Immobilized enzymatic membrane surfaces for biocatalytic organics removal and fouling resistance.

This research reported on the immobilization of environmentally friendly enzymes, such as horseradish peroxidase (HRP) and laccase (L), along with the hydrophilic zwitterionic compound l-DOPA on nano-filtration (NF) membranes. This approach introduced biocatalytic membranes, leveraging combined effects between membranes and enzymes. The aim was to systematically assess the efficacy of the enzymatic modified membrane (HRP-NF) in degrading colors in the wastewater, as well as enhancing the membrane resistance toward organic fouling. The enzymatic immobilized membrane demonstrated 96.3 ± 1.8% to 96.6 ± 1.9% removal of colors, and 65.2 ± 1.3% to 67.2 ± 1.3% removal of TOC. This result was underpinned by the insights obtained from the radical scavenger coumarin, which was employed to trap and confirm the formation of PRs through the reaction of enzymes and H2O2. Furthermore, membranes modified with enzymes exhibited significantly improved antifouling properties. The HRP-NF membrane experienced an 8% decline in flux, while the co-immobilized HRP-L-NF membrane demonstrated as low as 6% flux decline, contributed by the synergistic effect of increased hydrophilicity and biocatalytic effects. These findings confirmed that the immobilized enzymatic surface has added function of degrading contaminants in addition to separation function of nanofiltration membrane. These l-DOPA-immobilized enzymatic membranes offered a promising hybrid biocatalytic membrane to eliminate dyes and mitigate membrane fouling, which can be applied in many industrial and domestic water and wastewater treatment.

Highly Permeable MoS2 Nanosheet Porous Membrane for Organic Matter Removal.

MoS2 nanosheets were synthesized by a bottom-up green chemical process where l-cysteine was used as a sulfur precursor. With specific concentrations, molar ratio of reactants, and pre-mixing conditions, MoS2 nanosheets of 200-300 nm in size and 4.2 nm in average thickness were successfully obtained. Porous membranes were then prepared by depositing the MoS2 nanosheet suspension on a 0.1 µm pore size poly(vinylidene difluoride) membrane filter in a multiple batch procedure. The membrane deposited with 12 batches of MoS2 nanosheets achieved 93.78% removal of bovine serum albumin. Acid red removal of 95.65% was also achieved after the second filtration pass. The porous MoS2 nanosheet membrane also demonstrated a high water flux of 182 ± 2.0 L/(m2 h). This result overcame the trade-off between selectivity and permeability faced by polymeric ultrafiltration membranes. The MoS2 nanosheets as building blocks formed not only intersheet slit pores with a narrow half-width to restrict the passage of organic molecules but also macro-channels allowing easy passage of water. The assembled MoS2 nanosheet membrane delivered promising separation of protein molecules and a high flux, attributing to its porous nanostructure, and could be a potential membrane for various water applications.

2D MoS2 nanoplatelets for fouling resistant membrane surface.

2D Molybdenum disulfide (MoS2) nanoplatelets were synthesized via a green bottom-up strategy using non-toxic l-Cysteine as sulfur source. Thehydrophobic MoS2 nanoplatelets assisted by hydrophilic 3-(3, 4-dihydroxyphenyl)-l-alanine (l-DOPA) were coated on a thin film composite nanofiltration (TFC-NFG) membrane. The accelerated fouling experiments were conducted by usingbovine serum albumin (BSA) asmodel organic foulant,and MoS2 coated membrane demonstrated excellent resistance with almost no flux decline within first hour of filtration, whereas the uncoated membrane showed flux decline immediately from the beginning of the experiment. After 5-hour filtration, the flux reduced by only 26% for MoS2 coated membrane with a higher flux recovery rate of 85.4% after washing by de-ionized (DI) water, whereas 45% flux decline was observed for uncoated membrane with lower flux recovery of 68%.These antifouling effects attributed by MoS2coated membrane were underpinned by combined unique interfacial properties offered by 2D tri-atomic layered MoS2morphology including dispersive surface tension, reduced surface roughness, weaker MoS2-foulant interactive forces, and negatively charged surface. This research positively confirms the role of 2D MoS2 nanoplatelets as an anti-fouling coating on membranes and brings up more possibility for applying other nanomaterials in 2D family in water applications such as desalination and water treatment.

Electrostatically coupled SiO2 nanoparticles/poly (L-DOPA) antifouling coating on a nanofiltration membrane.

In this work, the fouling resistance of TFC (thin film composite) nanofiltration membranes have been enhanced using an electrostatically coupled SiO2 (silica dioxide) nanoparticles/poly(L-DOPA) (3-(3,4-dihydroxyphenyl)-l-alanine) antifouling coating. SiO2 nanoparticles were synthesized in different size ranges and combined with L-DOPA; and then coated as an anti-fouling layer on the membrane surface by recirculated deposition. Membranes were coated with S-NP (silica nanoparticles) in small (19.8 nm), medium (31.6 nm) and large (110.1 nm) sizes. The zwitterionic compound L-DOPA in the form of self-polymerized poly(L-DOPA) (PDOPA) helped with the attachment of the S-NP to the membrane surface. It was confirmed by AFM (atomic force microscopy) measurement that coating of membranes led to an increase in hydrophilicity and reduction in surface roughness, which in turn led to a 60% reduction in the adhesion force of the foulant on the membrane as compared to the neat membrane. The modified membranes experienced almost no flux decline during the filtration experimental period, whereas the unmodified membrane showed a sharp flux decline. The best coating conditions of silica nanoparticles resulting in enhanced anti-fouling properties were identified. The biofouling film formation on the membranes was evaluated quantitatively using the flow cytometry method. The results indicated that the modified membranes had 50% lower microbial population growth in terms of total event count compared to the neat membrane. Overall, the experimental results have confirmed that the coating of electrostatically coupled SiO2 nanoparticles and PDOPA (S-NP/PDOPA) on TFC-NF (nanofiltration) membrane surfaces is effective in improving the fouling resistance of the membranes. This result has positive implications for reducing membrane fouling in desalination and industrial wastewater treatment applications.