Electrochemistry for the Environment

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Nowadays the degradation of the environment has reached such an extent that the health and even the survival of many people and ecosystems are threatened. This is why it is imperative to search for alternatives to better our environment and aspire to "performance without pollution". Electrochemistry has a very important role in the protection of our environment, monitoring the pollution, developing modern techniques to improve electrical energy storage and conversion and avoiding pollution.
Electrochemical technology continues to make many contributions in many domains, such as:
o Monitoring the pollutants and reagent levels in process streams, gaseous emissions and rinse sections;
o Converting chemical energy to electrochemical energy using fuel cells and photovoltaic cells;
o Treatment of water by electrochemical methods, such as chlorination of swimming pools;
o Removal of environmental contaminants, such as metal ions and organics from industrial process streams;
o Avoidance of corrosion by electrochemically coat metals with protective layers.
In the way of minimization of waste emissions, raw material usage and energy consumption, electrochemical technologies offers many advantages, such as:
o Low costs; electrons have the lowest cost per unit of charge and the electrochemical reactors are mechanically simple and of low maintenance;
o Multiple uses; the same electrochemical reactor can be used for several reactions;
o Electrochemical reactions do not require high temperature or pressure;
o The experimental conditions and the reactor's characteristics can be selected so that the energy waste and the by-product formation are minimum;
But there are also some disadvantages or limitations of the electrochemical technologies, including:
o The corrosion of the electrodes affects the performance and longevity;
o From the decomposition of water there may result an explosive mixture of hydrogen and oxygen;
o The best electrodes are made from precious metals, and this increases the investment;
o The limited knowledge of electrochemistry in some sectors;
o The cost of electricity may be high in some areas.
Many of the substances considered polluting factors can be either oxidized or reduced and in this way they are transformed in non-polluting substances, and this means that electron transfer can dramatically reduce the toxicity of a substance. This electron transfer can be achieved using an electrode. For example a compound containing highly oxidized chromium is very toxic, but the reduced form is considered a nutrient.
Many polluting substances can be detected and monitored using electroanalytical techniques, such as voltammetry, polarography, chronoamperometry, etc. In the field of instrumental analysis, electrochemistry offers better accuracy and detection limits as other techniques as well as lower costs.
DIRECT AND INDIRECT PROCESSES
The process in which electrons are transfered directly from the electrode to an electroactive species is called a "direct process". But there are also several cases where the targeted species is not electroactive or the rate of reaction is low.
To diminish these problems mediators, or active species, can be generated, these being capable of moving into the solution and reacting there with the pollutant, these being the "indirect processes". The process is called "reversible" if the mediator can be regenerated and "irreversible" otherwise.(see Fig 1.)
Fig. 1. Electron-transfer schemes for direct (left) and indirect (right) processes. (From:
"Environmental Electrochemistry" K. Rajeshwar and J. G. Ibanez, Academic Press, 1997.)
Direct processes
Direct oxidation
By direct oxidation at anodes many organic and also inorganic pollutants can be treated. Problems such as costs, composition and pH of the medium, stability and accessibility and environmental compatibility should be taken into account when choosing the materials. Another serious issue which increases costs ad decreases yields is water decomposition. This reaction can also generate explosive mixtures of hydrogen and oxygen. Electrodes that make this reaction very slow and require overpotential were developed in order to avoid this oxidation. A good example is the boron-doped diamond electrode. This electrode has a large overpotential for oxygen formation, making possible the oxidation of substances with higher potential than water, is very durable and resists oxidation.
Many organic substances have been treaded using this technique, including aldehydes, fecal wastes, dyes, phenols, halogenated compounds etc. Cyanide is one of the most common inorganic compound that has been treated using direct oxidation, giving the cyanide ion, a compound less toxic than its oxidized form.


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