Tight ultrafiltration and loose nanofiltration membranes by concentration polarization-driven fast layer-by-layer self-assembly for fractionation of dye/salt

Low Fouling Molecular Selective Channels through Self-assembly of Cross-linked Block Copolymer Micelles for Selective Separation of Dye and Salt

We report the solvent-evaporation and ionic cross-linking mediated self-assembly of the shell cross-linked micelles of the amphiphilic triblock copolymer containing middle poly(methyl methacrylate) block (hydrophobic) and poly(2-dimethylamino)ethyl methacrylate end blocks (hydrophilic) on the membrane substrate to create molecular selective channels. The formation of selective channels on the substrate is attributed to the local increase of micelle concentration upon solvent evaporation, which leads to the core-core hydrophobic interaction. The post-ionic cross-linking of the shell part further reduces the intermicelle distance, thereby creating interstices for selective separation. The TUF-1:1 membrane prepared by the self-assembly of the cross-linked micelles (triblock copolymer:halide-terminated PEG-based = 1:1 w w-1) and by the post-ionic cross-linking shows molecular weight cutoff of 3000 g mol-1 and pure water permeance of 52 L m-2 h-1 bar-1. The membrane shows 99.5-99.9% rejection of Congo red and Direct red-80 in the presence or absence of salts and Na2SO4 to dye separation factor of about 900. The added functionality (PEG) in the micelle structure provides good fouling-resistant properties toward dye and bovine serum albumin. This work provides the membrane formation mechanism and the advantages of the membrane for fractionation and resource recovery applications.

Low Fouling Polyelectrolyte Layer-by-Layer Self-Assembled Membrane for High Performance Dye/Salt Fractionation: Sequence Effect of Self-Assembly

Layer-by-layer (LbL) self-assembly of oppositely charged polyelectrolytes (PEs) is usually performed on a conventional ultrafiltration base substrate (negative zeta potential) by depositing a cationic PE as a first layer. Herein, we report the facile and fast formation of high performance molecular selective membrane by the nonelectrostatic adsorption of anionic PE on the polyvinylidene fluoride (PVDF, zeta potential -17 mV) substrate followed by the electrostatic LbL assembly. Loose nanofiltration membranes have been prepared via both concentration-polarization (CP-LbL, under applied pressure) driven and conventional (C-LbL, dipping) LbL self-assembly. When the first layer is poly(styrene sodium) sulfonic acid, the LbL assembled membrane contains free -SO3- groups and exhibits higher rejection of Na2SO4 and lower rejection of MgCl2. The reversal of salt rejection occurs when the first layer is quaternized polyvinyl imidazole (PVIm-Me). The membrane (five layers) prepared by first depositing PStSO3Na shows higher rejection of several dyes (97.9 to >99.9%), higher NaCl to dye separation factor (52-1800), and higher dye antifouling performance as compared to the membrane prepared by first depositing PVIm-Me (97.5-99.5% dye rejection, separation factor ∼40-200). However, the C-LbL membrane requires a longer time of self-assembly or higher PE concentration to reach a performance close to the CP-LbL membranes. The membranes exhibit excellent pressure, pH (3-12), and salt (60 g L-1) stability. This work provides an insight for the construction of low fouling and high-performance membranes for the fractionation of dye and salt based on the LbL self-assembly sequence.

Recent advances in cellulose- and alginate-based hydrogels for water and wastewater treatment: A review

From the environmental perspective, it is essential to develop cheap, eco-friendly, and highly efficient materials for water and wastewater treatment. In this regard, hydrogels and hydrogel-based composites have been widely employed to mitigate global water pollution as this methodology is simple and free from harmful by-products. Notably, alginate and cellulose, which are natural carbohydrate polymers, have gained great attention for their availability, price competitiveness, excellent biodegradability, biocompatibility, hydrophilicity, and superior physicochemical performance in water treatment. This review outlined the recent progress in developing and applying alginate- and cellulose-based hydrogels to remove various pollutants such as dyes, heavy metals, oils, pharmaceutical contaminants, and pesticides from wastewater streams. This review also highlighted the effects of various physical or chemical methods, such as crosslinking, grafting, the addition of fillers, nanoparticle incorporation, and polymer blending, on the physiochemical and adsorption properties of hydrogels. In addition, this review covered the alginate- and cellulose-based hydrogels' current limitations such as low mechanical performance and poor stability, while presenting strategies to improve the drawbacks of the hydrogels. Lastly, we discussed the prospects and future directions of alginate- and cellulose-based hydrogels. We hope this review provides valuable insights into the efficient preparations and applications of hydrogels.

Colloidal 2D Covalent Organic Framework-Tailored Nanofiltration Membranes for Precise Molecular Sieving

Covalent organic frameworks (COFs) with tunable pore sizes and ordered structures are ideal materials for engineering nanofiltration (NF) membranes. However, most of the COFs prepared by solvothermal synthesis are unprocessable powders and fail to form well-structured membranes, which seriously hinders the development of COF NF membranes. Herein, colloidal 2D-COFs with processable membrane formation ability were synthesized by oil-in-water emulsion interfacial polymerization technology. COF NF membranes with tailored thickness and surface charge were fabricated via a layer-by-layer (LBL) assembly strategy. The prepared COF NF membrane achieved precise sieving of dye molecules with high permeance (85 L·m-2·h-1·bar-1). In this work, the strategy of prepared COF NF membranes based on colloid 2D-COF LBL assembly is proposed for the first time, which provides a new idea for the on-demand design and preparation of COF membranes for precise molecular sieving.