Reactivity Fate and Lifetime
Nanoparticles can sequester groundwater contaminants (via adsorption or complexation), making them immobile, or can degrade or transform them to innocuous compounds. Contaminant transformations by nano-iron, which is a strong reductant, are typically redox reactions. When the oxidant or reductant is the nanoparticle itself, it is considered a reactive nanoparticle (Figure 8.2). A good example of a reactive nanoparticle is TCE dechlorination by nanoiron. Oxidation of Fe0 in the particle provides electrons for the reduction of TCE, which acts as the oxidant. The Fe0 core shrinks and is used up in the reaction. The particles are no longer active once all of the Fe0 is oxidized (Figure 8.2a). Nanoparticles that catalyze redox reactions but are not themselves transformed are catalytic nanoparticles, which requires an additional reagent that serves as the reductant or oxidant (e.g., Pd nanoparticles require H2 as a reduc-tant [Lowry and Reinhard 1999]). H2 (the reductant) is activated by the
- Figure 8.2a Reactive nanoparticles. TCE dechlorination by nanoiron. Fe oxidation provides electrons for the reduction of TCE. The Fe0 core shrinks while the Fe3O4 oxide shell grows. The particles are no longer active once all of the Fe0 is oxidized.
Figure 8.2b Catalytic nanoparticles. TCE dechlorination by catalytic Pd particles. H2 is supplied as the reductant for TCE dechlorination. In principle, the Pd catalyst is not altered by the reaction and can remain active as long as H2 is supplied. In practice, catalyst deactivation occurs and the particle lifetime is finite. Catalyst regeneration may extend the life of the particle.
Figure 8.2b Catalytic nanoparticles. TCE dechlorination by catalytic Pd particles. H2 is supplied as the reductant for TCE dechlorination. In principle, the Pd catalyst is not altered by the reaction and can remain active as long as H2 is supplied. In practice, catalyst deactivation occurs and the particle lifetime is finite. Catalyst regeneration may extend the life of the particle.
Pd to form adsorbed reactive H species. TCE adsorbs to the Pd surface where it is reduced by the reactive H species on the Pd surface (Figure 8.2b). In principle, the catalyst can repeat this indefinitely so long as reductant is continually supplied. In practice, precipitation of minerals or natural organic matter on the Pd surface or adsorption of reduced sulfur species deactivate the catalyst and it has a finite lifetime (Lowry and Reinhard 2000). Some nanomaterials are engineered to strongly sequester contaminants (Figure 8.2c). The high affinity for the contaminant allows the nanoparticle to significantly lower the aqueous phase concentrations, to out-compete natural geosorbents such as organic carbon, and serves to concentrate the contaminants onto the particles. Once concentrated onto the nanoparticles, the contaminants can be removed along with the nanoparticles. This can be highly effective for hydrophobic organic contaminants such as PCBs and PAHs and for heavy metals. For in situ
Figure 8.2c Adsorbent nanoparticles. Nanoparticles engineered as very strong sorbents can be used to strongly sequester organic or inorganic contaminants. Once adsorbed, the contaminants are no longer bioavailable.
remediation with any type of nanoparticle, it is important to know which groundwater contaminants will respond to the treatment and which will not. It is also important to know how long the reactive or catalytic particles will remain active as this will determine important operation decisions such as how much to inject and when reinjection may be necessary.
Degradation of halogenated hydrocarbons, particularly chlorinated solvents, occurs via a reductive process. The Fe0 in the nanoiron is oxidized by the chlorinated solvent, which is subsequently reduced. For chlorinated hydrocarbons, the reduction typically results in the replacement of a chlorine atom with a hydrogen atom. For heavy metals, the metal, such as Pb(II) or Cr(VI), is reduced to its zerovalent form on the nanoiron surface, or forms mixed (Fe-Metal) precipitates that are highly insoluble (Ponder et al. 2000). The general half-reactions for the oxidation of iron and the reduction of chlorinated organic compounds (COC) or heavy metals are given in Eqs. 1 to 3, where Me is a metal ion of charge a.
In the case of nanoiron, or Fe0-based bimetallics, the reduction of the contaminant is surface-mediated, and the particle itself is the reductant. The attractiveness of nanoiron is that the particles have a high surface-to-volume ratio and therefore have high reactivity with the target contaminants. The following generalizations can be made about the reactivity and lifetime of all nanoparticulate remedial agents that are themselves the reactive material—that is, not true catalysts according to the formal definition of a catalyst:
■ Any process that affects the surface properties of the particles (e.g., formation of an Fe-oxide on the surface) can affect their reactivity.
■ Any oxidant (e.g., O2or NO3) competing with the target contaminant will utilize electrons and may lower the rate and efficiency of the nanoiron treatment for the target contaminants.
■ Reactive nanoparticles that serve as a reactant rather than a catalyst will have a finite lifetime, the length of which depends on the concentration of the target contaminant, the presence of competing oxidants, and the selectivity of the particles for the desired reaction.
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