Reactions in the Soil

Chemical Reactions in the Soil. — Bacteriology is constantly being enriched by the discovery of new species of bacteria that have been isolated from the soil, and most of these are already well studied. However, it was quickly found that our artificial cultures do not lend themselves to the isolation of all species. The transformations which are accomplished in the soil result almost entirely from actions of symbiosis, which offers to the different micro-organisms a medium appropriate-to each, and it is often very difficult to reproduce these media 

I- Media exclusively inorganic, containing ammonia or nitrites  

Media rich in carbohydrates, but not containing combined nitrogen  

The following are two examples of the first type of culture medium  

WiXH AioioNiA Base. g With Nitrite Base. g- 

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Potassium nitrate is being used increasingly on intensive crops such as tomatoes, potatoes, tobacco, leafy vegetables, citms, and peaches. The properties that make it particularly desirable for these crops are low salt index, nitrate nitrogen, favorable N K20 ratio, negligible CU content, and alkaline residual reaction in the soil. The low hygroscopicity of KNO (Table 9) leads to its use in direct appHcation and in mixtures. It is an excellent fertilizer but the high cost of production limits its use to specialty fertilizers. 

Inorganic reactions in the soil interstitial waters also influence dissolved P concentrations. These reactions include the dissolution or precipitation of P-containing minerals or the adsorption and desorption of P onto and from mineral surfaces. As discussed above, the inorganic reactivity of phosphate is strongly dependent on pH. In alkaline systems, apatite solubility should limit groundwater phosphate whereas in acidic soils, aluminum phosphates should dominate. Adsorption of phosphate onto mineral surfaces, such as iron or aluminum oxyhydroxides and clays, is favored by low solution pH and may influence soil interstitial water concentrations. Phosphorus will be exchanged between organic materials, soil inter-... 

Measnrements of Ea are usually made with a platinum electrode placed in the soil solntion together with a reference half cell electrode of known potential. The platinnm electrode transfers electrons to and from the soil solution withont reacting with it. Reducing half reactions in the soil tend to transfer electrons to the platinum electrode and oxidizing half reactions to remove them. At eqnilibrinm no electrons flow and the electric potential difference between the half cell comprising the platinnm electrode and the soil solntion and the half cell comprising the reference electrode is recorded. 

System failure for in situ bioremediation efforts is often the result of ineffective transport of nutrients and electron acceptors due to channeling into preferential flow paths, heterogeneities, adsorption, biological utilization, and/or chemical reactions in the soil. Many of these problems can be overcome using electric fields for transport and injection instead of conventional groundwater injection by hydraulic techniques. 

Equation 8.9 shows that when NH3 is introduced to an acid solution, it reacts directly with the acid and produces the ammonium ion (NH4) (see Chapter 12). Concurrent with Equation 8.9, NH3 may associate itself with several water molecules (NH3nH20) without coordinating another H+. This hydrated NH3 is commonly referred to as unionized ammonia and is toxic to aquatic life forms at low concentrations. Because NH3 is a volatile gas, some of it may be lost directly to the atmosphere (volatilization) without dissolving in solution. On the other hand, the ammonium ion may undergo various reactions in the soil water that may alter its availability to plants and/or other organisms. These reactions include formation of metal-ammine complexes, adsorption on to mineral surfaces, and chemical reactions with organic matter. 

FIGURE 3.2 Solute phases and reactions in the soil postulated by the model. The solute can be in solution (C) sorbed reversibly in the soil (Se) in equilibrium with C sorbed reversibly and reacting kinetically (S,) sorbed reversibly and slowly reacting kinetically (S2) strongly and reversibly sorbed slowly reacting kinetically (S() or sorbed irreversibly in the soil (Sirr). 

There are virtually no sources of drinking water on Earth that are not contaminated with xenobiotics. Rain water cleanses the atmosphere as it forms and falls. As a result, it contains dissolved acids, organic compounds, and heavy metals such as mercury and selenium in many areas. Surface collection basins from which potable water is drawn—rivers, streams, and lakes—accumulate ground level pollutants in addition to those carried in rain water. Underground water, which is somewhat filtered and generally contains lesser quantities of pollutants than surface water, may itself be contaminated by ground releases of toxicants and by contaminants produced by chemical reactions in the soil and water. 

A large increase in pH, as much of the acidity is neutralized by buffering reactions in the soil... 

Redox reactions in the soil are mostly the result of a cycle started by photosynthesis. One part of the reaction is... 

Acid deposition undergoes many reactions in the soil, and leads to a change in the soil solution composition. The exchange complex of the soil becomes dominated by aluminum, the exchange acidity increases, bases are leached in association with acid anions, and the chemistry of the surface waters is changed. Increased deposition (wet and dry) of acid or potentially acidifying compounds (e.g., ammonia/ammonium) as well as decreased deposition of alkaline or acid-neutralizing compounds may decrease the soil pH. 

The application of an electric flied to moisten a porous matrix also induces chemical reactions in the soil and upon the electrodes. Chemical reactions include acid-alkaline reactions, redox reactions, adsorption-desorption reactions, and dissolution-precipitation reactions. Such reactions dramatically affect the speciation of the contaminants and therefore affect the transportation and contaminant removal efficiency [4]. 

Many inorganic and organic chemicals can undergo oxidation or reduction reactions in the soil. An indicator of a compound s abihty to be oxidized or reduced is provided by its oxidation potential (EO), which is the voltage at which it is transformed to its reduced state. A similar measure of a soil s ability to reduce a compound is provided by the redox potential (pE), which is a measure of electron activity. Redox potentials are relatively high and positive in oxidized environments (e.g., surface waters), and low and negative in reduced environments (e.g., aquatic sediments and the subsurface soil layers). These environmental conditions are especially important for inorganic chemicals that are rarely present in their elemental form in the environment. Arsenic, for example, exists primarily in its oxidized form (arsenate) in the atmosphere and in surface waters and in its reduced form (arsenite) in sediments. 

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