P.S.C. Rao

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.

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.

Partitioning of inorganic orthophosphate in soil‐water systems∗

The fate of phosphorus in terrestrial ecosystems is of concern because it has been identified as a major contributor to eutrophication of surface water supplies. In order to ascertain the impact of inorganic phosphate on water quality, the chemical speciation, concentration, bio‐availability, and distribution in the soil‐water system are prerequisites. Environmental regulations and land disposal of organic wastes also require quantifying the partitioning of inorganic phosphate between solid and solution phases in soil‐water systems. In order to develop a partition coefficient or a partition function, several aspects of the solution chemistry of inorganic phosphate were reviewed. Several sorption mechanisms and mathematical models have been used to describe equilibrium sorption and the time‐dependent phosphate removal from solution. The most frequently applied equilibrium model was the Langmuir equation in the single‐surface form. Because of the amount of Langmuir data available, Langmuir parameters were c...

Simulating solute transport in an aggregated soil with the dual-porosity model: measured and optimized parameter values

The capability of the first-order, dual-porosity model, which explicitly accounts for non-ideal transport caused by the presence of ‘immobile’ water, to predict the non-ideal transport of non-sorbing solute in a constructed aggregated soil has been investigated. Miscible-displacement experiments performed with a well-characterized aggregated soil and a non-reactive tracer (pentafluorobenzoate) served as the source of the data. Values for the input parameters associated with physical non-equilibrium were determined independently and compared with values obtained by curve fitting of the experimental measurements. The calculated and optimized values compared well, suggesting that the non-equilibrium parameters represent actual physical phenomena

Vapor-phase adsorption of alkylbenzenes and water on soils and clays

Abstract Vapor-phase adsorption of water and three alkylbenzenes (toluene, p-xylene, and ethylbenzene) on bentonite, kaolin, two soil samples, an aquifer material, and a silica gel was measured by a procedure in which vapor-phase concentrations were measured by trapping the vapors in methanol and adsorbed-phase concentrations were measured by methanol-extraction. Water in the methanol was measured by titration with Karl Fischer reagent; alkylbenzenes were measured by UV spectrophotometry and/or HPLC analysis. Bentonite and the two soil samples adsorbed much more water than alkylbenzene. Kaolin adsorbed nearly equal amounts of water and alkylbenzene, while silica gel adsorbed nearly twice as much of the alkylbenzenes as water. With the exception of water adsorption on bentonite and alkylbenzene adsorption on Oldsmar soil (Alfic Arenic Haplaquod), adsorption conformed to type-II adsorption isotherms which were adequately described by the BET adsorption model. Surface areas calculated from BET monolayer capacities indicated that the surface available for alkylbenzene adsorption was essentially that measured by N 2 adsorption except for silica gel. Values for the relative adsorption (the amount adsorbed divided by the monolayer capacity as determined by the BET model) of water and p-xylene were similar for several of the adsorbents used.

Competitive adsorption of alkylbenzene and water vapors on predominantly mineral surfaces

Abstract Competitive adsorption of ethylbenzene (EB) and water on bentonite and of p-xylene (pXYL) and water on kaolin and silica gel was studied using a technique that allowed the amount of adsorbed water and alkylbenzene to be measured independently. EB adsorption on bentonite was not affected by water at relative humidities (RH) near 0.23 but was reduced significantly at RHs near 0.50. pXYL adsorption on kaolin and silica gel decreased with increasing RH, especially above a RH of about 0.2. Increasing RH not only decreased the amount of alkylbenzene adsorption but also resulted in a change from Type-II adsorption isotherms to ones that were essentially linear. Linear isotherms for the adsorption of hydrophobic organic compounds on hydrated soil have generally been attributed to partitioning into organic carbon (OC). However, since the clays and oxide used here had very low to trace amounts of OC, it is suggested that processes involving only mineral surfaces can give rise to linear isotherms. Based on solubility considerations alone, partitioning of EB and pXYL into adsorbed water films was not considered to be an important adsorption mechanism in this study. The effect of cation hydration on the amount of water adsorbed from a mixture of water and pXYL vapors was evaluated by comparing adsorption on Li- and Na-saturated kaolin.

Solvophobic approach for predicting sorption of hydrophobic organic chemicals on synthetic sorbents and soils

Abstract The application of a solvophobic approach for predicting the sorption of hydrophobic organic compounds (HOC) was evaluated with data collected using synthetic sorbents and soils. The experimental data consisted of batch equilibrium sorption coefficients ( K D ), as well as soil-TLC and reversed-phase liquid chromatographic (RPLC) retention factors (κ′). All data were collected using aqueous solutions and binary or ternary solvent mixtures of water, methanol, acetone, and acetonitrile. As predicted by the theory, the chromatographic retention factors and sorption coefficients for HOC decreased log-linearly with increasing fraction of organic cosolvent in binary solvents. Model parameters estimated from the binary solvent data could be used to predict sorption (or retention) from ternary solvents. Reasonable agreement was found between model parameters reported in the literature and those estimated using the data from batch sorption, soil-TLC, and RPLC studies.

Retention of hydrophobic solutes on reversed-phase liquid chromatography supports: Correlation with solute topology and hydrophobicity indices

Abstract Retention of several nonpolar solutes on two reversed-phase liquid chromatography (RPLC) supports was examined during isocratic, isothermal elution with binary mixtures of methanol/water and acetonitrile/water. Changes in chromatographic retention factors (k) were correlated with the following indices of solute hydrophobicity and molecular topology: octanol-water partition coefficient (Kow), hydrophobic surface area (HSA), and first-order molecular connectivity (1χ). For each sorbent-solvent combination, one regression equation was required to describe the data for polycyclic aromatic hydrocarbons and halobenzenes, and another for the alkylbenzenes. This distinctive behavior was attributed to the differences in the nature of the interaction of these two groups of solutes with the bonded, n-alkyl chains. Solute molecular size, shape, and conformation as well as hydrophobicity appear to be the dominant factors controlling solute retention by RPLC supports.