Pathway transformation

Klupinski TP, Y-P Chin (2003) Abiotic degradation of trifluralin by Fe(II) kinetics and transformation pathways. Environ Sci Technol 37 1311-1318. 

Figure 1. Hydrolysis pH-rate profiles of phenyl acetate (lower) and a substituted 2-phenyl-l,3-dioxane (HND). Phenyl acetate profile constructed from data of Mabey and Mill (32), HND profile from data of Bender and Silver (33). Phenyl acetate reacts via specific-acid catalyzed, neutral, and base-catalyzed transformation pathways. The pseudo-first-order rate constant is given by Kobs = K(h+) [H+] + Kn + K(qh-) [0H—]. HND hydrolyzes only via an acid-catalyzed pathway the phenolate anion is some 867 times more reactive than its conjugate acid.

As reported earlier by Han (1998) and Han et al. (2001c), heavy metals were transferred and redistributed from the soluble and labile forms into the stable forms with time. The transformation pathways of Cu, Ni and Zn in soils were strongly affected by the soil moisture regime, while Cd and... 

The transformation pathway(s) and kinetics of Mn in the solid-phase of two Israeli arid-zone soils incubated under saturated paste and field capacity conditions for a prolonged period of time is discussed below. 

However, the two soils differed significantly in their detailed transformation pathways. During saturation in the sandy soil, Mn was transformed from the ERO and the OM fractions predominately into the EXC fraction (Figs. 6.19 and 6.21). In the loessial soil, Mn was transformed... 

The section below will briefly address the distribution of Co, its transformation pathway(s), and kinetics among solid-phases in two selected Israeli arid-zone soils incubated under saturated paste conditions for prolonged periods of time (Han et al., 2002b). The parallel relationships of Co and Mn transformations among their solid-phases in soils are demonstrated. 

The initial transformations of Co among solid-phase fractions in two arid soils during incubation were concomitant with changes in Eh and pH. A rapid initial stage of transformation of Co was observed, mainly from the ERO fraction, and to some extent, from the RO and the OM fractions to the CARB fraction. Transformation pathways of Co followed those of Mn in the soils during incubation (Fig. 6.29). Most of the changes were completed within three days in the sandy soil, but these changes lasted 18 days in the... 

The analysis of environmental TPs has become a major trend in environmental chemistry, and increasingly, researchers are taking this a step further in proposing complex transformation pathways. It is expected to see a gradual shift from parent compound analysis to the analysis of metabolites and TPs. It is evident that more research is needed to determine the breakdown pathways and to evaluate the fate of TPs. Therefore, development of future generic analytical protocols should permit the simultaneous determination of parent compounds and their metabolites. 

The degradation activity depended on the anaerobic conditions and ranged between complete inhibition of biotransformation and mineralization of the herbicide. The degradation and transformation pathways that were observed under different reducing conditions are summarized in Fig. 28. 

Carbon tetrachloride slowly reacts with hydrogen sulfide in aqueous solution yielding carbon dioxide via the intermediate carbon disulfide. However, in the presence of two micaceous minerals (biotite and vermiculite) and amorphous silica, the rate transformation increases. At 25 °C and a hydrogen sulfide concentration of 0.001 M, the half-lives of carbon tetrachloride were calculated to be 2,600, 160, and 50 d for the silica, vermiculite, and biotite studies, respectively. In all three studies, the major transformation pathway is the formation of carbon disulfide. This compound is... 

Dodd MC, Shah AD, Von Gunten U, Huang C-H (2005) Interactions of fluoroquinolone antibacterial agents with aqueous chlorine reaction kinetics, mechanisms, and transformation pathways. Environ Sci Technol 39 7065-7076... 

Fig. 13.6 Redox transformation pathways of aldicarb. (Macalady et al. 1986 Wolfe et al. 1986)...

A heterogeneous natural system such as the subsurface contains a variety of solid surfaces and dissolved constituents that can catalyze transformation reactions of contaminants. In addition to catalytically induced oxidation of synthetic organic pollutants, which are enhanced mainly by the presence of clay minerals, transformation of metals and metalloids occurs with the presence of catalysts such as Mn-oxides and Fe-containing minerals. These species can alter transformation pathways and rates through phase partitioning and acid-base and metal catalysis. 

Klok, J. Baas, M. Cox, H.C. de Leeuw, J.W. Schenck, P.A. (1984) Loliolide and dihydroactinidiolide in a recent marine sediment probably indicate a major transformation pathway of carot ioids. Tetrahedron Lett., 25, 5577-80. 

Another intensively studied element in speciation analysis is arsenic. The biological and environmental effects of arsenic species and their transformation pathways have been studied in numerous papers.40- 42 Both arsenite and arsenate accumulate in living tissues because of their affinity for proteins, lipids and other cellular compounds.43 Arsenic species can undergo transformation via... 

When comparing the hydrolysis of methyl bromide with its reaction with Cl under the same conditions (i.e., [Cl-] = 100 mM, see Illustrative Example 13.2), we see that from a thermodynamic point of view, the hydrolysis reaction is heavily favored (compare ArG° values). This does not mean that the methyl bromide present is primarily transformed into methanol instead of methyl chloride (which it would be, if the reaction were to be thermodynamically controlled). In fact, in this and all other cases discussed in this chapter, we will assume that the reactions considered will be kinetically controlled that is, the relative importance of the various transformation pathways of a given compound will be determined by the relative reaction rates and not by the respective ArG° values. Thus, in our example, because CE is about a 103 times better nucleophile as compared to water (see Section 13.2) and because its concentration is about 103 times smaller than that of water (0.05 M versus 55.3 M), the two reactions would be of about equal importance under the conditions prevailing in this groundwater. Note that the product methyl chloride would subsequently also hydrolyze to yield methanol, though at a much slower rate. We will come back to this problem in Section 13.2 (Illustrative Example 13.2). 

Thus sorption, followed by intra-particle diffusion of the dyes, causes rapid initial loss followed by slow long term loss. Transformation pathways probably involve azo reduction. Diffusion may limit the dyes transformation rates because of the large size of the molecules. 

The hypothesized transformation pathways of CT and CF to methane are shown in Figure 2. The transformation of CT and CF to methane in the Pd/alumina system, despite the low reactivity of MeCl, indicates that the reactions do not involve sequential dehalogenation of CF to methane (i.e. MeCl is not an intermediate). The formation of C2 and C3 compounds during the transformation of both CT and CF indicates the existence of a radical pathway. However, the production of ethane (12-14%) and CF (18-23%) from CT was much lower than that of methane (51-60%). This implies that the main transformation pathway is a direct reaction of CT to methane, with a secondary pathway involving a trichloromethyl radical which then reacts to form CF and C2 and C3 species. Similarly, the relatively low production of ethane (<1%) from CF indicates that the major pathway for the reaction of CF to ethane occurs through direct transformation to methane, rather than through a dichloromethyl radical species. (Lowry and Reinhard 1999)... 

Figure 2. Hypothesized transformation pathways of chlorinated methanes, (based on Lowry...

Figure 4. Possible transformation pathway of DBCP. (based on Siantar et al., 1996)...

Figure 5. Transformation pathway of lindane to benzene. Reprinted from Applied Catalysis B Environmental, Vol. 18, Schtlth and Reinhard, Hydrodechlorination and Hydrogenation of Aromatic Compounds over Palladium on Alumina in Hydrogen-Saturated Water, pp. 219, Copyright 1998, with permission from Elsevier Science.

FIGURE 1 Fate and major transformation pathways of phytoplankton- and macrophyte-derived DOM in aquatic systems. Arrows indicate fluxes POC denotes particulate organic matter LMW and HMW DOM refer to the monomeric (low molecular weight) and polymeric (high molecular weight) fractions, respectively. 

There are four major transformation pathways leading from the DOM pool into the microbial loop direct uptake and photolysis-, ectoenzyme-, and sorption-mediated uptake (Fig. 1). Each of these pathways or processes is regulated by a combination of intrinsic and extrinsic factors. Intrinsic factors are elements of the pathway itself and include DOM characteristics, enzyme kinetics, and microbial diversity. For instance, the uptake characteristics of the resident microbial community will affect which monomers are assimilated from the pool of DOM. Conversely, the composition of the DOM pool is likely to affect which microbial consortia are present and active at any given time. 

Wolfe, N.L., Zepp, R.G., Schlotzhauer, P, Sink, M. (1982) Transformation pathways of hexachlorocyclopentadiene in the aquatic environment. Chemosphere 11, 91-101. 

Very significant photochemical processes can take place in various environmental compartments and account for the transformation of organic and inorganic compounds, including pollutants released by human activities. In many cases pollutant transformation is beneficial to the environment and to human health because it decreases the lifetime and hence the possible impact of harmful compounds. However, in some cases the environmental transformation of pollutants and of some otherwise harmless xenobiotics can yield compounds having much higher impact than the parent compounds (e.g., the case of carbamazepine transformation into acridine). It is therefore very important to assess the transformation pathways, including the photochemical ones, of compounds naturally present in the environment or released by human activities. 

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