Evaluating the effects of sodium and magnesium on the interaction processes of humic acid and ultrafiltration membrane surfaces

Employing Atomic Force Microscopy (AFM) for Microscale Investigation of Interfaces and Interactions in Membrane Fouling Processes: New Perspectives and Prospects

Membrane fouling presents a significant challenge in the treatment of wastewater. Several detection methods have been used to interpret membrane fouling processes. Compared with other analysis and detection methods, atomic force microscopy (AFM) is widely used because of its advantages in liquid-phase in situ 3D imaging, ability to measure interactive forces, and mild testing conditions. Although AFM has been widely used in the study of membrane fouling, the current literature has not fully explored its potential. This review aims to uncover and provide a new perspective on the application of AFM technology in future studies on membrane fouling. Initially, a rigorous review was conducted on the morphology, roughness, and interaction forces of AFM in situ characterization of membranes and foulants. Then, the application of AFM in the process of changing membrane fouling factors was reviewed based on its in situ measurement capability, and it was found that changes in ionic conditions, pH, voltage, and even time can cause changes in membrane fouling morphology and forces. Existing membrane fouling models are then discussed, and the role of AFM in predicting and testing these models is presented. Finally, the potential of the improved AFM techniques to be applied in the field of membrane fouling has been underestimated. In this paper, we have fully elucidated the potentials of the improved AFM techniques to be applied in the process of membrane fouling, and we have presented the current challenges and the directions for the future development in an attempt to provide new insights into this field.

Role of inorganic foulants in the aging and deterioration of low-pressure membranes during the chemical cleaning process in surface water treatment: A review

Low-pressure membrane (LPM) filtration, including microfiltration (MF) and ultrafiltration (UF), is a promising technology for the treatment of surface water for drinking and other purposes. Various configurations and operational sequences have been developed to ensure the sustainable provision of clean water by overcoming fouling problems. In the literature, various periodic physical and/or chemical approaches to the cleaning of LPMs have been reported, but little data is available on the aging of MF/UF membranes that results from the interaction between the foulants and the cleaning agent. Periodic physical cleaning of the membrane is expected to return the membrane to its original performance capacity, but it only recovers to a certain level because the remaining foulants cause irreversible fouling. Chemical cleaning can then be employed to recover the membrane from this irreversible fouling but, in the process, it can cause irrecoverable damage to the membrane. In this review, the foulants responsible for irrecoverable damage to MF/UF membranes are summarized, and their interaction with cleaning agents and other foulants is described. The impact of these foulants on various membrane parameters, including filtration efficiency, flux decline, permeability, membrane characterization, and membrane integrity are also summarized and discussed in detail. In addition, mitigation options and future prospects are also discussed with regard to increasing the operational life span of a membrane in a cost-effective manner. Ultimately, this review suggests an advanced control system based on membrane-foulant interactions under the impact of various operational parameters to mitigate the integrity loss of membranes.

Evaluation of Organic and Inorganic Foulant Interaction Using Modified Fouling Models in Constant Flux Dead-End Operation with Microfiltration Membranes

The goal of this study was to elucidate the interaction of complex feed solutions under modified membrane fouling models for constant flux operation. The polyvinylidene fluoride membrane (PVDF) was tested for three types of solutions containing inorganic foulants (Al, Mn, and Fe), organic foulants, and suspended solids at 0.5 mM Ca2+ ionic strength. The membrane's performance was evaluated by measuring the increase in transmembrane pressure (TMP) during two different filtration scenarios: continuous filtration lasting 1 h and cyclic filtration lasting 12 min, with 3 min backwashing cycles included. Statistical analysis (linear regression results (R2), p-value) was used to verify the fouling model propagation along with the determination of the contributing constant of each fouling model. An increasing TMP percentage of 164-302%, 155-300%, and 208-378% for S1 (HA + Ca2+), S2 (inorganics + kaolin + Ca2+), and S3 (HA + inorganics + kaolin + Ca2+) was recorded for 1 h filtration, respectively. Furthermore, a five percent increase in irreversible resistance was noted for the S3 solution due to the strong adsorption potential of foulants for the PVDF membrane caused by the electrostatic and hydration forces of foulants. In addition to that, the participation equation elucidated the contribution of the fouling model and confirmed that complete blocking and cake layer contribution were dominant for the S1 and S3 solutions, while standard blocking was dominant for the S2 solution with a high significance ratio. Moreover, R2 and cyclic filtration analysis also confirmed the propagation of these fouling models. The statistical confirmation and regression results analysis of the modified model gave comparative results and satisfied the filtration mechanism and can be used for the constant flux dead filtration analysis of water treatment.

How and why does time matter - A comparison of fouling caused by organic substances on membranes over adsorption durations

This study investigated the effect of time on the severity of adsorptive fouling on polyvinylidene fluoride (PVDF) membrane surface. Sodium alginate (SA), bovine serum albumin (BSA), and humic acid (HA) were selected as representative membrane foulants. We examined the fouling behavior of these three selected model foulants over different adsorption durations (i.e., ~2300 and ~20,000 s). The fouling experiments were performed under conditions with and without the presence of Ca²⁺. For the SA-Ca²⁺ system, a longer adsorption duration slightly increased adsorption amount of SA but sharply reduced the reversibility (from 86.8 % to 12.9 %). For BSA-Ca²⁺, extended time did not change the deposition amount of BSA on the membrane surface, but led to more residual BSA after cleaning (reversibility decreased from 11.3 % to 4.5 %). Similarly, in the HA-Ca²⁺ system, adsorption duration barely influenced the adsorption amount of HA, while reduced its reversibility from 39.4 to 32.2 %. Therefore, time duration significantly influenced the amount and reversibility of membrane fouling depending on their chemical property. Corresponding results can be well reflected by a selected mathematical model. Further investigation on relevant mechanisms was conducted, quartz crystal microbalance with dissipation (QCM-D) and atomic force microscope (AFM) measurements indicated that longer adsorption duration resulted in more compacted fouling layer and stronger foulant-membrane interaction force. Our results suggest that time (adsorption duration) plays an important role in determining the reversibility of membrane fouling, while the severity is related to the inherent characteristics of foulants.

Sustainable and efficient technologies for removal and recovery of toxic and valuable metals from wastewater: Recent progress, challenges, and future perspectives

Due to their numerous effects on human health and the natural environment, water contamination with heavy metals and metalloids, caused by their extensive use in various technologies and industrial applications, continues to be a huge ecological issue that needs to be urgently tackled. Additionally, within the circular economy management framework, the recovery and recycling of metals-based waste as high value-added products (VAPs) is of great interest, owing to their high cost and the continuous depletion of their reserves and natural sources. This paper reviews the state-of-the-art technologies developed for the removal and recovery of metal pollutants from wastewater by providing an in-depth understanding of their remediation mechanisms, while analyzing and critically discussing the recent key advances regarding these treatment methods, their practical implementation and integration, as well as evaluating their advantages and remaining limitations. Herein, various treatment techniques are covered, including adsorption, reduction/oxidation, ion exchange, membrane separation technologies, solvents extraction, chemical precipitation/co-precipitation, coagulation-flocculation, flotation, and bioremediation. A particular emphasis is placed on full recovery of the captured metal pollutants in various reusable forms as metal-based VAPs, mainly as solid precipitates, which is a powerful tool that offers substantial enhancement of the remediation processes' sustainability and cost-effectiveness. At the end, we have identified some prospective research directions for future work on this topic, while presenting some recommendations that can promote sustainability and economic feasibility of the existing treatment technologies.

Exploring the influence mechanism of ozonation on protein fouling of ultrafiltration membranes as a result of the interfacial interaction of foulants at the membrane surface

Ozonation was widely used before ultrafiltration processes, but its effect mechanism on protein fouling is still controversial. Ozonation of bovine serum albumin (BSA) solutions was performed in the present work. The interfacial forces of BSA at the membrane surface were measured before and after ozonation. The adsorption behaviour of BSA onto the membrane surface and the fouling layer structures under different ozone dosages were also investigated. These results were combined with the membrane fouling behaviour to identify the effect of ozonation on protein fouling. The results showed that ozonation could weaken the interaction forces between the membrane and BSA effectively, but this did not have any effect on membrane fouling. In contrast, in terms of membrane fouling behaviour after pre-ozonation, the contribution of the changes in the covalent disulfide bonds between BSA molecules outweighs those of the non-covalent bonds. The number of disulfide bonds gradually increased as the O₃:DOC ratio increased from 0 to 0.3, and began to decline when the O₃:DOC ratio was further increased to 0.45 and 0.6. This could have altered the deposition rate of foulants onto the membrane surface and the structure of the fouling layers, and may have caused the membrane fouling first to be enhanced and then to decline with increasing ozone dosages.

New mechanistic insights into the effect of cations on membrane fouling caused by anionic polyacrylamide

HYPOTHESIS

Understanding the effect of cations on membrane fouling is crucial for the widespread application of the membrane technology. However, contradictory results have been reported based on different studies. Moreover, although the effect of the ionic strength has been studied extensively, limited information is available on the effect of the ion type on membrane fouling.

EXPERIMENTS

The physicochemical properties of the membrane and anionic polyacrylamide (APAM) were evaluated to calculate the APAM-membrane and APAM-APAM interfacial interaction energies under different conditions. Moreover, a series of microfiltration (MF) experiments was conducted to investigate the effects of the ionic conditions on the flux decline, pore blockage and cake layer resistances, and the flux recovery rate of APAM during the MF process.

FINDINGS

As the ionic strength increased, the rate of decrease in the normalized flux increased, the total and cake layer resistances increased significantly, the pore blockage resistance was affected slightly, and the recovery rates of the water flux after physical and chemical cleaning decreased gradually, which could be clearly explained using the Derjaguin-Landau-Verwey-Overbeek theory. Furthermore, compared with Na⁺, Ca²⁺ could effectively mitigate the membrane fouling at an identical ionic strength, which is attributed to the hydration forces of APAM-membrane and APAM-APAM.

Stable Forward Osmosis Nanocomposite Membrane Doped with Sulfonated Graphene Oxide@Metal-Organic Frameworks for Heavy Metal Removal

A sulfonated graphene oxide@metal-organic framework-modified forward osmosis nanocomposite (SGO@UiO-66-TFN) membrane was developed to improve stability and heavy metal removal performance. An in situ growth method was applied to uniformly distribute UiO-66 nanomaterial with a frame structure on SGO nanosheets to form SGO@UiO-66 composite nanomaterial. This nanomaterial was then added to a polyamide layer using interfacial polymerization. The cross-linking between SGO@UiO-66 and m-phenylenediamine improved the stability of the nanomaterial in the membrane. Additionally, the water permeability was improved because of additional water channels introduced by SGO@UiO-66. SGO, with its lamellar structure, and UiO-66, with its frame structure, made the diffusion path of the solute more circuitous, which improved the heavy metal removal and salt rejection performances. Moreover, the hydrophilic layer of the SGO@UiO-66-TFN membrane could block contaminants and loosen the structure of the pollution layer, ensuring that the membrane maintained a high removal rate. The water flux and reverse solute flux of the SGO@UiO-66-TFN membrane reached 14.77 LMH and 2.95 gMH, and compared with the thin-film composite membrane, these values were increased by 41 and 64%, respectively. The membrane also demonstrated a good heavy metal ion removal performance. In 2 h, the heavy metal ion removal rate (2000 ppm Cu²⁺ and Pb²⁺) was greater than 99.4%, and in 10 h the removal rate was greater than 97.5%.

Impacts of Natural Organic Matter Adhesion on Irreversible Membrane Fouling during Surface Water Treatment Using Ultrafiltration

To understand impacts of organic adhesion on membrane fouling, ultrafiltration (UF) membrane fouling by dissolved natural organic matter (NOM) was investigated in the presence of background cations (Na⁺ and Ca²⁺) at typical concentrations in surface water. Moreover, NOM adhesion on the UF membrane was investigated using atomic force microscopy (AFM) with colloidal probes and a quartz crystal microbalance with dissipation monitoring (QCM-D). The results indicated that the adhesion forces at the NOM-membrane interface increased in the presence of background cations, particularly Ca²⁺, and that the amount of adhered NOM increased due to reduced electrostatic repulsion. However, the membrane permeability was almost not affected by background cations in the pore blocking-dominated phase but was aggravated to some extent in the cake filtration-governed phase. More importantly, the irreversible NOM fouling was not correlated with the amount of adhered NOM. The assumption for membrane autopsies is doubtful that retained or adsorbed organic materials are necessarily a primary cause of membrane fouling, particularly the irreversible fouling.

Quantitative analysis of cake characteristics based on SEM imaging during coagulation-ultrafiltration process

Cake formed by flocs is a crucial factor to affect membrane fouling during coagulation-ultrafiltration process. To investigate the role of floc properties on cake, cake characteristics under various coagulant dosage conditions were calculated by scanning electron microscope (SEM) imaging. Results found that one SEM image with × 5000 magnification could accurately estimate cake porosity with relative error lower than 5.00% for all conditions, whereas more SEM images with × 10,000 magnification or × 20,000 magnification should be applied to calculate cake porosity precisely. This could be explained by different pore information of SEM images with various magnifications. Compared to single SEM image with × 10,000 magnification and × 20,000 magnification, single SEM image with × 5000 magnification contained the most comprehensive pore information and slightly overestimated pore area for pore smaller than 0.4 μm² due to lower resolution. To verify feasibility by SEM image evaluating cake characteristics, cake porosity calculated by SEM image and Carman-Kozeny equation were analyzed. The results showed that cake porosity estimated by these two methods were nearly the same, proving the feasibility of this method. Moreover, with the increase of coagulant dosage, cake porosity presented similar variation with floc average size, indicating that floc average size was likely to dominate cake porosity in this study. For pore characteristics, pore average characteristic length and pore average area were in accordance with floc fractal dimension, whereas pore fractal dimension and pore amount were consistent with floc average size. This gives specific information about the relation between floc properties and cake characteristics.

New insights into the humic acid fouling mechanism of ultrafiltration membranes for different Ca2+ dosage ranges: results from micro- and macro-level analyses

To reveal the mechanisms of the influence of Ca²⁺ on membrane fouling with humic acid (HA), the adhesion forces of HA with both other HA molecules and the membrane, the HA fouling layer structure, HA fouling experiments, and the HA rejections at a wide range of Ca²⁺ dosages were investigated. The results indicated that the effect of Ca²⁺ on HA fouling can be divided into three stages. At lower ionic strength (IS) of CaCl₂, the change in electrostatic forces is the main factor in controlling HA fouling behavior; i.e., increasing Ca²⁺ dosages resulted in more serious membrane fouling. When the IS of CaCl₂ reached 10 mM, HA aggregates became the dominant factor in the fouling process, which could result in a porous fouling layer accompanied by less membrane fouling. Interestingly, much weaker membrane fouling was observed when the IS increased to 100 mM and the HA rejection began to decline. This was because a stronger hydration repulsion force was generated, which could weaken the compactness of the fouling layer and the adhesion forces of HA with both the membrane and HA, while enabling smaller-sized HA to pass more easily into the permeate, which led to less membrane fouling and a lower HA rejection.