Mark L. Brusseau

Sorption nonideality during organic contaminant transport in porous media

In modeling subsurface contaminant transport, sorption is often simplified by assuming instantaneous equilibrium, isotherm linearity, and sorption‐desorption singularity. Data exhibiting behavior that deviates from that predicted by this simple model have been reported, challenging the validity of these assumptions. This nonideal sorptive behavior has been attributed to several different factors, including kinetic sorption reactions, diffusive mass transfer resistances, isotherm nonlinearity, and sorption‐desorption nonsingularity. These factors are examined and their relative impact on contaminant transport is evaluated. For hydrophobic organic compounds, physical nonequilibrium (i.e., rate‐limited mass‐transfer in aggregated or layered systems) and intraorganic matter diffusion (rate‐limited diffusion within the sorbent organic matter matrix) are probably the predominant factors causing nonideality. The relative importance of these factors is scale‐dependent. For smaller scale systems, mass‐transfer lim...

Modeling solute transport in structured soils: a review

Abstract Solute transport in structured soils is and has been the focus of a significant research effort. Transport in such systems is often characterized by nonideal phenomena. These phenomena have usually been ascribed to the presence of immobile domains within the porous medium, which results in physical nonequilibrium. The phenomena can also, in some cases, be attributed to rate-limited sorption (i.e., sorption nonequilibrium). These processes are often modeled using a bicontinuum approach. It might be expected, for fieldscale problems, that nonequilibrium may be the result of more than one contributing process. A model that explicitly accounts for multiple sources of nonequilibrium would be preferable for such cases. The various models developed for simulating solute transport under nonequilibrium conditions are reviewed.

The influence of sorbate-organic matter interactions on sorption nonequilibrium

Abstract Literature data on nonequilibrium sorption of organic solutes by natural sorbents were compiled. The solutes included a broad spectrum of nonpolar, hydrophobic organic chemicals (Type I) and polar/ionizable organic chemicals (Type II). Values for equilibrium sorption coefficient (K p ) and sorption rate constant (k) were determined for the data and were analyzed with the Linear Free Energy Relationship approach. The compiled data spanned approximately seven orders-of-magnitude in K p values and approximately six orders-of-magnitude in k values. An inverse linear relationship was found between log k and log K p . The intercept value for the Type I chemicals was larger than that for the Type II chemicals, which suggests that the sorption dynamics for the Type II chemicals were constrained to a greater degree than that for the Type I chemicals. After examining the experimental conditions under which the data were collected, and considering the nature of the sorbents and the sorbates, it was postulated that the following processes were responsible for the observed sorption nonequilibrium: intraorganic matter diffusion for Type I chemicals; intraorganic matter diffusion and chemical nonequilibrium for Type II chemicals. The rationale for the elimination of other mechanisms is discussed. A regression equation, relating k and K p , is presented that may be used to estimate approximate values of the sorption rate constant for organic solutes. This provides a means to evaluate nonequilibrium potential and to attempt to predict nonequilibrium behavior.

Biosurfactant-enhanced removal of residual hydrocarbon from soil

Abstract An anionic monorhamnolipid biosurfactant produced by Pseudomonas aeruginosa was investigated for its potential to remove residual hexadecane from sand columns. In a series of column experiments, residual hexadecane saturation was established by pumping 14C-hexadecane into water-saturated sand columns and then flushing with water at a velocity of 25 cm h−1. Monorhamnolipid solutions of varying concentration were then applied to the columns at a velocity of 15 cm H−1 to remove the residual hexadecane. Of the rhamnolipid concentrations tested, which ranged from 40 to 1500 mg l−1, the optimal concentration for residual removal was 500 mg l−1, approximately ten times the critical micelle concentration (cmc). Approximately 84% of the residual was removed from the column packed with 20 30 mesh sand, and 22% was removed from the 40 50 mesh column. The primary mechanism for residual removal was mobilization (displacement and dispersion), whereas solubilization was found to be insignificant. The performance of monorhamnolipid was compared with that of two synthetic surfactant solutions on a mass basis (500 mg l−1) for the 40 50 mesh sand. Sodium dodecyl sulfate (0.2 X cmc), and polyoxyethylene (20) sorbitan monooleate (38 × cmc), removed 0% and 6.1% of the residual saturation, respectively.

Flow interruption: A method for investigating sorption nonequilibrium

Abstract A flow-interruption method for investigating sorption nonequilibrium is presented. The technique has a greater sensitivity to nonequilibrium than to traditional column experiments and hence provides a greater capacity to investigate nonequilibrium. The technique may provide more reliable determinations of kinetic parameter values. In some situations, the technique may be used to assist in the delineation of mechanism(s) responsible for sorption nonequilibrium. Use of the technique is demonstrated with miscible displacement of the herbicide 2,4-D through saturated soil columns. The sorption nonequilibrium exhibited by 2,4-D is suggested to be a result of intraorganic matter diffusion. Experimentally determined values of the sorption rate constant show an inverse relationship to organic matter content.

Rate‐limited sorption and nonequilibrium transport of organic chemicals in low organic carbon aquifer materials

The rate-limited sorption and nonequilibrium transport of several hydrophobic organic chemicals in three low-organic carbon (< 0.025%) aquifer materials was investigated. Results of miscible displacement experiments performed at two pore water velocities and with very low solution-phase concentrations (30-60 {mu}g L{sup {minus}1}) were analyzed using a first-order mass transfer nonequilibrium model, as well as a model employing the local equilibrium assumption. Results of the analyses revealed sorption to be significantly rate limited, possibly by a diffusion-limited mechanism. The impact of rate-limited sorption on transport was dependent upon pore water velocity. The experiments performed at a faster velocity ({approximately} 1 cm/h) could be successfully simulated only with the nonequilibrium model, whereas the equilibrium model was adequate for the slower-velocity ({approximately} 0.2 cm/h) experiments. Comparison of experimental results to those reported in the literature revealed that time scale has a significant impact on the degree of nonequilibrium observed in, and on the values of rate constants determined from, experiments. Regression equations were developed for the two kinetics-associated parameters contained in the nonequilibrium model: first-order mass transfer constant and fraction of instantaneous sorption.

Transport of reactive contaminants in heterogeneous porous media

The potential for human activities to adversely affect the environment has become of increasing concern during the past two decades. Concomitantly, the transport and fate of contaminants in subsurface systems has become one of the major research areas in the environmental, hydrological, and Earth sciences. An understanding of how contaminants move in the subsurface is needed to evaluate the probability of contaminants associated with a chemical spill reaching an aquifer and contaminating groundwater. This knowledge is also required to develop and evaluate methods for cleaning up contaminated soils and aquifers. Just as importantly, knowledge of contaminant transport and fate is necessary to design “pollution prevention” strategies. A tremendous body of literature on contaminant transport has been generated in response to these needs. This literature consists primarily of results obtained by theoretical, experimental, and mathematical modeling based investigations and, to a much lesser extent, field experiments. This paper consists of a brief review of some of the major aspects associated with the transport of reactive contaminants in heterogeneous subsurface environments. It begins with a review of basic concepts related to contaminant transport, followed by a discussion of the results obtained from some of the few well-controlled field experiments designed to investigate transport of reactive contaminants in the subsurface. Some of the major factors controlling contaminant transport will then be discussed, followed by a review of conceptual and mathematical approaches used to represent those factors in mathematical models. A brief overview of future needs and opportunities in contaminant transport will close the discussion.

Transport of Organic Chemicals by Gas Advection in Structured or Heterogeneous Porous Media: Development of a Model and Application to Column Experiments

The use of soil venting or vacuum extraction for remediation of contaminated soil has lead to an increased interest in modeling gas-phase processes. The majority of existing transport models have been developed assuming homogeneous porous media properties. It is well known, however, that the assumption of homogeneity will almost always be invalid for field systems. In addition, sorption has been described using the local equilibrium assumption. This assumption has also been shown to be invalid under certain conditions. A one-dimensional model is presented that accounts for a structured or heterogeneous porous medium and for rate-limited sorption. The model is designed for cases where transport occurs by advection and dispersion in the gas phase and where the liquid phase is immobile. A sensitivity analysis is presented, and parameter determination is discussed. The performance of the model was evaluated by comparing predicted simulations to data obtained from the literature. The model performed well, especially considering that the values for all input parameters were obtained independent of curve fitting.

Using flow interruption to identify factors causing nonideal contaminant transport

The transport and fate of many contaminants in subsurface systems can be influenced by several rate-limited processes, such as rate-limited sorption, diffusional mass transfer, and transformation reactions. Identification of the controlling process in such systems is often difficult, and is confounded by the possible influence of additional factors such as nonlinear or hysteretic sorption. We present a relatively simple method, flow interruption, that can be used to discriminate between various sets of processes. The application of the method is illustrated with results obtained from experiments performed for selected systems. Specific process-pairs investigated include physical nonequilibrium vs. physical heterogeneity, rate-limited sorption vs. nonlinear sorption, and sorption vs. transformation reactions. The results show that, while both physical nonequilibrium and physical heterogeneity can cause enhanced spreading or dispersion, only the former causes a noticeable concentration perturbation upon imposition of flow interruption under typical conditions. In addition, while both rate-limited sorption and nonlinear sorption can cause breakthrough curves to exhibit tailing, only rate-limited sorption induces a concentration perturbation upon imposition of flow interruption. The information obtained from applying flow interruption can be used to assist in the planning of additional, process-specific experiments and to help identify appropriate mathematical models to be used for transport simulation.

Enhanced Transport of Low-Polarity Organic Compounds through Soil by Cyclodextrin.

The removal of low-polarity organic compounds from soils and aquifers by water flushing is often constrained by sorption interactions. There is great interest in developing systems that can enhance the transport of organic compounds through porous media, thus facilitating remediation. We investigated the potential of hydroxypropyl-[beta]-cyclodextrin (HPCD), a microbially produced compound, to reduce the sorption and to enhance the transport of several low-polarity organic compounds. The results show that cyclodextrin does not interact with the two porous media used in the study. As a result, there is no retardation of cyclodextrin during transport. The retardation of compounds such as anthracene, pyrene, and trichlorobiphenyl was significantly (orders of magnitude) reduced in the presence of cyclodextrin. The enhancement effect of the cyclodextrin was predictable with a simple equation based on three-phase partitioning. The nonreactive nature of cyclodextrin combined with its large affinity for low-polarity organic compounds makes cyclodextrin a possible candidate for use in in-situ remediation efforts. 22 refs., 6 figs., 3 tabs.