Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
EXAFS and DFT study of the cadmium and lead adsorption on modified silica nanoparticles
Graphical abstract
Introduction
Effluents from industrial applications including mining, refining and production of textiles, paints and dyes may contain heavy metal ions at high concentrations [1]. In particular, lead and cadmium are considered priority metals in the framework of the European water policy (Directive 2013/39/EU [2]), because they present a significant risk to biota and humans, given its persistence, toxicity and bioaccumulation characteristics.
A wide variety of techniques to remove heavy metals from water is available, such as ion exchange, reverse osmosis and nanofiltration, precipitation, coagulation/co-precipitation and sorption [3]. Among them, adsorption is considered as one of the most suitable methods due to its ease of operation and the availability of a wide range of adsorbents. Organofunctionalization of adsorbents for the removal of heavy metal ions from water has attracted great research interest in the last decade due to the advantages of achieving high efficiency and good selectivity [4], [5]. In particular, amines have been used to functionalize silica [5] to synthesize sorbents for heavy metal ions removal from aqueous media. Mesoporous silica materials chemically modified with aminopropyl, [2-aminoethylamino]-propyl, and [(2-aminoethylamino)-ethylamino]-propyl [6] were recently employed to sorb divalent copper, nickel, lead, cadmium and zinc from aqueous solution.
In this context we here prepared 7 nm-diameter silica nanoparticles organofunctionalized with aminopropyl groups to sorb divalent lead and cadmium ions from aqueous solution. Sorption isotherms for these cations on the silica particles were favorably obtained. Complexation of Pb2+ with the silica nanoparticles was confirmed by EXAFS (Extended X-ray Absorption Fine Structure) with synchrotron light measurements. To compare predicted Pb–N and Pb–C distances in the modified nanoparticles bonded to Pb2+ with the EXAFS determinations, quantum-chemical calculations were carried out.
Separation of small silica nanoparticles by filtration or ultracentrifugation can be a cumbersome procedure. Thus, to facilitate the separation of the nanomaterial by filtration, after sorption of the metal ions an alternative novel method involving the synthesis of bigger composite particles (NPNH2@MEH-PPV) of the copolymer poly[2-methoxy-5-(2-ethylhexyl-oxy)-1,4-phenylene-vinylene] (MEH-PPV, Fig. 1a) and the modified silica nanoparticles was employed here.
Section snippets
Materials
Fumed silica (Sigma, specific surface area; SSA = (390 ± 40) m2 g−1, particle diameter estimated from the SSA = 7 nm and confirmed by TEM images [7]), toluene (Baker, p.a.), ethyl acetate (Ciccarelli, p.a.), CaH2 (Fluka), Pb(NO3)2 (Timper), tetrahydrofurane (THF) Biopack, and CdSO4·8/3H2O, CaCl2, (3-aminopropyl) triethoxysilane (APTES) and poly[2-methoxy-5-(2-ethylhexyl-oxy)-1,4-phenylene-vinylene] (MEH-PPV) from Sigma–Aldrich were used without further purification. Distilled water (>18 MΩ cm−1, <20 μg L−1
Infrared spectroscopy
The FTIR spectrum of NPNH2 shows additional peaks compared to those reported for bare silica nanoparticles [7], [21] (see Fig. 2 and Table 1). These bands are also present in the spectrum of APTES. The bands in the 3000–2800 cm−1 region are assigned to the CH2 stretching [22]. Absorption bands in the region 1575–1450 cm−1 may be assigned to N−H vibrations in protonated amines [23]. The N–H stretch absorption over the 3300–3400 cm−1 present in APTES [24] is masked by the O–H absorption present in
Conclusion
Silica nanoparticles of 7 nm diameter were modified with (3-aminopropyl) triethoxysilane (APTES) and characterized by CP-MAS 13C and 29Si NMR, FTIR, zeta potential measurements, and thermogravimetry. Isotherms for the adsorption of divalent lead and cadmium ions from aqueous solution were obtained. Lead complexation with these silica nanoparticles was clearly confirmed by EXAFS. Predicted Pb–N and Pb–C distances obtained from quantum-chemical calculations are in very good agreement with the
Acknowledgments
This work was supported by Grant PICT 2008 # 0686 from Agencia Nacional de Promoción Científica y Tecnológica, (ANPCyT, Argentina), PIP # 01035 (CONICET, Argentina) and LNLS (Brasil, Project XAFS1-14534). M.M., J.M.R.L., C.J.C. and E.R.D. are researchers from CONICET. V.B.A. and D.O.M. are researchers from Comisión de Investigaciones Científicas de la Provincia de Buenos Aires.
The authors thank Drs. J.C. Combo and C. Bilos from LAQAB (Universidad Nacional de La Plata) for the nitrogen
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