MICROGRAVITY COMPATIBLE REAGENTLESS INSTRUMENTATION FOR DETECTION OF DISSOLVED ORGANIC ACIDS AND ALCOHOLS IN WATER
James R. Akse, James E. Atwater, and John T. Holtsnider
UMPQUA Research Company (URC).
Organic acids and alcohols occur in humidity condensate, urine distillate, and composite wastewaters aboard spacecraft. Current water recovery processes which are shown in Figure 1 sequester organic acids on anion exchange media located in the multifiltration (MF) sorbent train. Alcohols and other highly polar, low molecular weight organics not removed in the MF train are catalytically oxidized in the volatile removal assembly (VRA), forming organic acids which are then separated using anion exchange resins. The ability to quantify both alcohols and organic acids in support of long-duration manned missions such as the International Space Station (ISS) will insure that proper operation of MF and VRA sub-systems can be verified and problems diagnosed. The determination of a sample's organic acid levels begins with 'reagentless' acidification by flow through a solid phase acidification (SPA) module as shown in Figure 2. Following acidification, carbon dioxide, the chief interferant, is selectively removed in the CO2 degassing module (CDM). Organic acids are then selectively transferred to the analytical stream in the organic acid transfer module (OATM) using a stopped flow protocol. The organic acid concentration is determined from the change in the specific conductance of the analytical stream. This methodology is easily adapted to quantitate primary alcohols following enzymatic oxidation using immobilized alcohol oxidase as a catalyst.
Figure 1. International Space Station Water Recovery System Schematic.
Figure 2. Organic Acid and Alcohol Monitor (OAAM) Schematic.
During the program's first year, improved organic alcohol and acid monitor (OAAM) sub-components were developed. These included: 1) a composite SPA module capable of lowering the pH of a flow-through stream to 1.75, allowing more efficient transfer of propionic, acetic and formic acids; 2) a more efficient CDM which completely removed CO2 from a sample after a contact time of 20 seconds, and; 3) an OATM module with a threefold increase in the transfer rates of organic acids.
CURRENT STATUS OF RESEARCH
The new composite SPA module consisted of a bed of calcium sulfate followed by a strong acid ion exchange resin. This module replaced a bed of crystalline MoO3 particles. The new composite SPA module produced an effluent pH of 1.75 compared to 3.0 for the MoO3 particulate bed. The fraction of formic acid, the most dissociated organic acid, protonated at this lower pH increased from 85 to 98 %, significantly increasing the volatile fraction.
The efficiency of the gas permeable sub-components, the CDM and the OATM, was also enhanced. In the CDM, the previously used PTFE membrane was replaced with a more permeable polydimethylsiloxane (PDMS) membrane. Results for the removal of CO2 by these two membranes is shown in Figure 3. The PDMS membrane completely removed CO2 after a contact time of 20 seconds, while retaining better than 95% of formic, acetic, and propionic acid as determined by total organic carbon (TOC) analysis. Further efficiency gains for the transfer of organic acids from the sample to the analytical stream were obtained by raising the OATM operating temperature. Figure 4 shows the transfer efficiency for formic acid as a function of the OATM temperature. Based on this data the OATM operating temperature was increased to 50C with a factor of 3 improvement in the formic acid transfer rate.
Figure 3. Carbon Dioxide Degassing for PTFE and PDMS Membranes in the CDM.
Figure 4. Effect of Temperature on OATM Transfer Efficiency for Formic Acid.
To further improve sensitivity, the OAAM was operated as a stopped flow analyzer. The continuous passage of a low pH sample through the OATM creates a pH driven volatile organic acid concentration gradient between sample and analytical streams. Volatile organic acids pass through the OATM microporous membrane and concentrate in the analytical stream. In conjunction with longer contact times, the concentration of ionized organic acids in the analytical stream increases, thereby reducing the detection limits. To investigate this effect, the conductiometric response for all three low molecular weight organic acids was determined as a function of stopped flow time. These results are shown in Figure 5. Based on these data, a stopped flow time of 10 minutes was selected as the best compromise between response time and sensitivity. Another sensitivity gain was made by the replacement of stainless steel surfaces with PEEK, after it was determined that stainless steel surfaces adsorbed low levels of organic acids. The mechanism probably involves adsorption of negatively charged formate, acetate, and propionate ions on the thin protective and positively charged oxide layer which protects the surface of stainless steels.
Figure 5. Conductiometric Response versus Stopped Flow Time for Various Organic Acids.
Stainless steel were replaced with PEEK components, and calibration curves were determined for formic, acetic, and propionic acids with the improved CDM and OATM operating conditions. The CDM consisted of a 3 m section of PDMS tube with an inner diameter of 0.305 mm and a 0.330 mm wall thickness. The OATM consisted of a 2 m section of hollow fiber microporous polypropylene membrane with a 0.300 mm inner diameter and a 0.028 mm wall thickness. The OATM outer tube was PEEK with an inner diameter of 0.762 mm. The quantitation of formic, acetic, and propionic acid standards was determined between 0.1 and 40 mg/L with detection limits of less than 0.005 mg/L. The calibration over the entire range is shown in Figure 6 with the concentration between 0.005 and 1.20 mg/L shown in Figure 7.
Figure 6. High Level Organic Acid Calibration Curve.
Figure 7. Low Level Organic Acid Calibration Curve.
Separation of organic acids from a mixture in unbuffered water was also demonstrated using strong acid ion exchange resin. The chromatogram is shown in Figure 8. The ability to separate organic acids in the analytical stream is a prerequisite for the quantitation of individual organic acids in organic acid mixtures.
Figure 8. Separation of Organic Acids on Strong Acid Exchange Media in Deionized Water.
These data indicate that significant progress has been made this year towards the development of an organic acid and alcohol analyzer which will provide accurate analysis over a large dynamic range with high sensitivity. In addition, this analyzer responds rapidly, uses virtually zero expendables, produces little waste, and is capable of on-line analysis.
In the next year, work will be directed towards improvement in the OAAM capabilites and the assembly of an integrated computer controlled instrument. These efforts will include: 1) the incorporation of enzyme and/or heterogeneous catalyst beds that will convert primary alcohols to the corresponding organic acid allowing the quantitation of alcohols in water; 2) the integration of control and data acquisition software with OAAM hardware resulting in a computer controlled analyzer; 3) the addition of a detection scheme for the selective quantitation of each organic acid (and alcohol) in a mixed sample, and; 4) microminiaturization of OATM and CDM modules to improve performance, reduce mass, volume, and enhance applicability to space based environments.