in underground sewer or storm piping networks. Instead of discharging runoff, a more practical solution would be to convey surface runoff to rain gardens or bioswales to promote local groundwater recharge and decrease street flooding. Additionally, urban stormwater could be captured and harvested for use during landscape irrigation.
Despite its potential for augmenting water supplies, particularly in urban cities that are prone to drought, urban stormwater contains elevated concentrations of trace contaminants that pose risks to human and aquatic ecosystems. According to King County, Washington Stormwater Services and Information, the impacts of urban stormwater in the Pacific Northwest are significant. For example, approximately 1/3 of the water pollution in Washington state comes from stormwater runoff. Contamination of the Puget Sound by stormwater runoff is particularly concerning for the security of the seafood industry as Washington is the nation’s #1 commercial producer of oysters, clams and mussels.
To help decrease the volume of urban stormwater runoff, city planners have installed green stormwater infrastructure (e.g., rain gardens, green roofs, bioswales) to convey runoff from the streets and reduce flooding. These infrastructure can also provide remediation of some contaminants present in urban stormwater. Unfortunately, there could be hundreds to thousands of trace levels of pollutants in runoff. In fact, our recent publication suggests that green stormwater infrastructure have low or varied efficacy for removing some contaminants. The data presented here is based on very limited information as most urban stormwater contaminants entering and leaving these systems is not available.
Therefore, our group is interested in the development and application of low-cost reactive media to remove contaminants in urban stormwater. Researchers will be investigating urban stormwater contaminant fate and transport where reactive media are employed in existing stormwater infrastructure. We seek to understand: (i) the lifetime of these materials under conditions representative of urban stormwater; (ii) hydraulic and mechanical properties of the materials to facilitate deployment in infrastructure; (iii) the range and types of contaminants treated by our materials; and, (iv) impacts to stormwater infrastructure where our media is implemented.
2. Selective removal of per- and polyfluoroalkyl substances (PFAS) in wastes
While media generated for urban stormwater treatment should be able to remove a broad suite of contaminants, these materials may not be appropriate for more toxic, persistent contaminants. For example, PFAS are organofluoro compounds that are extremely stable and resist chemical and biological degradation. PFAS were originally developed as active ingredients in fire-fighting foams used to extinguish hydrocarbon fuels. Their applications as flame retardants and in water-resistant industrial products are resulting in the widespread presence of PFAS in wastewater and drinking water sources.
In order to treat PFAS, it is useful to separate PFAS from water to prevent interference from co-contaminants, dissolved organic carbon and other aqueous species. Granular activated carbon, an inexpensive and commercially available adsorbent is typically applied to remove PFAS in water. However, PFAS typically exist in low concentrations in water sources and waste streams, and often in the presence of co-contaminants. This is a problematic because in addition to PFAS, activated carbon is a highly efficient adsorbent for other trace organic compounds, trace metals, and other aqueous species. Therefore, PFAS removal by activated carbon may become ineffective due to lack of selectivity and competition for adsorption sites.
Our group is investigating the synthesis of PFAS selective polymer composites. By incorporating the target compound during synthesis, the resulting polymer operates in a lock-and-key mechanism for selective adsorption. We will examine composite affinity and selectivity for PFAS compared to existing technologies to treat complex, waste streams and water sources to protect environmental and human health. We are also exploring opportunities to release adsorbed PFAS and regenerate our selective polymers for multiple cycles of treatment.
3. Substrate-facilitated degradation of PFAS
Selective removal is only one half of the solution for complete remediation of toxic and recalcitrant contaminants like PFAS. Typical chemical oxidants used to degrade trace organic compounds during wastewater treatment are not powerful enough to completely degrade PFAS. The carbon-fluoride bonds in PFAS are have high thermodynamic stability and the large reduction potentials of halogens (like fluoride) are difficult to oxidize. Furthermore, PFAS cannot be naturally degraded by microorganisms.
Instead of relying on oxidants to degrade PFAS, recent studies suggest that PFAS destruction can be facilitated by chemical reduction. Advanced reduction processes that produce solvated electrons, e–aq, can cleave carbon-fluoride bonds to chemically breakdown PFAS into benign species. Ultraviolet (UV) light excitation of chemical mediators like 3-indole-acetic-acid can generate high yields of solvated electrons in solution. However, literature suggests that specific conditions like high pH and anoxic conditions are needed to prevent solvated electrons from being scavenged.
To maximize the effectiveness of reductive treatments, it is beneficial to adsorb the targeted pollutant onto a substrate to co-localize the solvated electrons and mitigate scavenging. Therefore, we are probing the use of nanocomposites for heterogeneous PFAS reduction. Our group will identify the reduction mechanisms via formation of reactive oxygen species, the formation of transformation products and characterize the evolution of the nanocomposites during treatment.