Salt Rejection Research Articles

Nanomaterials-modified reverse osmosis membranes: a comprehensive review.

Because of its great efficiency and widespread application, reverse osmosis (RO) is a popular tool for water desalination and purification. However, traditional RO membranes have a short lifespan due to membrane fouling, deterioration, decreased salt rejection rate, and the low water flux with aging. As a result, membrane modification has received a lot of attention recently, with nanomaterials being extensively researched to improve membrane efficacy and lifespan. Herein, we present an in-depth analysis of recent advances of RO membranes modification utilizing nanomaterials. An overview of the various nanomaterials used for membrane modification, including metal oxides, zeolites, and carbon nanomaterials, is provided. The synthesis techniques and methods of integrating these nanomaterials into RO membranes are also discussed. The impacts of nanomaterial change on the performance of RO membranes are addressed. The underlying mechanisms responsible for RO membrane enhancements by nanomaterials, such as improved surface hydrophilicity, reduced membrane fouling via surface repulsion and anti-adhesion properties, and enhanced structural stability, are discussed. Furthermore, the review provides a critical analysis of the challenges and limitations associated with the use of nanomaterials to modify RO membranes. Overall, this review provides valuable insights into the modification of RO membranes with nanomaterials, providing a full grasp of the benefits, challenges, and future prospects of this challenging topic.

Enhanced desalination with polyamide thin-film membranes using ensemble ML chemometric methods and SHAP analysis.

Addressing global freshwater scarcity requires innovative technological solutions, among which desalination through thin-film composite polyamide membranes stands out. The performance of these membranes plays a vital role in desalination, necessitating advanced predictive modeling for optimization. This study harnesses machine learning (ML) algorithms, including support vector machine (SVM), neural networks (NN), linear regression (LR), and multivariate linear regression (MLR), alongside their ensemble techniques to predict and enhance average water flux (AWF) and average salt rejection (ASR) essential metrics of desalination efficiency. To ensure model interpretability and feature importance analysis, SHapley Additive exPlanations (SHAP) were employed, providing both global and local insights into feature contributions. Initially, the individual models were validated, with NN demonstrating superior performance for both AWF and ASR, achieving the lowest mean absolute error (MAE = 0.001) and root mean squared error (RMSE = 0.0111) for AWF and an MAE = 0.0107 and RMSE = 0.0982 for ASR. The accuracy of predictions improved significantly with ensemble models, as evidenced by the near-perfect Nash-Sutcliffe efficiency (NSE) values. Specifically, the NN ensemble (NN-E) and Linear Regression ensemble (LR-E) reached an MAE and RMSE of 0.001 and 0.0111, respectively, for AWF. For ASR, NN-E reduced the MAE to 0.0013 and the RMSE to 0.0089, while LR-E maintained competitive performance with an MAE of 0.0133 and an RMSE of 0.0936. SHAP analysis revealed that features such as MDP and TMC were critical drivers of performance, with MDP showing the most significant positive impact on ASR. These findings demonstrate the dominance of ensemble methods over individual algorithms in predicting key desalination parameters. The enhanced precision in estimating AWF and ASR offered by these neuro-intelligent ensembles, combined with the interpretability provided by SHAP analysis, can lead to significant environmental and operational improvements in membrane performance, optimizing resource usage and minimizing ecological impacts. This study paves the way for integrating intelligent ML ensembles and SHAP-based interpretability into the practical field of membrane technology, marking a step forward toward sustainable and efficient desalination processes.

One-step rejuvenation for prolonging running lifespan of nanofiltration membranes

Commercial nanofiltration (NF) membranes based on polyamides may experience a decline in permeation performance after prolonged operation. The short lifespan of NF membranes will lead to waste and additional carbon emissions. Thus, rejuvenating membranes and extending their lifespan seem more meaningful than investigating new materials. In this paper, polyamide NF membranes were modified with various polyphenol monomers to improve their permeation performance. The effects of different polyphenols on pore size, surface morphology, and permeation performance of the NF membranes were thoroughly investigated. After modification with tannic acid, the NF membrane exhibits improved salt rejection while experiencing an acceptable decrease in water flux. It should be noted that the commercial NF membrane element fabricated by Koch can recover its Na2SO4 rejection from 83.0% to 94.2% and demonstrate long-term stability after rejuvenation with tannic acid. Combined with the environmental friendliness of polyphenols, this straightforward modification method has the potential for prolonging the operational lifespan of industrial NF membrane products.

Reduction-enhanced water flux through layered graphene oxide (GO) membranes stabilized with H3O+ and OH- ions.

Graphene oxide (GO) is one of the most promising candidates for next generation of atomically thin membranes. Nevertheless, one of the major issues for real world application of GO membranes is their undesirable swelling in an aqueous environment. Recently, we demonstrated that generation of H3O+ and OH- ions (e.g., with an external electric field) in the interlayer gallery could impart aqueous stability to the layered GO membranes (A. Gogoi, ACS Appl. Mater. Interfaces, 2022, 14, 34946). This, however, compromises the water flux through the membrane. In this study, we report on reducing the GO nanosheets as a solution to this issue. With the reduction of the GO nanosheets, the water flux through the layered GO membrane initially increases and then decreases again beyond a certain degree of reduction. Here, two key factors are at play. Firstly, the instability of the H-bond network between water molecules and the GO nanosheets, which increases the water flux. Secondly, the pore size reduction in the interlayer gallery of the membranes, which decreases the water flux. We also observe a significant improvement in the salt rejection of the membranes, due to the dissociation of water molecules in the interlayer gallery. In particular, for the case of 10% water dissociation, the water flux through the membranes can be enhanced without altering its selectivity. This is an encouraging observation as it breaks the traditional tradeoff between water flux and salt rejection of a membrane.

Graphene oxide incorporated thin film nanocomposite nanofiltration membrane to enhance permeation and antifouling properties

Aiming at the problems of low permeation flux and low salt rejection of polyamide (PA) nanofiltration (NF) membrane, this study improved the surface properties and structure of PA layer by adding hydrophilic GO nanosheets. Polyethylimide (PEI), GO and trimethylchloride (TMC) were prepared on the surface of polyether sulfone (PES) substrate by interfacial polymerization. After adding GO, the hydrophilicity and zeta potential of NF membrane were improved. The NF test showed that when the amount of GO was 0.1%, the permeation flux of the membrane was increased from 7.06 to 12.285 L·m−2·h−1 (improve 73.94%), and the rejection of MgSO4 was increased from 90.23% to 95.9%. In the test of repulsion of univalent ion and bivalent ion, the repulsion value of bivalent ion is above 95%, and the repulsion degree of univalent ion is moderate. Compared with the original TFC membrane, the composite membrane also showed higher anti-fouling. This study provides a shortcut for improving the hydrophilicity and anti-fouling properties of NF membranes by using GO nanosheets.

Hydrolyzed polyacrylonitrile nanofibers as interlayers for ultrathin nanofiltration membranes of high permeance and salt rejection

Interfacial polymerization (IP) is one of the most common approaches to prepare polyamide (PA) nanofiltration (NF) membranes. However, these membranes usually lack permeability because of their dense PA layers created by superfast IP. A strategy to use interlayers has gained widespread popularity because it can regulate IP processes and consequently make loose and thin PA layers. Polymer interlayers are usually thick and dense, lowering the permeance of the NF membranes. Herein, we demonstrate a novel and versatile approach to prepare hydrolyzed polyacrylonitrile (HPAN) nanofiber interlayers that are thin and porous, resulting in highly permeable ultrathin PA NF membranes. HPAN nanofibers were prepared by hydrolysis of PAN and the innovative approach of fast freeze-extraction. These interlayers help to retain more piperazine (PIP) monomers by electrostatic attraction, slow down the IP rate by impeding diffusion of the PIP monomers towards the oil−water interface, and consequently create ultrathin and wrinkled PA layers. For example, ultrathin PA NF membrane (∼34 nm thickness, 13.85 cm2 membrane area) using 20 mL nanofiber dispersion as interlayer has a favorable water permeance (33.7 L m−2 h−1 bar−1) and remarkable salt rejections (98.9 % for Na2SO4 and 99.7 % for MgSO4), suggesting great potential for rapid desalination.

Quantum dot- β-Cyclodextrin nanofiller decorated thin film nanocomposite membrane for removal of cationic and anionic dyes from aqueous solution

Graphene oxide quantum dots (GOQDs) are finding tremendous attention as nanofillers in separation processes owing to their excellent chemical and mechanical robustness. GOQDs-functionalized thin film nanocomposite (TFN) membranes were fabricated via low pressure mediated interfacial polymerization (IP) over a polysulfone support, where β-Cyclodextrin (β-CD) was taken as an aqueous monomer with a fractional amount of GOQDs and tetraethylenepentamine (TEPA) as the organic monomer. The resultant TFN membranes showed improved hydrophilicity over its pristine polymeric analogue. The best performing membrane was obtained for MPG2, which had a mass loading of 3 % of β-CD, 0.3 % of TEPA and 0.4 % of GOQD during the interfacial polymerization. The membrane showed pure water flux that was two fold higher than that of the pristine polymeric membrane, with a dye rejection of 99.6 % for congo red and 96 % for methyl blue. This excellent rejection for the anionic dye can be attributed to the Donnan exclusion imparted by the negative charges on the membrane surface. The salt rejection properties of the membranes were also analyzed and the MPG2 membrane exhibited a high rejection rate of 96 % and 99.1 % for the salt containing Cl− and NO3− ions, respectively.

Surface-engineered TiO2 nanoparticles incorporated Chitosan polymer membrane for seawater desalination: Fabrication, characterization, and performance evaluation

The effect of surface coating over titanium dioxide nanoparticles (TiO2-NPs) incorporated with chitosan (TiO2-NPs/chitosan) was evaluated as a reverse osmosis membrane (RO) for enhanced performance on seawater desalination. The impact of surface coating on the chitosan membrane performance in seawater reverse osmosis (SWRO) was investigated by altering the mass of TiO2-NPs (0.25 g and 0.5 g) used for the surface coating RO membrane. TiO2-NPs were applied to the membranes using a surface coating technique and dried to create a sturdy polymer structure. The characteristic of fabricated membranes shows the function group reflects on organic compounds from /chitosan membranes polymer (–OH, -CH, C=O, C-O-C, -CH3, C-O, and NH2). In addition, TiO2-NPs are expressed in the wavenumber range of 850-500 cm-1, which characterizes the presence of Ti-O-Ti bonds. Morphological and crystal analyses of TiO2-NPs incorporated in chitosan membrane show significantly smaller pores formed because TiO2-NPs are essential in the high permeability performance under the amorphous phase structure. Also, the high performance of fabricated membranes was evaluated against water flux and salt. Adding TiO2-NPs can decrease the water flux value by 23 L m-2 h-1 and increase salt rejection by 52.94%. In optimized pH, the seawater desalination had efficient recovery.

Superhydrophobic nanofiber membranes with 3D-network reinforced pore structure for membrane distillation

Electrospun nanofiber membranes (ENMs) have garnered significant attention among researchers for membrane distillation (MD) in treating high-salinity wastewater due to their high water flux and low mass transfer resistance. However, a significant challenge in using ENMs for MD is membrane wetting, which results from the deformation of their weakly randomly overlaid nanofibers structure due to water flow shear forces. Herein, we focused on the membrane's structure and successfully prepared superhydrophobic nanofiber membranes with 3D-network reinforced pore structure by immersing PVDF ENMs in a mixed solution of polydimethylsiloxane (PDMS) and titanium dioxide nanoparticles (TNPs) after a hot-pressing treatment for impregnation and coating. The resulting membrane exhibits exceptional properties, including a high water contact angle (WCA) of 160.3°, a high liquid entry pressure (LEP) of 1.57 bar, and sustained structural integrity with an unchanged pore size distribution even after 5 h of water scouring. These characteristics contribute to a water flux of 39.3 kg·m−2·h−1 and an unexpectedly long durability of up to 102 h, along with an outstanding salt rejection exceeding 99.99 %. Moreover, the membrane demonstrates stable water flux and resistance to wetting when processing a 3.5 wt% NaCl solution containing 0.1 mM sodium dodecyl sulfate (SDS), underscoring its significant potential for MD applications.

Dewatering poly(acrylic acid-co-acrylamide) hydrogels by ammonium bicarbonate for desalination

Charged hydrogels can swell in salty water, while a portion of salt ions are repelled by the fixed charged groups in the hydrogels. This characteristic of charged hydrogels grants them with excellent potential for desalination. Attributed to their high osmotic pressure, hydrogels can absorb large amounts of water. Conversely, when a swollen hydrogel is exposed to an environment with higher osmotic pressure, it releases absorbed water, yielding comparatively dilute product water. In this study, ammonium bicarbonate (NH4HCO3), which can be regenerated and recycled by waste heat, was introduced to provide high osmotic pressure to dewater swollen poly (acrylic acid-co-acrylamide) (PAAM) hydrogels. A water recovery of nearly 48 % and a salt rejection of 58 % were obtained for 0.85 g/L NaCl solutions. Even in real well water (~1.3 g/L NaCl), the hydrogel-NH4HCO3 system recovered 43 % of water and rejected 56 % of salt. Moreover, this method proved to be chemically gentle with regards to the hydrogel composition and stability; observations showed consistent water recovery and salt rejection values across >20 cycles. The mechanisms behind the dewatering and desalination process were elucidated through the calculation of osmotic pressure and Donnan ion distribution, offering valuable insights for the future utilization of hydrogels in desalination applications.

Comparison of Anion-Exchange Membranes for Diffusion Dialysis of Mixtures of Acids and Their Iron Salts.

This study presents the possibility of using diffusion dialysis for the separation of inorganic acids (hydrochloric, nitric, and hydrofluoric) and their ferric salts whose composition corresponds to that of real spent pickling solutions. At a steady state, the transport properties of three different anion-exchange membranes (Fumasep-FAD, Neosepta-AFN, and Neosepta-AHA) are compared using a continuous counter-current dialyzer. At a constant composition of the solutions (acid concentration 3 mol L-1 and iron concentration 30-40 g L-1), the effects of volumetric liquid flow rates on the transport rate of H+ and Fe3+ ions through the membrane are studied. The dialysis process is characterized by the recovery of acids and the rejection of salts. Furthermore, the values of the dialysis coefficients of acids, iron, and the acid/iron separation factors are calculated and compared. The volumetric flow rates of the inlet streams change in limits from 3 × 10-8 to 6 × 10-8 m3 s-1 (from 3 to 6 L h-1 m-2, relative to the membrane area). A comparison of the tested membranes shows slightly better results for acid recovery, iron rejection, and acid/iron separation factors for the Fumasep-FAD membrane than for the Neosepta-AFN membrane. However, the results obtained show that both of these anion-exchange membranes can be considered good separators for tested mixtures that simulate real spent pickling solutions, and there is a good precondition for using diffusion dialysis for processing these solutions in industrial practice. On the contrary, very low values of acid recovery and the overall dialysis coefficient of acid are found for the Neosepta-AHA membrane in the test range of the volumetric flow rate, and, thus, this membrane is insufficient for the adequate separation of these acids and iron salts.

Solar Desalination with Monolithic Foams Containing a Combination of Graphite and SnSe Featured with a Hierarchical Structure and Bimodal Pores for Spontaneous Salt Rejection

The increasing interest in solar water evaporation technology is due to its potential to replace conventional desalination systems. In this study, we suggest a new solar desalination system that has a hierarchical photothermal structure and is structurally monolithic. The overall membrane is made up of a macroscale porous melamine foam frame that acts as a porous insulator and a mass transfer layer. The mass transfer layer is designed with a bimodal porous structure of melamine foam, with large pores coated with a composite of graphite and SnSe (G-SnSe) and small pores. This simple structure provides many benefits. First, it combines two photothermal mechanisms, thermal vibration in graphite and non-radiative relaxation in SnSe, to achieve synergies in the conversion of light to heat energy. Second, compared to the single materials, graphite or SnSe alone, the nanocomposite of G-SnSe has a higher effective surface area, resulting in increased solar absorption. Finally, the melamine foam structure with bimodal pores enhances seawater wicking and has a high salt rejection capacity due to its volumetric porosity bimodal configuration of up to 35%. The monolithic melamine foam with a hierarchical photothermal structure minimizes interference with water transport due to the resistance-free presence of a continuous interfacial layer.The monolithic structure evaporaters (MSE) achieved a high evaporation rate of 1.303 kg m-2h-1 under 1 sun condition due to increased capillarity by the small-scale pores in the photothermal layer. Furthermore, spontaneous salt rejection was observed during solar desalination, attributed to the presence of intrinsic large-scale pores in melamine foam for draining the concentrated salt water from the surface. The feasibility of outdoor solar desalination was established, with a high evaporation rate of 1.259 kg m-2h-1. The mechanical stability of MSE was achieved by incorporating a small amount of binder (less than 3% of total photothermal materials), maintaining efficient performance for up to 30 days without degradation. The proposed strategy in the simple monolithic structure with bimodal porosity provides a general toolkit for designing high-performance solar desalination devices due to high evaporation rate and spontaneous salt rejection capability.In summary, the solar water evaporation technology is becoming increasingly popular as a possible replacement for traditional desalination systems. We propose a novel monolithic solar desalination system with a hierarchical photothermal structure, which is composed of a macroscale porous melamine foam as a skeleton which has both roles : an insulator and a mass transfer layer. The bimodal porous structure of melamine foam with large pores coated with a composite of graphite and SnSe provides many benefits, including increased solar absorption and a high salt rejection capacity. The MSE structure achieves a high evaporation rate and spontaneous salt rejection, making it a feasible option for outdoor solar desalination. The proposed strategy provides a general toolkit for designing high-performance solar desalination devices.

Facile preparation of sodium alginate/poly(vinyl alcohol)/graphite hybrid porous hydrogel for efficient solar desalination

Porous hydrogel incorporated with the photothermal conversion materials has been the key component of the interfacial solar evaporation device. However, the preparation of high-performance porous hydrogel for the interfacial solar evaporation device still existed some limitation, like the widely available and cheap photothermal conversion material, the ease and scalability of porous structure construction in the hydrogel. In this work, a porous biomass-derived hydrogel evaporator with interconnected pore structure were developed via the simple and effective foaming-freezing-crosslinking three-step method using sodium alginate and poly(vinyl alcohol) as matrix materials and commercial graphite as the photothermal material. It has excellent photothermal conversion and water transport capabilities, with an evaporation rate of 3.15 kg m-2h−1 and an evaporation efficiency of 91 % at a solar irradiation intensity of 1 kW m−2. The evaporator also has excellent salt rejection performance due to its unique porosity. The evaporator shows good stability and long-term durability when evaporating in brine cycles. This easily prepared, inexpensive and biocompatible hydrogel evaporator has great potential in the field of solar desalination.

Comprehensive Characterization of Commercial Reverse Osmosis Membranes through High-Temperature Cross-Flow Filtration.

Developing thermally stable reverse osmosis membranes is a potential game-changer in high-temperature water treatment. In this work, the performance of three commercial reverse osmosis membranes was evaluated with a series of high-temperature filtrations. The membranes were tested with different filtration methodologies: long-term operation, cyclic tests, controlled stepwise temperature increment, and permeability tests. The morphological and physiochemical characterizations were performed to study the impact of high-temperature filtration on the membranes' chemical composition and morphological characteristics. An increase in the temperature deteriorated the membrane performance in terms of water flux and salt rejection. Flux decline at high temperatures was recognized as the primary concern for high-temperature filtrations, restricting the applications of commercial membranes for long-term operations. This research provides valuable insights for researchers aiming to thoroughly characterize reverse osmosis membranes at high temperatures.

Solar driven thermal responsive polyionic liquid/PDDA semi-IPN hydrogel for near-room temperature membrane-free osmotic desalination

The designed thermally responsive polyionic liquid (TPIL)/ poly (diallyl dimethylammonium chloride) (PDDA) hydrogel with a semi-interpenetrating (semi-IPN) structure, containing highly charged chloride anions, was fabricated to explore the comprehensive capabilities of solar driven membrane-free osmotic desalination. Introducing linear PDDA polyelectrolyte with higher charge density has effectively excluded salt ions from entering the TPIL hydrogel matrix due to the strong charge concentration gradient established between the hydrogel and the saline reservoir. Meanwhile, the thermo-sensitivity of the polyionic liquid/PDDA semi-IPN (PNCl) hydrogel enabled the release of absorbed salt-depleted water at a temperature above the Lower Critical Solution Temperature (LCST). The multiple purification process established by partial submersion of PNCl hydrogel in saline enabled continual reduction of the salt ion concentration during the upwards transportation of water along the polyelectrolyte network. Moreover, a mushroom-shaped hydrogel demonstrated enhanced the effectiveness in long-term cyclic desalination. The “mushroom cap” was loaded with photo-thermal conversion fillers (Ti3C2Tx MXene) to achieve higher photo-thermal conversion efficiency for water recovery. A record high salt rejection rate of more than 93% was obtained for membrane-free osmotic desalination. More importantly the whole desalination/water absorption/release process was solely supported by gently heating the PNCl hydrogel under the illumination of renewable solar energy.

A holistic framework for improving the prediction of reverse osmosis membrane performance using machine learning

Accurate prediction and modeling of RO membranes performance is crucial in desalination processes for proper process control and operation. Existing models do not consider all process parameters, leading to less understanding of the parameter's importance. In this study, 5 non-ensemble and 7 ensemble machine learning models were employed to predict the performance of RO membrane. Data from a modern RO desalination plant in the UAE was utilized for the models' building. Thirteen input parameters, including operational parameters, water characteristic parameters, and time-dependent parameters, were used to predict salt rejection. The results suggested that ensemble techniques are more capable of predicting the performance of RO membranes. Among ensemble methods, the XGBoost model was found to outperform other models. Recursive feature elimination was integrated with Shapley additive explanation analysis to gain insights into the most influential predictors and confirm the model's ability to comprehend the RO membrane mechanism. The findings highlighted that five parameters are critical for predicting RO membrane performance and could be prioritized for future monitoring to provide timely membrane performance warnings: the membrane's age, feed water temperature, pressure, feed water flow, and chloride concentration. It also indicated that maintaining lower temperatures, increasing feed water pressure, and increasing feed flow can improve process efficiency. The optimal XGBoost model was found to have an outstanding predictive performance with a high R2 (94.75) and a low RMSE (0.181). Ultimately, the framework proposed by this study can serve as a tool to simplify and understand complex membrane processes.

Amino acid-bonded Ti3C2Tx MXene nanofiltration membranes with superior antifouling property for enhanced water purification

MXene nanosheets have been identified as promising building blocks for high-performance nanofiltration (NF) membranes in water purification. Herein, we report a series of amino acid-bonded Ti3C2Tx MXene membranes with enhanced NF performance and antifouling properties via glycine (Gly), l-glutamic acid (L-Glu), l-lysine (L-Lys) or l-cysteine (L-Cys) modification. The physicochemical properties, separation performances and anti-fouling behaviors of the pure Ti3C2Tx MXene membrane, Gly-, L-Glu-, L-Lys- and L-Cys-bonded Ti3C2Tx MXene membranes were comparatively investigated. The stable Ti–N bonds were found between the amino acids and the MXene nanosheets and different amino acids lead to different properties of the corresponding membranes. While maintaining ultra-high rejections to various organic dyes, all the amino acid functionalized membranes exhibited enhanced water permeability (7.17–13.11 L·m−2·h−1·bar−1) compared with that of the pure MXene membrane. Importantly, the Gly-bonded membrane showed the optimal separation performance with high salt rejections of 86.28%, 81.36%, 72.88% and 66.82% to MgSO4, MgCl2, Na2SO4 and NaCl, respectively, and the membrane also exhibited superior anti-fouling property with a flux recovery rate of 80.59% to bovine serum albumin (BSA). The separation mechanism was further revealed which may offer a simple yet efficient way for enhancing 2D MXene-based NF membranes.

Preparation of non-woven supported ECTFE membrane by cast-coating method for membrane distillation

This work presents a novel preparation method, based on cast-coating, of a non-woven supported low melting point (LMP) poly (ethylene chlorotrifluoroethylene) (ECTFE) (Halar®) membrane. In particular, a LMP ECTFE dope solution was prepared via thermally induced phase separation (TIPS) method. Diethyl adipate(DEA) was used as solvent and diethylene glycol (DEG) was used as coagulation bath. Three types of non-woven material were used as support and the prepared membranes were characterized by SEM, pore size, porosity, contact angle, etc. Suitable membranes were selected and tested in pure water membrane distillation (MD) process, and also in 0.6 M NaCl solution to assess the recovery ability of the membrane. These prepared membranes were also compared with a commercial polypropylene (PP) membrane in terms of MD performance. The membranes showed a flux from 5 to 7 L/m2 h at the feed temperature of 55 °C and the salt rejection was above 99.9% when 0.6 M NaCl solution was used as feed, thus indicating them as promising candidates for the MD process.

Thermal treatment enhanced the hydrophobicity of PVDF hollow fibers membrane for brine treatment using membrane distillation

The objective of this research was to explore the impact of thermal treatment on the characteristics and performance of Poly (vinylidene fluoride) (PVDF) hollow fiber (HF) membranes utilized in direct contact membrane distillation (DCMD). To assess the effects of thermal treatment, the PVDF polymer's allotropy and crystallinity were analyzed using Fourier transform infrared ATR-FTIR and Differential scanning calorimetry (DSC). The thermal treatment predominantly affected the α-phase of the membrane, leading to increased hydrophobicity. On the other hand, Atomic Force Microscopy (AFM) characterized the membrane surface morphology. The AFM characterization revealed an increment in surface membrane roughness leading to an improvement in membrane hydrophobicity. This straightforward technique of thermal treatment resulted in improved membrane properties for DCMD application, such as increased LEP, porosity, and hydrophobicity, as well as reduced water uptake. The DCMD's efficacy was then evaluated using seawater reverse osmosis (SWRO) brine and it was observed that thermal treatment improved flux while slightly reducing salt rejection. In conclusion, these findings display the potential of thermal treatment to enhance the performance of PVDF HF membranes for DCMD applications.

Valorization of acid mine drainage into potable water and valuable minerals through membrane distillation crystallization

Owing to the rise in population growth and demands for the current living standards, mineral and water resources are depleting at the alarming rate. Also, the mineral–water-energy nexus raises a concern of high-quality water and mineral production at low costs. Membrane distillation crystallization (MDC) has emerged as a promising technology capable of simultaneous water and mineral recovery from wastewater or industrially discharged brines. Herein, the MDC was evaluated for recovery of water and precious minerals from acid mine drainage discharged from the mine tailings. High quality fresh water with 99.9 % salt rejection and permeate flux of 2.32 kg·m−2·h−1 was recovered at 52 °C feed inlet temperature. Also, feed temperatures were varied to determine their impact on mineral crystal morphology, size distribution and growth rate. Predominantly recovered minerals were gypsum, manganese fluoride, potassium iron oxide, nickel oxide hydroxide, sodium sulphate and dihydrogen sulphate. Orthorhombic crystal structure of mineral salts was attained at low feed inlet temperature (34 °C). Higher feed inlet temperatures (i.e. 41° C and 52° C) promoted co-crystallization giving rise to orthorhombic polymorph structures. Narrow crystal size distribution was reported at low water recovery factor (initial growth) for all temperatures. However, the particle size distribution broadened as the recovery factor increased. The crystal sizes ranged from 3.91 ± 0.97 µm to 14.04 ± 3.31. Remarkably, crystal growth rate increased with an increase in feed inlet temperature. These findings open a research direction towards economic recovery of mineral and water to supplement the existing shortages. With a myriad of crystals obtained from the MDC process, an additional operational unit is required to selectively recover high economic minerals.