The use of functionalized aerogels as a low level chromium scavenger

https://doi.org/10.1016/j.micromeso.2014.10.004Get rights and content

Highlights

  • High sorption capacity and fast removal of sorbed ions.

  • Single step synthesis and easy handling.

  • No swelling in solvent and high thermodynamic stability.

  • Can be used in radioactive and chemical environment.

  • Regeneration and complete recovery of the sorbed ions.

Abstract

This paper communicates synthesis of aminopropyl functionalized aerogel by grafting 3-aminopropyltrimethoxy silane (APTMS) onto the surface of nano sol–gel silica by post synthetic grafting procedure. The physico-chemical behavior of the grafted silica is investigated by SEM, XRD, FTIR and TGA techniques. The BET surface area analysis technique is used for the determination of the surface area, total pore volume and pore diameter. The effectiveness of chromium removal was systematically monitored as a function of the pH of the medium, agitation time, Cr(III) ions concentration and functionalized silica dosage. The chromium uptake followed Freundlich, Langmuir and D–R isotherm models and sorption capacity computed from the Langmuir model is 1.67 mmol g−1. The effect of temperature on sorption is also checked and various thermodynamic parameters like ΔH, ΔS, and ΔG are calculated and discussed. Regeneration studies show that the sorbent is reusable. The effect of interfering cations and anions is also investigated.

Introduction

Silica aerogels are highly porous structures and have been the subject of extensive research since their discovery [1]. The attractive properties of these materials like high specific surface area, low density and greater chemical, mechanical and thermal stability make them potential candidates in many practical applications like catalysis and sorption [2], [3], [4], [5], [6]. The unique property of these materials is their three dimensional network system accessible to sorbed molecules through their highly porous structure. Sol–gel is a commonly used synthesis technique for these materials and has received tremendous technical and commercial importance [7], [8], [9]. Mesoporous silica’s with tailored functional groups are excellent nanostructures, which combine the inherent properties of inorganic backbone and the specific selectivity of organic functional groups. The surface functionalization of these structures can be accomplished by incorporating chemical groups into their porous three dimensional networks directly [10], [11] or through post grafting procedure. Functionalized aerogel is a potential candidate for the cleaning of the low level impurities from aqueous, petrochemicals and gaseous media. Modified silica have attracted special attention in different areas like metal ions preconcentration, ion exchange, biotechnology, catalysis and green chemistry [12], [13], [14], [15], [16].

Literature reveals that amine functionalized silica is extensively studied in solid phase extraction [17] either covalently attached to silica or simply impregnated on silica surface for heavy metals like Cu(II), Cd(II), Hg(II) Co(II), Ni(II), Zn(II) and Pb(II) [18], [19], [20], [21], [22], [23], [24], [25], [26]. The complexation ability of amine bearing silica is also utilized for the voltammetric detection of trace Cu(II) [27].

Chromium compounds are used in several industrial applications such as leather tanning, metal finishing, pigment manufacturing and electroplating [28]. Exposure to chromium compounds causes a wide range of clinical/health disorders [29]. Cr(VI) is more toxic than Cr(III), where the leather industry produces a large amount of Cr(III) rich waste that could be potentially oxidized to Cr(VI). Therefore, removal/recovery of Cr(III) from the industrial wastewater is important from both environmental and economic point of view. It may reduce the risk of polluting environment while the recovered Cr(III) can be reused [30], [31].

The removal of chromium ions from water is a serious issue in Pakistan. There is a large number of tanneries, where leather is the second largest export earning and has 5% share in the GDP. The tanneries in Pakistan discharge their wastes untreated to water bodies. The literature reveals that aminated silica is an effective sorbent for the separation of various heavy metals, but very little is reported about its efficiency in the separation/preconcentration of chromium. In this work amine functionalized aerogels are synthesized and their efficiency for the removal/preconcentration of Cr(III) ions is investigated.

Section snippets

Reagents and chemicals

All the reagents in this study were of analytical grade and used without any further purification. The buffer solution (1–8) with an ionic strength of 10 mmol was prepared using an appropriate volumes of solutions of KCl and HCl (pH 1–2), sodium acetate and acetic acid (pH 3–6), Boric acid and sodium hydroxide (pH 7–8).

The radiotracer 51Cr was prepared by irradiating spec pure metal in swimming pool type research reactor PARR-1 of PINSTECH at a neutron flux of 7 × 1013 s−1 for 12 h. The irradiated

Characterization of the sorbent

X-ray diffraction (XRD) pattern of virgin aerogel and APTMS modified aerogels are shown in Fig. 3. The broad horn peak at 22° is the distinguishing silica peak and represents amorphous nature of silica [33] and broad peaks at small edges are due to the absence of long-range crystallographic order. After calcinations at 900 °C the original broad peak at 2θ, 22° was reduced and intense peaks appeared at 22°, 29°, 32°, 36° showing some changes in the crystalline domain of silica.

After

Conclusions

These batch experiments conclude that silica aerogels with nitrogen atoms functional groups can effectively remove Cr(III) ions from chromium spiked and drinking water samples. The functionalized sorbent offered fast kinetics for Cr(III) removal with a high sorption capacity of 1.66 mmol/g. Sorption follows intra particle diffusion mechanisms and first order kinetics. The sorption of Cr(III) onto functionalized aerogels obey Langmuir, Freundlich and D–R isotherm over the entire range of

Acknowledgments

The authors highly acknowledge Pakistan Institute of Nuclear Science and Technology (PINSTECH) for providing laboratory facilities and Higher Education Commission (HEC) of Pakistan for providing financial assistance.

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