Aminopropyl-modified mesoporous silica SBA-15 as recovery agents of Cu(II)-sulfate solutions: Adsorption efficiency, functional stability and reusability aspects

doi:10.1016/j.jhazmat.2012.04.049

Journal of Hazardous Materials

Volumes 223–224, 15 July 2012, Pages 53-62

https://doi.org/10.1016/j.jhazmat.2012.04.049 Get rights and content

Abstract

Hybrid mesoporous materials are potentially useful for metal ion scavenging and retrieval because of their high surface areas, controlled accessibility and tailored functionalization. Some aspects that are linked to the performance of HMM include pore accessibility, stability of the organic functions and reusability. Knowledge of these aspects is critical in the design of adsorption–desorption protocols. In this work we produce and characterize propylamino-substituted large pore silica (SBA-15-N), which is submitted to Cu(II) adsorption from copper sulfate solutions, followed by desorption in acid media and material regeneration. We find that the hybrid material is an efficient adsorbent (1.15–1.75 mmol Cu(II) g⁻¹), although a fraction of the organic groups is lost during the adsorption process. An X-ray photoelectron spectroscopy (XPS) study demonstrates that the contents of amino groups are higher in the material surface, leading to different behaviors in Cu(II) complexation along the material. These materials can be regenerated by exposure to acidic media. Thermal processing of the hybrid materials leads to better durability in aqueous solutions during reprocessing, due to enhanced polycondensation of the inorganic framework. Thermally treated samples, once regenerated, are efficient adsorbents in a second step of Cu(II) adsorption. We discuss the materials processing factors involved in the improved adsorption of Cu(II), its quantitative release and reusability of the material.

Highlights

► We produce mesoporous amino-silica as Cu(II) adsorbent (1.15–1.75 mmol Cu(II) g⁻¹). ► Elemental analysis and XPS demonstrate that amino groups concentrate at the material surface. ► The integrity of the adsorbent through the adsorption, desorption and recycling processes is assessed. ► These materials can be regenerated by exposure to acidic media. ► A careful thermal processing of the material is central to better durability during reprocessing.

Introduction

Metal ion scavenging and retrieval from water is one of the central problems in natural resources management, and has been subject of study in many aspects during the last decade. Heavy metals present in water from industrial applications (including mining, refining and production of textiles, paints and dyes) are pollutants sometimes present in very low concentrations. A wide variety of techniques to remove these chemical species are available, such as ion exchange, reverse osmosis and nanofiltration, precipitation, coagulation/co-precipitation and adsorption [1]. This last technique is one of the most effective and several approaches have been developed for aqueous efficient separation and remediation. Materials tested include natural or synthetic inorganic solids (zeolites, clays, metal oxides), natural organic matter (polysaccharide-based materials), biosorbents [2], as well as advanced materials such as functional polymers, organic-inorganic hybrids, porous carbons, etc. [3]. Nanoparticle-based materials are potentially interesting adsorbents due to their high surface to volume ratios, although they present limitations associated with their size and dispersability:

(a)
Nanoparticle packing leads to the loss of the effective exchange area and high pressure drops, limiting their use in compact reactors.
(b)
Handling, retrieval or separation of nanoparticles from slurries is difficult.
(c)
Nanoparticles are highly reactive, and tend to readily dissolve under real operation conditions.
(d)
Potential health hazards due to dealing with reactive dispersed species.

In this context, mesoporous materials present an advantage: their high specific surface area (200–1500 m²/g) is contained within the material, which is generally in the shape of micronic or submicronic particles, which are easier to process into columns, for example. Their porosity in the nanometer scale (2–50 nm) should grant accessibility to the surface, while keeping a robust framework. Therefore, silica-based organic–inorganic hybrids and, more recently, ordered mesoporous organosilica materials have been developed for adsorption procedures [4], [5], [6].

Silica-based templated materials with organized, large-size monodisperse mesopores [7], [8] display high specific surface areas in the order of 600–1000 m²/g and highly controlled porous structures (up to 1 cm³/g or even more) [9]. A functionalized hybrid material can be achieved combining sol–gel techniques, self-assembly of appropriate surfactant and selective surface modification with specific organic functions [10], [11], [12]. This kind of composites is attractive since they combine in a single solid phase both the properties of a rigid three-dimensional silica network and the particular chemical reactivity of the organic component [13]. A great variety of mesoporous functional adsorbents have thus been described in the literature, either organically modified mesoporous oxides or periodic mesoporous organosilica [14], [15]. However, the use of these sophisticated materials in real situations is limited by their cost. Therefore, interest in robust, reusable adsorbents is growing. Functionalized mesoporous adsorbents are interesting phases, because of their inorganic skeleton that can withstand adverse environmental conditions, as well as repeated adsorption–regeneration cycles. Most of the reported work deals with the synthesis and characterization of the adsorbent hybrid material, and the testing of a variety of heavy metals, kinetics of the processes and eventually the selectivity between different metals [16], [17], [18], [19]. Fewer efforts are dedicated to explaining the actual mechanism of adsorption/desorption, and, more importantly, the mechanisms of degradation of the material under operation conditions, which is essential to their reusability [10], [11], [14], [15], [20], [21]. Among the mesoporous silica materials, SBA-15 (Santa Barbara Amorphous), prepared using poly(alkene oxide) triblock-copolymer as structure directing agent, has large pores and robust thick walls that make it an ideal candidate for designing general purpose adsorbents once functionalized with organic groups or even polymers [8], [22], [23], [24].

In this work, we produce and characterize propylamino-substituted SBA-15 (SBA-15-N). We test the material towards Cu(II) adsorption, followed by desorption in acid media and regeneration. We find that a fraction of the organic groups is lost during the adsorption process, but improved material stability is gained by thermal processing. Chemical analysis of the bulk material and X-ray photoelectron spectroscopy (XPS) characterization of the surface indicate that the surface and interior of the materials present a different behavior towards Cu(II) adsorption. We discuss the factors involved in the adsorption of Cu(II), its quantitative release and the regeneration process in the reusability of the material, focusing on aspects that are relevant in the design of a practical adsorbent.

Section snippets

Synthesis of SBA-15, SBA-15-N and SBA-15-N-T

Preparation of the SBA-15 sample was performed following the method reported by Zhao et al. [8] 12.6 g of Pluronic P123 (Aldrich) was dissolved in 89.6 g of water with stirring at 38 °C. The temperature was lowered to 35 °C following the addition of 359.6 g of 2 M HCl solution (prepared from 37% fuming hydrochloric acid, Merck) and 25.6 g of tetraethyl orthosilicate (TEOS, Fluka, 98%). The solution was kept under stirring at 35 °C for 20 h. The mixture was aged at 90 °C for 24 h without stirring. The

Synthesis and characterization of the mesoporous hybrid matrices

High quality SBA-15 powder was obtained by precipitation, after controlled hydrolysis-condensation of silica in acidic medium in the presence of P123 template, as described in Section 2.

SAXS patterns of calcined SBA-15 powders show a high degree of ordering, presenting at least six peaks due to the mesostructure (Fig. 1A). Peak distance ratios correspond well with a 2D hexagonal structure (space group p6m), details of the indexing are supplied in the ESI.

The cell parameter (a) obtained from Eq.

Conclusions

This work demonstrates that amino post-grafted mesoporous silica with channel-like structure is a strong, fast, efficient (1.15–1.75 mmol Cu(II) g⁻¹) and potentially reusable adsorbent for Cu(II) ions, even in dilute solutions. The role of the counter ion (in this case, sulfate) is important, for it mediates in the adsorption behavior of the cation of interest. We demonstrated that the post-grafting procedure leads to high N loadings that take place mostly in the micropores; amino groups

Acknowledgements

Authors thank ABTLUS for funding access to the LNLS synchrotron facility (proposals D11A-SAXS1-11060 and D04A-SXS-10808). Work funded by ANPCyT (grants PICT 1848, PAE 2004 22711, PME 00038, FONARSEC FSNANO 2010/007). VL thanks CONICET and Rhein Chemie Argentina for a graduate student fellowship. GJAASI is a CONICET staff researcher. Authors thank Dr. M.C. Marchi for her assistance in electronic microscopy measurements, and Drs. H. Westfahl and M. Cardoso for their assistance in SAXS.

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Citation Excerpt :
Moreover, the S contents in ATBS (Table 1) were comparable to those in TFS (Table S3), indicating no significant change in thiol groups occurred during the post-grafting process. In addition, two peaks at around 399.4 eV and 401.3 eV being found in XPS N 1s spectra (Fig. 2c) evidenced the successful introduction of amino and ammonium groups onto the ATBS surface, in accordance with the FTIR results and reported XPS information on amino and ammonium groups (Lombardo et al., 2012). The N contents in ATBS are listed in Table 1.
In this study, a two-step functionalizing strategy by combining co-condensation with grafting procedures was employed to synthesize well-ordered Amino- and Thiol-Bifunctionalized SBA-15 (ATBS) mesoporous silica. Its physicochemical properties, performance, and mechanisms in immobilization of toxic metals Pb and Cd in water and soil were investigated. After bi-functionalization, X-ray diffractometer, transmission electron microscope, and N₂ adsorption-desorption measurements confirmed that the ATBS maintained a highly-ordered mesoporous structure, large surface area and pore volume. The elemental analysis, Fourier transform infrared spectroscopy and X-ray Photoelectron Spectroscopy (XPS) evidenced the successful incorporation of amine and thiol groups into ATBS. These structure and functional characteristics of ATBS benefited Pb and Cd sorption. Sorption isotherms of Pb and Cd were better fit with Sips and Redlich-Peterson models. Sorption kinetics suggested that Pb sorption was mainly regulated by chemical reactions, whereas both diffusion process and chemical reactions were rate-regulating steps in Cd sorption. ATBS showed the maximum sorption capacities for Pb and Cd at 120 and 38 mg g⁻¹, respectively. The sorption mechanisms revealed by XPS measurements suggested that Cd sorption was mainly attributed to thiol groups while Pb was efficiently bond to both thiol and amino groups. High and stable sorption efficiencies were attained in the pH range of 4–6, with a higher affinity towards Pb than Cd. Furthermore, its ability to immobilize Pb and Cd in soils was examined with an incubation experiment, which showed that ATBS reduced 30–56% of MgCl₂-extractable Pb and Cd in a contaminated soil. The synthesized sorbent via the two-step functionalizing strategy shows high sorption efficiency towards Pb and Cd, and thus it has potential application in remediating Pb and Cd contaminated water and soils.

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Aminopropyl-modified mesoporous silica SBA-15 as recovery agents of Cu(II)-sulfate solutions: Adsorption efficiency, functional stability and reusability aspects

Abstract

Highlights

Introduction

Section snippets

Synthesis of SBA-15, SBA-15-N and SBA-15-N-T

Synthesis and characterization of the mesoporous hybrid matrices

Conclusions

Acknowledgements

J. Hazard. Mater.

J. Hazard. Mater.

Water Res.

Talanta

Desalination

Inorg. Chem. Commun.

Appl. Surf. Sci.

J. Hazard. Mater.

Chem. Eng. J.

J. Hazard. Mater.

J. Non-Cryst. Solids

Stud. Surf. Sci. Catal.

J. Non-Cryst. Solids

Microporous Mesoporous Mater.

Chem. Eng. J.

J. Hazard. Mater.

Appl. Surf. Sci.

J. Colloid Interface Sci.

Desalination

J. Membr. Sci.

Chem. Mater.

J. Mater. Chem.

Periodic mesoporous silica-based organic–inorganic nanocomposite materials

Chem. Mater.

Silica-based mesoporous organic–inorganic hybrid materials

Angew. Chem. Int. Ed

Mesoporous organosilica adsorbents: nanoengineered materials for removal of organic and inorganic pollutants

J. Mater. Chem.