Depletion of Biocidal Iodine in a Stainless Steel Water System

Iodine depletion in a simulated water storage tank and distribution system was examined to support a larger research program aimed at developing disinfection methods for spacecraft potable water systems. The main objective of this study was to determine the rate of iodine depletion with respect to the surface area of the stainless steel components contacting iodinated water. Two model configurations were tested. The first, representing a storage and distribution system, consisted of a stainless steel bellows tank, a coil of stainless steel tubing and valves to isolate the components. The second represented segments of a water distribution system and consisted of eight individual lengths of 21-6-9 stainless tubing similar to that used in the Shuttle Orbiter. The tubing has a relatively high and constant surface area to volume ratio (S/V) and the bellows tank a lower and variable S/V. The test apparatus was cleaned and filled with a 10 mg/L iodine solution prepared with distilled-deionized water. Sampling from the tank, coil, and the series of tubes was performed periodically, and the major iodine species were monitored to determine the rate of decay of iodine and its fate in the system.

Results of this study show that the iodine demand of stainless steel was significant. Decay rates were higher than anticipated in tubing that had not been previously exposed to iodine. The iodine (I₂) concentration in the tank contents decreased to less than 1 mg/L in 10 weeks, whereas none was detected in water from the tubes after 1 week. Also, trace metals were measured in the samples by graphite furnace atomic absorption spectrometry and found to increase with contact time. A subsequent attempt to passivate the inside surface of the tubing via a 4-hour soak with 30 mg/L iodinated water reduced the rate but did not eliminate iodine loss. Ramifications of these findings to the current Shuttle potable water system and the design of spacecraft water systems in general are discussed.