Rejection and transport of chromate (Cr (VI)), arsenate (As (V)), and perchlorate (CIO4-) by and through reverse osmosis, nanofiltration, and ultrafiltration were found to be affected by solution pH and ionic strength. In this study, the rejection of these target ionic contaminants increased with increasing pH at the same conductivity (ionic strength) for the tested membranes, regardless of salt type. General trends showed that the rejection followed the order: CaCl2 < KCI ≤ K2SO4, at the same pH and conductivity condition. The rejection also significantly decreased as conductivity was increased regardless of the salt type. These results support the notion that a more negative surface charge due to increasing pH and/or decreasing ionic strength (conductivity) enhances rejection; lower molecular weight cut-off also enhances rejection. Measured values of hindered diffusion coefficient through the membrane pores (Dp) for the target contaminants significantly decreased as pH was increased, regardless of salt type at the same conductivity. At the same pH and conductivity, the Dp of the target contaminants follows the order: CaCl2 > KCI > K2SO4, in the presence of different types of salts. The Dp of the target contaminants significantly increased as conductivity was increased regardless of the salt type. The rejection data were modeled by application of a non-equilibrium thermodynamic model. The rejection model has five transport parameters: the molecular transport coefficient (ω), osmotic pressure gradient (Δ∏), molecular reflection coefficient (σ), the average bulk fluid interfacial concentration between the feed and permeate side (Cavg), and the solvent flux (Jv). The transport parameters were determined by independent measurements (and calculation with minimum assumptions.) Measurements and predictions of the target contaminant transport were in good agreement.
- Hindered diffusion
- Negatively charged membrane