Hydrometallurgy 96 (2009) 27–34

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Hydrometallurgy

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Extraction and separation studies of silver(I) and copper(II) from their aqueoussolution using chemically modified melamine resins

M.A. Abd El-Ghaffar a,⁎, Z.H. Abdel-Wahab b, K.Z. Elwakeel c

a Department of Polymers and Pigments, National Research Centre, Dokki, Cairo, Egyptb Department of Chemistry, Faculty of Science (Girls), Al-Azhar University, Cairo, Egyptc Egyptian Water and Wastewater Regulatory Agency, Masraweya District, 5th Community, New Cairo City, Egypt

⁎ Corresponding author. Tel.: +20 12 7901129; fax: +2E-mail addresses: mghaffar50@yahoo.com (M.A. Abd

khalid_Elwakeel@yahoo.com (K.Z. Elwakeel).

0304-386X/$ – see front matter © 2008 Elsevier B.V. Adoi:10.1016/j.hydromet.2008.07.008

a b s t r a c t

a r t i c l e i n f o

Article history:

Two modified melamine re Received 9 February 2008Received in revised form 5 July 2008Accepted 23 July 2008Available online 29 July 2008

Keywords:Modified melamine resinsCopper and silver separation

sins has been prepared and investigated, the modification process took placethrough the treatment with thiourea to produce R1 or tetraoxalyl ethylenediamine to produce R2. Theadsorption behavior of the obtained resins towards Ag(I) and Cu(II) from their aqueous solutions has beenstudied using batch method. The obtained resins were tested for selective separation between Cu(II) and Ag(I)form their binarymixtures. Resin R1 showedhigh selectivity towards Ag(I) fromCu(II) solution,while R2 gave apromising results for the selective separation of Cu(II) from Ag(I) solution. Both kinetic and thermodynamicparameters of the adsorption process were obtained. Thermodynamic data indicated that the adsorptionprocess is endothermic spontaneous reaction. Kinetic analysis showed that the adsorption of Ag(I) on R1 isperfectly fit pseudo-first order model, while the adsorption of Cu(II) on R2 is perfectly fit pseudo-second orderone. The interaction mechanism between metal ion and active sites has been interpreted as chelation.Durability of resins was estimated from column studies. These parameters indicated that the prepared resinshave a good efficiency for repeated use.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Silver(I) is generally found in the combined state in nature, usuallyin copper(II) or lead(II) mineralization (Butterman and Hilliar, 2005).Solvent extraction was used in the recovery of silver(I) from chemicalsolutions (Wan and Miller, 1986). The small but finite aqueoussolubility of extractants, diluents and modifiers is a major disadvan-tage of solvent extraction. The loss of organics by evaporation andentrainment is also a potential problem. Adsorption has attractedattention because of new material types available for the recoveryprocess. Some of these materials are zeolits, activated carbon, fly ash,biosorbents, resins (ion exchanger and chelating compounds) andothers. Chelating resins can be produced by attaching or constructingchelating groups corresponding to known analytical reagents ontoinsoluble polymeric materials (Donia et al., 2005a,b; Donia et al.,2008). The nature of the functional groups gives an idea about theselectivity of the resin towards metal ions (Katarina et al., 2006).Chelating resins have many practical applications in the fields ofchemical analysis and environmental protection (Pohl and Prusisz,

0 2 3370931.El-Ghaffar),

ll rights reserved.

2004; Pan et al., 2006; Chen et al., 2007). Several chelating resins withdifferent functionalities were used for concentrating and retrieving ofsilver(1) from solutions (Donia et al., 2007; Yirikoglu and Gülfen,2008). Thiourea resins showed higher adsorbing capacity, rate, andselectivity for Ag(I) than for the other ions (Ni et al., 2001). Melamine-formaldehyde (MF) resins were not widely studied for metal ionsremoval purpose. Few publications were reported in this area ofapplication, for example MF resins were used for the separation andpreconcentration of Cr(VI)/Cr(III) (Dixon et al., 1947; Demirata, 1996)and Fe(III)/Fe(II) from natural water (Filik et al., 1997). MF-diethylene-triaminepentaacetic acid resin was prepared as a new adsorbent forremoving heavy metals from wastewater effluents (Baraka et al.,2007). The importance of this type of resins comes from their highthermal stability and possible modification of their physical andchemical properties to fulfill the application needed allowing theproduction of a good adsorbent (Baraka et al., 2006, 2007). Theultimate goal of this study is to modify MF resin with differentchelating reagents to be amenable for the extraction and separation ofsilver(I) or copper(II) from their aqueous solution, therefore MF wasmodified with thiourea (to produce Ag(I) selective resin) or tetraoxalylethylenediamine (to produce Cu(II) selective resin), thus we canseparate Ag(I) or Cu(II) from their mixture using any one of the twoadsorbents. The application of modified melamine resins for theselective separation between Ag(I) and Cu(II) from their aqueoussolutions was studied. Thermodynamic and kinetic analysis foradsorption process has also been carried out.

28 M.A. Abd El-Ghaffar et al. / Hydrometallurgy 96 (2009) 27–34

2. Experimental

2.1. Chemicals

Melamine, formaldehyde (37%), thiourea, were Aldrich products. Allother chemicalswere Prolabo products andwere used as received. Silvernitrate and copper acetate were used as a source for Ag(I) and Cu(II)respectively.

2.2. Preparation of MF-thiourea resin (R1)

R1 was prepared from the reaction of melamine, thiourea andformaldehyde at molar ratio of (1.5:1:1), respectively according to thefollowing method: in 250 mL round flask 0.76 g of thiourea wasdissolved in 100mLdistilledwater, thereafter 1.89 gmelamine and 8mLformaldehyde were added, then few drops of ammonia was added toreach pH 9. The reaction mixture was heated at 90 °C for 6 h withstirring. The precipitate produced was collected, washed thoroughlywith hot water, cold water, ethanol then acetone and air dried.

2.3. Preparation of tetraoxalyl ethylenediamine (TOED)

Tetraoxalyl ethylenediamine (TOED)was prepared according to thepreviously reported method (Naser et al., 1980). Oxalic acid andethylenediamine with molar ratio (4:1) were mixed in 250 mL round-bottom flask fitted with Dean & Stark apparatus. The mixture washeated under reflux with xylene until the theoretical amount of waterhas been collected. The reaction mixture was then cooled to roomtemperature, filtered and dried at 50 °C under vacuum. The solidcollected was recrystallised from a suitable solvent.

2.4. Preparation of MF-TOED resin (R2)

R2 was prepared through the reaction of melamine, Tetraoxalylethylenediamine and formaldehyde at molar ratio of (1.5:1:1),respectively. In 250 mL round flask 1.89 g of melamine was dissolvedin 100 mL water, then 3.48 g of TOED was added to the flask, thecontents of the flask was heated inwater bathwith stirring at 75 °C for5 h. Then 8mL formaldehydewere added to the reactionmixture withcontinuous stirring. Thereafter the temperaturewas raised to 90 °C for6 h. The precipitate gel particles was collected, washed thoroughlywith hot water, cold water, ethanol then acetone and air dried.

2.5. Characterization of chelating resin

2.5.1. Water regainWater regain factor, W%, represents the percentage of water held

intrinsically by the resin. For water regain determination, resinsamples were centrifuged for one h at 1000 rpm to remove excesswater and then weighed. These samples were then dried at 50–60 °Cuntil complete dryness then weighed again. To calculate this factor,the following equation was applied:

W% ¼ 100 Ww−Wdð ÞWw

ð1Þ

where Ww and Wd are weights (g) of the wet and dried resinrespectively. Water regain values are around 56±3% and 50±3% for R1and R2, respectively with insignificant differences when changingconditions. This value reflects the hydrophilic character of the resintype.

2.5.2. Surface areaThe surface area of the prepared resins was measured by

methylene blue adsorption as this material is known to be adsorbedas a monolayer only on solid sorbents. A standard solution of this

material was prepared (0.02 g/L). A calibration curve for methyleneblue was drawn (λ=660 nm) by measuring dilutions from standardstock. To calculate the surface area, 0.1 g of resin was treated with25 mL of methylene blue of concentration 0.02 g/L. The treatmentlasted until there was no further decrease in absorbance. The amountof methylene blue adsorbed was calculated based on concentrationdifference between the initial and equilibrium values, which weremeasured by DR 5000 spectrophotomer (HACH) USA. The surface areaof the resin was calculated using the following equation (Baraka et al.,2006):

As ¼ GNAvF10−20

MMWð2Þ

whereAs is the gel resin surface area inm2/g,G the amount ofmethyleneblue adsorbed (g), NAv the Avogadro's number (6.02×1023), Ø themethylene blue molecular crosssection (197.2 Å2), MW the molecularweight of methylene blue (373.9 g/mol) andM is the mass of adsorbent(g). The surface area of the prepared resinswas calculated fromEq. (2) tobe 208 and 160 m2/g for R1 and R2, respectively.

2.5.3. Pore Size determinationThe average pore sizes of the prepared resins were measured using

High Pressure Mercury Pore Size Analyzer, Micromeritics 9320, USA.The average pore diameters were found to be 0.12 and 0.09 (µm) for R1and R2, respectively.

2.5.4. FT-IR spectraFT-IR spectra of the synthesized resin were performed using

Nicolet IR200 FT-IR Spectrometer.

2.5.5. Determination of active sits concentrationElemental analysis was used for sulphur content determination in

R1 and found to be 8.1%, which is equivalent to 2.5 (mmol sulphur/gresin).

Carboxyl content of R2 was determined according to the earlyreported method (Atia et al., 2005a,b). One gram of R2 was shaked in100 mL of 0.1 M NaOH at room temperature for 6 h. The mixture thenwas filtered and the concentration of unreacted NaOH was deter-mined through the titration of 5 mL of the filtrate with 0.1 M HCl.Therefore, the concentration of carboxyl group (mmol/g) wascalculated by the following mass balance equation:

Carboxyl group content mmol=gð Þ ¼ M1−M2ð Þ � VW

ð3Þ

WhereM1 andM2 are the initial and final concentrations of NaOH,V is the volume of NaOH, W is the weight of the resin. The concen-tration of carboxyl groups in R2 found to be 3.6 mmol/g.

2.6. Preparation of solutions

Stock solutions of silver nitrate and copper acetate (2×10−2 M)were prepared. A stock solution of EDTA (5×10−3 M)was prepared andstandardized against a solution of MgSO4·7H2O using EriochromeBlack-T (EBT). HNO3 and NaOH were used to change the acidity of themedium. Thiourea (0.5 M) acidified with H2SO4 (0.2 M) was used as aneluent for stripping Ag(I) adsorbed on R1 and 0.5 M of HCl forstripping Cu(II) adsorbed on R2.

2.7. Uptake measurements

2.7.1. Effect of equilibrium pH on the uptakeUptake experiments under controlled pH were carried out by

introducing 0.1 g of dry resin in a series of flasks containing 100 mL ofAg(I) or Cu(II) solutions at initial concentrations of 1×10−2 M. Thedesired pH was controlled using HNO3 and NaOH and measured using

29M.A. Abd El-Ghaffar et al. / Hydrometallurgy 96 (2009) 27–34

pH meter model HI 255 (HANNA instruments). The flasks wereconditioned on a vibromatic shaker at 300 rpm at 25±1 °C for 3 h. Fivemilliliters of the solution were taken and filtered off where residualconcentration of metal ion was determined. Copper(II) concentrationwas determined via the titration against 5 × 10−3 M EDTA usingmurexide as an indicator (Bassett et al., 1978) while Ag(I) concentra-tion was determined by the replacement titration method usingpotassium tetracyanonickelate(II) (Bassett et al., 1978). Studying theadsorption of Ag(I) or Cu(II) in strong basic media was avoided due tothe precipitation of metal hydroxide.

2.7.2. Selectivity studiesThe uptake behavior of Ag(I) from binary mixtures with Cu(II), Pb

(II), Cd(II), Zn(II) or Ca(II) was studied at normal pH using R1. There-fore 0.1 g of dry resin was introduced in a series of flasks, each flaskcontaining 100 mL of 1×10−2 M Ag(I) solution in a binary mixturewith 1×10−2 M Cu(II), Pb(II), Cd(II), Zn(II) or Ca(II). Five millilitres ofthe solution was taken after the equilibration time then filtered off,where the residual concentration of metal ion was determined viathe titration against 5×10−3 M EDTA using murexide as an indicatorfor Cu(II). EBT was used as an indicator for Ca(II), Mg(II), Cd(II), Zn(II)and Pb(II). Also the recovery of Cu(II) or Pb(II) from Ag(I) solution wasstudied using R2 at normal pH, initial concentration of 1×10−2 M andthe same other adsorption conditions.

2.7.3. Kinetic studiesMetal ionuptake atdifferent time intervalsweredoneby introducing

0.1 g of R1 in a flask containing 100 mL of Ag(I) and 0.1 g of R2 in a flaskcontaining 100mLof Cu(II) solution at initial concentration of 1×10−2Mat normal pH. The contents of the flask were equilibrated on aVibromatic-384 shaker at 300 rpm and at 25±1 °C. Five milliliters of

Fig. 1. FT-IR spectra of the

the solution were taken at different time intervals where the residualconcentration of the metal ions was determined.

2.7.4. Adsorption isothermsComplete adsorption isothermswere obtained by introducing 0.1 g

resin in a series of flasks. To each flask, 100 mL of Cu(II) or Ag(I)solution at the desired initial concentration were added. The flaskswere conditioned at 300 rpmwhile keeping the temperature at 25, 35or 45 °C for 3 h. Later on, the residual concentration was determinedwhere the metal ion uptake was estimated.

2.7.5. Column experimentsColumn experiments were performed in a plastic column with a

length of 10 cm and a diameter of 1 cm. A small amount of glass woolwas placed at the bottom of the column to keep the contents. Onegram of resin was introduced into the column. Metal ion solutionshaving an initial concentration of 4×10−3 M were flowed downwardthrough the column at flow rate of 1 mL/min. Samples were collectedfrom the outlet of the column at different time intervals and analyzedfor metal ion concentration. The operation of the column wasterminated when the outlet metal ion concentration matches itsinitial concentration. The outlet metal ion concentrations were plottedversus time to give the breakthrough curves.

2.7.6. ElutionThe resin loaded by metal ion in the previous column was then

subjected for elution. 100 mL of thiourea (0.5) acidified with H2SO4

(0.2 M) was used for stripping Ag(I) adsorbed on the resin and 0.5 M ofHCl was used for stripping Cu(II). After elution of the resin, the columnwas carefully washed with water, dilute alkali and finally with distilledwater to become ready for reuse. This process was repeated for 3 cycles.

investigated resins.

Fig. 2. Effect of pH on the uptake of Ag(I) by R1 and Cu(II) by R2 from initial

Scheme 1. Proposed structures of the studied chelating resin.

30 M.A. Abd El-Ghaffar et al. / Hydrometallurgy 96 (2009) 27–34

3. Results and discussions

3.1. FT-IR spectra

The obtained resins were characterized by FT-IR spectroscopy. Asshown in Fig. 1 the spectra of the investigated resins showed commonpeaks due to similar groups present in both resins, such as 3317 cm−1

which can be assigned to the N–H stretch of secondary amine attachedto themethylene bridge. The peak at 1553 cm−1 can be assigned to N–Hbend of bridging secondary amine. The spectrum of R1 showed acharacteristic beak at 1156 cm−1. This beak is attributed to –N–CfSgroup. For R2 spectrum, the amide carbonyl (CfO) stretch appeared at1650 cm-1, amide N–H out of plane appeared at 717 cm-1 and carboxylicin plane O–H bend appeared at 1339 cm-1. The suggested structures ofthe resins are presented in Scheme 1.

3.2. Uptake studies using batch method

3.2.1. Uptake as a function of equilibrium pHMetal ions adsorption from aqueous solution into a resin is a pH

dependent process due to pH influence on both the chemistry of metalions and resin functional groups. The data of the effect of variation ofequilibrium pH on the uptake of Ag(I) or Cu(II) is shown in Fig. 2. It isseen that the uptake of Ag(I) by R1 slightly increases as theequilibrium pH increases in the pH range 2.0 to 7.2. The maximumuptake value was reported at initial pH 7.2. The dominant mechanismof interaction is probably due to the presence of free lone pairs ofelectrons on nitrogen or sulfur atoms that are suitable for coordinationwith Ag(I) to give the corresponding resin–metal complex. In acidicmedia, HNO3 may compete with AgNO3 for interaction with the resinas follows:

Polymeric matrix� ðN=SÞ þ HNO3 ¼ Polymeric matrix� ðN=SÞHþNO−3

Polymeric matrix� ðN=SÞ þ AgNO3 ¼ Polymeric matrix� ðN=SÞAgNO3

According to the Hard and Soft Acid and Bases (HSAB) theory(Myasoedova et al., 1985), the sulfur and the nitrogen atoms inthiourea moiety have an easily polarizable lone pairs of electrons andis considered as a ‘‘soft base’’, which would have a tendency to createcomplex with silver ions (considered ‘‘soft acids’’). HSAB theoryconfirms that, the selective resins containing N or S donor atomsinteract strongly with soft acids like Ag(I) even in the presence ofhigh concentration of protons. The observed decrease in the uptakeof Ag(I) prior to initial pH 8 may be due to the masking of Ag(I) ionsin the form of soluble anions. At pHN8, almost all of the Ag(I) ions areprecipitated in the form of AgOH (Atia et al., 2005a,b). It is also worthmentioning that the equilibrium pH is lower than initial pH in allstudied rang. The released H+ can be attributed to the deprotonationof thiol form of thiourea moiety at the end of reaction as shownbelow

It is reported that thiourea forms stable complexes with severalmetal ions such as Ag(I), Au(III), Cu(II), Hg(II), … etc. (Bombicz et al.,2004; Donia et al., 2007, 2008), therefore the observed weakadsorption of Cu(II) on R1 (thiourea containing resin) is astonishingphenomena, this phenomena may be attributed to the nature of theformed melamine–thiourea–formaldehyde resin (i.e. types of the newformed chelating moieties).

TOED molecule has EDTA-like (Ethylenediaminetetra-actic acid)structure, therefore it was expected that TOED moiety will behaveEDTA behaviour towards Ag(I) and Cu(II) ions (EDTA does not interactwith Ag(I) ions and forms stable complexes with Cu(II) ions (West,1969)). This behaviour is clear in Fig. 2 where a very low uptake(0.05 mmol/g) was recorded for Ag(I) adsorption by R2 (TOEDcontaining resin) at all studied pH rang (1.0–7.0). While the adsorptionof Cu(II) on R2 was increased dramatically with increasing pH untilreaching a maximum value (0.5 mmol/g) at equilibrium pH 4.7, thismay be attributed to the partial deprotonation of coordinating groups(carboxylate and amine). In neutral or slightly acidicmedia, the uptakecould be attributed to the formation of resin–metal ion complex,which cause increase of the acidity of the medium due to release of H+

from coordinating carboxylate groups.From the observed adsoption behaviour of R1 and R2 towards Ag(I)

and Cu(II) at different pHs, we can conclude that R1 is suitable foruptake of Ag(I) while R2 is suitable for uptake of Cu(II) from theiraqueous solutions.

concentration 1×10−2 M at 25 °C.

Fig. 4. (a) Pseudo first-order and (b) Pseudo second-order kinetics of the uptake of Ag(I)by R1 and Cu(II) by R2.

Table 1Separation factor for the studied metal ions at normal pH and initial concentration1×10−2 M

R1 R2

Separation factor SA/B Separation factor SB/A

SAg/Cu 19.68 SCu/Ag 10.30SAg/Pb 15.98 SPb/Ag 13.15SAg/Cd 16.78SAg/Zn 19.44SAg/Ca 100.00

31M.A. Abd El-Ghaffar et al. / Hydrometallurgy 96 (2009) 27–34

3.2.2. Selectivity studiesSelective separation of Ag(I) from binary mixtures with Cu(II), Pb(II),

Cd(II), Zn(II) or Ca(II) was studied at normal pH and same otheradsorption conditions. The separation factors for silver (I) ions overcopper (II), Zinc (II) and calcium (II) ions were calculated from theadsorption data using the following equation (Ni et al., 2001)

Separation factor SA=B� � ¼ CA1−CA2ð ÞxCB2

CB1−CB2ð ÞxCA2 ð4Þ

where CA1 and CA2 stand for the concentrations of metal ion A (Ag(I))before and after adsorption, respectively, and CB1 and CB2 stand for theconcentrations of metal ion B (Cu(II), Pb(II), Cd(II), Zn(II) or Ca(II))before and after adsorption. The values of the separation factor (SA/B)are reported in Table 1. From Table 1, the values of the separationfactor for the adsorption of Ag(I) over Cu(II) is 19.68, this valuerepresents 20 fold, which mean that R1 has selectivity for Ag(I) ≈20times Cu(II) ions. The data reported in Table 1 also show excellentselective separation for Ag(I) from the other studied metal ions usingR1. The selective separation of Cu(II) or Pb(II) from Ag(I) solution wasstudied using R2. As reported in Table 1, the separation factor of Cu(II)or Pb(II) over Ag(I) is more than 10, which indicates that R2 has highselectivity for Cu(II) or Pb(II) over Ag(I) ions.

3.2.3. Kinetic studiesFig. 3 shows the change in the uptake of Ag(I) by R1 and Cu(II) by R2

as a function of time at initial concentration of 1×10−2 M. Obviously, theequilibrium for the uptake of Ag(I) and Cu(II) was reachedwithin 65 and30 min, respectively where 90% of the maximum uptake capacity wasachieved.

The adsorption-time data in Fig. 3were treated according to pseudo-first order (Lagergren, 1898), pseudo-second order (Ho and McKay,

Fig. 3. Effect of time on the uptake of Ag(I) by R1 and Cu(II) by R2 from initialconcentration 1×10−2 M at normal pH and 25 °C.

1999), intraparticle diffusion (Guibal et al., 1998) and Elovich equation(Cheung et al., 2003). The pseudo-first order model is expressed as:

log qe−qtð Þ ¼ log qe−k1

2:303

� �t ð5Þ

where k1 is the pseudo-first order rate constant (min−1) of adsorptionand qe and qt (mmol/g) are the amounts of metal ion adsorbed atequilibrium and time t (min), respectively. On the other hand, thepseudo-second order model is expressed as:

tqt

¼ 1k2q2e

þ 1qe

� �t ð6Þ

where k2 is the pseudo-second order rate constant of adsorption(g mmol−1 min−1). The aforementioned two models basically considerexternal film diffusion, intraparticle diffusion and interaction step foradsorption process. The rate determining step of the adsorption

Table 2Parameters of the pseudo-first order and pseudo-second order for the adsorption of Ag(I)on R1 and Cu(II) on R2

Metalion

qe, exp(mmol/g)

Pseudo-first order Pseudo-second order

k1(min−1)

qe(mmol/g)

R2 k2 (g/mmolmin)

qe(mmol/g)

R2

Ag(I) 0.95 0.035 0.996 0.996 0.028 1.253 0.983Cu(II) 0.50 0.082 0.524 0.991 0.120 0.64 0.993

Fig. 6. The Elovich kinetics model for the adsorption of Ag(I) on R1 and Cu(II) on R2.

32 M.A. Abd El-Ghaffar et al. / Hydrometallurgy 96 (2009) 27–34

reactionmay be one of the above three steps. Stirringmayminimize tocertain extent the external film diffusion. So, the adsorption rate maybe controlled by intraparticle diffusion or interaction step. The kineticparameters for the pseudo-first and pseudo-second models aredetermined from the linear plots of log (qe−qt) vs t (Fig. 4a) or (t/qt)vs t (Fig. 4b), respectively. The validity of each model could be checkedby the fitness of the straight lines (R2 values). Accordingly as shown inTable 2, the adsorption of Ag(I) on R1 is perfectly fit pseudo-first ordermodel rather than pseudo-second order one, while the adsorption ofCu(II) on R1 is perfectly fit pseudo-second order model rather thanpseudo-first order one. This kinetic behavior confirms the effective roleof textural properties of the resins on the rate of sorption reaction. Thiseffect was confirmed from the plot between the quantity of metal ionsorbed at different time intervals (qt) and square root of time (t0.5),according to the following equation (it originates from Fick's secondlaw):

qt ¼ X þ Kdif t0:5 ð7Þ

where X represents the boundary layer diffusion effects (external filmresistance); Kdif is the intraparticle diffusion rate constant (mmol.g−1

min−1), which is usually used to compare mass transfer rates. Thestraight line obtained in Fig. 5 indicates that the rate of adsorption iscontrolled by intraparticle diffusion process (Guibal et al., 1998). Asthe value of X decreases the effect of external diffusion on thereaction rate decreases. As shown in Fig. 5, the negative value ofintercept (X) for the adsorption of Ag(I) on R1 indicates that theboundary layer diffusion effects (external film resistance) has nosignificant effect on the diffusion rate. The reaction is mainlycontrolled by intraparticle diffusion and interaction step (Guibal etal., 1998). This may be attributed to the higher hydrophilicity of theresin due to the presence of (S andN atoms). The calculated (X) for theadsorption of Cu(II) on R2 is a positive value, this value for (X) in the2nd case indicates the dependence of the reaction rate to a smallextent on external film diffusion. This may be attributed to the lowerhydrophilicity of TOEDmoiety present on the resin. This external filmdiffusion in 2nd case shifts the reaction rate of the adsorption of Cu(II)on R2 towards pseudo-second order one.

The Elovich equationwas also applied to the sorption of Ag(I) on R1and Cu(II) on R2. The Elovich equation is given as follows (Cheung et al.,2003):

dqt=dt ¼ α exp −βqtð Þ ð8Þ

where q is the sorption capacity at time t and α the initial sorptionrate (mmol/g min) and β the desorption constant (g mmol−1). To

Fig. 5. The intraparticle diffusion kinetics model for the adsorption of Ag(I) on R1 andCu(II) on R2.

simplify Elovich equation, it is assumed that αβt≫1 and by applyingthe boundary conditions qt=0 at t=0, this equation becomes

qt ¼ 1=β lnαβð Þ þ 1=β ln tð Þ ð9Þ

Thus, the constants can be obtained from the slope and intercept ofa straight line plot of qt versus ln t. The linearity of the equation givesthe rate of reaction, which allows obtaining the initial sorption rate, α(mmol/g min) from the intercept of a straight line plot of qt versus ln tFig. 6. The values ofα for the adsorption of Ag(I) on R1 and Cu(II) on R2

Fig. 7. Adsorption isotherms for the adsorption of Ag(I) on R1 and Cu(II) on R2 atdifferent temperatures and normal pH.

Table 3Langmuir constants for adsorption of Ag(I) on R1 and Cu(II) on R2

Temp.(°C)

Ag(I) on R1 Cu(II) on R2

Qmax

(mmol/g)KL

(L/mmol)R2 Qmax

(mmol/g)KL

(L/mmol)R2

25 0.951 2.212 0.999 0.511 1.538 0.99935 1.033 2.293 0.999 0.637 1.801 0.99845 1.140 2.313 0.997 0.715 1.942 0.999

Table 5Free energy change for adsorption of Ag(I) on R1 and Cu(II) on R2 at different temperatures

Temperature(K)

Ag(I) on R1 Cu(II) on R2

ΔG° (kJ/mol) TΔS° (kJ/mol) ΔG° (kJ/mol) TΔS° (kJ/mol)

298 −18.82 20.86 −18.2 27.416308 −19.52 21.56 −19.12 28.336318 −20.22 22.26 −20.04 29.256

33M.A. Abd El-Ghaffar et al. / Hydrometallurgy 96 (2009) 27–34

are 0.039 and 0.095 (mmol/gmin), respectively. The values of β for theadsorption of Ag(I) on R1 and Cu(II) on R2 were also obtained and arefound to be 4.201 and 6.66 (g/mmol), respectively. The values of R2 arefound to be 0.983 and 0.969 for Ag(I) and Cu(II) respectively. This datasuggest the applicability of Elovich kinetic model for the adsorption ofAg(I) on R1 and Cu(II) on R2.

3.2.4. Adsorption isothermsFig. 7a, b show the isotherms of adsorption of Ag(I) on R1 and Cu(II)

on R2, respectively at different temperatures. The adsorption curvesshow maximum uptake values for Ag(I) and Cu(II) at 1.10 and0.68 mmol/g at 45 °C, respectively. The adsorption data were plottedaccording to Langmuir equation

Ce

qe¼ Ce

Qmaxþ 1KLQmax

ð10Þ

where Ce is the concentration of metal ions in solution (mmol/L), qethe metal ions concentration in the resin phase (mmol/g), Qmax themaximumadsorption capacity (mmol/g) andKL is the Langmuir bindingconstant which is related to the energy of adsorption (L/mmol). Allconcentrations refer to equilibrium conditions. Plotting Ce/qe against Cegives a straight line with a slope and an intercept equals 1/Qmax and1/KLQmax, respectively. The values of KL and Qmax at differenttemperatures for adsorption of Ag(I) and Cu(II) are reported in Table 3.It is seen that the valueofQmax (obtained fromLangmuir plots) at 25 °C ismainly consistentwith that experimentally obtained, indicating that theadsorption process is mainly monolayer.

The degree of suitability of resin towards metal ions was estimatedfrom the values of separation factor constant (RL), which givesindication for the possibility of the adsorption process to proceed.RLN1.0 unsuitable; RL=1 linear; 0bRLb1 suitable; RL=0 irreversible(Qi and Xu, 2004). The value of RL can be calculated from the relation

RL ¼ 11þ KLC0

ð11Þ

where KL (L/mmol) is the Langmuir equilibrium constant and C0(mmol/L) is the initial concentration of metal ion. The values of RL liesbetween 0.041 and 0.394 indicating the suitability of the studiedresins as adsorbents for Ag(I) or Cu(II) from aqueous solutions.

The observed increase in both values of Qmax and KL at elevatedtemperature indicates the endothermic nature of the adsorptionprocess. The values of KL at different temperatures were treatedaccording to van't Hoff equation (Tellinghuisen, 2006)

lnKL ¼ −ΔH∘

RTþ ΔS∘

Rð12Þ

where ΔH° (J/mol) and ΔS° (J/mol K) are enthalpy and entropy changes,respectively, R is the universal gas constant (8.314 J/mol K) and T is the

Table 4Enthalpy and entropy changes for adsorption of Ag(I) on R1 and Cu(II) on R2

Metal ion ΔH° (kJ/mol) ΔS° (J/mol.K)

Ag(I) 2.046 70.95Cu(II) 9.21 92.01

absolute temperature (in Kelvin). Plotting ln KL against 1/T gives astraight line with slope and intercept equal to −ΔH°/R and ΔS°/R,respectively. The values ofΔH° and ΔS° were calculated and reported inTable 4. The positive values ofΔH°confirming theendothermic nature ofadsorption process whereas the positive values of ΔS° suggest the highdegree of freedom of adsorption system at equilibrium due to theinteraction between active sites and metal ion. The observed greaterpositive value of ΔS° for Cu(II) than that of Ag(I) may be related to themechanism of interaction between metal ion and active sites. Thechelation mechanism for Ag(I) is a substitution reaction of water ofhydration bonded to the metal ion by active sites and release of oneproton, whereas the chelation of Cu(II) by the TOED group immobilizedon theR2 results in the release of twohydrogen ions for eachCu(II) ion inaddition to water of hydration. These results are in a good agreementwith the data obtained by Baraka et al. (2006). Gibbs free energy ofadsorption (ΔG°) was calculated from the following relation

ΔG∘ ¼ ΔH∘−TΔS∘ ð13Þ

The values of ΔG° at different temperatures were calculated andreported in Table 5. The negative value of ΔG° indicates that theadsorption reaction is spontaneous. The observed increase in negativevalues of ΔG° with increasing temperature implies that the adsorptionbecomesmore favorable at higher temperature. The data given in Table 5also showedan increase in thevaluesof TΔS°with increasing temperatureand |ΔH°|b |TΔS°|. This indicates that the adsorptionprocess is dominatedby entropic rather than enthalpic changes (Donia et al., 2007).

3.3. Column studies

Fig. 8 shows the breakthrough curves for the adsorption of Ag(I) onR1 and Cu(II) on R2. The Breakthrough and exhaustion occurred fasterfor the adsorption of Cu(II) on R2, also the adsorption of Cu(II) on R2showed sharper breakthrough curve than that of the adsorption of Ag(I)on R1. This difference is mainly is attributed to:

(i) Nature of the chelating group in each resin.(ii) Nature of the adsorbed metal ion.

Fig. 8. Breakthrough curves for the recovery of Ag(I) by R1 and Cu(II) by R2 at flow rateof 1 mL/min.

Fig. 9. Effect of successive desorption cycles on the breakthrough curves for the recoveryof Ag(I) by R1 and Cu(II) by R2 at flow rate of 1 mL/min.

34 M.A. Abd El-Ghaffar et al. / Hydrometallurgy 96 (2009) 27–34

(iii) The difference in pore size distribution in both resins as well asthe higher hydrophilicity of R1 in addition to the external filmdiffusion found in case of R2 as reported in kinetic studies.

Thus we can separate between Ag(I) and Cu(II) from their binarymixture using any one of the studied two resins. It is obvious from thecomparison between the efficiency of the studied resins that R1 ismore economical resin for the separation between Ag(I)/Cu(II)mixture than R2.

3.4. Elution and regeneration cycles

Sorption/desorption cycles up to three runswere carriedout for Ag(I)on R1 and Cu(II) on R2. The elution of Ag(I)was performed using 100mLof thiourea (0.5) acidified with H2SO4 (0.2 M). 100 mL of 0.5 M HCl wasused for stripping Cu(II). As shown in Fig. 9a,b, the breakthrough curvesfor recovery of Ag(I) and Cu(II) showed no characteristic changes duringsuccessive cycles. This indicates that the resin has a good durability aswell as good efficiency for repeated use.

4. Conclusions

Modified melamine resins were obtained through the treatmentwith thiourea (R1) or tetraoxalyl ethylenediamine (R2), The obtainedresinswere tested for selective separation between Cu(II) andAg(I) formtheir binary mixtures. R1 showed high selectivity for Ag(I) from Cu(II)solution, while R2 succeeded for the selective separation of Cu(II) fromAg(I) in their binary mixtures. Kinetic studies indicated that the adsorp-tion reaction of Ag(I) on R1 is perfectly fit pseudo-first order modelrather than pseudo-second order one, while the adsorption of Cu(II) on

R2 is perfectly fit pseudo second order model rather than pseudo firstorder one. Thermodynamic parameters obtained indicated that theadsorption process is spontaneous and endothermic. Themechanism ofinteraction between metal ion and resin was also clarified. The resinobtained showed good durability and easy regeneration.

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