Abiotic pathways

In wetland systems, abiotic reactions can take place in solution and at the sediment-water interface. Hydrolysis, photolysis, sorption, and redox reactions are the most common abiotic reactions. 

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HgCl2 in the presence of estuarine sediments showed distinct seasonal variation, whose rhythm apparently relates to the seasonal ability of the microorganisms to carry out biomethylation (755, 156). The ability of different river sediments to perform biomethylation depended markedly on the nature of the sediments (157, 158) thus, organic sediments reacted appreciably faster than coarse sand. Activated sludge will methylate inorganic mercury compounds under both sterile and nonsterile conditions, suggesting the presence of both biotic and abiotic pathways (159). 

All three chloroacetic acids (chloroacetic acid [MCA], dichloroacetic acid [DCA], and trichloroacetic acid [TCA]) are naturally occurring (7), with TCA being identified in the environment most frequently (reviews (278, 405 108)). However, these chlorinated acetic acids also have anthropogenic sources. The major source of natural TCA appears to be the enzymatic (chloroperoxidase) or abiotic degradation of humic and fulvic acids, which ultimately leads to chloroform and TCA. Early studies (409) and subsequent work confirm both a biogenic and an abiotic pathway. Model experiments with soil humic and fulvic acids, chloroperoxidase, chloride, and hydrogen peroxide show the formation of TCA, chloroform, and other chlorinated compounds (317, 410-412). Other studies reveal an abiotic source of TCA (412, 413). 

Figure 2.8. N Is XANES spectra of (a) fulvic acid isolated from a glucose-glycine-8-Mn02 system and (b) the lyophilized solid phase. The peaks are assigned to pyridinic (398.6eV), pyridone (400.7 eV), amide (401.3 eV), and pyrrolic (402.0 eV) moieties. Reprinted from Jokic, A., Schulten, H.-R., Cutler, J. N., et al. (2004). A significant abiotic pathway for the formation of unknown nitrogen in nature. Geophys. Res. Lett. 31, L05502, with permission from the American Geophysical Union.

The study of Jokic et al. (2004a) reveals that the Maillard (sugar-amino acid condensation) reaction, catalyzed by birnessite, is an abiotic pathway for the forma-... 

Jokic, A., Schulten, H.-R., Cutler, J. N., Schnitzer, M., and Huang P. M. (2004a). A significant abiotic pathway for the formation of unknown nitrogen in nature. Geophys. Res. Lett. 31, L05502. 

Jokic, A., Wang, M. C., Liu, C. Frenkel, A. I., and Huang, P. M. (2004b). Integration of the polyphenol and Maillard reactions into a unified abiotic pathway for humification in nature the role of 8-Mn02. Org. Geochem. 35, 747-762. 

Photocatalysis is mostly thought of in terms of the photodegradation of molecules initiated either by oxidative or by reductive processes. However, photosynthesis may also result from a photocatalytic process, as observed for the reduction of C02 to formic acid by natural minerals. This reaction is the first step in an abiotic pathway for the synthesis of organic molecules and has been proposed to have played a role in the origin of life on earth [9]. 

Weber, E. J. (1994) Abiotic Pathways of Organic Chemicals in Aquatic Ecosystems. In Chemistry of Aquatic Systems Local and Global Perspectives, G. Bidoglio and W. Stumm, Eds., Kluwer Academic, Norwell, MA. 

Oxidation Kinetics of Mn(II). This section addresses the question of whether the Mn(II) oxidation rates shown in Figure 4 can be explained by microbiological or abiotic pathways. Several incubation studies of Mn(II) with natural water, natural particulate matter, or pure cultures reported evidence for microbial catalysis of Mn(II) oxidation (4, 18, 54-58). In bottom waters of Lake Zurich (58) and in water samples from the marine fjord of Saanich Inlet (18) maximum Mn oxidation occurred at around 33 and 20 °C, respectively. These results strongly suggest microbial catalysis. In the case of abiotic catalysis, a steady increase in the oxidation rate with temperature is to be expected (16). Working with water samples from the bottom of Lake Zurich that were spiked with Mn(II) at 2 or 10 xM, Diem (58) found a Michaelis-Menten-type rate law for Mn(II) oxidation ... 

Methanethlol (MeS) and 3-mercaptopropionate (MP) were by far the dominant thiols (Figure 2). Biochemical formation pathways are feasible for these thiols, although the importance of these pathways relative to abiotic pathways is presently unknown, especially for 3-mercaptopropionate, as summarized in Figure 4. The relative importance of these pathways are discussed in the following sections. 

The anthropomorphic question of the connection and relevance of interstellar chemistry to the existence and origin of life on earth is not touched on in this discussion, although a high premium has been placed on establishing this connection and thereby creating relevance. The earth has conditions and chemistries vastly different from those dealt with in this chapter. Whether the molecular forms synthesized by clearly abiotic pathways survive the condensation of the solar nebula, or whether molecular synthesis will occur essentially from the elements, is an interesting question on which to conclude. 

Non-peer-reviewed publications are still available on the isotopic effects related to the two main abiotic pathways for ethers, oxidation and hydrolysis under acidic conditions. However, preliminary results on isotopic fractionation associated with the reaction of potassium permanganate (KMn04) with MTBE showed a carbon enrichment factor between -4.2 and -4.9%o [18]. These eC values seem higher than the ones reported upon aerobic biodegradation of MTBE (from -0.28 to -2.4%o) suggesting potentially different reaction pathways and opening new lines of research and discussion for the future. 

While the importance of enzymes such as phosphatases, nucleases and phytase in the breakdown of organic phosphorus substrates is well documented (see Quiquam-poix and Mousain, Chapter 5, this volume), abiotic pathways for the degradation of these compounds have received substantially less attention. 

We thank our colleagues Dr David Jones, Dr Lynne Coleman and Dr Matthew Inman for their contributions to the understanding of abiotic pathways for the degradation of organic phosphorus, particularly metal-ion-facilitated hydrolysis of phosphate esters. 

List important abiotic pathway or processes involved in transformation of toxic organics in wetlands. 

Newman et al. (56), and Rochette et al. (68) suggest that the reduction of arsenate by dissolved sulfide is very slow at circumneutral pH values. However, at pH values less than 5, the reduction rates of arsenate due to sulfide may be significant in natural systems, where half-lives as short as 21 hr have been reported (68) for this abiotic pathway (Table 3). Rochette et al. (68) also revealed the potential importance of intermediate As-O-S species in electron transfer reactions between sulfide and arsenate, such as H2 As OsS H2As 02S , and H2 As OS2. It is not known whether these chemical species may also serve as important redox active species for microbial metabolism. These authors have also compared the rates of As(V) reduction in the presence of sulfide versus those rates expected via dissimilatory reduction by an arsenate-respiring organism (strain SES-3) (54) and for those measured in lake sediments (69) at pH values less than 5, reduction rates due to dissolved sulfide can become more significant than reduction rates due to anaerobic respiration where As(V) is used as the terminal electron acceptor (Fig. 8). 

Table 4 Approximate Time Scales of Abiotic Pathways that May Contribute to Oxidation of As(III) to As(V) in Soils and Natural Waters...

I asked Severo whether it would be worthwhile to study non-enzymic reactions involving formaldehyde, certain one-carbon compounds and other simple organic molecules in the hope of discovering abiotic pathways for the synthesis of the most important blocks of biological macromolecules. His answer was quite encouraging, and although I was not fully prepared to start work in this field immediately, it helped me to plan the program of research which I was to carry out later upon the completion of my doctoral studies. 

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