2.2 Review of Nanoparticles as Adsorbents for Metal Ion Removal
2.2.1.3 Modified iron-based nanoparticles
Many researchers often apply post-synthesis treatment to modify the pristine iron- based nanoparticles. Functional materials such as stable noble metal, metal oxide, low molecular weight organic molecules and polymers are often added to complement the pristine nanoparticles or augment additional functionality which enables the resulting iron-based nanoparticles to deliver better heavy metal remediation (higher adsorption capacity) or optimal process characteristics (increased lifespan for cyclic reuse). One of the most feasible routes has been to fabricate iron oxide-based nanoparticles (e.g.
Fe3O4 or Fe2O3) and apply them directly for metal ion removal. Due to their inertness, these oxide nanoparticles often undergo further chemical modification to enhance their adsorption performance.
By varying the types and concentrations of precursor metal salts during the controlled chemical co-precipitation, different nanoscale ferrites (MeFe2O4, Me = Mn, Co, Cu, Mg, Zn, Ni) have been obtained (Hu et al., 2007). It was demonstrated that the adsorption performance of nanoscale ferrites could be maximized by optimizing the elemental composition of the nanoparticles. It was determined that the highest adsorption capacity of 31 mg/g for Cr(VI) was achieved for jacobsite nanoparticle (MnFe2O4). Regeneration of the nanoscale ferrites in their work was investigated and it was found that the chemical redox reaction between the Mn(II) and the Cr(VI) caused the poor recovery. Regeneration of nanoscale adsorbents is not widely investigated as most of the iron-based nanoparticles remove heavy metal ions through
a combination of both surface adsorption and redox reaction. Remedial capacity and performance would undoubtedly degrade during treatment and eventually becomes exhausted. Simple pretreatment was attempted successfully by Hu et al. (2005b) to convert all lower valent Mn to its highest valence. It was reported that the modified jacobsite nanoparticles could be used and regenerated for multiple cycle of use without drop in performance in successive runs.
Binding sites found on iron-based nanoparticles consist mostly of oxygen-donor- atoms or protonated ion-exchange sites, which has little or no affinity towards some of the more toxic heavy metals, for example, Hg(II) and Pb(II). Surface modifications via various “wet-chemistry” routes are often carried out. These modified nanoparticles could serve as nanoscale carrier for specific metal-coordinating ligands which are softer Lewis base, such as ligands containing either sulfur-donor-atom (e.g. thiol groups) or nitrogen-donor-atom (e.g. amine groups). Yantasee, et al. (2007) obtained surface-modified iron nanoparticles carrying dimercaptosuccinic acid, via the ligand- exchange method. The modified nanoparticles showed a high adsorption capacity of 227 mg/g-nanoadsorbent for Hg(II). High heavy metal affinity of the modified nanoparticles also promoted a more rapid adsorption kinetics, for instance, 99 wt% of 1 ppm Pb(II) was removed within a minute.
Another type of hybrids iron-based nanoparticles was obtained by encapsulating pre- synthesized iron nanoparticles with a polymer that carries desired heavy metal- selective binding sites. Shih and Jang (2007) utilized Fe3O4 nanoparticles as seed and allowed the precursor monomer ethylenedioxythiophene to adsorb onto the seed nanoparticles under acid etching-mediated conditions, followed by polymerization.
The poly(3,4-ethylenedioxythiophene) (PEDOT) polymer encapsulated the parent seeds and the resulting hybrid nanoparticles displayed remarkably high heavy metal sequestration capacity. For instance, the maximum adsorption capacity for Hg(II) could exceed 400 mg/g-modified nanoparticles.
Without further chemical modification, iron nanoparticles, in general, have low adsorption capacity for metal ion removal. Bulk of the binding sites available for metal ion adsorption are located on the external surface of the nanoparticles, which may be easily lost as a result of irreversible aggregation of these nanoparticles. These nanoparticles therefore display low potential to function as effective nanoadsorbent for palladium sequestration and recovery.
Table 2.1 Survey of literature on iron-based nanoparticles and related metal ion removal applications.
Type of nanoparticles Particle size
(nm) Target metal ion
Major binding sites or functional groups for metal ion removal
Reference Zero-valent iron nanoparticles
Nanoscale zero-valent iron (NZVIs) 1-120 As(III), As(V) Initially the sites are amorphous Fe(II)/Fe(III), magnetite (maghemite). As treatment proceeds, initial reactive sites gradually transform into lepidocrocite and more crystalline magnetite.
Kanel et al., 2005;
Kanel et al., 2006.
Maghemite nanoparticles ~10 Cr(VI) The major binding site is hydroxyl groups of iron oxide.
Hu et al., 2005a.
Core/shell nanoscale zero-valent iron (Fe(0)/Fe(III))
10-200 Zn(II), Cd(II), Pb(II), Ni(II), Cu(II), Ag(I), Cr(VI), Hg(II)
Simultaneous adsorption and co-precipitation may occur, depending on the standard redox potential difference between the Fe0 and the incoming heavy metal. Major binding site is
≡Fe-OH.
Li and Zhang, 2006; Li and Zhang, 2007.
Modified iron-based nanoparticles
Modified jacobsite (MnFe2O4) ~10 Cr(VI) The major adsorptive components are MnO2
and Fe2O3.
Hu et al., 2005b.
Nanoscale ferrites, MeFe2O4 (Me = Mn, Co, Cu, Mg, Zn, Ni)
~20 Cr(VI) For MnFe2O4, the major driving force for adsorption is chemical redox reaction between Mn(II) and incoming Cr(VI).
Hu et al., 2007.
Magnetic nanoparticles encapsulated by Poly(3,4-ethylenedioxythiophene) (PEDOT)
~11 Ag(I), Hg(II), Pb(II)
The major binding sites are the O-donor- atoms and S-donor-atoms of the PEDOT.
Shih and Jang, 2007.
Magnetite nanoparticles modified with dimercaptosuccinic acid
~6 Hg(II), Co(II), Cu(II), As(V),
Ag(I), Cd(II), Tl(III), Pb(II)
The major binding site responsible for remarkable heavy metal binding is thiol groups originated from the dimercaptosuccinic acid added.
Yantasee et al., 2007.
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