rapid synthesis of ultrahigh adsorption capacity zirconia by a solution combustion technique

Published: February 28, 2011

r 2011 American Chemical Society 3578 dx.doi.org/10.1021/la104674k | Langmuir 2011, 27, 3578–3587

ARTICLE

pubs.acs.org/Langmuir

Rapid Synthesis of Ultrahigh Adsorption Capacity Zirconiaby a Solution Combustion TechniqueParag A. Deshpande, Sneha Polisetti, and Giridhar Madras*

Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India

bS Supporting Information

’ INTRODUCTION

The synthesis of new materials for the efficient processing oforganic effluents from industry is important owing to theimplications on the environment. Adsorption is one of the mostwidely used primary processes for the treatment of pollutants athigh concentrations. Several classes of materials including acti-vated carbon, zeolites, and clays have been used for adsorption.Activated carbon has been widely used for the liquid-phaseadsorption of a large number of organic effluents.1 However,the activation of carbon is a tedious process involving hightemperature and expensive chemicals, and thus activated carbonhas been reported to be one of the most expensive adsorbents.2

A lot of attention has been focused on the synthesis ofcomposite materials owing to advantages such as tunable proper-ties and synergistic effects. Composite materials have also beensynthesized for adsorption applications with enhanced adsorp-tion capacity. Chitosan-based polymeric composite adsorbentshave been used for the removal of organics and ions fromaqueous media.3-6 Salehi et al.7 have recently reported biocom-patible chitosan-zinc oxide composites for the adsorption ofdyes. The biocompatibility of the adsorbent material is also animportant factor considering the possible entry of the material inthe food chain because of the inefficient separation of theadsorbent from treated water. In this study, we report the rapidsynthesis of zirconia for adsorption applications. Zirconia hasbeen reported to be bioinert, and its composites are suitable forimplant materials.8 It is a commonly used dentistry material9

and has been reported to be cytocompatible10 and hemo-compatible.11

The dispersion of carbon in a material depends upon theidentity of the material and the associated interactions. A highdispersion of both carbon and the material is important to thefinal properties of the adsorbent. Xiao et al.12 have reported the

synthesis of coke-supported V2O5 for catalytic applications.V2O5 was dispersed in activated coke using a two-step methodinvolving the activation of coke followed by impregnation ofV2O5 using the pore volume impregnation method. Followingthemethod used in this study, it was possible to obtain pure ZrO2

as well as a fine dispersion of carbon in ZrO2, and bothcompounds were found to be good adsorbents. Adsorption hasbeen reported to be greatly influenced by the presence ofheteroatoms in the system.13 Functional groups in the adsorbateinteract with the heteroatoms, resulting in the adsorption of thespecies over the surface. Different types of sites in ZrO2,including the Lewis acid and Bronsted acid sites and thoseassociated with different forms of carbon, can be vital toadsorption.

Apart from the synthesis of good adsorbents, it is important todetermine the kinetics of the adsorption. For the efficientoperation of an adsorption system, a knowledge of the relativemagnitudes of the various diffusion processes is required. Theaim of this study was 2-fold: (i) the synthesis and characterizationof high-capacity zirconia and carbon-zirconia composite foradsorption applications and (ii) a detailed investigation of thekinetics of the adsorption process.

’EXPERIMENTAL SECTION

Synthesis of Zirconia and Carbon-Zirconia Composite.Zirconia and carbon-dispersed zirconia were synthesized using thesolution combustion technique. Zirconium nitrate (Zr(NO3)4 3 5H2O,Rolex, India) and glycine (C2H5NO2, Rolex, India) were dissolved in

Received: November 24, 2010Revised: January 6, 2011

ABSTRACT: Tetragonal ZrO2 was synthesized by the solution combustion technique usingglycine as the fuel. The compound was characterized by X-ray diffraction, scanning electronmicroscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, and BET surface areaanalysis. The ability of this compound to adsorb dyes was investigated, and the compound hada higher adsorption capacity than commercially activated carbon. Infrared spectroscopicobservations were used to determine the various interactions and the groups responsible forthe adsorption activity of the compound. The effects of the initial concentration of the dye,temperature, adsorbent concentration, and pH of the solution were studied. The kinetics ofadsorption was described as a first-order process, and the relative magnitudes of internal andexternal mass transfer processes were determined. The equilibrium adsorption was alsodetermined and modeled by a composite Langmuir-Freundlich isotherm.

3579 dx.doi.org/10.1021/la104674k |Langmuir 2011, 27, 3578–3587

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just enough water to obtain a clear solution. The amounts of precursorswere determined by making a balance over the oxidizing and reducingvalences of the precursors in solution.14 In a typical synthesis, 2 g ofzirconium nitrate and 0.76 g of glycine were taken and 20 mL ofdeionized water was added. Sufficient time was provided for the solids todissolve completely and obtain a clear solution. The synthesis wascarried out in open crystallizing dishes made up of borosilicate glass. Thesolution was made to obtain a homogeneous mixture of the reactants.Loss of water from the solution took place by evaporation in the furnace,which was at a temperature of 350 �C. Then the solids were observed tocatch fire. All of these processes were carried out in a single step. Thistechnique is an advancement over the established self-propagating high-temperature synthesis techniques and can be used to synthesizenanocrystalline oxides. The advantages of this technique over the othertechniques can be found elsewhere.14

The synthesis process was complete within 5 min. The solid productwas very light brown in color, and this color could be attributed todispersed carbon. The light-brown solid was then finely ground and wastested for adsorption. This composite is referred to as ZrO2-S in thetext. Part of the compound was calcined at 500 �C for 24 h (referred to asZrO2-C). Both of the finely ground compounds were used for adsorp-tion experiments without further treatment.Characterization of Compounds. The composites were char-

acterized by a number of techniques for a complete chemical andstructural description. X-ray diffraction (XRD) studies were carriedout using a Philips X’pert diffractometer with Cu KR radiation. Scanningelectron microscopy (SEM) was carried out on Quanta 200 ESEM (FEIQuanta). X-ray photoelectron spectra (XPS) were recorded on ThermoFisher Scientific Multilab 2000 spectroscope. FT infrared spectroscopywas carried out using a Perkin-Elmer instrument. UV-vis spectra wererecorded on a Lambda 32 Perkin-Elmer spectrophotometer. A Micro-meritics ASAP 2020 apparatus with N2 adsorption-desorption cycleswas used to determine the surface area.Adsorption Experiments.The adsorption of dyes was carried out

over the synthesized composites. Four cationic dyes, viz., methyl green(MG, C27H35Cl2N3, C.I. number 42585, Loba Chemie, India), brilliantgreen (BG, C27H33N2 3HSO4, C.I. number 42040, Rolex, India),malachite green (MCG, C23H25ClN2, C.I. number 42000, SD. FineChem, India), and methyl violet 2B (MV, C24H28ClN3, C.I. number42535, SD. Fine Chem, India) were used for adsorption experiments. Allof the chemicals used were analytical grade, and they were used withoutfurther purification. A 50 mL portion of a 50 ppm dye solution wasplaced in a beaker, and 10 mg of the adsorbent was added to it. Thesolution was stirred continuously using a magnetic stirrer. Samples weretaken at regular intervals and centrifuged immediately to remove thesuspended adsorbent particles. The concentration of the dye in thesolution was determined using a UV-vis spectrophotometer(Shimadzu, 1700). All of the experiments were carried out in the darkto ensure that no degradation of the dye took place because of light. Tostudy the effect of temperature on adsorption, experiments were carriedout in a three-necked round-bottomed flask with a reflux condenser. Thetemperature of the dye solution wasmaintained using a PID temperaturecontroller in which the temperature could be maintained to within(2 �C. The sensor of the temperature controller was dipped in the dyesolution. The samples were cooled in an ice-cold water bath andcentrifuged to remove the suspended adsorbent particles. Experimentswere carried out in solutions with different pH values. Buffer capsules(Merck, India) were used to make solutions at different pH. Buffercapsules at 4.0 pH consisted of potassium hydrogen phthalate; buffercapsules at pH 7.0 consisted of potassium dihydrogen phosphate/disodium hydrogen phosphate; and buffer capsules at 9.0 pH consistedof a boric acid/potassium chloride/sodium hydroxide mixture (buffercomposition provided by the manufacturer). The variation of pH withtime during the reaction was monitored using a pH meter (Waterproof

pH Testr 30, Eutech instruments, Singapore). The adsorption activity ofthe synthesized compounds was compared against that of the commer-cially activated carbon (Sarabhai M Chemicals Ltd., India).

’RESULTS AND DISCUSSION

Structural Studies. X-ray Diffraction. ZrO2 exists in mainlythree phases, viz., monoclinic, tetragonal, and cubic. Crystal-lization of ZrO2 in a particular phase depends upon the tem-perature conditions employed during the synthesis and the timeof reaction. The monoclinic phase is the low-temperature stablephase, and the cubic phase is the high-temperature stable phase,with tetragonal ZrO2 being stable at intermediate temperatures.All three phases have been reported using differenttechniques.15-17 XRD patterns of all of the synthesized com-pounds were recorded to determine the crystallinity and crystalstructure of ZrO2. XRD patterns for both as-synthesized andcalcined compounds are shown in Figure 1. ZrO2 in the ZrO2-Scomposite crystallized in the tetragonal structure (Figure 1a).Similarly, the XRD patterns of the calcined compound (ZrO2-C) also corresponded to ZrO2 in the tetragonal structure(Figure 1b). Peaks in ZrO2-S were found to be broad comparedto the peaks in the calcined sample. This could be due to eitherthe smaller crystallite size or lattice strains. An increase in thecrystallite size is possible on calcination. On exposure of thecrystallites to high temperature for a long time, an increase in thesize of the crystallites takes place owing to solid-state diffusion.Loss of activity due to sintering of the particles for catalyticprocesses is an established mechanism for deactivation of the cata-lysts. Following the same mechanism, an increase in the size ofthe crystallites takes place on calcination. The crystallite size wasfound to increase from 32 to 55 nm, as determined by theScherrer formula, on calcination. However, the possibility ofdifferential lattice strains cannot be neglected for the two cases.Broadening of the peaks is observed in the crystals with latticestrain. During the solution combustion synthesis, a rapid de-crease in the temperature of the sample occurs after thecombustion of the samples. This results in strain in the crystals,and peaks with higher widths may be observed. In the sampleshaving carbon, heat is quickly lost to the surroundings because ofthe good thermal conductivity of carbon, resulting in crystalswith high strain. However, in the absence of carbon in the sample,ZrO2 has a relatively smaller thermal conductivity, and theresulting strains are lower owing to the longer timescales forthe temperature loss of the sample. Furthermore, there is also thepossibility to substitute a small amount of carbon into the ZrO2

Figure 1. XRD patterns of (a) ZrO2-S and (b) ZrO2-C.


Langmuir ARTICLE

lattice during the synthesis. This phenomenon has been reportedfor the synthesis of TiO2 in the presence of natural gas flames.18

Because a large amount of carbon is actually present in thecomposite (discussed later), lattice strains due to substitution arealso possible. Nakamura et al.19 have shown the effect ofsubstitution on lattice strain and XRD peak broadening foralloys. Therefore, broadening of the peaks can also be attributedto the lattice strains in ZrO2 crystals in ZrO2-S.Scanning Electron Microscopy. SEM images of ZrO2-C are

shown in Figure 2a. It can be seen from the images that ZrO2

particles, which are very porous in nature, were obtained. Highlyinterconnected pores of size less than 1 μm were observed.Flakelike morphology of the particles can be observed in theimages, and flakelike morphology of ZrO2-based compounds hasbeen observed in the studies by Ramos-Bristo et al.20 and Nayaket al.21 Porous structure with interconnected network pores wasalso observed in their studies. However, the porosity andmorphology of the ZrO2-S sample were found to be entirelydifferent. It can be seen from Figure 2b that most of the poreswere occupied by carbon. Very fine ZrO2 particles can also beobserved in the images. Therefore, ZrO2-S was actually acomposite of ZrO2 with amorphous carbon, with carbon beingfinely distributed in the porous structure of ZrO2.Gravimetric Studies. To determine the amount of carbon in

ZrO2-S, gravimetric analysis was carried out. A known amount(250 mg) of the compound was taken in a ceramic crucible. Thesample was heated in a muffle furnace at 800 �C for 24 h. Thesample was then cooled to 25 �C and weighed. No lattice oxygenloss was possible because Zr was found to be in theþ4 state in all

compounds, as confirmed by XPS (discussed later). Therefore, areduction in the weight gave the amount of carbon present in thesample. Nearly 20% carbon was found to be present in ZrO2-S.X-ray Photoelectron Spectroscopy. XPS of Zr 3d and C 1s

were recorded to determine the oxidation state of Zr in variouscomposites and the different types of carbon present in thecompounds. XPS of Zr 3d in the two compounds is shown inFigure S1 (Supporting Information). It is clear that Zr waspresent in the þ4 state in all of the compounds.22 Smalldifferences in the binding energies of less than 0.5 eV wereobserved, and Zr can therefore be said to be present as Zr4þ. TheXPS of C 1s is shown in Figure 3. Broad spectra were observed forall of the compounds. Several other peaks were also observed inthe spectra apart from the peak for graphitic carbon near 285 eV.It is to be noted that C 1s signals were observed for bothcompounds. Peaks corresponding to carbon of the CdO andC-(OH) type are observed around 286.5 and 288 eV. There-fore, the spectra in Figure 3 suggest the presence of differentfunctional groups in the compound. The presence of thesedifferent groups can be vital to the adsorption properties of thecompound. Dieckhoff et al.23 have analyzed in detail the C 1sspectra and have found peaks over a large range of bindingenergies, but the peaks were observed mainly until 290 eV andshake-up satellites were observed beyond 290 eV. We have alsoobserved the shake-up satellites in all of the spectra.Infrared Spectroscopy. FTIR spectra of the compounds were

recorded to determine the various groups present in the compo-sites. Figure 4 shows the spectra of the compounds,MG, andMGadsorbed over ZrO2-C. The spectra of ZrO2-S and ZrO2-Cwere similar and showed characteristic peaks mainly at 3445,1645, and 1400 cm-1 (Figure 4a,b). A broad band with a peak at3445 cm-1 can be attributed to the presence of hydroxyl groups

Figure 2. SEM images of (a) ZrO2-C and (b) ZrO2-S.

Figure 3. XPS of C 1s for (a) ZrO2-S and (b) ZrO2-C.

Figure 4. FTIR spectra of (a) ZrO2-S, (b) ZrO2-C, (c) MG, and (d)MG adsorbed over ZrO2-S. Asterisks show the peaks corresponding toadsorbed MG.


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in the compounds. The number of hydroxyl groups was relativelysmaller in ZrO2-C because of calcination at high temperatures,but hydroxyl groups were indeed present even in ZrO2-C. Thepeak at 1645 cm-1 can be attributed to the presence of carbon asCdC. It is interesting that CdC groups were present in all of thesamples. This shows that combustion-synthesized compoundsinherently have a tendency to retain carbon and the amount ofcarbon in the sample is dependent upon the fuel used. Theunsaturation in the compound and carbon formation overthe synthesized compound depend upon the attainment of thetemperature during synthesis, which in turn depends upon thefuel used. Therefore, the detection of CdC groups indeed showsthe carbon formation and dispersion in the compound during thesynthesis. This process may be considered to be analogous to thesoot-formation process resulting in carbon formation fromhydrocarbons. The observance of CdO group is consistent withthe XPS observation. An intense peak at 1400-1375 cm-1 wasalso observed in all of the samples. This may correspond tohydroxyl groups bonded directly to the metal (Zr-OH).24

Therefore, it can be concluded that hydroxyl and carbonyl groupswere inherently present in all of the compounds synthesized.The spectrum of MG is shown in Figure 4c. Peaks at 1590,

1380, and 1175 cm-1 were observed, and the peaks can beattributed to the presence of C-N, CdO, NdC, and N-Cbonds, which are consistent with the structure of MG. For theMG-adsorbed ZrO2-S sample, some of the peaks correspondingtoMG could be observed (Figure 4d). TheMGpeaks aremarkedwith asterisks in the Figure. An extended spectrum can be foundin Supporting Information Figure S2. This was the confirmationof the adsorption of the dye over the compound. The adsorptionof the dye was further confirmed by recording the UV-visspectra of ZrO2-S before and after adsorption. A peak corre-sponding to the adsorbed dye can be clearly seen in the spectrumof the compound after adsorption (Supporting InformationFigure S3).BET Surface Area Analysis.N2 adsorption-desorption curves

for the compounds are shown in Figure S4 (Supporting In-formation). The BET surface area of ZrO2-S was 13 m2/g. Oncalcination, no substantial change in the surface area of thecompound was observed, and the surface area of ZrO2-C was12 m2/g. Average pore sizes of 21 and 36 nm were observed forZrO2-S and ZrO2-C, respectively. Clearly, from the aboveinformation and the SEM images (Figure 2), the pores of ZrO2

particles were occupied by carbon in ZrO2-S. N2 adsorption-desorption was also used to determine the average particle size.The average particle sizes were found to be 0.53 and 0.46 μm forZrO2-S and ZrO2-C, respectively. From all of the aboveanalysis, it can be inferred that combustion synthesis led to theformation of highly porous ZrO2, with the porosity beinggoverned by the amount of carbon present in the composite.The commercial activated carbon used in this study had anaverage pore size of 0.58 nm and a surface area of 600 m2/g.Adsorption Studies.MGwas chosen as a model dye to assess

the adsorption capacity of the synthesized compounds. Figure 5shows the variation ofMG concentration with time over differentcompounds with an adsorbent concentration of 0.2 g/L. Theadsorption of MG over both of the synthesized compounds wasobserved, and 90% ormore adsorption was observed over both ofthe compounds within 4 h of adsorption. A very rapid decrease inthe concentration of the dye in the solution was observed uponaddition of the adsorbent in the solution. Nearly 10% adsorptionwithin 6 s and nearly 40% adsorption within 15 min were

observed over all of the compounds. No appreciable differencesin the activities of ZrO2-S and ZrO2-Cwere observed. Becausethe activities of both of the compounds were similar and theZrO2-S composite could be prepared without any post-treat-ment, the effect of different parameters on the adsorption of MGwas studied over ZrO2-S.The adsorption of MG was also carried out over activated

carbon. It can be seen from Figure 5 that the initial adsorption ofdye over both commercial and synthesized compounds wascomparable. However, in later stages, adsorption was found tobe higher over the adsorbents synthesized in this study, indicat-ing the high uptake capacity of the combustion-synthesizedcompounds.Effect of Initial Dye Concentration. Adsorption experiments

were carried out with different initial dye concentrations. Theexperiments were carried out in two different ranges of initialconcentrations: the low concentration range from 20 to 100 ppmand the high concentration range from 100 to 900 ppm. This wascarried out to check the adsorption capacity and the range ofconcentration in which the adsorbent remains effective. Thevariation of dye concentration in both ranges is shown inFigure 6a,b. It can be observed that the efficient adsorption ofMG was possible over the entire range of the dye concentrationstudied. When the initial concentrations were 100 ppm or less,more than 95% adsorption was observed for all initial concentra-tions. However, at higher concentrations, equilibrium was ob-served to be attained. A large decrease in the concentration of thedye was observed in the initial stages of adsorption. However, noconsiderable change in the concentration of the dye in thesolution was observed after 2 h of stirring, and equilibriumadsorption was attained (Figure 6b).The q factor, signifying the amount of adsorbate adsorbed per

unit mass of the adsorbent, is often used to trace the course ofadsorption. The variation of q with time for both low and highranges of concentration is shown in Figure S5a,b (SupportingInformation). Large values of q in the Figures signify the largecapacity of the adsorbent for the adsorption of the dye. It can beseen from the Figure that a q as high as 2500 mg/g of adsorbentwas obtained, which is extremely high, showing the highlyeffective adsorption of MG over C-ZrO2. Xu et al.25 havereported the adsorption of MG over cross-linked amphotericstarch. The q values obtained in their studies were limited to 120.Similarly, Guiza et al.26 observed an uptake of 427 mg/g for theadsorption of MG over natural clays. For an initial dye

Figure 5. Variation of the concentration of MG with time over thedifferent adsorbents at 25 �C, natural pH of the solution, and anadsorbent loading of 0.2 g/L.


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concentration of 100 ppm, uptake over commercially activatedcarbon was limited to 385 mg/g whereas the uptake for synthe-sized compounds was 480 mg/g. Therefore, the adsorptioncapacity of the synthesized compounds was nearly 25% higherthan that of the commercial adsorbent.Effect of Initial pH of the Solution. All of the reactions, unless

otherwise mentioned, were carried out at the natural pH of thedye solution, which was found to be nearly 5.2. On addition of theadsorbent to the solution, an increase in the pHwas observed andthe pH of the solution was found be nearly 7.6. The effect of thepH of the solution on the adsorption of the dye was studied byconducting the reaction at acidic (∼4.0), natural (near neutral),and basic pH (∼9.0). For all acidic and neutral pH, no appreci-able change in the concentration of the dye was observed in theabsence of the adsorbent. When basic pH was used, a drop in theconcentration of the dye was indeed observed when the solutionwas kept for 24 h. A control experiment was then carried out, andthe change in the concentration of the dye in basic pH solutionwas traced for 60 min. The change in the concentration waslimited to 10% against a nearly 95% decrease in the case of theexperiment with the adsorbent. This clearly showed the effect ofthe adsorbent on the adsorption of the dye. The variation of dyeconcentration with time at different pH values is shown inFigure 7. A large change in the activity of the catalyst can beobserved with the change in the pH of the solution. At pH 4, noadsorption of the dye was observed. The concentration of the dyewas found to be constant within experimental error. However, alarge increase in adsorption was observed at basic pH. Whereasnearly 80% adsorption was observed at the natural pH of the

solution in 90 min, nearly 97% adsorption was observed in basicpH. The initial rates of adsorption were also very different atdifferent pH values. Whereas nearly 70% adsorption was ob-served within 15 min at basic pH, only 35% adsorption wasobserved at neutral pH. The variation of pH with time fordifferent pH solutions is shown in the Supporting Information(Figure S6). It can be observed that no considerable change inthe pH took place during adsorption when the initial pH of thesolution was either acidic or basic. However, for the case ofneutral pH, the solution pH increased from 5.2 to 7.6 on additionof the adsorbent. According to Crini et al,27 the functional groupspresent in the dye get protonated at acidic pH, thereby increasingthe effective positive charge around the dye molecule. Similarobservations have also been reported byGarg et al.28 and Elaigwuet al.29 At basic pH, more negatively charged hydroxyl groups arepresent. Furthermore, the various acidic groups present in thezirconia part of the composite may be deprotonated,30 as a resultof which negative groups may be formed. All of these result in anincreased positive charge in acidic pH and a negative charge inbasic pH. Because the dyes are cationic, exhibiting positive chargein the solution, an enhanced adsorption of the dye is observed atbasic pH. High electrostatic repulsion at lower pH resulted in acomplete loss of adsorption activity of the compound, as can beseen from Figure 7. Furthermore, the electrostatic nature of theprocess was also confirmed by carrying out the experiment withan anionic dye, orangeG, which exhibited a negative charge in thesolution. No adsorption of the dye over the compound con-firmed the electrostatic nature of the adsorption process.Furthermore, the various groups present over the adsorbent,observed from XPS and FTIR, and the partial charge present inthem can be considered to be responsible for the high adsorptionactivity of the compound.Effect of Adsorbent Concentration. The amount of adsorbent

in the solution was varied from 0.2 to 2 g/L. It can be seen fromFigure 8 that the adsorption increased with an increase inadsorbent concentration. Nearly 30% adsorption was observedin 15 min with an adsorbent concentration of 0.2 g/L whereasmore that 65% adsorption was observed with an adsorbentconcentration of 0.5 g/L and more than 90% adsorption wasobserved with an adsorbent concentration of 1 g/L concentra-tion and above. The variation in the initial rate of adsorption withadsorbent concentration is shown in Figure S7 (SupportingInformation). Because a large decrease in the concentration ofthe dye was observed on the addition of the adsorbent, the initialrate of adsorption was calculated for the first 5 min of adsorption.

Figure 7. Variation of MG concentration with time over Zr-S atdifferent pH values. Conditions: 25 �C and adsorbent loading = 0.2 g/L.

Figure 6. (a) Variation of the concentration of MG with time overZrO2-S at low values of the initial dye concentration. (b) Variation ofthe concentration of MG with time over ZrO2-S at high values of theinitial dye concentration. Conditions: 25 �C, natural pH, adsorbentloading = 0.2 g/L.


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A linear increase in the initial rate was observed until anadsorbent concentration of 1 g/L was reached, but a saturationof the rate was observed at higher adsorbent concentrations.Effect of Adsorption Temperature.Adsorption was carried out

at different temperatures to study the effect of temperature onadsorption. The variation of MG concentration with time atdifferent temperatures is shown in Figure S8 (SupportingInformation). To ensure that the decrease in the concentrationof the dye was due to adsorption and not due to the thermaldegradation of the dye, a control experiment was carried out at60 �C in the absence of the adsorbent and the variation of the dyeconcentration was monitored. No change in the dye concentra-tion was observed. In the presence of the adsorbent, an increasein the adsorption activity was observed with an increase intemperature. This showed that adsorption was endothermic. Amonotonous increase in adsorption activity was observed with anincrease in temperature in the temperature range of study.Effect of Different Dyes.Having established the high activity of

the adsorbent for the adsorption of MG, experiments werecarried out with the other three dyes. The dye uptake resultsare shown in Figure S9 (Supporting Information) for theadsorption of all dyes over ZrO2-S. The adsorption followedthe order MG > BG ≈ MCG > MV. These results clearly showthat the adsorbent is capable of adsorbing different dyes.As discussed earlier, the adsorption of dyes over the com-

pounds originates from the electrostatic forces existing in thesolution between the dye and the surface of the compound. ThepH at the zero-point charge for the synthesized compounds wasfound to be 3.9, as determined by the pH drift method. Thisresulted in the net negative surface charges in solution. Cationicdyes exhibit a net positive charge in solution. Therefore, surfaceswith negative charge groups result in attractive forces, as a resultof which the migration of the dye molecules from the bulk to thesurface of the compound can take place. A large number ofsurface hydroxyl groups in the compounds, as determined byXPS and FTIR and several other groups, detected in the XPS ofC 1s, make the surface of the compound charged. Partial chargesdeveloped in C-O groups due to differences in the electro-negativities of the compounds aid the electrostatic interactionsand enhance adsorption. The differences in the extent ofadsorption of different dyes as well as different compounds canbe attributed to the differences in the number of charged speciespresent on the dyes or the materials.Kinetic Models for Adsorption. The kinetics of adsorption

over different compounds can be described as nth-order process

with the variation of concentration, C(t), and q(t) with timegiven by the following equations31

dCðtÞdt

¼ knfCðtÞ- Cegn ð1Þ

dqðtÞdt

¼ knfqe - qðtÞgn ð2Þ

where Ce is the equilibrium concentration, qe is the correspond-ing value of q at equilibrium, n is the order of adsorption, and kn isthe nth-order adsorption rate constant. Using the data fromFigure 6, we can determine the order of adsorption and the rateconstant. For a first-order process, the following equation isapplicable:

qðtÞ ¼ qef1- expð- k1tÞg ð3ÞThe equilibrium value qe and first-order adsorption rate

constant k1 were determined for eq 3 using Origin 7.5 softwarewith a Levenberg-Marquardt algorithm. Table 1 shows thepredicted equilibrium concentration of the dye in the solutionafter adsorption and the corresponding rate parameters. The dyeuptake and the extent of adsorption were found to be almost thesame over ZrO2-S and ZrO2-C. The initial rates of adsorptionwere found to be slightly higher over ZrO2-C. The initial ratesof adsorption over activated carbon were found to be higher thanthat of the synthesized compounds. However, it can be seen fromTable 1 that the equilibrium dye uptake over activated carbonwas found to be less than that for the synthesized compounds.Both experimental and predicted dye uptakes show a nearly 25%higher uptake over the synthesized compounds.Apart from the first-order adsorption kinetics, the second-

order kinetics was also tested. However, the possibility of second-order kinetics was discarded owing to large errors in theestimated parameters. Similarly, order n in eq 2 was treated asa variable, and the parameters were estimated. The possibility ofnth-order kinetics was also discarded because physically unac-ceptable parameters were obtained. This also confirmed thatadsorption followed first-order kinetics. Furthermore, the ki-netics of the adsorption process was also tested by treating theequilibrium dye concentration in solution as fixed, as determinedby the experiment. However, no significant difference in the rateparameters was observed, and only first-order kinetics describedthe adsorption accurately. The values of rate parameters withfixed equilibrium concentrations are given in the SupportingInformation, Table S1.Internal and External Diffusion. Mass-transfer resistances

during the adsorption of the dye in a porous adsorbent mayconsist of the resistances for the diffusion of the dye in the filmand the resistances for the diffusion of the dye in the pores. This isvery often referred to as intraparticle diffusion, and the relationbetween dye uptake and diffusion time is given by27

qðtÞ ¼ kDt1=2 ð4Þ

Table 1. Predictions of the Equilibrium Concentration andRate Parameter for the Adsorption of MG over DifferentCompounds Following First-Order Kinetics

sample no. compound qe (mg/g) Ce (ppm) k1 (� 103) (min-1) R2

1 ZrO2-S 464( 17 7.1( 3.4 31.1( 3.8 0.973

2 ZrO2-C 455( 21 9.0( 4.3 40.0 ( 6.5 0.941

3 activated carbon 375( 12 25( 2.4 63.5( 7.3 0.964

Figure 8. Variation of MG concentration with time over Zr-S atdifferent adsorbent loadings. Conditions: 25 �C and natural pH.


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where kD is the mass-transfer coefficient and for different timeintervals the value changes depending upon the relative magni-tudes of film and pore diffusion resistances. Whereas filmdiffusion dominates over small initial periods of diffusion, porediffusion becomes significant at later time periods. The equationsgoverning the variation of q(t) with time are the following.32-34

Film diffusion:

qðtÞ ¼ 6D1

πa2

� �1=2

t1=2 ð5Þ

Pore diffusion:

ln 1-qðtÞqe

!¼ ln

6π2

-D2π2

a2

!t ð6Þ

The slope of the plot of q(t) with t1/2 (Figure 9a) gives an ideaof the magnitude of D1, and the slope of the plot of ln(1-(q-(t))/(qe)) with t (Figure 9b) gives an idea of the magnitude ofD2. It can be seen from Figure 9a that the linearity of the plot wasmaintained for a long time and deviation from nonlinear behaviorwas observed after 2 h, showing the presence of strong intra-particle diffusion at later stages of adsorption. The slopes ofcurves defined by eqs 5 and 6 are given as

R1 ¼ 6D1

πa2

� �1=2

ð7Þ

R2 ¼ D2π2

a2

!ð8Þ

We define the parameter β, signifying the relative diffusionresistance parameter, as follows:

β ¼ R1

R2¼ π3R2

1

36R2ð9Þ

β is independent of the particle diameter, and we use the aboveparameter to determine the relative magnitudes of mass-transferresistances. Table 2 gives the various parameters associated withthe film and pore diffusion for the different compounds synthe-sized in this study. The value of β(t) > 1 shows that intraparticlediffusion resistances are high. However, at time t = 0, it can beseen that whereas β(0) < 1 for ZrO2-S, β(0) > 1 for ZrO2-C.This shows that at the moment the adsorbent is added to thesolution, film diffusion resistances are high for the ZrO2-Ssample. It was seen in the SEM images of the calcined samplethat large porous structure is present and the pores are filled withdispersed carbon particles in ZrO2-S. This resulted in a reduc-tion in the porosity of the compound; therefore, the filmresistances were found to be high. Because of the highly porousstructure of ZrO2-C, the film resistances were smaller. How-ever, a sharp change in the slope took place within a very shortinterval of time and the pore diffusion resistance started domi-nating. This showed that the adsorption of the dye over theadsorbent was instantaneous and a large accumulation of the dyeover the surface of the adsorbent took place within a very shortinterval of time. This process was extremely fast, and during thistime, the resistance to diffusion was offered by the mass transferacross the film. An appreciable concentration of the dye was builtup at the surface of the adsorbent, and the diffusion of the dyetook place in the bulk of the adsorbent. In this regime, all ofthe compounds showed similar behavior signified by similarvalues of β(t).Although the analysis of mass-transfer control regimes are

well described using the methods described above, studies byRudzinski and Plazinski35-37 have shown the necessity of furtheranalysis using first-order kinetics and isotherm analysis. We havecarried out a similar analysis for the present case. The change inthe concentration of the dye was modeled by the followingequation35

lnfqe - qðtÞg ¼ β- kt ð10Þwhere β and κ are the rate parameters. The differentiationbetween the external and intraparticle diffusion limitation canbe obtained by the relative magnitudes of parameters β, βL, andβD, where βL equals ln(qe) andβD equals ln(qe)þ ln(6/π2). Thedetails of the analysis and the underlying assumptions can befound elsewhere.35

Table S2 gives the magnitudes of β, βL, and βD determined forthe adsorption of MG over ZrO2-S, ZrO2-C, and AC. From

Figure 9. Dependence of q on t following the intraparticle diffusionmodel. (a) Film diffusion regime and (b) intraparticle diffusion regime.

Table 2. Relative Magnitudes of Film and Intraparticle Dif-fusion Coefficients for the Adsorption of MG over DifferentCompounds

ZrO2-S ZrO2-C activated carbon

R1 0.097 0.107 0.136

R2(0) 0.022 0.366 0.228

R2(t) 0.024 0.026 0.031

β(0) 4.41 0.29 0.60

β(t) 4.04 4.12 4.39


Langmuir ARTICLE

the similarity of parameter β with βL or βD, the relativemagnitude of diffusion limitations can be determined. ForZrO2-S, β is close to βL. This showed that intraparticle diffusionis not limiting at longer exposure times. This is in accordancewith our previous analysis. However, for ZrO2-C and AC, theproximity of β to both βL and βD was close; therefore, bothresistances can be considered to be comparable at longer times.Isotherms. The analysis of the isotherms is important to the

design of the columns because they provide information on themaximum adsorption capacity of the adsorbent. The isothermsfor the adsorption of MG over ZrO2-C were analyzed. Iso-therms give the variation of equilibrium dye uptake qe with theequilibrium dye concentration in the solution Ce. Five differenttypes of isotherms have been proposed, depending upon the typeof mechanism dominating the adsorption process. In the con-centration range used in this study, adsorption was found tofollow isotherm type I, which is the generally observed isothermfor the case of microporous adsorbents.34 To describe thevariation of qe withCe, a number of relations have been proposed.Langmuir and Freundlich isotherms are classical isotherms andare the most commonly used isotherms to describe the adsorp-tion process.39-41 The Langmuir isotherm assumes monolayeradsorption of the adsorbate molecules on the adsorbent, withall of the adsorption sites being energetically equivalent.40

Freundlich proposed an empirical relation for adsorption overheterogeneous surfaces, known as the Freundlich isotherm,where the heterogeneity of the system is quantified by the valueof the parameter n.41 To incorporate the advantages of both theisotherms, a composite isotherm known as the Langmuir-Freundlich isotherm can be used.

qe ¼ qmklCe

1þ klCeð11Þ

qe ¼ kfCe1=nc ð12Þ

qe ¼ qmðklfCeÞβ

1þ ðklfCeÞβð13Þ

Equations 11 and 12 represent the equations for the Lang-muir and Freundlich isotherms, respectively. The compositeLangmuir-Freundlich isotherm is given by eq 13. In the limitingcase of β = 1, the isotherm reduces to the Langmuir isotherm,and at very low concentrations, the isotherm reduces to theFreundlich isotherm.

More adsorption experiments were carried out with a numberof different initialMG concentrations to establish the equilibriumparameters. For these experiments, the dye solution was stirredin the dark, exactly in the manner described in the ExperimentalSection. However, the time variation of the dye concentrationwas not followed. After allowing a sufficiently long time (5-8 h),the final concentration of the dye in the solution was determinedand this information was used to determine the adsorptionisotherms. These additional experiments increased the numberof data points in the isotherm and improved the reliabilityparameters in the regression. The isotherms based on theexperimental data and the correlations by various models areshown in Figure 10, and the parameters obtained from nonlinearregression are given in Table S3 (Supporting Information). It canbe seen that the Langmuir-Freundlich isotherm best describedthe process of adsorption. As stated earlier, the correlations bythe Frendlich isotherm were satisfactory at lower equilibriumconcentrations but were unsatisfactory at high concentrations.The correlations by the Langmuir isotherm were also notsatisfactory at intermediate dye concentrations. However, theLangmuir-Freundlich isotherm was found to correlate the dataover nearly the entire range of concentration.The composite Langmuir-Freundlich isotherm has been

used by several investigators to describe the adsorption of a largevariety of adsorbents and adsorbates. For a type I isotherm, theparameter nf in the Freundlich isotherm has a value of greaterthan 1, and the value of the parameter β describing theLangmuir-Freundlich isotherm generally has a value of lessthan 1. However, β is more of a fitting parameter combining theLangmuir isotherm with the Freundlich isotherm, and β > 1 isindeed possible. Nam et al.42 have reported similar observationsfor adsorption over zeolites. In their case also, a type I adsorptionisotherm was observed. Similarly, Oubagaranadin and Murthy43

have observed index β to be both greater than and less than 1 forthe adsorption of ions over activated clays. Therefore, it can beconcluded that the adsorption over microporous materials suchas the ones reported in this study can be best described by theLangmuir-Freundlich isotherm.One of the advantages of the use of the Langmuir isotherm is

gaining information about the feasibility of the adsorptionprocess over a particular adsorbent. Parameter Rl given by thefollowing relation is used to determine the feasibility of adsorp-tion in a given concentration range over a particular adsorbent.

Rl ¼ 11þ klCo

ð14ÞThe values of Rl over the entire range of concentration studied

are shown in Figure S10 (Supporting Information). It can beseen that 0 < Rl < 1 over the entire range of concentration,indicating that the adsorption of the dye over the adsorbent wasreversible and favorable.39

In this study, we report for the first time the adsorption activityof combustion-synthesized ZrO2. The compounds were porous,but the surface area of the compound was much less than that ofthe commercial adsorbents. However, it is important that thesurface area is one of the important parameters affecting theadsorption activity of the adsorbent. A high surface area aids thehigher accessibility of the adsorbate molecules to the absorbentactive sites. However, a high surface area alone cannot guaranteea high adsorption activity, and many other factors are to beconsidered. Electrostatic forces as the origin for the adsorptionhave been established in this study as well as in other studies. The

Figure 10. Variation of qe with Ce following different isotherms.


Langmuir ARTICLE

surface charge density plays an important role in determining theactivity of the compound. Surface modification and functionali-zation have been used to enhance the adsorption activity of theadsorbents.44-47 The advantage of the compounds synthesizedin this study is that no post-treatment steps are required and themethod provides a rapid way to obtain high-capacity compoundsowing to the various surface groups. However, the surface area ofthe compounds was found to be low. New methods of synthesiscan be devised to make the surface area of this material compar-able to that of commercially activated carbon. In such a case, theadvantages of high surface area and functionality can be attainedand adsorbents with an even higher capacity can be expected.

’CONCLUSIONS

Rapid synthesis (<5 min) of ZrO2 was accomplished by asolution combustion technique using glycine as a fuel. Both theas-synthesized compound and the calcined compound were wellcharacterized, and both materials showed a very high capacity forthe adsorption of dyes. The effects of the initial concentration ofthe dye, temperature, adsorbent concentration, and pH of thesolution on the adsorption capacity were studied. The analysis ofthe kinetic data showed that adsorption of the dye followed first-order kinetics. The composite Langmuir-Freundlich isothermwas used to correlate the isotherms obtained experimentally.

’ASSOCIATED CONTENT

bS Supporting Information. First-order rate constantswhen the equilibrium constants are fixed, as obtained from theexperiments. Analysis of mass-transfer limitations using Lagerg-ren first-order rate parameters. Isotherm constants correspond-ing to the different isotherms. Structure of the different dyes usedin the adsorption study. XPS of Zr 3d in ZrO2-S and ZrO2-C.FTIR spectra of MG adsorbed over ZrO2-S with asterisks in theFigure showing the peaks corresponding to adsorbed MG. UV-vis spectra of ZrO2-S before and after the adsorption of MG.N2 adsorption-desorption curves for ZrO2-S and ZrO2-C.Variation of q with time for adsorption at low and highconcentrations. Variation of the pH of the dye solution withtime during adsorption over ZrO2-S. Variation of the initial rateof adsorption of dye with time for the initial dye concentration.Variation in the concentration of MG with time over Zr-S atdifferent temperatures. Variation in the concentration of differentdyes with time over ZrO2-S. Variation of the Rl factor with theinitial dye concentration. This material is available free of chargevia the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected]. Tel: þ91 80 22932321. Fax: þ91 80 2360 0683.

’ACKNOWLEDGMENT

G.M. gratefully acknowledges the Department of Science andTechnology, Government of India, for a Swarnajayanti fellow-ship. P.A.D. and S.P. gratefully acknowledge the help provided byAnkur Goswami of Materials Research Center, IISc, in recordingSEM images and also gratefully acknowledge Prof. N. Munichan-driaah and Tirupati of Inorganic and Physical Chemistry, IISc, forsurface area analysis.

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rapid synthesis of ultrahigh adsorption capacity zirconia by a solution combustion technique

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