Cadmium (II) and lead (II) transport in a polymer inclusion membrane using tributyl phosphate as mobile carrier and CuFeO2 as a polarized photo electrode
Introduction
The separation processes [1] based on membrane technologies represent a sophisticated way for that purpose. The conventional routes, concern the synthesis of porous inorganic or hybrid membranes of well-defined pore size [2] or the modification of the surface properties in order to introduce specific interactions or electrostatic repulsive/attractive effects as in the case of membranes for electro dialysis [3].
The membranes modified by adsorption of polyethyleneimine and macrocyclic polyethers under the effect of an electric field showed an enhancement on their transport properties toward monovalent ions compared to divalent ones [4], [5], [6].
A solvent polymeric membrane which consists of a plasticizer polymer film has been used as example of liquid membrane containing a mobile carrier. Facilitated transport of metal ions through polymer inclusion membranes (PIM) has resulted in good selectivity in ion separations with vast improvements in membrane stability compared to over liquid membranes and polymer stabilized liquid membranes [7], [8]. The transport studies through cellulose triacetate (CTA) membranes as polymeric matrix with high flux and good stability have been recently reported [9], [10], [11], [12]. We have developed a new plasticized cellulose triacetate membranes modified by carrier incorporation that are selectively permeable to copper and silver cations.
The separation and removal of toxic metal cations and neutral chemicals from water has frequently been addressed in membrane separation systems. Environmentally damaging and toxic cations have received significantly less attention primarily due to the challenging nature of selectively binding cations [13].
Tributyl phosphate (TBP) is widely used to separate toxic metal ions, particularly iron, zinc, nickel, chromium and copper [14].
Tributyl phosphate has been used to improve the wetting ability of mercerizing liquids and the film strength of lubricating oils [15]. Tributyl phosphate is a plasticizer and/or solvents for cellulose esters, lacquers, plastic and vinyl resins [16]. It is used as a solvent extractant of rare earth metals from ores [17]. This is an organic phosphor compound, which forms stable, hydrophobic complexes with metals such as cadmium and lead [18].
CTA membranes containing tributyl phosphate as carrier and 2- nitrophenyloctyl ether (NPOE) or tris ethylhexyl phosphate (TEHP) as plasticizers were prepared according to the procedure reported by Sugiura et al. [19], [20], [21]. The membranes polymer + plasticizer + carrier were characterized using chemical techniques as well as Fourier transform infrared (FTIR), X-ray diffraction and scanning electron microscopy (SEM). The permeation of lead and cadmium ions through CTA + TBP + NPOE membrane was comparable to that through a membrane in the presence of CuFeO2 as polarized electrode.
Such study has been previously done using commercial cation exchange membrane (noticed CRA) and CdS as photo electrode for the separation and recovery of some metallic ions [22]. The behavior of such membrane has been investigated in different experimental conditions, for example in electrodialysis by using titane/platinium electrode [23], [24].
On the other hand, there is an increasing interest in the photo electrochemically functional oxide materials. Considerable attention has been focused on developing new semiconductors (SC) for the photo electrochemical (PEC) conversion [25], [26], [27], [28]. The delafossite CuFeO2 has a gap Eg of 1.3 eV and absorbs in the whole sun spectrum. Additionally, it is low cost, non-toxic and exhibits a chemical stability over the entire pH range [29], these characteristics make it attractive for PEC applications.
The photo catalytic reactions are kinetically speeded down by a relatively slow charge transfer resulting in low conversion efficiencies. This occurs because the diffusion length, which is an intrinsic property, is small compared to the crystallite size and the lifetime of the minority carriers is not long enough to reach the space charge region by diffusion. Therefore, it is of interest to use CuFeO2, elaborated by chemical route. The electrons must be captured by the metal ions and the distance they have to diffuse before reaching the interface is reduced below the diffusion length.
Section snippets
Chemicals
Pb(NO3)2 (≥99%), Cd(NO3)2 (99.999%), chloroform (GC ≥ 99%), CTA (pure) and 2NPOE (GC ≥ 99.5%) were analytical grade reagents purchased from Fluka, tris ethylhexyl phosphate (GC ≥ 98%) was product of Merck. Whereas tributyl phosphate (GC ≥ 98%) was purchased from Aldrich. All reagents were used as received without further purification. The aqueous solutions were prepared by dissolving the different reagents in distilled water.
Membranes preparation
The CTA membranes were prepared according to the procedure reported by Sugiura
Physical and chemical characteristics of cellulose triacetate membranes
In Table 1, some characteristics of the membrane made up of carriers have been listed in comparison with those of the reference CTA membrane. As the carrier molecules (ionophore and plasticiser) are hydrophobic, the location of carrier molecules at the surface of the CTA modified membranes should modify the contact angle which is an indicative parameter of the wetting character of the material.
As shown in Table 1, the inclusion of carriers into CTA membrane induced an increase of its thickness.
Conclusion
A cellulose triacetate membrane containing a tributyl phosphate as a carrier and 2-nitrophenyloctyl ether or tris ethylhexyl phosphate as plasticizers has been synthesized. These CTA + plasticizer + carrier membranes were characterized using chemical techniques as well as Fourier transform infrared (FTIR), X-ray diffraction and scanning electron microscopy. The systems constituted by the mixture of CTA + NPOE, CTA + NPOE + TBP, CTA + TEHP and CTA + TEHP + TBP do not give any diffraction peak. It can be due to
Acknowledgement
The authors are indebted to Dr. S. Omeiri for preparing CuFeO2 electrode.
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