Equilibrium Sorption Coefficient Research Articles

Sorption of 32Si and 45Ca by isotopic exchange during recrystallisation of cement phases

The uptake kinetics of 32Si and 45Ca on cement minerals including C-S-H phases, portlandite, AFm phases, ettringite, and aged hardened cement paste were determined through batch sorption experiments. A two-step uptake kinetics was observed, with a fast initial step during ∼1 day followed by a much slower second step, not yet completed after one year. The working hypothesis that the fast uptake is caused by exchange of the radioisotopes with stable isotopes adsorbed on the mineral surface, whereas the slow uptake step is due to uptake in the crystal lattice during recrystallisation, was tested with the help of phenomenological models that combine surface adsorption and homogeneous recrystallisation. The experimental data could be reproduced satisfactorily using a refined version of the formerly published continuous homogeneous recrystallisation (CHOR) model, supporting the working hypothesis and allowing equilibrium sorption coefficients (Rd; L kg−1) for these radionuclides to be calculated on a mechanistic basis. This provides insight into the intrinsic rates and mechanisms of interaction between cementitious materials and their pore fluids which contain dissolved calcium and silicon.

Differential Sorption of Short-Chain versus Long-Chain Anionic Per- and Poly-Fluoroalkyl Substances by Soils

The impact of chain length on the sorption of anionic PFAS by soils and sediments was investigated by aggregating and synthesizing data sets from the literature. Quantitative structure/property relationship (QSPR) analysis was applied to characterize the influence of molecular size and soil properties on sorption. The log of the organic carbon-normalized equilibrium sorption coefficient (Koc) exhibited a biphasic relationship with molar volume, wherein the log Koc values for the short-chain PFAS were generally greater than would be predicted using the QSPR correlation determined for the long-chain PFAS. This enhanced differential sorption is observed to different degrees for all studies, which are compiled and synthesized for the first time. The results reveal remarkable congruency across a wide array of soils comprising a large range of properties and indicate that the observed enhanced differential sorption of short-chain PFAS is a prevalent phenomenon. Aggregating the long-chain PFAS data for all soils and sediments with organic carbon contents > 1% produced a strong correlation, indicating that the resultant QSPR model can produce representative log Koc values irrespective of the other properties of the medium. Silt+clay content was shown to be an important soil component for the short-chain PFAS for most soils, as well as the long-chain PFAS for soils with organic carbon contents < 1%. The results indicate that while the simple Koc-foc approach may produce reasonable estimates of Kd values for long-chain anionic PFAS, particularly for soils and sediments with larger organic carbon contents, it is unlikely to do so for short-chain anionic PFAS.

The influence of chain length on the sorption of C4-C10 perfluorocarboxylic acids during transport in a sand

The impact of chain length on the retention and transport of perfluorocarboxylic acids (PFCAs) in a quartz sand was investigated. Short-chain (C4–C7: PFBA, PFPeA, PFHxA, PFHpA) and long-chain (C8–C10: PFOA, PFNA, PFDA) PFCAs were selected as a representative homologous series. Miscible-displacement transport experiments were conducted under saturated conditions to characterize the magnitudes of sorption mediating retention and transport. Quantitative-structure/property-relationship (QSPR) analysis was applied to characterize the influence of molecular size on sorption. The transport of the long-chain PFCAs exhibited greater retardation than the short-chain PFCAs. The log of the equilibrium sorption coefficient (Kd) exhibited a biphasic relationship with carbon number and molar volume, with the magnitude of measured sorption for the short-chain PFCAs significantly greater than would be predicted using the QSPR regression developed for the long-chain PFCAs. This is consistent with batch-measured data reported in the literature, and likely reflects the relative influence of different sorption mechanisms for the short-chain vs long-chain PFCAs.

1,4-Dioxane cosolvency impacts on trichloroethene dissolution and sorption

Solvent stabilizer 1,4-dioxane, an emerging recalcitrant groundwater contaminant, was commonly added to chlorinated solvents such as trichloroethene (TCE), and the impact of co-disposal on contaminant transport processes remains uncertain. A series of batch equilibrium experiments was conducted with variations of 1,4-dioxane and TCE composition to evaluate aqueous dissolution of the two components and their sorption to aquifer sediments. The solubility of TCE increased with increasing amounts of 1,4-dioxane, indicating that 1,4-dioxane acts as a cosolvent causing solubility enhancement of co-contaminants. The solubilization results compared favorably with predictions using the log-linear cosolvency model. Equilibrium sorption coefficients (Kd and Kf) were also measured for different 1,4-dioxane and TCE compositions, and the findings indicate that both contaminants adsorb to aquifer sediments and TCE Kd values increased with increasing organic matter content. However, the Kd for TCE decreased with increases in 1,4-dioxane concentration, which was attributed to cosolvency impacts on TCE solubility. These findings further advance our understanding of the mass-transfer processes controlling groundwater plumes containing 1,4-dioxane, and also have implications for the remediation of 1,4-dioxane contamination.

Sorption of Molecular Oxygen by Metal–Ion Exchanger Nanocomposites

Kinetic features are studied of the chemisorption and reduction of molecular oxygen from water by metal–ion exchanger nanocomposites that differ in the nature of the dispersed metal and state of oxidation. In the Pd < Ag < Cu series, the increasing chemical activity of metal nanoparticles raises the degree of oxygen sorption due to its chemisorption and subsequent reduction, while the role of the molecular chemisorption stage increases in the Cu < Ag < Pd series. Metal particles or their oxides are shown to act as adsorption sites on the surface and in the pores of the ion-exchanger matrix; the equilibrium sorption coefficient for oxygen dissolved in water ranges from 20 to 50, depending on the nature and oxidation state of the metal component.

Sorption of the nematicide fluensulfone in six UK arable soils – implications for control of the potato cyst nematode Globodera pallida

Batch adsorption experiments were performed to determine the sorption of the nematicide fluensulfone as a technical-grade and a granular formulation (as Nimitz 15G) in six UK arable soils. The Freundlich and equilibrium sorption coefficients and , respectively, were generally low. and correlated positively with soil organic matter in all instances. The sorption kinetics was similar for both forms, but the was about four times lower for Nimitz 15G than the technical-grade, suggesting concentration dependency of fluensulfone sorption. The low sorption of fluensulfone across the soils indicates that partitioning of fluensulfone to the soil liquid phase may be unlimited. Therefore, substantial availability in the soil to be effective is likely. Sorption, therefore, may not limit fluensulfone efficacy. Nonetheless, these results call for cautious use of the nematicide because leaching is possible.

Effects of aged sorption on pesticide leaching to groundwater simulated with PEARL

Leaching to groundwater is an important element of the regulatory risk assessment of pesticides in western countries. Including aged sorption in this assessment is relevant because there is ample evidence of this process and because it leads to a decrease in simulated leaching. This work assesses the likely magnitude of this decrease for four groundwater scenarios used for regulatory purpose in the EU (from the UK, Portugal, Austria and Greece) and for ranges of aged-sorption parameters and substance properties using the PEARL model. Three aged-sorption parameters sets were derived from literature, representing approximately 5th, 50th and 95th percentile cases for the magnitude of the effect of aged sorption on leaching concentrations (called S, M and L, respectively). The selection of these percentile cases was based only on the fNE parameter (i.e. the ratio of the aged sorption and the equilibrium sorption coefficients) because leaching was much more affected by the uncertainty in this parameter than by the uncertainty in the desorption rate coefficient of these sites (kd). For the UK scenario, the annual flux concentration of pesticide leaching at 1m depth decreased by typically a factor of 5, 30 and >1000 for the S, M and L parameter sets, respectively. This decrease by a factor of 30 for the M parameter set appeared to be approximately valid also for the other three scenarios. Decreasing the Freundlich exponent N from 0.9 into 0.7 for the M parameter set, increased this factor of 30 into a factor of typically 1000, considering all four scenarios. The aged-sorption sites were close to their equilibrium conditions during the leaching simulations for two of the four scenarios (for all substances considered and the M parameter set), but this was not the case for the other two scenarios.

Predicting suitability of different scale-dependent dispersivities for reactive solute transport through stratified porous media

In this paper, the behavior of breakthrough curves (BTCs) for reactive solute transport through stratified porous media is investigated. A physical laboratory model for layered porous media was constructed, in which thin layer of gravel was sandwiched in between two thick layers of natural soil. Gravel layer and natural soil layers were hydraulically connected as single porous continuum. A constant source of tracer was connected through gravel layer and elucidated at different sampling points in the direction of flow. Flexible multiprocess non-equilibrium (MPNE) transport equation with scale-dependent dispersivity function was used to simulate experimental BTCs of reactive solute transport through layered porous media. The values of equilibrium sorption coefficient and other input parameters were obtained experimentally. The simulation of BTC was performed using MPNE model with scale-dependent dispersivity. The simulation of different scale-dependent dispersivities was then compared and it was found that for field scale of estimation of dispersivity, asymptotic and exponential dispersivity functions performed better. In continuation to the comparison of simulated BTCs obtained using different models, spatial moment analysis of each aforesaid scale-dependent dispersivity model was also done. Spatial moment analysis provides the information related to mean solute mass, rate of mass travel, and mean plume dispersion. Linear and constant dispersivities showed higher variance as compared to asymptotic and exponential dispersion functions. This supports the field applicability of asymptotic and exponential dispersivity functions. The BTCs were also found to elucidate a nonzero concentration with time, which was clearly affected by physical non-equilibrium. In natural condition, such information is required in effective aquifer remediation process.

Temporal moments for solute transport through fractured porous media

Transport of reactive chemical through fractured porous media is described by advection, dispersion, diffusion and sorption processes. Hence, an advective dispersive transport equation, including equilibrium sorption and first-order degradation coefficient, is considered for solute in the fracture and simultaneously a diffusive transport equation is considered for porous media. A numerical implicit finite difference method is used to solve the transport equation for solute in the fracture. Finally, numerical results of various temporal moments have been predicted to investigate the behaviour of reactive solute in the fracture. It was found that the behaviour of zeroth temporal moment, mean arrival time and second temporal moments are non-linear along the travel distance for solute in the fracture in the presence of matrix diffusion and decay rate coefficient. It was also seen that the higher values of matrix diffusion, retardation factor and decay rate coefficients reduce solute mass recovery in a fracture along the travel distance.

Acceptability of inversely-modelled parameters for non-equilibrium sorption of pesticides in soil

Simulation of the increase of sorption in time is one of the options in higher tiers of pesticide regulatory leaching assessments to obtain more realistic leaching estimates. Therefore, accurate estimates of non-equilibrium sorption parameters are required as input for the pesticide leaching scenarios. Usually, non-equilibrium sorption is described with a two-site equilibrium/non-equilibrium model in which the non-equilibrium sorption is described with two parameters (i.e. the desorption rate coefficient of the non-equilibrium site and the Freundlich sorption coefficient of this site). Estimates of these parameters can be obtained with inverse modelling techniques. At the moment, there is little understanding about whether the confidence intervals provided by inverse modelling can be used as measure of the likely accuracy (i.e. how close the estimated value is to the true value) of these estimates. We set up a semi-global inverse modelling exercise for a large number of parameter sets (i.e. different pesticides) using simulated datasets. Inverse modelling of non-equilibrium parameters demonstrated decreasing accuracy of the estimates for decreasing values of the non-equilibrium sorption parameters and the equilibrium sorption coefficient. Furthermore, we found a relationship between the accuracy of a parameter estimate and its CV (coefficient of variation) provided by the inverse modelling technique. Using this relationship we calculated the likelihood of rightly or wrongly accepting or rejecting a parameter estimate as a function of this CV. We recommend to use this likelihood as the basis of communication with decision makers on how to decide on accepting or rejecting parameter estimates.

Measuring equilibrium sorption coefficients with the miscible-displacement method

The miscible-displacement method is one commonly used approach for measuring equilibrium sorption coefficients. The objective of this research was to examine the impact of experiment conditions (detection limit, input-pulse size, input concentration) on the measurement of retardation factors and sorption coefficients for a system that exhibits significant nonideal sorption behavior. A series of miscible-displacement experiments was conducted wherein effluent solute concentrations were monitored over a range of approximately seven orders of magnitude, allowing characterization of asymptotic tailing phenomenon, which was significant. The magnitude of Kd increased asymptotically with the increase in the extent of the elution tail measured. The results also showed that the fraction of the tail required to obtain close-to-maximum measures of Kd is greater for smaller input pulses. Investigating the impact of analytical detection limit (equivalent to relative concentrations of 10−3 and 10−7) revealed that the magnitude of Kd was invariant with input pulse for the 10−7 detection limit. Conversely, the measured Kd values were significantly smaller at low input pulses for the 10−3 detection limit.

Volatilization Modeling of Two Herbicides from Soil in a Wind Tunnel Experiment under Varying Humidity Conditions

Volatilization of pesticides from the bare soil surface is drastically reduced when the soil is under dry conditions (i.e., water content lower than the permanent wilting point). This effect is caused by the hydrated mineral surfaces that become available as additional sorption sites under dry conditions. However, established volatilization models do not explicitly consider the hydrated mineral surfaces as an independent sorption compartment and cannot correctly cover the moisture effect on volatilization. Here we integrated the existing mechanistic understanding of sorption of organic compounds to mineral surfaces and its dependence on the hydration status into a simple volatilization model. The resulting model was tested with reported experimental data for two herbicides from a wind tunnel experiment under various well-defined humidity conditions. The required equilibrium sorption coefficients of triallate and trifluralin to the mineral surfaces, K(min/air), at 60% relative humidity were fitted to experimental data and extrapolated to other humidity conditions. The model captures the general trend of the volatilization in different humidity scenarios. The results reveal that it is essential to have high quality input data for K(min/air), the available specific surface area (SSA), the penetration depth of the applied pesticide solution, and the humidity conditions in the soil. The model approach presented here in combination with an improved description of the humidity conditions under dry conditions can be integrated into existing volatilization models that already work well for humid conditions but still lack the mechanistically based description of the volatilization process under dry conditions.

Slow and Fast Transport in Heap Leaching of Precious Metals

Heap leaching of precious metals from low-grade ores is modeled using the slow and fast zone concept of liquid movement in porous media (Kartha and Srivastava Transport in Porous Media 75, 227–247, 2008). The heap is conceptualized as a porous medium with the pores filled by liquid and/or air. The liquid phase is further divided into three zones—immobile, slow moving, and fast moving, with only the immobile and slow-moving liquids interacting with the solid particles. The conceptual model is applied to simulate two- dimensional leaching of a shallow gold ore heap. Temporal evolution of gold concentration in the solid and the liquid phases are analyzed to ascertain the effect of several parameters on the leaching process. Sensitivity analyses are performed with respect to the irrigation rate, the equilibrium sorption coefficient, and the fractional volume of the slow-moving liquid. For a constant volume of the leaching solution and for the same duration of leaching, the metal extraction is higher when a larger number of irrigation cycles are used. The metal extraction is found to be smaller for higher values of the sorption coefficient.

Temperature-dependent sorption of polycyclic aromatic hydrocarbons on natural and treated sediments

In aqueous environment temperature is considered to play a significant role in the sorption process of polycyclic aromatic hydrocarbons (PAHs) and its influence on the sorption equilibrium is indicative of sorption energies and mechanisms. In this study, sorptions of five PAHs on three heterogeneous sorbents including one river sediment (YHR), one estuary sediment (YRD) and one treated sediment with organic matter removed (IM) were carried out at a range of temperature from 5 °C to 35 °C. Stronger sorptions were observed at lower temperatures, with the equilibrium sorption coefficient K d increasing 2–5 times as the temperature decreases 30 °C. The increase of K d value was attributed primarily to the change of PAH water solubility, which predicted 40–75% of the increase of K d in the sorption process. To provide insight into the sorption mechanism, enthalpy change (Δ H S) for the sorption process was calculated and the values were observed to be negative for all of the interactions, suggesting that the exothermal sorption of PAHs inversely dependents on temperature. Based on the values of Δ H S, van der Waals forces were inferred as the main sorption mechanism for the PAHs, especially on the YHR sediment which contained more organic matter. For sorption of larger size PAHs on the sorbents with low organic matter, specific interactions were deduced to contribute to the overall sorption.

Predicting mobility of alkylimidazolium ionic liquids in soils

Background, aim, and scope Ionic liquids (ILs) are a new class of alternative solvents that make ideal non-volatile media for a variety of industrial processes such as organic synthesis and biocatalysis, as alternative electrolytes, as phases and phase modifications in separation techniques, and as alternative lubricants. Once the large-scale implementation of ILs begins, the industrial application will follow. In view of their great stability, they could slip through classical treatment systems to become persistent components of the environment, where the long-term consequences of their presence are still unknown. Sorption on soils has a critical effect on the transport, reactivity, and bioavailability of organic compounds in the environment. So far, the IL sorption mechanism was investigated solely on the basis of batch experiments, which precluded any assessment of the dynamics of the process. An understanding of the mobility of ILs in soil columns is crucial for an accurate prediction of their fate in the soil. The aim of this study therefore was to investigate in detail the mobility of selected imidazolium ILs on three soil types. Moreover, it was decided to study these processes in soils from the coastal region (Gdansk, Poland), which usually constitute a very important geochemical compartment, participating in the transport of contaminants on their way to the sea. Materials and methods The mobility of alkylimidazolium ILs was investigated in columns containing soils from the coastal area. In addition, the sorption processes in all the soil systems studied were described isothermally and the equilibrium sorption coefficient was evaluated. The sorption capacities were determined according to OECD guidelines. Sorption dynamics was studied with use of polypropylene columns (diameter—10 mm, height— 100 mm) packed with 10 g of soil. The ionic liquid solution was then injected into the soil column and left for 24 h to equilibrate. After this, a solution of 0.01 mM CaCl2 was pumped through the column at a rate of 0.3 ml min –1 . Effluents were collected from the bottom of the column and analyzed by HPLC. Results Sorption was strongest on the Miocene silt and the alluvial agricultural soil and weakest on the podsolic soil and Warthanian glacial till. The Kd value of long-chain ILs was far higher than that of the short-chain ones. Among the substances tested, hydroxylated ILs were usually more weakly sorbed. Desorption of ILs is inversely correlated with sorption intensity. The experimental results of the column tests correlate well with those from batch experiments. In the cases of weakly binding soils, ILs were

Sorption Kinetics and Equilibria of Organic Pesticides in Carbonatic Soils from South Florida

A batch reactor was used to determine sorption kinetic parameters (k2, F, and K*) and the equilibrium sorption coefficient (K). The two-site nonequilibrium (TSNE) batch sorption kinetics model was used to calculate the kinetic parameters. Two probe organic pesticides, atrazine [2-chloro-4-ethylamino-6-isopropylamino-s-triazine] and diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] were studied using three carbonatic soils from South Florida (Chekika, Perrine, and Krome), one noncarbonatic soil from Iowa (Webster), and one organic soil (Lauderhill) from South Florida. Carbonatic soils contained more than 600 g kg(-1) CaCO3. Sorption is initially very fast up to 3 h and then slowly reaches equilibrium. All soil-chemical combinations reached sorption equilibrium after about 24 h and all sorption isotherms were linear. The sorption kinetics data were well described by the TSNE model for all soil-chemical combinations except for the marl soil data (Perrine-Atrazine), which were better described by the one-site nonequilibrium (OSNE) model. Diuron, with higher K, undergoes slower sorption kinetics than atrazine. The Lauderhill soil containing organic carbon (OC) of 450 g kg(-1) exhibited slowest sorption kinetics for both pesticides. An inverse relationship between k3 and K was observed for atrazine and diuron separately in Chekika, Webster, and Lauderhill soils but not in Perrine and Krome soils. The sorption kinetic parameters were used to distinguish the sorption behavior between atrazine and diuron and to identify differences between soils. Normalizing the sorption coefficient (K) to OC showed that atrazine and diuron had K oc values in carbonatic soils that were a third of reported literature values for noncarbonatic soils. Using existing literature K oc values in solute transport models will most likely underestimate the mobility of atrazine, diuron, and other neutral organic chemicals in carbonatic soils.

KINETICS OF CADMIUM ION SORPTION ON ION EXCHANGE AND CHELATING RESINS

ABSTRACT Cadmium sorption from aqueous solutions on sulfonic (C-150) and iminodiacetic (S-930) Purolite macroporous resins was investigated. The influence of operating variables such as initial pH, Cd(H) concentration, time and temperature on the equilibrium parameters was measured. The ion preference and sorption ability of resins, i.e. binding constant (b) and saturation capacity {xm), derived from sorption isotherm, depend on the functional group structure. The Cd(II) uptake is only particle diffusion controlled. The kinetic parameters, i.e. t1/2 values for 50 % attainment of equilibrium sorption, rate constant (K¯ ) and diffusion coefficient (D¯) are higher on the sulfonic resin. The “moving boundary” particle diffusion model fits the entire ion sorption process on chelating resin, but only the initial sorption on the sulfonic resin, confirming the difference in chemistry between chelation and ion exchange.

Organic cosolvent effects on the sorption and transport of neutral organic chemicals

Soil column miscible displacement techniques were used to investigate the effects of an organic cosolvent (methanol) on the sorption and transport of three neutral organic chemicals, naphthalene, phenanthrene, and the herbicide diuron, through a sandy surface soil. A two-domain, or bicontinuum, first-order mass transfer model described the experimental data well. For the three solutes used in this study, the equilibrium sorption coefficient (K) decreased log-linearly as the volume fraction of methanol (f c) increased. The physical properties calculator of the SPARC computer model was used for generating solute solubility profiles to estimate the slope of the Log K-f c relationship.

Modeling and risk assessment of a 30‐Year‐old subsurface radioactive‐liquid drain field

The contamination from a 30‐year‐old radioactive liquid drain field was assessed for movement in the subsurface and potential risks to humans. This assessment included determining field concentrations of cesium 137 (137Cs) and other inorganic contaminants and modeling of the flow and transport of the liquid waste that was sent to the drain field. The field investigation detected no contamination deeper than 15 feet (4.6 m) from the bottom of the drain field. Prediction of the water content of the vadose zone showed no saturated conditions for times greater than 10 years after the known infiltration. Sensitivity analysis of the modeling parameters showed the equilibrium sorption coefficient to be the most important factor in predicting the contaminant plumes. Calibration of modeling results with field data gave a 137Cs sorption coefficient that is within the range of values found in the literature. The risk assessment for the site showed that the contamination poses no significant risk to human health.

Sorption of Nonionic, Hydrophobic Organic Chemicals to Mineral Surfaces

Equilibrium sorption coefficients (Kd) of a series of polycyclic aromatic hydrocarbons (PAHs) and chlorobenzenes were measured on the pure mineral oxides α-Al2O3 and α-Fe2O3 using column chromatography. Sorbent surface area did not account for differences in the measured distribution coefficients (Kd) for a given compound on α-Fe2O3 and α-Al2O3. This observation indicates that the type of mineral surface is important in the sorption of nonionic, hydrophobic organic chemicals (HOCs). On both minerals, sorption was reversible as sorption isotherms of the PAHs were linear to at least 50% of solute aqueous solubility and the measured enthalpy of sorption similar to the compounds negative enthalpy of dissolution. Solution pH and the sign and magnitude of sorbent surface charge did not affect sorption, nor did a factor 50 change in ionic strength. The relationship between Kd and the aqueous activity coefficient, γsatw , was log Kd = 0.70 log γsatw + −10.68R 2 = 0.94 on α-Al2O3 and log Kd = 0.98 log γsatw + −11....