Research▼Watch Perchlorate impact quantified The levels of perchlorate found in U.S. drinking water are unlikely to affect human thyroid functioning, according to a group of scientists led by Monte Greer of Oregon Health & Science University. Their results show that effects are only produced at doses more than an order of magnitude higher than the 5–20 parts-per-billion (ppb) levels generally found in groundwater in the southwestern United States. Drinking water has been contaminated by ammonium perchlorate from solid rocket fuel production, and this low level of contamination has been associated with abnormal thyroid activity in Arizona newborns (Environ. Sci. Technol. 2000, 34, 374A). Perchlorate is known to impair thyroid function by interfering with the gland’s uptake of iodide. The thyroid gland regulates oxygen consumption and metabolism throughout the body, and if a pregnant woman’s thyroid function is depressed early in her pregnancy, her fetus’s mental and physical development may be harmed. Greer and his collaborators determined the dose of perchlorate in drinking water necessary to trigger an effect on iodide uptake by having 37 male and female adult human volunteers drink 400 milliliters of water laced with varying concentrations of pharmaceutical-grade potassium perchlorate for 14 days. Doses ranged from 7 to 500 ppb. The scientists measured each subject’s iodide uptake by having them ingest 123I, a radioactive isotope of iodine, before, during, and 15 days after they consumed the perchlorate. The subjects’ 123I levels were measured both later in the day that the tracer was administered and on the following day. Thyroid hormone levels were only depressed in the test subjects who drank water with 500 ppb of perchlorate during the study period, and all recovered within 15 days, the researchers found. They observed no differences based on sex or cumulative exposure.
On the basis of their observations, Greer and his peers calculated a noobservable-effect level (NOEL) of 0.007 milligrams per kilogram of body weight. From that, they determined that perchlorate concentrations ranging from 180 to 220 ppb, and possibly much higher, “should be of no health concern in iodine-sufficient populations.” They note that the U.S. population’s iodine intake has declined in the past 25 years, but they don’t believe that this has caused general iodine deficiency. (Environ. Health Perspect. 2002, 110, 927–937)
Renewable sources of hydrogen Hydrogen can be produced efficiently from renewable biomass products like sugars and alcohols with the help of a platinum-based catalyst, report James Dumesic and colleagues at the University of Wisconsin–Madison. Although such catalysts are currently too expensive for the process to be economically viable, the research demonstrates an important proof of concept and highlights future research needs for developing sustainable fuels. Hydrogen production typically involves the conversion of petroleum hydrocarbons at high temperatures using catalytic steam-reforming technology. Instead of fossil fuels and high temperatures, Dumesic and colleagues use an aqueous-phase reforming process to convert glucose or glycerol to hydrogen and gaseous alkanes at moderate temperatures near 500 K. The amount of hydrogen produced with the new process is dependent on the starting material, with glycerol providing better yields than glucose. The researchers believe that improvements in catalyst performance and new reactor designs that maximize the number of catalytically active sites could enhance the amount of hydrogen produced directly from reforming sugars. Although the search continues for a lower-cost catalyst that produces higher yields of hydrogen, the work provides a first
step toward generating hydrogen-rich fuels from renewable biomass materials. (Nature 2002, 418, 964–967)
Marine source of odd nitrogen Researchers in the United Kingdom have found that surface waters near the equator are a large natural source of alkyl nitrates. Because such compounds were previously thought to be exclusively of anthropogenic origin, the findings raise questions about the formation of nitrogen compounds in remote marine environments and could have important implications for the formation and destruction of tropospheric ozone. Alkyl nitrates are a significant component of the so-called odd nitrogen (NOy ) reservoir, a group of inorganic and organic nitrogen species formed in the atmosphere during the conversion of short-lived NOx. NOy plays an important role in regulating ozone levels because tropospheric ozone is produced photochemically in the presence of oxidized nitrogen species. In remote regions where NOx levels are low, alkyl nitrate levels are also expected to be low. But that is the opposite of what Adele Chuck and colleagues at the University of East Anglia in Norwich, England, found when they measured methyl and ethyl nitrates in air and water samples along two Atlantic Ocean transects. To their surprise, the researchers found that surface waters near the equator were supersaturated (up to 800%) with respect to both species, whereas waters in more temperate regions were closer to equilibrium. The results provide direct evidence for an oceanic source of alkyl nitrates in equatorial marine environments, but the mechanism of formation is still unclear. In some cases, high concentrations of alkyl nitrates coincided with the chlorophyll maximum, suggesting a biological source; however, photochemical production also appears to be important, particularly in coastal regions. (Science 2002, 297, 1151–1154)
NOVEMBER 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
■
407 A
