# Enhanced silver recovery from electronic wastes using ionic liquid-integrated nanocomposite membrane

Bo Han, Ziyang Liu, Dong Xia, Guillaume Zante, Qingyu Yan, Jean-Christophe Gabriel

► To cite this version:

Bo Han, Ziyang Liu, Dong Xia, Guillaume Zante, Qingyu Yan, et al.. Enhanced silver recovery from electronic wastes using ionic liquid-integrated nanocomposite membrane. Separation and Purification Technology, 2025, Special issue "Ionic liquids-assisted advanced separation technology", 366, pp.132689. 10.1016/j.seppur.2025.132689. cea-05025441

HAL Id: cea-05025441

https://cea.hal.science/cea-05025441v1

Submitted on 8 Apr 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# Enhanced silver recovery from electronic wastes using ionic liquid-integrated nanocomposite membrane☆

Bo Han<sup>a, *,</sup> Ziyang Liu<sup>b</sup>, Dong Xia<sup>a</sup>, Guillaume Zante<sup>c</sup>, Qingyu Yan<sup>a, b</sup>, Jean-Christophe P. Gabriel<sup>a, c, *</sup>

## ARTICLE INFO

Handling editor: M Freire

**Keywords:**

Ionic liquid (IL)

Ag recovery

Electronic waste treatment

Metal organic framework (MOF)

Nanofiltration

Thin-film nanocomposite flexible membrane

## ABSTRACT

The rapid iteration of electronic technology has exacerbated the environmental challenges posed by electronic waste containing valuable and often critical metals such as silver (Ag). Efficient recovery of Ag from waste streams is critical to mitigate environmental impacts and promote the circular economy as it can be found in a large majority of all electronic components, due to its favorable electrical properties. In this study, we present an innovative integration of ionic liquids (ILs), specifically tetrabutylphosphonium hydroxide ([TBPH][OH]), with metal-organic framework MOF-74 (Mg) in thin-film nanocomposite (TFN) membranes for enhanced Ag recovery. The encapsulation of ILs in MOF-74 (Mg) addresses key challenges such as IL instability and detachment, while utilizing the high porosity, hydro stability, and coordinative unsaturated sites of the MOF to improve overall membrane performance. The resulting IL-MOF TFN membrane demonstrates superior selectivity, stability, and efficiency in the recovery of Ag (99.4%) from Ag-bearing solution leachated ceramic capacitors. This work highlights a synergistic approach combining the unique properties of ILs and MOFs to develop sustainable and high performance solutions for metal recovery applications.

## 1. Introduction

The rapid growth of electronic technology has contributed significantly to the global shift towards renewable energy [1]. However, electronic waste is a growing concern [2]. Electronic waste contains valuable metals, particularly silver (Ag) [3], a precious resource with extensive industrial applications [4]. Recovering Ag from electronic waste is critical, not only to mitigate the environmental impact (such as toxic pollution, resource depletion, ecosystem and health risks, energy and carbon footprint from the silver mining and refining process), but also to recycle valuable materials and support the circular economy [5].

Ionic liquids (ILs) have emerged as promising agents for metal recovery due to their unique properties such as high thermal stability, low volatility and tunable solubility [6–8]. These properties make ILs ideal for the selective extraction and recovery of metals such as Ag from complex waste streams [9]. In addition, Thin-film nanocomposite (TFN) membranes are advanced separation materials that integrate nanomaterials into a thin polymer matrix to enhance their performance.

These membranes exhibit superior properties such as high mechanical strength, improved chemical stability, and enhanced selectivity due to the incorporation of functional nanomaterials such as ionic liquids. TFN membranes also offer high permeability, reduced fouling and excellent scalability for industrial applications. Their ability to be tailored to specific separation processes, such as metal recovery, makes them particularly suitable for challenging applications such as the selective extraction of Ag from complex waste streams. When combined with TFN membranes, ILs allow for their tailoring, hence, enhancing their selectivity and permeability [10], and has resulted in efficient and targeted Ag recovery from electronic waste leachates [11]. The synergy between ILs and TFN membranes provides a high-performance solution that outperforms traditional metal recovery methods [12].

However although ILs offer numerous benefits, their application in TFN membranes also presents certain challenges, that currently hinders their commercialization. One of the main drawbacks is the potential instability of ILs under certain operating conditions [13], which can lead to performance degradation over time. In addition, ILs can detach from the membrane matrix, resulting in reduced recovery efficiency and the

<footnote><sup>a</sup> SCARCE Laboratory, Energy Research Institute @ NTU (ERI@N), Nanyang Technological University, Singapore 637459 Singapore</footnote>

<footnote><sup>b</sup> School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore</footnote>

<footnote><sup>c</sup> LICSEN, NIMBE, CNRS, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France</footnote>

<footnote>* This article is part of a special issue entitled: 'ILs-ass adv sep sep tech' published in Separation and Purification Technology.</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: han.bo@ntu.edu.sg (B. Han), jean-christophe.gabriel@cea.fr (J.-C.P. Gabriel).</footnote>
Nomenclature

BSA Bovine Serum Albumin

BET Brunauer-Emmett-Teller

CUS Coordinatively unsaturated site

DMF Dimethylformamide

EDX Energy dispersive X-ray spectroscopy

FESEM Field Emission Scanning Electron Microscopy

FRR Flux Recovery Rate

FTIR Fourier Transform Infrared Spectroscopy

HRTEM High-resolution transmission electron microscopy

ICP-OES Inductively Coupled Plasma Optical Emission Spectroscopy

MOF Metal-Organic Framework

NCM Non-contact mode

NPs Nanoparticles

PCPs Porous Coordination Polymers

PA Polyamide

PV Photovoltaic

PVDF Polyvinylidene fluoride

SLCC Single-layer ceramic capacitor

TFC Thin film composite

TFN Thin-film nanocomposite

TGA Thermalgravimetric Analysis

WCA Water contact angle

XRD X-ray diffraction

need for frequent membrane replacement or regeneration [14,15]. This instability and detachment are critical issues that need to be addressed to fully realize the potential of ILs in membrane-based metal recovery applications.

The encapsulation of ILs within metal-organic frameworks (MOFs) represents a significant advance in addressing the challenges associated with ILs [16]. On the one hand, MOFs themselves have excellent properties for nanofiltration as nanofillers (NPs), such as high surface area [17,18], chemical stability [19,20] and the ability to selectively adsorb specific ions [21,22], making them highly effective in the capture and recovery of valuable metal ions [23]. On the other hand, MOFs offer tunable pore sizes [24,25] and chemical functionalities [26,27], providing an ideal platform for stabilizing ILs within the membrane composite matrix [28]. This encapsulation not only enhances the stability of the ILs, preventing their detachment, but also improves the overall performance of the TFN membrane in silver recovery [29]. Furthermore, IL-MOF may be incorporated in the TFN membrane by the interfacial polymerization (IP) process. We have already shown that IP plays a critical role in the fabrication of TFN membranes, offering excellent control over the formation of ultrathin selective layers with enhanced mechanical resistance, which is essential for maintaining membrane integrity and durability under operational conditions [10].

Hence, this work further explores this approach with the innovative integration of tetrabutylphosphonium hydroxide ([TBPH]) type IL with MOF-74 (Mg) in TFN membranes for Ag recovery. Silver is targeted here as we noticed that silver is fairly ubiquitous in electronic components from printed circuit boards at an average of ~ 0.1 wt%[30]. We therefore used our single layer ceramic capacitors (SLCCs) and its waste leachate as a case study to demonstrate the interest of such membranes for silver recovery from electronic wastes. Indeed, our previous study on neodymium recovery indicated that such SLCC leachates were particularly rich in Ag⁺ and could undergo further purification toward Ag recovery [31]. [TBPH] IL was selected for its high ionic conductivity [32], competent durability [33] and promising compatibility with various NPs [34]. In addition, [TBPH] IL is considered as a green and sustainable additive with strong interaction with metal cations. Furthermore, MOF-74 (Mg) was chosen as host for its large porosity [35], promising hydro stability [36,37] and abundance of coordinative unsaturated metal sites (CUS) for IL encapsulation [38]. The motivation for this research stems from the need to develop more economically viable, efficient, stable and sustainable methods for recovering valuable Ag metals from waste streams. Overall, this research builds upon the combination of ILs and MOFs to enhance the performance and durability of TFN membranes in Ag recovery applications. By focusing on the encapsulation of ILs within MOFs, this work addresses the key challenges of IL instability and detachment, whilst utilizing the superior properties of MOFs for nanofiltration.

## 2. Experimental

### 2.1. Materials and chemicals

Magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O, 99 %), 2,5-dihydroxyterephthalic acid (DHTA, 98 %), tetrabutylphosphonium hydroxide solution ([TBP]⁺/[OH]⁻ or [TBPH], 40 wt% in H₂O), piperazine (C₄H₁₀N₂, 99 %, PIP), 1,3,5-Benzenetricarbonyl Trichloride (C₆H₃(COCl)₃, 98 %, TMC), dimethylformamide (DMF, 99.8 %), ethanol (C₂H₅OH, ≥ 99.8 %) and hexane (CH₃(CH₂)₄CH₃, ≥ 97 %) were purchased from Merck. All chemicals were of analytical grade and used without further purification. Commercially available nanoporous-hydrophilic polyvinylidene fluoride (PVDF) substrates were purchased from United Scientific for fabricating the TFN membranes.

### 2.2. IL-MOF synthesis

#### 2.2.1. Pristine MOF-74 (Mg)

As shown in Fig. 1(a), Mg(NO₃)₂·6H₂O (0.475 g, 1.85 mmol) and DHTA (0.112 g, 0.570 mmol)) were dissolved in a mixture of water (3 mL), ethanol (4 mL) and DMF (45 mL). The solution was then transferred to a Teflon-lined autoclave (inner volume 125 mL) and heated at 125 °C for 24 h. After cooling, the solid product was collected and washed several times with methanol to remove unreacted precursors and excess solvent. After washing, the material was dried overnight at 80 °C. To activate the MOF, the material was further treated at 150 °C under vacuum for 12 h.

#### 2.2.2. Solvent exchange

Standard solvent exchange method is used as described in reference [39]. The synthesized MOF-74 (Mg) was soaked in ethanol and stirred at 150 rpm for 72 h (Fig. 1(b)). The solvent was replaced several times with fresh ethanol. The aim of this process is to replace the strongly bound solvents (e.g. DMF) with a more volatile one (e.g. ethanol).

#### 2.2.3. Vacuum activation (dehydration)

After solvent exchange, the sample was first dried at 80°C, then transferred to a vacuum oven and heated at 150°C for 20 h under vacuum conditions. This dehydration step removes the solvent molecules coordinated to the Mg²⁺ ions, leaving them unsaturated and generating CUS.

#### 2.2.4. Ionic liquid encapsulation

A wet impregnation technique was used to encapsulate ILs on MOF-74 (Mg) [40]. The standard procedure involved dispersing 0.5 g of activated dehydrated MOF-74 (Mg) and 0.5 g of [TBPH] in 100 mL of ethanol liquor. The solution was stirred at 200 rpm for 24 h at room pressure and temperature, and the ILs were deposited on the CUS of dehydrated MOF. The IL-MOFs sample was then subjected to filtration,
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Schematic synthesis process for this work: (a) synthesis of the pristine MOF-74 (Mg), (b) synthesis of the ionic liquid encapsulated MOF composite, (c) fabrication of IL-MOF-TFN membrane, (d) nanofiltration for silver recovery from the waste leachate.</figcaption></figure>

2 x 50 ml ethanol washing, drying at 80 °C overnight and activation at 160 °C for 5 h. Thus, a composite material consisting of [TBP]$^+$[OH]$^-$ ILs embedded in MOF-74 (Mg) with an impregnation weight ratio of 1:1 was synthesized and designated as “1:1 [TBPH]-MOF-74 (Mg)” (Fig. 1(b)). The encapsulation technique kept the same for all IL-MOFs composites, except for the variations in the IL amount. The amount of IL was changed to 0.25 g and 1.0 g for the preparation of 1:2 [TBPH]-MOF-74 (Mg) and 2:1 [TBPH]-MOF-74 (Mg), respectively. The appearance of the pristine and IL-encapsulated MOFs is shown in Fig. S1.

## 2.3. TFN membrane fabrication

The preparation of the TFN membrane consisted of four main steps: (i) Solution preparation: Solution A was prepared by sonicating 100 mL of an aqueous solution containing 0.1 % (w/v) of pristine MOF or IL-MOF composite, 0.2 % (w/v) piperazine (PIP) and deionized water. Solution B was prepared by dissolving 1,3,5-benzenetricarbonyl chloride (0.15 % w/v) in hexane and then vacuum filtering 100 mL of the resulting organic solution. (ii) Vacuum filtration: 10 mL of solution A was transferred to the vacuum filtration vessel and passed through the PVDF substrate. (iii) Interfacial polymerization (IP): 6 ml of solution B was introduced onto the PVDF substrate containing PIP-MOFs and allowed to sit for 1 min. Thus, the polyamide layer of the TFN membrane was formed by IP of the PIP/MOF aqueous phase and the TMC organic phase. (iv) Thermal treatment: Membrane was then subjected to thermal treatment by removing excess organic liquids and heating at 70 °C for 5 min. Fig. 1(c) shows the schematic composition of the as-prepared TFN membrane. To investigate the effect of the MOF loading concentration, different amounts of MOF NPs were added to the PVDF substrate by

adjusting the volume of solution A (5 mL, 10 mL, and 15 mL). The resulting MOF-TFN membranes were designated as M5, M10 and M15 respectively. It should also be noted that in each experiment, multiple membranes were produced and tested to ensure reproducibility of results. In summary, the IP process was carefully optimized by adjusting key parameters to ensure the formation of a uniform and defect-free polyamide layer: (i) The monomer concentrations were controlled by using 0.2 wt% PIP in the aqueous phase and 0.15 wt% TMC in the organic phase, which allowed the formation of an ultrathin selective layer without excessive cross-linking that could hinder permeability. (ii) The reaction time was precisely maintained at 60 s to prevent over-polymerization, ensuring that the selective layer remained thin while providing sufficient mechanical stability. (iii) Thermal post-treatment at 70 °C for 5 min further enhanced cross-linking, improving the structural integrity of the membrane without compromising its selectivity. (iv) To effectively integrate the IL-MOF nanoparticles into the membrane, the dispersion of the MOF nanoparticles in the aqueous phase was carefully optimized to prevent aggregation and ensure uniform incorporation into the polyamide matrix. The presence of IL-MOF not only contributed to the enhanced rejection of Cu$^{2+}$ through electrostatic repulsion, but also provided additional transport pathways for Ag$^{+}$, resulting in improved selectivity. By carefully tuning these parameters, the IP process facilitated the fabrication of a TFN membrane with high mechanical strength, excellent separation performance, and long-term stability.

## 2.4. Characterization

### 2.4.1. IL-MOF characterization

XRD (X-ray diffraction, XRD Bruker D8 Advance) analysis was used
to determine the IL-MOF crystallography. FTIR (Fourier Transform Infrared Spectroscopy, PerkinElmer Frontier) was applied for chemical functional group identification on IL-MOF. The elemental composition and chemical states of the IL-MOF were investigated by XPS (X-ray photoelectron spectroscopy) analysis. Specifically, XPS measurements were carried out using a Kratos Axis Supra spectrometer (Shimadzu Corporation) equipped with an Al Kα X-ray source.

In addition, the thermal stability of the sample was detected by TGA (Thermalgravimetric Analysis, TA Instruments TGA 2950). The porosities (such as pore volume and BET surface area) of IL-MOF were evaluated by nitrogen adsorption (VacPrep 061/TriStar II analyzer) at 77 K. Prior to the N₂ adsorption experiment, the sample is outgassed at 160°C for 6 h. The N₂ isotherm data can be deduced to calculate the BET (Brunuer-Emmet-Teller) surface area by using the BET equation, and the total pore volume under saturation conditions (P/P<sub>s</sub>=1) is obtained, which is equivalent to the total liquid N₂ adsorbed in the MOF sample, and it can be calculated by V<sub>liq</sub> = P<sub>a</sub>V<sub>ads</sub>V<sub>m</sub>/RT, where P<sub>a</sub> is the ambient pressure, V<sub>ads</sub> is the volume of adsorbed N₂ and V<sub>m</sub> is the molar volume of liquid of N₂ with a value of 3.47 × 10<sup>-5</sup>m<sup>3</sup>/mol, R and T are the gas constant and temperature, respectively. Furthermore, the average pore size (r<sub>p</sub>) is calculated by r<sub>p</sub> = 2V<sub>liq</sub>/A<sub>s</sub>, where A<sub>s</sub> is the BET surface area of the adsorbent sample. The charge property of the IL-MOF was tested by Zeta potential (Malvern Zetasizer Nano Instrument) characterization. Furthermore, the particle size, microstructure and morphology of zeolite is measured by FESEM (Field Emission Scanning Electron Microscopy, JEOL JSM-7600F) and TEM (Transmission electron microscopy, JEM-2100F).

### 2.4.2. IL-MOF-TFN membrane

XRD and XPS techniques were used to check the deposition of the IL-MOF polyamide selective layer on the PVDF substrate. ATR-FTIR (attenuated total reflectance-Fourier transform infrared spectroscopy) was used to detect the chemical functional groups of the TFN membrane in the 400 – 4000 cm<sup>-1</sup> range.

In addition, the surface roughness of the TFN membrane was investigated by AFM (NX10 Atomic force microscopy) technique with a scan size of 20 µm × 20 µm and NCM (non-contact) mode. Here, the surface roughness was assessed by root mean square roughness (R<sub>ms</sub>), average roughness (R<sub>a</sub>) and maximum vertical distance (R<sub>z</sub>). The wettability (surface hydrophilicity) of the TFN membrane was evaluated by sessile drop method with a contact angle goniometer apparatus. Typically, RO water was applied as probe liquid and the average WCA (Water Contact Angle, Dataphysics, OCA15) is calculated with respect to over 3 independent WCA measurements for uncertainty minimization purpose. Furthermore, the thickness and surface morphology of the membrane was characterized by FESEM. The cross-sectional delamination of the TFN membrane was obtained by slicing the membrane in liquid nitrogen environment. Besides that, EDX (energy dispersive X-ray) spectroscope attached to the FESEM was used for elemental mapping analysis.

## 2.5. Ag recovery experiments

### 2.5.1. Nanofiltration

The schematic process of Ag recovery via nanofiltration is shown in Fig. 1(d). The metal ion separation (Ag⁺ and Cu²⁺) experiments were carried out using a high pressure stirred cell apparatus (Fig. S3 and Fig. S4). In general, the TFN membrane was positioned on the porous stainless steel membrane support and fixed at the bottom of the cell. SLCC waste leachate was introduced as the feed solution into the main body of the cell matrix (The detailed SLCC waste leachate preparation process is shown in S1.3 SLCC leachate preparation) [31]. Once the entire configuration was properly sealed, dry nitrogen gas with a purity of 99.999 % and a pressure between 2.5 and 15 bar was introduced from the top of the cell. The feed solution was then filtered through the

membrane, passed through the permeate tube and collected in the beaker as the permeate solution. Throughout the nanofiltration process, the beaker was placed on the balance, which was connected to an integrated weight/time analysis software. The change in weight of the permeate solution was automatically recorded as a function of time for the further derivation of water flux and permeability. Finally, the metal ion concentration in the permeate solution was determined by ICP (ICP-OES; Perkin Elmer Avio 200) analysis.

### 2.5.2. Regeneration and fouling investigation

Following the metal ion separation experiments, the used TFN membrane was immersed in deionized water and agitated at room temperature at 150 rpm for 1 h for regeneration. Agitation in deionized water induces hydrodynamic shear stress, which leads to the detachment of contaminants from both the membrane surface and the pores of the NPs. After that, the regenerated membrane is ready for the next cycle. In addition, comparison study on the water permeability of the TFN membrane before and after each regeneration cycle were conducted at a pressure of 7.5 bar. Furthermore, the fouling performance of TFN membranes was mainly influenced by two factors: hydrodynamic operation conditions (such as cross-flow velocity) and the composition of the feed solution (concentration/type of foulant). Therefore, the fouling investigation of this study was conducted by continuously exposing the TFN membrane to a multi-ionic solution containing 200 mg/L bovine serum albumin (BSA) as foulant at a pressure of 10 bar for 4 h. The contaminated membrane was then rinsed several times in water for one hour. The water flux of the TFN membrane before and after fouling were tested for comparison.

### 2.5.3. Aging evaluation

The detachment tendency of MOF NPs was assessed using a modified quantification technique described previously [10]. Typically, the MOF-TFN membrane (M15) was immersed in 100 mL of deionized water and agitated continuously for 12 h at 150 rpm under ambient conditions. The aim of this procedure was to investigate the stability and adhesion properties of MOF NPs in TFN membranes. Insufficient interaction leads to the detachment of NPs because of the hydrodynamic shear stress generated by agitation. The magnesium ion concentration (C<sub>Mg</sub>) in the solution was measured every 1 h by ICP analysis. Knowing that the weight percentage of Mg (Mg%) in MOF-74 (Mg) (Mg2(dobdc), 388.9 g/mol, H<sub>4</sub>dobdc = 2,5-dihydroxyterephthalic acid) is 12.5 % and the total mass of MOF-74 (Mg) NPs (M<sub>MOF</sub>) on the M15 TFN membrane is 15 mg, the detachment percentage of MOF (D<sub>MOF</sub> (%)) is evaluated as

$$D_{\text{MOF}}(\%) = \frac{C_{\text{Mg}} \cdot V_{\text{H}_2\text{O}}}{\text{Mg}\% \cdot M_{\text{MOF}}} \times 100\% \quad (1)$$

## 2.6. TFN membrane performance

**Water permeability, flux and flux recovery rate.**

Water permeability (A, L/(m²·h·bar)) is calculated with Eq. (2):

$$A = \frac{\Delta V}{(A_m \cdot \Delta t \cdot \Delta P)} \quad (2)$$

where ΔV is the volume of the permeate solution in liters, A<sub>m</sub> is the effective surface area of the membrane in square meters (here: 0.0009 m²), Δt is the permeation time in seconds and ΔP is the pressure of the system in bar. Water flux or LMH, (J<sub>w</sub>, L/(m²·h)) is calculated with Eq. (3):

$$J_w = \frac{\Delta V}{A_m \cdot \Delta t} \quad (3)$$

where the variable ΔV represents the changing volume of the permeate solution (L), A<sub>m</sub> represents the effective membrane surface area (m²), and Δt represents the permeation duration (s). The flux recovery rate (FRR, %) is calculated using Eq. (4):
$$FRR = \left( \frac{J_{w2}}{J_{w1}} \right) \times 100 \qquad (4)$$

where the variable $J_{w1}$ indicates the initial water flux of a fresh TFN membrane before fouling, while $J_{w2}$ indicates the subsequent water flux of the TFN membrane.

# 3. Results and discussion

## 3.1. IL-MOF characterization

Comprehensive characterizations were performed to investigate two things: (i) the successful encapsulation of IL within the MOF structure, and (ii) the structure integrity of both the pristine MOF and IL-MOF composite.

Fig. 2(a) shows the XRD profiles of the pristine and IL encapsulated MOFs. The XRD patterns of the pristine MOF are in good agreement with those reported in the literature [41], confirming the successful replication of the crystal structure. In addition, the XRD patterns show distinct peaks corresponding to the crystalline structure of the MOF-74 framework that are present in all IL-MOF samples. The presence of these peaks in the modified samples suggests that the fundamental crystallinity of the MOF-74 (Mg) structure is largely retained even with increasing IL loading. This retention of crystallinity indicates that the incorporation of IL into the MOF pores has not disrupted the primary structural framework of the MOF.

However, with higher IL content, particularly in the 2:1 IL-MOF sample, there are noticeable changes in the intensity of specific diffraction peaks. The decrease in peak intensity with increasing IL loading, as observed in the 1:1 and 2:1 IL-MOF samples, can be attributed to the occupation of MOF pores by IL molecules, which can attenuate X-ray scattering from the framework. This observation is consistent with the hypothesis that the encapsulated IL progressively fills the pores, affecting the overall electron density within the MOF structure and subsequently altering the diffraction intensity. Furthermore, the broadening of peaks, particularly evident in the 2:1 IL-MOF, may indicate partial structural distortion due to higher IL inclusion, suggesting that excessive IL loading may lead to minor changes in the pore structure of the MOF. This effect could also imply a reduction in the degree of long-range order within the framework, possibly due to interactions between the IL and the MOF backbone, which become more pronounced at higher loadings. In summary, the XRD profiles show that while the encapsulation of IL in MOF-74 (Mg) does not fundamentally alter the crystalline structure of the MOF, increasing the IL content influences the diffraction intensity and, potentially, the framework order.

Fig. 2(b) shows the FTIR spectrum of the pristine and IL encapsulated MOFs. The FTIR spectrum of the pristine MOF-74 (Mg) sample closely matches those reported in the literature [42,43], indicating consistent structural properties and confirming the successful synthesis of the un-modified framework according to established standards. In the pristine MOF-74 structure, characteristic absorption bands associated with the Mg-O bonds (around 595 cm⁻¹) and organic linkers are observed. With increasing IL content, subtle shifts and intensity variations of certain peaks are observed, indicating interactions between the encapsulated IL and the MOF framework.

Distinct and pronounced absorption peaks at 1522, 1420, 890, and 823 cm⁻¹ were identified and attributed to v(–C=O), v(–COO–), and v(C–H) of the benzene rings in H₄dhtp [42]. The prominent and subdued peak at 1240 cm⁻¹ is attributed to the stretching vibration of the C-O bond in the phenolate group [42,43]. The broadening of the peaks in this region for the 1:1 and 2:1 IL-MOF samples suggest the presence of hydrogen bonding or electrostatic interactions between the IL and the organic linkers of the MOF, which could modify the local electronic environment around these bonds. In addition, the appearance of new absorption bands or shifts in existing bands could indicate successful IL encapsulation, as the IL molecules introduce their own characteristic vibrations into the system. In addition, IL incorporation affects the bands around 3,000 cm⁻¹, where –OH or N–H stretching from the IL can overlap with or enhance the existing bands of the MOF structure. This effect is more pronounced in the 2:1 IL-MOF sample, where the intensity of these bands appears significantly altered, suggesting a higher IL interaction within the MOF pores.

In Fig. 3(a), the XPS spectra for the Mg 1 s core level show distinct differences between the pristine MOF-74 (Mg) and the IL-encapsulated samples. The pristine MOF-74 (black line) exhibits a characteristic Mg 1 s binding energy peak centered around 1305 eV, corresponding to the Mg-O interactions typical of the MOF-74 framework. After encapsulation with the ionic liquid, the Mg 1 s peaks for the IL-MOF samples (1:2 IL-MOF, 1:1 IL-MOF, and 2:1 IL-MOF) show a gradual shift toward a higher binding energy, which is attributed to changes in the electronic environment surrounding the Mg atoms. The observed shift suggests that the ionic liquid interacts with the Mg centers, changing the local electron density and affecting the coordination environment. However, the intensity of the Mg 1 s peak remains relatively stable, indicating that the total Mg content does not change significantly with IL encapsulation.

Fig. 3(b) shows the XPS spectra for the P 2p nuclear level. As expected, no P 2p signal is observed in the pristine MOF-74 (Mg), confirming the absence of phosphorus species. In contrast, the IL-MOF samples show distinct P 2p peaks around 133–134 eV, corresponding to the phosphonium cations ([TBP]⁺) from the encapsulated IL. The

<figure><img src="image_4.png" /><figcaption>Fig. 2. Characterization of the pristine and ionic liquid encapsulated MOFs: (a) XRD and (b) FTIR.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 3. (a) Mg 1 s and (b) P 2p XPS spectra of the pristine MOF and ionic liquid encapsulated MOFs.</figcaption></figure>

intensity of the P 2p peak increases progressively from the 1:2 IL-MOF to the 2:1 IL-MOF sample, indicating a higher concentration of phosphonium groups in the material. This trend directly correlates with the increasing IL loading, demonstrating that the encapsulated ionic liquid is effectively retained within the MOF structure. The XPS results provide strong evidence that IL incorporation modifies the electronic properties of the MOF while maintaining the balance of the elemental composition.

Fig. 4(a) shows the TGA profiles of the pristine MOF-74 (Mg) and the IL-encapsulated MOFs. The pristine MOF-74 (Mg) exhibits an initial weight loss below 150 °C, which can be attributed to the desorption of physically adsorbed moisture and residual solvents. A second major weight loss step occurs between approximately 300 °C and 450 °C, corresponding to the decomposition of the organic linkers and the collapse of the MOF framework. In particular, the pristine MOF-74 (Mg) undergoes a higher weight loss in this temperature range compared to the IL-MOF samples, suggesting that the framework decomposition is more pronounced in the absence of ionic liquid encapsulation.

For the IL-modified MOFs (1:2 IL-MOF, 1:1 IL-MOF, and 2:1 IL-MOF), a gradual weight loss is observed between 150 °C and 300 °C, which is not present in the pristine MOF. This additional weight loss is associated with the decomposition of the encapsulated IL ([TBP]⁺[OH]⁻). As the IL

loading increases from 1:2 IL-MOF to 2:1 IL-MOF, the magnitude of the weight loss also increases in this range, confirming the progressive incorporation of the IL. However, in the 300–450 °C range, the IL-MOF samples exhibit less weight loss than the pristine MOF-74 (Mg), suggesting that the presence of the IL may help stabilize the MOF structure and reduce the extent of thermal degradation in this region. This could be attributed to the interactions between the IL and the MOF framework, which may strengthen the structure or alter the degradation pathways of the organic linkers.

Beyond 450 °C, the weight loss continues, corresponding to the complete decomposition of the MOF and carbonization of the organic components. The observed differences in weight loss behavior indicate that while IL encapsulation introduces an additional degradation step at lower temperatures, it also appears to mitigate the extent of framework decomposition at higher temperatures.

Fig. 4(b) shows the nitrogen adsorption isotherms of the pristine MOF-74 (Mg) and the IL-modified MOFs at 77 K. The pristine MOF-74 (Mg) exhibits a type I isotherm, characteristic of microporous materials, with a steep increase in nitrogen uptake at low relative pressures ($P/P_0 < 0.1$), indicating a high surface area and a well-defined microporous structure. The total nitrogen uptake for the pristine MOF is

<figure><img src="image_6.png" /><figcaption>Fig. 4. Characterization of the pristine and ionic liquid encapsulated MOFs: (a) TGA and (b) N₂ adsorption isotherm.</figcaption></figure>
relatively high, reflecting its significant porosity and available adsorption sites.

In contrast, the IL-modified MOFs (1:2 IL-MOF, 1:1 IL-MOF, and 2:1 IL-MOF) show a significant decrease in nitrogen uptake with increasing amount of encapsulated ionic liquid. This decrease in gas adsorption capacity is indicative of partial pore blockage or a decrease in accessible surface area caused by the incorporation of the ionic liquid. The presence of the IL within the MOF framework likely occupies some of the internal pore spaces, thereby reducing the availability of adsorption sites for nitrogen molecules. In addition, the shape of the isotherms for the IL-MOF samples remains consistent with a microporous structure but with a lower overall adsorption capacity, highlighting the effect of IL encapsulation on the textural properties of the MOF.

The pore-reduction effect of IL is further demonstrated by porosity analysis. As shown in Fig. 5(a), the encapsulation of IL in the MOF-74 framework results in a systematic reduction of the BET surface area, total pore volume and micropore volume. For example, under the highest IL encapsulation condition (2):1 IL-MOF), a 10.9 % decrease in BET surface area (from 1311.1 m²/g to 1168.5 m²/g), 12.5 % decrease in total pore volume (from 0.63 cm³/g to 0.55 cm³/g), and 10.1 % decrease in micropore volume (from 0.29 cm³/g to 0.26 cm³/g) is found for the pristine MOF. This effect is due to the physical presence of IL molecules occupying the MOF pores, thereby reducing the accessible surface area and pore volume for gas or ion adsorption. The reduction in micropore volume suggests that IL preferentially enters the micropores, limiting their accessibility. At higher IL loadings (e.g., 2:1 IL-MOF), the more extensive pore occupation exacerbates this effect, leading to a significant decrease in overall porosity. In addition, pore size distribution (PSD) analysis of pristine MOF-74 (Mg) and IL-modified MOF samples (1:2 IL-MOF, 1:1 IL-MOF, and 2:1 IL-MOF) is shown in Fig. S5. The data show a primary peak centered around 0.7–1.2 nm, indicating the dominant microporous nature of MOF-74 (Mg). Upon encapsulation of the IL, notable changes in the pore size distribution are observed. The intensity of the primary peak systematically decreases from the pristine MOF-74 to the 2:1 IL-MOF, suggesting a progressive reduction in the accessible micropore volume due to partial pore occupation by the IL. This trend confirms that the IL molecules are successfully incorporated into the framework, rather than simply adsorbed on the outer surface. In addition, a broadening of the distribution at larger pore sizes (~1.5–3.0 nm) in IL-modified samples implies the formation of mesoporous voids, possibly due to structural rearrangements or partial aggregation of IL within the pores.

The charge property analysis of the as-prepared MOFs is shown in Fig. 5(b). It is found that the pristine MOF exhibits a predominantly negative zeta potential over the entire pH range, with the magnitude of negativity increasing at higher pH levels (above pH 6). However, at low pH (e.g. pH 1–3), all IL-MOFs show positive zeta potential compared to the pristine MOF. This indicates the influence of the positively charged

tetrabutylphosphonium ([TBP]⁺) ions from the IL. Since the pH of the SLCC leachate used in this work is 1.3, the enhanced positive charge of the NPs brings a favorable effect on Cu²⁺ rejection. Specifically, at pH 1.3, the pristine MOF exhibits a slightly negative zeta potential, indicating a tendency to attract positively charged metal ions such as Cu²⁺. This could lead to non-selective adsorption of both Ag⁺ and Cu²⁺ on the membrane, reducing the separation efficiency. In contrast, the IL-MOFs exhibit an enhanced positive zeta potential at pH 1.3. This positively charged surface introduces an electrostatic repulsion effect for other positively charged species in the feed solution, especially for the divalent Cu²⁺. On the other hand, Ag⁺, which have a lower charge density compared to Cu²⁺, experience weaker electrostatic repulsion. This allows the membrane to selectively transport Ag⁺ ions through the pores, facilitating effective separation of Ag⁺ from Cu²⁺. It is noted that the IL-to-MOF ratios of 1:2, 1:1, and 2:1 represent the theoretical encapsulation amounts, i.e., these ratios were used as the initial loading amounts during the preparation process with the expectation that the added IL would be fully incorporated into the MOF structure. However, under practical experimental conditions, it is difficult to accurately quantify the exact amount of IL that is successfully encapsulated within the MOF due to potential variations in adsorption efficiency, partial retention of IL within the solvent medium, or interactions that may not result in complete incorporation. Although direct quantification of encapsulated IL remains challenging, our characterization results (TGA, XPS and N₂ adsorption) provide strong evidence of increasing IL content within the nanoparticles of 1:2 IL-MOF, 1:1 IL-MOF, and 2:1 IL-MOF.

Fig. 6(a-d) shows the SEM images of the original and modified MOF-74 (Mg). In Fig. 6(a), the pristine MOF-74 (Mg) shows well-defined crystallites with a uniform and ordered arrangement. The surface of the crystals appears smooth and distinct with minimal agglomeration. The characteristic morphology of MOF-74 (Mg) is indicative of the crystalline nature and well-preserved framework structure formed during the solvothermal synthesis process.

The encapsulation of the ionic liquid induces moderate morphological changes in the structure of MOF-74 (Mg), as shown in Fig. 6(b) to 6 (d). In Fig. 6(b), the 1:2 IL-MOF shows slight structural distortion compared to the pristine MOF. The overall shape of the crystallites remains recognizable, although there is some surface roughening and minor aggregation, indicating that the ionic liquid has a subtle effect on the crystal morphology. With increasing IL content in Fig. 6(c) for the 1:1 IL-MOF, the morphological changes become more pronounced, but the crystallites still retain much of their original shape. The surface appears rougher, and some disruption of the ordered arrangement is evident, although the framework remains relatively intact. The particles show moderate aggregation, suggesting that while the ionic liquid is affecting the structure, the effect is not too severe. In Fig. 6(d), the 2:1 IL-MOF shows further changes, but the crystallites are still visible. The surface irregularities and aggregation are more pronounced, but the

<figure><img src="image_7.png" /><figcaption>Fig. 5. Characterization of the pristine and ionic liquid encapsulated MOFs: (a) porosity analysis and (b) Zeta-potential.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 6. SEM images of (a) pristine MOF-74 (Mg), (b) 1:2 IL-MOF, (c) 1:1 IL-MOF and (d) 2:1 IL-MOF.</figcaption></figure>

overall morphology does not collapse or become unrecognizable. The moderate effect of the ionic liquid encapsulation is evident, indicating that while the structural cohesion is affected, the framework retains some stability. This suggests that the ionic liquid influences the surface properties and aggregation behavior of the MOF, but the extent of structural disruption is limited. In addition, lower-magnification images with multiple crystals are shown in Fig. S6 for comprehensive understanding.

The TEM and STEM-EDS images of the pristine MOF-74 (Mg) and the ionic liquid encapsulated MOF (1:1 IL-MOF as an example) shown in Fig. 7(a) to 7(h). Fig. 7(a) shows the TEM image of pristine MOF-74 (Mg), which shows well-defined crystalline particles with clear edges and regular morphology. The HRTEM images in Fig. 7(b) and 7(c) show the crystalline lattice fringes of the MOF framework, confirming its ordered and porous structure. These images highlight the high degree of crystallinity and structural integrity of pristine MOF-74 (Mg). In addition, the STEM image in Fig. 7(d.1) further illustrates the overall morphology of the pristine MOF particle. The corresponding EDS elemental mapping in Fig. 7(d.2) to 7(d.4) shows uniform distributions of carbon (C), oxygen (O) and magnesium (Mg) throughout the MOF framework, indicating the homogeneous composition of the material.

Fig. 7(e) shows the TEM image of the 1:1 IL-MOF, which still retains a well-defined morphology similar to the pristine MOF, but with some observable changes. The encapsulation of the ionic liquid leads to a slight increase in particle thickness, suggesting the successful incorporation of the IL. The HRTEM images in Fig. 7(f) and 7(g) show the

presence of lattice fringes, but with reduced clarity compared to the pristine MOF, which is mainly due to the interaction between the ionic liquid and the MOF framework. Furthermore, the STEM image in Fig. 7 (h.1) illustrates the overall morphology of the 1:1 IL-MOF particle, while the EDS elemental mapping in Fig. 7(h.2) to 7(h.5) provides detailed insight into the elemental distribution. Carbon (C), oxygen (O) and magnesium (Mg) are uniformly distributed as shown in Figures (h.2), (h.3) and (h.4), similar to the pristine MOF. Notably, the presence of phosphorus (P) in Fig. 7(h.5) confirms the successful encapsulation of the ionic liquid ([TBP]$^+$[OH]$^-$) within the MOF structure.

## 3.2. IL-MOF-TFN membrane characterization

### 3.2.1. Chemical structure

Fig. 8(a) shows the XRD patterns of the PVDF substrate, the MOF-TFN membrane containing pristine MOF-74 (Mg) nanoparticles, and the IL-MOF-TFN membrane containing ionic liquid encapsulated MOF-74 (Mg). Here, the 1:1 IL-MOF-TFN membrane is used as an example for all characterization discussions in this section. The XRD pattern of the pure PVDF substrate shows characteristic peaks corresponding to the crystalline α-phase of polyvinylidene fluoride, indicating its semicrystalline nature. These peaks are consistent with the known crystalline forms of PVDF and confirm the integrity of the polymer structure. In addition, the MOF-TFN membrane fabricated from pristine MOF-74 (Mg) exhibits additional diffraction peaks corresponding to the crystalline planes of MOF-74 (Mg), indicating successful incorporation of the
<figure><img src="image_9.png" /><figcaption>Fig. 7. (a) TEM image of the pristine MOF-74 (Mg), (b-c) the corresponding HRTEM images, (d.1) STEM image and (d.2-d.4) the corresponding EDS elemental mapping of the pristine MOF-74 (Mg); (e) TEM image of the ionic liquid encapsulated MOF (1:1 IL-MOF), (f-g) the corresponding HRTEM images, (h.1) STEM image and (h.2-h.5) the corresponding EDS elemental mapping of the ionic liquid encapsulated MOF (1:1 IL-MOF).</figcaption></figure>

MOF particles into the membrane matrix. The retention of these distinct diffraction peaks suggests that the MOF framework retains its crystalline structure, and that the dispersion of the MOF nanoparticles does not significantly disrupt the crystalline order of the PVDF substrate.

For the IL-MOF-TFN membrane, the XRD pattern shows similar MOF-related peaks; however, some slight shifts and peak broadening are

observed. These changes in the diffraction pattern can be attributed to the interaction between the ionic liquid and the MOF framework. The encapsulation of the ionic liquid would induce some degree of lattice distortion or strain, resulting in these observed variations. Nevertheless, the presence of MOF-specific peaks confirms that the MOF structure is retained after ionic liquid encapsulation, albeit with some changes in the
<figure><img src="image_10.png" /><figcaption>Fig. 8. Characterization of the PVDF substrate, MOF-TFN membrane and IL-MOF-TFN membrane: (a) XRD and (b) ATR-FTIR.</figcaption></figure>

crystalline environment.

Fig. 8(b) shows the ATR-FTIR spectra of the PVDF substrate, the MOF-TFN membrane, and the IL-MOF-TFN membrane. The spectrum of the pure PVDF substrate shows characteristic absorption bands associated with the α-phase of PVDF, including prominent peaks at about 840 cm⁻¹, 1070 cm⁻¹, and 1400 cm⁻¹, corresponding to C-F stretching and CH₂ bending vibrations.

The MOF-TFN membrane shows additional absorption bands that can be attributed to the vibrational modes of the MOF-74 (Mg) framework. New peaks are observed in the regions corresponding to metal-oxygen (Mg-O) bonds and C-O vibrations from the organic linkers of the MOF. These features confirm the successful integration of MOF-74 (Mg) into the PVDF membrane. Importantly, the characteristic bands

of PVDF are still present, indicating that the polymer matrix retains its chemical structure despite the incorporation of the MOF particles.

Furthermore, other changes are observed in the spectrum of the IL-MOF-TFN membrane, including shifts and intensity variations in some absorption bands. New peaks associated with the ionic liquid, such as those corresponding to the P-O and C-N stretching vibrations of the tetrabutylphosphonium cation ([TBP]⁺), appear in the spectrum. These new features confirm the presence of the ionic liquid in the MOF-TFN membrane. The interactions between the ionic liquid and the MOF framework would change the vibrational environment, leading to these spectral changes. Nevertheless, the main absorption bands of PVDF remain identifiable, suggesting that the encapsulation of the ionic liquid does not significantly disrupt the polymer matrix.

<figure><img src="image_11.png" /><figcaption>Fig. 9. (a) Mg 1 s and (b) P 2p XPS spectra of the PVDF substrate, MOF-TFN membrane and IL-MOF-TFN membrane.</figcaption></figure>
The XPS spectra shown in Fig. 9(a) and Fig. 9(b) provide an analysis of the surface elemental composition and electronic states of the PVDF substrate, MOF-TFN membrane, and IL-MOF-TFN membrane. Fig. 9(a) shows the Mg 1 s XPS spectra. The PVDF substrate shows no peak in this region, as expected, indicating the absence of magnesium in the native polymer. In the MOF-TFN membrane, a distinct Mg 1 s peak is observed at a binding energy of about 1305 eV, confirming the successful incorporation of MOF-74 (Mg) nanoparticles into the TFN membrane. The IL-MOF-TFN membrane exhibits a similar Mg 1 s peak, but with a slight shift in binding energy and variation in intensity, suggesting that the encapsulation of the ionic liquid modifies the electronic environment around the magnesium atoms.

In addition, Fig. 9(b) shows the P 2p XPS spectra. The PVDF substrate and the MOF-TFN membrane show no peak in this region, consistent with the absence of phosphorus in these samples. However, a distinct P 2p peak is observed in the IL-MOF-TFN membrane at around 133 eV, which is characteristic of the phosphorus species in the tetrabutylphosphonium cation ([TBP]⁺) from the encapsulated IL. This confirms the successful incorporation of the ionic liquid into the MOF-TFN membrane.

### 3.2.2. Surface morphology and thickness

Fig. 10 shows the thickness and surface morphology analysis of the TFN membranes fabricated with 1:1 IL-MOF nanoparticles and PVDF substrate. The membranes are designated M5, M10, and M15, corresponding to different loading of the 1:1 IL-MOF nanoparticles. Fig. 10 (a.1) shows the cross-sectional SEM image of the TFN M5 membrane, highlighting a clear interface between the PVDF substrate and the polyamide (PA) selective layer. The PA layer has an approximate thickness of 65.63 µm, while the PVDF substrate measures approximately 59.53 µm. EDS elemental mapping shows the distribution of fluorine (F) from the PVDF substrate and magnesium (Mg) from the

embedded MOF nanoparticles, confirming the presence of MOF-74 (Mg) within the selective layer. Fig. 10(a.2) shows the cross section of the TFN M10 membrane where the PA selective layer is thicker at approximately 95.16 µm and the PVDF substrate is approximately 46.41 µm. The EDS mapping shows a more pronounced presence of Mg in the selective layer, indicating an increased loading of MOF nanoparticles. In addition, Fig. 10(a.3) shows the cross-sectional SEM image of the TFN M15 membrane, which has the thickest PA selective layer at approximately 110.63 µm, while the thickness of the PVDF substrate is reduced to 44.53 µm. The EDS elemental mapping again confirms the incorporation of MOF nanoparticles, with a higher density of Mg signals reflecting the higher MOF content.

The distinctive valley and ridge type of morphology is found for the IL-MOF based TFN membrane surface (Fig. 10(b-d)), which is consistent with other NPs embedded TFN membrane as reported in the literature [44]. Relatively uniform encapsulation of IL-MOF on membrane surface is observed for all the TFN membranes (M5, M10 and M15), which indicates a good compatibility between polyamide matrix and IL-MOF NPs. Specifically, Fig. 10(b.1) shows the surface morphology of the TFN M5 membrane, showing a relatively rough and textured surface indicative of the embedded MOF nanoparticles. Fig. 10(b.2) shows the surface morphology of the TFN M10 membrane, showing a more pronounced roughness and denser coverage of the MOF nanoparticles. The surface appears more uniform compared to M5, likely due to the increased loading of 1:1 IL-MOF nanoparticles, which contributes to a more interconnected texture. Notably, the surface features are moderately distributed, with some visible aggregation of the IL-MOF NPs for TFN membrane M15 (Fig. 10(b.3)). These agglomerated regions are indicative of uneven dispersion of the MOF particles, which is due to the high loading of the ionic liquid encapsulated MOF (therefore, increased van der Waals attractive force between NPs). Agglomeration can have several adverse effects, including a reduction in the effective surface

<figure><img src="image_12.png" /><figcaption>Fig. 10. (a.1-a.3) Cross-sectional SEM images with EDS elemental mapping analysis for 1:1 IL-MOF-TFN membrane M5, M10 and M15, respectively. (b.1-b.3) Surface morphologies of 1:1 IL-MOF-TFN membrane M5, M10 and M15, respectively.</figcaption></figure>
area available for separation, potential formation of non-selective pathways, and degradation of membrane performance. These aggregated regions can hinder the uniform flow of feed solutions across the membrane, reducing the overall separation efficiency and selectivity for metal ions.

### 3.2.3. Surface roughness and wettability

The surface roughness parameters ($R_{ms}$, $R_a$ and $R_z$) are furnished in Table S1, the overall AFM 3-D images are shown in Fig. S7, and the representative AFM 3-D images of the TFN membranes are shown in Fig. 11 (a.1-a.3). For both pristine MOF-TFN and IL-MOF-TFN membranes, the surface roughness increases with higher NP loadings (from M5 to M15). This trend reflects the progressive incorporation of nanoparticles into the polyamide (PA) selective layer, which contributes to a more textured and heterogeneous surface. At lower loadings (M5), the surface roughness is moderate, indicating a relatively uniform dispersion of NPs. At intermediate (M10) and higher loadings (M15), the surface becomes increasingly rough, with more pronounced peaks and valleys. This higher roughness indicates greater NP incorporation, but also increases the possibility of aggregation, especially at the highest loading (M15), which could affect filtration performance by introducing non-selective pathways or increasing fouling susceptibility. Such phenomenon is also proved by SEM characterization as shown in Fig. 10(b).

The water contact angles of PVDF substrate and TFN membranes are furnished in Table S2, the overall water contact angles of all the membranes are shown in Fig. S8, and the representative screenshots are shown in Fig. 11(b.1-b.3). Smaller contact angle (i.e., more hydrophilic) of TFN membrane is observed as compared with that of the PVDF substrate. This is due to the fact that the MOF NPs enhance the surface free energy and polarity of the membrane surface; hence, the surface wettability (hydrophilicity) is increased for TFN membrane. The wettability is future improved for the IL-MOF based TFN membrane, which is mainly attributed to the introduction of the hydrophilic IL additives. On the one hand, ILs can reduce the surface tension between the membrane and water by introducing functional groups that interact

with water molecules more effectively. On the other hand, when ionic liquids are evenly distributed in the TFN membrane, they create more hydrophilic sites across the membrane surface, which further promote the water transfer.

Based on the Wenzel's Law $cos\theta^* = r \cdot cos\theta$ (where r represents roughness factor, $\theta$ indicates the Young angle, $\theta^*$ refers to the measured contact angle), the surface hydrophilicity of membrane is also related to the surface roughness. It is known that r is greater than 1 for hydrophilic and rough surface, the $\theta^*$ is negatively correlated with r. In other words, the rougher the membrane surface, the smaller of the contact angle and the more hydrophilic of the membrane [45]. Based on the surface roughness and wettability analysis, IL-MOF based TFN membrane exhibits rougher and more hydrophilic surface as compared with those of the PVDF substrate as well as the pure MOF based TFN membrane, which will promote the permeability. Based on the TFN membrane characterization results discussed above, M10 exhibits the relatively uniform NPs on the TFN membrane surface, with moderate surface roughness and high wettability. Therefore, M10 TFN membranes are used for the Ag recovery by nanofiltration experiment.

### 3.3. Ag recovery performance

Fig. 12 (a) and (b) shows the concentration of $Ag^+$ and $Cu^{2+}$ in the permeate solution over successive separation cycles for TFN membranes (M10) with various NPs (pristine MOF and 1:2/1:1/2:1 IL-MOF). The 1:1 IL-MOF TFN membrane was found to have the best Ag recovery (99.4 %) and Cu rejection (99.8 %) over six cycles. A clear divergence in the behavior of $Ag^+$ and $Cu^{2+}$ is evident, with the $Ag^+$ preferentially permeating while the $Cu^{2+}$ are largely rejected. In particular, the permeate $Ag^+$ concentration remains high over cycles, indicating a preferential permeation of $Ag^+$ through the membrane. The $Ag^+$ concentration stabilizes after the fourth cycle, reflecting the consistent separation performance of the TFN membrane. On the other hand, the $Cu^{2+}$ ion concentration in the permeate decreases sharply after the first cycle and continues to decrease with subsequent cycles. This trend

<figure><img src="image_13.png" /><figcaption>Fig. 11. (a.1-a.3) AFM images and (b.1-b.3) water contact angle screenshot for pure PVDF substrate, MOF-TFN membrane (10 M) and 1:1 IL-MOF TFN membrane (10 M).</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 12. (a) Ag⁺ and (b) Cu²⁺ concentration of permeate solution after each separation cycle for 1:1 IL-MOF TFN membrane M10.</figcaption></figure>

highlights the ability of the TFN membrane to effectively retain Cu²⁺ and ensure minimal passage into the permeate solution.

The Ag recovery and Cu rejection performance of the pristine MOF TFN membrane are 97.9 % and 56.5 %, respectively. The improved separation performance of the 1:1 IL-MOF TFN membrane can be attributed to the synergy between the MOF-74 (Mg) framework and the encapsulated IL. The molecular size and hydration radius of metal ions play a critical role in their separation. The hydrated ion radius of Ag⁺ is reported to be between 1.02 Å [46] and 3.41 Å [47] (depending on the

computational model and testing method), while Cu²⁺ has a larger hydrated radius of 4.19 Å [48]. The MOF-74 (Mg) framework, with its tunable pore size, provides selective transport channels that favor smaller Ag⁺ over larger Cu²⁺. The encapsulation of IL further influences the pore size and polarity, reinforcing this effect. In addition, Ag⁺ diffuses faster than Cu²⁺ in aqueous systems due to its lower charge density and smaller hydration shell. It is reported that the diffusion coefficients at 25 °C in infinite dilution for Ag⁺ and Cu²⁺ are 1.68 × 10⁻⁹ m²/s [49] and 0.71 × 10⁻⁹ m²/s [50], respectively. Therefore, the higher diffusion

<figure><img src="image_15.png" /><figcaption>Fig. 13. (a) working mechanism of TFN membrane, including pore sieving and electrostatic repulsion effect, (b) schematic diagram of TFN membrane with insufficient/appropriate/excess MOF NPs loading, (c) schematic diagram of TFN membrane with insufficient/appropriate/excess IL encapsulation.</figcaption></figure>
rate of Ag⁺ will further enhance its preferential transport across the membrane over Cu²⁺. The presence of [TBP][OH] modifies the pore environment and surface charge of MOF-74, favoring the transport of monovalent Ag⁺ while hindering the passage of divalent Cu²⁺ due to enhanced pore sieving and electrostatic repulsion (Fig. 13(a)). In addition, IL also enhances the hydrophilic properties and reduces the resistance to ion diffusion of the TFN membrane, further improving the permeability of Ag⁺. Based on the Zeta potential analysis, the surface charge of TFN membranes varies with pH, which directly impacts the metal ion separation efficiency. Under the working condition of this work (pH = 1.3), positively charged surface is observed for the IL-MOF based TFN membrane. As shown in Fig. 13(a), such surface will provide strongly electrostatic repulsion to the divalent metal ions (such as Cu²⁺) as compared to the monovalent metal ions (such as Ag⁺), making Cu²⁺ less permeable to the membrane while allowing Ag⁺ to pass through more easily.

The effect of the loading amount of MOF NPs on the TFN membranes is also shown schematically in Fig. 13(b). TFN membranes with low NP loading are structurally weaker and do not exhibit the high selectivity and separation efficiency required for effective ion separation. At low NP loading, the membrane exhibits incomplete rejection of Cu²⁺ while Ag recovery is suboptimal. This behavior is attributed to the presence of voids or unmodified regions on the membrane surface. When the NP loading is optimized, the membrane achieves nearly perfect Ag recovery and high Cu rejection. At this loading level, the NPs are uniformly dispersed and embedded in the polymer matrix of the PVDF substrate, resulting in a well-distributed and compact membrane structure. In addition, at high NP loading, overloading of the NPs leads to their agglomeration, especially on the surface of the TFN membrane. This defect results in non-uniform membrane morphology and blocked active sites, which also leads to a decrease in Cu rejection efficiency.

Fig. 12 also shows that the performance of the TFN (M10) membranes fabricated with IL encapsulated MOF varies significantly depending on the amount of IL encapsulation in the MOF framework. The encapsulation ratios (1:2, 1:1, and 2:1 IL-MOF) have distinct effects on the Ag recovery and Cu rejection performance during the nanofiltration process. For example, Ag recovery is highest for the 1:1 IL-MOF TFN membrane (99.4 %), indicating that moderate IL encapsulation achieves the optimal balance for selective Ag⁺ transport. The 1:2 IL-MOF TFN membrane shows slightly lower Ag recovery (98.7 %), probably due to insufficient IL encapsulation, which may limit the formation of favorable interactions with Ag⁺. However, for the 2:1 IL-MOF TFN membrane, Ag recovery drops significantly to 86.3 %, suggesting that excessive IL encapsulation hinders the selective transport of Ag⁺. In addition, the Cu rejection performance is nearly perfect (99.8 %) for both the 1:1 and 2:1 IL-MOF TFN membranes, indicating that these membranes effectively block Cu²⁺ due to steric hindrance and electrostatic interactions. However, for the 1:2 IL-MOF TFN membrane, the Cu rejection significantly decreases to 91.1 %, suggesting that insufficient IL encapsulation reduces the TFN membrane's ability to effectively reject Cu²⁺.

The effect of the amount of IL encapsulation is shown in Fig. 13 (c). Insufficient IL encapsulation (1:2 ratio) results in incomplete pore modification, leading to suboptimal rejection of Cu²⁺. Appropriate IL encapsulation (1:1 ratio) optimally modifies the pore environment and surface charge of the MOF, favoring Ag⁺ transport while rejecting Cu²⁺ due to their different sizes and charge densities. However, excessive IL encapsulation (2:1 ratio) can block or narrow the MOF pores, reducing permeability and hindering Ag⁺ recovery while maintaining Cu²⁺ rejection.

The selectivity of the IL-MOF-TFN system is determined by a combination of several factors, including size exclusion, electrostatic interactions, coordination chemistry, and specific affinity between membrane components and target ions.

The ability to selectively adsorb specific ions can be tuned by modifying the MOF structure, changing the IL composition, and

optimizing the membrane fabrication process. The MOF framework itself plays a crucial role as its pore size and functional groups determine the accessibility and binding strength of different ions. By selecting MOFs with appropriate pore sizes and functionalizing them with chelating or polar groups, the selection preference can be shifted toward desired cations or anions. For example, functionalization with carboxylate groups can enhance selectivity toward multivalent cations such as Fe³⁺ [51] or Pb²⁺ [52,53], while amine-functionalized MOFs can exhibit stronger affinity for sulfide [54].

In addition, the IL encapsulated within the MOF further refines the ion selectivity by providing a tunable chemical environment. The cation and anion components of the IL can be carefully chosen to induce specific interactions with target ions. For example, phosphonium-based ILs can facilitate preferential transport of soft metal cations such as Ag⁺ [55], while imidazolium-based ILs can enhance selectivity for transition metal ions such as Cu²⁺ [56] and Zn²⁺ [57]. In addition, ILs influence the hydration layer and diffusion kinetics of ions [58], which affects their mobility across the membrane.

On the other hand, the TFN membrane architecture integrates these selective mechanisms into a practical separation platform. The distribution of IL-MOF nanoparticles within the polyamide matrix allows the formation of preferential transport pathways while maintaining mechanical stability and antifouling properties. The charge distribution at the membrane interface, influenced by both the MOF and IL components, contributes to electrostatic repulsion or attraction effects that further define the selectivity. This synergy between the IL, MOF, and TFN structure allows fine tuning of the adsorption behavior for a wide range of ionic species, making the membrane adaptable for separation applications such as heavy metal removal, lithium extraction, and nutrient recovery.

### 3.4. Water permeance and regeneration performance

The effects of both the NPs loading and IL encapsulation on water flux and permeability are investigated. Fig. 14(a.1) and Fig. 14(a.2) show the water flux of pristine MOF-TFN membrane and IL-TFN membrane, respectively. It can be seen from Fig. 14(a.1) that for the pristine MOF-TFN membrane, the water flux increases with increasing MOF nanoparticle loading from M5 to M10. This increase is due to the increased hydrophilicity and porosity imparted by the MOF nanoparticles, which improves water transport pathways within the polyamide selective layer. However, the water flux becomes less pronounced at higher loadings (M10 to M15), indicating aggregation of the NPs, which can partially block the pores and reduce the improvement. Similarly, for IL-MOF-TFN membranes (Fig. 14(b.1)), the water flux increases with increasing NP loading, with the highest flux observed for M10 membranes. However, the magnitude of the improvement is greater for IL-MOF membranes compared to pure MOF membranes. This suggests that IL encapsulation enhances hydrophilicity, allowing for more efficient water transport.

Fig. 14(a.2) shows that for the pristine MOF-TFN membrane, the permeability follows a similar trend as the water flux, improving with increasing NPs loading (form M5 to M10). However, at higher loadings (M15), the permeability starts to decrease, which is mainly due to partial pore blockage or reduced uniformity of nanoparticle dispersion. In addition, IL-MOF membranes show consistently higher permeability at all loadings compared to pristine MOF membranes (Fig. 14(b.2)). This improvement reflects the synergistic effect of IL encapsulation, which not only improves water transport properties, but also reduces fouling potential by creating a smoother and more hydrophilic surface.

In addition, it is found that the water flux increases linearly with pressure for all TFN membranes (Fig. 14(a.1) and (b.1)). This linear relationship reflects the dominance of pressure-driven flow through the membrane, where the hydraulic force enhances the movement of water molecules through the selective layer. In addition, the permeability values for TFN membranes are relatively consistent at higher pressures,
<figure><img src="image_16.png" /><figcaption>**Fig. 14.** (a) water flux evaluation of pristine MOF (a.1) and IL-MOF (a.2) based TFN membrane; (b) permeability evaluation of pristine MOF (b.1) and IL-MOF (b.2) based TFN membrane.</figcaption></figure>

with only minor variations (Fig. 14(a.2) and (b.2)). This stability suggests that the selective layer maintains its integrity under pressure, and the contribution of MOF/IL-MOF NPs to water transport remains effective.

Fig. 15 shows the permeability change over several nanofiltration cycles, and it also illustrates the surface morphology of the 1:1 IL-MOF-TFN membrane at different stages of the nanofiltration and regeneration processes. Before nanofiltration, the pristine 1:1 IL-MOF-TFN membrane exhibits its maximum permeability (47.8 L/(m²·h·bar)), reflecting the clean, open structure of the polyamide selective layer with well-dispersed IL-MOF nanoparticles. The hydrophilic and porous nature of the nanoparticles contributes to efficient water transport pathways. Such phenomenon is supported by the surface morphology images at Fig. 15(a.1), where the surface exhibits a uniform distribution of nanoparticles without significant aggregation or fouling.

After filtering 0.5 L of SLCC waste leachate, the membrane surface begins to show signs of fouling (Fig. 15(a.2)). A thin layer of contaminants is visible, partially covering the nanoparticle-rich surface. This initial fouling layer is mainly attributed to the adsorbed metal ions ($Cu^{2+}$) and other feed solution components. Therefore, the permeability

decreased to 40.9 L/(m²·h·bar). Next, after filtering 0.75 L of feed solution, the fouling layer becomes more pronounced. The membrane surface appears more covered, with fewer exposed nanoparticles visible (Fig. 15(a.3)). The accumulation of contaminants likely creates additional resistance to water flow, resulting in a minimum permeability of 37.0 L/(m²·h·bar).

After regeneration, the membrane surface regains much of its original morphology (as shown in Fig. 15(a.4)), resembling the pre-nanofiltration state Fig. 15(a.1). The fouling layer is effectively removed, leaving a clean surface with re-exposed IL-MOF nanoparticles. Therefore, the permeability is resumed to 47.2 L/(m²·h·bar). This indicates the effectiveness of the regeneration process, which uses hydrodynamic shear stress to detach contaminants from both the surface and the nanopores of the membrane. The restored morphology suggests that the membrane retains its functional performance for subsequent filtration cycles.

In addition, the TFN membrane stability test in terms of detachment of NPs from the TFN membrane surface as well as leaching of IL from the MOF frameworks is shown in Fig. S7 and Fig. S8, respectively. It is found that the detachment percentage of the MOF is as low as 0.14 % over a

<figure><img src="image_17.png" /><figcaption>**Fig. 15.** Regeneration performance (in terms of permeability) with surface morphology of the TFN membrane evaluation ((a.1) before nanofiltration, (a.2-a.3) after nanofiltration, and (a.4) after regeneration)).</figcaption></figure>
period of 24 h (Fig. S9). In addition, a distinct P 2p peak is observed in the IL-MOF-TFN membrane at around 133 eV ((referred to [TBP]$^+$)) after the cyclic nanofiltration (Fig. S10)), which proves the existence of IL within the MOF structure after the Ag recovery experiment. All these results underline the robust adhesion between the substrate, the PA matrix and the MOF NPs, as well as the stable encapsulation of IL within the MOF.

In this study (shown in Fig. 15), membrane performance is tested over five consecutive filtration cycles in a single test run, during which silver recovery efficiency remained above 99 % with negligible changes in separation performance. However, it is important to note that the membrane was not limited to five cycles. In practice, the same membrane has been reused in multiple rounds of testing, each involving multiple sets of filtration cycles, and its performance parameters, including permeability, selectivity, and antifouling characteristics, have been found to remain stable over extended use, suggesting that the membrane has a much longer service life.

### 3.5. Antiaging and antifouling performance

The antifouling performance of the 1:1 IL-MOF-TFN membrane in terms of FRR is shown in Fig. 16(a), where a continuous decrease in FRR (from 85.3 % to 70.0 %) is observed with pressure increases from 2.5 bar

to 15 bar. This indicates that the membrane experiences minimal irreversible fouling under low pressure conditions, likely due to reduced compressive forces on foulants and a reduced tendency to adhere to the surface. At higher pressures, foulants such as BSA in the feed solution can be compressed onto the membrane surface and into its pores, leading to greater adhesion and more difficult removal during regeneration. This compression effect is more pronounced at the highest pressures tested, where the water flux shows a significant decrease. For example, at a pressure of 15 bar, the water flux of the TFN membrane decreases from 669.6 L/(m²·h) to 468.8 L/(m²·h) before and after the fouling test.

The effect of the foulant on the TFN membrane could also be explained by the characterization of the surface morphology. As shown in Fig. 16(b.1), the initial TFN membrane surface before fouling is rough and well textured, characterized by evenly distributed IL-MOF NPs. The absence of any impurities indicates a clean, functional surface ideal for filtration. After 1 h of fouling (Fig. 16(b.2)), a thin fouling layer begins to form on the surface, consisting mainly of adsorbed foulants such as BSA from the feed solution. The NP-rich texture of the surface remains partially visible, indicating minimal fouling at this stage. As the fouling process progresses, the fouling layer becomes more pronounced and covers a larger portion of the TFN membrane surface (Fig. 16(b.3)). The adsorbed foulants obscure more of the nanoparticle-rich structure,

<figure><img src="image_18.png" /><figcaption>**Fig. 16.** (a) antifouling performance in terms of FRR, (b) surface morphology of the TFN membrane during the anti-fouling testing process ((d.1) before fouling, (d.2-d.5) after fouling, and (d.6) after regeneration).</figcaption></figure>
indicating increased surface fouling. Next, fouling continues to progress up to 3 h, resulting in a denser and thicker foulant layer (Fig. 16(b.4)). Furthermore, after 4 h of fouling, the fouling layer appears to be the most severe, with significant coverage of the TFN membrane surface (Fig. 16(b.5)). The aggregation of foulants forms larger clusters, potentially reducing the TFN membrane permeability and water flux. After the regeneration process (Fig. 16(b.6)), the surface morphology is largely restored to its pre-fouling state. The fouling layer is effectively removed, re-exposing the IL-MOF NPs and the textured structure of the polyamide selective layer. This demonstrates the effectiveness of the regeneration process in removing foulants from the surface and restoring TFN membrane functionality.

While our current results demonstrate that the TFN membrane maintains high performance over multiple cycles, several strategies could be explored to improve its long-term reusability. One key approach is to optimize the stability of the IL within the MOF framework to minimize potential leaching. This can be achieved by strengthening the interaction between the IL and the MOF through covalent bonding or coordination strategies to ensure that the IL remains firmly retained within the structure even after extended filtration cycles [59,60].

Another avenue for improvement lies in refining the antifouling properties of the membrane. While the IL-MOF already helps reduce membrane fouling, additional surface modifications such as the introduction of hydrophilic coatings [61] or zwitterionic functional groups [62,63] could further mitigate the accumulation of contaminants, thereby extending the effective lifetime of the membrane. In addition, optimization of the regeneration process itself could further improve recycling performance. Implementing electrochemical cleaning [64] protocols tailored to remove residual contaminants without compromising membrane integrity would allow repeated reuse with minimal loss of performance. In addition, structural improvements to the TFN membrane, such as optimizing the distribution and loading of the IL-MOFs within the polyamide matrix, could improve both selectivity and mechanical durability, thereby contributing to prolonged operational stability.

## 4. Conclusions

This study presents a novel approach for silver recovery from electronic waste leachate using IL encapsulated MOF-based TFN membranes. By encapsulating [TBPH][OH] IL in MOF-74 (Mg), the key challenges of IL instability and detachment were effectively addressed. The IL encapsulation not only stabilized the IL within the membrane matrix, but also utilized the superior properties of MOF-74 (Mg), including its high porosity, hydro stability, and abundant coordinative unsaturated sites, to enhance the nanofiltration performance. The resulting IL-MOF TFN membrane demonstrated excellent selectivity and efficiency in separating Ag from complex feed solutions, making it a promising solution for sustainable metal recovery applications. This selective separation is highly advantageous for recovering pure Ag⁺ from e-waste leachate, a critical step in sustainable resource recovery. The ability to consistently recover Ag⁺ with minimal Cu²⁺ contamination demonstrates the potential of the membrane for industrial metal recovery applications, especially in the case of electronic components from printed circuit boards that are rich in both copper and silver, but usually with a much large amount of copper. Furthermore, this work highlights the potential of integrating ILs and MOFs to develop advanced, durable, and efficient TFN membranes that contribute to the circular economy and address the environmental challenges posed by e-waste. Future efforts should focus on scaling up this technology and exploring its applicability to other valuable metals in diverse waste streams as well as its economic viability.

### Claim

The English writing is grammatically checked by the AI-assisted tool DeepL.

### CRediT authorship contribution statement

**Bo Han:** Conceptualization, Methodology, Investigation, Writing – original draft. **Ziyang Liu:** Methodology. **Dong Xia:** Methodology, Writing – review & editing. **Guillaume Zante:** Methodology, Supervision, Writing – review & editing. **Qingyu Yan:** Funding acquisition, Project administration & supervision, Writing – review & editing. **Jean-Christophe P. Gabriel:** Conceptualisation, Methodology, Funding acquisition, Project administration & supervision, Writing – review & editing.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: [Jean-Christophe P. Gabriel reports financial support was provided by National Environment Agency. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper].

### Acknowledgements

The authors acknowledge financial support from the SCARCE project, which (i) phase 1 (2018-2023) was supported by the National Research Foundation, Prime Minister's Office, Singapore, the Ministry of National Development, Singapore, National Environment Agency, Singapore, and the Ministry of the Environment and Water Resource, Singapore under the Closing the Waste Loop R&D Initiative as part of the Urban Solutions & Sustainability – Integration Fund [Award No. USS-IF-2018-4] and; (ii) Phase 2 is currently supported by the National Research Foundation, Singapore, and National Environment Agency, Singapore under its Closing the Waste Loop Funding Initiative (Award No. CTRL-2023-1D-01).

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.seppur.2025.132689.

## References

[1] A.I. Khan, A. Keshavarzi, S. Datta, The future of ferroelectric field-effect transistor technology, Nat. Electron. 3 (10) (2020) 588–597.

[2] A.K. Awasthi, J. Li, L. Koh, O.A. Ogunseitan, Circular economy and electronic waste, Nat. Electron. 2 (3) (2019) 86–89.

[3] V. Fiandra, L. Sannino, C. Andreozzi, Photovoltaic waste as source of valuable materials: a new recovery mechanical approach, J. Clean. Prod. 385 (2023) 135702.

[4] T. Bruna, F. Maldonado-Bravo, P. Jara, N. Caro, Silver nanoparticles and their antibacterial applications, Int. J. Mol. Sci. 22 (13) (2021) 7202.

[5] H. Trivedi, A. Meshram, R. Gupta, Recycling of photovoltaic modules for recovery and repurposing of materials, J. Environ. Chem. Eng. 11 (2) (2023) 109501.

[6] G.S. Dobhal, L.N. Pham, S.A. Tawfik, C. Pozo-Gonzalo, T.R. Walsh, Exploring coordination of neodymium in ionic liquid: insights to guide sustainable recovery, ACS Sustain. Chem. Eng. 11 (39) (2023) 14614–14621.

[7] G. Kaur, H. Kumar, M. Singla, Diverse applications of ionic liquids: a comprehensive review, J. Mol. Liq. 351 (2022) 118556.

[8] S.S. de Jesus, R. Maciel Filho, Are ionic liquids eco-friendly? Renew. Sustain. Energy Rev. 157 (2022) 112039.

[9] G. Zante, A. Braun, A. Masmoudi, R. Barillon, D. Trébouet, M. Boltoeva, Solvent extraction fractionation of manganese, cobalt, nickel and lithium using ionic liquids and deep eutectic solvents, Miner. Eng. 156 (2020) 106512.

[10] B. Han, S.M. Chevrier, Q. Yan, J.-C.-P. Gabriel, Tailorable metal–organic framework based thin film nanocomposite membrane for lithium recovery from wasted batteries, Sep. Purif. Technol. 334 (2024) 125943.

[11] S. Imdad, R.K. Dohare, A critical review on heavy metals removal using ionic liquid membranes from the industrial wastewater, Chemical Engineering and Processing-Process Intensification 173 (2022) 108812.

[12] H. Wu, Y. Lin, W. Feng, T. Liu, L. Wang, H. Yao, X. Wang, A novel nanofiltration membrane with [MimAP][Tf2N] ionic liquid for utilization of lithium from brines with high Mg2+/Li+ ratio, J. Membr. Sci. 603 (2020) 117997.
[13] G. Zante, M. Boltoeva, A. Masmoudi, R. Barillon, D. Trébouet, Supported ionic liquid and polymer inclusion membranes for metal separation, Sep. Purif. Rev. 51 (1) (2022) 100-116.

[14] S.-J. Choi, S. Kim, S.-J. Im, A. Jang, D.S. Hwang, S. Kang, Ionic fluid as a novel cleaning agent for the control of irreversible fouling in reverse osmosis membrane processes, Water Res. 224 (2022) 119063.

[15] Y. Jia, F. Shi, H. Li, Z. Yan, J. Xu, J. Gao, X. Wu, Y. Li, J. Wang, B. Zhang, Facile ionization of the nanochannels of lamellar membranes for stable ionic liquid immobilization and efficient CO2 separation, ACS Nano 16 (9) (2022) 14379-14389.

[16] W.L. Xue, W.H. Deng, H. Chen, R.H. Liu, J.M. Taylor, Y.K. Li, L. Wang, Y.H. Deng, W.H. Li, Y.Y. Wen, MOF-directed synthesis of crystalline ionic liquids with enhanced proton conduction, Angew. Chem. Int. Ed. 60 (3) (2021) 1290-1297.

[17] B. Han, A. Chakraborty, Ligand extension of aluminum fumarate metal-organic framework in transferring higher water for adsorption desalination, Desalination (2024) 118135.

[18] B. Han, E. Hu, B.-E. Jia, Z. Liu, S. Tso, A. Sumboja, I.T. Anggraninrum, A. Z. Syahrial, Q. Yan, Metal-organic frameworks (MOFs) in aqueous batteries (ABs): unlocking potential through innovative materials design, Sci. China Chem. (2025) 1-35.

[19] T. He, X.-J. Kong, J.-R. Li, Chemically stable metal-organic frameworks: rational construction and application expansion, Acc. Chem. Res. 54 (15) (2021) 3083-3094.

[20] B. Han, J. Liu, C. Lee, C. Lv, Q. Yan, Recent advances in metal-organic framework-based nanomaterials for electrocatalytic nitrogen reduction, Small Methods 7 (9) (2023).

[21] L. Rani, J. Kaushal, A.L. Srivastav, P. Mahajan, A critical review on recent developments in MOF adsorbents for the elimination of toxic heavy metals from aqueous solutions, Environ. Sci. Pollut. Res. 27 (2020) 44771-44796.

[22] B. Han, A. Chakraborty, Recent advances in metal-organic frameworks for adsorption heat transformations, Renew. Sustain. Energy Rev. 198 (2024) 114411.

[23] B. Wang, Y. Ma, W. Xu, K. Tang, A novel S, N-rich MOF for efficient recovery of Au (III): performance and mechanism, J. Hazard. Mater. 451 (2023) 131051.

[24] M. Kim, R. Xin, J. Earnshaw, J. Tang, J. P. Hill, A. Ashok, A.K. Nanjundan, J. Kim, C. Young, Y. Sugahara, MOF-derived nanoporous carbons with diverse tunable nanoarchitectures, Nat. Protoc. 17 (12) (2022) 2990-3027.

[25] B. Han, A. Chakraborty, Functionalization on metal-organic frameworks to enhance water adsorption uptakes and kinetics for cooling applications, Advancements in Non- conventional Cooling and Thermal Storage Strategies (2024) 65-103.

[26] B. Han, L. Zhong, C. Chen, J. Ding, C. Lee, J. Liu, M. Chen, S. Tso, Y. Hu, C. Lv, Tuning main group element-based metal-organic framework to boost electrocatalytic nitrogen reduction under ambient conditions, Small (2023).

[27] B. Han, A. Chakraborty, Synthesis and characterization of various MOFs and MOFs- zeolite composites: water adsorption and rapid cooling production, in: Rapid Refrigeration and Water Protection: next Generation Adsorbents, Springer, 2022, pp. 29-85.

[28] Z. Guo, W. Zheng, X. Yan, Y. Dai, X. Ruan, X. Yang, X. Li, N. Zhang, G. He, Ionic liquid tuning nanocage size of MOFs through a two-step adsorption/infiltration strategy for enhanced gas screening of mixed-matrix membranes, J. Membr. Sci. 605 (2020) 118101.

[29] B. Han, J.-C.-P. Gabriel, Thin-film nanocomposite (TFN) membrane technologies for the removal of emerging contaminants from wastewater, J. Clean. Prod. (2024) 144043.

[30] N.M. Charpentier, D. Xia, J.-C.-P. Gabriel, Printed circuit board recycling: a focus on a novel, efficient and sustainable process for spent critical metals recovery, C. R. Chim. 27 (S4) (2024) 1-11.

[31] D. Xia, N.M. Charpentier, A.A. Maurice, A. Brambilla, Q. Yan, J.-C.-P. Gabriel, Sustainable route for Nd recycling from waste electronic components featured with unique element-specific sorting enabling simplified hydrometallurgy, Chem. Eng. J. 441 (2022) 135886.

[32] M.N. Nguyen, T.T. Tran, Q.T. Nguyen, T. Tran Thi, B.C. Nguyen, D. Hollmann, Simple synthesis of cellulose hydrogels based on the direct dissolution of cellulose in tetrabutylphosphonium hydroxide followed by crosslinking, Polym. Adv. Technol. 33 (10) (2022) 3376-3385.

[33] A. Baranov, F. Sommerhoff, B. Duchemin, O. Curnow, M.P. Staiger, Toward a facile fabrication route for all-cellulose composite laminates via partial dissolution in aqueous tetrabutylphosphonium hydroxide solution, Compos. A Appl. Sci. Manuf. 140 (2021) 106148.

[34] F. Cheng, P. Zhao, T. Ouyang, J. Sun, Y. Wu, Comprehensive utilization strategy of cellulose in a facile, controllable, high-yield preparation process of cellulose nanocrystals using aqueous tetrabutylphosphonium hydroxide solution, Green Chem. 23 (4) (2021) 1805-1815.

[35] H. Kim, C.S. Hong, MOF-74-type frameworks: Tunable pore environment and functionality through metal and ligand modification, CrstEngComm 23 (6) (2021) 1377-1387.

[36] A.A. Voskanyan, V.G. Goncharov, N. Novendra, X. Guo, A. Navrotsky, Thermodynamics drives the stability of the MOF-74 family in water, ACS Omega 5 (22) (2020) 13158-13163.

[37] B. Han, A. Chakraborty, B.B. Saha, Isothermal heats and entropy of adsorption in Henry's law region for carbon and MOFs structures for energy conversion applications, Int. J. Heat Mass Transf. 182 (2022) 122000.

[38] Y. Jiao, C.R. Morelock, N.C. Burtch, W.P. Mounfield III, J.T. Hungerford, K. S. Walton, Tuning the kinetic water stability and adsorption interactions of Mg-MOF-74 by partial substitution with Co or Ni, Ind. Eng. Chem. Res. 54 (49) (2015) 12408-12414.

[39] B. Han, A. Chakraborty, Synthesis and characteristics of ionic liquid-implanted HKUST-1 metal-organic frameworks for transforming heat into extraordinary water transfer, ACS Sustain. Chem. Eng. (2024).

[40] B. Han, A. Chakraborty, Synergistic ionic liquid encapsulated MIL-101 (Cr) metal- organic frameworks for an innovative adsorption desalination system, J. Clean. Prod. (2024) 143565.

[41] L.J. Wang, H. Deng, H. Furukawa, F. Gándara, K.E. Cordova, D. Peri, O.M. Yaghi, Synthesis and characterization of metal-organic framework-74 containing 2, 4, 6, 8, and 10 different metals, Inorg. Chem. 53 (12) (2014) 5881-5883.

[42] J. Ling, A. Zhou, W. Wang, X. Jia, M. Ma, Y. Li, One-pot method synthesis of bimetallic MgCu-MOF-74 and its CO2 adsorption under visible light, ACS Omega 7 (23) (2022) 19920-19929.

[43] M. Ranjbar, M.A. Taher, A. Sam, Mg-MOF-74 nanostructures: facile synthesis and characterization with aid of 2, 6-pyridinedicarboxylic acid ammonium, J. Mater. Sci. Mater. Electron. 27 (2016) 1449-1456.

[44] I.H. Aljundi, Desalination characteristics of TFN-RO membrane incorporated with ZIF-8 nanoparticles, Desalination 420 (2017) 12-20.

[45] S. Yang, Q. Jiang, K. Zhang, Few-layers 2D O-MoS2 TFN nanofiltration membranes for future desalination, J. Membr. Sci. 604 (2020) 118052.

[46] Y. Marcus, Ionic radii in aqueous solutions, Chem. Rev. 88 (8) (1988) 1475-1498.

[47] Radii, I., *Diameters from Several Sources*. Priega per interneta: http://www.electrotronics.org/ionic_radii.pdf.

[48] Musumba, G., C. Nakiguli, C. Lubanga, and N. Emmanuel, Adsorption of lead (II) and copper (II) ions from mono synthetic aqueous solutions using bio-char from Ficus natalensis fruits. 2020.

[49] Vanýsek, P., Ionic conductivity and diffusion at infinite dilution. CRC handbook of chemistry and physics, 1993. 94.

[50] Parkhurst, D. and C. Appelo, THREEQC (Version 2)-A Computer Program for Speciation, Batch-Reaction, One-Dimensional Transport, and Inverse Geochemical Calculations, v2. 14.02, 2007 [accessed December 2007]. US Geological Survey (USGS): Denver.

[51] C.-H. Chen, X.-S. Wang, L. Li, Y.-B. Huang, R. Cao, Highly selective sensing of Fe 3+ by an anionic metal-organic framework containing uncoordinated nitrogen and carboxylate oxygen sites, Dalton Trans. 47 (10) (2018) 3452-3458.

[52] J.-X. Hou, J.-P. Gao, J. Liu, X. Jing, L.-J. Li, J.-L. Du, Highly selective and sensitive detection of Pb2+ and UO22+ ions based on a carboxyl-functionalized Zn (II)-MOF platform, Dyes Pigm. 160 (2019) 159-164.

[53] X. Zhao, Y. Wang, Y. Li, W. Xue, J. Li, H. Wu, Y. Zhang, B. Li, W. Liu, Z. Gao, Synergy effect of pore structure and amount of carboxyl site for effective removal of Pb2+ in metal-organic frameworks, J. Chem. Eng. Data 64 (6) (2019) 2728-2735.

[54] H.-Y. Zhang, C. Yang, Q. Geng, H.-L. Fan, B.-J. Wang, M.-M. Wu, Z. Tian, Adsorption of hydrogen sulfide by amine-functionalized metal organic framework (MOF-199): an experimental and simulation study, Appl. Surf. Sci. 497 (2019) 143815.

[55] R.M. Rios-Vera, A. Chagnes, L. Hernández-Perales, D.E. Martínez-Rodríguez, D. L. Navarro-Segura, L. Gaillon, J. Sirieix-Plènet, C. Rizzi, A.L. Rollet, M. Avila-Rodríguez, Trihexyl (tetradecyl) phosphonium bis-2, 4, 4-(trimethylpentyl) phosphinate micellar behavior in the extraction of Ag (I) from acidic nitrate media, J. Mol. Liq. 358 (2022) 119132.

[56] F. Zou, H. Ling, L. Zhou, F. Wang, Y. Li, Construction and photoluminescence properties of octaimidazolium-based polyhedral oligomeric silsesquioxanes hybrids through the 'bridge' of ionic liquids: Chemical sensing for Cu2+, Dyes Pigm. 184 (2021) 108840.

[57] R. Katiyar, P. Sappidi, Molecular simulations of understanding the Zn2+ ion structure, dynamics and thermodynamic properties in water in ionic liquids, Chem. Phys. 587 (2024) 112424.

[58] X. Li, K. Chen, R. Guo, Z. Wei, Ionic liquids functionalized MOFs for adsorption, Chem. Rev. 123 (16) (2023) 10432-10467.

[59] M. Zeeshan, V. Nozari, S. Keskin, A. Uzun, Structural factors determining thermal stability limits of ionic liquid/MOF composites: imidazolium ionic liquids combined with CuBTC and ZIF-8, Ind. Eng. Chem. Res. 58 (31) (2019) 14124-14138.

[60] H. Abroshan, H.J. Kim, On the structural stability of ionic liquid-IRMOF composites: a computational study, PCCP 17 (9) (2015) 6248-6254.

[61] Z. Wu, J. Tian, L. Wu, C. Zhang, A hydrophilic coating capable of withstanding acid and alkali to modify PVDF membrane, J. Water Process Eng. 45 (2022) 102519.

[62] M. He, K. Gao, L. Zhou, Z. Jiao, M. Wu, J. Cao, X. You, Z. Cai, Y. Su, Z. Jiang, Zwitterionic materials for antifouling membrane surface construction, Acta Biomater. 40 (2016) 142-152.

[63] H.Z. Shafi, Z. Khan, R. Yang, K.K. Gleason, Surface modification of reverse osmosis membranes with zwitterionic coating for improved resistance to fouling, Desalination 362 (2015) 93-103.

[64] Y. Jing, L. Guo, B.P. Chaplin, Electrochemical impedance spectroscopy study of membrane fouling and electrochemical regeneration at a sub-stoichiometric TiO2 reactive electrochemical membrane, J. Membr. Sci. 510 (2016) 510-523.