Estimation of nanofiltration membrane transport parameters for cobalt ions removal from aqueous solutions

In the present work, three types of hollow fiber nanofiltration membranes (NF1, NF2 and NF3) were employed for the rejection and permeation flux of cobalt ions from aqueous wastewater. The operating variables-initial ion concentration (10-250 ppm), feed solution pH (5.5-6.5), pressure applied (1 bar) and feed flow rate (0.6 L/min)-were studied. It is observed that the obtained cobalt ion rejection values increase with the decrease in initial concentration and increase in pH at constant feed flow rate. The maximum observed rejection of the metal is found to be 77%, 50.2% and 46.8% and 38%, 30% and 29% for NF1, NF2 and NF3 for the initial feed concentration in the 10-250 ppm range, respectively. In addition, the flux decreases with the increase in both pH and initial concentration. Combined film theory-solution diffusion (CFSD), combined film theory-Spiegler-Kedem (CFSK) and combined film theory-finely porous (CFFP) membrane transport models were employed to estimate membrane transport parameters and mass transfer coefficient (k). Moreover, enrichment factor, concentration polarization modulus and Peclet number were estimated using various parameters. Analysis of the experimental data using CFSD, CFSK and CFFP models showed good agreement between theoretical and experimental results. Effective membrane thickness and active skin layer thickness were evaluated using CFFP model, indicating that the Peclet number is important for determining the mechanism of separation by diffusion.