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Pb2+ adsorption by ethylenediamine-modified pectins and their adsorption mechanisms
Carbohydrate Polymers 234 (2020) 115911
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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Pb2+ adsorption by ethylenediamine-modified pectins and their adsorption mechanisms
T
Rui-hong Lianga, Ya Lia, Li Huangb, Xue-dong Wanga, Xiao-xue Hua, Cheng-mei Liua, Ming-shun Chena,*, Jun Chena,* a b
State Key Laboratory of Food Science and Technology, Nanchang University, Nanchang, Jiangxi, 330047, China Nanchang Institute for Control of Food and Drug, Nanchang, Jiangxi, 330047, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Pectin Ethylenediamine Modification Pb2+ Adsorption
Ethylenediamine-modified pectins (EPs) with different degrees of amidation (DA) were prepared and characterized by Fourier Transform Infrared spectra (FTIR), elemental analysis, X-ray photoelectron spectroscopy (XPS). The prepared EPs were then used to remove Pb2+ from the aqueous solution. It was found that EPs with the highest DA (EP48) exhibited great removal efficiency of Pb2+ (≥94 %) at low concentrations of 40−80 mg/ L. The zeta potential analysis showed that EP48 had the fastest increase in zeta potential when Pb2+ was continuously added and was the first to be electroneutralized. Particle size analysis further confirmed that EP48 was the first precipitated and formed a larger EP48-Pb2+ complex. The FTIR and XPS analyses indicated that Pb2+ was adsorbed via the ion exchange of carboxylic groups and chelation with acylamino and amino groups. These results suggested that the EP48 might be a promising adsorbent for the removal of low concentrations of Pb2+ in contaminated water.
1. Introduction Heavy metal pollution of the water body has become an increasingly serious problem, which affects the ecological environment as well as the human health (Liu, Jia, Liu, & Liang, 2018; Wang, Liu, Duan, Sun, & Xu, 2019). These heavy metals may be enriched in organisms and enter the human body through the food chain (Liu, Pang et al., 2018; Pal, Majumder, Sengupta, Das, & Bandyopadhyay, 2017). Lead, as a heavy metal extensively used in industrial production, is one of the most hazardous elements to human beings. The excessive intake of Pb2+ mainly harms the central nervous system, liver and kidney of the human body (Huang & Zhu, 2013; Tao, Yuan, Xiaona, & Wei, 2012; Wang, Huang, Tu, Ruan, & Lin, 2016). Hence, Pb2+ removal from water bodies has been a priority in water-treatment plants. A series of conventional treatments including chemical precipitation, ion exchange, chemical oxidation, membrane filtration and adsorption were used for Pb2+ removal (Deng et al., 2017; Onditi, Adelodun, Changamu, & Ngila, 2016). Among these, adsorption is a widely used process due to its low cost (Wang, Miao, He, & Shen, 2011). However, adsorption by activated carbon is often limited by their several disadvantages, e.g., ineffective, expensive when used in low concentration heavy metal solution processing (Wang & Chen, 2006) and large-amount sludge creation.
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Because of the shortages of traditional activated carbon, new alternative materials should be extensively studied. Pectin is an important natural anionic polysaccharide extracted from cell walls of higher plants. This polysaccharide is known as a gelling/thickening agent, emulsifying agent, as well as stabilizer in the food industry (Liu, Guo, Liang, Liu, & Chen, 2017). Moreover, pectin can be regarded as an attractive novel biopolymer material in wastewater treatment that facilitates metal chelation with a backbone of (1–4) α-D-galacturonic acids (Celus et al., 2017). Diana, Jaime, LópezMaldonado, and Oropeza-Guzmán (2017) reported that pectin extracted from nopal and used as a coagulant-flocculant agent to remove 99 % of all metallic ions. Some scholar has proposed that chemical modifications could tune the pectin structure to adjust its heavy metal adsorption capability due to the formation of specific structure with the particular groups (Wang, Liang et al., 2019). As reported by Li, Yang, Zhao, and Xu (2007), the modification of pectin by cross-linked with adipic acid could increase its stability and adsorption performance, the saturated loading capacity for Pb2+ reached 1.82 mmol/g. Many studies have reported that amino groups have good chelating ability of heavy metals (Li et al., 2018; Li, Ye, Fang, & Liu, 2019; Qin et al., 2019). Ethylenediamine is a metal chelating agent which is rich in amino groups. Li et al. (2009) prepared modified activated carbon with
Corresponding authors at: Nanchang University, 235 Nanjing East Road, Nanchang, China. E-mail addresses: [email protected] (M.-s. Chen), [email protected] (J. Chen).
https://doi.org/10.1016/j.carbpol.2020.115911 Received 8 October 2019; Received in revised form 28 December 2019; Accepted 23 January 2020 Available online 25 January 2020 0144-8617/ © 2020 Elsevier Ltd. All rights reserved.
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ethylenediamine and found that the new sorbent has high analytical potential for the preconcentration of trace metal ions. The percentage or degree of amidation (DA) is an important characteristic of pectin that may determine the capacity of interaction with metallic ions. However, to our knowledge, there are no reports on the graft of ethylenediamine on pectin. The effect of the amide group on the Pb2+ adsorption capacity of modified pectin has not been reported either. In this study, ethylenediamine-modified pectins (EPs) with different DA were prepared by amidation of pectin with ethylenediamine. The purpose of the study is to evaluate whether the EPs can be used as adsorbents of Pb2+ and to explore their adsorption mechanisms in aqueous solution.
DA=
MCR 6 × ×100 MCP K
(3)
where MN and MC are contents of nitrogen and carbon (%), respectively. MCR and MCP are contents of carbon (%), 12/14 is the ratio of the carbon to the nitrogen atomic mass, 6 is the sum of carbons in the galacturonic unit of pectin, K and I are the numbers of carbons and nitrogens in the amine molecule, respectively. In this study, K = 2 and I = 2. The EPs with different DA were named EPx, where EP stood for ethylenediamine-modified pectin and x was the DA.
2.5. XPS analysis 2. Materials and methods The surface chemical composition of samples was determined by the X-ray photoelectron spectroscopy (ESCALAB250Xi, Thermo Fisher Scientific, Madison, USA). The binding energy scale for final calibration was corrected by the C1s peak corresponding to 284.8 eV (Zhang, Li, Li, & Yang, 2017).
2.1. Materials Citrus pectin (Commercial name 13CG, Mw = 527 KDa, with a degree of methylation of 41.46 % and galacturonic acid content of 87.73 %), was supplied by the general agent of CPkelco (Shanghai, China). Ethylenediamine, N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC), N-hydroxysuccinimide (NHS, 98 %) and Pb (NO3)2 were purchased from Aladdin Reagent Company (Shanghai, China). All reagents were of analytical reagent grade and used as received. All aqueous solutions were prepared with ultrapure water (organic free, 18.2 MΩ cm resistance) from a Milli-Q system (Millipore, Billerica, USA).
2.6. Zeta potential analysis The zeta potential measurements were done at room temperature in porcelain cuvettes by Malvern Zetasizer Nano ZSP (Malvern Instruments Ltd., Worcestershire, UK) according to Pal et al. (2017).
2.7. Particle size analysis 2.2. Preparation of EPs The Z-average particle size of EPs solution (0.1 %) before and after adsorption of different concentrations of Pb2+ were monitored using a laser light scattering (Mastersizer 2000, Malvern Instruments Ltd., Worcestershire, UK).
The synthesis was carried out according to the method of Huang and Wang (2017) with some modifications. Firstly, 10.00 g of pectin powder was dissolved in 1000 mL ultrapure water, then 2.88 g NHS and 9.59 g EDC were added, followed with the addition of 0.1 M HNO3 until pH to 4.0. Shortly afterward, different amounts of ethylenediamine (0.05, 0.20, 0.50 and 2.00 mL) were added. Then the solution was magnetic stirred at room temperature for 8 h. The solution was purified by dialysis through a 10000-8000 molecular weight cut-off dialysis bag (MD1477, Yuanye Bio-Technology Co., Ltd, Shanghai, China) for three days. Finally, products were freeze-dried with a lyophilizer (Alpha12LD, Christ, Gottingen, Germany). All the EPs prepared in our experiments were soluble in water.
2.8. Batch adsorption studies Stock solution (1.0 g/L) of Pb2+ was prepared by dissolving 1.599 g Pb (NO3)2 in 1000 mL distilled water. The solutions of different concentrations used in various experiments were obtained by dilution of the stock solutions. The effect of pH on the adsorption was conducted at solution pH ranging from 2.0 to 6.0. The initial pH of the Pb2+ solution was adjusted by 0.1 M HNO3 and 0.1 M NaOH aqueous solutions. 30.0 mg of adsorbents were added to 30.0 mL of 150.0 mg/L Pb2+ solution. The mixture was stirred for 1 h at 25 °C, then the mixture was centrifuged at 40000 rpm for 20 min and the resulting supernatant was filtered with 0.45 μm filter. The concentration of Pb2+ in the filtrate was measured by atomic absorption spectrophotometer (AAS, A3AFG-12, Puxi, Beijing, China). After adsorption, the final pH was also measured for investigating the interaction between the adsorbents and the Pb2+. To explore the effect of Pb2+ concentrations on the adsorption, the adsorption experiments were also operated at the Pb2+ concentrations varying from 20.0 to 250.0 mg/L at pH 5.0 (optimized pH). 30.0 mg of adsorbents were put into a 30.0 mL Pb2+ solution, and the mixture was shaken in a water bath 25 °C for 1 h. The adsorbent was removed after centrifugation 40,000 rpm, 20 min. Finally, the Pb2+ concentrations in filtrate were determined by AAS. The removal rate (%) was calculated as follows:
2.3. FTIR spectra analysis The FTIR spectra were collected on a Nicolet 5700 spectrometer (Thermo Fisher Scientific, Madison, USA) using KBr pellets according to Diana et al. (2017). The samples (1.0 mg) were mixed with KBr in a ratio of 1:100. The pills were formed at 6000 Psi of pressure in a hydraulic press manual. Spectra were recorded in the absorption mode 4000 to 400 cm−1 with a resolution of 2 cm−1. 2.4. Elemental analysis The elemental analysis (C, H and N) was carried out with an Elemental analyzer (Vario Micro cube Elementar, Langenselbold, Germany) according to the work of Li et al. (2009). The degrees of amidation (DA) of the products were calculated from the elemental composition according to the following formulas (Šimkovic, Synytsya, Uhliariková, & Čopíková, 2009):
MCR = MN ×
12 K × 14 I
MCP = MC − MCR
R= (1)
C 0 − Ce × 100 C0
(4)
where C0 and Ce (mg/L) are the initial and equilibrium concentrations of Pb2+ solution, respectively.
(2) 2
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Fig. 1. FTIR spectra of pectin and EPx (A). FTIR spectra of pectin-Pb2+ and EPx-Pb2+ (B).
acid residues in the amidated pectin (Rolin & De Vries, 1990). The DA of EPs were obtained from elemental analysis and were listed in Table 1. It was found that the DA increased with the addition of ethylenediamine. When ethylenediamine was added at 0.05, 0.20, 0.05 and 2.00 mL, DA was 31.62 %, 38.94 %, 43.13 % and 48.31 %, respectively. Thus, the four derivatives were named as EP32, EP39, EP43 and EP48. It can be found that low ethylenediamine concentration showed better modification efficiency, which may be because the EDC and NHS were sufficient at those concentrations when employed as cross-linkers in the amidation reaction (Huang et al., 2018). At higher ethylenediamine concentrations, the DA did not significantly increase, possibly due to fewer carboxyl sites that could be modified as well as the deficiency of EDC and NHS.
2.9. Statistical analysis All of the experiments were done in triplicate. Statistical analysis was carried out using SPSS (version 16.0, Chicago, United States). The results were expressed as mean ± standard deviations and compared using the Tukey test at 5 % confidence level. 3. Results and discussion 3.1. Characteristics of the EPs 3.1.1. FTIR spectroscopy analysis of EPs The FTIR spectra of EPs and original pectin were shown in Fig. 1A. The original pectin showed a broad band at about 3415 cm−1, assigned to stretching vibration modes of OeH groups. The weak peak toward 2938 cm−1 was attributed to the C–H antisymmetrical stretching vibration (Huang et al., 2018). Two peaks at about 1747 and 1623 cm−1 typically represent the protonated carboxylic acid groups and carboxylic anion, respectively (Liang, Wang, Chen, Liu, & Liu, 2015). In comparison with the original pectin, EPs showed an enhanced absorption peak at about 1644 cm−1 which represented the carbonyl stretching vibration of the amide bonds, and the peak at about 1587 cm−1 which was related to the NeH bending vibration of amide (Ren, Abbood, He, Peng, & Huang, 2013). The peak of amide I and amide II are not obvious, so we decomposed FITR spectroscopies between 1500 cm−1 and 1800 cm−1 by peak fitting and it was shown in the Supplementary Fig. 1. It was found that new amide bands at about 1650 cm−1 (amide I) and 1560 cm−1 (amide II) appeared. Šimkovic et al. (2009) also found new amide bands appeared at about 1652 cm−1 and 1551 cm−1 for pectin propylamide derivatives. The formation of amide bonds suggested that ethylenediamine was grafted on pectin through the amidation reaction.
3.1.3. Zeta potential vs. pH plots for the EPs Surface charge of biological polymer is an important parameter determining the interaction between the adsorbent and metal ions (Guiza, 2017). The zeta potential plots of EPs were performed to evaluate the influence of pH over the surface charge, in order to determine the carboxyl groups deprotonation capacity and amine protonation capacity. The change of zeta potential (ζ) of EPs and original pectin with pH were studied in a pH range of 2.0–7.0, and the results were shown in Fig. 2. The original pectin presented negative surface charge in the whole pH range, which may be attributed to the large amount of COO− (Diana et al., 2017). However, EPs have a positive charge at low pH because of the presence of NH3+. A shift of the isoelectric point (IEP) was observed depending on the DA of derivatives. The IEP of EP32, EP39, EP43 and EP48 are 1.80, 1.95, 2.45 and 3.20, respectively. It was reported that the surface charge of polysacchariderich materials (positively or negatively charged) depending on the pH of the solution (Onditi et al., 2016). If pH > IEP, the material is promoted to be negatively charged, which creates vacant sites for binding of positively charged metal ions. It is assumed that the lower IEP, the higher adsorption capacity of adsorbent towards Pb2+.
3.1.2. Degree of amidation of EPs The DA represents the number of acidylated C6 per 100 galacturonic Table 1 Degrees of amidation (DA) of ethylenediamine-modified pectins. Samples
ethylenediamine(mL)
N(%)
Original pectin EP32 EP39 EP43 EP48
0 0.05 0.20 0.50 2.00
< 0.3 4.31 ± 5.10 ± 5.50 ± 5.94 ±
C(%)
0.00a 0.01b 0.01c 0.01d
36.97 38.75 38.02 37.51 36.68
DA(%) ± ± ± ± ±
0.00a 0.03b 0.01c 0.07d 0.08e
— 31.62 38.94 43.13 48.31
± ± ± ±
0.02a 0.04b 0.00c 0.17d
Mean values in the same column with different letters are significantly different (Tukey test, p < 0.05). Data are presented as means ± standard deviations of triplicate measurements. EPx: EP stands for ethylenediamine-modified pectin and x represents the percentage or degree of amidation. 3
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trend as original pectin with the increase of pH, but EP43 and EP48 showed a significantly lower removal rate. This phenomenon may be due to these two materials were not suitable at this high Pb2+ concentration (150 mg/L). Therefore, the effect of initial Pb2+ concentration on the adsorption was further studied in the next section. The pH of Pb2+ solutions before and after Pb2+ adsorption on the adsorbents were shown in Fig. 3B. When the pH was between 4 and 6, the ionization of carboxyl groups happened and mainly existed in the form of −COO−. EP−COO− can interact with Pb2+ to form (EP−COO)2Pb according to Eq. 6. Along with the formation of (EP−COO)2Pb, the amount of EP−COO− was reduced, leading the Eq. 5 to the positive direction and resulting in more release of H+. The decline of pH (it was found that except for EP48, the final pH of solutions decreased to 3.0–4.5 after adsorption from the initial pH range of 4.0–6.0) confirmed the existence of EP−COOH in the system. In addition, EP−COOH could also react with Pb2+ according to Eq. 7 by ion exchange. Similar chemical reactions were reported by Wang, Zhou, Peng, Yu, and Yang (2007).
Fig. 2. Profile the ζ = f (pH) of pectin and EPx in water. Sample concentration is 0.1 % w/v. 2+
3.2. Pb
adsorption properties of EPs
EP−COOH ⇌ EP−COO− + H+
(5)
EP−COO− + Pb2+ → (EP−COO)2Pb
(6)
EP−COOH + Pb2+ → (EP−COO)2Pb + 2H+
3.2.1. Effect of initial pH on the adsorption The pH of the aqueous solution is an important operational parameter in the adsorption process, since the pH affect not only the protonation of the functional groups of the adsorbent, but also the speciation of heavy metal ions in the solution (Teh, Budiman, Shak, & Wu, 2016). The pH in the following experiments was set to be less than 6.0 due to the probability of forming a metal hydroxide precipitates of Pb2+ at a higher pH (Akinyeye, Ibigbami, & Odeja, 2016). The effect of pH on the removal rate of Pb2+ by original pectin and EPs are presented in Fig. 3A. In the case of original pectin, it can be seen that the removal rate increased rapidly from 60 % to 90 % as the pH increased from 2.3 to 4.0, and then gently reach a plateau at pH beyond 4.0. There was a competition between Pb2+ and the protons at low pH (2.3–4.0), while the increase of OH− concentration improved the Pb2+ removal rate at higher pH (Pal & Pal, 2017). In addition, the swelling behavior of both pectin and EPs may change along with pH and consequently influence its adsorption towards Pb2+. At low pH, the swelling of pectin may be low due to a complex association inside the polyelectrolyte developed through a random hydrogen bonding between −OH and −COOH (Pal et al., 2017). It may reduce the adsorption of Pb2+ because the main reaction site (−COOH) was not available for interaction. Once the pH was increased, it encouraged the ionization of −COOH and reduced the entanglement density through a repulsive interaction between −COO−. Changes in the pectin chain structure may result in a higher swelling and promote the adsorption of Pb2+. EPs presented a similar
(7) 2+
adAs for EP48, there was no significant change in pH after Pb sorption, which might be because approximately 83 % of carboxyl sites were occupied by ethylenediamine after modification, resulting not much −COOH available for ion exchange. 3.2.2. Effect of initial concentration of Pb2+ on the adsorption Generally, it was found that at Pb2+ concentration ranged from 20.0 to 250.0 mg/L, the Pb2+ removal rate increased with the increase of initial Pb2+ concentration (< 150.0 mg/L), but gradually decreased at higher concentrations (150.0–250.0 mg/L) (Fig. 4). At low Pb2+ concentration, the increase of Pb2+ removal rate may be due to the unsaturation of the binding site. Whereas the decrease of Pb2+ removal rate when Pb2+ concentration further increased might be caused by the lack of the available binding sites (Liang et al., 2018). Although EP48 showed significantly lower removal rate at high Pb2+ concentration (150 mg/L), it exhibited great removal efficiency (≥94 %) of Pb2+ at initial Pb2+ concentrations as low as 40−80 mg/L. On the contrary, the original pectin, EP32, EP39 and EP43 had removal rates of less than 45 % at initial Pb2+ concentrations of 20.0–60.0 mg/L. This phenomenon is quite interesting and may be related to the adsorption mechanisms of adsorbents, which was studied in the next section. In general, pectin compounds are effective substances for the collection of metal ions in high metal concentrations (Khotimchenko, Kovalev, & Khotimchenko, 2007), however, many papers mentioned
Fig. 3. Effect of pH on adsorption of Pb2+ by pectin and EPx (A). Comparison of pH of Pb2+ solutions before and after Pb2+ adsorption (B). (Initial concentration of Pb2+: 150.0 mg/L, dose: 1 g/L, temperature: 25 ± 2 °C, contact time: 1 h). 4
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Fig. 5. Relative zeta potential (ζ) response of pectin and EPx in Pb2+ solution. 2+
2+
Fig. 4. Effect of initial Pb concentrations on adsorption of Pb by pectin and EPx. (dose: 1 g/L, temperature: 25 ± 2 °C, pH: 5.0, contact time: 1 h).
It was noted that the formation of precipitation when the ζ of EP48 solution reached −17 mV with the Pb2+ concentration at 40 mg/L. EP32, EP39 and pectin solution had no visible precipitation until Pb2+ concentration up to 150 mg/L. There is a large amount of negative charge on the surface of original pectin, EP32, EP39 and EP43. Therefore, a small amount of positive Pb2+ at low concentration is not enough to neutralize its negative charge, so as to aggregate with each other and form precipitation. That's why these samples are less effective in low concentrations of Pb2+. In the case of EP48, it has the highest IEP, a small amount of Pb2+ is able to neutralize its negative COO−, showing its advantage for Pb2+ removal in low concentration.
that the removal of heavy metal by pectin also was not very efficient at Pb2+ of low concentration. (Diana et al., 2017). The main drawbacks of the conventional treatment included the low efficiency at low concentration of heavy metals and might be very expensive (Abdolali et al., 2015, 2017; Castro, Blázquez, González, Muñoz, & Ballester, 2017). In this study, it was found that Pb2+ concentration can be reduced to less than 1 mg/L by EP48 when the initial Pb2+ concentration is 50−80 mg/ L. The removal rate of Pb2+ by EP48 was compared with other adsorbents in the Supplementary Table 1. and was found that the adsorption efficiency of EP48 was generally higher than previously reported values, indicating EP48 may be an excellent adsorbent for removing Pb2+ of low concentrations. Lead wastewaters mainly originate from mining activities, metal plating, smelting, battery manufacture etc. A kind of acid mine drainage sample from the low grade sulphur deposit in the Sarcheshmeh copper mine was found to contain 55.1 mg/L of Pb2+ (Beygli, Mohaghegh, & Rahimi, 2019). This Pb2+ concentration is within the best adsorption range of EP48. Therefore, EP48 may be a potential adsorbent used to remove Pb2+ from such mine drainage.
3.3.2. Z-average particle size analysis of EP-Pb2+ The Z-average particle sizes of EPs were measured. It could be seen from Fig. 6 that the average particle size of EPs increased with the increase of DA. The increase of particle size may be due to the introduction of new functional groups on the carboxyl group. In case that these amide groups are blockwise distributed within the pectins molecules (Racape, Thibault, Reitsma, & Pilnik, 1989), this would allow the formation of larger areas of compatibility. And according to EinhornStoll and Kunzek (2009), the amidation of pectin caused an increased calcium reactivity. Yang, Li, and Chen (2013) also reported that the particle sizes of polyaluminum chloride-modified etherified citrus pectin increased after etherification, and added potential binding groups to the citrus pectin surface. In order to further study the EP-Pb2+ interaction and understand
3.3. Adsorption mechanisms of Pb2+ 3.3.1. Influence of Pb2+ on the zeta potential of the EPs In order to study the chemical affinity between EPs and Pb2+, the pectin and EPs solution (0.1 %) were titrated by successive increment of 1.0 g/L Pb2+ until the final concentration of Pb2+ in pectin and EPs solution were 10.0, 20.0, 40.0, 60.0, 80.0, 100.0, 150.0 and 200.0 mg/ L. The ζ of the pectin and EPs solution were determined during the addition of Pb2+ and the changes in the ζ were registered in Fig. 5. As can be seen, the addition of Pb2+ led to an increase in the ζ of pectin solution (ζ from −45 mV to −10 mV). The ζ of the EPs solution also presented a growing trend. It is worth mentioning that EP48 showed an extremely rapid increase in potential, and even became positively charged. Since carboxylic acid groups are important contributors for negative charge on the surface of pectin biopolymers (Balaria & Schiewer, 2008), and the EP48 consumed the most carboxyl group during modification, which explained why ζ of EP48 increased more quickly. Furthermore, the reason why the lower removal rate by EP48 at high Pb2+ concentrations of 150.0 mg/L may be because the electrostatic repulsion between Pb2+ and the surface positive charge of EP48 hindered the adsorption of Pb2+. However, compared with carboxyl groups, amino groups seem to be easier associated with Pb2+ at low Pb2+ concentration, and EPs with higher DA is more likely to precipitate or to form an EP-Pb2+ complex.
Fig. 6. Z-average particle size of pectin-Pb2+ and EPx-Pb2+. Sample concentration is 0.1 % w/v. 5
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Fig. 7. Wide-scan XPS spectra of pectin, EP48, and EP48 adsorbed with Pb2+ (A). High-resolution XPS spectra of Pb4f in EP48 adsorbed with Pb2+ (B). High-resolution XPS spectra of C1s in pectin (C), EP48 (D) and EP48 adsorbed with Pb2+ (E); High-resolution XPS spectra of O1s in pectin (F), EP48 (G) and EP48 adsorbed with Pb2+ (H); High-resolution XPS spectra of N1s in EP48 (I) and EP48 adsorbed with Pb2+ (J).
6
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the state of EP-Pb2+ complex, the particle size of the EP-Pb2+ complex was determined and was shown in Fig. 6. At a low concentration of Pb2+, it was found that the particle size of the EP32-Pb2+, EP39-Pb2+, EP43-Pb2+ complex was very small and with no significant (p > 0.05) difference from that of the corresponding modified pectin. The complex was uniformly dispersed in the solution and invisible by the naked eye. With the increase of Pb2+ concentration, the association enhanced and formed more and stable junctions evident as larger and more prominent aggregate-like structures. Therefore, the particle size of EP-Pb2+ complexes increased and even produced visible precipitation. The addition of Pb2+ could induce an association between chains of galacturonic acid and similar to the formation of “egg-box” model between pectin and calcium. The original pectin would not precipitate until the Pb2+ concentration reached 150 mg/g. Nevertheless, the EP48 was the first to precipitate and formed a larger EP-Pb2+ complex at Pb2+ concentration of 40 mg/L. The different precipitation behavior of EP48 and original pectin, suggesting that the amino groups might directly take part in the interchain association to form the complex.
144.1 eV and 139.1 eV were assigned to Pb4f5/2 and Pb4f7/2, respectively (Fig. 7B). According to the report of Wang et al. (2007), the Pb4f7/2 peak at 139.3 eV can be attributed to the eCOOPb+, (eCOO)2Pb and eOPb+. It is indicated that the surface functional groups, such as carboxyl group and hydroxyl group, had the function of electron donor for Pb2+ adsorption. As also can be seen in the highresolution spectrum of O1s in EP48 (Fig. 7G), the two peaks lying at 532.20 eV and 532.80 eV were observed due to the O-containing groups (OeC]O, OeCeO and CeOH). After Pb2+ adsorption, the peak shifted to 532.40 eV and 533.00 eV (Fig. 7H). In the case of the high-resolution spectrum of N1s, the peak corresponding to eNH2 with a binding energy of 399.63 eV and the peak corresponding to eCONHe with a binding energy of 401.10 eV (Fig. 7I) increased to 399.79 eV and 401.32 eV respectively (Fig. 7J). As suggested by Li et al. (2018), the increase of binding energy confirmed that the O, N atoms contributed to the formation of Pb2+-related coordination complexes. It was reported that the shared bond between the N atom and metal obtained a lone pair of electrons from the N atom, which decreased the electron cloud density on the N atom, and increased the binding energy (Deng et al., 2017). This finding suggested that eNH2 and eCONHe might also be an important and effective functional group for Pb2+ adsorption. From the variation of pH, XPS and FTIR analysis of EP48 before and after Pb2+ adsorption, it was confirmed that the adsorption mechanisms of Pb2+ into EPs were a complicated process, in which the ion exchange of eCOOH and chelation with eCONHe and eNH2 were involved.
3.3.3. FTIR spectroscopy analysis of EP-Pb2+ FTIR analysis was further used to acquire the information of possible interaction between the functional groups in adsorbent and the Pb2+. Fig. 1B presents the FTIR spectra of EPs and original pectin after adsorption of Pb2+. The FTIR spectra comparisons of pectin versus pectin-Pb2+ revealed that the absorption peak corresponding to the carboxylate (−COO−) groups shifted to a higher frequency (from about 1623 cm−1 to about 1633 cm−1) after adsorption of Pb2+. This shift in wave number corresponds to a change in bonding energy corresponding to the functional group, which further reflects that the bonding pattern of carboxylate groups changes after biosorption, and confirmed the involvement of carboxylic acid groups in binding of Pb2+ (Balaria & Schiewer, 2008). In the case of EPs, shifts in the peak from 1644∼1629 cm−1 to 1623∼1614 cm−1 can be observed for EP32, EP39, EP43 and EP48, suggesting that both the amide bond and carboxylic acid groups changed after adsorption of Pb2+. In addition, the intensity of the peak at about 1587 cm−1 assigned to the NeH bending vibration is weakened or even disappeared, revealing that the amino group was also involved in the binding of Pb2+. No significant shift in the peak corresponding to the ester group (−COOR) was observed in either case due to absence of replaceable proton. Furthermore, an additional peak at about 1384 cm−1 appeared after Pb2+ adsorption. This peak indicates the presence of a pronounced carboxylate ion in the vicinity of nitrogen atoms, which could belong to the nitrate ions present in the system, as suggested by Minamisawa, Minamisawa, Yoshida, and Takai (2005) who also observed a similar peak for Pb2+ biosorption by lemon-gel.
4. Conclusions EPs with different DA were prepared by modification of pectin with ethylenediamine and used for the adsorption of Pb2+. The zeta potential analysis showed that the modification of ethylenediamine increased the alkalinity of the pectin. When Pb2+ was added continuously, the zeta potential of EP48 increased the most rapidly and was the first to be electroneutralized. The average particle size of EPs increased with the increase of DA, and EP48 was the first precipitated and formed visible large EP48-Pb2+ complex at Pb2+ concentration of 40 mg/L. EP48 exhibited great removal efficiency of Pb2+ (≥94 %) at low Pb2+ concentrations. The study of adsorption mechanisms indicated that Pb2+ was adsorbed via the ion exchange of eCOOH and chelation with eCONHe and eNH2. The amino groups played an important role in the adsorption of Pb2+ at low Pb2+ concentration. This study provided a new idea for the modification of natural polymers adsorbents in effluent treatment. Amidation of pectin by ethylenediamine exhibited a potential adsorption capacity in removing low concentration Pb2+ from wastewater. Thus, the application of pectin in low concentration Pb2+ wastewater was expanded.
3.3.4. XPS spectroscopy analysis of EP-Pb2+ XPS analysis was conducted to understand the interactions between the Pb2+ and adsorbent based on the binding energy of the species involved. Fig. 7A represents typical wide scan XPS spectra of original pectin, EP48, and EP48 adsorbed with Pb2+. It can be seen that peaks at about 400 eV belonging to N1s appeared after modification, indicating the ethylenediamine was successfully modified on pectin. Comparing the spectra of pectin before (Fig. 7C) and after (Fig. 7D) modification in the spectra of C1s, the peaks at binding energy of 286.00 eV and 288.35 eV can be assigned to the CeNH2 and NeC]O group, respectively, which further confirmed the success of modification. In addition, it was found that the C1s spectrum of the EP48 did not change significantly after Pb2+ adsorption (Fig. 7E) as compared to that before adsorption (Fig. 7D), indicating that these C-containing functional groups might not have a remarkable effect on Pb2+ adsorption. The peaks at about 139 eV in the wide scan XPS spectra (Fig. 7A) corresponding to Pb4f was identified, which confirmed the distribution of the Pb2+ on the surface of the adsorbent. The latter high-resolution XPS spectra of Pb4f analysis showed that the strong peaks appeared at
Acknowledgements The authors would like to thank the Center of Analysis and Testing of Nanchang University and State Key Laboratory of Food Science and Technology for their expert technical assistance. This study was supported financially by the National Natural Science Foundation of China (Nr 31660488, 31571875) and Postgraduate Innovative Funds of Nanchang University (CX2016212). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.carbpol.2020.115911. References Abdolali, A., Ngo, H. H., Guo, W., Zhou, J. L., Du, B., Wei, Q., et al. (2015).
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Pb2+ adsorption by ethylenediamine-modified pectins and their adsorption mechanisms
T
Rui-hong Lianga, Ya Lia, Li Huangb, Xue-dong Wanga, Xiao-xue Hua, Cheng-mei Liua, Ming-shun Chena,*, Jun Chena,* a b
State Key Laboratory of Food Science and Technology, Nanchang University, Nanchang, Jiangxi, 330047, China Nanchang Institute for Control of Food and Drug, Nanchang, Jiangxi, 330047, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Pectin Ethylenediamine Modification Pb2+ Adsorption
Ethylenediamine-modified pectins (EPs) with different degrees of amidation (DA) were prepared and characterized by Fourier Transform Infrared spectra (FTIR), elemental analysis, X-ray photoelectron spectroscopy (XPS). The prepared EPs were then used to remove Pb2+ from the aqueous solution. It was found that EPs with the highest DA (EP48) exhibited great removal efficiency of Pb2+ (≥94 %) at low concentrations of 40−80 mg/ L. The zeta potential analysis showed that EP48 had the fastest increase in zeta potential when Pb2+ was continuously added and was the first to be electroneutralized. Particle size analysis further confirmed that EP48 was the first precipitated and formed a larger EP48-Pb2+ complex. The FTIR and XPS analyses indicated that Pb2+ was adsorbed via the ion exchange of carboxylic groups and chelation with acylamino and amino groups. These results suggested that the EP48 might be a promising adsorbent for the removal of low concentrations of Pb2+ in contaminated water.
1. Introduction Heavy metal pollution of the water body has become an increasingly serious problem, which affects the ecological environment as well as the human health (Liu, Jia, Liu, & Liang, 2018; Wang, Liu, Duan, Sun, & Xu, 2019). These heavy metals may be enriched in organisms and enter the human body through the food chain (Liu, Pang et al., 2018; Pal, Majumder, Sengupta, Das, & Bandyopadhyay, 2017). Lead, as a heavy metal extensively used in industrial production, is one of the most hazardous elements to human beings. The excessive intake of Pb2+ mainly harms the central nervous system, liver and kidney of the human body (Huang & Zhu, 2013; Tao, Yuan, Xiaona, & Wei, 2012; Wang, Huang, Tu, Ruan, & Lin, 2016). Hence, Pb2+ removal from water bodies has been a priority in water-treatment plants. A series of conventional treatments including chemical precipitation, ion exchange, chemical oxidation, membrane filtration and adsorption were used for Pb2+ removal (Deng et al., 2017; Onditi, Adelodun, Changamu, & Ngila, 2016). Among these, adsorption is a widely used process due to its low cost (Wang, Miao, He, & Shen, 2011). However, adsorption by activated carbon is often limited by their several disadvantages, e.g., ineffective, expensive when used in low concentration heavy metal solution processing (Wang & Chen, 2006) and large-amount sludge creation.
⁎
Because of the shortages of traditional activated carbon, new alternative materials should be extensively studied. Pectin is an important natural anionic polysaccharide extracted from cell walls of higher plants. This polysaccharide is known as a gelling/thickening agent, emulsifying agent, as well as stabilizer in the food industry (Liu, Guo, Liang, Liu, & Chen, 2017). Moreover, pectin can be regarded as an attractive novel biopolymer material in wastewater treatment that facilitates metal chelation with a backbone of (1–4) α-D-galacturonic acids (Celus et al., 2017). Diana, Jaime, LópezMaldonado, and Oropeza-Guzmán (2017) reported that pectin extracted from nopal and used as a coagulant-flocculant agent to remove 99 % of all metallic ions. Some scholar has proposed that chemical modifications could tune the pectin structure to adjust its heavy metal adsorption capability due to the formation of specific structure with the particular groups (Wang, Liang et al., 2019). As reported by Li, Yang, Zhao, and Xu (2007), the modification of pectin by cross-linked with adipic acid could increase its stability and adsorption performance, the saturated loading capacity for Pb2+ reached 1.82 mmol/g. Many studies have reported that amino groups have good chelating ability of heavy metals (Li et al., 2018; Li, Ye, Fang, & Liu, 2019; Qin et al., 2019). Ethylenediamine is a metal chelating agent which is rich in amino groups. Li et al. (2009) prepared modified activated carbon with
Corresponding authors at: Nanchang University, 235 Nanjing East Road, Nanchang, China. E-mail addresses: [email protected] (M.-s. Chen), [email protected] (J. Chen).
https://doi.org/10.1016/j.carbpol.2020.115911 Received 8 October 2019; Received in revised form 28 December 2019; Accepted 23 January 2020 Available online 25 January 2020 0144-8617/ © 2020 Elsevier Ltd. All rights reserved.
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ethylenediamine and found that the new sorbent has high analytical potential for the preconcentration of trace metal ions. The percentage or degree of amidation (DA) is an important characteristic of pectin that may determine the capacity of interaction with metallic ions. However, to our knowledge, there are no reports on the graft of ethylenediamine on pectin. The effect of the amide group on the Pb2+ adsorption capacity of modified pectin has not been reported either. In this study, ethylenediamine-modified pectins (EPs) with different DA were prepared by amidation of pectin with ethylenediamine. The purpose of the study is to evaluate whether the EPs can be used as adsorbents of Pb2+ and to explore their adsorption mechanisms in aqueous solution.
DA=
MCR 6 × ×100 MCP K
(3)
where MN and MC are contents of nitrogen and carbon (%), respectively. MCR and MCP are contents of carbon (%), 12/14 is the ratio of the carbon to the nitrogen atomic mass, 6 is the sum of carbons in the galacturonic unit of pectin, K and I are the numbers of carbons and nitrogens in the amine molecule, respectively. In this study, K = 2 and I = 2. The EPs with different DA were named EPx, where EP stood for ethylenediamine-modified pectin and x was the DA.
2.5. XPS analysis 2. Materials and methods The surface chemical composition of samples was determined by the X-ray photoelectron spectroscopy (ESCALAB250Xi, Thermo Fisher Scientific, Madison, USA). The binding energy scale for final calibration was corrected by the C1s peak corresponding to 284.8 eV (Zhang, Li, Li, & Yang, 2017).
2.1. Materials Citrus pectin (Commercial name 13CG, Mw = 527 KDa, with a degree of methylation of 41.46 % and galacturonic acid content of 87.73 %), was supplied by the general agent of CPkelco (Shanghai, China). Ethylenediamine, N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC), N-hydroxysuccinimide (NHS, 98 %) and Pb (NO3)2 were purchased from Aladdin Reagent Company (Shanghai, China). All reagents were of analytical reagent grade and used as received. All aqueous solutions were prepared with ultrapure water (organic free, 18.2 MΩ cm resistance) from a Milli-Q system (Millipore, Billerica, USA).
2.6. Zeta potential analysis The zeta potential measurements were done at room temperature in porcelain cuvettes by Malvern Zetasizer Nano ZSP (Malvern Instruments Ltd., Worcestershire, UK) according to Pal et al. (2017).
2.7. Particle size analysis 2.2. Preparation of EPs The Z-average particle size of EPs solution (0.1 %) before and after adsorption of different concentrations of Pb2+ were monitored using a laser light scattering (Mastersizer 2000, Malvern Instruments Ltd., Worcestershire, UK).
The synthesis was carried out according to the method of Huang and Wang (2017) with some modifications. Firstly, 10.00 g of pectin powder was dissolved in 1000 mL ultrapure water, then 2.88 g NHS and 9.59 g EDC were added, followed with the addition of 0.1 M HNO3 until pH to 4.0. Shortly afterward, different amounts of ethylenediamine (0.05, 0.20, 0.50 and 2.00 mL) were added. Then the solution was magnetic stirred at room temperature for 8 h. The solution was purified by dialysis through a 10000-8000 molecular weight cut-off dialysis bag (MD1477, Yuanye Bio-Technology Co., Ltd, Shanghai, China) for three days. Finally, products were freeze-dried with a lyophilizer (Alpha12LD, Christ, Gottingen, Germany). All the EPs prepared in our experiments were soluble in water.
2.8. Batch adsorption studies Stock solution (1.0 g/L) of Pb2+ was prepared by dissolving 1.599 g Pb (NO3)2 in 1000 mL distilled water. The solutions of different concentrations used in various experiments were obtained by dilution of the stock solutions. The effect of pH on the adsorption was conducted at solution pH ranging from 2.0 to 6.0. The initial pH of the Pb2+ solution was adjusted by 0.1 M HNO3 and 0.1 M NaOH aqueous solutions. 30.0 mg of adsorbents were added to 30.0 mL of 150.0 mg/L Pb2+ solution. The mixture was stirred for 1 h at 25 °C, then the mixture was centrifuged at 40000 rpm for 20 min and the resulting supernatant was filtered with 0.45 μm filter. The concentration of Pb2+ in the filtrate was measured by atomic absorption spectrophotometer (AAS, A3AFG-12, Puxi, Beijing, China). After adsorption, the final pH was also measured for investigating the interaction between the adsorbents and the Pb2+. To explore the effect of Pb2+ concentrations on the adsorption, the adsorption experiments were also operated at the Pb2+ concentrations varying from 20.0 to 250.0 mg/L at pH 5.0 (optimized pH). 30.0 mg of adsorbents were put into a 30.0 mL Pb2+ solution, and the mixture was shaken in a water bath 25 °C for 1 h. The adsorbent was removed after centrifugation 40,000 rpm, 20 min. Finally, the Pb2+ concentrations in filtrate were determined by AAS. The removal rate (%) was calculated as follows:
2.3. FTIR spectra analysis The FTIR spectra were collected on a Nicolet 5700 spectrometer (Thermo Fisher Scientific, Madison, USA) using KBr pellets according to Diana et al. (2017). The samples (1.0 mg) were mixed with KBr in a ratio of 1:100. The pills were formed at 6000 Psi of pressure in a hydraulic press manual. Spectra were recorded in the absorption mode 4000 to 400 cm−1 with a resolution of 2 cm−1. 2.4. Elemental analysis The elemental analysis (C, H and N) was carried out with an Elemental analyzer (Vario Micro cube Elementar, Langenselbold, Germany) according to the work of Li et al. (2009). The degrees of amidation (DA) of the products were calculated from the elemental composition according to the following formulas (Šimkovic, Synytsya, Uhliariková, & Čopíková, 2009):
MCR = MN ×
12 K × 14 I
MCP = MC − MCR
R= (1)
C 0 − Ce × 100 C0
(4)
where C0 and Ce (mg/L) are the initial and equilibrium concentrations of Pb2+ solution, respectively.
(2) 2
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Fig. 1. FTIR spectra of pectin and EPx (A). FTIR spectra of pectin-Pb2+ and EPx-Pb2+ (B).
acid residues in the amidated pectin (Rolin & De Vries, 1990). The DA of EPs were obtained from elemental analysis and were listed in Table 1. It was found that the DA increased with the addition of ethylenediamine. When ethylenediamine was added at 0.05, 0.20, 0.05 and 2.00 mL, DA was 31.62 %, 38.94 %, 43.13 % and 48.31 %, respectively. Thus, the four derivatives were named as EP32, EP39, EP43 and EP48. It can be found that low ethylenediamine concentration showed better modification efficiency, which may be because the EDC and NHS were sufficient at those concentrations when employed as cross-linkers in the amidation reaction (Huang et al., 2018). At higher ethylenediamine concentrations, the DA did not significantly increase, possibly due to fewer carboxyl sites that could be modified as well as the deficiency of EDC and NHS.
2.9. Statistical analysis All of the experiments were done in triplicate. Statistical analysis was carried out using SPSS (version 16.0, Chicago, United States). The results were expressed as mean ± standard deviations and compared using the Tukey test at 5 % confidence level. 3. Results and discussion 3.1. Characteristics of the EPs 3.1.1. FTIR spectroscopy analysis of EPs The FTIR spectra of EPs and original pectin were shown in Fig. 1A. The original pectin showed a broad band at about 3415 cm−1, assigned to stretching vibration modes of OeH groups. The weak peak toward 2938 cm−1 was attributed to the C–H antisymmetrical stretching vibration (Huang et al., 2018). Two peaks at about 1747 and 1623 cm−1 typically represent the protonated carboxylic acid groups and carboxylic anion, respectively (Liang, Wang, Chen, Liu, & Liu, 2015). In comparison with the original pectin, EPs showed an enhanced absorption peak at about 1644 cm−1 which represented the carbonyl stretching vibration of the amide bonds, and the peak at about 1587 cm−1 which was related to the NeH bending vibration of amide (Ren, Abbood, He, Peng, & Huang, 2013). The peak of amide I and amide II are not obvious, so we decomposed FITR spectroscopies between 1500 cm−1 and 1800 cm−1 by peak fitting and it was shown in the Supplementary Fig. 1. It was found that new amide bands at about 1650 cm−1 (amide I) and 1560 cm−1 (amide II) appeared. Šimkovic et al. (2009) also found new amide bands appeared at about 1652 cm−1 and 1551 cm−1 for pectin propylamide derivatives. The formation of amide bonds suggested that ethylenediamine was grafted on pectin through the amidation reaction.
3.1.3. Zeta potential vs. pH plots for the EPs Surface charge of biological polymer is an important parameter determining the interaction between the adsorbent and metal ions (Guiza, 2017). The zeta potential plots of EPs were performed to evaluate the influence of pH over the surface charge, in order to determine the carboxyl groups deprotonation capacity and amine protonation capacity. The change of zeta potential (ζ) of EPs and original pectin with pH were studied in a pH range of 2.0–7.0, and the results were shown in Fig. 2. The original pectin presented negative surface charge in the whole pH range, which may be attributed to the large amount of COO− (Diana et al., 2017). However, EPs have a positive charge at low pH because of the presence of NH3+. A shift of the isoelectric point (IEP) was observed depending on the DA of derivatives. The IEP of EP32, EP39, EP43 and EP48 are 1.80, 1.95, 2.45 and 3.20, respectively. It was reported that the surface charge of polysacchariderich materials (positively or negatively charged) depending on the pH of the solution (Onditi et al., 2016). If pH > IEP, the material is promoted to be negatively charged, which creates vacant sites for binding of positively charged metal ions. It is assumed that the lower IEP, the higher adsorption capacity of adsorbent towards Pb2+.
3.1.2. Degree of amidation of EPs The DA represents the number of acidylated C6 per 100 galacturonic Table 1 Degrees of amidation (DA) of ethylenediamine-modified pectins. Samples
ethylenediamine(mL)
N(%)
Original pectin EP32 EP39 EP43 EP48
0 0.05 0.20 0.50 2.00
< 0.3 4.31 ± 5.10 ± 5.50 ± 5.94 ±
C(%)
0.00a 0.01b 0.01c 0.01d
36.97 38.75 38.02 37.51 36.68
DA(%) ± ± ± ± ±
0.00a 0.03b 0.01c 0.07d 0.08e
— 31.62 38.94 43.13 48.31
± ± ± ±
0.02a 0.04b 0.00c 0.17d
Mean values in the same column with different letters are significantly different (Tukey test, p < 0.05). Data are presented as means ± standard deviations of triplicate measurements. EPx: EP stands for ethylenediamine-modified pectin and x represents the percentage or degree of amidation. 3
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trend as original pectin with the increase of pH, but EP43 and EP48 showed a significantly lower removal rate. This phenomenon may be due to these two materials were not suitable at this high Pb2+ concentration (150 mg/L). Therefore, the effect of initial Pb2+ concentration on the adsorption was further studied in the next section. The pH of Pb2+ solutions before and after Pb2+ adsorption on the adsorbents were shown in Fig. 3B. When the pH was between 4 and 6, the ionization of carboxyl groups happened and mainly existed in the form of −COO−. EP−COO− can interact with Pb2+ to form (EP−COO)2Pb according to Eq. 6. Along with the formation of (EP−COO)2Pb, the amount of EP−COO− was reduced, leading the Eq. 5 to the positive direction and resulting in more release of H+. The decline of pH (it was found that except for EP48, the final pH of solutions decreased to 3.0–4.5 after adsorption from the initial pH range of 4.0–6.0) confirmed the existence of EP−COOH in the system. In addition, EP−COOH could also react with Pb2+ according to Eq. 7 by ion exchange. Similar chemical reactions were reported by Wang, Zhou, Peng, Yu, and Yang (2007).
Fig. 2. Profile the ζ = f (pH) of pectin and EPx in water. Sample concentration is 0.1 % w/v. 2+
3.2. Pb
adsorption properties of EPs
EP−COOH ⇌ EP−COO− + H+
(5)
EP−COO− + Pb2+ → (EP−COO)2Pb
(6)
EP−COOH + Pb2+ → (EP−COO)2Pb + 2H+
3.2.1. Effect of initial pH on the adsorption The pH of the aqueous solution is an important operational parameter in the adsorption process, since the pH affect not only the protonation of the functional groups of the adsorbent, but also the speciation of heavy metal ions in the solution (Teh, Budiman, Shak, & Wu, 2016). The pH in the following experiments was set to be less than 6.0 due to the probability of forming a metal hydroxide precipitates of Pb2+ at a higher pH (Akinyeye, Ibigbami, & Odeja, 2016). The effect of pH on the removal rate of Pb2+ by original pectin and EPs are presented in Fig. 3A. In the case of original pectin, it can be seen that the removal rate increased rapidly from 60 % to 90 % as the pH increased from 2.3 to 4.0, and then gently reach a plateau at pH beyond 4.0. There was a competition between Pb2+ and the protons at low pH (2.3–4.0), while the increase of OH− concentration improved the Pb2+ removal rate at higher pH (Pal & Pal, 2017). In addition, the swelling behavior of both pectin and EPs may change along with pH and consequently influence its adsorption towards Pb2+. At low pH, the swelling of pectin may be low due to a complex association inside the polyelectrolyte developed through a random hydrogen bonding between −OH and −COOH (Pal et al., 2017). It may reduce the adsorption of Pb2+ because the main reaction site (−COOH) was not available for interaction. Once the pH was increased, it encouraged the ionization of −COOH and reduced the entanglement density through a repulsive interaction between −COO−. Changes in the pectin chain structure may result in a higher swelling and promote the adsorption of Pb2+. EPs presented a similar
(7) 2+
adAs for EP48, there was no significant change in pH after Pb sorption, which might be because approximately 83 % of carboxyl sites were occupied by ethylenediamine after modification, resulting not much −COOH available for ion exchange. 3.2.2. Effect of initial concentration of Pb2+ on the adsorption Generally, it was found that at Pb2+ concentration ranged from 20.0 to 250.0 mg/L, the Pb2+ removal rate increased with the increase of initial Pb2+ concentration (< 150.0 mg/L), but gradually decreased at higher concentrations (150.0–250.0 mg/L) (Fig. 4). At low Pb2+ concentration, the increase of Pb2+ removal rate may be due to the unsaturation of the binding site. Whereas the decrease of Pb2+ removal rate when Pb2+ concentration further increased might be caused by the lack of the available binding sites (Liang et al., 2018). Although EP48 showed significantly lower removal rate at high Pb2+ concentration (150 mg/L), it exhibited great removal efficiency (≥94 %) of Pb2+ at initial Pb2+ concentrations as low as 40−80 mg/L. On the contrary, the original pectin, EP32, EP39 and EP43 had removal rates of less than 45 % at initial Pb2+ concentrations of 20.0–60.0 mg/L. This phenomenon is quite interesting and may be related to the adsorption mechanisms of adsorbents, which was studied in the next section. In general, pectin compounds are effective substances for the collection of metal ions in high metal concentrations (Khotimchenko, Kovalev, & Khotimchenko, 2007), however, many papers mentioned
Fig. 3. Effect of pH on adsorption of Pb2+ by pectin and EPx (A). Comparison of pH of Pb2+ solutions before and after Pb2+ adsorption (B). (Initial concentration of Pb2+: 150.0 mg/L, dose: 1 g/L, temperature: 25 ± 2 °C, contact time: 1 h). 4
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Fig. 5. Relative zeta potential (ζ) response of pectin and EPx in Pb2+ solution. 2+
2+
Fig. 4. Effect of initial Pb concentrations on adsorption of Pb by pectin and EPx. (dose: 1 g/L, temperature: 25 ± 2 °C, pH: 5.0, contact time: 1 h).
It was noted that the formation of precipitation when the ζ of EP48 solution reached −17 mV with the Pb2+ concentration at 40 mg/L. EP32, EP39 and pectin solution had no visible precipitation until Pb2+ concentration up to 150 mg/L. There is a large amount of negative charge on the surface of original pectin, EP32, EP39 and EP43. Therefore, a small amount of positive Pb2+ at low concentration is not enough to neutralize its negative charge, so as to aggregate with each other and form precipitation. That's why these samples are less effective in low concentrations of Pb2+. In the case of EP48, it has the highest IEP, a small amount of Pb2+ is able to neutralize its negative COO−, showing its advantage for Pb2+ removal in low concentration.
that the removal of heavy metal by pectin also was not very efficient at Pb2+ of low concentration. (Diana et al., 2017). The main drawbacks of the conventional treatment included the low efficiency at low concentration of heavy metals and might be very expensive (Abdolali et al., 2015, 2017; Castro, Blázquez, González, Muñoz, & Ballester, 2017). In this study, it was found that Pb2+ concentration can be reduced to less than 1 mg/L by EP48 when the initial Pb2+ concentration is 50−80 mg/ L. The removal rate of Pb2+ by EP48 was compared with other adsorbents in the Supplementary Table 1. and was found that the adsorption efficiency of EP48 was generally higher than previously reported values, indicating EP48 may be an excellent adsorbent for removing Pb2+ of low concentrations. Lead wastewaters mainly originate from mining activities, metal plating, smelting, battery manufacture etc. A kind of acid mine drainage sample from the low grade sulphur deposit in the Sarcheshmeh copper mine was found to contain 55.1 mg/L of Pb2+ (Beygli, Mohaghegh, & Rahimi, 2019). This Pb2+ concentration is within the best adsorption range of EP48. Therefore, EP48 may be a potential adsorbent used to remove Pb2+ from such mine drainage.
3.3.2. Z-average particle size analysis of EP-Pb2+ The Z-average particle sizes of EPs were measured. It could be seen from Fig. 6 that the average particle size of EPs increased with the increase of DA. The increase of particle size may be due to the introduction of new functional groups on the carboxyl group. In case that these amide groups are blockwise distributed within the pectins molecules (Racape, Thibault, Reitsma, & Pilnik, 1989), this would allow the formation of larger areas of compatibility. And according to EinhornStoll and Kunzek (2009), the amidation of pectin caused an increased calcium reactivity. Yang, Li, and Chen (2013) also reported that the particle sizes of polyaluminum chloride-modified etherified citrus pectin increased after etherification, and added potential binding groups to the citrus pectin surface. In order to further study the EP-Pb2+ interaction and understand
3.3. Adsorption mechanisms of Pb2+ 3.3.1. Influence of Pb2+ on the zeta potential of the EPs In order to study the chemical affinity between EPs and Pb2+, the pectin and EPs solution (0.1 %) were titrated by successive increment of 1.0 g/L Pb2+ until the final concentration of Pb2+ in pectin and EPs solution were 10.0, 20.0, 40.0, 60.0, 80.0, 100.0, 150.0 and 200.0 mg/ L. The ζ of the pectin and EPs solution were determined during the addition of Pb2+ and the changes in the ζ were registered in Fig. 5. As can be seen, the addition of Pb2+ led to an increase in the ζ of pectin solution (ζ from −45 mV to −10 mV). The ζ of the EPs solution also presented a growing trend. It is worth mentioning that EP48 showed an extremely rapid increase in potential, and even became positively charged. Since carboxylic acid groups are important contributors for negative charge on the surface of pectin biopolymers (Balaria & Schiewer, 2008), and the EP48 consumed the most carboxyl group during modification, which explained why ζ of EP48 increased more quickly. Furthermore, the reason why the lower removal rate by EP48 at high Pb2+ concentrations of 150.0 mg/L may be because the electrostatic repulsion between Pb2+ and the surface positive charge of EP48 hindered the adsorption of Pb2+. However, compared with carboxyl groups, amino groups seem to be easier associated with Pb2+ at low Pb2+ concentration, and EPs with higher DA is more likely to precipitate or to form an EP-Pb2+ complex.
Fig. 6. Z-average particle size of pectin-Pb2+ and EPx-Pb2+. Sample concentration is 0.1 % w/v. 5
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Fig. 7. Wide-scan XPS spectra of pectin, EP48, and EP48 adsorbed with Pb2+ (A). High-resolution XPS spectra of Pb4f in EP48 adsorbed with Pb2+ (B). High-resolution XPS spectra of C1s in pectin (C), EP48 (D) and EP48 adsorbed with Pb2+ (E); High-resolution XPS spectra of O1s in pectin (F), EP48 (G) and EP48 adsorbed with Pb2+ (H); High-resolution XPS spectra of N1s in EP48 (I) and EP48 adsorbed with Pb2+ (J).
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the state of EP-Pb2+ complex, the particle size of the EP-Pb2+ complex was determined and was shown in Fig. 6. At a low concentration of Pb2+, it was found that the particle size of the EP32-Pb2+, EP39-Pb2+, EP43-Pb2+ complex was very small and with no significant (p > 0.05) difference from that of the corresponding modified pectin. The complex was uniformly dispersed in the solution and invisible by the naked eye. With the increase of Pb2+ concentration, the association enhanced and formed more and stable junctions evident as larger and more prominent aggregate-like structures. Therefore, the particle size of EP-Pb2+ complexes increased and even produced visible precipitation. The addition of Pb2+ could induce an association between chains of galacturonic acid and similar to the formation of “egg-box” model between pectin and calcium. The original pectin would not precipitate until the Pb2+ concentration reached 150 mg/g. Nevertheless, the EP48 was the first to precipitate and formed a larger EP-Pb2+ complex at Pb2+ concentration of 40 mg/L. The different precipitation behavior of EP48 and original pectin, suggesting that the amino groups might directly take part in the interchain association to form the complex.
144.1 eV and 139.1 eV were assigned to Pb4f5/2 and Pb4f7/2, respectively (Fig. 7B). According to the report of Wang et al. (2007), the Pb4f7/2 peak at 139.3 eV can be attributed to the eCOOPb+, (eCOO)2Pb and eOPb+. It is indicated that the surface functional groups, such as carboxyl group and hydroxyl group, had the function of electron donor for Pb2+ adsorption. As also can be seen in the highresolution spectrum of O1s in EP48 (Fig. 7G), the two peaks lying at 532.20 eV and 532.80 eV were observed due to the O-containing groups (OeC]O, OeCeO and CeOH). After Pb2+ adsorption, the peak shifted to 532.40 eV and 533.00 eV (Fig. 7H). In the case of the high-resolution spectrum of N1s, the peak corresponding to eNH2 with a binding energy of 399.63 eV and the peak corresponding to eCONHe with a binding energy of 401.10 eV (Fig. 7I) increased to 399.79 eV and 401.32 eV respectively (Fig. 7J). As suggested by Li et al. (2018), the increase of binding energy confirmed that the O, N atoms contributed to the formation of Pb2+-related coordination complexes. It was reported that the shared bond between the N atom and metal obtained a lone pair of electrons from the N atom, which decreased the electron cloud density on the N atom, and increased the binding energy (Deng et al., 2017). This finding suggested that eNH2 and eCONHe might also be an important and effective functional group for Pb2+ adsorption. From the variation of pH, XPS and FTIR analysis of EP48 before and after Pb2+ adsorption, it was confirmed that the adsorption mechanisms of Pb2+ into EPs were a complicated process, in which the ion exchange of eCOOH and chelation with eCONHe and eNH2 were involved.
3.3.3. FTIR spectroscopy analysis of EP-Pb2+ FTIR analysis was further used to acquire the information of possible interaction between the functional groups in adsorbent and the Pb2+. Fig. 1B presents the FTIR spectra of EPs and original pectin after adsorption of Pb2+. The FTIR spectra comparisons of pectin versus pectin-Pb2+ revealed that the absorption peak corresponding to the carboxylate (−COO−) groups shifted to a higher frequency (from about 1623 cm−1 to about 1633 cm−1) after adsorption of Pb2+. This shift in wave number corresponds to a change in bonding energy corresponding to the functional group, which further reflects that the bonding pattern of carboxylate groups changes after biosorption, and confirmed the involvement of carboxylic acid groups in binding of Pb2+ (Balaria & Schiewer, 2008). In the case of EPs, shifts in the peak from 1644∼1629 cm−1 to 1623∼1614 cm−1 can be observed for EP32, EP39, EP43 and EP48, suggesting that both the amide bond and carboxylic acid groups changed after adsorption of Pb2+. In addition, the intensity of the peak at about 1587 cm−1 assigned to the NeH bending vibration is weakened or even disappeared, revealing that the amino group was also involved in the binding of Pb2+. No significant shift in the peak corresponding to the ester group (−COOR) was observed in either case due to absence of replaceable proton. Furthermore, an additional peak at about 1384 cm−1 appeared after Pb2+ adsorption. This peak indicates the presence of a pronounced carboxylate ion in the vicinity of nitrogen atoms, which could belong to the nitrate ions present in the system, as suggested by Minamisawa, Minamisawa, Yoshida, and Takai (2005) who also observed a similar peak for Pb2+ biosorption by lemon-gel.
4. Conclusions EPs with different DA were prepared by modification of pectin with ethylenediamine and used for the adsorption of Pb2+. The zeta potential analysis showed that the modification of ethylenediamine increased the alkalinity of the pectin. When Pb2+ was added continuously, the zeta potential of EP48 increased the most rapidly and was the first to be electroneutralized. The average particle size of EPs increased with the increase of DA, and EP48 was the first precipitated and formed visible large EP48-Pb2+ complex at Pb2+ concentration of 40 mg/L. EP48 exhibited great removal efficiency of Pb2+ (≥94 %) at low Pb2+ concentrations. The study of adsorption mechanisms indicated that Pb2+ was adsorbed via the ion exchange of eCOOH and chelation with eCONHe and eNH2. The amino groups played an important role in the adsorption of Pb2+ at low Pb2+ concentration. This study provided a new idea for the modification of natural polymers adsorbents in effluent treatment. Amidation of pectin by ethylenediamine exhibited a potential adsorption capacity in removing low concentration Pb2+ from wastewater. Thus, the application of pectin in low concentration Pb2+ wastewater was expanded.
3.3.4. XPS spectroscopy analysis of EP-Pb2+ XPS analysis was conducted to understand the interactions between the Pb2+ and adsorbent based on the binding energy of the species involved. Fig. 7A represents typical wide scan XPS spectra of original pectin, EP48, and EP48 adsorbed with Pb2+. It can be seen that peaks at about 400 eV belonging to N1s appeared after modification, indicating the ethylenediamine was successfully modified on pectin. Comparing the spectra of pectin before (Fig. 7C) and after (Fig. 7D) modification in the spectra of C1s, the peaks at binding energy of 286.00 eV and 288.35 eV can be assigned to the CeNH2 and NeC]O group, respectively, which further confirmed the success of modification. In addition, it was found that the C1s spectrum of the EP48 did not change significantly after Pb2+ adsorption (Fig. 7E) as compared to that before adsorption (Fig. 7D), indicating that these C-containing functional groups might not have a remarkable effect on Pb2+ adsorption. The peaks at about 139 eV in the wide scan XPS spectra (Fig. 7A) corresponding to Pb4f was identified, which confirmed the distribution of the Pb2+ on the surface of the adsorbent. The latter high-resolution XPS spectra of Pb4f analysis showed that the strong peaks appeared at
Acknowledgements The authors would like to thank the Center of Analysis and Testing of Nanchang University and State Key Laboratory of Food Science and Technology for their expert technical assistance. This study was supported financially by the National Natural Science Foundation of China (Nr 31660488, 31571875) and Postgraduate Innovative Funds of Nanchang University (CX2016212). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.carbpol.2020.115911. References Abdolali, A., Ngo, H. H., Guo, W., Zhou, J. L., Du, B., Wei, Q., et al. (2015).
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