Performance of nanofiltration membranes on water demineralization : assessment and comparative study
Performance of nanofiltration membranes on water demineralization : assessment and comparative study
College of Engineering / University of Anbar
https://www.ijacskros.com/6%20Volume%204%20Issue/DOI%2010.22607IJACS.2018.604001.pdf
Ions transport in nanofiltration (NF) membranes was investigated by means of extended Nernst–Planck (ENP) equation and Donnan relationship using as main variables, ions strength, membrane charge density, membrane thickness, and with a permeation flux range of 10−6–10−5 m/s. Four different salt solutions were considered: NaCl, Na2SO4, NaF, and MgSO4. Salt rejection coefficient is the main index used for analyzing NF performance for both positive and negative charged membranes. The model calculation showed an excellent agreement with experimental data and high rejection coefficient for Na2SO4 has been obtained. Donnan effect was the major mechanism of rejection for different salt solutions.
The model calculations and predictions for different types of charged solutes. In general, the rejection is observed to decrease with increasing feed concentration. As seen, the negatively charged membrane followed the salt rejection sequence Na2SO4 > NaF > NaCl >MgSO4, which is typical for this type of membrane where Donnan potential plays an important role in its selectivity. Based on ion valence, Donnan exclusion elucidates the salt selectivity sequence observed in Figure 3. For example, R (Na2SO4) > R(NaCl) because the divalent anion, SO 42-, is strongly rejected by the negatively charged membrane compared to the monovalent anion Cl−1 for the same counterion Na+1. Similarly, R(Na2SO4) > R(MgSO4) because Donnan exclusion results in stronger attraction of divalent counterions Mg2+ to the membrane compared to monovalent Na1+ for the same coion SO42-. To help to understand the membrane rejection behavior, the membrane hindrance coefficient and salt diffusivity are presented in Table 1. With Na+ as the common monovalent counterion, the negatively charged membrane showed the rejection sequence R (Na 2SO4) > R(NaF) > R(NaCl), which is inversely related to the diffusivity demonstrating the significance of steric effects on its selectivity toward electrolytes with the same valence.