Coupled-diffusion transport of Cr(VI) across anion-exchange membranes prepared by physical and chemical immobilization methods

Anion-exchange membranes with different configurations were examined for the removal of Cr(VI) from aqueous medium. The polymer inclusion membranes (PIM) were formed by physical immobilization of an organic anion-exchanger Aliquat-336 into the matrix of plasticized cellulose triacetate (CTA). The supported liquid membrane (SLM) was prepared by impregnating microporous poly(propylene) host membrane with an organic phase containing Aliquat-336. The pore-filled membrane (PFM) was prepared by anchoring poly(vinyl benzyl chloride) (PVBCl) within the pores of a poly(propylene) microporous host membrane by in situ crosslinking of PVBCl with a diamine 1,4-diazabicyclo[2.2.2.]octane (DABCO). The resulting PVBCl-filled precursor membrane was converted to anion-exchange membrane by reacting it with excess of DABCO followed by alkylation with α,α′-dibromo-p-xylene. The permeability of Cr(VI) across these membranes from aqueous feed having pH 2 or 8 was studied. The choice of these two pH was based on the fact that Cr(VI) exists in aqueous medium as HCrO4−/Cr2O72− at pH 2 and CrO42− at pH 8. In the concentration range of Cr(VI) used in the permeation experiment (≈10−6 mol/L), HCrO4− is expected to be the predominant Cr(VI) species in the aqueous solution at pH 2. However, the IR spectra of membrane samples loaded with different amounts of Cr(VI) from aqueous solution containing 10−6 mol/L of Cr(VI) at pH 2 indicated that HCrO4− dimerizes to Cr2O72− in the membrane phase. The permeability coefficients (P) of Cr(VI) across PIM, SLM and PFM were measured using 1 mol/L NaNO3 as a receiver phase. The permeability coefficient (P) values of Cr(VI) were found to be P(PFM) ≈ P(SLM) > P(PIM) at pH 2 and, P(PFM) > P(SLM) ≈ P(PIM) at pH 8. It was also observed that the P-values for Cr(VI) across PFM were similar from aqueous feed at pH 2 and 8. However, the P-value of Cr(VI) across PIM and SLM from aqueous feed at pH 2 was found to be significantly higher than that at pH 8. In order to understand the variations in P-values of Cr(VI) in these membranes, the self-diffusion coefficients of Cr(VI) were measured in these membranes by radiotracer isotopic-exchange method. In order to optimize PIM, the permeability of Cr(VI) across PIMs made up of different plasticizers were studied. An attempt was made to explain the permeability of Cr(VI) in different PIMs in terms of dielectric constant of the plasticizer which plays an important role in Cr(VI) transport across the PIM. The PIM and PFM were found to be effective for recovery of Cr(VI) from municipal water. However, only PIM could quantitatively recover Cr(VI) from seawater.