Modelling the long-term performance of zero-valent iron using a spatio-temporal approach for iron aging

Zero-valent iron (ZVI) permeable reactive barriers (PRBs) have become popular for the degradation of chlorinated ethenes (CEs) in groundwater. However, a knowledge gap exists pertaining to the longevity of ZVI. The present investigation addresses this situation by suggesting a numerical simulation model that is intended to be used in conjunction with field or column tests in order to describe long-term ZVI performance at individual sites. As ZVI aging processes are not yet completely understood and are still subject to research, we propose a phenomenological modelling technique instead of a common process-based approach. We describe ZVI aging by parameters that characterise the extent and rate of ZVI reactivity change depending on the propagation of the precipitation front through ZVI. We approximate degradation of CEs by pseudo-first order kinetics accounting for the formation of partially dechlorinated products, and describe ZVI reactivity change by scaling the degradation rate constants. Three independent modelling studies were carried out to test the suitability of the conceptual and numerical model to describe the observations of accelerated column tests. All three tests indicated that ZVI reactivity declined with an increasing number of exchanged pore volumes. Measured and modelled concentrations showed good agreement, thereby proving that resolving spatial as well as temporal changes in ZVI reactivity is reasonable.     

Diet-mediated effects of heavy metal pollution on growth and immune response in the geometrid moth Epirrita autumnata

The potential capacity of larval growth and immune response traits of the autumnal moth to adapt to heavy metal polluted environment was tested experimentally. Both the relative growth rate (RGR) and pupal weight were significantly higher in control trees than on polluted trees, indicating that metal pollution prevented the insect from achieving maximal growth on birch leaves. Larval growth rates of different broods differed significantly between metal contaminated and control birches. However, pupal weight of broods, which is considered more important for fitness than growth rate, in response to pollution did not differ. Immune response was significantly higher in moths exposed to pollution than in moths that were exposed to control environment suggesting that pollution enhances the immune defense of defoliators. Encapsulation rate tended to differ between broods indicating that the immune function has potential to respond to selection.     

Seasonal differences and within-canopy variations of antioxidants in mature spruce (Picea abies) trees under elevated ozone in a free-air exposure system

The effect of free-air ozone fumigation and crown position on antioxidants were determined in old-growth spruce (Picea abies) trees in the seasonal course of two consecutive years (2003 and 2004). Levels of total ascorbate and its redox state in the apoplastic washing fluid (AWF) were increased under double ambient ozone concentrations (2xO3), whilst ascorbate concentrations in needle extracts were unchanged. Concentrations of apoplastic and symplastic ascorbate were significantly higher in 2003 compared to 2004 indicating a combined effect of the drought conditions in 2003 with enhanced ozone exposure. Elevated ozone had only weak effects on total glutathione levels in needle extracts, phloem exudates and xylem saps. Total and oxidised glutathione concentrations were higher in 2004 compared to 2003 and seemed to be more affected by enhanced ozone influx in the more humid year 2004 compared to the combined effect of elevated ozone and drought in 2003 as observed for ascorbate.     

Susceptibility to acidic precipitation contributes to the decline of the terricolous lichens Cetraria aculeata and Cetraria islandica in central Europe

The effective quantum yield of photochemical energy conversion in photosystem II (Phi2) was shown to be reduced in the terricolous lichens Cetraria aculeata and Cetraria islandica by short-term exposure to aqueous SO2 at pH values occurring in the precipitation of areas with high SO2 pollution. Significant reduction of Phi2 was found at pH相似文献   

Selenium oxyanion inhibition of hydrogenotrophic and acetoclastic methanogenesis

Inhibitory effects of selenite and selenate towards hydrogenotrophic and acetoclastic methanogenesis were evaluated in anaerobic toxicity assays. The 50% inhibitory concentration (IC(50)) of both selenium oxyanions was below 6.1x10(-5)M in hydrogenotrophic assays, whereas acetoclastic methanogens were less inhibited: IC(50)=8.3x10(-5)M and 5.5x10(-4)M for selenite and selenate, respectively. Selenite completely inhibits methanogenesis from both substrates tested at concentrations 10(-3)M selenite, while only marginal methanogenic activities occur at equimolar concentrations of selenate. Selenite becomes irreversibly inhibitory upon a single exposure, whereas selenate inhibits methanogens upon repeated exposure. Consequently, methane recovery can be seriously hampered or even impossible during anaerobic treatment of highly selenium contaminated waste streams.     

Effective rates of heavy metal release from alkaline wastes--quantified by column outflow experiments and inverse simulations

Column outflow experiments operated at steady state flow conditions do not allow the identification of rate limited release processes. This requires an alternative experimental methodology. In this study, the aim was to apply such a methodology in order to identify and quantify effective release rates of heavy metals from granular wastes. Column experiments were conducted with demolition waste and municipal waste incineration (MSWI) bottom ash using different flow velocities and multiple flow interruptions. The effluent was analyzed for heavy metals, DOC, electrical conductivity and pH. The breakthrough-curves were inversely modeled with a numerical code based on the advection–dispersion equation with first order mass-transfer and nonlinear interaction terms. Chromium, Copper, Nickel and Arsenic are usually released under non-equilibrium conditions. DOC might play a role as carrier for those trace metals. By inverse simulations, generally good model fits are derived. Although some parameters are correlated and some model deficiencies can be revealed, we are able to deduce physically reasonable release-mass-transfer time scales. Applying forward simulations, the parameter space with equifinal parameter sets was delineated. The results demonstrate that the presented experimental design is capable of identifying and quantifying non-equilibrium conditions. They show also that the possibility of rate limited release must not be neglected in release and transport studies involving inorganic contaminants.     

Modelling heavy metal fluxes from traffic into the environment

A new method is presented which allows emissions of traffic into the environment to be described as a function of road distance. The method distinguishes different types of emissions (runoff, spray and drift), which are determined by measurements and mass balances of a specified road section. The measurement of two-dimensional pollutant concentrations in the road shoulder is an important part of the method. In a case study performed at Burgdorf, Switzerland, the method was applied to the determination of the spatial distribution of heavy metal emissions. The results show that between 36 and 65% of the heavy metals Cd, Cr, Cu, Pb and Zn are present in runoff and spray and between 35 and 64% are dispersed diffusely in the environment (defined as drift). The runoff infiltrates into the vegetated road shoulder up to a distance of approx. 1 m from the road. The distribution of spray shows a maximum at 1 m and decreases steadily up to a distance of 5 m. This information can serve as a basis for the quantitative evaluation of road-runoff treatment scenarios. Although the results of the Burgdorf study are case-specific, several general guidelines for the reduction of traffic-related emissions can be derived from it.     

Robust trend tests with application to toxicology

In most real data situations in the one-way design both the underlying distribution and the shape of the dose-response curve are a priori unknown. The power of a trend test strongly depends on both. However, tests which are routinely used to analyze toxicological assays must be robust. We use nonparametric tests with different scores—powerful for different distributions—and different contrasts—powerful for different shapes—and use the maximum of all test statistics as a new test statistic. Simulation results indicate that this maximum test, which is a nonparametric multiple contrast test, stabilizes the power for various shapes and distributions. The investigated tests are applied to the data of a toxicological assay.     

Regio- and stereoselective isomerization of hexabromocyclododecanes (HBCDs): kinetics and mechanism of beta-HBCD racemization

Hexabromocyclododecanes (HBCDs) are high production volume chemicals currently produced in quantities exceeding 20000ty(-1). They are used as flame retardants for plastics and textiles. HBCDs are thermally labile compounds, rapidly decomposing at temperatures above 250 degrees C to form bromine radicals, which scavenge other radicals formed during pyrolysis. But certain HBCD stereoisomers must reach the environment without decomposition, because their levels in soils, sediments, and biota are increasing worldwide. The fate of individual HBCD stereoisomers during production, product use, disposal, and transformation in the environment remains unclear. Herein we report on the thermally induced, highly selective isomerization of (+) and (-)beta-HBCD. Regio- and stereoselective migration of only two of the six bromine atoms resulted in the racemization of both beta-HBCDs. First order rate constants (k(rac)) increased from 0.005, 0.011, 0.021, to 0.055min(-1) at 130, 140, 150, and 160 degrees C, corresponding to half life times tau(1/2) of 143, 63, 29, and 14min, respectively. From the deduced kinetic model, we conclude that any thermal treatment of enantiomerically enriched beta-HBCDs in the range of 100-160 degrees C will result in a loss of most optical activity within few hours. The simultaneous inversion of two asymmetric centers occurred with perfect stereocontrol. Selectively, vicinal dibromides with the RR- and the SS-configurations migrated at these temperatures. An intramolecular reaction mechanism with a four-center transition state is postulated, based on the obtained stereoisomer pattern and the observed reaction kinetics. Crystal structure analysis revealed that all vicinal dibromides in beta-HBCDs prefer synclinal (gauche) conformations. However, an antiperiplanar (staggered) conformation is assumed to facilitate the concerted 1.2-shifts of both bromine atoms, resulting in an inversion of both neighboring carbon atoms. First experiments with other HBCD stereoisomers suggest that the presented isomerization mechanism is of relevance for those stereoisomers as well.     

Harmonized assessment of nutrient pollution from urban systems including losses from sewer exfiltration: a case study in Germany

Environmental Science and Pollution Research - A growing literature indicates that untreated wastewater from leaky sewers stands among major sources of pollution to water resources of urban...