Impact of acids on the structure and composition of Linde Type A zeolites for use in reverse osmosis membranes for recovery of urine-containing wastewaters

On the International Space Station (ISS) there are low pH, urine containing wastewaters that need to be recycled. We evaluated the effects of synthetic ISS wastewater on Linde Type A (LTA) zeolites to assess their applicability in membrane separations to produce potable water. Specifically, we studied the impact of the four acid-producing components found in the wastewater: chromium(VI) oxide, potassium phosphate monobasic, sodium phosphate monobasic dihydrate and sulfuric acid. We characterized effects of the acids on crystal structure, bonding, surface morphology and solid phase composition of the LTA zeolites. After 24 h of exposure to synthetic wastewater brine (pH 1.94), the zeolites sustained some damage with extensive structural degradation occurring after 128 h. The crystal structure of the LTA particles completely degraded in all acid-type solutions with a pH below 1 and in phosphate solutions with a pH below 5. The structural degradation resulted from the removal of aluminum from the zeolite framework. Additionally, we observed that phosphate caused changes in bonding of the zeolite, through removal of silicon from the framework and formation of aluminum phosphate precipitates. Ultimately, we found that the presence of acidic anions (specifically dihydrogen phosphate anions that catalyzed aluminum and silicon removal), rather than pH, was the most significant factor causing degradation of LTA in dihydrogen phosphate-containing solutions. Our results indicate that Linde Type A zeolites are inappropriate for use with low-pH, dihydrogen phosphate containing solutions.