Ptm

SPD

h

•

•

•

•

ARRAA

KISTSM

0h

Journal of Hazardous Materials 260 (2013) 313– 322

Contents lists available at SciVerse ScienceDirect

Journal of Hazardous Materials

jou rn al hom epage: www.elsev ier .com/ locate / jhazmat

reparation and characterization of 5-sulphosalicylic acid dopedetraethoxysilane composite ion-exchange material by sol–gel

ethod

uhail-ul Rehman, Nasarul Islam, Sozia Ahad, Syed Zeeshan Fatima, Altaf Hussainandith ∗

epartment of Chemistry, University of Kashmir, Hazratbal, Srinagar 190 006, J&K, India

i g h l i g h t s

Sulphosalicylic acid dopedtetraethoxysilane composite isprepared by sol–gel method.Its X-ray diffraction studies suggestthat it is crystalline in nature.This material shows selectivity forMg(II) and Ni(II) ions in aqueous solu-tions.Separation of Ni(II) from binary mix-tures was successfully achieved onthis material.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

rticle history:eceived 17 December 2012eceived in revised form 20 May 2013ccepted 21 May 2013vailable online 27 May 2013

eywords:on exchangersilicatesetraethoxysilane

a b s t r a c t

In this manuscript, we report the preparation and characterization of sulphosalicylic doped tetraethoxysi-lane (SATEOS), composite material by sol–gel method as a new ion exchanger for the removal of Ni(II) fromaqueous solution. The fine granular material was prepared by acid catalyzed condensation polymerizationthrough sol–gel mechanism in the presence of cationic surfactant. The material has an ion exchange capac-ity of 0.64 mequiv./g(dry) for sodium ions, 0.60 mequiv./g(dry) for potassium ions, 1.84 mequiv./g(dry)for magnesium ions, 1.08 mequiv./g(dry) for calcium ions and 1.36 mequiv./g(dry) for strontium ions. ItsX-ray diffraction studies suggest that it is crystalline in nature. The material has been characterized bySEM, IR, TGA and DTG so as to identify the various functional groups and ion exchange sites present inthis material. Quantum chemical computations at DFT/B3LYP/6-311G (d,p) level on model systems were

ulphosalicylic acidaleic acid

performed to substantiate the structural conclusions based ion instrumental techniques. Investigationsinto the elution behaviour, ion exchange reversibility and distribution capacities of this material towardscertain environmentally hazardous metal ions are also performed. The material shows good chemicalstability towards acidic conditions and exhibits fast elution of exchangeable H+ ions under neutral con-ditions. This material shows remarkable selectivity for Ni(II) and on the basis of its Kd value (4 × 102 in0.01 M HClO4) some binary separations of Ni(II) from other metal ions are performed.

© 2013 Elsevier B.V. All rights reserved.

∗ Corresponding author. Tel.: +91 194 2424900; fax: +91 194 2421357.E-mail address: altafpandit23@gmail.com (A.H. Pandith).

304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2013.05.036

1. Introduction

The ever increasing economic growth and industrialization hasushered the human civilization into a new era of consumerismand urbanization, thereby posing a severe threat to our environ-ment and ecology. The unabated pollution of our water bodies,

3 ardou

pmwaCrfipaesepNapdscttctammchtiasm

ssmeonmvehihfihdshisonasni

mipta

14 S.-u. Rehman et al. / Journal of Haz

articularly, warrants the development of efficient and cheapethods of analysis and monitoring of pollution levels of theseater bodies. Ions of heavy metals such as nickel, copper, lead,

nd zinc have a significant impact on our aqueous environments.ontamination of aquatic media by heavy metals is a serious envi-onmental problem, mainly introduced into water bodies in theorm of effluents from nickel plating plants, silver refineries, zincndustries, storage batteries and oil industry [1–3]. Nickel com-lexes are mostly soluble in water and, therefore, may be readilybsorbed into living organisms. Nickel is the fifth most abundantlement in the earth’s crust by weight after iron, oxygen, magne-ium and silicon, comprising about 3% of the composition of thearth [4]. More recently, nickel is reported to be used in nuclearower plants, gas turbine engines and cryogenic containers [5].ickel is a potent carcinogen and causes cancer in lungs, nose, stom-ch and bone. Prolonged contact of skin with nickel can cause a veryainful disease known as nickel itch which may result in suddeneath [6]. Several methods such as evaporation, electro-deposition,olvent extraction, reverse osmosis, membrane separation pro-ess and activated carbon adsorption have been employed forhe removal of heavy metal ions from wastewater [7]. Conven-ional methods for the removal of Ni(II) from wastewaters includehemical precipitation, chemical reduction, flocculation, filtra-ion, evaporation, solvent extraction, biosorption, activated carbondsorption, ion-exchange, reverse osmosis, electro-dialysis andembrane separation processes. The chemical precipitation is theost cost-effective treatment technology. The possibility to pre-

ipitate metals in the form of insoluble compounds, mostly metalydroxides, in solutions containing complexing agents depends onhe stability constant of the complex and the hydroxide solubil-ty product [8]. A broad range of biomass types including bacteria,lgae, yeast, fungi, activated sludge, anaerobic sludge, digestedludge, peat have also been used as bio-sorbents to remove Ni(II)etals from aqueous solution [9–16].Over the years, hybrid materials have been the objects of con-

iderable interest, because of their excellent chemical and thermaltability and their potential application in the field of environ-ental analysis and monitoring. Their use in the field of ion

xchange, intercalation, catalysis, ionic conductivity and recoveryf domestic and industrial wastes from aqueous systems stimulatesew research on this class of compounds. Intercalation of organicolecules/polymeric materials into an inorganic host matrix pro-

ides new class of hybrid ion exchangers with enhanced ionxchange capacity, high stability, reproducibility, and selectivity foreavy metal ions. These materials may possess improved mechan-

cal strengths, chemical inertness, reproducibility, selectivity andigher stability at elevated temperatures and ionizing radiationelds. Fibrous and membranous hybrid ion exchange materialsave found wider applications in chemical analysis and can be pro-uced in many forms such as thin films, conveyer belts and as ionelective electrodes. Recently, organic–inorganic hybrid materialsave attracted much interest as intercalation and non-intercalation

on-exchangers and sorbents for applications in chemical analysis,eparation processes, and waste cleanup under harsh conditionsf pH, temperature and ionizing radiation fields [17–21]. Largeumber of hybrid materials using organic moieties such as n-butylcetate [22], acrylonitrile [23], acrylamide[24], polyaniline [25],tyrene [26], triethylammonium [27], polymethylacrylate [28],ylon 6-6 [29,30] and EDTA [31] have been synthesized and used

n separation, purification and detection of heavy metal ions.Organic–inorganic hybrid materials prepared by the sol–gel

ethod have rapidly become a fascinating new field of research

n material sciences. The explosion of activity in this area in theast decade has made tremendous progress in both the fundamen-al understanding of the sol–gel process and the development andpplications of new organic–inorganic hybrid materials [32–34].

s Materials 260 (2013) 313– 322

The advantage of sol–gel process is that tailor made size, shape,and charge selective materials having specialized applications inchemical analysis can be easily prepared by simple manipulationof reaction conditions. The sol–gel process has so far been one ofthe most attractive ways of synthesizing these porous materialsbecause of the mild synthesis conditions. Ion-exchange sites canbe incorporated into the inorganic skeleton for the preparationof highly selective sorbents for preferential uptake of metal ions.The mild reaction conditions provide the possibility to incorporatevarious functionalized moieties and molecules such as proteins,enzymes, dyes, organic and organo-metallic reagents into the inor-ganic host matrix [35–40]. This is achieved by doping the reagentinto the sol prior to its gelation [41] or by using organo-siliconderivatives [42–44]. The materials with enhanced chemical andmechanical stabilities, better optical properties, controlled poresize, and pore size distribution, surface area, and polarity can beprepared in a variety of sizes and shapes, including thin films,monoliths, fibres and powders [45]. In recent past, many newadsorbent and ion exchange materials have been developed for theremoval of heavy metal ions such as lead, cadmium and mercuryfrom wastewaters [46–48]. However, to our best information, nomajor works are reported in the literature regarding the prepara-tion of hybrid ion-exchangers/adsorbents for selective removal ofNi(II) from aqueous solutions.

Therefore, the main motivation of this work has been to synthe-size cheap and cost-effective crystalline hybrid composite material,by using the simple synthetic methods, for the removal of nickelfrom drinking and industrial wastewaters. By using sol–gel method,we have synthesized a new composite material by incorporat-ing 5-sulphosalicylic acid into tetraethoxysilane matrix, which iscrystalline in nature and shows improved functionalities in termsof ion-exchange and adsorption behaviour. The material is a rel-atively low cost ion-exchanger/adsorbent having promising ionexchange/adsorption characteristics. We find that the material ishighly selective for Ni(II) absorption from aqueous solutions and,therefore, may have potential applications in the recovery andremoval of Ni(II) from aqueous media

2. Materials and methods

2.1. Materials

Tetraethoxysilane and hexadecyltrimethylammonium chloridewere obtained from Merck, Germany. Sulphosalicylic acid andethyl alcohol were obtained from Glaxo Laboratories, India. Otherreagents and chemicals were of analytical grade and were usedwithout any further purification.

2.2. Instrumentation

pH measurements were performed using an Eutech InstrumentPC 5500. X-ray diffraction spectra were recorded on a Bruker AXSD8 Advance diffractometer. Analysis of C, H, S and N were deter-mined on an Elemental Vario Micro CHNS analyser. IR spectrawere recorded on Interspec-2020 FTIR spectrophotometer. Elec-tron micrographs were recorded with a Hitchi-S3000H scanningelectron microscope. An incubator shaker Yellow Line OSC with atemperature variation of ±0.5 ◦C was used for equilibrium studies.Exstar 6000 TGA/DTG instrument from SIINT, Japan was used forthermal studies.

2.3. Synthesis of the ion exchange material

A number of samples of the composite ion exchange materialwere prepared from silica sol using sol–gel method. The silica solwas prepared from tetraethoxysilane, deionised water, ethanol,

ardou

haTlwtwcTaaftmv

2

mdNpoat(gpw

2

phatepoHa1ws

2

N0scswavois

2

N

frequencies. To obtain the real time spectral frequencies, all the

S.-u. Rehman et al. / Journal of Haz

exadecyltrimethylammonium chloride keeping the molar ratios 4:200:50:1 respectively and using 0.01 M HCl as the catalyst.he resulting solution was heated at 35 ◦C with continuous stirringeading to the formation of a gel. Prior to gelation, organic dopants

ere physically doped into the sol. The gelation time was main-ained at 24–36 h. The gel was dried in an oven at 50 ◦C. The productas then grounded and studied as xerogel powder. The xerogel was

onverted into the H+ form by treating with 1.0 M HNO3 for 24 h.he material obtained was finally washed with deionised waternd then dried at 45 ◦C. On the basis of Na+ ion exchange capacity,ppearance and percent yield, sample no. 22 (Table S1) was selectedor further studies. The reproducibility was checked by preparinghe samples several times following the same procedure and deter-

ining the ion exchange capacity of the material every time, whicharied negligibly.

.4. FTIR, XRD, thermal and microscopic analyses

FTIR spectra of the SATEOS samples were recorded by KBr discethod. X-ray powder diffraction patterns were obtained using a

iffractometer (Bruker AXS D8 Advance) at 40 kV and 40 mA withi-filtered Cu-K� radiation of wavelength (� = 1.54056 A). The XRDatterns were recorded over the range 10–70◦ in 2� with scan ratef 2◦ min−1. Simultaneous TGA and DTG curves were recorded withn automatic thermo balance on heating the material from 0 ◦Co 1000 ◦C at a constant rate (10 ◦C min−1) in the air atmosphereair flow rate of 200 ml min −1). For scanning electron microscopy,old sputter coatings were carried out on the SATEOS samples atressure of 10−6 Pa. Images were recorded at 10−6 Pa EHT 15.00 kVith 300 V collector bias using Hitchi-S3000H microscope.

.5. Ion-exchange studies

To determine the ion-exchange capacity, 1.0 g dry compositeolymer material in the H+ form was taken into a glass columnaving an internal diameter ∼1.0 cm and fitted with a sintered disct the bottom. 250 ml of 1.0 M metal nitrate solutions was passedhrough it maintaining a very slow flow rate (0.5 ml min−1). Theffluent was titrated against a standard 0.01 M NaOH solution usinghenolphthalein as indicator. For evaluating the elution behaviourf SATEOS the column containing one gram of the material in the+ form was eluted with 1.0 M Mg(NO3)2 solution (100 ml) having

standard flow rate of 0.50 ml min−1. The effluent was collected in0 ml fractions and the amount of H+ ions released in each fractionas determined titrimetrically using standard sodium hydroxide

olution.

.6. Reversibility of the exchange materials

500 mg portions of SATEOS were mixed with 100 ml of 0.01 MaCl solution. This mixture was kept for 1 h and titrated against.01 M solutions of the respective alkali, recording the pH of theolution after each addition of 1.5 ml of the titrant till the pH becameonstant. The back-titration was then carried out by adding theame fractions of 0.01 M HNO3 to the solution. A blank pH titrationas also run by titrating the salt solutions against respective base

nd HNO3 solution before the exchanger was added. From the pHalues before and after the exchange process, the milli-equivalentsf OH− ion consumed were determined. Milli equivalents of OH−

ons consumed by the exchanger were plotted against the corre-ponding (initial) pH values.

.7. Distribution studies

The distribution coefficient (Kd) of metal ions Mg2+, Ca2+, Cd2+,i2+ were determined by batch method in different solvents of

s Materials 260 (2013) 313– 322 315

analytical interest. Various portions of 100 mg SATEOS in H+ formwere taken in several Erlenmeyer flasks and mixed with 10 ml ofdifferent metal nitrate solution in the required medium and subse-quently shaken for 4 h in a temperature controlled shaker at 19 ◦Cto attain the equilibrium. The amount of metal ions before and afterthe equilibrium was determined by EDTA titration. The distributioncoefficient is the measure of a fractional uptake of metal ions com-peting for H+ ions from a solution by an ion exchange material andhence mathematically can be calculated using the formula givenas:

Kd =[

I − F

F

]× V

W(mL g−1) (1)

where I is the initial amount of metal ion in the aqueous phase, F isthe final amount of metal ion in the aqueous phase, V is the volumeof the solution (ml), and W is the amount of cation-exchanger (g).

2.8. Separation of binary mixtures

1.0 g of the material (100–200 mesh) in H+ form was used forthe column separation in a glass column having 100 cm length and1.0 cm internal diameter. The column was first washed thoroughlywith double distilled water and then saturated with 0.01 M HClO4solution before loading the mixture on it. A definite volume of amixture of two metal ion solutions, each with initial concentra-tion of 0.1 M, was loaded on top of the column. The elution wasdone maintaining a flow rate of 2–3 drops min−1. The separationwas achieved by passing a suitable solvent through the columnas eluent with the help of a separating funnel. The metal ions inthe 10 ml fractions of the effluent were determined by the EDTAtitrations.

2.9. Water content of SATEOS

Swelling characteristics of the SATEOS composite material wasmeasured as water content (%) absorbed by the dry material underequilibrium conditions. The dried particles of known weight ingram (Wdry) were suspended in distilled water under agitation for24 h and then decanted. The surface water was then removed bysubjecting the decanted sample to centrifugation at 13,400 rpm for10 min and weight of the wet particles in gram (Wwet) was recorded.The percent water content in the material was calculated using thefollowing equation:

water content(%) = Wwet − Wdry

Wdry× 100 (2)

2.10. Computational details

The density functional theoretical (DFT) computations were per-formed at B3LYP/6-311G (d,p) [49,50] level of theory by usingthe GAUSSIAN 03 [51] set of codes to get the optimized geom-etry and vibrational wavenumbers of the normal modes of themodel compounds. DFT calculations were carried out with Becke’sthree-parameter hybrid model using the Lee–Yang–Parr correla-tion functional (B3LYP) method. Molecular geometries were fullyoptimized by Berny’s optimization algorithm using redundantinternal coordinates. The frequency calculations were carried outon the optimized geometries at the same level of theory. All thegeometries were characterized as minima with zero imaginary

calculated frequencies were uniformly scaled by 0.97 for all themodel compounds. The assignment of the calculated wavenum-bers was performed by means of visual inspection of the animatedvibrations with the help of Gauss view 03 software.

316 S.-u. Rehman et al. / Journal of Hazardous Materials 260 (2013) 313– 322

ne (SA

3

wphsaalihttsiwcStaarSic

psFdSetcagsiSgiugaI

Fig. 1. FTIR spectrum of (a) sulphosalicylic acid doped tetraethoxysila

. Results and discussion

In this study, a number of samples of new hybrid materialsere prepared by the sol–gel method. The sol–gel method for thereparation of hybrid materials involves a simple procedure ofydrolysis followed by condensation of suitable metal alkoxidesuch as tetraethoxysilane, mediated by simple catalysts such as ancid, base or a simple nucleophile. The hydrolysis reaction undercidic condition involves protonation of the alkoxide group fol-owed by nucleophilic attack by water to form a pentacoordinatentermediate which ultimately completes with the release of alco-ol molecules. During the sol–gel transformation, the viscosity ofhe solution gradually increases as the sol becomes interconnectedo form a rigid porous network – the gel. In order to incorporatepecific ion-exchange sites into the silica host structure, a var-ed number of organic molecules with acidic functional groups

ere physically doped into the hydrolysed gel and the resultantomposites were tested for ion-exchange characteristics (see Table1 in the Supporting Information File). Amongst many availableechniques, the simple doping of sol–gel precursor with function-lized dopants gives unique advantages of generality, simplicitynd effective maintenance of the original properties of the dopedeagents in the immobilized state. Among the different samples ofATEOS prepared, the sample-22 possessed relatively better Na+

on exchange capacity (0.64 mequiv./g(dry)) and was, therefore,hosen for detailed studies and characterization.

To confirm the presence of various functional groups and com-osite nature of the synthesized material, we recorded the IRpectrum of the composite material at room temperature (Fig. 1a).or comparison, the experimental IR of free 5-sulphosalicylic acidehydrates in the region 4000–400 cm−1 is given in Fig. 1b [52].ome specific bands in Fig. 1a clearly point out towards the exist-nce of the 5-sulfosalicylic acid and tetraethoxysilane moieties inhe composite polymer. At 2927 cm−1 an intense band appears,haracteristic of a stretching vibration of the OH groups of silanols,s well as the ethanol and the water occluded in the gels after itselation. Generally this band is present near 3400 cm−1 [53], buthifts to the low energy region in the doped material, perhaps due tontermolecular hydrogen bonding with the dopant, that weaken theiO2 OH bonds. The presence of the hydrogen-bonded hydroxylroups on the oxide surface (or the adsorbed molecular water)s indicated by the shift of their IR peaks relative to the isolated

nperturbed hydroxyl groups on the un-doped substrate. Thus, ineneral, involvement of an OH group in a hydrogen-bonding inter-ction causes broadening and shift to a lower wave number of theR band associated with the OH stretching vibration. The medium

TEOS) and (b) free sulphosalicylic acid in the region 4000–400 cm−1.

intensity band at 2854 cm−1 in the IR spectrum are assigned tothe C H stretching modes of the 5-sulfosalicylic acid moiety. Thevibrational band due to C O stretching mode is the most char-acteristic band in the IR spectra of the compounds containing aCOOH group and is expected in the range 1660–1700 cm−1. Thischaracteristic band due to �(C O) of 5-sulfosalicylic acid moiety inthe composite material is observed as relatively a medium inten-sity band at 1682 cm−1 in the IR spectrum, close to the position of�(C O) of free 5-sulphosalicylic acid [52,54]. At about 1614 cm−1,an �(OH) peak characteristic of the intercalated water moleculesand hydroxylated silanol groups on the silica surface is observed.This band is important because it also provides information on theamount of water and ethanol occluded in the gel. This band mayalso have some contribution from the phenyl stretch vibrations ofthe sulphosalicylic group. The narrow intense peak at 1479 cm−1

may be due to the �(C O) stretching motion of the sulphosalicylicgroup. The low intensity peaks due to the in-plane bending modes�(OH) of COOH, referred as (�(OH)c) and �(OH) of the hydroxylgroup attached to the phenyl ring, referred as (�(OH)h) in sulphos-alicylic part are assigned at 1347 cm−1 and 1288 cm−1 respectively.The group of peak between 1348 cm−1 and 1480 cm−1 may also bedue to the vibrations excited in the benzene ring. The group of over-lapping peaks which constitute a broad band between 1077 cm−1

and 1220 cm−1 is characteristic of the SiO2 group in different typesof silicate matrix. The partially resolved peak near 1021 cm−1 isassigned to anti-symmetrical stretching of the bridging oxygensalong a parallel line to Si O Si group with a substantial amountof silicon motion. At 966 cm−1, the characteristic peak of silanolgroups (Si OH) appears at relatively higher intensity. The out-of-plane bending modes �(OH)c and �(OH)h are assigned at in theregion 914 cm−1 and 664 cm−1, respectively. The peak with mod-erate intensity at 599 cm−1 may be due to SO2 scissoring or waggingvibration. At 461 cm−1 stretching motion of all bridging is observed,corresponding to a completely symmetric stretching motion of allbridging oxygen atoms along the bisectors of the Si O Si angles[53].

We also performed quantum chemical computations on threemodel systems of SATEOS and free 5-sulphosalicylic acid in orderto simulate IR spectra for comparison with the experimental one,using density functional theory. In the first model compound, M-SATEOS-1, we have taken two sulphosalicylic acid molecules on thesilica backbone resulting due to complete hydrolysis and conden-

sation of two TEOS molecules, wherein the two sulphosalicylic acidmolecules are bonded with the silica skeleton through the sulfonicand carboxylic acid groups. The second model compound con-sists of one sulphosalicylic group attached to partially hydrolysed

S.-u. Rehman et al. / Journal of Hazardous Materials 260 (2013) 313– 322 317

Fig. 2. Optimized geometries of Model-SATEOS-1, Model-SATEOS-1, and Model-SATEOS-1 calculated by employing DFT level of theory using B3LYP/6-311G (d,p).

F in thel ylic ac

Tbioitc

ig. 3. FTIR spectrum of Model-SATEOS-1, Model-SATEOS-1, and Model-SATEOS-1evel of theory using B3LYP/6-311G (d,p). Simulated IR spectrum of free sulphosalic

EOS, wherein the sulphosalicylic group is bonded through car-oxylic linkage only, while keeping sulfonic group free for metal

on exchange. We have also tried to see the solvent effect on the IR

f the model compounds by explicitly putting two water moleculesn the third model compound, M-SATEOS-3. The optimized geome-ries of the three model compounds are shown in Fig. 2 and theiralculated IR spectra are shown in Fig. 3(a andb). A comparison

region (a) 800–1200 cm−1 and (b) 1200–3500 cm−1 calculated by employing DFTid in the inset.

of the main vibrational frequencies for the experimental IR spec-trum of SATEOS and the three model compounds along with theirassignments are reported in Table S2 in Supporting Information file.

A good agreement is observed in the position of essential IR peaksbetween the experimental IR spectra and the simulated ones. Thetheoretical frequency values shown are for the gas phase geome-tries, which may be the reason for minor disagreements between

318 S.-u. Rehman et al. / Journal of Hazardous Materials 260 (2013) 313– 322

Fig. 4. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) curves of sulphosalicylic acid doped tetraethoxysilane (SATEOS). Temperature range:0–1000 ◦C, heating rate: 10 ◦C min−1.

id dop

tqt

mwcspoopawbts

Fig. 5. XRD of sulphosalicylic ac

heory and experiment, in addition to the inherent tendency of theuantum chemical methods to overestimate the force constants athe equilibrium geometry.

In order to analytically characterize the sample, thermogravi-etric analysis (TGA) as well as differential thermal analysis (DTA)as performed. Fig. 4 shows the thermogravimetric analysis (TGA)

urve (temperature range: 20–900 ◦C), which exhibits four distincttages of weight loss. The first stage of weight loss of about 5% com-letes at a temperature of 100 ◦C, which stems from the evaporationf external water molecules and residual solvent from the surfacef the composite material. The second, observed between a tem-erature range of 150 and 250 ◦C, exhibit a further weight loss ofbout 15%, which may be due to the loss of interstitial/intercalated

ater molecules. The third stage of sharp weight loss of about 60%

etween a temperature range of 250 and 400 ◦C may be attributedo the degradation of the sulphosalicylic acid moiety, incorporatedurfactant molecules and the total removal of the residual organic

ed tetraethoxysilane (SATEOS).

composites from the material. Further weight loss beyond 400 ◦Cmay be due to the condensation process taking place and seems tobe completed at 900 ◦C. The DTG curve (Fig. 4) shows four distinctendothermic peaks at 100 ◦C, 200 ◦C, and 300 ◦C, corresponding tofour stages of weight loss during temperature rise from 20 to 900 ◦C.

XRD analysis was carried out for structural identification of thecomposite material in the 2� range between 10◦ and 70◦, as shownin Fig. 5. The X-ray diffraction spectrum of the composite polymermaterial confirms the crystalline nature of the material. The diffrac-togram was indexed using powder-X software. Calculation of cellparameters reveals that this crystal belongs to hexagonal crystalstructure. The cell parameters for this crystal are a = b = 8.9876 A,c = 10.87654 A, = = 90◦, and � 120◦.

Scanning electron microscopy was performed to gain infor-mation about the morphological characteristics of the material.The SEM pictures of the composite polymer material at two dif-ferent magnifications (10 �m and 20 �m) show the presence of

S.-u. Rehman et al. / Journal of Hazardous Materials 260 (2013) 313– 322 319

oxysil

slttsa4Thm1

nswisaaap

portog(oi

1

2

rtei

Fig. 6. SEM pictures of sulphosalicylic acid doped tetraeth

heet like structures (Fig. 6). This indicates that at micro-structureevel, the material possess a layered type three dimensional struc-ure. We also performed chemical analysis in order to investigatehe elemental composition of this material. The chemical analy-is reveals the presence of nitrogen, carbon, hydrogen and sulphurs 1.46%, 39.07%, 5.988%, and 5.266% respectively. The residual8.21 wt% composition may be due to Si and coordinated water.he small amounts of nitrogen may be due to the embeddedexadecyltrimethylammonium chloride. These results indicate theolar ratio of N, C, H, and S in the composite polymer material is

:60:40:2.The material exhibits exceptionally high chemical stability as

o appreciable dissolution of the material was observed even in aolution of 6 M HNO3. Swelling properties of the composite materialere assessed by estimating absorbed water content, after keep-

ng the samples in contact with water for 24 h. We found that theamples underwent appreciable swelling upon water absorptionnd showed 61% increase in weight. The water may be physicallybsorbed within the ultraporous gel structure and/or chemicallybsorbed through hydrogen bonding with the hydroxyl groupsresent on the exposed porous gel surface.

In order to investigate the working capacity of the compositeolymer material as an ion-exchanger, the ion exchange capacitiesf some monovalent and divalent cations were investigated and theesults are summarized in Table 1. The ion exchange process seemso occur at the ionogenic sites provided by the sulphonic groupsf the sulphosalicylic acid moiety and/or the silanol (Si O H)roups on the surface of the pores in the three dimensional matrixScheme 1). The preferential absorption of one cation over the othern a cation-exchanger depends on the nature of the exchanging ionn the following manner [55]

. At low aqueous concentrations and at ordinary temperaturesthe extent of exchange increases with increasing charge of theexchanging ion.

. Under similar conditions and constant charge, for singly chargedions the extent of exchange increases with decrease in the size ofhydrated cations, while for doubly charged ions the preferentialadsorption depends on other factors in addition to the ionic size.

The interesting feature shown by this composite polymer mate-

ial is that exchange capacity of alkaline earth metals is greaterhan alkali ions (Table 1). This is quite expected because, in gen-ral, the distribution coefficient between silica based materialss larger for high-valent ions than for low valent ions at a given

ane (SATEOS) at (a) 10 �m and (b) 20 �m magnifications.

acidity of the solution. Also, the extent of exchange on this mate-rial increases along a group with decrease in hydrated ionic radiifor alkali and alkaline earth metal ions, except in case of Mg2+

ions. In fact, the ion exchange capacity is mainly dependent on thetype and size of the pores and channels containing ion-exchangesites and, therefore, also on the ability of the hydrated cations toreach the proper site. In addition to this, like an ordinary chem-ical reaction, the extent of ion-exchange for a certain type ofmetal ion is governed by the thermodynamics of the exchangeprocess involved. On this material, the enthalpy changes for theexchange reactions are expected to have three major contribu-tions: (1) the heat consumed in breaking of the S3O H/SiO Hcovalent bonds, (2) the heat released in the formation of bondsto the incoming cations, and (3) the enthalpy changes accompa-nying hydration and dehydration of the exchanging ions. Sincethe enthalpy changes accompanying O H bond breaking and thehydration of the proton remain relatively constant throughout theexchange reaction, the heat released during the binding of an iondepends upon the amount of water the ion must give up in orderto accommodate itself to the cavity size. Thus, the ion exchangereaction will be less exothermic with the ions of larger hydratedradii than that with the similar ones. Moreover, as exchange pro-ceeds and smaller cavities are occupied, the more loosely heldwater by the larger cations is squeezed out and a progressiveincrease in entropy occurs. In contrast, the smaller ions bind thewater molecules strongly, resulting in a decrease in entropy. Hence,exchange of less hydrated ions is thermodynamically favourable. Ahydrated cation gives up most of its water at the surface and dif-fuses into the cavity as either an unhydrated or partially hydratedspecies.

The efficiency of the exchange process on this material in thecolumn form is studied by evaluating the elution behaviour of 1 gcolumn bed (Figure S1 in the Supporting Information File). Wefound that the exchange is quite fast at the beginning of the process,as most of the exchangeable H+ ions are eluted out in the first 70 mlof the effluent. In order to check the dependence of exchange onthe pH of the medium and reversibility of the exchange process onthis material, the pH titrations were performed. The plots of milli-equivalents of OH− ions consumed in the solution due to release ofH+ ions during the metal ion uptake vs pH of the solution, obtainedduring the forward and reverse pH titration under non-equilibrium

conditions are shown in Fig. 7. The milli-equivalents of OH− ionsconsumed due to release of H+ ions by the material were calculatedat different pH (initial) values from the difference of change in pHof the solution upon addition of NaOH or HNO3, in the presence

320 S.-u. Rehman et al. / Journal of Hazardous Materials 260 (2013) 313– 322

Table 1Ion exchange capacity of sulphosalicylic acid doped tetraethoxysilane composite material for different metal ions.

S. No. Cation Salt used Ionic radii (A) Pauling radii (A)a Hydrated ionic radii (A) IEC (mequiv./g)

1 Na+ NaNO3 0.97 0.976 2.76 0.642 K+ KNO3 1.33 1.33 2.32 0.683 Mg2+ Mg(NO3)2 0.78 0.65 7.00 1.844 Ca2+ Ca(NO3)2 0.94 0.99 6.30 1.085 Sr2+ Sr(NO3)2 1.06 1.13 6.10 1.36

a Ref. [56].

Scheme 1. Representation of ionogenic sites provided by the sulphonic groups of the sulphosalicylic acid moiety and/or the silanol (Si O H) groups on the surface of thepores in the three dimensional matrix.

Ft

atmi

Table 2Kd values of some metal ions on the sulphosalicyclic acid dopes tetraethoxysilanecomposite materials in nitric acids and perchloric acid solutions.

Metal ion 0.1 M HNO3 0.01 M HNO3 0.01 M HClO4

Ca2+ 2 × 102 1.5 × 102 1.5 × 102

Mg2+ 2 × 102 1.5 × 102 2.33 × 102

Ni2+ 1.5 × 102 4 × 102 9 × 102

Cd2+ 1.5 × 102 2.33 × 102 4 × 102

ig. 7. Non equilibrium pH titration curve of sulphosalicylic acid dopedetraethoxysilane (SATEOS).

nd in the absence of the material. It is interesting to note thathe number of milli-equivalents consumed in the solution due to

etal uptake increase at higher pH values and decreases, accord-ngly, at lower pH upon addition of HNO3. This shows that the metal

uptake by the material by ion-exchange is more favourable in thebasic medium, probably due to the removal of H+ ions from the sur-rounding medium. The first inflexion point in the forward pH plotis observed near pH 7, after which the curve rises smoothly up topH ≈ 11.5, which suggests that the ion exchange process probablyoccurs at chemically two different types of ion exchange sites. It isalso evident from the pH titration plots that the cation exchangereaction is almost reversible.

The distribution studies (Table 2) for a number of metals indi-cate that the composite polymer material is highly selective forNi(II) in comparison to the other metal ions studied in different

solvent systems. It is interesting to note that the value of Paulingionic radius of Ni(II) is comparably nearer to that that of Mg(II) ionthan the other cations investigated, such as Ca(II) and Cd(II) [56](Table 1). In general, the differential selectivity of the orthosilicate

S.-u. Rehman et al. / Journal of Hazardous Materials 260 (2013) 313– 322 321

and (

boaabmsactbpatmtawpha

oqcsaqltfawtprcfimbn(fidarai

Fig. 8. Separation of (a) Ni2+–Mg2+, (b) Ni2+–Cd2+

ased sol–gel hybrid materials towards different metal ions dependn a variety of factors, such as size, charge, complex formationnd the chemical conditions such as the nature of the solventnd the pH. The greater selectivity for Ni(II) and Mg(II) ions maye mainly due to the reason that the size of these cations justatches the size of the pores of matrix, leading to enhanced acces-

ibility of these ions to the ionogenic sites. As a result, the Ni(II)nd Mg(II) ions exhibit a preferred binding to the matrix over theations with inappropriate sizes, which form weaker bonds withhe polymer framework. Although the typical pore sizes of silicaased sol–gel network ranges from 10 to 100 A, with an averageore radius of the acid-catalyzed dry alkoxide silica gels beingbout 23 A [57,58], the tailored made pore sizes, pore size distribu-ions and shapes in the final material can be achieved by carefully

anipulating reaction conditions such as pH, H2O:Si mole ratio,emperature, solvents, the nature and concentration of the cat-lyst and the alkoxide precursors used. In general, low pH, lowater preparations lead to denser materials with smaller averageore size. By carefully controlling the synthetic reaction conditions,igher levels of reproducibility of pore sizes and shapes can bechieved.

In order to investigate further the selective adsorption of Ni(II)n this material for potential practical applications we performeduantitative separations of binary mixtures under different elutiononditions. The experimental results of the binary separations arehown in Fig. 8. By using 0.01 M HClO4 as an eluent, the wholemount of Mg(II), Ca(II) and Cd(II) ions loaded on the material isuantitatively eluted from the column, which indicates that neg-

igible sorption of these ions occurred at this acidity. However, athis concentration of perchloric acid, the distribution coefficientor Ni(II) is much larger than these ions and, therefore, wholemount of the Ni(II) is retained on the column bed. Interestingly,e found that it was not possible to elute out the Ni(II) ions from

he exchanger bed even up to 3 M HNO3 and the faint blue colourersisted in the exchanger bed till the molarity of nitric acid wasaised up to 4.5. The Ni(II) was quantitatively eluted out from theolumn using 4.5 M HNO3, as expected from its distribution coef-cient at higher acidity with HNO3. These results indicate that theetal ion separation on this silicate based composite materials can

e approximated by taking place through ion-exchange mecha-ism, whereby the materials behave as weakly acidic exchangersSi O H groups) and, more importantly, the distribution coef-cient of metal ions increases strongly with increasing pH andecreases sharply at higher acid concentrations. Further, the Ni(II)

dsorption through ion-exchange is highly efficient on this mate-ial which makes this material a promising candidate for potentialpplications in removal and recovery of Ni(II) from drinking andndustrial wastewaters.

[

c) Ni2+–Ca2+ from binary mixtures on (SATEOS).

4. Conclusion

The new composite materials, SATEOS, synthesized in this workby sol–gel method, possess some novel characteristics as an ion-exchanger and an adsorbent. The material is crystalline in natureand shows higher degree of selectivity for Mg(II) ions among thealkali and alkaline earth metals and for Ni(II) ions among thetransition metal ions. The material is expected to be a promisingcandidate for scavenging of these metal ions from laboratory wastesolutions and other polluted water samples.

Acknowledgement

We are thankful to the Head, Department of Chemistry, Univer-sity of Kashmir, for providing the necessary laboratory facility forcarrying out this work. A.H.P. thanks the University Grants Com-mission (UGC), Government of India for research grant.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2013.05.036.

References

[1] M. Monier, D.M. Ayad, Y. Wei, A.A. Sarhan, Adsorption of Cu (II), Co (II), andNi(II) ions by modified magnetic chitosan chelating resin, J. Hazard. Mater. 177(2010) 962–967.

[2] N. Kuyucak, Acid mine drainage prevention and control options, CIM Bull. 95(1999) 96–102.

[3] L.H. Filipek, C. Hatton, J. Gusek, T. Tsukamoto, Passive treatment of acid rockdrainage (ARD): state of the practice, in: Proceedings of the Tenth Interna-tional Conference on Tailings and Mine Waste, Colorado, USA, October, 2003,pp. 293–303.

[4] V.R. Kinhikar, Removal of nickel from aqueous solutions by adsorption withgranular activated carbon (GAC), Res. J. Chem. Sci. 2 (6) (2012) 6–11.

[5] A.H. Elshazly, K.H. Konsowa, Removal of nickel ions from waste water using acation exchange resin in a batch-stirred tank reactor, Desalination 158 (2003)189–195.

[6] A.L. Mukerjee, Environmental Pollution and Health Hazards — Causes and Con-trol, Golgotia Publications, New Delhi, India, 1986.

[7] O. Gyliene, J. Aikaite, O. Nivinskiene, Recycling of Ni(II) citrate complexes usingprecipitation in alkaline solutions, J. Hazard. Mater. 109 (2004) 105–111.

[8] E. Malkoc, Y. Nuhoglu, Removal of Ni(II) ions from aqueous solutions usingwaste of tea factory: adsorption on a fixed-bed column, J. Hazard. Mater. 135(2006) 328–336.

[9] Y.S. Ho, D.A.J. Wase, C.F. Forster, Batch nickel removal from aqueous solutionby Sphagnum moss peat, Water Res. 29 (5) (1995) 1327–1332.

10] A. Selatnia, A. Madani, M.Z. Bakhti, L. Kertous, Y. Mansouri, R. Yous, Biosorptionof Ni2+ from aqueous solution by NaOH-treated bacterial dead Streptomycesrimosus biomass, Miner. Eng. 17 (7–8) (2004) 903–911.

3 ardou

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

22 S.-u. Rehman et al. / Journal of Haz

11] F.A. Abu Al-Rub, M.H. El-Nass, F. Benyahia, I. Ashour, Biosorption of nickel onblank alginate beads, free and immobilized algal cells, Process Biochem. 39 (11)(2004) 1767–1773.

12] S. Kalyani, P. Srinivasa Rao, A. Krishnaiah, Removal of nickel (II) from aqueoussolutions using marine macroalgae as the sorbing biomass, Chemosphere 57(9) (2004) 1225–1229.

13] C.E. Borba, R. Guirardello, E.A. Silva, M.T. Veit, C.R.G. Tavares, Removal of nickel(II) ions from aqueous solution by biosorption in a fixed bed column: exper-imental and theoretical breakthrough curves, Biochem. Eng. J. 30 (2) (2006)184–191.

14] H. Xu, Y. Liu, J.H. Tay, Effect of pH on nickel biosorption by aerobic granularsludge, Bioresour. Technol. 97 (3) (2006) 359–363.

15] P.K. Wong, K.Y. Fung, Removal and recovery of nickel ion (Ni2+) from aque-ous solution by magnetite-immobilized cells of Enterobacter sp. 4-2, EnzymeMicrob. Technol. 20 (2) (1997) 116–121.

16] Y.S. Ho, G. McKay, Pseudo-second order model for sorption processes, ProcessBiochem. 34 (5) (1999) 451–465.

17] J.K. Moon, K.W. Kim, C.H. Jung, Y.G. Shul, E.H. Lee, Preparation oforganic–inorganic composite adsorbent beads for removal of radionuclide’sand heavy metal ions, J. Radioanal. Nucl. Chem. 246 (2004) 299–307.

18] A.M. Liu, K. Hidujat, S. Kawi, D.Y. Zhao, New class of hybrid mesoporous mate-rials with functionalized organic monolayers for selective adsorption of heavymetal ions, Chem. Commun. 10 (2000) 1145.

19] S.E. Artemenko, M.M. Kardash, O.E. Taraskina, A.A. Fedorchenko, Removal ofsurfactants from industrial sewage with hybrid ion-exchange composite mate-rials, Fiber Chem. 29 (1997) 261.

20] K.G. Varshney, A. Agarwal, S.C. Mojundar, Pectin based cerium (IV) and thorium(IV) phosphates as novel hybrid fibrous ion exchangers synthesis, characteri-zation and thermal behavior, J. Therm. Anal. Calorim. 81 (2005) 183.

21] M. Lakraimi, A. Legrouri, A. Barroug, A.D. Roy, J.P. Besse, Preparation of a newstable hybrid material by chloride-2,4-dichlorophenoxyacetate ion exchangeinto the zinc–aluminium-chloride layered double hydroxide, J. Mater. Chem.10 (2002) 1007.

22] K.G. Varshney, M.Z.A. Rafiquee, A. Somya, M. Drabik, Synthesis and charac-terization of a Hg (II) selective n-butyl acetate cerium (IV) phosphate as anew intercalated fibrous ion exchanger: effect of surfactants on the adsorptionbehavior, J. Chem. 45 (2006) 1856.

23] (a) K.G. Varshney, A.H. Pandith, Synthesis and ion exchange behavior of acry-lonitrile based zirconium phosphate, a new hybrid cation exchanger, J. IndianChem. Soc. 78 (2001) 250;(b) A. Nilchi, A. Kanchi, H. Atashi, A. Bagheoi, L. Nematollahi, The applicationand properties of composite sorbents of inorganic ion exchangers and poly-acrylonitrile binding matrix, J. Hazard. Mater. 137 (2006) 1271.

24] K.G. Varshney, P. Gupta, Synthesis, characterization and applications of a newphase of crystalline and mercury selective acrylamide cerium (IV) phosphate:a novel fibrous ion exchanger, Indian J. Chem. 42 (2003) 89–93.

25] (a) A.A. Khan, M.M. Alam, Synthesis, characterization and analytical applica-tions of a new and novel organic–inorganic composite material as a cationexchanger and Cd(II) ion selective membrane electrode; polyaniline Sn (iv)tungstoarsenate, React. Funct. Polym. 55 (2003) 277;(b) A.A. Khan, Inamouddin, Applications of Hg (II) sensitive polyaniline Sn(IV)phosphate composite cation-exchange material in determination of Hg2+ fromaqueous solutions and in making ion-selective membrane electrode, Sens.Actuators 120 (2006) 10.

26] K.G. Varshney, A.H. Pandith, Synthesis and characterization of styrene sup-ported zirconium phosphate; a new hybrid ion-exchange material, J. Chem.Environ. Res. 5 (1996) 141.

27] D.K. Singh, S. Singh, B. Sirvastava, Synthesis, characterization and analyticalapplication of Zr(IV) triethylammonium phosphate, Indian J. Chem. 41 (2002)2526.

28] W.A. Siddiqui, S.A. Khan, Inamouddin, Synthesis, characterization andion-exchange properties of a new and novel ‘organic–inorganic’ hybrid cation-exchanger: poly (methyl methacrylate) Zr (IV) phosphate, Colloids Surf. A:Physicochem. Eng. Asp. 295 (2006) 193.

29] Z.A. Al-Othmana, Mu. Naushada, Inamouddin, Organic–inorganic type com-posite cation exchanger poly-o-toluidine Zr(IV) tungstate: preparation,physicochemical characterization and its analytical application in separationof heavy metals, Chem. Eng. J. 172 (2011) 369–375.

30] Inamuddin, S.A. Khan, W.A. Siddiqui, A.A. Khan, Synthesis, characterization andion-exchange properties of a new and novel ‘organic–inorganic’ hybrid cation-exchanger: Nylon-6,6, Zr(IV) phosphate, Talanta 71 (2007) 841–847.

31] S.A. Nabi, A.H. Shalla, EDTA-stannic (IV) iodate: preparation, characterizationand its analytical applications for metal content determination in real and

synthetic samples, J. Porous Mater. 16 (2009) 587.

32] S. Prakash, M. Kumar, B.P. Tripathi, V.K. Shahi, Sol–gel derived poly(vinylalcohol)-3-(2-aminoethylamino) propyl trimethoxysilane: cross-linkedorganic–inorganic hybrid beads for the removal of Pb(II) from aqueoussolution, Chem. Eng. J. 162 (2010) 28–36.

[

[

s Materials 260 (2013) 313– 322

33] L.L. Hench, J.K. West, The sol–gel process, Chem. Rev. 90 (1990) 33.34] M. Kumar, B.P. Tripathi, V.K. Shahi, Crosslinked chitosan/polyvinyl alcohol

blend beads for removal and recovery of Cd(II) from wastewater, J. Hazard.Mater. 172 (2009) 1041–1048.

35] D. Avnir, S. Braun, O. Lev, M. Ottolenghi, Enzymes and other proteins entrappedin sol–gel materials, Chem. Mater. 6 (1994) 1605.

36] H. Zou, S. Wu, J. Shen, Polymer/silica nanocomposites: preparation, character-ization, properties, and applications, Chem. Rev. 108 (2008) 3893–3957.

37] L. Depre, M. Ingram, C. Poinsignon, M. Popall, Proton conducting sul-fon/sulfonamide functionalized materials based on inorganic–organic matri-ces, Electrochim. Acta 45 (2000) 1377–1383.

38] V.V. Binsu, R.K. Nagarale, V.K. Shahi, Phosphonic acid functionalized amino-propyl triethoxysilane–PVA composite material: organic–inorganic hybridproton-exchange membranes in aqueous media, J. Mater. Chem. 15 (2005)4823–4831.

39] A. Walcarius, Analytical applications of silica-modified electrodes—a compre-hensive review, Electroanalysis 10 (1998) 1217.

40] E.H. Lan, B.C. Dave, J.M. Fukoto, B. Dunn, J.I. Zink, J.S. Valentine, Synthesis ofsol–gel encapsulated heme proteins with chemical sensing properties, J. Mater.Chem. 9 (1999) 45.

41] Y.J. Wu, X.Q. Ren, Y.D. Lu, J. Wang, Rapid synthesis of zeolite MCM-22 byacid-catalyzed hydrolysis of tetraethylorthosilicate, Mater. Lett. 62 (2008)317–319.

42] S. Cabrera, J. El Haskouri, C. Guillem, J. Latorre, A. Beltran-Porter, D. Beltran-Porter, M.D. Marcos, P. Amoros, Generalised synthesis of ordered mesoporousoxides: the atrane route, Solid State Sci. 2 (2000) 405–420.

43] U. Schubert, N. Husing, Hybrid inorganic–organic materials by sol–gelprocessing of organofunctional metal alkoxides lorenz, Chem. Mater. 7 (1995)2010–2027.

44] X.Y. Bao, X. Li, X.S. Zhao, Synthesis of large-pore methylene bridged periodicmesoporous organosilicas and its implications, J. Phys. Chem. B 110 (2006)2656–2661.

45] M.M. Collinson, Sol–gel strategies for the preparation of selective materials forchemical analysis, Crit. Rev. Anal. Chem. 29 (1999) 289–311.

46] X.J. Zhang, T.Y. Ma, Z.Y. Yuan, Titania–phosphonate hybrid porous materials:preparation, photocatalytic activity and heavy metal ion adsorption, J. Mater.Chem. 18 (2008) 2003–2010.

47] J. Liu, X. Wang, T. Xu, G. Shao, Novel negatively charged hybrids. 1. Copolymers:preparation and adsorption properties, Sep. Purif. Technol. 66 (2009) 135–142.

48] C. Xiong, C. Yao, Synthesis, characterization and application of triethylenete-tramine modified polystyrene resin in removal of mercury, cadmium and leadfrom aqueous solutions, Chem. Eng. J. 155 (2009) 844–850.

49] A.D. Becke, Density-functional thermochemistry. III. The role of exact exchange,J. Chem. Phys. 98 (1993) 5648–5652.

50] C. Lee, W. Yang, G.R. Parr, Development of the Colle–Salvetti conelation energyformula into a functional of the electron density, Phys. Rev. B 37 (1988)785–789.

51] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheese-man, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S.Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P.Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Strat-mann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K.Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich,A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari,J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Ste-fanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith,M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. John-son, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, GAUSSIAN 03 Revision B.03,Gaussian, Inc, Wallingford, CT, 2004.

52] H.T. Varghese, C.Y. Panicker, D. Philip, IR, Raman and SERS spectra of 5-sulphosalicylic acid dehydrate, J. Raman Spectrosc. 38 (2007) 309–315.

53] T. Lopez, E. Sanchez, E. Poulain, Novaro, Experimental and theoretical studiesof tetraethoxysilane gelation doped with Mn (II), Mater. Chem. Phys. 32 (1992)286–294.

54] K.G. Varshney, A.H. Pandith, U. Gupta, Synthesis and characterization of zirco-nium alumino phosphate: a new cation exchanger, Langmuir 14 (1998) 7357.

55] G.H. Jeffery, J. Bassett, J. Mendham, R.C. Denneny, Vogel’s Textbook of Quan-titative Chemical Analysis, 5th ed., John Wiley and Sons, New York, 1999, pp.192.

56] F.A. Cotton, G. Wilikinson, P. Gaus, Basic Inorganic Chemistry, 3rd ed., JohnWiley and Sons, New York, 1995, pp. 133.

57] D.A. Loy, K.J. Shea, Bridged polysilsesquioxanes. Highly porous hybridorganic–inorganic material, Chem. Rev. 95 (1995) 1431–1442.

58] Y. Shi, C.J. Seliskar, Optically transparent polyelectrolyte–silica compositematerials; preparation, characterization and application in optical chemicalsensing, Chem. Mater. 9 (1997) 821–829.