(56b) Continuous Flow Catalytic Hydrogenation of Groundwater Nitrate in a Membrane Catalyst-Film Reactor

Nitrate is the most commonly occurring anthropogenic groundwater pollutant in the US, due largely to over application of nitrogen-based fertilizers. In drinking water, nitrate consumption can cause oxygen deficiency in the bloodstream; an especially dangerous occurrence for infants. Accordingly, the US EPA has imposed an MCL of 10 mg NO₃-N/L. Existing technologies to treat nitrate focus on separating nitrate from water using ion exchange or reverse osmosis that produce brine waste streams that require disposal. Also, a few full-scale treatment systems currently operate using biological denitrification for drinking water treatment wherein nitrate is converted to innocuous by-products.

Non-biological catalytic hydrogenation rapidly converts nitrate to nitrogen gas using hydrogen gas (H₂) and a solid-phase catalyst. Unlike biological systems, these abiotic catalytic hydrogenation can be turned on or off with near zero start-up time. However, 80% of publications regarding catalytic hydrogenation treatment option focus on material development and employ batch slurry reactor with a catalyst in suspension and uncontrolled hydrogen flow, which is not scalable for drinking water treatment.

The presented technology uses hollow fibers as a method of controlling hydrogen delivery in a continuous flow reactor. This method improves the efficiency of hydrogen delivered, which results in an order of magnitude decrease in required hydrogen flow rate and cost. Previously studied nitrate reduction systems have incorporated catalysts in suspension, but those configurations risk metal nanoparticles carrying over into drinking water. To resolve this technology barrier, the hollow fibers also act as a surface attachment point for the catalyst. After analyzing various catalyst coating methods, we have found a method to irreversibly attach nano-catalysts to the H₂-permeable hollow-fibers. The research focuses on a bimetallic Palladium-Indium catalyst because it has the highest selective reduction of nitrate to nitrogen gas without leaching. The parameters of the synthesized Pd-In catalyst and the reaction result in up to four times the nitrate reduction activity as similar catalysts while still achieving nitrogen selectivity above 95%. Reactor parameters and the corresponding effects on activity and selectivity of the catalyst will be discussed; particularly relating to scale up from batch to continuous flow. The reactor system is operated for extended periods to determine any sources of long term catalyst deactivation. This presentation focuses on the development of a scalable technology for groundwater nitrate treatment with high selectivity to desired nitrogen gas and minimized catalyst deactivation over time.