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The Effects of Diluents on Sodium Ion Transport through Bulk Liquid Membrane
with Crown Ether Carriers
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The Ef fects of Dil u ents on So dium Ion Trans port through
Bulk Liq uid Mem brane with Crown Ether Car ri ers
by S. Altin1*, &I. Peker2 and N. Demircio(glu3
1Z. Karaelmas Uni ver sity, De part ment of En vi ron men tal En gi neer ing, 67100 Zonguldak, Tur key 2
Erciyes Uni ver sity, De part ment of En vi ron men tal En gi neer ing, 38039 Kayseri, Tur key
3
Atatürk Uni ver sity, De part ment of En vi ron men tal En gi neer ing, 25240 Erzurum, Tur key *Fax: +90 (372) 257 4023; e-mail: saltin@karaelmas.edu.tr
(Re ceived Feb ru ary 25th, 2008; re vised manu script Au gust 26th, 2008)
In this study, the ef fects of the dif fer ent mem brane dil u ents, car ri ers and vari a tions in the con cen tra tions of the car ri ers on the so dium ion trans port were ex am ined in bulk liq uid mem brane sys tems. The ki netic anal y sis of the so dium ion trans port in the liq uid mem -brane was per formed through the im ple men ta tion of two con sec u tive ki netic equa tions of first or der ir re vers ible re ac tions and the rel e vant ki netic pa ram e ters (k1, k2a, k2m, Rmax, tmax) were de ter mined. To lu ene, n-hex ane, 1-octanol, chlo ro form and di chloro methane were used as mem brane dil u ents, while 18crown6 (18C6), dicyclohexyl 18crown6 (DC18C6) and dibenzo 18crown6 (DB18C6) were used as car ri ers. The re sults re veal that the best so dium ion trans port was re al ized when to lu ene-di chloro methane (90%–10%) was used as the mem brane, and 1×10–3 M DB18C6 was used as the car rier. Key words: bulk liquid membrane, diluent’s effect, crown ethers, sodium, kinetic analysis
In re cent years liq uid mem branes have drawn much at ten tion for ap pli ca tions in sep a ra tion and en rich ment. Liq uid mem brane sys tems have high chem i cal se lec tiv ity; there fore, they were pre ferred for sep a ra tion op er a tions re quir ing chem i cal re ac -tions. The fa cil i tated trans port in liq uid mem brane sys tems can be ex pressed as the trans port of re quired ma te ri als with the aid of a complexing agent [1]. Within the scope of the study, the ef fects of the mem brane dil u ent, the car rier and its con cen tra tion on the trans port of the so dium ion in the bulk liq uid mem brane sys tems were ex -am ined. The char ac ter is tics of the mem brane dil u ent af fects the trans port ac tiv ity in the liq uid mem branes to a great de gree [2,3]. Mem brane dil u ents hav ing high di elec -tric permittivities, high boil ing points, low vis cos ity and low sol u bil ity are pre ferred. On the other hand, in ter ac tions among car riersol utedil u ent are also im por tant fac -tors for sol ute trans port in the liq uid mem branes. There fore, ex per i ments in the liq uid mem brane stud ies should be fo cused on the se lec tion of the dil u ents. Some stud ies of the ef fects of dil u ents on the trans port ef fi ciency were per formed by us ing dif fer ent dil u ents such as chlo ro form, di chloro methane, to lu ene, xylene, ker o sene, n-oc tane, mesitylene [3–6]. It is also nec es sary that the se lected mem brane dil u ent should not evap o rate at the re ac tor tem per a ture, and should be a good dil u ent for the car rier [7,8]. Fur ther more, fac tors which in clude struc tural prop er ties of dil u ent, the length
of the car bon chain and the com bi na tion of the dil u ents se lected from dif fer ent dil u -ent classes have a sig nif i cant ef fect on the trans port of so dium ions [2,3].
In or der to main tain the fa cil i tated trans port in the liq uid mem branes, a suit able complexing agent (a car rier) should be added to the mem brane phase. By se lect ing an ap pro pri ate car rier, a highse lec tiv ity sep a ra tion is pos si ble. When se lect ing a suit able car rier, im por tant fac tors in clude physicochemical prop er ties of the car rier; at -tached func tional groups, chain struc ture and the bonds that form a com plex [9,10]. In many stud ies, crown ether struc tures were used as car rier for the trans port of al kali met als [10–12]. The crown ether part of the macrocycle com pounds can take the al -kali metal cat ions se lec tively within the crown cav ity [8,11].
The con cen tra tion of the car rier is as im por tant as the type of the car rier. How -ever, it is dif fi cult to de ter mine a gen eral re la tion ship be tween the con cen tra tion of the car rier and the ef fi ciency of the trans port [9,11]. There fore, the most suit able con -cen tra tion for the car rier should be de ter mined by ex per i men tal stud ies.
In the treat ment of sa line wa ter, the re moval of Na+ ion by fil tra tion sys tems is very ex pen sive be cause it is most re sis tant ion for fil tra tion sys tems. Aim of this study is to de ter mine the ef fect of mem brane dil u ent, car rier type and con cen tra tion of the car rier on so dium trans port ef fi ciency of the liq uid mem brane sys tem, and to per form ki netic anal y sis of ex per i men tal data by us ing a suit able ki netic ap proach.
EX PER I MEN TAL
Ma te ri als. The fol low ing re agents were used in the ex per i ments: so dium chlo ride (Merck) as a do -nor phase, sul fu ric acid (Merck) as an ac cep tor phase, to lu ene (Merck), 1-octanol (Merck), n-hex ane (Riedel), di chloro methane (Riedel), chlo ro form (Merck) as mem brane phase. 18C6 (18crown6) (Merck, pu rity; 98%), DC18C6 (dicyclohexyl 18crown6) (Merck, pu rity: 98%), DB18C6 (dibenzo 18crown6) (Merck, pu rity: 99%) as a car rier. The aque ous so lu tions were pre pared by us ing de min er al ized wa ter.
Meth ods. The ex per i ments were per formed at the same con di tions as those used in the sup ported liq -uid mem brane stud ies in the pre vi ous pa per [13]. In this study, in ad di tion to the mem brane liq -uids used in [13], 1-octanol, n-hex ane, to lu ene-chlo ro form, 1-octanol-di chloro methane, 1-octanol-chlo ro form,
nhex anedi chloro methane, nhex anechlo ro form sin gle or bi nary mem branes were ex am ined. In the ex
-per i ments us ing dif fer ent mem brane dil u ents, the car rier 1×10–3
M DB18C6, the ac cep tor phase 2 M H2SO4, and the do nor phase 500 mg/L Na+ in form of NaCl were used. Since 1octanol and nhex ane can -not dis solve DB18C6 com pletely, the trans port rate of the sys tems us ing these dil u ents was poor. In this work, in ad di tion to our pre vi ous works [13], DC18C6 and 18C6 car ri ers and their con cen tra tion ef fects on the trans port were in ves ti gated. For eval u a tion of ob tained data in the ex per i ments, nonsteady-state anal y sis [6,13,14] was used. The fi nal equa tions con sid ered in this work are given be low.
Rd = exp(–k1t) (1) Rm = k km k 1 1 2 [exp(–k1t) – exp(k2mt)] (2) Ra = 1 – 1 1 k2a-k [k2a exp(–k1t) – k1exp(–k2at)] (3)
The de tails of the ki netic ap proach and the def i ni tion of sym bols can be found in [13]. 276 S. Altin, &I. Peker and N. Demircioglu(
RE SULTS AND DIS CUS SION
Ac cord ing to the data ob tained from ex per i men tal stud ies for each pa ram e ter, the ef fect of pa ram e ters on the re duced so dium con cen tra tion in the do nor phase (Rd), the
mem brane phase (Rm) and the ac cep tor phase (Ra) as a func tion of time are shown in
Fig ures 1–3. The ki netic val ues of all pa ram e ters are given in Ta bles 1–3.
Ef fects of the mem brane dil u ent. By con sid er ing the re sults given in the pre vi ous
works, ac cord ing to the ob tained trans port ef fi cien cies, the dil u ents form the fol low ing or der: to lu enedi chloro methane > to lu ene > to lu enechlo ro form > octanoldi chloro -methane > octanol-chlo ro form > hex ane-di chloro -methane > hex ane-chlo ro form.
With re spect to di elec tric permittivities, the dil u ents can be listed in the fol low ing or der: 1-octanol > di chloro methane > chlo ro form > to lu ene > hex ane. The great est ef fi ciency was ex pected for 1-octanol, how ever, since the car rier is not dis solved com pletely in 1-octanol, the ex pected ef fi ciency could not be achieved. In or der to en able the dis so lu tion of the car rier and to fa cil i tate its mo bil ity, di chloro methane was added. As the den sity of the mem brane liq uid was re quired to be lower than 1 due to the de sign of the liq uid mem brane sys tem, only 10% of di chloro methane could be added. At such con cen tra tion it had not much ef fect on the ef fi ciency.
Ta ble 1. Ki netic pa ram e ters that were ob tained from the ki netic anal y sis of liq uid mem brane sys tems with dif fer ent dil u ents.
Dil u ent (90%–10%; v/v) k1×10–2 (h–1) k2a×10–1 (h–1) k2m×10–1 (h–1) Rmax×10 –1 tmax (h) k1/k2m×10 –1 Na+ trans port % (in 24 h) To lu ene-chloroform 6.28 2.17 2.47 1.84 7.42 2.54 42 Hex ane-chloroform 2.079 3.17 1.31 1.11 16.62 1.57 30 Hex ane-dichloromethane 2.25 1.58 1.55 1.33 14.50 1.44 30 Octanol-chloroform 2.91 1.49 1.56 1.26 13.22 1.86 38 Octanol-dichloromethane 5.78 1.79 1.81 1.24 9.23 3.18 48
With the ad di tion of 10% of di chloro methane to to lu ene, the ob tained mix ture has greater sol u bil ity for the car rier and has a higher di elec tric permittivity, the ef fi ciency of the trans port in creases. Sim i larly, Mohapatra et al. [15] re ported that the ex trac tion of the ion-pairs is not fa vored when dil u ents of low po lar ity is used. On the other hand, dil u ents of very high po lar ity such as nitrobenzene can lead to the desta -bilization of the ion-pair in the or ganic phase lead ing to dis so ci a tion in the in di vid ual ions. As these two fac tors are op pos ing to each other, an op ti mum value in di elec tric permittivity could re sult in higher ex trac tion/trans port ef fi ciency [15]. Crown ether and cat ion-crown ether par ti tion ing are re lated to sol vent di elec tric permittivity and di pole mo ment [16]. When the dil u ent mix tures are eval u ated, it is ob served that the mem branes in clud ing di chloro methane were more ef fi cient than the mem branes in -clud ing chlo ro form. In pre vi ous stud ies, Izatt et al. [16], Araiki and Tsukube [17] and Bartsch et al. [3] dem on strated that di chloro methane pro vides a better trans port rate
than that of chlo ro form. For mer stud ies dem on strated that the use of dil u ent mix tures as mem branes rather than us ing mem branes con sist ing of only one dil u ent could be more ef fi cient [2,18–21].
Tak ing into ac count k1, k2 (k2a and k2m) con stants, Rmax and tmax (Ta ble 1), it can
be seen that k1 con stant is quite high for to lu ene and to lu enedi chloro methane mem
-branes. Lower tmax val ues of these mem branes in di cate that the trans port pro cess was
faster. Fur ther more, higher k1/k2m ra tio of the to lu ene-di chloro methane mem brane
also in di cates that a better sep a ra tion is pos si ble with this mem brane.
In many dil u ent ex trac tion stud ies, to in crease both the trans port rate and sol u bil -ity of car rier com plex in or ganic phase, mod i fier has been added into dil u ent. But, the add ing of mod i fier has not al ways pos i tive re sults for liq uid mem branes. Be cause, the trans port rate may be af fected neg a tively, due to ei ther the re ac tion be tween the mod i -fier and the sol ute or complexing/decomplexing re ac tions at the in ter faces [22,23].
278 S. Altin, &I. Peker and N. Demircioglu(
Fig ure 1. The ef fect of dil u ents upon re duced so dium mole num bers in the do nor phase (Rd), mem brane
phase (Rm) and ac cep tor phase (Ra) as a func tion of time. Rm
To lu ene
To lu ene+di chloro methane To lu ene+chlo ro form Hex ane+di chloro methane Hex ane+chlo ro form Octanol+di chloro methane Octanol+chlo ro form Rd
Ef fects of car rier types. For the ex am i na tion of the crown ether struc ture and the
func tional groups at tached to this struc ture, 18C6, DC18C6 and DB18C6 were used as the car rier. In the ex per i ments, the con cen tra tion of the car rier was 1×10–3 M, the sol vent mem brane was to lu ene-di chloro methane.
Ta ble 2. Ki netic pa ram e ters ob tained from the sys tems in which dif fer ent car ri ers were used (mem brane: to lu ene-di chloro methane).
Car rier k1×10–2 (h–1) k2a×10–1 (h–1) k2m×10–1 (h–1) Rmax×10–1 tmax (h) k1/k2m×10–1
18C6 8.33 2.35 2.35 1.51 6.82 3.35
DC18C6 2.92 1.52 1.30 1.00 14.80 2.24
DB18C6 17.76 2.39 2.20 0.42 5.04 8.06
Fig ure 2. The ef fect of car rier upon re duced so dium mole num bers in the do nor phase (Rd), mem brane
phase (Rm) and ac cep tor phase (Ra) as a func tion of time. Rd
Ra
When Fig ure 2 and Ta ble 2 are ex am ined, it is seen that the best so dium trans port ef fi ciency is ob tained for the sys tems hav ing DB18C6 car rier. The higher val ues of k1/k2m re sult in a shorter time to reach to Rmax, thereby more ef fi cient trans por ta tion
oc curs. For the sys tem in clud ing DC18C6, the lower val ues of Rmax and k1/k2m ra tio,
and the higher val ues for tmax in di cate poor trans por ta tion ef fi ciency. The lower k1
value in this sys tem shows that the car rier-cat ion com plex for ma tion is not ad e quate. On the other hand, the higher tmax value is a re sult of the poor mo bil ity of the car
riercat ion com plex due to poor dif fu sion in the mem brane. Gaikwad et al. [24] dem -on strated that DB18C6 was more ef fi cient than DC18C6 in a study of the trans port of the K+ ion. The good in ter ac tion be tween the benzyl groups in the struc ture of DB18C6 and the an ion bonded to the so dium has a sig nif i cant ef fect on the trans port rate. Fur ther more, since the sol u bil ity of DB18C6 is low, the car rier be ing lost due in the aque ous phase can be ne glected; the car rier con cen tra tion in the mem brane does not de crease in time. In this way, the amount of car rier in the trans port stays con stant and the ef fi ciency is not neg a tively af fected. Due to the fact that the sol u bil ity of 18C6 car rier is high, the trans mis sion to the aque ous phase may be pos si ble. This causes a de crease of the ef fi ciency in time.
Ef fects of car rier con cen tra tion. The con cen tra tion of the car rier in flu ences the
ef fi ciency of the trans port just as much as the type of the car rier used in the liq uid mem brane sys tems [11,25,26]. To ex am ine the ef fects of the con cen tra tion of the car -rier, three dif fer ent con cen tra tions of the car ri ers were tested. The ex per i ments were per formed us ing to lu ene-di chloro methane liq uid mem branes (Fig. 3 and Tab. 3).
When the con cen tra tions of the car ri ers were al tered, the best con cen tra tion value for DB18C6 was 1×10–3 M. There are two dif fer ent ef fects re sult ing from the rise of the car rier con cen tra tion: (i) The value of dis tri bu tion co ef fi cient in creased pro por tion ally with in crease of the car rier con cen tra tion, as a re sult, the flux reached a max i -mum value for both the do nor phase/the mem brane phase and the mem brane phase/the ac cep tor phase in ter faces. (ii) Af ter, the max i mum was reached, the flux de creased due to the in cre ment of vis cos ity, and ag gre ga tion of mi cro emul sions [23,27].
CON CLU SIONS
In this study, the bulk liq uid mem brane sys tem and the ex per i mental con di tions used for the in ves ti ga tion of mem brane sol vents and car ri ers are the same as those used in our ear lier work [13]. There fore, the data given in pre vi ous work for to lu ene, to lu ene-di chloro methane mem brane liq uids and DB18C6 are also eval u ated here. The fea tures of the mem brane liq uids are im por tant in terms of the mo bil ity of the car -rier in side the mem brane. The re sults show that the ad di tion of an other dil u ent to mem brane liq uid can in crease the mo bil ity of such car ri ers. In this study, the ad di tion of di chloro methane into octanol and to lu ene, the ad di tion of chlo ro form into hex ane dil u ent pos i tively af fects the trans port.
Con se quently, the use of multi-com po nent in mem brane sys tems is an ad van tage when in ter ac tion oc curs be tween them. It is also re ported [28] that multi-com po nent mem branes gen er ally are more ad van tage com pared to a sin gle com po nent mem brane. Our work clearly pres ents the sig nif i cance of the mem brane dil u ents for op ti -mum trans port ef fi ciency in liq uid mem brane sys tems.
Ta ble 3. Ki netic pa ram e ters ob tained from ex per i ments at dif fer ent car rier con cen tra tions. Con cen tra tion
of car rier k1×10 –2 (h–1) k2a×10–1 (h–1) k2m×10–1 (h–1) Rmax×10–1 tmax (h) k1/k2m×10–1 1×10–2 M DB18C6 5.11 0.93 0.79 0.28 15.51 6.40 1×10–3 M DB18C6 17.76 2.39 2.20 0.42 5.04 8.06 1×10–4 M DB18C6 1.85 2.31 2.26 2.08 12.02 0.81
Fig ure 3. The ef fect of car rier con cen tra tion, upon re duced so dium mole num bers in the do nor phase (Rd), the mem brane phase (Rm) and ac cep tor phase (Ra) as a func tion of time.
Rd
Ra
All car ri ers used in the ex per i ments have the 18C6 struc ture of crown ether group. It is known that trans port oc curs in the cav ity of crown struc tures. Al though cav ity struc tures in all car ri ers are sim i lar, var i ous func tional groups at tached to crown struc ture af fect the mo bil ity and complexation prop er ties of car rier-com plex within the mem brane liq uid. Con se quently, the trans port rate of sol ute in the mem -brane may be changed.
In or der to eval u ate the use of liq uid mem brane in large scale, more ex per i men tal works are re quired. The re sults of this work re veal that the mem brane dil u ent, car rier con cen tra tion and struc ture have sig nif i cant ef fect on the ef fi ciency of liq uid mem -brane sys tems.
Ac knowl edg ments
This study was sup ported by Re search Fund of Cumhuriyet Uni ver sity, Sivas, Tur key (Pro ject Num ber M-183).
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