Use of Bimetallic Nanoparticles in the Catalytic Hydrogel Membrane Reactor to Treat Nitrate in Drinking Water

The catalytic hydrogel membrane reactor (CHMR) is a promising reactor design for contaminants in water using heterogeneous hydrogenation catalysis. It allows for control of reaction conditions and byproduct selectivity. The CHMR has previously been investigated using palladium (Pd) catalysts for treating nitrite. The primary goal of this thesis was to expand the CHMR technology to successfully treat nitrate (NO3-) in water by using bimetallic catalysts.

The research objectives were to (1) develop and calibrate a computational model of the CHMR to predict the effects of reactor conditions on catalytic activity, (2) develop an ex-situ method to incorporate nanoparticles into the hydrogel without impacting stability, and (3) demonstrate the efficacy of a bimetallic catalyst in the CHMR to treat NO3- in water.

Results from the model indicate that hydrogel thickness plays a large role in reaction flux with thick hydrogels being limited by diffusion. The model also shows that too thin of hydrogels reduce the activity due to a limited amount of available Pd active sites and result in wasted H2 into the bulk solution. The effect of hydrogel thickness was verified experimentally. The Pd mass-normalized rate constant decreased from 0.030 to 0.007 L mol Pd-1 s-1 when the hydrogel thickness increased from 150 to 450 μm using a palladium-indium (Pd-In) catalyst.

Nanoparticles were synthesized outside of the hydrogel (ex-situ) and then stabilized with alginate to prevent aggregation. Ex-situ catalysts incorporated into the hydrogel were shown to be more active than in-situ synthesized catalysts (99.6% vs 78.7% nitrite (NO2-) removal). Alginate stabilization also benefited catalyst protection from oxidation. After aging 30 days, stabilized nanoparticles had a higher reaction rate (1.27 x 10-4 s-1) for NO2- reduction than non-stabilized nanoparticles (5.88 x 10-5 s-1).

Pd-In bimetallic nanoparticles successfully reduced NO3-, though had a poor N2 selectivity, in the CHMR. In a six-hour reaction in the CHMR, 43% NO3- removal and 14% N2 selectivity were observed. The type of bimetallic and synthesis procedure affected NO3- removal and selectivity. These results are promising and suggest the CHMR is capable of efficient NO3- hydrogenation with an improved catalyst.