Oxidation of organic matter

In addition to the acid—base components shown in Figure 9, various organic acids are often found. Many of these acids are by-products of the atmospheric oxidation of organic matter released into the atmosphere. Of special interest are formic, acetic, oxaUc, and benzoic acids, which have been found in rainwater in concentrations occasionally exceeding a few micromoles per Hter. 

Menzel, D. W. (1974). Primary productivity, dissolved and particulate organic matter and the sites of oxidation of organic matter. In The Sea," Vol. 5 (E. D. Goldberg, ed.). Wiley, New York. 

Froelich, P. M., Klinkhammer, G. P., Bender, M. L. et al. (1979). Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic suboxic diagenesis, Geochem. Cosmochim. Acta 43, 1075-1090. 

The loss of residual oxidants does not correspond exclusively to bromate formation, and other reactions, including oxidation of organic matter, also take place. The rate and extent of bromate formation depend on the intensity of sunlight. 

There is considerable interest in the role of formic acid and other volatile fatty acids in the early diagnosis of organic matter in lacustrine and marine sediments. Formic acid is an important fermentation product or substrate for many aerobic and anaerobic bacteria and for some yeasts, hi the atmosphere, formic acid is an important product in the photochemical oxidation of organic matter. 

Chapter 8 describes a similar one-dimensional chain of identical reservoirs, but one that contains several interacting species. The example illustrated here is the composition of the pore waters in carbonate sediments in which dissolution is occurring as a result of the oxidation of organic matter. I calculate the concentrations of total dissolved carbon and calcium ions and the isotope ratio as functions of depth in the sediments. I present... 

I consider a system in which organic matter is oxidized at a steady rate that is a specified function of depth in uniform calcium carbonate sediments. The oxidation of organic matter increases the total dissolved carbon in the pore water of the sediment. The resultant increase in acidity causes the dissolution of calcium carbonate and a consequent increase in alkalinity as well as another increase in total dissolved carbon. The total dissolved carbon and alkalinity are transported by diffusion between different depths in the sediment. 

The microbial catabolic processes, which proceed in wastewater, provide the biomass with energy. These processes include two process steps oxidation of organic matter and reduction of an electron acceptor. The entire oxidation-reduction process, or redox process, consists basically of transfer of electrons from the electron donor (the organic matter) to the relevant electron acceptor, i.e., from the oxidation step to the reduction step. 

Example 2.2 shows how the oxidation of organic matter (the electron donor) is balanced by using the production of electrons as the central element. The cases in Examples 2.3 to 2.5 are balances for electron acceptor reductions under aerobic, anoxic and anaerobic conditions, respectively (all are relevant for processes in wastewater of sewer networks). 

Most oxidation reactions are between specific metal cations or metal oxy-anions and cations. The problem that arises when applying oxidation-reduction reactions to soils is that all soils contain a complex mixture of oxidizable and reducible cations, anions, and organic matter, which means that it is impossible to determine which is being titrated. An exception to this is the oxidation of organic matter where an oxidation-reduction titration is routinely carried out. Organic matter determination will be discussed in Section 10.3. 

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],... 

In this equation, C is the concentration of element i in pore water at depth z below the seafloor and A is a reaction (sink and source) term. For reactions involving the oxidation of organic matter, A can be evaluated independently. For constant porosity , the sulfate transport equation becomes... 

Heterotrophic activity A mode of nutrition based on the oxidation of organic matter. 

The chemical equation that describes the aerobic oxidation of organic matter is ... 

By catalyzing the oxidation of organic matter, the redox reactions conducted by the heterotrophs serve to restore the reduced atoms in the organic compounds back to their oxidized forms. During these reactions, electrons supplied by the organic matter cause O2, NO3, SO4 , and CO2 to be reduced to H2O, N2, H2S, and CH4, respectively. The oxidation of organic matter regenerates the CO2 required for photosynthesis. 

The reduced compounds produced from the oxidation of organic matter are thermodynamically imstable in the presence of O2. Their ensuing oxidation by O2 regenerates the oxidized species (NO3, SO4 , and CO2) needed to reinitiate the cycle. Water is also regenerated and, hence, is made available to participate in photosynthesis. 

Many of the chemical reactions that occur in sediments during diagenesis are mediated by marine organisms or are a consequence of biotic activities. Most are energy-yielding redox reactions driven by the oxidation of organic matter and, hence, represent a critical metabolic resource to benthic organisms. 

Phosphate is remineralized during the oxidation of organic matter and dissolution of hard parts, such as bones and teeth, that are composed of the minerals hydroxyapatite and fluoroapatite. Unlike the other products of remineralization, pore-water phosphate concentrations are regulated only by mineral solubility, such as through vivianite (iron phosphate) and francolite (carbonate fluoroapatite). Redox reactions are not significant because phosphorus exists nearly entirely in the h-5 oxidation state. 

Nitrogen uptake that results in the formation of new biomolecules is termed an assimilation process, such as assimilatory nitrogen reduction. The processes that result in the release of DIN into seawater are referred to as dissimilations, such as dissimi-latory nitrogen reduction. An example of the latter is denitrification, in which nitrate and nitrite obtained from seawater serve as electron acceptors to enable the oxidation of organic matter. This causes the nitrate and nitrite to be transformed into reduced species, such as N2O and N2, which are released back into seawater. 

Dissolntion and reduction of crystalline Fe(III) minerals is accelerated by chelation with carboxylate ligands in the presence of Fe(ll) (Zinder et al, 1986 Blesa et al, 1987 Phillips et al, 1993 Kostka and Lnther, 1994). Therefore as soil reduction proceeds and carboxylates formed in oxidation of organic matter accumulate in solution together with Fe +, dissolntion and rednction of crystalline Fe(lll) will commence. Dissolution of oxyhydroxide coatings will therefore lag behind the initial reduction of Fe(lll). 

Figure 4.3 Free energy changes in redox reactions mediated by microbes, (a) Oxidation of reduced inorganic compounds linked to reduction of O2. (b) Oxidation of organic matter CH2O linked to reduction of various organic and inorganic oxidants. pH = 7 and unit oxidant and reductant activities except (Mn +) = 0.2mM and (Fe +) = ImM...

Figure 4.15 indicates the range of rates of O2 consumption in different soils. Oxygen is consumed in oxidation of inorganic reductants, such as Fe(II), as well as in oxidation of organic matter by microbes. Bouldin (1968) and Howeler and Bouldin (1971) compared measured rates of O2 movement into anaerobic soil cores with the predictions of various models, and obtained the best fits with a model allowing for both microbial respiration and abiotic oxidation of mobile and immobile reductants abiotic oxidation accounted for about half the O2 consumed. The kinetics of the abiotic reactions are complicated. They often depend on the adsorption of the reductant on solid surfaces as, for example, in... 

The distribution of 5 C-values with water depth is mainly controlled by biological processes Conversion of CO2 into organic matter removes C resulting in a C enrichment of the residual DIG. In turn, the oxidation of organic matter releases C-enriched carbon back into the inorganic reservoir, which results into a depth-dependent isotope profile. A typical example is shown in Fig. 3.21. 

North Atlantic Deep Water (NADW), which is formed with an initial 5 C-value between 1.0 and 1.5%c, becomes gradually depleted in C as it travels southward and mixes with Antarctic bottom water, which has an average 8 C-value of 0.3%c (Kroopnick 1985). As this deep water travels to the Pacific Ocean, its C/ C ratio is further reduced by 0.5%o by the continuous flux and oxidation of organic matter in the water column. This is the basis for using 8 C-values as a tracer of paleo-oceanographic changes in deep water circulation (e.g., Curry et al. 1988). 

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