- Email: [email protected]
Removal of mercury ions in a simulated wastewater using functionalized poly(glycidyl methacrylate)
G Model JIEC 3225 No. of Pages 5
Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Removal of mercury ions in a simulated wastewater using functionalized poly(glycidyl methacrylate) Hyun Hee Kim, Tai Gyu Lee* Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea
A R T I C L E I N F O
Article history: Received 15 November 2016 Received in revised form 16 December 2016 Accepted 16 December 2016 Available online xxx Keywords: W/O PGMA Emulsion polymerization L-cysteine Thiol
A B S T R A C T
Porous poly(glycidyl methacrylate) (PGMA) was prepared via water-in-oil (W/O) emulsion polymerization and was grafted with thiol groups. The removal of mercury (Hg) ions from water was evaluated using the functionalized PGMA. First, PGMA was synthesized and functionalized. Then, FTIR was used to assess the bonding of thiol groups with PGMA. SEM was used to analyze the porous characteristics of the material. To determine the effect of the pH on the Hg adsorption to functionalized PGMA, Hg adsorption tests were performed by varying the solution pH from 2 to 10. Additionally, a Hg adsorption test was performed by changing the initial Hg concentration. © 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Mercury (Hg) exists in various forms as metallic Hg, inorganic Hg salts and organic Hg compounds in the environment. Hg bioaccumulates up the food chain. Hg accumulation beyond a certain level in the human body damages the nerves and kidneys and causes other harmful effects. Therefore, efforts to suppress the discharge of Hg into the environment are underway. The Minamata Convention on Mercury was ratified in 2013 and is expected to be operational in 2016 [1–3]. Therefore, a safe management plan for Hg and Hg compounds is required in response to this agreement. It is difficult to separate heavy metal ions in wastewater discharged as the anthropogenic emissions from industrial facilities from water. Chemical, biological and physical treatments have been used to separate the heavy metal ions in wastewater. Adsorption is a physical method and is the most effective in terms of efficiency, stability and cost [4]. Various types of inorganic adsorbents have been studied, such as activated carbon [5–7], zeolite [8,9], and silica gel [10,11], in addition to biochemical organic materials from seaweed [12], wood [13], and polymer adsorbents [14]. Adsorption efficiency is determined by the physical structure of the pore, the specific surface area, and the porosity. Physical adsorption involves the combination with adsorbates through van der Waals forces, forming weak bonds
with the adsorbates. Chemical adsorption is caused by moving electrons, and the resulting bonding force is stronger than physical adsorption bonding [15]. In this study, porous poly(glycidyl methacrylate) (PGMA) was developed as a polymer adsorbent. PGMA forms pores via emulsion polymerization, which can control the structure and properties of the polymer material in the fabrication process. In addition, thiol groups that form stable bonds with Hg were used to functionalize PGMA [16]. A polymeric material with porous PGMA is easily functionalized with other materials because it contains highly reactive epoxy groups [17]. Epoxy groups are reactive with amines or carboxylic acids. Based on these facts, PGMA was functionalized with L-cysteine, which possesses carboxyl groups, amine groups and thiol groups [18,19]. High selectivity and efficiency are expected for the removal of Hg ions from water when using thiol groups as the PGMA functional groups. The chemical bonds between the thiol groups and PGMA were measured using Fourier transform infrared (FTIR) spectroscopy. The physical characteristics of the functionalized PGMA were identified using field emission scanning electron microscopy (FESEM) and N2 sorption analysis, and the properties of Hg ion adsorption were determined.
Materials and methods Reagents
* Corresponding author. Fax: +82 2 6008 0560. E-mail addresses: [email protected], http://[email protected] (T.G. Lee).
Glycidyl methacrylate (95%, Daejung, Siheung, Korea), divinylbenzene (80%, Sigma Aldrich, Saint Louis, USA), Span80 (Daejung,
http://dx.doi.org/10.1016/j.jiec.2016.12.019 1226-086X/© 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: H.H. Kim, T.G. Lee, Removal of mercury ions in a simulated wastewater using functionalized poly(glycidyl methacrylate), J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2016.12.019
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Siheung, Korea), potassium sulfate (99%, Daejung, Siheung, Korea), potassium persulfate (99%, Sigma Aldrich, Saint Louis, USA), magnesium chloride (98%, Siheung, Daejung, Korea), L-cysteine (95.5%, Sigma Aldrich, Saint Louis, USA), ethyl alcohol (94.5%, Daejung, Siheung, Korea), Hg(II) chloride (99.5%, Samchun Chemical, Seoul, Korea), stannous chloride dihydrate (Junsei Chemical Co., Ltd., Tokyo, Japan), and SRM3133 (9.954 mg/kg 0.053, NIST, Gaithersburg, USA) were used.
100 C for 48 h. Using this process, monolith PGMA was obtained. The preparation scheme for porous PGMA is shown in Fig. 1. Preparation of functionalized porous PGMA After mixing 0.5 g of L-cysteine, 0.1 g of MgCl2, and 1 g of PGMA in 100 mL of water, the mixture was reacted for 24 h at 100 C. After 24 h, the product was washed and dried at 60 C.
Preparation of porous PGMA
Preparation of the Hg ion solution and measurements
The water phase was prepared by mixing 0.11 g of potassium persulfate (K2S2O8), 0.55 g of potassium sulfate (K2SO4) and 45 mL of water. The organic phase consisted of 3.0 g of glycidyl methacrylate (GMA), 2.0 g of divinylbenzene (DVB), and 4.0 g of Span80. The organic phase was stirred at 600 rpm, and during stirring, the water phase slowly formed small droplets in the organic phase. The two-phase materials were mixed to result in an emulsion polymerization process, and the material formed a white emulsion. The emulsion was introduced into a mold and was polymerized at 60 C in an oven for 48 h. After 48 h, the product was washed with ethyl alcohol and water and dried in an oven at
A Hg(II) chloride (HgCl2) solution was used in the Hg adsorption test. A stock solution was prepared at a concentration of 1,000 mg/ L, which was diluted to 50, 100, 200, 300, 400, and 500 mg/L. A volume of 50 mL of the Hg solution was used in the adsorption experiments. After adding 0.1 g of the adsorbent to the 50 mg/L Hg solution while stirring, the Hg concentration in the solution after adsorption was analyzed using a CVAAS (Cold Vapor Atomic Absorption Spectrometry)-type Hg analyzer (RA-915+/RP-91, Lumex Ltd., St. Petersburg, Russia). The amount of Hg adsorbed by the adsorbent was obtained by measuring the Hg concentration of the Hg solution before and after the adsorption. Hg analysis A solution of 10% stannous chloride was added to reduce all the Hg(II) ions in the solution to Hg0 prior to the measurement. The concentrations before and after Hg adsorption were measured in triplicate. The adsorption amount was calculated using Eq. (1): qe¼
ðC 0 C e Þ V m
ð1Þ
where qe is the amount of Hg adsorbed at equilibrium, C0 is the initial Hg ion concentration (mg/L), Ce is the equilibrium Hg ion concentration (mg/L), m is the weight of adsorbent (g), and V is the volume of the Hg solution (L). The concentration range of the calibration curve was from 0 to 10,000 ng/L for the Hg solution, prepared by SRM3133. The calibration curve was only used when the coefficient of determination (R2) was greater than 99%. Each concentration in the calibration reflects the average value obtained by measuring the Hg solution three times. Results and discussion Characteristics of the thiol-grafted porous PGMA Chemical structure FTIR spectroscopy (Spectrum 100 Series, PerkinElmer, Waltham, USA) was used to identify the presence of thiol groups in PGMAs (Fig. 2). For PGMA, peaks of an epoxy and an OH group are observed at 910 and 3400 cm1, respectively [20,19]. Meanwhile, new peaks, such as a C–O stretching vibration (1345 cm1) [21], a C–N bond (1095 cm1) [22,23], a C–S bond (665 cm1) [24], and a –SH group (2540 cm1) [22], are observed in the functionalized PGMA spectrum. The peak for C–O stretching appeared due to bond breakage in the epoxy group during the L-cysteine grafting process. A C–N bond and a C–S bond are also formed, and a –SH group bond is clearly formed via a reaction with L-cysteine, verifying that PGMA is functionalized with thiol groups.
Fig. 1. Preparation of porous PGMA.
Morphology and specific surface area The morphologies of PGMAs were characterized by FESEM (Model 7001F, JEOL Ltd., Tokyo, Japan). Because pores were produced by emulsion polymerization, two samples wereconfirmed in the form of a porous open cell structure possessing a shape
Please cite this article in press as: H.H. Kim, T.G. Lee, Removal of mercury ions in a simulated wastewater using functionalized poly(glycidyl methacrylate), J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2016.12.019
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Fig. 3. FESEM image of (a) PGMA and (b) functionalized PGMA.
Fig. 2. (a) FTIR spectra of PGMA and functionalized PGMA. (b) Magnified FTIR spectra of functionalized PGMA.
similar to foam. The FESEM image (Fig. 3) revealed that the functionalized PGMA formed more smooth pores than those in PGMA. The specific surface area, total pore volume, and pore size were measured by nitrogen adsorption (BELSORP-max, MicrotracBEL, Osaka, Japan) at 77 K (Table 1). For PGMA, the specific surface area was 5.1073 m2/g, and the total pore volume was 0.0118 cm3/g. The specific surface area and total pore volume of functionalized PGMA were 3.0223 m2/g and 0.0077 cm3/g, respectively. PGMAs have small values for specific surface area and total pore volume because the pore sizes of both materials (PGMA and functionalized PGMA) are larger than mesopores (2–50 nm), as determined by analyzing the pore size distribution. The average pore size of the two materials is approximately 90 nm, and they have high macroporosity, small surface areas and low pore volumes. Although the difference in the total pore volume of the two materials is small, the total pore volume of PGMA is larger than
3
that of functionalized PGMA because PGMA has more pores that are <6 nm in size, while functionalized PGMA has more pores >6 nm in size. PGMAs with small values of specific surface area and pore volume along with an adsorbent that has high microporosity, as examined in this study, can be easily found in previous studies [25–30]. The specific surface area and total pore volume of PGMA were larger than the values for functionalized PGMA because the PGMA surface was covered with thiol groups from the reaction with L-cysteine. Hg adsorption occurs simultaneously with physical adsorption to PGMA pores and chemical adsorption to thiol groups. Even when pure PGMA is used, Hg is removed from solution, as confirmed by the experiments. PGMA adsorbs 4.98 mg/g of Hg under the following conditions: pH 7, initial Hg concentration of 50 mg/L and 24 h of contact time. However, using functionalized PGMA, 12.90 mg/g of Hg is adsorbed, which is approximately 3 times more Hg than that absorbed by pure PGMA. To test the effect of varying the L-cysteine dose on Hg adsorption, the L-cysteine weights of 0.1,
Table 1 The characteristics of pores measured by nitrogen adsorption. Samples
Adsorptive
Adsorption temperature (K)
Specific surface area (m2/g)
Total pore volume (cm3/g)
PGMA Functionalized PGMA
N2
77
5.1073 3.0223
0.0118 0.0077
Please cite this article in press as: H.H. Kim, T.G. Lee, Removal of mercury ions in a simulated wastewater using functionalized poly(glycidyl methacrylate), J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2016.12.019
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0.5, 1.0, 1.5, and 2.0 g were used; the amount of absorbed Hg under each condition was 6.24, 6.81, 3.59, 2.31, and 2.72 mg/g, respectively, at pH 3, an initial Hg concentration of 50 mg/L and 24 h of contact time. When a substantial amount of L-cysteine reacts with PGMA, Hg adsorption tends to decrease due to the blockage of pores necessary for physical adsorption [31,32]. Effect of pH To confirm the effect of pH on the adsorption of Hg to functionalized PGMA, a Hg adsorption experiment was performed while adjusting the initial pH of the Hg solution from pH 2 to 10 and maintaining this value for 24 h. The pH values were controlled using 0.1 M NaOH and 0.01 M HNO3. In this experiment, the initial Hg concentration of the solution was 50 mg/L. The highest adsorption amount was measured when the initial pH of the Hg solution was set to 7. The adsorption amount at pH values lower than 7 was reduced because hydrogen ions compete with Hg ions for binding sites in acidic solution, causing low Hg adsorption efficiency. The Hg adsorption rate at a high pH was also decreased because the Hg ions are converted to hydroxo complexes in basic solution at high pH [19]. A graph of the pH effect is shown in Fig. 4. Effect of contact time The effect of contact time on Hg adsorption was evaluated using the adsorption isotherms of functionalized PGMA (Fig. 5). Measurements were obtained after 1, 2, 3, 4, 6, 8, 12, and 24 h of contact time. The initial concentration of the Hg solution was adjusted to 50, 100, and 300 mg/L, and the initial pH was 7, at which the highest amount of Hg adsorption was observed. Adsorption equilibrium was achieved within 4 h at all initial Hg concentrations. For initial Hg concentrations of 50, 100, and 300 mg/L, the maximum adsorption amounts that can be expected based on Fig. 5 were calculated as 12.90, 28.53, and 50.91 mg/g, respectively. Regarding the effect of contact time on the Hg adsorption amount, although the adsorption rate is rapid, more time is required to reach the adsorption equilibrium than that required by other conventional adsorbents [33–36] because physical adsorption occurs rapidly prior to chemical adsorption to thiol groups.
Fig. 5. The effect of contact time on functionalized PGMA at pH 7 and C0 of 50, 100, and 300 mg/L for 24 h.
Effect of the initial concentration of Hg ions Experiments were performed to confirm the effect of the initial Hg concentration and to obtain adsorption isotherms of Hg ions. The initial concentration of the Hg solution was controlled at 50, 100, 200, 300, 400, and 500 mg/L. The Hg ions were adsorbed for 24 h at pH 7. The maximum adsorption amount was 50.91 mg/g from an initial concentration of 300 mg/L of the Hg solution. The adsorption amount increased as the initial Hg concentration increased. However, the adsorption amount did not increase further at initial concentrations above 300 mg/L (Fig. 6). Kinetic studies To estimate the mechanism of Hg ion adsorption on functionalized PGMA, two common kinetic models, pseudo-1st-order and pseudo-2nd-order, were examined using the data on the adsorption amount relative to the contact time. Both kinetic models can be expressed according to Eqs. (2) and (3): ln ðqe qt Þ ¼ ln qe k1 t
Fig. 4. Effect of initial pH value of thiol-grafted PGMA at 50 mg/L initial Hg concentration.
ð2Þ
Fig. 6. Effect of the initial concentration of Hg ions on functionalized PGMA at pH 7 and contact time 24 h.
Please cite this article in press as: H.H. Kim, T.G. Lee, Removal of mercury ions in a simulated wastewater using functionalized poly(glycidyl methacrylate), J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2016.12.019
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R2 (%) qe k
q (mg/g)
Pseudo-1st-order
Pseudo-2nd-order
50.91
43.0 7.39 0.104
99.9 51.02 0.070
Table 3 Langmuir and Freundlich parameters for Hg(II) ions adsorption. Langmuir isotherm parameter
Freundlich isotherm parameter
q0 (mg/g) KL (L/mg) R2 (%)
KF n R2
56.18 0.025 97.5
5.030 2.381 73.8
5
thiol groups on the functionalized PGMA pore surface. According to the initial pH, the Hg adsorption was most efficient at pH 7. The number of Hg ions decreased at a low pH because of competition with hydrogen ions in acidic solution during the adsorption process. Hg ions are converted to hydroxo complexes in basic solution at a high pH. The variation of Hg adsorption onto functionalized PGMA with contact time was confirmed. The adsorption quantity drastically increased within the first 3 h and then remained constant after many Hg ions settled onto the pore surface of the adsorbent. The experimental data were calculated using pseudo-1st-order and pseudo-2nd-order models, and the latter model provided a better fit. The Hg adsorption of thiolgrafted PGMA also fit well with the Langmuir isotherm model. Acknowledgment
t 1 1 ¼ þ t qt k2 q2e qe
ð3Þ
where k1 (1/h) and k2 (g/mg/h) are pseudo-1st-order and pseudo2nd-order rate constants, respectively, and qe and qt are the adsorbed Hg amounts at equilibrium and at time, t, respectively. The correlation coefficient (R2) of the pseudo-2nd-order model was 99.95%, which is higher than that of the pseudo-1st-order model (42.96%). Parameters of both kinetic models are shown in Table 2. Adsorption isotherms The experimental data from the adsorption equilibrium isotherms were applied to the Langmuir isotherm (Eq. (4)) and the Freundlich isotherm equations (Eq. (5)): Ce Ce 1 ¼ þ qe q0 q0 K L
ln qe ¼ ln K F þ
ð4Þ
1 ln C e n
ð5Þ
where Ce is the equilibrium Hg concentration (mg/L), qe is the equilibrium Hg adsorption amount, q0 is the maximum Hg adsorption capacity, n is the adsorption intensity, KL is the Langmuir isotherm constant (L/mg), and KF is the Freundlich isotherm constant. The correlation coefficient (R2) of the Langmuir equation was 97.51%, which is higher than that of the Freundlich model (R2 = 73.84%). Hence, the Langmuir model provides a better fit than the Freundlich model for Hg adsorption on thiol-grafted PGMA. The Hg adsorption capacity calculated by the Langmuir equation was 56.18 mg/g, which is close to the experimental data value of 50.91 mg/g. Each parameter of the Hg adsorption isotherm is shown in Table 3. Conclusions Thiol-grafted porous PGMA is a functionalized adsorbent for Hg in aqueous solution. The open cell structure of thiol-grafted porous PGMA produced by emulsion polymerization was evaluated using FESEM. The surface area and total pore volume of functionalized PGMA were lower than those of PGMA because of the presence of
This work was supported by the Ministry of Environment-Korea as the Advanced Industrial Technology Development Project (No. E315-00014-0404-0). References [1] J.H. Cho, Y. Eom, S.H. Jeon, T.G. Lee, J. Ind. Eng. Chem. 19 (2013) 144. [2] United Nations Environment Program, Minamata Convention on Mercury— Text and Annexes, (2013) . [3] J.H. Won, J.Y. Lee, D. Chung, S.K. Shin, T.G. Lee, J. Ind. Eng. Chem. 19 (2013) 1560. [4] S. Babel, T.A. Kurniawan, J. Hazard Mater. 97 (2003) 219. [5] K. Kadirvelu, K. Thamaraiselvi, C. Namasivayam, Bioresour. Technol. 76 (2001) 63. [6] H. ShamsiJazeyi, T. Kaghazchi, J. Ind. Eng. Chem. 16 (2010) 852. [7] T.S. Anirudhan, S.S. Sreekumari, J. Environ. Sci. 23 (2011) 1989. [8] T. Gebremedhin-Haile, M.T. Olguín, M. Solache-Ríos, Water Air Soil Pollut. 148 (2003) 179. [9] E. Erdem, N. Karapinar, R. Donat, J. Colloid. Interface Sci. 280 (2004) 309. [10] M.E. Mahmoud, M.M. Osman, M.E. Amer, Anal. Chim. Acta 415 (2000) 33. [11] M. Najafi, Y. Yousefi, A.A. Rafati, Sep. Purif. Technol. 85 (2012) 193. [12] T.A. Davis, B. Volesky, A. Mucci, Water Res. 37 (2003) 4311. [13] A. Saeed, M.W. Akhter, M. Iqbal, Sep. Purif. Technol. 45 (2005) 25. [14] M.A. Abd El-Ghaffar, M.H. Mohamed, K.Z. Elwakeel, J. Chem. Eng. 151 (2009) 30. [15] Z. Aksu, J. Yener, Waste Manage. 21 (2001) 695. [16] J.D. Merrifield, W.G. Davids, J.D. MacRae, A. Amirbahman, Water Res. 38 (2004) 3132. ska, in: A. Kilisliog lu (Ed.), Ion Exchange Technologies, [17] Z. Hubicki, D. Kołodyn INTECH Open Access Publisher, Turkey, 2012, pp. 193. [18] L. Shechter, J. Wynstra, R.P. Kurkjy, Ind. Eng. Chem. 48 (1956) 94. [19] J. Han, Z. Du, W. Zou, H. Li, C. Zhang, J. Hazard Mater. 276 (2014) 225. [20] T. Caykara, F. Çakar, S. Demirci, Polym. Int. 60 (2011) 141. [21] A. Chowdhury, A. Bhowal, S. Datta, Water 4 (2012) 37. [22] R.B.D. Alencastro, C. Sandorfy, Can. J. Chem. 50 (1972) 3594. [23] S.A. McCarthy, G.L. Davies, Y.K. Gun’ko, Nat. Protoc. 7 (2012) 1677. [24] I.L. Lagadic, M.K. Mitchell, B.D. Payne, Environ. Sci. Technol. 35 (2001) 984. [25] H. Mouaziz, K. Lacki, A. Larsson, D.C. Sherrington, J. Mater. Chem. 14 (2004) 2421. [26] M.E. Paez-Hernandez, K. Aguilar-Arteaga, C.A. Galan-Vidal, M. PalomarPardave, M. Romero-Romo, M.T. Ramirez-Silva, Environ. Sci. Technol. 39 (2005) 7667. [27] S. Dubinsky, H. Zhang, Z. Nie, I. Gourevich, D. Voicu, M. Deetz, E. Kumacheva, Macromolecules 41 (2008) 3555. [28] M. Kim, S.H. Yoon, E. Choi, B. Gil, LWT–Food Sci. Technol. 41 (2008) 701. [29] M. Clauss, U.F. Tafin, B. Betrisey, N. van Garderen, A. Trampuz, T. Ilchmann, M. Bohner, Eur. Cell Mater. 28 (2014) 39. [30] G.M. Nisola, L.A. Limjuco, E.L. Vivas, C.P. Lawagon, M.J. Park, H.K. Shon, W. Chung, J. Chem. Eng. 280 (2015) 536. [31] Y. Ito, Y. Ochiai, Y.S. Park, Y. Imanishi, J. Am. Chem. Soc. 119 (1997) 1619. [32] J. Pieracci, D.W. Wood, J.V. Crivello, G. Belfort, Chem. Mater. 12 (2000) 2123. [33] A. Denizli, K. Kesenci, Y. Arica, E. Pişkin, React. Funct. Polym. 44 (2000) 235. [34] K. Kadirvelu, M. Kavipriya, C. Karthika, N. Vennilamani, S. Pattabhi, Carbon 42 (2004) 745. [35] I. Tuzun, G. Bayramoglu, E. Yalcin, G. Basaran, G. Celik, M.Y. Arica, J. Environ. Manage. 77 (2005) 85. [36] J.C. Igwe, A.A. Abia, C.A. Ibeh, Int. J. Environ. Sci. Technol. 5 (2008) 83.
Please cite this article in press as: H.H. Kim, T.G. Lee, Removal of mercury ions in a simulated wastewater using functionalized poly(glycidyl methacrylate), J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2016.12.019
Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Removal of mercury ions in a simulated wastewater using functionalized poly(glycidyl methacrylate) Hyun Hee Kim, Tai Gyu Lee* Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea
A R T I C L E I N F O
Article history: Received 15 November 2016 Received in revised form 16 December 2016 Accepted 16 December 2016 Available online xxx Keywords: W/O PGMA Emulsion polymerization L-cysteine Thiol
A B S T R A C T
Porous poly(glycidyl methacrylate) (PGMA) was prepared via water-in-oil (W/O) emulsion polymerization and was grafted with thiol groups. The removal of mercury (Hg) ions from water was evaluated using the functionalized PGMA. First, PGMA was synthesized and functionalized. Then, FTIR was used to assess the bonding of thiol groups with PGMA. SEM was used to analyze the porous characteristics of the material. To determine the effect of the pH on the Hg adsorption to functionalized PGMA, Hg adsorption tests were performed by varying the solution pH from 2 to 10. Additionally, a Hg adsorption test was performed by changing the initial Hg concentration. © 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Mercury (Hg) exists in various forms as metallic Hg, inorganic Hg salts and organic Hg compounds in the environment. Hg bioaccumulates up the food chain. Hg accumulation beyond a certain level in the human body damages the nerves and kidneys and causes other harmful effects. Therefore, efforts to suppress the discharge of Hg into the environment are underway. The Minamata Convention on Mercury was ratified in 2013 and is expected to be operational in 2016 [1–3]. Therefore, a safe management plan for Hg and Hg compounds is required in response to this agreement. It is difficult to separate heavy metal ions in wastewater discharged as the anthropogenic emissions from industrial facilities from water. Chemical, biological and physical treatments have been used to separate the heavy metal ions in wastewater. Adsorption is a physical method and is the most effective in terms of efficiency, stability and cost [4]. Various types of inorganic adsorbents have been studied, such as activated carbon [5–7], zeolite [8,9], and silica gel [10,11], in addition to biochemical organic materials from seaweed [12], wood [13], and polymer adsorbents [14]. Adsorption efficiency is determined by the physical structure of the pore, the specific surface area, and the porosity. Physical adsorption involves the combination with adsorbates through van der Waals forces, forming weak bonds
with the adsorbates. Chemical adsorption is caused by moving electrons, and the resulting bonding force is stronger than physical adsorption bonding [15]. In this study, porous poly(glycidyl methacrylate) (PGMA) was developed as a polymer adsorbent. PGMA forms pores via emulsion polymerization, which can control the structure and properties of the polymer material in the fabrication process. In addition, thiol groups that form stable bonds with Hg were used to functionalize PGMA [16]. A polymeric material with porous PGMA is easily functionalized with other materials because it contains highly reactive epoxy groups [17]. Epoxy groups are reactive with amines or carboxylic acids. Based on these facts, PGMA was functionalized with L-cysteine, which possesses carboxyl groups, amine groups and thiol groups [18,19]. High selectivity and efficiency are expected for the removal of Hg ions from water when using thiol groups as the PGMA functional groups. The chemical bonds between the thiol groups and PGMA were measured using Fourier transform infrared (FTIR) spectroscopy. The physical characteristics of the functionalized PGMA were identified using field emission scanning electron microscopy (FESEM) and N2 sorption analysis, and the properties of Hg ion adsorption were determined.
Materials and methods Reagents
* Corresponding author. Fax: +82 2 6008 0560. E-mail addresses: [email protected], http://[email protected] (T.G. Lee).
Glycidyl methacrylate (95%, Daejung, Siheung, Korea), divinylbenzene (80%, Sigma Aldrich, Saint Louis, USA), Span80 (Daejung,
http://dx.doi.org/10.1016/j.jiec.2016.12.019 1226-086X/© 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: H.H. Kim, T.G. Lee, Removal of mercury ions in a simulated wastewater using functionalized poly(glycidyl methacrylate), J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2016.12.019
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Siheung, Korea), potassium sulfate (99%, Daejung, Siheung, Korea), potassium persulfate (99%, Sigma Aldrich, Saint Louis, USA), magnesium chloride (98%, Siheung, Daejung, Korea), L-cysteine (95.5%, Sigma Aldrich, Saint Louis, USA), ethyl alcohol (94.5%, Daejung, Siheung, Korea), Hg(II) chloride (99.5%, Samchun Chemical, Seoul, Korea), stannous chloride dihydrate (Junsei Chemical Co., Ltd., Tokyo, Japan), and SRM3133 (9.954 mg/kg 0.053, NIST, Gaithersburg, USA) were used.
100 C for 48 h. Using this process, monolith PGMA was obtained. The preparation scheme for porous PGMA is shown in Fig. 1. Preparation of functionalized porous PGMA After mixing 0.5 g of L-cysteine, 0.1 g of MgCl2, and 1 g of PGMA in 100 mL of water, the mixture was reacted for 24 h at 100 C. After 24 h, the product was washed and dried at 60 C.
Preparation of porous PGMA
Preparation of the Hg ion solution and measurements
The water phase was prepared by mixing 0.11 g of potassium persulfate (K2S2O8), 0.55 g of potassium sulfate (K2SO4) and 45 mL of water. The organic phase consisted of 3.0 g of glycidyl methacrylate (GMA), 2.0 g of divinylbenzene (DVB), and 4.0 g of Span80. The organic phase was stirred at 600 rpm, and during stirring, the water phase slowly formed small droplets in the organic phase. The two-phase materials were mixed to result in an emulsion polymerization process, and the material formed a white emulsion. The emulsion was introduced into a mold and was polymerized at 60 C in an oven for 48 h. After 48 h, the product was washed with ethyl alcohol and water and dried in an oven at
A Hg(II) chloride (HgCl2) solution was used in the Hg adsorption test. A stock solution was prepared at a concentration of 1,000 mg/ L, which was diluted to 50, 100, 200, 300, 400, and 500 mg/L. A volume of 50 mL of the Hg solution was used in the adsorption experiments. After adding 0.1 g of the adsorbent to the 50 mg/L Hg solution while stirring, the Hg concentration in the solution after adsorption was analyzed using a CVAAS (Cold Vapor Atomic Absorption Spectrometry)-type Hg analyzer (RA-915+/RP-91, Lumex Ltd., St. Petersburg, Russia). The amount of Hg adsorbed by the adsorbent was obtained by measuring the Hg concentration of the Hg solution before and after the adsorption. Hg analysis A solution of 10% stannous chloride was added to reduce all the Hg(II) ions in the solution to Hg0 prior to the measurement. The concentrations before and after Hg adsorption were measured in triplicate. The adsorption amount was calculated using Eq. (1): qe¼
ðC 0 C e Þ V m
ð1Þ
where qe is the amount of Hg adsorbed at equilibrium, C0 is the initial Hg ion concentration (mg/L), Ce is the equilibrium Hg ion concentration (mg/L), m is the weight of adsorbent (g), and V is the volume of the Hg solution (L). The concentration range of the calibration curve was from 0 to 10,000 ng/L for the Hg solution, prepared by SRM3133. The calibration curve was only used when the coefficient of determination (R2) was greater than 99%. Each concentration in the calibration reflects the average value obtained by measuring the Hg solution three times. Results and discussion Characteristics of the thiol-grafted porous PGMA Chemical structure FTIR spectroscopy (Spectrum 100 Series, PerkinElmer, Waltham, USA) was used to identify the presence of thiol groups in PGMAs (Fig. 2). For PGMA, peaks of an epoxy and an OH group are observed at 910 and 3400 cm1, respectively [20,19]. Meanwhile, new peaks, such as a C–O stretching vibration (1345 cm1) [21], a C–N bond (1095 cm1) [22,23], a C–S bond (665 cm1) [24], and a –SH group (2540 cm1) [22], are observed in the functionalized PGMA spectrum. The peak for C–O stretching appeared due to bond breakage in the epoxy group during the L-cysteine grafting process. A C–N bond and a C–S bond are also formed, and a –SH group bond is clearly formed via a reaction with L-cysteine, verifying that PGMA is functionalized with thiol groups.
Fig. 1. Preparation of porous PGMA.
Morphology and specific surface area The morphologies of PGMAs were characterized by FESEM (Model 7001F, JEOL Ltd., Tokyo, Japan). Because pores were produced by emulsion polymerization, two samples wereconfirmed in the form of a porous open cell structure possessing a shape
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Fig. 3. FESEM image of (a) PGMA and (b) functionalized PGMA.
Fig. 2. (a) FTIR spectra of PGMA and functionalized PGMA. (b) Magnified FTIR spectra of functionalized PGMA.
similar to foam. The FESEM image (Fig. 3) revealed that the functionalized PGMA formed more smooth pores than those in PGMA. The specific surface area, total pore volume, and pore size were measured by nitrogen adsorption (BELSORP-max, MicrotracBEL, Osaka, Japan) at 77 K (Table 1). For PGMA, the specific surface area was 5.1073 m2/g, and the total pore volume was 0.0118 cm3/g. The specific surface area and total pore volume of functionalized PGMA were 3.0223 m2/g and 0.0077 cm3/g, respectively. PGMAs have small values for specific surface area and total pore volume because the pore sizes of both materials (PGMA and functionalized PGMA) are larger than mesopores (2–50 nm), as determined by analyzing the pore size distribution. The average pore size of the two materials is approximately 90 nm, and they have high macroporosity, small surface areas and low pore volumes. Although the difference in the total pore volume of the two materials is small, the total pore volume of PGMA is larger than
3
that of functionalized PGMA because PGMA has more pores that are <6 nm in size, while functionalized PGMA has more pores >6 nm in size. PGMAs with small values of specific surface area and pore volume along with an adsorbent that has high microporosity, as examined in this study, can be easily found in previous studies [25–30]. The specific surface area and total pore volume of PGMA were larger than the values for functionalized PGMA because the PGMA surface was covered with thiol groups from the reaction with L-cysteine. Hg adsorption occurs simultaneously with physical adsorption to PGMA pores and chemical adsorption to thiol groups. Even when pure PGMA is used, Hg is removed from solution, as confirmed by the experiments. PGMA adsorbs 4.98 mg/g of Hg under the following conditions: pH 7, initial Hg concentration of 50 mg/L and 24 h of contact time. However, using functionalized PGMA, 12.90 mg/g of Hg is adsorbed, which is approximately 3 times more Hg than that absorbed by pure PGMA. To test the effect of varying the L-cysteine dose on Hg adsorption, the L-cysteine weights of 0.1,
Table 1 The characteristics of pores measured by nitrogen adsorption. Samples
Adsorptive
Adsorption temperature (K)
Specific surface area (m2/g)
Total pore volume (cm3/g)
PGMA Functionalized PGMA
N2
77
5.1073 3.0223
0.0118 0.0077
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0.5, 1.0, 1.5, and 2.0 g were used; the amount of absorbed Hg under each condition was 6.24, 6.81, 3.59, 2.31, and 2.72 mg/g, respectively, at pH 3, an initial Hg concentration of 50 mg/L and 24 h of contact time. When a substantial amount of L-cysteine reacts with PGMA, Hg adsorption tends to decrease due to the blockage of pores necessary for physical adsorption [31,32]. Effect of pH To confirm the effect of pH on the adsorption of Hg to functionalized PGMA, a Hg adsorption experiment was performed while adjusting the initial pH of the Hg solution from pH 2 to 10 and maintaining this value for 24 h. The pH values were controlled using 0.1 M NaOH and 0.01 M HNO3. In this experiment, the initial Hg concentration of the solution was 50 mg/L. The highest adsorption amount was measured when the initial pH of the Hg solution was set to 7. The adsorption amount at pH values lower than 7 was reduced because hydrogen ions compete with Hg ions for binding sites in acidic solution, causing low Hg adsorption efficiency. The Hg adsorption rate at a high pH was also decreased because the Hg ions are converted to hydroxo complexes in basic solution at high pH [19]. A graph of the pH effect is shown in Fig. 4. Effect of contact time The effect of contact time on Hg adsorption was evaluated using the adsorption isotherms of functionalized PGMA (Fig. 5). Measurements were obtained after 1, 2, 3, 4, 6, 8, 12, and 24 h of contact time. The initial concentration of the Hg solution was adjusted to 50, 100, and 300 mg/L, and the initial pH was 7, at which the highest amount of Hg adsorption was observed. Adsorption equilibrium was achieved within 4 h at all initial Hg concentrations. For initial Hg concentrations of 50, 100, and 300 mg/L, the maximum adsorption amounts that can be expected based on Fig. 5 were calculated as 12.90, 28.53, and 50.91 mg/g, respectively. Regarding the effect of contact time on the Hg adsorption amount, although the adsorption rate is rapid, more time is required to reach the adsorption equilibrium than that required by other conventional adsorbents [33–36] because physical adsorption occurs rapidly prior to chemical adsorption to thiol groups.
Fig. 5. The effect of contact time on functionalized PGMA at pH 7 and C0 of 50, 100, and 300 mg/L for 24 h.
Effect of the initial concentration of Hg ions Experiments were performed to confirm the effect of the initial Hg concentration and to obtain adsorption isotherms of Hg ions. The initial concentration of the Hg solution was controlled at 50, 100, 200, 300, 400, and 500 mg/L. The Hg ions were adsorbed for 24 h at pH 7. The maximum adsorption amount was 50.91 mg/g from an initial concentration of 300 mg/L of the Hg solution. The adsorption amount increased as the initial Hg concentration increased. However, the adsorption amount did not increase further at initial concentrations above 300 mg/L (Fig. 6). Kinetic studies To estimate the mechanism of Hg ion adsorption on functionalized PGMA, two common kinetic models, pseudo-1st-order and pseudo-2nd-order, were examined using the data on the adsorption amount relative to the contact time. Both kinetic models can be expressed according to Eqs. (2) and (3): ln ðqe qt Þ ¼ ln qe k1 t
Fig. 4. Effect of initial pH value of thiol-grafted PGMA at 50 mg/L initial Hg concentration.
ð2Þ
Fig. 6. Effect of the initial concentration of Hg ions on functionalized PGMA at pH 7 and contact time 24 h.
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H.H. Kim, T.G. Lee / Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx Table 2 Parameters of the pseudo-first order and pseudo-second order for the adsorption of Hg ions on functionalized PGMA.
R2 (%) qe k
q (mg/g)
Pseudo-1st-order
Pseudo-2nd-order
50.91
43.0 7.39 0.104
99.9 51.02 0.070
Table 3 Langmuir and Freundlich parameters for Hg(II) ions adsorption. Langmuir isotherm parameter
Freundlich isotherm parameter
q0 (mg/g) KL (L/mg) R2 (%)
KF n R2
56.18 0.025 97.5
5.030 2.381 73.8
5
thiol groups on the functionalized PGMA pore surface. According to the initial pH, the Hg adsorption was most efficient at pH 7. The number of Hg ions decreased at a low pH because of competition with hydrogen ions in acidic solution during the adsorption process. Hg ions are converted to hydroxo complexes in basic solution at a high pH. The variation of Hg adsorption onto functionalized PGMA with contact time was confirmed. The adsorption quantity drastically increased within the first 3 h and then remained constant after many Hg ions settled onto the pore surface of the adsorbent. The experimental data were calculated using pseudo-1st-order and pseudo-2nd-order models, and the latter model provided a better fit. The Hg adsorption of thiolgrafted PGMA also fit well with the Langmuir isotherm model. Acknowledgment
t 1 1 ¼ þ t qt k2 q2e qe
ð3Þ
where k1 (1/h) and k2 (g/mg/h) are pseudo-1st-order and pseudo2nd-order rate constants, respectively, and qe and qt are the adsorbed Hg amounts at equilibrium and at time, t, respectively. The correlation coefficient (R2) of the pseudo-2nd-order model was 99.95%, which is higher than that of the pseudo-1st-order model (42.96%). Parameters of both kinetic models are shown in Table 2. Adsorption isotherms The experimental data from the adsorption equilibrium isotherms were applied to the Langmuir isotherm (Eq. (4)) and the Freundlich isotherm equations (Eq. (5)): Ce Ce 1 ¼ þ qe q0 q0 K L
ln qe ¼ ln K F þ
ð4Þ
1 ln C e n
ð5Þ
where Ce is the equilibrium Hg concentration (mg/L), qe is the equilibrium Hg adsorption amount, q0 is the maximum Hg adsorption capacity, n is the adsorption intensity, KL is the Langmuir isotherm constant (L/mg), and KF is the Freundlich isotherm constant. The correlation coefficient (R2) of the Langmuir equation was 97.51%, which is higher than that of the Freundlich model (R2 = 73.84%). Hence, the Langmuir model provides a better fit than the Freundlich model for Hg adsorption on thiol-grafted PGMA. The Hg adsorption capacity calculated by the Langmuir equation was 56.18 mg/g, which is close to the experimental data value of 50.91 mg/g. Each parameter of the Hg adsorption isotherm is shown in Table 3. Conclusions Thiol-grafted porous PGMA is a functionalized adsorbent for Hg in aqueous solution. The open cell structure of thiol-grafted porous PGMA produced by emulsion polymerization was evaluated using FESEM. The surface area and total pore volume of functionalized PGMA were lower than those of PGMA because of the presence of
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Please cite this article in press as: H.H. Kim, T.G. Lee, Removal of mercury ions in a simulated wastewater using functionalized poly(glycidyl methacrylate), J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2016.12.019