(508c) Molecular Dynamics Study for the Removal of Dimethylsilanediol from Water Using Ionic Liquids As Extraction Agent

Since 2010, the high levels of total organic carbon measured in the on-orbit product water from the Water Processor Assembly (WPA) in the International Space Station (ISS) have been attributed to the presence of dimethylsilanediol (DMSD), a hydrolysis product of poly(dimethyl)siloxanes (PDMS). Although DMSD does not pose health hazards, the presence of DMSD is detrimental to a number of downstream processes. For example, the resupply mass of the multifiltration bed increases because DMSD can saturate the bed prematurely, which necessitates that the MF beds are replaced more frequently than the intended life of the bed. DMSD can also negatively impact the performance of the WPA catalytic reactor and lead to degradation in the functioning of the Oxygen Generation System, which utilizes the WPA product water for electrolysis. It has also been demonstrated that high amounts of DMSD in wastewater can mask the detection of other contaminants. Current approaches for the removal of DMSD include a search for appropriate adsorbents that can effectively remove DMSD from the wastewater streams and conversion of DMSD via catalytic reduction.

In this presentation, we will show our approach for using ionic liquids to decrease the solubility of DMSD relative to water. We consider two sets of ionic liquids with different cations combined with four environmentally friendly and biodegradable anions. The selection of ionic liquid, its aqueous concentration and temperature for the process follow a two-step strategy: First, an optimal ionic liquid is identified based on the ratio of the infinite dilute activity coefficients of DMSD and water in various ionic liquids by calculating the infinite dilute chemical potential from free-energy based molecular dynamics simulations. The interaction potential parameters between the ionic liquid and the solute are tuned to minimize the error between the predictions from molecular simulations and experimental measurements. The optimized set of parameters are then employed to predict concentration-dependent activity coefficients of water and DMSD in the optimal ionic liquid-water mixtures from molecular simulations at a range of temperatures. By computing the ratios of activity coefficients, the ionic liquid concentration and temperature are determined for the desired separation.