Soil organic matter oxidation

Finally, mention must be made of possible conjugation reactions in which a covalent bond is formed between a contaminant molecule and a second contami nant molecule or soil organic matter. Oxidative coupling reactions of phenolics and aromatic amines are catalyzed by extracellular enzymes, clays, and oxides (Wang et al., 1986 Liu et al., 1987 Fluang, 1990). The bioavailability of the synthetic organic within the product is reduced or possibly eliminated (Dec et al., 1990 Allard et al., 1994). 

Lorenz, K., Preston, C.M., Kandeler, E. 2006. Soil organic matter in urban soils Estimation of elemental carbon by thermal oxidation and characterization of organic matter by solid-state 13C NMR spectroscopy. Geoderma, 130, 312-323. 

The real problem is the complete oxidation of all of the carbon present. As noted previously, soil organic matter can be associated with mineral surfaces or in pores with limited access. For these reasons, simple oxidation will not be sufficient and drastic oxidation procedures are necessary. 

For these and other reasons, direct oxidation of soil organic matter using high temperature and atmospheric air, although commonly used, is not the best procedure for determining soil organic matter content. 

There are still other methods of oxidizing soil organic matter. These are not generally or commonly used for a variety of reasons, which can be found by investigating them in the literature [22] (see also Chapter 10 for further details). 

Soil and soil suspensions are colored and hard to see through. Thus, it is hard to directly titrate them using colored indicators. There are typically only two cases where direct titrations of soil are carried out. The first is to determine the amount of amendment needed to bring the soil to a desired pH. The second is in the determination of soil organic matter where organic matter is oxidized with chromate and the unreacted chromate is titrated (actually called a back titration) to determine, by subtraction, the amount of dichromate reduced and thus the amount of organic matter present. 

Other methods for determining the amount of soil organic matter are available [4], However, they are not as commonly used as is chromate oxidation, which is commonly called the Watley-Black method. Usually, these other methods are both more time-consuming and less accurate than is the dichromate oxidation method. The dichromate oxidation of organic matter is the standard by which all other methods of determining soil organic matter must be compared [4,5],... 

Caution Chromates, including potassium and sodium dichromate, are hazardous materials, as are sulfuric and phosphoric acid used in the oxidation of soil organic matter. Great care must be exercised in using these chemicals and in disposing of the waste generated. 

In a standard method [15, 19] soil organic matter is almost completely oxidized by boiling gently with a solution of potassium dichromate, sulphuric acid and phosphoric acid. Excess dichromate is determined by titration with standard ferrous sulphate solution. 

In a further study the oxidation of the methyl carbon of methanearsonate was associated with the oxidation of soil organic matter in a number of soils. Additions of organic matter to a Norfolk loamy sand greatly increased the decomposition of methanearsonate. In three of the soils, there was no evidence of microbiological adaptation to methanearsonate. In Norfolk loamy sand, however, increasing decomposition of methanearsonate relative to soil organic matter occurred with time of incubation. 

Abiontic, involving free extracellular enzymes or solubilizing agents, enzymes bound to soil surfaces, enzymes within dead or non-proliferating cells, or enzymes associated with dead cell fragments. Extracellular enzymes are important in the initial stages of organic matter oxidation, in which polysaccharides and proteins are hydrolysed to soluble compounds that can be absorbed by microbial cells and further oxidized in biotic processes. 

Fig. 8.34 Parathion adsorption from aqueous solutions by three soils, before and after oxidation, as weU as on clay and organic matter. Reprinted with permission from Saltzman S, Kliger L, Yaron B (1972) Adsorption-desorption of parathion as affected by soil organic matter. J Agric Food Chem 20 1224-1227. Copyright 1972 American Chemical Society...

Retention of organic contaminants on subsurface solid phase constituents in general is not completely reversible, so that release isotherms differ from retention isotherms. As a consequence, the extent of sorption depends on the nature of the sorbent. Subsurface constituents as well as the types of bonding mechanisms between contaminants and the sohd phase are factors that control the release of adsorbed organic contaminants. Saltzman et al. (1972) demonstrated the influence of soil organic matter on the extent of hysteresis. Adsorption isotherms of parathion showed hysteresis (or apparent hysteresis) in its adsorption and desorption in a water solution. In contrast, smaller differences between the two processes were observed when the soils were pretreated with hydrogen peroxide (oxidized subsamples) to reduce initial organic matter content. The parathion content of the natural... 

---