T

Sa

b

a

ARRAA

KSSMT

1

aahisitdtir

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sis

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0d

Journal of Membrane Science 389 (2012) 287– 293

Contents lists available at SciVerse ScienceDirect

Journal of Membrane Science

jo u rn al hom epa ge: www.elsev ier .com/ locate /memsci

ransport of Ag+ through tri-n-dodecylamine supported liquid membranes

aeed ur Rehmana,∗, Gul Akhtara, M. Ashraf Chaudrya, Khurshid Alia, Najeeb Ullahb

Institute of Chemical Sciences, University of Peshawar, PakistanUniversity of Engineering and Technology Peshawar, Peshawar 25120, Khyber Pakhtunkhwa, Pakistan

r t i c l e i n f o

rticle history:eceived 15 May 2011eceived in revised form 25 October 2011ccepted 28 October 2011vailable online 3 November 2011

a b s t r a c t

Silver (I) has been transported from feed in to strip solution via tri-n-dodecylamine-cyclohexane-polypropylene supported liquid membrane (SLM). The transport of Ag+ has been found to be dependenton different parameters such as concentration of H+ in feed solution, tri-n-dodecylamine (TDDA) con-centration in membrane phase, stripping phase composition and membrane thickness. The optimized

+ 3

eywords:ilverLMembrane

conditions obtained for Ag transport are: 0.75 mol/dm of HNO3 in feed solution, 0.788 M of TDDA incyclohexane in membrane phase and 1.0 mol/dm3 of NH3(aq) in stripping phase. The stoichiometry of thecomplex was calculated from the flux data of Ag+ transport across the membrane. The complex respon-sible for the transport of Ag+ has been investigated to be (LH)·Ag(NO3)2. Recovery of more than 98% ofsilver has been observed from feed solution at optimized conditions. Stability of this SLM system has

table

DDA been studied and found s

. Introduction

Silver is one of the most ancient metals used in jewellery andrts. The various types of alloys used in dentistry contain silver, goldnd copper with a trace amount of palladium and platinum. Silveras been used in the field of communication, aerospace, chemical

ndustry, electroplating and photographic materials. Furthermore,ilver has been reported to be used in catalysts, mirrors, cloud seed-ng, disinfection of water and some medicines [1–4]. It is estimatedhat about 12% of the world’s silver resources are used in the pro-uction of light sensitive devices. Silver can enter the environmenthrough industrial water because it is often present as an impurityn Cu, Zn, As, and Sb ores. Being precious and toxic, silver must beecovered and separated from industrial effluents [5–7].

Different techniques have been used for the extraction of silveruch as solvent extraction [8–10], adsorption [11,12], cloud pointxtraction [13], emulsion liquid membrane (ELM) [14,15], bulk liq-id membrane (BLM) [16,17]. Traditional solvent extraction andther conventional methods may not be appropriate for large scalerocesses due to their major shortcomings such as time consuming,equirements of costly solvents and equipments and generation ofoxic sludge [18,19].

Facilitated transport of cations, especially of metal ions using

upported liquid membrane is one of the useful techniquesn separation science [20,21]. Supported liquid membrane con-ists of hydrophobic polymeric microporous thin sheet which is

∗ Corresponding author. Tel.: +92 91 5641658; fax: +92 91 9216687.E-mail addresses: srachem@yahoo.com (S.u. Rehman), gul akhtar72@yahoo.com

G. Akhtar).

376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2011.10.040

for 120 h.© 2011 Elsevier B.V. All rights reserved.

impregnated with hydrophobic organic carrier. In this method theextraction from donor phase and stripping in receiving takes placesimultaneously [19]. There is a limited study for extraction of Ag(I)by SLM. Gherrou et al. [22] have studied the transport of Ag(I) fromacidic thiourea solution via SLM containing D2EHPA as Ag(I) ioncarrier and it has been found that Ag(I) species predominate at verylow thiourea concentration (10−5 to 10−4 M). Shamsipur et al. [23]have reported the selective transport of Ag(I) through SLM com-posed of aza-thioether crown containing 1,10-phenanthroline as aspecific ion carrier. It has been observed that the presence of thio-sulfate as a metal ion acceptor in the strip solution, enhance thetransport of Ag ion and almost all the silver was recovered after3 h. Selective transport of Ag(I) and Hg(II) through two microporoussupported membrane loaded with a mixed N/O/S donor macrocy-cle aza-thioether crown containing 1,10-phenanthroline sub unitand terathia has been reported by Shamsipur et al. [24]. Amiri et al.[25] have investigated selective transport of silver ion through sup-ported liquid membrane using calix [4] pyrroles as a suitable ioncarrier. Furthermore, it has been reported that various parameterssuch as carrier concentration in the membrane phase, thiosulfateconcentration in strip phase, picric acid concentration in the feedphase, stirring speed and the type of solvent affect the transport ofsilver ion.

Earlier workers have successfully extracted Ag(I) through SLMusing triethanolamine as a carrier and KCN as a stripping agent[26]. However, the reported method has limitations since KCN instripping phase is not environmental friendly and its disposal is one

of the serious issues.

In the present work we report a new supported liquid mem-brane system containing tri-n-dodeclyamine as a carrier dissolvedin cyclohexane and immobilized in a thin polypropylene film. The

288 S.u. Rehman et al. / Journal of Membra

Ag+

nH+ NH3(aq)

[Ag(NH3)2]+

(LH )n.Ag(NO3)n+1

nL (n+1)NO3

-

H2O + (n+1)NO3-

NtcpfAit

2

opfdm

bmatc

A

b

n

wpr

A

isfdt[btcbwch

K

As there is no extraction from the strip to the membrane phase,

Fig. 1. Schematic diagram of Ag+ transport.

H3(aq) in stripping phase has been found to play significant role inhe transport of Ag+. The various process parameters such as acidoncentration in feed solution, TDDA concentration in membranehase, NH3(aq) concentration in strip phase etc. were first optimizedor Ag+ transport. The optimized SLM was then used for recovery ofg+ from silver plating and photographic waste solution. The sto-

chiometry of the chemical reactions as well as mechanism of Ag+

ransport has also been evaluated.

. Theory

In the present study the supported liquid membrane is madef tri-n-dodecylamine in cyclohexane supported in microporousolymeric polypropylene film. Being hydrophilic in nature Ag+

rom feed phase cannot enter the liquid membrane organic phaseirectly and hence the direct transport of silver ions through theembrane is not possible.Tri-n-dodecylamine may be indicated as L. The TDDA that is

asic carrier and its pKa value is 11.3 [27]. The TDDA at the feedembrane interface may be protonated to LH+ in acidic medium

s nitrogen atom of TDDA has lone pair of electrons. It is assumedhat in the presence of HNO3 in feed solution the silver nitrate isonverted to [Ag(NO3)n+1]n− [26].

gNO3 aq + nNO3 aq− � [Ag(NO3)n+1]aq

n− (1)

The species LH+ and [Ag(NO3)n+1]n− then react at the feed mem-rane interface and form the complex as shown below.

LH+org + [Ag(NO3)n+1]aq

n− � (LH)n · Ag(NO3)n+1 org (2)

here the subscript org is the organic phase and aq is the aqueoushase. To show the contribution of H+ and NO3

−, Eq. (2) may beepresented as:

gaq+(n + 1)(NO3)aq

− + nLorg + nHaq+ � (LH)n · Ag(NO3)n+1 org(3)

The visualized transport mechanism of Ag+ is shown in Fig. 1,n which H+, NO3

− and Ag+ move in the same direction towardstripping phase. The complex formed as per Eq. (3) diffuses fromeed membrane interface into liquid membrane phase and then itiffuses to membrane–strip interface. At membrane–strip interfacehe complex dissociates due to NH3(aq) in stripping phase and formAg(NH3)2]+ [28]. The free carrier (L) diffuses back to feed mem-rane interface, and net result is the transport of Ag+ from feedo strip phase. According to Wilke-Chang [29], the diffusion coeffi-ient of the backward transported free carrier (L) molecules shoulde much greater as compared to the diffusion coefficient of the for-ard transported complex (LH)n·Ag(NO3)n+1. Due to this reason the

oncentration of TDDA at feed membrane interface will always beigher as compared to complex.

The equilibrium constant of Eq. (3) can be written as under.

Ag =[(LH)n · Ag(NO3)n+1]org

[Ag+]aq · [NO3−]aq

n+1 · [H+]aqn · [L]org

n(4)

ne Science 389 (2012) 287– 293

If �Ag stands for the distribution coefficient of Ag+ for distribu-tion between the membrane and aqueous phase then,

�Ag =[(LH)n · Ag(NO3)n+1]org

[Ag+]aq(5)

and Eq. (4) can be written as:

KAg = �Ag

[NO−3 ]aq

n+1 · [H+]aqn[L]org

n(6)

and on re-arranging Eq. (6), we obtain

�Ag = KAg · [NO3−]aq

n+1 · [H+]aqn · [L]org

n (7)

In the above equations the brackets indicate the concentration.According to the Fick’s law, the rate of diffusion of a solute dN/dt(where N is amount of substance and t stands for time), across anarea A is known as diffusion flux and given by the symbol J.

J = 1A

dN

dt(8)

J is proportional to the concentration gradient dc/dx. Since the con-centration gradient is negative in the direction of flux i.e. −dc/dx,thus

J = −Ddc

dx(9)

where J is the flux of species under transport and dc is the differencein concentration of that species across a very small segment of themembrane thickness dx and D is the diffusion coefficient.

It is supposed that the x-axis is perpendicular to the face bound-aries of the membrane. Then, at the feed side boundary of themembrane x = 0, at the strip side boundary of the membrane x = �,and Cfm and Csm are the membrane phase concentrations of Ag+ atfeed and strip side respectively. Assuming a linear concentrationgradient for Ag+ within the membrane one can write:

J ∝ Cfm − Csm

�(10)

Hence

Cfm − Csm

�= − dc

dx(11)

Eq. (9) then becomes,

J = D(

Cfm − Csm

�

)(12)

where, as mentioned earlier, D is the diffusion coefficient. Sincedistribution coefficient of Ag+ at the membrane surface in the feedside and strip side can be given by �f and �s respectively.i.e.

�f = Cfm

Cfand �s = Csm

Cs(13)

where Cf and Cs represent bulk feed and strip concentration of Ag+

respectively.From relation (13) we obtain

Cfm = �f · Cf and Csm = �s · Cs (14)

and with the help of Eq. (14), Eq. (12) becomes

J = D�f · Cf − �s · Cs

�(15)

so �s → 0 and as a result, �s Cs ≈ 0 and Eq. (15) transforms to

J = D · �f · Cf

�(16)

embrane Science 389 (2012) 287– 293 289

J

D

wa

J

J

o

l

t

l

wslfsd

3

3

6bctocw

Table 1Parameters of microporous polymeric support.

Membrane Thickness (�m) Porosity (%) Material

Celgard 2500 25 45 PolypropyleneCelgard 2502 50 45 Polypropylene

tion2) ×

S.u. Rehman et al. / Journal of M

Here in our case �f = �Ag as given by Eq. (7), thus Eq. (16) becomes

= D · KAg · [NO3−]aq

n+1 · [H+]aqn · [L]org

nCf

�(17)

According to the Wilke-Chang relation [30].

= k′ · T

�(18)

here T is the absolute temperature, � is viscosity and k′ is constantnd so

= k′ · T · KAg · [NO3−]aq

n+1 · [H+]aqn · [L]org

n · Cf

� · �(19)

Since k′ and KAg is constant equal to Y, a new constant.Therefore,

= Y · T · [NO3−]aq

n+1 · [H+]aqn · [L]org

n · Cf

� · �(20)

However, if Cf does not change rapidly, then for a short intervalf time, Cf may be taken nearly as constant.

Taking log of Eq. (20)

og J� = log Y + log T + (n + 1) log[NO3−]aq + n log[H+]aq + n log[L]org + log Cf

�

(21)

As Y, T, and the thickness � of the membrane is constant, andhen Eq. (21) becomes

og J� = constant + (n + 1) log[(NO3)n−]aq + n log[H+]aq + n log[L]org + log Cf

�

(22)

Eq. (22) can be used to determine the number of H+ associatedith L in the form LH+. This can be done in different ways, one

imilar way is to keep [NO3−], [L], � and Cf constant and plotting

og J� versus log [H+], the slope of the curve will give the “n” valueor H+. Similarly by keeping [NO3

−], [H+], � and Cf constant, thelope of log J� versus log [L] will give number of moles of tri-n-odecylamine taking part in complexation.

. Experimental

.1. Chemicals and reagents

The silver nitrate (Scharlau 99.99%) in HNO3 (Fisher Scientific9–72%) was used as feed in given concentration. The liquid mem-rane composed of TDDA (Merck ≥95%) as a metal ion carrier inyclohexane (Panreac 99.5%) as a diluent to get various composi-ion of liquid membrane phase. The stripping solutions were madef NH3(aq) (35% BDH) in given concentration. All experiments werearried out in double distilled deionized water. All other chemicalsere used of analytical grade.

Flux = concentration change of Ag+ion(mol/dm3) × solueffective membrane area (m

P = [ln (initial feed concentration/feed concentreffective membr

Celgard 4400 175 68 Polypropylene

3.2. Membranes

Three types of Celgard polypropylene hydrophobic membranes2500, 2502 and 4400 were used as a solid support for liquid mem-brane. The specifications of membranes are given in Table 1. Thesupported liquid membranes were prepared by soaking the sup-port in a predetermined concentration of TDDA in cyclohexane for24 h. The membrane was taken out from the organic solution andallowed to drain off for 5 min to remove excess amount of carrierand diluent.

3.3. Apparatus/instruments

The atomic absorption spectrometer Perkin Elmer model 400was used for determination of metal ions concentration in feedand strip solutions. pH meter Metrohm model 827 was used formeasurement of pH. Brookfield viscometer/rheometer LVDV-IIIwas used for viscosity measurement of TDDA in cyclohexane. Thesodium and potassium metal ions were analyzed with industrialflame photometer JENWAY, model PFP7 UK.

3.4. Membrane cell

All the metal ion permeation experiments were performed in atwo compartment cell as shown in our previous study [30]. Eachcompartment of membrane cell had a volume capacity of 250 cm3.The effective membrane area was 23.75 cm2. The solution in bothcompartments of the cell was stirred at 1500 rpm by synchronousmotors at a temperature of 25 ◦C ± 1 ◦C. The stirring speed was opti-mized for similar type of carrier group [30] and cell, hence all studywas performed at this stirring speed (1500 rpm).

3.5. Permeation study

The feed and strip solutions, 250 cm3 each, were added to bothcompartments of the cell simultaneously. Both compartments ofthe cell were then covered and agitated continuously to avoid con-centration polarization at membrane faces. Samples of 1.0 cm3 fromfeed and strip solution were drawn after regular time intervalsand analyzed for metal ion concentration. The experimental condi-tions for most of the experiments were: initial Ag+ concentration infeed solution was 7.42 × 10−4 mol/dm3, the diluent was cyclohex-ane and the membrane was celgard 2500 (except for membranethickness study on transport of Ag+). The flux was calculated usingrelation [30]:

volume in feed or strip (dm3) �t

(23)

and permeability was calculated using following formula [31]:

ation at time(t))] × volume in feed phase

ane area × �t

(24)

where �t indicates time interval in seconds.

290 S.u. Rehman et al. / Journal of Membrane Science 389 (2012) 287– 293

0

20

40

60

80

100

120

400350300250200150100500

Time (min)

[Ag]

(%ag

e)

0.158M TDDA

0.315M TDDA

0.473M TDDA

0.630M TDDA

0.788M TDDA

0.946M TDDA

1.103M TDDA

1.261M TDDA

Fig. 2. Effect of TDDA concentration on recovery of Ag+ in stripping phase withtime (HNO3 concentration in feed solution = 0.75 mol/dm3, TDDA concentrationit

4

4

vwaat

oficbT(T

inrcc[

bpcc

Fm

0

1

2

3

4

5

6

1.210.80.60.40.20

Visc

osity

(cP)

n membrane phase = 0.158 M–1.261 M, NH3(aq) concentration in stripping solu-ion = 1.0 mol/dm3).

. Results and discussion

.1. Effect of TDDA concentration

To study the effect of concentration of TDDA on transport of Ag+

arious concentration of TDDA ranging from 0.158 M to 1.261 Mere used. During this study the concentration of AgNO3 was kept

t 7.42 × 10−4 mol/dm3 in 0.75 mol/dm3 of HNO3 in feed solutionnd concentration of NH3(aq) was fixed at 1.0 mol/dm3 in strip solu-ion.

Fig. 2 indicates the effect of carrier concentration on extractionf Ag+ in stripping phase with respect to time. It is clear from thisgure that extraction of Ag+ increases from 46.49% to 98.65% as theoncentration of TDDA increases from 0.158 M to 0.788 M in mem-rane phase. The extraction of Ag+ decreases beyond 0.788 M ofDDA. Similar trend of increase and then decrease for permeabilityp) and flux (J) of Ag+ is also observed for various concentration ofDDA as shown in Fig. 3.

As expected and as per Eq. (22), that as concentration of TDDAncreases in membrane phase, the flux of Ag+ also increases as moreumbers of TDDA interact with H+ to form LH+, which on furthereaction with anions ([Ag(NO3)n+1]n−) enhance the formation ofomplex. The increase in metal ion transport with increasing carrieroncentration is available in literature for similar type of carrier31,32].

The decrease in transport of Ag+ beyond 0.788 M of TDDA coulde explained by an increase in the viscosity of the liquid membranehase. As the concentration of TDDA in cyclohexane increases, vis-

osity of liquid membrane also increases. The viscosity of variousomposition of TDDA is shown in Fig. 4. The diffusion coefficient of

0

1

2

3

4

5

6

7

8

9

10

1.41.210.80.60.40.20

TDDA (mol/dm3)

J (1

0-10 m

ol/m

2 .s)

-0.8

0.2

1.2

2.2

3.2

4.2

5.2

6.2

p (1

0-6 m

/s)

Flux (J)

Permea bili ty (p)

ig. 3. Flux (J) and permeability (p) variation at various concentrations of TDDA inembrane phase (Same operating conditions as given in Fig. 2).

[TDDA] mol/dm3

Fig. 4. Viscosity of TDDA–cyclohexane solutions.

the solute across the liquid membrane is shown by the followingStokes–Einstein equation [33]:

D = KT

6��r(25)

where T is the absolute temperature, K is the Boltzmann constant,r is the ionic radius of solute and � is the viscosity of the organicphase in cP, equilibrated with the aqueous phase. Since viscosityis inversely proportional to diffusity, that is why diffusion coeffi-cient of complex (LH)n·Ag(NO3)n+1 in membrane phase decreases,and results in decrease in extraction of Ag+. The decrease in metalion transport with increasing viscosity of liquid membrane phasehas already been observed in our previous study for Mn(II), W(VI)and Pd(II) transport [30,32,34]. Hence 0.788 M of TDDA was con-sidered to be the optimum concentration for extraction of Ag+ forsubsequent study to optimize various parameters.

The number (n) of TDDA(L) involved in complex of(LH)n·Ag(NO3)n+1 can be determined by plotting log [TDDA]versus log J� as shown in Fig. 5. The slope of the graph calculated1.0023, this means that one molecule of TDDA is involved in thecomplex formation.

4.2. Effect of HNO3 concentration

HNO3 plays a significant role in extraction of Ag+, because itprovide H+ and NO3

− for the formation of LH+ and [Ag(NO3)n+1]n−

respectively. During this study the concentration of HNO3 infeed solution was varied from 0.25 mol/dm3 to 1.25 mol/dm3,while concentration of TDDA in membrane phase and that ofNH3(aq) in stripping phase was fixed at 0.788 M and 1.0 mol/dm3

respectively. Fig. 6 shows that the flux of Ag+ increases from4.682 × 10−10 mol/m2 s to 8.901 × 10−10 mol/m2 s with increase inthe concentration of HNO3 from 0.25 mol/dm3 to 0.75 mol/dm3

and becomes maximum at 0.75 mol/dm3 of HNO3. This behavioris in accordance with Eq. (22) where flux is directly related to [H+].Further increase in concentration of HNO3 beyond 0.75 mol/dm3

results in decrease in the flux of Ag+. This can be attributed to theformation of HnAg(NO3)n+1 instead of (LH)n·Ag(NO3)n+1 due to largenumber of H+ and NO3

−, and Eq. (1) is inhibited in forward direc-tion. Thus 0.75 mol/dm3 of HNO3 in feed solution was considered

y = 1.0023x - 8.7182

R = 0.9978

-9.6

-9.5

-9.4

-9.3

-9.2

-9.1

-9

-8.9

-8.8

-8.7

-8.6

0.10-0.1-0.2-0.3-0.4-0.5-0.6-0.7-0.8-0.9

log [TDDA]

log

Jη

Fig. 5. Plot of log [TDDA] versus log J� (same operating conditions as given in Fig. 2).

S.u. Rehman et al. / Journal of Membrane Science 389 (2012) 287– 293 291

0

1

2

3

4

5

6

7

8

9

10

1.41.210.80.60.40.20

HNO3 (mol/dm3 )

J (1

0-10 m

ol/m

2 .s)

Fig. 6. Effect of HNO3 concentration in feed solution on flux (J) of Ag+ (HNO3

concentration in feed solution = 0.25–1.25 mol/dm3, TDDA concentration in mem-brane phase = 0.788 M, NH3(aq) concentration in stripping solution = 1.0 mol/dm3,time = 4.0 h).

y = 1.0013x - 8.9088

R = 0.9986

-9.5

-9.3

-9.1

-8.9

-8.7

-8.5

0.20.10-0.1-0.2-0.3-0.4-0.5-0.6-0.7

log [HNO3]lo

g Jη

F

as

cam

siofcfpH

4

ioc

Ftc

Table 2Effect of membrane thickness on flux of Ag+.

Membranes Flux of Ag+ (×10−10 mol/m2 s)

2500 8.9012502 8.0534400 7.231

3

-9.7

ig. 7. Plot of log [HNO3] versus log J� (same operating conditions as given in Fig. 6).

s the optimum concentration for transport of Ag+ for this SLMystem.

Eq. (22) was used to determine the number of H+ in the proposedomplex of (LH)n·Ag(NO3)n+1 by plotting log [HNO3] versus log J�s shown in Fig. 7. The slope calculated 1.0013, indicates that oneole of H+ is involved in complex formation.To study the transport of H+, pH of the feed solution was mea-

ured after regular interval of times as shown in Fig. 8. Increasen pH of the feed solution was observed, this shows that protonsf feed solution are being utilized by carrier (TDDA) molecules toorm LH+ species at feed membrane interface. An experiment wasonducted to show the transport of H+ in absence of Ag(NO3)aq ineed solution at optimum experimental conditions. No change inH of feed and strip solution was observed, indicates no transfer of+ towards the stripping phase.

.3. Effect of stripping phase concentration

NH was used as strippant with different concentration rang-

3(aq)ng from 0.25 mol/dm3 to 1.5 mol/dm3 whereas the concentrationf HNO3 was kept at 0.75 mol/dm3 in feed solution and TDDA con-entration at 0.788 M in membrane phase. It was found that in

0

0.5

1

1.5

2

2.5

300250200150100500

Time (min)

pH

ig. 8. pH variation in the feed solution (HNO3 concentration in feed solu-ion = 0.75 mol/dm3, TDDA concentration in membrane phase = 0.788 M, NH3(aq)

oncentration in stripping solution = 1.0 mol/dm3).

Conditions: (HNO3 concentration in feed solution = 0.75 mol/dm , TDDA con-centration in membrane phase = 0.788 M, NH3(aq) concentration in strippingsolution = 1.0 mol/dm3, time = 4.0 h).

absence of strippant no transport of Ag+ was observed even afterconducting the experiment for long time (8 h). This indicates thatstrippant plays a vital role in transport of Ag+. It is seen fromFig. 9 that as the concentration of NH3(aq) increases the permeabil-ity of Ag+ also increases and becomes maximum at 1.0 mol/dm3 ofNH3(aq). The extraction phenomena with NH3(aq) can be explainedby the following two reasons. Firstly, the OH− of NH3(aq) can reactwith the H+ of the complex (LH)n·Ag(NO3)n+1 which was formed asaccording to Eq. (3) and it results in the decomposition of complexin the reverse way as shown below.

(LH)n · Ag(NO3)n+1 + OH− → nL + [Ag(NO3)n+1]n− + H2O (26)

Secondly the free NH3 of NH3(aq) can react with [Ag(NO3)n+1]n−

and can form soluble diammine–argentate complex ion [28,35].

[Ag(NO3)n+1]n− + 2NH3 � Ag(NH3)+2 + (n + 1)NO3

− Kf = 2.5 × 107 (27)

To investigate the transport of H+ towards strip solution, the pHof the strip solution was measured. The pH of the strip solution atthe start of the experiment at optimum conditions was 10.55, whileafter 4 h the pH decreased to 9.08. This decrease in pH may be dueto that OH− of NH3(aq) are consumed in the neutralization of H+ ofthe complex (LH)n·Ag (NO3)n+1 as per Eq. (26).

4.4. Effect of Ag+ concentration in feed solution

To investigate the effect of Ag+ concentration on transport ofAg+, concentration range of 3.71 × 10−4 mol/dm3 to 22.26 × 10−4

of Ag+ was used in feed solution. During this study the concen-tration of HNO3 in feed solution was adjusted at 0.75 mol/dm3,NH3(aq) in stripping solution at 1.0 mol/dm3 and TDDA in mem-brane phase at 0.788 M. As shown in Fig. 10, that flux of Ag+

increases from 4. 463 × 10−10 mol/m2 s to 26.788 × 10−10 mol/m2 sas the concentration of Ag+ increases from 3.71× 10−4 mol/dm3 to22.26× 10−4 mol/dm3 in feed solution. This behavior is in accor-dance with Eq. (22), where flux is directly proportional to feedconcentration (Cf). No metal loading behavior for carrier up to theconcentration of 22.26 × 10−4 mol/dm3 of Ag+ in feed solution was

observed. This phenomenon is unlike our earlier observation [30]with Triethanolamine SLM, where transport of Mn(VII) was limiteddue to carrier loading effect.

0

1

2

3

4

5

6

1.61.41.210.80.60.40.20

(NH3(aq) mol/dm 3)

p (1

0 m

/s)

Fig. 9. Effect of NH3(aq) concentration on permeability of Ag+ (HNO3 concentrationin feed solution = 0.5 mol/dm3, TDDA concentration in membrane phase = 0.788 M,NH3(aq) concentration in stripping solution = 0.25–1.5 mol/dm3, time = 4.0 h).

292 S.u. Rehman et al. / Journal of Membrane Science 389 (2012) 287– 293

Table 3Composition of silver plating and photographic waste solution.

Sample Metal ions [M] in feed solution (×10−4 mol/dm3) [M] in strip solution (×10−4 mol/dm3)

Silver plating Ag 4.339 4.335Cr Nil NilZn Nil NilCu Nil Nil

Photographic waste Ag 2.887 2.885K 23.75 NilMg 1.22 NilNa 21.65 NilFe 3.345 Nil

0

5

10

15

20

25

30

2520151050

[Ag ]x10 mol/dm

J (1

0 m

ol/m

. s)

Fig. 10. Effect of Ag+ concentration in feed solution on its flux (initial Ag+ concen-tration in feed = 3.71 × 10−4 mol/dm3 to 22.26 × 10−4 mol/dm3, HNO3 concentrationin feed solution = 0.75 mol/dm3, TDDA concentration in membrane phase = 0.788 M,time = 4.0 h).

0

1

2

3

4

5

6

7

8

9

10

6543210

No. of runs

J (1

0 m

ol/m

.s)

FiN

4

A2aAgfltiAr

5

ttt

ig. 11. Stability of SLM, number of runs versus permeability (HNO3 concentrationn feed solution = 0.75 mol/dm3, TDDA concentration in membrane phase = 0.788 M,H3(aq) concentration in stripping solution = 1.0 mol/dm3).

.5. Effect of membrane thickness on transport of Ag+

To investigate the effect of membrane thickness on transport ofg+, three microporous hydrophobic membranes of celgard 2500,502 and 4400 were used. The specifications of the membranesre given in Table 1. It can be seen from Table 2 that the flux ofg+ decreases as thickness of membrane film increases. The cel-ard 4400 membrane with the highest thickness shows the lowestux of 7.231 × 10−10 mol/m2 s. These results are in accordance withhe proposed theoretical model shown in Eq. (22), where the fluxs inversely proportional to membrane thickness. This decrease ing+ transport with membrane thickness is comparable to thoseeported earlier for supported liquid membranes [22,36,37].

. Stability of SLM

The supported liquid membrane is one of the promisingechniques for separation and recovery of metal ions; never-heless their applications were limited for two decades dueo instability problems. The main degradation or instability

reasons are: shear induced emulsion formation in membranephase, Pore blocking of membrane by precipitation of car-rier complex, solubility of carrier or solvent in aqueous phase,osmotic pressure and wettability of support pores by aqueousphase [38].

To show the long term stability of this SLM, it was impregnatedonce in 0.788 M of TDDA in cyclohexane and was used continu-ously for 5 runs. Each run was of 4 h duration and time betweensuccessive run was of 24 h. Fig. 11 shows that permeability is con-sistent for continuous operation of 120 h and recovery of Ag+ foreach run was more than 98%. The study replicated twice with rel-ative standard deviation ± 1%. This study indicates that this SLMconfiguration is quite stable and can be scaled up at industriallevel.

6. Recovery of silver from silver plating and photographicwaste solution

The tri-n-dodecylamine SLM has the most effective transportability for Ag+. To show practical utilization of this SLM system,it was used for recovery of Ag+ from silver plating and photo-graphic waste solution. The conditions used for extraction of Ag+

were: 0.75 mol/dm3 of HNO3 in feed solution, 0.788 M of TDDA inmembrane phase and 1.0 mol/dm3 of NH3(aq) in stripping solution.Table 3 shows almost complete recovery of Ag+ from silver platingwaste solution. To confirm further application of this technique,the optimized SLM was used for recovery of Ag+ from photographicwaste solution. The results indicate that only silver is transportedwhich show the selectivity and efficiency of this method for silverrecovery. The composition of silver plating and photographic wastesolution is shown in Table 3.

7. Conclusions

(1) Ag+ can be transported through tri-n-dodecylamine flat sheetsupported liquid membrane.

(2) The transport of Ag+ was found to be co-ions coupling transportmechanism, as H+, NO3

− and Ag+ move in same direction.(3) One mole of TDDA and one mole of H+ associate with one mole

of Ag+ forming a complex (LH)·Ag(NO3)2 responsible for trans-port of Ag+.

(4) The optimum conditions found for this SLM system are:0.75 mol/dm3 of HNO3 in feed solution, 0.788 M of TDDAin membrane phase and 1.0 mol/dm3 of NH3(aq) in strippingphase.

(5) This SLM system was found quite stable for 120 h.(6) Almost all Ag+ have been removed when this SLM sys-

tem was applied to silver plating and photographicwaste solution, which concludes that this techniquecan be used for recovery of Ag+ from industrial wasteseffluent.

S.u. Rehman et al. / Journal of Membra

Nomenclature

TDDA, L tri-n-dodecylamineD2EHPA bis-(2-ethylhexyl) phosphoric acidSLM supported liquid membraneLM liquid membraneELM emulsion liquid membraneBLM bulk liquid membrane[M] metal ion concentrationCf concentration of Ag+ in the bulk feed (mol/dm3)Cs concentration of Ag+ in the stripping solution

(mol/dm3)Cfm concentration of Ag+ in the membrane phase on feed

side of membraneCsm concentration of Ag+ in the membrane phase on

strip side of membraneD diffusion coefficient (m2 s−1)J flux (mol/m2 s)p permeability (m/s)n number of moles of TOA or H+ associated in the com-

plexA area of the membranet time (s)T absolute temperature (K)K boltzmann constantr ionic radius of soluteKAg equilibrium constant of silver� membrane thickness

Greek symbols� viscosity (cp)�Ag distribution coefficient of silver�f distribution coefficient of metal ions into membrane

from feed solution�s distribution coefficient of metal ions into membrane

from strip solution

Subscriptsaq. aqueousorg. organicm membrane

R

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eferences

[1] P.N.J. Dennis, Photodetectors, Plenum Press, New York, 1986.[2] X. Wang, L. Zhang, C. Ma, R. Song, H. Hou, D. Li, Enrichment and separation of sil-

ver from waste solutions by metal ion imprinted membrane, Hydrometallurgy100 (2009) 82–86.

[3] S. Song, C. Ji, M. Wang, C. Wang, C. Sun, R. Qu, C. Wang, H. Chen, Adsorption ofsilver (I) from aqueous solution by chelating resins with 3-aminopyridine andhydrophilic spacer arms: equilibrium kinetic, thermodynamic and mechanismstudies, J. Chem. Eng. Data 56 (2011) 1001–1008.

[4] R.J. White, An historical overview of the use of silver in wound management,Br. J. Nurs. 10 (2) (2001) 3–8.

[5] E. Main (Ed.), Metals and their Compounds in Environment, VCH, New York,1991.

[6] D.E. Kimbrough, P.W. Wong, J.K. Biscoe, A critical review of photographicand radiographic silver recycling, J. Solid Waste Technol. Manage. 23 (1996)197–207.

[7] F.Z.E. Aamrani, A. Kumar, L. Beyer, A. Florido, A.M. Sastre, Mechanistic studyof active transport of silver (I) using sulfur containing novel carriers across aliquid membrane, J. Membr. Sci. 152 (1999) 263.

[8] K.C. Sole, T.L. Ferguson, J.B. Hiskey, Solvent extraction of silver by cyanex 272,cyanex 302 and cyanex 301, Solvent Extr. Ion Exch. 12 (1994) 1033.

[9] D. Sevdic, H. Meider, D. Parkovic, Solvent extraction of silver and mercury byO-dibutyl phenylamino-phenylmethanethio phosphonate from chloride solu-tions, Solvent Extr. Ion Exch. 8 (1990) 643.

[

ne Science 389 (2012) 287– 293 293

10] V. Stankovic, L. Outarra, F. Zonnevijlle, C. Comninellis, Solvent extraction ofsilver from nitric acid solutions by calix [4] arene amide derivatives, Sep. Purif.Technol. 61 (3) (2008) 366–374.

11] M.A. El-Ghaffar, M.H. Mohamed, K.Z. Elwakeel, Adsorption of silver (I) onsynthetic chelating polymer derived from 3-amino-1,2,4-triazole-5-thiol andglutaraldehyde, Chem. Eng. J. 151 (1-3) (2009) 30–38.

12] M. Tuzen, M. Soylak, Column solid phase extraction of nickel and silver inenvironmental samples prior to their flame atomic absorption spectrometricdeterminations, J. Hazard. Mater. 164 (2–3) (2009) 1428–1432.

13] M. Ghaedi, A. Shokrollahi, K. Niknam, E. Niknam, A. Najibi, M. Soylak, Cloudpoint extraction and flame atomic absorption spectrometric determination ofcadmium (II), lead (II), palladium (II) and silver (I) in environmental samples, J.Hazard. Mater. 168 (2009) 1022–1027.

14] S.C. Lee, Y.S. Mok, W.K. Lee, Application of emulsion liquid membrane technol-ogy to the extraction of silver, J. Ind. Eng. Chem. 2 (1996) 130–136.

15] N. Othman, H. Mat, M. Goto, Separation of silver from photographic wastes byemulsion liquid membrane system, J. Membr. Sci. 282 (1–2) (2006) 171–177.

16] G.H. Rounaghi, M.S. Kazemi, H. Sadeghian, Transport of silver ion through bulkliquid membrane using macrocyclic and acyclic ligands as carriers in organicsolvents, J. Incl. Phenom. Macrocycl. Chem. 60 (2008) 79–83.

17] K. Farhadi, S. Bahar, R. Maleki, Separation study of silver (I) ion through abulk liquid membrane containing meloxican, J. Braz. Chem. Soc. 18 (3) (2007)595–600.

18] N.M. Kocherginsky, Q. Yang, Big carrousel mechanism of copper removal fromammonical wastewater through supported liquid membrane, Sep. Purif. Tech-nol. 54 (2007) 104–116.

19] J.D. Gyves, E.R. San-Miguel, Metal ion separations by supported liquid mem-branes, Ind. Eng. Chem. Res. 38 (1999) 2182–2202.

20] N. Bukhari, M.A. Chaudry, M. Mazhar, Triethanolamine-cyclohexanone sup-ported liquid membranes study for extraction and removal of nickel ions fromnickel plating wastes, J. Membr. Sci. 283 (2006) 82–189.

21] B. Swain, J. Jeong, J.C. Lee, G.H. Lee, Separation of Co (II) and Li (I) by supportedliquid membrane using cyanex 272 as mobile carrier, J. Membr. Sci. 297 (2007)253–261.

22] A. Gherrou, H. Kerdjoudj, R. Molinari, E. Drioli, Removal of silver and copper ionsfrom acidic thiourea solutions with a supported liquid membrane containingD2EHPA as carrier, Sep. Purif. Technol. 28 (2002) 235–244.

23] M. Shamsipur, S.Y. Kazemi, G. Azimi, S.S. Madaeni, V. Lippolis, A. Garau, F. Isaia,Selective transport of silver ion through a supported liquid membrane usingsome mixed aza-thioether crowns containing a 1,10-phenanthroline sub-unitas specific ion carriers, J. Membr. Sci. 215 (2003) 87–93.

24] M. Shamsipur, O.R. Hashemi, V. Lippolis, A supported liquid membrane sys-tem for simultaneous separation of silver (I) and mercury (II) from dilute feedsolutions, J. Membr. Sci. 282 (2006) 322–327.

25] A.A. Amiri, A. Safavi, A.R. Hasaninejad, H. Shrghi, M. Shamsipur, Highly selectivetransport of silver ion through a supported liquid membrane using calix [4]pyrroles as suitable ion carriers, J. Membr. Sci. 325 (2008) 295–300.

26] M.A. Chaudry, N. Bukhari, M. Mazhar, Coupled transport of Ag (I) ionsthrough triethanolamine-cyclohexanone-based supported liquid membranes,J. Membr. Sci. 320 (2008) 93–100.

27] Z. Pawelka, T.Z. Huyskens, Alkyl substituent effect on the polarity ofphenols–tri-n-alkylamine complexes, Can. J. Chem. 81 (2003) 1012–1018.

28] G. Svehla, Vogel’s Qualitative Inorganic Analysis, seventh ed., Pearson Educa-tion Publisher, 2007, pp. 83–84.

29] S.A. Ansari, P.K. Mohapatra, V.K. Manchanda, Recovery of actinides and lan-thanides from high level waste using hollow- fiber supported liquid membranewith TODGA as the carrier, Ind. Eng. Chem. Res. 48 (2009) 8605–8612.

30] S. Rehman, G. Akhtar, M.A. Chaudry, N. Bukhari, Najebullah, N. Ali, Mn (VII) ionstransport by triethanolamine cyclohexanone based supported liquid mem-brane and recovery of Mn (II) ions from discharged zinc carbon dry batterycell, J. Membr. Sci. 366 (2011) 125–131.

31] K. Chakrabarty, P. Saha, A.K. Ghoshal, Simultaneous separation of mercury andlignosulfonate from aqueous solution using supported liquid membrane, J.Membr. Sci. 346 (2010) 37–44.

32] M.A. Chaudry, M.T. Malik, M. Ahmed, Supported liquid membrane extractionof W(VI) ions using tri-n-octylamine as carrier, J. Radioanal. Nucl. Chem. 150(2) (1991) 383–396.

33] S. Panja, R. Ruhela, S.K. Misra, J.N. Sharma, S.C. Tripathi, A. Dakshinamoorthy,Facilitated transport of Am(III) through a flat-sheet supported liquid mem-brane. (FSSLM) containing tetra(2-ethyl hexyl) diglycolamide (TEHDGA) ascarrier, J. Membr. Sci. 325 (2008) 158–165.

34] M.A. Chaudry, N.U. Islam, N.U. Rehman, Extraction of Pd (II) ions using tri-n-octylamine xylene base supported liquid membranes, J. Radioanal. Nucl. Chem.218 (1) (1997) 53–60.

35] D. Gary, Christan, Analytical Chemistry, sixth ed., John Wiley & Sons, 2004, pp.295–296.

36] B. Swain, J. Jeong, J.C. Lee, G.H. Lee, Extraction of Co (II) by supported liq-uid membrane and solvent extraction using Cyanex 272 as an extractant: acomparison study, J. Membr. Sci. 288 (2007) 139–148.

37] S. Sriram, P.K. Mohapatra, A.K. Pandey, V.K. Manchanda, L.P. Bad-

heka, Facilitated transport of americium(III) from nitric acid mediausing dimethyldibutyltetradecyl-1,3-malonamide, J. Membr. Sci. 177 (2000)163–175.

38] N.M. Kocherginsky, Q. Yang, L. Seelam, Recent advances in supported liquidmembrane technology, Sep. Purif. Technol. 53 (2007) 171–177.