(430a) Impact of Acids On the Structure of Linde Type A Zeolites for Use in Reverse Osmosis Membranes for Wastewater Recovery

Abstract:

        Membrane separations and osmotic processes are attractive options for wastewater reclamation, and with growing concerns over a depleting global water supply such technologies are rapidly becoming more valuable. Previously, hydrophilic Linde Type-A (LTA) zeolites have been incorporated into polyamide reverse osmosis membranes, creating high flux membranes called Thin Film Nanocomposite membranes^{[1], [2]}.  In our ongoing work, we are exploring the use of zeolite nanocomposite membranes for the recovery of potable water from wastewaters.  We are specifically interested in the wastewaters generated by NASA on the International Space Station (ISS).  Currently, only 70% of the wastewater generated on the ISS is recovered; the remaining 30% of the water is stored for later disposal in the form of brine^[3]. With the retirement of the NASA space shuttle fleet, the need to minimize use of ground-based resources is a high priority; as a result, NASA desires to maximize water recovery^[4]. Our research focuses on using LTA zeolite nanocomposite membranes to recover water from the urine and urine brine wastewaters produced on the ISS. The small pore size of the LTA zeolite should make it ideally suited to allow passage of water, but rejection of small neutral organic molecules such as urea, a main component of urine^[5].

        In this talk, we will present our results evaluating the resistance of LTA zeolites to the components NASA uses to acidify the urine solutions. NASA treats the urine wastewaters with a multi-component acidic solution to inhibit bacterial growth. We have created synthetic urine brines in our lab and found that these have adverse effects on the crystal structure of the LTA zeolites. We produced solutions of potassium phosphate at pH 4.2 and 5, and sulfuric acid, and chromium trioxide solutions at pH values varying from 0.5 to 5 to identify the most damaging acid component. We exposed zeolites to the synthetic urine, and acid-specific solutions for 1 to 18 days to test the effects of pH and acid type over time. We characterized the samples with X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy to evaluate any structural changes in the zeolites. Significant degradation was observed over a period of 9 to 18 days at pH values below 2 for all acids, and immediate dissolution occurred at or below pH 1. Potassium phosphate showed completely amorphous structure at pH 4.2, likely due to formation of aluminum phosphate. We will discuss the potential mechanisms for zeolite degradation, including acid hydrolysis at low pH. The results of this experimentation suggest that LTA zeolites are inappropriate for use in low pH environments.

References:

[1] Jeong, B. -H. et al. Interfacial polymerization of thin film nanocomposites: A new concept for reverse osmosis membranes. Journal of Membrane Science. 294, 1-7 (2007).

[2] Lind, M. L. et al. Influence of zeolite crystal size on zeolite-polyamide thin film nanocomposite membranes. Langmuir: the ACS Journal of Surfaces and Colloids. 25, 10139-10145 (2009).

[3] Carter, L., Marshall, N. & Flight, S. Status of the regenerative ECLS water recovery system. AIAA International Conference on Environmental Systems; 11-16 Jul. 2010; Barcelona; Spain1-11 (2010). 

[4] Link, D. E., Carter, D. L. & Higbie, S. Development of an advanced recycle filter tank assembly for the ISS Urine Processor Assembly. American Institute of Aeronautics and Astronautics1-8.

[5] Database of Zeolite Structures. International Zeolite Association. http://www.iza-structure.org/databases/, IZA.