Solvent activation before heat-treatment for improving reverse osmosis membrane performance

Impact of Polymer Chain Rearrangements in the PA Structure of RO Membranes on Water Permeability and N-Nitrosamine Rejection

The use of solvents is overall recognized as an efficient method to improve the water permeability of polyamide thin film composite membranes (PA-TFC). The objective of this work was to test the performance of the membranes after exposing them to n-propanol (n-PrOH) to improve the permeability of the membranes while maintaining the rejection factor for small uncharged organic molecules, namely N-nitrosamines (NTRs). After the membranes were exposed to n-PrOH, the water permeability of the UTC73AC membrane increased by 98%, with minimal change in rejection. N-nitrosodiethylamine (NDEA) rejection decreased (3.4%), while N-nitrosodi-n-propylamine (NDPA) and N-nitrosodi-n-butylamine (NDBA) rejection increased by 0.9% and 2.8%, respectively. In contrast, for the BW30LE membrane, water permeability decreased (by 38.7%), while rejection factors increased by 14.5% for NDEA, 6.2% for NDPA, and 15.0% for NDBA. In addition, the morphology of the membrane surface before and after exposure to n-PrOH was analyzed. This result and the pore size distribution (PSD) curves obtained indicate that the rearrangement of polymer chains affects the network or aggregate pores in the PA layer, implying that a change in pore size or a change in pore size distribution could improve the permeability of water molecules, while the rejection factor for NTRs is not significantly affected.

Modulating the Asymmetry of the Active Layer in Pursuit of Nanofiltration Selectivity via Differentiating Interfacial Reactions of Piperazine

Nanofiltration (NF), highly prospective for drinking water treatment, faces a challenge in simultaneously removing emerging contaminants while maintaining mineral salts, particularly divalent cations. To overcome this challenge, NF membranes possessing small pores concomitant with highly negatively charged surfaces were synthesized via a two-step fabrication strategy. The key is to generate a polyamide active layer having a loose and carboxyl group-abundant segment on top and a dense barrier segment underneath. This was achieved by restrained interfacial polymerization between trimesoyl chloride and partly protonated piperazine to form a highly depth-heterogeneous polyamide network, followed by second amidation in an organic environment to remove untethered polyamide fragments and associate malonyl chlorides with reserved amine groups to introduce more negative charges. Most importantly, on first-principle engineering the spatial architecture of the polyamide layer, amplifying asymmetric charge distribution was paired with the thinning of the vertical structure. The optimized membrane exhibits high salt/organic rejection selectivity and water permeance superior to most NF membranes reported previously. The rejections of eight emerging contaminants were in the range of 66.0-94.4%, much higher than the MgCl₂ rejection of 41.1%. This new fabrication strategy, suitable for various diacyl chlorides, along with the new membranes so produced, offers a novel option for NF in potable water systems.

Linear versus Nonlinear Aromatic Polyamides: The Role of Backbone Geometry in Thin Film Salt Exclusion Membranes

Two aromatic polyamides─poly(3,3'-dihydroxybenzidine terephthalamide) (DHTA) and poly(3,3'-dihydroxybenzidine isophthalamide) (DHIA)─are compared for their ability to remove salts from water. DHTA is linear and rigid whereas DHIA is nonlinear and semirigid. DHTA and DHIA were selected as they allow us to investigate the effect of polymer backbone geometry on salt exclusion in a non-crosslinked thin film membrane, independently of the backbone chemistry. Because of their differences in solution viscosity, spin coating parameters for DHTA and DHIA solutions were optimized separately to produce thin film composites (TFCs) with reproducible membrane properties. The resulting DHTA TFCs displayed salt rejections of 87.8% (NaCl), 97.0% (MgSO4), and 80.3% (CaCl2). In comparison, DHIA TFCs demonstrated poor salt rejections of 21.0% (NaCl), 29.3% (MgSO4), and 15.4% (CaCl2). Cross-sectional SEM images of DHTA and DHIA films reveal that DHTA has a stratified (layered) morphology whereas DHIA exhibits a dense, featureless morphology. Both DHTA and DHIA TFCs exhibit similar surface morphology, contact angle, surface charge, and water uptake. PEG rejection experiments indicate that the average pore size of DHTA TFCs is ∼2 nm while DHIA TFCs have an average pore size of ∼3 nm. Our findings illustrate that using a rigid, linear aromatic polyamide gives an active layer with a stratified morphology, uniplanar orientation, smaller pores, and higher salt rejection, whereas the nonlinear aromatic polyamide analogue results in an isotropic active layer with larger pores and lower salt rejection.

Exploitation of Amine Groups Cooped up in Polyamide Nanofiltration Membranes to Achieve High Rejection of Micropollutants and High Permeance of Divalent Cations

To enhance the use of nanofiltration in the production of quality drinking water, particularly through the efficient removal of micropollutants yet still preserving essential minerals, the targeted nanofiltration membranes (NFMs) are required to have small pore dimensions coupled with a high, net-negative charge density. Herein, after the formation of a separation layer using piperazine interfacially polymerized with trimesoyl chloride, the exploitation of residual amine groups was systematically investigated by different diacyl chlorides in an organic milieu, which caused the upper part of the final separation layer to be denser and highly negatively charged. Hence, this protocol offers a novel means to fabricate NFMs simultaneously endowed with a low molecular cutoff (MWCO) of 145-238 Da and a reduced rejection of MgCl₂ (48%-80%) as well as a competitive water permeance. Those features are ideally applicable to the goal of removing small micropollutants while preserving mineral ions, as needed for the energy-efficient production of safe, quality drinking water. Furthermore, an attempt was made to correlate MWCO with MgCl₂ rejection, which provides some insights on the nexus of the electrostatic effects constrained by size exclusion. The significance of residual amine groups and the modification environment was unveiled, and this method paves a new avenue for designing functional NFMs.

Tailored design of nanofiltration membranes for water treatment based on synthesis-property-performance relationships

Tailored design of high-performance nanofiltration (NF) membranes is desirable because the requirements for membrane performance, particularly ion/salt rejection and selectivity, differ among the various applications of NF technology ranging from drinking water production to resource mining. However, this customization greatly relies on a comprehensive understanding of the influence of membrane fabrication methods and conditions on membrane properties and the relationships between the membrane structural and physicochemical properties and membrane performance. Since the inception of NF, much progress has been made in forming the foundation of tailored design of NF membranes and the underlying governing principles. This progress includes theories regarding NF mass transfer and solute rejection, further exploitation of the classical interfacial polymerization technique, and development of novel materials and membrane fabrication methods. In this critical review, we first summarize the progress made in controllable design of NF membrane properties in recent years from the perspective of optimizing interfacial polymerization techniques and adopting new manufacturing processes and materials. We then discuss the property-performance relationships based on solvent/solute mass transfer theories and mathematical models, and draw conclusions on membrane structural and physicochemical parameter regulation by modifying the fabrication process to improve membrane separation performance. Next, existing and potential applications of these NF membranes in water treatment processes are systematically discussed according to the different separation requirements. Finally, we point out the prospects and challenges of tailored design of NF membranes for water treatment applications. This review bridges the long-existing gaps between the pressing demand for suitable NF membranes from the industrial community and the surge of publications by the scientific community in recent years.

Application of Desalination Membranes to Nuclide (Cs, Sr, and Co) Separation

Desalination and nuclide separation, with cesium (Cs), strontium (Sr), and cobalt (Co), using commercial polymeric membranes are investigated under room temperature (298 K) to elucidate the permeation mechanism and possibility of applying commercial membranes to the separation of radioactive nuclides. The physicochemical properties of membranes are characterized by multiple techniques. The thickness of the selective layer and the boundary between the layers of membranes are observed by scanning electron microscopy. The chemical structure of selective and support layers is assessed by direct Fourier transform infrared/attenuated total reflection measurements on membrane samples. Thermogravimetric analysis demonstrates the composition comparison between membranes, which describes the relative amount of selective layers consisting of polyamide. The separation performance of polyamide-based commercial membranes is tested on simulated seawater (35,000 ppm of NaCl) and single- and multi-component aqueous nuclide solutions (10 ppm). Nanofiltration (NF) membranes exhibit a high flux of 160-210 L m^-2 h^-1 with low 31-64% rejection on the permeation of simulated seawater, while reverse osmosis (RO) membranes display a low flux of 13-22 L m^-2 h^-1 with nearly 80% rejection. This reveals RO membranes to be more effective for the rejecting nuclides (Cs, Sr, and Co) in dilute aqueous solutions, and NF membranes have advantage on high throughput. RO membranes reject above 93% for single components and even higher for mixed nuclide separation (>98%), and NF membranes permeate high flux above 230 L m^-2 h^-1. This study indicates that the desalination membranes (NF and RO) can be potential candidates for nuclide separation with combination.

Fabrication of High-Performance Thin-Film Composite Nanofiltration Membrane by Dynamic Calcium-Carboxyl Intra-Bridging during Post-Treatment

Widespread applications of nanofiltration (NF) and reverse osmosis (RO)-based processes for water purification and desalination call for high-performance thin-film composite (TFC) membranes. In this work, a novel and facile modification method was proposed to fabricate high-performance thin-film composite nanofiltration membrane by introducing Ca2+ in the heat post-treatment. The introduction of Ca2+ induced in situ Ca2+-carboxyl intra-bridging, leading to the embedment of Ca2+ in the polyamide (PA) layer. This post modification enhanced the hydrophilicity and surface charge of NF membranes compared to the pristine membrane. More interestingly, the modified membrane had more nodules and exhibited rougher morphology. Such changes brought by the addition of Ca2+ enabled the significant increase of water permeability (increasing from 17.9 L·m-2·h-1·bar-1 to 29.8 L·m-2·h-1·bar-1) while maintaining a high selectivity (Na2SO4 rejection rate of 98.0%). Furthermore, the intra-bridging between calcium and carboxyl imparted the NF membranes with evident antifouling properties, exhibiting milder permeability decline of 4.2% (compared to 16.7% of NF-control) during filtration of sodium alginate solution. The results highlight the potential of using Ca2+-carboxyl intra-bridging post-treatment to fabricate high-performance TFC membranes for water purification and desalination.

Custom-Tailoring Loose Nanofiltration Membrane for Precise Biomolecule Fractionation: New Insight into Post-Treatment Mechanisms

Loose nanofiltration (NF) membranes with diverse selectivity can meet the great demands in various bioseparation applications. Thus, a facile strategy to tune the properties such as pore size, surface charge, and hydrophilicity of the NF membrane is required to produce tailor-made loose NF membranes without changing the existing production line. Herein, we systematically investigated the post-treatment of the nascent poly(piperazine amide) NF membranes using different reagents (organic acids, weak bases, organic solvents and ionic liquid (IL)). Various characterizations revealed that the skin/separation layer became looser and permeance was promoted with the decrease of salt rejection in varying degrees. It was found that the O/N ratio did not rigorously represent the cross-linking degree of the skin layer, because besides the hydrolysis of the residual acyl chloride impeding the amido bond formation, the breaking of existing amido bonds and the grafting of free trimesoyl chloride molecules on the nascent membranes could also increase the O/N ratio during post-treatments. Then three mechanisms including hydrolysis, swelling rearrangement and capping reaction effects were proposed to better understand the membrane properties variations. All these effects resulted in larger pore size of the NF membrane, and the hydrolysis/capping effect might increase negative charge and hydrophilicity on the membrane, while the swelling rearrangement could produce less defective skin structure. These three effects might be involved together during a single treatment. Finally, the NF membrane post-treated by N-hexane could efficiently separate antibiotics and NaCl with the highest permeate flux, whereas the one post-treated by ionic liquid outperformed others for the decoloration of cane molasses (much more efficient than NF270, DL, and NTR7450 membranes). The long-term operating stability of the post-treated membranes selected was also confirmed by a continuous crossflow filtration for 15 h with regular alkaline cleaning.