Numerical study on kinetic/equilibrium behaviour of dissolution of toluene under variable subsurface conditions

Abstract: Understanding dissolution dynamics of hazardous compounds from complex gasoline mixtures is a key to long-term predictions of groundwater risks. The aim of this study was to investigate if the local equilibrium assumption for BTEX and TMBs (trimethylbenzenes) dissolution was valid under variable saturation in two dimensional flow conditions and evaluate the impact of local heterogeneities when equilibrium is verified at the scale of investigation. An initial residual gasoline saturation was established over the upper two-thirds of a water saturated sand pack. A constant horizontal pore velocity was maintained and water samples were recovered across 38 sampling ports over 141 days. Inside the residual NAPL zone, BTEX and TMBs dissolution curves were in agreement with the TMVOC model based on the local equilibrium assumption. Results compared to previous numerical studies suggest the presence of small scale dissolution fingering created perpendicular to the horizontal dissolution front, mainly triggered by heterogeneities in the medium structure and the local NAPL residual saturation. In the transition zone, TMVOC was able to represent a range of behaviours exhibited by the data, confirming equilibrium or near-equilibrium dissolution at the scale of investigation. The model locally showed discrepancies with the most soluble compounds, i.e. benzene and toluene, due to local heterogeneities exhibiting that at lower scale flow bypassing and channelling may have occurred. In these conditions mass transfer rates were still high enough to fall under the equilibrium assumption in TMVOC at the scale of investigation. Comparisons with other models involving upscaled mass transfer rates demonstrated that such approximations with TMVOC could lead to overestimate BTEX dissolution rates and underestimate the total remediation time.

Abstract: The focus of this study was to investigate the fate and transport of toluene, a light nonaqueous-phase liquids (LNAPLs) in the subsurface region under dynamic groundwater table conditions. ...

Abstract: India’s soil-water systems provide a vital source of freshwater and sustain the drinking water supply for the world’s second-largest population. However, groundwater within the large geogra...

Abstract: Dissolution from residual source zones of benzene poses serious threat to the groundwater quality. Proper understanding of fate and migration of dissolved benzene is a prerequisite for planning remediation strategies to reduce groundwater contamination. In the present study, an attempt has been made to numerically model the dissolution of benzene and to investigate the transport of aqueous phase benzene in a saturated fracture-matrix system under steady-state flow condition. In addition to dissolution mass transfer, advection, dispersion and matrix diffusion of aqueous benzene has been considered along the fracture. In the present numerical model, residual phase benzene is considered to be present along the entire length of the fracture and residual phase benzene is assumed to be immobile for the flow conditions considered in the analysis. Transport equations for fracture and rock-matrix are solved using implicit finite difference method. Transport equation for aqueous benzene within the fracture has been solved in a one-dimensional domain and transport equation for aqueous benzene within rock-matrix has been solved in a pseudo-two-dimensional domain. Sensitivity studies have been conducted to investigate the impact of variation of flow velocity, dispersivity, fracture aperture, inlet benzene concentration, rock-matrix diffusion coefficient, and half fracture spacing on transport of aqueous benzene concentration within the fracture. From the present study, it can be concluded that the addition of slow liquid benzene dissolution into aqueous phase influences the benzene breakthrough curves/profiles at different velocity, fracture aperture, initial benzene concentration, mass transfer rate, rock dispersivity and half fracture spacing.

Abstract: When the non-aqueous phase liquid (NAPL) contaminant is entrapped in soil pores, their release to the subsurface environment is mainly limited by interphase mass transfer such as dissolution and sorption. Sorption onto the aquifer material plays an important role as a retardation mechanism in subsurface contaminant transport processes. Considering the heterogeneity associated with aquifer properties as well as contaminant source distribution, long-term contamination is an inevitable consequence of mass transfer limitation of residual mass from soil micropores. A comprehensive numerical model is presented in this study for understanding the kinetic nature of dissolution and sorption of hydrophobic hydrocarbons and their interactions in order to estimate the rate and extent of mass removal from different phases. The results showed that sorption by soils and sediments having different physicochemical properties generally follow non-linear behavior. It is observed from the study that sorption non-linearity can be effectively incorporated by the two-site kinetic model rather than combination of linear and non-linear sorption isotherms. Even though initial phase of mass transfer is dissolution-dominated, the extended tailing of concentration at higher pore volumes is controlled by non-equilibrium sorption, which is better explained by the two-site kinetic model.

Abstract: This work focuses on the experimental measurement and mathematical modeling of processes affecting the dissolution of nonaqueous phase liquids (NAPLs) entrapped in sandy porous media. Results of a series of laboratory-scale one-dimensional column dissolution experiments indicate that the length of time required to dissolve NAPLs and substantially reduce aqueous phase effluent concentrations is many times greater than predicted by equilibrium calculations. Experimental measurements clearly show an influence of both grain size and grain size distribution on the evolution of effluent concentrations. The longer cleaning times associated with coarse or graded media are attributed to the larger and more amorphous NAPL blobs associated with these media. A general correlation for transient dissolution rates is proposed which incorporates porous medium properties, Reynolds number, and volumetric fraction of NAPL. The model is calibrated with results from styrene dissolution experiments and is shown to adequately predict trichloroethylene dissolution rates in the same sandy media over the period of time required to dissolve the NAPL.

Abstract: The attenuation of gamma radiation was utilized to measure changing residual trichloroethylene (TCE) saturation in an otherwise water-saturated porous medium as clean water was flushed through the medium. A front over which dissolution actively occurred was observed. Once developed, this front varied in length from ≈11 mm to ≈21 mm, lengthening as it moved through the porous medium. Gamma attenuation measurements and analyses of effluent water samples indicate that there was minimal if any transport of TCE as colloidal droplets. Even as trapped TCE ganglia decreased in size due to dissolution, there is no evidence that they became mobile and advected downgradient. An extraction of the porous medium at the completion of one experiment indicated that less than 0.002% of the original TCE mass remained, suggesting that minimal amounts of separate phase TCE remained trapped within the medium after flushing with 290 pore volumes. Mass transfer rate coefficients were computed and are shown to be a function of Darcy flux, TCE volumetric content, and distance into the region of residual TCE.

Abstract: Results of dissolution experiments with trapped nonaqueous phase liquids (NAPLs) are modeled by a mass transfer analysis. The model represents the NAPL as isolated spheres that shrink with dissolution and uses a mass transfer coefficient correlation reported in the literature for dissolving spherical solids. The model accounts for the reduced permeability of a region of residual NAPL relative to the permeability of the surrounding clean media that causes the flowing water to partially bypass the residual NAPL. The dissolution experiments with toluene alone and a benzene-toluene mixture were conducted in a water-saturated column of homogeneous glass beads over a range of Darcy velocities from 0.5 to 10 m d(-1). The model could represent the observed effluent concentrations as the NAPL underwent complete dissolution. The changing pressure drop across the column was predicted following an initial period of NAPL reconfiguration. The fitted NAPL sphere diameters of 0.15 to 0.40 cm are consistent with the size of NAPL ganglia observed by others and are the smallest at the largest flow velocity.

Abstract: A number of previous studies are reviewed to examine the actual reduction of NAPL from source zones and the effectiveness of the specific technique of remediation used at sites under study. It has been shown that complete removal of the NAPL in free phase or residual is not possible due to the complex entrapment architecture of NAPLs at field sites. Consequently, the assessment of remediation efficiency should not be solely based on the reduction of entrapped NAPL mass from source zone. Instead, it should be based on the reduction of risk achieved through the lowering of the concentration of the dissolved constituents emanating from the entrapped NAPL during source zone clean-up. The prediction of the concentration in the plume requires a knowledge of the dissolution of NAPLs in the source zone. Attention is directed to the need for the understanding the mass transfer from entrapped NAPLs in the source zone before and after remediation. In this paper, the current knowledge of mass transfer processes from the non-aqueous phase to the aqueous phase is summarised and the use of mass flux measurements (monitoring the concentration of contaminants in aqueous phase due to source zone NAPL-groundwater mass transfer) is introduced as a potential tool to assess the efficiency of technologies used in source zone remediation. Preliminary results of numerical simulations reveal that factors such as source zone morphology as determined by the heterogeneity of the formation control the post-remediation dissolution behaviour, than the local mass transfer. Thus, accurate site characterization is essential for predicting NAPL dissolution and mass flux relationships as well as for assigning site-specific remediation target values.

Abstract: Under some conditions, a first-order kinetic model is a poor representation of biodegradation in contaminated aquifers. Although it is well known that the assumption of first-order kinetics is valid only when substrate concentration, S, is much less than the half-saturation constant, K{sub S}, this assumption is often made without verification of this condition. The authors present a formal error analysis showing that the relative error in the first-order approximation is S/K{sub S} and in the zero-order approximation the error is K{sub S}/S. They then examine the problems that arise when the first-order approximation is used outside the range for which it is valid. A series of numerical simulations comparing results of first- and zero-order rate approximations to Monod kinetics for a real data set illustrates that if concentrations observed in the field are higher than K{sub S}, it may be better to model degradation using a zero-order rate expression. Compared with Monod kinetics, extrapolation of a first-order rate to lower concentrations under-predicts the biotransformation potential, while extrapolation to higher concentrations may grossly over-predict the transformation rate. A summary of solubilities and Monod parameters for aerobic benzene, toluene, and xylene (BTX) degradation shows that the a priori assumption of first-order degradationmore » kinetics at sites contaminated with these compounds is not valid. In particular, out of six published values of K{sub S} for toluene, only one is greater than 2 mg/L, indicating that when toluene is present in concentrations greater than about a part per million, the assumption of first-order kinetics may be invalid. Finally, the authors apply an existing analytical solution for steady-state one-dimensional advective transport with Monod degradation kinetics to a field data set.« less