A HOLLOW FIBER MEMBRANE EMBEDDED WITH Fe3O4 / MWCNTS-COOH NANOHYBRID FOR HEAVY METALS REMOVAL FROM CONTAMINATED WATER

Abstract

Provided herein is a Fe3O4 / MWCNTs-COOH nanohybrid incorporated into Psf Hollow Fiber membrane for As3+ and Cd2+ ions separation from contaminated water. Also provided herein a method for fabricating the Fe3O4 / MWCNTs-COOH nanohybrid incorporated Psf Hollow Fiber membrane. The membrane has the potential to remove toxic metals from aqueous solutions, making it a promising candidate for water treatment applications.

Description

Field of InventionThe present invention relates to ultrafiltration membranes for removal of toxic materials from contaminated water. Particularly, the present invention relates to a nanohybrid integrated hollow fiber membrane for the separation of heavy metal ions.Background of Invention and Prior artHeavy metal contamination is a major environmental and public health concern, primarily arising from a variety of anthropogenic activities such as industrial manufacturing, mining operations, metal smelting, electroplating, chemical production, and agriculture. These sectors often release untreated or inadequately treated wastewater containing toxic heavy metals into natural water bodies, including rivers, lakes, and groundwater sources. Commonly encountered heavy metals in contaminated waters include arsenic, cadmium, lead, mercury, chromium, nickel, copper, and zinc. These metals are known for their high toxicity, non-biodegradability, and tendency to bioaccumulate, making them persistent environmental pollutants. The U.S. Environmental Protection Agency (US EPA) has established strict limits for heavy metals in drinking water due to their harmful effects on human health and ecosystems. Exposure to even trace amounts can result in chronic toxicity, neurological disorders, organ damage, and carcinogenic effects. The persistent nature of heavy metals demands the development of effective and sustainable removal technologies to safeguard environmental and public health.Conventional treatment methods for heavy metal removal include membrane separation, electrochemical precipitation, adsorption using activated carbon, and ion exchange resins. Among these, membrane-based separation techniques have gained significant attention due to their operational efficiency, cost-effectiveness, and scalability. Polymeric membranes, in particular, offer advantages over inorganic membranes because of their ease of fabrication, tunable properties, and lower production costs. Common polymers used in membrane fabrication include polysulfone (Psf), polyether sulfone (PES), polyacrylonitrile (PAN), and cellulose acetate (CA).Despite these advantages, traditional polymeric membranes face significant limitations. Their inherent hydrophobicity often leads to low water permeability, high fouling tendencies, and poor flux recovery, reducing their long-term operational performance. Membrane processes like nanofiltration (NF) and reverse osmosis (RO) can effectively remove a wide range of heavy metals, but they typically require high transmembrane pressures and offer limited throughput, increasing energy consumption and operating costs. In contrast, ultrafiltration (UF) membranes operate under lower pressure conditions and are more energy-efficient; however, they are generally ineffective for directly removing dissolved heavy metal ions due to their larger pore sizes.Given these drawbacks, there remains a pressing need for the development of next-generation membrane materials and configurations that combine high selectivity for a broad range of heavy metals, improved hydrophilicity, enhanced permeability, antifouling characteristics, and energy-efficient operation. Advancing such membrane technologies is essential for meeting the growing demand for safe, clean water and for addressing the limitations of existing treatment systems.CN110963547A describes the preparation of a hollow fiber membrane by grinding commercially available ion exchange resin to a particle size of 300-500 mesh. The membrane was fabricated by blending 20-30% polymer material A, 5-15% polyvinylpyrrolidone, 0.1-10% of the ground ion exchange resin, and 60-70% organic solvent (by mass). The resulting membrane was tested in a module setup under a transmembrane pressure of 1 bar. This membrane was specifically designed for the removal of heavy metals such as Cr6+, Hg2+, and Pb2+ from industrial wastewater. The innovation lies in its blended composition, which integrates ion exchange resins directly into the membrane matrix, thereby enhancing adsorption capacity for heavy metals. The membrane demonstrated a high pure water flux of 521 L.m-2.h-1.bar-1 and excellent rejection rates: 99.1% for Pb2+, 98.9% for Hg2+, and 98.3% for Cr6+.IN202241012215A discloses an ultrafiltration membrane composed of a base polymer, activated charcoal, and Al₂O3 metal oxide nanoparticles. The membrane is prepared using a non-solvent induced phase separation (NIPS) method. In the process, activated carbon is first acid-treated, followed by doping with Al₂O3 nanoparticles. The resulting mixture is blended with a base polymer to form a casting solution. Polyphenylsulfone (PPSU) is used as the base polymer, and N-methyl-2-pyrrolidone (NMP) serves as the solvent. A flat sheet membrane is fabricated from this solution. The resulting membrane demonstrated a pure water flux of 112.76 L.m-2.h-1.bar-1 under dead-end filtration. The PA-3 membrane variant showed promising heavy metal removal efficiency, with >80% rejection for Pb²⁺ and >70% for Cd²⁺ ions. The integration of Al₂O3 and acid-treated activated carbon within the membrane matrix significantly enhanced its adsorption and separation performance for toxic metal ions.CN112827358B describes a polysulfone (Psf)-based hollow fiber ultrafiltration membrane designed for the selective removal of heavy metals. The membrane formulation includes PVP K30 as a pore former and polyvinyl imidazole to introduce functional groups that enhance affinity for metal ions. The membrane achieves >95% removal of Pb²⁺, Hg²⁺, and Cr⁶⁺, while allowing essential minerals (Na⁺, K⁺, Ca2+, Mg2+) to pass, with permeability ranging from 100-600 L.m-2.h-1.bar-1. Though effective, the use of functional additives and selective targeting adds complexity to the formulation.CN104587852B describes a heavy metal ion-adsorbing polysulfone (PSf) hollow fiber ultrafiltration membrane and its preparation method. The membrane is fabricated using the dry-wet spinning technique, where the spinning dope consists of polysulfone as the base polymer, PEG 1000 as the pore-forming agent, NMP as the solvent, and ion exchange resin (chitosan, ~10 μm particle size) as the functional additive. Compared to existing technologies, this membrane features a self-supporting hollow fiber structure with a high surface area, excellent water flux (200 L / m²·h·bar), and strong pressure resistance. Its enhanced heavy metal adsorption is attributed to the embedded ion exchange resin. The preparation process is straightforward, stable, and scalable for industrial production. In heavy metal adsorption tests involving wastewater contaminated with copper, lead, nickel, and chromium, the membrane achieved respective removal efficiencies of 25% (Cu2+), 22% (Pb2+), 25% (Ni2+), and 20% (Cr⁶⁺), demonstrating its potential for selective heavy metal removal.CN109692577A discloses a hollow fiber ultrafiltration membrane was fabricated using polysulfone (PSf) as the base polymer, PVP K15 as the pore-forming agent, and DMF as the solvent. For surface modification, a crosslinking solution was prepared containing 2.4% polyethyleneimine (PEI, molecular weight 70,000), 0.6% polyvinylpyrrolidone, 0.024% glutaraldehyde, and 0.006% potassium peroxydisulfate, all dissolved in deionized water and mixed thoroughly. The prepared hollow fiber membranes were immersed in the crosslinking solution for 2 hours, then removed and heat-treated at 80 °C for 1 hour to initiate crosslinking reactions on the membrane surface. This process resulted in the formation of a hydrogel layer capable of adsorbing heavy metal ions. After rinsing and drying, the modified membranes exhibited improved functionality for copper ion removal. The final membrane demonstrated a pure water flux of 64.8 L / m²·h·bar and achieved a Cu2+ rejection rate of 83%.CN117000045B discloses a loose nanofiltration membrane for heavy metal removal and its preparation method, falling under polymer-based membrane technology. The process starts by dispersing graphene oxide into a lignin-tetrahydrofuran solution to form lignin-graphene oxide composite nanospheres (LGNs). At the same time, styrene maleic anhydride, additives, and polymer materials are dissolved in an organic solvent to produce a homogeneous casting solution. This solution is used to fabricate a surface anhydride ultrafiltration base membrane via phase inversion. To convert this into a nanofiltration membrane, a fixed amount of LGNs is dispersed in a polyethyleneimine aqueous solution, which is then applied to the surface of the base membrane. Crosslinking is carried out for 2 hours to form the active separation layer. The final membrane shows excellent removal performance for heavy metals such as cadmium (Cd), mercury (Hg), lead (Pb), copper (Cu), zinc (Zn), chromium (Cr), and arsenic (As), while maintaining good water permeability.CN110917897B presents a composite nanofiltration membrane and its preparation method, designed for efficient heavy metal removal. The membrane consists of a PVDF-based ultrafiltration flat sheet base, modified with a surface active layer formed from chitosan quaternary ammonium salt, β-cyclodextrin, and a cross-linking agent. The preparation involves two steps: (1) treating the PVDF ultrafiltration membrane with an aqueous solution containing β-cyclodextrin at varying concentrations, and (2) forming a surface layer through crosslinking with chitosan derivatives. This surface modification enhances the membrane's selectivity and adsorption properties. In performance testing, the composite nanofiltration membrane demonstrated a zinc ion (Zn²⁺) rejection rate of 87%, indicating strong heavy metal removal capabilities. Despite the high rejection, the membrane maintained moderate permeability, achieving a pure water flux of 15 L.m-2.h-1.bar-1. The combination of β-cyclodextrin's cavity structure and chitosan's functional groups contributes to improved metal ion interaction, making this membrane suitable for water purification applications.US11420163B2 discloses a composite nanofiltration membrane and its preparation method aimed at effective heavy metal removal. The membrane consists of an ultrafiltration base layer modified with a surface-active layer. This surface layer is formed from the reaction product of chitosan quaternary ammonium salt, β-cyclodextrin, and a cross-linking agent. The preparation process involves two main steps: (1) treating the ultrafiltration membrane with a surface-active layer solution, which includes chitosan quaternary ammonium salt, optionally β-cyclodextrin, additives, surfactants, and catalysts dissolved in water; and (2) applying a cross-linking agent solution-prepared in water, an organic solvent, or a mixture of both-to initiate crosslinking and form the final surface layer. This functional layer enhances the membrane's selectivity and adsorption capability. Performance evaluation revealed a copper ion (Cu2+) rejection rate of 97%, with a water permeability of 12 L.m-2.h-1.bar-1, indicating strong heavy metal removal with moderate flux performance.WO2018129859A1 describes a chelating microfiltration membrane, along with its preparation, regeneration method, and application for heavy metal removal from wastewater. The membrane is fabricated using a thermally induced phase separation (TIPS) technique, starting from a polyvinylidene fluoride (PVDF) casting solution to form a flat sheet membrane. To enhance its functionality, the PVDF membrane undergoes defluorination using a basic potassium permanganate solution, followed by grafting with glycidyl methacrylate (GMA) to lengthen the carbon chain. Subsequently, a nucleophilic substitution reaction between melamine and GMA introduces chelating sites onto the membrane surface. This modification enables the membrane to effectively capture and retain heavy metal ions. The resulting chelating microfiltration membrane demonstrates a high pure water flux of 398 L / m²·h·bar. In heavy metal rejection studies, it achieved removal efficiencies of Cu2+ (100%), Cd2+ (98.6%), Zn2+ (99.8%), and Pb2+ (97.6%), highlighting its excellent potential for wastewater treatment applications.CN101961648A discloses a membrane adsorbent developed for heavy metal ion removal, using polyethersulfone (PES) as the base polymer and incorporating macroporous strong acid cation exchange resin powder (~10 μm particle size) as the functional additive. The membrane is fabricated via phase inversion using DMF as the solvent. It demonstrates high permeability, with a pure water flux of 250 L / m²·h·bar, and is effective in removing divalent heavy metals such as Pb2+, Cu2+, and Hg2+. This design integrates adsorption and filtration into a single membrane-based process.CN103203188B discloses a polyvinyl chloride (PVC)-based ultrafiltration membrane designed for the removal of heavy metal ions from water. The membrane comprises a blend of PVC, an EVA-VOH copolymer to enhance flexibility and hydrophilicity, and nano-hydroxyapatite, a biocompatible additive known for its strong metal-binding properties. Fabricated via phase inversion, the membrane features a porous structure with robust mechanical strength. In performance evaluation, it achieved a high pure water flux of 630 L / m²·h·bar, along with notable heavy metal rejection rates-90% for Cu²⁺ and 80% for Pb²⁺-demonstrating both high throughput and effective metal ion removal.US10556222B2 presents a membrane filtration technology using thiol-functionalized polysaccharide nanofibers for the removal of heavy metal ions. These nanofibers (3-50 nm diameter, 100-5000 nm length) are produced from polysaccharides such as cellulose or chitin via oxidative pretreatment (e.g., TEMPO-mediated oxidation), followed by functionalization with thiol-containing compounds (e.g., cysteine). The membranes demonstrated high rejection rates-97% for Pb²⁺ and 96.3% for Cr⁶⁺-and a high pure water permeability of 206 L.m-2.h-1.bar-1. However, the preparation process is multi-step and chemically intensive, requiring surface oxidation and post-synthesis thiolation using catalysts like EDC / NHS, which adds complexity and cost.Jie Gao, Shi-Peng Sun, Wen-Ping Zhu and Tai-Shung Chung. "Green- modification of outer selective P84 nanofiltration (NF) hollow fiber membranes for cadmium removal." (https: / / doi.org / 10.1016 / j.memsci.2015.10.051) presents a green fabrication method for outer-selective thin-film composite (TFC) hollow fiber membranes using a P84 polyimide substrate. By sequentially modifying the surface with polyethyleneimine (PEI) and water-soluble crosslinkers such as glutaraldehyde (GA) and epichlorohydrin (ECH), the researchers eliminate the need for traditional interfacial polymerization involving toxic alkanes. These modifications reduce pore size and adjust surface charge, enabling NaCl rejection >90%, Cd²⁺ rejection of 94%, and a pure water permeability (PWP) of 1.74 ± 0.01 L.m-2.h-1.bar-1. While environmentally friendly, this approach involves multi-step chemical modifications that add complexity and may limit large-scale scalability.Wen-Ping Zhu, Shi-Peng Sun, Jie Gao, Feng-Jiang Fu, Tai-Shung Chung. "Dual-layer polybenzimidazole / polyethersulfone (PBI / PES) nanofiltration (NF) hollow fiber membranes for heavy metals removal from wastewater." (https: / / doi.org / 10.1016 / j.memsci.2014.01.001) presents a dual-layer nanofiltration hollow fiber membrane fabricated through simultaneous co-extrusion of polybenzimidazole (PBI) as the selective layer and polyethersulfone (PES) / polyvinylpyrrolidone (PVP) as the support. PBI imparts chemical resistance and favorable charge properties, while the PES / PVP layer ensures mechanical strength and porosity. The resulting membrane demonstrates Cd²⁺ rejection of 95%. However, its pure water permeability is low at 0.829 L / m²·h·bar, limiting throughput and operational efficiency.Wen-Ping Zhu, Jie Gao, Shi-Peng Sun, Sui Zhang, Tai-Shung Chung. "Poly(amidoamine)-dendrimer (PAMAM) grafted on thin film composite (TFC) nanofiltration (NF) hollow fiber membranes for heavy metal removal." (https: / / doi.org / 10.1016 / j.memsci.2015.03.033) introduces a PAMAM dendrimer-grafted thin-film composite (TFC) nanofiltration (NF) membrane, wherein PAMAM is covalently bonded to the interfacial polymerization layer on a polyethersulfone (PES) support. This functionalization imparts positively charged amino groups that enhance metal ion affinity while slightly reducing pore size to increase selectivity. The modified membrane exhibits high rejection (99%) for heavy metals such as Cd2+ and As3+ and achieves a pure water permeability of 3.6 L.m-2.h-1.bar-1at 10 bar. However, the process involves multiple chemical grafting steps and interfacial polymerization, which add complexity and increase fabrication cost.Mrinmoy Mondal, Madhurima Dutta, Sirshendu De. "A novel ultrafiltration grade nickel iron oxide doped hollow fiber mixed matrix membrane: Spinning, characterization and application in heavy metal removal." (https: / / doi.org / 10.1016 / j.seppur.2017.07.013) discloses nickel iron oxide (NFO) nanoparticles were incorporated into hollow fiber mixed matrix membranes to improve permeability, porosity, hydrophilicity, and heavy metal adsorption. The membrane with 3 wt% NFO exhibited a twofold increase in permeability, reducing the contact angle from 77° to 64° and increasing MWCO from 17 kDa to 34 kDa. Its maximum adsorption capacity for Cd²⁺ was 24 mg / g, with corresponding removal efficiency of ~71%, and a pure water permeability of 43.5 L / m²·h·barYoussef-Amine Boussouga, Malini Bangalore Mohankumar, Akhil Gopalakrishnan, Alexander Welle, Andrea I. Schafer. "Removal of arsenic(III) via nanofiltration: contribution of organic matter interactions." (https: / / doi.org / 10.1016 / j.watres.2021.117315) provides nanofiltration (NF) membranes (NF270 and NF90) were evaluated for As(III) removal, with results showing that retention was strongly pH-dependent, increasing significantly between pH 9 and 12 due to the deprotonation of As(III) and enhanced Donnan exclusion. Moreover, As(III) retention was improved by up to 40% only after the addition of high concentrations (100 mgC / L) of humic acid (HA), which enabled complexation-driven rejection mechanisms. However, such enhancements required feed pre-treatment and elevated operating pH, limiting practical applicability.Junwen Lv, Kai Yu Wang, Tai-Shung Chung. "Investigation of amphoteric polybenzimidazole (PBI) nanofiltration hollow fiber membrane for both cation and anions removal. (https: / / doi.org / 10.1016 / j.memsci.2007.11.050)" reports on a lab-developed amphoteric PBI nanofiltration (NF) hollow fiber membrane designed for dual cation and anion removal, including arsenite (As3+), phosphate, and copper ions. While this membrane demonstrated moderate As3+ rejection of 66% and a pure water permeability (PWP) of 1.86 L.m-2.h-1.bar-1, its performance was highly dependent on feed solution pH, ion speciation, and concentration, owing to the isoelectric point of PBI (~pH 7) which alters membrane charge properties under different conditions.The widespread contamination of water bodies with toxic heavy metals poses a critical challenge to environmental sustainability and public health. There remains a need to develop membrane technologies for heavy metal removal which overcome the limitations such as poor hydrophilicity, low water permeability, high fouling tendencies, and insufficient selectivity toward a broad range of metal ions.Object of the InventionIt is an object of the present invention to synthesize a nanohybrid (Fe3O4 / MWCNTs-COOH) with enhanced dispersibility and adsorption properties. It is another object of the present invention to integrate this nanohybrid uniformly within a hollow fiber membrane matrix.It is a further object of the present invention to improve the membrane's hydrophilicity, permeability, and antifouling performance.It is yet another object to enable efficient and selective removal of multiple heavy metal ions from water. It is an object to provide a scalable, cost-effective solution for advanced water purification. Summary of the InventionIn an aspect of the present invention, there is provided a membrane for removal of toxic metal ions from contaminated water sources, comprising a Fe3O4 / MWCNTs-COOH nanohybrid incorporated in a hollow fiber membrane.In another aspect of the present invention, there is provided a method of preparing a membrane for removal of toxic metal ions from contaminated water sources, the method comprising the steps of:a. preparing a Fe3O4 / MWCNTs-COOH nanohybrid;b. dispersing the Fe3O4 / MWCNTs-COOH nanohybrid in a solvent to obtain a dispersion;c. adding Polysulfone to the dispersion of step (b) to obtain a casting solution; andd. fabricating the casting solution of step (c) into the membrane.In a further aspect of the present invention, there is provided membrane obtained by the method as described herein, comprising Fe3O4 / MWCNTs-COOH nanohybrid integrated in the Psf hollow fiber membrane.Brief Description of Accompanying DrawingsFig. 1 illustrates a schematic preparation of Fe3O4 / MWCNTs-COOH nanohybrid (Mahdavi et al., 2013a, 2013b)Fig. 2 illustrates Characterization of Fe3O4 / MWCNTs-COOH Nanohybrid: (A) FTIR spectra confirming the incorporation of Fe3O4 (449 cm⁻¹, 619 cm⁻¹) and MWCNTs-COOH (1744 cm⁻¹, 1625 cm⁻¹) in the nanohybrid. (B) XRD patterns showing peaks at 25.8° and 42.87° for MWCNT-COOH, and at 30.17°, 35.28°, 53.54°, and 63.06° for Fe3O4, indicating successful hybrid formation. (C) Raman spectra highlighting key peaks from MWCNT-COOH (1578 cm⁻¹, 1345 cm⁻¹, 2692 cm⁻¹) and Fe3O4 (684 cm⁻¹), further confirming the integration of both components. (D) TEM image of Fe3O4, (E) MWCNTs-COOH TEM image, (F) Fe3O4 / MWCNTs-COOH nanohybrid, showing successful attachment of Fe3O4nanoparticles. (G) SEM image shows Fe3O4nanoparticles. (H) SEM image depicting MWCNTs-COOH. (I) SEM image confirming the formation of Fe3O4 / MWCNTs-COOH nanohybrid.Fig. 3. illustrates SEM images depict various HFM samples (P, PFC-25, PFC-50), showcasing their overall structures (Figure A, D and G), cross-sectional view (Figure B, E and F), and inner pore (Figure C, F and I) morphology. The images reveal concentrically formed membranes with distinctive finger-like structures. Furthermore, an increase in nanohybrid concentration is correlated with enhanced porosity, as evidenced by the inner pore structures. In addition, (K) illustrates the mixed elements within the cross section of the PFC-50 membrane, while (J) provides a detailed representation of individual elemental mapping and elements present in the same cross section. (L) presents a comprehensive table detailing the weight and atomic percentages of each element within the cross section. In nutshell, the presence of Fe in the cross section confirms the successful incorporation of the Fe3O4 / MWCNTs-COOH nanohybrid in the HFMs.Fig. 4 illustrates (A) ATR-FTIR spectra of plain and modified membranes, revealing peaks at 457 and 559 cm-1 corresponding to the Fe-O bond in the Fe3O4 nanoparticles. (B) Water contact angle measurements of various hollow fiber membranes indicated an increase in hydrophilicity with the addition of the nanohybrid. (C) Pore size distribution showing mean pore sizes (dashed line) of 10.83 nm (P), 13.2 nm (PFC-25), and 16.48 nm (PFC-50), with mode pore sizes (dotted line) of 10.83 nm, 13.97 nm, and 14 nm, respectively. The differences between the mean and mode values are attributed to the distribution tail. (D) Zeta potential measurements demonstrated an increase with the addition of the Fe3O4 / MWCNTs-COOH nanohybrid at both pH 7 and 11.Fig. 5 illustrates pure water permeability and antifouling of HFMs: Bar charts represent the (A) The initial (without fouling) and (with fouling) PWP, it shows addition of Fe3O4 / MWCNTs-COOH nanohybrid significantly increased membrane pure water permeability. PFC-50 exhibited nearly 5 times higher pure water permeability (PWP) compared to the pristine membrane, while PFC-25 showed around 2 times higher PWP. (B) Shows the addition of nanohybrid improved the antifouling property of the membranes. The FRR values increased with the nanohybrid addition, reaching 82.50% for PFC-50, demonstrating enhanced antifouling characteristics compared to the pristine membrane (51.6%).Fig. 6 illustrates adsorption isotherm for PFC-50 HFMs containing nanohybrid (A) Adsorption isotherm for Cd2+ and (B) Adsorption isotherm for As3+. The maximum adsorption capacity was observed for Cd2+ at 85.06 ± 7.12 mg / g, followed by As3+ at 54 ± 4.13 mg / g.Fig. 7 illustrates removal performance of PFC-50 membrane in the presence of Na+, Cu2+, Pb2+, As3+, Cd2+, and a mixture of all ions. Slight reduction in the removal of As3+ and Cd2+ ions is seen in the presence of Na+ ion. This reduction further reduces with an increase in the valency of other ions. This observation suggests that the presence of other metal ions may lead to a competition for active sites on the membrane, potentially leaving fewer active sites for the interaction of As3+ and Cd2+ ions.Fig. 8 illustrates (A) and (B) the long-term study of Cd²⁺ and As³⁺ ions in lab water. After 23 hours for Cd²⁺ and 26 hours for As³⁺, the removal rate begins to decrease rapidly, indicating that all active sites on the membrane are occupied by adsorbed ions. (C) and (D) the reusability of the PFC-50 membrane over 5 cycles for spiked lab water and lake water, respectively. After the 5th cycle, the removal values for Cd²⁺ were 84.12 ± 1.48% (lab-made water) and 55.44 ± 2.93% (lake water). For As³⁺, the removal values were 56 ± 5.48% (lab-made water) and 44.54 ± 0.56% (lake water).Fig 9 illustrates schematic representation of the present inventionDetailed description of the InventionThe present invention provides a hollow fiber ultrafiltration membrane that incorporates a Fe3O4 / MWCNTs-COOH nanohybrid. This nanohybrid enhances nanoparticle dispersion, increases membrane hydrophilicity, and improves adsorption capacity for diverse heavy metals. The invention specifically targets the efficient removal of toxic metal ions-including but not limited to arsenic (As³⁺), cadmium (Cd²⁺), lead (Pb²⁺), nickel (Ni²⁺), and copper (Cu²⁺)-from contaminated water sources.The present invention overcomes the drawbacks associated with incorporation of promising nanomaterials like Fe3O4 into polymer matrices, that is hindered by nanoparticle agglomeration, which compromises membrane performance and uniformity.This invention addresses a critical technological gap and offers a practical, high-performance approach for mitigating heavy metal pollution in water treatment applications.The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the present disclosure as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the various embodiments described herein can be made without departing from the scope and spirit of the present disclosure. In addition, descriptions of well-known functions and constructions may be omitted for clarity and conciseness.It is to be understood that the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a component surface" includes reference to one or more of such component surfaces.The terminology used herein is for the purpose of describing particular various embodiments only and is not intended to be limiting of various embodiments. It will be further understood that the terms "comprises" and / or "comprising" used herein specify the presence of stated features, integers, steps, operations, members, components, and / or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, members, components, and / or groups thereof. Also, expressions such as "at least one of," when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list.Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The system, methods, and examples provided herein are illustrative only and not intended to be limiting.The present invention provides a membrane for removal of toxic metal ions from contaminated water sources, the membrane comprising a Fe3O4 / MWCNTs-COOH nanohybrid incorporated in a hollow fiber membrane.Hollow fiber membranes (HFMs) can be fabricated from various polymeric materials. In one embodiment, the hollow fiber membrane is made of a polymer material selected from Polysulfone (Psf), polyethersulfone (PES), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyetherimide (PEI), polycaprolactone (PCL), and polylactic acid (PLA). In a preferred embodiment, the polymer material is Polysulfone.In one embodiment, the nanohybrid Fe3O4 / MWCNTs-COOH has an initial concentration in the range of 0-5 wt% with respect to a polymer solution for making the membrane. In one embodiment, the nanohybrid Fe3O4 / MWCNTs-COOH has an initial concentration selected from 0.25 wt% or 0.50 wt% with respect to a polymer solution for making the membrane. In a preferred embodiment, the nanohybrid Fe3O4 / MWCNTs-COOH has an initial concentration of 0.5 wt% with respect to a polymer solution for making the membrane. In one embodiment, the polymer material of the membrane is polysulfone. In one embodiment, the polysulfone has an initial concentration in the range of 10 to 25 wt% with respect to the solution for making the membrane. In a preferred embodiment, the polysulfone has an initial concentration of 18 wt% with respect to the solution for making the membrane. In a preferred embodiment, the Fe3O4 / MWCNTs-COOH nanohybrid is incorporated into Psf HFM membrane for As3+ and Cd2+ ions separation. A pure water permeability (PWP) and antifouling test showed that with an increasing concentration of Fe3O4 / MWCNTs-COOH nanohybrid, the PWP also increased. Overall, the incorporation of the nanohybrid in the membrane significantly changed its physicochemical properties. The PFC-50 membrane exhibited excellent permeation (64.42 ± 7.12 L / (m2.h.bar)) and excellent antifouling properties (flux recovery 82.50 %). In addition, the adsorption capacity of the modified membrane is investigated for As3+ and Cd2+, which were found to be 54 ± 4.13 mg / g and 85.06 ± 7.12 mg / g, respectively. A significantly high removal of Cd2+ (99.19 ± 0.25 % for lab and 79.51 ± 4.21 % for Lake water) and As3+ (76.89 ± 1.056 % for lab and 64.14 ± 4.58 % for Lake water) was observed. Furthermore, a long-term study revealed that the adsorption sites became saturated after 23 hours for Cd2+ ions and 26 hours for As3+ ions. The reusability study, conducted over five cycles, indicated that the membrane can be easily revived and used for further studies. In addition, we conducted a comparative analysis with relevant literature, revealing that the PFC-50 membrane exhibits superior removal efficiency and a more favorable trade-off between permeability and removal than other reported membranes. Based on the results, it can be that the modified membrane has the potential to remove toxic metals from aqueous solutions, making it a promising candidate for water treatment applications. The present invention also provides a method of preparing the membrane for removal of toxic metal ions from contaminated water sources. The method comprises the steps of:a. preparing a Fe3O4 / MWCNTs-COOH nanohybrid;b. dispersing the Fe3O4 / MWCNTs-COOH nanohybrid in a solvent to obtain a dispersion;c. adding Polysulfone to the dispersion of step (b) to obtain a casting solution;d. fabricating the casting solution of step (c) into the membrane.In one embodiment of the method, in step (a) Fe3O4 / MWCNTs-COOH nanohybrid is prepared by dissolving ferric chloride hexahydrate (FeCl3·6H₂O) in DI water followed by the addition of MWCNTs-COOH. Separately, ferrous chloride tetrahydrate (FeCl₂·4H₂O) is added to the mixture. The pH is gradually adjusted to promoting the co-precipitation of Fe²⁺ and Fe³⁺ as Fe(OH)₂ and Fe(OH)3, which subsequently formed Fe3O4 nanoparticles. The resulting nanoparticles are washed alternately with ethanol and water to remove impurities and dried in an oven at for 24 hours to form the Fe3O4 / MWCNTs-COOH nanohybrid.In one embodiment of the method, in step (b) the solvent can be selected from N-methyl-2-pyrrolidone (NMP), N,N-Dimethylformamide (DMF), N,N-Dimethylacetamide (DMAc), Dimethyl sulfoxide (DMSO), Tetrahydrofuran (THF). In a preferred embodiment, the solvent is NMP.In one embodiment of the method, in step (b) Fe3O4 / MWCNTs-COOH nanohybrid is dispersed in the solvent in a concentration in the range of 0-5 wt%. In one embodiment of the method, in step (b) Fe3O4 / MWCNTs-COOH nanohybrid is dispersed in the solvent in a concentration selected from 0.25 wt% or 0.50 wt%. In a preferred embodiment, 0.5 wt% of Fe3O4 / MWCNTs-COOH nanohybrid is dispersed in the solvent. The Fe3O4 / MWCNTs-COOH nanohybrid dispersed in the solvent is sonicated to obtain a uniform dispersion.In one embodiment of the method, in step (c) Polysulfone is added to the uniform dispersion of Fe3O4 / MWCNTs-COOH nanohybrid in the solvent. In one embodiment, 10 to 25 wt% of Polysulfone is added to the uniform dispersion of Fe3O4 / MWCNTs-COOH nanohybrid in the solvent. In a preferred embodiment, 18 wt% of Polysulfone is added to the uniform dispersion of Fe3O4 / MWCNTs-COOH nanohybrid in the solvent.The mixture was stirred for 24 hours to obtain a homogeneous casting solution. In one embodiment, the homogeneous casting solution was degassed to remove air bubbles.In one embodiment of the method, in step (d) the casting solution is fabricated into the membrane. In the membrane fabrication process, deionized (DI) water is used as the bore fluid and coagulant. The casting solution and bore fluid flow rates are controlled within the range of 0-20 mL / min, while the air gap distance is maintained between 0 and 50 cm. Both the coagulation and rinsing baths consisted of DI water, with temperatures maintained between 0 and 40 °C. After spinning, the HFMs are immersed in DI water for 24 hours to remove residual solvent and then stored in 10-50% glycerol to prevent pore collapse and maintain membrane integrity.In one embodiment, there is provided a membrane for removal of toxic metal ions from contaminated water sources comprising Fe3O4 / MWCNTs-COOH nanohybrid integrated in the Psf hollow fiber membrane.In a preferred embodiment, there is provided a membrane for removal of toxic metal ions from contaminated water sources comprising Fe3O4 / MWCNTs-COOH nanohybrid integrated in the Psf hollow fiber membrane, such that the Fe3O4 / MWCNTs-COOH nanohybrid has an initial concentration of 0.5 wt% with respect to the solution making the membrane.The present invention introduces an advanced approach to heavy metal removal by developing hollow fiber ultrafiltration membranes integrated with a Fe3O4 / MWCNTs-COOH nanohybrid. This innovative design delivers multiple technical advantages over conventional membrane systems, as detailed below:1. First-of-its-kind membrane composition: This is the first reported instance of incorporating a Fe3O4 / MWCNTs-COOH nanohybrid into a hollow fiber membrane matrix. This unique integration synergistically combines the magnetic, adsorptive, and hydrophilic properties of Fe3O4 and MWCNTs-COOH to significantly enhance membrane performance2.Substantial improvement in permeabilityThe modified membranes exhibited a five-fold increase in pure water permeability (PWP), achieving 64.42 ± 7.12 L.m-2.h-1.bar-1) compared to only 12.18 ± 0.47 L.m-2.h-1.bar-1) for unmodified membranes. This improvement enables higher water throughput and lower energy consumption.3. Superior antifouling performance The nanohybrid-modified membranes demonstrated excellent antifouling properties, with a flux recovery ratio of 82.50%, significantly higher than the 51.6% observed for pristine membranes. This leads to reduced cleaning frequency and longer operational lifespan.4. High heavy metal removal efficiency:- Cd²⁺ removal: 99.19 ± 0.25% in laboratory water and 79.51 ± 4.21% in real lake water.- As³⁺ removal: 76.89 ± 1.06% in laboratory water and 64.14 ± 4.58% in lake water. These results highlight the membrane's effectiveness even in complex, real-world water matrices.5. Demonstrated long-term adsorption capacity: Extended operation studies revealed that the membrane maintained adsorption capability for up to 23 hours for Cd2+ and 26 hours for As3+ before active sites reached saturation-indicating strong binding affinity and extended use potential.6. Good reusability and stability: The membranes exhibited strong regeneration potential, maintaining performance over five reuse cycles. This indicates the membrane's robustness and suitability for sustainable, long-term application in water treatment systems.The present invention is now being illustrated by way of non-limiting examples. The examples are intended to be purely exemplary of the invention and should, therefore, not be considered to limit the invention in any way. Efforts have been made to ensure accuracy with respect to the numbers used, but some experimental errors and deviations should be accounted for.ExamplesSynthesis of Fe3O4 / MWCNTs-COOHAll chemicals and reagents used for the synthesis of Fe3O4 / MWCNTs-COOH nanohybrids were obtained from Sigma-Aldrich (USA), and all solutions were prepared using deionized (DI) water. A schematic of the synthesis process is shown in Fig. 1. Briefly, 5.35 g of ferric chloride hexahydrate (FeCl3·6H₂O, 46 mmol) was dissolved in 300 mL of DI water and stirred for 15 minutes, followed by the addition of 0.5 g of MWCNTs-COOH. Separately, 1.968 g of ferrous chloride tetrahydrate (FeCl₂·4H₂O, 23 mmol) was sonicated and then added to the mixture. The solution was left to mix for 1 hour. The pH was gradually adjusted to 10 using 1 M NaOH, promoting the co-precipitation of Fe²⁺ and Fe³⁺ as Fe(OH)₂ and Fe(OH)3, which subsequently formed Fe3O4 nanoparticles. The resulting nanoparticles were washed alternately with ethanol and water to remove impurities and dried in an oven at 80 °C for 24 hours, as described by Mahdavi et al. (2013)Physiochemical characteristic of Fe3O4 / MWCNTs-COOH nanohybridTo confirm and validate the formation of the Fe3O4 nanohybrid, Fe3O4 / MWCNTs-COOH, and the incorporation of Fe3O4 / MWCNTs-COOH into the hollow fiber membrane, various characterization techniques were employed. These included Fourier Transmission Electron Microscopy (FTIR), X-ray Diffraction (XRD), Raman spectroscopy, Transmission Electron Microscopy (TEM), and Scanning Electron Microscopy (SEM).For TEM analysis, a sample was prepared by dispersing a small quantity of the sample in ethanol and sonicating it in an ultrasonic bath (Bransonic, CPX2800-E, USA) for 1 hour. Then, 1-2 drops of the prepared sample were placed on a TEM grid and left to dry for 1 hour at room temperature. Fourier Transform Infrared Spectroscopy (ATR-FTIR) was performed to identify the various functional groups in the nanoparticles, while XRD analysis was conducted to analyze the crystalline and amorphous structure of the nanoparticles.Fig. 2-A presented the FTIR spectra of Fe3O4 / MWCNTs-COOH, Fe3O4, and MWCNTs-COOH. In the Fe3O4spectrum, the bands at 449 cm⁻¹ and 619 cm⁻¹ corresponded to the Fe-O bond (Ghasemy-Piranloo et al., 2022; Sadegh et al., 2018). The band around 1630 cm⁻¹ was attributed to the bending vibrational modes of H-O-H from water molecules. In the MWCNTs-COOH spectrum, the peak at 1744 cm⁻¹ was associated with the C=O stretch mode in carboxylic acid groups, while the peak at 1625 cm⁻¹ was due to backbone vibrations (Bhatia et al., 2019; Chen et al., 2015; Zhang et al., 2018). The Fe3O4 / MWCNTs-COOH spectrum exhibited characteristic peaks of both Fe3O4 (449 cm⁻¹ and 619 cm⁻¹) and MWCNTs-COOH (1744 cm⁻¹ and 1625 cm⁻¹), confirming the successful synthesis of Fe3O4 / MWCNTs-COOH. Additionally, in all spectra, peaks at 2920 cm⁻¹ and 2948 cm⁻¹ indicated the presence of C-H groups, while the broad band at 3417 cm⁻¹ corresponded to O-H stretching.The XRD patterns of MWCNTs-COOH, Fe3O4, and the Fe3O4 / MWCNTs-COOH nanohybrid were shown in Fig. 2-B. The characteristic peaks of MWCNTs-COOH were observed at 25.8° and 42.87°, corresponding to the (002) and (100) planes, respectively (Abdel-Ghani et al., 2015; Nie et al., 2015). For Fe3O4, peaks were observed at 30.17°, 35.28°, 43.22°, 53.54°, and 63.06°, corresponding to the (220), (311), (400), (422), and (511) planes, respectively (Liu et al., 2019; Suwattanamala et al., 2017). These characteristic peaks, with slight intensity variations, were also found in the XRD pattern of the Fe3O4 / MWCNTs-COOH nanohybrid, indicating the successful incorporation of both MWCNTs-COOH and Fe3O4in the hybrid material.The Raman spectra shown in Fig. 2-C provided strong evidence for the successful formation of the Fe3O4 / MWCNTs-COOH nanohybrid, as the characteristic peaks of both Fe3O4and MWCNTs-COOH were clearly present in the nanohybrid spectrum. In the spectrum of pure MWCNTs-COOH, strong peaks were observed at 1578 cm⁻¹ (G mode) and 1345 cm⁻¹ (D mode), along with a secondary Raman signal at 2692 cm⁻¹ (G' mode) (Patole and Lubineau, 2015; Zhou et al., 2016) . The Raman spectrum of Fe3O4 displayed a prominent peak at 684 cm⁻¹, with additional peaks at 221 cm⁻¹ and 286 cm⁻¹, likely resulting from oxidation during the Raman experiment (Yew et al., 2017; Zhou et al., 2016). The ID / IG ratio, which reflected the degree of functionalization in MWCNTs-COOH (Zhou et al., 2016) , was approximately 1.08 for pure MWCNTs-COOH and 1.07 for the Fe3O4 / MWCNTs-COOH nanohybrid. These ratios indicated that MWCNTs-COOH in its pure form was slightly more defective than in the nanohybrid structure.The morphologies of Fe3O4, MWCNTs-COOH, and the Fe3O4 / MWCNTs-COOH nanohybrid were observed through TEM, as depicted in Fig. 2 (D-F). In Fig. 2-E, the dark spots on MWCNT-COOH are likely due to the functionalization process, indicating defects or residual impurities from carboxylation. (Van Trinh et al., 2017). Fig. 2-F shows a cluster of Fe3O4nanoparticles intricately entwined within the network of MWCNTs-COOH, forming a distinctive nanohybrid structure. Additionally, SEM images in Fig. 2 (G-I) illustrate Fe3O4, MWCNTs-COOH, and the Fe3O4 / MWCNTs-COOH nanohybrid, respectively.Fabrication of hollow fiber membraneFe3O4 / MWCNTs-COOH nanohybrids were dispersed in NMP solvent at varying concentrations (0.25 and 0.5 wt%) and sonicated for uniform dispersion. Polysulfone was then added, and the mixture was stirred for 24 hours to obtain a homogeneous casting solution. To remove air bubbles, the solution was degassed for 15 minutes. Hollow fiber membranes (HFMs) were then fabricated using this dope solution under the conditions listed in Table 1. The as-spun membranes were immersed in water for 24 hours to extract residual NMP and subsequently stored in 50% glycerol.Table 1: Process parameter for fabrication of HFMsPhysiochemical characterization of HFMsTo characterize the physicochemical properties of the plain and Fe3O4 / MWCNTs-COOH incorporated hollow fiber membranes (HFMs), ATR-FTIR, SEM, and Energy Dispersive X-ray Spectroscopy (EDS) were employed. ATR-FTIR analysis was carried out using a Bruker Tensor 27 instrument (United States) to confirm the incorporation of Fe3O4 / MWCNTs-COOH into the membrane matrix. SEM was used to examine the morphology of the plain and Fe3O4 / MWCNTs-COOH embedded membranes. Prior to analysis, the membranes were immersed in liquid nitrogen, carefully cut, and placed on carbon tape-coated copper sheets. They were then sputter-coated with iridium to obtain a 10 nm thickness on the HFMs. EDS mapping analysis (X-Max 20 mm2 EDS system, Oxford Instruments, United Kingdom) was performed to observe the distribution of Fe, O, and C in the HFMs. The water contact angle of the membranes was determined using a contact angle analyzer (GBX digidrop, GBX instrument, France). The method outlined by Izni Yusoff et al. (2017) (Izni Yusoff et al., 2017) was followed for determining the molecular weight cut-off (MWCO) of hollow fiber membranes (HFMs). Briefly, a series of Polyethylene Glycol (PEG) polymers of varying molecular weights was used. The PEG solution was filtered through the membrane, and the rejection rate of PEG in the filtrate was calculated using the formula in equation (5). The PEG concentration was measured colorimetrically at a wavelength of 535 nm. The MWCO value of the membranes was determined as the molecular weight at which the membrane achieved a 90% rejection.SEM analysis was conducted to examine the morphology of the prepared hollow fiber membranes (HFMs). In Fig. 3, SEM images illustrate various samples of hollow fiber membranes (HFMs) - P, PFC-25, and PFC-50, presenting an overview of their structure, cross-section, and inner surface view. The overview images demonstrate the co-centricity of the fibers, ensuring a well-structured membrane. The cross-section images (Fig. 3-B, E and H) reveal a finger-like structure that facilitates efficient water transport. Furthermore, the inner surface of the HFMs exhibited increased porosity with the addition of Fe3O4 / MWCNT-COOH nanohybrid concentration. This increase in porosity contributes to the improved hydrophilicity of the membranes, promoting better water permeability. To confirm the presence of iron oxide in the HFMs, EDS (Energy Dispersive Spectroscopy) and mapping techniques were employed as given in Fig. 3 (J-L). These analyses provide conclusive evidence of the presence of Fe3O4 / MWCNT-COOH nanohybrid in the membranes, further validating their composition.The surface functional groups on the different HFM samples were analyzed using ATR-FTIR. The ATR-FTIR spectrum of pristine (P) and modified nanohybrid (PFC) HFMs, presented in the Fig. 4-A, showed characteristic peaks at 1153, 1244, and 1325 cm⁻¹, corresponding to the sulfonyl (O=S=O) group. The peak at 1296 cm⁻¹ was attributed to the aromatic ether (C-O-C) group (Sadare and Daramola, 2021; Tshabalala et al., 2016). Additionally, aromatic C-H bending and rocking vibrations were observed at 688, 832, and 871 cm⁻¹ ((Badrinezhad et al., 2018; Ghanbari et al., 2015; Singh et al., 2021). Peaks at 1011 and 1235 cm⁻¹ were associated with asymmetric C-O-C stretching vibrations (Anadão et al., 2013; Singh et al., 2021). Aromatic C=C stretching appeared at 1484 and 1582 cm⁻¹ (Anastasiou et al., 2018; Dehghani Kiadehi et al., 2015), while CH3-C-CH3 stretching was observed at 1504 cm⁻¹ (Dehghani Kiadehi et al., 2015; Singh et al., 2021). and peak at 559 cm⁻¹ was attributed to the sp2 aromatic C-H bending vibration (Modi and Bellare, 2020a). In the modified membrane, characteristic peaks of the Fe3O4 / MWCNTs-COOH nanohybrid were observed, with Fe-O vibrations at 457 cm⁻¹ and increased intensity at 559 cm⁻¹ (Modi and Bellare, 2020a, 2019a). Additionally, slight increases in peak intensity at 1648 cm⁻¹ and 3082 cm⁻¹ indicated C=C stretching vibrations in graphitic structures and the carboxylic group in MWCNT-COOH, respectively (Modi and Bellare, 2019b; Zheng et al., 2006) (Kumari et al., 2020).The contact angle was also measured for prepared HFM samples (Fig. 4-B). There is inverse relationship between contact angle and hydrophilicity, lesser the contact angle, more the hydrophilic nature and vice-versa. In a pristine hollow fiber membrane, 77.51 ± 2.003° contact angle was measured. It was found that with the addition of Fe3O4 / MWCNTs-COOH nanohybrid, the contact angle reduced to 47.33 ± 1.100°. The pore size distribution from SEM images (Fig. 4-C) revealed mean pore sizes of 10.83 nm for P, 13.2 nm for PFC-25, and 16.48 nm for PFC-50, with mode pore sizes of 10.83 nm for P, 13.97 nm for PFC-25, and 14 nm for PFC-50. The variation between mean and mode values is due to the distribution tail extending toward larger pores. The incorporation of Fe3O4 / MWCNTs-COOH nanohybrids increased the mean pore size, significantly enhancing the PWP, as discussed later. Zeta potential measurements were carried out on the HFM membranes (Fig. 4-D), and the addition of nanohybrids led to an increase in zeta potential. At pH 7, the zeta potential values were recorded as -13.04 ± 0.53 mV for P, -22.2 ± 1.67 mV for PFC-25, and -28.93 ± 3.63 mV for PFC-50 membranes. These values exhibited further elevation at a basic pH of 11, with zeta potentials measuring -13.6 ± 0.127 mV for P, -26.48 ± 0.94 mV for PFC-25, and -36.61 ± 1.09 mV for PFC-50.PWP and antifouling study:To determine the pure water permeability (PWP) of the hollow fiber membranes (HFMs), a pressure of 0.5 bar was applied, and the permeate volume was measured over time using deionized (DI) water (natural pH). The PWP was calculated using the following equation:PWP =Q / (A xtx ∆P) (1)Where Q represents the permeated volume in liters (L), A is the effective surface area of the membrane in square meters (m²), t is the time in hours (h), and ΔP is the transmembrane pressure in bars.For the antifouling study, a 1000 mg / L bovine serum albumin (BSA) solution was passed through the membrane. Subsequently, the membranes were rinsed with DI water for 30 minutes. The PWP value was then calculated again for the HFMs sample, indicating the fouled PWPf. Using the PWP value, the flux recovery ratio (FRR) and flux reduction (FR) were calculated using the following equations:FRR=(PWP_f) / (PWP_i )x100 % (2)FR=(1-(PWP_f) / (PWP_i )x100 %) (3)The performance of membranes in wastewater treatment depends largely on their water permeability and antifouling ability. In this study, Fe3O4 / MWCNTs-COOH nanohybrids were incorporated into hollow fiber membranes (HFMs) to enhance these properties. As shown in Fig. 5-A, pure water permeability (PWP) significantly increased from 12.48 ± 0.52 L / (m²·h·bar) in the pristine membrane to 26.16 ± 4.9 and 64.42 ± 7.12 L / (m²·h·bar) for PFC-25 and PFC-50, respectively - nearly 2- and 5-fold improvements. This enhancement is due to increased membrane hydrophilicity. After fouling, PWP values decreased but remained higher in modified membranes: 6.44 ± 0.59 (pristine), 19.9 ± 2.86 (PFC-25), and 52.66 ± 7.6 L / (m²·h·bar) (PFC-50). Fig. 5-B shows improved antifouling performance, with flux recovery ratios (FRR) of 51.6% (pristine), 76.36% (PFC-25), and 82.5% (PFC-50), confirming effective fouling resistance and stable membrane performance after modification.Adsorption study:Based on its superior pure water permeability (PWP) and antifouling properties, the PFC-50 hollow fiber membrane (HFM) was chosen for adsorption and further studies. A measured quantity of the membrane fiber was immersed in 50 mL of Cd²⁺ and As³⁺ ion solutions, prepared using cadmium nitrate and sodium arsenite, respectively, with concentrations ranging from 10 to 100 mg / L. The mixtures were shaken at 150 rpm for 24 hours. The pH was left unadjusted for Cd²⁺ adsorption, while it was set to 11 for As³⁺. Metal ion concentrations before and after adsorption were measured using either an ICP-AES (SPECTRO Analytical Instrument GmbH, Germany) or a high-resolution ICP-MS (Element XR, Thermo Fisher Scientific, Germany). The adsorption capacity was then calculated using the following equation:q_e=((C_o-C_e)xV) / m (4)Here, Co and Ce are the initial and equilibrium concentration (mg / L), respectively. qe (mg / g) denotes the equilibrium adsorption capacity, V(L) denotes the volume of feed of feed solution, m (g) indicates the dry weight of HFMs.The adsorption study of Cd2+ and As3+ was conducted using PFC-50 HFMs at their respective natural pH for Cd2+ and at pH 11 for As3+, as illustrated in Fig. 6, Among the various model, the Langmuir model best fitted the adsorption data. Langmuir model equation as given below;q_e = (k x q_maxxC_e) / (1+kxC_e )Where qe is the indicate the amount of metals ion adsorbed (mg / g), and Ce represent the equilibrium concentration (mg / L). qmax and k represent the maximum adsorption capacity (mg / g) and adsorption constant (L / mg), respectively.The Fe3O4 / MWCNTs-COOH-modified HFMs exhibited high adsorption capacities for Cd²⁺ (85.06 ± 7.12 mg / g) and As³⁺ (54 ± 4.13 mg / g), as shown in Fig. 4-D. This performance is attributed to strong electrostatic interactions between the positively charged metal ions and the negatively charged membrane surface, enhanced by the -COO⁻ groups of MWCNTs-COOH. The iron oxide component provided Fe-OH surface sites, promoting inner-sphere complexation and increasing surface negativity ((Kumari et al., 2015); (Nguyen et al., 2015). At pH >9, As³⁺ converts into charged species (H₂AsO3⁻, HAsO3²⁻), improving removal through Donnan exclusion Cd²⁺ showed greater adsorption than As³⁺, explained by its lower hydration energy (1575 vs. 2735 kJ / mol) (Lv et al., 2008a; Sato et al., 2002; Urase et al., 1998), favoring surface interaction (Mondal et al., 2017) (Fricke, 1975).Heavy metal ions removal study:A solution of Cd2+ and As3+ ions was prepared using deionized (DI) water, with initial concentrations of 4.24 ppm and 5.57 ppm, respectively. It is important to note that throughout the removal and reusability studies, the pH of the Cd²⁺ solution was kept at its natural level, while the pH of the As³⁺ solution was adjusted to 11 by adding 1M NaOH. It is worth noting that a pH of 11 was selected because As3+ ions remain uncharged at pH levels below 9, and previous studies have reported higher removal rates at pH 11 (Zhu et al., 2015).The solution was then passed through PFC-50 membrane modules, and the concentration of both the feed (Cf) and permeate (Cp) was determined using ICP-AES and ICP-MS. In addition, a removal test was conducted using lake water from Powai lake. The lake water was left undisturbed for 12 hours, filtered, and used to prepare the feed solution. To determine the removal percentage (Rexp), following equation was utilized.R_exp =(1-C_p / C_f )x100% (5)The removal efficiencies for Cd2+ were 19.89 ± 1.43% for P, 66.65 ± 2.34% for PFC-25, and 99.19 ± 0.25% for PFC-50. For As3+, the removal percentages were 12.32 ± 1.44% for P, 42.14 ± 1.43% for PFC-25, and 76.89 ± 1.04% for PFC-50. The significantly improved performance of PFC-50 is attributed to its highly negatively charged surface, enabled by the incorporation of the Fe3O4 / MWCNT-COOH nanohybrid, as corroborated by the zeta potential data shown in Fig. 4-D.To demonstrate the influence of other ions on the removal performance of the PFC-50 membrane, a mixed-ion study was conducted. The results clearly show that the presence of ions such as Na+, Cu2+, Pb2+, and their mixtures reduced the overall removal efficiency (Fig. 7). This reduction can be attributed to the interaction of these ions with the membrane surface, which inhibits further binding of target ions. Once these ions form complexes with the active functional groups on the membrane surface, fewer active sites remain available for further interaction. As a result, the removal performance of PFC-50 HFMs declined (Ibrahim et al., 2018; Modi and Bellare, 2020b). When tested with a mixture of both Cd2+ and As3+ ions, or a combination of all five ions (Na+, Cu2+, Pb2+, Cd2+, and As3+), Cd2+ was preferentially rejected over As3+.A long duration study was conducted using the PFC-50 membrane to evaluate its effectiveness in removing Cd2+ and As3+ ions from lab water. As shown in Fig. 8 (A and B), the permeate flux profile shows a gradual decrease over time. This decline is primarily due to the adsorption of heavy metal ions onto the membrane surface, which increases resistance and consequently reduces the flux. Initially, the membrane performs well, with effective removal of the heavy metal ions. However, as the operation continues, the active sites on the membrane, which are responsible for ion adsorption, gradually become saturated. This saturation point is critical because once all the active sites are occupied by the metal ions, the ability of membrane to continue rejecting these ions diminishes rapidly. The study found that the removal rate for Cd2+ ions begin to decline significantly after approximately 23 hours, while for As3+ ions, this decline starts after around 26 hours. This indicates that by these times, the active sites of membrane have become fully saturated, and its effectiveness in removing the metal ions is markedly reduced.Reusability study of membraneTo assess economic feasibility, a membrane reusability study was conducted over five cycles using nitric acid (0.2 M HNO3) for regeneration. Cd²⁺ and As³⁺ solutions (4.24 ppm and 5.57 ppm, respectively) were filtered under a transmembrane pressure of 0.5 bar and a flow rate of 60 mL / min for 120 minutes. After each cycle, the membranes were washed with nitric acid followed by DI water, and their removal performance was re-evaluated. The same procedure was applied using feedwater collected from Powai Lake.1.7. Characterization of nanoparticles and membranes A membrane is economically viable if it can be reused multiple times. To prove the membrane reusability capacity, the PFC-50 membrane was used for 5 cycles. For the feed prepared from lab water, the removal value measured after the 5th cycle was 84.12 ± 1.48% for Cd2+ and 56 ± 5.48% for As3+ (Fig. 8-C). For the feed prepared from lake water, the removal values were 55.44 ± 2.93% for Cd2+ and 44.54 ± 0.56% for As3+ (Fig. 8-D).Comparison of the membrane with prior artThe performance of the PFC-50 membrane was evaluated by comparing various parameters, including PWP, adsorption capacity, and the efficiency of heavy metal ion removal during filtration. The assessment was conducted in reference to the value reported in relevant literatures, as outlined in Table 2. Notably, a previous study (Zhu et al., 2015) reported higher As3+ removal compared to our findings. However, it is important to highlight that our PWP value, which was measured at 64.42 ± 7.12 L / (m2.h.bar), was significantly higher than their recorded value of 3.6 L / (m2.h.bar). This suggests that the novel HFMs (Psf blended with Fe3O4 / MWCNTs-COOH) have the potential to be effectively used in the treatment of natural and real-life wastewater contaminated with heavy metals.Table 2: Performance comparison with different literature reported membraneThe present invention is not to be limited in scope by the specific embodiments described herein. Indeed, various modifications of the invention, in addition to those described herein, will become apparent to those skilled in the art from the foregoing description and accompanying figures. Such modifications are intended to fall within the scope of the appended claims. The invention is, therefore, to be limited only by the terms of the appended claims along with the full scope of equivalents to which the claims are entitled.

Claims

1. A membrane for removal of toxic metal ions from contaminated water sources, comprising a Fe3O4 / MWCNTs-COOH nanohybrid incorporated in a hollow fiber membrane.

2. The membrane as claimed in claim 1, wherein the hollow fiber membrane is made of a polymer material selected from Polysulfone (Psf), polyethersulfone (PES), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyetherimide (PEI), polycaprolactone (PCL), and polylactic acid (PLA).

3. The membrane as claimed in claim 1, wherein the hollow fiber membrane is made of Polysulfone.

4. The membrane as claimed in claim 1, wherein the membrane comprises the nanohybrid Fe3O4 / MWCNTs-COOH has an initial concentration in the range of 0-5 wt% with respect to a polymer solution for making the membrane.

5. The membrane as claimed in claim 1, wherein the membrane comprises the nanohybrid Fe3O4 / MWCNTs-COOH has an initial concentration selected from 0.25% or 0.50% with respect to a polymer solution for making the membrane.

6. The membrane as claimed in claim 3, wherein polysulfone has an initial concentration in the range of 10 to 25 wt% with respect to a polymer solution for making the membrane.

7. The membrane as claimed in claim 1, wherein the membrane comprises Fe3O4 / MWCNTs-COOH nanohybrid incorporated into Polysulfone hollow fiber membrane.

8. A method of preparing a membrane for removal of toxic metal ions from contaminated water sources, the method comprising the steps of: a. preparing a Fe3O4 / MWCNTs-COOH nanohybrid; b. dispersing the Fe3O4 / MWCNTs-COOH nanohybrid in a solvent to obtain a dispersion; c. adding Polysulfone to the dispersion of step (b) to obtain a casting solution; and d. fabricating the casting solution of step (c) into the membrane.

9. The method as claimed in claim 8, wherein in step (a) Fe3O4 / MWCNTs-COOH nanohybrid is prepared by dissolving ferric chloride hexahydrate (FeCl3·6H₂O) in DI water followed by the addition of MWCNTs-COOH.

10. The method as claimed in claim 9, wherein pH is adjusted and Fe3O4 nanoparticles are formed.

11. The method as claimed in claim 8, wherein the solvent is selected from N-methyl-2-pyrrolidone (NMP), N,N-Dimethylformamide (DMF), N,N-Dimethylacetamide (DMAc), Dimethyl sulfoxide (DMSO), and Tetrahydrofuran (THF).

12. The method as claimed in claim 11, wherein the solvent is NMP.

13. The method as claimed in claim 8, wherein in step (b) Fe3O4 / MWCNTs-COOH nanohybrid is dispersed in a solvent in a concentration in the range of 0-5 wt%.

14. The method as claimed in claim 8, wherein in step (b) Fe3O4 / MWCNTs-COOH nanohybrid is dispersed in the solvent in a concentration selected from 0.25 wt% or 0.50 wt%.

15. The method as claimed in claim 8, wherein the Fe3O4 / MWCNTs-COOH nanohybrid dispersed in NMP solvent is sonicated to obtain a uniform dispersion.

16. The method as claimed in claim 8, wherein in step (c) 10 wt% to 25 wt% of Polysulfone is added to the dispersion of Fe3O4 / MWCNTs-COOH nanohybrid in the solvent.

17. The method as claimed in claim 8, wherein in step (c) 18 wt% of Polysulfone is added to the dispersion of Fe3O4 / MWCNTs-COOH nanohybrid in the solvent.

18. The method as claimed in claim 8, wherein the casting solution is degassed before fabrication into the membrane.

19. A membrane obtained by the method as claimed in claim 8, comprising Fe3O4 / MWCNTs-COOH nanohybrid integrated in the Psf hollow fiber membrane.

20. The membrane as claimed in claim 19, wherein the membrane can separate As3+ and Cd2+ ions from water sources.