Microwave Assisted Preparation of Thio-Functionalized

Research Article pubs.acs.org/journal/ascecg

Microwave Assisted Preparation of Thio-Functionalized Polyacrylonitrile Fiber for the Selective and Enhanced Adsorption of Mercury and Cadmium from Water Sheng Deng,†,‡ Guangshan Zhang,*,†,‡ Shuang Liang,† and Peng Wang*,†,‡ †

State Key Laboratory of Urban Water Resource and Environment, Harbin 150090, PR China School of Environment, Harbin Institute of Technology, Harbin 150090, PR China

‡

S Supporting Information *

ABSTRACT: In order to overcome the drawbacks of conventional heating for performing surface modification process, such as long synthesis time, low ef ciency, and adsorption capacity, we reported a novel approach by introducing a microwave (MW) assisted method to prepare a thio-functionalized fibrous adsorbent (PANMW-Thio) for the enhanced and selective removal of Hg2+ and Cd2+ from water in this study. The properties and morphologies of the adsorbent prepared by the MW-assisted method and conventional heating were compared through FTIR, XRD, SEM characterization, and mechanical tests, and the results revealed that the MW assisted method is a satisfactory technology to enable a higher sulfur content and reinforced mechanical property for the adsorbent. The selective adsorption experiment indicated that PANMW-Thio fibers possess higher affinity toward both Hg2+ and Cd2+ in the mixed solution with Pb2+, Cu2+, Ni2+, and Zn2+. The optimum pH value for the adsorption of Hg2+ and Cd2+ was found to be 7 with maximum adsorption capacities of 322.8 mg·g−1 and 350.6 mg·g−1, respectively. The pseudo-second-order model and Langmuir model have revealed good fitness with the experimental data. The XPS analysis of the PANMW-Thio fibers before and after metal adsorption demonstrated the chelation adsorption mechanism between metal ions and sulfur, and parts of the metals were believed to be converted to metallic sulfate or sulfides on the surface of the fibrous adsorbent. This fibrous adsorbent could still retain more than 80% of its original adsorption capacity when regenerated by HCl solution. The facile and rapid preparation protocol, high adsorption capacity and highly retained mechanical property of PANMW-Thio indicates its possible application in selective removal of Hg2+ and Cd2+ ions from water. KEYWORDS: Microwave assisted, Sulfur containing, Selective removal, Hg2+, Cd2+

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INTRODUCTION Among many heavy metal ions, mercury (Hg) and cadmium (Cd) present severe environmental concerns on account of their high toxicity, bioaccumulation, and biomagnified character in aquatic food chains.1 Furthermore, mercury is feasible to be transformed through both biotic and abiotic methylation processes to methylmercury after being released to the environment, which is much more toxic than inorganic mercury.2 Cadmium, in particular, can cause muscular cramps, renal degradation, skeletal deformities, chronic pulmonary problems, erythrocyte destruction, and diarrhea if enriched in the human body.3 Thus, to protect ecosystems, the discharge of Hg2+ and Cd2+ are under strict regulatory limits, and the permissible concentration levels of mercury and cadmium in drinking water are 2 μg·L−1 and 5 μg·L−1, respectively.4,5 Generally, the source of heavy metal pollution can be divided into general accumulation created by anthropogenic activities and sudden pollution accidents.6 In contrast to common water pollution, sudden water pollution events can cause rapid © 2017 American Chemical Society

deterioration of water quality, and the severe impact on the health of humans and socio-economic activities could be both short-term and long-term. Therefore, rapid responses to emergency environmental incidents related to heavy metal pollution are requisite. Currently, the precipitation/adsorption methods are the most commonly used process to treat heavy metal sudden pollution accidents, which are usually costly, of low capacity, and technologically challenging for implementation.7 In response to these challenges, a variety of novel adsorbents have been developed for removing the Hg2+ and Cd2+ from aqueous solution. He et al. prepared a (3-mercaptopropyl)trimethoxysilan functionalized Zn-doped biomagnetite nanostructured particles which could be used as a high-capacity and collectable adsorbent for the removal of Hg2+ from water.8 Received: March 29, 2017 Revised: May 24, 2017 Published: June 4, 2017 6054

DOI: 10.1021/acssuschemeng.7b00917 ACS Sustainable Chem. Eng. 2017, 5, 6054−6063

Research Article

ACS Sustainable Chemistry & Engineering

Polyacrylonitrile fibers (PANF) made of 100% acrylonitrile with a diameter of 10 ± 0.5 μm were commercially available from Beijing Rongnai Industry Material Co., Ltd. (Beijing, PR China). Stock solution of metal ions were prepared from mercury chloride (Hg(Cl)2), cadmium chloride dehydrate (CdCl2·2.5H2O), lead nitrate (Pb(NO3)2), and copper nitrate (Cu(NO3)2·3H2O) (Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, PR China) in deionized (DI) water. All of the other reagents were of analytical purity and used without further purification. Cross-Linked Reaction of PANF. The PANF were dried in an oven overnight before use and cut into 5 cm lengths, to prevent them from entwining during the reaction with stirring. Then, 1.0 g of PANF, 40 mL of DETA, and 20 mL of DI water were added into a 250 mL round bottomed flask and then sonicated for 5 min. Subsequently, the flask was moved into a specialized commercial microwave reactor (COOLPEX-E) with a maximum power range of 1200 W. Figure S1 shows the components of the MW reactor. The refluxed reaction takes place by setting the power of MW at 200 W and 20 min reaction time in total. However, for the sake of overheating, an intermittent heating pattern was applied, which means that the reaction time was divided into 10 groups and that there is a 30 s break off between each group. After the reaction, the cross-linked fiber was washed with hot distilled water until it was neutral and dried in a vacuum at 343 K overnight. The grafting rate percentage (GP %) was calculated by gravimetry through eq 1:

Sorption of Hg2+ to the nanostructured particles was much faster than other commercial sorbents, and the maximum capacity of Hg2+ was reached around 416 mg·g−1. Hua et al. fabricated a new hydrous Zr(IV) oxide-based nanocomposite for the enhanced removal of Cd2+ from water.9 The synthesized adsorbent exhibited both preferable adsorption ability toward Cd2+ and satisfactory regeneration performance without significant capacity loss. Unfortunately, these adsorbents are usually designed in granular form, and the porous area on their site are inclined to be impeded by diffusion processes, which could limit metal ions transported onto the adsorption site. Moreover, the preparation processes usually involved a long time interval, which is inappropriate for the treatment of sudden pollution. Recently, fibrous materials have been well studied because of their ultralarge special area, low flow resistance, and many kinds of application forms, for instance, being knitted into a fibrous net or curtain.10−12 Compared with granular adsorbents, the fibrous adsorbents are barely porous, while different functional groups anchored on the surface of the fibers could interact with metal ions directly and efficiently. This unique feature enables the elevated adsorption amount and selective adsorption ability for practical applications. A microwave (MW) assisted method, applied in both polymer syntheses and modification areas, has been of continuous growing interest since the beginning of the millennium.13−15 MW-assisted heating is different from conventional heating, based on the dielectric character of microwaves that excite polar molecules on account of their dipolar polarization or to conduct charged particles.16 Because of the direct interaction of the electromagnetic irradiation with the molecules, several advantages such as shortened reaction time, increased yields, and reduced side reactions were offered by the MW-assisted method.17 Surface modification of different matrixes under MW irradiation, such as activated carbon,18 chitosan,19 multiwall carbon nanotubes,20 biochar,21 cellulose fiber,22,23 and polyacrylonitrile fiber,24−26 has been reported to remove metal ions from aqueous solution. Herein, we described a thio-functionalized polyacrylonitrile fiber for Hg2+ and Cd2+ removal. The fibrous functional material was prepared via the MW-assisted method. The MWassisted approach has several advantages, including high grafting rate (leads to high adsorption capacity), low energy input (remarkably reduced synthesis time), enhanced mechanical property, and good regeneration ability. In addition, the novelty of this method from our previous studies includes the application of an environmentally friendly solvent instead of toxic organic solvent for the coating of the thio ligand onto the surface of the fibers. It is firmly believed that the thio functionalized fiber may have strong and selective complexation affinity for Hg2+ and Cd2+, as a consequence of a soft Lewis acid−base theory. Accordingly, the modified fiber was carefully characterized, and the adsorption performance, including pH effect, adsorption kinetics and isotherm, adsorption selectivity, recycling properties, and adsorption mechanism were thoroughly investigated.

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GP% =

m1 − m0 × 100% m0

(1)

where m0 and m1 are the weights of raw PANF and cross-linked fiber, respectively. The 40% weight gain jonquille fiber was obtained and named as PANMW-DETA. Preparation of Thio-Functionalized Fibers. The functionalization reaction was carried out in a phosphate buffer solution with a pH value around 6.8−6.9. Briefly, 1.0 g of PANMW-DETA, 5.5 g of Na2S· 9H2O, and 100 mL of buffer solution were mixed homogeneously, and the reaction was performed in the MW reactor under the same intermittent heating pattern; the preparation parameters are the power of MW at 200 W and 5 min of reaction time. The modified fiber was washed with DI water until neutral and suction filtered. Finally, the carmine fiber with 17% weight fiber was prepared and named as PANMW-Thio. Figure 1 illustrated the preparation process for PANMWThio fibers by the MW-assisted method. For comparison, the chelating fiber (PANCV-Thio) was also prepared by a conventional method

EXPERIMENTAL SECTION

Chemical Reagent. Diethylenetriamine (DETA), disodium hydrogen phosphate (Na 2 HPO 4 ), sodium dihydrogen phosphate (NaH2PO4), sodium sulfide nonahydrate (Na2S·9H2O), and N,Ndimethylformamide (DMF) were of analytical grade and purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, PR China).

Figure 1. Schematic of PANMW-Thio fiber preparation by the MWassisted method. 6055



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according to the references, and the synthesis details are described in the Supporting Information. Characterization. Fourier transform infrared spectroscopy (FTIR) experiments were performed via a FT-IR spectrometer (PerkinElmer spectrum 100), and the spectra were recorded in the wavenumber ranging from 400 to 4000 cm−1. Zeta potential measurements were conducted with a Zeta voltmeter (Zetasizer Nano ZS90). The X-ray powder diffraction spectra were recorded with a X’PERT PRO MPD X-ray diffractometer (Panalytical Corporation). Tensile mechanical properties were evaluated using a YG001A electronic fiber tensile strength tester at a loading rate of 10 mm· L−1 under room temperature. Surface morphology and element composition were observed by scanning electron microscope spectroscopy (SEM; FEI QUANTA 200). The X-ray photoelectron spectroscopy (XPS) measurements were conducted using an XPS spectrometer (Thermo Fisher Scientific, ESCALAB 250Xi), with monochromatized Al Kα X-rays. Heavy Metal Adsorption onto PANMW-Thio Fibers. The selective adsorption property of PANMW-Thio fibers was investigated under competitive conditions, and approximately 0.1 g of fiber was placed into the 100 mL ion mixture solution at pH 7.0. More specifically, two mixed solutions were prepared, in which Hg2+ or Cd2+ were the target ions, while Pb2+, Cu2+, Ni2+, and Zn2+ were the coexisting ions. The initial concentration for the ions in both solution 1 and solution 2 was 500 mg·L−1, and the mixed solution was shaken overnight at 25 °C until an equilibrium capacity was reached. The concentration variations of different metal ions were determined using ICP-OES. The amount of adsorbed metal ions onto the modified fiber can be obtained by eq 2

qe =

(Co − Ce) × V m

Research Article

RESULTS AND DISCUSSION Characterization: Comparison of the MW-Assisted Method and Conventional Heating. Surface modification of PANF with the thio ligand was successfully performed both by the MW-assisted method and conventional heating, as shown by FTIR analysis of the materials in Figure 2. The

Figure 2. FT-IR spectra of PANF, PANMW-DETA, PANCV-Thio, and PANMW-Thio fibers.

characteristic peak of the PANF framework at wavenumber 2243 cm−1 was observed, which can be attributed to the stretching modes of −CN.27 However, the adsorption band at 1683 cm−1 still confirms the existence of methyl acrylate or methyl methacrylate in the fiber. The PANF cross-linked with DETA gave rise to a spectrum similar to that of the reference. In particular, the adsorption band at 2243 cm−1 was remarkably reduced, and the 1681 cm−1 adsorption band disappeared completely after cross-linking, while the stretching band of -CO and -NH2 in the amide group was observed at 1651 and 1566 cm−1,28,29 respectively. Furthermore, the peaks at 3400 cm−1 were assigned to the combination of adsorbed water and -NH2, therefore, confirming the successful preparation of PANMW-DETA fiber. After reaction with sodium sulfide, the −CN adsorption band was further declined. The newly appeared peak at 1373, 1224, and 884 cm−1 corresponded to the stretching vibration of SC-NH, -CS, and bending vibration of -CS,25,30,31 indicating that the thio group was anchored to the fiber. Compared to the specific adsorption band of these two fibers, the peaks at 1373, 1224, and 884 cm−1 are sharper and more intensified, and the reduction of the −CN group is more obvious, which may result from the increased conversion ratio of −CN into SC-NH in the alkalescence condition. These results were also confirmed by elemental analysis experiments. As illustrated in Table 1, the sulfur content in the PANMW-Thio fiber is almost twice as much as that of the PANCV-Thio fibers. The promising increment of

(2)

where qe is the equilibrium amount of PANMW-Thio fibers, Ce and C0 are the concentration of metal ions remaining in solution phase and at initial phase, V is the volume of the aqueous phase, and m is the weight of PANMW-Thio fibers put into the solution. The adsorption efficiencies of prepared PANMW-Thio fibers toward Hg2+ and Cd2+were evaluated by batch adsorption experiments, and the influence of several parameters, such as pH value of the solution, contact time, and initial concentrations of metal ions have been investigated accordingly. Briefly, 0.1 g of PANMW-Thio fibers were added into each 100 mL metal ion solution in the 250 mL sealed bottle with the concentration ranging from 30 to 500 mg·L−1. Then, the bottle was oscillated on a model SHA-C shaker (Ronghua instrumental manufactory Co., Ltd., China) with a shaking speed of 100 rpm at different temperatures until the equilibrium was established. The fiber was filtered, and the concentration of the metal ions before and after adsorption was measured with ICP-OES. The pH effect for the capacities toward different ions of PANMWThio fibers were performed by adjusting the pH value of the solution from 2.0 to 7.0. The initial concentrations of different metal ions were kept at 500 mg·L−1, and the adsorption amount under different pH conditions was also calculated according to eq 2. The adsorption kinetics experiments were conducted with the same operation conditions as the equilibrium studies. Generally, a 0.1 g portion of fibers was added to 100 mL of metal ion solution with the initial phase of 500 mg·L−1 with the pH at 7.0. Afterward, aliquots of 1.0 mL solution were taken out at different predetermined intervals, and the amount of metal ions was measured. For regeneration experiments, the PANMW-Thio fibers were retrieved by filtration from metalions solution and washed with DI water to remove any residual solution. Subsequently, the metal−fiber composite was immersed into a 0.1 M HCl solution and shaken at 100 rpm for 30 min. The desorbed metal ions in the solution were determined by ICP-OES, and the desorption efficiency (%) was calculated based on the ratio between desorbed and preadsorbed amounts of metal ions. The adsorption/desorption cycle was repeated five times using the same PANMW-Thio fibers to estimate the regeneration performance.

Table 1. Elemental Analysis of the PANF, PANMW-DETA, PANMW-Thio, and PANMW-Thio Fibers

6056

sample

C (%)

N (%)

H (%)

S (%)

PANF PANMW-DETA PANMW-Thio PANCV-Thio

65.86 56.41 60.71 62.16

25.96 20.35 13.58 17.24

5.62 7.18 4.06 6.95

17.83 9.62


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Figure 3. (a) XRD pattern of PANF, PANMW-Thio, and PANCV-Thio fibers. (b−d) The breaking strength, modulus, and rupture elongation of PANF, PANMW-Thio, and PANCV-Thio fibers.

gradually, while this effect becomes more and more severe in a long time duration (>12 h) by the conventional heating. To further prove this assumption, the mechanical properties of PANF, PANMW-Thio, and PANCV-Thio were tested by fiber electronic tensile strength tester, and the results are elucidated in Figure 3b−d. Basically, the high value of breaking strength, modulus, and low profile of rupture elongation imply a highly crystallization of molecules, thus leading to high strength fiber.34 The breaking strength, modulus, and rupture elongation of the untreated fibers were 927.3 MPa, 12.5 GPa, and 18.06%, respectively. After modification by the MW-assisted method, the breaking strength, modulus, and rupture elongation values of PANMW-Thio fibers changed to 733.5 MPa, 10.7 GPa, and 26.5%, which indicated that the modification process mostly happened in the side chain of the fiber resulting in the high maintenance of the mechanical property of the fiber. However, by using conventional heating, due to the prolonged time of reaction, the fiber was swollen and modified simultaneously, so the crystalline region was fractured, and the ratio of noncrystalline region increased gradually. The higher the ratio of noncrystalline region holds, the easier can be its rupture. Consequently, the breaking strength and modulus of PANCVThio fibers declined to 321.7 MPa and 5.15 GPa, respectively, while the rupture elongation increased to 41.36%. The surface features and morphological structure of the fibers were observed with SEM. In the SEM image of the PANF shown in Figure 4, one can notice that the fibers have a flatter, dense, and neat surface, with the average fiber diameter of (13 ± 0.4) μm. After surface functionalization with thio groups by applying the MW-assisted method, it was clearly observed that the diameter of the fiber has increased considerably and that the average fiber diameter reached 25 ± 0.6 μm. It was also

sulfur may be attributed to a direct and significant interaction between the MW and nitrile/carbonyl dipoles in the fiber, which causes the direct heating, as opposed to indirect, thermal heating by contact with the solvent molecules. To further improve this, the water has also been applied in conventional heating. However, almost no sulfur content has been discovered by elemental analysis, which means the grafting process cannot be implemented in water solution by conductive heating. It is then speculated that this could explain why MW heating not only is faster but also leads to higher grafting content of sulfur onto PANMW-Thio fiber. Additionally, the mechanical property of the fibrous adsorbent prepared by two methods has been investigated and compared. The powder XRD patterns of PANF, PANMWThio, and PANCV-Thio fibers are shown in Figure 3a. The strong and sharp diffraction peak at 16.9° corresponds to the (100) diffraction of the hexagonal lattice formed by parallel close packing of molecule rods, and it is believed that a rod-like conformation was formed in the fiber matrix as a consequence of the intermolecular repulsion of the −CN dipoles.32,33 After modification with the MW-assisted method, the diffraction peak of PANMW-Thio fiber decreases a certain amount, which is mostly due to the reaction of −CN in the molecular chain with the functional reagent. However, the inner crystalline structure of the fiber has slightly been wrecked. Comparatively, the crystallization rate of PANCV-Thio fibers prepared through conventional heating reduced drastically. Apparently, this does not indicate the high ratio conversion of the −CN group based on the result of elemental analysis. Since the DMF possesses the ability to dissolve PANF, the most likely reason for this phenomenon is the penetration effect of DMF solvent into the inner molecular chain and the destruction of the crystal region 6057


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Figure 4. SEM images of PANF, PANMW-Thio, and PANCV-Thio fibers.

noticeable that the fiber was wrinkled after modification. However, no clear fracture was visualized on its surface. Although the grafting effect can hardly be distinguished through SEM images, it is still confirmative to see that the PAMMW-Thio fiber retained its original conformation to a great extent. Comparatively, the SEM images of PANCV-Thio fibers have revealed much more rough surface morphology, and crackles appear to emerge on the surface extensively, which results in the poor mechanical property when subject to practical application. The consistent SEM results with mechanical property tests have further indicated that the MW-assisted method is a promising technology to overcome the liability problem that results from conventional heating and improves the range of possible application of the modified fibers. Adsorption Performance Studies. Our previous studies have confirmed that original PAN fibers do not show any adsorption effect toward metal ions.24 Thus, the adsorption experiments were only performed with modified fibers. Selectivity Adsorption. The selective adsorption property of the PANMW-Thio fibers toward cation heavy metal ions makes them promising candidates for the removal of Hg2+ and Cd2+ from ion mixtures. In order to investigate the favorable ability,

the experiments were performed by add Hg2+ or Cd2+ into the mixture solution of Pb2+, Cu2+, Ni2+, and Zn2+ ions with the initial concentration of 500 mg·L−1, and the adsorption was tested without adjusting the pH of the solution. The results shown in Figure 5 indicate that in the five ions combination system, the PANMW-Thio fibers have exhibited specific adsorption ability for both Hg2+ and Cd2+ ions. Specifically, in the Hg2+ system, the adsorption affinity of PAMMW-Thio fibers toward metal ions was in the order of Hg2+ > Pb2+ > Cu2+ > Zn2+ > Ni2+ with the adsorption capacity of 218.7 mg·g−1, 52.42 mg·g−1, 25.71 mg·g−1, 12.94 mg·g−1, and 8.5 mg·g−1, respectively. While in the Cd2+ system, almost the same tendency was found, and the sequence of adsorption was Cd2+ > Pb2+ > Cu2+ > Ni2+ > Zn2+. The distribution coefficient (Kd) represented the affinity of an adsorbent, and values for both Hg2+ and Cd2+ were calculated, and the results are given in Table 2. Obviously, a higher adsorption amount would lead to higher Kd values, and for this reason, the Hg2+ demonstrated the highest Kd value of 777.5 mL·g−1, while the Kd value of PANMW-Thio fibers for Cd2+ also reached 564.9 mL·g−1. The superior binding feature of PANMW-Thio fibers toward Hg2+ ions may be due to Pearson’s hard−soft-acid−base (HSAB) 6058


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Figure 6. Effect of solution pH on Hg2+ and Cd2+ adsorption onto PANMW-Thio fibers. Figure 5. Selective adsorption capacity of PANMW-Thio fibers for different metal ions.

reduction of adsorption amount of PANMW-Thio fibers at lower pH value may be attributed to (i) higher hydrated species (Hg2+, Cd2+) having low mobility, (ii) the protonation increasing the positive charge density in the fibrous adsorbent (−NH2+H+=-NH3+) and enhancing electrostatic repulsion for metal ions, and (iii) the excessive existence of hydrogen ion competing with metal ions. An appreciable increase in the sorption rate was observed as the pH increased, and the maximum sorption efficiency for Hg2+ and Cd2+ were obtained at pH 7.0. This phenomenon could be explicated by that fact that at higher pH values, more functional groups are deprotonated and become more accessible for metal ions, leading to higher adsorption capacities. In order to confirm this assumption, the zeta potential of PANF and PANMW-Thio fibers (Figure S2, Supporting Information) was measured as a function of different pH values. The pHPZC value increased from 3.7 to 5.5 by anchoring thio groups onto the surface of the fiber, which means the adsorbent is negatively charged when pH > 5.5. This result further elucidated the elevated adsorption performance of PANMW-Thio fibers when pH increased from 2 to 7. Consequently, pH 7.0 was chose to be the optimum pH for further studies. Adsorption Kinetics. The kinetic studies of Hg2+ and Cd2+ onto the PANMW-Thio fibers were performed at room temperature (298 K), and the results are present in Figure 7. The adsorption amount increased rapidly during the first 60 min, and over 50% of the equilibrium adsorption was obtained. Afterward, the adsorption rate became gentle, and the equilibrium was accessed within 4 h. Initially, the fast adsorption rate may be attributed to the high concentration of metal ions, along with the large amount of active adsorption sites within PANMW-Thio fibers. As the adsorption amount of Hg2+ or Cd2+ onto the adsorbent increased, the repulsive forces between the adsorbed species are boosted, and adsorption resistance for free metal ions is exacerbated accordingly. To describe the adsorption process quantitatively, adsorption data for Hg2+ and Cd2+ were subjected to three commonly used kinetics fittings, and details are provided in Supporting Information. The parameters calculated based on a nonlinear approach as well as the correlation coefficient (R2) were summarized in Table 3. The considerable high value of R2 (>0.998) indicated that the pseudo-second-order model was able to exhibited better approximation for the kinetics adsorption for both Hg2+ and Cd2+ onto the PANMW-Thio fiber as compared to the other two models. Thus, the adsorption process would be determined

Table 2. Distribution coefficient of PAMMW-Thio fibers for different metal ions Kd (mL·g−1) matrix

Hg(II)

solution 1 solution 2

777.5

Cd(II)

Pb(II)

Cu(II)

Ni(II)

Zn(II)

564.9

117.1 197.3

54.28 58.65

27.57 32.20

17.29 33.91

that soft ligands (thio) are more favorable to bind soft metal ions. Theoretically, the thioamide group was initially protonated and then loses the proton after incorporation with Hg2+. Additionally, according to the report of Gomez-Serrano et al.,35 the majority of dissolved mercury chloride in the solution is in the form of neutral HgCl2, while less than 2% undergoes primary dissociation (HgCl2 = [HgCl]+ + Cl−) and even more smaller rate of secondary dissociation ([HgCl]+ = Hg2+ + Cl−). As a general rule, compared with dissolved metal ions, neutral species are softer acids and that suggested higher affinity of Hg2+ for PANMW-Thio fibers. As for CdCl2, in the absence of excessive Cl−, the main dissociation equilibrium would be CdCl2 = Cd2+ + 2Cl−. Although some reports indicated that the sulfur anchored adsorbent exhibited higher adsorption performance for Pb2+ than Cd2+,36 the reverse results in our study may be attributed to the synergistic chelation effect of the thio group and partially hydrolyzed carboxyl groups on the fibers. Effect of pH. The change of pH could affect the speciation of metal ions and the surface properties of modified fibers. Referring to the literature, when the pH is more than 7.0, Hg(OH)2 and Cd(OH)2 become the main existing compound and begin to precipitate out of the solution.37,38 In this case, the pH value of the solution was chosen from 2.0 to 7.0. As shown in Figure 6, the adsorption rate was found to increase from pH 2.0 to 7.0. This trend might relate to the property of the adsorbent along with the different species of metal ions. The speciation studies have confirmed that the mercury exists as Hg2+ at pH < 3 and that HgCl2, (HgCl2)2, Hg(OH)+, and HgOHCl are present in the pH range 3.0−7.0.39,40 While at pH < 7, the predominate species of cadmium is Cd2+.41 Moreover, metal ions have a strong tendency to form hydrated composition, and the smaller effective hydrated size exhibits high mobility in comparison with that of larger hydrated species. The effective sizes of hydrated species are in the order of M2+> M(OH)+1> M(OH)2. Consequently, the observed 6059


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Figure 7. Adsorption kinetics curves of Hg2+ and Cd2+ on PANMW-Thio fibers.

Table 3. Kinetic Parameters for Hg2+ and Cd2+ Uptake onto PANMW-Thio Fibers at 298 K pseudo-first-order metal ions 2+

Hg Cd2+

pseudo-second-order

Weber-Morris

qe (mg·g−1)

k1 (min−1)

R2

qe (mg·g−1)

k2 × 104 (mg·g−1·min−1)

R2

k3 (mg·g−1·min−1/2)

R2

C

310.1 334.45

0.03764 0.03269

0.9884 0.9894

325.4 362.7

1.172 0.8728

0.9996 0.9987

22.87 25.45

0.8981 0.9456

51.19 41.76

Figure 8. Adsorption isotherm curves of Hg2+ and Cd2+ on PANMW-Thio fibers.

Table 4. Isotherm Parameters for Hg2+ and Cd2+ Uptake onto PANMW-Thio Fibers at 298 K Langmuir model

Freundlich model

Temkin model

metal ions

qm (mg·g−1)

KL (L·mg−1)

R2

n

Kf (mg·g−1)

R2

KT (L·g−1)

R2

BT (kJ·mol−1)

Hg2+ Cd2+

342.5 368.8

0.2322 0.04942

0.9930 0.9945

51.19 106.7

0.3877 0.2646

0.9242 0.8817

0.5359 3.728

0.9678 0.9678

29.82 38.21

by the square of the number of free binding sites over the modified fiber and metal ion concentration, which, in another way, is a rate-limiting chemical adsorption step. Concededly, the adsorption mechanism is mainly attributed to the metal− ligand complex formation between Hg2+/Cd2+ and the thio group. This result appears to follow a similar adsorption behavior to that of divalent transition metal ions onto other sulfur-containing chelating fibers. Adsorption Isotherms. The maximum adsorption capacity is the most important indicator for the adsorbent. In order to evaluate the maximum adsorption capacities of PANMW-Thio fibers toward Hg2+ and Cd2+, the adsorption isotherms were obtained for an initial concentration of 20−500 mg·L−1 with the pH at 7. The result shown in Figure 8 indicated that the adsorption amount for both metal ions improved with the increase of initial concentration until the plateau was reached gradually. The equilibrium data were fitted by using the

Langmuir, Freundlich, and Temkin sorption mode, and the details of various isotherm models can be explored in the Supporting Information. The parameters obtained from nonlinear regression using adsorption models are shown in Table 4. The experimental results for Hg 2+ and Cd 2+ uptake versus equilibrium concentration show an excellent fit with the Langmuir adsorption model for PANMW-Thio fiber, compared to that of the Freundlich and Temkin model. The correlation coefficients (R2) are more than 0.99, and the maximum adsorption capacities are 342.5 mg·g−1 and 368.8 mg·g−1 for Hg2+ and Cd2+, respectively. The good agreement between the data and the Langmuir adsorption model indicates that the extent of metal ion adsorption is a function of specific binding sites, a limited number of which are located on the sorbent surface. The isotherm results further suggest that the large number of accessible thio ligands on the PANMW-Thio surface give rise to 6060


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Figure 9. (a) Overall XPS scan of PANMW-Thio, Hg2+, and Cd2+ adsorbed PANMW-Thio fibers. Detailed analysis of (b) Hg 4f, (c) Cd 3d, and (d) S 2p.

the large sorption amount toward Hg2+ and Cd2+. Furthermore, we also evaluated the effect of temperature (Table S1) and adsorbent dosage (Figure S3) on the adsorption performance of PANMW-Thio fiber and the results are provided in Supporting Information. Adsorption Mechanism. The surface chemical nature of modified fibers before and after adsorption was determined by X-ray photoelectron spectroscopy. Figure 9a shows the XPS spectra of PANF modified with thio groups with the MWassisted method and clearly shows the presence of sulfur. The peaks for S 2s and S 2p at 226.4 and 162.5 eV and 168.2 eV, were observed in the PANMW-Thio fibers, respectively.42,43 After mercury and cadmium adsorbed onto the fibers, the peaks for both ions were detected on the XPS spectra. More specifically, as depicted in Figure 9b, the doublet Hg2+ peaks at 101.5 and 104.8 eV are associated with Hg 4f7/2 and Hg 4f5/2, respectively. These results are coordinated with previous literature reports and indicate that the loaded Hg2+ might exist in the form of the HgSO4, HgCl2, or HgO complex.36,44 Similarly, the adsorption interaction with thio ligand was also confirmed for cadmium, and the peaks appearing at 404.5 and 412.8 eV are assigned to Cd 3d5/2 and Cd 3d3/2, as shown in Figure 9c, which are believed to be CdSO4, CdCl2, or CdO complex formation.45 To further identify whether the thio ligand is responsible for the uptake of Hg2+ and Cd2+ ions, the S 2p core-level spectra of PANMW-Thio fibers before and after adsorption were analyzed. As shown in Figure 9d, the higher signal at 162.5 eV indicates the presence of sulfur in the −2 oxidation state,46 and after the adsorption of Hg2+ or Cd2+ ions, the intensity of this peak decreased considerably which is attributed to the chelating effect of sulfur atom with metal ions.47 Likewise, the S 2p

component with a higher oxidation state was revealed at 168.3 eV, and the much lower signal suggests that the metal ions bound to sulfur in the outer part of the surface.48 In addition, the comparative low intensities of these two peaks after Hg2+ adsorption suggest that the affinity of PANMW-Thio fibers toward metal ions is in the order of Hg2+> Cd2+. Regeneration Studies. It is economized to investigate the reliability of the adsorbent in terms of long-term usage. Thus, further adsorption−desorption experiments were conducted, and the PANMW‑Thio fibers were regenerated by 0.1 M HCl after the equilibrium adsorption. The recycling efficiencies of PANMW-Thio fibers for Hg2+ and Cd2+ were collected, and the results are shown in Figure 10. It was notable that both ions can be desorbed efficiently by hydrochloric acid, which could be

Figure 10. Adsorption capacities of Hg2+ and Cd2+ on PANMW-Thio fibers after five times regeneration. 6061


Research Article

ACS Sustainable Chemistry & Engineering due to the comparatively high affinity feature of H+, and the metal ions were replaced from the fibers subsequently. The desorption efficiencies for Hg2+ and Cd2+ range from 98.6 to 88.1% and 94.6−76.1%, respectively. These results were coordinated to the inferior adsorption feature at lower pH value that we have addressed previously. Meanwhile, the readsorption experiments revealed that regenerated fibers could still capture Hg2+ and Cd2+ ions significantly in a five round usage and that the adsorption capacities retained 89.3% and 82.2% of their original amount for Hg2+ and Cd2+, respectively. The highly preserved macromolecular structure of PANMW‑Thio fibers by the MW-assisted preparation method can endow advanced resistance of acid solution during the regeneration process.

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AUTHOR INFORMATION

Corresponding Authors

*(G.Z.) Tel: +86−15245095893. E-mail: gszhanghit@gmail. com. *(P.W.) Tel: +86-451-86283557. E-mail: [email protected]. com. ORCID

Guangshan Zhang: 0000-0002-7190-415X Peng Wang: 0000-0003-4465-6207

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Notes

CONCLUSIONS In this work, a thio-functionalized fibrous adsorbent was prepared by a two step modification process through a MWassisted method that simultaneously enhanced the sulfur content onto the surface of fiber and preserved the mechanical property commendably. The MW-assisted method involves a more “green” and highly efficient approach to fulfill the surface functionalized process by replacing the DMF solvent with water. The element analysis results confirmed that the sulfur content of PANMW-Thio fibers was almost two times that of PANCV-Thio fibers. The breaking strength, modulus, and rupture elongation values of PANMW-Thio fibers were 733.5 MPa, 10.7 GPa, and 26.5%, respectively, while these values for PANCV-Thio fibers were 321.7 MPa, 5.15 GPa, and 41.36%, respectively. The SEM images showed a wrinkled morphology of the PANMW-Thio fibers, and the surface of PANCV-Thio fibers was more rough and covered with crackles. Selective adsorption tests were performed by using Hg2+, Cd2+, Pb2+, Cu2+, Ni2+, and Zn2+ as the target ions and the PANMW-Thio fibers demonstrated high selectively property toward Hg2+ and Cd2+ ions, which was mainly due to the higher affinity of these two ions with the thio group on the fibrous adsorbent. The influence of pH value, time, and initial concentration for Hg2+ and Cd2+ adsorption has been further investigated. The kinetics and isotherms of adsorption indicated the better fitness of pseudo-second-order and Langmuir model, and the maximum adsorption amount of PANMW-Thio fibers were reached to 322.8 mg·g−1 and 350.6 mg·g−1 for Hg2+ and Cd2+ at pH 7, respectively. XPS analysis confirmed that the extraction of Hg2+ and Cd2+ ions from aqueous solution was attributed to the chelation effect between sulfur and metal ions. Furthermore, the adsorbent possesses a good reusability after five times of regeneration. We believe that these facile-prepared and high adsorption capacity fibrous adsorbents may find potential application in the field of sudden heavy metal removal and other environmental protection systems.

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namics investigation of PANMW-Thio fibers and effect of adsorbent dose on the adsorption of Hg2+ and Cd2+ by PANMW-Thio fibers (PDF)

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51678185), State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (2017DX11), and the Ninth Special Financial Grant from the China Postdoctoral Science Foundation (2016T90304).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00917. Feature of the MW reactor and the procedure for the preparation of PANCV-Thio fibers through conventional heating; determination of the pHpzc of PANF and PANMW-Thio fibers; equation description of adsorption kinetics and adsorption thermals; adsorption thermody6062


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