Kinetics of Chromium(VI) transport from mineral acids across cellulose triacetate (CTA) plasticized membranes immobilized by tri-n-octylamine

The extraction of chromium(VI) from an aqueous acidic solution into a coexisting organic phase that contained tri-n-octylamine (TOA) and solvent (o-nitrophenyl pentyl ether) was studied. The calculated logarithm of the extraction constant (log K-ex) value for this extraction system was 6.03 +/- 0.03. For transport experiments, the polymer inclusion membranes (PIMs) were prepared by physical immobilization of TOA as the carrier and o-nitrophenyl pentyl ether in cellulose triacetate as the polymer matrix. The competitive transport of Cr(VI) ions from mineral acid solutions (chloride, sulfate, nitrate) media through a PIM with TOA was investigated. The selectivity of the process is described by the sequence of ions removed from the source phase: HCrO4- > NO3- > Cl- > SO42-. This order of permeability limits seems to correlate with the molar Gibbs energy of hydration of transported anions. An activation energy of 30.5 kJ/mol was also determined for Cr(VI) permeability across a PIM with TOA, which suggests that the transport of Cr(VI) ions is controlled by a membrane diffusion mechanism. A presented model describes the transport mechanism, which involves diffusion via a fast interfacial chemical reaction, as well as diffusion of a carrier and its metal complex through the plasticized membrane. The diffusion coefficients of the Cr(VI) complex were calculated as a function of the carrier concentration and membrane thickness; their values were 2.34 x 10(-9) and 1.22 x 10(-10) cm(2)/s, respectively.