Taking chlorine out of tough pollutants.
In the process of bleaching wood pulp to press out pearly reams, paper mills may create up to 250 different types of chlorinated contaminants (SN: 5/12/90, p.303). The most tenacious include a class of halogenated aromatic compounds called trichlorophenols (TCPs). At the heart of a TCP molecule lies a tightly bound ring of atoms that includes three chlorines.
The structure of TCPs allows them to stand up to nature's degradative forces, presenting soil-borne microorganisms with an indigestible meal. Their resistance to decomposition causes trouble for the paper industry, which must safely dispose of the long-lasting waste.
Offering a potential solution to this problem, chemists Alexander Sorokin and Bernard Meunier at the National Scientific Research Center (CNRS) in Toulouse Cedex, France, and Jean-Louis Seris of GRL-Biotechnology in Artix describe a new type of catalytic system for breaking up TCPs.
Using hydrogen peroxide, a relatively safe and environmentally benign agent, coupled with a readily available iron-based catalyst, the new system oxidizes TCP molecules by breaking open the chlorinated rings at their cores. The chemical compounds resulting from the reaction can then undergo natural degradation, the team reports in the May 26 Science.
"Many of these pollutants can be converted into less dangerous organic products and can be eventually degraded by different microorganisms," the researchers point out. "Systems that can remove halogen substitutents from [TCPs] may produce compounds that can be more easily biodegraded.
"In this case," they add, "the use of chemical catalysts to convert recalcitrant pollutants to more degradable molecules by microorganisms would be beneficial."
The iron-based catalyst crucial to the new dechlorination technique bears the unwieldy name 2,9,16,23- tetrasulfophthalocyanine. Its power lies in its ability to "cleave" the aromatic chlorine-containing rings of the otherwise impenetrable TCPs, Meunier says.
Upon mixing the difficult-to-destroy trichlorophenols with hydrogen peroxide, then adding the iron-containing catalyst, the chemists found that the chlorine atoms freed by TCP breakup linked with other molecules to form four relatively benign products.
Moreover, they found that it took only a little catalyst to trigger the desired reactions. And the process goes speedily: In several tests of the catalytic system, the chemists converted all of the toxic compounds into tolerable ones in less than 5 minutes, they report.
This system offers several potential advantages to industry, Meunier says. Both hydrogen peroxide and the catalyst are relatively "clean," easy to make, and inexpensive.
"The chemical industry has been worrying about the accumulation of these compounds that don't break down," he says. "But if you make them biodegradable, then microorganisms will take care of them."
"This catalytic process is quite general," Meunier adds. "You could use it to treat many chemicals. We use trichlorophenol mainly to show that the method works."
The new technique could prove useful for processing contaminants at toxic waste sites, the researchers say. "Now we have to try other molecules and demonstrate that the system works on a large scale."
"We've had a good start," observes Meunier, who adds that he and his colleagues are exploring potential applications with industry.
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|Article Type:||Brief Article|
|Date:||May 27, 1995|
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