Nitrate is the most common groundwater contaminant in the United States and is regulated in drinking water by the EPA due to its harmful health impacts.Ion exchange (IX) is frequently used to treat nitrate and is very effective, but suffers from inefficiencies associated with the regeneration process.When the IX resin is saturated, it requires regeneration, which is accomplished through back-washing with a high-salt brine, which treatment plants use only one time prior to disposal.The cost of salt to make fresh brine and disposal requirements of waste brine are expensive processes for the water treatment plant.Additionally, this process merely transfers the nitrate to another phase (from resin to brine) rather than destroying it, leading to significant environmental impacts of the brine disposal process.Recently, a hybrid system that incorporates catalytic denitrification of IX waste brine has been shown to be technically feasible.Using a bimetallic palladium-indium on activated carbon catalyst, nitrate in waste brine can be selectively reduced to inert dinitrogen gas.This technology has the potential to significantly reduce the cost and environmental burden of the convention IX process for nitrate treatment.In order to improve the hybrid-IX system, the following research objectives were pursued: (1) Using an experimental and modeling approach, determine whether the accumulation of bicarbonate and sulfate in reused waste brine will negatively impact the hybrid system performance and model key IX system variables using a case-study approach, (2) Evaluate reactor performance in continuously stirred and fixed bed reactors; and optimize a fixed bed reactor to reduce hydrogen mass transfer limitations to the catalyst surface, and (3) Evaluate selectivity of Pd-In/AC catalyst using different reactor types and matrix conditions.A model of the IX-catalyst system was developed, calibrated and validated using experimental data.Results from modeling simulations show that concentrations of non-target ions like sulfate and bicarbonate will buildup in waste brines over repeated cycles of reuse, but this buildup will not negatively impact IX performance or lead to permanent deactivation of the Pd metal catalyst.IX columns were tested experimentally to verify the modeling results.The key IX variables evaluated using the model and case study approach based on data from Chino, CA were resin regeneration length, treatment time, and addition of make-up salt.Overall, salt costs and waste brine volumes can be decreased by up to 80% with the hybrid system.A fixed-bed catalytic reactor was used to evaluate a real brine from Chino, CA and demonstrated consistent reduction, however the overall activity was very low due to hydrogen mass transfer limitations.This led to prohibitively high predicted catalyst costs for a commercial-scale hybrid system, leading to a focus on reactor design in an attempt to reduce mass transfer limitations.Based on its significant use in industrial catalytic applications and the known ability to facilitate high mass transfer rates, a trickle bed reactor (TBR) was chosen as the new reactor design for use in the hybrid IX-catalyst system.The 2” ID TBR with two 10” beds of catalyst and recycling flow was designed in accordance with reactor design guidelines and evaluated across a range of liquid and gas superficial velocities.Synthetic waste brines were treated with two Pd-In/AC catalysts that had different support sizes.In comparison to a previously tested up-flow fixed-bed reactor, the same catalyst in the TBR demonstrated c. 300% higher activity.While the results showed a major step forward in reactor performance, the TBR activity was only 8.3% of the activity found in batch, indicating significant mass transfer limitations remained.The impact of catalyst dilution was evaluated in the TBR and had been previously shown to improve reactor performance.The catalyst was diluted at a ratio of 1 part catalyst : 2 parts inert support.Continuous flow experiments using the diluted TBR did not demonstrate better performance and, to the contrary, showed a significant decrease in selectivity of catalyst.The TBR with non-diluted catalyst resulted in c. 50% selectivity towards N2, which is the desired end product due to its inert nature.The TBR with 1:2 diluted catalyst resulted in selectivity of nearly 100% towards NH4+.To better understand reduction mechanisms and selectivity, a series of experiments were performed and it was found the support had no direct role in selectivity.Rather, the change in selectivity was due to high hydrogen concentrations on the catalyst surface.In the diluted catalyst bed, reactive metal surfaces were geographically dispersed, allowing more time for hydrogen mass transfer from the gas to liquid phase.This led to higher hydrogen concentrations on the catalyst surface, which altered the N:H ratio and shifted selectivity towards NH4+.In contrast, in the non-diluted catalyst bed, reactive metal surfaces are found throughout the reactor, leaving less time for hydrogen mass transfer and resulting in a lower concentration of hydrogen on each metal surface.Overall, this thesis advanced the state of the art for a hybrid IX-catalyst system and brought the system closer to economic feasibility.The modeling and experimental approaches served to more thoroughly evaluate the system and provide focus on the remaining barriers to increased improvement. This thesis also highlighted the critical role hydrogen mass transfer played as a barrier to a significant step forward in technology development.Reactor design contributed to improvements in the catalytic system, but were unable to completely overcome mass transfer limitations thus far.The findings from this thesis supported additional research directions regarding hydrogen delivery, reactor design and techno-economic analysis.
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Development of a hybrid ion exchange-catalyst system to denitrify ion exchange waste brine
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