Sorption processes partition coefficients

The concentration in the membrane depends on the outside activity and the sorption or partition coefficient of the species, the mobifity on the nature of the membrane. The driving force for a component is a function of the process parameters, e.g. temperature, pressure, and concentration. In a pervaporation process usually the minor component is removed from a mixture. For the retained major component the driving force will always be higher than for the transported one. The selectivity of the membrane is then determined by the differences in the product of mobility and concentration and not by a difference in the driving force. 

Another feature of the process is that the sorption capacity of type II organoclays is inversely related to the aqueous solubility of the NOCs (Chiou 1989). For example, the affinity of HDTMA-smectite for various phenols increases in the order phenol < chlorophenol < dichlorophenol < trichlorophenol since phenol is the most water-soluble while trichlorophe-nol is the most hydrophobic (Mortland et al. 1986, Lo et al. 1998). The relationship between the distribution (partition) coefficient in a type II organoclay and water-solubility is illustrated in Fig. 5 for a range of nonionic organic pollutants. 

Once estrogens and progestagens have reached the waterways, a series of processes, such as, photolysis, biodegradation, and sorption to bed-sediments, can contribute to their elimination from the environmental water. Given the relatively low polarity of these compounds, with octanol-water partition coefficients mostly between 103 and 105, sorption to bed-sediments appears to be a likely process. Kd values calculated for estriol, norethindrone, and progesterone in a Spanish river (479,128, and 204, respectively) as the ratio between the sediment concentration (ng kg-1) and the water concentration (ng L 1) indicate that, in fact, these compounds exhibit a general tendency to accumulate in sediments. 

The dominant transport process from water is volatilization. Based on mathematical models developed by the EPA, the half-life for M-hexane in bodies of water with any degree of turbulent mixing (e.g., rivers) would be less than 3 hours. For standing bodies of water (e.g., small ponds), a half-life no longer than one week (6.8 days) is estimated (ASTER 1995 EPA 1987a). Based on the log octanol/water partition coefficient (i.e., log[Kow]) and the estimated log sorption coefficient (i.e., log[Koc]) (see Table 3-2), ii-hexane is not expected to become concentrated in biota (Swann et al. 1983). A calculated bioconcentration factor (BCF) of 453 for a fathead minnow (ASTER 1995) further suggests a low potential for -hcxanc to bioconcentrate or bioaccumulate in trophic food chains. 

Pollutants with high VP tend to concentrate more in the vapor phase as compared to soil or water. Therefore, VP is a key physicochemical property essential for the assessment of chemical distribution in the environment. This property is also used in the design of various chemical engineering processes [49]. Additionally, VP can be used for the estimation of other important physicochemical properties. For example, one can calculate Henry s law constant, soil sorption coefficient, and partition coefficient from VP and aqueous solubility. We were therefore interested to model this important physicochemical property using quantitative structure-property relationships (QSPRs) based on calculated molecular descriptors [27]. 

Despite these reservations, environmental distribution values may be considered valid for the sorption process, to a first approximation. On this basis, it can be concluded that detected environmental partition coefficients show the clear affinity of surfactants to particulate material. The affinity is higher for cationic surfactants than for other surfactants, as shown by the high partition coefficient values (Table 5.4.1). Partition coefficients are also higher for the water column than for sediments (Table 5.4.1), and it is difficult to offer an explanation for this, bearing in mind the many factors affecting the partition coefficient in both natural water and sediment. 

The partition coefficients for different LAS homologues (Table 5.4.2) are higher in the marine environment due to the higher ionic strength that promotes sorption of anionic surfactants [14] and an increase in the partition coefficient with the alkylic chain length has been observed (Table 5.4.2). The evolution of the concentration of the various homologues of LAS in solids in suspension (cf. Fig. 5.4.2) is similar to that found in water and, in the process of adsorption, an increase can be observed in line with the chain length, as commented on previously. 

Processing of the triolein alone (i.e., without analyzing the LDPE as well) is not encouraged for a number of reasons. First of all, the LDPE constitutes a significant part of the total SPMD sorption capacity, in contrast to biota, where the sorption capacity of the non-lipid phase is often considered to be negligible. Data on membrane-lipid partition coefficients (/fmc) is very limited, but the available... 

In a sediment system, the hydrolysis rate constant of an organic contaminant is affected by its retention and release with the sohd phase. Wolfe (1989) proposed the hydrolysis mechanism shown in Fig. 13.4, where P is the organic compound, S is the sediment, P S is the compound in the sorbed phase, k and k" are the sorption and desorption rate constants, respectively, and k and k are the hydrolysis rate constants. In this proposed model, sorption of the compound to the sediment organic carbon is by a hydrophobic mechanism, described by a partition coefficient. The organic matrix can be a reactive or nonreactive sink, as a function of the hydrolytic process. Laboratory studies of kinetics (e.g., Macalady and Wolfe 1983, 1985 Burkhard and Guth 1981), using different organic compounds, show that hydrolysis is retarded in the sohd-associated phase, while alkaline and neutral hydrolysis is unaffected and acid hydrolysis is accelerated. 

From Fig. 19.3a-c, and as opposed to purely sorption controlled processes, it can be seen that during pervaporation both sorption and diffusion control the process performance because the membrane is a transport barrier. As a consequence, the flux 7i of solute i across the membrane is expressed as the product of both the sorption (partition) coefficient S, and the membrane diffusion coefficient Di, the so-called membrane permeability U, divided by the membrane thickness f and times the driving force, which maybe expressed as a gradient of partial pressures in place of chemical potentials [6] ... 

The ability to predict the behavior of a chemical substance in a biological or environmental system largely depends on knowledge of the physical-chemical properties and reactivity of that compound or closely related compounds. Chemical properties frequently used in environmental assessment include melting/boiling temperature, vapor pressure, various partition coefficients, water solubility, Henry s Law constant, sorption coefficient, bioconcentration factor, and diffusion properties. Reactivities by processes such as biodegradation, hydrolysis, photolysis, and oxidation/reduction are also critical determinants of environmental fate and such information may be needed for modeling. Unfortunately, measured values often are not available and, even if they are, the reported values may be inconsistent or of doubtful validity. In this situation it may be appropriate or even essential to use estimation methods. 

In any evaluation of a remediation scheme utilizing surfactants, the effect of dose on HOC distribution coefficients must be quantified. Very often, only one coefficient value for HOC partitioning to sorbed surfactants has been reported in the literature, presumably because the experimental data covers only the sorption regions where the surfactant molecule interactions dominate at the surface (Nayyar et al., 1994 Park and Jaffe, 1993). However, all of the characteristic sorption regions will develop during an in-situ SEAR application as the surfactant front (i.e., mass transfer zone) advances through the porous medium. Therefore, the relative role ofregional HOC partition coefficients to sorbed surfactant should be considered in any remediation process. Finally, the porosity or solid volume fraction for the particular subsurface system must be taken into account when surfactant sorption is quantified. 

Sorption is most commonly quantified using distribution coefficients (Kd), which simplistically model the sorption process as a partitioning of the chemical between homogeneous solid and solution phases. Sorption is also commonly quantified using sorption isotherms, which allow variation in sorption intensity with triazine concentration in solution. Sorption isotherms are generally modeled using the empirical Freundlich equation, S = K CUn, in which S is the sorbed concentration after equilibration, C is the solution concentration after equilibration, and Kt and 1 In are empirical constants. Kd and K are used to compare sorption of different chemicals on one soil or sorbent, or of one chemical on several sorbents. Kd and K are also commonly used in solute leaching models to predict triazine interactions with soils under various environmental conditions. 

Solute retention in reversed-phase HPLC is dependent on the different distribution coefficients established between a polar mobile and a nonpolar stationary phase by the peptidic components of a mixture. Although there are many similarities between reversed-phase HPLC separations of peptides and the classical liquid-liquid partition chromatographic methods, it is debatable whether the sorption process in reversed-phase HPLC arises due to partition or adsorption events, i.e., whether the nonpolar stationary phase functions as a bulk liquid or as an adsorptive monolayer. These aspects and the theoretical models for reversed-phase HPLC are discussed in a subsequent section. 

The log -octanol-water partition coefficient (log Kow) is a measure of the lipophilicity of a substance. As such, log Kow is a key parameter in the assessment of environmental fate. Many distribution processes are driven by log Kow, e.g. sorption to soil and sediment and bioconcentration in organisms. 

Soil sorption is an important detoxification process for pollutants of aquifer waters however, sediment-column experiments to determine the soil sorption capacity are tedious, expensive and their results are affected by various extraneous factors. It was proposed to estimate the adsorption coefficients KA based on their correlation with the capacity factors, k, measured by isocratic RP-HPLC108. A QSAR estimation study was carried out of soil sorption coefficients, K0c, of polar organic chemicals such as substituted anilines and phenols, based on descriptors such as n-octanol-water partition coefficients, Kow,... 

When sorption-desorption processes are fast relative to particle settling, the effectiveness of the downward transport is related to the particle concentration, downward particle flux, and the particle-water partition coefficient for the pollutant in question. Rates of sorption-desorption may be limiting in some instances characteristic times for sorption-desorption then become important, along with the contact time available for interaction with particles. 

Polymer-incorporated CPs could also be released during recycling of plastics, which may involve processes such as chopping, grinding and washing. If released as dust from these operations, the CPs would be adsorbed to particles because of high sorption and octanol-air partition coefficients. 

A leaching model based on a sorption-desorption process is used for describing the contaminant release from the waste disposal. The leachate concentration, CL (Bq.m3), is determined by the a distribution or a partition coefficient, Kd (cnr .g1) which describes the relative transport speed of the contaminant to the water existing in the pores soil properties such as bulk density, p (g.cm3), and water content, 0, affect the extent of contamination, described by the contaminated zone thickness, xt (m), area, A (m2) and the amount of contaminant in the source, It (Bq), (EPA 1996 Hung 2000) ... 

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