Soil Reaction Coefficients

Hard Lewis acids and bases have inflexible electron orbitals that form ionic bonds. The electron orbitals of soft Lewis acids and bases are more polarizable and more likely to form covalent bonds. Soft Lewis acids and bases are also called covalent-bonding ions and are siderophile (sulfur-loving) ions in the geology literature. Organic ligands and soil organic matter range from hard to soft Lewis bases.  

This classification explains why, for example, Fe3+ reacts differently than Fe2+ in soils. Reduced oxidation states tend to be softer Lewis acids and bases. Hard and soft also explains why Cd24 reacts quite differently than other cations of similar charge and size such as Ca, and why soil organic matter reacts with soft Lewis acids and also contributes greatly to the exchange capacity of hard Lewis acids. 

Many of the hydroxyoxides listed in Table 3.4 exist in soils. Their ion activity products, as well as those of phosphates, carbonates, sulfides, and silicates, have been measured in soil solutions. Unfortunately, tire ion activity products often differ widely from accepted solubility products in pure solutions, and also from soil to soil. The differences between ion activity products and solubility products is due 

Phosphate is associated with many phases of the soil, including organic matter. None of these phases predominates in all soils, and all have different dissociation strengths for phosphate. Hence, each should support a different phosphate concentration, and the strength of association decreases as the phosphate concentration increases for all of the phases. As a result, phosphate ions should distribute themselves among the various retention sites until, at equilibrium, all the ions have the same dissociation energy. The speed of these transformations may control soil phosphate concentrations lather than the equilibrium solubility of this distribution. 

The rates of soil phosphate reactions also may differ from the rates of phosphate uptake by plants and of phosphate release by organic matter decay. This phosphate turnover would further upset soil phosphate equilibria. If a steady state (concentration is constant with time) existed between the soil and dissolved phosphate ions, it might be described by a reaction such as 

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Knowledge of soil surfaces is still very poor, as there is limited information on basic parameters such as light intensity profiles in the soil, extinction coefficients and reaction quantum yields. Because light intensity decreases... 

Equation 3.41 requires that the standard states of the products and reactants be known, that the components can be defined quantitively and in a thermodynamic sense. In soils and much of nature these definitions are rarely possible. The states of ions or molecules in soil systems, and in probably all colloidal systems, are ill-defined thermodynamically. In rigorous thermodynamic terms even ions are undefined. Soil reactions, because of the nonequilibrium in soils and the lack of defined standard states, yield reaction coefficients, rather than reaction constants, and their values vary with soil conditions. 

Trichloroethane will enter the atmosphere from its use in the manufacture of vinylidene chloride and its use as a solvent. Once in the atmosphere, 1,1,2-trichloroethane will photodegrade slowly by reaction with hydroxyl radicals (half-life 24 to 50 days in unpolluted atmospheres and within a few days in polluted atmospheres). The soil partition coefficient of 1,2-trichloroethane is low and it will readily leach in the case of eventual, very slow biodegradation. Bioconcentration is not a significant process. It will also be discharged in wastewater associated with these uses and in... 

Available Chlorine Test. The chlorine germicidal equivalent concentration test is a practical-type test. It is called a capacity test. Under practical conditions of use, a container of disinfectant might receive many soiled, contaminated instniments or other items to be disinfected. Eventually, the capacity of the disinfectant to serve its function would be overloaded due to reaction with the accumulated organic matter and organisms. The chlorine germicidal equivalent concentration test compares the load of a culture of bacteria that a concentration of a disinfectant will absorb and still kill bacteria, as compared to standard concentrations of sodium hypochlorite tested similarly. In the test, 10 successive additions of the test culture are added to each of 3 concentrations of the hypochlorite. One min after each addition a sample is transferred to the subculture medium and the next addition is made 1.5 min after the previous one. The disinfectant is then evaluated in a manner similar to the phenol coefficient test. For equivalence, the disinfectant must yield the same number of negative tubes as one of the chlorine standards. 

Such simulations suggest that because of their relatively high water solubility which in combination with low vapor pressure causes low air-water partition coefficients, the phenols tend to remain in water or in soil and show little tendency to evaporate. Their environmental fate tends to be dominated by reaction in soil and water, and for the more sorptive species, in sediments. Their half-lives are relatively short, because of their susceptibility to degradation. 

Sorption of pharmaceuticals onto the surface of particulate matter or their distribution between two phases (water and either sludge, sediment or soil) depends on many factors, the most important being liquid phase pH and redox potential, the stereochemical structure and chemical nature of both the pharmaceutical compound and the sorbent, the lipophilicity of the sorbed molecules (excellent sorption at log Kov > 4, low sorption at log < 2.4), the sludge-water distribution coefficient Kd Kd > 2 L g SS good sorption, < 0.3 L g SS low sorption), the extent of neutral and ioiuc species present in the wastewater and the characteristics of the suspended particles. Moreover, the presence of humic and fulvic substances may alter the surface properties of the sludge, as well as the number of sites available for sorption and reactions, thereby enhancing or suppressing sorption of PhCs [38, 55, 61]. 

Irrespective of the sources of phenolic compounds in soil, adsorption and desorption from soil colloids will determine their solution-phase concentration. Both processes are described by the same mathematical models, but they are not necessarily completely reversible. Complete reversibility refers to singular adsorption-desorption, an equilibrium in which the adsorbate is fully desorbed, with release as easy as retention. In non-singular adsorption-desorption equilibria, the release of the adsorbate may involve a different mechanism requiring a higher activation energy, resulting in different reaction kinetics and desorption coefficients. This phenomenon is commonly observed with pesticides (41, 42). An acute need exists for experimental data on the adsorption, desorption, and equilibria for phenolic compounds to properly assess their environmental chemistry in soil. 

An early spectrophotometric method [ 1 ] for aluminium in soil involves the use of a Technicon sample changer, proportioning pump and automatic colorimeter. The method is based on the measurement of the rate of colour development in the reaction between aluminium and xylenol orange in ethanolic media. The calibration graph is rectilinear up to 2.7 mg/1 aluminium and the coefficient of variation is 4.5%. 

The 2-bromoethanol produced in this reaction is analysed by gas chromatography using an election capture detector. At the 10 mg/kg level in soil a standard deviation of 0.34 mg/kg was obtained, i.e., a coefficient of variation of 3%. Recoveries from soil were 81-94%. 

A number of equations have been used to describe the kinetics of soil chemical processes (see, e.g., Sparks, 1985, 1986). Many of these equations offer a means of calculating rate coefficients, which then can be used to determine energies of activation (E), which reveal information concerning rate-limiting steps. Energies of activation measure the magnitude of forces that must be overcome during a reaction process, and they vary inversely with reaction rate. 

With all batch techniques, there is the common problem of not removing the desorbed species. This can cause an inhibition of further adsorbate release (Sparks, 1985, 1987a), promote hysteretic reactions, and create secondary precipitation during dissolution of soil minerals (Chou and Wollast, 1984). However, one can use either exchange resins or sodium tetraphenylboron, which is quite specific for precipitating released potassium, as sinks for desorbed species and still employ a batch technique (Sparks, 1986). Also, since in most batch methods the reverse reactions are not controlled, problems are created in calculating rate coefficients. This is particularly true for heterogeneous systems such as soils. 

TABLE 6.3 Sorption Rate Coefficients (k) and Energies of Activation (E) for Initial Reaction between Pesticides and Soil Components... 

Wu and Gschwend (1986) reviewed and evaluated several kinetic models to investigate sorption kinetics of hydrophobic organic substances on sediments and soils. They evaluated a first-order model (one-box) where the reaction is evaluated with one rate coefficient (k) as well as a two-site model (two-box) whereby there are two classes of sorbing sites, two chemical reactions in series, or a sorbent with easily accessible sites and difficultly accessible sites. Unfortunately, the latter model has three independent fitting parameters kx, the exchange rate from the solution to the first (accessible sites) box k2, the exchange rate from the first box to the... 

A deeper perception of the mechanistic implications of equation (9.2) can be had if the rational activity coefficients are described on the molecular level using the methods of statistical mechanics. This approach is the analogue of the statistical mechanical theory of activity coefficients for species in aqueous solution (Sposito, 1983). Fundamental to it is the prescription of surface speciation and the dependence of the rational activity coefficient on surface characteristics. Three representative molecular models of adsorption following this paradigm are summarised in Table 9.8. Each has been applied with success to describe the surface reactions of soil colloids (Goldberg, 1992). 

Of course, redox reactions do not occur in isolation but are coupled through complexation reactions to other species. For example, Eq. 2.30 could include terms for nitrate and amine complexes, in addition to those for free nitrate, nitrite, and ammonium ions, if a typical soil solution were under consideration. The calculation of nitrogen speciation then would proceed just as described above. Indeed, redox reactions introduce no new mathematical elements into a speciation computation, any more than would the consideration of, for example, C02 reactions. The only new item brought in is an additional variable, the pE value, which, like the partial pressure of C02(g), must be specified in order to solve mole balance equations for distribution coefficients. 

The generalization of Eqs. 4.96 and 5.24 to include the possibility of imbibed water in an exchanger (thus making it a three-component mixture) is described in Chap. 5 of G. Sposito, The Thermodynamics of Soil Solutions, Clarendon Press, Oxford, 1981. The presence of charge fractions in Eq. 5.25 instead of mole fractions, as in Eq. 4.11, derives from the possible inequality of the stoichiometric coefficients, a and b, in the cation exchange reaction (cf. Eqs. 4.6 and 5.9). For homovalent exchange reactions, only mole fractions appear in the expressions for the adsorbate species activity coefficients. 

Transport and Transformation of Chemicals A Perspective. - Transport Processes in Air. - Solubility, Partition Coefficients, Volatility, and Evaporation Rates. - Adsorption Processes in Soil. - Sedimentation Processes in the Sea. - Chemical and Photo Oxidatioa - Atmospheric Photochemistry. -Photochemistry at Surfaces and Interphases. -Microbial Metabolism. - Plant Uptake, Transport and Metabolism. - Metabolism and Distribution by Aquatic Animals. - Laboratory Microecosystems. - Reaction Types in the Environment. -Subject Index. 

An important component of homovalent exchange is the magnitude of the exchange selectivity coefficient. Commonly, homovalent cation-exchange reactions in soils or soil minerals exhibit a selectivity coefficient somewhere around 1 (Table 4.2). This value signifies that the soil mineral surface does not show any particular adsorption preference for either of the two cations. However, for a mineral where the A Ca Mg is... 

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