Nanofiltration of Magnesium Chloride, Sodium Carbonate, and Calcium Sulphate in Salt Solutions

Multi-ionic electrolytes and E.coli removal from wastewater using chitosan-based in-situ mediated thin film composite nanofiltration membrane

This work presents the experimental investigation of flat sheet composite nanofiltration membrane synthesized with chitosan nanoparticles through interfacial polymerization of piperazine with trimesoyl chloride on polyethersulfone/sulfonated polysulfone substrates. The synthesized membrane was tested in wastewater treatment containing inorganic salts and E.Coli. Single binary electrolyte solution of KCl, MgCl₂, MgSO_4, and Na₂SO₄, ternary electrolyte solution, containing a combination of MgCl₂ and MgSO₄, KCl and MgCl₂ and quaternary electrolyte solution of KCl, MgCl_2, and MgSO₄ as feed were treated in crossflow membrane cell for the water flux and species rejection in the permeate under operating pressure up to 0.5 MPa. The rejection of Na¹⁺, K¹⁺, Mg²⁺, Cl^1-, and SO₄^2- was observed to be 81, 28, 87, 96, and 98%, respectively with an average water flux up to 214 ± 10 L m⁻².hr⁻¹ in the permeate for the binary electrolyte solution. Similarly, the rejection for K¹⁺, Mg²⁺, Cl^1- and SO₄^2- was noted to be 33, 94, 97, and 99%, respectively, for ternary electrolyte solution with an average water flux up to 211 ± 10 L m^-2.hr^-1. The quaternary ion system in the feed resulted in an average water flux up to 198 ± 12 L m⁻².hr⁻¹ with the rejection of K⁺, Mg⁺², Cl^- and SO₄^-2 as 35, 87, 96, and 99%, respectively. The model feed solution of E. coli after passing through the membrane achieved an E. coli rejection (99%) with water flux up to 220 L m^-2.hr^-1.

Effect of Chemical Structure on the Performance of Sulfonated Poly(aryl ether sulfone) Composite Nanofiltration Membranes

This paper discusses the effect of the chemical structure of sulfonated poly(aryl ether sulfone) on the performance of composite nanofiltration membranes. The composite nanofiltration membranes were fabricated by coating sulfonated poly(aryl ether sulfone) solution onto the top surface of poly(phthalazinone ether sulfone ketone) support membranes. Three kinds of sulfonated poly(aryl ether sulfone)s with different amounts of phthalazinone moieties, namely, sulfonated poly(phthalazinone ether sulfone) (SPPES), sulfonated poly(phthalazinone biphenyl ether sulfone) (SPPBES), and sulfonated poly(phthalazinone hydroquinone ether sulfone)s (SPPHES), were used as coating materials. The solvents used in preparing the coating solution were investigated and optimized. The separation properties, thermal stability, and chlorine resistance of composite membranes were determined. The structures and morphologies of membranes were characterized with FTIR and SEM, respectively. The membrane prepared from SPPES with more phthalazinone moiety groups showed high water flux and salt rejection. The salt rejection of composite membranes followed the order SPPES > SPPHES > SPPBES. The rejection of the three composite membranes decreased slightly with the solution temperature rising from 20 to 90 °C, while the composite membrane with SPPES as the active layer showed a higher increase in flux than others. The results indicate that SPPES composite membranes show better thermal stability than others.

Influence of Active Layer on Separation Potentials of Nanofiltration Membranes for Inorganic Ions

Active layers of two fully aromatic and two semi-aromatic nanofiltration membranes were studied along with surface charge at different electrolyte composition and effective pore size to elucidate their influence on separation mechanisms for inorganic ions by steric, charge, and dielectric exclusion. The membrane potential method used for pore size measurement is underlined as the most appropriate measurement technique for this application owing to its dependence on the diffusional potentials of inorganic ions. Crossflow rejection experiments with dilute feed composition indicate that both fully aromatic membranes achieved similar rejection despite the differences in surface charge, which suggests that rejection by these membranes is exclusively dependent on size exclusion and the contribution of charge exclusion is weak. Rejection experiments with higher ionic strength and different composition of the feed solution confirmed this hypothesis. On the other hand, increase in the ionic strength of feed solution when the charge exclusion effects are negligible due to charge screening strongly influenced ion rejection by semi-aromatic membranes. The experimental results confirmed that charge exclusion contributes significantly to the performance of semi-aromatic membranes in addition to size exclusion. The contribution of dielectric exclusion to overall ion rejection would be more significant for fully aromatic membranes.

Deviations from Electroneutrality in Membrane Barrier Layers: A Possible Mechanism Underlying High Salt Rejections

Reverse osmosis and nanofiltration (NF) employ composite membranes whose ultrathin barrier layers are significantly more permeable to water than to salts. Although solution-diffusion models of salt transport through barrier layers typically assume ubiquitous electroneutrality, in the case of ultrathin selective skins and low ion partition coefficients, space-charge regions may occupy a significant fraction of the membrane barrier layer. This work investigates the implications of these deviations from electroneutrality on salt transport. Both immobile external surface charge and unequal cation and anion solvation energies in the barrier layer lead to regions with excess mobile charge, and the size of these regions increases with decreasing values of either feed concentrations or ion partition coefficients. Moreover, the low concentration of the more excluded ion in the space-charge region can greatly increase resistance to salt transport to enhance salt rejection during NF. These effects are especially pronounced for membranes with a fixed external surface charge density whose sign is the same as that of the more excluded ion in a salt. Because of the space-charge regions, the barrier-layer resistance to salt transport initially rises rapidly with increasing barrier thickness and then plateaus or even declines within a certain thickness range. This trend in resistance implies that thin, defect-free barrier layers will exhibit higher salt rejections than thicker layers during NF at a fixed transmembrane pressure. Deviations from electroneutrality are consistent with both changes in NF salt rejections that occur upon changing the sign of the membrane fixed external surface charge, and CaCl2 rejections that in some cases may first decrease, then increase and then decrease again with increasing CaCl2 concentrations in NF feed solutions.