Catalytic reduction of N-nitrosodimethylamine: Selecting a metal catalyst, promoter metal influence and field applicability

This research characterizes the ability of mono- and bimetallic catalysts to reduce N-nitrosodimethylamine (NDMA) in laboratory water samples using hydrogen gas as the reducing agent and evaluates the efficacy of reductive catalysis as a potential remediation technology for contaminated sites. NDMA is a major contaminant of concern because it is being detected at elevated levels in drinking and reclaimed water and has an extremely low public health goal, 0.7 ng L-1. Catalytic destruction of NDMA has only recently emerged as a potential remediation strategy, and has not been studied at environmentally relevant concentrations or using the promising metal catalysts that have been studied for destruction of other trace contaminants. This work provides the scientific basis for further evaluation of reductive catalysis by demonstrating that NDMA is efficiently reduced by palladium (Pd) based catalysts with an optimal bimetallic mixture of Pd and indium (In) on high surface area support material (alumina) providing the fastest kinetics. Specifically, In was evaluated as a promoter metal for Pd catalysts and NDMA reduction was most rapid using a bimetallic catalyst composed of 5% Pd and 1% In by weight---five times faster than monometallic 5% Pd by weight catalyst. Data suggest that hydrogen activation occurred only on Pd surfaces and that In activated NDMA 14 times more effectively than Pd on a metal mass basis. The only products detected were dimethylamine and ammonium with carbon and nitrogen balances in excess of 92%, consistent with a mechanism involving reductive N--N bond cleavage. A field study using monometallic Pd catalyst for destruction of trichloroethylene (TCE) was completed at Edwards Air Force Base to evaluate the longevity and efficacy of Pd catalyst in field applications. Using a flow-through column reactor packed with Pd coated alumina beads and supplied with excess dihydrogen, TCE concentrations in groundwater were reduced by 2--3 orders of magnitude (>99%) after a single pass with 1.1 min contact time. Operational parameters such as oxidative regeneration strength and frequency were optimized so that the effluent water consistently met the maximum contaminant level and catalyst activity could be maintained for extended periods (years). Results of the studies included in this dissertation serve as the basis for further optimizing bimetallic Pd-In catalysts for NDMA reduction and piloting the technology for field implementation treating NDMA contaminated waters.