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Evaluation of Modeling Approaches for Sorption-Desorption Processes in Flow-Through Soil Columns
Abstract
The influence of soil organic matter and pore water velocity on the transport of naphthalene in flow-through columns was investigated. Pulse injection experiments were conducted using three different soils (with 0%, 1.9%, and 3.9% organic content by weight) and flow rates (0.05, 0.1, and 0.2 mL=min) and solvent extractions were performed to measure the nondesorbable naphthalene fraction. To describe interactions between contaminants in the aqueous and solid phases, observed breakthrough data were described using two-site models, four different formulations of a three-site model, and two fully kinetic models. While the two-site models did not adequately describe the breakthrough data for all cases, simulations based on the three-site models matched the observations well with the exception of high organic content soils and high flow rates. For soils with high organic content, the fully kinetic models described the observed data better compared to the three-site models. Results indicate that as long as the equilibrium, rate-limited, and irreversible sorption domains are included in the models, different conceptualizations about how contaminants interact in the aqueous and solid phases do not produce significantly different results.
... Different models, such as pseudofirst and pseudosecond-order, exponential, and Elovich, have been used to describe adsorption kinetics for various adsorbents (ash, sewage sludge, leaves, charcoal, minerals, peat, tropical soils, and biopolymers) and adsorbates (metal ions, dyes, organic compounds, and pesticides), as summarized by Ho and Mckay (1999), Hamdaoui and Naffrechoux (2007), Liu and Shen (2008), Wu et al. (2009), Plazinski et al. (2009, Kumar andPorkodi (2009), Haerifar andAzizian (2013), Pal et al. (2016), Vega and Boscov (2016), Bashiri and Javanmardi (2017), Dickson et al. (2017), and Eris and Azizian (2017). Raganati et al. (2020), Hu and Zhang (2020), Phanikumar et al. (2022), andHu et al. (2022) conducted a comprehensive review of the kinetic models developed to identify the selection criteria and mathematical characteristics of each type. Azizian (2004) suggested a new general adsorption kinetics model, which he solved analytically for two particular cases. ...
Petroleum hydrocarbon (PHC) contamination is a global environmental issue. Understanding the key factors and mechanisms controlling the fate and mobility of PHCs in soils and aquifers is critical for environmental risk assessment, the development of remediation strategies, and policy decisions. This study focuses on the effects of soil composition and temperature on the sorption and desorption of two representative aromatic PHC compounds: naphthalene and benzene. The experiments were carried out using artificial sandy loam soil mixtures with temperatures ranging from 3 to 25°C. As expected, the sorption capacities of the soils were primarily controlled by the organic carbon content, while barely affected by the clay content. The sorption data for benzene and naphthalene followed linear to near-linear isotherms. Naphthalene sorption further increased with decreasing temperature, whereas temperature had little effect on benzene sorption. The latter was consistent with the very small magnitude of the sorption enthalpy of benzene. Under imposed dynamic temperature fluctuations, naphthalene sorption and desorption were shown to be reversible: model simulations demonstrated minimal kinetic limitation of the temperature-dependent soil-water partitioning. Our results imply that even in simple artificial soil systems, temperature variations can have complex, but predictable, effects on the soil-pore water partitioning of PHCs and, hence, on their mobility and bioavailability. Understanding the role of temperature is thus a prerequisite to unraveling the coupled abiotic and biotic processes that modulate the fate of PHCs in real-world soils.
Core Ideas
Vegetative buffer strip (VBS) soils sorb imidacloprid (ICD) more than cropland soils due to greater SOC.
Riparian VBS soils retard ICD leaching more than cropland and grass VBS soils.
Results indicate ICD sorption to two or three sites, likely through different mechanisms.
A VBS may reduce the mobility of ICD in agroecosystems.
An understanding of neonicotinoid sorption and transport in soil is critical for determining and mitigating environmental risk associated with the most widely used class of insecticides. The objective of this study was to evaluate mobility and transport of the neonicotinoid imidacloprid (ICD) in soils collected from cropland, grass vegetative buffer strip (VBS), and riparian VBS soils. Soils were collected at six randomly chosen sites within grids that encompassed all three land uses. Single‐point equilibrium batch sorption experiments were conducted using radio‐labeled ( ¹⁴ C) ICD to determine solid–solution partition coefficients ( K d ). Column experiments were conducted using soils collected from the three vegetation treatments at one site by packing soil into glass columns. Water flow was characterized by applying Br ⁻ as a nonreactive tracer. A single pulse of ¹⁴ C‐ICD was then applied, and ICD leaching was monitored for up to 45 d. Bromide and ICD breakthrough curves for each column were simulated using CXTFIT and HYDRUS‐1D models. Sorption results indicated that ICD sorbs more strongly to riparian VBS ( K d = 22.6 L kg ⁻¹ ) than crop ( K d = 11.3 L kg ⁻¹ ) soils. Soil organic C was the strongest predictor of ICD sorption ( p < 0.0001). The column transport study found mean peak concentrations of ICD at 5.83, 10.84, and 23.8 pore volumes for crop, grass VBS, and riparian VBS soils, respectively. HYDRUS‐1D results indicated that the two‐site, one‐rate linear reversible model best described results of the breakthrough curves, indicating the complexity of ICD sorption and demonstrating its mobility in soil. Greater sorption and longer retention by the grass and riparian VBS soils than the cropland soil suggests that VBS may be a viable means to mitigate ICD loss from agroecosystems, thereby preventing ICD transport into surface water, groundwater, or drinking water resources.
Understanding the dynamics of sorption and bioavailability is crucial to
the success of transport models as bioavailability often limits the
complete bioremediation of contaminated soils. This paper examines the
interplay between sorption and bioavailability with pulsed injection of
nutrients based on a mechanistic model of microbially mediated reactive
transport. We used a dimensionless parametric approach based on
nondimensional groups such as the Damköhler and Peclet numbers to
assess the relative importance of processes and rates. We consider case
studies involving the biodegradation of carbon tetrachloride (CT) as
well as a chemically induced degradation system to evaluate the effects
of bioavailability. We first used these two cases to explore the effects
of selectively activating the degradation terms in the aqueous and
sorbed phases. The models for these two cases consistently predict that
degradation is insensitive to retardation if degradation terms are not
included for the sorbed phase. A specific mass removal rate was
developed as an efficiency metric to explore the effects of pulsed
nutrient injection on contaminant degradation and to estimate an optimal
injection interval. The contaminant mass degraded per unit pumping was
shown to be significantly higher for pulsed injection of substrates than
with continuous injection. The presented results clearly indicate that
considerations of bioavailability profoundly alter model predictions of
degradation as well as parameter estimation results.
This paper evaluates the microbial transport and degradation processes associated with carbon tetrachloride (CT) biodegradation in laboratory aquifer columns operated with a pulsed microbial feeding strategy. A seven component reactive transport model based on modified saturation kinetics and on a two-site sorption model was developed to describe the linked physical, chemical, and biological processes involved in CT degradation by Pseudomonas stutzeri KC, a denitrifying bacterium that cometabolically converts CT to harmless end products. After evaluating several expressions for attachment and detachment, we selected a dynamic partitioning model in which strain KC detachment decreases at low substrate concentrations. The resulting model enabled improved understanding of the complex coupled processes operative within our system and enabled us to test a model for field-scale design and transport studies. Batch studies were used to identify initial degradation and microbial transport processes, and constrained optimization methods were used to estimate a set of reaction rates that best describe the column experiment data. The optimal set of parameters for one column provided a reasonable prediction of solute and microbial concentrations in a second column operated under different conditions, providing an initial test of the model. This modeling strategy improved our understanding of biodegradation processes and rates. The CT degradation rate in the columns was lower than values obtained from batch studies, and processes in addition to the growth and decay of strain KC cells (due to native flora) are necessary to describe the observed nitrate consumption.
A comparison is made between observed tritium effluent concentra-tion distributions and those calculated with a previously published an-alytical solution for the movement of chemicals through unsaturated, aggregated sorbing media. In the analytical model the liquid phase of the soil is divided into mobile and immobile regions, with transfer be-tween the two regions diffusion controlled. Effluent data obtained from several displacements of tritium through 30-cm long columns of Glendale clay loam were used to determine the different parameters in the analytical solution by curve fitting. The data indicate some adsorp-tion or isotopic exchange of tritium during its flow through the soil col-umns. The amount of immobile water increases with decreasing flow velocity and increasing aggregate size, and varies between 6 and 45% of the total volume of water in the columns. The analytical solution provides an excellent description of the experimental effluent data, and shows that tailing can be explained satisfactorily by diffusional ex-change of tritium between mobile and immobile regions of the soil. Additional Index Words: intra-aggregate diffusion, immobile water, miscible displacement
Breakthrough curves (BTC's) of 3H2O and
36Cl simultaneously displaced through columns of
various-sized aggregates of an Ione Oxisol soil were measured under
water-saturated steady flow conditions. The data were simulated using
two conceptual models. In model I, all soil water was assumed to be
mobile with a physical equilibrium existing in the system. For model II,
soil water was partitioned into mobile and immobile regions. Convective
diffusive solute transport was limited to the mobile water region.
Transfer of a tracer between the two soil water regions was assumed to
occur at a rate proportional to the difference in tracer concentration
between the two regions. Sorption Of 3H2O and
36Cl was considered to be an instantaneous linear and
reversible process. The two unknown parameters in model I and the four
unknown parameters in model II were estimated by fitting model
predictions to the experimental data. Model I could only describe BTC's
obtained from columns packed with small aggregates (0.5-1 mm) and for
displacements run at small fluxes (0.2 cm/h), whereas model II described
all the BTC's well. Peclet numbers P in model II, as measured on each
separate column, were essentially constant, indicating a linear
relationship between the apparent diffusion coefficient D and the mobile
pore water velocity vm. The fraction of soil water that is
mobile, Φ, and the mass transfer coefficient α were found to
be a function of the physical and chemical properties of the porous
medium (aggregate size, pore water velocity, and solution
concentration).
Analytical solutions are presented for two convection-dispersion type transport models useful for studying simultaneous pesticide sorption and degradation. One solution is for the familiar two-site sorption model in which adsorption-desorption proceeds kinetically on one fraction of the sorption sites, and at equilibrium on the re-maining sites. Another solution holds for two-region (or mobile-im-mobile liquid phase) transport appropriate for aggregated or frac-tured media. The transport models account for degradation in both the solution and sorbed phases. The dimensionless analytical so-lutions for the two-site and two-region models are shown to be iden-tical; they contain up to six independent dimensionless parameters: a column Peclet number, a retardation factor, a coefficient parti-tioning the soil/chemical system in equilibrium and nonequilibrium parts, a rate coefficient, and two dimensionless degradation coeffi-cients. One of the two independent degradation coefficients may be eliminated when the solution and sorbed phase degradation rate coef-ficients are assumed to be identical, or when, with additional but reasonable assumptions, adsorbed phase degradation is assumed to be negligible.
vegetation by moving to off-site areas, and may contami- nate drinkable water sources. Conversely, persistent We investigated atrazine (2-chloro-4-(ethylamino)-6-(isopropyl- nonselective herbicides that are immobile in soils can amino)-s-triazine) leaching, sorption, and desorption in a Brazilian Oxisol under no-till (NT) and conventional (CON) agricultural man- cause problems with carryover. Therefore, the mobility agement. The specific objectives were (i) to infer characteristics of of herbicides in soils is both of agronomic and of envi- the sorption process (equilibrium vs. nonequilibrium sorption, revers- ronmental concern. Scientists have concentrated their ible vs. nonreversible sorption) from breakthrough experiments, (ii) efforts on the elucidation of herbicide mobility in differ- to compare sorption parameters derived from breakthrough and batch ent studies on sorption, dissipation, and leaching of pes- experiments, (iii) to predict the distribution of atrazine on reversible ticides using soil samples in the laboratory or under and irreversible sorption sites within the soil columns using transport field conditions. parameters derived from breakthrough data, and (iv) to evaluate the Atrazine is a herbicide widely used in Brazil to control atrazine metabolites during breakthrough experiments. Breakthrough broadleaf weeds in corn (Zea mays L.) and sorghum curves of radiolabeled atrazine and of a nonreactive tracer, Br, were (Sorghum bicolor (L.) Moench) under both NT and measured during miscible displacement experiments in unsaturated soil columns. The concentration of metabolites in the leachate was CON agricultural managements. It is applied in pre- determined at three different times. At the end of the displacement emergence treatment and inhibits the Hill reaction in experiment, which lasted 494 h, the distributions of atrazine and its photosynthesis (Rodrigues and Almeida, 1995). metabolites within the soil columns and their partitioning in de- Atrazine and its dealkylated metabolites deethyl- sorbable, extractable and nonextractable fractions were determined. atrazine (DEA) and deisopropylatrazine (DIA) have Atrazine breakthrough curves were fitted with a three-site chemical been detected in surface and groundwater (Thurman et nonequilibrium convective dispersive transport model considering ir- al., 1991, 1994; Cai et al., 1994; Lerch et al., 1998). To reversible sorption. The three-site chemical nonequilibrium model date there are no data for Brazilian conditions. The predicted that around 40% of the applied atrazine was irreversibly principal causes for finding of atrazine in streams and sorbed at the end of the leaching experiment. This corresponded with shallow groundwater are its widespread use in conjunc- sum of the measured extractable and nonextractable fractions. The Kd values for reversible sorption sites derived from the BTCs were tion with its moderate persistence in soil (Mersie and similar to those derived from 24-h batch adsorption experiments. Seybold, 1996). However, irreversible sorption was not observed in a four-step consec- Several studies of atrazine behavior in soils have utive batch desorption experiment that lasted 120 h in total. Differ- shown a wide range in the sorption partition coefficient ences in desorption between the column and batch experiments were (Kd), which was found to be proportional to the soil probably due to differences in time-scale and sorption-desorption organic C content (e.g., Mersie and Seybold, 1996; Mo- conditions. More than 90% of the radioactivity in the leachate and reau and Mouvet, 1997; Moorman et al., 2001). In the 80% remaining in the soil column, respectively, were characterized database compiled by Wauchope et al. (1992), an aver- as atrazine, indicating a slow decay. Hydroxyatrazine was the most
1] Bioremediation can be a cost-effective approach to clean up groundwater contaminants, especially in cases where numerical models have been used to evaluate microbial processes and design remediation strategies. This paper describes the three-dimensional reactive transport modeling of carbon tetrachloride (CT) bioremediation at the Schoolcraft site in western Michigan. A ''biocurtain'' was created to remediate this site using a recirculation well gallery installed normal to groundwater flow, which was inoculated with nonnative microbes and fed with weekly additions of electron donor (acetate) and nutrients. The model simulates the transport and reactions of aqueous and sorbed phase CT, acetate, electron acceptor (nitrate), mobile and immobile microbes, and tracer (bromide). Simulated microbial processes include growth, decay, attachment, detachment, and endogenous respiration. This model was used to predict solute concentrations across the site using laboratory-based reaction parameters and to evaluate changes in rates from the laboratory to the field. A reasonable agreement was found between predicted and observed acetate and nitrate concentrations; however, a lower CT degradation rate was needed to describe the CT concentrations observed after the inoculation event.
Bioavailability of pesticides sorbed to soils is an important determinant of their environmental fate and impact. Mineralization of sorbed atrazine was studied in soil and clay slurries, and a desorption-biodegradation-mineralization (DBM) model was developed to quantitatively evaluate the bioavailability of sorbed atrazine. Three atrazine-degrading bacteria that utilized atrazine as a sole N source (Pseudomonas sp. strain ADP, Agrobacterium radiobacter strain J14a, and Ralstonia sp. strain M91-3) were used in the bioavailability assays. Assays involved establishing sorption equilibrium in sterile soil slurries, inoculating the system with organisms, and measuring the CO(2) production over time. Sorption and desorption isotherm analyses were performed to evaluate distribution coefficients and desorption parameters, which consisted of three desorption site fractions and desorption rate coefficients. Atrazine sorption isotherms were linear for mineral and organic soils but displayed some nonlinearity for K-saturated montmorillonite. The desorption profiles were well described by the three-site desorption model. In many instances, the mineralization of atrazine was accurately predicted by the DBM model, which accounts for the extents and rates of sorption/desorption processes and assumes biodegradation of liquid-phase, but not sorbed, atrazine. However, for the Houghton muck soil, which manifested the highest sorbed atrazine concentrations, enhanced mineralization rates, i.e., greater than those expected on the basis of aqueous-phase atrazine concentration, were observed. Even the assumption of instantaneous desorption could not account for the elevated rates. A plausible explanation for enhanced bioavailability is that bacteria access the localized regions where atrazine is sorbed and that the concentrations found support higher mineralization rates than predicted on the basis of aqueous-phase concentrations. Characteristics of high sorbed-phase concentration, chemotaxis, and attachment of cells to soil particles seem to contribute to the bioavailability of soil-sorbed atrazine.
Although microbial reductive dechlorination (MRD) has proven to be an effective approach for in situ treatment of chlorinated ethenes, field implementation of this technology is complicated by many factors, including subsurface heterogeneity, electron donor availability, and distribution of microbial populations. This work presents a coupled experimental and mathematical modeling study designed to explore the influence of heterogeneity on MRD and to assess the suitability of microcosm-derived rate parameters for modeling complex heterogeneous systems. A Monod-based model is applied to simulate a bioremediation experiment conducted in a laboratory-scale aquifer cell packed with aquifer material from the Commerce Street Superfund site in Williston, VT. Results reveal that (uncalibrated) model application of microcosm-derived dechlorination and microbial growth rates for transformation of trichloroethene (TCE), cis-1,2-dichloroethene (cis-DCE), and vinyl chloride (VC) reproduced observed aquifer cell concentration levels and trends. Mean relative errors between predicted and measured effluent concentrations were quantified as 6.7%, 27.0%, 41.5%, 32.0% and 21.6% over time for TCE, cis-DCE, VC, ethene and total volatile fatty acids (fermentable electron donor substrate and carbon source), respectively. The time-averaged extent of MRD (i.e., ethene formation) was well-predicted (4% underprediction), with modeled MRD exhibiting increased deviation from measured values under electron donor limiting conditions (maximum discrepancy of 14%). In contrast, simulations employing a homogeneous (uniform flow) domain resulted in underprediction of MRD extent by an average of 13%, with a maximum discrepancy of 45%. Model sensitivity analysis suggested that trace amounts of natural dissolved organic carbon served as an important fermentable substrate, providing up to 69% of the reducing equivalents consumed for MRD under donor-limiting conditions. Aquifer cell port concentration data and model simulations revealed that ethene formation varied spatially within the domain and was associated with regions of longer residence times. These results demonstrate the strong influence of subsurface heterogeneity on the accuracy of MRD predictions, and highlight the importance of subsurface characterization and the incorporation of flow field uncertainty in model applications for successful design and assessment of in situ bioremediation.
Measured (or empirically fitted) reaction rates at groundwater remediation sites are typically much lower than those found in the same material at the batch- or laboratory-scale. The reduced rates are commonly attributed to poorer mixing at the larger scales. A variety of methods have been proposed to account for this scaling effect in reactive transport. In this study, we use the Lagrangian particle tracking and reaction (PTR) method to simulate a field bioremediation experiment at the Schoolcraft, Michigan site. A denitrifying bacterium, Pseudomonas Stutzeri strain KC (KC), was injected to the aquifer, along with sufficient substrate, to degrade the contaminant, Carbon Tetrachloride (CT), under anaerobic conditions. The PTR method simulates chemical reactions through probabilistic rules of particle collisions, interactions, and transformations to address the scale effect (lower apparent reaction rates for each level of upscaling, from batch- to column- to field-scale). In contrast to a prior Eulerian reaction model, the PTR method is able to match the field-scale experiment using the rate coefficients obtained from batch experiments.
Solute breakthrough curves (BTC) resulting from miscible displacement of ³HâO in undisturbed soil columns under a range of soil-water tensions were evaluated in terms of the mobile-immobile (MIM) water model and the convective-dispersive (CD) model. The BTC performed under tensions > 0.1 kPa were approximately symmetric in shape and accurately described by the CD model, whereas BTC performed under tensions of 0 or 0.1 kPa were quite asymmetric and better described by the MIM model. Application of tension resulted in about a 10- to 20-fold decrease in hydraulic conductivity (K(Phi)) with relatively little change in soil-water content (Phi). Highly asymmetric BTC were attributed to bypassing or preferential flow along macropores. Thus, in terms of model selection, bypassing was significant only when soil-water contents were at or very near saturation. Rhodamine B dye patterns obtained under saturated conditions showed the soil-water flow was confined to small regions within the columns. However, easily identifiable, discrete channels were not observed in these regions. It appears that flow was conducted via a series of relatively large pores, or continuous pore sequences. Application of tension appears to have disconnected the most rapidly conducting pore sequence, thus reducing the asymmetry in measured BTC and compressing the dye solute front. Even so, dye patterns showed that flow as very heterogeneous, as was also evidenced by high dispersion coefficients in the ³HâO experiments. Comparison of the frequency distributions of field-measured saturated hydraulic conductivity values and measured rainfall intensities indicated that saturated conditions, and hence significant bypassing, are not expected to occur in this soil under field conditions.
Batch and column techniques were utilized to determine retardation coefficients of two nonionic organic compounds: benzene and dimethylphthalate. Three sandy soil materials with medium to high organic C content were used as sorbents. Batch data consistently overestimated retardation coefficients of the two compounds. Several previously reported factors that may cause discrepancy between the two techniques were experimentally investigated. Sorption nonsingularity, sorption nonequilibrium, presence of immobile water regions in the column, and reduction in particle spacing in the column were eliminated as the cause of this discrepancy. Loss of sorbent from the column is unlikely to be the cause of the discrepancy. Although rapidly mixed batch tubes are subjected to soil abrasion, it causes no apparent impact on the value of sorption distribution coefficients. Batch isotherms at two solids concentration were identical, indicating that differences in solids concentration between batch and column setup is not a significant factor. This was also confirmed by a column study. Sorption nonlinearity was found to have a minor impact. Column residence time had a major impact on one of the cases studied, but no effect on the other cases. When column residence time was accounted for, some differences in sorption distribution coefficients, which seemed to be independent of the organic C content, were noticed. In this study, all previously suggested causes for the discrepancy between batch- and column-determined retardation coefficients were investigated and rejected. It remains unclear why the values determined by the two techniques were different.
Breakthrough curves (BTC) from miscible displacement of two pesticides through three soils were measured for two input concentrations of each pesticide. These BTC data were used to evaluate two conceptual models for describing the nonequilibrium adsorption-desorption of pesticides in soils under steady-state water flow conditions. In both models, adsorption on one group of sites was assumed to be instantaneous, while the rate of adsorption on the second group of sites followed either nonlinear reversible kinetics (Model I) or was a diffusion-controlled process (Model II). Parameters in both models were estimated by curve-fitting model predictions to one set of measured BTC data using a nonlinear least-squares optimization procedure. These parameter values were used to verify the conceptual models by comparing simulated and measured BTC for a different input concentration.
Soil contact time with organic chemicals (aging) has been found creased with increased aging (McCall and Agin, 1985; to cause increased sorption coefficients and reduced isotherm linearity as well as increased environmental sequestration and persistence. In Pavlostathis, 1992; Pignatello, 1990). From these exam- this study, the sorption/desorption behavior of chlorobenzene (CB) ples, it seems apparent that soil-chemical contact time on four soils was evaluated after aging periods of 24 h and 14 mo. does have an effect on sorption/desorption behavior, The four soils ranged in organic matter content from 0.69 to 13.4%. hence fate, and that these processes are not always rapid Sorption isotherms were performed after each aging period to observe and reversible. The specific effects of aging time on con- changes in CB uptake. Desorption kinetic profiles were generated taminant behavior, however, are not well defined or after each aging period to observe changes in CB release from soil, understood. with desorption followed for a period of four months. These data Despite empirical evidence that aging can affect sorp- demonstrated no increase in CB uptake by three soils between the tion/desorption processes, there are relatively few long- 24-h and 14-mo sorption periods and only a slight increase (22%) term, laboratory-controlled aging studies that explicitly
Three models for sorption/desorption of polycyclic aromatic hydrocarbon (PAH) contaminants from soil were compared for their ability to predict the transport of PAH in soil: a “gamma” model, a “two-site/two-region” nonequilibrium model, and a “hybrid” model. In the “hybrid” model, soil organic matter was conceptually divided into two compartments; a fraction with rapid sorption/desorption kinetics and a compartment with mass-transfer-limited kinetics. Contaminant sorbed in the rapid compartment was assumed to be in instantaneous equilibrium with the aqueous phase, while the release of contaminant from the slow fraction was assumed to be governed by a gamma distribution of rate coefficients. The “hybrid” model successfully described the initial rapid release of a model PAH contaminant, naphthalene, from a sieved soil sample of moderate organic content (2.3%) as well as the following slow release observed over 25 days in batch desorption experiments. Other necessary model parameters, such as the hydrodynamic dispersion coefficient of naphthalene and the macropore porosity, were evaluated in separate experiments. A transport model incorporating the “hybrid” model for naphthalene sorption/desorption successfully predicted the elution profile of naphthalene in independent soil-column experiments with no adjustable parameters. The success of the hybrid model suggests that a wide array of rate controls govern PAH desorption. This conclusion is consistent with the view of soils as consisting of a mix of different sorptive constituents and heterogeneous physical constraints on PAH release.
The desorption kinetics of PCBs and chlorobenzenes have been studied at 5, 20, and 60 °C for model sorbents in which either micropore diffusion (zeolite, montmorillonite, and XAD-8) or organic matrix diffusion/entrapment (rubbery polyacetal and glassy polystyrene) could occur. Also, a sediment was studied whose organic matter (OM) had been completely removed. All sorbents exhibited slow desorption (rate constants (1−5 × 10-3 h-1). The sediment without OM showed significantly smaller slowly desorbing fractions (factor 3−8) than the original sediment (about 6% OM). Sorbent−water distribution ratios of the microporous sorbents and the sediment without OM were 10−100 times lower than the ones of the original sediment. So, although the presence of both mineral micropores and/or OM can result in slow desorption behavior of organic compounds from soils and sediments, OM is more important for slow desorption than mineral micropores in sediments with more than about 0.1−0.5% OM. The sorption and desorption parameters measured for the sorbents were compared to the ones measured for sediment. This analysis showed that the observations for XAD-8 (in which slow desorption is assumed to be caused by slow diffusion along hydrophobic pore walls) were most similar to the ones for the sediment, indicating that diffusion through pores in the organic matter or pores coated with organic material play roles in slow desorption.
Release rates of naphthalene from suspensions of freshly contaminated (days to weeks) and aged (approximately 30 years) soil samples were obtained using a gas purge method. A continuously increasing resistance to desorption was observed with increasing purge time. Initial desorption rates were similar to those estimated using available empirical relationships, but subsequent desorption rates were lower by more than 1 order of magnitude. A model incorporating a continuum of compartments with a gamma ([Gamma]) distribution of rate coefficients was postulated to describe the experimental data. An analytical equation with two adjustable parameters was obtained for the mass fraction desorbed. Release profiles with this [open quotes][Gamma] model[close quotes] were able to describe the experimental release profiles for long term desorption experiments. An implication of the gamma model is that increased incubation time will allow organic compounds to be sorbed to compartments or regions in the sorbent that exhibit slow adsorption/desorption kinetics. This has important implications for the fate and remediation of sites that have been contaminated with hydrophobic organic compounds for extended time periods. 31 refs., 6 figs., 3 tabs.
This study describes the formation of slowly reversible sorbed fractions of various halogenated alkanes and alkenes in two surface soils. After the initial sorption equilibration period, the compounds were desorbed by one of two methods. The first used repetitive batch extraction with water. After 16 extractions of 24 to 72 h each, concentrations in the aqueous phase reached low values and a slow desorbing, residual fraction remained in the soil. The residual fraction in the soil was determined independently after extraction with hot acetone, whose efficacy was demonstrated. During desorption, apparent soil-water distribution coefficients increased progressively to as much as 200 times greater than equilibrium sorption coefficients, Kd, obtained separately from sorption isotherms. With increasing sorption equilibration time, the residual became greater in magnitude and less mobile. The second desorption method simulated desorption to infinite dilution over a 96 h period by using Tenax GC polymeric adsorbent beads included in the suspension as a sink for desorbed chemical. Control experiments proved the usefulness of Tenax and showed that desorption from the soil was rate limiting. All compounds studied formed slowly reversible fractions in the soils. This fraction amounted to several percent of the total sorbed from solution. The results indicate that formation of slowly reversible fractions is probably typical of nonpolar organic compounds, including those with weak sorbing tendencies.
The influence of organic carbon content at various flow rates on the transport of aqueous benzene in a sandy soil was investigated. Column experiments were conducted for a sandy soil with different carbon contents (0%, 0·5%, and 2% by weight) and flow rates (0·25, 0·625, and 1·25 ml min−1) to observe breakthrough curves of KCl and benzene using a step application. Three types of convection–dispersion transport model (equilibrium, reversible two-site, and reversible–irreversible sorption) were used to determine the appropriate model that best described the observed benzene transport. The modelling results revealed that the reversible–irreversible transport model was suitable for describing the transport and sorption behaviour of benzene in the sandy soil studied. Retardation and irreversible sorption coefficients increased with increasing carbon content, since the increment of carbon content resulted in the enhancement of sorption capacity for benzene. With increasing flow rate, decreased retardation of aqueous benzene was observed due to less reaction time; however, the model parameter for irreversible sorption increased unrealistically. This was attributed to the interference of velocity change on the irreversible sorption coefficient during parameter estimation. In order to avoid this problem, we introduced a Damköhler number (Da1), which took into account the velocity changes in estimating the irreversible sorption coefficient. A reasonable relationship was then found between Da1 and flow rate, which showed a decrease of Da1 with the increase in flow rate. Copyright © 2006 John Wiley & Sons, Ltd.
Soil–chemical contact time (aging) is an important determinant of the sorption and desorption characteristics of the organic contaminants and pesticides in the environment. The effects of aging on mechanism-specific sorption and desorption of atrazine were studied in soil and clay slurries. Sorption isotherm and desorption kinetic experiments were performed, and soil–water distribution coefficients and desorption rate parameters were evaluated using linear and non-linear sorption equations and a three-site desorption model, respectively. Aging time for sorption of atrazine in sterilized soil and clay slurries ranged from 2 days to 8 months. Atrazine sorption isotherms were nearly linear (r2>0.97) and sorption coefficients were strongly correlated to soil organic carbon content. Sorption distribution coefficients (Kd) increased with increase in age in all five soils studied, but not for K-montmorillonite. Sorption non-linearity did not increase with increase in age except for the Houghton muck soil. Desorption profiles were well described by the three-site desorption model. The equilibrium site fraction (feq) decreased and the non-desorbable site fraction (fnd) increased as a function of aging time in all soils. For K-montmorillonite, fnd≈0 regardless of aging, showing that aging phenomena are sorbent/mechanism specific. In all soils, it was found that when normalized to soil organic matter content, the concentration of atrazine in desorbable sites was relatively constant, whereas that in non-desorbable site increased. This, and the lack of aging effects on desorption from montmorillonite, suggests that sorption into non-desorbable sites of soil organic matter is primary source of increased atrazine sorption in soils during aging.
Rates and extents of phenanthrene desorption were studied for more than 250 days as functions of sorbent type, initial loading level, and aging. Apparent first-order desorption rate constants for the slowly desorbing fraction were found to (i) range from 0.00086 to 0.148 days-1 for geosorbents that contain geologically mature kerogen and less rigid humic-type soil organic matter, respectively, (ii) decrease by as much as an order of magnitude with decreasing initial sorbed solid-phase phenanthrene concentration, (iii) decrease by a factor of 2 with increasing aging time for a humic topsoil but remain unaffected by aging time beyond 3 months for a shale, and (iv) be 1-2 orders of magnitude lower than rate constants for the rapidly desorbing phenanthrene fractions for any given contaminated sample. Six models were used to fit the desorption rate data. Biphasic diffusion and biphasic first-order models with three fitting parameters possess broad utility and are potentially useful in a variety of environmental applications. Disadvantages of a five-parameter triphasic first-order desorption model, a two-parameter gamma-function model, and a one- or two-parameter pore diffusion model are also discussed.
The degradation of naphthalene was studied in soil-slurry systems, and a quantitative model was developed to evaluate the bioavailability of sorbed-phase contaminant. Four soils with different organic matter contents were used as sorbents. Two naphthalene-degrading organisms, Pseudomonas putida G7 and NCIB 9816-4, were also selected. Sorption isotherms and single and series dilution desorption studies were conducted to evaluate distribution coefficients, desorption parameters, and the amount of non-desorbable naphthalene. Biodegradation kinetics were measured in soil extract solutions and rate parameters estimated. Bioavailability assays involved establishing sorption equilibrium, inoculating the systems with organisms, and measuring naphthalene concentrations in both sorbed and dissolved phases over time. For all four soils, the sorption isotherms were linear, and desorption could be described by a model involving three types of sites: equilibrium, nonequilibrium, and non-desorption. Enhanced bioavailability, as evidenced by faster than expected degradation rates based on liquid-phase concentrations, were observed in soils with the higher sorption distribution coefficients. These observations could be described using model formulations that included solid-phase degradation. In all soils studied, degradation of non-desorbable naphthalene was observed.
The degradation of naphthalene in soil-slurry systems was studied using four different organisms and two soils. Organisms with zero-order, first-order, and Michaelis-Menten rates were selected. The soils had substantially different sorption distribution coefficients. Sorption and desorption was evaluated in abiotic soil-slurry systems. The desorption process was described by a model that accounts for equilibrium, rate-limited and non-desorbing sites. Biodegradation parameters were measured in soil-extract solutions. Bioavailability assays, inoculated soil slurries, were conducted and both liquid- and sorbed-phase naphthalene concentrations were measured over time. For the less sorptive soil, the results could be explained by sequential desorption and degradation processes. For the other soil, enhanced degradation was clearly observed for the organisms with first-order and Michaelis-Menten rates. Several explanations are explored for these observations including direct sorbed-phase degradation and the development of elevated substrate concentrations at the organism/sorbent interface. No enhancement was found for the organism with zero-order kinetics.
Uncertainties in model structures have been recognised often to be the main source of uncertainty in predictive model simulations. Despite this knowledge, uncertainty studies are traditionally limited to a single deterministic model and the uncertainty addressed by a parameter uncertainty study. The extent to which a parameter uncertainty study may encompass model structure errors in a groundwater model is studied in a case study. Three groundwater models were constructed on the basis of three different hydrogeological interpretations. Each of the models was calibrated inversely against groundwater heads and streamflows. A parameter uncertainty analysis was carried out for each of the three conceptual models by Monte Carlo simulations. A comparison of the predictive uncertainties for the three conceptual models showed large differences between the uncertainty intervals. Most discrepancies were observed for data types not used in the model calibration. Thus uncertainties in the conceptual models become of increasing importance when predictive simulations consider data types that are extrapolates from the data types used for calibration.
Antibiotics, such as sulfadiazine, reach agricultural soils directly through manure of grazing livestock or indirectly through the spreading of manure or sewage sludge on the field. Knowledge about the fate of antibiotics in soils is crucial for assessing the environmental risk of these compounds, including possible transport to the groundwater. Transport of (14)C-labelled sulfadiazine was investigated in disturbed soil columns at a constant flow rate of 0.26 cm h(-1) near saturation. Sulfadiazine was applied in different concentrations for either a short or a long pulse duration. Breakthrough curves of sulfadiazine and the non-reactive tracer chloride were measured. At the end of the leaching period the soil concentration profiles were determined. The peak maxima of the breakthrough curves were delayed by a factor of 2 to 5 compared to chloride and the decreasing limbs are characterized by an extended tailing. However, the maximum relative concentrations differed as well as the eluted mass fractions, ranging from 18 to 83% after 500 h of leaching. To identify relevant sorption processes, breakthrough curves of sulfadiazine were fitted with a convective-dispersive transport model, considering different sorption concepts with one, two and three sorption sites. Breakthrough curves can be fitted best with a three-site sorption model, which includes two reversible kinetic and one irreversible sorption site. However, the simulated soil concentration profiles did not match the observations for all of the used models. Despite this incomplete process description, the obtained results have implications for the transport behavior of sulfadiazine in the field. Its leaching may be enhanced if it is frequently applied at higher concentrations.
Desorption of organic contaminants from soil can be modeled by dividing the desorption time-concentration profile into three distinct regimes. These are characterized by desorption that occurs faster than the experimental sampling scheme, at a rate that is captured by it, and at a rate for which the duration of the experiment and data uncertainty obscures the rate. Batch desorption curves for atrazine and naphthalene on four soils were experimentally generated to demonstrate the existence of discrete observational desorption regimes. Nine mathematical models, each containing mechanisms formulated to describe at least one of the three regimes, were fit to each contaminant-soil combination using the Gauss-Newton method for parameter estimation. Each of the nine models was ranked using the small-sample-corrected Akaike information criterion (AICc). By interpretation of the AICc values, the atrazine desorption data were best described by three regimes, while the naphthalene desorption data were best described by two regimes. Furthermore, for a given number of regimes, we could find no general basis to suggest that a particular type of rate model (chemical, physical, kinetic, or statistical) is intrinsically superior over another.
Field studies have demonstrated that prolonged pesticide-soil contact times (aging) may lead to unexpected persistence of these compounds in the environment. Although this phenomenon is well documented in the field, there have been very few controlled laboratory studies that have tested the effects of long-term aging and the role of differing sorbates on contaminant sorption-desorption behavior and fate in soils. This study examines the sorption-desorption behavior of chlorobenzene, ethylene dibromide (1,2-dibromomethane), atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), and 2, 4-D (2,4-dichlorophenoxyacetic acid) on one soil type after 1 d, 30 d, and 14 mo of aging. Sorption isotherms were evaluated after each aging period to observe changes in the uptake of each compound by soil. Desorption kinetic data were generated after each aging period to observe changes in release from soil, and desorption parameters were evaluated using a three-site desorption model that includes equilibrium, nonequilibrium, and nondesorption sites. The data indicate no statistically significant increase in sorption for ethylene dibromide or chlorobenzene from 1 to 30 d, although sorption of 2,4-D increased slightly, and sorption of atrazine decreased slightly. Statistically significant increases in linear sorption coefficients (Kd), from 1 d to 14 mo of aging, were apparent for ethylene dibromide and 2,4-D. The Kd values for chlorobenzene, measured after 1 d, 30 d, and 14 mo of aging, were statistically indistinguishable. Aging affected the distribution of chemicals within sorption sites. With aging, the desorbable fraction decreased and the nondesorbable fraction, which was apparent after only 1 d of pesticide-soil contact, increased for all chemicals studied.
Modeling desorption kinetics in soil columns
- I Aslam
Aslam, I. 2006. "Modeling desorption kinetics in soil columns." Ph.D.
dissertation, Dept. of Civil and Environmental Engineering, Michigan
State Univ.
Temperature dependence of slow adsorption and desorption kinetics of organic compounds in sediments
- G Cornelissen
- P C M Van Noort
- J R Parsons
- H A J Govers
Cornelissen, G., P. C. M. Van Noort, J. R. Parsons, and H. A. J. Govers.
1997. "Temperature dependence of slow adsorption and desorption
kinetics of organic compounds in sediments." Environ. Sci. Technol.
31 (2): 454-460. https://doi.org/10.1021/es960300+.
Transport of dissolved volatile organic compounds in the unsaturated zone
- M A Maraqa
Maraqa, M. A. 1995. "Transport of dissolved volatile organic compounds
in the unsaturated zone." Ph.D. dissertation, Dept. of Civil and Environmental Engineering, Michigan State Univ.
Bioavailability of sorbed organic contaminants
- J H Park
Park, J. H. 2000. "Bioavailability of sorbed organic contaminants."
Ph.D. dissertation, Dept. of Civil and Environmental Engineering,
Michigan State Univ.
Aging effect on mechanism-specific sorption and desorption of atrazine
- J H Park
- Y C Feng
- T C Voice
- S A Boyd
- Park J. H.
Sorption and desorption of the model aromatic hydrocarbons naphthalene and benzene: Effects of temperature and soil composition
- B Shi
- S K Ngueleu
- F Rezanezhad
- S Slowinski
- G J Pronk
- C M Smeaton
- K Stevenson
- R I Al-Raoush
- P Van Cappellen
- Shi B.