Synthesis and characterization of an insoluble polymer based on polyamidoamine: applications for the decontamination of metals in aqueous systems.

Journal of Environmental Management xxx (2014) 1e9

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Synthesis and characterization of an insoluble polymer based on polyamidoamine: Applications for the decontamination of metals in aqueous systems s a, Claudia E. Vergara b, Maria B. Camarada c, d, 1, Oscar Valde nchez a, b, Fabiane M. Nachtigall a, Jaime Tapia b, Veronica Carrasco-Sa d, e, 1, 2 lez-Nilo c, f, Leonardo S. Santos a, b, * , F.D. Gonza Rainer Fischer a

Fraunhofer Chile Research Foundation-Center System Biotechnology, FCR-CSB, Nanobiotechnology Division at University of Talca, P.O. Box 747, Talca, Chile Laboratory of Asymmetric Synthesis, Chemistry Institute of Natural Resources, University of Talca, P.O. Box 747, Talca, Chile c Universidad Andres Bello, Facultad de Biología, Center for Bioinformatics and Integrative Biology (CBIB), República 239, Santiago, Chile d Fraunhofer Institute for Molecular Biology and Applied Ecology, Aachen, Germany e Institute of Molecular Biotechnology e RWTH Aachen University, Germany f nchez Fontecilla 310 piso 14, Las Condes, Chile Fraunhofer Chile Research Foundation, M. Sa b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 June 2014 Received in revised form 9 September 2014 Accepted 14 September 2014 Available online xxx

We present a novel, insoluble, low-generation polyamidoamine (PAMAM)-based polymer. The monomer and polymer were characterized by fourier transform infrared spectroscopy, electrospray ionization mass spectrometry and thermogravimetric measurement, revealing that G0 acryloyl-terminated PAMAM were synthesized and polymerized using ammonium persulfate as an initiator, producing a high-density PAMAM derivative (PAMAM-HD). PAMAM-HD was tested for its ability to remove Na(I), K(I), Ca(II), Mg(II), Cu(II), Mn(II), Cd(II), Pb(II) and Zn(II) ions from acidic, neutral and basic aqueous solutions. PAMAM-HD efficiently removed metals ions from all three solutions. The greatest absorption efficiency at neutral pH was observed against Cu(II), Cd(II) and Pb(II), and the experimental data were supported by the calculated Kd values. Our data could have a significant impact on water purification by providing an inexpensive and efficient polymer for the removal of metal ions. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Dendrimers PAMAM Heavy metals Mass spectrometry

1. Introduction Most heavy metals in the environment originate from industrial processes, including automobile emissions, mining, battery manufacturing, fossil fuels, metal plating, and the electronics industry (Guilherme et al., 2007). Many technologies can be used to remove heavy metal ions, including ion exchange chromatography, alkaline precipitation, electrochemical extraction, membrane retention and filtration (Akpor and Muchie, 2010). However, these methods suffer from drawbacks such as the production of solid residues, poor selectivity, relatively low loading capacities and the release of dangerous ionic metals and/or organic compounds.

* Corresponding author. Fraunhofer Chile Research Foundation-Center System Biotechnology, FCR-CSB, Nanobiotechnology Division at University of Talca, P.O. Box 747, Talca, Chile. E-mail address: [email protected] (L.S. Santos). 1 Tel.: þ49 241 6085 12153; fax: þ49 241 6085 10000. 2 Tel.: þ49 (0)241 802 663; fax: þ49 241 871 062.

These challenges have been addressed by the development of a promising novel insoluble polymer based on a modified lowgeneration form of polyamidoamine (PAMAM). Specifically, PAMAM dendrimers have the ability to chelate metal ions from solutions (Tomalia et al., 1985). This attribute has primarily been exploited to synthesize metal nanoparticles (Huang et al., 2008; Witham et al., 2010; Li et al., 2011), but dendrimer based chelation has recently attracted interest. Chelation is a low-cost and environmentally-friendly technique that has the potential to overcome the limitations of other removal strategies (Reed, 2001; Roundhill, 2004). Many materials have been used for chelation, including polymers, activated carbon, metal oxides, silica, and ion exchange resins (Duran et al., 2008; Cavus and Gurdag, 2008; Copello et al., 2008; Uguzdogan et al., 2009). Studies with PAMAM dendrimers and derivatives have provided excellent preliminary results (Gugliotta et al., 2010; Rether and Schuster, 2003; Zhang et al., 2013; Li et al., 2011). PAMAM is commercial dendrimer that was synthesized and investigated for the first time by Tomalia et al., (1985). Dendrimers

http://dx.doi.org/10.1016/j.jenvman.2014.09.021 0301-4797/© 2014 Elsevier Ltd. All rights reserved.

s, O., et al., Synthesis and characterization of an insoluble polymer based on polyamidoamine: Please cite this article in press as: Valde Applications for the decontamination of metals in aqueous systems, Journal of Environmental Management (2014), http://dx.doi.org/ 10.1016/j.jenvman.2014.09.021

O. Valdes et al. / Journal of Environmental Management xxx (2014) 1e9

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are macromolecules that have recently generated a great deal of interest, reflecting their potential as templates for the controlled fabrication of nanosized metal particles that can be used for catalysis and to create electronic devices with specific functions that take advantage of their energy-harvesting and light-emitting properties (Laurent and Caminade, 2011; Kumar et al., 2012; Lo et al., 2009). Such dendrimers consist of globular macromolecules with three covalently-bonded components: an ethylenediamine core, interior branch units, and terminal branch units. The nanocavities and straightforward functionalization of terminal groups offer unique characteristics in a wide range of applications, such as drug delivery (Zhu and Shi, 2013), biomedical engineering (Rajasekhar and Liao, 2012), gene therapy (Santander-Ortega et al., 2012), catalysis (Myers et al., 2011), biosensing (Sarkar Abhijit Kaganove et al., 2005), water purification (Rether and Schuster, 2003; Zhang et al., 2013) photonics and electronics (Astruc et al., 2010). Here we describe the synthesis of a novel, low-cost polymer based on PAMAM G0 using simple organic transformations, and the quantitative evaluation of its ability to remove metal ions from water. We also carried out calculations to study the energetics and structural properties of metal complexes, which supported our empirical analysis of complex formation and confirmed that multiple factors in addition to electronic and electrostatic interactions must be considered to achieve better predictions.

addition of a suitable amine initiator core with MA, and exhaustive amidation of the resulting esters with large excess of EDA, as reported previously (Tomalia et al., 1985). This strategy for the preparation of dendrimers is known as the divergent synthesis method. The details of PAMAM G0 synthesis and characterizations are described in Text S1 in the SI. 2.3. Synthesis and characterization of acryloyl-terminated PAMAM G0 (PAMAM-acryloyl) PAMAM-acryloyl used for the syntheses of PAMAM-HD was generated by the nucleophilic substitution of the amines with acryloyl chloride as described in Scheme 1B (Uguzdogan and Kabasakal, 2010). PAMAM G0 obtained as described above (3.87 105 mol) was dissolved in 5.0 mL DMF in the presence of TEA (21.59 mL) and cooled in an ice bath. Freshly distilled acryloyl chloride (12.53 mL) was added dropwise to the above solution while stirring. The reaction mixture was brought to room temperature and stirred overnight before pouring into a large excess of diethyl ether. The precipitated PAMAM-acryloyl was separated, rotoevaporated to remove the residual diethyl ether and dried under vacuum until the weight was constant (yield NeH stretching vibration. Moreover, the spectra also feature characteristic absorptions at 1662 and 1554 cm1, representing amide I (primarily C]O stretch), and amide II (a combination of NeH inplane bend and CeN stretch). It is important to note that these absorption intensities increased for PAMAM-acryloyl, reflecting the acylation of PAMAM. Another remarkable difference compared to the PAMAM G0 spectra is the presence of a band at 1628 cm1 corresponding to C]C stretch. The area of the fingerprint region from 600 to 1200 cm1 is not identical between the two molecules, but the two spectra are nearly indistinguishable in the CeH/NeH stretch region. Specifically, one peak at 1244 cm1 in the spectrum emerges as consequences of the modification of PAMAM G0. Further intense signals were observed at 3072, 2983 and 2870 cm1, corresponding to the antisymmetric and symmetric


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Fig. 1. Spectroscopic characterization of the precursor reagents. (a) FT-IR spectra of PAMAM-acryloyl. (b) ESI-MS of PAMAM-acryloyl. The spectra contain signals resulting from the loss of the R group or the formation of intramolecular loops (R ¼ eCH2CH2CONHCH2CH2NHCOCHCH2, 169 Da). M represents a PAMAM-acryloyl dendrimer with an ideal, defectfree structure.

eCH2 groups. Finally, the signals that appear at 1433 cm1 were assigned to the CeN bending. The above results led us to the conclusion that the nucleophilic substitution reaction was carried out successfully in PAMAM G0. We also investigated the average molecular weights and polydispersity of PAMAM-acryloyl dendrimers. An isotopically-resolved mass spectrum of PAMAM-acryloyl is shown in Fig. 1b. The ion with the highest m/z (733) corresponds to the protonated molecule of the ideal (defect-free) [MIS þ H]þ (see Scheme S1A in the SI). Additional signals were observed at m/z values lower than MIS in integral units of 169 Da [MIS 169 þ H]þ. These signals may reflect the fragmentation of the protonated PAMAM-acryloyl ions, e.g. the loss of 169 mass units, or the presence of so-called “missing arm” defects (-CH2CH2CONHCH2CH2NHCOCHCH2) inside dendrimers (see Scheme S1B in the SI), or the formation of an intramolecular loop during the amidation reaction [MIS þ 1 loop] (see Scheme S1C in the SI). These kinds of structures represent subproducts of the PAMAM G0 synthesis and are normally found in the PAMAM-acryloyl ESI-MS. The evidence that such signals arise from inherent defects in the dendrimers rather than fragmentation of the gas-phase ion is that the relative abundances of each ion series are not a function of the ionization mechanism. “Missing-arm” defects in dendrimers are a natural consequence of the divergent synthesis strategy, which involves multiple steps proceeding outward from a central core. Further signals are observed at m/z values lower than MIS [MIS 54 þ H]þ. These signals could represent small quantities of PAMAM G0 tri-substituted with a vinyl group. Finally, the signals with m/z values greater than MIS in integral units of 39 Da [MIS þ K]þ represent potassium (K) salts (ionization by K is due to random K impurities in the samples). The calculated mass numbers of the structural errors agree with those derived from the spectra. Finally, to verify that the structure and composition of the obtained materials affect thermal degradation, thermal analysis of PAMAM-acryloyl was carried out in the interval 20e600 C. The weight loss (TGA) and derivate (DTG) curves are presented in Fig. 2. The thermogram of PAMAM-acryloyl indicates four degradation stages, represented by four peaks in the DTG curve. The main difference compared to the PAMAM G0 thermogram curve is the weight loss onset temperature of 164 C, which is attributed to the acryloyl groups. PAMAM-acryloyl therefore has a lower thermal stability than PAMAM G0. This may be explained by the formation of hydrogen bonds between the amine groups present in PAMAM G0. Considering that the first weight loss is attributed to the

experimental conditions, we can conclude that our findings agree with earlier reports (Zheng et al., 2009) showing that PAMAM dendrimer TGA curves show two stages of mass loss. As shown in Fig. 2, 9% weight loss occurred in the temperature range 0e90 C corresponding to the loss of TEA and water traces present in sample. Two further decompositions occurred in two steps starting at 132, 211 and 319 C, with weight losses of 23, 38 and 25%, respectively. The total weight loss was about 95%. The degradation mechanism involves random chain scissions during the first step (retro-Michael processes and transamidation) and ammonium salt decomposition during the second, highertemperature step. 3.2. Preparation of the polymer The radical polymerization of the monomer PAMAM-acryloyl produced the polymer PAMAM-HD, which was insoluble in water and all other common organic solvents (both polar and nonpolar) such as methanol, ethanol, tetrahydrofuran, nitrobenzene, dichloromethane, dimethyl formamide and dimethyl sulfoxide. The insolubility of PAMAM-HD is a great advantage as it simplifies its separation from solvents following applications in metal ion removal. The structure of PAMAM-HD was investigated by FT-IR spectroscopy (Fig. 3). The polymer revealed the same characteristic

Fig. 2. TGA/DTG plot of PAMAM-acryloyl in an N2 atmosphere with a 10 C/min heating rate.



amine absorption bands found in the monomer. For example, the signals in the region 3100e3400 cm1 can be attributed to NeH stretching vibration in the amide group. The peaks with maximum absorption at 3084 and 2939 cm1 correspond to the stretching vibration of the CeH bond. Moreover, characteristic absorptions at 1655 and 1556 cm1 correspond to C]O stretching and a combination of NeH in-plane bending and CeN stretching, respectively. The remarkable difference compared to the PAMAM-acryloyl spectrum is the absence of the band in 1628 cm1 corresponding to C]C stretching. Specifically, one peak at 1111 cm1 in the spectrum appears to reflect the consequences of the polymerization. Following the same protocol, the thermal behavior of PAMAMHD was investigated. As shown in Fig. 3b, the weight loss curves for the thermal degradation of PAMAM-HD closely follow the TG and DTG curves of the PAMAM-acryloyl monomer, and PAMAM-HD decomposes in four steps with a total mass loss of 77%. The overall decomposition process consisted of a minor dehydration followed by depolymerization to the monomers, which started at 184 C. The mechanisms for the next three steps appear the same as those suggested for the decomposition of PAMAM-acryloyl. The thermograms reveal a weight loss to total weight ratio at the lowtemperature stage (180e283 C) of 32%, and at the hightemperature stage (283e450 C) of 34%. The weight loss appears to stop at the high temperature of 450e600 C, since the ratio of 23% to the total weight ratio is hydrocarbon black. Therefore, PAMAM-HD is more thermoresistant than the precursors PAMAMacryloyl and PAMAM G0. 3.3. Efficiency of PAMAM-HD-mediated metal ion removal from aqueous solution The preliminary studies of the efficiency of PAMAM-HDmediated metal ion removal from aqueous solution varying pH are shown in Fig. 4. Fig 4 reflect the selectivity of PAMAM-HD for K(I), Na(I), Ca(II), Mg(II), Mn (II), Cu(II), Cd(II), Pb(II) and Zn(II). It is important to note that the metal concentrations did not exceed 0.5 mg/L in our experiments. Encapsulation of metal ions by PAMAM-HD involves the coordination of each metal ion with amide sites in the polymer, and additional physical retention achieved by the three-dimensional crosslinking of PAMAM-HD. This phenomenon can be attributed to ligand-to-metal charge-transfer, i.e. a transfer of electrons from high-electron-density groups of the polymer to the metal center. Fig. 4 clearly shows that metal complexes did not form efficiently at low pH values, although Cu reached 9%. Moreover, PAMAM-HD showed no affinity for K(I),

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Fig. 4. Removal efficiency of different metal ions by PAMAM-HD at three pH values (4, 7 and 9).

Na(I), Mg(II) or Ca(II). This is a useful observation since these metals are important biologically. The retention curves in Fig. 4 also indicate that PAMAM-HD remove the metals at pH 7 with the following order of affinity: Cu(II) > Cd(II) > Pb(II) > Zn(II) > Mn(II). The quantity of metal ions retained by PAMAM-HD increases at alkaline pH values, and the order of affinity was modified. This suggests that the removal of metal ions with PAMAM-HD is dependent on the pH, which can affect the surface charge of the adsorbent and the degree of ionization of PAMAM-HD (Petrov and Nenov, 2004). Internal tertiary amines of the polymer could be protonated, resulting in direct competition between protons and metal ions for complex formation. The more efficient removal of metal ions at higher pH values may be associated with the pH-dependent solubility of metal hydroxides, resulting in differences in the solubility product constant (Ksp). For example, precipitation of Cd(OH)2 is relatively difficult, compared to other metals hydroxides, because of its higher Ksp value (Belgin, 2002). Based on these preliminary studies and the experimental conditions, we determined the amount of absorbed metal (G), the removal efficiency (h) and metal ion distribution coefficient (Kd) for each metal at pH 7. These parameters are often used to describe the potential of insoluble materials such as cross-linked polymers, hydrogels and magnetic nanoparticles for the removal of metal contaminants. Table 1 summarizes the preliminary values obtained for each metal at pH 7. The amount of metal absorbed by PAMAM-HD was

Fig. 3. (a) Characterization of PAMAM-HD: FT-IR spectra of PAMAM-HD. (b) TG/DTA plot of PAMAM-HD in an N2 atmosphere, at a heating rate of 10 C/min.



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Table 1 Amounts of absorbed metal (G), removal efficiencies (h) and metal ion distribution coefficients (Kd) calculated for 0.7 mg of PAMAM-HD at pH 7 and metal concentrations below 0.5 mg/L. Metals

G (mg/g)

h (%)

Kd

K(I) Na(I) Ca(II) Mg(II) Cu(II) Mn(II) Pb(II) Zn(II) Cd(II)

e e e e 2.1 1.8 2.8 2.5 2.9

e e e e 63.0 36.7 49.1 40.7 52.2

e e e e 19,513.9 6625.9 12,998.7 9180.4 14,548.9

similar for all elements (1.8e2.9 mg/g). These values indicate that the amount of metal absorbed by PAMAM-HD is not dependent on the nature of the metal. Finally, Table 1 also shows the distribution coefficients (Kd) of different metal ions. PAMAM-HD achieved high selectivity for Cu(II), Cd(II) and Pb(II) with Kd values greater than 12,998.7. All these ions, with the exception of Cu(II), are toxic towards humans. Specifically PAMAM-HD showed a large Kd of 19,513.9 for Cu(II), confirming its strong binding and excellent selectivity for Cu(II). 3.4. Computational results Theoretical calculations were carried out to study the electronic, geometrical and energetic parameters involved in the formation of metal ion/PAMAM-HD complexes. Only those metals effectively entrapped by the polymeric substrate under alkaline pH conditions were considered, i.e. Cu(II), Zn(II), Mn(II), Pb(II) and Cd(II). Because PAMAM-HD is large and has several degrees of freedom, first principle studies are not suitable to investigate metal ion coordination. The monomeric repetitive unit of the polymer was therefore selected as an approximation of the entire polymeric system (Fig. 5a). Some potential coordination sites in PAMAM-type dendrimers possess high electron density, allowing a positively-charged metal ion to attach and generate a complex (Petrucci et al., 2007; Cross et al., 2004; Kaczorowska and Cooper, 2009; Rether and Schuster, 2003). Amide and amine sites act as potent chelating agents for metal cations. Previous studies have suggested that metal ions in a tetragonal field preferentially coordinate, in the case of low generations, to the core site of the dendrimer (see Fig. 4a), i.e. to the tertiary nitrogen sites of the ethylenediamine core and oxygen

atoms from the carbonyl site of the amide groups (Crooks et al., 2001; Krot et al., 2005; Ottaviani et al., 2002; Diallo et al., 2004, 1999; Ottaviani et al., 1997; Ottaviani et al., 1994) forming a fourmembered ring. However, theoretical studies by TarazonaVasquez and coworkers (Tarazona-Vasquez and Balbuena, 2004) confirmed that the most stable site for metal ion coordination is the core site of PAMAM G0 dendrimers. Computational analysis of PAMAM-HD/M2þ complexes was therefore carried out using starting geometries for optimization designed by locating metal ions next to the complex-forming region of the core site of the ligand. The PAMAM-HD structure was optimized (B3LYP/6-311g (d,p)), characterized as a stationary point using second derivative calculations and used as reference for the calculation of binding energies. Several initial conformations were tested to search for alternative local minima. Fig. 5b shows the optimized monomer geometry. Internal hydrogen bonds that can be established between amide groups from different branches contribute to dendrimer stabilization. These cross-linked interactions, when extrapolated to the polymeric system, generate a compact and high-density matrix in which metal ions are easily trapped. Simulations were carried out with different metal ions located next to the core complex-forming region of the optimized PAMAMHD dendrimer. Full optimization was done using the hybrid exchange correlation B3LYP function and the 6-311G(d,p)//LANL2DZ basis sets. Second derivative calculations were performed on the complexes to confirm the presence of stable minima. The optimized complexes for the different metal ions are shown in SI, Figure S1. Table 2 uses the binding energy (Ebind) to quantify the coordination strength of each metal ion in the PAMAM-HD core site. It is important to note that the reference system is arbitrary and Ebind therefore accounts for the relative interaction strength of attachment without necessarily implying exothermic or endothermic adsorption processes. A positive Ebind value thus indicates weak interactions but does not confirm an endothermic adsorption process. Fig. 6a shows the ranking of Ebind values: Cu(II) > Zn(II) > Mn(II) > Cd(II) > Pb(II). The copper ion complex exhibited the most negative values of binding energy and consequently achieved stronger interactions with the dendrimer. Table 2 also summarizes the bond distances between the metal ion (M) and the ligand atoms bonded to it (see Fig. 5a): the nitrogen atoms of the ethylenediamine core (N1 and N2) and the carbonyl oxygen atoms of the amide group (O1 and O2). Fig. 6b shows that the shortest metal-ligand (ML) distance in all systems corresponds to the bond with the amide oxygen atoms. Metal cation binding

Fig. 5. (a) PAMAM-HD structure. M2þ represents a metal ion coordinated in the core site of the dendrimer. (b) Optimized structure of PAMAM-HD calculated at B3LYP/6-311G(d,p) level of theory.



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Table 2 Multiplicity and binding energies (Ebind) in kcal mol1 for the coordination of metal ions at the PAMAM-HDm core site, showing the bond distance (Å) and absolute NPA charge (a.u.) between the metal ion (M) and the ligand (see Fig. 5a for atomic labeling) at the B3LYP/6-311g(d,p)//LANL2DZ level of theory. m

2 1 1 6 1

Metals

Cu(II) Zn(II) Cd(II) Mn(II) Pb(II)

Ebind

440.184 382.635 328.115 350.740 291.137

Bond distance

Charge

O1

O2

N1

N2

M

O1

O2

N1

N2

1.979 2.014 2.043 2.201 2.31

1.962 2.000 2.023 2.183 2.263

2.063 2.138 2.212 2.348 2.528

2.09 2.174 2.256 2.400 2.576

1.028 1.501 1.351 1.53 1.537

0.707 0.823 0.777 0.822 0.836

0.688 0.797 0.759 0.795 0.796

0.561 0.657 0.628 0.642 0.621

0.557 0.662 0.618 0.643 0.626

strength is directly related to the bond distances, because the bond distances become longer as the binding becomes weaker. Cu(II) has the shortest bond distances of the metal series indicating stronger interactions with the ligand than the other metals. Another important property of metal complexes is the atomic charge, which can be calculated using the natural population analysis (Reed et al., 1985) (NPA) method as implemented in Gaussian 09 (Table 2). Chemical bonds in transition metal complexes are usually described in terms of ionic and covalent interactions between the metal and ligands. The ionic contribution is associated with the atomic charges of the metal and the ligand, whereas the covalent effect is reflected in the orbital mixing. Fig. 6c shows a plot of the absolute atomic charge of the metal (M), the bonded atoms of the ligand (N1, N2, O1 and O2) and the difference in charge between the metal and the average charge of the ligand atoms (D). The atomic charge distribution indicates that electron transfer occurs from the ligand atoms to the metal ion, resulting in a ligand-to-metal charge-transfer (LMCT) complex as discussed above. Zn(II), Pb(II) and Cd(II) complexes had similar charges between the metal and the ligand atoms (D), and the values were higher compared to complexes containing Cu(II) and Mn(II). This indicates that the ionic contribution to the M-L bond energy is lower in the latter complexes, and therefore the Zn(II), Pb(II) and Cd(II) complexes have greater ionic characteristics. In these complexes, all of the d orbitals of the metal ion are doubly occupied (d10) in contrast to copper and manganese cations (d9 and d5, respectively). Consequently, the p contribution to covalent bonding is canceled and the covalent interaction between the d orbitals and the ligand orbitals becomes small. The only remaining covalent interactions come from inner metal orbitals. Metal ion complex formation depends on covalent and electrostatic interactions at the atomic level, which in turn reflect metal ion solubility and ligand-chelating ability. In this sense, our

computational analysis is approximate and does not take into account all the different factors that affect this process, such as the simultaneous coordination of different metal cations, complex formation at dendrimer sites outside the core, solvent molecules effects, hydroxide precipitation, and retention inside the polymeric matrix. Despite the approximations introduced into the model, it is possible to correlate the trend in Ebind values, i.e. Cu(II) > Zn(II) > Mn(II) > Cd(II) > Pb(II), with the empirical trend in metal retention by PAMAM-HD in alkaline solutions, i.e. Cu(II) > Zn(II) > Mn(II) > Pb(II) > Cd(II). The latter tendency is related to the stability of each complex (among other contributing factors) and can therefore be associated with the theoretical energetic results, with strong agreement. Differences in the order of Pb(II) and Cd(II) confirm that factors other than electronic interactions also influence complex formation. As already stated, Cu(II) achieved the highest retention value, which is directly related to the stability of the complex, reflecting the stronger binding energy, shorter bond distances between the metal center and the ligand, and greater contribution of covalent interactions based on the more prevalent orbital mixing. Despite all the simplifications introduced into this theoretical model, essential information about the electronic and electrostatic effects that influence PAMAM-HD/metal coordination can be interpreted. The mode could be improved further by incorporating factors such as simultaneous coordination, solvation effects and the impact of a larger PAMAM-HD matrix, therefore leading to better predictions of retention trends. 4. Conclusions A novel metal-chelating polymer was synthesized by the radical polymerization of a PAMAM G0 dendrimer modified with acryloyl chloride. Remediation of Na(I), K(I), Ca(II), Mg(II), Cu(II), Mn(II),

Fig. 6. (a) Binding energy of metal ions to the PAMAM-HD core site; (b) bond distance; and (c) NPA absolute atomic charge on the metal center (M) and the ligand atoms (see Fig. 5a for labeling).



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Cd(II), Pb(II) and Zn(II) ions from aqueous solutions at acid, neutral and basic pH values was successfully achieved using PAMAM-HD. The results showed that metals do not form complexes with this dendrimer at acidic pH values with the exception of copper, whereas complexes are readily formed in neutral and alkaline pH solutions. Theoretical studies delivered essential information about the electronic and electrostatic effects that influence PAMAM-HD/ metal coordination. The experimental retention trend was well correlated with these studies. Modifications that could be introduced to improve the model and achieve better predictions of retention trends include simultaneous coordination, solvation effects and the consideration of a larger PAMAM-HD matrix. Notes The authors declare no competing financial interest. Acknowledgments The authors acknowledge InnovaChile CORFO Code FCR-CSB 09CEII-6991 for supporting this research activity. C. E. Vegara thanks CONICYT for PhD fellowship (21120473). LSS thanks additional support from PIEI (Quimica y Bio-organica en Recursos Naturales) Universidad de Talca. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2014.09.021. References Akpor, O.B., Muchie, M., 2010. Remediation of heavy metals in drinking water and wastewater treatment system: processes and applications. Int. J. Phys. Sci. 5, 1807e1817. Astruc, D., Boisselier, B., Ornelas, C., 2010. Dendrimers designed for functions: from physical, photophysical, and supramolecular properties to applications in sensing, catalysis, molecular electronics, photonics, and nanomedicine. Chem. Rev. 110, 1857e1959. Becke, A.D., 1993. Density-Functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 98, 5648e5652. Belgin, B.J., 2002. Comparative study of adsorption properties of Turkish fly ashes: I. The case of nickel(II), copper(II) and zinc(II). J. Hazard. Mater. 95, 251e273. Cavus, S., Gurdag, G., 2008. Competitive heavy metal removal by poly(2acrylamido-2-methyl-1-propane sulfonic acid-co-itaconic acid). Polym. Adv. Technol. 19, 1209e1217. Copello, G.J., Varela, F., Vivot, R.M., Diaz, L.E., 2008. Immobilized chitosan as biosorbent for the removal of Cd(II), Cr(III) and Cr(VI) from aqueous solutions. Bioresour. Technol. 99, 6538e6544. Crooks, R.M., Zhao, M., Sun, L., Chechik, V., Yeung, L.K., 2001. Dendrimer-Encapsulated metal nanoparticles: synthesis, characterization, and applications to catalysis. Acc. Chem. Res. 34, 181e190. Cross, J.P., Lauz, M., Badger, P.D., Petoud, S.P., 2004. Polymetallic lanthanide complexes with PAMAM-Naphthalimide dendritic ligands: luminescent lanthanide complexes formed in solution. J. Am. Chem. Soc. 126, 16278e16279. Diallo, M.S., Balogh, L., Shafagati, A., Johnson, J.H., Goddard, W.A., Tomalia, D.A., 1999. Poly(amidoamine) dendrimers: a new class of high capacity chelating agents for Cu(II) ions. Environ. Sci. Technol. 33, 820e824. Diallo, M.S., Christie, S., Swaminathan, P., Balogh, L., Shi, X., Um, W., Papelis, C., Goddard, W.A., Johnson, J.H., 2004. Dendritic chelating agents. 1. Cu(II) binding to ethylene diamine core Poly(amidoamine) dendrimers in aqueous solutions. Langmuir 20, 2640e2651. Duran, A., Soylak, M., Tuncel, S.A., 2008. Poly(vinylpyridine-poly ethylene glycol methacrylate-ethylene glycol dimethacrylate) beads for heavy metal removal. J. Hazard. Mater. 155, 114e120. Frisch, M.J., Trucks, G.W., Schlegel, H.B., Scuseria, G.E., Robb, M.A., Cheeseman, J.R., Scalmani, G., Barone, V., Mennucci, B., Petersson, G.A., Nakatsuji, H., Caricato, M., Li, X., Hratchian, H.P., Izmaylov, A.F., Bloino, J., Zheng, G., Sonnenberg, J.L., Hada, M., Ehara, M., Toyota, K., Fukuda, R., Hasegawa, J., Ishida, M., Nakajima, T., Honda, Y., Kitao, O., Nakai, H., Vreven, T., Montgomery, J.A., Peralta, J.E., Ogliaro, F., Bearpark, M., Heyd, J.J., Brothers, E., Kudin, K.N., Staroverov, V.N., Kobayashi, R., Normand, J., Raghavachari, K., Rendell, A., Burant, J.C., Iyengar, S.S., Tomasi, J., Cossi, M., Rega, N., Millam, J.M., Klene, M., Knox, J.E., Cross, J.B., Bakken, V., Adamo, C., Jaramillo, J., Gomperts, R., Stratmann, R.E.,

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