Rejection of perchlorate by reverse osmosis (RO), nanofiltration (NF), and ultrafiltration (UF) membranes: Mechanisms and modeling

The mechanisms of perchlorate transport through RO, NF, and UF membranes were investigated and perchlorate transport was predicted by a non-equilibrium thermodynamic model. The objectives of this research were to: (i) determine the removal/rejection of perchlorate ion (ClO₄−) by pressure-driven (ΔP) membranes; (ii) evaluate the effects of water quality parameters, pH, ionic strength (conductivity), and co- and counter ions, on process performance; (iii) study membrane operational conditions such as the J_o/ k ratio (ratio of initial flux to back-diffusional transport), associated with the goal of maximizing system (feed water) recovery and minimizing the volume of retentate; and (iv) provide/develop transport parameters/models.; Measurements of rejection of the perchlorate anion have been performed using two RO (polyamide TFC), three NF (polyamide TFC), and four UF membranes. The UF membranes are all from the same manufacturer and from the same polymer-material family. These were chosen to systematically change the membrane's steric properties while maintaining the material chemistry (i.e., the enthalpic interactions) unchanged. In general, the results obtained from this study indicate that, in a pure component system, target ions (in this case ClO₄ −) can be electrostatically excluded from (negatively) charged membranes that have pores that are large with respect to the size of the ion, but this rejection capability is quickly lost in the presence of a sufficient amount of other ions that can screen the apparent electrostatic force field.; The transport of perchlorate anion through NF and UF membranes was predicted by a non-equilibrium thermodynamic model. The flux of perchlorate was observed in the presence of three electrolytes, KCl, K₂SO₄, and CaCl₂, under varying pH conditions (4, 6, 8, and 10) to determine their effects on perchlorate transport, which is influenced by diffusion and convection forces. The model was characterized by five transport parameters: molecular transport coefficient (ω), osmotic pressure gradient ( ΔΠ), molecular reflection coefficient (σ), average bulk fluid interfacial concentration between feed and permeate side ( C_avg), and solvent flux (J_v). The transport parameters were determined by independent measurements (and calculation with minimum assumptions). For example, the molecular transport coefficient was obtained by diffusion cell measurements under varying pH and conductivity. The results indicate that measured and predicted perchlorate flux results are shown to be in good agreement. The results also show that perchlorate flux by convection is significantly larger than that by diffusion for the relatively large pore membranes.