Studies on Liq uid/liq uid Ex trac tion of Cop per Ion with Room Tem per a ture Ionic Liquid

Transcription

1 Jour nal of the Chi nese Chem i cal So ci ety, 2003, 50, Studies on Liq uid/liq uid Ex trac tion of Cop per Ion with Room Tem per a ture Ionic Liquid Guor-Tzo Wei* ( ), Jin-Chu Chen ( ) and Zusing Yang ( ) Na tional Chung Cheng Uni ver sity, De part ment of Chem is try and Bio chem is try, Chia-Yi 621, Tai wan, R.O.C. Room tem per a ture ionic liq uids are re garded as Green sol vents for their non vol a tile and ther mally sta - ble prop er ties. They are em ployed to re place tra di tional vol a tile or ganic sol vents in or ganic syn the sis, sol vent ex trac tion, and elec tro chem is try. In this work, a wa ter im mis ci ble room tem per a ture ionic liq uid, 1- butyl- 3-methylimidazolium hexafluorophosphate [C 4mim][PF6], was used as an al ter na tive sol vent for liq uid/liq uid ex trac tion of cop per ions. Metal chelators, in clud ing dithizone, 8-hydroxyquinoline, and 1-(2-pyridylazo)- 2- naphthol, were em ployed to form neu tral metal-chelate com plexes with cop per ions so that cop per ions were ex tracted from aque ous so lu tion into [C4mim][PF6]. The pa ram e ters that af fect the ex trac tion of cop per ions with this biphasic sys tem were in ves ti gated. The ex trac tion be hav ior in this novel biphasic sys tem is shown to be con sis tent with that of tra di tional sol vents. For ex am ple, the ex trac tion with this biphasic sys tem is strongly ph de pend ent. So, the ex trac tion ef fi ciency of cop pers ion from an aque ous phase can be ma nip u lated by tai lor ing the ph value of the ex trac tion sys tem. Hence, the ex trac tion, sep a ra tion and pre con centrac tion of cop per ions can be ac com plished by con trol ling the ph value of the ex trac tion sys tem. It ap pears that the use of ionic liq uid as an al ter nate sol vent sys tem in liq uid/liq uid ex trac tion of cop per ions is very prom is ing. Key words: Room tem per a ture ionic liq uid; 1-Butyl-3-methylimidazolium hexafluorophosphate; Liq - uid- liquid ex trac tion; Cop per ion ex trac tion; Preconcentration. INTRODUCTION Liq uid-liquid (or sol vent) ex trac tion has been ex ten - sively em ployed in the re cov ery and sep a ra tion of metal ions. Re cov ery of met als from an aque ous phase by sol vent ex trac - tion is nor mally achieved by con tact ing the aque ous phase with an or ganic phase con tain ing a metal se lec tive metal chelator. The or ganic phases used in this ex trac tion are gen er - ally flam ma ble and vol a tile or ganic com pounds (VOCs). The use of VOCs faces en vi ron men tal and safety con cerns of emis sion loss and fire haz ard. Al ter nate ma te ri als are of par - tic u lar in ter est in pro vid ing po ten tial so lu tions to re duce these prob lems. Re cently, room tem per a ture ionic liq uids (RTILs) have at tracted wide in ter est for their prom is ing role as al ter na tive me dia in re ac tion, 1-11 sep a ra tion, and elec tro chem is try 15 as a re sult of their unique chem i cal and phys i cal prop er - ties They have no de tect able va por pres sure and are rel - a tively ther mal sta ble. Fur ther, they can dis solve a wide spec trum of or ganic, organometallic, and in or ganic com - pounds There fore, they are pro posed as novel sol vent sys tems to re place tra di tional VOCs. Since there is no loss of sol vent through evap o ra tion with ionic RTILs, they will avoid en vi ron men tal and safety prob lems due to vol a til iza - tion, as is the case of VOCs. There fore, RTILs are re garded as Green sol vents Most of the work re gard ing RTILs con cern their use as al ter na tive sol vents for syn the sis and elec tro lytes for elec tro - chem is try. 1-15,23 Nev er the less, the ap pli ca tions of RTILs in an a lyt i cal chem is try have also re ceived at ten tion re - cently For sol vent ex trac tion, RTILs were used for the sep a ra tion of or ganic com pounds and or ganic me tal lic com - pound from or ganic phase. Ionic liq uid was shown to ex tract butanol from an aque ous so lu tion of al co hol. 14 The use of ionic liq uids for the ex trac tion of sub sti tuted-benzene de riv a - tive from aque ous phase was pre sented by Huddleston et al. 12 RTIL was com bined with super criti cal car bon di ox ide to re - cover polyaromatic hy dro car bons from ionic liq uid, hence IL can be re used. 13 A sim i lar ap proach by comb ing RTILs with super criti cal car bon di ox ide was also em ployed for ex tract - ing a va ri ety of or ganic re ac tion prod ucts from RTILs that were used as re ac tion me dia. 27 RTILs were em ployed to re - * Cor re sponding au thor. Tel: ; fax: ; chegtw@ccunix.ccu.edu.tw

2 1124 J. Chin. Chem. Soc., Vol. 50, No. 6, 2003 Wei et al. cover dyes from aque ous so lu tion by ad just ing the ph value of the aque ous so lu tion. 28 For the ex trac tion of metal ions with RTILs, sev eral groups re ported the use of crown ethers for the ex trac tion of group 1 and 2 ions from aque ous phase RTILs were fur - ther ex tended to the ex trac tion of heavy metal ions Rog - ers et al. in cor po rated thiourea, thioeter, and urea into deri - vatized imidazolium cat ions and used these functionized ionic liq uids (ILs) as the extractant in liq uid/liq uid ex trac tion of Hg 2+ and Cd In that re port, low dis tri bu tion ra tios were ob tained with out the use of functionized ILs as extractant for Hg 2+ and Cd 2+. The dis tri bu tion ra tios were highly im proved when functionized ILs were mixed with [C4mim][PF6]. There fore, functionized ILs played the role of extractant and hy dro pho bic phase. Rog ers and co work ers also uti lized less ex pen sive [C 4 mim][pf 6 ] with or ganic and in or ganic ex tract - ant for liq uid/liq uid ex trac tion of heavy metal ions. 33 Both or - ganic metal chelators and an ions as extractant were stud ied to re veal the ex trac tion be hav ior of heavy metal ions with ionic liq uid. Ra dio iso topes of metal ions were em ployed to ob tain the dis tri bu tion ra tios. Even with the use of or ganic metal chelators as extractants, ex trac tion ef fi cien cies were still low for some heavy metal ions. In our re cent work, dithizone was uti lized as an or ganic extractant for liq uid/liq uid ex trac tion of heavy metal ions with ionic liq uid-[c4mim][pf6]. 34 UVvis spec trom e ter and atomic ab sorp tion spec trom e try were em ployed to ex am ine the po ten tial use of this ap proach, in - stead of radiotracers, for ex trac tion of heavy metal ions. 34 Pre lim i nary re sults are prom is ing. How ever, fur ther in ves ti - ga tion of the ex trac tion of heavy metal ions with these novel me dia is re quired to un der stand the ex trac tion be hav iors of heavy metal ions in RTILs and their po ten tial in an a lyt i cal ap - pli ca tions. Liq uid-liquid ex trac tion of cop per with [C4mim] [PF 6] was stud ied in this work. Three metal chelators were em ployed to form cop per-chelate com plexes. Then, the pa - ram e ters that af fect the ex trac tion of cop per-chelate com - plexes were in ves ti gated to ex am ine the ex trac tion be hav ior of ionic liq uid. Also, the ex trac tions of cop per ions with VOC and ionic liq uid were com pared. These re sults are used to as - sess the fea si bil ity of RTILs as al ter na tive sol vents to re place con ven tional sol vents in liq uid/liq uid ex trac tion of heavy metal ions. EXPERIMENTAL Chemicals Di chloro methane, acetronitrile, ethyl ac e tate, tris(hy - droxy methyl)-amionomethane, cit ric acid, ph 4.00 and ph buffer so lu tions, and all metal ions stan dard so lu tion (1000 ppm) were sup plied by Merck (Darmstadt, Ger many). Dithizone was ob tained from Riedel-deHaen (Sleeze, Ger - many) and 1-chlorobutane was ob tained from TEDIA (Fairfield, OH, USA). 1-methylimidazole, 8-hydroxy quino - line (Oxine), and 1-(2-pyridylazo)-2-naphthol (PAN) were ob tained from ACROSS (Bel gium), and po tas sium hexafluo - rophosphate was sup plied by ProChem (Rockford, IL, USA). So dium hy drox ide and an hy dride mag ne sium sul fate were ob tained from Hanawa (Osaka, Ja pan). Ni tric acid (70%) was ob tained from J. T. Baker. [C 4 mim][pf 6 ] was pre pared in house us ing the re ported pro ce dure. 12,34 Apparatus Atomic ab sorp tion (AA) mea sure ments were per formed with a Perkin-Elmer AA 3110 spec trom e ter (Uerlingen, Ger - many). UV-visible spec tra were ob tained from a Cary 3E (Varian, Palo Alto, CA, USA). The pu rity of ionic liq uid was con firmed with the spec trum ob tained from a Varian Gem ini 200 MHz NMR. The ph value of the ex trac tion sys tems were mea sured with an Orion model 420A ph me ter (Boston, MA, USA) that was cal i brated with a ph 4.00 and a ph buffer stan dard. Liquid/liquid extraction of copper-chelate complexes Metal chelators were dis solved in 4 ml wa ter with 1 ml acetonitrile to form an M che late so lu tion. UV-vis mea sure ments were per formed to ob tain the ab sorp tion spec - trum of metal chelators. Then, 5 ppm of Cu 2+ was added to each chelator so lu tion and the UV-vis spec trum of Cu com - plex was re corded. Next, ex trac tion of cop per-chelator com - plex was per formed by mix ing 1 ml ionic liq uid with cop - per- chelator com plex fol lowed by shak ing for about 2 min and then cen tri fug ing to sep a rate aque ous-rtil phase. The UV-vis spec trum of aque ous phase was re corded. Extraction efficiencies of RTIL with AA One mil li li ter of extractant, a metal chelator dis solved in RTIL at the con cen tra tion of M, was mixed in a 5 ml 5 ppm Cu 2+ so lu tion that was ad justed to the de sired ph value to form the liq uid-liquid ex trac tion sys tem. The ef fects of other ions co-existed in the aque ous phase on the ex trac - tion of metal ions were ex am ined by mix ing 100 ppm of dif - fer ent metal ions or an ions in the aque ous phase be fore mix - ing with IL. The biphasic sys tem was shaken and cen tri fuged to en sure it was fully mixed and then sep a rated af ter ex trac - tion. The up per aque ous phase was taken out and mea sured with AA spec trom e try to de ter mine the con cen tra tion of metal ion that was left in the aque ous phase. A sim i lar ap -

3 Ex trac tion with Room Tem per a ture Ionic Liq uid J. Chin. Chem. Soc., Vol. 50, No. 6, Table 1. Conditionsof AA for the Determinationsof Metal Ions in Aqueous Phase Cu 2+ Cd 2+ Ag + Zn 2+ Ca 2+ Wavelength (nm) Slit width (mm) Read time (sec.) Sample replicates proach was em ployed for other metal ions. The con di tions for AA mea sure ment are shown in Ta ble 1. Ex trac tion ef fi cien - cies (E) were cal cu lated by was mixed with 1 ml of extractant, and then shaken and cen - tri fuged. The aque ous phase was then re moved and mea sured with AA to de ter mine cop per con cen tra tion. Aque ous cop per ion so lu tion was ex tracted to the ionic liq uid phase with op ti mized con di tion. Dif fer ent vol ume of 10% ni tric acid so lu tion was then mixed with the ionic liq uid phase to strip the cop per ion from ionic liq uid phase. Next, the acid so lu tion was re moved, di luted (if nec es sary), and mea sured with AA to de ter mine Cu 2+ in aque ous phase. The re cov ery fac tor was es ti mated from the ra tio of metal ion in aque ous so lu tion to acid so lu tion, cor rected with di lu tion fac - tor. E(%) ( C ) ( C ) i aq f aq ( C ) i aq 100% (1) RESULTS AND DISCUSSION where (C i ) aq and (C f ) aq are the con cen tra tion of metal ions in aque ous phase be fore and af ter ex trac tion, re spec tively. Separation and preconcentration of copper ion by liq - uid/liquid ex trac tion with RTIL Sep a ra tion of cop per ion was per formed by tak ing 5 ppm Cu 2+ with 5 ppm of other metal ions, such as Zn 2+, Cd 2+, or Ag +, and ad justed to the de sired ph. This aque ous phase Examining the formation and extraction of copperchelate complex with UV-Vis measurement All metal-chelators used in this work are colorimetric re agents. These metal chelators are widely used for the ex - trac tion and de ter mi na tion of metal ions in aque ous phase. 35 They form color metal-chelate com plexes with cop per ion and can be ex tracted with or ganic phase, as shown in Fig. 1. A sche matic ex pres sion for liq uid-liquid ex trac tion of cop per Fig. 1. Com plex for ma tion of cop per ions with metal or ganic chelator.

4 1126 J. Chin. Chem. Soc., Vol. 50, No. 6, 2003 Wei et al. with ionic liq uid extractant con tain ing metal chelator can be rep re sented by Fig Dithizone is used as an ex am ple to re - veal the for ma tion and ex trac tion of Cu 2+ with ionic liq uid. Fig. 3(A) in di cates two ab sorp tion peaks of dithizone in aque ous phase. With the ad di tion of Cu 2+ into the aque ous dithi zone so lu tion, the re ac tion of dithizone with Cu 2+ forms a cop per-dithizone com plex. So a new ab sorp tion max i mum at about 570 nm is ob served, as shown in Fig. 3(B). This in di - cates the for ma tion of cop per-dithizonate. Once the so lu tion of cop per-dithizone com plex was mixed with [C 4mim][PF 6], cop per-dithizonate was re moved from aque ous phase. There - fore, the char ac ter is tic ab sorp tion peak of cop per-dithizone com plex in aque ous phase dis ap peared, as shown in Fig. 3(C). Spec trum of Fig. 3(B) and 3(C) clearly in di cate the for - ma tion of cop per-dithizone com plex and the ex trac tion of this com plex from aque ous phase into [C 4 mim][pf 6 ]. Here, ad di tion of a small amount of acetonitrile to the aque ous phase fa cil i tates the dis so lu tion of metal chelators and cop - per-chelate com plexes in aque ous phase so that the ab sorp - tion spec trum of chelator and cop per-chelate com plex in aque ous phase can be ob tained. Sim i lar re sults for the for ma - tion of cop per- chelate com plex of cop per with PAN and Oxine and the ex trac tion of these cop per-chelate com plexes by ionic liq uid were ob tained (data not shown here). UV-vis spec trom e ter was also di rectly em ployed to obtain the ab sorp tion spec trum of dithizone and cop perdithizone com plex in [C 4 mim][pf 6 ], as shown in Fig. 3(D) and 3(E), re spec tively. Spec tro pho to met ric spec trum of metal chelator and cop per-chelate com plex in [C 4 mim][pf 6 ] are strongly over lapped. There fore, it is im pos si ble to use a spectrophotometer for the determination of metal-chelator com plex in ionic liq uid with ex cess of metal chelator in the ex trac tion sys tem. It has to be em pha sized that the ab sorp tion spec trum of dithizone in ionic liq uid is dif fer ent from that in other me dia. Normally metal chelator and metal-chelate com - plex have dif fer ent spec trum in tra di tional or ganic sol vents. Hence, a spectrophotometer can be used to de ter mine the con cen tra tion of metal com plexes. 36 The ef fect of RTILs on the mo lec u lar spec trum has been a sub ject of in ter est em - ployed to probe the po lar ity of RTILs. 17 Influence of ph value on the extraction of metal ion with IL and dichloromethane AA, in stead of us ing ra dio iso tope tracer, 33 was uti lized to de ter mine the con cen tra tion of metal ions in aque ous so lu - tion. The ex trac tion ef fi cien cies of cop per ions at dif fer ent con di tions are ob tained by AA mea sure ment of cop per ion in aque ous and es ti mated with Eq. 1. The in flu ence of ph value on the ex trac tion of cop per-chelate com plex with tra di tional sol vent (dichchloromethane) and IL are com pared and shown in Fig. 4. Both biphasic sys tems show sim i lar be hav ior that the ex trac tion is strongly de pend ent on the ph value of the ex - trac tion sys tem, as shown in Fig. 1. At low ph, the ma jor ity of metal chelator is in HL (neu tral) form that pre fers to dis - trib ute in or ganic phase. This leads to a very low con cen tra - tion of cop per-chelate com plex be cause there is only a lim - ited amount of deprotonated chelator for the for ma tion of cop per-chelate com plex. Also highly hy dro philic metal ions pre fer par ti tion in aque ous phase and this re sults in low ex - trac tion ef fi ciency at low ph. On the con trary, high con cen - tra tion of deprotonated chelator at high ph leads to high con - Fig. 2. Il lus tra tion of var i ous equilibra in volved in metal ion ex trac tion with ionic liq uid. Fig. 3. UV-vis ab sorp tion spec trum of (A) dithizone in aque ous so lu tion con tain ing acetonitrile, (B) cop per-dithizone in aque ous so lu tion con tain - ing acetonitrile, (C) aque ous so lu tion af ter IL ex trac tion, (D) dithizone in IL, and (E) cop perdithiozone com plex in IL.

5 Ex trac tion with Room Tem per a ture Ionic Liq uid J. Chin. Chem. Soc., Vol. 50, No. 6, cen tra tion of cop per-chelate com plex and re sults in high extrac tion ef fi ciency, due to the high sol u bil ity of cop perchelate com plex in or ganic phase. A care ful ex am i na tion of Fig. 4 re veals that the ex trac - tion ef fi ciency of ionic liq uid is higher than that of di chloro - methane at low ph. A pos si ble rea son may be a higher con - cen tra tion of deprotonated dithizone in [C4mim][PF6], com - pared to that in di chloro methane. Ionic liq uid fa cil i tates the for ma tion of deprotonated metal chelator be cause IL has higher sol va tion en ergy than that of di chloro methane. 31 There fore, the use of RTIL as the sol vent for liq uid/liq uid ex - trac tion of metal ions al lows the ex trac tion to be per formed at lower ph. It will re duce the pos si bil ity of metal hy drox ides for ma tion. An other ad van tage of us ing RTIL in liq uid/liq uid ex trac tion of metal ion is that this method elim i nates the use of chlo ri nated sol vents such as di chloro methane. Di chloro - methane is highly toxic and listed as a po ten tial car cin o gen. Pro longed or re peated overexposure leads to liver and kid ney dam age and an in creased risk of can cer. Fig. 5~7 show the ph ef fect on the ex trac tion of dif fer - ent metal ions with [C 4mim][PF6] for three metal chelators. The ex trac tion of metal ions with RTIL is strongly ph de - pend ent. The ex trac tion of metal ions also de pends on the type of or ganic metal chelator em ployed. The char ac ter is tic ph with 50 per cent age of ex trac tion ef fi ciency de pends on the sta bil ity of metal-chelate com plex (Kf). Complete ex trac - tion of metal ions can be ac com plished at lower ph for metal ions with more sta ble metal-chelate com plex (higher Kf). Con se quently, the sep a ra tion of dif fer ent metal ions by tai lor - ing the type of metal chelator and the ph value of the sep a rat - ing me dium can be achieved. Interference in the ex trac tion of cop per ion with ionic liq uid Fig. 5~7 also im ply that ex trac tion of cop per ion with ionic liq uid may be in ter fered with by other metal ions. Fig. 8 dis plays the ef fect of cat ions on the ex trac tion of Cu 2+ with dithizone. The data in di cate that the ex is tence of Ag + re duces the ex trac tion ef fi ciency of Cu 2+ in the case of dithizone as the chelator re agent. The ef fect of Ag + on the ex trac tion of Cu 2+ is con cen tra tion de pend ent. 34 This is at trib uted to the greater for ma tion con stant of Ag + with dithizone than that of Cu 2+. So dithizone pre fers to form a neu tral metal com plex with Ag + rather than with Cu 2+ when they co ex ist, and the con cen tra tion of dithizone is less than stoichiometric con cen - Fig. 5. The ef fect of ph value on the ex trac tion ef fi - cien cies of metal ions with dithizone by IL. Fig. 4. Com paring the ex trac tion of cop per ions in ionic liq uid and di chloro methane with PAN. Fig. 6. The ef fect of ph value on the ex trac tion ef fi - cien cies of metal ions with PAN by IL.

6 1128 J. Chin. Chem. Soc., Vol. 50, No. 6, 2003 Wei et al. tra tion of metal ions. Fig. 8 also in di cates that larger in ter fer - ence was ob served with dichrolometane, as com pared with RTIL at the same ex trac tion con di tion. There were sim i lar re - sults for the ef fect of cat ions on the ex trac tion of Cu 2+ with PAN and Oxine (data not shown here). The re sults of Fig. 9 dis play the ef fect of an ions on the ex trac tion of Cu 2+ with PAN. Here, the an ions are as sumed to com pete with or ganic chelator for com plex for ma tion and may lead to in ter fer ence. Less an ion in ter fer ence was ob - served for the ex trac tion of cop per with RTIL than that of with di chloro methane. The pro posed rea son for less an ion in ter fer ence in the ex trac tion of cop per with ionic liq uid is that an ion (PF6 - ) is part of an ionic liq uid mol e cule. A large amount of an ion in ionic liq uid may thwart the ef fect of an - ions in aque ous phase. Yet, the re sults show that the mag - Fig. 7. The ef fect of ph value on the ex trac tion ef fi - cien cies of metal ions with Oxine by IL. ni tude of an ions ef fect on the cop per ex trac tion is metal chelator de pend ent. Ap pli ca tions of liq uid/liq uid ex trac tion of cop per ion with ionic liquid The fea si bil ity for the sep a ra tion of metal ions in liq - uid/liq uid ex trac tion with IL was ex am ined. A mix ture con - tain ing 5 ppm Cu 2+ and 5 ppm other metal ions was ex tracted with an aque ous-[c 4 mim][pf 6 ] sys tem. The metal ion con - cen tra tion in aque ous phase was mea sured af ter ex trac tion to es ti mate the ex trac tion ef fi ciency. The re sults are shown in Ta ble 2. In the mix ture of Ag + and Cu 2+ at ph 2.2, only Ag + re - mains in the aque ous phase. In the mix ture of Zn 2+ and Cu 2+ at ph 5.5, only Zn 2+ re mains in the aque ous phase. In the mix - ture of Cd 2+ and Cu 2+ at ph 6.9, only Cd 2+ re mains in the aque ous phase. These ex am ples clearly in di cate that IL can be uti lized for the sep a ra tion of metal ions by tai lor ing the ph value of the aque ous phase. One of the ap pli ca tions for liq uid-liquid ex trac tions of metal ions with metal chelator is the preconcentration of metal ions to in crease the con cen tra tion of metal ions so that the de tec tion sen si tiv ity can be im proved. Also, the ex trac - tion pro cess re moves the in ter fer ence el e ments and re - duce/elim i nate the anal y sis in ter fer ence. Trace cop per ion can be re moved from aque ous so lu tion into the or ganic phase by the for ma tion of neu tral metal-chelate com plex. With ad - di tion of acid to de crease the ph value to a cer tain value, the ex trac tion ef fi ciency is re duced. So cop per ions can be stripped from IL phase into aque ous phase by the dis so ci a tion of cop per-chelate com plex. Cop per ions in a large vol ume of aque ous phase are trans ferred to a small vol ume of 10% ni tric acid so lu tion by us ing a large vol ume of aque ous phase with small value of RTIL dur ing the ex trac tion and a small vol ume of acid so lu tion in the strip ping step. This re sults in an in - Fig. 8. Cat ion af fects on the ex trac tion of cop per ions with dithizone by [C4mim][PF6]. 100 ppm of in - ter fer ence ion was added to ph 6.5, 5 ppm cop - per so lu tion and ex tracted with M dithizone. Fig. 9. An ion af fects on the ex trac tion of cop per ions with PAN by [C 4mim][PF6]. 100 ppm of in ter - fer ence ion was added to ph 6.5, 5 ppm cop per so lu tion and ex tracted with M PAN.

7 Ex trac tion with Room Tem per a ture Ionic Liq uid J. Chin. Chem. Soc., Vol. 50, No. 6, Table 2. Separation of Copper Ions from Other Metal Ionsby Liquid-liquid Extraction with Oxine in [C 4 mim][pf 6 ] Condition crease of cop per ion con cen tra tion. The mag ni tude of the con cen tra tion fac tor de pends on the vol ume ra tio of orig i nal aque ous so lu tion to the strip ping acid so lu tion. Re sults for the pre-concentration of Cu 2+ with Oxine are shown in Ta ble 3. These data show that the preconcentration and re cov ery of cop per ions with ionic liq uid from di luted so lu tions are pos si - ble. Recycling ionic liquid for reuse One of the prin ci ples of Green Chem is try is the use of re us able ma te rial. 16 RTIL is re garded as a Green Sol - vent The fea si bil ity of re cy cling RTIL was ex am ined. Ex trac tion of 5 ppm Cu 2+ at about ph 3.5 with Oxine was per - formed. Af ter the ex trac tion, the ionic liq uid phase was mixed with 0.1 M HNO 3 so lu tion to re move Cu 2+ from the ionic liq uid phase. This ionic liq uid was re cy cled for re use. This ex trac tion and strip ping pro cess was re peated for five cy cles. AA was used to mea sure the con cen tra tion of Cu 2+ in acidic aque ous phase af ter each strip ping pro cess. Ex trac tion ef fi cien cies of % were ob tained. Also, the ex trac - tion pro cess of the ex trac tion is shown to be rea son ably re - pro duc ible from these data. CONCLUSION Percentage extracted Ag + and Cu 2+ at ph 2.2 Ag +, 78 (%) Cu 2+, 0 (%) Zn 2+ and Cu 2+ at ph 5.5 Zn 2+, 0 (%) Cu 2+, 88 (%) Cd 2+ and Cu 2+ at ph 6.9 Cd 2+, 0 (%) Cu 2+, 89 (%) Table 3. Preconcentration of Cu 2+ by Liquid/liquid Extraction with [C 4 mim][pf 6 ] Concentration Theoretical Experimental Recovery Factor value (ppm) value (ppm) (%) P.S. 5 ppm of Cu 2+ in ph 7.0. The ex trac tion of cop per ion with metal chelators in a biphasic sys tem con sist ing of a room tem per a ture ionic liq uid and an aque ous phase with an atomic ab sorp tion spec trom e - ter was dem on strated. Ex trac tion be hav iors are sim i lar to those of tra di tional or ganic sol vent. The ex trac tion of cop per ion with [C4mim][PF6] tends to have less in ter fer ence by other cat ions and an ions. This ex trac tion pro cess can be used to sep a rate cop per ions from other metal ions. Also, it can be used to con cen trate cop per ions from di luted aque ous so lu - tions. RTILs are en vi ron men tally be nign be cause of their non vol a tile and non flam ma ble prop er ties. Re cy cling for re - use is shown to be pos si ble. Hence, they have the po ten tial to re place tra di tional vol a tile or ganic sol vents in liq uid/liq uid ex trac tion of metal ions. ACKNOWLEDGMENTS The au thors would like to thank Na tional Chung Cheng Uni ver sity and the Na tional Sci ence Coun cil, Tai wan for fi - nan cially sup port ing this re search with grant (NSC M ). Thanks to Pro fes sor I. W. Sun, De part ment of Chem is try, Na tional Cheng Kung Uni ver sity for his help in the prep a ra tion of ionic liq uid. Re ceived January 20, REFERENCES 1. Wilkes, J. S.; Zaworotko, M. J. J. Chem. Soc., Chem. Commun. 1992, Suarez, P. A. Z.; Dullius, J. E. L.; Einloft, S.; de Souza, R. F. Poly he dron 1996, 15, Adam, C. J.; Earle, M. J.; Rob ert, G.; Sneddon, K. R. Chem. Commun. 1998, Dyson, P. J.; Ellis, D. J.; Parker, D. C.; Welton, T. Chem. Commun. 1999, Adam, C. J.; Earle, M. J.; Sneddon, K. R. Green Chem. 2000, 2, Toma, G.; Gotov, B.; Solcaniova, E. Green Chem. 2000, 2, Song, C. E.; Roh, E. J. Chem. Commun. 2000, Rosa, J. N.; Afonso, C. A. M.; Santos, A. G. Tet ra he dron 2001, 57, Bauer, D. J.; Kottsieper, K. W.; Liek, C.; Stelzer, O.; Waffenschmidt, H.; Wasserscheid, P. J. Organomet. Chem. 2001, 630, Handy, S. T.; Zhang, X. L. Org. Lett. 2001, 3, Favre, F.; Olivierbourbigou, H.; Commercuc, D.; Saussine, L. Chem. Commun. 2001, Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rog ers, R. D. Chem. Commun. 1998, Blanchard, L. A.; Hancut, D.; Beckman, E. J.; Brennecke, J.

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