SAC and SBA CO2 removal problem

[bfinger35]

Hope someone can help me figure this out. We are investigating the separate SAC/SBA ix bed configuration with a membrance contactor located between the beds for removal of CO2 (using vacuum desorb). This is being investigated as part of a spacecraft wastewater processing study, where up to 50% of the anion load is bicarbonate. When runing in this configuration, SBA resin (UP4000, Rohm and Haas) capacity is very close to the predicted capacity of 1.1 eq/liter-resin. However, when we run a control using mixed-media [1 part SBA and 2 parts SAC (UP1400)], I am getting almost the same SBA utilization rate. In other words, the "apparent" capacity of the SBA is almost 2 eq/l-resin. I cannot figure out where the bicarbonate/TIC is going, as my water quality analysis shows very little TIC in the effluent. If the pH is ~ 5, is pehaps the TIC not loading the AIX resin because it is primarily CO2? If so, should I see lots of gas bubbles in the effluent? Any help is greatly appreciated.