Soil sorption from water partitioning

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]. 

Soil sorption partition coefficient (or soil-water partition coefficient), denoted as Kqc or logKoc accounts for sorption from water into soil. Because this often depends primarily on the soil s organic carbon content, measured values are usually normalized for the organic carbon (OC) content of soil, in which case the soil sorption equilibrium constant is expressed as... 

Several studies have shown that sorption of various organic compounds on solid phases could be depicted as an accumulation at hydrophobic sites at the OM/water interface in a way similar to surface active agents. In addition Hansch s constants [19,199-201], derived from the partition distribution between 1-octanol and water, expressed this behavior better than other parameters. Excellent linear correlations between Koc and Kow were found for a variety of nonpolar organic compounds, including various pesticides, phenols, PCBs, PAHs, and halogenated alkenes and benzenes, and various soils and sediments that were investigated for sorption [19,76,80,199-201]. 

It has been argued that the Kqa values can be used as a unifying property for describing volatilization of POPs from soils and sorption to aerosols. The limited experimentally obtained values typically are supplemented by estimates from octanol-water and air-water partition coefficients. The value of condensation temperature lies in its ability to estimate sorption of atmospheric contaminants to aerosols (Bidleman, 1988). At Tc, the chemical is equally partitioned between the gas phase and aerosols. Since POPs exist in the atmosphere both as gases (vapor phase), and in condensed form adsorbed to aerosol particles, the characteristic temperature of... 

Values of Koc reported for the association of 1,2,4-trichlorobenzene (TCB) with a variety of soils and organic macromolecules are shown in Table I (4, 6-8). Such collections of experimental data for sorption of a particular solute by a variety of different soils and sediments have been used to develop correlations with which Koc values are then predicted from certain chemical properties of a solute, such as the octanol-water partition coefficient. 

Several models for estimating soil sorption coefficients take advantage of the correlation between Koc and other experimental partition coefficients, specially Kow For example, Kqc values have been estimated from experimental octanol-water partition coefficients by... 

The Henry s law constant is an important criteria in determining the extent to which an organic compound will distribute into the atmosphere from water and vice versa. Another important environmental process is the extent to which a compound will distribute into a hydrophobic region. It will be shown that soil organic matter controls sorption of organic compounds in soil, while movement into an organism involves passage across the hydrophobic barrier of the membrane. This tendency is indicated by the octanol-water partition coefficient, Xqw... 

Vapor-phase sorption of carbon tetrachloride and benzene onto a peat soil (fom = 0.864) give linear isotherms indicating partitioning into the organic matter (Fig. 3.18). The sorption of these two compounds on the same soil from water is only 40-50% less than that observed from the vapor phase. This effect is much less than that observed when the mineral fraction is the primary absorbent. 

The base set of partition coefficients needed for most multimedia models include the Henry s law constant (JCh) to describe partitioning between air and water, partition coefficients between water and various solid phases in soils, sediments, and particulate matter in the water coliunn (fCa), and a coefficient describing partitioning between air-borne particles and air (Kp). If not available from direct experimental measurements, soUd phase-water partition coefficients are often derived from the organic carbon-water partition coefficient, Kqc (Eq. 4), the underlying assumption being that sorption into organic matter dominates the overall sorption to bulk solid material. 

The sorption to soil components is a determinant factor for the mobility of contaminants, accounting for their distribution among soil, sediment and water phases. The extent to which chemicals partition between the solid and solution phases in soil, or between water and sediment in aquatic ecosystems, determines the likelihood of the contaminants leaching through the soil or being immobile. The soil sorption hence influences the elution of compounds into groundwater bodies as well as their availability for transformation by soil microbes, their volatilization from soil surfaces and their bioavailability for exposed organisms. 

From an environmental impact standpoint, the four most important physical properties of PCBs are very low water solubUlty and vapor pressure, high octanol/water partition coefficients, and stability or persistence (NRC, 1979). The five mechanisms by which PCBs can be transported in the soil are as a dissolved material in the water by sorption as an emulsion with water as an immiscible oUy Uquid phase and as a discrete fluid. 

Chemicals in water can sorb to sediment or soil in a reversible process that reflects the attraction and adhesion of molecules to solids. (Less commonly considered in environmental mass balances, some air pollutants can sorb to particulates in the atmosphere.) The n-octanol/water partition coefficient (K ) of a substance provides a crude indication of the tendency to partition to solids from water a high value indicates that a substance is hydro-phobic/lipophilic and would tend to sorb to solids. More sophisticated tests determine a distribution coefficient (KJ or adsorption isotherm to relate the concentration in solution to the concentration sorbed to solids. The sorption coefficient is the ratio between the concentration of a chemical in soil to the concentration in water which is in contact with the soil. Normalized to the organic carbon content of the soil, this coefficient becomes (K = I i/ fraction organic carbon in soil) [4]. 

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