Development of a sustainable water treatment technology for oxyanions using palladium-based catalysts: catalyst design, reaction mechanisms, and life cycle assessment

Perchlorate and nitrate are oxyanion contaminants found in many drinking water sources, causing human health risks when consumed even at low concentrations. These oxyanions are not removed via conventional drinking water treatment processes, and require specialized treatment. One emerging technology for destroying oxyanion water contaminants is catalytic reduction using supported Pd-based catalysts and an electron donor. While the technology is promising, several challenges need to be addressed before it is adapted by water treatment utilities. The overall goal of my thesis is to contribute to the development of catalytic treatment processes for removing oxyanion contaminants, specifically perchlorate and nitrate, from drinking water, either as a stand-alone system or in combination with ion exchange (IX), and to compare the overall costs and environmental sustainability of the technology with other available oxyanion treatment technologies. My thesis work specifically contributed to three areas of study: 1) elucidation of perchlorate reduction mechanisms using X-ray spectroscopic characterization to identify the chemical states and coordination of Re species in carbon supported Re-Pd catalysts (Re-Pd/C), 2) comparative assessment of environmental sustainability of the catalytic treatment technology with alternative perchlorate treatment technologies such as IX and biological reduction, and 3) evaluation of the applicability and environmental benefits of recycling spent IX brines via catalytic reduction using pelletized carbon supported Pd-In catalysts for removal of nitrate in drinking water. Results from the 1st study showed that Re in Re-Pd/C catalyst exists as ReVII species under oxic conditions and transforms to a mixture of two Re species under reducing solution conditions induced by H2 sparging. These Re species support a revised mechanism for catalytic reduction of perchlorate involving a series of oxygen atom transfer reactions between rhenium species and perchlorate. Results from the 2nd study showed catalytic treatment using Re- ii Pd/C catalyst has a higher (ca. 4,600 times) environmental impact than other perchlorate treatment technologies, but is within 0.9-30 times the impact of IX with a newly developed ligand-complexed Re-Pd catalyst suggesting catalytic reduction can be competitive with increased activity. Results from the 3rd study indicated the hybrid IX/catalyst system is more environmentally sustainable than the conventional IX for nitrate removal in drinking water, but the environmental impacts of the system are sensitive to brine conditions (e.g., presence of sulfate and bicarbonate) that influence catalyst activity. Overall, catalytic treatment technology showed the promise as an environmentally sustainable oxyanion treatment technology option for drinking water