Theoretical Analysis on Interacting Effect of Steric and Donnan Factors in Nanofiltration Membrane Separation System

Abstract

Prestasi pemisahan sistem membran penuras nano didapati sangat bergantung kepada kesan sterik dan cas. Kajian terdahulu telah menunjukkan bahawa kombinasi yang tepat di antara saiz liang (kesan sterik) dan ketumpatan cas berkesannya (kesan Donnan) akan memberikan prestasi pemisahan yang optimum. Justeru itu, keupayaan menghasilkan membran penuras nano bersifat optimum akan memberikan pembaikan yang signifikan dari aspek prestasi membran dan pengurangan kos pemprosesan. Oleh itu, objektif utama kajian teoritikal ini ialah untuk menyelidik kesan interaktif saiz liang (r_p) dan ketumpatan cas membran, (X_d) terhadap prestasi pemisahan. Dua parameter tanpa dimensi telah digunakan iaitu λ (λ = r_s/r_p) dan ζ(ζ = X_d/C_b). Pengangkutan elektrolit melalui membran penuras nano tersebut ditentukan dengan menggunakan model liang sterik-donnan (DSPM). Pelbagai larutan garam dengan pelbagai saiz liang dan kesan cas diuji dengan menggunakan model tersebut. Kajian menunjukkan bahawa untuk kes membran dengan kapasiti cas berkesan yang tinggi, faktor Donnan didapati lebih penting untuk membran yang longgar strukturnya. Sumbangan faktor Donnan ini berkurangan dengan peningkatan saiz liang membran. Sebaliknya, untuk membran berketumpatan cas yang rendah, kesan faktor Donnan terhadap tolakan ion boleh diabaikan, tanpa bergantung kepada saiz liang. Kata kunci: Penurasan nano, DSPM, ketumpatan caj berkesan, saiz liang, teoritikal The separation performance of NF membrane system is found to be significantly dependent on the steric and charge effects. Previous studies have shown that the right combination of membrane pore size (steric effect) and its effective charge density (donnan effect) will lead to an optimum separation performance. Hence, the ability to produce nanofiltration membranes with optimized properties will certainly provide significant improvement in terms of membrane performance and processing cost reduction. Therefore, the main objective of this theoretical study is to investigate the interactive effect of membrane pore size (r_p) and membrane charge density, (X_d) towards the separation performance. Two general dimensionless parameters were used namely λ (λ = r_s/r_p) and ζ(ζ = X_d/C_b). The electrolyte transport through nanofiltration membranes is determined using a model based on the application of the extended Nernst–Planck equation coupled with Donnan–steric pore model (DSPM). A variety of salt solution with various pore size and charge effect was tested using the above–mentioned model. It was found that for the case of membranes with high effective charge density, the Donnan factor is more prominent for loose membranes. The contribution of the factor decreased with the increase of membrane pore size. Vice versa, for the lower charge density membrane, the effect of Donnan factor towards ion rejection could be neglected, regardless of membrane pore sizes. Key words: Nanofiltration, DSPM, membrane charge density, pore size, theoretical