Soil Charge

In practice, it is not very usual to find in a single colloidal substance (either mineral or organic) all the contributions detailed in the preceding discussion (Equation 5.7). Only a particular type of mineral (some phyllosilicates, such as micas or smectites) show significant amounts of fixed charge. In other cases, inner sphere complexes are not important. On the contrary, almost all soil colloids (and most other colloids 

Diffuse layer cations z = +l Diffuse layer anion z = -l (hydration layer not shown) 

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Atlantic soils Charge properties and nitrate retention kinetics. Soil Sci. Soc. Am. J. 

These reactions determine the charge variability of most oxide minerals and consequently the pH dependence of the whole soil charge, for oxide-rich soils (Section 5.1.5). 

It neutralises the acidity of the soil by removing the hydrogen ions on the soil charged sites and replacing them with calcium or magnesium. 

Antistats such as polyoxyethylenes (151,152) and A/-alkyl polycarbonamide (153) are added to nylon to reduce static charge and improve moisture transport and soil release in fabrics. These additives also alter the luster of fiber spun from bright polymer. Static reduction in carpets is achieved primarily by the use of fibers modified with conductive carbon black (see Antistatic agents Carbon, carbon black). 

Of the various inorganic soil constituents, smectites (montmorillonite clays) have the greatest potential for sorption of pesticides on account of their large surface area and abundance in soils. Weak base pesticides, both protonated and neutral species, have been shown to be sorbed as interlayer complexes. Sorption of atrazine on smectites ranges from 0 to 100% of added atrazine, depending on the surface charge density of the smectite (36). 

Textile finishing encompasses a broad range of approaches and may be directed toward needed properties such as shrinkage control or smooth-dry performance or toward developing properties for specific end uses such as flame retardance, soil release, smolder resistance, weather resistance, or control of static charges. 

Soiling of Antistatic Finishes. Soiling of fabrics having a tendency for accumulation of charges has been assumed to be an electrostatic phenomenon, and therefore it follows that if static is eliininated, soiling will be reduced. However, most antistatic agents have been developed and used for reasons other than the reduction of soiling. 

In general, builders supply alkalinity to the wash Hquor and thus function also as alkaHes. In addition, they can exert a suspending (antiredeposition) effect and keep detached soil from depositing on the fabric builder ions with multiple charges are especially effective in this area. 

Adsorption of bath components is a necessary and possibly the most important and fundamental detergency effect. Adsorption (qv) is the mechanism whereby the interfacial free energy values between the bath and the soHd components (sofld soil and substrate) of the system are lowered, thereby increasing the tendency of the bath to separate the soHd components from one another. Furthermore, the soHd components acquire electrical charges that tend to keep them separated, or acquire a layer of strongly solvated radicals that have the same effect. If it were possible to foUow the adsorption effects in a detersive system, in all their complex ramifications and interactions, the molecular picture of soil removal would be greatly clarified. 

The oxidation products are almost insoluble and lead to the formation of protective films. They promote aeration cells if these products do not cover the metal surface uniformly. Ions of soluble salts play an important role in these cells. In the schematic diagram in Fig. 4-1 it is assumed that from the start the two corrosion partial reactions are taking place at two entirely separate locations. This process must quickly come to a complete standstill if soluble salts are absent, because otherwise the ions produced according to Eqs. (2-21) and (2-17) would form a local space charge. Corrosion in salt-free water is only possible if the two partial reactions are not spatially separated, but occur at the same place with equivalent current densities. The reaction products then react according to Eq. (4-2) and in the subsequent reactions (4-3a) and (4-3b) to form protective films. Similar behavior occurs in salt-free sandy soils. 

Anion A negatively charged ion in an electrolyte solution, attracted to the anode under the influence of a difference in electrical potential. Chloride is an anion. Anion exchange capacity Sum total of exchangeable anions that a soil can adsorb. Expressed as centimoles of negative charge per kilogram of soil. 

Oxides, non-crystalline minerals, and humified organic matter can also develop charges at their surfaces by reactions with the soil solution. In this case, the surface can have positive (CEC)... 

Fig. 8-3 Diagram illustrating the development of positively and negatively charged sites on surfaces of soil constituents, at low and high pH. (Reproduced with permission from R. L. Parfitt (1980). Chemical properties of variable charge soils. In "Soils with Variable Charge" (B. K. G. Theng, ed.), p. 168. New Zealand Society of Soil Science Offset Publications.)...

Another factor that can influence the environmental distribution of a chemical is the presence of charged groups. Some pollutants, such as the sodium or potassium salts of phenoxyalkanoic herbicides, dinitrophenols, and tetra- or penta-chlo-rophenol, exist as anions in solution. Others, such as the bipyridyl herbicides diquat and paraquat, are present as cations. In either case, the ions may become bound to organic macromolecules or minerals of soils or sediments that bear the opposite... 

In soil, the chances that any enzyme will retain its activity are very slim indeed, because inactivation can occur by denaturation, microbial degradation, and sorption (61,62), although it is possible that sorption may protect an enzyme from microbial degradation or chemical hydrolysis and retain its activity. The nature of most enzymes, particularly size and charge characteristics, is such that they would have very low mobility in soils, so that if a secreted enzyme is to have any effect, it must operate close to the point of secretion and its substrate must be able to diffuse to the enzyme. Secretory acid phosphatase was found to be produced in response to P-deficiency stress by epidermal cells of the main tap roots of white lupin and in the cell walls and intercellular spaces of lateral roots (63). Such apoplastic phosphatase is safe from soil but can be effective only when presented with soluble organophosphates, which are often present in the soil. solution (64). However, because the phosphatase activity in the rhizo-sphere originates from a number of sources (65), mostly microbial, and is much higher in the rhizosphere than in bulk soil (66), it seems curious that plants would have a need to secrete phosphatase at all. 

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