ARTICLE pubs.acs.org/Langmuir
pH Manipulation: A Facile Method for Lowering Oxidation State and Keeping Good Yield of Poly(m-phenylenediamine) and Its Powerful Ag+ Adsorption Ability Liyuan Zhang, Liyuan Chai, Jin Liu, Haiying Wang,* Wanting Yu, and Peilun Sang Department of Environmental Engineering, School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China
bS Supporting Information ABSTRACT: A method of pH manipulation has been used to improve chemically oxidative polymerization of m-phenylenediamine (mPD) through concurrent addition of NaOH when adding oxidant (NH4)2S2O8. pH detection and open-circuit potential technique were adopted to monitor the polymerization process of mPD and to explain the oxidation statepH and yieldpH relationships. Results from Fourier transformed infrared (FTIR) and X-ray photoelectron (XPS) spectroscopies indicate that a low oxidation state is under control by regulating NaOH concentration. At 2.5 M NaOH, the oxidation state of poly(m-phenylenediamine) (PmPD) is 64.7 mol % (measured by molar content of quinoid imine from XPS), while the yield is 84%. The synthesized PmPD possesses better Ag+ adsorption performance when lowering its oxidation state. Moreover, the Ag+ adsorbance of PmPD can reach 1693 mg g1. Meanwhile, Ag+ adsorption mechanism was studied by pH tracking, X-ray diffraction (XRD) patterns, and X-ray photoelectron spectroscopy. The adsorption process includes redox reaction, chelation, and physical adsorption.
1. INTRODUCTION Ag+ pollution treatment has gradually become an important topic in environmental protection area due to the emission of Ag+-containing wastewater from industries and high value of recovering Ag.1 At present, one of the most adopted techniques for treating Ag-containing wastewater is adsorption.1 Most recently, to develop effective adsorption materials, aromatic amine/diamine polymers were taken as excellent candidates due to their powerful redox reversibility and chelation ability.2 For these polymers, poly(phenylenediamine)s are considered to own a brilliant prospect owing to the advantage of cost-effective production3 and their good performance to treat metal ions. For instance, Li et al. have found that the poly(phenylenediamine)s have strong ability to adsorb not only Pb2+, Hg2+ but also Ag+.46 Particularly, as a substantial member of poly(phenylenediamine)s, poly(m-phenylenediamine) is outstanding because of its convenient preparation by chemically oxidative polymerization7,8 and the best Ag+ adsorption ability among the poly(phenylenediamine)s.2 But the shortage is that PmPD’s performance for Ag+ adsorption is relatively poor2 compared with other well-known materials, e.g., melamine,9 poly(naphthalenediamine)s,10,11 etc. Generally, aromatic amine/diamine polymers are believed to consist of quinoid imine-like (y mol %) and benzenoid amine-like (1 y mol %) units.12 The molar content (y mol %) of quinoid imine-like units can be used to measure the oxidation state of the polymer;13,14 e.g., increasing y mol % rises the oxidation state. r 2011 American Chemical Society
Recent studies have demonstrated that lowering their oxidation state is beneficial for enhancing the adsorption performance of aromatic amine/diamine polymers for metal ions with oxidizabillity.15 Li et al. have reported that Ag+ adsorptivity of poly(o-phenylenediamine) and Cr(VI) adsorptivity of polyaniline were improved when lowering their oxidation state.3,16 However, one sticky problem still existed in current researches that the chemically oxidative polymerization, the most suitable way to prepare aromatic amine/diamine polymers adsorbents,10,11 cannot maintain its native advantage of high yield synthesis when lowering the oxidation state.3,1720 Herein pH manipulation experiment was designed to improve the chemically oxidative polymerization of mPD. The function of pH manipulation was elucidated with the aid of pH tracking and the open-circuit potential technique. Ag+ adsorption with PmPD and the adsorption mechanism were investigated.
2. EXPERIMENTAL SECTION 2.1. Materials. Ammonium persulfate (APS), m-phenylenediamine (mPD), sodium hydroxide (NaOH), hydrochloric acid (HCl), N-methyl2-pyrrolindone (NMP), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), Received: March 21, 2011 Revised: September 28, 2011 Published: October 04, 2011 13729
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Scheme 1. Ideal Chemically Oxidative Polymerization of m-Phenylenediamine without Denitrogenation and pH Manipulation Based on Previous Research
dimethylformamide (DMF), ethanol (EtOH), ammoniawater, and silver nitrate were of analytical grade.
2.2. Polymerization of m-Phenylenediamine with the Aid of pH Manipulation. The typical procedure for the preparation is as follows: 10.0 g of mPD was dissolved in 175 mL of distilled water in a 500 mL of five-necked round-bottom flask at 25 °C, and the monomer solution was stirred for 30 min prior to the polymerization. 21.1 g of APS (oxidant/monomer ratio is ∼1) was added to 55 mL of distilled water to give an oxidant solution. The polymerization was initiated by the addition of oxidant solution at a rate of one drop per second. Synchronously, 55 mL of acid or base solution was added dropwise into the reaction system to manipulate the polymerization pH. Then, the reaction mixture was stirred for another 5 h at 25 °C. Simplified equipment drawing is shown in Figure S1 (Supporting Information). The precipitate was collected by filtration and rinsed with distilled water, ammoniawater (1:1, v/v), and absolute ethanol to remove the oligomers, inorganic salts, and other impurities, respectively. Thereafter, the product was dried at 60 °C under vacuum for 12 h. According to the acid or base used, the product was denoted as PmPD(HClx) or PmPD(NaOHx), respectively, where x represents the concentration of the acid or base. The polymer synthesized with no pH manipulation was named as PmPD(NM). The reaction between oxidant and monomer without pH manipulation was briefly illustrated in Scheme 1 based on the previous literatures.13,17,2125 In order to monitor the process of polymerization under the interference of pH manipulation, a pH digital meter and an open-circuit potential (OCP) technique using a saturated calomel electrode as reference electrode and a Pt electrode as working electrode were employed during the polymerization. 2.3. Ag+ Adsorption. The investigation for the capability of PmPD for the Ag+ adsorption was conducted. 25 mg of PmPD was added into 20 mL of AgNO3 aqueous solution with a specific concentration at 30 °C for 24 h. The initial pH of the solution was controlled with HNO3 or NaOH. Afterward, the solution containing the solid matters was filtrated to separate the filtrate and filter residue. The Ag+ concentration in the filtrate was detected by inductively coupled plasma when its concentration was lower than 10 mM or by the Volhard titration method when higher than 10 mM. The filter residue was collected for further characterization. It is worthy of note that PmPD(NaOH2.5)PmPD(NaOH5) are slightly soluble in Ag+ solution as indicated by the light brown color of the filtrate. This is negative for the adsorption since it would introduce the impurity into the solution. Also, the soluble polymers in the filtrate do disturb the detection of the Ag+ concentration by inductively coupled plasma. Hence, the investigation of the adsorption performance of PmPD(NaOH2.5)PmPD(NaOH5) was not involved in current study.
Competitive adsorption was carried out with identical conditions as above besides introducing coexisting metal ions. The initial concentration of the metal ions is 10 mM. The concentration of metal ions was measured with inductively coupled plasma. 2.4. Characterization. Fourier transformed infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were applied to analyze the structure of the product. For FTIR the PmPD powder was blended with KBr to be pressed into a disk, and the FTIR spectra were recorded on a Nicolet IS10 spectrometer at 4 cm1 resolution. The XPS spectra were obtained in an ESCALab220i-XL analyzer with an Al KR X-ray source (power: 300 W). All binding energies were referenced to the neutral C 1s peak at 284.6 eV to compensate for the surface charging effect. The software Xpeak 4.1 was applied to fit the spectra.26 The full width at half-maximum for N and Ag element was kept at 1.5 and 1.2, respectively.27,28 Meanwhile, thermal stability of the product was investigated by the SETSYS Evolution thermo analyzer under N2 and air atmosphere at a heating rate of 20 °C min1. The XRD pattern was collected on a D/ Max 2500 VB+X X-ray diffractometer using Cu (40 kV, 300 mA) radiation. The solubility for the product was semiquantitatively examined as follows:29 25 mg of polymer was added into 10 mL of solvent which was shaken for 24 h at room temperature. The solubility was calculated by comparing the weights of the initial polymer with the soluble part. Molecular weight was measured with gel permeation chromatography (GPC) by PL-GPC120 instrument by using DMF as mobile phase and polystyrene as standard samples. Surface area of PmPD was measured by adsorption of ultrapure N2 on the ASAP2020M+C analyzer by the BET method. An electrochemical workstation (LK2005) was purchased from Tianjin Lanlike Co. Ltd. pH digital indicator (pHS-3C) was purchased from Shanghai Leichi Co. Ltd.
3. RESULTS AND DISCUSSION 3.1. Function of pH Manipulation. Figure 1 gives pH (A) and OCP (B) curves of polymerization with and without pH manipulation. As shown in Figure 1A, without pH manipulation the reaction pH rapidly dropped from 8.5 to 1. This is owing to the H+ release during the polymerization,22 as illustrated in Scheme 1. When introducing pH manipulation via adding HCl, pH decrement was intensified. But this decrement was restrained by adding NaOH. Generally, the rise in acidity makes the protonation of monomers easier, which apparently lowers the electron density of mPD and then their oxidation reactivity.30 Consequently, raising the solution pH via NaOH addition inevitably tends to increase the monomer reactivity and in turn 13730
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Figure 1. pH (A) and open-circuit potential (B) versus reaction time. The color correlates the lines with the concentration of HCl or NaOH used. NM represents no pH manipulation.
facilitates the oxidation polymerization.31 In other words, adding NaOH for pH manipulation may be beneficial to maintain a good yield of PmPD. More information on the function of pH manipulation was given from the discussion of the OCP data (Figure 1B) with the help of the Nernst equation32,33 OCP ¼ E0 þ ðRT=2FÞ ln½ðCpersulfate =Csulfate 2 ÞCred þ 2 0:05916pH
ð1Þ
where the equation establishment is ideally based in Scheme 1A, E0 is the standard polymerization potential, R is gas constant, 2 is electron number, T is absolute temperature, and Cpersulfate, Csulfate, and Cred are the bulk concentrations of persulfate, sulfate, and reducers, respectively.34 Polymers are hypothesized to be insoluble due to their highly limited solubility in water according to previous literature.12 Thus, its bulk concentration is 1. As seen in Figure 1B, the polymerization OCP without pH manipulation increased with a few drops of oxidant initially (070 s) and dropped (70250 s). Then it increased again and reached maximum 0.4 V at 1230 s, and at this time all of the oxidant was added. Thereafter the OCP declined to a constant value 0.35 V. As a whole, the overall OCP increased with adding oxidant. Additionally the pH decreased during the polymerization. Hence, as deduced from eq 1, item (Cpersulfate/Csulfate2)Cred increased when adding oxidant. Because of the oxidation of monomers, the Cred should constantly decrease, and therefore the item Cpersulfate/Csulfate2 accordingly increased. This indicates the bulk concentration of persulfate and sulfate changed during the polymerization, which may alter the oxidation property of persulfate according to the Nernst equation Er ¼ E01 þ ðRT=2FÞ lnðCpersulfate =Csulfate 2 Þ
ð2Þ
Figure 2. FTIR characterization of PmPD synthesized by chemically oxidative polymerization with pH manipulation. Labels a, b, and c indicate the peaks at ∼1620, ∼1500, and ∼1257 cm1, respectively. The color correlates the lines with the concentration of HCl or NaOH used. NM represents no pH manipulation.
where the equation establishment was based on Scheme 1B. The representation of R, T, and F is the same as that of eq 1. E01 is the standard potential of persulfate. Cpersulfate and Csulfate are the bulk concentrations of persulfate and sulfate, respectively. Er is the reduction potential of persulfate. The Er is directly affected by Cpersulfate and Csulfate,35 and from eq 2 the increase of Cpersulfate/Csulfate2 means the rise of persulfate’s reduction potential. Moreover, when the pH decrement was intensified by HCl addition, the overall OCP was increased compared with the polymerization without pH manipulation, indicating the oxidizability of persulfate became higher than that without manipulation. In contrast, when restraining the pH decrement, typically by increasing the NaOH concentration, the overall OCP became lower comparing to the polymerization without pH manipulation. That is to say the persulfate’s oxidizability was comparatively dropped with increasing the NaOH concentration during pH manipulation. It is commonly believed that the extent of the oxidation of benzenoid amine to quinoid imine weaken when the oxidizability of oxidant decrease.10 Hence, the oxidation state of PmPD obtained was potentially increased by promoting the persulfate’s oxidizability as adding HCl to strengthen pH decrement, while it can be possibly lowered as the oxidizability was decreased by adding NaOH to restrain pH decrement. Wholly speaking, pH manipulation through adding NaOH can possibly improve the polymerization to lower the oxidation state and maintain a good yield of PmPD. 3.2. Efficiency of pH Manipulation on Improving the Chemically Oxidative Polymerization. Herein to directly study the effect of pH manipulation on the polymers, FTIR and XPS 13731
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Figure 3. XPS N 1s core-level spectra of PmPD(NM), PmPD(NaOH2.5), and PmPD(NaOH5).
were applied to investigate the PmPD’s oxidation state. Figure 2 gives the FTIR spectra of PmPD. The broad adsorption between 3500 and 3000 cm1 represents the stretching vibration of NH.3638 The two peaks around 1620 and 1500 cm1 are owing to the stretching mode of quinoid imine and benzenoid amine units, respectively.12 The adsorption band at ∼1257 cm1 is attributed to the CN stretching vibration in the benzenoid amine units.39 Profoundly, with restraining pH decrement by increasing NaOH concentration, the intensity of the benzenoid amine and CN stretching mode of PmPD increased relatively as comparing to that of the quinoid imine. However, the reverse tendency happens when adding HCl for pH manipulation. This is strongly indicative that pH manipulation through NaOH addition has a prominent effect on lowering the oxidation state of PmPD. Furthermore, the XPS technique was taken to examine the N element of PmPD(NM) (A), PmPD(NaOH2.5) (B), and PmPD(NaOH5) (C) to study the relative molar content of benzenoid amine and quinoid imine (Figure 3). As seen in Figure 3, the peak at 399.2 eV is attributed to the neutral N= in the quinoid imine units while the one at 400.2 eV is due to the NH in the benzenoid amine units.27 According to the quantitative calculation of the molar content of NH and N=, the molar percentage of NH and N= for PmPD(NM) is 27.8 and 72.2 mol %, respectively. However, as for other two samples “PmPD(NaOH2.5), Figure 3B, and PmPD(NaOH5), Figure 3C”, the molar percentage of NH increased apparently to 35.3 and 43.3 mol %, respectively, and that of N= decreased accordingly to 64.7 and 56.7 mol %. Evidently, the higher NaOH concentration to restrain the pH decrement contributes to the marked decrement of the oxidation state.
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Besides the oxidation state, the yield of PmPD was calculated following the equation described in the literature.2 The data are listed in Table 1. The yield increased to 72% when using 1 M NaOH for manipulation and further to 84% when adding 2.5 M NaOH, compared to the 65% for PmPD(HCl1) and 69% for PmPD(NM). This, as stated above, is owing to the restraint of pH decrement by NaOH, which promotes the monomer reactivity to significantly raises the polymerization efficiency of mPD and thus the yield of PmPD.30 Nevertheless, when the NaOH concentration increased to 3.0, 4.0, and 5.0 M, the yield drops from 75% [PmPD(NaOH3)] to 64% [PmPD(NaOH4)] and to 45% [PmPD(NaOH5)]. That means further restraining the pH decrement by increasing the NaOH concentration is not beneficial to improve the polymerization. The overincrement of solution pH probably makes the persulfate’s oxidizability too weak to effectively initiate the polymerization and thus lower the yield. Anyway, according to the analysis of FTIR and XPS and the calculation of yield, applying pH manipulation to appropriately restrain pH decrement can satisfactorily improve the chemical polymerization for retaining good yield of PmPD and lowering its oxidation state. Previously, the pH-stat techniques were used to restrain the pH decrement during the chemically oxidative polymerization.4047 However, those researches focused on polyaniline nanostructures or polymerization kinetic of aniline and did not involve the chemical polymerization of aromatic diamine. The yieldpH and oxidation statepH relationship of aromatic diamine polymers, typically PmPD, was not considered in their studies. 3.3. Physical Characteristics of Poly(m-phenylenediamine). Supermolecular Structure. Macromolecular arrangement has a great impact on the adsorption performance of the polymers.2,3 Herein, XRD characterization of PmPD was performed. As revealed clearly from Figure 4, the nine polymers exhibit a broad diffraction peak between 18° and 25°. This is the typical characteristics for amorphous structure.2 The diffraction peaks for PmPD(HCl1) and PmPD(NM) are the same centered at ∼23°. The broad peak shifts to ∼19° for PmPD(NaOH1)PmPD(NaOH5), suggesting the drop of the accumulating density and thus the increasing spacing and the amorphousness of the macromolecular chains compared with PmPD(HCl1) and PmPD(NM).29 It is commonly believed that amorphous structure favors the penetration and then adsorption of ions onto the macromolecules mainly attributing to the loose and disordered piles of the polymeric chains in the amorphous state.2,3 Hereby, the enhanced amorphousness for PmPD(NaOHx) than that for PmPD(HCl1) and PmPD(NM) may be more suitable adsorption conditions. Specific Surface Area. Besides the supermolecular structure, specific surface area is another substantial parameter for adsorption application. Whereby, the surface area of PmPD was tested and the result is given in Table 1. As shown in Table 1, the specific surface area of PmPD(HCl1), PmPD(NM), and PmPD(NaOH1) PmPD(NaOH2) was fluctuated in the range 1416.5 m2 g1. That means increasing NaOH concentration for restraining pH decrement has a slight influence on the surface area. However, the surface area of PmPD(NaOH2.5)PmPD(NaOH5) was on average 7 m2 g1, and this may be ascribed to the overincrease of NaOH concentration for pH manipulation. 3.4. Solubility and Thermal Stability of Poly(m-phenylenediamine). Solubility and thermal stability are both substantial properties for aromatic amine/diamine polymers. So these two properties of PmPD are investigated in this section. Solubility. Table 1 gives the solubility of PmPD in EtOH, THF, DMSO, NMP, and DMF. As can be seen, the solubility of 13732
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Table 1. Yield, Surface Area, Solubility, and Performance of PmPD Synthesized by Chemically Oxidative Polymerization with pH Manipulation adsorbabilitya
solubility (%) yield (%)
surface area (m2 g1)
HCl1
65
14.1
0
NM
69
14.6
0
NaOH1 NaOH1.5
72 77
13.9 16.4
NaOH2
81
NaOH2.5
84
NaOH3 NaOH4 NaOH5
PmPD(name)
EtOH
THF
DMSO
DMF
0
0
0
0
18
0
2
0
0
22.7
292
0 0
0 0
1.2 2.4
3.6 2
0 0
36 41.1
464 537
13.7
0
0
22
9.2
1.7
49.9
4.6
0
0
46.4
44.4
3.1
75
7.2
3
3.2
50.4
54
25
b
64
7.9
3.6
4.4
70
64.8
41.1
b
45
6.3
3
3.2
100
93.2
b
100
adsorptivity (%)
adsorbance (mg g1)
NMP
231
640 b
a
The conditions for the adsorption of Ag+ onto PmPD particles: 15 mM of Ag+ solution (20 mL), initial solution pH 5, adsorption time 24 h, temperature 30 °C, dosage of PmPD 25 mg. b The performance of PmPD(NaOH2.5)PmPD(NaOH5) was not tested in current researches due to the increased solubility of them in Ag+ solution.
Table 2. Gel-Permeation Chromatography Data of PmPD(NaOH3)PmPD(NaOH5)
Figure 4. X-ray diffractograms of PmPD in various oxidation states.
the nine samples is very poor in EtOH and THF, which on average is less than 3%. When using DMSO, NMP, and DMF as solvents, the solubility for PmPD(HCl1), PmPD(NM), and PmPD(NaOH1)PmPD(NaOH1.5) was not improved. But as for PmPD(NaOH2)PmPD(NaOH5), their solubility in these solvents has an obvious promotion. This is particularly true for PmPD(NaOH3)PmPD(NaOH5). The solubility reached 51% in NMP, 54% in DMSO, and 25% in DMF for PmPD(NaOH3) and increased to 70% in NMP, 65% in DMSO, and 41.1% in DMF for PmPD(NaOH4). For PmPD(NaOH5), their solubility in these solvents then all significantly surpassed 90% at room temperature. This indicates NMP, DMSO, and DMF are all good solvents for PmPD synthesized by pH manipulation using a high concentration of NaOH. It is generally accepted that molecular weight is one of the important factors affecting the solubility of conducting polymers.3,12 Hence, the molecular weight of PmPD(NaOH3) PmPD(NaOH5) was examined by gel-permeation chromatography,
PmPD(name)
Mn
Mw
PD
NaOH3
3984
6365
1.59
NaOH4
2556
3830
1.49
NaOH5
2264
3329
1.47
and data are listed in Table 2. The three samples have relatively low molecular weight, which is less than 4000 in Mn and 7000 in Mw. The Mn and Mw of PmPD decreased gradually from 3984 and 6365 [PmPD(NaOH3)] to 2556 and 3830 [PmPD(NaOH4)] and further to 2264 and 3329 [PmPD(NaOH5)], respectively. Meanwhile, the PD decreased from 1.59 [PmPD(NaOH3)] to 1.49 [PmPD(NaOH4)] and to 1.47 [PmPD(NaOH5)]. In common, the solubility of conjugated materials becomes higher when their molecular weight drops. So the decreased molecular weight of PmPD prepared by adding high concentration of NaOH is the main reason for the solubility improvement. This is mainly because of the low oxidizability of oxidant when using high NaOH concentrations for pH manipulation, which lowers the polymerization efficiency. That is to say, the high concentration of NaOH is not beneficial for obtaining the product with a high polymerization degree. Thermal Stability. Figures S2 and S3 (Supporting Information) provide the TG (A) and DTA (B) curves of PmPD under N2 and air atmosphere, respectively. From Figure S2, the TG curve (A) of PmPD(HCl1) shows slight weight loss between 50 and 180 °C, owing to the evaporation of water from the polymer.39,48,49 Then, another weight loss occurs at the temperature range 1801200 °C and with a mass loss averaged 29% at ∼700 °C and 43% at ∼1050 °C. This weight loss is associated with the degradation of main chains.39 Compared with PmPD(HCl1), other PmPDs exhibit the similar TG curves. Meanwhile, the nine DTA curves (Figure S2B) show an endothermic peak among 50180 °C, proving the evaporation of water.17 Also, a broad exothermic peak at ∼1000 °C was found. The exothermic peak should be ascribed to the complex chemical reaction, causing the degradation of PmPD. By contrast with that in N2, the thermal behavior of PmPD in air exhibits a similar mass loss at 13733
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Table 3. Effect of Coexisting Ions on Ag+ Adsorption of PmPD(NaOH2)a ion concentration [mM] ions
before adsorption
after adsorption
ion adsorptivity [%]
Ag+
10
3.65
63.5
Ag+/Pb2+
10/10
4.78/8.48
52.2/15.2
Ag+/Zn2+ Ag+/Li+
10/10 10/10
3.8/9.08 3.82/9.92
62/9.2 61.8/0.8
a Adsorption conditions: 20 mL solution, 25 mg of PmPD(NaOH2), 24 h, 30 °C.
+
Figure 5. Effect of initial pH on Ag adsorption with PmPD(NM) at 30 °C in AgNO3 solution of 20 mL with sorbent dosage of 25 mg for 24 h.
Figure 6. Effect of initial Ag+ concentration on the Ag+ adsorption with PmPD(NaOH1)PmPD(NaOH2) at 30 °C in AgNO3 solution of 20 mL with initial pH 5 and sorbent dosage of 0.025 g for 24 h.
the range 50180 °C, ascribing to the water evaporation, but a lower degradation temperature from 350 to 730 °C (Figure S3A). Correspondingly, an endothermic peak at ∼150 °C representing water evaporation and an exothermic peak at ∼650 °C standing for chain degradation were observed from DTA of PmPD (Figure S3B). According to the analysis, the thermostability of PmPD in N2 and air is good. But the product in air is less thermostable than that in N2. 3.5. Ag+ Adsorption Ability of Poly(m-phenylenediamine). Effect of Initial pH. Ag+ adsorption was measured with initial pH value from 2.0 to 7.0 (Figure 5). The adsorptivity was calculated according to the equations described in the literature.50 As shown in Figure 5, when lowering the pH from 3.9 to 2.0, the adsorptivity declines obviously from 22.7% to only 11%. It means that the acidic condition is not beneficial for Ag+ removal. But the difference of the adsorptivity at pH 3.9 and 7.0 is only 1%, suggesting the weak acidic condition influences slightly the Ag+ adsorption. Consequently, the solution pH at ∼5.0 is suitable for the Ag+ adsorption.3 Ag+ Adsorption Ability of Different Poly(m-phenylenediamine). PmPD(HCl1), PmPD(NM), and PmPD(NaOH1)PmPD(NaOH2) were taken to treat the Ag+ solution (Table 1). In addition to adsorptivity, the adsorbance was also calculated based on the literature.50 As listed in Table 1, the adsorptivity of
PmPD(HCl1) and PmPD(NM) is ∼20% while the adsorbance is about 262 mg g1. Yet, the Ag+ adsorptivity enhances distinctly in sequence from 22.7% [PmPD(NM)] to 36% [PmPD(NaOH1)], to 41.1% [PmPD(NaOH1.5)], and further to 49.9% [PmPD(NaOH2)]. Typically, the highest adsorbance among these five products was calculated to be 640 mg g1 for PmPD(NaOH2), which is 2 times more than that of PmPD(NM) and PmPD(HCl1). It suggests that PmPD synthesized with the aid of pH manipulation through adding NaOH obviously improves the Ag+ adsorption performance. Meanwhile, the physical characteristics and oxidation state of conducting polymers both influence the adsorption performance. These five samples bear similar physical characteristics (supermolecular structures and surface areas) but different oxidation state, indicating that the oxidation state is one main reason for different performance of various samples.3 Effect of Initial Ag+ Concentration. Figure 6 shows the Ag+ removal ability of PmPD(NaOH2), PmPD(NaOH1.5), and PmPD(NaOH1) under different initial Ag+ concentration (5200 mM). For PmPD(NaOH2) the adsorbance increases apparently from 262 to 725 mg g1 in the concentration range 525 mM. Then the enhancement of the adsorbance gradually decreases when further increasing the concentration from 25 to 200 mM. The adsorbance can increase to 1110 mg g1 (90 mM) and finally reach 1693 mg g1 (200 mM). For PmPD(NaOH1.5) and PmPD(NaOH1), the adsorbance also increases under relatively high Ag+ concentrations. Nonetheless, the highest adsorbance for PmPD(NaOH1.5) and PmPD(NaOH1) was 1338 and 1049 mg g1, respectively, which were lower than that of PmPD(NaOH2). Some materials have been used for Ag+ adsorption, such as bisthiourea-formaldehyde resin (745 mg g1),51 electrooxidized carbon fiber (399.6 mg g1),52 poly(p-/o-phenylenediamine)s (373/540 mg g1),2,3 and polyaniline (106 mg g1)53 and so on. These materials, however, possess apparently lower adsorbance, which is commonly less than 800 mg g1. Moreover, the 1693 mg g1 of PmPD(NaOH2) is close to those of sulfodiphenylamine and diaminonaphthalene copolymer microparticles and poly(anilineco-5-sulfo-2-anisidine) nanosorbents having the highest Ag(I) adsorbance of 2000 and 2034 mg g1.11,54 Consequently, with the low cost of raw materials for the synthesis under pH manipulation, a much more promising prospective of PmPD, especially PmPD(NaOH2), for Ag+ adsorption could be anticipated. Competitive Adsorption. The effect of coexisting ions (Pb2+, Zn2+, and Li+) on Ag+ adsorption was examined (Table 3). PmPD(NaOH2) was used here considering its better performance. Since the adsorbance of PmPD(NaOH2) is high, there will still leave many adsorption sites after treating one kind of metal ion in low concentration. This would possibly promise a good performance for other metal ions, which interferes the 13734
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Figure 7. XPS N 1s core-level spectra of PmPD(NM) adsorbing Ag+ (A), PmPD(NaOH2) before (C) and after (D) adsorbing Ag+; XPS Ag 3d5/2 corelevel spectra of PmPD(NM) (B) and PmPD(NaOH2) (E) adsorbing Ag+. Adsorption conditions: 20 mL of 15 mM Ag+, 25 mg of PmPD(NaOH2), 24 h, 30 °C.
Table 4. Amount of Ag+ and Ag on PmPD Calculated from XPS N 1s Data and Ag 3d5/2 Dataa from N 1s data (mg)
from Ag 3d5/2 data (mg)
+
Ag on
Ag on
Ag+ on
Ag on
PmPD
PmPD
PmPD
PmPD
PmPD(NM)
0.8
1.6
4
PmPD(NaOH2)
2.3
6
1.8
PmPD(name)
3.2 14.2
a Adsorption conditions: 20 mL of 15 mM Ag+, 25 mg of PmPD(NaOH2), 24 h, 30 °C.
judgment of the competitive ability of the target ions. Therefore, to evaluate the competiveness of Ag+ during the adsorption, a relatively high concentration (10 mM) of these four ions was selected. It can be seen from Table 3 that for Ag+/Zn2+ and Ag+/ Li+ removal the adsorptivity of Zn2+ and Li+ is lower than 10%, while that of Ag+ was averagely 61.9%, which is close to 63.5% in the condition without introducing other ions. When taking Pb2+ as competitive ions, the Pb2+ adosorptivity is only 15.2% but the Ag+ adsorptivity decreased about 10%. That means Pb2+ can moderately affect the Ag+ adsorption. Anyway, it can be concluded that PmPD(NaOH2) synthesized in this paper exhibits a good selectivity for Ag+ adsorption but Pb2+ has a gentle impact on Ag+ adsorption. Mechanism of Ag+ Adsorption with PmPDs. To clarify the adsorption mechanism and moreover the influence of oxidation state on the mechanism, PmPD(NM) and PmPD(NaOH2) before and after adsorption were characterized by XPS combining auxiliary methods, e.g., pH-tracking and XRD. The Ag 3d5/2 and N 1s data (XPS) of samples before and after adsorption and relevant calculation are given in Figure 7 and Table 4 (note: calculations were done by reference to the Stejskal and Li methods3,55).
Ag+ adsorption by PmPD(NM) in relatively high oxidation state was first discussed. A major adsorption condition was initial Ag+ concentration of 15 mM and initial pH of 5. As from Figures 3A and 7A, 3.2 mol % of benzenoid amine was transformed to quinoid imine after adsorption, proving the redox reaction between benzenoid amine and Ag+,5666 which was depicted in Scheme 2A. This can be further demonstrated by pH tracking and XRD.67 As given in the Supporting Information, after Ag+ adsorption the solution pH dropped obviously (Table S1) and the XRD pattern of PmPD displays clear signals of Ag crystals (Figure S4). The decrease of pH is from the oxidation of benzenoid amine with H+ release while Ag is the result of Ag+ reduction. In addition, 3.1 mol % of N+= was found from Figure 7A, indicating imine took part in adsorption through chelation,6874 as illustrated in Scheme 2B. According to reactions in Scheme 2A,B and variation (mol %) of benzenoid amine and N+=, the calculation was carried out following eqs 3 and 4 WAg ¼ 4:8 104 ΔðNHÞ 107:87 1000
ð3Þ
WAgþ ¼ 4:8 104 CðNþ ¼ Þ 107:87 1000 0:5
ð4Þ
where the deduction of equation establishment is shown in the Supporting Information “Calculation Method Deduction”, WAg and WAg+ are the amount of Ag+ and Ag on PmPD (mg), respectively, Δ(NH) is the difference value of amine content (mol %, XPS) before and after adsorption (Figures 3A and 7A,C,D), C(N+=) is the content of imine participating in the adsorption (mol %, XPS) (Figure 7A,D), and 107.87 is the atomic weight of Ag. Results showed that 1.6 and 0.8 mg Ag+ were theoretically reduced and chelated, respectively, as shown in Table 4 (supposed no imine was protonated by H+). From Figure 7B the molar content of Ag and Ag+ on PmPD(NM) was 44.1 and 55.9 mol %, respectively. On the basis of these data and Ag+ adsorbance of PmPD(NM), the real amount 13735
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Scheme 2. Reaction of Benzenoid Amine and Quinoid Imine with Ag+
of Ag and Ag+ on PmPD(NM) was calculated with eqs 5 and 6 W 0Ag ¼
M 25 CAg 1000
W 0Agþ ¼
M 25 CAg 1000
ð5Þ
ð6Þ
where W0 Ag+ and W0 Ag are the real amount of Ag+ and Ag on PmPD (mg), respectively, M is adsorbance (mg g1), 25 is the dosage of PmPD (mg), and CAg+ and CAg are content of Ag+ and Ag on PmPD detected by XPS (mol %) (Figure 7B,E). As given in Table 4, the results, however, were 3.2 mg of Ag and 4 mg of Ag+ on PmPD, respectively. They obviously exceed that reckoned from XPS N 1s data. On one hand, the polymers are the only materials to reduce Ag+ to metallic Ag. But the polymers cannot reduce that much Ag+ for only one time. Recently, Huang et al. found that benzenoid amine can be oxidized by metal ions to quinoid imine, but quinoid imine can return to benzenoid amine by H+ protonation to again reduce the metal ions.7578 We believe that these “oxidationreduction cycles” should exist in this research and be a major form of redox reaction. On the other hand, only 3.1 mol % of N= became N+=, demonstrating Ag+ chelation is relatively minor and the abundant Ag+ (55.9 mol %) on PmPD(NM) should be due to the physical adsorption.10 With similar techniques, analysis, and calculation, Ag+ adsorption by PmPD(NaOH2) in relatively low oxidation state is proved to be composed of redox reaction (oxidationreduction cycles), chelation, and physical adsorption (Figure 7C,D,E and Table 4). However, if no H+ protonation occurs to imine, the N+= content (Figure 7D) is equal to chelate 2.3 mg of Ag+, which is more than the real value 1.8 mg of Ag+ chelated by PmPD(NaOH2), as given in Table 4. This indicates that physical adsorption is weak and imine protonation by H+ exists. Moreover, the high Ag content (88.9 mol %) but low Ag+ content (11.1 mol %) on PmPD(NaOH2) (Figure 7E) suggests the redox reaction containing oxidationreduction cycles is the major Ag+ adsorption mechanism. The role of physical adsorption and chelation becomes minor. Similar adsorption manners were found when using PmPD(NaOH2) to treat 90 mM Ag+ (Table S2, Supporting Information). This is quite different from the Ag+ adsorption with PmPD(NM). The low oxidation state of PmPD(NaOH2) should be the key reason since decreasing oxidation
state is highly beneficial for PmPD reducing Ag+, which is similar to Li’s research.3 Based on the analysis, redox reaction including oxidation reduction cycles, chelation, and physical adsorption are the mechanisms for adsorbing Ag+. As for PmPD(NM), redox reaction and physical adsorption are both the dominating mechanisms. But only the redox reaction is the primary Ag+ adsorption manner for PmPD(NaOH2). The chelation is minor for PmPD(NM) and PmPD(NaOH2) adsorbing Ag+.
4. CONCLUSION A new method, pH manipulation, was successfully applied to improve the chemically oxidative polymerization through concurrently adding oxidant and NaOH or HCl. Specifically, when using NaOH for manipulation, the oxidation state of PmPD can be lowered effectively without losing a good yield. Moreover, the oxidation state of PmPD can be lowered in a controllable manner by increasing NaOH concentration. The Ag+ adsorption performance becomes better when lowering the oxidation state. Typically PmPD(NaOH2) in relatively low oxidation state exhibited 2 times stronger Ag+ adsorption performance than PmPD(NM) or PmPD(HCl1) in relatively high oxidation state. The adsorbance of PmPD in our study can reach 1693 mg g1, approximating that of poly(naphthalenediamine) and much higher than that of many other reported materials. PmPD showed the ability of strong selective Ag+ adsorption. Moreover, PmPD prepared by pH manipulation, typically PmPD(NaOH2), displayed a good capability for reducing Ag+ during adsorption. This can form the conjugated polymer/Ag composite, which has a potential for biosensors, etc.7983 The pH manipulation here is facile, effective, large scale, and low cost and may be propitious to other aromatic diamine polymers. Also, PmPD in low oxidation state is suitable to adsorb the Cr(VI), Hg2+, Pd2+, and Au3+ ions, and the relevant studies are still ongoing. ’ ASSOCIATED CONTENT
bS
Supporting Information. Calculation method deduction; Figure S1: simplified setup diagram for pH manipulation; Figures S2 and S3: TG and DTA of PmPD in N2 and air, respectively; Figure S4: XRD pattern of PmPD adsorbing Ag+; Table S1: solution pH before and after adsorption; Table S2: the amount of Ag+/Ag on PmPD after adsorbing Ag+ with initial concentration of 90 mM. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: +86-13874882823; Fax: +86-731-88830875; e-mail: haiyw25@ yahoo.com.
’ ACKNOWLEDGMENT This work was financially supported by Key Project for Water Pollution Control and Management Technology of China (2009ZX07212-001-01), National Science Foundation for Distinguished Young Scholars (50925417), Key Project for National Natural Science of China (50830301), and Public Welfare Project for Ministry of Environmental Protection of China (2011467062). The authors thank Professor Yu-De Shu from Department of Environmental Engineering and Dr. Fangyang Liu 13736
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Langmuir from Department of Light Metals and Industrial Electrochemistry, Central South University, for their helpful discussions on data analysis of the open-circuit potential, Dr. Feng Chen from Department of Chemistry, Hunan Normal University, China, for her important help in arranging the titration method and analyzing the Ag+ concentration, and Dr. Lang Jiang from Institute of Chemistry, Chinese Academy of Sciences, for his important help in XPS characterization. Meanwhile, we must give our thanks to editor and reviewers for their critical and helpful advice to improve this work.
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