Characterization of adsorptive fouling on ultrafiltration membranes by peptides mixtures using contact angle measurements

Surface energy-induced anti-wetting and anti-fouling enhancement of Janus membrane for membrane distillation

Membrane distillation (MD) presents a promising alternative to conventional desalination systems, particularly for the treatment of hypersaline wastewater. However, the large-scale application of MD is hindered by challenges such as membrane wetting, membrane fouling, and low permeate flux. Herein, we proposed an air/liquid interface deposition method to fabricate a Janus membrane, termed the PVDF-PDA/PEI-Si membrane. The membrane featured a nanosieving, superhydrophilic polydopamine/polyethylenimine (PDA/PEI) layer decorated with silica nanoparticles, coupled with a microporous, hydrophobic polyvinylidene fluoride (PVDF) layer. The introduction of a dense PDA/PEI-Si layer featuring high surface energy significantly enhanced the wetting and fouling resistance of the membrane, with a minor effect on the permeate flux. The performance enhancement was particularly evident when hypersaline water containing sodium dodecyl sulfate (SDS) and oily contaminants was used as the feed. The interactions between the membrane and contaminants were calculated using the XDLVO theory and molecular dynamics simulations to elucidate the mechanisms underlying the enhanced anti-wetting and anti-fouling properties, respectively. According to the XDLVO theory, a large energy barrier must be overcome for the SDS to attach onto the PDA/PEI-Si surface. Meanwhile, molecular dynamics simulations confirmed the weak interaction energy between the oily foulants and the PVDF-PDA/PEI-Si membrane due to its high surface energy. This study presents a promising approach for the fabrication of high-performance MD membranes and provides new insights into the mechanisms underlying the enhanced anti-wetting and anti-fouling properties.

Control ultrafiltration membrane fouling in Chlorella-laden water treatment by integrated heat-activated peroxydisulfate pre-oxidation and coagulation treatment

The membrane fouling induced by algal extracellular organic matter (EOM) remain a bottleneck in restricting ultrafiltration (UF) application during harmful algal-water treatment. In current study, the application of heat-activated peroxydisulfate (PMS) and coagulation (Aluminum chlorohydrate, PACI) on membrane fouling behavior during Chlorella-laden water treatment was investigated. The membrane fouling mechanism was analyzed using the extended Derjaguin-Landau-Verwey-Over-beek (XDLVO) theory. The results revealed that separated heat-activated PMS could enhance the filtration flux of EOM at high PMS does >0.2 mM, whereas the membrane fouling was further alleviated by combined heat-activated PMS (0.2-1.0 mM) and PACI (20 mg/L) treatment, especially at low PMS dose. Combined heat-activated PMS and PACI pretreatment could effectively increase the adhesive repulsion between membrane and foulants and reduce the cohesion free energies between organic foulants than those by separated heat-activated PMS treatment, making the initial filtration flux reduced and the cake layer looser. Moreover, the organic foulants of proteins, polysaccharides, and humic-like organics were removed. Cake formation was the major fouling mechanism when EOM was treated with/without separated heat-activated PMS treatment, whereas the membrane fouling mechanism was changed from cake layer formation to pore blocking after combined heat-activated PMS and PACI treatment. Overall, this research provided a feasible method in membrane fouling control during Chlorella -laden water treatment.

Quantitative Assessment of Interfacial Interactions Governing Ultrafiltration Membrane Fouling by the Mixture of Silica Nanoparticles (SiO2 NPs) and Natural Organic Matter (NOM): Effects of Solution Chemistry

Mixtures of silica nanoparticles (SiO₂ NPs) and natural organic matter (NOM) are ubiquitous in natural aquatic environments and pose risks to organisms. Ultrafiltration (UF) membranes can effectively remove SiO₂ NP-NOM mixtures. However, the corresponding membrane fouling mechanisms, particularly under different solution conditions, have not yet been studied. In this work, the effect of solution chemistry on polyethersulfone (PES) UF membrane fouling caused by a SiO₂ NP-NOM mixture was investigated at different pH levels, ionic strengths, and calcium concentrations. The corresponding membrane fouling mechanisms, i.e., Lifshitz-van der Waals (LW), electrostatic (EL), and acid-base (AB) interactions, were quantitatively evaluated using the extended Derjaguin-Landau-Verwey-Overbeek (xDLVO) theory. It was found that the extent of membrane fouling increased with decreasing pH, increasing ionic strength, and increasing calcium concentration. The attractive AB interaction between the clean/fouled membrane and foulant was the major fouling mechanism in both the initial adhesion and later cohesion stages, while the attractive LW and repulsive EL interactions were less important. The change of fouling potential with solution chemistry was negatively correlated with the calculated interaction energy, indicating that the UF membrane fouling behavior under different solution conditions can be effectively explained and predicted using the xDLVO theory.

Critical minority fractions causing membrane fouling in reclaimed water: Fouling characteristics, mechanisms and control strategies

In regard to membrane-based technologies of wastewater reclamation, the reported key foulants were faced with dilemma that they could not be effectively separated and extracted from reclaimed water for thorough investigation. In this study, the crucial foulants were proposed as "critical minority fraction (F_CM)", representing the fraction with molecular weight (MW) > 100 kDa which could be easily separated by physical filtration using MW cut-off membrane of 100 kDa with fairly high recovery ratio. F_CM with low dissolved organic carbon (DOC) concentration (∼1 mg/L) accounted for less than 20% of the total DOC in reclaimed water, while contributed to more than 90% of the membrane fouling, and thus F_CM could be considered as a "perfect criminal" causing membrane fouling. Furthermore, pivotal fouling mechanism was attributed to the significant attractive force between F_CM and membranes, which led to severe fouling development due to the aggregation of F_CM on membrane surface. Fluorescent chromophores of F_CM were concentrated in regions of proteins and soluble microbial products, with proteins and polysaccharides accounted for 45.2% and 25.1% of the total DOC, specifically. F_CM was further fractionated into six fractions, among which hydrophobic acids and hydrophobic neutrals were the dominant components in terms of DOC content (∼80%) as well as fouling contribution. Regarding to these pronounced properties of F_CM, targeted fouling control strategies including ozonation and coagulation were applied and proved to achieve remarkable fouling control effect. High-performance size-exclusion chromatography results suggested that ozonation achieved distinct transformation of F_CM into low MW fractions, while coagulation removed F_CM directly, thus leading to effective fouling alleviation. Therefore, the investigation of the critical foulants was expected to help glean valuable insight into the fouling mechanism and develop targeted fouling control technologies in practical applications.

Extracellular polymeric substance profiling and biophysical analysis reveal influence factors of spontaneous flocculation in rich lipid alga Heveochlorella sp. Yu

Microalgae harvest and lipid accumulation were important factors influencing commercialized development of microalgae biodiesel. Spontaneous flocculation was an ideal method in microalgae harvest, but few rich lipid microalgae could be harvested by spontaneous flocculation. Rich lipid alga Heveochlorella sp. Yu has a characteristic of spontaneous flocculation to be harvested. Heveochlorella sp. Yu has high lipid productivity (105.24 mg L^-1 d^-1) and fine spontaneous flocculation efficiency (82.93 %, 2 h) on early stationary phase (day 9). The polysaccharides consisting of glucose, mannose, galactose, rhamnose and fructose (8.67:4.90:3.27:2.16:1) in loose-bound extracellular polymeric substance (LB-EPS) might make great contribution in microalgae flocculation. Meanwhile, the zeta potential close to zero was also beneficial to microalgae flocculation. Besides, the adhesion free energy related with cells adhesion was detected by thermomechanical analysis. Afterward, Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory was utilized to quantitatively evaluate short-range interactions involved in the spontaneous aggregation among cells. Collectively, biophysical analyses indicated that content and composition of EPS, Zeta potential, thermodynamic parameter and total interaction based on XDLVO theory were closely connected with spontaneous flocculation in microalga Yu. Our study provided a harvest-simplified process of rich microalgae, which proposes a new idea for commercial development of microalgae biodiesel.

Effects of graphene derivatives on polyvinylidene fluoride membrane modification evaluated with XDLVO theory and quartz crystal microbalance with dissipation

In this study, the different graphene derivatives, graphene oxide (GO), carboxylic acid-modified graphene (G-COOH), and amine-modified graphene (G-NH₂ ), were used to prepare polyvinylidene fluoride (PVDF) composite membranes. The membrane modification performance was evaluated using the extended Derjaguin-Landau-Verwey-Overbeek theory and quartz crystal microbalance dissipation monitoring. The results show that the addition of low-dose GO and G-NH₂ can improve membrane surface porosity and permeability. The hydrophilicity and electron donor monopolarity of PVDF/GO composite membranes were enhanced by adding more than 0.024 wt% GO, thus improving its antifouling ability. In addition, the enhancement of hydrophilicity, free energy of cohesion, and antifouling ability of composite membrane modified with G-COOH and G-NH₂ was more significant compared with that of GO with the same dosage, which implies the important role of functional group in additives. This study provides new insights for the blending modification of PVDF membranes by systematically comparing the addition of graphene derivatives with different functional groups. PRACTITIONER POINTS: The comprehensive comparison of membrane modification with different graphene derivatives was investigated. The enhancement of hydrophilicity and antifouling ability of membranes modified with G-COOH and G-NH₂ was more significant than that of GO. The free energy of cohesion of nanocomposite membrane was affected by the functional group of additives. G-NH₂ composite membrane had the best comprehensive performance with great hydrophilicity, permeability, and antifouling performance.

Nanoparticle-free and self-healing amphiphobic membrane for anti-surfactant-wetting membrane distillation

In membrane distillation (MD), complicated feed water with amphiphilic contaminants induces fouling/wetting of the MD membrane and can even lead to process failure. This study reports a facile approach to fabricate robust and self-healing hybrid amphiphobic membranes for anti-surfactant-wetting MD based on the ultra-low surface energy of fluorinated polyhedral oligomeric silsesquioxanes (F-POSS) and its thermal induced motivation and rotation. The thermal treatment makes the membranes achieving amphiphobicity at a very low cost of F-POSS (13.04 wt.%), which is about 1/3 of without thermal treatment. The prepared membrane exhibits excellent amphiphobicity, i.e. ethanol contact angle of 120.3°, without using environmentally toxic fluorinated nanoparticles. Robust MD performance was observed for the amphiphobic membrane in concentrated sodium dodecyl sulfate (SDS) feed solutions. Furthermore, the fabricated membrane exhibited stable amphiphobicity even in extreme environments, including strong acid or alkaline solutions. In the event of a damaged or abraded membrane surface where the F-POSS can be removed, the amphiphobic membrane exhibits self-healing ability with additional thermal treatment. This simple approach without the use of nanoparticles provides an environmentally friendly way for fabrication of amphiphobic membranes for anti-surfactant-wetting membrane distillation.

Investigation of membrane fouling mechanism of intracellular organic matter during ultrafiltration

This study investigated the ultrafiltration (UF) membrane fouling mechanism of intracellular organic matter (IOM) from Chlorella vulgaris (CV) and Microcystis aeruginosa (MA). Both CV- and MA-IOM caused severe membrane fouling during UF; however, there were significant differences in the membrane fouling by these two materials. Neutral hydrophilic (N-HPI) compounds were the organics that caused the most severe membrane fouling during CV-IOM filtration, whereas the MA-IOM membrane fouling was induced by mainly hydrophobic (HPO) organics. From an analysis based on Derjaguin–Landau–Verwey–Overbeek theory, it was found that the interaction energy between the membrane and foulants in the later stage of filtration was the major factor determining the efficiency of filtration for both CV-IOM and MA-IOM. The TPI organics in CV-IOM fouled the membrane to a more severe degree during the initial filtration flux; however, when the membrane surface was covered with CV-IOM foulants, the N-HPI fraction of CV-IOM caused the most severe membrane fouling because its attractive energy with the membrane was the highest. For MA-IOM, regardless of the initial filtration flux or the late stage of filtration, the HPO organics fouled the membrane to the greatest extent. An analysis of modified filtration models revealed that cake layer formation played a more important role than other fouling mechanisms during the filtration of CV-IOM and MA-IOM. This study provides a significant understanding of the membrane fouling mechanism of IOM and is beneficial for developing some strategies for membrane fouling control when treating MA and CV algae-laden waters.

Evaluation of the fouling potential of sludge in a membrane bioreactor integrated with microbial fuel cell

This study focused on the fouling characteristics evaluation of the sludge in a membrane bioreactor integrated with microbial fuel cell (MFC-MBR) to reveal the mechanisms of membrane fouling mitigation. The filtration of soluble microbial products (SMPs) in MFC-MBR showed lower flux decline rate than those in the control system (C-MBR). Based on the extended Derjaguin-Landau-Verwey-Overbeek analysis, decreases in free energies of adhesion between the SMPs and clean membrane or SMP-fouled membrane were observed in MFC-MBR. When approaching the clean membrane or SMP-fouled membrane, the SMPs in MFC-MBR had to overcome a higher energy barrier compared to those in C-MBR, indicating the inhibition of adsorption of SMPs on the membrane surface in MFC-MBR. Additionally, sludge flocs in MFC-MBR exhibited lower hydrophobicity and were less negative surface charged in comparison to those in the C-MBR. In MFC-MBR, the sludge flocs approaching the clean membrane, SMP-fouled membrane and cake layer all experienced higher energy barriers and lower secondary energy minimums compared to those in C-MBR, exhibiting the lower potential of cake layer formation. These results confirmed that decreases of the fouling potentials of SMPs and sludge flocs were essential for the membrane fouling mitigation in the MFC-MBR.

Constructing zwitterionic polymer brush layer to enhance gravity-driven membrane performance by governing biofilm formation

In this study, zwitterionic polymer brushes with controlled architecture were grafted on the surface of gravity-driven membrane (GDM) via surface-initiated reaction to impart antifouling property. A variety of membrane characterization techniques were conducted to demonstrate the successful functionalization of zwitterionic polymers on PVDF hollow fiber membrane. The membrane underwent 90 min of reaction time possessing strong hydrophilicity and high permeability was determined as the optimal modified membrane. Long-term GDM dynamic fouling experiments operated for 30 days using sewage wastewater as feed solution indicated zwitterionic polymer modified membrane exhibit excellent membrane fouling resistance thus enhanced stable flux. Confocal laser scanning microscopy (CLSM) imaging implied that zwitterionic polymer modification significantly inhibit the adsorption of extracellular polymeric substances (EPS) which dominates fouling propensity, resulting in the formation of a thin biofilm with high porosity under synthetic functions of foulants deposition and microbial activities. Interfacial free energy prediction affirmed the presence of zwitterionic functional layer on membrane surface could substantially decrease the interactions (e.g., electrostatic attractions and hydrophobic effects) between membrane and foulants, thereby reduced flux decline and high stable flux. Our study suggests surface hydrophilic functionalization shows promising potential for improving the performance of ultra-low pressure filtration.

Effect of metabolic uncoupler, 3,3′,4′,5-tetrachlorosalicylanilide (TCS) on Bacillus subtilis: biofilm formation, flocculability and surface characteristics

In order to understand the inhibitory mechanism of metabolic uncoupler in biofilm, this study investigated the effect of TCS on B. subtilis biofilm formation, flocculability, surface characteristics and thermodynamic properties. An optimal concentration of TCS, a metabolic uncoupler, was observed to substantially inhibit biofilm formation and the secretion of extracellular polymeric substances (EPS). The effect of TCS on the zeta potential and flocculability of bacterial suspension implied the addition of 100 μg L⁻¹ TCS increased the net negative charge of cell surface which induced the reduction of B. subtilis flocculability. Meanwhile, the effects of TCS on bacterial surfacial thermodynamic properties were analyzed by the Derjaguin–Landau–Verwey–Overbeek (DLVO) and extend DLVO (XDLVO) theories. As DLVO and XDLVO predicted, the primary energy barrier between bacterial cells incubated with 100 μg L⁻¹ TCS were increased compared to that of control, indicating that B. subtilis incubated with 100 μg L⁻¹ TCS must consume more energy to aggregate or form biofilm.

This study aimed to investigate the inhibitory mechanism of metabolic uncoupler on biofilm formation through surface characteristics and thermodynamics analysis.

Natural organic matter fouling behaviors on superwetting nanofiltration membranes

Nanofiltration has been widely recognized as a promising technology for the removal of micro-molecular organic components from natural water. Natural organic matter (NOM), a very important precursor of disinfection by-products, is currently considered as the major cause of membrane fouling. It is necessary to develop a membrane with both high NOM rejection and anti-NOM fouling properties. In this study, both superhydrophilic and superhydrophobic nanofiltration membranes for NOM removal have been fabricated. The fouling behavior of NOM on superwetting nanofiltration membranes has been extensively investigated by using humic acid (HA) as the model foulant. The extended Derjaguin-Landau-Verwey-Overbeek approach and nanoindentor scratch tests suggested that the superhydrophilic membrane had the strongest repulsion force to HA due to the highest positive total interaction energy (ΔG(TOT)) value and the lowest critical load. Excitation emission matrix analyses of natural water also indicated that the superhydrophilic membrane showed resistance to fouling by hydrophobic substances and therefore high removal thereof. Conversely, the superhydrophobic membrane showed resistance to fouling by hydrophilic substances and therefore high removal capacity. Long-term operation suggested that the superhydrophilic membrane had high stability due to its anti-NOM fouling capacity. Based on the different anti-fouling properties of the studied superwetting membranes, a combination of superhydrophilic and superhydrophobic membranes was examined to further improve the removal of both hydrophobic and hydrophilic pollutants. With a combination of superhydrophilic and superhydrophobic membranes, the NOM rejection (RUV254) and DOC removal rates (RDOC) could be increased to 83.6% and 73.3%, respectively.

Understanding the fouling of algogenic organic matter in microfiltration using membrane-foulant interaction energy analysis: effects of organic hydrophobicity

Fouling caused by algogenic organic matter (AOM) in membrane filtration is a critical problem in algae-rich waters, and understanding fouling mechanisms, particularly by identifying the predominant membrane foulants, could have significant effects on algal fouling prediction and pretreatment. In this work, the fouling behavior of Aphanizomenon flos-aquae (APF)- and Anabaena flos-aquae (ANF)-AOM fractions was analyzed using the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. The results show that the interfacial energy of membranes and foulants could be used for AOM membrane fouling analysis. The attractive energy was highest between the membrane and the neutral hydrophilic fractions (N-HPI) on clean membrane surfaces, followed by the energy associated with the hydrophobic fractions (HPO) and the transphilic fractions (TPI) in both of the AOMs; on the other hand, the negatively charged hydrophilic organics (C-HPI) in the APF-AOM suffered from repulsive interactions when nearing the membrane surface, which was consistent with their initial filtration flux. After the formation of an initial fouling layer on the membrane surface, membrane fouling was controlled mainly by the cohesion free energy between the approaching foulants and the foulants on the fouled membranes. In addition, it was observed that the interfacial energy between foulants was the dominant factor controlling membrane fouling in AOM filtration. Finally, the interfacial energies between the N-HPI fractions had the greatest effect on both APF-AOM and ANF-AOM membrane fouling.

Antifouling behaviours of PVDF/nano-TiO2 composite membranes revealed by surface energetics and quartz crystal microbalance monitoring

Introducing nano-TiO₂ improved the interaction energy between the membrane surface and foulant; however, aggregation of nano-TiO₂ facilitated foulant adsorption on pore walls.

Identification of the change in fouling potential of soluble microbial products (SMP) in membrane bioreactor coupled with worm reactor

This study focused on the effect of predated sludge recycle on the fouling potential of soluble microbial products (SMP) in the MBR coupled with Static Sequencing Batch Worm Reactor (SSBWR-MBR). The S-SMP (SMP in SSBWR-MBR) filtration showed slower diminishing rate of flux than C-SMP (SMP in Control-MBR) filtration, and the standard blocking model showed the most excellent fit (R² = 0.9999) for both C-SMP and S-SMP filtration, confirming that hydrophobic/hydrophilic attractive force was supposed to play a major role in SMP filtration. Based on the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) analysis, the decrease in the hydrophobic interactions between SMP and membrane (Adhesion) and between the SMP themselves (Cohesion) was found. The structural parameters analysis indicated the S-SMP fouling layer showed a higher porosity, lower biovolume and thinner average thickness than the C-SMP fouling layer at the end of filtration. Further investigations demonstrated that these changes could be attributed to the lower hydrophobic interaction as the result of the decrease in the relative abundance of unsaturated groups (aromatic protein-like substances) in the S-SMP.

Food materials adhesion: a review

This article reviews the various theories of adhesion mechanism and, more specifically, studies concerning foodstuffs adhesion to industrial equipment and packaging surfaces. Adhesion is governed by mechanical interlocking, wetting, electrostatic and chemical forces, and diffusion. Direct conclusions about the validity of one of these theories were seldom made in the empirical studies reviewed. The different food adhesion determination methods were detailed: direct observations, evaluations (weighting, UV absorbance, and adhesive loss), adhesion strength measurements, and indirect measurements via the wetting theory (tilted plane method, contact angle, and surface tension). The importance of proteins, product rheological properties, solid surface rugosity, and wetting phenomena in many adhesion cases is highlighted. Conclusions were made that fundamental mechanisms of food-contact surfaces interactions still need to be investigated to improve understanding in the science of food materials.