The application status and potential of three chemical oxidation treatment methods which generate powerful oxidants (hydroxyl radicals): ultraviolet light (UV)/hydrogen peroxide (H2O2) process, Fenton's reagent treatment, and titanium dioxide (TiO2)-assisted photocatalytic degradation, are described and discussed. These oxidation methods are known to effectively degrade and, in several cases, mineralize contaminants ranging from inorganic compounds (such as cyanides) to chlorinated aliphatic compounds and complex aromatic compounds in reaction times on the order of a few minutes to a few hours. Of the three oxidation systems discussed, the technology for the UV/H2O2 process is the most advanced, with numerous successful full-scale treatment units already in existence. Applications of both the Fenton's reagent and TiO2-assisted photodegradation processes are currently being developed, with the concepts proven in numerous laboratory-scale studies for a wide range of contaminants. However, both of t...

Chemical Oxidation Technologies: Ultraviolet Light/Hydrogen Peroxide, Fenton's Reagent, and Titanium Dioxide-Assisted Photocatalysis

A commercial chromia-alumina catalyst was investigated for an extended period of time for the oxidation of two gas streams containing volatile organic compounds (VOCs) with compositions similar to those expected from groundwater air strippers. The first gas stream ( chlorinated stream ) contained 500 ppm of C1 to C2 chlorohydrocarbons, while the second stream (mixed stream) contained 450 ppm of C5 to C9 hydrocarbons plus 50 ppm of trichloroethylene. Catalyst deactivation was studied at constant conversion of the main reaction, achieved by increasing the temperature, when needed, to compensate for the loss of intrinsic catalyst activity. The catalytic activity and selectivity were found to be a function of the type of feed stream and the reactor configuration. Although the catalyst used for the oxidation of the chlorinated stream did not require any temperature increase when used fore 153 days-on-stream in a fixed bed reactor, the results suggest progressive loss in catalyst activity down the catalyst bed. A fluid bed reactor was found to be more effective than a fixed bed reactor in maintaining the catalyst activity for the oxidation of the chlorinated stream. The physical attrition of catalyst particles in the fluid bed reactor accompanied with the loss of chromium, via oxychloride formation, appears to be beneficial in maintaining the catalyst activity by constantly exposing the fresh catalyst surface. The catalyst used for mixed stream oxidation required a temperature increase of 33°C over 210 days-on-stream to maintain constant conversion. This decrease in catalytic activity was related to a decrease in BET surface area.

Catalyst deactivation during deep oxidation of chlorohydrocarbons

First row transition metal oxides supported on carbonaceous materials are shown to be effective in the low temperature deep oxidation of hexane, butane and toluene to carbon dioxide and water. The oxides of manganese, cobalt, copper and zinc were supported on several different types of carbon, out of which it was found that manganese and cobalt oxide supported on Ambersorb® 572 were most durable. In addition, Ambersorb® 563 and Ambersorb® 572 were thermally stable under the reaction conditions. Mechanistic studies in cobalt oxide/Ambersorb® 572 indicate a synergism between the metal oxide and carbonaceous material. A catalytic cycle has been described, involving a CoII/CoIII couple, and the stabilizing effect of the support on the reduced form of the metal oxide.

Low-temperature deep oxidation of hydrocarbons by metal oxides supported on carbonaceous materials

The activity of two commercial oxidation catalysts was investigated for the oxidation of a humidified air stream containing 500 ppm of a mixture of C5 to C9 hydrocarbons over an extended period of time (ten months). The composition of the feed stream was similar to that obtained from air-strippers used for groundwater clean-up. The reaction was conducted continuously in fixed bed reactors at constant total conversion (> 99%). The temperature was increased, when needed, to compensate for the catalyst deactivation and maintain a constant conversion. A 0.1% Pt/3% Ni/Al2O3 catalyst (G-43A, United Catalysts, Inc., Louisville, KY) did not lose any activity at 430 ° C when used continuously for 253 days-on-stream. On the other hand, a ceria-promoted hopcalite catalyst suffered considerable deactivation and required a temperature increase of 85° C over 297 days-on-stream to maintain the total conversion above 99%. However, the final temperature (400 ° C) required to maintain > 99% conversion over the hopcalite catalyst was still lower than the initial temperature required for the operation of the Pt/Ni catalyst (430° C). No partial oxidation products were formed over either of the catalysts. Also, the only total carbon oxidation product was carbon dioxide. A simple first order deactivation model fits the time-temperature relationship over the hopcalite catalyst and predicts a hopcalite catalyst lifetime of 362 days for a maximum operating temperature of 500° C.

Deep oxidation of hydrocarbons

A new and effective method has been developed for the treatment of VOC's, PCB's and other toxic organics. Direct UV photolysis of organics is achieved with this new method with the use of high intensity ultraviolet light of a broad UV spectrum. Standard and novel UV flashlamps can be used for generation of this broad UV spectrum. The pulsing nature of such spectrum helps to increase the efficiency of destruction of toxics. The final products of this destruction process are non-toxic simple compounds. The energy efficiency of this new process exceeds that the traditional UV aided processes with medium pressure mercury lamps. This article reviews the Direct UV Photolysis Process, gives experimental results, and provides recommendations for applications in the treatment of groundwater, wastewater, contaminated air and soil.

Treatment of contaminated water, air and soil with UV flashlamps

Air emissions from the treatment of volatile organic compound (VOC) - contaminated groundwater are a growing problem in the US. Historically, air stripping has been used to remove VOCs from contaminated groundwater. Air stripping technology is a cross media treatment technique, i.e., it solves a groundwater problem by transferring contamination to the atmosphere. In response to the air pollution problem created by air stripping, the public, air quality regulatory agencies, the federal government and private industry are exerting pressure to eliminate and/or reduce air emissions from the clean-up of contaminated groundwater. These forces make it desirable to consider alternative and innovative technologies for the treatment of groundwater contaminated with VOCs.

Geoffrey H. Swett

Papers

Groundwater treatment with zero air emissions