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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.

Two widespread practices in water treatment are, removal of pollutants via adsorption onto activated carbon, and, oxidation of pollutants using a photocatalyst slurry and ultraviolet radiation. The ultimate goal of this research is to combine the adsorptive properties of carbon and the oxidative properties of titanium dioxide (TiO2), and construct a photocatalytically regenerative carbon filter for 100% water recovery. The premise is that the activated carbon, coated with TiO2, will capture the compounds through traditional filtration and adsorption. Once the carbon becomes exhausted, it can be regenerated in-situ by turning on the UV lamps thereby activating the photocatalyst.

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.

A magnetically agitated photocatalytic reactor (MAPR) has been developed and tested as a post-processor in the past using phenol and reactive red dye to simulate these waste components, yet these components ignore factors that may hinder a photocatalytic post processor including competitive adsorption of various organic compounds and their oxidation byproducts and the demonstrated detrimental effect of inorganic compounds such as ammonium bicarbonate on photocatalytic oxidation. To assess these effects, this work looks at photocatalytic oxidation of air evaporation subsystem (AES) ersatz water while modifying the photocatalyst mass, magnetic field current and frequency to find the optimal conditions. Additionally, the magnetic photocatalyst has been characterized to observe the assembled structures formed when exposed to the magnetic field array in the MAPR and the crystallinity of the titanium dioxide coating.

Two annular continuous flow reactors with nominal volumes of 400 mL and 150 mL were packed with silica gel pellets that were doped with titania (TiO2) (12 wt%). The reactors were configured with UV lamps in the center of the reactors. SOC oxidation experiments were performed in a single-pass mode with bicarbonate ions present and in a low dissolved oxygen environment. SOC concentrations decreased (ranging from 40% to 95%) without bicarbonate present. These removal efficiencies were not affected by moderate bicarbonate concentrations (up to 200 mg/L as NaHCO3) or low dissolved oxygen levels (2 mg/L). Microbial experiments were performed for the inactivation of selected viruses and bacteria. The log [N0/N] values resulting from two hours of 254-nm UV irradiation for the bacteriophages ФX-174, PRD-1, and MS-2 were 1.67, 1.43, and 1.65, respectively.

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.

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.