Sensing and Mitigating Water Contaminants with Engineered Gold-Based Nanostructures
Yin, Yiyuan Ben
Wong, Michael S.
Doctor of Philosophy
Decentralized or distributed water monitoring and treatment systems are expected to be a fundamental tenant of the next generation of solutions aimed at addressing the global water crisis. Within this context, emerging nanotechnology-based options provide a promising approach to overcome many of the shortcomings associated with traditional, centralized water treatment technologies. For example, in the realm of water monitoring, gold-based nanostructures exhibit unique optical properties compared with bulk Au which can be employed as more rapid and affordable options for contaminant detection than conventional technologies (e.g., ion chromatography). However, there are still many technological challenges that limit practical implementation. Typically, Au nanostructures lack the sensitivity to detect contaminants at low concentration levels relevant to drinking water standards, and/or display insufficient selectivity to find practical use in real complex water matrices. Similarly, regarding remediation, Au-based nanostructures display promising catalytic performance towards the degradation of contaminants but are often negatively impacted by other co-present species in the water matrix. This is particularly problematic in the treatment of highly complex waters such as oil and gas hydraulic fracturing produced waters (HFPW). Here, two Au-bases nanostructures, luminescent gold nanoclusters (Au NCs) and bimetallic palladium gold nanoparticles (PdAu NPs), were investigated for their ability to circumvent these limitations for contaminant sensing and catalytic water treatment, respectively. Hexavalent chromium (Cr(VI)) is a carcinogenic contaminant regulated by the United States Environmental Protection Agency (US EPA) with the maximum contaminant level (MCL) of 100 ppb in drinking water. As Cr(VI) detection probes, novel Au nanostructures with Au NCs encapsulated in silica-coated microcapsule structures were developed, which have a 5× luminescence enhancement compared to traditional luminescent Au NCs, and can detect Cr(VI) at concentrations as low as 6 ppb. The Au microcapsules can also be successfully extended to a simple test strip system, similar to that of a pH indicator paper. The reuse of HFPW requires the removal of organic compounds. Bimetallic PdAu NPs were investigated as catalysts to degrade phenol, a model organic compound, at room temperature and atmospheric pressure via the in-situ catalytic formation of H2O2. PdAu showed the highest rate of phenol degradation compared with pure Pd and Au in simulated HFPW with total dissolved solid (TDS) concentration as high as ~16,000 ppm. However, while viable, performance was still negatively impacted by neutral pH and elevated TDS concentrations. It was further found that with a cost-effective additive, bimetallic PdAu NPs can degrade phenol with a TDS of ~160,000 ppm even at neutral pH. Again compared to pure Pd and Au, bimetallic PdAu NPs showed greater resistance to high TDS and lower byproduct formation rates. As a proof of concept, a scaled-up model of this technology was built implementing a circulating trickle bed reactor to successfully treat model HFPW.
Sensing; Catalysis; Nanostructures; Gold; Water