Bulk electron donor

It is known that the decolorization rate of azo dyes is increased by using redox mediators, which speed up the reaction rate by shuttling electrons from the biological oxidation of primary electron donors or from bulk electron donors to the electron-accepting azo dyes [21, 31, 40]. But continuous dosing of the dissolved redox mediators implies continuous expenses related to procurement of the... 

Vitamin B12 catalyzed also the dechlorination of tetrachloroethene (PCE) to tri-chloroethene (TCE) and 1,2-dichloroethene (DCE) in the presence of dithiothreitol or Ti(III) citrate [137-141], but zero-valent metals have also been used as bulk electron donors [142, 143]. With vitamin B12, carbon mass recoveries were 81-84% for PCE reduction and 89% for TCE reduction cis-l,2-DCE, ethene, and ethyne were the main products [138, 139]. Using Ni(II) humic acid complexes, TCE reduction was more rapid, leading to ethane and ethene as the primary products [144, 145]. Angst, Schwarzenbach and colleagues [140, 141] have shown that the corrinoid-catalyzed dechlorinations of the DCE isomers and vinyl chloride (VC) to ethene and ethyne were pH-dependent, and showed the reactivity order 1,1-DCE>VC> trans-DCE>cis-DCE. Similar results have been obtained by Lesage and colleagues [146]. Dror and Schlautmann [147, 148] have demonstrated the importance of specific core metals and their solubility for the reactivity of a porphyrin complex. 

Figure 14.5 Relative initial rates of reduction [relative to nit-To-benzene (NB)] of 2-methyl-(2-CHj-NB), 4-chloro-(4-Ct-NB), and 4-acetyl- (4-Ac-NB) nitrobenzene (a) by dissolved natural organic matter constituents (DOM) in aqueous solution in the presence of hydrogen sulfide as bulk electron donor (Dunnivant et al.,...

An additional consideration in formulating redox reactions is the possibility of catalysis by substances that mediate the transfer of electrons between the bulk reductant (or oxidant) and the substrate being transformed. Such considerations arise frequently in many areas of chemistry, especially electrochemistry and biochemistry (e.g., 97). In environmental applications, the most common model for mediated electron transfer involves a rapid and reversible redox couple that shuttles electrons from a bulk electron donor to a contaminant that is transformed by reduction. 

Figure 9 H3fpothesized mechanism for the reductive transformation of a pesticide compound through the transfer of electrons from a bulk electron donor (e.g., FeS (Kenneke and Weber, 2003)) to a pesticide molecule (e.g., methyl parathion (Tratnyek and Macalady, 1989)) by an electron carrier (e.g., hydro-quinone (Schwarzenbach et al., 1990 reproduced by permission of American Chemical Society from Environ. Sci. TechnoL, 1990, 24, 1566-1574).

Some reactants may play different roles in their interactions with pesticide compounds, depending on the geochemical conditions. Metals and their complexes, for example, may react with pesticide compounds as hydrolysis catalysts (e.g., Mortland and Raman, 1967 Smolen and Stone, 1997 Huang and Stone, 2000), direct reductants (e.g., Castro and Kray, 1963, 1966 Mochida et al., 1977 Strathmann and Stone, 2001, 2002a,b), or bulk electron donors through electron-transfer agents such as hydroquinones (e.g., Tratnyek and Macalady, 1989 Curtis and Reinhard,... 

Most of what little information is available on how different metals and their complexes vary in their ability to promote transformations of pesticide compounds, however, focuses on their roles as direct reductants or hydrolysis catalysts (see references cited above) reduced iron appears to be the only metal that has been examined as a potential bulk electron donor in natural systems (Glass, 1972). 

In this scenario, the bulk electron donor or reductant rapidly reduces an electron carrier or mediator, which in turn transfers electrons to the pollutant of interest. The oxidized electron mediator is then rapidly reduced again by the bulk reductant, which enables the redox cycle to continue. 

Other reports on the abiotic reduction of nitro groups indicated the participation of a mediator, a quinone compound or an iron porphyrin, which served as an electron carrier between a bulk electron donor like hydrogen sulfide and different nitrobenzenes (Fig.5) (16, 46). Furthermore, picric acid is abiotically reduced initially to picramic acid under anaerobic conditions by Fe(II) with dithiothreitol as the bulk reductant (Rieger, unpublished). Heijman et al. (20) described the abiotic reduction of 4-chloronitrobenzene with Fe(II), associated with or bound to the surface of magnetite particles in an iron-reducing enrichment culture. The abiotic reduction of TNT by sulfide was observed by Preuss et al. (39) and will be described in section 3. 

Reduction of NACs in Homogeneous Aqueous Solution By NOM as ElectronTransfer Mediator and H2S as Bulk Electron Donor... 

In bulk chemical reactions, an oxidizer (electron acceptor) and fuel (electron donor) react to form products resulting in direct electron transfer and the release or absorption of energy as heat. By special arrangements of reactants in devices called batteries, it is possible to control the rate of reaction and to accomplish the direct release of chemical energy in the form of electricity on demand without intermediate processes. 

Further studies were carried out on the Pd/Mo(l 1 0), Pd/Ru(0001), and Cu/Mo(l 10) systems. The shifts in core-level binding energies indicate that adatoms in a monolayer of Cu or Pd are electronically perturbed with respect to surface atoms of Cu(lOO) or Pd(lOO). By comparing these results with those previously presented in the literature for adlayers of Pd or Cu, a simple theory is developed that explains the nature of electron donor-electron acceptor interactions in metal overlayer formation of surface metal-metal bonds leads to a gain in electrons by the element initially having the larger fraction of empty states in its valence band. This behavior indicates that the electro-negativities of the surface atoms are substantially different from those of the bulk [65]. 

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

As previously mentioned, all biologically initiated reactions are basically heterogeneous. However, for practical reasons, the processes in the suspended phase can be considered homogeneous. Processes in biofilms proceed by exchange of electron donors and electron acceptors with the surrounding bulk water phase. These processes are, therefore, heterogeneous. 

If the surface complex is the chromophore, then the photochemical reductive dissolution occurs as a unimolecular process alternatively, if the bulk iron(III)(hydr)-oxide is the chromophore, then it is a bimolecular process. Irrespective of whether the surface complex or the bulk iron(IIl)(hydr)oxide acts as the chromophore, the rate of dissolved iron(II) formation depends on the surface concentration of the specifically adsorbed electron donor e.g. compare Eqs. (10.11) and (10.18). It has been shown experimentally with various electron donors that the rate of dissolved iron(II) formation under the influence of light is a Langmuir-type function of the dissolved electron donor concentration (Waite, 1986). 

Cytochromes serve as electron donors and electron acceptors in biological electron transfer chains, and with >75,000 members (53) they provide the bulk of natural heme proteins in biology. Cytochromes may be fixed into place within an extended electron transfer chain, such as the membrane-bound 6l and 6h of the cytochrome bci complex, or may be soluble and act as mobile electron carriers between proteins, for example, cytochrome c (54). In either role, the cytochrome may be classified by the peripheral architecture of the porphyrin macrocycle. Figure 1 shows the dominant heme types in biological systems, which are hemes a, b, c, and d, with cytochomes b and c being most prevalent. The self-association of a protein with heme via two axial ligands is a... 

---