Electrochemical remediation methods require an ion-conducting medium to perform their function of oxidizing or reducing polluting species. For this reason, gaseous mixtures must be normally absorbed first in aqueous solutions to be treated. This can be accomplished either by using an absorption medium inside an electrochemical cell ( inner-cell process) or by first absorbing the gas and then transferring the absorption medium into the electrochemical cell for treatment (outer-cell process). Also, in such methods the polluting species can either undergo electron transfer on an electrode surface (direct electrolysis), or electrons can be shuttled to/from the electrode by an electron carrier or mediator (indirect electrolysis). Some examples of gases treated electrochemi-cally are discussed in Chapter 10.

Hydrogen sulfide is a well-known pollutant produced in considerable amounts by sulfate reduc tion in organic-rich (anaerobic) environments, from heavy oil desulfurization processes, oil recovery operations, coal gasification/liquefaction processes, etc. It is frequently treated by absorption in basic scrubbing solutions of different amines whereby the scrubbing liquor can be regenerated, providing a concentrated stream of H2S that unfortunately requires further treatment. Alternatively, the Claus process can be used:

This presents the following disadvantages: 1) hydrogen is essentially wasted, because H2O is produced from it, 2) the high temperatures and catalysts required do not offer flexible adjustment to varying concentrations of H2S, 3) a pre-treatment for the separation of companion hydrocarbons and H2 is required, and 4) a post-treatment is required since the Claus process converts only 90-98% of the initial H2S content.

In this experiment, H2S will be produced and then absorbed with simultaneous reaction, whereby sulfide ions are oxidized by a chemical oxidant (mediator) to elemental sulfur. Then, elemental hydrogen will be produced at the cathode of an electrolytic cell and the oxidant will be simultaneously regenerated at the anode. The net result is rather uncommon: the decomposition of a pollutant (H2S) into its components in their pure, useful elemental forms. The reactions are: a) Chemical:

(in the aqueous solution) (3)

b) Electrochemical:

31" 2H4

(at the anode) (at the cathode)

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