Reduction of organic pollutants

The reaction during reduction of organic pollutants by Fe° has two parts. The anodic half-reaction produces Fe2+ from Fe° and causes the iron metal to corrode (Agrawal and Tratnyek, 1996) ... 

Numerous studies have demonstrated that the facile reduction of organic pollutants such as nitroaromatics, azo compounds, and polyhalogenated hydrocarbons can occur in anaerobic environments with exceedingly fast reaction rates. The most abundant electron donors in these systems include reduced iron and sulfur species. Laboratory studies, however, indicate that the rates of reduction by these bulk reductants are much slower than those observed in natural reducing systems. The addition of electron carriers or mediators to the laboratory systems is found to greatly accelerate the measured reduction rates. An electron shuttle system has been postulated to account for the enhanced reactivity observed in these laboratory studies and presumably in reducing natural systems (Dunnivant et al., 1992). 

As in stoichiometric organic reactions, the application of nonvolatile ionic liquids can contribute to the reduction of atmospheric pollution. This is of special relevance for non-continuous reactions, in which complete recovery of a volatile organic solvent is usually difficult to integrate into the process. 

The communities include in particular bacteria, lower aquatic plants (algae), higher aquatic plants, organisms fish feed on (e.g. water flea, amphipods etc.) and fish. They participate in the self purification of waters (reduction of residual pollution from effluent discharges like industrial drainage) and maintain the natural biological equilibrium. 

These processes can occur by a direct electron transfer reaction to (reduction) or from (oxidation) the present organic pollutant, or by a chemical reaction of the pollutant with previously electrogenerated species. The mechanism is generally viewed as a direct anodic oxidation of organic pollutant involving its reduction by direct electron transfer from organic molecule to the electrode to form a radical cation that readily deprotonates, equation (37) ... 

This requires a biomass which can be metabolized. The process usually involves enzymatic transfer of electrons by micro-organisms from the decomposing biomass (represented in the above equation as CH2O) to the Fe " in Fe " oxides. As seen from eq.16.3, reduction consumes protons and is, therefore, favoured, the lower the pH (see also Chap. 12). It usually takes place when all pores are filled with water (see reviews by Fischer, 1988 and Van Breemen, 1988). Biotic reduction of Fe oxides is now recognized as an important process in the oxidation (metabolism) of organic pollutants in soils by dissimilatory, iron-reducing bacteria. 

Figure 14.1 Schematic representation depicting the importance of electron transfer mediators as well as the concurrence of microbial and abiotic processes for reductive transformations of organic pollutants. Adapted from Schwarzen-bach et al. (1997).

Let us now take a brief look at some important redox reactions of organic pollutants that may occur abiotically in the environment. We first note that only a few functional groups are oxidized or reduced abiotically. This contrasts with biologically mediated redox processes by which organic pollutants may be completely mineralized to C02, HzO and so on. Table 14.1 gives some examples of functional groups that may be involved in chemical redox reactions. We discuss some of these reactions in detail later. In Table 14.1 only overall reactions are indicated, and the species that act as a sink or source of the electrons (i.e., the oxidants or reductants, respectively) are not specified. Hence, Table 14.1 gives no information about the actual reaction mechanism that may consist of several reaction steps. 

You work in a research laboratory and your job is to investigate the microbial degradation of organic pollutants in laboratory aquifer column systems. You supply a column continuously with a synthetic groundwater containing 0.3 mM 02, 0.5 mM NOj, 0.5 mM SO -, and 1 mM HCOj, as well as 0.1 mM benzoic acid butyl ester, which is easily mineralized to C02 and H20. The temperature is 20°C and the pH is 7.3 (well buffered). Would you expect sulfate reduction or even methanogenesis to occur in this column Establish an electron balance to answer this question. 

The discharge of organic pollutants into lakes or declines in the concentrations of copper, zinc, and other heavy metal toxins may promote the growth of phytoplankton (e.g. algal blooms ). Greater biological activity may then increase anoxic conditions in lake bottoms, which stimulate the reductive dissolution of (oxy)(hydr)oxides and increase the mobilization of arsenic. In particular, Martin and Pedersen (2002) concluded that reduced discharges of copper, zinc, and nickel to Balmer Lake, Ontario, Canada, increased phytoplankton production and arsenic mobility in the lake. 

Alatorre Ordaz, A., Manriquez Rocha, J., Acevedo Aguilar, F.J., Gutierrez Granados, S. and Bedioui, F. (2000) Electrocatalysis of the reduction of organic halide derivatives at modified electrodes coated by cobalt and iron macrocyclic complex-based films Application to the electrochemical determination of pollutants. Analusis 28, 238-244. 

Schnabel, C., Womer, M., Gonzalez, B., Del Olmo, I. and Braun, A.M., (2001) Photoelectrochem-ical characterization of p- and n-doped single crystalline silicon carbide and photoinduced reductive dehalogenation of organic pollutants at p-doped silicon carbide. Electrochim. Acta... 

The direct reduction of pollutants also includes other inorganic compounds such as chromates, oxy-chlorinated species (e.g., chlorites and chlorates) and oxynitrogenated ions (nitrates and nitrites), as well as organic compounds (e.g., the dehalogena-tion of chlorinated hydrocarbons and the reduction of organic acids to the corresponding alcohols or phenols). 

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