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.

Application of moving bed biofilm reactor - nanofiltration - membrane bioreactor with loose nanofiltration hollow fiber membranes for synthetic roxithromycin-containing wastewater treatment: Long-term performance, membrane fouling and microbial community.

The present study operated the novel moving bed biofilm reactor-nanofiltration-membrane bioreactor (MBBR-NF-MBR) with loose polyamide NF membranes for the first time to treat roxithromycin (ROX) wastewater. Results showed that both MBBR-NF-MBRs achieved superior COD removal of 98.4% and 97.2% and excellent removal of ROX at 74.1% and 65.5%, respectively. The main membrane fouling mechanism was reversible fouling caused by the combination of abundant polysaccharides, proteins and Ca-P precipitates, which could be effectively removed by acidic cleaning. Sorption and biodegradation were the main removal routes of ROX in MBBR. Partial retention of loose NF membrane contributed to microbial metabolism and increased microbial diversity, especially the genera Hyphomicrobium in attached biofilm, which was reasonable for ROX removal. The cleavage of cladinose, demethylation, phosphorylation and ß-oxidation in macrolactone ring were the main biotransformation reactions of ROX. This study provides novel insights for micropollutants wastewater treatment by using loose NF membrane in MBR.

Design of microstructure for hollow fiber loose nanofiltration separation layer and its compactness-tailoring mechanism.

In order to promote the application of membrane technology in the treatment of textile wastewater containing small molecule dye, fabricating a hollow fiber loose nanofiltration (LNF) with a thin and compact separation layer and deepening the understanding of compactness-tailoring mechanism in chemical crosslinking are essential. Firstly, the mechanisms of synergistic crosslinking of PEI-70K and PEI-10K, along with a weakening of the PEI hydration by ethanol, were expounded in primary crosslinking. Then, some LNF separation layers with different compactness were prepared through crosslinking with different crosslinkers to further reduce pore size, which resulted in the efficient removal (~100%) of a small molecular dye (methyl orange (MO), M = 327 g mol-1). The removal of methyl orange is mainly caused by size sieving. The relationship among the pore size, the Mw of the secondary crosslinkers, and the pore size reduction rate was interpreted by comparing the pore size reduction rate of three secondary crosslinkers with different molecular weights. In addition, the as-prepared separation layer exhibited excellent dimensional stability and solvent resistance. This paper not only provides a reference for fabricating hollow fiber LNF with better purification performance, but also shows their potential in developing solvent resistant nanofiltration.

Metal-polyphenol cross-linked titanium carbide membranes with stable interlayer spacing for efficient wastewater treatment.

Membranes based on transition metal carbides/nitrides (MXenes) have significant water treatment potential because of their unique molecular sieving properties and excellent permeation performance. However, hydrophilic MXenes swell upon water immersion, and improving their stability remains challenging. In this study, a Fe3+-tannic acid (TA) complex was used as a cross-linker and surface modifier to prepare high-performance titanium carbide (Ti3C2Tx) MXene laminar membranes. Fe3+-TA formation on the nanosheets increased the interlayer spacing and stabilized the laminar structure. The membrane with the highest performance among the as-prepared membranes exhibited a high water permeance of 90.5 L/m-2(-|-)h-1 bar-1 (which is twice that of the pristine Ti3C2Tx membrane) and good separation efficiency (methyl blue rejection rate: ∼99.8 %; Na2SO4 rejection rate: ∼5.0 %). Furthermore, the Fe3+-TA complex enhanced the membrane hydrophilicity, resulting in excellent antifouling properties. This study provides an environmentally friendly and facile method for fabricating two-dimensional loose nanofiltration membranes for textile wastewater treatment.

Custom-Tailoring Loose Nanofiltration Membrane for Precise Biomolecule Fractionation: New Insight into Post-Treatment Mechanisms.

Loose nanofiltration (NF) membranes with diverse selectivity can meet the great demands in various bioseparation applications. Thus, a facile strategy to tune the properties such as pore size, surface charge, and hydrophilicity of the NF membrane is required to produce tailor-made loose NF membranes without changing the existing production line. Herein, we systematically investigated the post-treatment of the nascent poly(piperazine amide) NF membranes using different reagents (organic acids, weak bases, organic solvents and ionic liquid (IL)). Various characterizations revealed that the skin/separation layer became looser and permeance was promoted with the decrease of salt rejection in varying degrees. It was found that the O/N ratio did not rigorously represent the cross-linking degree of the skin layer, because besides the hydrolysis of the residual acyl chloride impeding the amido bond formation, the breaking of existing amido bonds and the grafting of free trimesoyl chloride molecules on the nascent membranes could also increase the O/N ratio during post-treatments. Then three mechanisms including hydrolysis, swelling rearrangement and capping reaction effects were proposed to better understand the membrane properties variations. All these effects resulted in larger pore size of the NF membrane, and the hydrolysis/capping effect might increase negative charge and hydrophilicity on the membrane, while the swelling rearrangement could produce less defective skin structure. These three effects might be involved together during a single treatment. Finally, the NF membrane post-treated by N-hexane could efficiently separate antibiotics and NaCl with the highest permeate flux, whereas the one post-treated by ionic liquid outperformed others for the decoloration of cane molasses (much more efficient than NF270, DL, and NTR7450 membranes). The long-term operating stability of the post-treated membranes selected was also confirmed by a continuous crossflow filtration for 15 h with regular alkaline cleaning.

Bio-inspired co-deposited preparation of GO composite loose nanofiltration membrane for dye contaminated wastewater sustainable treatment.

The fully separation of dye/salt through loose nanofiltration membranes is of great significance for the sustainable development paradigm of textile wastewater. However, the current loose nanofiltration membranes suffer low separation efficiency and complex preparation. Herein, by one-step co-deposition, we develop graphene oxide (GO) composite loose nanofiltration membranes with low negatively charged surface. Our membrane possesses unconventional high pure water permeation of 71.7 LMH/bar, 92.9 % rejection for Methyl blue (MB) and 98.8 % rejection for Congo red (CR). Benefiting from the large interlayer distance of GO nanosheets and low negatively charged surface, membrane achieves high dyes/salts separation with satisfactory permeation to salts (94.3 % of Na2SO4, 97.6 % of MgSO4, 98.3 % of MgCl2 and 99.0 % of NaCl). The CR/salt mixed solutions exhibit similar removal rates to their constituents' single dye or salt solutions (CR rejection is up to more than 97 % and the permeations of all salts are above 93 %). At the same time, binary dyes mixtures (Congo red and Methyl orange) can also be effectively separated. Furthermore, the membrane shows a relatively desirable antifouling property. The flux recovery still remains at 85.9 % after three cycling filtrations. This study provides a facile approach to prepare highly-efficient loose nanofiltration membranes for wastewater sustainable remediation.

PIP/TMC Interfacial Polymerization with Electrospray: Novel Loose Nanofiltration Membrane for Dye Wastewater Treatment.

A loose nanofiltration (NF) membrane with excellent dye rejection and high permeation of inorganic salt is required to fractionate dye/salt mixture in dye wastewater treatment. In this study, we fabricated the loose NF membrane by using the electrospray interfacial polymerization (EIP) method. It is a novel and facile interfacial polymerization method, which controls the thickness of the poly(piperazine-amide) (PPA) layer in nanometers (1 nm/min) and changes cross-linking degree of PPA layer and pore size by varying the electrospray time; consequently, water permeance and dye/salt rejection ratio can be handled. The fabricated EIP membrane with an optimized fabrication condition (M30, electrospray time was 30 min) possessed excellent pure water permeance (20.2 LMH/bar), high dye rejection (e.g., 99.6% for congo red (CR)), and low salt rejection (e.g., 6.3% for NaCl). Moreover, the EIP membrane exhibited enhanced antifouling property than commercial NF membrane (NF90) with a high flux recovery rate (FRR) of 87.1% and low irreversible fouling (Rir) of 12.9% after fouled by bovine serum albumin (BSA) due to its great smooth surface (average roughness (Ra) is 12.2 nm), hydrophilicity property, enhanced zeta potential, and low protein adsorption. The results indicate that the EIP loose NF membrane had a high potential for dye wastewater treatment.

Sustainable management of landfill leachate concentrate through recovering humic substance as liquid fertilizer by loose nanofiltration.

The hybrid membrane bioreactor - nanofiltration treatment process can be an effective approach for treating the landfill leachate, but the residual leachate concentrate highly loaded with the humic substance and salts remains an environmental concern. Herein, a loose nanofiltration membrane (molecular weight cut-off of 860 Da) was used to recover the humic substance, which can act as a key component of organic fertilizer, from the leachate concentrate. The loose nanofiltration membrane showed the high permeation fluxes and high transmissions (>94.7%) for most inorganic ions (i.e., Na+, K+, Cl-, and NO3-), while retaining 95.7 ±â€¯0.3% of the humic substance, demonstrating its great potential in effective fractionation of humic substance from inorganic salts in the leachate concentrate. The operation conditions, i.e., cross-flow rates and temperatures, had more pronounced impacts on the filtration performance of the loose nanofiltration membrane. Increasing cross-flow rates from 60 to 260 L h-1 resulted in an improvement of ca. 7.3% in the humic substance rejection, mainly due to the reduced concentration polarization effect. In contrast, the solute rejection of the nanofiltration membrane was negatively dependent on the temperature. The rejection of humic substance decreased from 96.3 ±â€¯0.3% to 92.0 ±â€¯0.4% with increasing the temperature from 23 to 35 °C, likely due to the enlargement of the membrane pore size and enhancement in solute diffusivity. The humic substance was enriched from 1735 to 15,287 mg L-1, yielding a 91.2% recovery ratio with 85.7% desalination efficiency at a concentration factor of 9.6. The recovered HS had significantly stimulated the seed germination and growth of the green mungbean plants with no obvious phytotoxicity. These results demonstrate that loose nanofiltration can be an effective promising technology to recover the humic substance as a valuable fertilizer component towards sustainable management of the landfill leachate concentrate.