Redox toxic organic compounds

One-electron reduction or oxidation of organic compounds provides a useful method for the generation of anion radicals or cation radicals, respectively. These methods are used as key processes in radical reactions. Redox properties of transition metals can be utilized for the efficient one-electron reduction or oxidation (Scheme 1). In particular, the redox function of early transition metals including titanium, vanadium, and manganese has been of synthetic potential from this point of view [1-8]. The synthetic limitation exists in the use of a stoichiometric or excess amount of metallic reductants or oxidants to complete the reaction. Generally, the construction of a catalytic redox cycle for one-electron reduction is difficult to achieve. A catalytic system should be constructed to avoid the use of such amounts of expensive and/or toxic metallic reagents. 

Abiotic transformation of contaminants in subsurface natural waters result mainly from hydrolysis or redox reactions and, to lesser extent, from photolysis reactions. Complexation with natnral or anthropogenic ligands, as well as differential volatilization of organic compounds from multicomponent hquids or mixing with toxic electrolyte aqueous solutions, may also lead to changes in contaminant properties and their environmental effects. Before presenting an overview of the reactions involved in contaminant transformations, we discuss the main chemical and environmental factors that control these processes. 

It is generally accepted that free ionic forms of heavy metals are generally more toxic to biota than chelated or precipitated forms. Several factors control metal bioavailability and, thus, toxicity in environmental samples. These factors include pH, redox potential, alkalinity, hardness, adsorption to suspended solids, cations and anions, as well as interaction with organic compounds (Kong et al., 1995). 

Thus, the synthesis of triphenylscandium is a salt-elimination reaction (or metathesis) whilst the route for the lanthanide phenyls involves a redox reaction. The former has the problem of producing LiCl, which is often significantly soluble in organic solvents and contaminates the desired product, whilst the latter involves disposal of mercury waste, as well as handling toxic organomercury compounds. 

Many redox reactions by colloidal nanoparticles have been reported. Three of the most-studied reactions are (1) the catalyzed electron transfer between ferricyanide and thiosulfate [8,19-21], (2) the catalytic reduction of fluorescent dyes by sodium borohydride [22, 23], and (3) the catalytic reduction of organic compounds (e.g., nitro-aryls [9] and alcohols [24]). These reactions have been studied extensively because they are easy to follow spectroscopically allowing for straightforward measurement of reaction kinetics. The third set of reactions has enormous industrial significance, where nitro compounds are reduced to their less toxic nitrate or amine counterparts [25, 26] and the electrooxidation of methanol is utilized for methanol fuel cells [27, 28]. 

Oxides of transition metals, mainly Cr, Mn, Co, Ni, Fe, Cu, and V are employed in the oxidation of organic compounds. Deep oxidation reactions over these metal oxides are considered to be catalyzed by lattice oxygen. A common feature of these metal oxides is the presence of multiple oxidation states. During catalysis, the metal may be reduced by the hydrocarbon and reoxidized by oxygen. It may cycle between two or more oxidation states thus operating in a redox cycle (Mars-van Krevelen mechanism) [12]. However, the actual mechanism of a working catalyst may involve many steps in a number of consecutive or parallel reactions. Because of its low volatility and low toxicity, MnOjc has received the attention of many researchers. 

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