2.2 Review of Nanoparticles as Adsorbents for Metal Ion Removal
2.2.3.2 Dendrimers for water/wastewater treatment or resource recovery
Poly(amidoamine) (PAMAM0 dendrimers are relatively more often investigated due to the rich abundance of chargeable amine groups throughout the dendrimers (Figure 2.4). The first environmental application of dendrimers was reported by Diallo et al.
(1999) at CalTech. The PAMAM dendrimers of various generations acted as high capacity chelating agents and effectively removed Cu(II) from synthetic wastewater.
Depending on the background electrolyte concentration, solution pH as well as the generation number of dendrimers, the Cu(II) binding capacity of PAMAM dendrimers was found to range from 308 to 451 mg/g, which is significantly higher than conventional ion-exchange or chelating resins (Diallo et al., 2003).
For environmental nanotechnology to become affordable and economically feasible, cost-effective manufacturing of these nanoparticles or related nanomaterials should be made available. Another critical step in ensuring the overall feasibility is to integrate a recovery and reuse operation with the nanoparticle-based technologies. Diallo et al.
(2005) investigated the “dendrimer-enhanced ultrafiltration” (DEUF), i.e. the coupling of ultrafiltration process with separation of dendrimer-Cu(II) complexation, and showed that the copper-laden dendrimers could be fully rejected by optimizing the types of membranes used as well as their MWCO. The retained Cu(II)-dendrimer complexes could be regenerated through pH adjustment.
The attractiveness of the DEUF owes to the defined shape and size of the dendrimer as well as its high intrinsic binding capacity. Unlike conventional low-molecular weight chelating agents such as ethylenediamine tetraacetic acid (EDTA), the DEUF process could recover the remedial agents by choosing ultrafiltration membrane with a greater
MWCO, without the use of high pressure membrane processes such as reverse osmosis and nanofiltration. Dendrimers have lower hydrodynamic volumes in comparison to their linear polymer analogue with the same molecular weight (Bosman et al., 1999). The viscosity of dendrimer solution would only increase with molecular weight up to certain generation, and it is always lower than their linear polymer analogue, which render them to be more energy efficient in tangential/cross-flow filtration than their polymeric counterpart, i.e. polymeric chelating agents of polymer- enhanced ultrafiltration process (PEUF).
Contrary to surfactant-based nanoparticles, i.e. micelles, the dendrimers acquire stable covalently globular nanostructures without thermodynamic restriction on the particle- forming threshold concentration, e.g. the CMC for the former. The leakage of dendrimers through membranes with an appropriate MWCO is highly unlikely. Thus the loss of dendrimers by shear-induced mechanical breakdown is minimum in DEUF, be it in dead-end or tangential filtration systems that are commonly found in water/wastewater treatment practice nowadays. Diallo et al. (2005) examined the fouling phenomenon of the DEUF system. More severe membrane fouling was observed for PAMAM-polyethersulfone membrane system whereas PAMAM- regenerated cellulose membrane system only experienced a slight drop in flux. The PAMAM dendrimers for generation 3, 4 and 5 have hydrodynamic diameters of 3.5, 5 and 5.4 nm respectively, which are comparatively larger than the equivalent pore size of the membrane used. Diallo et al. (2005) attributed the fouling in the case of polyethersulfone membrane to particle deposition, whereas surface adsorption was the main fouling mechanism for the regenerated cellulose membrane. As a high pressure of 5 bar was required to obtain acceptable flux in the DEUF system, the claimed
advantages such as reduced operating costs and lower energy expenditure are still in doubts. In fact, the membrane molecular weight cut-off investigated is comparable to those of MEUF as discussed before. Theoretically, higher flux would only be achieved by selecting membranes with higher MWCO. However, this would require the use of higher generation dendrimers (i.e. larger dendrimers). As the globular-shape dendrimers become more rigid as the generation number increases, the accessibility of interior binding sites is reduced (Ottaviani et al., 1994).
Rether and Shuster (2003) explored the modification of the sequestration selectivity of the PAMAM dendrimers of generation 2.5, by functionalizing the surface groups with benzoylthiourea groups. In contrast to the original PAMAM dendrimers, the benzoylthiourea-modified PAMAM dendrimers could retain Cu(II) with a retention of
>90% even at pH = 4. In the presence of various water-soluble complexing agents such as ammonia and triethanolamine, Cu(II) and Ni(II) would still be fully removed by the modified PAMAM dendrimers. Their work showed that post-synthesis chemical modification of the dendrimers was feasible but there was insufficient data to show whether the reactions were strictly selective towards the surface terminal groups.
Unlike Cu(II) removal in DEUF as discussed before, removal of metal ions from metal-laden soil or sand will only utilize surface or near-surface functional groups of dendrimers because the binding to interior groups is diffusion-controlled. Situation is further exacerbated when the dendrimer becomes larger as generation increases (Ottaviani et al., 1994; Ottaviani et al., 1997). Xu and Zhao (2005) explored the efficacy of PAMAM dendrimers of different terminal groups as recoverable extracting
agents for in-situ flushing of Cu(II)-contaminated soil. Fixed-bed column elution studies with copper-laden sandy soil were carried out to compare the copper elution and recovery by different types of dendrimers. Almost 90% of Cu(II) was removed using 66 bed volumes with PAMAM dendrimers with terminal carboxylic acid groups at 0.10% (w/w) and solution pH 6.0. In addition, it was also shown that dendrimers of lower generation could remove soil-sorbed Cu(II) more effectively than that of higher generation, on a equal weight basis. This is probably due to that the heavy metals removal from contaminated soil is a non-equilibrium process; hence the mass transfer of dendrimers as well as the availability of accessible binding sites have significant influence on the dynamic column elution’s performance. Hence, the less bulkier the dendrimers are, the higher Cu(II) removal could be achieved at the same elution rate.
As discussed earlier, dendrimers possess larger size and well-defined shape, in contrast to other low molecular weight extracting agents such as pyridine-2,6- dicarboxylic acid (DPA), aminoethyl ethanolamine (AEEA), tetramethyl ethylenediamine (TEMED) and EDTA. About 72% recovery of the spent dendrimers of generation 4 was achieved by using a centrifugal nanofiltration filter (Xu and Zhao, 2005). This possibly implies that some dendrimers disintegrated during the column elution test. Hence the smaller fragments passed through the nanofiltration filter and were not recovered.
With the aid of EPR spectroscopy, Ottaviani et al. (1997) reported that the dendritic nanostructures of all generation eventually degraded after 60 days of aging at neutral solution pH, whereas complete decomposition was observed for dendrimers of generation 4 and lower. Stability of such precisely engineered highly branched
macromolecules is therefore questioned. The overall high cost of the dendrimers (Dendritech, 2009) could have thwarted any further efforts for a full-scale environmental remediation application. By sacrificing the symmetry as well as the precisely engineered microstructures of the dendrimers, polymeric nanoparticles of bigger or comparable dimensions as well as rich chemical functionalities can be another economically viable candidates for environmental applications.
Table 2.3 Survey of literature on dendrimers and related metal ion removal applications.
Generation number and terminal groups Target metal ions
Remark Reference Gn-NH2 (n: 3,4,5,6,7,8) Cu(II) The first study that reports significant enhanced chelating capacity in
nanostructured polymer. Diallo et al., 1999.
Gn-NH2 (n: 3,4,5) Cu(II), Co(II), Ni(II), Na(I), Mg(II), Ca(II)
RC and PES membrane (MWCO: 5, 10kDa) was used in ultrafiltration. Fouling by dendrimers on polymeric membrane is examined. Particle-deposition model is proposed.
Diallo et al., 2005.
Gn-NH2 (n: 1, 1.5, 4); Gm-COOH, (m: 1.5, 4.5);
G4-OH Cu(II) Centrifugal PES ultrafilter (MWCO: 1kDa) The first study that reports the use of dendrimers as recoverable chelating agents for soil washing.
Xu and Zhao, 2005.
Gn-NH2 (n: 1, 4); G1.5-COONa Pb(II) 97% of dendrimers is recovered using nanofiltration device
(MWCO: 1kDa) Xu and Zhao,
2006.
G2.5-NH2 Cu(II), Co(II),
Hg(II), Ni(II), Pb(II), Zn(II)
Cellulose membrane (MWCO: 3kDa), the dendrimer was modified with benzoylthiourea for selective and enhanced removal of specific heavy metals Competitive binding between common water soluble complexing agents and modified dendrimer was investigated. No study on membrane fouling.
Rether and Schuster, 2003.
Notes: Gn: generation number -NH2: amine group
-COOH: carboxylic acid group -OH: hydroxyl group
-Sac: succinamic acid group -Gly: glycidyol group -Ac: acetamide group
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