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Feb 11, 2014 - Single electron transfer living radical polymerization (SET-LRP), reported by Percec in the polymerization of nonactivated monomer viny...
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Sweet Potato Starch Residue as Starting Material To Prepare Polyacrylonitrile Adsorbent via SI-SET-LRP Zhihai Hao, Dongju Wang, Hou Chen,* Jinming Sun, and Yuanyuan Xu School of Chemistry and Materials Science, Ludong University, Yantai 264025, China ABSTRACT: Sweet potato starch residue (SPSR) was used as starting material to prepare an eco-friendly adsorbent. SPSR was modified by bromoacetyl bromide to obtain a macroinitiator for surface-initiated single electron transfer-living radical polymerization (SI-SET-LRP) of acrylonitrile (AN) catalyzed by La(0)/hexamethylenetetramine (HMTA) in N,Ndimethylformamide (DMF) in the presence of ascorbic acid (VC). The amidoxime (AO) adsorbent was prepared by the reaction of the graft copolymer bromoactylated sweet potato starch (BSPS)/polyacrylonitrile (BSPS-g-PAN) with hydroxylamine. The maximum adsorption capacity for Hg(II) was 4.03 mmol·g−1. This simple method provided a novel approach to recycle and reuse agricultural residues for controlling heavy metal pollution. KEYWORDS: sweet potato starch residue, SI-SET-LRP, acrylonitrile, Hg(II), adsorption

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INTRODUCTION

The past decade has witnessed a significant improvement in the field of controlled radical polymerization. Single electron transfer living radical polymerization (SET-LRP), reported by Percec in the polymerization of nonactivated monomer vinyl chloride,20 is applicable to a wide range of monomers and a variety of reaction conditions. It can produce a number of complex polymer architectures.21−28 In SET-LRP, activation of dormant chains occurs through the heterolytic bond cleavage of the carbon halide bond via a heterogeneous Cu(0) catalyzed outer-sphere single-electron transfer process.29−32 The versatility and controllability of SET-LRP open up a range of possibilities of site-specific controlled grafting from functionalized sites on macromolecules. However, there are few reports on adsorbent production via SET-LRP initiated by the modified SPSR. In this paper, the modified SPSR was first used as a macroinitiator for surface-initiated single electron transfer living radical polymerization (SI-SET-LRP) of acrylonitrile (AN). The polymerization was conducted with La(0) powder as a catalyst, bromoactylated sweet potato starch (BSPS) as a macroinitiator, N,N-dimethylformamide (DMF) as a solvent, and hexamethylenetetramine (HMTA) as a ligand in the presence of ascorbic acid (VC). Graft copolymer bromoactylated sweet potato starch (BSPS)/polyacrylonitrile (BSPS-gPAN) was modified to produce the amidoxime (AO) adsorbent (AO BSPS-g-PAN) by the reaction with hydroxylamine. Then, the adsorption capacity of the resin for Hg(II) was investigated. In this work, agricultural residues were used as raw materials to produce adsorbents, truly transforming agricultural residues into “treasure”.

Sweet potato is one of the major tuber crops. It provides a valuable source of food, animal feed, and industrial raw materials.1,2 Sweet potato is mainly used to produce starch and starchy foods. Sweet potato starch residue (SPSR) is produced as a byproduct. Due to poor water solubility of the residues, the vast majority of them are directly thrown out and consequently pollute the environment.3 Recycling the starchy materials not only lowers the pollution but also reduces the waste of resources. Starch has attracted more and more attention.4,5 It consists of interconnected anhydroglucose units, each of which contains three hydroxyl groups. The properties of starches can be modified by graft polymerization to convert hydroxyl groups to other functionalities.6−9 Suwanmala et al. synthesized starchbased metal adsorbent via radiation-induced graft copolymerization of methyl acrylate (MA) onto cassava starch.10 Guo et al. prepared cross-linked porous starch (CPS) for removal of methylene blue (MB) from an aqueous solution.11 The possibility of producing adsorbents from the starch in the sweet potato would render starch residues to biodegradable and economical materials.12−14 Recently, there has been a lot of research interest in the removal of heavy metal ions, especially for selective recovery and enrichment of Hg(II) ions.15,16 Hg(II) exists in aqueous media in various forms and is the main species which can easily bind to organic and inorganic substances. Amidoximecontaining adsorbents have been synthesized and used for metal ion removal.17 Methacrylic acid (MAA) and 2hydroxyethyl methacrylate (HEMA) were cografted with acrylonitrile (AN) onto polyethylene fiber by radiation-induced graft polymerization. The resulting cyano groups were converted to amidoxime groups by reaction with hydroxylamine to recover uranium in seawater.18 Polyacrylonitrile (PAN) is an important precursor to polymer materials. Its pendant cyano groups can be easily converted to amidoxime groups for adsorption purpose.19 © 2014 American Chemical Society

Received: Revised: Accepted: Published: 1765

June 20, 2013 January 16, 2014 February 11, 2014 February 11, 2014 dx.doi.org/10.1021/jf4048397 | J. Agric. Food Chem. 2014, 62, 1765−1770

Journal of Agricultural and Food Chemistry

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Article

EXPERIMENTAL SECTION

Materials. SPSR and AN were acquired from Henan Shizhong Shuye Ltd. (Henan, China) and Tianjin Fu Chen Chemical Reagents (Tianjin, China), respectively. Methanol, hydroxylamine hydrochloride, VC, and DMF were obtained from Tianjin Chemical Reagents (Tianjin, China). HMTA, bromoacetyl bromide, dimethylacetamide (DMAc), pyridine, LiCl, and La(0) powder were purchased from Aladdin Chemistry. Methods. Infrared spectra (IR) were collected on a Perkin-Elmer Spectrum 2000 FTIR. The surface morphologies were studied using a scanning electron microscope, JSF5600LV, JEOL, Japan. Preparation of AO BSPS-g-PAN. SPSR was bromoactylated using the method described in the literature33 to prepare the bromoactylated sweet potato starch (BSPS). SI-SET-LRP of AN on the modified BSPS surface was achieved in DMF in the presence of VC with La(0) powder as catalyst and HMTA as ligand. The graft polymer BSPS-gPAN was obtained. The modified resin (AO BSPS-g-PAN) was prepared by the reaction of BSPS-g-PAN with hydroxylamine hydrochloride (NH2OH·HCl). Adsorption Characteristics of AO BSPS-g-PAN for Hg(II). The concentration of Hg(II) was determined by atomic adsorption spectroscopy. AO BSPS-g-PAN adsorbent was added to a buffered Hg(II) solution of certain pH to discuss adsorption behavior. Adsorption efficiency under situations mimicking real conditions was studied with the initial concentration of Hg(II) at 1.7 × 10−5 mol·L−1, which was close to the levels found in industrial wastewater samples.34

Figure 1. First-order kinetic plots for the SI-SET-LRP of AN.

with SPSR (Figure 2a), the FT-IR spectrum of BSPS (Figure 2b) shows a very sharp absorption band at 1749 cm−1 attributable to the carbonyl groups after esterification. A sharp peak at 2244 cm−1, corresponding to the stretching of the CN bond, is present in Figure 2c but absent in Figure 2b. This indicates that PAN has been grafted onto BSPS using SETLRP. The SEM images of (a) SPS, (b) macroinitiator, (c) BSPS-gPAN, and (d) AO BSPS-g-PAN are shown in Figure 3. Significant changes in morphology can be seen from PAN-gBSPS (Figure 3c) to AO BSPS-g-PAN (Figure 3d), indicating successful modification. Moreover, the surface of BSPS changed significantly after the esterification reaction, which demonstrated the success of the grafting reaction. Compared with the macroinitiator, the surface of BSPS-g-PAN is rougher, confirming that organic moieties were anchored successfully on the surface. All these results indicated that PAN adsorbent was successfully prepared. Adsorption Properties. Effect of pH on Adsorption. Figure 4 shows the effect of pH on the adsorption of Hg(II) by AO BSPS-g-PAN. With increasing pH of the adsorption medium, the uptake value increased and then decreased. When the pH was at 4.0, the uptake value reached a maximum of 4.03 mmol·g−1. This maximum Hg(II) adsorption capacity value is higher than that for sulfur-containing Hg(II)-imprinted thiol-functionalized mesoporous sorbent (0.39 mmol·g−1)37 and that for silica gel supported amidoxime adsorbents (1.28 mmol·g−1).38 The decrease of the uptake in the acidic media may be attributed to the protonation of nitrogen, hindering the complex formation. When the pH reached about 5 or higher, this condition is often not optimal. It was doubtful to attribute this decrease to the formation of metal hydroxide species.39 Adsorption Kinetics. As shown in Figure 5, the kinetic curves at four different temperatures follow an ascending trend with time, especially at the beginning. Then the increase becomes gentle and finally reaches equilibrium at around 4 h. In the temperature range from 15 to 45 °C, higher temperature corresponds to higher adsorption. This suggests that the adsorption was an endothermic process. Adsorption Efficiency under Situations Mimicking Real Conditions. As shown in Figure 6, the adsorption of Hg(II) per unit weight of adsorbent decreases with increasing adsorbent load. When the amount of adsorbent reached 0.10 g, the percent adsorption maximized at approximately 100%. The pseudo-first- and -second-order models were used to describe the adsorption process of Hg(II) on AO BSPS-g-PAN, as expressed in eq 1 and eq 2.

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RESULTS AND DISCUSSION Surface-Initiated SET-LRP on the Bromoactylated Sweet Potato Starch Surface. Polymerization of AN via SI-SET-LRP initiated by BSPS was carried out in DMF with La(0) powder as the catalyst in the presence of VC at 65 °C (Scheme 1). The novel catalyst system based on La(0)/HMTA Scheme 1. Modification of BSPS Macroinitiator and Polymerization of AN on BSPS Surface by SI-SET-LRP

had been developed by our laboratory, and proven to be superior to the traditional catalyst system.35 In addition, we had also reported that the addition of a small amount of VC to the reaction mixture significantly increased the reaction rate and the controllability of the polymer.36 The monomer conversion (C%) increased reaching about 23% within 24 h. Figure 1 shows kinetic plots of ln([M]0/[M]) versus reaction time. The apparent rate constant for SET-LRP of AN was calculated to be 2.9 × 10−6 s−1. The linearity of the plot indicated that the polymerization was approximately first-order with respect to the monomer concentration. Characterization. Figure 2 shows the infrared spectra of the (a) SPSR, (b) macroinitiator, (c) BSPS-g-PAN, and (d) AO BSPS-g-PAN. As shown in Figure 2c, the spectrum of the modified resins exhibits two absorption peaks at 1636 and 920 cm−1, corresponding to the stretching vibrations of C−N and N−O bonds of AO groups, respectively. In Figure 2d, the band at 2244 cm−1 related to the CN group is absent. Compared 1766

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Figure 2. FTIR spectra of (a) SPS, (b) BSPS, (c) BSPS-g-PAN, and (d) AO BSPS-g-PAN.

Figure 4. Effect of pH on the adsorption of AO BSPS-g-PAN for Hg(II).

Figure 5. Adsorption kinetics of Hg(II) on AO BSPS-g-PAN.

t 1 t = + 2 qt qe k 2qe

where k1 and k2 are the pseudo-first-order and the pseudosecond-order rate constants (min−1), respectively; qe and qt (mmol·g−1) are the amounts of Hg(II) adsorbed at equilibrium and time t (min), respectively. The value of ln(qe − qt) was calculated from the experimental results and plotted against t (min). The slope and intercept of the linear plot t/qt versus t yielded the values of qe and k2. The pseudo-first-order kinetic plots and pseudo-secondorder kinetic plots for the adsorption of Hg(II) onto AO PANg-BSPS are shown in Figures 7 and 8, respectively, and the fitting results are summarized in Table 1. Results suggest that

Figure 3. SEM photographs of (a) SPS, (b) BSPS, (C) BSPS-g-PAN, and (d) AO BSPS-g-PAN.

ln

(qe − qt) qe

= − k1t

(2)

(1) 1767

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Ce Ce 1 = + qe q0 q0KL

ln qe = ln KF +

(3)

1 ln Ce n

(4) −1

where qe is the adsorption capacity, mmol·g ; Ce is the equilibrium concentration of Hg(II), mol·cm−3; q0 is the adsorption capacity, mmol·g−1; KL is an empirical parameter; KF is the binding energy constant reflecting the affinity of the adsorbents to Hg(II); and n is the Freundlich exponent related to adsorption intensity. The adsorption isotherms of AO BSPS-g-PAN for Hg(II) in the temperature range from 15 to 45 °C are presented in Figure 9. The figures and parameters of the Langmuir and Freundlich

Figure 6. Effect of the amount of adsorbent on percent adsorption of Hg(II) ions.

the pseudo-second-order model was more suitable to describe the adsorption process.

Figure 9. Adsorption isotherms of AO BSPS-g-PAN beads for Hg(II). Figure 7. Pseudo-first-order kinetic plots for the adsorption of Hg(II) onto AO BSPS-g-PAN.

Figure 10. Langmuir isotherms of Hg(II) adsorbed on AO BSPS-gPAN at different temperatures.

Figure 8. Pseudo-second-order kinetic plots for the adsorption of Hg(II) onto AO BSPS-g-PAN.

models are shown in Figure 10, Figure 11, and Table 2. The Langmuir model fits the experimental results better than Freundlich model as the values of RL2 are higher than those of RF2. The uptake of metal ions is applicable to homogeneous adsorption, where each molecule of adsorbate on the surface has equal adsorption activation energy. Thus, the adsorptions of

Adsorption isotherms. The adsorption experimental data were analyzed by Langmuir model (eq 3) and Freundlich model (eq 4), respectively.

Table 1. Kinetic Parameters for Hg(II) Adsorption onto AO BSPS-g-PAN at Different Temperatures pseudo-first-order kinetics

pseudo-second-order kinetics

T (°C)

qe(exp) (mmol·g−1)

k1 (min−1)

qe(cal) (mmol·g−1)

R21

15 25 35 45

3.46 3.76 3.85 3.98

0.0156 0.0213 0.0220 0.0201

1.27 1.28 1.15 1.06

0.9527 0.9757 0.9723 0.9099 1768

k2 (g·mmol−1·min 0.031 0.043 0.052 0.051

−1

)

qe(cal) (mmol·g−1)

R22

3.55 3.83 3.91 4.04

0.9996 0.9999 0.9999 0.9999

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Notes

The authors declare no competing financial interest.

■ Figure 11. Freundlich isotherms of Hg(II) adsorbed on AO BSPS-gPAN at different temperatures.

Table 2. Langmuir and Freundlich Constants for Hg(II) Adsorption on AO BSPS-g-PAN at Different Temperatures Langmuir params

Freundlich params

T (°C)

q0(mmol·g−1)

KL

RL2

KF

1/n

RF2

15 25 35 45

5.21 4.87 4.68 4.53

1019 1526 2218 3077

0.9902 0.9897 0.9952 0.9979

147 88 85 61

0.6028 0.5128 0.4929 0.4348

0.9582 0.9484 0.9254 0.9346

Hg(II) on AO BSPS-g-PAN follow the mechanism of monolayer adsorption. Adsorption Selectivity. A series of representative binary metal ions of Hg(II)−Ag(I), Hg(II)−Pb(II), and Hg(II)− Ni(II) systems were chosen to investigate the adsorption selectivity of AO PAN-g-BSPS for Hg(II), and the results are shown in Table 3. As shown in Table 3, Hg(II) can be readily Table 3. The Adsorption Selectivity of AO BSPS-g-PAN for Hg(II) in Binary Ions Systems at 25 °C system

metal ion

adsorption capacity (mmol·g−1)

Hg(II)−Ag(I)

Hg(II) Ag(I) Hg(II) Pb(II) Hg(II) Ni(II)

4.02 0.41 4.07 0.45 3.99 0.36

Hg(II)−Pb(II) Hg(II)−Ni(II)

adsorbed by AO BSPS-g-PAN in the binary systems of Hg(II)− Ag(I), Hg(II)−Pb(II), and Hg(II)−Ni(II). The percent adsorptions for Hg(II) are more than 98%, indicating that AO BSPS-g-PAN exhibited good selectivity for Hg(II).

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 535 6697933. Fax: +86 535 6696162. E-mail: [email protected]. Funding

The authors are grateful for the financial support by the National Natural Scientific Foundation of China (No. 20904018), the Program for New Century Excellent Talents in University (No. NCET-11-1028), the Natural Science Foundation for Distinguished Young Scholars of Shandong province (No. JQ201203), the Shandong Provincial Natural Science Foundation of China (No. ZR2010BQ007), and the Program for Scientific Research Innovation Team in Colleges and universities of Shandong Province. 1769

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