Insight into Highly Efficient Coremoval of Copper and p-Nitrophenol by

Oct 28, 2013 - Highly efficient coremoval of Cu(II) and p-nitrophenol (PNP) was accomplished using a newly synthesized polyamine chelating resin (CEAD...
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Insight into Highly Efficient Coremoval of Copper and p‑Nitrophenol by a Newly Synthesized Polyamine Chelating Resin from Aqueous Media: Competition and Enhancement Effect upon Site Recognition Taipeng Chen,† Fuqiang Liu,*,† Chen Ling,† Jie Gao,† Chao Xu,† Lanjuan Li,‡ and Aimin Li† †

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, P. R. China ‡ Jiangsu Provincial Department of Housing and Urban-Rural Development Municipal Water Conservation Office, Nanjing 210000, P. R. China S Supporting Information *

ABSTRACT: Highly efficient coremoval of Cu(II) and pnitrophenol (PNP) was accomplished using a newly synthesized polyamine chelating resin (CEAD) as compared to three other commercial resins. The mutual effects and inner mechanisms of their adsorption onto CEAD were systematically investigated by binary, preloading, thermodynamic, and dynamic adsorption procedures. PNP was adsorbed onto both hydrophobic and hydrophilic sites, while Cu(II) only interacted with hydrophilic amine group sites. In both preloading and binary systems, the adsorption of PNP was inhibited to the same degree by the presence of Cu(II) because of selective recognition and direct competition. On the other hand, the presence of PNP obviously enhanced the adsorption of Cu(II) by more than 7%, which was related to PNP loading on the hydrophobic surface. As proved by structural characterization, hydroxyl groups facing outward create new sites for coordination with Cu(II). Moreover, ionic strength exerted some positive influence on the properties of CEAD. Finally, more than 98% of PNP and 99% of Cu(II) could be sequentially recovered with dilute NaClO3 and HCl. These superior properties demonstrated with CEAD indicate that it could be applied to wastewaters containing both heavy metals and PNP, even for high saline aqueous media.

1. INTRODUCTION Organic and inorganic pollutants such as nitroaromatic compounds (NACs) and heavy metals (HMs) are common in the environment and pose a serious toxicological threat to the ecosystem and human health. For example, p-nitrophenol (PNP) may cause cancer in human beings as a hydrolytic product from the degradation of organophosphorus pesticides.1 HMs have been mainly used in industrial operations such as metal plating/coating and agricultural industries.2 The coexistence of NACs and HMs as complex solute mixtures in contaminated water has significantly influenced humans and the ecosystem because of their relative mobility, combined toxicity, and carcinogenic properties.3 Such pollutants accumulate in aquatic environments as a result of widespread usage in printing and from dye wastewater, agriculture production, and biotreated effluent. For example, Cu(II) and PNP are both utilized in dye production as raw materials and catalysts, respectively. Consequently, the effective control of these combined pollutants has attracted continuous concern recently. The traditional methods of removing HMs © 2013 American Chemical Society

and NACs involve chemical precipitation, advanced oxidation, membrane treatment, and biological treatment,4−7 which suffer from disadvantages such as high cost, low efficiency, and secondary pollution.8,9 As a green technology, adsorption has been widely utilized in recent years for the successful removal of HMs10 and NACs.11 Various adsorbents such as activated carbon, soil, biosorbents, chitosan, and polymeric resins have been applied.11−13 Among them, synthetic polymers have proved be superior to modified soil and biosorbents because of advantages such as the simple and high capacity for modification of functional groups, excellent stability, and easy regeneration.14 Particularly, polyamine resins have potential for removal of both HMs and NACs because amine groups could chelate with HMs and synchronously remove phenols through physical or Received: Revised: Accepted: Published: 13652

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electrostatic interaction.15−17 However, there have been only a few studies on the simultaneous removal and the mutual interactions of multiple contaminations on a microinterface. Similarly, little is known about the regeneration of adsorption materials as well as recovery of pollutants. Furthermore, the inhibited sorption of NACs by HMs was previously reported, which was generally attributed to the competition for binding sites. A typical example is alginate/mauritanian clay,18 which was used to remove Cu(II) and PNP, but an obvious suppression of the adsorption of PNP was observed. Similar results involving Cu(II) and 2,4,6-trichlorophenol were also obtained due primarily to competition for adsorption sites.19 Hence, it might be extremely difficult to coremove HMs and NACs. Yet, evidence for coordination between PNP and Cu(II) on two typical Chinese soils was provided by Pei et al.,20 hinting at the probability of bridge enhancement on solid phase and the potential for simultaneous removal. In this work, polyamine groups and ethyl 2-aminothiazole-4acetate were introduced for the modification of the hydrophobic matrix of chloromethyl polystyrene resin. This novel polyamine chelating resin was synthesized to efficiently coremove both Cu(II) and PNP from aqueous solutions, which could simultaneously show additional affinity to Cu(II) and PNP through coordination as well as hydrogen bonding. The adsorption and desorption behavior onto the selfsynthesized resin was systematically investigated by carrying out static and column adsorption from sole, preloaded, binary, and especially saline systems. Furthermore, Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) were selected to help investigate the mutual effects involving PNP and Cu(II), for further illumination of the site recognition processes, direct competition on hydrophilic sites, and selective enhancement on hydrophobic sites.

adjusted to 4.0 with HCl or NaOH, which was determined in the preliminary experiments (Figure S3, Supporting Information). The flasks were completely sealed and agitated in an incubator shaker at 303 K and 140 rpm for 36 h. Then the concentrations of PNP and Cu(II) in the residual aqueous phase were determined with a UV−vis spectrophotometer (Agilent 8453, Santa Clara, CA) and atomic adsorption spectrophotometer (AAS, Thermo, Chelmsford, MA), respectively. In the preloading adsorption experiments, fresh CEAD was first equilibrated with 40−500 mg/L of PNP or 0.5−5 mmol/L of Cu(II), and then the preloaded CEAD was conversely fed by Cu(II) or PNP at various concentrations to obtain a second equilibra. 2.3.2. Dynamic Adsorption and Desorption. The dynamic adsorption procedures were carried out in a water-jacketed glass column (⌀10 × 240 mm). A dose of 0.500 g of CEAD particles was put into the column. The solution of PNP and Cu(II) with the initial concentrations at 400 mg/L and 2.0 mmol/L was adjusted to the initial pH value of 4.0 and then pumped into the column in a down-flow direction using a peristaltic pump at the desired flow rate of 10 BV/h (bed volume per hour) at 303 K. After the adsorption of the first added adsorbate reached saturation, a certain amount of the second adsorbate was alternately introduced into the glass column. Regeneration of the particles loaded with PNP and Cu(II) was carried out with 1.5 mol/L NaClO3 solution at a pH of 8, followed by 4 mol/L HCl solution. 2.4. Analysis Procedures. The concentrations of PNP and Cu(II) in the equilibrium solutions were determined with UV− vis at an absorption wavelength of 316 nm and with AAS at λCu = 324.75 nm, respectively. The adsorption amounts for PNP and Cu(II) were calculated from the differences between the initial and residual concentrations. The recoveries of PNP in control groups without adsorbents were all near 100%, indicating no significant degradation or other loss during the adsorption experiments. 2.5. Characterization for CEAD. Fourier transform infrared spectrometry (Thermo Nicolet Nexus 870 FTIR) and X-ray photoelectron spectroscopy (ESCALAB-2, UK) were applied to determine the differences in solid phase before and after PNP and Cu(II) adsorption. All spectra were calibrated with graphitic carbon as the reference at a binding energy of 284.6 eV to compensate for the surface charging effects. The XPSPEAK41 software was used to fit the XPS spectra of each relevant element into subcomponents.21

2. MATERIALS AND METHODS 2.1. Preparation of Polyamine Chelating Resin. The polyamine chelating resin, called CEAD, was synthesized through the modification of the chloromethyl polystyrene resin (CMPS) with polyamine groups. The detailed synthetic process is shown in Text S1 of Supporting Information. The key structural information for CEAD, including EA, SEM, and contact angle, are respectively shown in Table S1, Figure S1, and Figure S2 of Supporting Information. Much evidence certified the successful graft of amine groups and numerous pores on the outside of the resin. The contact angle demonstrated that CEAD may be more hydrophilic than CMPS, implying that CEAD could contain both hydrophilic and hydrophobic sites. 2.2. Materials. The main information on the characteristics of tested adsorbents such as S984, S910, NDA88, and CEAD are tabulated in Table S2, Supporting Information. p-Nitrophenol (p-NO2-C6H4-OH), p-chlorophenol, p-hydroxybenzaldehyde, and pentachlorophenol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, P. R. China) and used directly without further purification. Cu(NO3)2, NaOH, HCl, HNO3, and NaClO3 were all reagent grade chemicals. 2.3. Batch Adsorption Studies. 2.3.1. Adsorption Isotherm. The equilibrium experiments were performed in a 150 mL conical flask by mixing 0.050 g of adsorbent and 50 mL of solution containing various concentrations of PNP and Cu(II). The concentration range of PNP and Cu(II) was 40−500 mg/ L and 0.5−5 mmol/L, respectively. The pH of the solution was

3. RESULTS AND DISCUSSION 3.1. Comparisons of Adsorption Properties for PNP and Cu(II) onto Different Resins. 3.1.1. Sole Systems. Three commercial resins including S984, S910, and NDA88 were selected to compare with the newly synthesized polyamine chelating resin CEAD, on the basis of their established applications for removing HMs and organic pollutants.2,22The adsorption isotherms of PNP for these four resins are presented in Figure S4a, Supporting Information. The adsorption capacities for PNP were in the following order: CEAD > NDA88 > S984 ≫ S910. CEAD exhibited the highest adsorption capacity, which could be partly due to π−π interaction in the polystyrene-divinylbenzene matrix. Furthermore, a larger number of amine groups in CEAD offers additional affinity and more available binding sites, through hydrogen bonding or electrostatic interaction, for PNP. The 13653

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among the tested resins. The total capacity of CEAD was around 1.4 times, 1.8 times, and 2.2 times that of S984, NDA88, and S910, respectively. Consequently, the superior properties of CEAD for adsorption of both Cu(II) and PNP demonstrate the potential for successful simultaneous removal of typical HMs and NACs and should be explored further to develop a method that is inexpensive and that results in high recovery. 3.2. Mutual Effects upon the Adsorption of Both PNP and Cu(II) onto CEAD. 3.2.1. Adsorption Isotherms. The adsorption isotherms for PNP with or without Cu(II) are evaluated in Figure 2a. The amounts of adsorbed PNP onto CEAD decreased with the increase in Cu(II) initial concentration from 0 to 2.0 mmol/L. As listed in Table S4 (Supporting Information), the Freundlich equation could fit well, indicating that the adsorption for PNP was primarily physical adsorption.26 The adsorption capacity of PNP declined by around 46% in the presence of 2.0 mmol/L Cu(II). The suppression impact might be attributed to the direct competition of both adsorbates for similar sites such as amine groups. The adsorption isotherms for Cu(II) from sole and binary systems are compared in Figure 2b, and the obtained characteristic parameters are tabulated in Table S4, Supporting Information. The Langmuir model gave a better fit than the Freundlich model. In comparison with the sole systems, the maximum adsorptive amount of Cu(II) decreased by 5% in the presence of PNP with the initial concentration of 100 mg/L. However, with the further increase in PNP concentration up to 400 mg/L, the adsorptive amount of Cu(II) obviously increased by 7%. The large difference suggested that multiple mechanisms might coexist herein. A competitive interaction would probably occur between Cu(II) and PNP on solid phase. Accordingly, a kind of new adsorption site for Cu(II) caused by the large number of PNP loaded on CEAD might account for the enhancement, especially in highly concentrated PNP systems. 3.2.2. Thermodynamic Investigations. The thermodynamic equation and parameters are listed in Text S3, Supporting Information. The vant’s Hoff plot (Figure S6, Supporting Information) is plotted as ln Kd versus 1/T to calculate ΔH° and ΔS°. The thermodynamic parameters for PNP and Cu(II) are summarized in Table S5, Supporting Information. The values of ΔH° for PNP were all negative, confirming that the adsorption of PNP onto CEAD was exothermic and physical in

electrostatic interaction is indicated by the less positive zeta potentials of CEAD at various pH values (2−8) as affected by the adsorption of PNP, as shown in Figure S5, Supporting Information. Langmuir and Freundlich models were both applied to describe isotherm data. The equations and parameters of these isotherm models are listed in Text S2, Supporting Information. The calculated parameters mentioned above are listed in Table S3. The Freundlich equation with higher correlation coefficients could better describe the adsorption isotherms for PNP than the Langmuir equation. According to the Kf values, CEAD had the highest adsorption capacity toward PNP among all tested resins. The adsorption isotherms for Cu(II) are shown in Figure S4b (Supporting Information), and the adsorption capacity was in the following order: CEAD > S984 > S910 ≫ NDA88. From Table S3 (Supporting Information), the Langmuir model with higher correlation coefficients gave a better fit to the equilibrium data than the Freundlich model, assuming that the adsorption of Cu(II) was mainly chemisorption.23 In addition, CEAD showed the highest maximum adsorption capacity mainly due to the highest chelating affinity of amine groups to Cu(II), as reported previously.24,25 3.1.2. Binary Systems. As shown in Figure 1, the adsorption for both PNP and Cu(II) onto the four adsorbents showed a

Figure 1. Adsorption amounts for PNP and Cu(II) onto CEAD, S984, NDA88, and S910 from both single and double systems.

similar trend: a significant decrease in the uptake of PNP and a slight increase in the adsorption of Cu(II). As expected, CEAD exhibited the highest adsorption involving PNP and Cu(II)

Figure 2. Adsorption isotherms for (a) PNP and (b) Cu(II) with interaction to different extents. 13654

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Figure 3. (a) Sequential adsorption of PNP and Cu(II) and (b) simultaneous and sequential adsorption of Cu(II) onto CEAD.

nature.27 The positive values of ΔS° implied an increased randomness at the solid−solution interface.28 The value of ΔG° is negative, indicating the spontaneous nature of the adsorption of PNP onto CEAD. On the contrary, all the values of ΔH° for Cu(II) were positive and in the range of 18−22 kJ/mol, indicating that the adsorption of Cu(II) onto CEAD is endothermic and driven by chemical reaction.29 Similarly, the negative value of ΔG° could suggest the feasibility and spontaneity toward Cu(II) adsorption. Furthermore, it was noted that ΔG° increased with temperature. It is well-known that a more negative ΔG° indicates a greater driving force.29 Therefore, the adsorption of Cu(II) was more favorable at 313 K, in accordance with ΔH°.30 3.2.3. Preloading Identification. To further identify the interaction mechanisms behind the coadsorption of Cu(II) and PNP onto CEAD, a series of preloading equilibrium experiments were also conducted. The sequential adsorption procedures were performed, and the results are shown in Figure 3a. One group of experiments, namely, CEAD-PNP + Cu, was conducted by adding Cu(II) solution to CEAD which had been fed PNP previously. The amount of PNP previously adsorbed onto CEAD was 1.94 mmol/g, indicated as bar A. After the introduction of Cu(II), it was found that 0.42 mmol/g of the adsorbed PNP was released to the solution, indicated as bar F. This release probably resulted from the displacement by 0.92 mmol/g of Cu(II), especially for hydrophilic sites, indicated as bar C. So 1.52 mmol/g of the remaining PNP on the solid phase of bar B was mainly adsorbed onto those hydrophobic sites. The total adsorption amount from the PNP preloading experiment was 2.44 mmol/g, which was the sum of bar B and bar C, and almost the same as that of the simultaneous adsorption study, 2.45 mmol/g, shown as the sum of bar D and bar E. The other group of tests, namely, CEADCu + PNP, was carried out by adding PNP to CEAD-Cu. The amount of Cu(II) adsorbed on the hydrophilic sites, indicated as bar G, was 0.93 mmol/g. After the introduction of PNP, there was almost no Cu(II) released into solution, suggesting that Cu(II) showed a higher affinity toward CEAD than PNP. Additionally, the PNP adsorption isotherms in preloading experiments also indicated that the suppression effect by Cu(II) upon PNP almost equaled that observed for simultaneously studied systems, as shown in Figure S7, Supporting Information. Hence, the results of sequential adsorption of PNP indicated that PNP effectively competed with Cu(II) for the same sorption sites. PNP could be readily displaced by

Cu(II), which possessed a stronger binding affinity and accordingly suppressed PNP adsorption. The effect of the initial concentration of PNP on Cu(II) adsorption in sequential and simultaneous adsorption studies is shown in Figure 3b. When PNP was preloaded on CEAD, the values of Kd‑pre and qe‑pre decreased with the increase in the initial concentration of PNP. As mentioned above, site competition possibly played a primary role in the preloading adsorption. Nonetheless, when Cu(II) and PNP were adsorbed simultaneously, the change in Kd‑co could be divided into two stages. In stage I, the value of Kd‑co declined and then slightly went up in stage II, suggesting that the adsorption of Cu(II) could be successfully enhanced by PNP, especially in the presence of high concentrations of PNP. In view of the foregoing, it could be predicted that CEAD had multiple active sites for the adsorption of Cu(II) and PNP. Although Cu(II) might directly compete with PNP for the hydrophilic sites, the uptake of Cu(II) could be well enhanced through bridge interaction by PNP at hydrophobic sites, which were unavailable for Cu(II). 3.3. Mechanisms of Direct Competition on Hydrophilic Sites (Site I). In brief, site recognition and competition could be responsible for the suppression of adsorption of PNP in the presence of Cu(II). In sole systems, PNP would be adsorbed onto site I due to amine and carbonyl groups, as well as the hydrophobic sites due to benzene rings (site II). However, a portion of PNP on site I could be displaced by Cu(II) in the sequential experiment, because Cu(II) exerts stronger binding affinity than PNP.31 Thus, on the basis of their different affinities toward adsorption sites, Cu(II) and PNP would recognize and occupy site I and site II in binary systems, respectively. Compared with the sole systems, the adsorption of PNP was suppressed in the binary systems due to the occupation of site I by Cu(II). Generally speaking, characterizations of the solid phase favored illuminate the interaction mechanisms at the microlevel. Thus, to reveal the preferential sites for PNP and Cu(II), FT-IR and XPS studies were performed. The FT-IR spectra of CEAD, CEAD-PNP, CEAD-Cu, and CEAD-PNP-Cu are shown in Figure S8, Supporting Information. From Figure S8b, FT-IR spectra of CEAD-PNP contain peaks attributable to the phenolic OH deformation vibration (1382.7 cm−1)19,32 and N−O asymmetric and symmetric stretching vibrations (1494.8 cm−1 and 1355.0 cm−1) of PNP.33 Meanwhile, the stretching vibration of the carbonyl group (CO) at 1647.3 cm−134 and the in-plane deformation vibration of −NH− at 1563.7 cm−135 13655

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Figure 4. XPS N1s and O1s spectra of CEAD, CEAD-PNP, CEAD-Cu, and CEAD-PNP-Cu.

shifted to 1652.6 cm−1 and 1580.2 cm−1, respectively, indicating that PNP was adsorbed on site I through carbonyl and amine groups. Similarly, in the spectra of CEAD-Cu from Figure S8c (Supporting Information), the amine group deformation vibrations shifted from 1563.7 cm−1 and 1511.5 cm−1 to 1553.6 cm−1 and 1512.9 cm−1, respectively, demonstrating that site I was also available for the adsorption of Cu(II).21 The appearance of a new peak at 1384.6 cm−1 was observed, which was attributed to the stretching vibration of NO2 in the NO3− group, indicating that some negatively charged nitrate was also adsorbed along with the Cu(II) adsorption.21,36 In addition, the XPS spectra of CEAD, CEAD-Cu, CEADPNP, and CEAD-PNP-Cu are shown in Figure 4. The N1s spectra of CEAD were dissected into two peaks at a binding energy of 399.21 and 401.02 eV, corresponding to the nitrogen in the neutral amine (−NH2 or −NH−) and protonated amine (−NH3+ or −NH2+−), respectively.37,38 As to the N1s spectra of CEAD-Cu, the peak at 399.21 eV shifted to 398.8 eV, and two new peaks at 399.8 and 406.44 eV could be assigned to the nitrogen coordinated with Cu(II) and the nitrogen in a nitrate

ion.39 These features demonstrate that amine groups participate in the adsorption of Cu(II). On the other hand, the N1s spectrum of CEAD-PNP exhibited a new peak at 406.1 eV, corresponding to the nitro group of PNP.40 Meanwhile, the peak of neutral and protonated amine shifted to 399.47 and 400.65 eV, respectively, indicating that amine groups also contribute to PNP uptake through hydrogen bonding and electrostatic interaction. Additionally, more obvious changes occurred in the spectra of CEAD-PNP-Cu because it behaved as a combination of the sole systems of Cu(II) and PNP. The above results give strong evidence that Cu and PNP compete on site I in simultaneous adsorption. The O1s spectrum of CEAD was mainly characteristic for the carbonyl group (CO). After PNP was adsorbed, two new peaks at 532.33 and 533.1 eV, related to the nitro group41 and hydroxyl group,42 appeared and the carbonyl group peak shifted from 531.76 to 531.3 eV, indicating that PNP reacts with the carbonyl group of CEAD through hydrogen bonding. The new peak at 532.41 eV in the CEAD-Cu spectrum corresponds to nitrate from Cu(NO3)2.43 13656

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Figure 5. Schematic for the synergic mechanisms behind coadsorption of Cu(II) and PNP on CEAD.

3.4. Mechanisms of Bridge Enhancement on Hydrophobic Sites (Site II). As shown in the binary systems, the adsorption of Cu(II) underwent both competition on site I and enhancement on site II. In stage I, the direct competition between Cu(II) and dilute PNP played a dominant role as the result of an obvious decrease in the distribution coefficient of Cu(II). In stage II, with more PNP molecules adsorbed, the uptake of Cu(II) would be facilitated by the interaction on new sites derived from the hydroxyl groups of PNP.19,44 However, in the preloading systems, the decline in both Kd‑pre and qe‑pre was the balance between bridge enhancement and site competition. On one hand, the hydroxyl groups of the preloading PNP, especially on site II, acted as a new kind of available site and thus improved the uptake of Cu(II). On the other hand, the competition on site I enabled the replacement of PNP by Cu(II) even if site I was occupied by PNP in advance. Although PNP showed less binding affinity on site I than Cu(II), PNP molecules could not be thoroughly displaced by Cu(II) in sequential systems, resulting in the ultimate decline of Kd‑pre and qe‑pre. The FT-IR spectrum of CEAD-PNP-Cu is shown in Figure S8d, Supporting Information. In the case of simultaneous introduction of PNP and Cu(II), there was a blue shift of the phenolic OH deformation vibration from 1382.7 cm−1 to 1385.5 cm−1 due to the formation of a Cu−OH complex.45,46 The bridge enhancement was also demonstrated in the O1s spectra of XPS. In binary systems, the binding energy of the phenolic OH at 533.1 eV shifted to 532.92 eV, demonstrating that OH of PNP reacted with Cu(II). 3.5. Interrelationship Simulation at the Molecular Level. In the sole system of PNP, namely CEAD-PNP, PNP was previously adsorbed on hydrophilic sites (site I, amine and carbonyl groups), through hydrogen bonding and electrostatic interaction, and hydrophobic sites (site II, benzene rings), through π−π interaction. Afterward, Cu(II) was introduced in the preloading system of PNP, called CEAD-PNP + Cu. PNP on site I would be displaced by Cu(II) because of site competition. The excess Cu(II) could be captured by coordinating with the hydroxyl groups of PNP on site II, therefore facilitating the adsorption of Cu(II). In the sole system of Cu(II), named CEAD-Cu, Cu(II) was adsorbed on

site I by coordination interaction. Then in the preloading system of Cu(II), named CEAD-Cu + PNP, PNP added subsequently could only recognize site II through π−π interaction, because Cu(II) showed higher adsorption affinity for site I than PNP. In binary systems, the combination of site competition, bridge enhancement, and site recognition was demonstrated to comprise adsorption mechanisms. When the concentration of PNP was low, because of an adsorption affinity of Cu(II) for amine groups higher than that of PNP, Cu(II) would occupy most of the amine active sites on the surface and PNP would only interact with benzene rings. Therefore, Cu(II) and PNP would be adsorbed onto site I and site II, respectively. When the amount of PNP increased, PNP that adsorbed onto CEAD could serve as new active sites in addition to the amine groups for Cu(II) and result in the enhancement of adsorption, which would be seen in the binary system of CEAD-PNP-Cu. Consequently, various mechanisms involving hydrogen bonding, electrostatic interaction, and π−π interaction dominated the adsorption of PNP. Cu(II) was mainly captured by coordinating with amine groups on site I as well as hydroxyl groups of PNP on site II. Hence, the interaction simulation is illustrated in Figure 5. To further quantify the different sites of CEAD, a series of dynamic experiments were carried out and results are gathered in Table S6, Supporting Information and are rationally described by eq 1 to eq 4. (a) qCEAD ‐ PNP = qΙ‐ PNP + qΙΙ‐ PNP = 1.24qII ‐ PNP + qΙΙ‐ PNP (1)

(b) qCEAD ‐ Cu = qΙ‐ Cu

(2)

(d) qCEAD ‐ Cu + PNP = qΙ‐ Cu + qΙΙ‐ PNP

(3)

(c) qCEAD ‐ PNP + Cu = (e) qCEAD ‐ PNP ‐ Cu = qΙ‐ Cu + qΙΙ‐ PNP + qΙΙ‐ PNP ‐ Cu = qΙ‐ Cu + qΙΙ‐ PNP + 13657

1 q 7.7 I ‐ Cu

(4)

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after 10 recycles, indicating the excellent adsorption stability of CEAD. 3.8. Environmental Implication. This study provided a novel polyamine chelating resin CEAD, which contained both hydrophilic and hydrophobic sites and exhibited excellent adsorption of Cu(II) and PNP. The results of batch adsorption tests showed that site recognition, site competition, and bridge enhancement were the predominant adsorption mechanisms. With an increase of PNP, the adsorption of Cu(II) was enhanced by more than 7% due to the bridge enhancement where hydroxyl groups of PNP could create additional chances to coordinate with Cu(II). Furthermore, this enhancement in the removal of Cu(II) could be easily extended to other phenol derivatives (such as p-chlorophenol, p-hydroxybenzaldehyde, pentachlorophenol, etc.) which contain hydroxyl groups, as shown in Figure S13, Supporting Information. Even more interesting was that ionic strength exerted some positive influence especially on the removal of Cu(II). In addition, PNP and Cu(II) could be individually and successfully recovered with dilute NaClO3 and HCl solutions. The superior properties demonstrated by CEAD indicated that it could be applied to wastewaters containing both Cu(II) and PNP, even for high saline aqueous media.

For PNP, the maximum adsorption capacities of CEDA-PNP + Cu and CEAD-PNP correspond to the number of site II and the total adsorption sites (site I + site II), respectively. The amount of site I was 1.24 times that of site II. For Cu(II), the maximum adsorption of CEAD-Cu and CEAD-PNP-Cu represents the amount of site I and the total sites (site I + new sites from PNP), respectively. The amount of adsorbed Cu(II) on site I was about 7.7 times that on site II. The exposed hydroxyl groups of four adjacent PNP molecules which adsorbed on site II would create new adsorption sites for Cu(II) by forming [PNP-Cu] complexes, which was in accordance with previous reports. 2,47 Nevertheless, the heterogeneity of surface functional groups of CEAD resulted in the random distribution of adsorbates, which was proved in the adsorption isotherms of PNP. The steric hindrance made it difficult for a part of PNP to participate effectively and thus strengthened the capture of Cu(II). Therefore, there was only one Cu(II) ion facilitated by PNP among nine adsorbed Cu(II), as shown in Figure S9, Supporting Information. 3.6. Influence of Ionic Strength. Considering that abundant salt additives are often present in wastewaters, it was important to examine the performance of CEAD under high ionic strength.48 As shown in Figure S10 (Supporting Information), the adsorption capacities for Cu(II) increased by above 54% and 32%, respectively, with the increase in concentration of NaNO3 and NaCl from sole and binary systems, which can be well explained by the dual function of anions such as Cl− and NO3−. First, the anions might improve Cu(II) adsorption by allowing Cu(II) to easily replace more of the neutralized hydronium ions from the ligands. Second, the anions probably increased the amount of active sites by maintaining electroneutrality along the ligands, thus preventing the repulsion between Cu(II) particularly from adjacent sites.49 Moreover, the variations of ionic strength put a small negative effect on the adsorption amount of PNP. This inhibition could also contribute to the replacement of adsorbed anionic PNP by anions. These promising trends were extremely interesting in regard to the coremoval PNP and Cu(II) with CEAD, especially from a high saline waste stream. 3.7. Dynamic Simultaneous Removal and Sequential Recovery of PNP and Cu(II). Simultaneous removal and sequential recovery of PNP and Cu(II) were observed in dynamic processes. Typical effluent concentration curves for PNP and Cu(II) with CEAD are shown in Figure S11a, Supporting Information. The saturation adsorption of PNP and Cu(II) was obtained at around 1140 and 3060 mL, respectively. The regeneration study of CEAD was implemented in which 1.5 mol/L of NaClO3 solution at pH of 8 was first applied to recycle PNP. Then Cu(II) was recovered with 4 mol/L of HCl solution. From Figure S11b (Supporting Information), in the salt-desorption stage, more than 98% of PNP on the solid phase was desorbed with 7500 mL of NaClO3 solution without Cu(II) being released. PNP may be replaced by a high concentration of ClO3−, leading to the successful separation of PNP molecules from resin matrix to the liquid phase. Subsequently, more than 99% of Cu(II) was desorbed with 1500 mL of HCl, as the protonation of NH2/NH in the presence of a high concentration of protons would be adverse to the uptake of Cu(II) from CEAD. Then 10 additional adsorption−regeneration cycles were performed, as shown in Figure S12, Supporting Information. The adsorption capacities for PNP and Cu(II) remained within the values of 0.874 and 1.001 mmol/g, and the desorption rates were still above 98%

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ASSOCIATED CONTENT

S Supporting Information *

Additional methods description and figures/tables. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 139 1387 1032; fax: +86 25 89680377; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge generous support provided by the National Natural Science Foundation of P. R. China (grant no. 51078178), the Resources Key Subject of National High Technology Research & Development Project (863 Project, grant nos. 2009AA06Z315 and SQ2009AA06XK1482331) of P. R. China, the State Key Program of National Natural Science (grant no. 50938004), and the Discipline Crossing Foundation of Nanjing University.

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