TECHNOLOGY UPDATE Heavy-metal sensors provide fast detection A new class of chemical sensors being developed at Sandia National Laboratories, Albuquerque, N.M., allows nearly instantaneous detection of heavy metals in solution in the parts-per-billion (ppb) range. The sensors are faster, smaller, less expensive, and more portable than existing heavy-metal sensors. After learning about their material's extreme heavy-metal-sensing sensitivity this past spring, scientists are refining the new biochemical technique in the laboratory. "The material can sense copper +2 down to the parts-per-billion level in less than a second," said Darryl Sasaki, who is leading the research effort. The heavy metal sensors available require a minutes-tohours time frame to measure concentrations at these levels. Because they are composed mainly of silicate materials, the sensors are inexpensive to build, and the optical sensors they require are also moderately priced. The biochemical sensing technique behind these sensors uses liposomes—microscopic, fluid-filled pouches formed by mixing lipids with water solutions—that are tailored to react with metal ions in solution. When copper ions are added to a solution containing the liposomes, the resulting liquid's color emission changes very rapidly and can be detected by a fluorescence spectrophotometer. The researchers believe that the fluorescence occurs because the liposomes alter their molecular arrangements to incorporate the metal ions. This characteristic response also occurs in the presence of other metals, including manganese, cobalt, calcium, and nickel. The extremely sensitive sensing capabilities of the liposomes were an unanticipated result of being combined with porous silica materials called sol-gels. Liposomes are chemically fragile and have short shelflives. Sasaki's team found that by entrapping the liposomes inside the
pores of the solid sol-gels, their sensitivity increased up to 50 times. The researchers are now tweaking the recognition sites on the liposome molecule so that it can detect heavy metals such as lead, mercury, and chromium. "The material basically is all the same. It's just changing a small molecule on top of the liposome," explained Sasaki. Thus far, they've managed to detect chromium +2 and +3 in the range of tens of parts per billion. The Sandia research team's goal is to build a sensor capable of detecting multiple metals. They recently began talks with a sensor company interested in licensing the technology. —KELLYN S. BETTS
Wastewater ion exchange with recycled farm products A U.S. Department of Agriculture (USDA) scientist is recycling agricultural byproducts to produce an inexpensive and efficient ion exchanger for treating wastewater. Jacob Lehrfeld of the USDA's Agricultural Research Service has chemically converted wastes, ranging from corn cobs to sugar-beet pulp, into resins capable of binding heavy metals and organic pesticides. "We can use corn bran, oat hulls, newspaper residues," said Lehrfeld, naming just a few of the substrates he has tested. "The key is that you add in phytic acid in a given ratio and heat them for a specific length of time." After having synthesized the ionexchanging material and proven its efficacy, Lehrfeld is doing selectivity studies to document the resin's ability to bind with heavy metals. He is also evaluating the material's success in removing organic materials, including polyaromatic hydrocarbons and phenolic materials, in the laboratory. With these data in hand, Lehrfeld is currently trying to lure waste management industrial cooperators with the low cost of using agricultural residues to create an ion-exchanging resin. "Phytic acid resin is a natural for
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cleaning wastewater in the chromium and copper plating industries," said Lehrfeld. "It binds nearly three times more heavy metal than a similar volume of the petroleum-based sulfonated styrene-divinylbenzene resin now widely used in wastewater treatment." —K.S.B.
Laser measures metals in off-gas emissions The Diagnostic Instrumentation and Analysis Laboratory at Mississippi State University (MSU) has demonstrated the use of a Continuum Surelite laser to measure the concentration of toxic metals in off-gas emissions. The scientists received approval to patent their laser-induced breakdown spectroscopy (LIBS) technique earlier this year, and they plan to test its ability to meet EPA's anticipated maximum achievable control technology requirements in September. The technique relies on a pulsed laser beam that focuses at a test point, producing a spark that generates a high-density plasma. The plasma excites the elements in the focal volume, and the resulting atomic emissions are collected with a collimating lens and the detection system. There, the intensity of the atomic emission lines is used to infer the concentration of atomic species. The system can detect six chemicals monitored in the parts-per-billion range: beryllium, chromium, cadmium, lead, arsenic, and mercury. "We can do the measurement online and in real time," said Jagdish P. Singh, a senior research scientist at MSU. He pointed out that the LIBS technique is both faster and more compact than the inductively coupled plasma (ICP) systems traditionally used to monitor off-gas emissions. The LIBS also offers the advantage of being able to help control the processes it is being used to monitor. At the Department of Energy's Savannah River site, the system was used to minimize toxic metal emissions during plasma torch waste remediation. —K.S.B.
VOL. 3 1 , NO. 9, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 3 9 9 A
