Silver deposition on wetted materials used in the potable water system of manned spacecrafts

Silver is used as the biocide for potable water on the Russian Segment of the International Space Station and is also intended to be used for NASA Orion spacecraft. Studies about the stability of water quality containing silver have demonstrated the depletion of silver from water even after short term storage in water tanks, implying potential microbial water contamination or proliferation. This paper investigates the loss of silver ions from water bulk when water is in contact with various materials typically used water systems for space. Silver ions concentration was equal to 10 or 0.5 mg Ag+/L. Afterwards, water was brought in contact with various materials at surface to volume ratio equal to 5.0 cm-1 and temperature 30oC, and was stored either for 7d (for 10 mg Ag+/L) or 28 d (for 0.5 mg Ag+/L). All exposed surfaces were either leached, in order to estimate the amount of deposited silver onto the surfaces, or subjected to XPS analysis to investigate the deposition mechanism. Silver losses from water containing 10 mg Ag+/L followed the order: PTFE (7.4%) < FEP (14.7%) < SS 316L NASA/JSC PRC-5002 passivated & electropolished (21%) < EPR Parker E0798‐70 (63.2%) < SS 15-5 pH, H1025 (78.4%) < 316L with GTAW welding (84.2%) < SS316L NASA/JSC SS PRC-5002 passivated (94.7%) < SS316L (96.8%) ~ Titanium 6AL-4V(96.8%). On the other hand, silver was completely depleted from water containing 0.5 mg Ag+/L after contact with all metallic surfaces. In this case of low Ag+ concentration , silver was partially (about 65%) removed when water was brought in contact with polymeric surfaces FEP and PTFE. Leaching of surfaces verified the deposition of silver onto materials, while the silver mass balance closed to better than 90% in all cases. XPS results showed that silver was deposited by different mechanisms which are discussed.