Solid phase organic carbon-water partition

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. 

As mentioned before, POP transport in the environment depends on their physicochemical properties [40-54], and these include saturated vapor pressure, solubility, Henry s law constant, octanol-water, octanol-air, and organic carbon-water partition coefficients. The saturated vapor pressure characterizes the capability of a substance to be transferred to the gaseous state. Eollowing the study of Wania and Mackay [40], the efficiency of POP condensation with subcooled liquid pressure (p°L) at 25°C above 1 Pa is very low. POPs with a vapor pressure between 1 and 10" Pa are condensed at a temperature of about -30°C and their deposition may be expected mostly in the polar latitudes. POPs with a vapor pressure of subcooled liquid from 10" to 10" Pa are condensed at a temperature above 0°C and they may reach to the middle latitudes. EinaUy, POPs of low volatility with a vapor pressure of subcooled liquid below 10" Pa are practically not vaporized and these substances may be transported and deposited as fine aerosols or coarse particles [39]. Using the vapor pressure of the subcooled liquid it is possible to characterize the partitioning of a POP between the gas phase and the solid phase of the atmospheric aerosol. The POPs having a lower vapor pressure are better bound with... 

A large number of organic chemical pollutants are hydrophobic, literally "afraid of water." These chemicals have limited solubility in water but do tend to dissolve easily into oils, fats, nonpolar organic solvents, and solid-phase organic carbon, which is ubiquitous in the environment. To a first approximation, the partition coefficient for many hydrophobic chemicals is not especially sensitive to the exact source or nature of the organic carbon. Accordingly, Kqc, the organic carbon-water partition coefficient, can be used to estimate the extent of sorption. Kqc can be expressed as... 

The most common partition coefficient encountered in environmental work (Sect. 2.1.4) is the octanol water partition coefficient (K0VI) and the solid phase carbon-water partition coefficient (Koc). A partition coefficient for dissolved organic matter-water (i. e., Kd ()M) or dissolved organic carbon-water (i. e., Kd oc) occasionally appears in the literature. In the case of PCBs, Boyd and Sun [378] defined a partition coefficient for residual transformer oil and water as Kd oil, while Sun and Boyd [379] defined a coefficient for PCB dielectric fluid-water as Kd pen-These authors [378,379] identified a total partition coefficient that com-... 

Fig. 2.26. Organic carbon-normalized partitioning coefficients (log K

The sorption coefficient, ATp—essentially a distribution coefficient (mol kg solid phase)/mol (liter water) —can be expressed as a function of organic carbon content,/oc (fraction by weight), and the octanol-water partition coefficient... 

The bioavailability of trace elements in soil-water-plant ecosystems of arid zones is largely determined by their partitioning between solution and solid-phase components. The redistribution and transformation of trace elements among solid-phase components under various biogeochemical conditions strongly adjust their lability and bioavailability. Soluble, exchangeable and organically bound forms are bioavailable to plants. The carbonate bound fraction represents a reservoir of potentially bio-available trace elements to plants. In paddy soils, the amorphous Fe/Al oxide fraction contributes to trace element availability as well. 

In this section solids represent a partitioning or adsorption phase such as soil, asphalt pavement, or granular activated carbon. In contrast to air, water, and octanol, solid phases are typically very complex and poorly characterized. For example, many studies have shown that soils and sediments are characterized by many different types and amounts of organic matter and minerals, and that these different environments have various affinities for an organic chemical. The fugacity for the solid or sorbed phase is expressed as follows ... 

The Hydrophobic Effect Hydrophobic Sorption Hydrophobic ( water-hating ) compounds, for example, hydrocarbons and chlorinated hydrocarbons such as the polychlorinated biphenyls are soluble in many nonpolar solvents but not readily soluble in water. Because of the incompatibility of the hydro-phobic substance with water, these substances have a tendency to avoid contact with water and seek to associate with nonpolar environments such as the surface of a mineral or an organic particle (Tanford, 1980). The sorption of hydro-phobic substances to solid materials (particles, soils, sediments) that contain organic carbon may be compared with the partitioning of a solute between two solvents—water and the organic phase. 

Metal sorption to solid particles depends on various factors e.g. the amount of Fe or Mn oxyhydroxides (FeOOH, MnOOH) mainly present in clay, the amount of organic carbon, pH and redox conditions. The equilibrium partitioning between the solid and water phase may be estimated (although with great uncertainties) by mathematical models. 

Almost all the separatory and clean-up methods relevant to this book are ultimately based on differences in partition coefficients of various compounds between two phases. The concept of a phase in the present context (see also Section 4.3.2e) is that of a portion of matter that is separated from others by a clearly defined boundary thus, solid, liquid and gas phases satisfy this definition, but it is possible to have two distinguishable liquid phases (e.g., water and carbon tetrachloride). A simple example of an application of the latter, that is familiar to anyone who has taken an organic chemistry laboratory course, is liquid-liquid extraction a first step in purification of a desired synthesized compound is often to place the crude... 

Figure 2. Distribution of DNOC (open symbols), and TNT (filled symbols) between aqueous and solid phases of typical aquifer and soil materials. Shown is the fraction of sorbed species as a function of pH and K" -saturation of the clay minerals. The relative importance of the two dominating sorption mechanisms are compared hydrophobic partitioning into sediment organic matter (O, ) and specific EDA-complex formation with clay mineral surfaces (O, ) triangles (A, A) represent the overall speciation for DNOC and TNT respectively. Sediment parameters were chosen as follows fraction of organic carbon 4. (soil) = 0.05 f (aquifer) = 0.001 fraction of clay minerals f igy (soil) = 0.35 fciay (aquifer) = 0.05 porosity e(soil) = 0.4 (aquifer) = 0.3 bulk density p(soil) = p(aquifer) = 2.5 kg L water saturation 0(soil) = 0.5 0(aquifer) = 1 fraction of K -saturation of clay minerals = 0.001 (when pH was used as system variable) pH = 7.0 (when f(, +, was used as system variable). Linear adsorption isotherms of TNT and... 

---