Accepted Manuscript Novel composites based on geopolymer for removal of Pb(II) Chunjie Yan, Liang Guo, Daming Ren, Ping Duan PII: DOI: Reference:
S0167-577X(18)32073-1 https://doi.org/10.1016/j.matlet.2018.12.105 MLBLUE 25507
To appear in:
Received Date: Revised Date: Accepted Date:
2 November 2018 29 November 2018 20 December 2018
Please cite this article as: C. Yan, L. Guo, D. Ren, P. Duan, Novel composites based on geopolymer for removal of Pb(II), Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet.2018.12.105
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Novel composites based on geopolymer for removal of Pb(II) Chunjie Yan, Liang Guo#, Daming Ren, Ping Duan* Faculty of Materials Science and Chemistry, Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, China Abstract: In this work, novel geopolymer-alginate-chitosan composites were synthesized from metakaolin geopolymer and mixtures of sodium alginate solution and chitosan for direct removal of Pb from wastewater. The results show that the equilibrium adsorption capacity of the composites for Pb removal was as high as 120.45-142.67 mg/g at pH 5 and 25 °C, and the adsorption kinetics can be described by pseudo-second-order kinetic model and Freundlich isotherm model. The composites preserved a crystal structure of hydrocerussite (Pb-rich phase) after adsorption. Pb2+ was attracted intensely by the functionalities from alginate-chitosan and then immobilized crystallization between OH- from geopolymer and dissolved CO2 from the air. Keywords: amorphous materials; composite materials; geopolymer; Pb removal; crystallization 1. Introduction Heavy metals pose severe negative impacts on the environment and human beings health . The most widely applied adsorbents for removal of heavy metals include activated carbon, carbon nanotubes, graphene oxide and clay minerals [2-5]. Using geopolymer to develop low-cost and efficient adsorbent was proposed only very recently [6-9]. Concerning the synthesizing of geopolymer, an interesting phenomenon attracted great attention: geopolymer was subjected to efflorescence after long time curing under humid environment. The excessive Na+ and OH- were transported to the surface via the pore network, and react with dissolved CO2 [10-11]. For geopolymer matrix, carbonation leads to a decrease of pH and therefore a less efficient activation of precursor such as metakaolin, which induces a reduction in mechanical strength and an increase in total porosity. It is one of the most harmful degradation processes and can
Corresponding author: [email protected]
; [email protected]
Tel: +86 15807165207; Fax: +86 27
Co-first author, this author contributes to this manuscript equally 1
drastically affect the long-term durability . Being inspired by the efflorescence phenomenon, a new theory of adsorbing and immobilizing heavy metals by introducing functional groups into geopolymer matrix is proposed, aiming at attracting heavy metal ions, and taking advantage of the character of the alkali for crystallization reaction. Alginate and chitosan are natural biomass that are effective for binding metallic ions. However, the use of sodium alginate and chitosan in geopolymer is limited. Only Ge et al.  reported that geopolymer prepared using metakaolin and sodium alginate increased adsorption capacity for Cu2+ ions. Li et al.  used chitosan in geopolymer but for the purpose of enhancing mechanical properties. This study intends to develop a new geopolymer based adsorbent by incorporating sodium alginate (SA) and chitosan (CS) to combine their special characteristics, i.e. ideally the heavy metals are gathered strongly by the functional groups of SA and CS, and then crystallized by efflorescence reaction. 2. Experimental (1) Preparation of slurries: the geopolymer slurries were prepared by mechanically mixing alkali activator solution (a combination of sodium silicate and sodium hydroxide, silicate modulus equals to 1.5), metakaolin powder, H2O2, sodium dodecyl sulfate (K12) and ultra-purified deionized (DI) water. The mass fractions of H2O2, K12 and DI in the slurries were 0.5%, 1.5% and 12.5%. Alkali activator/metakaolin mass ratio was 0.5. The composition of metakaolin by wt% was: CaO 0.09%; MgO 0.21%; SiO2 53.32%; Fe2O3 2.01%; Al2O3 42.09%; Na2O 0.49%; K2O 0.64%; TiO2 0.33% and LOI 0.08%. (2) Injection: the slurries were continuously injected into a poly (ethylene glycol) (PEG-600) medium in 80 °C water bath by using syringes to prepare the geopolymeric spheres. (3) Solidification: the spheres in the medium were kept for 12 h to completely solidify. After that, the spheres were collected, washed to neutral by ethyl alcohol and water, filtrated, dried at 105 °C for 12 h and ground into powder. (4) Preparation of hybrid spheres: the sodium alginic acid solution (SAS) was prepared by dissolving 20 g sodium alginic acid into 980 g water in a 40 °C water bath with magnetic stirring, and sealed for 24 h before using. The chitosan solution (CSS) was prepared by dissolving 2 g of chitosan into 1000 mL of 2% acetic acid solution. 10 ml CSS and 10 2
ml SAS were mixed prior to usage. The 5 wt% CaCl2 solution was prepared by dissolving 50 g CaCl2 into 950 g water. Two types of spheres were fabricated according to the different cross-linking solutions (CLS): 1) 100 ml 5 wt% CaCl2 solution (CLS (-1)); 2) 0.3 ml 25 wt% glutaraldehyde solution and 0.1 M NaOH was added to 100 ml 5 wt% CaCl 2 solution to make CLS (-2). The pH of the mixed solution was also adjusted to precipitate cross-linking chitosan. 5 g porous geopolymer spheres (PGS) powder was blended with the 20 ml mixtures of sodium alginate solution and chitosan for 0.5 h. Afterwards, the mixture was dropwise injected into the prepared 100 ml cross-linking solution medium by using syringes at a uniform injection speed to control the size of spheres at 4 mm in diameter. The hybrid spheres were kept in CLS (-1) and CLS (-2) for 24 h at 30 °C water bath to solidification. The spheres were obtained after filtration, washed and freeze dried for 12 h, and denoted as GACS-1 and GACS-2, respectively. For adsorption test, the pH of solution was adjusted to make sure it lower than the precipitation condition of Pb 2+ (pH = 7.95, 25°C, initial concentration = 300 mg/L) . 3. Results and discussion The morphology of PGS, surface and cross-section of GACS-2 are presented in Fig. 1. PGS shows distinct amorphous structure inserted by rodlike particles of crystalline halloysite phase (Fig. 1a). The compact geopolymer matrix structure and granular Ca-alginate on the surface of GACS-2 are clearly illustrated in Fig. 1c. The surface of GACS-2 is rough, as shown in the optical microscope of Fig. 1b. Results in Fig. 1d exhibit cross section irregular rough fluffy structure of GACS-2 before adsorption. After the contacting with Pb2+ solution for a certain period of time, formation of hydrocerussite (Fig. 1e) takes place. GACS is consequently covered with a layer of white crystal rich in Pb (Fig. 1f). The porous properties of GACS samples were characterized using AutoPore IV 9500 mercury intrusion porosimeter (a pressurization from 0.004 MPa to 200 MPa and then a depressurization from 200 MPa to 0.15 MPa) and the results are presented in Fig. 2. In general, there is no significant difference between GACS-1 and GACS-2 according to BET surface area and median pore diameter. The porosity of GACS-1 and GACS-2 is 49.48% and 56.40%, respectively, which is beneficial for the adsorption of Pb 2+. The maximal pore size of GACS-1 and GACS-2 is about 23.7 μm and 34 3
μm, respectively, which indicates the easy access of Pb2+ onto the pore surfaces. In order to understand the adsorption mechanism of Pb2+ on GACS-1 and GACS-2, pseudo-first-order and pseudo-second-order equations are adopted to fit the experimental data. The results of kinetic parameters (k1, k2, qe,calc,1, and qe,calc,2) for Pb2+ adsorption and the corresponding correlation coefficients (R2) are listed in Table 1. The nonlinear simulation plots of adsorption data are provided in Figs. 3a-b. For GACS-1 and GACS-2, pseudo-second-order model suits better than pseudo-first-order model. The adsorption capacities predicted by pseudo-second-order model are in accordance with the experimentally obtained values. The linear plots of Freundlich isotherm and Langmuir isotherm model are displayed in Figs. 3c-d. The fitting parameters and the corresponding correlation coefficients (R2) are also summarized in Table 1. It is noticeable that Freundlich isotherm model describes Pb2+ sorption onto GACS-1 and GACS-2 more exactly than Langmuir isotherm model, indicating that the surface of GACS-1 and GACS-2 can be regarded as heterogeneous [16-17]. It is not only determined by the specificity of ionic bonds, but also the superficial electric field distribution generated by the charged functional groups, including carboxyl, hydroxyl and amino groups . The first adsorbed layer of Pb2+ ions cannot completely impede the penetration of Pb2+ ions into the inner layer, therefore, Pb2+ ions is attracted by the second layer although the intrinsic repulsion exists between Pb 2+ ions and the first layer . SEM images of GACS-2 after adsorption (Fig. 1) indicate a new crystal growth covered on the surface during the contacting with Pb2+. After a certain period of time, a transformation of hydrocerussite takes place. This phenomenon can be elucidated that the functional groups of sodium alginate and chitosan dispersed in GACS capture metallic cations through adsorption via electrophilic-nucleophilic interactions and ion exchange, and excess OH- of geopolymer leaches out to the surface through the pore network, promoting an irreversible crystallization process. As a consequence, GACS is covered with a layer of white crystal. Furthermore, benefitting from the porous structure, the adsorption process occurs at both the superficial and interior levels of GACS, owing to the penetration of ions towards the inner parts of matrix.
4. Conclusions This study develops geopolymer-alginate-chitosan composites with excellent Pb2+ adsorption and immobilization capacity. The kinetics are better described by pseudo-second-order kinetic model and Freundlich model. The hydrocerussite crystal grows on the composites after Pb(II) adsorption, resulting from the three-joint actions by OHfrom geopolymer, CO2 from the air and Pb(II) from aqueous solution. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51502272), the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan). References  Da'na E. Microporous and Mesoporous Materials 2017; 247: 145-157.  Li J, Xing X, Li J, Shi M, Lin A, Xu C, Zheng J, Li R. Environmental Pollution 2018; 234: 677-683.  Ranjan B, Pillai S, Permaul K, Singh S. Journal of Hazardous Materials 2019; 363: 73-80.  Peng W, Li H, Liu Y, Song S. Journal of Molecular Liquids 2017; 230: 496-504.  Uddin MK. Chemical Engineering Journal 2017; 308: 438-462.  Villaquirán-Caicedo MA, Mejía de Gutiérrez R. Materials Letters 2018; 230: 300-304.  Kara İ, Yilmazer D, Akar ST. Applied Clay Science 2017; 139: 54-63.  Duan P, Yan C, Zhou W, Ren D. Ceramics International 2016; 42: 13507-13518.  Fernández-Pereira C, Luna-Galiano Y, Pérez-Clemente M, Leiva C, Arroyo F, Villegas R, Vilches LF. Materials Letters 2018; 227: 184-186.  Zhang Z, Provis JL, Ma X, Reid A, Wang H. Cement and Concrete Composites, 2018, 92: 165-177.  Xue X, Liu Y, Dai J, Poon CS, Zhang W, Zhang P. Cement and Concrete Composites, 2018, 94: 43-52.  Pouhet R, Cyr M. Cement and Concrete Research, 2016, 88: 227-235.  Ge Y, Cui X, Liao C, Li Z. Chemical Engineering Journal, 2017, 311: 126-134.
 Li Z, Chen R, Zhang L. Journal of Materials Science, 2013, 48(22): 7986-7993.  Wang X, Chen Z, Yang S. Journal of Molecular Liquids 2015; 211: 957-964.
Fig. 1. (a) SEM images of PGS; (b) optical microscope photo of GACS-2; (c) SEM images of surface and (d) cross-section of GACS-2 before adsorption; (e) SEM images of GACS-2 after adsorption and (f) the corresponding elemental EDS mapping image of Pb.
Fig. 2. The pore parameters and pore size distribution of GACS-1 and GACS-2.
Fig. 3. Adsorption kinetic data fitting to pseudo-first order model (a) and pseudo-second-order model (b); the adsorption isotherm data fitting to Langmuir model (c) and Freundlich model (d). 7
Table 1 Kinetic parameters and adsorption isotherm constants for Pb2+ adsorption Langmuir isotherm parameters
Freundlich isotherm parameters
kinetic model k1
kinetic model -3
k2 (×10 ) -1
(g·mg h )
GACS-1 qe,exp (mg/g) 120.45 GACS-2 qe,exp (mg/g) 142.67
 Al-Harahsheh MS, Al Zboon K, Al-Makhadmeh L, Hararah M, Mahasneh M. Journal of Environmental Chemical Engineering 2015; 3: 1669-1677.  Gupta VK, Ali I. Journal of Colloid and Interface Science 2004; 271: 321-328.  Mousa NE, Simonescu CM, Pătescu RE, Onose C, Tardei C, Culiţă DC, Oprea O, Patroi D, Lavric V. Reactive and Functional Polymers 2016; 109: 137-150.
Declaration of Interest Statement The article is original, has been written by the stated authors who are all aware of its content and approve its submission, has not been published previously, it is not under consideration for publication elsewhere, no conflict of interest exists, and if accepted, the article will not be published elsewhere in the same form, in any language, without the written consent of the publisher.
Highlights ► A novel geopolymer-alginate-chitosan composites was proposed. ► Equilibrium adsorption capacity for Pb(II) as high as 120.45-142.67 mg/g. ► Adsorption described by pseudo-second-order kinetic and Freundlich isotherm model. ► Pb(II) gathered by the functional groups of SA and CS, and crystallized by efflorescence reaction.