Soybean hulls, the seed's fiber coat, show promise as a new filter for removing toxic metals from industrial wastewater, an Agricultural Research Service scientist reported at the American Chemical Society's national meeting in San Diego, Calif.

According to ARS chemist Lynda Wartelle, soybean hulls are a low-value, high-volume agricultural waste that can be rendered into metal adsorbents comparable to commercial products called ion exchange resins. The key lies in a new process she and colleagues devised that changes the hull's properties and surface charge using food-grade citric acid combined with a heating step.

Wartelle is based at the ARS Southern Regional Research Center, New Orleans, La. Her colleagues are research chemist Wayne Marshall, also at SRRC in New Orleans; cost engineer Andrew McAloon, ARS Eastern Regional Research Center, Wyndmoor, Pa.; and engineering technician Alexandra Chatters, currently at Xavier University of Louisiana.

Soybean hulls, produced in the United States at the rate of 10 billion to 15 billion pounds annually, are typically sold to animal feed supplement producers for $40 a ton. But scientists envision increasing the hulls' worth by turning them into adsorbent filters used by electroplaters, jewelers and other industries that generate wastewater with metal contaminants.

Most commercial ion exchange resins cost between $2 and $20 a pound, depending on whether they're synthetic or cellulose-based, according to Marshall. But in studies Wartelle reported at the ACS meeting, the team calculates that making adsorbents from 22,000 pounds of soybean hulls per day costs about 53 cents per pound. And, in trials with solutions containing cadmium, copper, lead, nickel and zinc, the modified hulls captured positively-charged ion forms of these metals at rates slightly above comparable commercial resins.

Peat Technologies, Inc., has a material transfer agreement with ARS to test the soybean hull adsorbents. ARS is USDA's chief scientific research agency.