10076 DOI: 10.1021/la100204d Langmuir 2010, 26(12), 10076–10083Published on Web 03/26/2010

pubs.acs.org/Langmuir

© 2010 American Chemical Society

New Ultrastable Mesoporous Adsorbent for the Removal of Mercury Ions

ElsDe Canck,† Linsey Lapeire,† Jeriffa DeClercq,‡ Francis Verpoort,† and Pascal VanDer Voort*,†

†Department of Inorganic and Physical Chemistry, Center for Ordered Materials, Organometallics andCatalysis (COMOC), Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium, and ‡Faculty of

Applied Engineering Sciences, University College Ghent, Schoonmeersstraat 52, 9000 Ghent, Belgium

Received January 15, 2010. Revised Manuscript Received March 10, 2010

To find a more stable adsorbent for the selective removal of mercury ions, a new mesoporous adsorbent is developedand compared with a number of carefully selected mesoporous silica adsorbents described in literature. This newadsorbent is based on a pure trans-ethene bridged periodic mesoporous organosilica (PMO) which is subsequentlymodified to obtain a suitable adsorbent. The outcome is a new thiol-containing ethene bridged PMOwhich combines theadsorption efficiency of the thiol group toward mercury ions with the stability of ethene bridged PMOs. During theadsorption process, this material not only maintains its mesoporous structure and ordering, it also completely preservesthe amount of organic functionalities, allowing recycling and reuse of the adsorbent. Additionally, this PMO is able toreduce the Hg2þ amount in aqueous solutions below 0.5 μg/L, and the adsorbent has a maximal adsorption capacity of64 mg/g which means an apparent 1:1 ratio mercury(II) ion to thiol.

1. Introduction

Periodic mesoporous organosilicas (PMOs)1,2 have drawnthe attention of many research groups during the latest decadebecause of their unique properties.3-5 The materials are synthe-sized using bridged bis-silanes, for example, (EtO)3-Si-R-Si-(OEt)3 and immediately incorporate the organic functionality Rin their structure. The bis-silane polycondensates around a non-ionic triblock copolymer, acting as a template. After formation ofthe PMO, the template is removed by extraction to reveal thepores. These mesoporous materials have well-defined pore sizesand pore shapes, high specific surface areas, and uniform dis-tribution of the functionalities. As a result, they combine thestrength of inorganic structures and the flexibility and chemicalversatility of the organic moieties.

Thesematerials have awide range of potential applications andare used as chromatographic packing materials,6-8 low-k de-vices,9 chemical sensors,3 catalysts,10 and host materials forbiomolecules and drug delivery.3 In addition, promising results

have been obtained in the field of environmental sciences, wherethe materials are applied as adsorbents.11,12

Extensive research13-36 has highlighted the importance ofadsorbents in the area of removing harmful or regaining valuablecomponents. In most cases, it is important that the adsorptionoccurs selectively. In this study, the focus is placed upon theremoval of mercury(II) ion, known for its major toxicity.37,38 Anexcellent adsorbent19-22 for mercury ions is characterized by,among other properties, (1) a high loading of the functionaladsorbing groups, (2) a uniform distribution of these groups, and

*Towhom correspondence should be addressed. Telephone:þ32 964 44 42.Fax: þ 32 9 264 49 83. E-mail: Pascal.VanDerVoort@UGent.be.(1) Asefa, T.; MacLachlan, M. J.; Coombos, N.; Ozin, G. A. Nature 1999,

402, 867.(2) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Teresaki, O. J. Am. Chem.

Soc. 1999, 121, 9611.(3) Van Der Voort, P.; Vercaemst, C.; Schaubroeck, D.; Verpoort, F. Phys.

Chem. Chem. Phys. 2008, 10(3), 347.(4) Fryxell, G. E. Inorg. Chem. Commun. 2006, 9(11), 1141.(5) Vercaemst, C.; Ide, M.; Friedrich, H.; de Jong, K. P.; Verpoort, F.; Van Der

Voort, P. J. Mater. Chem. 2009, 19(46), 8839.(6) Cho, E. B.; Kim, D.; Jaroniec, M. Langmuir 2007, 23(23), 11844.(7) Park, S. S.; Kim, S. J.; Seo, Y. K.; Park, D. H. 4th International Symposium

on Nanoporous Materials, Niagara Falls, Canada; Sayari, A., Jaroniec, M., Eds.;Elsevier Science Bv: The Netherlands, 2005; Vol. 156, pp 183-190.(8) Rebbin, V.; Schmidt, R.; Fr€oba, M. Angew. Chem., Int. Ed. 2006, 45(31),

5210.(9) Goethals, F.; Meeus, B.; Verberckmoes, A.; Van Der Voort, P.; Van

Driessche, I. J. Mater. Chem. 2010, 20, 1709.(10) Shylesh, S.; Samuel, P. P.; Sisodiya, S.; Singh, A. P. Catal. Surv. Asia 2008,

12(4), 266.(11) Zhong, Z. X.; Wei, Q.; Wang, F.; Li, Q. Y.; Nie, Z. R.; Zou, J. X. J. Inorg.

Mater. 2008, 23, 408.(12) Wu, H. Y.; Liao, C. H.; Pan, Y. C.; Yeh, C. L.; Kao, H. M. Microporous

Mesoporous Mater. 2009, 119, 109.

(13) Zhang, L.; Zhang, W.; Shi, J.; Hua, Z.; Li, Y.; Yan, J. Chem. Commun.2003, 210.

(14) Lee, B.; Kim, Y.; Lee, H.; Yi, J. Microporous Mesoporous Mater. 2001,50, 77.

(15) Yang, H. J. Hazard. Mater. 2008, 152, 690.(16) Perez-Quintanilla, D.; del Hierro, I.; Fajardo,M.; Sierra, I. J.Mater. Chem.

2006, 16, 1757.(17) Perez-Quintanilla, D.; del Hierro, I.; Fajardo, M.; Sierra, I. J. Hazard.

Mater. 2006, 134, 245.(18) Perez-Quintanilla, D.; del Hierro, I.; Fajardo, M.; Sierra, I. Mater. Res.

Bull. 2007, 42, 1518.(19) Delacote, C.; Gaslain, F. O. M.; Lebeau, B.; Walcarius, A. Talanta 2009,

79, 877.(20) Walcarius, A.; Etienne, M.; Bessi�ere, J. Chem. Mater. 2002, 14, 2757.(21) Walcarius, A.; Etienne, M.; Lebeau, B. Chem. Mater. 2003, 15, 2161.(22) Walcarius, A.; Delacote, C. Chem. Mater. 2003, 15, 4181.(23) Brown, J.; Richer, R.; Mercier, L. Microporous Mesoporous Mater. 2000,

37, 41.(24) Puanngam, M.; Unob, F. J. Hazard. Mater. 2008, 154, 578.(25) Walcarius, A.; Delacote, C. Anal. Chim. Acta 2005, 547, 3.(26) Zhang, L. X.; Zhang, W. H.; Shi, J. L.; Hua, Z.; Li, Y. S.; Yan, J. Chem.

Commun. 2003, 2, 210.(27) Brown, J.; Richer, R.; Mercier, L. Microporous Mesoporous Mater. 2000,

37, 41.(28) Bibby, A.; Mercier, L. Chem. Mater. 2002, 14, 1591.(29) Brown, J.; Mercier, L.; Pinnavaia, T. J. Chem. Commun. 1999, 1, 69.(30) Aguado, J.; Arsuaga, J. M.; Arencibia, A. Ind. Eng. Chem. Res. 2005,

44, 3665.(31) Aguado, J.; Arsuaga, J. M.; Arencibia, A.Microporous MesoporousMater.

2008, 109, 513.(32) Walcarius, A.; Delacote, C. Anal. Chim. Acta 2005, 547, 3.(33) Mattigod, S. V.; Feng, X. D.; Fryxell, G. E.; Liu, J.; Gong, M. L. Sep. Sci.

Technol. 1999, 34, 2329.(34) Olkhovyk, O.; Jaroniec, M. J. Am. Chem. Soc. 2005, 127, 60.(35) Olkhovyk, O.; Pikus, P.; Jaroniec, M. J. Mater. Chem. 2005, 15, 1517.(36) Olhovyk, O.; Jaroniec, M. Ind. Eng. Chem. Res. 2007, 46, 1745.(37) Jarup., L. Br. Med. Bull. 2003, 68, 167.(38) Guzzi, G.; La Porta, C. A. M. Toxicology 2008, 244, 1.

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(3) a great stability during the adsorption and the regeneration inacidic media.23-26

The sulfur functionality has already performed excellently inthe removal of mercury ions. Mercier and co-workers27,28 havedescribed mesoporous silica materials synthesized by the co-condensation of tetraethoxyorthosilicate (TEOS) and 3-(mercapto-propyl)trimethoxysilane (MPTMS). The materials with differentsulfur content exhibited specific surface areas between 763 and1176 m2/g and possessed approximately 0.47-2.3 mmol of thiolgroupsper gramofmaterial.Mercury(II) ion uptakebetween 0.90and 2.3 mmol/g was observed. When these materials were testedin the presence of other metal ions such as Pb2þ, Cd2þ, Zn2þ,Ni2þ, and Cu2þ, the results showed that the thiol functionalizedadsorbents have a more pronounced affinity for the mercury(II)ion than for the other metals. This was consistent with previouslypublished findings.29-31 Aguado et al.30,31 also reportedmaterialsprepared by the co-condensation of TEOS and MPTMS, andthese materials possess a remarkably high maximal adsorptioncapacity up to 4.1 mmol of mercury(II) ion per gram of mesopor-ous adsorbent.

Walcarius andDelacote32 grafted thiolpropyl groups onMCMmaterials with SBET= 1000m2/g and thiol amounts of 0.84-2.26mmol/g. No influence of other metals (Agþ, Cu2þ, Ni2þ, Cd2þ,Zn2þ, Bi2þ) was observed when the mercury(II) adsorption wasexamined.

In addition to the uptake of Hg2þ in aqueous solutions, theremoval of strongly complexed mercury(II) was examined byMattigod et al.33Mesoporous self-assembled silica materials withsurface areas higher than 800 m2/g have shown a mercury ionadsorption of 2.8mmol/g fromKI/I2 lixiviant streams which evencontain Kþ, I-, I2, Fe

2þ, SO42-, and Ca2þ.

This mercaptopropyl unit can also be combined with a largeheterocyclic bridging group such as isocyanurate.34 Jaroniecet al.35 investigated the mercury ion adsorption of these materialssynthesized by the co-condensation of tris[3-(trimethoxysilyl)-propyl]isocyanurate, MPTMS, and TEOS. Mesoporous materi-als with specific surface areas of approximately 500 m2/g andnarrow pore size distributions were obtained. The materialsexhibited around 0.89mmol of SH groups per gram. The mixtureof isocyanurate and thiol resulted in a very high mercury ionadsorption. These co-condensed materials could adsorb up to5.64 mmol/g Hg2þ. This research group also introduced otherPMO materials.36 The isocyanurate moiety was combined withbis(3-(triethoxysilylpropyl)tetrasulfide), N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole, or ureidopropyltrimethoxysilane. Materi-als were prepared with specific surface areas ranging from 668 to707 m2/g. They could adsorb up to 1.37 mmol Hg2þ/g.

To achieve not only a selective but also a stable adsorbent, thisstudy develops a new periodic mesoporous organosilica startingfrom a pure trans-ethene PMO.39-41 A sulfur group is introducedby covalent bonding to the CdC. After characterization, thismaterial is tested for its stability in a set of mercury(II) ionadsorption experiments and compared with other functionalizedsilica materials available in literature. Equally, two differentpostsynthetic pathways have been followed to graft a functionalityon the surface of amesoporous silicamaterial, in this case SBA-15.Also, a one-pot synthesis has beenused to forma silicamaterial viaa co-condensation process. Next to the adsorption kinetics and

isotherm, special emphasis is on the long-term stability andrecyclability in acidic medium of this PMO adsorbent.

2. Experimental Section

Vinyltriethoxysilane (VTES; 97%), (3-mercaptopropyl)-triethoxysilane (MPTES; 98%), and (3-mercaptopropyl)tri-methoxysilane (MPTMS; 98%) were purchased from ABCR.Tetraethoxyorthosilicate (TEOS; 98%), the pluronic PEO20P-PO70PEO20 (P123), Grubbs’ first generation catalyst, 3-chloro-1-propanethiol (98%), triethylamine (>99%), magnesium, iron-(III) chloride (98%; anhydrous), hydrochloric acid (37%; p.a.),acetonitrile (99,5%; p.a.), acetone (>99,5%), ethanol (96%), andtetrahydrofuran (p.a.) were acquired from Sigma-Aldrich. Hg-(NO3)2 in HNO3 (2 M) was purchased from VWR. Triethyla-mine, acetonitrile, and tetrahydrofuran were dried and degassedbefore use.

2.1. Synthesis of SBA-15. The mesoporous material issynthesized following the procedure first published by Zhaoand co-workers.42,43 An amount of 4 g of P123 is dissolved into120mLof2Mhydrochloric acid and30mLofdistilledwater. Themixture is stirred at room temperature until the surfactantcompletely dissolves. An amount of 9 mL of TEOS is added,and the temperature is increased to 45 �C for 5 h under stirring. Awhite precipitation is formed. Subsequently, the temperature israised to 90 �C for 16 h under static conditions. Before filtering ofthe solids, the mixture is allowed to cool down slowly to roomtemperature. The solids are subsequently washed 3 times with15 mL of water and 15 mL of acetone. Finally, the material(denoted as SBA-15) is calcined at 550 �C for 6 h.

2.2. Functionalization of SBA-15. 2.2.1. WithNEt3.Atotal of 0.7 g of SBA-15 is mounted into a Schlenk flask under aninert atmosphere. Then 10 mL of acetonitrile and 2.7 mL oftriethylamine, behaving as an activator,44 are added. The reactionmixture is stirred for 2 h at room temperature, and subsequentlythe mixture of triethylamine and acetonitrile is removed underinert atmosphere. Volumes of 10mLof acetonitrile and 5.0mLofMPTES are added to thematerial. Themixture is allowed to reactfor 2 h at 65 �C before the solid is filtered and washed three timeswith 15 mL of acetonitrile and 15 mL of acetone. The material(SBA-SH-NEt3) is dried at 90 �C under vacuum for 16 h.

2.2.2. With HCl. A total of 0.3 g of SBA-15 material is put ina flask. Then 10 mL of acetonitrile, 0.89 mL of hydrochloric acid(2M), and 0.44mLofMPTES are added. The reactionmixture isstirred for 96 h at 50 �C before the solids are filtered and washedthree times with 15 mL of acetonitrile and 15 mL of acetone. Thematerial (SBA-SH-HCl) is dried at 90 �C under vacuum for 16 h.

2.3. One-Step Synthesis of a Functionalized SBA-15-like

Material. The synthesis is based on the recipes published byMercier et al.27 and Aguado et al.31 A total of 1 g of P123 isallowed to dissolve in 30 mL of 2 M hydrochloric acid for 45 minat room temperature. Following that, 2.10 mL of TEOS is addedand stirred for 45min at 40 �C.Then 0.20mLofMPTMS is addedand stirred for 20h.Awhite precipitation is formedand is allowedto age at 100 �C for 24 h. After filtering and washing several timeswith water and acetone, the surfactant P123 is extracted usingSoxhlet extraction with a mixture of ethanol/hydrochloric acid.The sample, listed below as Co-Con, is dried at 90 �C undervacuum for 16 h.

2.4. Synthesis of Ethene Bridged Periodic Mesoporous

Organosilica (PMO). The ethene bridged PMO is synthesizedusing the recipe previously published by our group.45 An amount

(39) Vercaemst, C.; Ide, M.; Wiper, P. V.; Jones, J. T. A.; Khimyak, Y. Z.;Verpoort, F.; Pascal Van Der Voort, P. Chem. Mater. 2009, 21(24), 5792.(40) Vercaemst, C.; de Jongh, P. E.; Meeldijk, J. D.; Goderis, B.; Verpoort, F.;

Van Der Voort, P. Chem. Commun. 2009, 27, 4052.(41) Vercaemst, C; Jones, J. T. A.; Khimyak, Y. Z.; Martins, J. C.; Verpoort, F.;

Van Der Voort, P. Phys. Chem. Chem. Phys. 2008, 10(35), 5349.

(42) Zhao, D. Y.; Huo, Q. S.; Feng, J. L.; Chmelka, B. F.; Stucky, G. D. J. Am.Chem. Soc. 1998, 120, 6024.

(43) Zhao, D. Y; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.;Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548.

(44) Blitz, J. P.; Murthy, R. S. S.; Leyden, D. E. J. Colloid Interface Sci. 1988,126, 387.

(45) Vercaemst, C.; Ide, M.; Allaert, B.; Ledoux, N.; Verpoort, F.; Van DerVoort, P. Chem. Commun. 2007, 22, 2261.

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of 1 g of P123 is dissolved in 47.8 mL of water, 3.42 mL ofconcentrated hydrochloric acid, and 2.45 mL of butanol. Themixture is vigorously stirred for 1.5 h.Then1.86mLofhomemade100% trans 1,2-bis(triethoxysilyl)ethene is added, and the tem-perature is raised to 45 �C. A white precipitation is formed. After6 h of stirring, the mixture is heated to 90 �C under staticconditions. After cooling down of the mixture, the solids arefiltered and washed with water (three times) and acetone (threetimes). To remove the template, a Soxhlet extraction is performedwith acetone for 5 h. Afterward, the PMO is dried. Subsequently,the pure trans-ethene PMO is functionalized by bromination. Thematerial (E-ePMO) is dried at 90 �Cunder vacuum for 16 h beforeadding Br2 in gaseous phase.46 The physisorbed bromine isremoved by drying the material at 90 �C for 24 h (Br-ePMO).The 100% trans 1,2-bis(triethoxysilyl)ethene is synthesized asfollows: 0.0535 g of Grubbs’ first generation catalyst(PCy3)2Cl2RudCH-Ph39,47 was added to a Schlenk flask underargon which contains 42.95 mL of vinyltriethoxysilane (VTES).Themixture is stirred for 1 h at roomtemperature and refluxed for1 h. Subsequently, the mixture is destilled to remove the remain-ing VTES.

2.5. Modification of Brominated Ethene Bridged PMO.Amixture of magnesium (0.74 g), iron(III) chloride (0.54 g), andtetrahydrofuran (30 mL) is prepared under an inert atmosphere.This viscous solution is allowed to stir for 30min at a temperatureof 50 �C to activate the magnesium. 3-Chloro-1-propanethiol(0.22 mL) is added, and the mixture is stirred for 2 h at roomtemperature. Immediately following, the mixture is added to aSchlenk flask containingBr-ePMO.After stirring for 5 h at 40 �C,filtration is executedwhere thematerial iswashed three timeswith15 mL of THF, 2 M HCl, water, and acetone. Finally, the solids(SH-ePMO) are collected and dried at 90 �C for 16 h.

2.6. Determination of Thiol Groups. The amount of reach-able thiol groups on the synthesized mesoporous materials isdetermined by silver titration.48 The thiol groups on the materialsreact with a known concentration of silver nitrate, and the excessof silver is titrated with potassium thiocyanate, using an ironindicator (FeNH4(SO4)2 3 12H2O in 0.3MHNO3). The number ofthiol groups can be calculated.

2.7. Mercury(II) Ion Adsorption Experiments in the Low

Concentration Range. A 150 mg amount of the material ismeasured out, and 50 mL of 10 μg/L mercury(II) solution isadded. The mixture is stirred at room temperature for 15min andfiltered, and then the amount of mercury ion in the filtrate isquantified with cold vapor atomic fluorescence spectroscopy(CV-AFS). The regeneration of the material is performed asfollows. A sample of the Hg2þ-loaded material is washed threetimeswith 10mLof 2MHCl and three times 10mLofwater. Theamount of Hg2þ leached out of the adsorbent was measured byCV-AFS. The materials are dried in a furnace at 90 �C for 1 hbefore reuse. Three regeneration cycles are performed on eachsample. The structures of the adsorbents are evaluated afterward.Nitrogen adsorption/desorption measurements are performedeach time, and the amount of thiol groups are specified.

2.8. Mercury(II) Ion Adsorption Experiments in the

High Concentration Range. Adsorbent equilibrium experi-ments were performed with 150 mg of mesoporous adsorbent(SH-ePMO) and 50 mL of Hg2þ solution with different concen-trations between 100 μg/L and 400mg/L. Themixture is stirred atroom temperature for 90min and filtered, and then the amount ofmercury ion in the filtrate is quantified with cold vapor atomicabsorption spectroscopy (CV-AAS). The adsorption experimentswere performed at the pH that resulted from the Hg2þ solution

without further adjustment. The initial and final pHof theseHg2þ

solutions were measured.For themercury(II) ion adsorptionkinetic experiments, 300mg

of mesoporous adsorbent (SH-ePMO) was contacted with 100mLof Hg2þ solution with Hg2þ concentrations of 10 and 100 mg/L.At predetermined intervals of time, samples of the mixture were

Table 1. Characteristics of theMesoporousAdsorbents Synthesized in

This Study

sampleSBET

(m2/g)aVp

(cm3/g)bdp

(nm)cmmolSH/gd

no.SH/nm2

SBA-15 865 0.84 3.12SBA-SH-HCl 286 0.28 2.74 10 7.0SBA-SH-NEt3 587 0.51 2.74 1.6 1.1Co-Con 704 0.78 2.74 1.1 1.6E-ePMO 1182 1.08 4.05Br-ePMO 558 0.55 3.55SH-ePMO 636 0.55 3.55 0.44 0.22

a SBET: the specific surface area calculated via the BET equation.bTotal pore volume. cPore diameter calculated from the adsorptionisotherm with the BJH method. dThe amount of thiol groups is deter-mined via thiol titration as described earlier.

Scheme 1. Pre-activation of the Silanol Group with Triethylamine

before Adding the Organosilane

Figure 1. DRIFT spectrum of (a) SBA-15, (b) SBA-SH-NEt3,(c) SBA-SH-HCl, and (d) Co-Con.

(46) Nakai, K.; Oumi, Y.; Horie, H.; Sano, T.; Yoshitake, H. MicroporousMesoporous Mater. 2007, 100, 328.(47) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118, 100.(48) Vogel, A. I. Quantitative Inorganic Analysis, 3rd ed.; Longmans: London,

1961; pp 790-791.

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withdrawn at suitable time intervals, filtered, and analyzed byCV-AAS.

2.9. Characterization Techniques. The specific surfacearea, pore volume, and pore radius are determined using nitrogenadsorption/desorption. The isotherms are recorded on BelsorpMini II equipment at -196 �C. The samples are pretreated at120 �C while degassing.

CHNS elemental analysis is executed by theCentre National dela Recherche Scientifique (CNRS, France).

Diffuse reflectance infrared Fourier transform (DRIFT) spec-troscopy is performed on a hybrid IR-RAMAN spectrophot-ometer, Equinox 55S (FRA 106; Bruker) with a MCT-detector,using aGraseby Specac diffuse reflectance cell, operating in vacuoand at 120 �C.

X-ray diffraction is performed with an ARL X0tra X-raydiffractometer of Thermo Scientific equipped with a Cu KR1tube and a Peltier cooled lithium drifted silicon solid stagedetector.

Cold vapor atomic fluorescence spectroscopy (CV-AFS) isperformed on a Mercur spectrophotometer (Analytik Jena) witha UV source of 253.7 nm. Experimental data are processed withWinAAS version 4.2.0.

Cold vapor atomic absorption spectroscopy (CV-AAS) isperformed on a GBC-933 instrument combined with GBC-HG3000 (GBC Scientific Equipment).

3. Results and Discussion

3.1. Adsorbent Characterization. The physical character-istics and the -SH loading of the functionalized ethene PMOtogether with a variety of mesoporous reference materials arepresented in Table 1.

The standard SBA-15 is functionalized by a grafting pro-cedure with MPTES. Two different pathways are used. Thefirst consists of the preactivation of the silanol groups on thesurface by triethylamine44 (SBA-SH-NEt3; Scheme 1). Whenusing the amine, the silanol group becomes more nucleophilicand an SN2 reaction with the organosilane occurs. This syntheticpathway results in a mesoporous material with 1.1 thiol groups/nm2. The DRIFT spectrum (Figure 1b) shows the absence of thesharp silanol peak at 3740 cm-1, indicating the complete reactionof all the silanol groups available on the surface with MPTES.Characteristic are the peaks for the propyl unit at 2980, 2935, and2896 cm-1.

The second pathway uses hydrochloric acid as an activator tospeed up the hydrolysis process (SBA-SH-HCl; Scheme 2). Theamount of thiol groups/nm2 is significantly higher compared tothe case of amine activation process. The high loading of 7.0thiol groups/nm2 indicates more organosilanes attached to thesurface than original silanol groups available. This is caused bya partial self-condensation of MPTES on the silica surface.Several MPTES units are attached to the surface via one silanolgroup. A network of condensed organosilanes is formed in thepores of the materials, lowering the specific surface area by

Scheme 2. Partial Self-Condensation of MPTES on the Silica

Surface

Scheme 3. (a) Schematic Presentation of the Ethene Bridged PMO with its Hexagonal Structure. (b) Functionalization of the Ethene Bond:

(i) Bromination with Br2(g); (ii) Substitution of the Bromine by 3-Chloro-1-propanethiol

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Article De Canck et al.

almost 600 m2/g. The DRIFT spectrum (Figure 1c) shows thatsilanol groups are still available on the surface: a small peak at3733 cm-1.

In addition to these two grafting procedures, another methodcan be used to synthesizemesoporousmaterials. Co-Conhas beenmade by the co-condensation of TEOS and MPTMS, and itscharacteristics and DRIFT spectrum are also shown in Table 1and Figure 1d. This material has the major advantage thatinclusion of the organosilane immediately occurs during synthesisof the material, without further functionalization steps. Theamount of thiol groups is comparable with the amount of SBA-SH-NEt3 and a high specific surface area is obtained.

The final material examined in this study was a functionalizedethene PMO, reported for the first time. The bromination of thepure trans-ethene PMO39 has already been described by ourgroup and yields almost 2 bromine groups/nm2. The SN2 substitu-tion with a Grignard reagent (Scheme 3) toward mercaptopropyl

groups is not 100% efficient; it yields 0.44 available surface SHfunctions per nm2 as determined by silver adsorption.3.2. Adsorption Experiments. For the development of a

good quality adsorbent, the regeneration ability is a key feature.Only materials which show high stability and do not collapsewhen used are good candidates. Therefore the stability of theadsorbent after a mercury ion adsorption/desorption cycle needsto be studied. In addition to the preservation of the structure, thematerials should keep their functionalities during the adsorptionof mercury ion and the regeneration process. Hence, the numberof functional groups is evaluated before and after the regenera-tion.

Preliminary mercury ion adsorption experiments were per-formed with the four adsorbents to determine the time necessaryfor eachmaterial to reach the equilibrium in an experiment with alow mercury(II) ion concentration. The material was stirred for acertain amount of time in amercury(II) solution.After removal of

Figure 2. N2adsorption/desorption isothermsof the samples. Every timebefore (9) and after three (4)mercury(II) ion adsorption cycles. (a)SBA-SH-HCl, (b) SBA-SH-NEt3, (c) Co-Con, and (d) SH-ePMO.

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the solids, the concentration of the mercury(II) was deter-minedwithCV-AFS.Whenusing a typical concentrationof 10μg/L, complete equilibrium was observed before 15 min for allmaterials.

The recyclability of these materials is of key importance in thetotal evaluation of an adsorbent. In the recycling experiments, alow concentration Hg2þ solution (10 μg/L) is added to theadsorbent and stirred for 15 min. After separation of the solids,the solids are washed several times with a 2 M hydrochloric acidsolution to recover the mercury ion.

These adsorption cycles with the low mercury ion solution of10 μg/L are repeated three times, and every mesoporous materialcould remove >99.99% of the mercury(II) ions. Thus, theiradsorption capacity is comparable after three cycles at 10 μg/Lmercury(II) ion concentration. However, remarkable differencescan be seen when their stability is examined. We will discussseparately the structural stability and the chemical stability of thematerials.

The overall mesoporous structure of the material is examinedwith nitrogen adsorption/desorption and X-ray diffraction mea-surements before and after adsorption. Significant differences canbe observed (Figure 2). Starting with SBA-SH-HCl, the isothermshows a change in structure of the material, where the specificsurface area has increased with approximately 220 m2/g. Thisdemonstrates the unblocking of the pores by removal of thefunctionalities. The SBA-SH-NEt3 material displays a similar

behavior. The specific surface area also increases, yet not asmuchas the SBA-SH-HCl. The isotherm maintains its typical type IVhysteresis, and thus, the material still exhibits its ordered struc-ture. When examining the isotherm of sample Co-Con, a struc-tural collapse can be observed. Whereas the material shows agood ordering before the adsorption experiments, it loses itsstructure upon adsorption and regeneration. For the PMOmaterial, the shape of the isotherm is similar, albeit slightly shiftedto higher volumes of adsorbed nitrogen gas. Consequently, thespecific surface area increases only 30 m2/g. This slight increasemight be caused by the opening of blocked micropores by usinghydrochloric acid during the rinsing step of the adsorptionexperiment. It is known that some micropores can be blockedwith the surfactant P123 since the surfactant extraction45 onlyremoves about 95% of the surfactant.

The XRD patterns of the adsorbents before and after threeregeneration cycles are displayed in Figure 3. The (100), (110),and (200) reflections, characteristic for a P6mm space group, canbe observed in the XRDpatterns of SBA-SH-NEt3 and SBA-SH-HCl. Co-Con possesses a sharp (100) reflection and less pro-nounced (110) and (200) reflections. e-PMO and SH-ePMO bothpossess these characteristic reflections even after performing thebromination and substitution with chloro-propylthiol.

The pattern of all the adsorbents slightly broadens whenmercury ion adsorption and regeneration of the materials areperformed. The (110) and (200) reflections of SBA-SH-NEt3 and

Figure 3. Powder X-ray diffraction patterns of (a) SBA-SH-NEt3, (b) SBA-SH-HCl, (c) Co-Con, and (d) SH-ePMO before and after threeregeneration cycles. The XRD pattern of ePMO is also displayed.

10082 DOI: 10.1021/la100204d Langmuir 2010, 26(12), 10076–10083

Article De Canck et al.

SBA-SH-HCl become smaller and especially the (110) reflection isless pronounced in the XRD pattern after the three regenerationcycles. This is consistent with the nitrogen adsorption desorptionmeasurement. The material made by the one-pot synthesis (Co-Con) does not completely maintain its ordered structure. The(100) reflection broadens after regeneration, and the (110) and(200) reflections disappear completely. The SH-ePMO adsorbenton the other hand preserves its ordered hexagonal structure; onlya slight broadening of the pattern can be observed.

To examine the stability of the PMO adsorbent in strong acidicsolution, thematerial was stirred for 96 h in 2Mhydrochloric acid.After filtering, washing, and drying the adsorbent, the materialwas characterized again to evaluate the stability of its structure.Figure 4 shows the nitrogen adsorption/desorption isotherms.Only a small difference can be seen in the isotherms, the result offurther unblocking of the micropores. This effect is heavilypronounced in the isotherm as the removal of surfactant alsomakes the sample lighter, and these isotherms are always expressedin milliliters of N2 adsorbed per gram of sample. This proves thatthe materials can resist acidic media without loss of structure.

The results of the thiol titration are presented in Table 2.When a comparison is made between the SBA-materials, the co-condensation and the PMO, the differences in the stabilities ofthe functional groups are remarkable. Whereas the periodic

mesoporous organosilica material retains 100% of the thiolgroups, the materials prepared via the postsynthetic route andthe one-pot synthesis both show significant loss of their function-alities. As shown in Table 2, sample SBA-SH-HCl even leaches itsthiol groups for 90%. This can be explained by the sensitivity ofthe siloxane bridges that are responsible for the attachment of thefunctionalities. During the grafting procedure, these bridges arevery sensitive to hydrolysis, especially when thematerial is treatedwith the acidified solution used for regeneration. Samples SBA-SH-NEt3 and Co-Con also suffer from functionality loss,although the loss is smaller. In the PMO, the propylthiol groupis attached to the surface bymeans of a C-C group. The strengthof this bond results in a material which does not leach anyfunctionalities.

The XRD, nitrogen adsorption/desorption measurements andthiol determination results clearly show the sustainability of thethiol functionalized PMO in comparison with the silica materialswhere the functionality is anchored via Si-O-Si bonds. Thestability of PMOs in general is also described by Burleigh et al.and recently by our research group.49,50 Furthermore, additionalmercury(II) ion adsorption experiments were performed to pro-vide a better insight into the adsorption behavior of theultrastableSH-ePMO.

The effects of the initial mercury(II) ion concentration on theadsorption and adsorptivity (% of the Hg2þ adsorbed) are shownin the Figure 5. The adsorptivity decreases with increasing Hg2þ

concentrations, whereas the adsorption capacity increases.The maximal adsorption capacity of the mesoporous adsor-

bent was approximately 64mg/g. This implies a 1:1 stoichiometryof the mercury(II) ion toward the thiol group of the mesoporousmaterial and is consistent with previously reported literature(Scheme 4).20,51,52 After adsorption, the pH of the solution

Figure 4. (left) N2 adsorption/desorption isotherm of SH-ePMO before (9) and after (4) 96 h of stirring in 2MHCl. (right) Powder X-raydiffraction patterns of SH-ePMO before and after 96 h of stirring in 2 M HCl.

Table 2. Characteristics of the Adsorbents after the Adsorption of

Mercury(II) Iona

sampleSBET

(nm2/g)bVp

(cm3/g)cdp

(nm)dmmolSH/ge

leaching(%)

SBA-SH-HCl 510 0.61 3.12 1.00 90SBA-SH-NEt3 614 0.57 3.12 0.76 53Co-Con 416 0.44 2.13 0.85 23SH-ePMO 666 0.61 3.55 0.44 0

aEvery time, three cycles were performed before examining thematerial by nitrogen adsorption measurements and thiol titration.b SBET: the specific surface area calculated via the BET equation. cTotalpore volume. dPore diameter calculated from the adsorption isothermwith the BJH method. eThe amount of thiol groups is determined withthe thiol titration as described earlier.

(49) Burleigh, M. C.; Markowitz, M. A.; Jayasundera, S.; Spector, M. S.;Thomas, C. W.; Gaber, B. P. J. Phys. Chem. B 2003, 107, 12628.

(50) Goethals, F.; Vercaemst, C.; Cloet, V.; Hoste, S.; Van Der Voort, P.;Van Driessche, I. Microporous Mesoporous Mater. 2010, 131, 68.

(51) Mercier, L.; Pinnavaia, T. J. Environ. Sci. Technol. 1998, 32, 2749.(52) Wu, H. Y.; Liao, C. H.; Pan, Y. C.; Yeh, C. L.; Kao, H. M. Microporous

Mesoporous Mater. 2009, 119, 109.

DOI: 10.1021/la100204d 10083Langmuir 2010, 26(12), 10076–10083

De Canck et al. Article

decreased; that is protons were released during the adsorp-tion, confirming the adsorption mechanism, earlier suggested inliterature.

The adsorption capacity as a function of time is shown inFigure 6. The kinetics of the adsorption of mercury(II) ion arebest described by a pseudo-second-order rate.

4. Concluding Remarks

The continuing search for newmetal adsorbents has resulted inthe development of a new functionalized periodic mesoporousorganosilica. The PMO material was compared with adsorbentsprepared via a postsynthetic route starting from SBA-15 and viaone-pot synthesis.

The outcome of modifying an ethene bridged PMO withpropylthiol is an ultrastable adsorbent for the adsorption ofmercury(II) ion. The material keeps its structure after multipleregeneration cycles and maintains its amount of thiol functiona-lities. The hydrochloric acid solution, necessary for the regenera-tion, does not affect the material on any level. The material wascompared with the three other mesoporous silica materials synthe-sized in this study and was proven to be by far the most stablematerial. Even when treated for a longer period in hydrochloricacid, the material sustains its mesoporous structure. All othermaterials (functionalized SBA-15, co-condensedmaterials) quicklylose their structure and/or functionalities during adsorption orregeneration. The thiol functionalized PMO showed a maximaladsorption capacity of 64 mg/g and a 1:1 ratio of Hg2þ/SH.

Acknowledgment. The authors acknowledge the Ghent Uni-versity for financial support. We thank Cindy Claeys and DannyVandeput from our Department of Inorganic and PhysicalChemistry (Ghent University) for performing the nitrogen ad-sorption/desorption measurements and Karen Leus for the XRDmeasurements.

Figure 5. Effect of the initial Hg2þ concentration on the equilibrium Hg2þ adsorption onto SH-ePMO (150 mg) in Hg(NO3)2 solution(50 mL) at 20 �C.

Figure 6. Effect of adsorption time onHg2þ adsorption onto SH-ePMO at initial Hg2þ concentration of 10 (b) and 100 (0) ppm.

Scheme 4. Proposed Adsorption Mechanism of Mercury(II) Ion on

the Mesoporous SH-ePMO Adsorbent