Effects Of Nonlinear Sorption Research Articles

Effect of nonlinear sorption on multispecies radionuclide transport in a coupled fracture-matrix system with variable fracture aperture: a numerical study

A one-dimensional numerical analysis on multispecies radionuclide transport in a single-horizontal coupled fracture-matrix system has been performed. Analysis considering linear and nonlinear adsorption cases with linear, Langmuir, and Freundlich adsorption models has been carried out. The results indicate that there is a significant change in the spatial distribution of radionuclides in a coupled fracture-matrix system when a nonlinear adsorption isotherm is considered as compared to the simplified linear sorption isotherm. Sensitivity analysis of Langmuir constant and Freundlich exponent has been performed, and the numerical results indicate that the behavior of radionuclide transport is strongly influenced by these nonlinear sorption isotherm parameters. In addition, an attempt has been made to consider the variation of fracture aperture thickness along its flow direction by introducing logarithmic distribution. A clear distinction in the spatial distribution of radionuclide concentration was observed when variable fracture aperture is considered as opposed to the conventional parallel plate model with a constant fracture aperture thickness. Further, the results suggest that there is an enhanced retardation of radionuclides within the high permeable fracture with varying fracture aperture.

Temporal moment analysis of solute transport in a coupled fracture-skin-matrix system

In the present study, method of temporal moments has been used to analyse the transport characteristics of reactive solute along fracture in a coupled fracture-skin-matrix system. In order to obtain the concentration distribution within the fracture, a system of coupled partial differential equations for fracture, fracture-skin and rock-matrix has been solved numerically in a pseudo two-dimensional domain using implicit finite difference method. Subsequently, lower order temporal moments of solute have been computed from the concentration distribution to analyse the transport characteristics of solutes in the fracture. This study has been done by considering an inlet boundary condition of constant continuous source in a single fracture. The effect of various fracture-skin parameters like porosity, thickness and diffusion coefficient on the transport of solutes have been studied by doing sensitivity analyses. The effect of nonlinear sorption and radioactive decay of solutes have also been analysed by carrying out simulations for different sorption intensities and decay constants. Numerical results suggested that the presence of fracture-skin significantly influences the transport characteristics of reactive solutes along the fracture.

Modeling reactive transport in heterogeneous saturated porous media. Effects of nonlinear sorption

Modeling reactive transport in heterogeneous saturated porous media. Effects of nonlinear sorption

Reactive transport of 85Sr in a chernobyl sand column: static and dynamic experiments and modeling

The effects of nonlinear sorption and competition with major cations present in the soil solution on radioactive strontium transport in an eolian sand were examined. Three laboratory techniques were used to identify and quantify the chemical and hydrodynamic processes involved in strontium transport: batch experiments, stirred flow-through reactor experiments and saturated laboratory columns. The major goal was to compare the results obtained under static and dynamic conditions and to describe in a deterministic manner the predominant processes involved in radioactive strontium transport in such systems. Experiments under dynamic conditions, namely flow-through reactor and column experiments, were in very good agreement even though the solid/liquid ratio was very different. The experimental data obtained from the flow-through reactor study pointed to a nonlinear, instantaneous and reversible sorption process. Miscible displacement experiments were conducted to demonstrate the competition between stable and radioactive strontium and to quantify its effect on the 85Sr retardation factor. The results were modeled using the PHREEQC computer code. A suitable cation-exchange model was used to describe the solute/soil reaction. The model successfully described the results of the entire set of miscible displacement experiments using the same set of parameter values for the reaction calculations. The column study revealed that the stable Sr aqueous concentration was the most sensitive variable of the model, and that the initial state of the sand/solution system had also to be controlled to explain and describe the measured retardation factor of radioactive strontium. From these observations, propositions can be made to explain the discrepancies observed between some data obtained from static (batches) and dynamic (reactor and column) experiments. Desorbed antecedent species (stable Sr) are removed from the column or reactor in the flow system but continue to compete for sorption sites in the batch system. Batch experiments are simple and fast, and provide a very useful means of multiplying data. However, interpretation becomes difficult when different species compete for sorption sites in the soil/solution system. A combination of batches, flow-through reactor and column experiments, coupled with hydrogeochemical modeling, would seem to offer a very powerful tool for identifying and quantifying the predominant processes on a cubic decimeter scale (dm 3) and for providing a range of radioactive strontium retardation factor as a function of the geochemistry of the soil/solution system.

Properties of exact and approximate traveling wave solutions for transport with nonlinear and nonequilibrium sorption

Nonlinear sorption leads to the existence of moving concentration fronts of substances transported in porous media that do not change shape. The cause for the existence of such traveling wave fronts is a balance between the self‐sharpening effect of nonlinear sorption and the spreading effects of dispersion and sorption kinetics. While analytical solutions for the transport of nonlinearly sorbing substances are usually not available, the asymptotic front speed and the shape of traveling waves can in some situations be determined analytically. This is done by deriving a traveling wave equation in a moving coordinate system. In the case of simultaneous linear equilibrium and nonlinear nonequilibrium sorption, this traveling wave equation is a second‐order differential equation. Because this equation cannot be solved analytically for typical nonlinear sorption isotherms, the second‐order term is usually neglected. In this paper the significance of the second‐order term is discussed for a simple piecewise linear sorption isotherm that allows the analytical solution of both the full and the simplified differential equations. These analytical solutions make it possible to calculate the approximation error as a function of flow and isotherm parameters and thus to localize domains where the error may be significant. In addition, the analytical solutions are used to discuss the identifiability of model parameters from asymptotic front shape data.

Sorption and desorption of arsenic from sandy soils: Column studies

Rate‐limited sorption/desorption can have a profound effect upon the transport of sorbing contaminants. Numerical and analytical models used to predict chemical movement through the subsurface rarely incorporate the effects of nonlinear sorption and desorption kinetics, resulting in potentially large overestimates of mass extractability. Mass transfer characteristics of arsenic‐contaminated soils at the site of a former arsenical herbicide manufacturer in Houston, Texas, were examined in the laboratory using soil columns. Unaffected soils comprised of silty sands to coarse sands were collected from the uppermost aquifer. Two soil columns were loaded with a known mass of mixed organic and inorganic forms of arsenic resident in site ground water. A third control column was prepared with dry 20 × 30 mesh ASTM silica sand. Leachate samples were collected from each void volume until arsenic breakthrough was achieved. The dynamic test applied a continuing head of water, operating in an upflow mode through 4‐in. diameter by 12‐in. long soil columns repacked to in situ density. A flow‐through velocity of one void volume per day was chosen for arsenic loading to the columns and 0.08 void volume per day during the desorption phase of the test. Uncontaminated ground water was then passed through the columns, and the tests were restarted in the desorption mode. Analysis of the leachate and resulting arsenic concentrations in the test columns allowed for the calculation of distribution coefficients that describe arsenic behavior. Measured distribution coefficients during desorption ranged from 0.26 after one void volume to 3.3 after six void volumes had been passed through the column.

The effect of nonlinear sorption on transformation of contaminants during transport in porous media : M. L. Brusseau, Journal of Contaminant Hydrology, 17(4), 1995, pp 277–291

The effect of nonlinear sorption on transformation of contaminants during transport in porous media : M. L. Brusseau, Journal of Contaminant Hydrology, 17(4), 1995, pp 277–291

The effect of nonlinear sorption on transformation of contaminants during transport in porous media

The transport of contaminants through porous media is influenced by several processes, two of the most important being sorption and transformation. Several mathematical models have been developed to investigate the effects of sorption and transformation on contaminant transport. Almost all of these models are based on the assumption of linear sorption. However, it is well known that sorption of reactive contaminants is often nonlinear. A mathematical model that describes the transport of solute undergoing nonlinear, rate-limited sorption and first-order transformation is used to investigate the effect of coupled transformation and non-linear sorption on contaminant transport. Results of the analyses show that a model based on linear sorption cannot provide an accurate simulation of the transformation and transport of nonlinearly sorbing solutes when n is less than ∼0.9. In addition, the relative impact of non-linear sorption on solute transport is mediated by the magnitude of transformation. The nondimensional time required for a specified fraction of solute mass to be transformed during transport is influenced by nonlinear sorption. These examples illustrate the intriguing effects that coupled processes can have on contaminant transport and which may be important for many contaminants of interest.

The effects of nonlinear sorption on the diffusion of volatile organic compounds from air-dry soils: A theoretical study

The effects of nonlinear (Brunauer-Emmett-Teller, BET) sorption on the rates of diffusion of volatile organic compounds (VOCs) within and from uniformly contaminated air-dry soils were evaluated by numerical solution of the one-dimensional gas-phase diffusion equation. A dimensionless sorption parameter, which accounts for the importance of BET sorption on the diffusive transport was identified. Numerical simulations demonstrated that the dimensionless gas-phase and total-soil concentration profiles, and the dimensionless emission rates of VOC from the soil surface, varied in response to changes in the initial concentration of the VOC in the soil, in marked contrast with the case of linear sorption of the VOC. The sensitivity of the numerical results towards the BET sorption parameters in the model was evaluated. A linearization step in the derivation of the continuity equation of diffusion and BET sorption was used to obtain an analytical solution for the soil surface flux of VOC to the atmosphere, by substituting a linear sorption isotherm for the BET isotherm. The errors associated with this step were evaluated and the deviations were found to be < 20% for all cases tested in comparison to the full numerical solutions.