Introduction

As discussed in Chapter 10, electrochemical processes can often be used to address environmental problems. Some of the main characteristics that make these techniques promising include their versatility, environmental compatibility, economic feasibility, ambient temperature and pressure requirements, and amenability to automation. Electrochemical approaches to remediation include direct and indirect electrolysis, membrane-based approaches (e.g., electrodialysis), electrokinetic remediation of soils, dissolution-electrolysis of gases, photoelectrochemical techniques, disinfection of water, etc.

The electrolytic production of an intermediate species for the oxidation/reduction of another substance is known as indirect electrolysis (this concept is discussed in Section 10.1). This process can be made reversible or irreversible, depending on whether the mediator (catalyst) can or cannot be electrochemically regenerated and recycled in the process. The equations that describe the reversible case (for an oxidation reaction) are:

where C is the redox mediator, Red is the pollutant species, Ox+ is its oxidized product and Ji\ and are the corresponding standard potentials for reactions 1 and 2 (here, E\ > 0). In addition to the reduction or oxidation of the target pollutant, other reactions can compete. The most likely cathodic parasitic reactions (reductions) in the presence of dioxygen in aqueous solutions are:

E° = 0.00 V (3a) (neutral or basic): ¡/2 02 + H20 + 2e~ = 20H~

whereas the most common competing anodic reaction (oxidation) is the reverse of:

In this experiment, students will produce a strong oxidizer [i.e., Co(III)] at the anode of a home-made electrolytic cell. This species will, in turn, oxidize a surrogate organic pollutant (e.g., ethylene glycol, ethanol, glycerol) to produce C02. The resulting gas is collected in a trap and its presence made evident by the appearance of a precipitate. Once reduced by its oxidizing action upon the pollutant, the mediator can be electrochemically regenerated so as to initiate the treatment cycle.

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