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The photocatalytic oxidation of VOCs was investigated using a novel countercurrent flow reactor designed to enable the treatment of toluene present in the gas and the aqueous phases simultaneously. The reactor was packed with silica-titania composites commingled with plastic pall rings. Using this mixed packing style was advantageous as it resulted in a higher UV penetration throughout the reactor. The average UV intensity in the reactor was determined to be 220 μW/g irradiated TiO2. It was found that under dry conditions, the STCs had a high adsorption capacity for toluene; however, this adsorption was completely hindered by the wetting of the STCs when the two phases were flowing simultaneously. The destruction of toluene in the aqueous phase was determined to follow a linear trend as a function of the contaminant concentration.

Silica-titania composites (STC), a novel sorbent and photocatalytic technology developed at the University of Florida in Gainesville, Florida have been evaluated for removal of volatile organic compounds (VOCs) from aircraft cabin air. Currently, activated carbon filters are used, but must be replaced frequently due to their limited adsorption capacity. These filters must be disposed of and cannot be regenerated and reused. The STC technology is a significant improvement upon the current control technology because of its high adsorption capacity and the ability to regenerate via photocatalytic oxidation (PCO). When the STC sorbent is irradiated with UV, adsorbed VOCs are mineralized to CO2 and H2O and the material is regenerated and ready for reuse multiple times.

The removal of volatile organic compounds (VOCs) from the air aboard spacecrafts is necessary to maintain the health of crewmembers. The use of photocatalysis has proven effective for the removal of VOCs. A majority of studies have focused on thin films, which have a low adsorption capacity for contaminants and intermediate oxidation byproducts. Thus, this study investigates the use of adsorbent materials impregnated or coated with titania to: (1) provide a system that can remove VOCs for a period of time in the absence of UV irradiation to reduce power requirements and/or offer contaminant removal in the event of lamp failure and (2) improve the photocatalytic oxidation efficiency by concentrating VOCs and intermediate oxidation byproducts near the surface of the photocatalyst. Two adsorbent materials (porous silica gel and BioNuchar120 activated carbon) and glass beads were tested as catalyst supports for the destruction of a target VOC, in this case methanol (Co = 50 ppmv).

Previous work demonstrated that the Photo-Cat® developed by Purifics is capable of reducing the total organic carbon (TOC) concentration of 51 mg/L to below 0.5 ppm using Degussa P25 titanium dioxide (TiO2) as a photocatalyst. The work also showed that ammonium bicarbonate had a detrimental effect on the rate of photocatalytic oxidation, but did not prevent the system from reaching the potable water specification. Nanometer sized Degussa P25 is very popular and quite frequently used as a benchmark of performance in literature, but it may not be the most effective for oxidizing all waste streams. It is critical that each component of the water recovery system be optimized for power consumption and the effectiveness of the photocatalyst plays an important role in accomplishing this.

Silica-titania composites have been engineered and proven to be a plausible post-processor technology for water recovery. The premise behind this technology is the adsorption and subsequent mineralization of organic compounds. Concurrent with this NASA-based research, faculty at the University of Florida have explored the efficacy of this technology for additional applications. Those that have shown the most promise include greater than 98% removal of volatile organic compounds (VOCs) and Hazardous Air Pollutants (HAPs) from pulp and paper mills and greater than 99% removal of mercury from coal-fired power plant flue gas. Both of these industries are facing pressing regulations that may be implemented as early as 2006.

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.

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.

Finding a manner to effectively filter water to the purest standards is an ongoing battle for various sectors of science. We present a set of experiments that will report the preparation of the photocatalytic component of our composite particle via sol-gel coatings with titanium n-butoxide with subsequent heat treatment at 500°C for three hours in Argon. Our ultimate goal is to create a particle with regenerative capabilities along with a surface enhanced Raman scattering effect. Characterization techniques were performed using SEM-EDS, and XRD.

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).

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.

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.