Organic solvent-free polyelectrolyte complex membrane preparation: Effect of monomer mixing ratio and casting solution temperature

Nanofiltration Membranes from Poly(sodium-p-styrenesulfonate)/Polyethylenimine Polyelectrolyte Complex Modified with Carbon Nanoparticles for Enhanced Water Treatment

Industrial wastewater poses a significant environmental challenge due to its harmful effects. The development of sustainable membrane processes for water treatment and the environmentally friendly production of polymer membranes is one of the major challenges of our time. An alternative approach is to prepare polyelectrolyte complex (PEC) membranes using the aqueous phase separation (APS) method without the use of toxic solvents. In this work, PEC nanofiltration membranes of poly(sodium-p-styrenesulfonate) (PSS)/polyethylenimine (PEI) modified with carbon nanoparticles (graphene oxide, polyhydroxylated fullerene (HF), multi-walled carbon nanotubes) were developed for enhanced water treatment from anionic food dyes and heavy metal ions. The effect of varying the PSS/PEI monomer ratio, carbon nanoparticles, the content of the optimal HF modifier, and the cross-linking agent on the membrane properties was studied in detail. The changes in the structure and physicochemical properties of the PEC-based membranes were investigated using spectroscopic, microscopic, thermogravimetric analysis methods, and contact angle measurements. The PSS and PEI interactions during PEC formation and the effect of PEI protonation on membrane properties were investigated using computational methods. The optimal cross-linked PEC/HF(1%) (1:1.75 PSS/PEI) membrane had more than 2 times higher permeability compared to the pristine PEC membrane, with dye and heavy metal ion rejection of 99.99 and >97%, respectively.

Polyelectrolyte complex-based materials for separations: progress, challenges and opportunities

Polyelectrolyte complex (PEC) based materials could provide a sustainable alternative to conventional materials, especially for separation applications. However, reproducible production remains a challenge due to the many parameters influencing the polyelectrolyte complexation process, eventually affecting the properties and performance of the final material. Here, we provide an overview of how different parameters affect polyelectrolyte complexation and discuss promising PEC-based materials for separation applications, i.e., porous membranes, functional and barrier coatings, adhesives, saloplastics, and extraction media. Additionally, we highlight the challenges and opportunities and discuss what is needed to get to the next level. We envision that collaboration between experimentalists and theoreticians can leverage experimental datasets with accurate descriptions of all the parameters for multiscale modelling, machine learning and artificial intelligence approaches that can be used to design PEC materials and predict their properties.

Analysis of Metal-Organic Framework and Polyamide Interfaces in Membranes for Water Treatment and Antibacterial Applications

Integrating biocidal nanoparticles (NPs) into polyamide (PA) membranes shows promise for enhancing resistance to biofouling. Incorporating techniques can tailor thin-film nanocomposite (TFN) membranes for specific water purification applications. In this study, silver-based metal-organic framework Ag-MOFs (using silver nitrate and 1,3,5-benzentricarboxylic acid as precursors) are incorporated into PA membranes via three different methods: i) incorporation, ii) dip-coating, and iii) in situ ultrasonic techniques. The characterizations, such as top-surface and cross-section scanning and transmission microscopy, reveal that the incorporation methods for the modified TFN membranes substantially control morphology and surface characteristics. For example, the in situ ultrasonically interlayered Ag-MOFs showed the largest pores (average pore diameter of 14 Å ± 0.1), resulting in the highest water permeance (water flux of 10.9 LMH/bar for Na2SO4). It also show superior antifouling and anti-biofouling performance, with a flux recovery ratio (FRR) of 94.1% in both fouling tests due to its improved surface hydrophilicity and the antibacterial properties of incorporated Ag-MOFs. Conversely, the surface-grafted dip-coated Ag-MOFs offered the highest salt rejection, attributed to its highly negatively charged surface and a dense PA network with narrow pores (average pore diameter of 10 Å ± 0.06).

Optimization of Polyelectrolyte Coacervate Membranes via Aqueous Phase Separation

Polymeric membranes fabricated via the nonsolvent-induced phase separation process rely heavily on toxic aprotic organic solvents, like N-methyl-pyrrolidine (NMP) and dimethylformamide. We suggest that the "saloplastic" nature of polyelectrolyte complexes (PECs) makes them an excellent candidate for fabricating next-generation water purification membranes that use a more sustainable aqueous phase separation process. In this study, we investigate how the properties of PECs and their interactions with salt can form pore-containing membranes from the strong polyelectrolytes poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC) in the presence of potassium bromide (KBr). How the single-phase polymer-rich (coacervate) dope solution and coagulation bath impacted the formation, morphology, and pure water permeance (PWP) of the membranes was systematically evaluated by using scanning electron microscopy and dead-end filtration tests. The impact of a salt annealing post-treatment process was also tested; these treated PEC membranes exhibited a PWP of 6.2 L m-2 h-1 bar-1 and a dye removal of 91.7% and 80.5% for methyl orange and methylene blue, respectively, which are on par with commercial poly(ether sulfone) nanofiltration membranes. For the first time, we have demonstrated the ability of the PEC membranes to repel Escherichia coli bacteria under static conditions. Our fundamental study of polyelectrolyte membrane pore-forming mechanisms and separation performance could help drive the future development of sustainable materials for membrane-based separations.

Biocatalytic PEI-PSS membranes through aqueous phase separation: influence of casting solution pH and operational temperature

Biocatalytic membranes combine the separation properties of membranes and the catalytic abilities of enzymes, holding great promise for industries where both purification and conversion are required. In this work, polyelectrolyte complex membranes incorporated with lysozyme were prepared using polyethyleneimine (PEI) and poly(sodium 4-styrenesulfonate) (PSS) through a one-step and mild pH shift aqueous phase separation (APS) approach. The effects of lysozyme addition and casting solution pH on the membrane properties were studied. All the membranes, both with and without added lysozyme, exhibited asymmetric structures with relatively dense top surfaces and porous cross-sections with finger-like macrovoids. The incorporation of lysozyme did not significantly influence the structure and permeability of the formed membranes. The PEI-PSS biocatalytic membranes exhibited temperature dependent enzymatic activity. The activity strongly increased with increased operational temperature, with the highest activity of 4.30 ± 0.15 U cm-2 at 45 °C. This indicates a responsive effect, where a higher temperature leads to some swelling of the polyelectrolyte complex membrane, making the enzyme more accessible to the used substrate. Moreover, the biocatalytic membranes demonstrate desirable enzymatic stability, maintaining 60% activity even after 60 days of storage. This study validates the potential of the water-based APS process as a straightforward approach for integrating enzymes into responsive biocatalytic membranes.

β-cyclodextrin modified GO ultrafiltration membranes with enhanced antifouling property for water purification

The study investigated the influence of additives on the fabrication of mixed matrix membranes comprising polyethersulfone (PES), with a specific focus on hydrophilicity, flux, morphology, and antifouling properties. Carboxymethyl modified β-cyclodextrin (CMβ-CD) was used to enhance the dispersion and hydrophilicity of graphene oxide (GO), leading to the formation of a hydrophilic and stable composite nanoparticle (CMCD@GO). The hydrophilicity (WCA <51.5°) and water flux (32.6 L.m-2.h-1) of the modified PES membranes (MCDGO-x) were improved by the incorporation of CMCD@GO nanoparticles, while that of PES membrane was 79.7° and 10.6 L.m-2.h-1. The rate of backscattered light intensity (ΔBS) of MCDGO-x suspensions remains stable, suggesting stable dispersion of CMCD@GO in organic solvents. Compared to the bare PES membrane, the MCDGO-x membrane exhibits a thinner active layer and a finger-like structure. The MCDGO-x membrane exhibited excellent naphthenic acids (NAs) rejection (> 93.2%) due to reduced roughness and higher hydrophilicity, while the GO-modified PES membrane (MGO-5) exhibited lower NAs rejection (87.2%). Furthermore, the MCDGO-5 membrane showed higher flux recovery ratio (FRR) of 79.3% compared to MGO-5 membrane (68.5%) after three cycles, indicating the antifouling performance of MCDGO-x for NAs was significantly improved. The combination of CMβ-CD and GO enhance the flux and antifouling properties of PES ultrafiltration membranes, suggesting significant potential for applications in the purification of oil sands process water and the treatment of oily wastewater.

Evaporation-Induced Polyelectrolyte Complexation: The Role of Base Volatility and Cosolvents

Film formation is a vital step for coating applications where a homogeneous, defect-free solid phase should be obtained, starting from a liquid casting formulation. Recently, an alternative waterborne-coating approach was proposed, based on the formation of a polyelectrolyte complex film. In this approach, an evaporating base induces a pH change during drying that initiates the complexation of oppositely charged polyelectrolytes, followed by further densification. In previous studies, ammonia was used as the evaporative base, leading to relatively fast evaporation and resulting in films showing significant brittleness, which tended to crack at low relative humidity or larger thicknesses. We hypothesize that slower complexation and/or evaporation can reduce the problematic stress build-up in the prepared polyelectrolyte complex coatings. For this reason, we studied the changes in the film formation process when there are different bases and cosolvents. We found that reducing the evaporation rate by changing ammonia to the slower evaporating dimethylamine or by adding DMSO as a cosolvent, led to less internal stress build-up during film formation, which could be beneficial for film application. Indeed, films prepared with ammonia showed cracking after 1 h, while films prepared with dimethylamine only showed cracking after one month. The fast evaporation of ammonia was also found to cause a temporary turbid phase, indicating phase separation, while for the slower evaporating bases, this did not occur. All prepared films remained sensitive to humidity, which poses the next challenge for these promising coatings.

"Biomass to Membrane": Sulfonated Kraft Lignin/PCL Superhydrophilic Electrospun Membrane for Gravity-Driven Oil-in-Water Emulsion Separation

Biobased membranes made with green solvents have numerous advantages in the water purification industry; however, their long-term use is impeded by severe membrane fouling and low structural stability. Herein, we proposed a facile and green approach to fabricate an eco-friendly and biodegradable electrospun membrane by simply blending polycaprolactone (PCL) with sulfonated kraft lignin (SKL) in a green solvent (i.e., acetic acid) without needing any additional post-treatment. We investigated the influence of SKL content on the surface morphology, chemical composition, and mechanical properties of the electrospun membrane. The SKL-modified membranes (L-5 and L-10) showed superhydrophilicity and underwater superoleophobicity with a water contact angle (WCA) of 0° (<3 s) and an underwater-oil contact angle (UWOCA) over 150° due to the combined effect of surface roughness and hydrophilic chemical functionality. Furthermore, the as-prepared membranes demonstrated excellent pure water flux of 800-900 LMH and an emulsion flux of 170-480 LMH during the gravity-driven filtration of three surfactant-stabilized oil-in-water emulsions, namely, mineral oil/water, gasoline/water, and n-hexadecane/water emulsions. In addition, these membranes exhibited superior antioil-fouling performance with excellent separation efficiency (97-99%) and a high flux recovery ratio (>98%). The 10 wt % SKL-incorporated membrane (L-10) also showed consistent separation performance after 10 cyclic tests, indicating its excellent reusability and recyclability. Furthermore, the stability of the membrane under harsh pH conditions was also evaluated and proved to be robust enough to maintain its wettability in a wide pH range (pH 1-10).