Microporous and Mesoporous Materials 116 (2008) 246–252

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Microporous and Mesoporous Materials

journal homepage: www.elsevier .com/locate /micromeso

Nickel(II) grafted MCM-41: A novel sorbent for the removal of Naproxen from water

Sindia M. Rivera-Jiménez, Arturo J. Hernández-Maldonado *

Department of Chemical Engineering, University of Puerto Rico-Mayagüez Campus, Mayagüez, PR 00681 9046, United States

a r t i c l e i n f o

Article history:Received 30 January 2008Received in revised form 29 March 2008Accepted 1 April 2008Available online 11 April 2008

Keywords:Mesoporous silicaSurface graftingAdsorptionPPCPsWater treatment

1387-1811/$ - see front matter Published by Elsevierdoi:10.1016/j.micromeso.2008.04.009

* Corresponding author. Tel.: +1 787 832 4040x374E-mail address: arturojh@uprm.edu (A.J. Hernánde

a b s t r a c t

MCM-41 type mesoporous silicates were functionalized with nickel species via thermal monolayer dis-persion and grafting techniques, respectively. The material potential to serve as sorbents for the selectiveremoval of a pharmaceutical drug (i = Naproxen) from water was tested via single component adsorptionequilibria measurements at ambient conditions and Ci = 18 ppm. In general, the grafting technique pro-vided the best strategy to avoid metal leaching. Although the anchoring of the nickel species was success-fully accomplished, analysis of the textural properties of the resulting sorbent indicated that aconsiderable reduction in average pore size (ca. 1 nm), and hence surface area, limited the ultimateuptake capacity. Nevertheless, after normalization with respect to accessible surface area, the grafted sor-bents offered capacities comparable to those exhibited by a traditional activated carbon (0.42 vs.0.67 lmol/m2). Ab initio and density functional theory calculations were employed to estimate the surfaceinteraction energies for single sorbent clusters and attempt to describe the adsorption mechanism. Fulloptimization of the structures showed that the sorbate–sorbent interactions were at the weak-chemi-sorption level (�15 kcal/mol), which is suitable for ultrapurification applications and sorbent regenera-tion via simple engineering means. In addition, a natural bond orbital analysis indicated that theinteraction between the Naproxen and the nickel functionality results in a redistribution of electronswithin the most prominent orbitals in the transition metal center (i.e., 4s and 3d). This suggests thatthe interaction between sorbate and sorbent could generate a metal complex, but further studies arerequired to validate this hypothesis and to determine a sorbent regeneration strategy.

Published by Elsevier Inc.

1. Introduction

The occurrence of Pharmaceutical and Personal Care Products(PPCPs) in the environment was first observed in the early 1980s[1]. The accelerated growth population has brought consequentlyan increase in PPCPs demand. To satisfy these needs, pharmaceuti-cals and personal care industries formulate their products withlong-shelf life even after these products have been used, makingPPCPs to be persistent in urban water system [2–4]. Unfortunately,traditional water treatment methods are inadequate for the re-moval of PPCPs resulting in their discharge into surface water[1,5–8]. Therefore, PPCPs have been identified as emerging con-taminants in our potable water supply and aquatic systems [9–11].

Current initiatives are focused on the possible removal of PPCPsusing adsorption methods, which could be easily implemented incurrent water treatment facilities. To date, there are only a few ac-counts of sorbents tested to address this or similar problems. Forinstance, activated carbon was screened for the removal of a se-lected list of PPCPs under deionized water conditions and at ppmlevels [1]. The observed uptake capacity could be attributed to

Inc.

8; fax: +1 787 834 3655.z-Maldonado).

van der Waals and electrostatic interactions [12]. However, in pilotscale experiments, the working capacity of activated carbon greatlydecreases in the presence of other natural organic substances, indi-cating considerable competition for adsorption sites. A number ofmembranes [13] and polymers [14] were also tested for theadsorption of various PPCPs. These studies concluded that their re-moval performance was highly dependent on the ionic form of thecompounds in the polar environment, suggesting that the selectedPPCPs were physisorbed, probably due to electrostatic and dipole–dipole interactions. It should be mentioned that some tests per-formed with silica, c-alumina and Parapak-P (non-polar organicmedium) showed that pH, hydrophobicity level of the pharmaceu-tical, and the nature of the surface charge, are important variablesto be considered during the adsorption process [15].

Considering that current PPCP levels are still well below theparts-per-million (ppm) mark, ultrapurification of water sourcesvia adsorption could only be achieved with materials based onstronger reversible chemical interactions. Evidently, the sorbentswill need to showcase a hydrophobic nature as well as large sur-face areas and pore volumes. These characteristics will producesuitable operation cycles and fewer regeneration steps. The currentwork focuses on the preparation, characterization and preliminarytesting of a potential sorbent material strategy to address the

S.M. Rivera-Jiménez, A.J. Hernández-Maldonado / Microporous and Mesoporous Materials 116 (2008) 246–252 247

previously mentioned requirements. Although the adsorption per-formance test will be limited to a single PPCP component in purewater, the results will serve as evidence of the strategy potentialto tailor the surface interactions for other sorbates/contaminantsand more complex matrices.

The sorbent material tested was a mesoporous silica-rich MCM-41 substrate functionalized with a nickel(II) complex (Ni(II) com-plex). A few investigations have reported employing mesoporousmaterials for organic adsorption in aqueous solution [16–19], butno previous studies have reported the use of MCM-41 for theadsorption of any PPCPs from aqueous solutions. Due to its highsurface area, pore volume, accessible pore channel and simple porechemistry, MCM-41 has been used as a model material for theincorporation of specific functionalities onto its surface in orderto add or increase the selectivity towards a target compound[18,20–28]. As described below, the incorporation of nickel(II)complexes onto the MCM-41 surface was accomplished via ther-mal monolayer dispersion and grafting techniques, respectively.The latter has proven to be an effective way of anchoring metalcomplexes, such as nickel, palladium, manganese, vanadium, andcopper onto the surface of siliceous materials and thereforediminish the potential leaching of the metal centers when exposedto aqueous phase processes [29].

2. Experimental

2.1. Materials

The following reagents were used for the synthesis and surfacemodification of MCM-41: cetyltrimethylammonium chloride(CTACl 25 wt%, Aldrich), hydrochloric acid (HCl, 37 wt%), tetraethylorthosilicate (TEOS 98wt%, Aldrich), ammonium hydroxide (30wt%,Fisher), nickel(II) chloride (98%, Aldrich), nickel sulfate hexahy-drate (Puris Reagent ACS, Riedel-de Haën), and anhydrous toluene(99.8%, Aldrich). For the adsorption experiments, Naproxen (Al-drich) and activated carbon (NORIT SX ULTRA 94029-5) were used.The latter was tested to obtain a performance benchmark baseline.All water used was distilled and deionized.

2.2. Synthesis of pure siliceous MCM-41

Pure siliceous MCM-41 was synthesized at room temperatureusing the liquid crystal template method reported in the literature[30]. Typically, CTACl surfactant (76.1 ml) was dissolved in 19.3 mlof aqueous hydrochloric acid under slow stirring. TEOS (21.41 ml)was added and the mixture was stirred for 75 min at room temper-ature. Subsequently, a 2 wt% ammonia solution was added drop-wise until the pH of the mixture was adjusted to ca. 4. The solidmaterial was then filtered, washed with copious amounts of deion-ized water, and dried overnight in a forced convection oven at100 �C. The surfactant template was removed via calcination inair at 550 �C for 6 h with a heating rate of 2.5 �C/min. The resultingmaterial was denominated MCM-41.

2.3. Metal incorporation onto the support surface

2.3.1. Spontaneous thermal monolayer dispersionA typical thermal monolayer dispersion procedure was started

by finely dividing and mixing nickel(II) chloride salt and MCM-41(BET SA � 1100 m2/g) at a weight ratio determined by the amountof metal required to form a monolayer coverage over the BET mea-sured support surface area [12,31,32]. The solid mixture was care-fully heated under nitrogen atmosphere to 400 �C, a temperaturethat lies between the Tammann temperature [31] and the meltingpoint of the salt. Due to the slow solid phase transport, this thermal

treatment was done for five consecutive days to ensure completedispersion of the metal salt over the support. The resulting mate-rial was denominated NiCl2_dm_MCM-41.

2.3.2. Metal immobilization over an amino-organic modified surface[29]

For the chemical incorporation of organo-amine molecules,MCM-41 (BET SA � 1100 m2/g) was pretreated in air at 110 �C for4hrs to remove any physisorbed water molecules. Then, 3 g ofthe dehydrated material were suspended in 75 ml of dry tolueneand stirred at room temperature for 1 h. An excess amount of 3-aminopropyltriethoxysilane in 25 ml toluene was added drop-wiseto the slurry and stirred under reflux at 100 �C for 24 hrs. Finallythe material was filtered, washed with dry toluene, and Soxhlet-extracted using a mixture of 100 ml of diethyl ether and 100 mldichloromethane for 24 h. The final solid was dried overnight inair at 90 �C. For the immobilization of Ni(II) complexes, 1.5 g oforganoamine-functionalized silica was mixed with a 0.02 M alco-holic solution of nickel(II) sulfate for 18 h at 60 �C under nitrogenatmosphere. The mixture was filtered, washed with copiousamounts of ethanol and then Soxhlet-extracted with ethanol for12 h. The latter step removed any unanchored Ni(II) complex. Fi-nally, the solid was dried in air at 80 �C for 3 h. This material wasdenominated NiNH2_g_MCM-41.

2.4. Characterization

Powder X-ray diffraction patterns were collected on a RigakuUltima III system using CuKa radiation (k = 1.5418 Å) and operatingat 40 kV and 44 mA. The data were recorded in the 2h range be-tween 1� and 7�, at a scanning rate of 0.2�/min. Nitrogen adsorp-tion isotherms at �196 �C (77 K) were measured using aMicromeritics ASAP 2020 volumetric porosimetry test instrument.All samples were activated for 3 h in situ at 200 �C under vacuumconditions prior to each adsorption test. The surface area was cal-culated using the Brunauer–Emmett–Teller (BET) isotherm model,while the Pore-Size Distribution (PSD) and pore volume were ob-tained by applying the Barret–Joyner–Halenda (BJH) pore analysisto the adsorption branch of the isotherms data. SEM micrographswere obtained using a JEOL-JSM-6930 scanning electron micro-scope operating with an accelerating voltage of 10 kV. Thermo-gravimetric Analyses (TGA) were done on a TA-Q500 system toquantify the amount of organosilane moieties present on theNiNH2_g_MCM-41 sorbents. The results were compared to theoret-ical values obtained assuming monolayer formation over the sup-port’s surface. For these experiments, the solid samples wereplaced in platinum holders and heated from 25 �C to 900 �C at aheating rate of 5 �C/min in air.

FTIR spectra of the solid samples were taken using a Varian 800FT-IR Scimitar Series instrument in the range of 4000–400 cm�1.The spectra were acquired at 4 cm�1 resolution and averaged over200 scans. For single point adsorption experiments, the equilib-rium concentration remaining in the aqueous phase was measuredusing a Shimadzu UV-2401 PC UV/visible spectrophotometer usingthe corresponding characteristic wavelength. The molecular struc-ture and the UV–visible spectra for Naproxen are shown in Fig. 1.

2.5. Adsorption experiments

The aqueous phase single point adsorption capacities for Na-proxen in MCM-41, NiNH2_dm_MCM-41, and NiNH2_g_MCM-41sorbents were obtained following procedures described elsewhere[1,18,20]. Briefly, the uptake experiments were performed by add-ing 150 mg of the adsorbent to 200 ml of an aqueous solution con-taining an initial concentration of 18 ppm Naproxen in pure water.For activated carbon, 10 mg of the adsorbent were added to 100 ml

Fig. 1. UV–vis spectrum for Naproxen in water at 18 ppm.

Fig. 2. Powder X-ray diffraction data.

248 S.M. Rivera-Jiménez, A.J. Hernández-Maldonado / Microporous and Mesoporous Materials 116 (2008) 246–252

of an aqueous solution of Naproxen with an initial concentration of18 ppm. Such concentration is high enough to allow one to tracethe sorbate using UV–visible techniques. The mixtures were thenplaced in a water bath shaker (operated at 75 rpm) and allowedto equilibrate at a constant temperature of 25 �C for ca. 24 h. Atthe end of the adsorption experiment, triplicate samples weretaken and filtered using 0.45 lm, 13 mm diameter syringe filtersto remove any solid particles. The equilibrium Naproxen aqueousphase concentration was measured using UV/visible spectroscopyas described before. Since each sorbent had a particular surfacearea (and density), the loading data were normalized using theBET surface area of the corresponding adsorbent.

2.6. Computational methods

Molecular orbital studies on reaction pathways [33] andadsorption mechanisms [34,35] of pharmaceuticals drugs havebeen studied before. In this work, the adsorption of Naproxen ontoNiCl2 and Nickel attached to the amino-organic moiety was stud-ied. All geometry optimizations and natural bond orbital (NBO)analyses were performed using Gaussian 03 [36] running on theNational Center for Supercomputing Applications (NCSA) at theUniversity of Illinois, Urbana-Champaign. The optimized structureswere used for adsorption energy calculations according to theequation:

Eads ¼ Eadsorbate þ Eadsorbent � Eadsorbent—adsorbate

where Eadsorbate is the minimum energy of the free adsorbate,Eadsorbent is the energy of free adsorbent, and Eadsorbent–adsorbate isthe energy of the adsorbate/adsorbent pair. Values of Eads lower that10 kcal/mol and above 20 kcal/mol correspond to physisorption andchemisorption, respectively. However, Eads values that fall betweenthose limits are considered to be weak chemisorption level surfaceinteractions, which are suitable for adsorption applications requir-ing more selectivity without compromising the sorbent regenera-tion options [12].

The natural bond orbital (NBO) analysis is a useful tool to de-scribe the electron distribution in metal containing compounds[37], especially for calculating localized weak interactions, suchas weak chemisorption and charge transfer. The NBO programavailable in Gaussian 03 was used to obtain an insight and under-standing of the possible adsorption mechanism pathways for theremoval of Naproxen in the presence of single Nickel clusters.Although the actual surface exhibits closely packed nickel-based

species, the use of representative, simplified clusters should helpin determining interaction tendency and order.

Ab initio molecular orbital (MO) calculations were used in theoptimization of the NiCl2 cluster model. Both geometry optimiza-tion and NBO analysis were performed at the restricted Hartree–Fock (HF) level and using the LAND2DZ basis set. This approachhas been used before to describe the molecular properties of tran-sition metal interactions [38]. Density functional theory (DFT) wasused instead for the optimization of the NiNH2CH3 cluster as re-ported before for similar systems [39–41]. The geometry optimiza-tion and NBO analysis for the amine-based system was performedusing the hybrid Becke’s three parameter exchange functional andthe gradient corrected functional of Lee, Yang, and Parr (B3LYP)[39–41]. All computations were performed with the standard basisset 6-31+G(d,p). The B3LYP method, also known as the self-consis-tent hybrid (SCH) approach, provides reliable property calculationssuch as geometric, and covalent, or weak non-covalent interac-tions. Also, the choice of the basis set was properly justified sinceit adds polarization functions to heavy metal atoms, adds diffusefunctions for systems with unpaired electrons, and it is accuratefor energy calculations.

3. Results and discussion

3.1. Powder X-ray diffraction and SEM

Fig. 2 shows well-defined X-ray diffraction peaks for MCM-41 inthe (100), (110), and (200) planes, typical of mesoporous materialwith hexagonal long-range arrays. A 2.9 nm average pore diameterwas calculated using interplanar data [42,43]. The XRD patternsand SEM images (Figs. 2 and 3) displayed evident differences inthe textural properties of the samples prior to and after functional-ization. A decrease in the intensity of the (100) peak and conse-quential loss of long-range order form can be observed (Fig. 2) inboth the NiCl2_dm_MCM-41 and NiNH2_g_MCM-41 samples. How-ever, the functionalization performed using the spontaneousmonolayer dispersion technique retains the mesopore structurecharacteristics, as shown by X-ray diffraction peaks and the rope-like particle morphology, respectively. The prominent reductionin the (100) peak intensity in the grafted samples may be attrib-uted to a partial collapse of the long range order of the materialdue to poor hydrothermal stability or to the flexibility of the sili-ceous framework produced by the stress applied by the anchoredmetal complex [29,44–47]. However, after calcination tests to

Fig. 3. Scanning electron micrographs for (A) MCM-41, (B) NiCl2_dm_MCM-41, and(C) NiNH2_g_MCM-41 sorbents.

S.M. Rivera-Jiménez, A.J. Hernández-Maldonado / Microporous and Mesoporous Materials 116 (2008) 246–252 249

remove the amino organic moiety anchored on MCM-41 surface,textural properties such as surface area increased by 33% whencompared to the uncalcined material. This is probably due torecovery of pore volume upon release of the organic parts.

3.2. FTIR spectroscopy and TGA

Infrared spectroscopy was used to characterize the surfaceproperties of silica-based samples. Several bands related to the sur-face and grafting species were observed (Fig. 4). For instance, the1070, 812 and 975 cm�1 bands correspond to the asymmetric Si–

O–Si vibrations, symmetric vibrations on the surface, and Si–OHvibrations, respectively. The IR data also suggest the presence ofsilanol groups within the MCM-41 pore channel system as evi-denced by a broad band in the 3600–3200 cm�1 range. After graft-ing of the amino-organic moiety, weak bands at 1560, 1650 and1747 cm�1 were observed due to NH2 vibrational modes [48,49].Also, the presence of a medium band appears at 1412 cm�1 possi-bly due to the Si–CH2 bending vibrations and it is maintain afterthe anchoring of the Ni species. In addition, bands at 2945 cm�1

and ca. 1200 cm�1 appear after grafting suggesting the presenceof different vibrational modes of C–H. These results confirm theanchoring of the aminopropyl species on the surface of the support.After the incorporation of the Ni groups, the bands assigned to theamine are shifted to lower wavenumber values suggesting theanchoring of the Ni(II) complexes to the active sites [47,50]. Bandsbetween wavenumbers of 3000 and 2900 cm�1 can be attributed tothe different vibrational modes of the CH groups present in theaminopropyl moiety. Bands that appear in all samples betweenthe wavelengths of 2400 and 2300 cm�1 are assigned to the pres-ence of CO2 during the sample collection.

TGA analysis provided a simple method to estimate the amountof silanes and weakly adsorbed water in the sorbents. For theMCM-41 sample (Fig. 5), a single weight loss region was observedbelow 100 �C, corresponding to the loss of weakly physisorbedwater inside the pore channels of the materials [51,52]. The ab-sence of further weight losses at higher temperatures confirmedthat all the surfactant moieties where removed from within thechannels during the calcination step. The TGA spectrum ofNiNH2_g_MCM-41 shows a sharp weight loss below 100 �C (stepI, Fig. 5), attributed to the water adsorbed inside the pores of themesoporous material. In addition, two other steps can be observedbetween 300 �C and 350 �C (step II), and 500 �C and 600 �C (stepIII). These two steps could be attributed to the decomposition ofthe amino-organic groups from the pore surface and the formationof the siloxane groups on the surface [29,53]. The overall weightloss was 36 wt%, which matches well with the theoretical amount(35 wt%) calculated assuming the formation of a monolayer overthe available substrate surface area.

3.3. Porosimetry tests

The equilibrium isotherms for the adsorption of N2 at �196 �Care shown in Fig. 6. The isotherms shape conforms to type IVaccording to the IUPAC classification, with no evidence of hystere-sis. For MCM-41, the equilibrium adsorption data yield a BET sur-face area and pore diameter of 1198 m2/g and 2.7 nm, respectively.Textural properties such as surface area, pore volume, and porewidth of other samples are summarized in Table 1. It should bementioned that mesoporous materials with pores measuring lessthan 4 nm in diameter usually do not exhibit a hysteresis behavior[43,54]. As shown in Fig. 6, the functionalization of the calcinedMCM-41 has a considerable effect on the original substrate surfacearea, as evidenced by the prominent reduction in the N2 adsorptioncapacity. For both spontaneous monolayer dispersion and graftingtechniques, the modification of the textural properties of the func-tionalized materials was attributed to the incorporation of newspecies inside the pores of the support. The pore volume (�99%)and surface area (�97%) reduction in NiNH2_g_MCM-41 samplecould be a consequence of the incorporation of large amino-organ-ic moieties inside the pore during the immobilization of the Ni(II)complex [29]. Theoretical calculations of the resulting pore widthwere performed assuming monolayer formation after the function-alization and using a BJH pore width (2.7 nm). For the deposition ofthe NiCl2 molecule (�0.3 nm), a close-packed monolayer modelcalculation indicates that the effective pore size should be reducedto ca. 2.1 nm, which matches well with the experimental datum.

Fig. 6. Nitrogen adsorption equil

Fig. 5. Thermal gravimetric analyses data.

Fig. 4. Fourier transform infrared spectra.

250 S.M. Rivera-Jiménez, A.J. Hernández-Maldonado / Microporous and Mesoporous Materials 116 (2008) 246–252

Similar calculations for the immobilization of the amino-organicgroups (�0.7 nm) indicated that the effective pore width shouldbe reduced to ca. 1 nm, which also matches well with the observedpore volume and surface area reductions. A 1 nm pore diameter isstill large enough to allow passage and adsorption of Naproxen sor-bate molecules onto the surface of the material.

3.4. Sorbent single-component adsorption performance

The normalized adsorption capacities obtained for the removalof Naproxen at 25 �C from water are shown in Fig. 7. For compar-ison, this figure also includes the adsorption capacity of activatedcarbon and MCM-41. It is evident that the NiNH2_g_MCM-41 sor-bent had a larger adsorption capacity per surface area(0.42 lmol/m2) when compared to both NiCl2_dm_MCM-41 andMCM-41 materials. The low adsorption capacity obtained for thematerial modified by spontaneous thermal monolayer dispersioncould be due to leaching of the metal salt in the aqueous mediumafter 24 h. In fact, UV–visible data (not shown here) showed a largeband corresponding to nickel ions dissolved in water during theadsorption tests.

The enhanced adsorption capacity observed for the NiNH2_g_MCM-41 sorbent probably resulted from covalent anchoring ofthe nickel functionality to the amino-organic moiety, leading tothe formation of exposed adsorption sites. Experiments for the re-moval of Naproxen using powdered activated carbon (SBET =1030 m2/g), a well known and commercial hydrophobic material,showed an adsorption capacity of 0.67 lmol/m2, which compareswell with the NiNH2_g_MCM-41 observed adsorption capacity.Although other sorbates and substrates are yet to be explored,the present results clearly support the potential of the metal graft-ing technique and provides a novel alternative to the design ofselective sorbents for the removal of emerging pharmaceuticalcontaminants in water.

3.5. Adsorption energy and NBO calculations

Ultra-deep-purification of water via adsorption can be achievedwith materials designed based on strong reversible chemical inter-actions energies between 10 and 20 kcal/mol. Interactions withinthis range are usually selective to the target compound and revers-ible enough to enable the material to be reactivated by simple

ibrium isotherms (�196 �C).

Table 1Summary of the porosimetry data for the sorbents

BET surfacearea (m2/g)

BJH pore volume(cm3/g)

BJH pore width(nm)

MCM-41 1198 1.138 2.7NiCl2_dm_MCM-41 600 0.135 1.9NiNH2_g_MCM-41 40 0.006 NdActivated carbon 1030 0.45 4.2

Fig. 7. Normalized adsorption capacities for the removal of Naproxen (Ci = 18 ppm)from water at 25 �C. Fig. 8. Optimized structures for Naproxen on (A) NiCl2 and (B) NiNH2CH3 repre-

sentative sorbent clusters, respectively.

S.M. Rivera-Jiménez, A.J. Hernández-Maldonado / Microporous and Mesoporous Materials 116 (2008) 246–252 251

engineering means [55]. Fig. 8 shows the optimized sorbate–sor-bent pairs while Table 2 summarizes the results obtained for thecomputational part of this work. It is very important to point outagain that the sorbent clusters used are based on single nickel-based species. In reality, the surface would be covered by a mono-layer of nickel species and this should result in Naproxeninteractions with multiple sites. As such, the results to be discussedshould be regarded as an initial approximation to the qualitativebehavior of the system.

Surprisingly, the interaction of both adsorbate–sorbent systemsare close to the 15 kcal/mol, suggesting that the adsorption processusing metal cations can provide a good alternative for targetingpharmaceutical compounds at ppm levels in water streams. How-ever, experimental results using NiCl2 salt thermally monodi-spersed over the support do not show the expected behavior,probably due to leaching of Ni complexes in the aqueous system.The anchoring of the Nickel complexes using an amino-organicmoiety seems to overcome the leaching problem and provides with

Table 3Changes in NBO electron occupancies of Ni

Eads (kcal/mol) Changes in NBO electron occupa

D4s D3dxy

Naproxen–NiCl2 16.562 0.0679 �0.0332Naproxen–NiNH2CH3 14.604 �0.3938 0.2087

Table 2Electron occupancies in the valence orbitals of Ni

Representative sorbent cluster 4s 3dxy

NiCl2 0.1993 1.9999NiNH2CH3 0.9333 1.7391

a novel adsorbent for the removal of pharmaceutical compoundsfrom water.

NBO analysis was performed to gain insight on the adsorptionmechanism during the process. Relevant electron occupancies be-fore and after adsorption are shown in Table 3. From this we canobserve that the electron occupancy of the 4s orbital of Ni in NiCl2

cluster increases, whereas the total occupancy in its 3d orbital de-creases. These results indicate a plausible p-complexation mecha-nism due to the r-donation from the p-bond of Naproxenmolecule to the 4s orbital of Ni2+, and the d–p backdonation fromthe 3d orbitals of the metal to the p-antibond orbitals of the sor-bate [12]. However, the larger net electron population increase inthe 4s orbital of Ni as compared to the 3d orbital indicates thestrength of the forward r-donation. For the NiNH2CH3 cluster,the electron occupancy of the 4s orbital is clearly smaller whencompared to that of the NiCl2 cluster. Yang and co-workers sug-gested that the electro-negativity of the anion closer to the metal

ncies in outer shell of Ni atom

D3dxz D3dyz D3dx2�y2 D3dz2

PD3d

�0.6194 �0.6209 �0.1311 1.4086 0.0041�0.0142 �0.0868 �0.0023 0.0495 0.1549

3dxz 3dyz 3dx2�y2 3dz2

1.9991 1.9991 1.9999 0.20311.9830 1.9994 1.6815 1.8095

252 S.M. Rivera-Jiménez, A.J. Hernández-Maldonado / Microporous and Mesoporous Materials 116 (2008) 246–252

could very well explain the observed reduction of the occupancy in4s orbital [37,56,57]. This tendency is observed in Table 2, wherethe electron occupancy of the 4s orbital of the NiNH2CH3 is smaller,probably due to the amine group, which is a larger anion whencompared to Cl�. As mentioned above, the adsorption of Naproxenonto NiNH2CH3 can be classified as weak chemisorption, howeverthe anchoring of the nickel in the amino organic moiety clearlyinfluences the final complex electronic configuration.

In general, the theoretical calculations indicate that the sor-bate–surface interactions form metal complexes based on p and/or r-type electron donations. Although more studies and probablylarger sorbent clusters are necessary to validate this proposedmechanism, it seems plausible to assume that these materialscould be easily regenerated via temperature swings. Studies madeby Yang and Hernández-Maldonado on the purification of com-mercial fuels via complexation with transition metal based zeoliteshave proven that such regeneration strategies are suitable for suc-cessful sorbent regeneration [58].

4. Conclusions

The present study indicates that the inclusion of a transitionmetal functionality onto the surface of MCM-41 enhances thematerial performance during the adsorption of Naproxen fromaqueous medium. Although the two surface functionalizationstrategies employed enhanced the surface interactions with Na-proxen sorbates, the nickel grafting option clearly overcame anymetal leaching due to the aqueous environment. Preliminary per-formance tests with the nickel anchored via an amino-organic moi-ety onto the surface of MCM-41 show that such strategy holdspromise for the adsorption of PPCPs from aqueous solution, espe-cially those present at ppm levels. According to ab initio DFT calcu-lations, this is plausible due to weak–chemical interactions at thesurface level. The theoretical calculations also indicate that theinteraction between the Naproxen and the nickel functionality re-sults in particular distributions of electrons within the most prom-inent orbitals in the metal center. This suggests that the interactionbetween sorbate and sorbent will generate a metal complex, butfurther studies will be necessary to validate this hypothesis anddetermine a viable sorbent regeneration strategy.

Acknowledgments

This work was supported by the National Science FoundationAwards CBET-0546370 and CBET-0619349. Support from the NSF-EPSCoR Puerto Rico Institute for Functional Materials is also grate-fully acknowledged. We wish to also acknowledge Professor CarlosRinaldi (UPRM Chemical Engineering) for providing access to FTIRand UV–visible analyses, and Professor Carlos Velázquez for provid-ing access to the SEM equipment. Computational resources wereprovided by the National Center for Super Computer Applications(Award CTS060048), University of Illinois at Urbana-Champaign.

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