MODELING SORPTION ISOTHERMS OF VOLATILE ORGANIC CHEMICAL MIXTURES IN MODEL AND NATURAL SOLIDS
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Parameters from single-component isotherm models were used in multicomponent isotherm models to predict the aqueous phase sorption of trichloroethylene (TCE) in the presence of tetrachloroethylene (PCE) in four zeolites, Tenax, and three natural solids. The Langmuir, the Polanyi-Dubinin, and the Freundlich or the Langmuir-Freundlich isotherm models were used to simulate single-component sorption in zeolites. The Langmuir two-site, the Polanyi-Dubinin two-site, and the Freundlich or the Langmuir-Freundlich isotherm models were used to simulate single-component sorption in Tenax and natural solids. Two-site models have been used previously to model sorption in soils and sediments, and they combine an adsorption component (e.g., Langmuir) with a linear partitioning component. By using parameters from the different single-component isotherm models, the multicomponent Langmuir, the ideal adsorbed solution theory, and the Polanyi theory were each used to predict multicomponent sorption. In general, the ability to predict TCE sorption in the presence of PCE depended more on the choice of the single-component model than the multicomponent model, and better results were obtained when the Freundlich or the Langmuir-Freundlich isotherm was used for single-component sorption. This suggests that the more mechanistically based Langmuir and Polanyi-type models may not adequately describe the distribution of adsorption sites in some model and natural solids. The Freundlich or the Langmuir-Freundlich model, although empirical, has greater flexibility in characterizing sorbent heterogeneity and results in better multicomponent model predictions. However, this last statement is tenuous, because more solids must be tested against various model combinations.Keywords:
Organic chemicals
Organic chemicals
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The sorption of components of crude oil to soils affects their transport and transformation.In this study,batch experiments were carried out to investigate the sorption and competitive sorption of benzene and toluene to soils with different amounts of organic carbon.The results of single-solute solution experiments showed that the sorption of single compound to soils could be described by a linear sorption isotherm,and the sorption capacity was proportional to the soil organic carbon content.In bi-solute solution experiments,competitive sorption of solutes was observed,which caused a decrease of sorption capacity of target compound in comparison with single-solute cases.In addition to partition process,the competitive sorption results indicated that the specific sites probably played an important role in the sorption of organic compounds on the solid surface.
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Langmuir adsorption model
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The sorption kinetic and isothermal behaviors of BDE-47 on three natural soils with different soil organic matter fractions(fOC) were studied. The results demonstrated that a two(fast and slow)-compartment first-order model was more appropriate for describing the sorption kinetic data, compared to a one-compartment first-order model, especially in the initial sorption stage within 49 h. The fast sorption was dominant during the whole sorption process from beginning to the apparent sorption equilibrium; while the contribution of the slow sorption to the total sorption amount gradually increased over time and then attained to a plateau at 97 h. The achieving time to the individual sorption capacity for the fast sorption was much shorter than that for the slow sorption. The contribution of the fast sorption to the increase in the total sorption amount of BDE-47 was prevailing at the start of sorption process from 3 h to 5 h; while the fraction of the slow sorption became principal in the subsequent stage of sorption process. Within the narrow equilibrium concentration range due to the fairly low aqueous solubility of BDE-47, the fitting results by the linear partitioning model were comparable with the linear portion by the nonlinear Freundlich model in this study. As for the Freundlich model, the nonlinear exponent(n) values of BDE-47 for the two samples with lower fOC(0.72%) or higher fOC(7.90%) were obviously lower than 1.0(0.75±0.03 and 0.74±0.02, respectively), suggesting somewhat nonlinear sorption characteristics in the studied range of apparent equilibrium concentrations of BDE-47; whereas the linear behavior of BDE-47 for the left sample with medium fOC(2.86%) was indicated by its n value much closed to 1.0(1.03±0.03).
Isothermal process
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In this paper,the container wall sorption percentage and the sorption ratio of Am betweensolution and such sorption materials as bentonite are studied with the HPGe detector connected-with S-85 multichannel system,the influence of container wall sorption on sorption ratio also isdiscussed.The results show that the container wall sorption with solid sorptive materials is lowerthan that without solid sorptive materials. The container wall sorption should be considered forthe sorptive materials with low sorption ratio values and can be neglected for those with highsorption ratio values.
Bentonite
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Nuclide
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The sorption kinetics and isotherms of BDE-28 on three natural soils with different soil organic matter fractions (f(oc)) were investigated. The results indicated that a two (fast and slow)-compartment first-order model was more appropriate for describing the sorption kinetic data, compared to a one-compartment first-order model, especially in the initial sorption stage within 25 h. The fast sorption was predominant during the whole sorption process from beginning to the apparent sorption equilibrium; while the contribution of the slow sorption to the total sorption amount gradually increased and then achieved a plateau at 49 h or 55 h. The approaching time to the individual sorption capacity for the fast sorption was much shorter than that for the slow sorption. The contribution of the fast sorption to the increase in the total sorption amount of BDE-28 was prevailing at the beginning of sorption process from 2.5 h to 4.5 h; whereas the fraction of the slow sorption became primary at the subsequent stage of sorption process. The fitting results by the Dubinin-Ashtakhov (DA) model were comparable with those by the Freundlich model in the range of apparent equilibrium concentration studied. As for the Freundlich model, the nonlinear exponent (n) values of BDE-28 for the two samples with lower f(oc) (0.72%) or higher f(oc) (7.90%) approached to 1.0 (1.03 +/- 0.05 and 1.00 +/- 0.05, respectively), suggesting the linear sorption characteristics in the studied range of apparent equilibrium concentrations of BDE-28; while the nonlinear behavior of BDE-28 for the left sample with medium f(oc) (4.42%) was indicated by its n value less than 1.0 (0.89 +/- 0.04).
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The kinetics characteristics and sorption amount under different pH conditions(i.e.pH=3.5,5.0,5.6 and 6.2) and the thermodynamics characteristics of norfloxacin(NOR) sorption on goethite(α-FeOOH) were determined by batch equilibrium experiments according to OECD Guideline 106 in this study.The sorption mechanisms were also found out.Results showed that the sorption of NOR on α-FeOOH began from a rapid stage with continuing for approximately 6 h,and then followed with a slow stage until a 48 h of sorption equilibrium time.Sorption kinetic curves fit a pseudo second-order kinetic model well at different pH.The sorption amount of different pH corresponded to the decreasing order of 5.6 5.0 6.2 3.5,which depended on the sum of the products concentrations obtained from different species of α-FeOOH and NOR at respective pH value.This result also indicated that the order of sorption amount was mostly attributed to the sorption of NORH2± on the surface of α-FeOOH.The Freundlich model could compatibly describe sorption isotherm of NOR on α-FeOOH.The 1/n of the isotherm fitted by the Freundlich model increased from 0.43 to 0.61 with increasing temperature from 15 ℃ to 35 ℃,which indicated that an increase in temperature could reduce the nonlinear sorption of NOR on α-FeOOH.Results with-39.45 kJ/mol of ΔH0 and less than 0 of ΔG0 revealed that the sorption of NOR on α-FeOOH was a spontaneously exothermic process.The exchange of ion was a dominant sorption mechanism for NOR sorption on α-FeOOH,while the H-bonding and dipole force were other possible sorption mechanisms.Less than 0 of ΔS0 indicated an entropy decrease during the sorption process.
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