PhD Dissertation Defense: Michael Fleming

This dissertation focused on ammonium (NH4+) recovery from agricultural waste by Donnan dialysis. The primary goals were to improve water quality and provide a sustainable source of ammonium-based fertilizers. Donnan dialysis is a separation process that exploits electrochemical potential gradients between a concentrated draw solution and a waste solution separated by a semipermeable, ion-exchange membrane. My research advanced the state of the art by employing phosphate-based draw solutions that allow for in situ precipitation of valuable fertilizers within the draw chamber of the Donnan dialysis reactor. In particular, the process was designed to recover struvite (MgNH4PO4·6H2O), a slow-release fertilizer that produces less nutrient runoff and provides better crop yields than conventional fertilizers. The broader impacts of my work will help to minimize our dependance on energy-intensive production of ammonia fertilizers and reduce eutrophication from agricultural runoff.

This dissertation had three specific aims. The first aim was to experimentally measure fundamental Donnan dialysis parameters, namely the separation factors and diffusion coefficients for ammonium in three different membranes: CMI-7000; Nafion 117; and Selemion CMVN. The CMI-7000 membrane had a higher ion-exchange capacity than Nafion 117, favoring NH4+ uptake; however, Nafion 117 was thinner than CMI-7000, suggesting faster diffusion. Selemion CMVN was a thinner membrane than Nafion 117 with an ion-exchange capacity similar to CMI-7000, which suggested better potential for NH4+ recovery. For the second aim, I incorporated the Selemion CMVN membrane into Donnan dialysis reactors to recover NH4+ from synthetic wastewater as struvite. Chelating agents were used to decrease recovery of calcium phosphate products, which are not useful fertilizers. 

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