Partition into Solid Phases

The transfer of molecules from solution into an environmental solid phase such as a soil or sediment is referred to as sorption, with the reverse process usually called desorption (Karickhoff, 1984 Weber et al., 1991). A variety of solid phases are available in the aquatic environment small suspended particles, both living and nonliving, the anatomical surfaces of larger biota such as fish, and bulk soils and bottom sediments. Even colloidal organic solutes such as humic macromolecules might be thought of as separate phases to which a dissolved molecule could be sorbed. Each of these surfaces may be thought of as a source or a sink for compounds in solution. 

Although distinctions are sometimes made between adsorption (uptake of compounds by the surface of a solid phase) and absorption (diffusion of molecules into the interior of a solid), it is usually not possible to distinguish between these cases in environmental situations. A complicating factor in sorption studies is that natural solid phases are not only not chemically and physically homogeneous, but are normally coated with extraneous materials such as transition metal oxides, microor- 

Sorption is important from the viewpoint of chemical reactivity, as well. A compound that is sorbed usually goes from a situation in which it is entirely surrounded by water molecules to one in which it is in a mineral environment rich in organic matter. In fact, a chemical substance in a suspension of natural particulate matter will exist in a complex equilibrium in which a fraction of the material is dispersed into several disparate phases that may contribute differently to the reactions the substance may undergo. 

Studies of the uptake of organic compounds by many types of natural solid phases (soils and sediments) in the presence of water have clearly shown that only two types of interactions are important first, a coulombic interaction, in which organic compounds of opposite (positive) charge are sometimes taken up by the (usually) negatively charged solid material and, generally more important, a hydrophobic interaction in which nonpolar organic compounds are attracted into the solid phase. 

In general, for both naturally occurring compounds and pollutants, 

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An element s tendency towards partitioning into solid phases is related to its fundamental atomic properties. These properties follow periodic trends, giving rise to the trends shown in Figure 11.5 for the oceanic residence times of the elements. In the broadest sense, these trends reflect the relative tendency of an element towards electrostatic versus covalent interactions. Chemists have devised various measures of this tendency, such as an element s electronegativity and its ionization potential. The latter is a predictor of electrostatic interactions and is defined as the ratio, z /r, where... 

The cleaning process proceeds by one of three primary mechanisms solubilization, emulsification, and roll-up [229]. In solubilization the oily phase partitions into surfactant micelles that desorb from the solid surface and diffuse into the bulk. As mentioned above, there is a body of theoretical work on solubilization [146, 147] and numerous experimental studies by a variety of spectroscopic techniques [143-145,230]. Emulsification involves the formation and removal of an emulsion at the oil-water interface the removal step may involve hydrodynamic as well as surface chemical forces. Emulsion formation is covered in Chapter XIV. In roll-up the surfactant reduces the contact angle of the liquid soil or the surface free energy of a solid particle aiding its detachment and subsequent removal by hydrodynamic forces. Adam and Stevenson s beautiful photographs illustrate roll-up of lanoline on wood fibers [231]. In order to achieve roll-up, one requires the surface free energies for soil detachment illustrated in Fig. XIII-14 to obey... 

A recent method, still in development, for determining total 4-nitrophenol in the urine of persons exposed to methyl parathion is based on solid phase microextraction (SPME) and GC/MS previously, the method has been used in the analysis of food and environmental samples (Guidotti et al. 1999). The method uses a solid phase microextraction fiber, is inserted into the urine sample that has been hydrolyzed with HCl at 50° C prior to mixing with distilled water and NaCl and then stirred (1,000 rpm). The fiber is left in the liquid for 30 minutes until a partitioning equilibrium is achieved, and then placed into the GC injector port to desorb. The method shows promise for use in determining exposures at low doses, as it is very sensitive. There is a need for additional development of this method, as the measurement of acetylcholinesterase, the enzyme inhibited by exposure to organophosphates such as methyl parathion, is not an effective indicator of low-dose exposures. 

Solid phase micro extraction (SPME) is a techniques in which a silica fiber coated with a thin film of polymer is brought into contact with an aqueous matrix where the organics in solution partition onto the fiber. The fiber is subsequently placed into the injector of a GC where the heat causes the release of analyte onto the column. This has been applied to endosulfan (a- and (3-) and endosulfan sulfate in water with limits of detection of less than 0.3 pg/L reported (Magdic and Pawliszyn 1996). 

Deposition of adamantane from petroleum streams is associated with phase transitions resulting from changes in temperature, pressure, and/or composition of reservoir fluid. Generally, these phase transitions result in a solid phase from a gas or a liquid petroleum fluid. Deposition problems are particularly cumbersome when the fluid stream is dry (i.e., low LPG content in the stream). Phase segregation of solids takes place when the fluid is cooled and/or depressurized. In a wet reservoir fluid (i.e., high LPG content in the stream) the diamondoids partition into the LPG-rich phase and the gas phase. Deposition of diamondoids from a wet reservoir fluid is not as problematic as in the case of dry streams [74, 75]. 

A soil sample (10 g) was extracted by mechanically shaking with methanol-deionized water. This mixture was filtered and a portion was removed for partitioning into toluene-hexane. A phenyl solid-phase extraction (SPE) cartridge was employed... 

Principles and Characteristics Solid-phase microextraction (SPME) is a patented microscale adsorp-tion/desorption technique developed by Pawliszyn et al. [525-531], which represents a recent development in sample preparation and sample concentration. In SPME analytes partition from a sample into a polymeric stationary phase that is thin-coated on a fused-silica rod (typically 1 cm x 100 p,m). Several configurations of SPME have been proposed including fibre, tubing, stirrer/fan, etc. SPME was introduced as a solvent-free sample preparation technique for GC. 

A number of mathematical models have been developed in recent years which attempt to predict the behavior of organic water pollutants. >2>3 Models assume that compounds will partition into various compartments in the environment such as air, water, biota, suspended solids and sediment. The input to the models includes the affinity of the compound for each of the compartments, the rate of transfer between the compartments, and the rates of various degradation processes in the various compartments. There is a growing body of data, however, which indicates that the models to date may have overlooked a small but significant interaction. A number of authors have suggested that a portion of the compounds in the aqueous phase may be bound to dissolved humic materials and are not therefore truly dissolved. 

Model of Dissolution. Based on the results described above, a model for the dissolution of phenolic resins in aqueous alkali solutions 1s proposed. The model 1s adapted from Ueberrelter s description for polymer dissolution 1n organic solvents (.21). In Ueberrelter s model, the dissolution process takes place 1n several steps with the formation of (a) a liquid layer containing the dissolved polymer, (b) a gel layer, (c) a solid swollen layer, (d) an infiltration layer, and (e) the unattacked polymer. The critical step which controls the dissolution process is the gel layer. In adapting h1s model to our case, we need to take into account the dependence of solvation on phenolate ion formation. There 1s a partition of the cation and the hydroxide ion between the aqueous solution and the solid phase. The... 

Interesting and interrelated with the previous case is one of enclosed partitions, when one of two partitions can be further divided into two partitions. An illustrative example is shown in Figure 9.17a. A granule of catalyst can be divided into two partitions porous aggregates (secondary particles—partition 1) and pores between the aggregates (partition 2). Partition 1 can also be divided into two partitions nonporous particles (primary particles—partition 11) and pores between particles (partition 12), excluding pores between aggregates. Another case of enclosed partitions has already been considered the case of a porous supported catalyst, which can be divided into pores and a solid phase, while the solid phase can be divided into the support and the active component. 

The mild conditions employed for the activation of sulfoxides, and the high reactivity of the glycosyl donor, suggested that the method might translate well to the solid phase. Moreover, the nonpolar nature of the sulfoxide, triflic anhydride, and DTBMP (solution-based reagents) should enable them to partition effectively into a non-polar, polystyrene-based resin. 

Lopez-Avila et al. [59] used microwave assisted extraction to assist the extraction of polyaromatic hydrocarbons from soils. Another extraction method was described by Hartmann [60] for the recovery of polyaromatic hydrocarbons in forest soils. The method included saponification of samples in an ultrasonic bath, partitioning of polyaromatic hydrocarbons into hexane, extract cleanup by using solid-phase extraction, and gas chromatography-mass spectrometric analysis using deuterated internal standards. Polyaromatic hydrocarbons were thermally desorbed from soils and sediments without pretreatment in another investigation [61]. 

Increasing water solubility corresponds to decreasing partitioning into the solid phase. 

Having chosen a particular model for the electrical properties of the interface, e.g., the TIM, it is necessary to incorporate the same model into the kinetic analysis. Just as electrical double layer (EDL) properties influence equilibrium partitioning between solid and liquid phases, they can also be expected to affect the rates of elementary reaction steps. An illustration of the effect of the EDL on adsorption/desorption reaction steps is shown schematically in Figure 7. In the case of lead ion adsorption onto a positively charged surface, the rate of adsorption is diminished and the rate of desorption enhanced relative to the case where there are no EDL effects. 

The molecular structure and conformation of an organic pollutant is a property which may affect adsorption onto a solid surface and/or partition into its organic lipid phase differently, thus hindering the expected correlation between KQM and K )W ... 

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