Electrochemical Removal of Contaminant Mixtures

The heavy metals are affected by the various geochemical processes due to change in the soil pH as presented in Parts I, II, and IV. The sorption and desorption processes of coexisting multiple metals is quite complicated. Some of the heavy metals are tightly held than the others. If cationic and anionic metals exist, the sorption behavior of them can be quite the opposite. Therefore, the extent of transport of multiple metals depends on their aqueous concentrations and ionic mobilities. In general, the applied electric field is distributed among the multiple metals, thereby resulting in lower transport of a metal in the mixture as compared with the transport 

The problem of soils contaminated with mixed heavy metals and organic compounds is even complex because of the different chemistry of heavy metals and organic compounds (Chapter 15). Some studies have shown that there may be some synergistic effects that retard the contaminant transport and removal, but few other studies show the behavior of heavy metals and organic compounds similar to that observed with either heavy metals or organic compounds. 

The heavy metals are removed predominantly by the electromigration process, while the organic contaminants are removed by electroosmosis. The presence of heavy metals causes the zeta potential of the soil to be less negative and even result in a positive value, affecting the electro-osmotic flow and sorption of the contaminants. 

For the simultaneous removal of organic compounds, these compounds are solubilized using different solubilizing agents (surfactants, cosolvents, and cyclo-dextrins). They are then transported and removed mainly by the electroosmosis process. It is essential to maintain all of the contaminants in soluble form and maintain electro-osmotic flow for the removal of both heavy metals and organic compounds. 

Sequential approaches are developed where (a) anionic metals are removed first and then cationic metals when mixed metal contamination is present and (b) organic compounds are removed first followed by the removal of heavy metals when coexisting heavy metal and organic contaminants are found. For example, the simultaneous electrokinetic removal of inorganic and organic pollutants (SEKRIOP) 

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High removal of multiple metals or coexisting metals and organic compounds (e.g. PAHs) is demonstrated in spiked soil conditions, but the performance was found inadequate in field soils from actual contaminated sites. Proper consideration should be made to reduce the treatment duration, proper handling of secondary liquid waste, and total cost in evaluating the electrochemical removal of contaminant mixtures. 

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