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A biomimetic SiO2@chitosan composite as highly-efficient adsorbent for removing heavy metal ions in drinking water
Chemosphere 214 (2019) 738e742
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A biomimetic SiO2@chitosan composite as highly-efficient adsorbent for removing heavy metal ions in drinking water Jinyun Liu a, *, Yu Chen a, Tianli Han b, Mengying Cheng a, Wen Zhang a, Jiawei Long a, Xiangqian Fu c a Key Laboratory of Functional Molecular Solids, Ministry of Education, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241002, PR China b College of Chemistry and Material Engineering, Chaohu University, Chaohu, Anhui 238000, PR China c Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A unique biomimetic SiO2@chitosan composite adsorbent is prepared. Leaf-like SiO2@chitosan shows high adsorption performance toward As(V) and Hg(II). Agglomeration and loss of adsorbents are reduced by biomimetic structure.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 July 2018 Received in revised form 19 September 2018 Accepted 29 September 2018 Available online 2 October 2018
Highly efficient adsorbents for drinking water purification are demanded since the contaminants are generally in a low concentration which makes it difficult for conventional adsorbents. Herein, we present a novel biomimetic SiO2@chitosan composite as adsorbent with a high adsorption capability towards heavy metal ions including As(V) and Hg(II). The hollow leaf-like SiO2 scaffold within the adsorbent has a stable chemical property; while on the surface SiO2, the chitosan nanoparticle provide a large amount of active sites such as amino and hydroxyl groups for adsorbing heavy metal ions. The special SiO2 structure also prevents the agglomeration and loss of chitosan, which enables the efficient contact between the functional groups of chitosan and heavy metal ions. The SiO2@chitosan composite exhibits maximum adsorption capacities of 204.1 and 198.6 mg g1 towards Hg(II) and As(V), respectively. In addition, the removal efficiency reaches over 60% within 2 min. The adsorption performance enables the presented biomimetic adsorbent suitable for adsorbing low-concentration heavy metal ions, especially possessing a promising potential for drinking water purification. © 2018 Elsevier Ltd. All rights reserved.
Handling Editor: T.S. Galloway Keywords: Adsorbents Composite Adsorption Heavy metal ion Drinking water
1. Introduction
* Corresponding author. E-mail address: [email protected] (J. Liu). https://doi.org/10.1016/j.chemosphere.2018.09.172 0045-6535/© 2018 Elsevier Ltd. All rights reserved.
Contaminants in drinking water are mainly in the form of micropollution with a low concentration, which is different from the
J. Liu et al. / Chemosphere 214 (2019) 738e742
industrial pollution containing a high concentration of toxic substances (Kokkinos et al., 2018; El-Moselhy et al., 2017; Al-Qodah and Al-Shannag, 2017). Among many contaminants, inorganic micro-pollutants mainly include heavy metal ions such as Pb(II), As(III), Hg(II), Cu(II), and Cr(V) which are highly harmful to human body. The sources of heavy metal ions in drinking water are extensive including industrial production and dissolution of heavy metals in natural materials (Sener et al., 2017). The purification process is commonly complicated, high-cost, and time-consuming. Therefore, how to efficiently remove the low-concentration heavy metal ions from drinking water resource remains a great challenge. Among many purification approaches, adsorption strategy using adsorbents is currently one of the most attractive one (Onur et al., 2018; Ko et al., 2018). However, because of the low concentration of heavy metal ions in the drinking water, it is difficult to achieve deep adsorption. Since that, emerging adsorbents with a highly-efficient adsorption performance are desired. Chitosan (CS), which is obtained from deacetylation of chitin, is widely existed in shells of crustacean shrimps and crabs, carapace of insects, cell walls of fungals and plants. CS has a good biocompatibility and biodegradability, and is broadly used in pharmaceutical, chemical, papermaking and environmental protection industries (Wu et al., 2018; Mittal et al., 2016). The solubility of CS is related to the degree of deacetylation, relative molecular mass, and viscosity. It contains a large number of functional groups including eNH2 and eOH, which enables a good adsorption capacity towards heavy metal ions. Researchers have reported many methods to modify CS such as introducing a cross-linking reaction to stabilize chitosan (Zhang et al., 2018; Taghizadeh and Hassanpour, 2017; Ngwabebhoh et al., 2016; El-Salam et al., 2017). In addition to the complex operation, the quantity of functional groups decreases, which reduces the adsorption capacity of CS. Here, we present a novel CS-based nanocomposite adsorbent consisting of a biomimetic SiO2 micro-/nanostructure and CS nanoparticles coating on the surface. As illustrated in Fig. 1, at first, a CdS micro/nanostructure with a biomimetic leaf-like morphology was synthesized. Then, a layer of SiO2 was coated on the surface. After the CdS core was removed by acid, a hollow SiO2 shell with a leaf-shaped structure was constructed. At last, on the surface of SiO2, a CS layer was coated through a dissolution-deposition mechanism, forming a biomimetic SiO2@CS nanocomposite. In this composite, the leaf-like structure enables the tip of each leaf contacts with each other in solution. Since that, a frame structure would be formed, resulting in a remained apace between each SiO2@CS structure, instead of severely overlapping by tight agglomeration. It enables the sufficient exposing of adsorption sites on the composite for capturing heavy metal ions in solution. The prepared SiO2@CS composite exhibits a high adsorption performance towards As(V) and Hg(II). The maximum adsorption capacities towards As(V) and Hg(II) are 198.6 and 204.1 mg g1, respectively, which are much exceeding many adsorbents such as iron-chitosan composite (Gupta et al., 2009), chitosan flakes (Gupta et al., 2012), cross-linked magnetic chitosan (Abou El-Reash et al., 2011), ethylenediamine-modified activated carbon (Li et al., 2009), etc.
Fig. 1. Illustration for the preparation of the SiO2@CS composite with a biomimetic leaf-like structure.
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2. Experimental 2.1. Synthesis of CdS micro-/nanostructure The biomimetic CdS micro-/nanostructure was prepared by using dimethyl sulfoxide (DMSO) as the growth template via a hydrothermal method. All the chemicals were purchased from purchased from Baierdi Chemical Technology Co., Ltd., and used without further purification. First, 1 mmol of CdCl2$5H2O and 1 mmol of thiourea (CH4N2O4S) were dissolved in 35 mL of deionized water and stirred at room temperature to form a homogeneous solution. Then, 0.15 mL of DMSO was added to the solution, stirring was continued for 10 min until the solution was homogeneous. Finally, the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and sealed, then maintained at 170 C for 8 h. After that, the autoclave was naturally cooled to room temperature. The orange precipitate at the bottom was collected, washed for several times with ethanol and Millipore water, and dried at 60 C for 6 h. 2.2. Preparation of hollow leaf-like SiO2 To achieve a hollow SiO2 structure as the carrier of CS, 0.04 g of CdS, 40 mL of ethanol, and 8 mL of deionized water were mixed, then 2 mL of aqueous ammonia solution was added and stirred for 30 min. Subsequently, 1.6 mL of ethyl silicate solution was slowly added, and kept stirring for 2 h. The sample was collected and centrifuged. The reddish-brown precipitate was washed alternately with water and ethanol for several times, and placed in a vacuum oven at 60 C for 6 h. Finally, the dried sample was dispersed in a diluted hydrochloric acid solution (0.2 mol L1) in a fume hood, and stirred for 15 min. The sample was centrifuged and washed with water and ethanol for several times. The product was dried in an oven at 60 C for 6 h. 2.3. Preparation of SiO2@CS nanocomposite The CS particles were loaded on the obtained SiO2 on the basis of a dissolution-deposition mechanism since the CS dissolves in acid solution and deposits in natural and alkaline surroundings. At first, 0.05 g of CS was added into 40 mL of acetic acid solution at a pH value of 3. After the CS was fully dissolved by sonication for 15 min, 0.01 g of the hollow SiO2 was added and continuously stirred for 2 h. A certain amount of Na2CO3 was added to adjust the pH value of the solution to ~8. At last, the sample was centrifuged and washed for several times with water, and dried in a vacuum oven at 60 C for 4 h. 2.4. Characterizations The morphology, structure and composition of the samples were characterized by using scanning electron microscopy (FEI Sirione200 fieldeemission scanning microscope microscope, FESEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM, JEM-2010 Microscope equipped with Oxford INCA EDX system, accelerating voltage 200 kV), photoelectron spectroscopy (XPS, ESCALab MK II, Mg Ka X-rays as excitation light source) and Philips X-ray scattering meter with Cu Ka1.5418 Å as Xray source). The XRD peaks of this material were compared to standard JCPDS (Joint Committee on Powder Diffraction Standards) cards. Fourier transform infrared spectroscopy (FTIR) was determined by a JASCO 410 spectrometer and by KBr pelleting. The concentration of the heavy metal ion was measured on an inductively coupled plasma (ICP) atomic emission spectrometer (JarrellAsh model ICAP 9000).
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2.5. Adsorption measurements Prior to adsorption, As(V) and Hg(II) aqueous solutions with a series of concentrations using sodium arsenate and mercury nitrate as ion sources were prepared, respectively. It should be noted that the pH value of the Hg(II) solution was adjusted to 6e7 in order to avoid the precipitation of Hg(II). At this pH value, the CS is stable with tiny dissolution. All adsorption experiments were performed at room temperature (298 ± 1 K). The amount of ion adsorbed (qe, unit: mg g1) can be calculated by the following formula:
qe ¼
ðC0 Ce Þ V m
(5-1)
where, qe represents the amount (mg g1) of As(V) and Hg(II) adsorbed on the nanocomposite at equilibrium, while Co and Ce represent the concentration of the target ions (mg L1) at the beginning and equilibrium, respectively. V represents the total volume (L) of the solution during the adsorption; while m represents the mass (mg) of the adsorbent. For the investigation of the kinetic adsorption property, 20 mg of the prepared SiO2@CS composite was dispersed into 60 mL of target ion solution at an initial concentration of 2 ppm. Then the solution was placed on a shaker and oscillated. Sample solutions were collected at regular intervals, rapidly centrifuged at a high speed, labeled for ICP measurements. For the adsorption isotherm measurement, 20 mg of SiO2@CS was dispersed into 30 mL of As(V) and Hg(II) solutions with different concentrations, respectively. The solution was shacked in a shaker for 6 h. The residual concentration of As(V) and Hg(II) was detected by ICP to calculate the maximum adsorption capacity. 3. Results and discussion 3.1. Structure and composition of the composites As seen in Fig. 2, leaf-like CdS structure was synthesized through the hydrothermal route. Each leaf has a length of about 3 mm and a thickness between 50 and 80 nm. A uniform layer of SiO2 was coated on the surface of the leaf-like CdS, as displayed in Fig. 3. The thickness of the SiO2 layer is about 50 nm. Then, the CdS@SiO2 was dispersed in an acid solution. Due to the reaction of between acid and CdS, a turbid yellow liquid was obtained after the reaction, indicating the dissolution of CdS, which is confirmed in Fig. 4, showing a hollow SiO2 with a leaf shape. As seen, the hollow SiO2 maintains a biomimetic leaf-like morphology. The shell thickness is about 50 nm. Furthermore, in order to verify the complete removal of CdS after dissolution with hydrochloric acid, XPS analysis was performed. As displayed in Fig. 5, there is no Cd signal, indicating the complete removal of CdS. Fig. 6 displays the SEM and TEM images of the samples after loading CS on SiO2. In the FESEM image (Fig. 6a and b), it can be
Fig. 2. (a) Low- and (b) high-magnification FESEM images of the leaf-like CdS.
Fig. 3. (aec) FESEM and (d), (e) TEM images of the CdS@SiO2.
Fig. 4. Hollow SiO2 after removing CdS: (a), (b) FESEM, and (c) TEM images.
seen that there are a large number of particles on the surface of leaf SiO2 which is initially transparent and smooth (Fig. 4), indicating the loading of CS. The diameter of the particles is between 20 and 60 nm. In the TEM image (Fig. 6c and d), the transparency of the leaf is significantly reduced, and the edge is obviously rough, demonstrating the coating of CS on the surface of. Furthermore, the infrared (IR) spectroscopy was used to analyze the composition of the composite. Fig. 7 shows that the IR spectrum of the SiO2@CS exhibits a broad absorption peak at 1379, 1416, 1580 cm1, and an absorption peak at around 2868e2911 cm1, which are the characteristic peaks of CS (Elwakeel et al., 2017; Jiang et al., 2018), further confirming the loading of CS on SiO2@CS composite. 3.2. Adsorption properties As the adsorption kinetics is one of the most important factors of the adsorbent, the time-dependent adsorption performance was
Fig. 5. XPS spectrum of the leaf-like CdS and the hollow SiO2.
J. Liu et al. / Chemosphere 214 (2019) 738e742
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time was 30 min, the removal efficiency becomes stable. The rapid increase of the adsorption efficiency at the initial stage is ascribed to the fact that the surface of SiO2@CS composite contains a large amount of functional groups (i.e., amine groups and hydroxyl groups) which interact with heavy metal ions. When the adsorption time is extended, the heavy metal ions adsorbed on the adsorbent surface would hinder the contact between the heavy metal ions and SiO2@CS composite. In this condition, the adsorption amount of As(V) and Hg(II) gradually reaches an equilibrium. In order to further explore the adsorption kinetics, a pseudosecond-order model was used to fit the kinetics. The secondorder kinetics equation is expressed as follows:
t 1 t ¼ þ qt k2 q2e qe
Fig. 6. (a), (b) FESEM and (c), (d) TEM images of the leaf-like SiO2@CS composite.
where qe and qt represent the adsorption capacity of the adsorbent at the equilibrium time and at the time t, respectively; k2 represents the rate constant for the second order reaction, and the unit is g mg1 min1. The value of k2 can be calculated by the slope and intercept of the line obtained by t/qt. As shown in Fig. 8b, the pseudo-second-order kinetic models are well fitted with the adsorption results in terms of the equations below: As(V): y ¼ 0.2453x þ 0.5829 r2 ¼ 0.9964; Hg(II): y ¼ 0.1981x þ 0.5851 r2 ¼ 0.9977 The adsorption isotherms of the SiO2@CS composites towards As(V) and Hg(II) are displayed in Fig. 9a. The adsorption capacity of SiO2@CS composites for Hg(II) is higher than that of As(V). Adsorption results were fitted using Langmuir and Freundlich adsorption models, respectively. The Langmuir model is used to describe the homogeneous adsorption of monolayers. The adsorption sites are uniformly distributed, and can be expressed by the following equation:
Ce 1 Ce ¼ þ qe qm KL qm where Ce represents the equilibrium concentration of heavy metal
Fig. 7. IR spectra of the hollow SiO2 and the SiO2@CS composite.
investigated. In Fig. 8a, the adsorption capacity rapidly increases at the initial stage of adsorption (~2 min). When the adsorption time is continuously extended to 20 min, the adsorption capacity increases slowly and tends to be equilibrium. When the adsorption
Fig. 8. (a) Adsorption kinetic curves of SiO2@CS towards As(V) and Hg(II), and (b) the corresponding pseudo-second-order kinetics models.
Fig. 9. (a) Adsorption isotherms of SiO2@CS composites towards As(V) and Hg(II); (b) Fitted Freundlich adsorption model towards As(V); (c) Fitted Langmuir adsorption model towards Hg(II).
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Table 1 Comparison of maximum adsorption capacity of some adsorbents. Heavy metal ions
Adsorbents
Maximum adsorption capacity (mg g1)
Ref.
As(V)
Magnetic chitosan/biochar composite Iron-chitosan composite Biogenic Fe(III) (oxy) hydroxides Chitosan flakes Chitosan thiomer Cross-linked magnetic chitosan CeeTi oxide Biomimetic SiO2@CS composite Mesoporous chitosan Poly-chitosan composite membranes Ethylenediamine-modified activated carbon Dithiocarbanate-magnetite particles Cross-linked chitosan-phenylthiourea resin Ethylhexadecyldimethyl ammonium bromide impregnated chitosan Biomimetic SiO2@CS composite
11.96 22.47 134.3 20 19 62.42 40.2 198.6 164 50 60.9 140 135 43.43 204.1
Liu et al., 2017 Gupta et al., 2009 Kleinert et al., 2011 Gupta et al., 2012 Singh et al., 2016 Abou El-Reash et al., 2011 Deng et al., 2010 Our study Fu et al., 2018 Genc et al., 2003 Li et al., 2009 Figueira et al., 2011 Monier and Abdel-Latif, 2012 Shekhawat et al., 2017 Our study
Hg(II)
ions in the remained solution with the units of mg L1, qe represents the equilibrium adsorption capacity with the unit of mg g1, qm represents the maximum adsorption capacity with the unit of mg g1, and KL represents the Langmuir adsorption constant related to the adsorption energy and the unit is L mg1. A straight line can be plotted from Ce by Ce/qe. The values of qm and KL can be calculated by the slope and intercept of the straight line. The Freundlich adsorption model is used to describe the heterogeneous adsorption process. The equation is expressed as follows:
In qe ¼
1 In Ce þ In KF n
where n and KF are Freundlich constants, respectively, which represent the adsorption strength and the adsorption capacity. From lnqe to lnCe, a straight line is obtained. From the slope and intercept of the line, the values of n and KF are calculated. As seen in Fig. 9, the adsorption model of SiO2@CS for As(V) is a Freundlich adsorption model; while the adsorption for Hg(II) is a Langmuir adsorption model. The maximum adsorption capacity of the SiO2@CS composite towards Hg(II) and As(V) are 204.1 and 198.6 mg g1, respectively. The comparison of adsorption performance with some other adsorbents is shown in Table 1, which shows that the presented leaf-like SiO2@CS has a competitive adsorption performance for removing Hg(II) and As(V). 4. Conclusions A novel biomimetic SiO2@CS composite is presented, which exhibits a high adsorption performance towards heavy metal ions As(V) and Hg(II) in solution. The SiO2 has good mechanical strength, stable chemical properties, and can effectively prevent the agglomeration of adsorbent; while the CS nanoparticles coated on SiO2 provide many functional groups including amino and hydroxyl groups for adsorbing heavy metal ions. The leaf-like SiO2@CS composite nanomaterials adsorbed quickly and efficiently in the water with low-concentration heavy metal ions. The SiO2@chitosan composite exhibits maximum adsorption capacities of 204.1 and 198.6 mg g1 towards Hg(II) and As(V), respectively. The removal efficiency reaches higher than 60% within 2 min of adsorption. The good adsorption properties show that the presented composite has a potential for removing heavy metal ions in drinking water, especially applied for rapidly adsorbing heavy metal ions with a low concentration. In addition, it is expected that through preparing biomimetic structure, along with some target-directed surface
modifications, some high-performance adsorbents will be developed for removing both inorganic and organic pollutants in water. Acknowledgements This work was supported by the National Natural Science Foundation of China (51672176, 21471005, 61203212, and 661573334), the Major Project of the Department of Education of Anhui Province (KJ2018ZD034), and the Intergovernmental International Scientific and Technological Cooperation of Shanghai (17520710200). References Abou El-Reash, Y.G., Otto, M., Kenawy, I.M., Ouf, A.M., 2011. Int. J. Biol. Macromol. 49, 513e522. Al-Qodah, Z., Al-Shannag, M., 2017. Separ. Sci. Technol. 52, 2649e2676. Deng, S., Li, Z., Huang, J., Yu, G., 2010. J. Hazard Mater. 179, 1014e1021. El-Moselhy, M.M., Ates, A., Celebi, A., 2017. J. Colloid Interface Sci. 488, 335e347. El-Salam, H.M., Emad, H.M., Ibrahim, M.S., 2017. J. Polym. Environ. 25, 973e982. Elwakeel, K.Z., El-Bindary, A.A., Kouta, E.Y., 2017. J. Environ. Chem. Eng 5, 3698e3710. Figueira, P., Lopes, C.B., Daniel-da-Silva, A.L., Pereira, E., Duarte, A.C., Trindade, T., 2011. Water Res. 45, 5773e5784. Fu, Y., Huang, Y., Hu, J.S., 2018. Royal Soc. Open Sci. 5, 171921. Genc, O., Soysal, L., Bayramoglu, G., Arica, M.Y., Bektas, S., 2003. J. Hazard Mater. 97, 111e125. Gupta, A., Chauhan, V.S., Sankararamakrishnan, N., 2009. Water Res. 43, 3862e3870. Gupta, A., Yunus, M., Sankararamakrishnan, N., 2012. J. Chem. Technol. Biotechnol. 87, 546e552. Jiang, T.S., Dong, M.F., Yan, L., Fang, M.L., Liu, R.F., 2018. J. Nanosci. Nanotechnol. 18, 4692e4699. Kleinert, S., Muehe, E.M., Posth, N.R., Dippon, U., Daus, B., Kappler, A., 2011. Environ. Sci. Technol. 45, 7533e7541. Ko, D., Mines, P.D., Jakobsen, M.H., Yavuz, C.T., Hansen, H.C.B., Andersen, H.R., 2018. Chem. Eng. J. 348, 685e692. Kokkinos, E., Soukakos, K., Kostoglou, M., Mitrakas, M., 2018. Environ. Sci. Pollut. Res. 25, 12263e12273. Li, Z., Chang, X.J., Zou, X.J., Zhu, X.B., Nie, R., Hu, Z., Li, R.J., 2009. Anal. Chim. Acta 632, 272e277. Liu, S.B., Huang, B.Y., Chai, L.Y., Liu, Y.G., Zeng, G.M., Wang, X., Zeng, W., Shang, M.R., Deng, J.Q., Zhou, Z., 2017. RSC Adv. 7, 10891e10900. Mittal, A., Ahmad, R., Hasan, I., 2016. Desalination Water Treat 57, 19820e19833. Monier, M., Abdel-Latif, D.A., 2012. J. Hazard Mater. 209, 240e249. Ngwabebhoh, F., Erdem, A., Yildiz, U., 2016. J. Appl. Polym. Sci. 133, 43664. Onur, A., Ng, A., Garnier, G., Batchelor, W., 2018. Separ. Purif. Technol. 203, 209e216. Sener, S., Sener, E., Davraz, A., 2017. J. Water Health 15, 112e132. Shekhawat, A., Kahu, S., Saravanan, D., Jugade, R., 2017. Int. J. Biol. Macromol. 104, 1556e1568. Singh, P., Chauhan, K., Priya, V., Singhal, R.K., 2016. RSC Adv. 6, 64946e64961. Taghizadeh, M., Hassanpour, S., 2017. Polymer 132, 1e11. Wu, D., Hu, L.H., Wang, Y.G., Wei, Q., Yan, L.G., Yan, T., Li, Y., Du, B., 2018. J. Colloid Interface Sci. 523, 56e64. Zhang, C.Z., Yuan, Y., Guo, Z.Y., 2018. Separ. Sci. Technol. 53, 1666e1677.
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A biomimetic SiO2@chitosan composite as highly-efficient adsorbent for removing heavy metal ions in drinking water Jinyun Liu a, *, Yu Chen a, Tianli Han b, Mengying Cheng a, Wen Zhang a, Jiawei Long a, Xiangqian Fu c a Key Laboratory of Functional Molecular Solids, Ministry of Education, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241002, PR China b College of Chemistry and Material Engineering, Chaohu University, Chaohu, Anhui 238000, PR China c Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A unique biomimetic SiO2@chitosan composite adsorbent is prepared. Leaf-like SiO2@chitosan shows high adsorption performance toward As(V) and Hg(II). Agglomeration and loss of adsorbents are reduced by biomimetic structure.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 July 2018 Received in revised form 19 September 2018 Accepted 29 September 2018 Available online 2 October 2018
Highly efficient adsorbents for drinking water purification are demanded since the contaminants are generally in a low concentration which makes it difficult for conventional adsorbents. Herein, we present a novel biomimetic SiO2@chitosan composite as adsorbent with a high adsorption capability towards heavy metal ions including As(V) and Hg(II). The hollow leaf-like SiO2 scaffold within the adsorbent has a stable chemical property; while on the surface SiO2, the chitosan nanoparticle provide a large amount of active sites such as amino and hydroxyl groups for adsorbing heavy metal ions. The special SiO2 structure also prevents the agglomeration and loss of chitosan, which enables the efficient contact between the functional groups of chitosan and heavy metal ions. The SiO2@chitosan composite exhibits maximum adsorption capacities of 204.1 and 198.6 mg g1 towards Hg(II) and As(V), respectively. In addition, the removal efficiency reaches over 60% within 2 min. The adsorption performance enables the presented biomimetic adsorbent suitable for adsorbing low-concentration heavy metal ions, especially possessing a promising potential for drinking water purification. © 2018 Elsevier Ltd. All rights reserved.
Handling Editor: T.S. Galloway Keywords: Adsorbents Composite Adsorption Heavy metal ion Drinking water
1. Introduction
* Corresponding author. E-mail address: [email protected] (J. Liu). https://doi.org/10.1016/j.chemosphere.2018.09.172 0045-6535/© 2018 Elsevier Ltd. All rights reserved.
Contaminants in drinking water are mainly in the form of micropollution with a low concentration, which is different from the
J. Liu et al. / Chemosphere 214 (2019) 738e742
industrial pollution containing a high concentration of toxic substances (Kokkinos et al., 2018; El-Moselhy et al., 2017; Al-Qodah and Al-Shannag, 2017). Among many contaminants, inorganic micro-pollutants mainly include heavy metal ions such as Pb(II), As(III), Hg(II), Cu(II), and Cr(V) which are highly harmful to human body. The sources of heavy metal ions in drinking water are extensive including industrial production and dissolution of heavy metals in natural materials (Sener et al., 2017). The purification process is commonly complicated, high-cost, and time-consuming. Therefore, how to efficiently remove the low-concentration heavy metal ions from drinking water resource remains a great challenge. Among many purification approaches, adsorption strategy using adsorbents is currently one of the most attractive one (Onur et al., 2018; Ko et al., 2018). However, because of the low concentration of heavy metal ions in the drinking water, it is difficult to achieve deep adsorption. Since that, emerging adsorbents with a highly-efficient adsorption performance are desired. Chitosan (CS), which is obtained from deacetylation of chitin, is widely existed in shells of crustacean shrimps and crabs, carapace of insects, cell walls of fungals and plants. CS has a good biocompatibility and biodegradability, and is broadly used in pharmaceutical, chemical, papermaking and environmental protection industries (Wu et al., 2018; Mittal et al., 2016). The solubility of CS is related to the degree of deacetylation, relative molecular mass, and viscosity. It contains a large number of functional groups including eNH2 and eOH, which enables a good adsorption capacity towards heavy metal ions. Researchers have reported many methods to modify CS such as introducing a cross-linking reaction to stabilize chitosan (Zhang et al., 2018; Taghizadeh and Hassanpour, 2017; Ngwabebhoh et al., 2016; El-Salam et al., 2017). In addition to the complex operation, the quantity of functional groups decreases, which reduces the adsorption capacity of CS. Here, we present a novel CS-based nanocomposite adsorbent consisting of a biomimetic SiO2 micro-/nanostructure and CS nanoparticles coating on the surface. As illustrated in Fig. 1, at first, a CdS micro/nanostructure with a biomimetic leaf-like morphology was synthesized. Then, a layer of SiO2 was coated on the surface. After the CdS core was removed by acid, a hollow SiO2 shell with a leaf-shaped structure was constructed. At last, on the surface of SiO2, a CS layer was coated through a dissolution-deposition mechanism, forming a biomimetic SiO2@CS nanocomposite. In this composite, the leaf-like structure enables the tip of each leaf contacts with each other in solution. Since that, a frame structure would be formed, resulting in a remained apace between each SiO2@CS structure, instead of severely overlapping by tight agglomeration. It enables the sufficient exposing of adsorption sites on the composite for capturing heavy metal ions in solution. The prepared SiO2@CS composite exhibits a high adsorption performance towards As(V) and Hg(II). The maximum adsorption capacities towards As(V) and Hg(II) are 198.6 and 204.1 mg g1, respectively, which are much exceeding many adsorbents such as iron-chitosan composite (Gupta et al., 2009), chitosan flakes (Gupta et al., 2012), cross-linked magnetic chitosan (Abou El-Reash et al., 2011), ethylenediamine-modified activated carbon (Li et al., 2009), etc.
Fig. 1. Illustration for the preparation of the SiO2@CS composite with a biomimetic leaf-like structure.
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2. Experimental 2.1. Synthesis of CdS micro-/nanostructure The biomimetic CdS micro-/nanostructure was prepared by using dimethyl sulfoxide (DMSO) as the growth template via a hydrothermal method. All the chemicals were purchased from purchased from Baierdi Chemical Technology Co., Ltd., and used without further purification. First, 1 mmol of CdCl2$5H2O and 1 mmol of thiourea (CH4N2O4S) were dissolved in 35 mL of deionized water and stirred at room temperature to form a homogeneous solution. Then, 0.15 mL of DMSO was added to the solution, stirring was continued for 10 min until the solution was homogeneous. Finally, the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and sealed, then maintained at 170 C for 8 h. After that, the autoclave was naturally cooled to room temperature. The orange precipitate at the bottom was collected, washed for several times with ethanol and Millipore water, and dried at 60 C for 6 h. 2.2. Preparation of hollow leaf-like SiO2 To achieve a hollow SiO2 structure as the carrier of CS, 0.04 g of CdS, 40 mL of ethanol, and 8 mL of deionized water were mixed, then 2 mL of aqueous ammonia solution was added and stirred for 30 min. Subsequently, 1.6 mL of ethyl silicate solution was slowly added, and kept stirring for 2 h. The sample was collected and centrifuged. The reddish-brown precipitate was washed alternately with water and ethanol for several times, and placed in a vacuum oven at 60 C for 6 h. Finally, the dried sample was dispersed in a diluted hydrochloric acid solution (0.2 mol L1) in a fume hood, and stirred for 15 min. The sample was centrifuged and washed with water and ethanol for several times. The product was dried in an oven at 60 C for 6 h. 2.3. Preparation of SiO2@CS nanocomposite The CS particles were loaded on the obtained SiO2 on the basis of a dissolution-deposition mechanism since the CS dissolves in acid solution and deposits in natural and alkaline surroundings. At first, 0.05 g of CS was added into 40 mL of acetic acid solution at a pH value of 3. After the CS was fully dissolved by sonication for 15 min, 0.01 g of the hollow SiO2 was added and continuously stirred for 2 h. A certain amount of Na2CO3 was added to adjust the pH value of the solution to ~8. At last, the sample was centrifuged and washed for several times with water, and dried in a vacuum oven at 60 C for 4 h. 2.4. Characterizations The morphology, structure and composition of the samples were characterized by using scanning electron microscopy (FEI Sirione200 fieldeemission scanning microscope microscope, FESEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM, JEM-2010 Microscope equipped with Oxford INCA EDX system, accelerating voltage 200 kV), photoelectron spectroscopy (XPS, ESCALab MK II, Mg Ka X-rays as excitation light source) and Philips X-ray scattering meter with Cu Ka1.5418 Å as Xray source). The XRD peaks of this material were compared to standard JCPDS (Joint Committee on Powder Diffraction Standards) cards. Fourier transform infrared spectroscopy (FTIR) was determined by a JASCO 410 spectrometer and by KBr pelleting. The concentration of the heavy metal ion was measured on an inductively coupled plasma (ICP) atomic emission spectrometer (JarrellAsh model ICAP 9000).
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2.5. Adsorption measurements Prior to adsorption, As(V) and Hg(II) aqueous solutions with a series of concentrations using sodium arsenate and mercury nitrate as ion sources were prepared, respectively. It should be noted that the pH value of the Hg(II) solution was adjusted to 6e7 in order to avoid the precipitation of Hg(II). At this pH value, the CS is stable with tiny dissolution. All adsorption experiments were performed at room temperature (298 ± 1 K). The amount of ion adsorbed (qe, unit: mg g1) can be calculated by the following formula:
qe ¼
ðC0 Ce Þ V m
(5-1)
where, qe represents the amount (mg g1) of As(V) and Hg(II) adsorbed on the nanocomposite at equilibrium, while Co and Ce represent the concentration of the target ions (mg L1) at the beginning and equilibrium, respectively. V represents the total volume (L) of the solution during the adsorption; while m represents the mass (mg) of the adsorbent. For the investigation of the kinetic adsorption property, 20 mg of the prepared SiO2@CS composite was dispersed into 60 mL of target ion solution at an initial concentration of 2 ppm. Then the solution was placed on a shaker and oscillated. Sample solutions were collected at regular intervals, rapidly centrifuged at a high speed, labeled for ICP measurements. For the adsorption isotherm measurement, 20 mg of SiO2@CS was dispersed into 30 mL of As(V) and Hg(II) solutions with different concentrations, respectively. The solution was shacked in a shaker for 6 h. The residual concentration of As(V) and Hg(II) was detected by ICP to calculate the maximum adsorption capacity. 3. Results and discussion 3.1. Structure and composition of the composites As seen in Fig. 2, leaf-like CdS structure was synthesized through the hydrothermal route. Each leaf has a length of about 3 mm and a thickness between 50 and 80 nm. A uniform layer of SiO2 was coated on the surface of the leaf-like CdS, as displayed in Fig. 3. The thickness of the SiO2 layer is about 50 nm. Then, the CdS@SiO2 was dispersed in an acid solution. Due to the reaction of between acid and CdS, a turbid yellow liquid was obtained after the reaction, indicating the dissolution of CdS, which is confirmed in Fig. 4, showing a hollow SiO2 with a leaf shape. As seen, the hollow SiO2 maintains a biomimetic leaf-like morphology. The shell thickness is about 50 nm. Furthermore, in order to verify the complete removal of CdS after dissolution with hydrochloric acid, XPS analysis was performed. As displayed in Fig. 5, there is no Cd signal, indicating the complete removal of CdS. Fig. 6 displays the SEM and TEM images of the samples after loading CS on SiO2. In the FESEM image (Fig. 6a and b), it can be
Fig. 2. (a) Low- and (b) high-magnification FESEM images of the leaf-like CdS.
Fig. 3. (aec) FESEM and (d), (e) TEM images of the CdS@SiO2.
Fig. 4. Hollow SiO2 after removing CdS: (a), (b) FESEM, and (c) TEM images.
seen that there are a large number of particles on the surface of leaf SiO2 which is initially transparent and smooth (Fig. 4), indicating the loading of CS. The diameter of the particles is between 20 and 60 nm. In the TEM image (Fig. 6c and d), the transparency of the leaf is significantly reduced, and the edge is obviously rough, demonstrating the coating of CS on the surface of. Furthermore, the infrared (IR) spectroscopy was used to analyze the composition of the composite. Fig. 7 shows that the IR spectrum of the SiO2@CS exhibits a broad absorption peak at 1379, 1416, 1580 cm1, and an absorption peak at around 2868e2911 cm1, which are the characteristic peaks of CS (Elwakeel et al., 2017; Jiang et al., 2018), further confirming the loading of CS on SiO2@CS composite. 3.2. Adsorption properties As the adsorption kinetics is one of the most important factors of the adsorbent, the time-dependent adsorption performance was
Fig. 5. XPS spectrum of the leaf-like CdS and the hollow SiO2.
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time was 30 min, the removal efficiency becomes stable. The rapid increase of the adsorption efficiency at the initial stage is ascribed to the fact that the surface of SiO2@CS composite contains a large amount of functional groups (i.e., amine groups and hydroxyl groups) which interact with heavy metal ions. When the adsorption time is extended, the heavy metal ions adsorbed on the adsorbent surface would hinder the contact between the heavy metal ions and SiO2@CS composite. In this condition, the adsorption amount of As(V) and Hg(II) gradually reaches an equilibrium. In order to further explore the adsorption kinetics, a pseudosecond-order model was used to fit the kinetics. The secondorder kinetics equation is expressed as follows:
t 1 t ¼ þ qt k2 q2e qe
Fig. 6. (a), (b) FESEM and (c), (d) TEM images of the leaf-like SiO2@CS composite.
where qe and qt represent the adsorption capacity of the adsorbent at the equilibrium time and at the time t, respectively; k2 represents the rate constant for the second order reaction, and the unit is g mg1 min1. The value of k2 can be calculated by the slope and intercept of the line obtained by t/qt. As shown in Fig. 8b, the pseudo-second-order kinetic models are well fitted with the adsorption results in terms of the equations below: As(V): y ¼ 0.2453x þ 0.5829 r2 ¼ 0.9964; Hg(II): y ¼ 0.1981x þ 0.5851 r2 ¼ 0.9977 The adsorption isotherms of the SiO2@CS composites towards As(V) and Hg(II) are displayed in Fig. 9a. The adsorption capacity of SiO2@CS composites for Hg(II) is higher than that of As(V). Adsorption results were fitted using Langmuir and Freundlich adsorption models, respectively. The Langmuir model is used to describe the homogeneous adsorption of monolayers. The adsorption sites are uniformly distributed, and can be expressed by the following equation:
Ce 1 Ce ¼ þ qe qm KL qm where Ce represents the equilibrium concentration of heavy metal
Fig. 7. IR spectra of the hollow SiO2 and the SiO2@CS composite.
investigated. In Fig. 8a, the adsorption capacity rapidly increases at the initial stage of adsorption (~2 min). When the adsorption time is continuously extended to 20 min, the adsorption capacity increases slowly and tends to be equilibrium. When the adsorption
Fig. 8. (a) Adsorption kinetic curves of SiO2@CS towards As(V) and Hg(II), and (b) the corresponding pseudo-second-order kinetics models.
Fig. 9. (a) Adsorption isotherms of SiO2@CS composites towards As(V) and Hg(II); (b) Fitted Freundlich adsorption model towards As(V); (c) Fitted Langmuir adsorption model towards Hg(II).
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Table 1 Comparison of maximum adsorption capacity of some adsorbents. Heavy metal ions
Adsorbents
Maximum adsorption capacity (mg g1)
Ref.
As(V)
Magnetic chitosan/biochar composite Iron-chitosan composite Biogenic Fe(III) (oxy) hydroxides Chitosan flakes Chitosan thiomer Cross-linked magnetic chitosan CeeTi oxide Biomimetic SiO2@CS composite Mesoporous chitosan Poly-chitosan composite membranes Ethylenediamine-modified activated carbon Dithiocarbanate-magnetite particles Cross-linked chitosan-phenylthiourea resin Ethylhexadecyldimethyl ammonium bromide impregnated chitosan Biomimetic SiO2@CS composite
11.96 22.47 134.3 20 19 62.42 40.2 198.6 164 50 60.9 140 135 43.43 204.1
Liu et al., 2017 Gupta et al., 2009 Kleinert et al., 2011 Gupta et al., 2012 Singh et al., 2016 Abou El-Reash et al., 2011 Deng et al., 2010 Our study Fu et al., 2018 Genc et al., 2003 Li et al., 2009 Figueira et al., 2011 Monier and Abdel-Latif, 2012 Shekhawat et al., 2017 Our study
Hg(II)
ions in the remained solution with the units of mg L1, qe represents the equilibrium adsorption capacity with the unit of mg g1, qm represents the maximum adsorption capacity with the unit of mg g1, and KL represents the Langmuir adsorption constant related to the adsorption energy and the unit is L mg1. A straight line can be plotted from Ce by Ce/qe. The values of qm and KL can be calculated by the slope and intercept of the straight line. The Freundlich adsorption model is used to describe the heterogeneous adsorption process. The equation is expressed as follows:
In qe ¼
1 In Ce þ In KF n
where n and KF are Freundlich constants, respectively, which represent the adsorption strength and the adsorption capacity. From lnqe to lnCe, a straight line is obtained. From the slope and intercept of the line, the values of n and KF are calculated. As seen in Fig. 9, the adsorption model of SiO2@CS for As(V) is a Freundlich adsorption model; while the adsorption for Hg(II) is a Langmuir adsorption model. The maximum adsorption capacity of the SiO2@CS composite towards Hg(II) and As(V) are 204.1 and 198.6 mg g1, respectively. The comparison of adsorption performance with some other adsorbents is shown in Table 1, which shows that the presented leaf-like SiO2@CS has a competitive adsorption performance for removing Hg(II) and As(V). 4. Conclusions A novel biomimetic SiO2@CS composite is presented, which exhibits a high adsorption performance towards heavy metal ions As(V) and Hg(II) in solution. The SiO2 has good mechanical strength, stable chemical properties, and can effectively prevent the agglomeration of adsorbent; while the CS nanoparticles coated on SiO2 provide many functional groups including amino and hydroxyl groups for adsorbing heavy metal ions. The leaf-like SiO2@CS composite nanomaterials adsorbed quickly and efficiently in the water with low-concentration heavy metal ions. The SiO2@chitosan composite exhibits maximum adsorption capacities of 204.1 and 198.6 mg g1 towards Hg(II) and As(V), respectively. The removal efficiency reaches higher than 60% within 2 min of adsorption. The good adsorption properties show that the presented composite has a potential for removing heavy metal ions in drinking water, especially applied for rapidly adsorbing heavy metal ions with a low concentration. In addition, it is expected that through preparing biomimetic structure, along with some target-directed surface
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