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A prototype reactor was designed and tested to oxidize synthetic organic chemicals (SOCs) without the use of expendable chemicals and without the need to separate the catalyst from the water after treatment. An annular continuous flow reactor with a nominal volume of 400 mL was packed with silica gel pellets that were doped with titania (TiO2) (12 wt%). The reactor was configured in a test stand with UV lamps in the center of the reactor. SOC oxidation experiments were performed in a recycle mode and in a single-pass mode. Five target analytes (acetone, chlorobenzene, ethyl acetate, toluene, and methylmethacrylate) were spiked (100 to 300 μg/L) into nano-pure water and recycled through the reactor until adsorption equilibrium was attained. UV lamps, which were shielded, were then uncovered, and effluent concentrations were monitored as a function of time. All of the compounds were degraded to below detection limit (5 μg/L) after an extended reaction period of 23 hours.

NASA will require a safe and efficient method for water recovery on long-term space missions. Photocatalysis represents a promising solution for part of a system designed for recovery of water from humidity condensate, urine, and shower waste. It eliminates the need for chemical oxidants that are dangerous and difficult to transport, and the considerable energy consumption of distillation. In terms of decreasing the equivalent system mass (ESM) with respect to these alternative technologies, considerations for the volume, mass, cooling and crew time are also important. This photocatalytic reactor generates the oxidant in the form of hydroxyl radicals and valence band holes by exposing silica-titania composite particles with a barium ferrite core to ultraviolet light. The magnetic core of the catalyst allows for separation, confinement, and agitation.

NASA has identified a number of water treatment options that have shown promise in space. The water recovery system on any particular mission will be a collection of individual treatment units, with each treatment unit focusing on a select group of contaminants. The project objective has been to develop a microgravity-compatible, compact post processor based on magnetic agitation that is safe and reliable and that provides product water that meets or exceeds NASA's potable water quality requirements. The micron-sized magnetic photocatalytic particles have been proven durable and capable of oxidizing synthetic organic chemicals. The reactor has been optimized with respect to agitation frequency (50 Hz), UV wavelength (312 nm), and geometry (circular coiled reactor cell).

Photocatalysis is used to mineralize water pollutants, providing water treatment without a waste stream. This water treatment method allows for a compact reactor design (i.e., reduced Equivalent Systems Mass (ESM)) that is applicable in future NASA missions that will require water recovery. The reactor would provide a post-processing unit to remove any organic contaminants (e.g., VOCs) not removed in prior water subsystems. Several approaches to the reactor design are being explored. Titanium dioxide (TiO2) is the chosen photocatalyst based on its proven performance and non-toxicity. Because of their propensity to adsorb pollutants, silica and activated carbon are being investigated as supporting materials for the titania. Three types of particles are being tested for their ability to destroy organic contaminants: silica gel doped with titania, activated carbon coated with titania, and silica gel doped with both activated carbon and titania. Each material has certain advantages.

Currently, proposed water recovery systems for baseline space missions consist of integrated technologies to remove contaminants from graywater for reuse. Lacking in these mission scenarios and in current research efforts is a solid understanding of how photocatalysis might perform as a primary and/or secondary processor. However, one of the major hurdles for slurry-based photocatalysis is the ability to separate the catalyst from solution after mineralization of pollutants is complete. Purifics, a Canadian engineering company, has solved this problem with a patented separation device utilizing a backpressure cycled membrane and automated system (Photo-Cat®). Purifics specifically designed a pilot unit to be used to solve the water recovery problem for long-term space missions. Operating Purifics’ Photo-Cat® as a secondary processor, with and without ammonium bicarbonate demonstrated that the TOC concentration could be reduced to below 0.5 ppm.