Soils surface oxidation

In waterlogged anaerobic soils, organic matter can be converted (via organic acids, carbon dioxide and hydrogen) to methane by methanogenic bacteria. A methane oxidizing flora at the soil surface oxidizes most of the methane to carbon dioxide. 

Surface oxidation short of combustion, or using nitric acid or potassium permanganate solutions, produces regenerated humic acids similar to those extracted from peat or soil. Further oxidation produces aromatic acids and oxaUc acid, but at least half of the carbon forms carbon dioxide. 

DBCP. The predictions suggest that DBCP is volatile and diffuses rapidly into the atmosphere and that it is also readily leached into the soil profile. In the model soil, its volatilization half-life was only 1.2 days when it was assumed to be evenly distributed into the top 10 cm of soil. However, DBCP could be leached as much as 50 cm deep by only 25 cm of water, and at this depth diffusion to the surface would be slow. From the literature study of transformation processes, we found no clear evidence for rapid oxidation or hydrolysis. Photolysis would not occur below the soil surface. No useable data for estimating biodegradation rates were found although Castro and Belser (28) showed that biodegradation did occur. The rate was assumed to be slow because all halogenated hydrocarbons degrade slowly. DBCP was therefore assumed to be persistent. 

Usually parts are soiled by dust, lint, mould release residues, greases, fingerprints... Cleaning eliminates these pollutants and, for some, the surface oxidation. .. 

In real soils where oxides, layer silicates, organic matter and other materials are present in intimate mixtures, with the oxides and organic matter often coating the surfaces of the other materials, the different functional groups interact with... 

Figure 3.17 Calculated changes in a soil solution upon rednction of Fe(III) oxide coatings on soil surfaces and structural Fe(III) in clay lattices without re-precipitation of Fe(II). and are exchangeable cations. Param-...

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. 

The oxidized form of As, arsenate, As(V), which is present as HAs04 at neutral pH (p f values in Table 7.8), is sorbed on soil surfaces in a similar way to orthophosphate. The reduced form arsenite, As(lll), which is present in solution largely as H3As03(p fi = 9.29), is only weakly sorbed, hence mobility tends to increase under reducing conditions. Mobility will also increase without reduction of As(V) because, as for phosphate, reductive dissolution of iron oxides results in desorption of HAs04 into the soil solution. Under prolonged submergence As(lll) may be co-precipitated with sulfides. 

The redox and sorption behaviour of Sb is similar, but no volatile forms are produced. The oxidized form of Sb, antimonite, Sb(V), has the anionic form Sb(OH)e at pH > 4, and is sorbed on oxides and sihcate clays. The reduced form, antimonite, Sb(III), is present as the uncharged Sb(OH)3 molecule except at very low or very high pH where the Sb(OH)2 cation and Sb(OH)4 anion form, respectively. The uncharged Sb(OH)3 is little sorbed on soil surfaces. 

These findings broadly agree with experimental observations. Measured rates of CH4 oxidation in the rice rhizosphere range widely from 5 to 90% of the CH4 transported (Holzapfel-Pschom et al 1985 Epp and Chanton, 1993 van der Gon and Neue, 1996). This agrees with the model. Rates of O2 flow through rice roots to the rhizosphere are of the order of a few mmol 02m (soil surface) h (Section 6.4), which is sufficient to account for the rates of oxidation calculated with the model. Measured differences in emissions between rice cultivars are largely due to differences in root biomass (Lu et al., 1999) the effects of differences in root porosity are smaller (Aulakh et al., 2001a,b). 

Iron oxides in soils have in common that they are of extremely small crystal size and/or low crystal order. This, in combination with their low concentration (only tens g kg in most soils) explains why soil iron oxides have escaped identification for a long time in spite of their obvious existence as seen from the soil colour. In the past, therefore, Fe oxides in surface environments have been considered to be amorphous to X-rays and often called limonite , which mineralogically, is an obsolete term. Furthermore, in order to identify the clay minerals in soils properly, Fe oxides are usually removed before X-ray diffraction methods are applied (Alexander et al., 1939 Mehra Jackson, 1960). 

Because Fe oxides are intimately associated with other soil components, it is not easy to determine the specific surface area of soil Fe oxides. An approximation can be obtained by attributing the surface area difference from before and after selective re-... 

Like chlorine dioxide, the chlorite ion is a strong oxidizer (Rav-Acha 1998). Since chlorite is an ionic species, it is not expected to volatilize and will not exist in the atmosphere in the vapor phase. Thus, volatilization of chlorite ions from moist soil and water surfaces or dry soil surfaces will not occur. 

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