Journal of Colloid and Interface Science 310 (2007) 353–361www.elsevier.com/locate/jcis

Removal of chromate from water by a new CTAB–silica gelatin composite

Francesco Venditti a, Andrea Ceglie a, Gerardo Palazzo b, Giuseppe Colafemmina b,Francesco Lopez a,∗

a Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase (CSGI), c/o Department of Food Technology (DISTAAM), Università del Molise,I-86100 Campobasso, Italy

b Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase (CSGI), c/o Università di Bari, I-70126 Bari, Italy

Received 21 September 2006; accepted 3 February 2007

Available online 13 February 2007

Abstract

A novel composite able to remove hexavalent chromium Cr(VI) from aqueous solutions was obtained by adding the silica precursortetraethoxysilane (TEOS) to the hexadecyltrimethylammonium bromide (CTAB) microemulsion-based gel. A physical characterization of thenew matrix revealed high stability of the silica gelatin composite in water at high temperatures and at neutral pH. Good efficiency in removingchromate from neutral solutions was also demonstrated by the adsorption kinetics. In particular, the adsorption data of chromate obtained withthe CTAB–silica gelatin composite at 25 ◦C and pH 7.5 are described by the Freundlich isotherm model. The specific role of CTAB in the silicagelatin composite was also evaluated by comparing the kinetics of the anionic AOT–silica gelatin composite to the CTAB one. The data collectedclearly showed that the positively charged surfactant was necessary to efficiently adsorb Cr(VI) from aqueous solutions. SEM and pulsed gradientspin–echo NMR analysis of the composite demonstrated that the silicon is well assembled in the gelatin network, in which water moleculesmaintain a high mobility. The diffusion coefficient of water in this system was shown to remain close to the value of pure water. Finally, X-rayanalyses of the elemental content in the CTAB–silica gelatin composite indicated no difference in terms of percentage of silica distributions indifferent areas of the matrix and suggested that chromium adsorption could take place in internal areas.© 2007 Elsevier Inc. All rights reserved.

Keywords: Organogel; Silica; Microemulsion; Chromate; Adsorption; Isotherm

1. Introduction

The presence of heavy metals in the environment has beenin the last few years and still is of major concern, becauseof their toxicity to many life forms [1]. Since the majority ofheavy metals do not degrade into harmless end products, theirconcentrations must be reduced to acceptable levels, and there-fore constantly monitored. Among these, chromium could posethreats to public health.

Emissions to air and water may result from leather tanningindustries, chemical manufacturing industries (e.g. dyes forpaints, rubber and plastic products), metal finishing industries(e.g. chrome plating), cement producing plants [2]. Chromiumis a naturally occurring element found in soil and groundwa-

* Corresponding author. Fax: +39 0874404652.E-mail address: lopez@unimol.it (F. Lopez).

0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2007.02.019

ter. Chromium is present in the environment in several differentforms. The most common oxidation states of chromium arefrom +2, to +6 with +3 and +6 giving rise to the prickliestissues in terms of environmental safety [3]. While Cr(VI) istoxic to animals and plants cells [4], Cr(III) is generally non-toxic and is believed to be essential in glucose metabolism inmammals [5,6]. In the face of the seriousness of chromiumcontaminations, efforts have been made to treat Cr(VI) conta-minated water with several methods, such as: adsorption andco-precipitation, transformation, ion exchange and reverse os-mosis. In the past decades, adsorption techniques have widelybeen studied and seem to be a reasonable choice for this matter.Several materials have been developed and tested, ranging fromlow cost waste material, such as: moss peat [7], sawdust [8],zeolites, clay [9], hazelnut shell [10], to more sophisticated ad-sorbents, such as: activated carbon [11], modified zeolite [12],

354 F. Venditti et al. / Journal of Colloid and Interface Science 310 (2007) 353–361

modified clay [13], modified steel slag [14], nanoscale magneticmaterial [15], chitosan based composite [16].

A number of significant applications stress the importanceof synthesizing hybrid materials that are self-supporting, with-out losing the properties of adsorbent devices which are alsoimportant to such processes. Possible use of biopolymers likegelatin associated with mineral phase seems to be an attractivegoal. Gelatin is a typical gelling agent in water. This polypep-tide is obtained by thermal degradation of collagen, a proteinwidely present in biological connective tissues. Gelation of col-lagen has extensively be used in food science, pharmaceuticaland industrial applications [17,18]. The ability of gelatin to jel-lify organic solvents, when dissolved in reverse micelles, waspreviously reported [19,20]. These systems are known as mi-croemulsions based gels (MBGs) or gelatin organogels and areusually prepared by mixing an aqueous gelatin solution anda w/o microemulsion. However, for biotechnological applica-tions in water solutions in general and in water treatment inparticular, these systems are affected by thermal and mechani-cal instability and, most of all, they can go through an extensiveswelling.

Interestingly, inorganic cross-linking agents can furthermodify the properties of gelatin organogels. Watzke and co-workers [21] characterized a novel nanocomposite obtainedemploying the silica sol–gel process in the AOT microemulsionorganogel. Due to the porous structures, this nanocompositewas proposed as carrier material with high specific area. Subse-quently Shuleit et al. also proposed to use this anionic surfactantAOT-based silica-biopolymer for biotechnological application[22]. The high frequency in content of proline and hydrox-iproline of this polypeptide suggested that the hydroxyl groupsinteract with water molecules and thus are available for silicainteractions.

Moreover, Liu et al. [23] in a recent paper reported thatthe interactions between gelatin and silicon leads to a sys-tem in which gelatin molecules are encapsulated by a shellof silica. The authors described the mechanical stabilizationof the gelatin polypeptide chain with the presence of a highdegree of interactions between a large number of hydroxylgroups on the pyrrolidine ring of the hydroxyproline residuesand the aqueous environment. This situation leads to a largeavailability of hydroxyl groups for further silica functionaliza-tion.

A current trend in the investigation over the remediationof chromium-contaminated environments calls for the de-velopment of new adsorbent materials containing positivelycharged surfactant, highlighting the crucial role of the surfac-tant [24–26]. However, as the literature reads, no direct evi-dence of the utilization of the silica gelatin composite in generaland the silica gelatin composite based on cationic surfactantsfor the removal process of the Cr(VI) are available. Usinga new composite containing biological (gelatin) and mineral(silicate) components in the presence of a positively chargedsurfactant for the remediation of chromium-contaminated watercould possibly be considered as a new effective method in thismatter. Furthermore, this kind of material is easy to handle andsuitable to storage and manipulation processes. In the current

work, we perform a physical characterization of the CTAB–silica gelatin composite obtained by hardening the organogelCTAB/water/hexane/pentanol/gelatin system and analyze thecomposite’s behavior in the removal process of hexavalentchromium from water at neutral pH.

2. Experimental

2.1. Materials

Hexadecyltrimethylammonium bromide (CTAB), 1-pent-anol, hexane, tetraethyl orthosilicate (TEOS), tetramethyl or-thosilicate (TMOS), gelatin type A from porcine skin (bloom300), were purchased from Sigma. CTAB was three times re-crystallized from anhydrous ethanol [27], and stored over driedsilica gel under vacuum. Sodium bis-(2-ethyl)hexylsulfoccinate(AOT), isooctane, sodium hydroxide, potassium chromate, am-monia solution 30 wt% were from Carlo Erba. Water was twicedistilled in all quartz devices.

2.2. Preparation of silica gelatin composites

2.2.1. CTAB–silica gelatin compositeTo define the microemulsion composition we use as parame-

ters the mole ratios: water/CTAB (W0), pentanol/CTAB, andas a third parameter we use the overall surfactant concentra-tion [CTAB]. The CTAB/ammonium solution/pentanol/hexanemicroemulsion was prepared by weighing in a volumetricflask the amounts of surfactant, alcohol, and 10 wt% ammo-nium solution in water needed to obtain the mole ratios wa-ter/CTAB (W0) = 5 and pentanol/CTAB = 4.9. Hexane wasadded to obtain a concentration of 100 mM CTAB. The CTABorganogels were prepared by adding 0.105 g of 10 wt% am-monium solution to 0.75 mL of the CTAB/ammonium solu-tion/hexane/pentanol microemulsion. Then 0.065 g of gelatinwere added to the solution. The mixtures were kept at 72 ◦Cfor 20 min and then cooled at 25 ◦C. All the steps were per-formed under stirring. Finally the samples were left standingat 25 ◦C without stirring. The CTAB organogel at this stage isa transparent and isotropic gel with compositions in terms W0and percentage of gelatin 80 and 7 wt%, respectively.

The silica gelatin composite were prepared starting fromthe organogels [28]. To define the amount of TEOS we useas parameter the molar ratio water/TEOS (r◦). The organogelswere brought at 42 ◦C and 0.225 mL of TEOS were added(for r◦ = 6). The resulting mixture was subsequently vigorousstirred cooled at 0 ◦C and left for 2 min. Now the CTAB–silicagelatin composite that appears as a white solid body was storedin 20 mL of hexane for twelve hours at 25 ◦C. This procedure isnecessary because in the first step of the reaction, the tetraethylorthosilicate molecule is hydrolyzed to form silicic acid andethanol. It was found that the presence of ethanol affects thefinal microstructure of the composite, thus hexane was used toremove ethanol from the composite. Successively the solid wascompletely dried and placed in water for five hours to permitthe hydratation before the Cr(VI) removal process.

F. Venditti et al. / Journal of Colloid and Interface Science 310 (2007) 353–361 355

2.2.2. AOT–silica gelatin compositeThe AOT/isooctane/sodium hydroxide solution microemul-

sion was prepared by adding amounts of a sodium hydroxidesolution (0.1 M) to a micellar solution of AOT 250 mM inisooctane to obtain a value of W0 = 60. For the preparation ofthe AOT–silica gelatin composite the same procedure describedabove was followed except that TMOS as precursor was used.

2.3. Kinetics experiments

All reactions were carried out at 25 ◦C and at pH 7.5; Cr(VI)decrement was determined by recording absorbance change atλmax = 370 nm with a Varian Cary 100 UV–vis spectropho-tometer equipped with thermostatted cells. K2CrO4 solutionswere prepared freshly. The reactions were started by adding0.12 g of the composite accurately weighted in 50 mL solu-tions containing potassium chromate. The samples were placedin a shaking water bath and stirred at 100 rpm. At regular timeintervals, samples of solutions were withdrawn from the reac-tion medium and spectrophotometrically analyzed. Chromateadsorption was determined from difference between initial andfinal solution concentrations with appropriate corrections basedon blanks. The adsorption isotherm was constructed by tun-ing the initial concentrations of K2CrO4 from 0.08 mM upto 9.93 mM. Reduction of Cr(VI) to Cr(III) was monitoredin CTAB–silica gelatin composite, taking pictures at differenttimes.

2.4. Morphology and element microanalysis

The SEM images and quantitative X-ray analysis were ob-tained using a Zeiss DSM 940 scanning electron microscopeequipped with a Link System inca energy dispersive X-ray an-alyzer (EDXRA). Scanning electron microscopic observationwas made operating at 10 kV to observe the morphology of thesamples. Different samples of CTAB–silica gelatin compositeat different content of silica were mounted and dried on alu-minum stubs, and were sputter coated with gold in a vacuumevaporator. Quantitative X-ray analysis was performed at 20 kVusing a working distance of 35 mm with a spot size of 5 µm andapplying the ZAF standard matrix correction procedure to con-vert X-ray intensities into weight percentage. Exposure of up30 min was necessary for the elemental distribution maps. Thesample the composite, after chromate adsorption, was cut forX-ray analysis and elemental distribution maps of internal andexternal faces were made.

2.5. Self diffusion study (PFGSE-NMR)

Self-diffusion coefficients measurements have been car-ried out by the Fourier transform NMR pulsed field gra-dient spin–echo (PFGSE-NMR) method [29] using a BS-587A NMR (Tesla) spectrometer operating at 80 MHz forthe proton, equipped with a pulsed field gradient unit (Au-todif 504, STELAR s.n.c.), interfaced through the HROCHsoftware to an IBM personal computer. The pulse sequenceemployed a slightly modified Stejskal–Tanner sequence [30],

90◦-τ -180◦-τ -echo with two rectangular field gradient pulsesof about 0.04 T m−1, separated by a constant interval �. Theecho amplitude recorded at 2τ is given by

(1)A(2τ) = A0 exp[−γ 2G2Dδ2(� − δ/3)

],

where γ is the gyromagnetic ratio of proton, D is the self-diffusion coefficient of the species responsible for the spin–echo decay, G is the strength of the magnetic field gradientpulses of duration δ and the effects of the transverse relaxationtime T2 are included in the constant A0. CTAB–silica gelatinecomposite of suitable size were gently transferred into NMRtubes (5 mm) and analyzed at 25 ◦C. The uncertainly of the selfdiffusion coefficients is 5%.

3. Results and discussion

3.1. Microstructure of the CTAB–silica gelatin composite

Recent data have demonstrated that the quaternary CTAB/pentanol/water/hexane microemulsion system allows the for-mation of organogels by dissolving appropriate amount ofgelatin in solution. The partial stability diagram of the systemCTAB + pentanol + water + hexane + gelatin was previ-ously determined. In this work, a hardening process was as-sessed to improve the matrix stability of the organogel in watersolutions. The hardening process follows two essential steps af-ter the initial collagen heat denaturation. First, the formationof aggregates via hydrophobic interactions; second, the stiff-ening through the inclusion of silicate inside the gelatin net-work. The polymerization of tetraethoxysilane (TEOS) insidethe CTAB organogel leads to a formation of a material, whichcan be considered as a silica gelatin composite. In this case,it is worth-noting that this material (organogel + silicate) iscompletely different from the one obtained by the hardeningprocess of hydrogel by silicate. In fact, in terms of stability,the cationic silica gelatin composite becomes almost tempera-ture and solvent-insensitive upon addition of adequate amountsof the precursor (TEOS). On the contrary, interaction betweengelatin/silicate in water leads to the formation of a compositewith stability lower than the biopolymer gel [31]. In order toanalyze the microstructure of the cationic silica gelatin compos-ite, we used different methodological approaches. One of thecrucial points in the characterization of a new composite wasto understand how the matrix is organized and consequentlyable to adsorb molecules. To this aim, the morphology and theparticle size of the composite were studied by SEM. Severalareas of the sample were examined under different magnifi-cation. In Figs. 1a and 1b microphotographs of the organogelwithout silicate are shown. These microphotographs indicatean open cell pore structure. The presence of strands of irreg-ular shape is also evident. In Figs. 1c and 1d microphotographsof the silica gelatin composite (TEOS 10%; r◦ = 9) show thepresence of particles connected each other. As reported by Iler[32] and Mukkamala [33] the connectivity between the spheri-cal particles can be attributed to the agglomeration effect of thesurfactant (CTAB), through which, the silica particles are heldtogether by micelles of the cationic surfactant.

356 F. Venditti et al. / Journal of Colloid and Interface Science 310 (2007) 353–361

Fig. 1. Microphotographs obtained by scanning electronic microscopy (SEM); (a and b) microphotographs of the organogel without silicate at different magnifica-tion; (c, d) microphotographs of the CTAB–silica gelatin composite at different magnification.

Table 1Compositions and diffusion coefficients of water (Dw ) of the CTAB–silicagelatin composite at different content of TEOS

TEOS Wet/dried(W/W)

r◦ TEOS/gelatin(W/W)

Dw ×10−9

(m2 s−1)

0%a – – – 2.310% 4.16 18 1.08 1.720% 3.88 9 2.15 1.730% 1.67 6 3.23 1.540% 1.07 4.5 4.30 1.5

a Refers to organogel without silica.

One of our main targets in the production of the new com-posite was the increase of the matrix stability in aqueous envi-ronment, i.e. its mechanical resistance. The role of the percent-age of TEOS in this process was next evaluated subsequently.The choice of the suitable amount of TEOS should be the resultof a reasonable compromise between the mechanical resistanceand the adsorption capability. In particular, the content of TEOSshould be high enough to obtain a stiff matrix without exceed-ing the right amount, because the TEOS/gelatin ratio and the r◦ratio should also be taken into account.

Table 1 reports the compositions of the composite in terms ofTEOS percentage, wet/dried ratio, r◦, TEOS/gelatin ratio and

diffusion coefficient (Dw) of water. Although the CTAB–silicagelatin composite turned particularly stable in water, it is worthseeing that a negligible amount of water shall be adsorbed fromthe aqueous solutions in the last step of its preparation (seeSection 2.2.1). The amount of water adsorbed in this step wasused as a parameter to define the hardening degree and the me-chanical resistance of the matrix. Fig. 2 depicts the swellingproperties of silica composite at different content of TEOS asa function of the ratio between the weight of the re-hydratedcomposite and the weight of the dried composite are reported.As shown, a significant decrease of this ratio is induced by in-creasing the amount of the precursor (TEOS), indicating that anadequate quantity of TEOS should be used (30%) for this ma-trix, to avoid a high degree of hydratation. In addition, these re-sults suggest that the ratio TEOS/gelatin should be higher than2 for a right preparation of the matrix, and the optimal r◦ valuein term of removal yields (see below) should be between 4 and9. Inset of Fig. 2 shows the linear correlation between the driedweight of the composite and the weight after hydratation forsamples having the same percentage of TEOS (10%). It seemsclear that no relevant change of swelling is detectable in theseexperimental conditions and the amplitude of this process ap-pears to be constant and proportional. Remarkably, these results

F. Venditti et al. / Journal of Colloid and Interface Science 310 (2007) 353–361 357

Fig. 2. Swelling properties of CTAB–silica gelatin composite at different content of TEOS as a function of the ratio between the weight of the hydrated compositeand the weight of the dried composite. In the inset the linear correlation between the dried weight of the composite and the weight after hydratation in samplesowing 10% of TEOS.

indicate a linear correspondence between quantities of gelatinand the swelling amplitude of this matrix and confirm the roleof silica in defining the mechanical resistance of these matrices.

To have an insight into the connectivity of aqueous domains,a study on the water diffusion through pulsed gradient spin–echo (PGSE) NMR was undertaken. Slice of dry compositeof suitable size were allowed to swell in distilled water for2 days. The samples were then carefully wiped and inserted inthe NMR tube. Although the water peak appears to be broaden(likely by a combination of poor field homogeneity and stronglyinteractions with the matrix) it was suitable for PGSE-NMR.Table 1 lists the diffusion coefficients (Dw) of water in samplescontaining the same amount of gelatin and different quantitiesof TEOS. Water diffusion remains high and close to the val-ues of neat water in all samples. It seems that an extended pathis still available to water diffusion within the composite body.This information seems to be very interesting, as to the mobilityof water/host molecules. In fact water molecules retains in thissystem a high value of diffusion coefficient not far from thatof “free water” (Dw = 2.3 × 10−9 m2 s−1). As far as the mi-crostructure is concerned, the composite made by gelatin andsilicate seems to have a continuous path available to water.

3.2. Adsorption of chromium by CTAB–silica gelatincomposite

The use of the previously described cationic silica gelatincomposite to remove hexavalent chromium Cr(VI) from theaqueous phase at neutral pH (7.5) was next investigated. Batchtechniques were used to determine the rate of chromium ad-sorption by the hardened composite. To choose the right contentof TEOS and test whether or not the content of the precursor(TEOS) can modify the physicochemical properties and thus

the kinetics of the reaction while keeping the amount of ini-tial Cr(VI) concentrations constant, kinetics at different r◦ wereperformed. In Fig. 3 kinetics of CTAB–silica gelatin compositesystems containing different r◦ are shown. The kinetics indicatethat this new composite is able to remove the toxic hexavalention Cr(VI) from the solution. In particular Fig. 3 shows thedetoxification process of Cr(VI) (0.96 mM) by the compositeat 25 ◦C as a function of time. As clearly indicated in the graph,the optimal r◦ is reached at 6. In fact, the composite with ther◦ = 6 was able to remove 45% of Cr(VI) from the solutionwithin 30 h. On the contrary, sample containing higher valuesof r◦ showed slowly lower adsorption (35% and 33% respec-tively for r◦ = 9 and r◦ = 18) see also Table 2.

These negligible differences in terms of removal of chromatecould be explained by a change of the physicochemical proper-ties occurring in the composite preparation. It seems that r◦ = 6is the right ratio to obtain a well hardened and homogeneouscomposite.

Fig. 4 shows the kinetics of chromate removal in solutions atdifferent content of Cr(VI) obtained in the presence of CTAB–silica gelatin composite. The rationale of the experiment wasto vary the initial Cr(VI) concentrations, while keeping theamount of the composite constant. The range concentration ofCr(VI) tested in our experiments was 0.08–9.93 mM, corre-sponding to 4–500 mg L−1.

As clearly shown in the graph, an initial drastic decrease inthe relative concentration of chromium can be detected. Thegreater part of adsorption occurs during the first 15 h andreaches a plateau within 30 h for almost all the conditionstested.

The analysis of data in terms of moles of chromium adsorbedper sample owing the same chromate concentration (0.96 mM)are reported in Table 2, where for samples of 0.120 g the moles

358 F. Venditti et al. / Journal of Colloid and Interface Science 310 (2007) 353–361

Fig. 3. Removal kinetics of Cr(VI) (0.96 mM) by CTAB–silica gelatin composite at 25 ◦C as a function of time at different r◦ (6, 9, 18).

Table 2Adsorption capacities for CTAB–silica gelatin composite (0.120 g) at differentwater/TEOS ratio (r◦)

Surfactant r◦ % Cr(VI)adsorbed at theequilibrium

Cr(VI) adsorbed(mol × 10−6)

CTAB 6 45 21.6CTAB 9 37.41 17.96CTAB 18 35.76 17.16AOT 6 6 2.88

Initial chromate concentration = 0.96 mM.

of the total chromium adsorbed are reported as function of dif-ferent r◦ values. Remarkably, these results indicate that 0.120 gof the CTAB–silica gelatin composite in chromate solutions of9.93 mM are able to remove 45 µmol of chromium from 50 mlof water in 30 h.

As already stated, the crucial point in the removal process ofchromate is given on the specific role of the CTAB in this silicagelatin composite. To this aim, the kinetics of the anionic AOT–silica gelatin composite was compared to the CTAB compositebatched in 50 mL of 0.96 mM Cr(VI) solutions at the same r◦value (r◦ = 6) (Fig. 5). Thus, the kinetics reported bears evi-dence to the fact that the composite without positively chargedsurfactant is scarcely able to adsorb Cr(VI) from aqueous so-lutions. As shown in Fig. 5, the amount of Cr(VI) removedafter 12 h (at the equilibrium) in the AOT–silica composite isalmost negligible compared to the CTAB–silica gelatin com-posite. In fact, the data reported in Table 2 clearly show that atequal amount of material (120 mg) and chromate (0.96 mM)the moles of chromium adsorbed in sample containing AOTis equal to 2.88 µmol versus 21.6 µmol in sample containingCTAB. This result is, in our opinion, a clear evidence of thespecific role of the cationic surfactant in the removal processof hexavalent chromium; in the absence of CTAB a negligibleamount of chromate can be removed. Moreover, it is worth-

noting that the data reported on the adsorption of chromate bymost of the proposed adsorbents were found to be pH dependentand reach the maximum effectiveness at very low pH values[15,26,34]. It should be stressed that our main target was con-cerned the use of a new composite to remove chromate fromneutral solutions, thus all the measurements here reported havebeen performed at pH 7.5. Therefore the CTAB–silica gelatincomposite can be considered a new adsorbent efficient in re-moving chromate from neutral solutions.

3.3. “In situ” chromium reduction

As stated above, the conversion of the chromium(VI) in itstrivalent form is a field of intensive research. However, it wouldbe desirable to achieve a reduction process in situ without fur-ther treatment. It is well known that the reduction of Cr(VI) ispH dependent, increasing with lower pH values. Under reduc-tion conditions, the hexavalent chromium will be reduced to thetrivalent form in soil by redox reaction with organic substanceslike protein and carbohydrates. On the other hand, the first iso-lation of Cr(III) species as a product of Cr(VI) was reported1989 [35]. Later, an intermediate complex with glutathione wasisolated the first time in 1990 by O’Brien et al. [36]. This com-plex is a green amorphous powder readily soluble in water.

A study of chromium reduction inside the new silica com-posite was also performed. In Fig. 6 we show that the compositecontaining the adsorbed hexavalent chromium left in water for30 days undergoes a change in color from yellow to green. Thischange is an indication of the reduction in situ of Cr(VI). Ascan be seen at time 0 (fresh prepared silica gelatin composite)the composite appears completely white, after the adsorptionthe composite becomes yellow, and after about 30 days with noadditional treatments the composite becomes green. The reduc-tion process of chromate takes place between Cr(VI) and theOH groups of hydroxyproline largely present in gelatin. In fact,in further experiments the same reduction process was observed

F. Venditti et al. / Journal of Colloid and Interface Science 310 (2007) 353–361 359

Fig. 4. Removal kinetics of Cr(VI) by CTAB–silica gelatin composite (r◦ = 6) at 25 ◦C as a function of time at different chromate concentrations.

Fig. 5. Removal kinetics of Cr(VI) (0.96 mM) by CTAB–silica gelatin composite and AOT–silica gelatin composite at 25 ◦C as a function of time (r◦ = 6).

Fig. 6. Pictures of the CTAB–silica gelatin composite at different times afterabsorption of chromate. White: fresh prepared CTAB–silica gelatin composite;yellow: composite containing the adsorbed hexavalent chromium; green: afterthe reduction of hexavalent chromium. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of thisarticle.)

by leaving fraction by leaving fraction of gelatin in potassiumchromate solution for approximately 30 days.

3.4. Adsorption isotherm

The experimental data reported in Fig. 4 were used for theelaboration of an adsorption isotherm. The absorption isothermof Cr(VI) on the CTAB–silica gelatin composite obtained atpH 7.5 and 25 ◦C is shown in Fig. 7. The data points on thebiding isotherm of Cr(VI) fitted well the Freundlich model rep-resented by Eq. (2)

(2)Qe = KC1/ne ,

360 F. Venditti et al. / Journal of Colloid and Interface Science 310 (2007) 353–361

Fig. 7. The adsorption isotherm of chromate obtained with the CTAB–silica gelatin composite (r◦ = 6) at 25 ◦C pH 7.5.

Fig. 8. Micrographs of the external (a) and internal (b) areas of the CTAB–silica gelatin composite after the reaction with chromium (green spots). In this figuresalso the silica (red color) distribution is reported. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of thisarticle.)

where Qe is the amount of chromium taken up per gram ofcomposite (mmol g−1) and Ce is the chromium concentrationleft in solution at the equilibrium (mmol L−1); K and n are theconstant isotherm parameters. As can be seen, the plot of logQe

versus the logCe was found to be linear (inset Fig. 7).This kind of binding model commonly describes the equi-

librium of the ion exchange relation between the liquid andsolid phases. The isotherm parameters obtained by fitting theFreundlich equation to experimental data are K = 0.17 andn = 2.87. Here K and n indicates the adsorption capacity andintensity of adsorption, respectively. In particular the value of n

is a very useful parameter. As a matter of fact, values of n lyingbetween 1 and 10 correspond to a good adsorption of chromiumon the composite. In our case the value of n equal to 2.87 makeswidely part of this range.

3.5. Elemental composition

Analysis for elemental content in the CTAB composite wasachieved by a quantitative X-ray. An attempt was made toidentify precisely the distribution of chromium in different ar-eas of the sample after the adsorption. To this aim, after thechromium adsorption, the CTAB–silica gelatin composite wascut and the internal and external surfaces were analyzed sepa-rately. Micrographs of the external and internal areas (Figs. 8aand 8b, respectively) allowed the identification of chromium(green spots) adsorbed in silica-gelatine composite. In this fig-ures also the silicon (red color) distribution is reported. As canbe seen, the silica is well assembled in the gelatin network.

In Table 3 the comparison of the external and internal areasin terms of elemental composition excluding carbon is reported.

F. Venditti et al. / Journal of Colloid and Interface Science 310 (2007) 353–361 361

Table 3Quantitative X-ray analysis: comparison of the external and internal areas ofCTAB–silica gelatin composite (r◦ = 6) after chromate adsorption

Element External area (%) Internal area (%)

O 46.9 56.6Si 31.7 36.8Cr 2.7 0.6Br 7.9 2.8

The percentage of silicon for the internal and external areas isfully comparable, giving the idea of a homogeneous matrix interms of silicate. The chromium percentage of the external areais equal to 2.7% versus 0.6% of the internal area. The data re-ported could indicate that this difference might be linked to theproblem of hindered diffusion of the chromate. To further inves-tigate this aspect, we analyzed the data comparing the contentof chromium and of bromide simultaneously for both areas.In Table 3 it is apparent a correlation between the content ofbromide and the content of chromium. Since the stoichiom-etry between the CTAB and bromide is 1:1 from this data arelation between CTAB and chromium may be easily assumed,thus confirming the key role of the positively charged surfac-tant in this process. The presence of bromide in the internal areaof the composite, in our opinion, demonstrates the presence ofCTAB inside the matrix feature larger interface area for the re-moval process. This evidence could be an attempt to improvethe performance of the CTAB-zeolite based systems reportedin literature, in which the absence of CTAB in the internal ar-eas was explained by the fact that the CTAB molecule is toolarge to enter into the internal position of the zeolite and as aconsequence the adsorption occurs only on the external area.However, the data obtained suggest that the adsorption perfor-mance of this matrix is satisfactory, pointing to a potential useof this composite for an anionic detoxification system.

4. Conclusions

The CTAB–silica gelatin composite can be used as a newcomposite to remove hexavalent chromium Cr(VI) from theaqueous solutions at neutral pH (7.5). SEM and NMR analy-sis showed that this material is composed by an intercon-nected network of gelatin, silicate and surfactants moleculesin which water molecules maintain a high mobility. Analy-ses of the elemental content in the CTAB–silica gelatin com-posite suggest that the adsorption of chromium takes alsoplaces in internal areas. Moreover, this adsorbent appears to beeasy to handle and thus suitable for storage and manipulationprocesses.

As for the biotechnological application of the material,it is worth noting that the absorption process of hexavalentchromium from water at pH 7.5 by the CTAB–silica gelatincomposite leads to a reasonable removal of the toxic contam-inant. In particular, the data presented indicate that 0.120 g ofthe composite are able to remove almost 50 µmol of chromiumfrom 50 ml of water.

In addition, kinetic experiments performed in comparison toa composite based on an anionic surfactant, pointed out the spe-

cific role of the cationic surfactant in this material as to theremoval of hexavalent chromium from water.

Acknowledgment

This work was supported by the MIUR of Italy (PRIN2003 NANOSCIENZE PER LO SVILUPPO DI NUOVE TEC-NOLOGIE) and by Consorzio Interuniversitario per lo sviluppodei Sistemi a Grande Interfase (CSGI-Firenze). Dr. LuciaMaiuro is gratefully acknowledged for the technical help withscanning electron microscopy.

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