Cadmium (II) and lead (II) transport in a polymer inclusion membrane using tributyl phosphate as mobile carrier and CuFeO₂ as a polarized photo electrode

Abstract

In this work, a development of polymeric inclusion membranes for the cations separation is reported. The membrane was made up of cellulose triacetate (CTA) with a tributyl phosphate (TBP) incorporated into the polymer as metal ions carrier. The transport of lead (II) and cadmium (II) ions in two membrane systems polymer inclusion membrane (PIM), PIM coupled with photo-chemical electrode using TBP as carrier and 2-nitro phenyl octyl ether (NPOE) or tris ethylhexyl phosphate (TEHP) as plasticizer have been investigated. The membranes: polymer + plasticizer + carrier were synthesized and characterized by FTIR, X-ray diffraction and scanning electron microscopy (SEM). Transports of lead and cadmium have been studied using these systems and the results were compared to commercial cation exchange membrane (CRA). The obtained results showed that for Pb²⁺ ion, the concentrations of the strip phase increases using synthesized membranes. The conduction band of the delafossite CuFeO₂ (−1.25 V_SCE) yields a thermodynamically M²⁺ (=Pb²⁺, Cd²⁺) photo electrodeposition and speeds up the diffusion process. In all the cases, the potential of the electrode M/M²⁺ in the feed compartment increases until a maximum value, reached at ∼100 min above which it undergoes a diminution.

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 E_g 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.