Membrane Surface Modification for the Recovery of Ammonium and Potassium

Ammonia (NH4+), phosphorous (P), and potassium (K+) are important contents of everyday fertilizers. This global fertilizer demand is worth about $200 billion dollars and is currently being met with traditional processes that are highly energy intensive. To meet this demand, alternative sources like anaerobic digestate, a nutrient-rich waste product, can be used. My research team aims to develop an integrated chemical precipitation and membrane separation technique to selectively separate these critical nutrients. Using ferrous iron, phosphorous can be precipitated out in the form of vivianite, this meets the demand for phosphorous, however, difficulty arrives when trying to separate NH4+ and K+. The main issues with separating NH4+ and K+ are that they have similar charges and are similar in size. For our approach, we are attempting to separate them based on their hydration energies and solvation structure, since NH4+ desolvates when forced to go through the crosslinked (confined) domain, thus reducing its effective size and allowing easy permeation through a membrane. In our current research we have investigated the NH4+/K+ selectivity for two hybrid polyelectrolyte systems consisting of linear, poly(allylamine hydrochloride) (PAH), and branched, polyethyleneimine (PEI). A greater emphasis has been placed on testing these membranes with increased concentrations (twice the original amount) for each respective polyelectrolyte to better understand the correlation between concentration of the individual polyelectrolyte and its effective separation of NH4+ and K+. We hypothesize that our hybrid system of PAH-PEI is better able to separate NH4+ and K+ due to greater creation of confined domains that aid in the permeation of NH4+ over K+. For our future work, we intend to continue altering the topology of the polyelectrolytes in our hybrid system as well as performing long term membrane tests using a continuous system.