Hydrophobic sinks

The pH of the pulp to the flotation cells is carefliUy controlled by the addition of lime, which optimizes the action of all reagents and is used to depress pyrite. A frother, such as pine oil or a long-chain alcohol, is added to produce the froth, an important part of the flotation process. The ore minerals, coated with an oily collected layer, are hydrophobic and collect on the air bubbles the desired minerals float while the gangue sinks. Typical collectors are xanthates, dithiophosphates, or xanthate derivatives, whereas typical depressants are calcium or sodium cyanide [143-33-9] NaCN, andlime. 

Figure 7.17 shows the asymmetry ratios of a series of compounds (acids, bases, and neutrals) determined at iso-pH 7.4, under the influence of sink conditions created not by pH, but by anionic surfactant added to the acceptor wells (discuss later in the chapter). The membrane barrier was constructed from 20% soy lecithin in dodecane. All molecules show an upward dependence on lipophilicity, as estimated by octanol-water apparent partition coefficients, log KdaA). The bases are extensively cationic at pH 7.4, as well as being lipophilic, and so display the highest responses to the sink condition. They are driven to interact with the surfactant by both hydrophobic and electrostatic forces. The anionic acids are largely indifferent... 

Sediments are important compartments for many organic contaminants in the aquatic environment, in particular for hydrophobic POPs such as PAHs and PCBs. Sediments have been recognised as important sinks for these compounds but with the reduction in levels of them in water, the question arises of whether the older highly contaminated sediments will function in the future as secondary sources of the compounds or whether burial by recent, cleaner sediment will prevent exchange with the water phase. This will depend on the strength of turbulence/bioturbation and on anthropogenic influences such as dredging. 

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. 

Hydrophobicity - Before plasma treatment, silica powder is highly hydrophilic and immediately sinks in water. After plasma film deposition, the material floats on water for several hours. A significant reduction in polarity and in surface energy compared to untreated silica is found, down to the range of 28.4-47.7 mJ/m2. The water penetration into powder beds of untreated and plasma-treated silica is shown in Fig. 7. The untreated silica absorbs water very fast, whereas the plasma-treated silicas show a significantly decreased water penetration rate. The lowest rate is found for the polythiophene-coated silica (PTh-silica). 

Hydrophobicity - After surface modification by plasma polyacetylene, sulfur floats on top of ethylene glycol, whereas the untreated sulfur sinks immediately. The surface energy of uncoated sulfur can therefore be scaled in the range of 47.7-50 mJ/m2 and polyacetylene-encapsulated sulfur in the range of 28.4 17.7 mJ/m2. 

Special rules apply to the world of hydrophilic polyurethanes. These alternate rules are based on the fact that hydrophilic polyurethanes can and should be processed in water. Rather than emulsifying a prepolymer with a polyol, as would be done with a hydrophobic polyurethane, hydrophilics are mixed with water. While the properties of the foam are governed loosely by the guidelines described above, one has more flexibility and control of the formulation and process by which the polyurethane is made. For example, the water can serve as a heat sink to closely control the temperature of the foam the water controls the rate of reaction. 

The hydrocarbon tails sink into the blob of grease up to the head groups. These hydrophilic groups remain on the surface of the blob and enable the entire unit to dissolve in water (Fig. 8.17). The grease is carried away in a casing of soap molecules called a micelle. The hydrophobic tail of each soap molecule sticks into the grease blob at the center and its hydrophilic head points out, toward the surrounding water molecules. 

Soo et al. (2002) studied tliB vitro release of hydrophobic Luorescent probes from PEO-b PCL micelles. Micelle solutions were placed in dialysis bags (MWCO 50,000) in a stirred water bath with a constant overLow of distilled water. This maintained the release environment at near perfect sink conditions, so the limited solubility ofthe probes in the medium did not affect release kinetics. Release was determined by removing aliquots ofthe dialysis bag contents and measuring Luorescently. Soo et al. found an initial burst release of probe followed by slow diffusional release. For the probes studies, benzopyrene and Cell-Tracker-CM-Dil, diffusion constants were ofthe order 10"15 cnnP/s. 

Although the simple bioconcentration model assumes relatively unhindered movement of a contaminant across the barriers between water and lipid tissue, such is often not the case. The uptake of an organic species can be a relatively complex process in which the chemical must traverse membranes in the gills and skin to reach a final lipid sink. A physiological component of the process by which a chemical species moves across membranes tends to cause bioconcentration to deviate from predictions based on hydrophobicity alone. 

Although a simple bioconcentration model assumes rapid movement of a hydrophobic contaminant through an organism, distribution may be relatively slow. The predominant limiting factor in this case is the blood flow. Slow transport to lipid tissue sinks can result in lower apparent BCF values than would be the case if true equilibrium were attained. 

Sediments can be defined as deposits of solid material laid down in water bodies. Minerals usually dominate sediments, but sediments also contain organic substances, including humic substances of different compositions. Sediments may be considered the ultimate sink for hydrophobic chemicals. Although sediments have many of the same properties as soils, the high water content and anaerobic conditions that typically occur within centimeters of the sediment-water interface distinguish this matrix into its own unique category. 

For air-water systems, this equation is known as Henry s law. For solids-water systems, the equilibrium constant is known as the partition coefficient (Ky) or distribution constant (KA). Partition coefficients are available for many organic chemicals from laboratory and field measurements. As organic carbon (OC) present in water (dissolved organic carbon, or DOC), sediment, or soil is the main sink for hydrophobic organic contaminants, the partition coefficients for these compounds are often adjusted (normalized) with respect to the organic carbon content of these compartments ... 

Mineral solubility and precipitation were discussed in Chapter 2 as techniques for predicting the release of ionic constituents to water or soil solutions or the removal of ionic constituents from water or soil solutions. In this chapter, a second source/sink for ionic constituents (e.g., contaminants or nutrients) is presented and the mechanisms controlling this sink are referred to as adsorption or sorption. Both terms denote the removal of solution chemical species from water by mineral surfaces (e.g., organics, metal oxides, and clays) and the distinction between the two terms is based on the mechanism(s) responsible for this removal. In adsorption, a chemical species may be adsorbed by a surface either electrostatically or chemically (electron sharing), whereas in sorption, a chemical species may accumulate on a minerars surface either through adsorption, hydrophobic interactions, and/or precipitation. 

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