A loose nano-filtration membrane prepared by coating HPAN UF membrane with modified PEI for dye reuse and desalination

Concentration and Recovery of Dyes from Textile Wastewater Using a Self-Standing, Support-Free Forward Osmosis Membrane

Forward osmosis (FO) can potentially treat textile wastewaters with less fouling than pressure-driven membrane processes such as reverse osmosis and nanofiltration. However, conventional FO membranes with asymmetric architecture experience severe flux decline caused by internal concentration polarization and fouling as dye molecules accumulate on the membrane surface. In this study, we present a new strategy for concentrating dye by using a self-standing, support-free FO membrane with a symmetric structure. The membrane was fabricated by a facile solution-casting approach based on a poly(triazole- co-oxadiazole- co-hydrazine) (PTAODH) skeleton. Due to its dense architecture, ultrasmooth surface, and high negative surface charge, the PTAODH membrane exhibits excellent FO performance with minimal fouling, low reverse salt flux, and negligible dye passage to the draw solution side. Cleaning with a 40% alcohol solution, after achieving a concentration factor of ∼10, resulted in high flux recovery ratio (98.7%) for the PTAODH membrane, whereas significant damage to the active layers of two commercial FO membranes was observed. Moreover, due to the existence of cytotoxic oxadiazole and triazole moieties in the polymer structure, our PTAODH membrane exhibited an outstanding antibacterial property with two model bacteria. Our results demonstrate the promising application of the symmetric PTAODH membrane for the concentration of textile wastewaters and its superior antifouling performance compared to state-of-the-art commercial FO membranes.

Free-standing graphene oxide membrane with tunable channels for efficient water pollution control

In this study, a graphene oxide (GO) membrane with tunable interlayer spacing was fabricated by a facile method combining the inter-layer modification with external treatment. Congo red (CR), a negatively charged dye with π-orbital-rich groups, was adsorbed on nonoxide regions (G regions) of GO nano-sheets; thus, the interlayers were cross-linked by Ca²⁺ ions through chelating reaction. GO@CR nano-sheets π-π stacking interactions were changed by thermal reduction of the GO/Ca/CR membrane using a hot-pressing method. A broader effective inter-layer spacing control of the GO membrane in wet condition was achieved (from 7.7 ± 0.2 Å to 11.7 ± 0.25 Å). With the decrease of effective inter-layer spacing, the rejection of dyes and heavy metal ions gradually increased (i.e., methylene blue (99.5%), Cu²⁺ (98.6%), Ni²⁺ (97.2%), Pb²⁺ (97.2%) and Cd²⁺ (99.1%) at 7.7 Å) and a sufficient permeation flux was also achieved (17.1 L/m²·h·bar). Meanwhile, the diffusion mechanism of water molecules inside the interlayer gallery of GO laminates was explored by climbing image nudged elastic band (cNEB) method. The hydrogen bonding between water molecules and hydroxyl groups constrained the diffusion of water molecules; consequently, partially reduced hybrid GO membrane can show a better permeability for water and superior rejection for heavy metal ions.

Preparation of the Sodium Alginate-g-(Polyacrylic Acid-co-Allyltrimethylammonium Chloride) Polyampholytic Superabsorbent Polymer and Its Dye Adsorption Property

A polyampholytic superabsorbent polymer (PASAP), sodium alginate-g-(polyacrylic acid-co-allyltrimethylammonium chloride) (SA-g-(PAA-co-PTM)), was prepared by free-radical graft copolymerization and characterized. The polymer exhibited pH-dependent swelling behaviors with extremely high swelling ratios, and was saline tolerant. The dye adsorption properties of SA-g-(PAA-co-PTM) were investigated using methylene blue (MB) as a cationic dye model. It was found that its dye adsorption capacity was significantly affected by the TM content in PASAP and pH of dye solution. The dye adsorption kinetics and isotherm obey the pseudo-second-order kinetic model and the Langmuir isotherm model, respectively, and the adsorption process is chemisorption in nature. In addition, SA-g-(PAA-co-PTM) exhibited high MB adsorption capacities in a wide pH range and reusability in at least five adsorption-desorption cycles, indicating its great application potentials as the adsorbent for dye removals from effluents.

Development of high flux nanofiltration membranes through single bilayer polyethyleneimine/alginate deposition

The aim of this study is to prepare high flux, stable, antifouling nanofiltration membranes through single bilayer polyelectrolyte deposition. To this end, a tight ultrafiltration support membrane was prepared from a polysulfone/sulfonated polyethersulfone blend. Deposition of a polyethyleneimine and alginate pair on this support has reduced the molecular weight cut off from 6 kDa to below 1 kDa. The pure water permeability and polyethylene glycol 1000 rejection of the coated membrane were found to be 15.5 ± 0.3 L/m²·h·bar and 90 ± 0.6%, respectively, by setting the deposition pH for each layer to 8 and the ionic strengths to 0.5 M and 0 M. This membrane has exhibited significantly higher permeability than commercial membranes with the same molecular weight cut off, retaining 98% of the initial flux during 15 h filtration of bovine serum albumine. In addition, the membrane has been able to completely remove anionic dyes from aqueous solution by showing 99.9% retentions to Reactive red 141, Brilliant blue G and Congo red with a 2 bar transmembrane pressure. High flux and membrane stability in acidic and salty environments have been achieved when deposition conditions favor high adsorption levels for the first layer and strong ionic cross-linking between the carboxyl group on the alginate and the amine groups on the polyethyleneimine.

Improving Water-Treatment Performance of Zirconium Metal-Organic Framework Membranes by Postsynthetic Defect Healing

Microporous metal-organic frameworks (MOFs) as building materials for molecular sieving membranes offer unique opportunities to tuning the pore size and chemical property. The recently reported polycrystalline Zr-MOF membranes have greatly expanded their applications from gas separation to water treatment. However, Zr-MOFs are notoriously known for their intrinsic defects caused by ligand/cluster missing, which may greatly affect the molecular sieving property of Zr-MOF membranes. Herein, we present the mitigation of ligand-missing defects in polycrystalline UiO-66(Zr)-(OH)₂ membranes by postsynthetic defect healing (PSDH), which can help in increasing the membranes' Na⁺ rejection rate by 74.9%. Intriguingly, the membranes also exhibit excellent hydrothermal stability in aqueous solutions (>600 h). Our study proves the feasibility of PSDH in improving the performance of polycrystalline Zr-MOF membranes for water-treatment applications.

Mussel-Inspired Architecture of High-Flux Loose Nanofiltration Membrane Functionalized with Antibacterial Reduced Graphene Oxide-Copper Nanocomposites

Graphene-based nanocomposites have a vast potential for wide-ranging antibacterial applications due to the inherently strong biocidal activity and versatile compatibility of such nanocomposites. Therefore, graphene-based functional nanomaterials can introduce enhanced antibiofouling and antimicrobial properties to polymeric membrane surfaces. In this study, reduced graphene oxide-copper (rGOC) nanocomposites were synthesized as newly robust biocides via in situ reduction. Inspired by the emerging method of bridging ultrafiltration membrane surface cavities, loose nanofiltration (NF) membranes were designed using a rapid (2 h) bioinspired strategy in which rGOC nanocomposites were firmly codeposited with polydopamine (PDA) onto an ultrafiltration support. A series of analyses (SEM, EDS, XRD, XPS, TEM, and AFM) confirmed the successful synthesis of the rGO-Cu nanocomposites. The secure loading of rGOC composites onto the membrane surfaces was also confirmed by SEM and AFM images. Water contact angle results display a high surface hydrophilicity of the modified membranes. The PDA-rGOC functionalization layer facilitated a high water permeability (22.8 L m^-2 h^-1 bar^-1). The PDA-rGOC modification additionally furnished the membrane with superior separation properties advantageous for various NF applications such as dye purification or desalination, as ultrahigh (99.4% for 0.5 g L^-1 reactive blue 2) dye retention and high salt permeation (7.4% for 1.0 g L^-1 Na₂SO₄, 2.5% for 1.0 g L^-1 NaCl) was achieved by the PDA-rGOC-modified membranes. Furthermore, after 3 h of contact with Escherichia coli (E. coli) bacteria, the rGOC-functionalized membranes exhibited a strong antibacterial performance with a 97.9% reduction in the number of live E. coli. This study highlights the use of rGOC composites for devising loose NF membranes with strong antibacterial and separation performance.

Fractionation and Concentration of High-Salinity Textile Wastewater using an Ultra-Permeable Sulfonated Thin-film Composite

A sulfonated thin-film composite (TFC) nanofiltration membrane was fabricated using 2,2'-benzidinedisulfonic acid (BDSA) and trimesoyl chloride (TMC) on a polyether sulfone substrate by conventional interfacial polymerization. Due to a nascent barrier layer with a loose architecture, the obtained TFC-BDSA-0.2 membrane showed an ultrahigh pure water permeability of 48.1 ± 2.1 L^-1 m^-2 h^-1 bar^-1, and a considerably low NaCl retention ability of <1.8% over a concentration range of 10-100 g L^-1. The membrane, which possesses a negatively charged surface, displayed an excellent rejection of over 99% toward Congo red (CR) and allowed the fast fractionation of high-salinity textile wastewater. The prepared membrane required only 3-fold water addition to accomplish the separation of multiple components, whereas the commercial NF270 (Dow) membrane required 4-fold water addition and almost double the length of time. Furthermore, the TFC-BDSA-0.2 membrane was subsequently tested for the dye concentration process. It maintained a high flux of 8.2 L^-1 m^-2 h^-1 bar^-1 and a negligible dye loss, even when the concentration factor reached ∼10. Finally, by using a 20% alcohol solution as a back-washing medium, a flux recovery ratio (FRR) of 95.6% was achieved with TFC-BDSA-0.2, and the CR rejection ability remained the same. These results prove the outstanding antifouling and solvent-resistant properties of the membrane.