Organic pollutants conversion

Microbiological conversion in molecular nitrogen by means of nitrification and denitrification, often in combination with the removal of organic pollutants... 

Membrane processes can not only produce a water quality that satisfies set standards but can simultaneously concentrate the pollutants in a small volume. This strongly promotes the possibility for recovery of pollutants such as ammonia and phosphate for reuse. Also biological conversion of dissolved organic pollutants becomes easier. 

In searching for chemical methods of destroying organic pollutants, chemists tend to look to oxidative methods, since the preferred end product is usually C02, but, as discussed below, special methods are usually necessary to bring about oxidation to acceptable products. In 1995, however, it was shown that elemental (zero-valent) metals such as ordinary iron filings can be effective in the reductive conversion of halocarbon pollutants such as trichloroethene in wastewater to innocuous hydrocarbons over a few days14 ... 

Comninellis C. Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for waste water treatment. Proceedings of the Symposium on Water Purification by Photocatalytic, Photoelectrochemical and Electrochemical Processes. Vol. 94-19. Pennington, NJ The Electrochemical Society, 1994 75-86. 

This paper has presented and briefly discussed the performance of different electrode materials for the electrochemical oxidation of organic pollutants for wastewater treatment. Literature results have demonstrated that anodes with low oxygen evolution overpotential, such as graphite, Ir02, Ru02, or Pt only permit the primary oxidation of organics (i.e., conversion), but not the complete mineralization, due to the accumulation of oxidation intermediates, mainly aliphatic acids, which are quite stable against further attack at these electrodes. 

Owing to its low cost, non-toxicity, extreme stability, wide availability, and other advantages (see discussion below), Ti02 is the semiconductor of choice for environmental applications. The main application is the mineralization of toxic organic pollutants in air, water, or soil matrices (i.e., their conversion into small, non-toxic inorganic molecules as CO2, H2O, HC1). In some cases it has been used for the reduction and removal of metal ions from aqueous solutions (e.g., Cu2+(aq) —> Cu+ — TiC>2(s)). In very few cases, both the anodic and cathodic reactions have been advantageously utilized (e.g., to remove a metal ion and to oxidize an organic molecule). 

These results show convincingly that a direct relationship exists between the number of OH radicals formed upon photoexcitation of Ti02 and the conversion of phenol, which is regarded as a model substrate for organic pollutants. Moreover, a synergistic effect in the photocatalytic abatement of organic pollutants was obtained by combined UV/MW excitation, which favors the formation of OH radicals as active species for the oxidative degradation of hydrocarbons. 

Comninellis CH. Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for waste water treatment. 

The data base CICLOPS (Computer interrogation of a Comprehensive List of Organic Pollutants) at December 1983 contained information on about 3.600 organic compounds with over 22.000 individual records. In order to make CICLOPS more easily accessible to participating laboratories and other establishments with legitimate interests, its conversion and loading as a subset of the environmental matrices files of the ECDIN (Environmental Chemicals Data and Information Network of the European Communities) data base has been investigated. 

Since the numerator in eq. 5.84 expresses the rate of reaction, will depend on the reactant concentration. As correctly noted by Braun and co-workers (1991) and emphasised by Cabrera et al (1994), only for a zero-order reaction is uniquely defined at the given wavelength A because when the reaction rate depends on the reactant concentration, falls off over time. In homogeneous photochemistry, this problem is normally overcome by determining at small (less than -10%) conversions of reactants, a point not often respected in heterogeneous photocatalysis, where the focus is often on complete mineralisation (100% transformation) of the substrate, at least in studies of environmental interest that focus on the total elimination of organic pollutants in water. 

Direct electroreduction methods are typically used for dechlorination of chlorinated pollutants in waters. The easy removal of Cl from chlorinated organics allows conversion of chlorofluorocarbons (CFCs) into hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and even fluorocarbons (FCs). ECFCs are much less destructive to the atmospheric ozone than CFCs, but HFCs and FCs are harmless to atmospheric ozone, although they may contribute to the greenhouse effect. 

If there is a strong electronic coupling between P and R in the transition state, one commonly speaks of an inner-sphere mechanism, and, conversely, if the interaction is weak, one uses the term outer-sphere mechanism. There are various theoretical approaches for quantifying the rates of redox reactions, including the so-called Marcus theory. For a description of these approaches, we refer to the literature (e.g., Eberson, 1987). For our discussion here, we content ourselves with trying to identify the factors that determine the rate at which a given organic pollutant is reduced or oxidized in the environment. 

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