A highly permeable loose nanofiltration membrane prepared via layer assembled in-situ mineralization

Construction of PDA-PEI/ZIF-L@PE tight ultra-filtration (TUF) membranes on porous polyethylene (PE) substrates for efficient dye/salt separation

Tight ultra-filtration (TUF) membranes were constructed by in situ growing zinc imidazole frameworks micro-crystalline leaves (ZIF-L) in polyethylene imine (PEI) and polydopamine (PDA) deposit layers on porous polyethylene (PE) substrates. The effects of preparation conditions on the surface physical and chemical structures as well as on the dye/salt separation performance of the formed TUF membranes were systematically investigated. By inserting selective water permeation channels and increasing contacting surface areas, in situ-grown ZIF-L arrays tightly cross-linked in the coating matrix greatly increased water permeation without trading off dye/salt retention selectivity. The morphology of the included ZIF-L particles could be varied by adjusting the ligand/Zn molar ratio (α) in the preparation processes. Optimized PDA-PEI/ZIF-L@PE TUF membranes containing ZIF-L of cross-cross block morphology showed very high pure water permeability of 180 ± 20 L·m-2·h-1·bar-1 (LMHB) and retention selectivity (SCR/Na2SO4 and SMB/Na2SO4) of 267 and 43, respectively, as well as excellent stability and anti-fouling properties.

Mixed scaling patterns and mechanisms of high-pressure nanofiltration in hypersaline wastewater desalination

Nanofiltration (NF) will play a crucial role in salt fractionation and recovery, but the complicated and severe mixed scaling is not yet fully understood. In this work, the mixed scaling patterns and mechanisms of high-pressure NF in zero-liquid discharge (ZLD) scenarios were investigated by disclosing the role of key foulants. The bulk crystallization of CaSO4 and Mg-Si complexes and the resultant pore blocking and cake formation under high pressure were the main scaling mechanisms in hypersaline desalination. The incipient scalants were Mg-Si hydrates, CaF2, CaCO3, and CaMg(CO3)2. Si deposited by adsorption and polymerization prior to and impeded Ca scaling when Mg was not added, thus pore blocking was the main mechanism. The amorphous Mg-Si hydrates contribute to dense cake formation under high hydraulic pressure and permeate drag force, causing rapid flux decline as Mg was added. Humic acid has a high affinity to Ca2+by complexation, which enhances incipient scaling by adsorption or lowers the energy barrier of nucleation but improves the interconnectivity of the foulants layer and inhibits bulk crystallization due to the chelation and directional adsorption. Bovine serum albumin promotes cake formation due to the low electrostatic repulsion and acts as a cement to particles by adsorption and bridging in bulk. This work fills the research gaps in mixed scaling of NF, which is believed to support the application of ZLD and shed light on scaling in hypersaline/ultra-hypersaline wastewater desalination applications.

Material Design Strategies for Recovery of Critical Resources from Water

Population growth, urbanization, and decarbonization efforts are collectively straining the supply of limited resources that are necessary to produce batteries, electronics, chemicals, fertilizers, and other important products. Securing the supply chains of these critical resources via the development of separation technologies for their recovery represents a major global challenge to ensure stability and security. Surface water, groundwater, and wastewater are emerging as potential new sources to bolster these supply chains. Recently, a variety of material-based technologies have been developed and employed for separations and resource recovery in water. Judicious selection and design of these materials to tune their properties for targeting specific solutes is central to realizing the potential of water as a source for critical resources. Here, the materials that are developed for membranes, sorbents, catalysts, electrodes, and interfacial solar steam generators that demonstrate promise for applications in critical resource recovery are reviewed. In addition, a critical perspective is offered on the grand challenges and key research directions that need to be addressed to improve their practical viability.

Removal of Methyl Red from Aqueous Solution Using Polyethyleneimine Crosslinked Alginate Beads with Waste Foundry Dust as a Magnetic Material

In this study, a cost-effective adsorbent based on sodium alginate (SA) with waste foundry dust (WFD) was fabricated for the removal of methyl red (MR) from aqueous media. However, the utilization of WFD/SA beads to remove anionic dyes (such as MR) from effluents has limitations associated with their functional groups. To improve the adsorption performance, WFD/SA-polyethyleneimine (PEI) beads were formed via PEI crosslinking onto WFD/SA beads, which could be attributed to the formation of amide bonds from the carboxyl and amino groups due to the change of N-H bonds in the reaction. The Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) results indicated that PEI was crosslinked on the WFD/SA via a chemical reaction. In the FTIR spectra of WFD/SA-PEI, peaks of the –COO (asymmetric) stretching vibration shifted to 1598 and 1395 cm⁻¹, which could be attributed to the hydrogen-bonding effect of the N–H groups in PEI. In the N1s spectrum, three deconvoluted peaks were assigned to N in –N= (398.2 eV), –NH/–NH₂ (399.6 eV), and NO₂ (405.2 eV). WFD/SA-PEI beads were assessed and optimized for aqueous MR adsorption. The WFD/SA-PEI beads showed a high removal efficiency for MR (89.1%) at an initial concentration of 1000 mg/L, and presented a maximum MR adsorption capacity of 672.7 mg/g MR. The adsorption process showed a good fit with the pseudo-second-order kinetic model and the Langmuir adsorption isotherm model. The amino and hydroxyl groups in the WFD/SA-PEI beads facilitate strong hydrogen bonding and electrostatic interactions. Moreover, these WFD/SA-PEI beads were easily recovered after the adsorption process.

Metal-Coordinated Nanofiltration Membranes Constructed on Metal Ions Blended Support toward Enhanced Dye/Salt Separation and Antifouling Performances

Metal-phenol coordination is a widely used method to prepare nanofiltration membrane. However, the facile, controllable and scaled fabrication remains a great challenge. Herein, a novel strategy was developed to fabricate a loose nanofiltration membrane via integrating blending and interfacial coordination strategy. Specifically, iron acetylacetonate was firstly blended in Polyether sulfone (PES) substrate via non-solvent induced phase separation (NIPS), and then the loose selective layer was formed on the membrane surface with tannic acid (TA) crosslinking reaction with Fe3+. The surface properties, morphologies, permeability and selectivity of the membranes were carefully investigated. The introduction of TA improved the surface hydrophilicity and negative charge. Moreover, the thickness of top layer increased about from ~30 nm to 119 nm with the increase of TA assembly time. Under the optimum preparation condition, the membrane with assembly 3 h (PES/Fe-TA3h) showed pure water flux of 175.8 L·m−2·h−1, dye rejections of 97.7%, 97.1% and 95.0% for Congo red (CR), Methyl blue (MB) and Eriochrome Black T (EBT), along with a salt penetration rate of 93.8%, 95.1%, 97.4% and 98.1% for Na2SO4, MgSO4, NaCl and MgCl2 at 0.2 MPa, respectively. Both static adhesion tests and dynamic fouling experiments implied that the TA modified membranes showed significantly reduced adsorption and high FRR for the dye solutions separation. The PES/Fe-TA3h membrane exhibited high FRR of 90.3%, 87.5% and 81.6% for CR, EBT and MB in the fouling test, stable CR rejection (>97.2%) and NaCl permeation (>94.6%) in 24 h continuous filtration test. The combination of blending and interfacial coordination assembly method could be expected to be a universal way to fabricate the loose nanofiltration membrane for effective fractionation of dyes and salts in the saline textile wastewater.

Current status and future prospects of membrane separation processes for value recovery from wastewater

Resource constraints and deteriorating environment have made it necessary to look for intensification of the industrial processes, to recover value from spent streams for reuse. The development of reverse osmosis has already established that water can be recovered from aqueous streams in a cost-effective and beneficial manner to the industries. With the development of several membrane processes and membrane materials, the possibility of recovering value from the effluents looks like a workable proposition. In this context, the potentialities of the different membrane processes in value recovery are presented. Among the pressure-driven processes, reverse osmosis can be used for the recovery of water as value. Nanofiltration has been used for the recovery of several dyes including crystal violet, congo red, methyl blue, etc., while ultrafiltration has been used in the fractionation of different solute species using membranes of different pore-size characteristics. Diffusion dialysis is found useful in the separation of acids from its salt solutions. Bipolar membrane electrodialysis has the potential to regenerate acid and base from salt solutions. Thermally driven membrane distillation can provide desalinated water, besides reducing the temperature of hot discharge streams. Passive membrane processes such as supported liquid membranes and membrane-assisted solvent extraction have been found useful in separating minor components from the wastewater streams. The details are discussed to drive home that membrane processes can be useful to achieve the objectives of value recovery, in a cost-effective manner through process intensification, as they are more compact and individual streams can be treated and value used seamlessly.

Construction of Loose Positively Charged NF Membrane by Layer-by-Layer Grafting of Polyphenol and Polyethyleneimine on the PES/Fe Substrate for Dye/Salt Separation

The effective separation of dyes and inorganic salts is highly desirable for recycling inorganic salts and water resource in printing and dyeing wastewater treatment. In this work, tannic acid (TA) and polyethyleneimine (PEI) were grafted on the PES/Fe ultrafiltration membrane via the coordination assembly and Michael addition strategy to fabricated a loose nanofiltration membrane (LNM). The effect of PEI concentration on membrane morphologies and properties was systematically investigated. The membrane surface becomes more hydrophilic and transforms into positive charge after the PEI grafting. The optimized PES/Fe-TA-PEI membrane possesses high pure water flux (124.6 L·m⁻²·h⁻¹) and excellent dye rejections (98.5%, 99.8%, 98.4%, and 86.4% for Congo red, Eriochrome black T, Alcian blue 8GX, and Bromophenol blue, respectively) under 2 bar operation pressure. Meanwhile, the LNM showed a high Alcian blue 8GX rejection (>98.4%) and low NaCl rejection (<5.3%) for the dye/salt mixed solutions separation. Moreover, the PES/Fe-TA-PEI LNM exhibited good antifouling performance and long-term performance stability. These results reveal that such LNM shows great potential for effective fractionation of dyes and salts and recycling of textile wastewater.

Self-healing and easy-to-shape mineralized hydrogels for iontronics

Hydrogel-based multifunctional materials have attracted much attention. In this work, novel mineralized hydrogels were fabricated through physically cross-linked polyvinylpyrrolidone (PVP) and CaCO3. The mineralized hydrogels were prepared by simply mixing CaCl2, Na2CO3, and PVP in aqueous solutions. The CO32- induced the aggregation of the PVP chains and the CaCO3 particles in situ generated in the aqueous solution worked as fillers to strengthen the hydrogels. Based on this method, other kinds of mineralized hydrogels were prepared by replacing the Ca2+ with different metal ions. The mineralized hydrogels displayed shapeable, self-healing and thixotropic properties. Moreover, the mineralized hydrogel-based sensor showed good and stable sensitivity to compressive pressure, and could be used to monitor human actions. This work presents a facile method for preparing mineralized hydrogels, which are promising for various applications due to their outstanding properties.

Preparation of a novel zwitterionic striped surface thin-film composite nanofiltration membrane with excellent salt separation performance and antifouling property

Thin-film composite (TFC) nanofiltration (NF) membranes were fabricated via the co-deposition of taurine, tannic acid (TA), and polyethyleneimine (PEI), followed by subsequent interfacial polymerization with trimesoyl chloride (TMC) on the surface of the polysulfone ultrafiltration substrates. The surface properties, including the roughness, hydrophilicity, surface potential, and NF performances were facilely tuned by varying the taurine content for the prepared TFC membranes. In addition, the as-prepared TFC NF membranes had an excellent antifouling property and flux recovery ratio (FRR) in humic acid (HA), bovine serum albumin (BSA) and sodium alginate (SA) filtration tests. These results also revealed that the taurine content controlled the formation of the striped surface. Thus, this work provided a viable strategy for fabricating TFC NF membranes with high selectivity and outstanding antifouling ability.

Thin-film composite (TFC) nanofiltration (NF) membranes with zwitterionic striped surface were fabricated via the co-deposition and interfacial polymerization.