Contaminated water exists in increasing quantities as a result of industrial, agricultural, and municipal wastes. Water treatment facilities often use powerful free-radical treatment processes (Advanced Oxidation Processes) that work well under normal circumstances because the electrophilic free radicals generated in those processes have a high ability to convert the easily oxidizable contaminants into less toxic substances. However, these processes do not efficiently remove classes of contaminants that are refractory to oxidation, particularly in the presence of other more-easily oxidized competing organic material, so that long reaction times and/or excessive amounts of chemicals are required.
Organic contaminants that contain electron-withdrawing substituents are difficult to oxidize (refractory) under normal oxidizing free-radical treatment conditions; however, the fact that they can readily accept an electron (i.e., be reduced), provides the basis for a more effective technology.
This class of contaminants includes difficult-to-oxidize byproducts that are formed from contaminants that are easily oxidized. Easily-reduced inorganic compounds can also be treated by this process. This new water treatment process successfully treats aqueous streams containing refractory contaminants that are difficult to destroy by oxidative free-radical treatment processes. These refractory contaminants include nitro-containing substances such as ordnance and energetic compounds, including dinitrobenzene, dinitrotoluene, trinitrobenzene, trinitrotoluene, and other nitro compounds, as well as polyhalogenated compounds such as carbon tetrachloride and tetrachloroethane. Polychlorinated or polybrominated biphenyls, diphenyl ethers, or dioxins, and other contaminants containing electron-withdrawing substituents such as sulfonate, nitrile, and carboxylate should be reduced as well.
How It Works
The contaminants are removed by producing a reactive intermediate from an additive. Oxidizing (electrophilic) free radicals are generated in solution (i.e., an Advanced Oxidation Process is used), and a reducing radical precursor is added to react with the electrophilic free radical to form the intermediate reactive species. The intermediate selectively reacts with (reduces) the refractory contaminant, producing a reduced species that is less toxic and/or more easily treated by other methods than the original contaminant. Many polychlorinated compounds will completely dehalogenate under these conditions, once the process is started. Since reduction of refractory compounds can be carried out simultaneously with oxidation of other components, the by-product from the refractory compound can be simultaneously oxidized to less toxic substances in the same reactor.
This process reduces the overall treatment time and eliminates the need for additional treatment while effectively removing all types of contaminants from the aqueous stream. Unlike existing reductive treatment processes that require that an expensive additive be photolyzed with ultraviolet radiation to yield an electron, this process uses inexpensive additives and takes place concurrently with, and within the same solution as, the destruction of other contaminants by oxidation. Within the aqueous stream, free radicals are produced rapidly enough to cause significant oxidation of typical contaminants into less harmful compounds. The aqueous stream should be low in oxygen content in order to restrict the reaction of oxygen with the reactive intermediate. A concentration of oxygen ranging from 0.1 to about 2 mg/L (or as low as feasible) is preferred. The aqueous stream can be deoxygenated by chemically treating it with a deoxygenation agent such as sulfite or sulfur dioxide. By-products of the oxidizing radical reactions also help to remove oxygen from the solution. Many groundwaters are already low in oxygen and may only need protection from air contact, rather than deoxygenation. When used in conjunction with free-radical processes that also generate hydrated electrons, carbon dioxide may be used as both the deoxygenation agent and as the reducing radical precursor.
This process can also be effective for treating water or aqueous alcohol or detergent solutions that have been used to extract nitro or halogenated contaminants from contaminated soil and/or oil, and shows promise for the in situ treatment of PCBs and other similar contaminants in groundwater, adsorbed to aquifer solids, or flushed from aquifers.