Phytoextraction of toxic metals: a review of biological mechanisms

Remediation of sites contaminated with toxic metals is particularly challenging. Unlike organic compounds, metals cannot be degraded, and the cleanup usually requires their removal. However, this energy-intensive approach can be prohibitively expensive. In addition, the metal removing process often employs stringent physicochemical agents which can dramatically inhibit soil fertility with subsequent negative impacts on the ecosystem. Phytoremediation has been proposed as a cost-effective, environmental-friendly alternative technology. A great deal of research indicates that plants have the genetic potential to remove many toxic metals from the soil. Despite this potential, phytoremediation is yet to become a commercially available technology. Progress in the field is hindered by a lack of understanding of complex interactions in the rhizosphere and plant-based mechanisms which allow metal translocation and accumulation in plants. In this paper, four research areas relevant to metal phytoextraction from contaminated soil are reviewed. The review concludes with an assessment of the current status of technology deployment and suggestions for future phytoremediation research.     

Vibrations in the orbwebs of Nuctenea sclopetaria (Araneidae)

Summary Orbweaving spiders obtain much information from web-borne vibrations. One of their major problems is to distinguish relevant vibrations, particularly prey-produced signals, from irrelevant vibrations, particularly wind noise. This problem was studied by measuring the spectral characteristics of prey-and wind-induced web vibrations, and the spider's responsiveness to sinusoidal stimuli. In wind-generated vibration (Fig. 1) signal amplitude is large below 10 Hz and decreases by 30 to 40 dB/decade above the lowest resonance frequency for each type of web vibration. The lowest resonance for transverse vibration is 3 Hz with a 200 mg spider, and 10 Hz for longitudinal and lateral vibration. Below 10 Hz transverse vibration is much larger than the other two types, but this difference decreases at higher frequencies. Most of the amplitude in prey signals is below 50 Hz (Fig. 2), except for buzzing insects such as flies, bees and mosquitoes, whose high wing beats generate significant motion above 100 Hz. The spider's response threshold for sinusoidal stimuli in the prey-catching region of the web (Fig. 3) declines at 20 dB/decade over the range that could be measured (10 to 3,200 Hz for longitudinal, 18 to 560 Hz for transverse and 18 to 320 Hz for lateral vibration). The threshold curve for longitudinal vibration is 10 to 20 dB lower than for the other two types and has a minimum of 7 m RMS near 300 Hz. This difference appears to have a mechanical rather than a sensory basis, due to the better transmission of longitudinal vibration through the web (Fig. 4A). If the effect of the spider on web vibration is also considered, the resulting adjusted threshold curve (Fig. 4B: data available for longitudinal vibration only), shows a continuous decline of approximately 30 dB/decade. A possible mechanism for recognizing prey signals based on spectral information is discussed.     

Monitoring individual exposure. Measurements of volatile organic compounds in breathing-zone air, drinking water, and exhaled breath

Methods for determining individual exposure to volatile organic compounds (VOC) during normal daily activities were field tested on university student volunteers in Texas and North Carolina. The equipment tested included a personal monitor employing Tenax GC® to collect organic vapors for later analysis by GC-MS, and a specially designed spirometer for collecting samples of expired human breath on duplicate Tenax cartridges. The personal monitor and spirometer proved feasible for collecting abundant quantitative data on most of the 15 target organics. Air exposures to many VOC varied widely, sometimes over three orders of magnitude, among students on the same campus who had been monitored over the same time period and day. A log-linear relationship between breathing-zone air exposures and concentrations in exhaled breath was suggested for three chemicals: tetrachloroethylene, 1,1,1-trichloroethane, and vinylidene chloride. Air was the main route of exposure for all target compounds except the two trihalomethanes (chloroform and bromodichloromethane), which were transmitted mainly through water. Estimated total daily intake through air and water of the target organics ranged from 0.3 to 12.6 mg, with 1,1,1-trichloroethane at the highest concentrations in both geographic areas.     

Sources and fate of nitrosodimethylamine and its precursors in municipal wastewater treatment plants.

To assess the occurrence and fate of nitrosodimethylamine (NDMA) and its precursors in wastewater treatment plants, samples from wastewater treatment plants and industrial sources were analyzed for NDMA, total NDMA precursors, and dimethylamine (DMA). The median concentration of NDMA in untreated wastewater was approximately 80 ng/L, with maximum concentrations up to 790 ng/L presumably occuring because of sources unrelated to domestic wastewater. Concentrations of DMA in untreated wastewater ranged from approximately 50 to 120 microg/L and accounted for a majority of the NDMA precursors. The removal of NDMA during secondary biological treatment exhibited considerable variability, with overall removal ranging from 0 to 75%. In contrast, removal of NDMA precursors and DMA generally exceeded 70%. The median concentration of NDMA in secondary effluent before disinfection was 46 ng/L. Although DMA was removed during secondary treatment, other NDMA precursors in wastewater effluent will result in formation of additional NDMA upon disinfection with chloramines.     

Hazardous Substance Research Centers program: An incubator for remediation research

The Hazardous Substance Research Center (HSRC) was established by the U.S. Environmental Protection Agency (EPA) to assist in the implementation of Superfund and to address major hazardous substance environmental problems at a regional level. Over the past 12 years, the HSRC program has produced more than 1,200 peer‐reviewed technical articles, 27 patents and licenses, 21 new technologies for the remediation marketplace, and provided technical assistance to more than 300 communities. Research, technology transfer, and training are conducted by five regional multi‐university centers, which focus on different aspects of hazardous substance management. Areas of focus include urban environments, contaminated sediments, natural remediation and restoration technologies, abandoned mine lands, and chlorinated solvents in groundwater. This article provides an overview of the five HSRC programs including current areas of research, field studies, and technology transfer Internet links to access research results and remediation technology information. © 2003 Wiley Periodicals, Inc.     

Uptake of cesium-137 and strontium-90 from contaminated soil by three plant species; application to phytoremediation

A field test was conducted to determine the ability of three plant species to extract 137Cs and 90Sr from contaminated soil. Redroot pigweed (Amaranthus retroflexus L.), Indian mustard [Brassica juncea (L.) Czern.], and tepary bean (Phaseolus acutifolius A. Gray) were planted in a series of spatially randomized cells in soil that was contaminated in the 1950s and 1960s. We examined the potential for phytoextraction of 90Sr and 137Cs by these three species. Concentration ratios (CR) for 137Cs for redroot pigweed, Indian mustard, and tepary bean were 2.58, 0.46, and 0.17, respectively. For 90Sr they were substantially higher: 6.5, 8.2, and 15.2, respectively. The greatest accumulation of both radionuclides was obtained with redroot pigweed, even though its CR for 90Sr was the lowest, because of its relatively large biomass. There was a linear relationship between the 137Cs concentration in plants and its concentration in soil only for redroot pigweed. Uptake of 90Sr exhibits no relationship to 90Sr concentrations in the soil. Estimates of time required for removal of 50% of the two contaminants, assuming two crops of redroot pigweed per year, are 7 yr for 90Sr and 18 yr for 137Cs.     

New Takes on Emerging Contaminants: Preface

<正>Since the 1990s,there has been a strong interest across the environmental science community in characterizing the threats to human health posed by emerging contaminants.Much of this research has focused on pharmaceuticals and personal care products in Europe and North America.However,the range of chemicals evaluated has expanded dramatically over the past three decades.This expansion raises a key