Dioxin and the clay that binds.

By treating an inexpensive, naturally occurring clay with a solution of water-soluble aluminum hydroxide, researchers at the University of Michigan in Ann Arbor have been able to modify the clay so that it binds with toxic organic chemicals, most notably 2, 3, 7, 8-TCDD, the most toxic species of dioxin. This modified clay could be used as a filter to sieve out small quantities of dioxin and other toxic chemicals from industrial wastewater or drinking water. Alternatively, it could be substituted for those clays now lining hazardous waste landfills as an improved barrier against migration of toxic chemicals into soil.

The hydroxy-aluminum treatment expands certain structures of montmorillonite clay, creating a labyrinth of engineered pores inside the clay (see diagram) that are large enough to let water pass through but small enough to trap dioxins, dibenzofurans and PCBs (polychlorinated biphenyls), explains Keeran Srinivasan, one of the project scientists.

Not only is this clay less expensive than the best available alternative -- activated carbon -- for water treatment, but it's also more effective, Srinivasan says. It can extract 3 milligrams of dioxin from 20 million gallons of water, a separate capability equal to finding 2 molecules of contaminant out every 10.sup.15 molecules of water, he says. He adds that the clay sieve might even be recyclable once it's collected its fill of contaminant. Because hydroxy-aluminum is a good catalyst, Srinivasan says, there are indications it might foster relatively low-temperature dechlorination (degradation) of the toxic organics--perhaps at 100[deg.]C or lower -- without affecting the clay.