Development of a continuously stirred flow cell for investigating sorption mass transfer

This paper introduces a new reversible-flow design for a continuously stirred reactor used to study sorption mass transfer in soil and solvent systems. The stirred reactor has potential advantages over conventional packed column or batch reactors because it isolates intraparticle sorption rate limitations from advective-dispersive transport, yet allows changes to flux through the reactor for analysis of sorption kinetics under dynamic conditions. Previously, stirred reactors have often failed due to clogging of sediment on the effluent frit. The reverse-flow backwashing design allows longer life and higher confidence in maintaining mixed conditions than previous designs. Mass transfer 'rate coefficients estimated from stirred and column experiments are compared; both techniques produced results consistent with a published correlation. The data also show that fitted sorption mass transfer coefficients can be strongly dependent on the choice of equilibrium partition coefficient (i.e. batch or first-moment derived values), and that the conventional two-site sorption kinetics model fails to accurately predict sorption mass transfer in the presence of changing solvent velocity through the reactor.     

A simplified approach to modelling underground migration of radionuclides from contaminated river sediments

Accidental releases of waste water from the first Czechoslovak nuclear power plant, A1, caused contamination of sediments of the Dudváh river, flowing into the Vah river, in Slovakia. Rather high concentrations of ¹³⁷Cs and ⁹⁰Sr (2150Bq dm⁻³ and 215Bq dm⁻³, respectively) were found in bottom sediments of a former channel of the re-engineered river body at a distance of about 250 m from a village, Siladice, with water-supply wells. In order to assess the possibility of contamination of the wells, underground migration of both radionuclides from the contaminated area was simulated using an original layered convection-diffusion model. K_d values determined in laboratory experiments were used. The analysis of the hydrological situation in the area reveals that the critical condition is a dominant horizontal groundwater flow near the water table in the direction from the Váh bank to Siladice, in the periods when the contaminated body lies under the water table. The simulation calculated under conservative conditions showed that the contamination of water-supply wells would not exceed permissible concentration limits.     

Intercomparison between TRIO-EF and IMPACT codes with reference to experimental strontium migration data

This work presents an intercomparison exercise between two geochemical migration codes, TRIO-EF (an object-oriented finite element code) and IMPACT (a chemical engineering code using mixing cells in series). The predictions of the two codes are compared with the reference experimental results obtained in a previous study of strontium transport in soil columns. This simulated geochemical system is well documented and includes ion exchange and dissolution-precipitation reactions. The solution transport is simulated by a one-dimensional advection-dispersion model. The predictions of TRIO-EF and IMPACT are both in good agreement with the experimental results. However, slight differences can be observed between the two codes, especially when concentration discontinuities are involved, such as precipitation fronts or changes in boundary conditions. These discrepancies between the two codes can mainly be attributed to the different discretisation approaches.     

Germination and seedling development of switchgrass and smooth bromegrass exposed to 2,4,6-trinitrotoluene

It is estimated that explosives contaminate approximately 0.82 million cubic metres of soil at former military installations throughout the US; major contaminants often include 2,4,6-trinitrotoluene (TNT) and its degradation products. At some sites, phytoremediation may be a viable option to incineration or other costly remediation treatments. Grasses may be particularly suited for remediation because of their growth habit and adaptability to a wide range of soil and climate conditions. We characterized the effects of TNT on germination and early seedling development of switchgrass and smooth bromegrass to evaluate their potential use on contaminated sites. Switchgrass and smooth bromegrass seeds were germinated in nutrient-free agar containing 0 to 60 mg TNT litre(-1). Smooth bromegrass germination decreased as TNT concentration increased, while switchgrass germination was unaffected by TNT. Concentrations up to 15 mg TNT litre(-1) did not affect switchgrass root growth rate, but bromegrass root growth was reduced at TNT concentrations above 7.5 mg litre(-1). At 7.5 mg TNT litre(-1), however, shoot growth rate was reduced in both species. Examination at 20-fold magnification revealed switchgrass radicles were unaffected by TNT, while smooth bromegrass radicles appeared slightly swollen. Results indicate switchgrass is more tolerant of TNT than smooth bromegrass, but the establishment of both species may be limited to soil containing less than 50 mg kg(-1) of extractable TNT.     

Identification of a multixenobiotic resistance mechanism in Xenopus laevis embryos

In the '90's a membrane-associated transport protein, discovered in aquatic organisms, was considered to be expressed in response to environmental xenobiotics. Like the multidrug resistance protein found in mammalian tumor cell lines, this protein confers resistance in organisms in polluted areas by binding xenobiotics and transporting them out of the cells in an energy-dependent manner.This study investigates the expression and the activity of a P-glycoprotein (Pgp) involved in a multixenobiotic resistance mechanism (MXRW) during the early developmental stages and in tissues of adult Xenopus laevis.     

Miscible fluid displacement stability in unconfined porous media:: Two-dimensional flow experiments and simulations

In situ flushing groundwater remediation technologies, such as cosolvent flushing, rely on the stability of the interface between the resident and displacing fluids for efficient removal of contaminants. Contrasts in density and viscosity between the resident and displacing fluids can adversely affect the stability of the displacement front. Petroleum engineers have developed techniques to describe these types of processes; however, their findings do not necessarily translate directly to aquifer remediation. The purpose of this laboratory study was to investigate how density and viscosity contrasts affected cosolvent displacements in unconfined porous media characterized by the presence of a capillary fringe. Two-dimensional flow laboratory experiments, which were partially scaled to a cosolvent flushing field experiment, were conducted to determine potential implications of flow instabilities in homogeneous sand packs. Numerical simulations were also conducted to investigate the differential impact of fluid property contrasts in unconfined and confined systems. The results from these experiments and simulations indicated that the presence of a capillary fringe was an important factor in the displacement efficiency. Buoyant forces can act to carry a lighter-than-water cosolvent preferentially into the capillary fringe during displacement of the resident groundwater. During subsequent water flooding, buoyancy forces can act to effectively trap the cosolvent in the capillary fringe, contributing to the inefficient removal of cosolvent from the aquifer.     

A laboratory exposure procedure for screening pulp and paper mill effluents for the potential of causing increased mixed function oxidase activity in fish

To better understand the relationships between pulp manufacturing processes and mixed function oxidase (MFO) enzyme induction in fish, a practical and standardized exposure procedure is required. This study was undertaken to develop a laboratory-based exposure procedure to quantify the relative MFO induction potencies of different types of pulp and paper mill effluents. One major consideration in developing the procedure was to ensure that the protocol was practical so that tests could be performed in a short time, with small volumes of effluents and using simple experimental conditions. A series of concentration-response and time-course experiments were conducted to find the minimum time and effluent concentration which could distinguish the ability of different effluents to cause significant MFO induction in rainbow trout in the laboratory. Experiments were also conducted to determine the effects of biotic and abiotic factors such as loading density, fish size and feeding regime. This study showed that the exposure of rainbow trout in the laboratory to 10% concentration of secondary-treated effluent for 96 h caused significant increases in hepatic MFO activity. The magnitude of MFO induction was comparable to other field and laboratory observations. While fish size, loading density and feeding regime were found to affect the test results, consistent responses within a laboratory using this protocol are possible, provided that these factors are standardized. Therefore, the short-term exposure approach described in this paper could be a relevant tool for assessing the ability of different types of pulp and paper mill effluents to cause MFO induction in fish.     

Linear least squares method for time series analysis with an application to a methane time series

A simple method of time series analysis, based upon linear least squares curve fitting, is developed. The method's advantages and disadvantages are discussed, and an example is presented using the Vostok Core methane record.     

Management options to limit nitrate leaching from grassland

Nitrate leaching can be reduced by the adoption of less intensive grassland systems which, though requiring a greater land area to achieve the same agricultural output, result in less nitrate leaching per unit of production than do intensively managed grasslands. The economic penalties associated with reductions in output can be partly offset by greater reliance on symbiotic nitrogen fixation and the use of clover-based swards in place of synthetic N fertilisers. Alternatively, specific measures can be adopted to improve the efficiency of nitrogen use in intensively managed systems in order to maintain high outputs but with reduced losses. Controls should take account of other forms of loss and flows of nitrogen between grassland and other components of the whole-farm system and, in most instances, should result in an overall reduction in nitrogen inputs. Removing stock from the fields earlier in the grazing season will reduce the accumulation of high concentrations of potentially leachable nitrate in the soil of grazed pastures but will increase the quantity of manure produced by housed animals and the need to recycle this effectively. Supplementing grass diets with low-nitrogen forages such as maize silage will reduce the quantity of nitrogen excreted by livestock but may increase the potential for nitrate leaching elsewhere on the farm if changes to cropping patterns involve more frequent cultivation of grassland. Improved utilisation by the sward of nitrogen in animal excreta and manures and released by mineralisation of soil organic matter will permit equivalent reductions to be made in fertiliser inputs, provided that adequate information is available about the supply of nitrogen from these non-fertiliser sources.