2.2 Review of Nanoparticles as Adsorbents for Metal Ion Removal
2.2.4.2 Polymeric nanoparticles for water/wastewater treatment
One of the first environmental applications of polymeric nanoparticles was investigated by Snowden and Vincent (1993) who have studied the applicability of using of poly(N-isopropylacrylamide) (PNIPAAm) colloidal microgels or nanoparticles for removal of heavy metals such as Pb(II) and Cd(II) (Table 2.4). The binding sites for metal ions were derived from the thermal initiator molecules used.
Hence the removal capacity for both heavy metals ions was limited as the ratio of initiator to N-isopropylacrylamide was kept small in all the synthesis. For instance, the adsorption capacity achieved for Pb(II) was only 0.4 mmole/g at solution pH 6.0. This shortcoming was overcome by Morris et al. (1997) by copolymerizing PNIPAAm nanoparticles with a co-monomer, acrylic acid. They reported an improved performance, for example, the specific adsorption capacity for Pb(II) at pH 8.0 to be 2.4 mmole/g, which they attributed to the favorable Coulumbic attraction between carboxylate groups and positively charged lead species. Kanazawa et al. (2004) experimentally demonstrated that the adsorption kinetic by nanoparticles prepared was significantly improved, as compared to their bulk analogue. For instance, the former took only 100 minutes to attain adsorption equilibrium, while the latter required 100 hours at least.
Antonietti et al. (1995) explored the synthesis of metal-chelating nanoparticles by one-step functionlization via miniemulsion polymerization approach. Polymeric nanoparticles of 13 to 19 nm were obtained successfully. The bipyridine-based metal- chelating groups are located on the surface of the nanoparticles and are accessible. It was observed that the complexation completed within minutes. In addition, the nanoparticle solution remained colloidally stable after metal ion adsorption, which
indicates that the cooperative binding event occurs between every two neighboring bipyridine groups solely.
Polymeric nanoparticles could acts as better nano-carrier than that of LM-MEUF as discussed earlier. Recent advances in polymer science and technology provide a great variety of efficient tool for surface-functionlization of polymeric nanoparticles or latexes (Wang et al., 2003). Specific ligands could be grafted or second polymer layer could be deposited onto the particle surfaces, resulting in a change in surface functionalities and morphologies. The grafted ligands are covalently bonded and highly accessible to the multivalent ions or molecules in surrounding environment;
whereas for LM-MEUF, the hydrophobic ligands are solubilized within the core of micelles and the sequestration kinetic is therefore lower. Amigoni-Gerbier et al. (1999;
2002) explored the synthesis and application of cyclam-grafted nanoparticles as recoverable chelating agents. Due to the small size (13 – 20 nm) of the nanoparticles, the nanoparticle suspension was stable and transparent. The cyclam ligands binds specifically copper ions with high selectivity, in the presence of other cations. The loading of cyclam ligands of 0.73 mmole/g is one-fold higher than those obtained by other approaches reported elsewhere (Amigoni-Gerbier et al., 2002). Fast chelation kinetics was observed and about 85-90% utilization of the available ligands is easily achieved. The nanoparticles remain stable after dialysis or Cu(II) complexation. The binding capacity of 0.6 mmole-Cu(II)/g of the resulting nanoparticles plus its ease of preparation demonstrates its competitive edge over others, for instance PAMAM dendrimers of generation 8, which has a Cu(II) binding capacity of 0.65 mmole/g (Diallo et al., 1999).
Chen et al. (2003) modified the surface-aminated polystyrene nanoparticles with azo- chromophore. It was found that the ligand-modified nanoparticles remain adsorptive towards Pb(II) after 3 cycles of adsorption/desorption processes. Bell et al. (2006) synthesized polymer nanoparticles with core/shell structure. They demonstrated that the heavy metals sequestration selectivity could be altered by grafting a macrocyclic ligand. The original core/shell nanoparticles is selective towards mercury ions solely;
whereas the modified nanoparticles would only adsorb Co(II) despite presence of thousand-fold excess of other heavy metals ions, such as mercury. Therefore, the polymeric nanoparticles could be conveniently engineered or tailored to display specific remedial function such as selective sequestration of heavy metals.
Another research area which receives considerable attention recently is the synthesis of biopolymer-based nanoparticles for environmental application (Qi and Xu, 2004;
Chang and Chen, 2005). Chitosan is a natural polysaccharide with rich functionalities.
Due to its non-toxicity and low cost, chitosan has been studied extensively in various areas such as biomedicine and water/wastewater treatment (Li and Bai, 2006). Chang and Chen (2005) carboxylated the chitosan polymer and later covalently bonded to magnetite nanoparticles, whereas Qi and Xu (2004) obtained the chitosan nanoparticles through simple ionic-gelation nucleation method, wherein tripolyphosphate (TPP) acts as ionic crosslinker. Both groups of authors reported high Cu(II) removal capacity. However, chitosan nanoparticles obtained via ionic-gelation has a critical drawback: they gradually disintegrate in aqueous media over days or aggregate in alkaline solution (pH 9.0) (Lόpez-Leόn et al., 2005). This is mainly due to the weak electrostatic interactions between chitosan chains and TPP molecules, as proven by the complete disintegration of spherical chitosan nanoparticles into
dissolved polymer chains in solution of high ionic strength (125 mM of KNO3). Like any other adsorbents, a systematic physicochemical examination should accompany the remedial assessment of nanoparticles in order to fully understand their potentials and pitfalls.
Polymeric nanoparticles resemble dendrimers minus of latter’s’ precise intramolecular structure and symmetry, and therefore they should perform as effective as dendrimers because of their high volumetric binding site density. Under right solution condition, these binding sites that reside within the polymeric nanoparticles could become available for sequestrating metal ions in surrounding aqueous solutions, for instance, when the polymeric nanoparticles are fully swollen.
Table 2.4 Survey of literature on polymeric nanoparticles and related metal ion removal applications.
Type of polymeric nanoparticles
Particle
size (nm) Target metal ions
Major functional groups for metal ion binding
Remarks Reference Core/shell polystyrene-graft-
poly(vinylamine) ~500 Pb(II), Cu(II) Bare nanoparticles: amine group and amide group. Modified nanoparticles: residual amine group and azo-chromophore (imine group, amide group and hydroxyl group)
Adsorption efficiency of Pb(II) was enhanced, but the resulting nanoparticles did not display exclusive selectivity towards Pb(II). This is because the copper ions have access to the amine groups buried beneath the corona.
Chen et al., 2003.
Core/shell polystyrene-co- poly((2-acetoacetoxy)ethyl methacrylate)
~70 Hg(II), Co(II) Bare nanoparticles: xanthate (thiolate group) and oxygen donor on the ester group. Modified nanoparticles: NH2-capten (secondary amine group and thiolether groups)
Selectivity in heavy metals sequestration (ppb level) can be engineered onto the particle surface by grafting a macrobicyclic ligand. Kinetic selective binding of Co(II) in the presence of 1000-fold excess of Cd(II), Pb(II), Hg(II) is achieved.
Bell et al., 2006.
Poly(N-isopropylacrylamide)
microgel (PNIPAAm) ~550 Pb(II), Cd(II) The electrostatic interaction conferred to the microgel comes from the thermal initiators which possess sulfate group, carboxylic group and amidine group respectively.
Desorption of metal ion could be triggered by heating. Both absorption as well as adsorption was proposed to account for the incomplete desorption.
Snowden et al., 1993.
Poly(N-isopropylacrylamide) microgel
~900 (>pH
=6), ~650 (<pH = 4)
Pb(II) Poly(acrylic acid) (carboxylic acid group)
Acrylic acid was copolymerized in the formulation to enhance the lead removal capacity. The presence of the additional acidic group within the gel network imparts pH dependence on microgel size.
Morris et al., 1997.
Poly(N-isopropylacrylamide)
microgel with metal chelating ~750 Cu(II) N-(4-vinyl)benzyl-ethylenediamine (amine group)
The thermosensitive microgel was incorporated metal ion chelation ability via copolymerization. The drastic increase in adsorption rate was observed, compared to bulk analogue in previous work. Molecular imprinting technique was employed in this work too.
Kanazawa et al., 2004.
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Polystyrene nanoparticles 13-19 Ni(II), Co(II), Cr(II), Cu(II),
Pd(II)
These co-monomer were synthesized and used in synthesis: 4- methyl-4’-2,2’-bipyridine, 6’methyl- 2,2’-bipyridine-6-ylmethyl
methacrylate, 4-(6’-methyl-2,2’- bipyridin-6-ylmethoxy)butyl methacrylate (bipyridine group)
Attempt was made to polymerize pyridine- containing monomer directly was unsuccessful. Instead, pyridine-containing nanoparticles were synthesized via copolymerization with styrene as main monomer. Results show that the pyridine moieties are located on the surface.
Antonietti et al., 1995.
Polystyrene nanoparticles 13, 15, 20 Cu(II) Vinylbenzyl-cyclam (cyclam:
1,4,8,11-tetraazacyclotetradecane) Fast adsorption kinetic towards copper ion was reported. Synthetic methodology adopted herein allows 1-fold higher of cyclam loading. 40-70% of cyclam is located on the particle surface and hence has good access to copper ions, which allows spontaneous and stochiometric binding between cyclam and copper ion. The remaining buried in core undergo diffusion- limited complexation.
Amigoni-Gerbier et al., 1999.
Polystyrene nanoparticles 13, 15, 20 Cu(II), Zn(II) Vinylbenzyl-cyclam (cyclam:
1,4,8,11-tetraazacyclotetradecane)
The UV/Vis spectroscopy revealed that the ligand accessibility is closely correlated with the particle size. Selective sequestration of copper ion in presence of large excess of competing ion does take place.
Amigoni-Gerbier et al., 2002.
Biopolymer—chitosan
nanoparticles 40, 100 Pb(II) Pristine amine groups and polyphosphoric groups derived from the ionic gelation
Freeze-drying was applied to lower the crystallinity of the chitosan nanoparticles, which led to better sorption performance.
Qi and Xu, 2004.
Magnetic nanoparticles coated
by chitosan 13.5 Cu(II) Pristine amine groups from chitosan A Cu(II) high adsorption capacity based on chitosan content is reported, attributed to high specific surface area.
Chang and Chen, 2005.
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