Chemistry in a spinneret – Formation of hollow fiber membranes with a cross-linked polyelectrolyte separation layer

Micropollutants removal from aquaculture water using layer-by-layer self-assembled nanofiltration membranes

Micropollutants (MPs) in aquaculture water are directly related to human health, but largely overlooked. The conventional water treatment technologies could not effectively remove MPs, and new technologies have been pursued with high MP removal rate, low cost and small footprint. This paper reported the first endeavor of using layer-by-layer (LBL) hollow fiber (HF) nanofiltration (NF) membranes to treat real aquaculture water. We compared three types of membranes with various surface chemical properties and pore sizes assembled by different polyelectrolytes (PEs). The MPs removal rates and operation stability of three NF membranes were studied in flocculation (Floc.)/ultrafiltration (UF)/NF, Floc./NF and direct NF processes. The best poly(styrene sulfonate)/poly(allylamine hydrochloride) (PSS/PAH) NF membrane showed almost 100 % MPs removal in Floc./UF/NF, and above 95 % in direct NF process. Stable permeance was observed in three 24 h cycles for three processes. The correlation hot spot analysis between the physical properties and removal rates of MPs confirmed importance of size exclusion as the important factor for removal of MPs. The organic humus, rather than inorganic silicon and calcium, was found as the main foulant, which increased slightly the pore size of the LBL membranes with small pores, but blocked pores of membranes with large pores, affected the rejection for ions as well. The cost assessment based on (PSS/PAH)2.5 showed a total cost below 0.75 ¥/ton water, which is feasible to efficiently remove MPs from aquaculture water by NF membrane process.

Incorporation of an Intermediate Polyelectrolyte Layer for Improved Interfacial Polymerization on PAI Hollow Fiber Membranes

In a single-step spinning process, we create a thin-walled, robust hollow fiber support made of Torlon® polyamide-imide featuring an intermediate polyethyleneimine (PEI) lumen layer to facilitate the integration and covalent attachment of a dense selective layer. Subsequently, interfacial polymerization of m-phenylenediamine and trimesoyl chloride forms a dense selective polyamide (PA) layer on the inside of the hollow fiber. The resulting thin-film composite hollow fiber membranes show high NaCl rejections of around 96% with a pure water permeability of 1.2 LMH/bar. The high success rate of fabricating the thin-film composite hollow fiber membrane proves our hypothesis of a supporting effect of the intermediate PEI layer on separation layer formation. This work marks a step towards the development of a robust method for the large-scale manufacturing of thin-film composite hollow fiber membranes for reverse osmosis and nanofiltration.

Effect of the Addition of Polyacrylic Acid of Different Molecular Weights to Coagulation Bath on the Structure and Performance of Polysulfone Ultrafiltration Membranes

Membrane fouling is a serious issue in membrane technology which cannot be completely avoided but can be diminished. The perspective technique of membrane modification is the introduction of hydrophilic polymers or polyelectrolytes into the coagulation bath during membrane preparation via non-solvent-induced phase separation. The influence of polyacrylic acid (PAA) molecular weight (100,000, 250,000 and 450,000 g·mol−1) added to the aqueous coagulation bath (0.4–2.0 wt.%) on the polysulfone membrane structure, surface roughness, water contact angle and zeta potential of the selective layer, as well as the separation and antifouling performance, was systematically studied. It was found that membranes obtained via the addition of PAA with higher molecular weight feature smaller pore size and porosity, extremely high hydrophilicity and higher values of negative charge of membrane surface. It was shown that the increase in PAA concentration from 0.4 wt.% to 2.0 wt.% for all studied PAA molecular weights yielded a substantial decrease in water contact angle compared with the reference membrane (65 ± 2°) (from 27 ± 2° to 17 ± 2° for PAA with Mn = 100,000 g·mol−1; from 25 ± 2° to 16 ± 2° for PAA with Mn = 250,000 g·mol−1; and from 19 ± 2° to 10 ± 2° for PAA with Mn = 450,000 g·mol−1). An increase in PAA molecular weight from 100,000 to 450,000 g·mol−1 led to a decrease in membrane permeability, an increase in rejection and tailoring excellent antifouling performance in the ultrafiltration of humic acid solutions. The fouling recovery ratio increased from 73% for the reference membrane up to 91%, 100% and 136% for membranes modified with the addition to the coagulation bath of 1.5 wt.% of PAA with molecular weights of 100,000 g·mol−1, 250,000 g·mol−1 and 450,000 g·mol−1, respectively. Overall, the addition of PAA of different molecular weights to the coagulation bath is an efficient tool to adjust membrane separation and antifouling properties for different separation tasks.

One-Step Water-Induced Phase Separation Simultaneously Triggering Polymer Solidification and Polyelectrolyte Complexation for Porous Ultrafiltration Membranes

Functional additives have been widely utilized for the membrane structure modulation and performance improvement during the nonsolvent-induced phase separation process, but the resulted membranes easily suffer from additives' inhomogeneous dispersity and compatibility with the polymer matrix. Herein, a facile and robust strategy, i.e., one-step water-induced phase separation, was proposed for the preparation of polyelectrolytes-contained composite membranes. Polyanion (dopamine modified polyacrylic acid) and polycation (quaternized chitosan paired with bis(trifluoromethane-sulfonyl)imide) were first premixed in dimethyl sulfoxide and used as polyelectrolyte additives in a polysulfone (PSF) solution, and then a uniform PSF-based casting solution was readily obtained. During the solvent-water exchange process, polymer solidification and polyelectrolyte complexation were simultaneously triggered, in situ generating a polyelectrolyte complex fixed within the membrane matrix. Ultrafiltration membranes with hierarchical structures were notably tailored through altering the concentration, molecular weight, and type of polyelectrolytes. The obtained membrane exhibited a water flux of 672 L·m^-2·h^-1, three times over the raw PSF membrane, while almost maintaining high bovine serum albumin (BSA) rejection. This work paves a straightforward and convenient path for the preparation of composite membranes with tunable architecture and properties.

Wetting-Induced Polyelectrolyte Pore Bridging

Active layers of ion separation membranes often consist of charged layers that retain ions based on electrostatic repulsion. Conventional fabrication of these layers, such as polyelectrolyte deposition, can in some cases lead to excess coating to prevent defects in the active layer. This excess deposition increases the overall membrane transport resistance. The study at hand presents a manufacturing procedure for controlled polyelectrolyte complexation in and on porous supports by support wetting control. Pre-wetting of the microfiltration membrane support, or even supports with larger pore sizes, leads to ternary phase boundaries of the support, the coating solution, and the pre-wetting agent. At these phase boundaries, polyelectrolytes can be complexated to form partially freestanding selective structures bridging the pores. This polyelectrolyte complex formation control allows the production of membranes with evenly distributed polyelectrolyte layers, providing (1) fewer coating steps needed for defect-free active layers, (2) larger support diameters that can be bridged, and (3) a precise position control of the formed polyelectrolyte multilayers. We further analyze the formed structures regarding their position, composition, and diffusion dialysis performance.

Recent developments in layer-by-layer assembled systems application in water purification

Electrostatically-based layer-by-layer (LbL) assembly is a versatile surface functionalization technique allowing the construction of complex three-dimensional architectures on virtually any type of material using various combinations of nano-bricks. One of the most promising applications of LbL assembled systems is in water purification. The main two strategies developed in this purpose consist in either enhancing the barrier properties of separation membranes and in the construction of core-shell organic/inorganic sorbents. In this review, the recent achievements in this topic are discussed with respect to the use of LbL-based composites in desalination and removal of heavy metal ions or organic pollutants. Finally, some works dealing with economic aspects of using LbL assemblies for water purification are presented, thus highlighting forthcoming strategies to develop economically-viable materials for such applications.