Reductive Remediation of Pollutants Using Metals

Abstract

Metal systems have been extensively used for the abiotic remediation of recalcitrant pollutants. Zero-valent metals such as iron (Fe 0 ), zinc (Zn 0 ), magnesium (Mg 0 ) have been used for the reductive dechlorination of organic compounds Nanoscale zero-valent iron particles have been reported to be very effective for the transformation and detoxification of a wide variety of contaminants, such as arsenic, hexavalent chromium, chlorinated organic solvents, nitroaromatic compounds and azo dyes. Zero-valent metals often result in the production of partially dechlorinated products which are sometimes more toxic than the parent compounds. Also the rate of reactions slow down due to passivation of metals and remediation process may take several days. In this context bimetallic systems offer advantages. Bimetallic systems combine the thermodynamic driving force associated with the corrosion of the reactive metals (like Mg 0 and Fe 0 ) with the catalytic hydrogenation characteristics of the second metal such as zero valent palladium or silver. Dechlorination of pollutants by bimetallic systems is presumed to occur via nucleophilic substitution reactions. The overall rate of the reaction is strongly influenced by the type of substituent groups on the aromatic ring. Commercial application of hydrogenating catalyst such as palladium is limited due to the fact that it is expensive and its recovery after the reduction reaction is very poor. Moreover the presence of trace concentrations of palladium in treated water may exert toxic effects on biological systems. Such problems can be overcome by immobilizing palladium onto suitable support matrices which would enhance its reuse and recovery. Important factors which should be taken into consideration while selecting a support is its stability to by-products, the ease and simplicity with which the catalyst can be immobilized.

Water Treatment Using Metallic Iron: A Tutorial Review

“…Conventional Fe 0 materials used for reductive transformation of contaminants often produce reaction products which are sometimes more toxic than the parent compounds [127]. On the other hand, the transformation process is slowed down as the natural oxide scale develops at the Fe 0 surface.…”

“…It is in this context that bimetallic systems were introduced, wherein a second metal is combined with Fe 0 [127,128]. The second metal primarily has three functions: (i) Acts as a hydrogenating catalyst, (ii) prevents the formation of the oxide film on the Fe 0 surface, and (iii) induces Fe 0 to release electrons due to the difference in reduction potentials [127,129,130]. However, the first function (hydrogenation catalyst) is questionable as H/H 2 has to migrate through the oxide film to the site where the contaminant is adsorbed (Figure 1-bottom).…”