ARTICLE pubs.acs.org/JPCC
Facile Synthesis of Polysulfoaminoanthraquinone Nanosorbents for Rapid Removal and Ultrasensitive Fluorescent Detection of Heavy Metal Ions Mei-Rong Huang,*,† Shao-Jun Huang,‡ and Xin-Gui Li*,† †
Institute of Materials Chemistry, Key Laboratory of Advanced Civil Engineering Materials, College of Materials Science & Engineering, Tongji University, 1239 Si-Ping Road, Shanghai 200092, P. R. China ‡ Department of Chemistry, Tongji University, Shanghai 200092, China
bS Supporting Information ABSTRACT: Poly(5-sulfo-1-aminoanthraquinone) nanoparticles were facilely synthesized by a chemical oxidative polymerization of 5-sulfo-1-aminoanthraquinone. The polymerization parameters such as oxidant species, acid species, acid concentration, oxidant/monomer ratio, polymerization time, and temperature were systematically studied to significantly optimize the synthetic yield, structure, and multifunctionalities of the target nanoparticles. The molecular structure, size distribution, morphology, and properties of the nanoparticles were detailedly analyzed by IR, UV-vis, and fluorescence spectroscopies, element analyses, MALDI-MS, X-ray diffraction, FESEM, AFM, laser particle analyzer, and simultaneous TG/ DSC. It is found that K2CrO4 oxidant and aqueous HClO4 without any external stabilizers are an optimal combination for synthesizing the nanoparticles with a clean surface, large πconjugation, narrow size distribution, intrinsic semiconductivity, blue fluorescence, and inherent self-stability that is ascribed to many negatively charged sulfonic groups on their macromolecular chains. In particular, the nanoparticles having a unique synergic combination of five kinds of active —NH—, —Nd, —NH2, dO, and —SO3H groups with an appropriate specific area of 115.15 m2 g-1 exhibit very high removal percentage of respective lead and mercury of 99.6 and 99.8% at initial concentrations of even up to 200 mg L-1, ultrarapid initial adsorption rate of up to 10350 (Pb(II)) and 14140 (Hg(II)) mg g-1 h-1, increased adsorbability order of Zn(II) < Fe(III) < Cu(II) , Ag(I) < Cd(II) < Pb(II) < Hg(II), and satisfactory removal of harmful heavy metal ions from ambient wastewaters, becoming ultrarapid chelate nanosorbents. Furthermore, PSA solution could be served as an advanced fluorescent chemosensor having ultrahigh sensitivity and high selectivity toward Pb(II) because of its very superior detection limit down to 1.0 10-10 M and strong anti-interference to almost all other metal ions.
’ INTRODUCTION Organic conducting polyaminoanthraquinones have attracted much attention because of their bulky three-dimensional large πconjugated system, namely, the long π-conjugated polyaniline skeleton1-4 and the rigid quinone rings, which endow with them some excellent properties, such as high electroactivity, unique electrochromism, good catalysis sensitivity, strong power storage stability, and high environmental, chemo-, and thermo-stability. Furthermore, the polyaminoanthraquinones have great potential as electrode materials for secondary batteries,5,6 heavy metal ion sorbents,7 electrochromic materials,8 modified electrodes for electrochemical determination of hemoglobin,9 and electrochemical capacitors.10 Polyaminoanthraquinones including poly(1aminoanthraquinone),5 poly(1,5-diaminoanthraquinone),6-8,10 and poly(2-aminoanthraquinone)9 have been synthesized by electrochemical or chemical oxidative polymerization of corresponding monomers in organic media. Because most of the aprotic solvents used in the preparation of polyaminoanthraquinones by chemical oxidative polymerization are environmentally hazardous, especially acetonitrile, it is important to synthesize the polymers in environmentally safe media. Also, the synthetic r 2011 American Chemical Society
methods used so far have some other intrinsic problems, like intricate synthetic procedures, tedious post-treatment, and high cost. Therefore, it is still a challenge to develop a facile, environmentally friendly, and economical method to efficiently prepare polyaminoanthraquinone or its derivatives. Cost-effective adsorptive removal of heavy metal ions from wastewater of various origins remains a challenge for environmental protection. An efficient sorbent with both high capacity and fast rate of adsorption should have the following two main characteristics: polyfunctional groups and large surface area. The polyfunctional groups provide a large number of active sites for the adsorption reaction,11 while the large surface area ensures a sufficient contact of the solid sorbent with metal ions.12 Unfortunately, most current sorbents rarely have both polyfunctional groups and large surface area at the same time. The typical sorbent is activated carbon, which has a high surface area but hardly ever has an adsorbing functional group.13 On the contrary, Received: October 18, 2010 Revised: January 13, 2011 Published: March 16, 2011 5301
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The Journal of Physical Chemistry C chelating resins are typically characterized by their polyfunctional groups, e.g., O, N, S, and P donor atoms which can coordinate to different metal ions, whereas their small specific area and low adsorption rate have limited their application. Polyphenylenediamine microparticles containing a large amount of amino and imino groups can effectively adsorb heavy metal ions, but their adsorption rate is not very fast due to their relative small specific area.14 Some thiol-modified mesoporous silica microspheres15 and humic acid-coating Fe3O4 magnetic nanoparticles16 that have functional groups and large specific area indeed demonstrate strong adsorbability toward heavy metal ions, but either their adsorption rate or stability is not good enough. Therefore, wholly new sorbents that simultaneously have both high capacity and fast rate of adsorption toward heavy metal ions are still expected. Similarly, it is of great significance to detect the quantity of heavy metals in environmental water. Fluorescent probes for sensing and monitoring heavy metal ions have been attracting more and more attention due to their accuracy and simplicity in the past decade. The Pb(II) fluorescent probes reported thus far include macrocyclic compounds linking fluorophore,17-19 rhodamine derivatives,20 DNA biomolecule,21-24 and organicinorganic hybrid fluorescent materials such as ferrocene25 and 26 L-cysteine capped CdSe. However, there still remain some drawbacks like unsatisfactory detection limit (ordinarily 10-710-9 M), severe interference by other metal ions, especially Hg(II) and Cd(II),27 and/or tedious and sophisticated preparation of the probes. Therefore, the design and synthesis of Pb(II) chemosensors which have excellent comprehensive performance including ultrahigh sensitivity and high selectivity toward Pb(II) even at a trace level, and low cost, still face challenges. Herein, we report the facile synthesis of poly(5-sulfo-1aminoanthraquinone) (PSA) nanoparticles through a chemical oxidative polymerization of ammonium 5-sulfonate-1-aminoanthraquinone in acidic aqueous media without any organic solvents or any external stabilizers or templates. The synthetic yield, large π-conjugation, size, thermal stability, conductivity, and fluorescence of the PSA particles have been optimized in detail. The introduction of sulfonic groups into the monomer has been proved to be the simplest and most effective method of cost-effectively synthesizing fine PSA nanoparticles in aqueous media. In particular, a large amount of —SO3-/—NH2/—NH —/—Nd/dO groups and high specific area endow them with fast and strong adsorbability toward heavy metal ions, and also ultrasensitive and selective fluorescent response toward Pb(II) ions.
’ EXPERIMENTAL SECTION Chemical Oxidative Polymerization of SA. The chemical oxidative polymerization of ammonium 5-sulfonate-1-aminoanthraquinone (SA) monomer for the synthesis of the PSA particles was carried out by following a typical procedure: SA monomer (1.0 g, 3.12 mmol) was added to 220 mL of distilled water in a 500 mL glass flask in a water bath at 25 °C and then stirred vigorously for 10 min. An oxidant solution was prepared separately by dissolving the oxidant CrO3, K2Cr2O7, or K2CrO4 (6.24 mmol) and 1.07 mL of 70% HClO4 in 30 mL of distilled water at 25 °C. The SA monomer solution was then treated with the oxidant solution in one portion. The reaction mixture was magnetically and continuously stirred for 72 h at 25 °C, accompanied by measurement of the open circuit potential (OCP) and
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Scheme 1. Nominal Chemical Oxidative Polymerization of SA
temperature of the polymerization solution. After that, the PSA salt particles as precipitates were isolated from the reaction mixture by centrifugation and washed with an excess of distilled water in order to remove the remaining monomer, residual oxidant, water-soluble oligomers, and water-soluble reduced byproduct. Then, a part of the polymer salts was redoped in 1.0 M HClO4 aqueous solution (20 mL) with stirring for a whole day. All of the final polymer particles were left to dry at 50 °C in ambient air for 3 days. The PSA polymers were obtained as very fine solid black powders that are quite different from relatively coarse reddish brown powders of the SA monomer. The black color is a characteristic indication of the polymer having a largely π-conjugated chain structure that has been further verified by UV-vis and IR spectroscopies, X-ray diffraction, and moderate electrical conductivity. The nominal oxidative polymerization is shown in Scheme 1. Measurements. The SA polymerization was followed by the OCP technique with the help of a saturated calomel electrode (SCE) as the reference electrode and a Pt electrode as the working electrode. UV-vis spectra of PSA polymers at a concentration of 10 mg L-1 in DMSO or 10 mM NaOH aqueous medium were taken on a 760CRT UV-vis spectrophotometer made by Shanghai Precision Scientific Instrument Co., Ltd., at a wavelength range of 900-200 nm at a scanning rate of 480 nm min-1. IR spectra were recorded on a Bruker Equinox 55(Germany)/Hyperion 2000 FT-IR spectrometer with a resolution of 3600:1 by transmittance and ATR methods. Elemental analysis of as-prepared PSA salts was carried out on a VARIO EL III element analyzer (ELEMENTAR Company). MALDI-TOF MS of polypyrene in THF with sinapinic acid as the matrix was recorded on a Waters Micromass MALDI micro MX mass spectrometer. Wideangle X-ray diffraction (WAXD) was performed with a D/ max2550VB3þ/PC X-ray diffractometer made by Rigaku International Corporation, Japan, with Cu KR radiation at a scanning rate of 10° min-1. The size and its distribution of the as-formed PSA salt particles were analyzed on a Beckman Coulter LS230 laser particle-size analyzer. The size and morphology of the particles were further observed by a Quanta 200 FEG fieldemission scanning electron microscope (FEI Company) and a SPA-300HV atomic force microscope (Seiko Instruments Inc., Japan). The apparent (bulk) density of the PSA nanoparticles was determined by the ratio of the mass to a given volume of 2 cm3. The fine particles were put into a plastic tube with a scale and stacked loosely and tightly for the determination of apparent and bulk densities, respectively. The chemoresistance of dry PSA particles was evaluated as follows: polymer powders of 2 mg were added into the solvent of 1 mL and dispersed thoroughly by shaking intermittently for 2.0 h at ambient temperature. Simultaneous TG and DSC measurements were accomplished in static 5302
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Table 1. Effect of Oxidant Species on the Synthetic Yield, Size, and Properties of PSA Salt Particles with an Oxidant/Monomer Molar Ratio of 2 oxidants HClO4 concentration (mM)
CrO3 0
CrO3 50
K2Cr2O7 50
K2CrO4 50
polymerization temperature (°C)/time (h)
15/48
25/72
25/72
25/72
synthetic yield (%)
5
43.4
49.8
34.9
Dn (nm)/PDI by LPA
353/1.14
368/1.08
382/1.12
particle size with Au/without Au (nm) by SEM
80-190/50-160
95-190/65-160
95-220/65-190
particle size (nm) by AFM bulk electrical conductivity (S cm-1)
20-43
24-43
20-43
virgin HClO4 doped salt 1 M HClO4 redoped salt
2.38 10-9
6.22 10-9
1.45 10-8
8.43 10-8
-6
-6
-6
3.60 10-5
3.25 10
5.80 10
5.90 10
temperature at maximal endothermicity (°C)
82.3
104.4
maximal endothermicity (W g-1)
0.01
3.21
71.9 1.33
decomposition temperature at 15% weight loss (°C)
299.8
349.4
341.9
temperature at the maximal exothermicity (°C)
397.3
386.9
396.9
maximal exothermicity (W g-1) temperature at first/second maximal weight-loss rates (°C)
90.9 82.3/392.3
114.0 104.4/381.9
76.2 81.9/391.9
first/second maximal weight-loss rates (% min-1)
0.11/1.34
0.08/1.49
0.09/1.06
char yield at 787 °C (wt %)
15.0
27.1
25.6
air with a sample size of 3 mg at a temperature range from room temperature to 787 °C at a heating rate of 10 °C min-1 by using an STA 449C Jupiter thermal analyzer (NETZSCH Company, Germany). Bulk electrical conductivity of as-formed PSA salts was measured by a two-disk method at 15-20 °C. The fluorescence excitation and emission spectra of the SA monomer and PSA solutions were obtained on a 970CRT fluorescence spectrophotometer made by Shanghai Precision Scientific Instrument Co., Ltd. The adsorption of Pb(II), Hg(II), Cd(II), Ag(I), Cu(II), Fe(III), and Zn(II) in aqueous solutions onto the PSA particles as a nanosorbent was performed in a batch experiment. Twenty-five mL metal ion aqueous solution in an ion concentration range from 0.42 to 200 mgL-1 was incubated with a given amount of the particles at a fixed temperature of 30 °C. After a desired treatment period, the particles were filtered from the solution and then the ion concentration in the filtrate was measured by molar titration at higher ion concentration and by inductively coupled plasma (ICP) analysis at lower ion concentration on a Thermo E. IRIS Duo ICP emission spectrometer. The concentration of various metal ions in real ambient wastewaters before and after the purification by using PSA nanoparticles as sorbents was simultaneously determined by the ICP analysis. The soluble fraction of 4.0 mg of PSA (synthesized with a K2CrO4/monomer molar ratio of 2 in 50 mM HClO4 at 25 °C for 72 h) in 200 mL of DMSO was used to fabricate a turn-off fluorescent chemosensor for sensing aqueous Pb(II) ions. The calibration curve was established by adding 0.5 mL of Pb(II) solution at a given concentration to 4.5 mL of PSA/DMSO solution prepared above. The fluorescence spectra of the mixed solutions were acquired on the fluorescence spectrometer at excitation at 358 nm.
’ RESULTS AND DISCUSSION Synthesis of the PSA. Selection of the Oxidant. The influence of the oxidant species and their standard reduction potential (RP) on the polymerization yield and conductivity of the PSA
polymers obtained has been systematically studied. It is found that the yield and conductivity vary significantly with the oxidant from FeCl3 (RP = 0.771), KClO4 (1.19), K2CrO4 (1.20), K2Cr2O7 (1.33), CrO3 (1.35), KMnO4 (1.51), NaClO (1.63), H2O2 (1.77), (NH4)2S2O8 (2.01), to Na2S2O8 (2.01) with different RP values. When FeCl3 or KClO4 with the lowest RP was used as the oxidant for the SA polymerization, the resultant polymerization solution did not turn dark; only a very little purple precipitate was obtained by filtration, suggesting the formation of some oligomers with very low molecular weight. No sufficient polymer was obtained or the polymerization solution did not turn dark if individually using KMnO4, NaClO, H2O2, (NH4)2S2O8, or Na2S2O8. Fortunately, another three oxidants, i.e., K2CrO4, K2Cr2O7, and CrO3, with moderate RP values ranging from 1.20 to 1.35 mV, can properly oxidize the SA monomer, successfully resulting in dark precipitates as polymerization products. That is to say, both the chemical composition and RP values of the oxidants play a key role in the polymerization of the SA monomer. Moreover, only the oxidants with the RP values that are neither too high nor too low are appropriate for the polymerization. Table 1 lists the influence of the oxidant species on the synthetic yield and electrical conductivity of the PSA polymers obtained. It seems that the yield is the highest if using CrO3, but K2CrO4 can afford the PSA with the highest conductivity. Since the high conductivity generally indicates a large π-conjugated structure and then high molecular weight of the PSA,2,4,28 the optimal oxidant is certainly K2CrO4 to prepare PSA having the highest conductivity in spite of the second highest yield. The PSA salt particles are an electrical semiconductor like other aromatic amine polymers obtained by oxidative polymerization.1,2 The variation in the conductivity of the PSAs with oxidant species suggests different π-conjugated structures. Insight of Polymerization Process. The polymerization process was followed by in situ measurement of the open circuit potential (OCP) and temperature of the polymerization solution during the initial polymerization time of 800 min. The solution 5303
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The Journal of Physical Chemistry C potential of the SA monomer in distilled water dramatically increased from the initial 115 mV to the maximum 673 mV vs SCE upon a sudden quick addition of K2CrO4 solution at 25 °C within the initial 1 min, as shown in Figure S1 of the Supporting Information. After passing through a relatively stable and maximal potential for around 100 min, a gradual potential decline to 650 mV vs SCE at a very low rate was observed after 100 min, implying that the polymerization reaches a summit at 12-50 min. Note that only a slight fluctuation of the polymerization solution temperature was detected, which is basically consistent with 1,5diaminoanthraquinone polymerization7 but different from aminoquinoline/phenetidine copolymerization,29 because the SA monomer (12.48 mM) and oxidant (24.96 mM) concentrations in this study are much lower than those in ref 29 with the same comonomer and oxidant concentration of up to 666.7 mM. As can be seen from Figure S1 of the Supporting Information, when K2Cr2O7 or CrO3 was used as the oxidant for the SA polymerization, the absolute OCP of the polymerization was higher because of their higher SP, but the decreasing rates of OCP or temperature are almost the same as that using K2CrO4. On the contrary, the absolute OCP of the polymerization is the lowest, reaches the maximum at the shortest polymerization time, decreases at the fastest rate after passing through the maximal OCP even using the same K2CrO4 oxidant if elevating polymerization temperature to 50 °C, signifying the fastest polymerization at 50 °C. Optimization of Polymerization Condition. It is seen from the effect of the polymerization time on SA polymerization in Figure 1a that, with an increase in the polymerization time from 0 to 72 h, the synthetic yield rapidly increases from 0 to 43.4%, but, as the polymerization time further increases from 72 to 168 h, the yield decreases down to 25.8%, while the conductivity of the polymer reaches the maximum at a polymerization time of only 24 h. That is to say, a relatively short time is favorable for the obtainment of the PSA with the highest conductivity because long π-conjugated sequences of the PSAs have already formed during the initial 24 h and would be broken to some extent by the hydrolysis or oxidation with further increasing polymerization time. The UV-vis spectra in Figure 2 of PSA prepared during different polymerization times can be used to confirm this conclusion. On the other hand, the polymerization rate is faster than its counterreaction because the SA monomer and oxidant have much higher concentration than the polymer in the reaction solution at the initial stage of the polymerization, but the counterreaction gradually becomes obvious with the consumption of the monomer and oxidant. As a result, the polymerization time of 24-72 h may be appropriate for the PSA synthesis. As shown in Figure 1b, with elevating polymerization temperature from 0 to 50 °C, the conductivity of the polymer exhibits a maximum of 2.36 10-7 S cm-1 at 15 °C, while the yield increases monotonically from 15.6 to 54.0%. Obviously, a relatively low temperature is advantageous to the growth of the polymer chains with large π-conjugated structure, because too high temperature would induce more and faster chain termination but too low temperature might cause less chain initiation and slower reaction. Certainly, 15 °C is the optimal temperature for the preparation of PSA with the highest conductivity, possibly due to the proper initiation and propagation rates of PSA chains. The effect of the oxidant/monomer molar ratio between 0 and 3 on SA polymerization is shown in Figure 1c, demonstrating that the yield and conductivity of the polymer reach the maximum
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Figure 1. The dependency of synthetic yield and bulk electrical conductivity of PSAs synthesized in 50 mM HClO4 on (a) polymerization time at a fixed K2CrO4/SA molar ratio of 2 at 25 °C, (b) polymerization temperature at a fixed K2CrO4/SA molar ratio of 2 for 72 h, and (c) K2CrO4/SA molar ratio at 15 °C for 48 h.
values at 1 and 2, respectively. At lower oxidant contents, the oxidant was consumed very quickly, so that insufficient oxidant was used to oxidize the residual monomers for further polymerization and chain propagation, consequently leading to lower yield and conductivity. When more oxidant was added, more monomers could be oxidized and then more chain initiation and propagation would occur, finally resulting in the formation of more polymers with higher conductivity. However, too much oxidant would produce many more water-soluble oligomers with much lower conductivity.4,28 Apparently, the optimal oxidant/ monomer molar ratio should be 2 for the synthesis of PSA with the highest conductivity and the second highest yield. The significant influence of the acid species at a constant acid concentration of 50 mM on the SA polymerization was listed in Table 2. The yield was found to be very low if using 50 mM H2SO4 solution as a polymerization medium because the 50 mM H2SO4 has the highest Hþ concentration in all four acid media used and high Hþ concentration may reduce the water solubility of the monomer SA that is disadvantageous to the polymerization. On the contrary, pure water without any acid is also not good for the SA polymerization (Table 1) because too low Hþ concentration consequently resulted in too slow initiation and 5304
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Figure 2. UV-vis absorption spectra of (a) SA monomer and PSAs prepared with an oxidant/SA molar ratio of 2 at 25 °C for 72 h; (b) PSAs prepared with K2CrO4 at an oxidant/SA molar ratio of 2 at 25 °C for different polymerization times; (c) PSAs prepared with a K2CrO4 oxidant/SA molar ratio of 2 in four 50 mM acid aqueous solutions at a constant temperature of 15 °C for 48 h; (d) PSAs prepared with K2CrO4 at an oxidant/SA molar ratio of 2 at different polymerization temperatures for 72 h; (e) PSAs prepared with K2CrO4 at different oxidant/SA molar ratios at 15 °C for 48 h, at a fixed 50 mM HClO4 concentration; and (f) PSAs prepared with a K2CrO4 oxidant/SA molar ratio of 2 in 10 and 50 mM HNO3 aqueous solutions at a constant temperature of 15 °C for 48 h. The solvent used for UV-vis spectral tests is (a) DMSO and (b-f) 10 mM NaOH.
Table 2. Effect of Acid Species on the Synthetic Yield and Bulk Electrical Conductivity of Virgin PSA Salts Prepared with an Oxidant/SA Molar Ratio of 2 in 50 mM Acid Aqueous Solutions acid species
H2SO4
HCl
HClO4
HNO3
oxidant
K2CrO4
K2CrO4
K2CrO4
K2CrO4
synthetic yield (%)
2.13
27.6
20.7
21.6
electrical conductivity (S cm-1)
3.39 10-9
9.30 10-8
1.64 10-7
2.50 10-7
At 15 °C for 48 h
At 50 °C for 24 h synthetic yield (%)
40.9
34.7
30.0
electrical conductivity (S cm-1)
1.70 10-8
2.40 10-8
3.40 10-8
propagation of the PSA chains. It seems that 50 mM HCl is the best for the productive synthesis of PSA, but 50 mM HNO3 is the optimal acid for the achievement of the PSA with the highest conductivity. In order to further testify whether HNO3 is the optimal acid, the reaction temperature and time were changed to 50 °C and 24 h, respectively. HNO3 was again demonstrated to be optimal for the synthesis of the PSA with the highest conductivity (see Table 2). Besides the nature of the acid, the acid concentration also remarkably affects the polymerization characteristics of SA monomer. Table S1 of the Supporting Information summarizes the effect of HNO3 concentration on the synthetic yield and conductivity of PSA salts. The polymerization cannot proceed in acid-free H2O because of the very weak oxidative ability of K2CrO4 in neutral water and the absence of the vital protonation of the -NH2 groups before the oxidative polymerization. On the other hand, the polymerization at a HNO3 concentration of 100 mM or higher does not produce any dark product, which is similar to the polymerization in 50 mM H2SO4, indicating that the higher acid concentration results in stronger chain termination due to too vigorous acidolysis. Another
crucial reason is that high acid concentration may reduce the water solubility of the monomer SA. The significant dependence of the synthetic yield and conductivity of the PSA on the HNO3 concentration indicates that two competing processes, namely, chain propagation and acidolysis, mainly control the polymerization. A small amount of PSA having a polymerization yield of 15.5% and conductivity of 6.20 10-8 S cm-1 can be synthesized in 10 mM HNO3. In particular, an appropriate HNO3 concentration is 50 mM for the acquirement of PSA salts with higher yield (21.6%) and higher conductivity (2.50 10-7 S cm-1) simultaneously. Molecular and Supramolecular Structure of the PSA. UVvis Spectra. The UV-vis absorption spectra of the SA monomer and PSA polymers in DMSO in Figure 2a suggest that all four PSAs exhibit quite different UV-vis spectra from that of the monomer. The strongest band I at 261 nm is due to the π f π* transition within benzene rings, as observed for both the SA monomer and PSAs. The moderate band II centered at 464 nm, ascribed to whole anthraquinone fused rings,30 has become very weak for the three PSAs. The PSAs obtained with CrO3, K2Cr2O7, and K2CrO4 display an additional unique band III 5305
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Table 3. Elemental Analysis and Proposed Chain Structures of Virgin PSA Synthesized with a K2CrO4/SA Molar Ratio of 2 in 50 mM HClO4 Aqueous Solution at 25 °C for 72 h
a The chromium content was determined by the ICP-AES method by digesting the PSA in 65% HNO3-30% H2O2 (3:2 V/V) at about 50 °C until a clear colorless final mixed solution was obtained.
centered at 654, 666, and 625 nm, respectively, which corresponds to the n f π* transition of the large π-conjugated system in polyaniline-like chains,2,10 because this band is not observed in the UV-vis spectrum of the SA monomer. So great a difference between the UV-vis spectra of the SA and PSAs must be attributed to a strong polymerization effect. In other words, the black solid powders obtained by the chemical oxidative polymerization of SA monomer are genuine polymers. In addition, it should be noted that the intensity ratio of band III over band I is directly proportional to the electrical conductivity. For example, the intensity ratio of the polymers obtained with CrO3, K2Cr2O7, and K2CrO4 are 0.435, 0.453, and 0.496, respectively, while their electrical conductivities are 6.22 10-9, 1.45 10-8, and 8.43 10-8 S cm-1, respectively; i.e., the maximal intensity ratio of the PSAs signifies the longest π-conjugation length. This should be the most important evidence of forming genuine polymers. With increasing polymerization time from 1 day to 7 days, both the wavelength/intensity of band III monotonically decrease from 611 nm/0.562 to 609 nm/0.467, as shown in Figure 2b, implying that the PSA obtained for 1 day possesses the longest π-conjugated length.31 This has been well confirmed by the highest conductivity of the PSA at 24 h (Figure 1a). Figure 2c shows UV-vis spectra of four PSAs prepared in different species of acid aqueous solution. The spectra are nearly equal regardless of the variation of reaction medium from HCl to HClO4 or HNO3 except for H2SO4, but the PSA obtained in H2SO4 displays the weakest band III. This spectral changing tendency is basically coincident with the conductivity variation in Table 2. As can be seen from Figure 2d, both the wavelength and intensity of the band III exhibit maxima of 611 nm and 0.602, respectively, at 15 °C in a polymerization temperature range from 0 to 50 °C, and the PSA synthesized at 50 °C exhibits the shortest wavelength 603 nm and weakest intensity 0.38 of band III. All of these have been perfectly proved by the highest and lowest conductivities of the PSAs gained at 15-50 °C in Figure 1b, respectively. Similarly, it is easily found from a comparison of the UV-vis spectra in Figure 2e that the wavelength and intensity of band III are the maximal values of 613 nm and 0.552, respectively, for the PSA with an oxidant/SA molar ratio of 2, but the smallest values of 604 nm and 0.414, respectively, for the PSA with the oxidant/SA molar ratio of 3. This variation order exactly coincides with the conductivity
change in Figure 1c. Figure 2f shows UV-vis spectra of two PSAs prepared in HNO3 aqueous solutions at 10 and 50 mM. The PSA synthesized in 50 mM HNO3 indeed has longer wavelength 613 nm and higher intensity 0.406 of band III than that in 10 mM HNO3 with shorter wavelength 611 nm and weaker intensity 0.361 of band III. This stronger band III of the PSA synthesized in 50 mM HNO3 is in a good coincidence with its higher conductivity in Table S1 of the Supporting Information. In summary, the PSA polymer salts synthesized under the following polymerization conditions: polymerization time of 24 h, polymerization temperature of 15 °C, oxidant/SA molar ratio of 2, reaction medium of 50 mM HNO3 and K2CrO4 as the oxidant, have the longest large π-conjugated length and the highest conductivity. IR Spectra. Figure S2 of the Supporting Information shows IR spectra for monomeric SA and the as-grown PSAs obtained by the oxidative polymerization of SA, and Table S2 of the Supporting Information summarizes their main IR bands and possible assignments. A strong and broad absorption centered at 3430 cm-1 due to -NH2 stretch in SA monomer has turned into a very broad absorption centered at 3410 cm-1 in the spectra of the three PSAs. Such broad absorption is frequently encountered in electrically conducting polymers because of an electronic transition involving promotion of the valence electrons to conduction bands.32 Only SA monomer displays a weak absorption at 1680 cm-1. Both SA and PSA display the same absorption at 1040 cm-1 due to OdSdO symmetrical stretch. However, other absorption bands significantly change their intensity and wavenumber upon polymerization of SA. For instance, the strongest band changes from 1040 to 1250 cm-1 after the SA polymerization. Reduction in CdC and CdO stretching frequencies may be an indication of the decreased electron density within the anthraquinone π-manifold.33 These assignments suggest that the PSAs were polymerized mainly at 1,4 positions through a bonding manner of head-to-tail structure in an electroactive polyaniline.1-4,28 Quite large differences between the IR spectra of the monomer and the polymer, as well as the formation of a large amount of quinoid and benzenoid structures in the polymerization products, indicate that the polymerization product is a real polymer rather than a simple chelate or mixture of monomers with some oligomers. Element Analysis. The macromolecular structure of PSA polymers synthesized with K2CrO4 has been conjectured from 5306
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Figure 3. WAXD diffractograms of SA monomer, PSA salts prepared with an oxidant/SA monomer molar ratio of 2 in 50 mM HClO4 at 25 °C for 72 h, and PSA salts after adsorption of Pb(II) and Hg(II) ions in Pb(NO3)2 and Hg(NO3)2 aqueous solutions (25 mL) at the same initial Pb(II) and Hg(II) concentration of 200 mg L-1 at 30 °C with 50 mg of sorbent for 24 h.
the C/H/N/S/O/Cr ratio determined by element analysis, as listed in Table 3. It seems that the PSA containing a linear and large π-conjugating polyaniline-like backbone with benzenoid and quinoid repetitive units satisfactorily matches the elemental composition, in which the PSA chains are coincident with UVvis and IR spectral results. Obviously, an elimination of ammonium (-NH4þ) happens during the oxidative polymerization because NH4þ ions in monomeric SA must have been replaced by Hþ ions in aqueous acids. By the way, the PSA chains have been doped by the HCrO2 that results from the reduction of K2CrO4 with the oxidative polymerization of SA monomer. Note that the virgin doped PSA salts can be dedoped into the lightly doped PSA salts with only 3.97 wt % Cr by thoroughly washing with 1.0 M HCl and then water. MALDI-TOF-MS. The MALDI-TOF mass spectrum in Figure S3 of the Supporting Information indicates that the PSA is mainly composed of at least 2 and even 4 repetitive aminoanthraquinone units in Table S3 of the Supporting Information. The PSA containing a linear and large π-conjugating polyaniline-like mainchain consisting of benzenoid and quinoid repetitive units is confirmed once again by the MALDI-TOF mass spectrum, which basically agrees with the UV-vis and IR spectral and elemental analytical results. The sulfonic groups sometimes exist as ammonium sulfonate, and the quinone and amino groups are sometimes ionized by Hþ, Naþ, and Kþ from the sinapic acid matrix or environment in order to form quasi-molecular ions or metal ion adducts because sulfonic groups, oxygen atoms, and amino/imino groups have a strong affinity with NH4þ, Hþ, Naþ, or Kþ. The -SO3NH4 or -SO3H is sometimes eliminated from the polymer chains because of laser irradiation. Wide-Angle X-ray Diffraction. Figure 3 illustrates the wideangle X-ray diffractograms of SA and the PSA obtained with three oxidants. All of three PSAs display a broad diffraction peak centered at 23.8, 25.4, and 24.5°, respectively, and another two very weak diffractions around 8.1 and 43°. Apparently, the PSAs are characterized by a typically amorphous structure that is quite different from the highly crystalline structures of SA monomer,
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1,4-DAAQ coordination oligomer,34 1,4-DAAQ chelate,35 and 1,5-DAAQ charge-transfer chelates.20 This is also important evidence for the formation of real oxidative PSA polymers. Size and Morphology of PSA Nanoparticles. Figures 4 and S4 of the Supporting Information and Table 1 show the size, its distribution, and morphology for PSA particles (dispersed in water) synthesized in 50 mM HClO4 aqueous solution without any external stabilizer or templates with three different oxidants by laser particle analyzer (LPA), field emission-scanning electron microscopy (FESEM), and atomic force microscopy (AFM). A slight influence of oxidant species on the particle size has been revealed, i.e., the number-average diameters (Dn) of three types of PSA particles are all of submicrometer level, ranging from 354 to 382 nm with a small size polydispersity index (PDI, Dw/Dn) of the PSA salt between 1.09 and 1.15, an indication of a narrow size distribution of the particles. Indeed, the dispersion of the PSA particles in pure water looks like a homogeneous solution that is stable for at least two months, substantially implying that the aqueous dispersed granules are nanoparticles or at least very small submicroparticles because the PSA particles are almost insoluble in pure water. In particular, the aqueous dispersed PSA particles synthesized with K2CrO4 have the second smallest size and the smallest PDI, suggesting that K2CrO4 is the optimal oxidant for the fabrication of uniform PSA nanoparticles that have intrinsic self-stability and relatively clean surface as compared with other conducting nanoparticles obtained by microemulsion or dispersion polymerizations. The FESEM images in Figure 4a-c display that the PSA salt particles synthesized with K2Cr2O7, K2CrO4, and CrO3 as an oxidant respectively look like irregular grains with a diameter of 80-220 nm with sputtering gold layer or 50-190 nm without sputtering gold layer in Table 1. A much smaller particle size of 20-43 nm has been seen by AFM in Figure 4d-i. The particle size revealed by FESEM and AFM is much smaller than that determined by LPA because of contraction and compression of the hydrophilic sulfonic groups containing PSA particles after the exclusion of water inside the particles during the drying for microscopic observations. Larger size by SEM than AFM should originate from the differences of the method and conditions of the particle observations. For example, the PSA particles have been directly observed by AFM without any sample pretreatment, but the sputtering of gold onto the PSA particle samples is necessary for clear SEM observation, consequently leading to bigger particles. Properties of PSA Nanoparticles. Herein, we report five types of new important properties of the PSA polymers formed by the oxidative polymerization, including solubility, thermostability, electric conductivity, fluorescence, Pb(II) and Hg(II) adsorbability, and practical purification of ambient wastewaters. Chemoresistance. As summarized in Table S4 of the Supporting Information, the PSA polymers demonstrate quite different chemoresistance as compared with SA monomer because the former have much higher molecular weight and thus much stronger interaction and cohesion among the PSA macromolecular chains. Almost all of the PSA polymers are slightly soluble or at most partly soluble in N,N-dimethylformamide (DMF), Nmethylpyrrolidone, dimethylsulfoxide (DMSO), propylene carbonate (PC), and 1.0 M HClO4, but the SA monomer is mostly or even completely soluble in these five solvents. The chemoresistance of the PSAs in water, CH3COOH, acetone, and tetrahydrofuran is always much better than that of the SA monomer in the four solvents, because the PSAs are totally insoluble in the five solvents. This good chemoresistance is one of the necessary 5307
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Figure 4. FESEM (a-c) and AFM (d-i) images of PSA particles prepared with (a, d, g) K2Cr2O7, (b, e, h) K2CrO4, and (c, f, i) CrO3 as oxidants, respectively, and an oxidant/SA molar ratio of 2 in 50 mM HClO4 aqueous solution at 25 °C for 72 h.
features of the PSAs as excellent sorbents discussed below. Of course, the PSAs and SA monomer are both totally soluble in concentrated inorganic acids like 17.8 M H2SO4 and 11.6 M HClO4, as well as even dilute alkali aqueous solutions like 10 mM NaOH and 200 mM NH4OH because there are so many sulfonic, ammonium sulfonate, and amino groups in the PSA and SA monomer. Apparently, the aqueous alkalis are stronger solvents than the aqueous acids, signifying that more sulfonic groups exist as side groups in the PSAs. On the other hand, it is interesting that the color of the PSA solution varied significantly with the different solvents, i.e., a novel solvatochromism. Almost all of the PSA polymers are generally cyan or dark cyan in DMSO, PC, 1.0 M HClO4, NaOH, and NH4OH, dark green in 95% H2SO4, and yellow in 11.6 M HClO4, possibly owing to the variation of PSA chain conformations or even configurations in various solvents. However, the PSA solution color hardly ever changes with the oxidant species used.
Thermostability. The differential scanning calorimetry (DSC) in Figure 5 reveals that the PSAs exhibit a very weak endothermic peak at 71.9-104.4 °C due to the endothermic volatilization of water absorbed in the PSAs and very strong and sharp exothermic peaks at 386.9-397.3 °C because of the thermo-oxidative breakage of the main chains or the rapid combustion of the PSAs, but no other thermal transitions, such as glass or melting transitions, have appeared. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves display two-stage thermal degradation at 81.9-104.4 and 381.9-391.9 °C due to water elimination and mainchain decomposition of the polymer, respectively. These twostage weight-loss temperatures are basically consistent with the endothermic and exothermic transition temperatures by DSC. Table 1 summarizes the thermal properties of the PSAs obtained with CrO3, K2Cr2O7, and K2CrO4. The PSA obtained with K2Cr2O7 has the highest initial decomposition temperature and char yield at elevated temperature, but the PSA obtained with 5308
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Figure 5. DSC, TG, and DTG curves in air for the PSA salt particles prepared with K2Cr2O7, K2CrO4, and CrO3 oxidant/SA molar ratios of 2 in 50 mM HClO4 at 25 °C for 72 h.
K2CrO4 has the highest temperature at the lowest second maximal weight-loss rate. Accordingly, the PSA powders are thermally stable at temperatures below 340 °C because of the high aromaticity and rigidity of the PSA skeletons. This high thermostability is one of the necessary features of the PSAs as excellent heat-resistant sorbents discussed below. Bulk Electrical Conductivity. The bulk electrical conductivity of the PSA polymers is summarized in Tables 1, 2, and S1 (Supporting Information) and Figure 1, which varies between 1.63 10-9 and 3.60 10-5 S cm-1, depending on the synthetic conditions and doping levels. Generally, shortening the polymerization time and elevating the doping level can both effectively enhance the conductivity of the PSAs obtained. The conductivity would rise steadily with changing oxidant from CrO3, K2Cr2O7, to K2CrO4 or acid medium from H2SO4, HCl, HClO4, to HNO3. In particular, 50 mM acid medium at a polymerization temperature of 15 °C and a K2CrO4/SA molar ratio of 2 are optimal for the synthesis of the PSA with the maximal conductivity. Therefore, it can be concluded that the PSAs exhibit the maximal conductivity when K2CrO4 acts as the oxidant and HClO4 as the redopant with an oxidant/monomer molar ratio of 2 at 15 °C in 1.0 M HNO3 aqueous solution. The wide controllability of the conductivity is attributive to the polymerization and doping conditions because the molecular weight, large π-conjugation length, and redox state of the PSAs are dramatically adjusted by the polymerization conditions and doping level. In other words, the conductivity could be satisfactorily optimized by facilely controlling and regulating the polymerization and doping conditions. Rapid Adsorption of Heavy Metal Ions onto PSA Nanosorbents. Adsorption of Lead and Mercury Ions. The PSA nanoparticles obtained by K2CrO4 exhibit a powerful adsorbability toward heavy metal ions like Pb(II) and Hg(II) ions because of the presence of a large amount of —NH—, —Nd, — NH2, dO, and —SO3H groups containing lone pair electrons on the PSA chains that are confirmed by the MALDI-TOF MS in
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Figure 6. Adsorption kinetics of (a) Pb(II) and (b) Hg(II) ions onto PSA nanosorbents at 30 °C in 25 mL of Pb(NO3)2 and Hg(NO3)2 solutions at the same initial Pb(II) and Hg(II) concentration of 200 mg L-1 with 50 mg of sorbent. Inset: Kinetics model plots of the adsorption of Pb(II) and Hg(II) onto the nanosorbents synthesized with a K2CrO4/ SA molar ratio of 2 in 50 mM HClO4 at 25 °C for 72 h.
Figure S3 and Table S3 of the Supporting Information. The groups can efficiently bind metal ions through sharing lone pair electrons to form metal ion chelates with stable multiple sixmember-ring structures illustrated in Scheme 1. The multi chelating sites in PSA can significantly facilitate the chelation with the metal ions. Although the monomer SA also contains — NH2, dO, and —SO3H4, it is water-soluble and has neither long chain nor multi chelating sites to firmly bind metal ions. Consequently, the monomer actually hardly ever possesses any practicable adsorbability for purifying water. Furthermore, the nanoeffect and large specific area of the PSA nanoparticles significantly enhance the adsorbability.7,36 As summarized in Table S5 of the Supporting Information, the Pb(II) and Hg(II) adsorption onto the PSA nanoparticles as nanosorbents is significantly affected by the oxidant species used. The PSA nanoparticles obtained by K2CrO4 have the strongest adsorbability due to their longest molecular chains or π-conjugated structures that have been confirmed by the strongest band III in Figure 2a, the highest conductivity in Table 1, and the second narrowest size distribution in Figure S4 of the Supporting Information and Table 1.37 Figure 6 shows representative plots of Pb(II) and Hg(II) adsorbance and adsorptivity versus adsorption time onto the PSA nanosorbents obtained by K2CrO4 to investigate the adsorption kinetics and discover the time of reaching equilibrium adsorption. The adsorbance and adsorptivity rise nonlinearly with increasing adsorption time, revealing an adsorption process comprising two steps: an initial primary rapid step and a subsequent secondary slow step. The adsorption rate of Pb(II) and Hg(II) ions onto the nanoparticles is very rapid during the initial 5 min. The Pb(II) adsorption capacity and adsorptivity at the adsorption time of 5 min are 81.5 mg g-1 and 81.5%, respectively. Moreover, the Hg(II) adsorption capacity and 5309
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Table 4. Kinetics Model Equations for Pb(II) and Hg(II) Adsorption on the PSA Nanoparticles Prepared with a K2CrO4/SA Molar Ratio of 2 in 50 mM HClO4 at 25 °C for 72 h Qe found by mathematical model
equation
pseudo-first-order pseudo-second-order
-ln(1 - Qt/Qe) = 2.65033t þ 2.52682 t/Qt = 0.01115t þ 9.66176 10-5
-1
-1
k1 (h ), k2 (g mg
-1
linear correlation
standard
coefficient R
deviation
h0 = k2Qe2 (mg g-1 h-1)
experiment
equation
0.29042 8.49 10-6
k1 = 2.65033 k2 = 1.28921
89.6 89.6
89.7
h ),
Pb(II) adsorption 0.96753 1
h0 = 1.035 104 Hg(II) adsorption pseudo-first-order
-ln(1 - Qt/Qe) = 4.30587t þ 2.71089
0.97782
0.38687
k1 = 4.30587
92.5
pseudo-second-order
t/Qt = 0.01079t þ 7.07291 10-5
0.99999
2.05 10-5
k2 = 1.65259
92.5
92.7
h0 = 1.414 104
Table 5. Adsorption Capacity and Adsorptivity of Six Metal Ions at an Initial Ion Concentration of 200 mg L-1 at an Adsorption Temperature of 30 °C for a Contact Time of 1 h and PSA Sorbent Dosage of 2.0 g L-1 metal ions metal ion
adsorption
solutions
capacity Qt (mg g-1)
theoretical selectivity coefficient
pH change conductivity after ion
adsorptivity (%)
Hg(II)
Pb(II)
Cd(II)
adsorption (S cm-1) -7
pHbefore
pHafter
ΔpH
HgNO3
92.4
92.4
1.00
0.96
0.94
5.85 10
3.02
2.77
-0.25
PbNO3
89.0
89.0
1.04
1.00
0.98
4.18 10-7
5.03
4.80
-0.23
CdSO4 CuSO4
87.5 28.3
87.5 28.3
1.06 3.27
1.02 3.14
1.00 3.08
4.35 10-7 1.26 10-7
6.61 5.55
6.42 5.40
-0.19 -0.15
FeCl3
15.1
15.1
6.12
5.89
5.79
1.10 10-7
2.51
2.38
-0.13
ZnSO4
9.6
9.6
9.63
9.27
9.11
9.77 10-8
6.23
6.13
-0.10
adsorptivity at 5 min are even as high as 87.9 mg g-1 and 87.9%, respectively, which are 95.0% of those at the equilibrium adsorption. In particular, the Pb(II) and Hg(II) adsorptivity will significantly rise to 99.6 and 99.8%, respectively, as listed in Table S5 of the Supporting Information, if doubling the PSA sorbent dose even for a much shorter adsorption time of 1 h. Thus, rapid adsorption should occur on the nanoparticle surfaces because of an immediate chelation between the ions and exposed active sites of the PSA chains. By contrast, the slow step may take place inside the nanoparticles, representing the diffusion of the ions into the interior of the nanoparticles. Due to the electrostatic repulsion between the free and adsorbed ions on the surface of the nanoparticles, it is difficult for the free and bulky ions to enter into the interior of the nanoparticles. In order to gain insight into the adsorption kinetics of the removal of Pb(II) and Hg(II) by PSA, the experimental data were fitted into the pseudo-first-order and pseudo-second-order kinetics equations.38 The curves of -ln(1 - Qt/Qe) and 1/Qt ∼ t in the inset of Figure 6 produce corresponding parameters in Table 4. It is found that the pseudo-second-order kinetics model better correlates the experimental data than the pseudo-firstorder model.39 It thus appears that the adsorption is very well described by the pseudo-second-order kinetics model based on the assumption of the rate limiting step by a chemical adsorption comprising valency forces through sharing electrons between sorbent and adsorbate. It is amazingly found that the adsorption time at equilibrium is even short down to 1.0 h for Pb(II) and 0.5 h for Hg(II). Further, the PSA sorbents show very rapid initial adsorption rates h of 10350 (Pb(II)) and 14140 (Hg(II)) mg g-1 h-1 according to the pseudo-second-order kinetics. No
higher h value than these could be found in the literature.11,12,40 The adsorption rate of Pb(II) and Hg(II) ions onto the nanosorbents is very rapid during the initial 5 min. Such a rapid adsorption is not only because of a great number of various functional groups including —NH—, —Nd, —NH2, dO, and —SO3H on the PSA nanoparticles but also because of their high porosity which is confirmed by low density (apparent density, 0.457 g cm-3; bulk density, 0.594 g cm-3) and high specific area (115.15 m2 g-1) in Figure S5 of the Supporting Information. By the way, the specific area of the PSA nanoparticles ranges between 155 m2 g-1 of multiwalled carbon nanotubes and 78 m2 g-1 of three-dimensional mesoporous chromium oxide as efficient sorbents for eliminating extremely toxic dioxin41 and toluene,42 respectively. The occurrence of the metal ion adsorption onto the PSA nanoparticles is further confirmed by a significant variation of IR spectra, wide-angle X-ray diffractograms, and electrical conductivity of nanoparticles adsorbing the ions. As compared with the diffraction curve before adsorption, in Figure 3, a weaker diffraction at 7.7°, a 0.4-1.7° shift of the strongest diffraction peak at 24.5°, and several sharp diffraction peaks at 25.6, 27.2, and 29.5° are all attributed to the ion adsorption onto the PSA nanoparticles. It should be noted that the three much stronger diffraction peaks indicate the existence of PbCrO4 (which corresponds to PDF#73-1068) from a reaction between Pb(II) and CrO42rather than Pb(NO3)2. The CrO42- ions could result from doping chromic acid. The weakened diffraction peaks at 7.7° and peak shift of the strongest diffraction peak around 24.5° are an indication of a chelation adsorption between the ions and PSA chains. There is no doubt that major chelation and minor precipitation adsorptions of the ions onto the nanoparticles take 5310
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Table 6. Competitive Adsorption of Six Metal Ions onto 50 mg of PSA at 30 °C in 25 mL Solution at the Same Initial Ion Concentration of 20 mg L-1 for Each Metal Ion for 1 h adsorptivity (%) mixed solution
metal ions
separated solution
mixed solution
adsorptivity difference (%)
Hg(NO3)2
Hg(II)
99.8
99.1
-0.7
Pb(NO3)2
Pb(II)
99.7
97.8
-1.9
Cu(NO3)2 FeCl3
Cu(II) Fe(III)
79.4 29.9
68.5 24.3
-10.9 -5.6
Zn(NO3)2
Zn(II)
25.6
22.1
-3.5
Hg(NO3)2
Hg(II)
99.8
99.3
-0.5
Pb(NO3)2
Pb(II)
99.7
98.5
-1.2
AgNO3
Ag(I)
94.8
86.7
-8.1
Cu(NO3)2
Cu(II)
79.4
70.8
-8.6
Zn(NO3)2
Zn(II)
25.6
38.6
13.0
place simultaneously.14 In particular, the electrical conductivity of the Pb(II)- and Hg(II)-adsorbing nanoparticles in Tables 5 and S5 (Supporting Information) is about 5 and 7 times higher than that of the virgin nanoparticles in Table 1, respectively. These results imply a real adsorption of the metal ions onto the PSA nanoparticles. Taking into account the presence of some dNþH— and —SO3H groups in the chain structure of PSA nanoparticles obtained in acidic aqueous solution, another possible route for the metal ion adsorption is ion exchange between Pb(II)/Hg(II) and Hþ ions on the dNþH—/—SO3H groups (Scheme 1). A slight decrease in the solution pH [ΔpH = -0.23 and -0.25 for Pb(II) and Hg(II) adsorption, respectively, in Table S5 of the Supporting Information] was observed once the metal ion solution was treated by the nanoparticles because of the release of Hþ ions from the dissociation of dNþH—/— SO3H groups. As summarized in Tables 5 and S5 (Supporting Information), lower ion adsorptivity exactly corresponds to the weaker pH decline, which is more evidence of the ion exchange adsorption. Further desorption by using enough HNO3 as a desorbent shows that only Pb(II) of 6.1% was desorbed from the nanoparticles, indicating the ion exchange, physical, and precipitation adsorptions only contribute 6.1% of the whole adsorption capacity. The fact that no Hg(I) and Fe(II) ions resulting from redox sorption were detected after the adsorption treatment strongly indicates no redox sorption during the Hg(II) and Fe(III) ion adsorption onto the nanoparticles. Therefore, it can be inferred that the chelation adsorption contributes 93.9%, which is substantially coincident with the existence of a large amount of chelation groups like —NH—, —Nd, —NH2, dO, and —SO3H, but with the existence of a very small amount of ion exchange groups including dNþH— and —SO3-Hþ. As a result, the chelation adsorption mechanism dominates the Pb(II) adsorption onto the nanoparticles. The Hg(II) adsorption onto the nanoparticles could consist of three adsorption mechanisms —chelation, physical, and ion exchange adsorptions—among which chelation may also be dominant during the adsorption. Competitive Adsorption of Metal Ions. As summarized in Table 5, the theoretical selectivity coefficient of the three heavier Pb(II), Hg(II), and Cd(II) ions over three lighter ions ranges from 3.08 to 9.63, signifying that coexisting lighter ions like Cu(II), Fe(III), and Zn(II) hardly ever influence the robust adsorbability of the PSA nanoparticles toward heavier Pb(II) and Hg(II) ions, as shown in Table 6. The adsorptivity of each ion in
the mixed ion solution is slightly lower than that in its pure solution because of their interferences in the mixed solution with one another. The adsorptivity order onto the nanoparticles was Hg(II) > Pb(II) > Cd(II) > Ag(I) > Cu(II) . Fe(III) > Zn(II). The most powerful adsorbability of Hg(II) ions onto the nanoparticles has not been affected by Pb(II) ions, and vice versa, because the Hg(II) adsorptivity is constantly higher than Pb(II) adsorptivity, just like the respective adsorptivity of Hg(II) and Pb(II) onto the nanoparticles in Tables 5 and S5 (Supporting Information). That is to say, although there is almost no competitive adsorption among Hg(II), Pb(II), Cd(II), Ag(I), and Cu(II) ions onto the nanoparticles, a good competitive adsorption between two groups of ions, i.e., Hg(II)/ Pb(II)/Cd(II)/Ag(I)/Cu(II) and Fe(III)/Zn(II) ions, is discovered. This competitive adsorption would play an important role in the perfect purification of real ambient wastewaters. Accordingly, the PSA nanoparticles are a very robust sorbent to selectively and efficiently remove the harmful Hg(II), Pb(II), Cd(II), Ag(I), and Cu(II) ions from their mixture containing the nutritious metal ions, as revealed below. Comparison of Pb(II) Adsorbability with Other Sorbents. Since the different adsorption conditions used by various investigators may cause the difference of the Pb(II) adsorbability, the comparison is divided into two parts: dilute and concentrated Pb(II) solutions. PSA nanoparticles possess stronger Pb(II) adsorbability than the other sorbents toward dilute Pb(II) solutions. The adsorptivity of Pb(II) at 20 mg L-1 onto the PSA nanoparticles at a dosage of 2.0 g L-1 can reach up to 99.7%, which could be realized only by using a much higher dose (5-10 g L-1) of previously reported efficient sorbents like mustard husk,43 celtek clay,44 lichen (Parmelina tiliaceae) biomass,45 and rice husk ash.46 On the basis of a careful comparison of the Pb(II) adsorbability onto several typical sorbents summarized in Table S6 of the Supporting Information,11,12,14,47-56 the PSA nanoparticles have an even stronger adsorbability toward concentrated Pb(II) solutions than the other sorbents reported so far to the best of our knowledge. The adsorptivity of activated carbon51 and chitosan-coated sand50 at the respective dosages of 20.0 and 83.3 g L-1 toward 100 and 500 mg L-1 Pb(II) is only 95.4 and 95.1%, respectively. However, the PSA nanoparticles at the dose of 1/10 and 1/42 of the activated carbon and chitosan-coated sand have achieved 89.6% adsorptivity. If the dose of the PSA nanoparticles is increased to 4.0 g L-1, the adsorptivity of the 5311
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Table 7. Removal Efficiency of Pb(II) from Ambient Polluted River and Printery Wastewater by Using the PSA Nanosorbents Obtained with K2CrO4 after first adsorption
after first/second adsorption
metal
concentration in
concentration
ions
polluted river (mg L-1)
(mg L-1)
adsorptivity (%)
wastewater (mg L-1)
concentration (mg L-1)
adsorptivity (%)
concentration in printery
Pb(II)
0.4242
,0.025
.94.1
6.796
99.6
Cu(II) Fe(III)
0.1385 0.1919
0.0831 0.1825
40.0 4.90
0.0490 0.1736
0.0442/0.0415 0.1642/0.1602
9.80/15.3 5.41/7.72
Zn(II)
0.0134
0.0133
0.746
0.5838
0.5811/0.5806
0.462/0.548
Figure 7. Fluorescent excitation and emission spectra obtained at respective slit widths of 5 and 2 nm of 20 mg L-1 SA and PSA solutions in DMSO. The PSAs were prepared with a K2CrO4/monomer molar ratio of 2 in 50 mM HClO4 at 25 °C for 72 h.
same 200 mg L-1 Pb(II) will significantly rise up to 99.6%. The fact that the PSA nanoparticles have powerful adsorbability toward concentrated Pb(II) solution of 200 mg L-1 implies that the nanoparticles will demonstrate very high treating capability and wonderful durability if treating dilute Pb(II) solution below 20 mg L-1. Besides, the initial adsorption rate and equilibrium adsorption time of Pb(II) onto the nanoparticles are both the highest heretofore. The PSA nanoparticles can accomplish equilibrium adsorption toward 200 mg L-1 Pb(II) solution in 1.0 h at a very fast initial adsorption of 10350 mg g-1 h-1, but most of the other sorbents could reach equilibrium adsorption in 1.04-24 h at a much slower initial adsorption rate. Thus, satisfactory Pb(II) adsorbability must be attributed to an optimal synergic combination of the five kinds of active groups with moderate specific area of the PSA nanoparticles. Therefore, high specific area is not necessary for advanced sorbents. Practicability of PSA Nanosorbents for Purifying Ambient Wastewaters. The four main metal ions and their concentrations in the ambient polluted river and Printing House wastewater
before and after adsorptions are summarized in Table 7. After the first adsorption, the Pb(II) concentration in ambient polluted river decreased remarkably to be below 0.025 mg L-1, realizing .94.1% Pb(II) adsorptivity. The Cu(II) concentration in ambient polluted river decreased significantly to 0.0831 mg L-1, realizing 40.0% Cu(II) adsorptivity. Fortunately, the concentration of two typical nutritious ions only slightly declined. Similarly, the Pb(II) concentration in printery wastewater containing many more Pb(II) ions decreased dramatically to be below 0.025 mg L-1, achieving .99.6% Pb(II) adsorptivity, whereas other metal ions were only slightly adsorbed onto the PSA nanosorbents. This significantly indicates that the PSA nanosorbents have a very high selectivity of adsorbing Pb(II) over Cu(II), Fe(III), and Zn(II) ions in real ambient wastewaters, demonstrating an adsorbability order of Pb(II) . Cu(II) > Fe(III) > Zn(II), which coincides well with the results in Tables 5 and 6. After only one-off adsorption, the Pb(II) concentration reduced to
