such as incineration, bioremediation, chemical treatment, and photolysis all have significant disadvantages, not the least of which is cost. R. J. Hilarides and colleagues evaluated gamma irradiation used to degrade dioxin in soil. They used 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as a model compound, applied it to soil, and exposed the soil to a dose of 800 kGy. Results indicated about 99% degradation. Additionally, a dose of 450 kGy reduced TCDD in a native soil by nearly 75%. The results demonstrated gamma irradiation as a potential alternative for degrading dioxins in soil. {Water Environ. Res. 1996, 68, 178-87)
Cr(VI) removal B. T. Nivas and co-workers developed a method to remediate chromate-contaminated subsurface environments. Cr(VI) is toxic and mutagenic, and in the subsurface, its highly mobile anionic forms pose significant risk of contaminating groundwater. The researchers achieved optimal removal of Cr(VI) from contaminated soil samples by using a combined micellar surfactant-complexing agent system. The extent of chromate elution by this system was 12 times that of water The authors determined that ion exchange is the dominant mechanism for Cr(VI) removal by surfactants alone Several other surfactants were also tested but they were more susceptible topreciDitation and sorption {Water Res 1996 30 511-20)
Modeling the behavior of humic substances M. F. Benedetti and co-workers used a Donnan model to consider electrostatic interactions and account for the effect of salt concentrations on the charge of humic acid molecules. They applied the model to interpret proton binding by humic and fulvic acids. In this approach, the volume of the Donnan phase was assumed to change with salt level, not charge. The authors found that the Donnan model was physically realistic for the swelling gel-like structure of humic acids but not for fulvic acids which have smaller molecules with limited swelling and shrinkage capabilities Results suggested that humic acid volumes may be more dependent on ionic strength than on pH The authors propospri that t h k information could promote understanding of cirganic pollutant infpractinnQ in natural
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Infants' DDT intakes Elevated levels of DDT and its degradation products DDE and DDD have been found in the sediments and fish of Washington's Yakima River. K. Marien and D. Laflamme examined the health implications of consuming DDT-contaminated fish from the river. They developed a tolerable daily intake for DDT and DDE for nursing infants based on epidemiological and toxicological literature and compared it with the infants' estimated daily intake level of DDT and DDE They estimated infant daily intake levels between 2 x 10~2 mg/kg-day and 2 5 x 10"2 mg/kg-dav which exceeded die tolerable daily intake level of 5 x 10"3 mg/kg-day for
f i n sri
SEM-to-AVS ratio of 1 0 did not provide additional information from the sediment sairmles The authors observed variability in the SEM-to-AVS ratio from sediments sampled at different locations and at different depths of the same location {Environ Toxicol Chem 1996 15 286-93)
,hic
SOILS these compounds. The results suggested that eating subsistence quantities of contaminated fish may expose newborn infants to unsafe levels of DDT and DDE. {Risifc Anal. 1995, 15, 709-17)
SEDIMENTS Metals extraction
RISK
Metal bioaccumulation J. M. Besser and colleagues studied the spatial distribution of acidvolatile sulfides (AVSs) and the simultaneously extracted metals (SEMs) copper and zinc in fresh water sediments from a river previously affected by mining. The growth and bioaccumulation of metals by midge larvae {Chironomus teutons) exposed to different sediment deptiis and metal concentrations indicated that the bioaccumulation of copper and zinc was associated with sediment SEM-to-AVS ratios. However, die prediction of heavy metal toxicity based
A new method of determining the bioavailability and toxicity of metals in anoxic sediments uses the ratio of acid-volatile sulfide (AVS) to simultaneously extracted metals (SEMs). W. G. Brumbaugh and I. W. Arms studied the method used to extract AVS and SEM to determine whether slight changes would affect the results. Slightly increasing the acid extractant resulted in increased extraction of low concentrations of AVS from the sediment, but this did not occur at high AVS concentrations. Although most SEMs were not affected by acid strength, copper concentrations increased with increased extractant acidity when sediments contained high concentrations of AVS In general increasing extractant acidity decreased the SEM-to-AVS ratio at low concentrations of AVS (Environ Toxicol Chem 1996 ,1 ,82-85)
2 3 4 A • VOL. 30, NO. 6, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
Inhibiting landfill methane Methane emission from landfills can be mitigated through oxidation by methanotrophs in the landfill cover soil. E Boeckx and O. Van Cleemput studied the effects of soil moisture content, temperature, and nitrogen nutrient sources on the efficiency of methane oxidation in landfill cover soil. They found optimal soil moisture content between 10 and 20%; methanotrophic activity was inhibited below this and methane diffusion in the soil was decreased above this range. Optimal temperatures were between 25 and 30 °C. Methane oxidation was inhibited by the addition of ammonium nitrogen or organic residues widi high carbon-to-nitrogen ratios because the ammonium competed with methane for active enzyme sites No inhibition occurred with the addition of nitrate nitrogen or organic residues with low carbon-to-nitroeen ratios IT Fnviron dual 1
