In order to further understand the removal mechanisms involved with nZVI and different metals, the standard electrode potentials (E0) were investigated for
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Cu, Pb and Zn as discussed in Li and Zhang (2007). The E0 can be used as an indication of the standard oxidation-reduction potentials and thus the potential ability of a species to gain electrons and be reduced. This is important in explaining why different metals react differently with different materials, in this case nZVI. As discussed in Section 5.1.1, the ability of nZVI to react with metal species so efficiently is its ability to donate electrons to the surface and thus, offer reduction potential to metals as well as offering surface complexation to its metal hydroxide surface in the form of adsorption (Figure 5.20). Different metals standard reduction potentials will determine which metals are able to accept electrons and be reduced and which metals have a lower reduction potential and thus are more typically adsorbed to nZVI. As reported in Stumm and Morgan (1996) and Li and Zhang (2007), the E0 for Cu is 0.34, Pb is -0.13 and Zn is -0.76 while Fe is -0.41. When E0 of the metal species involved is close to or more negative than Fe, which is the case for Zn, then reduction is not possible and adsorption is the dominant removal mechanism (Li and Zhang 2007). When E0 of the metal species is significantly more positive than that of nZVI, in the case of Cu, then the high potential to receive electrons makes reduction the key removal mechanism (Li and Zhang 2007). Finally, when the E0 is only slightly more positive than nZVI, such as in the case of Pb, then both mechanisms of reduction and adsorption have the potential to remove the metal species (Li and Zhang 2007). This can be visualized in Figure 5.20.
Figure 5.20. Schematic explaining different removal mechanisms involved between nZVI and different metal species. Adapted from Li, 2007.
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Once the E0 are taken into consideration, the expected and observed effective removal of Pb within the nZVI columns can be explained by the combination of both reduction and adsorption of Pb onto nZVI. Along with precipitation due to supersaturation in the solution and adsorption onto sand, this can therefore explain the better removal of Pb over Cu or Zn. On the contrary, the lack in variety of mechanisms available for Zn immobilization may explain the reduced removal rates observed. Considering E0 of Zn allowing for adsorption onto nZVI only, as well as the undersaturated SI not allowing for Zn removal due to precipitation in the system, the removal mechanisms available for Zn removal are adsorption onto nZVI or sand only. Since adsorption is the dominant removal mechanism in the Zn columns that demonstrated the worst removal of metals, it is believed that sorption based mechanisms are less effective at removal of metals than reduction based mechanisms for nZVI. Finally, in addition to saturation driven precipitation and possible adsorption onto sand the E0 for Cu would follow removal by nZVI due to reduction. The possible mechanisms involved for heavy metal immobilization in the sand and nZVI systems is summarized in Table 5.5 following from a combination of PHREEQC analysis and E0 analysis.
Table 5.5. Possible removal mechanisms for Cu, Pb and Zn according to PHREEQC and standard electrode potentials (E0)
It is not known why exactly Cu retention would improve in a multi-element solution, though specifics of solution chemistry and geochemical parameters must have altered the solutions towards an ideal environment for Cu complexation. While Cu removal is increased in the multi-element solution and Zn removal is decreased in the multi-element solution, an explanation could be that the competition between the two mechanisms clearly favors reduction over adsorption. Thus, it is hypothesized that when competition between metals in the multi-element solution occurs, reduction is the prominent mechanism while the adsorption capacity seems to be diminished. This explanation is also supplemented in that Pb removal (in which both adsorption and reduction
Possible Removal Mechanisms Cu Pb Zn PHREEQC Adsorption onto sand
PHREEQC Saturation driven precipitation E0 Adsorption onto nZVI
E0 Reduction due to nZVI
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contribute) was able to maintain similar retention rates between the multiple and single metal experiments even though different shape breakthrough curves were seen. It is possible that the different shape curves between the single and multi-elemental solution could be an indication of competition between removal mechanisms.