Persulfate-based ISCO for field-scale remediation of NAPL-contaminated soil: Column experiments and modeling.

An experimental and computational investigation of in situ chemical oxidation (ISCO) of weathered diesel fuel in soil columns was undertaken to validate a reactive-transport model capable of predicting contaminant mass reduction from a residual source zone. Reactivity tests with contaminated groundwater in batch reactors were used to estimate a priori the kinetic parameters of a phenomenological model of the oxidation of petroleum hydrocarbon (PHC) mixture fractions. The transport model, which incorporated groundwater flow, dissolution of main PHC fractions, and homogeneous reaction in the aqueous phase, was subsequently validated against experimental data of ISCO in soil columns using repetitive treatments with unactivated and alkaline-activated persulfate. No significant effect of the initial concentration of persulfate on the remediation performance was observed in the batch system, but alkaline activation significantly improved performance. The alkaline-activated persulfate treatment achieved ∼80% removal of the initial NAPL mass in soil columns. The combination of models and experiments described herein should enable the rational design of field-scale advanced oxidation strategies for the removal of weathered petroleum hydrocarbons. This expectation was supported by a comprehensive demonstration study at a historical site contaminated by weathered diesel fuel present as a residual source within the soil and dissolved within groundwater.

Modeling of adsorption dynamics at air-liquid interfaces using statistical rate theory (SRT).

A large number of natural and technological processes involve mass transfer at interfaces. Interfacial properties, e.g., adsorption, play a key role in such applications as wetting, foaming, coating, and stabilizing of liquid films. The mechanistic understanding of surface adsorption often assumes molecular diffusion in the bulk liquid and subsequent adsorption at the interface. Diffusion is well described by Fick's law, while adsorption kinetics is less understood and is commonly described using Langmuir-type empirical equations. In this study, a general theoretical model for adsorption kinetics/dynamics at the air-liquid interface is developed; in particular, a new kinetic equation based on the statistical rate theory (SRT) is derived. Similar to many reported kinetic equations, the new kinetic equation also involves a number of parameters, but all these parameters are theoretically obtainable. In the present model, the adsorption dynamics is governed by three dimensionless numbers: psi (ratio of adsorption thickness to diffusion length), lambda (ratio of square of the adsorption thickness to the ratio of adsorption to desorption rate constant), and Nk (ratio of the adsorption rate constant to the product of diffusion coefficient and bulk concentration). Numerical simulations for surface adsorption using the proposed model are carried out and verified. The difference in surface adsorption between the general and the diffusion controlled model is estimated and presented graphically as contours of deviation. Three different regions of adsorption dynamics are identified: diffusion controlled (deviation less than 10%), mixed diffusion and transfer controlled (deviation in the range of 10-90%), and transfer controlled (deviation more than 90%). These three different modes predominantly depend on the value of Nk. The corresponding ranges of Nk for the studied values of psi (10(-2)

Angstrom-to-millimeter characterization of sedimentary rock microstructure.

Backscatter SEM imaging and small-angle neutron scattering (SANS) data are combined within a statistical framework to quantify the microstructure of a porous solid in terms of a continuous pore-size distribution spanning over five orders of magnitude of length scale, from 10 A to 500 microm. The method is demonstrated on a sample of natural sandstone and the results are tested against mercury porosimetry (MP) and nuclear magnetic resonance (NMR) relaxation data. The rock microstructure is fractal (D=2.47) in the pore-size range 10 A-50 microm and Euclidean for larger length scales. The pore-size distribution is consistent with that determined by MP. The NMR data show a bimodal distribution of proton T(2) relaxation times, which is interpreted quantitatively using a model of relaxation in fractal pores. Pore-length scales derived from the NMR data are consistent with the geometrical parameters derived from both the SEM/SANS and MP data. The combined SANS/BSEM method furnishes new microstructural information that should facilitate the study of capillary phenomena in hydrocarbon reservoir rocks and other porous solids exhibiting broad pore-size distributions.

Stochastic reconstruction of particulate media from two-dimensional images.

In this contribution we address the problem of reconstructing particulate media from limited morphological information that may be readily extracted from 2D images of their microstructure. Sixty-five backscatter SEM images of the microstructure of a lightly consolidated pack of glass spheres are analyzed to determine morphological descriptors, such as the pore-pore autocorrelation function and pore and solid phase chord distributions. This information is then used to constrain the stochastic reconstruction of the glass sphere packing in two dimensions using a simulated annealing method. The results obtained demonstrate that the solid-phase chord distribution contains additional information that is critical for the reconstruction of the morphology of particulate media exhibiting short-range order. We further confirm this finding by successfully reconstructing the microstructure of a pack of irregular silica particles.

Magnetization evolution in network models of porous rock under conditions of drainage and imbibition.

Surface-enhanced relaxation of nuclear magnetization in fully and partially saturated water-wet porous media is studied using a pore network simulator. The simulator is based on a description of the pore space in terms of a regular cubic lattice of pores and throats following respective size distributions. The latter are obtained from geometric characterization of 3D stochastic replicas of real porous rock samples. Concepts of percolation theory are used to simulate the pore-scale distribution of a strongly wetting phase under conditions of quasistatic drainage by or imbibition against a nonwetting phase. The results of these simulations compare favorably with experimental mercury intrusion/retraction curves. The equations governing magnetization evolution in a connected pore system are then solved by matrix diagonalization for different values of wetting-phase saturation along the primary drainage and secondary imbibition paths. Analysis of the corresponding spectra of decay rates provides new insights regarding the influence of pore structure (pore and throat size distributions, spatial correlation), surface relaxation strength, and fluid distribution on diffusive coupling between pores. For pore and throat size distributions and surface relaxation strength representative of sandstones, diffusive coupling is quite important, especially under conditions of partial saturation. Remarkably, simulations show that correlated heterogeneity is the main reason pores appear poorly coupled with respect to NMR relaxation-an assumption underlying the correspondence between pore size and relaxation time distributions. Finally, for a given value of wetting-phase saturation, the history of saturation change (drainage or imbibition) is shown to have a profound effect on the spectrum of decay rates.