Evaluation of the Influence of Ion-exchange-membrane Properties, Chemical Catalysis, and Liquid Flow Rate on the Rate of CO2 Extraction from Varying Salinity Synthetic Ocean Water

This thesis discusses efforts made to develop a gas-liquid membrane contactor for direct oceanic CO2 capture and describes the synthesis and characterization of the photoacid 6-hydroxypyrene-1-sufonate (HPMS). In the first study, efforts are made to increase CO2 extraction yields from ocean water by studying the influence of various catalysts, ion-exchange membranes, and buffers on rates of mass transfer and mass action interconversion of bicarbonate and CO2(aq).In the second study, HPMS is developed because its pKa values are suitable to photogenerate both H+ and OH- in aqueous media and its single sulfonate group will aid in controllable modification of surface and polymer characteristics that could potentially aid in light-driven water desalination. The first study focuses on maximizing CO2 extraction yields from ocean water by using a gas-liquid membrane contactor with various catalysts, buffers, and ion-exchange membranes, as well as varying flow rates. When atmospheric CO2 dissolves in the ocean it reacts with water to yield carbonic acid, which dissociates into bicarbonate and solvated protons at the ocean water pH of 8.1. These events have caused the pH of the ocean to decrease. The goal of this project is to deacidify the ocean by extracting CO2(aq) by catalyzing the interconversion between bicarbonate and CO2. This work shows it is possible to extract CO2 from ocean water using ion-exchange membranes, and that extraction yield can be maximized by coating the membranes with various molecular catalysts or polymers, such as zinc-cyclen (a carbonic anhydrase mimic), and polyethylenimine. This work also demonstrates the influence of high salt concentration on oceanic CO2 capture when using ion-exchange membranes with no added catalyst. In addition, the effect of liquid flow rate on extraction yield and flux of CO2 was evaluated using a single porous PTFE fiber membrane contactor. Lastly, this work documents analytical calibrations made on the Hidden HPR 20-QIC Vacuum System mass spectrometer used for the studies. The second study entails the synthesis, purification, and characterization of the photoacid HPMS. Photoacids are species that become more acidic upon the absorption of light. Light-driven electronic excitation results in a decrease of their pKa value, which allows them to donate a proton to their surroundings. These characteristics may allow photoacids to be used in technologies for light-driven water desalination. The rate of proton transfer to and from water may be adjusted based on the solution pH, intensity of light, and photoacid pKa and concentration. HPMS has been reported to possess pKa values suitable for desired proton transfer rates with water. The synthesis for HPMS has been previously reported, but the work reported herein includes a new multi-step purification protocol to isolate a compound whose 1H NMR spectrum differs from that in the prior literature report but is consistent with that expected for HPMS. Steady state absorbance and photoluminescence spectroscopy measurements were conducted to obtain ground-state and excited-state pKa values at different aqueous ionic strengths. At aqueous 100 mM supporting electrolyte the ground-state pKa value of HPMS was measured to be 8.7. At aqueous 1 M supporting electrolyte, the ground-state and excited-state pKa values of this dye were measured to be 8.2 and 1.1, respectively. This work is important because knowledge of pKa values allow the prediction of proton transfer rates in aqueous media