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School of Chemistry, University College of
14155-6455, Tehran, Iran. E-mail: She
66405141; Tel: +98 21 61112481
Cite this: Anal. Methods, 2014, 6, 1875
Received 10th October 2013Accepted 7th January 2014
DOI: 10.1039/c3ay41782d
www.rsc.org/methods
This journal is © The Royal Society of C
Applicability of diclofenac–montmorillonite as aselective sorbent for adsorption of palladium(II);kinetic and thermodynamic studies
Mostafa Hossein Baki, Farzaneh Shemirani,* Rouhollah Khani and Mehrnoosh Bayat
A novel organo–nanoclay, i.e., diclofenac modified montmorillonite was used as a green and selective
support for the adsorption of Pd(II) ions from aqueous solution. The diclofenac–montmorillonite
composite was prepared and characterized by Fourier transform infrared (FT-IR) spectroscopy, elemental
analysis, scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. Batch experiments
were carried out with an equilibrium time of 30 min and the kinetics models of the interaction were
studied. The selectivity of the sorbent towards Pd(II) was extremely good at a pH of 6.0–7.0, while the
adsorption of other cations was low. The adsorption data were measured at room temperature and
the yielded Langmuir monolayer capacity was 20.0 mg g�1. The adsorption reaction was exothermic and
the thermodynamic parameters, DH, DS and DG, at room temperature were �34.37 kJ mol�1, �116.56 J
K�1 mol�1, and +0.96 kJ mol�1, respectively. The specific surface area increased from 35.8 m2 g�1 to
80.6 m2 g�1 for the modified clay, suggesting that intercalation creates a porous framework thereby
increasing the surface area and increasing the selectivity of this sorbent for the adsorption of Pd(II) ions.
In order to evaluate the applicability of this support for the uptake of palladium from complex matrices,
different real samples such as: road dust, rice, urine and water samples were analyzed.
Introduction
Palladium is a noble metal from the platinum group ofelements, it is especially resistant to acids, heat and corrosion.1
Palladium and its alloys have an extensive range of applicationsin jewellery, dentistry, metallurgy, coating agents, brazing alloys,petroleum and chemical industries.2,3 The most signicant andprevalent application of palladium is in the manufacture ofautomotive catalytic converters that exhibit a more efficientelimination of harmful and toxic components of exhaust gasesoriginating from diesel and petrol-fuelled vehicles and somehousehold utensils. Therefore the concentration of palladiumhas been continuously increasing in the environment.4 At thesame time, palladium compounds are considered to be highlytoxic and carcinogenic and available evidence indicates thatpalladium is easily transported into biological materials throughwater and plant roots. An excessive exposure to palladium causesadverse health effects in humans, such as primary skin prob-lems, eye irritations, substantial degradation of DNA and cellmitochondria, aggravation of hydroxyl radical damage, andinhibition of enzyme activity.5,6 The concentration of palladiumin environmental samples is very low. Moreover, high spectraland non-spectral interferences occur in the monitoring of
Science, University of Tehran, P.O. Box
[email protected]; Fax: +98 21
hemistry 2014
palladium in environmental samples, the quality control ofindustrial products, as well as palladium ore exploration.Therefore the development of separation methods for theaccurate and precise determination of palladium is important.7
Extensive research work has been done on the adsorption ofheavy metals by techniques such as chemical precipitation,8 ionexchange, electrochemical treatment, solvent extraction, solidphase extraction (SPE), membrane separation, sensors, ltra-tion, reverse osmosis and sedimentation. However, some ofthese techniques have major disadvantages such as incompletemetal uptake at low levels and high operational costs.9,10 Incomparison with the mentioned techniques, SPE reducessolvent usage, solvent exposure, disposal costs and extractiontime for sample preparation.11–13 The most widely used adsor-bents in SPE methods including: biosorbents,14 silicagel,15,16
natrolite zeolite and resins,17–19 and nanoparticles.20–22 Amongthem, nanoclay plays an important role in the environment byacting as a natural scavenger of pollutants by taking up cationsand anions through ion exchange, adsorption or both. Theproperties such as, the large specic surface area, chemical andmechanical stability, layered structure, high cation exchangecapacity (CEC), make clays excellent adsorbents.23,24 Clayminerals can react with different types of organic compounds inseveral ways. Intercalation is one of these modication tech-niques, which includes the penetration of organic moleculesinto the interlayer space of the clay minerals. These modiedorganoclays are used in a wide range of particular applications,
Anal. Methods, 2014, 6, 1875–1883 | 1875
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such as reinforcing ller plastics, catalysts25 and adsorbents formetal ions26–33 The basal spacing and increase in the adsorptionsites of montmorillonite (MMT), which can occur aer theintercalation process, results in a low equilibrium time. More-over, the formation of a porous framework can increase theselectivity of the sorbent for palladium adsorption.
A method for the adsorption of palladium ions on MMT–drug composite together with a study of the kinetic and ther-modynamic parameters for palladium–MMT interaction havenot been reported previously. Therefore, it was decided todevelop an easy, cheap and environmentally friendly method-ology for the selective uptake of palladium ions from aqueoussolutions using a green adsorbent, diclofenac–MMT. Diclofenacsodium (DFS), 2-(2-(2,6-dichlorophenylamino) phenyl) acetate,is a potent non-steroidal anti-inammatory drug (NSAID),which can be complexed with palladium ions.34 The effect oftime, temperature and other effective parameters on the selec-tive adsorption of palladium from water samples, road dust,urine and rice as test samples with complicated matrices, wasinvestigated.
ExperimentalReagents and solutions
Working solutions of palladium were prepared immediatelybefore use from 1000 mg L�1 standard atomic absorptionsolutions (Merck, Darmstadt, Germany). pH adjustments wereperformed with acetate buffer solutions. DFS with a purity of99.98% was supplied by Hakim Pharmaceutical (Tehran, Iran;http://www.hakimpharma.com) and used without further puri-cation. The nanoclay particles were Na–MMT in a platelet formand were supplied by Southern Clay Products, USA (http://www.matweb.com/search/datasheet.asp). The cation exchangecapacity and the basal spacing of MMT as reported by thesuppliers were 92.6 meq per 100 g and 11.7 A, respectively.
Fig. 1 Methodology employed to prepare the intercalated MMT.
Apparatus
A Varian model AA-400 atomic absorption spectrometer (VarianAustralia Pty Ltd, Musgrave), equipped with a deuterium lampbackground and palladium hollow cathode lamp was used fordetermination of palladium. The lamp was operated at 5 mA,using a wavelength of 244.8 nm, spectral bandwidth of 0.2 nm,burner height of 13mm and acetylene gas ow rate of 1.5 Lmin�1.All the measurements carried out in the peak height mode. Adigital pH-meter (model 692, Metrohm, Herisau, Switzerland),equipped with a glass-combination electrode was used for the pHadjustment. Separation was assisted by using a refrigeratedcentrifuge (Hettich, Universal 320 R) equipped with an angle rotor(6-place, 9000 rpm, Cat. no. 1620A). The intercalation of DFS intointerlayers of MMT was examined by powder X-ray diffraction (P-XRD) analysis. P-XRD was carried out on a Phillips powderdiffractometer X0Pert MPD using PW3123/00 curved Cu-lteredCu-Ka (l ¼ 1.540589 A) radiation in the 2q range of 2–20�. Fouriertransform infrared spectra (FT-IR) weremeasured with an Equinox55 Bruker with ATR over the wavelength range of 400–4000 cm�1.Surface morphology analysis of the adsorbents was carried out
1876 | Anal. Methods, 2014, 6, 1875–1883
using a eld emission scanning electron microscope (FESEM),model S-4160 (http://www.hitachi.com/procurement/network/japan). Elemental analysis was carried out using an ECS 4010CHNSO analyzer (Costech Company).
Preparation of intercalated nanoclay
About 0.9 g of DFS, (taken as twice the amount required basedon the CEC value for 5.0 g of MMT) was protonated in 100 mL of1 mol L�1 HCl for 1 h at room temperature with vigorous stir-ring.35,36 The protonated amine was transferred into a askalong with the addition of 100 mL of a mixture of ethanol anddistilled water (50 : 50 volumetric ratio). The solution was keptat 80 �C for 20 min along with magnetic stirring. Then, ultra-sonically dispersed MMT solution (2% w, 5.0 g in 200 mL ofwater) was added slowly into the ammonium solution alongwith stirring. Stirring was carried out for a period of 2 h.Subsequently, the solution containing the organically modiedmontmorillonite (OMMT) was centrifuged for 3 min at 5000rpm and washed with a solution of distilled water and ethanol.Aer drying for 8 h at 80 �C, it was milled into a ne powder andstored until used. The general procedure for the preparation ofOMMT is schematically illustrated in Fig. 1.
Batch sorption experiments
Due to the simplicity of the batch method, this operation modewas used for the adsorption of palladium by OMMT at roomtemperature. An aliquot of ion solution, 5–15 mg of Pd(II), wasput into a beaker. Then 30.0 mg of the adsorbent was added andthe pH of this solution was adjusted to 6.0 using acetate buffersolution, followed by dilution to 50 mL with distilled water.Aer stirring for 30 min the suspension was separated bycentrifugation at 5000 rpm for 3 min. Finally, the separatedsolid was shacken vigorously with 5.0 mL of 3.0 mol L�1 HClsolution. The palladium content of the nal solution wasdetermined by ame atomic absorption spectrometry (FAAS).The same procedure was applied to the analysis of blanksolution.
Preparation of real samples
The practical applicability of the proposed method was inves-tigated by adsorption of palladium from various real samples,such as rice, urine, road dust (collected from busy streets of
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Fig. 2 XRD patterns for Na–MMT and organically modifiedmontmorillonite.
Fig. 3 FT-IR spectra of MMT, DFS and the intercalated substance.
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Tehran) and different water samples including spring water andsea water (Caspian Sea).
Preparation of water samples
For the preparation of water samples, 500 mL of sea or springwater was adjusted to pH 2.0 with nitric acid aer samplecollection, in order to prevent adsorption of the ions on theask walls. The samples were then ltered through lter paperto remove any suspended particles and analyzed by thedescribed method.
Preparation of rice, urine and road dust samples
A 1.0 g portion of rice sample was placed into a furnace andfurther heated at 90 �C for 2 h until a constant weight wasobtained. Aer adding 20 mL of 65% nitric acid the sample washeated for 45 min at 90 �C, the temperature was then increasedto150 �C and the sample was boiled for at least 10 min until theevolution of brown fumes had ceased, followed by the additionof 10 mL of 30% hydrogen peroxide. The digestion ask washeated slightly on a hot plate until the solution became clear(the volume reached about 2–3 mL). This solution was cooled toroom temperature and made up to 80 mL with distilled water.The pH of the solution was raised from 1.5 to about 4 by thedropwise addition of concentrated ammonia. Then, 5 mL ofacetate buffer solution was added and the pH was xed to 6.Finally the solution was made up to volume in a 100 mL ask.For the preparation of urine samples, 10 mL of urine was placedin a ask. 5 mL of 65% nitric acid was added and the solutionwas heated for 10 min at 70 �C. Then, the temperature wasincreased to150 �C and the sample was boiled for 10 min fol-lowed by the addition of 5 mL of 30% hydrogen peroxide. Thesolution was heated to near dryness. Aer adjustment of the pHthe volume was made up to 100 mL. Road dust samples wereprepared by the reuxing method. For this purpose about 1.0 gof homogenized dust sample was weighed accurately anddigested with 20 mL of 3 : 1 HCl–HNO3 by reuxing the mixturefor 5 h. Then 5mL of 30% hydrogen peroxide was added and thesolution was heated to near dryness. The pH of this sample wasadjusted as for the rice samples and then made up to 200 mL.The concentration of palladium ions was determined by thedescribed procedure. In addition, the method was validated byspiking of Pd(II) ions in a 50 mL volume of the samples.
Results and discussionCharacterization of prepared composite
Fig. 2 shows the XRD patterns of pure MMT and DFS–MMT. Thebasal spacing (001) of MMT and DFS–MMT was found to be12.01 A� (2q ¼ 7.35) and 13.38 A� (2q ¼ 6.60), respectively.According to Bragg's equation: l ¼ 2d sin q, the peak shiing toa lower diffraction angle is due to the d-spacing increase.Compared to pure MMT, the DFS–MMT composite has a moreopen structure at the plane of (001). It is evidentthat DFSsuccessfully intercalated into the interlayer of MMT.
The FT-IR spectrum of MMT (Fig. 3), shows characteristicbands at: 3630 cm�1 (lattice hydroxyls OH stretching mode);
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3440 cm�1 (H–OH stretching vibration of free H2O onto theMMT structure); 1670 cm�1 (water crystallization bendingvibration); 1115 cm�1 (Si–O bending vibration); 1060 cm�1
(Si–O–Si stretching vibration); 920 and 850 cm�1 (Al–OH andMg–Al–OH vibration); 800 cm�1 (vibrations of quartz), 523 cm�1
and 467 cm�1 (bending vibrations of Al–O–Si and Si–O–Si).Characteristic peaks of DFS at: 3430 cm�1 (N–H stretching);1570 cm�1 (N–H bending); 1505 cm�1(N–H bending);1676 cm�1 (C]O stretching); 1452 cm�1 and 1400 cm�1 (CH2
CH3 deformation, CH2 bending of ortho-substitutedbenzene);1305 cm�1 and 1150 cm�1 (C–O-bending), 1585 cm�1
(benzene ring carbon–carbon vibration) and 748 cm�1 (C–Clstretching) were observed.
Generally, in the spectra of DFS–MMT particles, certainfeatures of DFS can be noticed. The spectrum is dominated bythe bands of the MMT, but the presence of DFS is also seen.Since the spectra used for comparison are not those of the
Anal. Methods, 2014, 6, 1875–1883 | 1877
Table 1 Elemental analyses data for MMT and MMT–DFS
Sample
Weight (%)
Carbon Hydrogen Nitrogen
MMT 6.68 0.36 —MMT–DFS 12.23 0.85 1.37
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protonated amine, shis in the position of certain bands are tobe expected. Nevertheless, closer inspection of the 1300–1700and 500 cm�1 of the OMMT and comparison with the samewavenumber interval of DFS and that of the MMT reveal thatguest ions are in the layers of MMT. Nevertheless, chemisorp-tions of the protonated amine on the outer surface of MMTcannot be ruled out completely. Moreover a band for aryl halideand benzene ring carbon–carbon vibration are observablebetween 1000 and 1600 cm�1. Bands in the 3500–2500 cm�1
region for R2N+H2 and –COOH groups are mostly observable in
the spectra of the intercalated material.Fig. 4 shows the SEM images of MMT before and aer
modication. It can be seen from Fig. 4a that raw MMT clayappears as small particles with nonporous and irregular plate-like shapes, moreover according to Fig. 4b the thickness ofMMT particles is in the nanoscale (30 nm). As shown in Fig. 4c,the original structure of MMT is changed, and the sharp sheetscould not be observed due to the coating of DFS on to thesurface of MMT but the thickness of the prepared composite isabout 50 nm. These results reveal that DFS slightly modied thestructure of MMT clay.
The amount of DFS intercalated onto MMT was calculatedfrom the contents of C, H and N elements. The results in Table 1indicate that about 9.78% of DFS is present in the layers of MMT(calculated based on the amount of C). These observationsverify that our attempt to prepare OMMT was successful.
Evaluation of sorbent surface area
The surface area of MMT and modied sorbent was estimatedaccording to Sears' method.37 0.5 g of sorbent was mixed with
Fig. 4 SEM image of MMT (a and b) and DFS–MMT (c and d).
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50 mL of 0.1 mol L�1 HCl and 10.0 g of NaCl. The mixture withpH 1.0 was titrated with a standard of 0.1 mol L�1 NaOH at 25 �Cto pH 4.0 and then to pH 9.0. The volume (V) required to raisethe pH from 4.0 to 9.0 was noted, and the surface areacomputed according to Sears' equation:
S(m2 g�1) ¼ 32V � 25 (1)
the MMT had a specic surface area of 35.8 m2 g�1. The specicsurface area increased to 80.6 m2 g�1 for OMMT, suggestingthat intercalation creates a porous framework therebyincreasing the surface area and increasing the selectivity of thissorbent for the adsorption of Pd(II) ions.
Effect of pH
In order to evaluate the inuence of this parameter on theadsorption, the experiments were carried out at a pH range of2.0–10.0 keeping the other variables constant. The results aredepicted in Fig. 5. About 0.03 g of the adsorbent was suspendedin 50 mL of 10 mg L�1 solution of palladium ions at several pHvalues. These samples were stirred for 30 min at 150 rpm. Thenthe samples were centrifuged at 5000 rpm for 3 min at roomtemperature to separate the adsorbent. The amount of palla-dium ions in the solution was determined by FAAS and qe wascalculated with the following equation:
qe ¼ (C0– Ce) V/m (2)
where C0 and Ce are the initial and equilibrium concentrations(mg L�1) of analyte ions in the solution, V is the sample volume(mL) and m is amount of sorbent (g). Maximum sorption ofpalladium on the modied sorbent was obtained in the pH of6.0–7.0. The dependence of metal sorption on pH is related toboth the metal chemistry in solution and the ionization state ofthe functional groups of the sorbent, which affects the avail-ability of binding sites. The increase of the sorption capacitywith increasing pH is due to the presence of a negative charge
Fig. 5 The effect of pH on palladium adsorption (sample volume,50 mL; contact time, 30 min; Pd(II) concentration, 10.0 mg L�1;sorbent, 0.6 g L�1, and pH 2–10).
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on the sorbent and deprotonated oxygen atoms in aluminol andsilanol groups of MMT that are available to retain the Pd(II) ions.Moreover, with an increase of the pH the acetate group in theligand is able to coordinate with the palladium ions. On furtherincrease of pH, adsorption decreases probably due to theformation of the hydroxide of palladium because of chemicalprecipitation. Therefore, at pH 6.0 good efficiency and selec-tivity can be achieved for palladium adsorption from complexmatrices.
Fig. 7 Lagergren pseudo-first order plot (a) and second order plot (b)for palladium ions adsorbed on OMMT, (sorbent, 0.6 g L�1; Pd(II),10 mg L�1; stirring time, 30 min; temperature, 303 K, and pH 6.0).
Kinetics of adsorption and effect of time
For quantitative extraction of the metal ions by the batchmethod, the time must be long enough for a complete extrac-tion, but must be short enough for an efficient adsorption step.In order to investigate the effect of the stirring time on theextraction efficiency, the extraction time was varied from 5 to 60min. qe increased rapidly in the rst 25 min and then, sloweddown as equilibrium was approached (Fig. 6). The increase in qewas not signicant aer 30 min. Therefore, a time of 30 min wasselected for further experiments. The high initial uptake ratemay be due to the availability of a large number of adsorptionsites. As the sites are gradually lled up, adsorption becomesslow and the kinetics is more dependent on the rate at whichthe analyte is transported from the bulk phase to the actualadsorption sites.
Assuming pseudo-rst order kinetics, the rate of theadsorptive interactions can be evaluated by using the linear
Fig. 6 The effect of stirring time on palladium adsorption (samplevolume, 50 mL; Pd(II) concentration, 10.0 mg L�1; sorbent, 0.6 g L�1;pH 6.0, and stirring time 5–60 min).
Table 2 First order rate (� 10�2 min�1) and second order rate (� 10�2 gLagergren and second order plots at 303 K (OMMT, 0.6 g L�1; Pd(II), 10
First order Second order qe (mg g�1)
k1 R2 k2 R2 Experimental Lage
7.7 0.98 9.8 0.99 2.02 3.45
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form of integrating the Lagergren equation for the boundarycondition t ¼ 0 to t ¼ t and qt ¼ 0 to qt ¼ qt:
ln(qe � qt) ¼ ln qe � k1t (3)
where k1 is the pseudo-rst order adsorption rate constant andqe, qt are the values of the amount adsorbed per unit mass atequilibrium and at any time t. The Lagergren plot of log(qe � qt)versus t (Fig. 7a) is linear (R2 ¼ 0.98) although linearity alonedoes not establish a rst order mechanism.38 The high differ-ence of 71.0% between the experimental qe values and thoseobtained from the Lagergren plots lead to almost total rejectionof the rst order kinetics. The integrated linear form of thepseudo-second order kinetic rate equation for the boundaryconditions is expressed as:
t/qt ¼ 1/(k2qe2) + (1/qe)t (4)
where k2 is the second order rate constant. The second orderplots of t/qe versus t (Fig. 7b) had better linearity (R2¼ 0.99). Thiswas observed when the qe values obtained from the plot werecompared with the experimental qe values (Table 2). The lowdifference of 8.9% between the experimental qe and thoseobtained from the Lagergren plots establish the second orderkinetics.
Due to rapid stirring in the batch method, metal ions aretransported from the aqueous phase to the surface of theadsorbent and subsequently diffuse into the interior of theporous particles.39 The intra-particle diffusion is governed bythe equation:
qt ¼ kit0.5 (5)
mg�1 min�1) constant and experimental – computed qe values frommg L�1, and pH 6.0)
rgren Deviation (%) Second order Deviation (%)
+71.0 2.2 +8.9
Anal. Methods, 2014, 6, 1875–1883 | 1879
Fig. 8 Plot of intra-particle diffusion (a) and liquid film diffusion (b) forpalladium ions adsorbed on OMMT (sorbent, 0.6 g L�1; Pd(II),10 mg L�1; stirring time, 30 min; temperature, 303 K, and pH 6.0).
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The plot of qt versus t0.5 (Fig. 8a), is approximately linear (R2 ¼
0.91), but with an intercept of �1.82 instead of zero. Porediffusion processes are therefore, unlikely to be rate controlling.The liquid lm diffusion model, which explains the role oftransport of the adsorbate from the liquid phase up to the solidphase boundary, can be expressed as:
ln(1 � F) ¼ �kfdt (6)
where F is the fractional attainment of equilibrium (F ¼ qt/qe)and kfd is the adsorption rate constant. Plotting �ln(1 � F)versus t (Fig. 8b) produced a linear (R2 ¼ 0.98) curve, but with anon-zero intercept (�2.37) against the predictions of the model.The small intercepts might point out limited applicability of themodel thus indicating the role of liquid phase transport ofpalladium ions to the sorbent surface in controlling thekinetics.
Effect of ionic strength
For adjustment of the ionic strength of the solution, differentamounts of NaNO3 (0.00–10 g), were added to 50 mL of 0.1 mgL�1 of palladium ion solution, the batch method was performedand the concentration of the desorbed palladium ions wasdetermined by FAAS. The recovery in almost all the cases wasapproximately constant (96–98%) up to 16% w/v of NaNO3 salt.For a higher ionic strength, the Pd(II) recovery and sorptiondecreased indicating that Na+ ions compete more effectivelywith Pd(II) for the negatively charged OMMT edge sites. Thisresult suggests signicant formation of inner sphere surfacecomplexes between palladium and OMMT. Therefore, thecoexistence of metal ions in solution does not signicantlyinterfere with the performance of the sorbent, and this sorbentcan be applied for palladium preconcentration from a compli-cated matrix with high ionic strength.
Adsorption isotherms
The effect of Pd(II) ion concentration on the sorbent wasanalyzed in terms of Langmuir and Freundlich equations (eqn(7) and (8)):
Ce/qe ¼ 1/(qmb) + Ce/qm (7)
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qe ¼ KfCe1/n (8)
where qe is the amount of metal ions sorbed per unit mass of thesorbent and Ce the amount of metal ions in the liquid phase atequilibrium. qm, b, and Kf, n are the Langmuir and Freundlichcoefficients, respectively.40,41 The Langmuir and Freundlichconstants were evaluated from the slopes and intercepts of thelinear plots. The empirical Freundlich isotherm yielded a linearplot (R2 ¼ 0.97) and the values of the coefficients were, n (0.47)and Kf (1.87 mg1�1/n L1/n g�1). This indicated that the sorbenthad good potential to be used as an adsorbent for Pd(II). TheLangmuir plots also had good linearity (R2 ¼ 0.985). The equi-librium coefficient, b, was 43.1 L g�1. The Langmuir monolayercapacity, qm, was 20 mg g�1. Therefore, the organoclay can takeup large amounts of palladium ions. The essential character-istics of the Langmuir isotherm can be explained in terms of adimensionless constant separation factor (RL), calculated by useof the equation:
RL ¼ 1/(1 + bCi) (9)
where Ci is the initial concentration of metal ions. RL describesthe type of Langmuir isotherm, to be irreversible (RL ¼ 0),favorable (0 < RL < 1), linear (RL ¼ 1) or unfavorable (RL > 1). AnRL value (0.14–0.69) of between 0 and 1 therefore indicates thatsorption of Pd(II) ions on OMMT is highly favorable. As seenfrom the results, the Langmuir isotherm ts well with theexperimental data. This may be due to the homogeneousdistribution of active sites on the sorbent, since the Langmuirequation assumes that the surface is homogeneous. It is wellknown that the Langmuir isotherm corresponds to a dominantion exchange mechanism while the Freundlich isotherm showsadsorption–complexation reactions taking place in the adsorp-tion process. Therefore, ion exchange and complexation reac-tions are involved in the adsorption mechanism for the SPE ofpalladium by DFS–MMT.
Thermodynamic studies
qe for Pd(II) decreased when the temperature was increased from303 to 313 K suggesting exothermic interactions. For example,Pd(II) adsorption on OMMT was 2.02, 1.75 and 1.5 mg g�1 at 303,308 and 313 K, respectively (Pd(II): 10 mg L�1). These resultsindicate that Pd(II) escapes to the solution phase from the solidphase with the rise in temperature, and the excess energy supplypromotes desorption. The thermodynamic parameters for theadsorption process, DH (kJ mol�1), DS (J K�1 mol�1) and DG(kJ mol�1) could be evaluated using the equations:
DG ¼ �RT ln Kd (10)
DG ¼ DH – TDS (11)
ln Kd ¼ DS/R � DH/RT (12)
where Kd, R and T are the distribution coefficients of the adsor-bate (qe/Ce), gas constant (8.314 � 10�3 kJ K�1 mol�1) andabsolute temperature (K) respectively. The plot of ln Kd versus 1/T
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Table 3 Thermodynamic data for the adsorption of Pd(II) (adsorbent,0.6 g L�1; Pd(II), 10 mg L�1; pH 6.0, and time 30 min)
DH(kJ mol�1)
DS(J K�1 mol�1)
DG (kJ mol�1)
303 K 308 K 313 K
�34.37 �116.56 +0.96 +1.61 +2.18
Table 5 Effect of interfering ions on % recovery (Pd2+, 5 mg andvolume, 50 mL)
Ion Ratio ion/Pd Recovery %
K+ 20 000 98 � 1.00Na+ 20 000 98.2 � 0.81Mg2+ 2000 98 � 1.00Ca2+ 2000 98 � 2.00Cl� 32 000 95 � 1.00PO4
3� 10 000 98.4 � 2.02NO3
� 10 000 98 � 1.00Cu2+ 1000 96 � 1.01Mn2+, Ni2+ 1000 95 � 0.70Co2+, Cd2+ 1000 97 � 2.00Cr3+ 1000 99 � 1.05Al3+ 1000 98.7 � 1.00Zr2+ 700 97 � 2.00Zn2+, Fe3+ 850 98 � 1.03Ag+ 500 95 � 0.80
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is linear with the slope and the intercept giving values of DH andDS. All these relations are valid when the enthalpy changeremains constant in the temperature range of the study.42 Pd(II)–OMMT interactions were accompanied by an increase in Gibbsenergy (Table 3), which made the interactions non-spontaneousand suggests that the process is not feasible at higher tempera-turesmoreover indicating the presence of an energy barrier in theadsorption process. The exothermic enthalpy change, DH, forPd(II) adsorption at 298 K, was �34.37 kJ mol�1. The magnitudeof the DH indicated moderately strong bonding between thepalladium and the OMMT. The entropy decreased for Pd(II)adsorption and stabilized the Pd(II)–clay adsorption complex. Thevalue of DS for this interaction was �116.56 J K�1 mol�1. Sincestability is associated with an ordered arrangement, it is obviousthat Pd(II) ions in aqueous solution are in a much more chaoticdistribution than they are in the adsorbed state, thus palladiumwill have a strong affinity towards modied nanoclay.
Desorption study
To make the sorption process more economical, it is necessaryto regenerate or desorb the sorbent material. Therefore,different eluents were tested and the results are shown in Table4. Observations indicated that by using a solution of 3 mol L�1
HCl the recovery was quantitative (98%). This result can beexplained by the fact that depending on the Pd(II) concentra-tion, chloride concentration, and pH of the solution, palladiumions can form stable chloro-complexes such as [PdCl]+, PdCl2,[PdCl3]
�, and [PdCl4]2�. Therefore, 5 mL of 3 mol L�1 HCl was
used as an eluent in subsequent experiments.
Effect of the sample volume
The effect of sample volumes from 20 to 1000 mL containing 5.0mg of Pd(II) were studied under optimum conditions. It was
Table 4 Effect of type and concentration of eluent on % recovery
Eluent (5 mL)Concentration(mol L�1)
Recovery(%)
H2SO4 3 75Na2SO3 1 80Na2SO3 3 702% Thiourea, HCl 1 631% Thiourea, HCl 2 902% Thiourea, HCl 2 97Na2SO3, thiourea 1, 0.3 90HCl, HNO3 2, 2 80HCl 3 986% Thiourea, HCl 3 99.1
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observed that Pd(II) ions were quantitatively recovered when thesample volume was less than 600 mL and aer elution with5 mL of the eluent, an enrichment factor of 120 was achieved.
Effect of diverse ions
In this experiment, the effect of other cations and anions onpalladium ion recovery was investigated under optimumconditions. The choice of cations and anions was madeaccording to their major abundance in real samples. A binarymixture (50 mL) of 5 mg of Pd(II) and each interfering cation, atdifferent concentration levels (10–100 mg L�1), was extracted bythe OMMT. The results are shown in Table 5. The tolerancelimit was set as the concentration of foreign substancesrequired to cause a � 5% error. The overall percent extractionsobtained were of a similar order (ranging from 95 to 100%).Moreover, for investigation of the specicity of OMMT towardspalladium ions, a solution of Cu(II), Cd(II), Co(II), Mn(II), Cr(III),Ni(II) and Pd(II) with a concentration of 0.1 mg L�1 of each of themetal ions, was adjusted to pH 6.0, the extraction was carriedout for 30 min, and the recovery was calculated. The recovery for
Table 6 Analytical results for the determination of Pd(II) in real samples(sample volume ¼ 50 mL and n ¼ 3)a
SampleAdd(mg)
Found(mg)
Recovery(%)
Sea water 0 ND —5 4.8 96 � 1
Spring water 0 ND —5 4.9 98 � 2
Rice 0 B.L.R —5 5.1 102 � 1
Road dust 0 0.18 —2.5 2.46 91 � 1
Urine 0 0.3 —5 5.18 97 � 1
a ND: not detected B.L.R: below of linear range.
Anal. Methods, 2014, 6, 1875–1883 | 1881
Table 7 Comparison of proposed method with reported methods in the literature for palladium preconcentration
MethodEnrichmentfactor
Detectionlimit (mg L�1)
RSD(%)
Linearrange
Adsorptioncapacity (mg g�1) Ref.
Ion-imprinted polymers-ICP-AES 100 0.36 3.2 — 26.71 1Octadecyl silica membranedisks-thioridazine – FAAS
100 12 1.2 — 88 2
5(Pdimethylaminobenzylidene)rhodanine silica–PEG
125 0.54 4 2–80 0.099 5
Multi-walled carbon nanotubes–FAAS 165 0.3 5.3 1–200 15.7 74-Amino-40-nitroazobenzene–chitosan–FAAS
— 15 4.33 — 58.58 10
Silica gel-bis(3-aminopropyl)amine–FAAS 80 0.36 — — 31.9 15Dimethylglyoxime–anchoredorgano-bentonite–FAAS
60 1.02 — — 8.73 33
Diclofenac–MMT–FAAS 120 0.52 1.5 2–200 20.0 This work
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palladium ions was more than 98% but for the other ions wasabout 40–65%. The possible reason for such selectivity for Pd(II)is mainly based on the fact that the Pd–OMMT complex is morestable than the other metal–OMMT complexes. These resultsindicate that all the studied interfering ions did not affect theextraction efficiency of Pd(II) and show that the selectivity of theproposed method is high for the adsorption of palladium fromreal samples with complex matrices.
Figures of merits
Under the optimized conditions, a calibration curve were con-structed for the determination of palladium according to thepreconcentration procedure. Linearity in the nal solution wasmaintained at 2–200 mg L�1 with a correlation factor of 0.999(A ¼ 0.33C + 0.001). The procedure was repeated six times andthe relative standard deviation (RSD) for the determination of0.1 mg L�1 of Pd(II) ion was found to be 1.5%. The limit ofdetection (LOD) was calculated as three times the standarddeviation of the blank signal (n ¼ 8) with the preconcentrationstep, and was 0.52 mg L�1.
Application of the method
In order to assess the applicability of the proposed method forthe analysis of a complex matrix, the adsorption of palladiumions from two natural water and acid digested samples wasperformed. The amount of palladium in road dust and urinesample was 0.72 mg g�1 and 6.0 mg L�1 respectively. According toTable 6, the recovery of the spiked samples was satisfactory(in the range of 91–102%), which indicates the capability of thismethod for the determination of palladium in real samples withdifferent matrices.
Conclusions
We have developed an efficient and environmentally friendlymethodology for the uptake of Pd(II) ions using organo-nano-clay. Pd(II)–OMMT interactions were accompanied by a decreasein the DS value and an exothermic enthalpy change, whichindicated the moderately strong affinity of palladium towards
1882 | Anal. Methods, 2014, 6, 1875–1883
the modied nanoclay. Moreover, the kinetics of the interactionhave been described by a pseudo-second order mechanism. InTable 7, this method has been compared with previouslyreported methods for the determination of palladium. Theperformance of this method was good with respect to efficiency,selectivity and preconcentration factors. The proposed systemenables the selective adsorption of trace amounts of Pd(II) ionsfrom different real samples with variable matrices. Diclofenac–MMT is a good choice for the separation and extraction ofpalladium from real samples as it is simple to prepare and islow cost when compared with different available sorbents.Therefore, this is a practical method for palladium trace anal-ysis in a variety of matrices in order to reduce its hazardousimpact on the ecosystem.
Acknowledgements
Support of this investigation by the Research Council ofUniversity of Tehran through grants, as well as proofreading byBarbora Ehrlichova is gratefully acknowledged.
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