References
  -  J. Kotas, Z. Stasicka, Chromium occurrence in the environment
    and methods of its speciation, Environ. Pollut., 107 (2000)
    263–283. 
-  V. Gomez, M.P. Callao, Chromium determination and speciation
    since 2000, Trends Anal. Chem., 25 (2006) 1006–1015. 
-  K.W. Bruland, J.R. Donat, D.A. Hutchins, Interactive influences
    of bioactive trace metals on biological production in oceanic
    waters, Limnol. Ocenaogr., 36 (1991) 1555–1577. 
-  G. Gurboga, H. Tel, Preparation of TiO2–SiO2 mixed gel spheres
    for strontium adsorption, J. Hazard. Mater., B120 (2005)
    135–142. 
-  S.P. Mishra, V.K. Singh, Radiotracer technique in adsorption
    study XIII. Adsorption of barium and strontium ions on
    chromium (IV) oxide powder, Appl. Radiat. Isot., 46 (1995)
    847–853. 
-  M. Torab-Mostaedi, A. Ghaemi, H. Ghassabzadeh, M. Ghannadi-
    Maragheh, Removal of strontium and barium from
    aqueous solutions by adsorption onto expanded Perlite, Can.
    J. Chem. Eng., 89 (2011) 1247–1254. 
-  F. Bari, N. Begum, S.B. Jamaludin, K. Hussin, Extraction and
    separation of Cu(II), Ni(II) and Zn(II) by sol-gel silica immobilized
    with Cyanex 272, Hydrometallurgy, 96 (2009) 140–147. 
-  J.L. Cortina, N. Miralles, M. Aguilar, A.M. Sastre, Solvent
    impregnated resins containing di(2-ethyl-hexyl)phosphoric
    acid. II. Study of the distribution equilibria of Zn(II), Cu(II)
    and Cd(II), Solvent Extr. Ion Exch., 12 (1994) 371–391. 
-  V.S. Kislik, Liquid Membranes. Principles and Application in
    Chemical Separation and Wastewater Treatment. Amsterdam,
    The Netherlands 2010. 
-  J. de Gyves, E.R. de S. Miguel, Metal ion separations by supported
    liquid membranes, Ind. Eng. Chem. Res., 38 (1999)
    2182–2202. 
-  P.K. Mohapatra, P.N. Pathak, A. Kelkar, V.K. Manchanda,
    Novel polymer inclusion membrane containing amacrocyclicionophore
    for selective removal of strontium from nuclear
    waste solution, New. J. Chem., 28 (2004) 1004–1009. 
-  S.D. Kolev, G. Argiropoulos, R.W. Cattrall, I.C. Hamilton, R.
    Paimin, Mathematical modelling of membrane extraction of
    gold(III) from hydrochloric acid solutions, J. Membr. Sci., 137
    (1997) 261–269. 
-  R. Tripathi, A.K. Pandey, S. Sodaye, B.S. Tomar, S.B. Manohar,
    S. Santra, K. Mahata, P. Singh, S. Kailas, Backscattering
    spectrometry studies on metal ion distribution in polymer
    inclusion membranes, Nucl. Instrum. Method. Phy. Res., B 211
    (2003) 138–144. 
-  L. Wang, R. Paimin, R.W. Cattrall, W. Shen, S.D. Kolev, The
    extraction of cadmium(II) and copper(II) from hydrochloric
    acid solutions using an Aliquat 336/PVC membrane, J. Membr.
    Sci., 176 (2000) 105–111. 
-  J. Konczyk, C. Kozlowski, W. Walkowiak, Removal of chromium(III) from acidic aqueous solution by polymer inclusion
    membranes with D2EHPA and Aliquat 336, Desalination, 263
    (2010) 211–216. 
-  A. Tor, G. Arslan, H. Muslu, A. Celiktas, Y. Cengeloglu,
    M. Ersoz, Facilitated transport of Cr(III) through polymer
    inclusion membrane with di(2-ethylhexyl)phosphoric acid
    (DEHPA), J. Membr. Sci., 329 (2009) 169–174. 
-  C.A. Kozlowski, W. Walkowiak, Competitive transport of
    cobalt-60, strontium-90, and cesium-137 across polymer inclusion
    membranes with DNNS, J. Membr. Sci., 297 (2007) 181–189. 
-  S.C. Lee, J.D. Lamb, M. Cai, J.T. Davis, Cs+ and Ba2+ selective
    transport by a novel self-assembled isoguanosine ionophore
    through polymer inclusion and bulk liquid membranes,
    J. Inclusion Phenom. Mac. Chem., 40 (2001) 51–57. 
-  B. Amiri-Eliasi, Metal ion separation by proton-ionizable macrocyclic
    and acyclic polyether ligands. Doctoral dissertation.
    Texas Tech University, Lubbock, USA, 2000. 
-  C. Kozlowski, W. Walkowiak, W. Pellowski, J. Koziol, Competitive
    transport of toxic metal ions by polymer inclusion membranes,
    J. Radioanal. Nucl. Chem., 253 (2002) 389–394. 
-  Y. Yildiz, A. Manzak, B. Aydýn, O. Tutkun, Preparation and
    application of polymer inclusion membranes (PIMs) including
    Alamine 336 for the extraction of metals from an aqueous solution,
    Mater. Technol., 48 (2014) 791–796. 
-  M. Ulewicz, J. Szczygelska-Tao, J.F. Biernat, Transport of Zn(II),
    Cd(II) and Pb(II) across polymer inclusion membrane doped
    with imidazole azocrown ethers, Desalination, 214 (2007) 352–364. 
-  N.S. Murthy, S.T. Correal, H. Minor, Structure of the amorphous
    phase in crystallisable polymers: Poly(ethyleneterephtalate),
    Macromolecules., 24 (1991) 1185–1189. 
-  G. Vancso, D. Snetvy, I. Tomka, Structural changes during
    polystyrene orientation: A study of optical birefringence
    and wide angle X-ray scattering, J. Appl. Polym. Sci., 42 (1991)
    1351–1359. 
-  G. Arslan, A. Tor, Y. Cengeloglu, M. Ersoz, Facilitated transport
    of Cr(III) through activated composite membrane containing
    di(2 ethylhexyl)phosphoric acid (DEHPA) as carrier
    agent, J. Hazard. Mater., 165 (2009) 729–735. 
-  R. Tayeb, C. Fontas, M. Dhahbi, S. Tingry, P. Seta, Cd(II) transport
    across supported liquid membranes (SLM) and polymeric
    plasticized membranes (PPM) mediated by Lasalocid
    A, J. Membr. Sci., 42 (2005) 189–193. 
-  M. Ulewicz, E. Radzymińska-Lenarcik, Transport of
    metal ions across polymer inclusion membrane with
    1-alkylimidazole, Physicochem. Probl. Miner. Process., 46
    (2011) 119–130. 
-  R.L. Ramos, L.F. Rubio, R.M.G. Coronado, J.M. Baron, Adsorption
    of trivalent chromium from aqueous solution onto activated
	  carbon, J. Chem. Technol. Biotechnol., 62 (1995) 64–67.