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Adsorption of Cu2+, Cd2+ and Pb2+ ions by layered double hydroxides intercalated with the chelating

Adsorption of Cu 2+,Cd 2+and Pb 2+ions by layered double hydroxides intercalated with the chelating agents diethylenetriaminepentaacetate and meso-2,3-dimercaptosuccinate

I.Pavlovic,M.R.Pérez,C.Barriga,M.A.Ulibarri ?

Departamento de Química Inorgánica e Ingeniería Química,Facultad de Ciencias,Campus de Rabanales,Ed.Marie Curie,Universidad de Córdoba,14071Córdoba,Spain

a b s t r a c t

a r t i c l e i n f o Article history:

Received 25March 2008

Received in revised form 22July 2008Accepted 23July 2008Available online xxxx Keywords:Hydrotalcite LDH

Heavy metal uptake Dmsa Dtpa

The hydrotalcite-like compound [Zn 2Al(OH)6]NO 3·n H 2O (ZnAl-NO 3)was intercalated with the chelating agents diethylenetriaminepentaacetic acid (dtpa)and meso-2,3-dimercaptosuccinic acid (dmsa)by anion-exchange to uptake Cu 2+,Cd 2+and Pb 2+from aqueous solutions.The amounts of heavy metals adsorbed at variable contact times and metal concentration were determined by atomic absorption spectrometry.The amounts removed of the three metal cations by both adsorbents were high.The shape of the adsorption isotherms obtained indicated speci ?c interactions and a high host –guest af ?nity.However,the metal ions were removed from solution not only by chelation,but also by precipitation or even by isomorphic substitution of Zn 2+by another metal ions in the brucite-like layer.

1.Introduction

Metal cations in aqueous solutions can interact in various ways with solid surfaces including precipitation,adsorption,ion exchange and absorption in soil pores.Knowledge of such interactions is important with a view to better understanding the way heavy metal ions behave in the environment and facilitating their removal.One effective way of removing metals is adsorption on various materials such as activated carbon (Chen and Wu,2004),biomaterials (Han et al.,2006;Gupta et al.,2006)and clay minerals.In recent years,clay minerals found increasing interest as adsorbents by virtue of their properties,which make them attractive materials for adsorbing heavy metal ions.Their abundance in nature,low cost and good cation adsorptive properties –a result of their negatively charged layers and high speci ?c surface areas –make them suitable for this purpose (Brigatti et al.,1996;Undabeytia et al.,1996;Stauton and Roubaud,1997).In any case,the selectivity of clay minerals for speci ?c metal ions can be improved by intercalating metal-chelating agents.For example,covalent grafting of ligands containing the thiol functionality has been found to increase the af ?nity of clay minerals for Hg (II)ions (Mercier and Detellier,1995;Celis et al.,2000;Lee et al.,2002).Presence of carboxyl groups in some clay minerals also improved adsorption of Pb (II)(Cruz-Guzmán et al.,2006).

Layered double hydroxides (LDHs)are the antitypes of clay minerals.A wide range of LDHs have been synthesized from various divalent (M II =Mg 2+,Zn 2+,Ni 2+)and trivalent cations (M III =Al 3+,Fe 3+,Cr 3+)in

variable M II /M III mole ratios and accompanied by interlayer anions of diverse nature.The ease of preparation and widely variable ways in which their components can be combined make LDHs useful for many applications (Cavani et al.,1991;Rives and Ulibarri,1999).However,only recently have hydrotalcite-like compounds been used as adsorbents for heavy metals (Gutmann et al.,2000;Tarasov et al.,2003;Li et al.,2004;Kameda et al.,2005;Pérez et al.,2006;Park et al.,2007).

We studied the uptake of metal cations (Cu 2+,Pb 2+and Cd 2+)from water solutions by ZnAl-LDH intercalated with the chelating agents dtpa and dmsa.These particular agents,which are widely used to adsorb metals in medical applications (Bucci et al.,2000;Crisponi et al.,2002;Lattuada and Lux,2003),were chosen on the grounds of the high af ?nity of carboxylates for LDH interlayers.The resulting anion-exchange products (“adsorbents ”)were designated ZnAl-dtpa and ZnAl-dmsa,respectively.2.Experimental

All synthetic reagents used were at least 98–99%pure.

The potential adsorbents ZnAl-dtpa and ZnAl-dmsa were obtained by anion-exchange of the precursor ZnAl-NO 3with dtpa and

dmsa.

The hydrotalcite-like compound ZnAl-NO 3was prepared by co-precipitation (Reichle,1986)in distilled,CO 2-free water at pH 8.The

Applied Clay Science xxx (2008)xxx –xxx

?Corresponding author.Tel.:+34957218648;fax:+34957218621.E-mail address:maulibarri@uco.es (M.A.Ulibarri).

CLAY-01502;No of Pages 5

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Applied Clay Science

j o u r n a l ho m e p a g e :w w w.e l se v i e r.c o m /l o c a t e /c l a y

ARTICLE IN PRESS

solution was purged with N2to avoid CO2uptake from atmosphere. Then,0.015mol of dtpa(dmsa)was dropped over150mL of ZnAl-NO3 suspension under N2stream at75°C.The pH was kept at 5.5 throughout the synthetic processes.Based on the aqueous dissociation constants for the ligands(Sillen and Martell,1964;Crisponi et al.,2002) (Table1),both chelating agents were partially dissociated and thus suitable for intercalation by anion-exchange.Also,pH5.5ensured minimal dissolution of atmospheric CO2.The end products,ZnAl-dtpa and ZnAl-dmsa,were separated by centrifugation,washed and dried at 60°C.

Adsorption of Cu2+,Pb2+and Cd2+by ZnAl-dtpa and ZnAl-dmsa was studied by suspending duplicate samples of0.1g of each adsorbent in 30mL of aqueous solutions of Cu2+,Pb2+and Cd2+nitrate salts.The suspensions were shaken at50rpm at room temperature,using an initial pH of3to avoid precipitation of metal hydroxides[Ks Cu(OH)2=1·10?20, Ks Cd(OH)2=3.2·10?14and Ks Pb(OH)2=2.5·10?16].The?nal pH,5±0.5in all cases,was rapidly reached.The resulting supernatants were used to determine the amount of each metal adsorbed by the LDHs,using atomic absorption spectrometry.Such an amount was calculated as the difference between the initial and?nal concentrations of metal ion in the supernatant.

Also,we performed the metal uptake experiment under the same

conditions and for initial concentration7mM on the precursor ZnAl-NO3.

X-ray diffraction patterns(XRD)were recorded on powder samples at room temperature under air conditions,using a Siemens D-5000 instrument with Cu Kαradiation.

FT-IR spectra were recorded by using the KBr disc method on a Perkin Elmer Spectrum One spectrophotometer.

Elemental chemical analyses for Zn,Al,Cd,Cu and Pb were done by atomic absorption spectrometry on a Perkin Elmer AA-3100instru-ment.Samples were dissolved in concentrated HCl prior to analysis. Carbon and nitrogen were determined on an Euro Elemental Analyze Eurovector instrument.

3.Results and discussion

3.1.Characterization of ZnAl-dtpa and ZnAl-dmsa

Table2shows the results of the chemical analysis of the precursor ZnAl-NO3and the adsorbents ZnAl-dtpa and ZnAl-dmsa.The inter-layer water content was determined thermogravimetrically from the mass loss observed between room temperature and150°C(diagrams not shown).The Zn/Al molar ratio in LDH-NO3was close to that in the starting solution,whereas those in ZnAl-dtpa and ZnAl-dmsa were somewhat lower.This was probably a result of partial dissolution of the hydroxyl layers during exchange of the anion(pH5.5),and also of the ligands(added in excess to the solution)reacting with Zn2+layer cations to form Zn–dmsa and Zn–dtpa soluble complexes.

Based on the XRD,FT-IR and elemental analysis results,dtpa and dmsa were intercalated in the LDH interlayer,as https://www.wendangku.net/doc/2a17058913.html,yer charge compensation was completed by a small amount of NO3?anions,as con?rmed by the weak bands at1385cm?1in FT-IR spectra for both adsorbents and,in ZnAl-dmsa,by the likely presence of some CO32?sharing the interlayer indicated by the excess amount of carbon found in the elemental analysis.The FT-IR spectrum for ZnAl-NO3(Fig.1)is typical of hydrotalcite-like compounds and similar to one previously reported by Cavani et al.(1991).The wide band at～3500cm?1 corresponds to vibrations of OH?groups in the brucite-like layers and water molecules;also,the water bending vibration at1623cm?1and the sharp peak at1383cm?1can be assigned to stretching vibrations of NO3?groups.

Based on the p K values(Table1),both ligands can be assumed to occur mainly as divalent anions at pH5.5.However,the results of the elemental analysis(Table2)suggest that dtpa is probably intercalated as [HL]4?and dmsa as[L]4?.The FT-IR spectra of the exchanged products (Fig.1and Table3)seem to con?rm this hypothesis,because of the presence of ionized carboxyl group bands in the form of an antisym-metric vibration and a symmetric one of–COO?at ca.1600cm?1and 1400cm?1,respectively(Nakamoto,1986;Bellamy,1975).On the other hand stretching vibration of unionized–COOH at ca.1700cm?1and the bands at2564and2537cm?1forνS–H present in the spectra for the pure

Table1

Ionization constants for dtpa and dmsa

p K1p K2p K3p K4p K5

dtpa 1.79 2.56 4.428.7610.42 dmsa 2.71(C1) 3.43(C2)9.65(S1)12.05(S2)–

Table2

Chemical analysis of ZnAl-NO3,ZnAl-dtpa and ZnAl-dmsa

Sorbent Wt(%)Atomic ratio Proposed formula Zn Al C S N Zn/Al Al/N Al/S

ZnAl-NO335.27.9–– 4.00 1.83––[Zn0.65Al0.35(OH)2]

(NO3)0.35·0.4H2O

ZnAl-dtpa23.48.312.7– 3.3 1.16 1.3–[Zn0.54Al0.46(OH)2]

(dtpa)0.12?x A?x·0.6H2O ZnAl-dmsa28.68.8 4.5 3.8– 1.33– 2.8[Zn0.57Al0.43(OH)2]

(dmsa)0.08?x A?x·0.3H2O A?x:NO?3and/or CO32?

anions.Fig.1.FT-IR spectra for(a)ZnAl-NO3,(b)dtpa,(c)ZnAl-dtpa,(d)dmsa and(e)ZnAl-dmsa.

Table3

Selected IR results obtained over the wavenumber range4000–1000cm?1 Wavenumber(cm?1)Assignment ZnAlNO3dtpa ZnAl-dtpa dmsa ZnAl-dmsa

～3500～3500～3500～3500–νOH

–3074,3010Not visible2971Not visibleνC–H

1623––H2O bend 1384–1384–1384νNO3

–––2564,2537νS–H

–1730,1693–1700–νC=O(COOH)–1595,1400–1577,1370νC=O(COO?)

2I.Pavlovic et al./Applied Clay Science xxx(2008)xxx–xxx

acids,were absent from the spectra for the LDH –ligand complexes which suggests deprotonation of –COOH and –SH groups in the LDH interlayers.Nevertheless,the band of an undissociated group –COOH in LDH –dtpa complex,which should be expected to be present,is not observed in the spectrum,probably due to its weakness and/or over-lapping by the wide –COO ?band,resembling to that reported in the case of a ZnAl-edta LDH (Pérez et al.,2006),and also Tarasov et al.(2003),where in the IR spectra of LiAl-(H 2edta)and LiAl-(Hedta)the band of undissociated carboxyl groups was not observed.

However,it was dif ?cult to estimate the extent of deprotonation of dtpa and dmsa in the interlayers owing to the high complexity of these systems,because of localized alkaline conditions in the solid (Park et al.,2007),the possibility of dmsa to form dimers and/or even poly-mers (Nakayama et al.,2007)and also the possibility to form com-plexes between a fraction of layer cations and complexing agents during the synthesis (Rojas et al.,data in preparation).

The XRD patterns for ZnAl-dtpa and dmsa (Fig.2)are both typical of hydrotalcite-like compounds (Cavani et al.,1991);however,their (00l )re ?ections differ from those for the precursor ZnAl-NO 3.The presence of (003),(006)and (009)re ?ections in these patterns suggests that the layered hydrotalcite structure was retained in spite of the decreased Zn/Al ratio resulting from layer erosion.

These patterns reveal that intercalation of the chelating agents in ZnAl-NO 3increased d (003)from 8.8?to 13.9?for dtpa and 10.5?for dmsa.Subtracting the thickness of the hydrotalcite-layer,4.8?,reveals that both ligands were probably accommodated in the interlayer,tilted to the layers (CS Chem 3D 5.0software package).A similar d 003value,11.6?,was previously obtained for dmsa intercalated in MgAl-LDH (Nakayama et al.,2007).On the other hand,d 110was slightly smaller for ZnAl-dtpa (1.520?)and ZnAl-dmsa (1.523?)than it was for the precursor (1.532?),which is consistent with the decreased Zn/Al molar ratio in the exchanged products.

3.2.Heavy metal ions adsorption on ZnAl-dtpa and ZnAl-dmsa The metal ions uptake experiments (kinetics and isotherms)were carried out at initial pH 3(?nal pH=5±0.5)and initial metal concentra-

tions (1–10mmol/L)in the order to minimize precipitation,in contrast to our previous work on the metal ion uptake by ZnAl-edta (Pérez et al.,2006),where the experiments of kinetics and adsorption isotherms were performed at higher initial pH (from 3until 6–8)and initial metal ion concentrations in the range of 1–40mmol/L.

The kinetic results of Fig.3indicated uptake of the metal cations from aqueous solution by both modi ?ed adsorbents,very similar to data reported before for ZnAl-edta (Pérez et al.,2006).The ?rst

step

Fig.2.XRD patterns for (a)ZnAl-NO 3,(b)ZnAl-dtpa and (c)

ZnAl-dmsa.

Fig.3.Uptake of metal cations by (a)ZnAl-dtpa and (b)ZnAl-dmsa.C i =7

mM.

Fig.4.Heavy metal adsorption isotherms for (a)ZnAl-dtpa and (b)ZnAl-dmsa.

I.Pavlovic et al./Applied Clay Science xxx (2008)xxx –xxx

involved fast adsorption of the almost all amounts of metal ions in solution,within a few hours,followed by slower,more gradual adsorption.Only Cu 2+was slowly adsorbed by ZnAl-dtpa,where equilibrium required 96h to be reached.Although the metals were presumably adsorbed by chelation with the ligands in the interlayer (Tarasov et al.,2003;Kameda et al.,2005),precipitation induced by a higher surface pH was also likely (Park et al.,2007).The slow uptake of Cu 2+from the solution may have resulted from the formation of some stable precipitate at the initial metal concentration used in the test (7mM)considering the high stability of Cu 2+hydroxide [Ks Cu(OH)2=1·10?20]relative to Pb 2+and Cd 2+hydroxide.Thus,covering of LDH particles by some Cu 2+hydroxide precipitate may hinder interaction with the ligand (Park et al.,2007).The different kinetics of Cu 2+adsorption on these adsorbents,(rapidly adsorbed on ZnAl-dmsa and gradually on ZnAl-dtpa),may have resulted from the presence of thiol groups in dmsa,which exhibit a higher af ?nity for Cu 2+than dtpa.

The adsorption isotherms shown in Fig.4,were of Langmuir H-type (Giles et al.,1960).This suggests a very high af ?nity between the adsorbent and adsorbate;in fact,the metal ions were nearly completely adsorbed when used at the lowest initial concentrations.

These results contradict those previously reported for the adsorption of heavy metals on –SH and –COO ?functionalized clay minerals (Mercier and Detellier,1995;Cruz-Guzmán et al.,2006),where only Hg 2+exhibited a high –SH af ?nity,Pb 2+was more ef ?ciently adsorbed on carboxyl-functionalized organoclays.Brown et al.(1999)ascribed the low af ?nity of Pb 2+and Cd 2+for thiol groups in the pores of functionalized clay minerals to their thermodynamic inability to coordinate within the space of the adsorbent pore channels.Based on our results,the –SH functionalized space in the Zn-Al-hydrotalcite seems to be more accessible to these cations.Also,in contrast with our results,Nakayama et al.(2007)reported that only small amounts of Cu 2+and Pb 2+were adsorbed on MgAl-dmsa.

Fig.5shows the XRD patterns for the products (C i =10mM);the (00l )re ?ections in the patterns indicate that the hydrotalcite structure was retained despite the low initial pH used in the adsorption tests.The patterns revealed poor crystallinity and turbostatic effects which made dif ?cult accurate identi ?cation of the re ?ections.Thus,metal adsorp-tion had virtually no effect on (00l )re ?ections,as also previously found

in heavy metals intercalated into Mg-Al-edta,Li-Al-edta and Zn-Al-edta (Kaneyoshi and Jones,2001;Lukashin et al.,2003;Li et al.,2004;Pérez et al.,2006).By contrast,Cd 2+adsorption on ZnAl-dtpa resulted in slight expansion of the interlayer space.This may have been a consequence of Cd 2+probably being removed from solution mainly by chelation;in fact.Because the formation constant of Cd 2+hydroxide is larger than those for Cu 2+and Pb 2+hydroxide (shown above),the fraction of cadmium removed by precipitation must have been lower.In any case,no increase in d(00l )was observed by Cd 2+adsorption on ZnAl-dmsa.

On the other hand,adsorption of Pb 2+on ZnAl-dmsa changed the (003)re ?ection from 10.3to 8.8?.This is suggestive of intercalation of NO 3?in the interlayer,accompanied by precipitation of Pb 2+as PbCO 3,which was also apparent from the XRD pattern.The pattern exhibited a (006)re ?ection for ZnAl-dmsa,which indicates both dmsa and NO 3?in the interlayer space.The presence of PbCO 3may have resulted from its higher stability relative to CuCO 3and CdCO 3[Ks PbCO3=1.5·10?13,Ks CuCO3=2.5·10?10and Ks CdCO3=2·10?12].Therefore,in this case Pb +2removed carbonate anions present in ZnAl-dmsa (Table 2)to form PbCO 3.Park et al.(2007)also found Pb 2+to precipitate with LDH interlayer carbonate and be replaced by chloride anions at the edges,which contradicts the high selectivity of LDH for carbonate anions.In the study of the uptake of Cu 2+and Cd 2+by ZnCr-LDH intercalated by nitriloacetate (nta)Gutmann et al.(2000)suggested that complex formation would cause an excessive positive charge in the LDH,which is likely to be compensated by intercalation of anions present in the solution.These authors found that degree of coordination of Cu 2+with intercalated nta was much higher when accompanied by intercalation of

NO 3?or Cl ?

into the interlayer.On the other hand,Tarasov et al.(2003)found that in the reactions of metal salts with LiAl-LDH intercalated with (Hedta 3?),the composition of the ?nal product depend of the anions,rather than of the metals taken for the treatment.They reported that the

treatment of that LDH with NO 3?or Cl ?

metal salts leads to the intercalation of the metals together with these anions as it seems to be the case in the present https://www.wendangku.net/doc/2a17058913.html,ly,in all the FT-IR spectra of ZnAl-dtpa(dmsa)-metal ?nal products,the sharp and intensive band of NO 3?was observed (data not shown),suggesting the possibility of co-intercalation of this anion in the LDH interlayer during the chelation reaction.However the amounts of

NO 3?and/or CO 32?

present were not high enough to be detected in XRD patterns,except in the case of ZnAl-dmsa-Pb (Fig.5).

Finally,we have also considered the possibility of “diadochy ”,i.e.isomorphic substitution of Zn 2+by Cu 2+in the layer,not necessarily accompanied by changes in the XRD patterns (Komarneni et al.,1998).We performed the metals uptake experiments on the precursor ZnAl-NO 3obtaining very low values of metals amount removed per gram of adsorbent:Cs=27,54and 80μmol/g for Cd 2+,Cu 2+and Pb 2+,respectively.However,it is dif ?cult to relate these values to those found for ZnAl-dmsa and ZnAl-dtpa.Komarneni et al.(1998)reported that transition metals can be uptaken by MgAl-LDH (intercalated by nitrate or carbonate)by diffusing through the interlayer space and isomorphic substitution of Mg 2+.Amount of transition metal ions bound in this way strongly depends on its concentration as well as on the nature of the interlayer anion.Kameda et al.(2005)also reported on the uptake of Cu 2+by MgAl-CO 3,and they state that the slight removal of this metal could be due to the coprecipitation of Cu 2+with the dissolved Al 3+to form Cu-Al-LDH.In any case,the low metal amounts we found to be removed from the solution by ZnAlNO 3compared to those removed by ZnAl-dtpa and ZnAl-dmsa,as well as by ZnAl-edta (Pérez et al.,2006)under the same conditions,could be indication of the important role of the coordination of these metals with the complexing agents into the interlayers.

Such a variety of possibilities makes it dif ?cult to identify all potential forms in which metals may be adsorbed.One plausible explanation for the adsorption behavior of the studied metals is based on the stability of their precipitates (hydroxides and carbonates)and of their dtpa and dmsa complexes.In any case,we found no direct relationship between the adsorption behavior and

formation

Fig.5.XRD patterns of sorption isotherm products (C i =10mM).

I.Pavlovic et al./Applied Clay Science xxx (2008)xxx –xxx

constants of the metal complexes with dtpa(viz.21.5,18.6,19.0and 19.7for Cu,Pb,Cd and Zn,respectively)(Jusys et al.,1999;Anderegg et al.,2005).Thus,we state that the stability of the metal–dtpa complexes was not the determining factor for metal uptake by the LDHs used here.Unfortunately,comparing the stability of the studied metal–dmsa complexes was dif?cult because their constants had scarcely been studied(Harris et al.,1991).

The FT-IR spectra of all end products(results not shown)exhibited only slight changes in position in the antisymmetric vibration of co-ordinated–COO?groups,which are sensitive to interactions between molecules which could be indication of chelating of the metals with the interlayers ligands(Nakamoto,1986;Kaneyoshi and Jones,2001).

The previous results suggest that ZnAl-LDH intercalated with the ligands dtpa and dmsa could provide a potential remedy for metal contamination in soils and waters.Thus,these materials might provide an effective solid matrix for immobilizing metals without substantially altering environmental pH values.Also,ZnAl-LDH may be a better support than MgAl-LDH with a view to designing potential barriers on account of its higher stability under acid conditions.In fact,layer erosion was restricted by effect of the increased stability of the Zn(OH)2 precipitate(Ks=4.5·10?17)relative to Mg(OH)2(Ks=3·10?10);thus,less than1%Zn2+was detected in the supernatants following adsorption of the metals.Strict control of Zn2+release from the adsorbents would be required.Although this metal is deemed non-toxic,it may be environmentally hazardous at suf?ciently high concentrations.

4.Conclusions

Two different ZnAl-LDHs intercalated with the chelating agents dtpa and dmsa were used to remove Cu2+Cd2+and Pb2+from solution. The adsorption process was performed at variable contact times and metal concentrations.Adsorption of these metal ions occurred mainly through chelation by interlayer ligands.

The adsorption isotherms indicated a high host–guest af?nity.The presence of dmsa in the hydrotalcite interlayer increased the af?nity for the metal ions relative to ZnAl-dtpa.Although Cu2+,Cd2+and Pb2+were mainly adsorbed by chelation,a certain amount may have been precipitated due to the higher surface alkalinity of the LDH.PbCO3was also detected by XRD.Precipitation of amorphous metal and isomor-phous substitution,might also be involved in the adsorption processes.

Acknowledgements

This work was co-funded by Spain's Ministry of Education and Science(MEC)(Projects AGL2005-05063-CO2-02and MAT2003-06605-C02-02),Junta de Andalucía(Research Group FQM-214)and Excellence Project(RNM-523).

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