Column Displacement Experiments Research Articles

Visualization and Characterization of Heterogeneous Water Flow in Double-Porosity Media by Means of X-ray Computed Tomography

Three-dimensional visualization of dynamic water transport process in soil by 1 computed tomography (CT) technique is still limited by its low temporal resolution. In order 2 to monitor dynamically water transport in soil, a compromise has to be found between water 3 flow velocity and CT acquisition time. Furthermore, an efficient image analysis method is 1 4 necessary. In this work, we followed the water transport in three dimensions by CT imaging 5 across a double-porosity media constituted of two distinct materials, i.e. sand and porous 6 clay spheres. The CT acquisition parameters were adjusted to the water pore velocity so that 7 we succeeded to register the water front displacement per time range of 25 min. We also used 8 the image subtraction method to extract water distribution evolution with time with a space 9 resolution of 6 × 10 −3 cm. Both time and space resolution are relatively high compared with 10 other dynamic studies. The water content profiles showed that the clay spheres remained 11 in their dry state during water infiltration, while the water transport only occurred in the 12 sand matrix. These results are consistent with macroscopic experiments. The water front 13 visualized by CT showed a non-symmetrical shape which was related to water transfer in 14 non-equilibrium as shown by column displacement experiments.

Reliability of the Multiple-Rate Adsorptive Model for Simulating Adsorptive Solute Transport in Soil Demonstrated by Pore-Scale Simulations

Minerals and organic matters in soils and sediments are chemically different and their ability to adsorb solute varies. Since individual minerals cannot be resolved in macroscopic models, these heterogeneous adsorption rates are often modelled as a random number with a pre-defined distribution function. Such an approach is also known as multiple-rate adsorption (MRA) models in reactive solute transport modelling. The MRA models have shown some improvement, but it is unclear that this improvement is due to their improved description of the statistics of the variable adsorption rates, or because of the extra parameters they introduced, which give them more freedom in curve-fitting. In this paper, we investigated this using pore-scale modelling. The pore-scale simulations were based on an idealised column packed with different minerals; solute concentration in the column was assumed to be low in comparison with the adsorptive capacity of the minerals. Hence, the adsorption of each mineral particle was assumed to be linear first-order kinetic. Both batch and column-displacement experiments at the pore scale were simulated using the lattice Boltzmann model. The simulated solute distributions were then volume-averaged to yield macroscopic concentration by treating the column as a continuum medium. The parameters in the MRA model were obtained from batch experiments and the dispersion coefficient was derived from displacement of an inert tracer. These parameters were then used to simulate the displacement of adsorptive solute under different water flow rates. The results showed that even though the MRA model accurately described the statistics of the heterogeneous adsorption rates, it failed to describe the macroscopic adsorption when the Pe number is high. We also found that the only way to improve the fitting is to increase the dispersion coefficient measured from the inert tracer. This yields a dispersion coefficient for adsorptive solute, which nonlinearly increases with the Pe number. To investigate whether this is a general phenomenon for adsorptive solute or only for solute under heterogeneous adsorption, we simulated solute displacement in the column filled by the same mineral. The results show that when the adsorption is homogeneous, the dispersion coefficients for adsorptive solute and inert tracer are the same. These findings imply that the MRA models should be used with care as the dispersion coefficient derived using inert tracers might not apply to adsorptive solute.

Development of a displacer-immobilized ligand docking scheme for displacer screening for protein displacement chromatography

Displacement chromatography is a powerful technique for protein purification, but the availability of high-efficacy displacers has greatly limited its applications. In this work, a displacer-immobilized ligand docking scheme was developed for the prediction of displacer efficacy and displacer screening for displacement chromatography of proteins. The structure of immobilized ligand was established by coupling a certain number of ligands to the 3D structure of agarose. A number of known cation, anion and hydrophobic displacers were docked to their respective immobilized ligands to verify the effectiveness of the scheme, and the Spearman ranking correlation coefficients of all cases were over 0.5. The scheme was then used to screen displacers for hydrophobic charge induction chromatography from over 1800 commercially available compounds. Column displacement experiments of several representative compounds showed that the identified displacers were efficacious in the displacement of single component and binary mixtures. It is expected that the combination of the docking scheme with the existing techniques for displacer discovery/design would greatly facilitate the discovery of high-affinity displacers for protein purification.

Evaluation of saline tracer performance during electrical conductivity groundwater monitoring

Saline solutions are the most commonly used hydrological tracers, because they can be easily and economically monitored by in situ instrumentation such as electrical conductivity (EC) loggers in wells or by geoelectrical measurements. Unfortunately, these low-cost techniques only provide information on the total concentration of ions in solution, i.e., they cannot resolve the ionic composition of the aqueous solution. This limitation can introduce a bias in the estimation of aquifer parameters where sorption phenomena between saline tracers and sediments become relevant. In general, only selected anions such as Cl − and Br − are recognised to be transported unretarded and they are referred to as conservative tracers or mobile anions. However, cations within the saline tracer may interact with the soil matrix through a range of processes such as ion exchange, surface complexation and via physical mass-transfer phenomena. Heterogeneous reactions with minerals or mineral surfaces may not be negligible where aquifers are composed of fine alluvial sediments. The focus of the present study was to examine and to quantify the bias between the aquifer parameters estimated during model-based interpretation of experimental data of EC measurements of saline tracer relative to the aquifer parameters found by specific measurements (i.e. via ionic chromatography, IC) of truly conservative species. To accomplish this, column displacement experiments with alluvial aquifer materials collected from the Po lowlands (Italy) were performed under water saturated conditions. The behaviour of six selected, commonly used saline tracers (i.e., LiCl, KCl, and NaCl; LiBr, KBr, and NaBr) was studied and the data analysed by inverse modelling. The results demonstrate that the use of EC as a tracer can lead to an erroneous parameterisation of the investigated porous media, if the reactions between solute and matrix are neglected. In general, errors were significant except for KCl and KBr, which is due to the weak interaction between dissolved K + and the sediment material. The study shows that laboratory scale pre-investigations can help with tracer selection and to optimise the concentration range targeted for in situ multilevel monitoring by unspecific geoelectrical instrumentation.

Chemical attenuation of arsenic by soils across two abandoned mine sites in Korea

The chemical attenuation of As by soils from abandoned mine sites was evaluated. Several soil samples, including As contaminated soil from the mine impacted areas, as well as As-free soils down-gradient from the mine sites, were collected across abandoned mine sites. Leaching and adsorption experiments were conducted under batch and 1-D water flow conditions. The cumulative As mass from 10 step sequential leaching experiments with six As contaminated soils, using 10 mM CaCl 2 solution, was less than 1% of the total As present in soils, indicating that As in contaminated soils is strongly adsorbed onto soil particles, which can serve as a long term potential As source. As adsorption by As-free soils was clearly nonlinear, with Freundlich N values (sorption nonlinearity) ranging from 0.56 to 0.87. Both the total As content in mine soils and the concentration-specific adsorption coefficient for arsine-free soils were best described by coupling the pH with various forms of Fe/Al oxides. In the breakthrough curves (BTCs) for As contaminated soils, an initial high concentration of As (called first-flush) was observed, and this flush export leveled off after the displacement of a few pore volumes. In the BTCs from layered soils, where clean down-gradient soils were overloaded above the mine soil, the appearance of measurable As was retarded, showing that the As attenuation by soils was effective in a flow water system. Also, the observed perturbation in the concentration of As during flow interruption supports that leaching/attenuation of As via flowing water occurs under nonequilibrium conditions. The results from both batch leaching/adsorption and column displacement experiments strongly suggested that the leaching of As from mine soils was rate limited and the risk of As leaching from soils can be mitigated by attenuation mechanisms, such as adsorption, provided by down-gradient clean soils.

Transport and degradation of metalaxyl and isoproturon in biopurification columns inoculated with pesticide-primed material

Laboratory column displacement experiments were performed to examine whether addition of pesticide-primed material to the matrix of an on-farm biopurification system (BPS), intended to remove pesticides from agricultural waste water, positively affects the degradation of mobile pesticides in the system. Percolated column microcosms with varying types and amounts of metalaxyl and/or isoproturon-primed material or non-primed material were irrigated with water artificially contaminated with isoproturon and/or metalaxyl. Transport of isoproturon was well described using the convection dispersion equation and no dissipation was observed, even in columns inoculated with isoproturon-primed material. On the other hand, delayed dissipation of metalaxyl, i.e., after an initial lag phase, was encountered in all columns receiving metalaxyl. In all systems, dissipation could be described using the Monod model indicating that a metalaxyl degrading population grew in the systems. There was a clear correlation between the lag phase and the amount of metalaxyl-primed material added to the system, i.e., increasing amounts of added material resulted into shorter lag phases and hence more rapid initiation of growth-associated metalaxyl degradation in the system. Our observations suggest that indeed pesticide-primed material can reduce the start-up phase of degradation of mobile pesticides in a BPS and as such can increase its efficiency. However, the primed material should be chosen carefully and preferentially beforehand tested for its capacity to degrade the pesticide.

Boron Transport in Soils as affected by Dissolved Organic Matter in Treated Sewage Effluent

Interaction of effluent dissolved organic matter (DOM) with native soil organic matter (OM), B complexation with DOM, and adsorption of B by soil all play an important role in assessing B transport in soil. However, no information is available yet on the impact of these reactions on B mobility in soil. The objective of the present study was to evaluate the effect of B‐DOM complexation on B mobility in soils irrigated with treated effluents. Soil column displacement experiments were conducted to measure the B mobility in the presence of DOM extracted from municipal treated sewage plants. To describe B transport in the presence of DOM, a traditional multicomponent transport model was used. The model took into account the B‐DOM‐soil reactions (obtained from batch experiments). It was observed that the presence of OM molecules in soil solution enhanced B transport in soil due to B‐OM complexation. The concentration of this complex in soil solution increases with the concentration of the dissolved OM (from both effluent and native soil).The model simulated well the effect of DOM on B transport in soil.

Characterizing pesticide sorption and degradation in macro scale biopurification systems using column displacement experiments

The efficiency of biopurification systems to treat pesticide-contaminated water was previously studied in microcosms. To validate the obtained results, macrocosm systems were set-up. Four pesticides (linuron, isoproturon, bentazone, and metalaxyl) were continuously applied to ten different organic substrate mixes. Retention of the pesticides was similar and in some cases slightly lower in the macrocosms compared to the microcosms. Differences in retention between the different mixes were however minimal. Moreover, the classification of the retention strength of the pesticides was identical to that observed in microcosms: linuron > isoproturon > metalaxyl > bentazone. Monod kinetics were used to describe delayed degradation, which occurred for isoproturon, metalaxyl and bentazone. No breakthrough of linuron was observed, thus, this pesticide was appointed as the most retained and/or degraded pesticide, followed by isoproturon, metalaxyl and bentazone. Finally, most of the matrix mixes efficient in degrading or retaining pesticides were mixes containing dried cow manure.

Synthesis and Characterization of Fluorescent Displacers for Online Monitoring of Displacement Chromatography

One of the major impediments to the implementation of displacement chromatography for the purification of biomolecules is the need to collect fractions from the column effluent for time-consuming offline analysis. The ability to employ direct online monitoring of displacement chromatography would have significant implications for applications ranging from analytical to preparative bioseparations. To this end, a set of novel fluorescent displacers were rationally designed using known chemically selective displacers as a template. Fluorescent cores were functionalized with different charge moieties, creating a homologous library of displacers. These compounds were then tested on two protein pairs, alpha-chymotrypsinogen A/ribonuclease A and cytochrome c/lysozyme, using batch and column displacement experiments. Of the synthesized displacers, two were found to be highly selective while one was determined to be a high-affinity displacer. Column displacements using one of the selective displacers yielded complete separation of both protein pairs while facilitating direct online detection using UV and fluorescence detection. Saturation transfer difference NMR was also carried out to investigate the binding of the fluorescent displacers to proteins. The results indicated a selective binding between the selective displacers and alpha-chymotrypsinogen A, while no binding was observed for ribonuclease A, confirming that protein-displacer binding is responsible for the selectivity in these systems. This work demonstrates the utility of fluorescent displacers to enable online monitoring of displacer breakthroughs while also acting as efficient displacers for protein purification.

Characterizing pesticide sorption and degradation in microscale biopurification systems using column displacement experiments

Biopurification systems treating pesticide contaminated water are very efficient, however they operate as a black box. Processes inside the system are not yet characterized. To optimize the performance, knowledge of degradation and retention processes needs to be generated. Therefore, displacement experiments were carried out for four pesticides (isoproturon, bentazone, metalaxyl, linuron) in columns containing different organic mixtures. Bromide, isoproturon and bentazone breakthrough curves (BTCs) were well described using the convection–dispersion equation (CDE) and a first-order degradation kinetic approach. Metalaxyl and linuron BTCs were well described using the CDE model expanded with Monod-type kinetics. Freundlich sorption, first-order degradation and Monod kinetics coefficients were fitted to the BTCs. Fitted values of the distribution coefficient K f,column were much lower than those determined from batch experiments. Based on mobility, pesticides were ranked as: bentazone > metalaxyl – isoproturon > linuron. Based on degradability, pesticides were ranked as: linuron > metalaxyl – isoproturon > bentazone.

Continuous time random walk model better describes the tailing of atrazine transport in soil

Contaminant transport in soils is complicated and involves some physical and chemical nonequilibrium processes. In this research, the soil column displacement experiments of Cl− and atrazine under different flow velocities were carried out. The data sets of Cl− transport in sandy loam fitted to the convection dispersion equation (CDE) and the two-region model (TRM) indicated that the effects of physical nonequilibrium process produced by immobile water on the breakthrough curves (BTCs) of Cl− and atrazine transport through the repacking soil columns were negligible. The two-site model (TSM) and the continuous time random walk (CTRW) were also used to fit atrazine transport behavior at the flow rate of 19.86cmh−1. The CTRW convincingly captured the full evolution of atrazine BTC in the soil column, especially for the part of long tailing. However, the TSM failed to characterize the tailing of atrazine BTC in the soil column. The calculated fraction of equilibrium sorption sites, F, ranging from 0.78 to 0.80 for all flow rates suggested the contribution of nonequilibrium sorption sites to the asymmetry of atrazine BTCs. Furthermore, the data sets for the flow rates of 6.68cmh−1 and 32.81cmh−1 were predicted by the TSM and the CTRW. As to the flow rate of 6.68cmh−1, the CTRW predicted the entire BTC of atrazine transport better than the TSM did. For the flow rate of 32.81cmh−1, the CTRW characterized the late part of the tail better, while the TSM failed to predict the tailings of atrazine BTC.

The effect of multi-component adsorption on selectivity in ion exchange displacement systems

This paper examines chemically selective displacement chromatography using affinity ranking plots, batch displacer screening experiments, column displacements, multi-component adsorption isotherms and spectroscopy. The affinity ranking plot indicated that the displacers, sucrose octasulfate (SOS) and tatrazine, should possess sufficient affinity to displace the proteins amyloglucosidase and apoferritin over a wide range of operating conditions. In addition, the plots indicated that the separation of these proteins by displacement chromatography would be extremely difficult. Further, the two proteins were shown to have very similar retention times under shallow linear gradient conditions. When batch displacement experiments were carried out, both tartrazine and SOS exhibited significant selectivity differences with respect to their ability to displace these two proteins, in contrast to the affinity ranking plot results. Column displacement experiments carried out with sucrose octasulfate agreed with the predictions of the affinity ranking plots, with both proteins being displaced but poorly resolved under several column displacement conditions. On the other hand, column displacement with tartrazine as the displacer resulted in the selective displacement and partial purification of apoferritin. Single- and multi-component isotherms of the proteins with or without the presence of displacers were determined and were used to help explain the selectivity reversals observed in the column and batch displacement experiments. In addition, fluorescence and CD spectra suggested that the displacers did not induce any structural changes to either of the proteins. The results in this paper indicate that multi-component adsorption behavior can be exploited for creating chemically selective displacement separations.

Deriving Boron Adsorption Isotherms from Soil Column Displacement Experiments

This study was conducted to elucidate the applicability of the column technique to obtain B adsorption isotherms. A column technique based on miscible displacement was used to investigate B equilibrium adsorption and B transport in soil at various pHs and B concentrations. Boron adsorption by soil at equilibrium was described by the Langmuir equation with the pH‐dependent apparent adsorption coefficient. This equation, combined with one‐dimensional convection–dispersion equation (CDE), was used to simulate B transport in soil columns. The results indicated that adsorption equilibrium was reached during the B transport in the homogeneously packed soil columns. The adsorption isotherms that were obtained from the column breakthrough curves (BTCs) and the batch experiments were identical at any given pH. This indicates that under equilibrium conditions, the column technique can be used for calculating B adsorption isotherm parameters for any given pH.

Deriving Boron Adsorption Isotherms from Soil Column Displacement Experiments

This study was conducted to elucidate the applicability of the column technique to obtain B adsorption isotherms. A column technique based on miscible displacement was used to investigate B equilibrium adsorption and B transport in soil at various pHs and B concentrations. Boron adsorption by soil at equilibrium was described by the Langmuir equation with the pH-dependent apparent adsorption coefficient. This equation, combined with one-dimensional convection–dispersion equation (CDE), was used to simulate B transport in soil columns. The results indicated that adsorption equilibrium was reached during the B transport in the homogeneously packed soil columns. The adsorption isotherms that were obtained from the column breakthrough curves (BTCs) and the batch experiments were identical at any given pH. This indicates that under equilibrium conditions, the column technique can be used for calculating B adsorption isotherm parameters for any given pH.

Transport of Triticonazole in Homogeneous Soil Columns Influence of Nonequilibrium Sorption

Nonequilibrium sorption of pesticides is frequently reported to greatly affect their transport and dissipation in soil. This study was aimed at evaluating the performances of equilibrium and two site–two region nonequilibrium convective–dispersive models for describing the sorption and decay characteristics during transport of triticonazole systemic fungicide in water‐saturated homogeneous soil. Chloride and 14C‐triticonazole column displacement experiments were carried out in a loamy clay soil under steady‐state water flow at high pore water velocities. The symmetrical breakthrough curves (BTC) obtained with the conservative tracer showed no significant physical nonequilibrium and were used to estimate a dispersivity of 0.06 cm. Compared with chloride, the 14C‐triticonazole BTC was strongly asymmetrical and shifted to the right, with a broad, extended tailing characteristic of sorption nonequilibrium. Chemical analysis of the soil after elution showed that bound residues were rapidly formed during transport. These bound residues were accounted for as decayed in the models. The two‐site model correctly described the first part of the tailing, with an estimated partition coefficient Kd of 1.5 L kg−1 for instantaneous sorption, and it predicted high values in the range of 58 d−1, and 7 d−1 for the sorption and decay first‐order rates, respectively. However, the model failed to describe the slower, extended release of 14C‐triticonazole. Nonequilibrium sorption and formation of bound residues of triticonazole were attributed to the rate‐limiting diffusive process. It was thus concluded that use of a single first‐order rate constant for description and prediction of both nonequilibrium sorption and dissipation of triticonazole in soil is not appropriate.

Column displacement experiments to validate hydrogeochemical models of seawater intrusions

Two sets of experimental data have been obtained flushing seawater through columns packed with sediment which had previously been in equilibrium with freshwater. The sets of data contain the ion concentrations of the outflow and the physical and chemical parameters needed for the theoretical simulation of marine intrusion using a hydrogeochemical model. The results show the decisive importance of the Péclet number in the shape of the curves of variation of the different ions with time.

Measured and predicted transport of two S-triazine herbicides through soil columns

Effluent concentration of chloride and two pesticides (prometon and atrazine) were measured during column displacement experiments at two water flow rates. A constant suction of approximately 1300 Pa was maintained in the packed soil columns which were positioned vertically on top of a vacuum chamber enclosing an automatic fraction collector. Measured breakthrough curves (BTC's) were analyzed in terms of two solute transport models: the standard two-parameter convection-dispersion equation (CDE), and a four-parameter two-site/two-region nonequilibrium model (TRM). Calculations obtained with the TRM model were found to be in better agreement with measured BTC's than predictions using the CDE model. Column retardation factors for prometon and artrazine calculated from equilibrium batch sorption coefficients were comparable to those estimated from the observed BTC's only when the nonequilibrium TRM model was used.

Comparison of Models for Describing the Transport of Dissolved Organic Carbon in Aquifer Columns

AbstractDissolved organic carbon (DOC) is a reactive constituent in aquifer and soil media and equilibrates between the mobile aqueous phase and the immobile solid phase. Since DOC is believed to accelerate the transport of associated contaminants, yet is known to interact with aquifer and soil material, our objective was to define and model those processes governing DOC adsorption to porous media that may affect the cotransport of contaminants. Column displacement experiments of DOC through aquifer sediments were modeled with various formulations of the convective‐dispersive equation, which considered time‐dependent adsorption reactions and linear or nonlinear adsorption processes. Batch equilibrium adsorption isotherm and kinetic studies for DOC interaction with the sediments were used to parameterize the transport models. The equilibrium DOC‐adsorption isotherm was nonlinear and was adequately described by the Langmuir equation. The adsorption of DOC to the aquifer sediment was also time dependent and the batch results could be described by two simultaneous reaction rates for solution concentrations ≥ 10 mg DOC L−1 and a single reaction rate for solution concentrations <10 mg DOC L−1. Observed DOC breakthrough curves (BTCs) with influent concentrations ≥ 10 mg DOC L−1, were adequately modeled as two‐site, non‐linear adsorption processes, with DOC interactions with both types of sites being time dependent. Batch adsorption and kinetic parameters were generally successful in describing DOC transport; however, the magnitude of the initial batch rate coefficient was significantly larger than that observed for the displacement experiments. The extended tailing of the observed DOC BTCs was influenced more by the slow, time‐dependent adsorption of DOC during transport than to the non‐linear features of the adsorption isotherms. Observed DOC BTCs with influent concentrations <10 mg DOC L−1 did not exhibit extensive tailing and were modeled as a one‐site, linear or nonlinear, time‐dependent adsorption process.

Modeling the Transport of Inorganic Ions Through Undisturbed Soil Columns from Two Contrasting Watersheds

AbstractThe transport of several inorganic cations and anions (i.e., Mg2+ NH+4, Br−, NO−3), via saturated flow, through large undisturbed soil columns from two contrasting watersheds was investigated. The mobility of nonreactive tracers was described well with the convection‐dispersion (CD) equation using flux‐averaged solute detection in the effluent. Reactive tracers were also modeled well using various linear and nonlinear, equilibrium and nonequilibrium, transport codes based on the CD equation. Appropriate independent estimates of model predicted chemical retardation were possible only through experimental development of “dynamic isotherms” using a steady flow method on small undisturbed soil columns. The latter condition can accurately predict solute retardation on large soil columns, since tracer movement is largely controlled by the physical aspects of the soil (e.g., preferential flow zones). Adsorption isotherms generated on disturbed soils using a batch equilibration method, overestimated the amount of solute retardation from displacement experiments on the large undisturbed soil columns. The use of batch adsorption measurements in curve‐fitting transport models to describe data from saturated column displacement experiments on undisturbed soils, may result in erroneous transport parameter estimates.