Abiotic mechanism

A major development since the previous review is the discovery that some orga-nohalogen compounds can form in soils by a purely abiotic mechanism involving a Fenton oxidation pathway and the concomitant reduction of Fe(III) to Fe(H) (2384-2386). The formation of alkyl halides by this mechanism is shown in Scheme 4.6 (2387). The rates of production from soils decreased in this order ... 

The C02 flux at the atmosphere vegetation cover boundary is determined in many respects by the soil processes involved in organic matter transformation. To better understand the biotic and abiotic mechanisms that control C02 emission from the soil, Jassal et al. (2005) compared measured C02 fluxes in a forest with their distribution profile in the soil of a 54-year-old coniferous forest on the eastern coast of Vancouver. It was established that C02 concentration grows at all depths of the soil layer with rising temperature and humidity. This is explained by soil diffusion reduction and changes in soil ecosystem functioning. It was noted that more than 75% of C02 emitted from the soil was generated at a depth of 20 cm, and almost total C02 flux forms from the 0 cm-50 cm layer. 

Photochemical degradation has been recognized as an abiotic mechanism that contributes to the loss of waste constituents in soils [71,72], It does not often result in complete degradation but allows further action of biotic processes. Phototransformation of soil-adsorbed pollutants takes place through two main pathways. 

It is interesting to compare the expected rates of Mn oxidation via abiotic mechanisms with the rates expected from the biological kinetic rate law described above. Abiotic Mn oxidation rates at pH 8.03 were measured in seawater by von Langen et al. (1997) who reported a first-order rate constant of l.lxlO-6 (normalized for Po2 = 1 atm and T = 25°C). At this pH and for similar conditions, the cell concentration of L. discophora required to obtain the same rate would be only 0.30 pg/1 (Zhang et al., 2002) (i.e., approximately 3x10s cells/1). It is reasonable to assume that cell populations of Mn-oxidizing bacteria far greater than this would be possible in natural environments. Even smaller population sizes would be required to match abiotic rates (if they could be measured) at lower pH values. 

The genesis of pyrite has claimed the attention of many workers. Pyrite has been synthesized chemically in the laboratory under a variety of temperatures and pressures (Berner, 1964a Roberts et al., 1969 Rickard, 1969). Pyrite framboids, so named because of their raspberry-like texture when viewed under a microscope, are found in clay sediments and silts, or as infillings of foram, diatom, or radiolarian tests. Biotic and abiotic mechanisms have been proposed for their formation (Schneiderhohn, 1923 Schou-ten, 1946 Love, 1965 Rickard, 1970). More recently, structures have been synthesized in the laboratory which resemble the pyrite framboids found in marine sediments (Berner, 1969 Farrand, 1970 Suna awa et al., 1971 Sweeney and Kaplan, 1973). On the basis of experiments with stable isotope... 

C.L. Wilson, N.W. Hinman, R.P. Sheridan (2000). Hydrogen peroxide formation and decay in iron-rich geothermal waters The relative roles of biotic and abiotic mechanisms. Photochem. Photobiol, 71,691-699. 

Abiotic mechanisms for degradation of herbicides in soil, discussion, 15 Abiotic transformation processes of pesticide dissipation in soil hydrolytic reactions, 5 role in pesticide degradation, 4-5... 

Explosive compounds may undergo extensive transformation in aquatic systems, by microbial attack or abiotic mechanisms such as hydrolysis, oxidation, phototransformation, and so forth. Therefore, aquatic receptors may be exposed not only to energetic compounds released to the environment but also to their numerous transformation products. The key biotic and abiotic transformations of major explosives are discussed in Chapter 2. 

Hg in waters originates from various sources, of which reduction of Hg(II) by aquatic microorganisms such as cyanobacteria is the most important (Mason etal. 1995a, b), but may also be mediated by abiotic mechanisms in the presence of humic acids (Allard and Arsenie 1991) or methyltin (Weber 1993), as by decomposition of orga-nomercury compounds (Mason and Fitzger-... 

Shoham D (1993). Biotic-abiotic mechanisms for long-term preservation and reemergence of influenza type A virus genes. Prog. Med. Virol. 40 178-192. 

H2 FROM THE PLANETARY MATRIX 2.1. Abiotic Mechanisms of H2 Production... 

Most synthetic carbon-based polymers are inert toward micro-organisms in the form in which they are initially produced. In order that the carbon nutrients which they contain can be made available to the biological cell, they must be first transformed by a variety of chemical reactions to low molecular weight metabolites that can be absorbed by the cell [1]. These reactions are sometimes induced by oxidative enzymes, following the colonization of cells on the surface of the polymer. This is in turn followed by surface erosion [2]. However, biodegradation does not need to be enzymically induced and in most cases is not [3] and the rate of the initiation process is governed by the normal abiotic mechanisms of organic and physical chemistry. 

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