Stability studies of poly(vinyl chloride)-based polymer inclusion membranes containing Aliquat 336 as a carrier

Abstract

The stability of poly(vinyl chloride)-based polymer inclusion membranes (PIMs) containing 40%(w/w) Aliquat 336 (R₃MeN⁺Cl⁻) and 60%(w/w) PVC is studied in detail when these membranes are immersed in deionized water and in aqueous solutions containing various salts or hydrochloric acid. It is shown that the mass loss is suppressed in the salt solutions and that the effectiveness of the suppression follows the order: NaClO₄ > NaNO₃ > NaSCN > NaCl ≈ LiCl ≈ HCl > Na₂SO₄ > CH₃COONa. It is also demonstrated that the distribution ratio of R₃MeN⁺X⁻ (where X⁻ is Cl⁻ or ${\text{NO}}_3^{-}$) between the PIM and the aqueous solution depends on the concentration of the “common anion” X⁻ in the solution. However, in every case, the PIM eventually reaches an equilibrium with the aqueous solution, and after that the PIM composition remains stable. The ion-exchange equilibrium constants between Cl⁻ on one hand and ${\text{NO}}_3^{-}$ or ${\text{SO}}_4^{2-}$ on the other are reported to be K_ie-NO3 = 1.8 × 10¹ and K_ie-SO4 = 1.5 × 10⁻², respectively.

The results reported in this study suggest that although PIMs are capable of losing some membrane liquid phase when exposed to aqueous solutions, this loss can be minimized or even eliminated by increasing the solution concentration of the common anion.

Introduction

Liquid membranes have attracted a great deal of the attention for the separation of various chemical species, such as small organic compounds, metal ions, and inorganic anions, in both industrial processes and in chemical analysis [1]. In separation techniques using liquid membranes, chemical species are extracted on the basis of the mechanisms similar to those in conventional liquid–liquid extraction; however large amounts of organic diluents, which are often volatile, flammable, and harmful, are not required. Among the types of liquid membranes, polymer inclusion membranes (PIMs) are becoming of great interest for the separation of chemical species [1]. PIMs are thin and flexible films that are easily prepared by casting an organic solution containing a base-polymer, such as poly(vinyl chloride) (PVC) or cellulose triacetate (CTA), an extractant as a carrier, and often a plasticizer or modifier. It has been recognized that PIMs have good stability because the rate of loss of the membrane liquid phase to the aqueous phase that the membrane is in contact with is small compared to other types of liquid membranes such as bulk liquid membranes, emulsion liquid membranes, and supported liquid membranes [1].

Aliquat 336, which is a mixture of long alkyl chain quaternary ammonium chlorides, is one of the most useful extractants for anionic species in PIMs. There are many reports using PVC-based or CTA-based PIMs containing Aliquat 336 [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. PIMs have been successfully applied to the extraction and/or transport of numerous chemical species including thiourea [2], some alkylsulfonates [3], Au(III) [4], [5], [6], Pd(II) [7], Cd(II) [8], [9], [10], [11], [12], [13], [14], Cu(II) [8], [12], [13], Zn(II) [14], Pt(IV) [15], [16], [17], Cr(VI) [18], [19], [20], [21], [22], Co(II) [23], [24], Cr(III) [25], U(VI) [26], As(V) [27], inorganic anions [28], [29], [30], [31] such as Cl⁻ [28], [29], [31], ${\text{NO}}_3^{-}$ [29], [31], SCN⁻ [29], [30], ${\text{ClO}}_4^{-}$ [28], [29], and ${\text{SO}}_4^{2-}$ [31] and organic anions such as acetate [29]. However, PIMs containing Aliquat 336 have exhibited some stability problems. In transport experiments for Cr(VI), the initial flux of Cr(VI) through the PIM decreased with increase in the number of transport cycles [19]. The suggestion was made that the decrease was caused by the partitioning of the carrier (Aliquat 336) between the membrane and the aqueous solutions. Kebiche-Senhadji et al. have also reported that a PIM containing Aliquat 336 lost 42% of its efficiency for Cr(VI) transport after 6 days [21]. In addition, Argiropoulos et al. reported that PVC-based PIMs containing 30–50%(w/w) of Aliquat 336 lost approximately a quarter to a third of their Aliquat 336 content in deionized water over 10 days, whereas the loss was only 2–7% in 2.5 mol L⁻¹ HCl [5]. Xu et al. have also confirmed the slight loss of Aliquat 336 into an aqueous phase containing 2 mol L⁻¹ HCl [32]. In these latter two studies [5], [32], the mass changes of the PIMs were only investigated by immersing PIMs in deionized water and HCl solutions, and the reasons for the different mass losses were not studied in detail. To the best of our knowledge, there are no reports of a systematic study of the loss of the liquid phase of PIMs containing Aliquat 336 as the carrier.

This paper describes a detailed investigation of the mass change of PIMs containing 40%(w/w) Aliquat 336 and 60%(w/w) PVC after immersion in deionized water, HCl solutions and solutions containing different Na and Li salts. These experiments have resulted in the replacement of the chloride anion of the Aliquat 336 quaternary ammonium chlorides with other anions, i.e. ${\text{NO}}_3^{-}\text{,}{\text{SO}}_4^{2-}$, CH₃COO⁻, SCN⁻, or ${\text{ClO}}_4^{-}$. The corresponding mixtures of quaternary ammonium salts are referred to in the current paper as Aliquat 336 nitrate, sulfate, acetate, thiocyanate, or perchlorate. The PIMs immersed in the salt solutions have also been observed using infrared (IR) spectrometry and scanning electron microscopy (SEM). The membrane mass change is discussed in terms of the anion exchange equilibrium between the membrane and the salt solution and the leakage of Aliquat 336 from PIMs before and after their contact with the aqueous solutions mentioned above.