Functionalized SBA-15 mesoporous silica by melamine-based dendrimer amines for adsorptive characteristics of Pb(II), Cu(II) and Cd(II) heavy metal ions in batch and fixed bed column
Introduction
Toxic heavy metals such as Pb(II), Cu(II) and Cd(II) in liquid effluents are considered persistent, non-biodegradable, bio-accumulative, thus they seriously threaten natural ecosystems and the health of human populations [1], [2], [3], [4]. Many industries such as mining, textiles, painting, electroplating, refining and pesticides discharge their wastewater with various heavy metals content into aquatic ecosystems. Some general techniques for heavy metals removal are precipitation, ion exchange, reverse osmosis, nanofiltration and adsorption. Meanwhile, chemists and environmental engineers involved in wastewater treatment have already been persuaded to find more efficient ways of removing heavy metals from aqueous solution and to improve wastewater treatment systems [5], [6], [7], [8]. Among the various techniques, adsorption is very popular due to its simplicity, low cost [9], [10], [11] and potential for overcoming the environmental problems [11]. Adsorbents with a higher surface area are undoubtedly the more efficient ones and therefore have been the subject of many studies [12]. Ordered nanoporous silicas (NPSs) are materials that not only have a large surface area but also high physico-chemical stability, uniform pore size distribution, long-range homogeneity of texture and modifiable surface chemistry via functionalization [12], [13], thus having great potential as adsorbents [14]. Within this group, SBA-15 is typical with its larger uniform pore size (4.7–8.9 nm) providing ample room for grafting functional groups, and its thick silica walls (3.1–6.4 nm) exhibiting enhanced hydrothermal stability compared to analogous materials, – properties that can be a reliable support for designing adsorbents [15], [16]. It appears that the increased diffusion of aqueous metal cations onto the SBA-15 pores play major roles in physical and chemical interactions between sorbate and sorbent that consequently result in increased heavy metal removal. The open-end pores in SBA-15 and its hydrophilic surface are partly responsible for such diffusion but these characterizations are not efficient enough for heavy metal removal from aqueous solution. The surface of the silica can be improved by functionalization to make it an ideal organic/inorganic hybrid adsorbent. Due to the binding preferences of metal cations and also to their co-ordination chemistry [17], more constrained diffusion of the solution into the pore and also more chemical interaction between the silica surface and the metal cations is created. It is generally accepted in the literature that metal cations seek out O–, N– or S– containing organic ligands [5], [18], [19], [20]. Pb(II), Cu(II) and Cd(II) are the borderline metals with ambivalent properties [17] that show a good affinity with aminopropyl-containing ligands [10], [21]. Researchers have used ligands containing multi-amine groups in a chain [5], [21], [22] or ligands involving dendrimer-amines [20] to increase removal efficiency of these cations into the NPS pores. Melamine-based dendrimer amines (MDAs) are such ideal dendrimer ligands due to intensively binding amine sites and also to their enhanced hydrophilic silica surface compared to analogous adsorbents. MDAs were first reported in 2000 by Simanek and Zhang [23], but they have not been applied for heavy metal removal, so far. Fixed bed column is one of the most effective configurations for cyclic adsorption–desorption, allowing more efficient use of the adsorbent. The performance of fixed bed columns is usually described through the concept of the breakthrough curve that is obtained by plotting the effluent concentration versus time. Despite the importance of column adsorption, a few studies have been reported on heavy metal removal from aqueous solution with modified mesoporous silicas employing a fixed bed column reactor [24], [25].
In this study, SBA-15 mesoporous silica was synthesized by a hydrothermal method and the functionalization of SBA-15 with 3-aminopropyltriethoxysilane (NH2–SBA-15) and melamine-based dendrimer amines (MDA–SBA-15) was attempted. SBA-15, NH2–SBA-15 and MDA–SBA-15 were used to remove Pb(II), Cu(II) and Cd(II) metal ions from aqueous solution in batch and fixed bed column systems. The effects on the adsorption behavior of the removal process parameters such as adsorbent dose, solution pH, metal ions concentration, and temperature were studied. Equilibrium data were fitted into the Langmuir and Freundlich isotherms to determine the correlation between the isotherms. The kinetics of adsorption of metal ions was also analyzed using pseudo-first and pseudo-second-order kinetics models and the kinetic constants were calculated depending on temperature. The thermodynamic parameters (enthalpy, entropy and adsorption free energy) were also calculated. Further, the investigation was aimed at removing Pb(II), Cu(II) and Cd(II) metal ions from aqueous solution by functionalized mesoporous silica SBA-15 with dendrimer amines as adsorbent in the fixed bed column and also to examine the effects of operational conditions such as bed height, flow rate, single and ternary ions solution on the column dynamics. The dynamics of the adsorption process was modeled by using Thomas and Yan approaches.
Section snippets
Chemicals and reagents
Poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymer non-ionic surfactant EO20PO70EO20 (Pluronic P123) was provided by Aldrich. Tetraethylorthosilicate (TEOS 98%), 3-aminopropyltrimethoxysilane (APTES 99%), N,N-di-isopropylethylamine (DIPEA), 2,4,6-trichloro-1,3,5-triazine (cyanuric chloride), ethylenediamine (EDA), hydrochloric acid (HCl, 37%), methanol, dichloromethane, tetrahydrofuran, toluene, Pb(NO3)2, Cu(NO3)2 and Cd(NO3)2 were purchased from Merck (Germany).
Synthesis of SBA-15 mesoporous silica
Characterization of adsorbents
The low-angle XRD spectra for SBA-15, NH2–SBA-15 and MDA–SBA-15 are shown in Fig. 2a. All three synthesized adsorbents exhibited a single strong peak (1 0 0) on its XRD pattern followed by two additional peaks (1 1 0, 2 0 0) which can be indexed on a hexagonal lattice [16]. Consequently, the anchoring of functionalized moieties (APTES) and further bonding of the melamine-based dendrimer amines complexes inside the mesoporous channels of SBA-15 do not seriously perturb the overall ordered structure of
Conclusion
This study demonstrates (to our knowledge for the first time) how the dendrimer amine grafted mesoporous silica (MDA–SBA-15), prepared by a post-grafting process and employed for the removal of Pb(II), Cu(II) and Cd(II) ions from aqueous solution in batch and upward flow fixed bed column, can be used as a more effective adsorbent than others already in use. This melamine-based dendrimer exhibits high efficiency for heavy metals removal in aqueous solution in the order of Pb(II) > Cu(II) > Cd(II),
Acknowledgements
Authors wish to thank the Iranian Nano Technology Initiative Council and Tarbiat Modares University that have financed this research work. The authors also wish to thank University of Tehran (School of Chemistry) for considerable laboratory support at the Lab of Dr. Badiei. We are also grateful to Mrs. Haghdost for assisting in laboratory works and Ellen Vuosalo Tavakoli (University of Mazandaran) for final editing of the English text.
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