Franklin W. Schwartz

Sorption of zn2+ and cd2+ on hydroxyapatite surfaces.

This study examines the mechanisms and kinetics of Zn 2+ and Cd 2+ sorption onto hydroxyapatite surfaces. The concentrations of Zn 2+ and Cd 2+ in the study rage from 0 to 2.5 mmol/L. Although the sorption data follow Langmuir isotherms, a detailed examination reveals that surface complexation and coprecipitation are the most important mechanisms with possibly ion exchange and solid diffusion also contributing to the overall sorption process. pH-controlled experiments point to significant deprotonation of hydroxyapatite surface and sorption by metal complexation with surface functional groups such as ≡POH. The major metal surface species are likely to be ≡POZn + and ≡POCd + .

Lead immobilization by hydroxyapatite in aqueous solutions

This study examines the possibilities for removing lead from water by reactions involving synthetic hydroxyapatite. batch experiments show that the rate of Pb2+ removal in these reactions is kinetically quite rapid. For example, ∼ 100 mg L−1 Pb2+ in the starting solution are reduced from the solutions to < 0.5 μg L−1 within several minutes. The reaction mechanisms are dominated by the dissolution of hydroxyapatite and the precipitation of lead apatites, namely hydroxypyromorphite and chloropyromorphite, both of which are very insoluble over the pH range of environmental concern. The results show that hydroxyapatite is capable of dissolving sufficient phosphate ions to promote lead precipitation and thus immobilized the lead from aqueous solutions. Further, evidence from the solid analysis shows how the morphology of the precipitates affects the reaction kinetics as well as the capacity of apatite to remove aqueous lead. For example, in the presence of chloride in the Pb2+-hydroxyapatite system. the formation of chloropyromorphite on the surface of hydroxyapatite provides an effective coating, which inhibits hydroxyapatite dissolution. While the potential for controlling lead exists, more work needs to be done to characterize the reactions fully. This study confirms the possibilities for using mineral apatite or natural phosphate rocks to treat lead-contaminated waste water or groundwater.

On the generation of instabilities in variable density flow

Interfacial or fingering instabilities have been studied recently in relation to contamination problems where a more dense plume is enclosed by and is moving along in a body of less dense fluid. Instabilities can play an important role in the mixing or dispersion process. Through the use of a variable density flow and transport code, we were able to study how the style of interfacial perturbation controls the pattern of instability development. Whether initial perturbations grow or decay depends mainly on the wavelength of the perturbing function. A critical perturbation wavelength must be exceeded for a perturbation to grow; otherwise the perturbation simply decays. Our work confirms earlier analyses that suggest that all stratified systems are inherently unstable, given some spectrum of the perturbing waves that exceed the critical wavelength. By implication, Rayleigh number stability criteria are inappropriate for evaluating the dense plume problem. Our study also demonstrates how numerical errors in a mass transport code can serve as a perturbing function and lead to the development of instabilities. However, these instabilities are not physically realistic and are essentially uncontrollable because their character depends on the extent to which numerical errors develop, as evidenced by the grid Peclet and Courant numbers.

Simulating the river-basin response to atmospheric forcing by linking a mesoscale meteorological model and hydrologic model system

Abstract The purpose of this article is to test the ability of a distributed meteorological/hydrologic model to simulate the hydrologic response to three single-storm events passing over the Upper West Branch of the Susquehanna River Basin. The high-resolution precipitation fields for three storms are provided by observations and by the Penn State–NCAR Mesoscale Meteorological Model (MM5) with three nested domains. The MM5 simulation successfully captures the storm patterns over the study area, although some temporal and spatial discrepancies exist between observed and simulated precipitation fields. Observed and simulated precipitation data for those storms are used to drive the Hydrologic Model System (HMS). The output from HMS is compared to the measured hydrographic streamflow at the outlet of the Upper West Branch. The Curve Number and Green-Ampt methods of rainfall-runoff partitioning are used in HMS and evaluated for streamflow simulation. The results of the hydrologic simulation compare well with observed data when using the Curve Number partitioning, but underestimate observed data when using the Green-Ampt. The likely cause is the lack of heterogeneity in hydraulic parameters. The simulated streamflow with the MM5-simulated precipitation is lower than the simulated streamflow with observed precipitation. The experiments suggest that the subgrid-scale spatial variability in precipitation and hydraulic parameters should be included in future model development

Instabilities in variable density flows: Stability and sensitivity analyses for homogeneous and heterogeneous media

This study improves our understanding of instability phenomena that may accompany the transport of dense plumes of dissolved contaminants. One major objective is to test how well analytic stability theory developed by List [1965] applies to the transport of dense plumes in both homogeneous and heterogeneous media. The data to test the prediction come from numerical model experiments in which instability growth is generated by perturbing the interface between fluids of differing density. Stability criteria, as determined by the transverse Rayleigh number, the ratio of transverse to longitudinal Rayleigh numbers, and the nondimensional wave number, compare very well with results observed in the numerical experiments for isotropic media. Comparisons involving correlated random fields were much less successful because plume stability is determined on a local basis as a function of the changing permeability field. Instabilities tend to dissipate in zones of lower permeability and grow in zones of higher permeability. Another objective of the study is to determine the factors that contribute to stability and instability in homogeneous and heterogeneous systems. Sensitivity analyses using a transport model within the framework of Lists stability theory show that stability is promoted by low medium permeability, small density differences, and significant dispersion. In heterogeneous media, stability is promoted by increased correlation length scales and increased log permeability variance. Furthermore, the simulations illustrate the intimate relationship that exists between instability growth and decay and the heterogeneous nature of the permeability field. Thus stability criteria that do not incorporate characteristics of the permeability field will not be suitable for natural or field-scale porous media.

Density-dependent solute transport in discretely-fractured geologic media : is prediction possible?

Abstract The development of a dense solute plume in a fractured geologic medium can be highly irregular due to both the complexity of the fracture network as well as the presence of convection cells that may arise as a result of the density contrast between the invading solute and the ambient groundwater. A two-dimensional numerical model has been developed here to investigate density-dependent groundwater flow and solute transport in geologic materials that contain discrete fractures in order to examine some of the complex forms into which plumes can evolve, particularly with regard to fracture–matrix interactions. Results from simulations which involve parallel vertical fractures show that the evolution of the solute plume is affected by the development of convection cells in the porous matrix blocks between the vertical fractures. In a geologic medium containing a network of regularly spaced horizontal and vertical fractures, complex migration pathways can develop that are unexpected even though the geometry and interconnectivity of the fractures are known a priori. Downward solute migration can occur in some vertical fractures, while upward migration of less dense fluid can occur in others with transient circulation patterns developing in the intervening porous matrix. Because of the inherent uncertainty associated with fracture delineation, and because of the irregular nature of unstable dense plumes, deterministic prediction of dense-plume migration pathways and travel times in fractured geologic media will be subject to considerable uncertainty.

On the application of image analysis to determine concentration distributions in laboratory experiments

Abstract Image analysis approaches provide an accurate and efficient way to generate detailed concentration distributions from photographic data of flow-tank experiments where the tracer can be observed through a glass or Plexiglas® wall. The technique is non-intrusive, does not disturb plume dynamics, and provides an essentially continuous distribution of sampling points over the whole plume, as observed at the tank wall. Computer processing of the scanned image is required to correct for spatial and temporal lighting nonuniformity, and for heterogeneous media, to account for different grain sizes, and thus differing dye thicknesses along tank walls. In our model study, utilizing a Plexiglas® flow tank (107 cm long, 71 cm high, 5 cm wide), individual concentrations are estimated for square areas approximately 1.3 by 1.3 mm in size over the entire area of a side wall of the tank. The negatives were scanned on an Eikonix® 78/99 digital scanning system at a resolution of 1024 by 1024 by 12 bits. The image analysis system is comprised of a Digital Equipment Corporation Microvax II® computer and a Gould® IP 9527 image processor. It transforms the scanned images, whose pixels represent dye intensity, into images whose pixels represent solute concentration.

Multispecies Contaminant Plumes in Variable Density Flow Systems

A finite element flow and transport code, highly optimized for a Cray Y-MP, is used to investigate the complex character of multispecies contaminant plumes that can develop in variable density flow systems. The reference data set for the simulation is based on a study of dense leachate plumes developed at Babylon, New York. A shallow unconfined aquifer receives leachates generated by annual recharge throughout the unlined landfill. The reference simulation generally reproduces the distribution of a density-determining species in the leachate for both intermittent and continuous sources. A sensitivity analysis illustrates how the initial source concentrations for both a density-determining species and a trace organic compound, the style of loading, and the reactive character of the organic compound can profoundly influence concentration distributions. For relatively large density contrasts between the leachate and the ambient groundwater, pockets of denser water sink to or near the bottom of the aquifer. A trace organic compound that is chromatographically separating from a dense plume due to sorption may follow different pathways as compared to the dense species.

DNAPL remediation with in situ chemical oxidation using potassium permanganate. Part I. Mineralogy of Mn oxide and its dissolution in organic acids.

Previous studies on in situ chemical oxidation of trichloroethylene (TCE) with potassium permanganate indicated that the solid reaction product, Mn oxide, could reduce the permeability of the porous medium and impact the success of dense non-aqueous phase liquid (DNAPL) removal. In order to address the issue of permeability reduction caused by precipitation, this study investigated the mineralogy of Mn oxides and the possibilities of removing the solid precipitates by dissolution. The solid reaction product from the oxidation of TCE by permanganate is semi-amorphous potassium-rich birnessite, which has a layered mineral structure with an interlayer spacing of 7.3 A. The chemical formula is K(0.854)Mn(1.786)O(4).1.55H(2)O. It has a relatively small specific surface area at 23.6+/-0.82 m(2)/g. Its point of zero charge (pzc) was measured as 3.7+/-0.4. This birnessite is a relatively active species and could participate in various reactions with existing organic and inorganic matter. The dissolution kinetics of Mn oxide was evaluated in batch experiments using solutions of citric acid, oxalic acid, and ethylenediaminetetraacetic acid (EDTA). Initial dissolution rates were determined to be 0.126 mM/m(2)/h for citric acid, 1.35 mM/m(2)/h for oxalic acid, and 5.176 mM/m(2)/h for EDTA. These rates compare with 0.0025 mM/m(2)/h for nitric acid at pH=2. Organic acids dissolve Mn oxide quickly. Reaction rates increase with acid concentration, as tested with citric acid. The dissolution mechanism likely involves proton and ligand-promoted dissolution and reductive dissolution. Citric and oxalic acid can induce ligand-promoted dissolution, while EDTA can induce ligand-promoted and reductive dissolutions. At low pH, proton-promoted dissolution seems to occur with all the acids tested, but this process is not dominant. Reductive dissolution appears to be the most effective process in dissolving the solid, followed by ligand-promoted dissolution. These experiments indicate the significant potential in using these organic acids to remove precipitates formed during the oxidation reaction.

Simulating the In Situ Oxidative Treatment of Chlorinated Ethylenes by Potassium Permanganate

Several laboratory and field studies have demonstrated the potential viability of oxidation schemes using MnO4− for the in situ treatment of source areas, which are contaminated by chlorinated ethylenes (PCE, TCE, and DCE). Chemically, the chlorinated ethylenes are oxidized to CO2, Cl−, and MnO2. The goal of this study was to develop a theoretical framework for the chemical and physical processes involved. To this end, a computer model was created to simulate the coupled processes of nonaqueous phase liquid (NAPL) dissolution, chemical reactions, and solute mass transport in the in situ chemical oxidization scheme. The model incorporates a kinetic description of reactions between the MnO4− and the chlorinated ethylenes and the rate of dissolution of the NAPL. A Strang operator-splitting method, which coupled the different physical and chemical processes and an exponentially expressed solution of the kinetic equations, led to a significant speed up in the solution process. The products were calculated based on the stoichiometry of the reaction. We demonstrated the capabilities of this code using already published results of column, test cell, and field experiments. Generally, the simulated results matched well with experimental measurements. The computer model provides a useful tool for assisting in the design and the prediction of the oxidization processes under field conditions.