# From electronic wastes to efficient and specific filtration membranes: A photovoltaic upcycling case enabling silver urban mining

Bo Han, Ying Sim, Qingyu Yan, Nripan Mathews, Jean-Christophe P Gabriel

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Bo Han, Ying Sim, Qingyu Yan, Nripan Mathews, Jean-Christophe P Gabriel. From electronic wastes to efficient and specific filtration membranes: A photovoltaic upcycling case enabling silver urban mining. Journal of Cleaner Production, 2025, 505, pp.145528. 10.1016/j.jclepro.2025.145528. hal-05044057

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# From electronic wastes to efficient and specific filtration membranes: A photovoltaic upcycling case enabling silver urban mining

Bo Han<sup>a,*</sup> <sup>†</sup>, Ying Sim<sup>a,†</sup>, Qingyu Yan<sup>a,b</sup>, Nripan Mathews<sup>a,b</sup>, Jean-Christophe P. Gabriel<sup>a,c,**</sup>

<figure><img src="image_3.png" /></figure>

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## ARTICLE INFO

Handling Editor: Dr. Sandra Eksioglu

**Keywords:**

Ag recovery

Photovoltaic waste treatment

Metal organic framework (MOF)

TFN membrane

Metal ions separation

## ABSTRACT

The increasing demand for sustainable and environmentally friendly solutions has driven research into the reuse of photovoltaic (PV) waste. In this study, a bimetallic metal-organic framework (Cu/Ag MOF) was synthesized directly from PV waste leachate and subsequently integrated into a thin-film nanocomposite (TFN) membrane for nanofiltration-based silver (Ag) recovery. The optimized TFN membrane demonstrated high selectivity of (< Si > / < i > > Ag*, Cu2* = 7.99) and recovery of 99.1 % for Ag over five repeated filtration cycles while maintaining the membrane integrity. The membrane also exhibited exceptional antifouling performance, with a flux recovery rate (FRR) of 92.8 %, and high stability with only 0.5 % detachment of Cu/Ag MOF after prolonged use. Additionally, the high water permeability (58.6 L/(m²·h·bar)) of the TFN membrane has enabled efficient filtration performance. These results highlight the feasibility of Cu/Ag MOF-based TFN membranes for sustainable Ag recovery from PV waste and likely other electronic waste leachates, contributing to circular economy initiatives and resource recovery technologies.

## 1. Introduction

Silver (Ag) is a precious metal found in limited quantities in the earth's crust (Chen et al., 2021). It is typically found in Ag-bearing ores, which together with copper (Cu), lead (Pb) and/or antimony (Sb) therefore necessitating purification to obtain high purity Ag (Shu et al., 2020). Despite its low abundance, Ag is used extensively in the photographic and imaging industries, and to a lesser extent in the manufacturing of jewelry, batteries, silverware and medical practices (Dawadi et al., 2021). Due to its excellent thermal and electrical conductivity, Ag is also favored as an electrical contact in many electronic and electrical devices, such as printed circuit boards in consumer products and in photovoltaics (PV) (Chakraborty et al., 2022). As a result of modernization and technological advances, the global Ag demand continues to rise, further straining the market supply and put Ag at risk of shortage (Zloty et al., 2024). Although many sectors have made extra efforts in cutting down or even replacing Ag, it remains insufficient to relieve the market strain. Instead, it is essential that we explore and develop strategies to recover Ag from above-ground Ag stocks to re-introduce it into the market and promote material circularity

(Grandell et al., 2014).

As the world progresses towards sustainability, there is a gradual shift from conventional energy sources such as fossil fuels to lower carbon emitting alternatives like nuclear, wind, hydro and solar energy (Paraschiv et al., 2023). Among these, PV is one of the emerging sources and predicted to account for approximately 20 % of the global energy sector by 2050 (Abdulla et al., 2024). The PV market is currently dominated by crystalline silicon-based (c-Si) PV (~90 % market share) (Bošnjaković et al., 2023; Wang et al., 2022). Such c-Si PV module has a typical lifetime of about 25 years before they gradually lose their efficiency and reach end-of-life (EOL) (Bošnjaković et al., 2023; Latunussa et al., 2016). The International Energy Agency (IEA) has modelled 2 scenarios - regular loss and early loss - and predicted the cumulative volume of EOL PV modules to reach 60 and 78 million metric tons by 2050 (Stéphanie et al., 2021). Current PV recycling attempts generally focus only on recovery of bulk items such as aluminum (Al) frames, junction boxes, cables and tempered glass to meet the European Union (EU) regulation (to recover >85 wt% of PV materials) (Wang et al., 2022; Paiano, 2015). There is no clear accounting of the remnants which contain high purity silicon (Si), polymers, Al, Cu, Pb and Ag. Although Ag is only present at < 0.1 wt% in a typical PV module, it accounts for

<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>* Corresponding author.</footnote>

<footnote>** Corresponding author. SCARCE Laboratory, Energy Research Institute @ NTU (ERI@N), Nanyang Technological University, Singapore, 637459, Singapore.
E-mail addresses: han.bo@ntu.edu.sg (B. Han), jean-christophe.gabriel@cea.fr (J.-C.P. Gabriel).</footnote>
Nomenclature

<table><tr><td>BSA</td><td>Bovine Serum Albumin</td></tr><tr><td>BET</td><td>Brunauer-Emmett-Teller</td></tr><tr><td>CNT</td><td>Carbon nanotube</td></tr><tr><td>CUS</td><td>Coordinatively unsaturated site</td></tr><tr><td>DMF</td><td>Dimethylformamide</td></tr><tr><td>EDX</td><td>Energy dispersive X-ray spectroscopy</td></tr><tr><td>FESEM</td><td>Field Emission Scanning Electron Microscopy</td></tr><tr><td>FRR</td><td>Flux Recovery Rate</td></tr><tr><td>FTIR</td><td>Fourier Transform Infrared Spectroscopy</td></tr><tr><td>GO</td><td>Graphene Oxide</td></tr><tr><td>HRTEM</td><td>High-resolution transmission electron microscopy</td></tr><tr><td>ICP-OES</td><td>Inductively Coupled Plasma Optical Emission</td></tr></table>

<table><tr><td></td><td>Spectroscopy</td></tr><tr><td>MOF</td><td>Metal-Organic Framework</td></tr><tr><td>NCM</td><td>Non-contact mode</td></tr><tr><td>NPs</td><td>Nanoparticles</td></tr><tr><td>PCPs</td><td>Porous Coordination Polymers</td></tr><tr><td>PA</td><td>Polyamide</td></tr><tr><td>PEI</td><td>polyethyleneimine</td></tr><tr><td>PV</td><td>Photovoltaic</td></tr><tr><td>PVDF</td><td>Polyvinylidene fluoride</td></tr><tr><td>TFC</td><td>Thin film composite</td></tr><tr><td>TFN</td><td>Thin-film nanocomposite</td></tr><tr><td>TGA</td><td>Thermalgravimetric Analysis</td></tr><tr><td>WCA</td><td>Water contact angle</td></tr><tr><td>XRD</td><td>X-ray diffraction</td></tr></table>

~47 % of the material value (Weckend et al., 2016). In addition, the PV industry has consumed approximately 12.7 % of the annual Ag production in 2020, a market share that is expected to continue growing over the next decades. However, current reported Ag recovery from EOL PV remains only at lab-scale or pilot trial settings. The proposed methodologies consume significant amounts of chemicals and are generally laborious - requiring either multi-step precipitation-oxidation-reduction processes, cementation, or careful control of parameters to avoid contamination and achieve high purity (Yashas et al., 2023; Wang et al., 2020; Cho et al., 2019). These shortcomings further increase the costs incurred, making adaptation of PV recycling even less attractive. Therefore, there is still a need to develop cost-effective and efficient approaches to recover Ag from EOL PV.

One possible solution is to use a suitable and selective membrane. The unique nature of membranes allows water penetration while selectively rejecting salt/heavy metal ions, making them widely employed in wastewater recovery and treatment applications. Since the first report by Cadotte et al., thin film composite (TFC) membranes have played an important role in water treatment (Cadotte et al., 1980). Structurally, the TFC membrane consists of a microporous substrate coated with a thin (0.1–1 µm) selective polyamide (PA) layer, where the interfacial polymerization of the liquid-organic phase is involved (Al Aani et al., 2018). The main drawback with TFC membranes is the irreconcilable trade-off between permeability and selectivity. To overcome this problem, Jeong et al. proposed the concept of TFN (thin film nanocomposite) membrane (Jeong et al., 2007), where nanoscale particles (NPs) are incorporated on the surface or inside the PA layer. In general, the NPs are hydrophilic materials with designable structures, stable frameworks and versatile porosity. Many studies have shown that the resultant TFN membrane exhibits improved antifouling performance, enhanced surface hydrophilicity and prolonged durability (Han et al., 2024a). In addition, it could possess both promising selectivity and permeability. Although various NPs such as TiO₂ (titanium dioxide) (Peyravi et al., 2014), ZnO (zinc oxide) (Etemadi et al., 2022), zeolite (Feng et al., 2024), GO (graphene oxide) (Li et al., 2019), CNT (carbon nanotube) (Wang et al., 2023) and titania nanotube (Azelee et al., 2017) have been employed for TFC membranes modifications to date, their progress have been hindered by the stringent requirements (e.g. cost, porosity, attachment, membrane capability and environmental concerns) needed for suitable NPs in TFN membrane.

In recent decades, a novel nanoscale composite, namely metal-organic frameworks (MOFs), has gathered considerable attention. MOFs are a class of materials constructed from metal nodes/clusters and organic ligands to form one-, two- or three-dimensional structures (Cheetham et al., 1999; Han et al., 2023). They have stable structures, ordered porous cavities, well-ordered geometry and tailorable frameworks with versatile functionalities (Han et al., 2024b, 2024c, 2025a; Arstad et al., 2008; Hwang et al., 2008). They also have potential to

create infinite homologies with different chemical composition (Liu et al., 2007; Ding et al., 2017; Han et al., 2024d, 2024e, 2025b; Luz et al., 2010) and crystal structures (Sutton et al., 2022; Han et al., 2022a, 2024f). Overall, they emerge as promising candidates to mitigate the above-mentioned problems associated with TFC/TFN membranes. However, MOFs cost relatively higher than other NPs hence hampering their progression and utilization (Rani et al., 2020; Han et al., 2022b). To lower such costs, we propose herein to upcycle Cu-rich PV waste as precursor for Cu-based MOFs. PV waste upcycling presents a transformative approach to sustainable materials science, where EOL panels are repurposed into high-value applications. Some examples demonstrated in academic studies are repurposing of Si into lithium-ion battery anodes (Boon Tay et al., 2022; Sim et al., 2023) or thermoelectric device (Cao et al., 2022). Another upcycling application is dye removal in wastewater remediation via the Al-based porous material obtained from polymeric backsheet and Al of EOL panels (Pham et al., 2022).

Among all the reported structures, Cu-BTC MOFs demonstrate great potential in the field of metal ion separation. Indeed, Cu-BTC (also known as HKUST-1, Basolite C300 and MOF-199), with the formula of $[Cu_3(TMA)_2(H_2O)_3]_n$ (where TMA is benzene-1,3,5-tricarboxylate), is a highly porous metal coordination polymer (Chui et al., 1999). Structurally, Cu-BTC has accessible main channels (9 Å) and tetrahedral side pockets (5 Å) with triangular windows (3 Å) (Kinik et al., 2017). Hence, it possesses a relatively large surface area (up to 2175 m²/g) and pore volume (about 0.75 cm³/g). Nowadays, Cu-BTC based membranes have been widely used in gas separation applications, such as $H_2/CH_4$ separation (Mao et al., 2014), methyl mercaptan removal (Ma et al., 2018), and $CO_2/N_2$ separation (Chuah et al., 2019). With these documented successful gas separations, as well as the lack of application in the wastewater treatment field, it prompts further investigation to assess the feasibility for metal ion separation and recovery.

This work aims to contribute to this burgeoning field by investigating the potential of using Cu/Ag MOF-based TFN membranes synthesized from PV wastes leachates towards the recovery of the same leachate. In other words, by exploiting the inherent properties of MOFs and integrating them into TFN membrane-based separation processes, this study strives to demonstrate a circular approach to maximize the utilization of PV waste while addressing critical environmental and resource sustainability concerns. Through a comprehensive exploration of the synthesis, characterization and performance evaluation of Cu/Ag MOF TFN membranes, this research seeks to advance the frontiers of sustainable PV waste management and resource recovery. Therefore, the main objectives of this work are: (i) preparation of Cu/Ag MOFs from PV leachate; (ii) preparation and characterization of leachate-synthesized Cu/Ag MOF based TFN membrane and (iii) Ag recovery from the leachate and performance evaluation using as-prepared TFN membrane.
## 2. Experimental section

### 2.1. Materials and chemicals

PV waste leachate preparation.

156 × 156 mm polycrystalline silicon-based solar cells were used as received. PV solder (coated with 60 %Sn 40 %Pb) was purchased from MSL Solar Energy Co. Ltd. The simulated PV waste matrix used in this study is prepared by mixing cells and solder with a fixed mass ratio of 4.47:1. Nitric acid (HNO₃, 70 wt %) and hydrochloric acid (HCl, 37 wt %) were purchased from Sigma-Aldrich Pte Ltd, Singapore and used as received to prepare dilute solutions for hydrometallurgical treatment. Hydrogen peroxide (H₂O₂; various concentrations of 5 % and 10 %) solutions were commercially purchased from Kimic Chemitech (S) Pte Ltd, Singapore. Purified water (type I) was obtained from Labconco WaterPro RO systems (Labconco Corporation).

#### 2.1.1. MOF synthesis

Copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O, 99 %), 1,3,5-Benzene-tricarbonyl Trichloride (C₆H₃(COCl)₃, 98 %, TMC), dimethylformamide (DMF, 99.8 %), ethanol (C₂H₅OH, <i ≥>>/i > 99.8 %) and hexane (CH₃(CH₂)₄CH₃, <i ≥>>/i > 97 %) were purchased from Merck.

#### 2.1.2. TFN membrane fabrication

Commercially available nanoporous hydrophilic polyvinylidene fluoride (PVDF) substrate was purchased from United Scientific for the fabrication of TFN membranes. Benzene-1,3,5-tricarboxylic acid (C₆H₃(CO₂H)₃, 95 %), piperazine (C₄H₁₀N₂, 99 %, PIP) were purchased from Merck. All chemicals were of analytical grade and were used without further purification.

### 2.2. Preparation of leachate

Simulated PV waste has been pre-treated to remove Al and Pb to minimize their interference in the MOF preparation step. HNO₃ is used to dissolve remaining Cu and Ag from the pre-treated samples (Fig. 1 (a)). Detailed procedure for the preparation of PV leachate is described in Section.

### 2.3. Syntheses of Cu-BTC and Cu/Ag-BTC MOFs

The synthesis of the pristine Cu-BTC MOF was modified from reported procedures (Chui et al., 1999; Morales et al., 2021; Lin et al., 2012) and described in the supporting document, S2.2 Synthesis of Cu-BTC MOF. (Fig. 1(b)). The MOFs generated from the PV waste leachate No. 1, No. 2 and No. 3 are referred to as leachate-synthesized MOF(1), MOF(2), and MOF(3), respectively. Pictures of all as-prepared MOF samples are shown in Fig. S3.

### 2.4. Fabrication of TFN membrane

Three main steps, namely solvent preparation, IP (interfacial polymerization) and heat treatment, were involved in the preparation of MOF-TFN membranes (Fig. 1(d)) (Han et al., 2024g). The detailed TFN membrane fabrication process is explained in the supporting documents, S2.3 Fabrication of TFN membrane. It should be noted that the Cu/Ag MOF-based TFN membranes made from different MOF loadings (5/10/15 mL aqueous solution) are hereafter referred to as TFN M5/M10/M15, respectively. The summary of all the TFN membrane fabricated in this work is furnished in Table S3 for comprehensive understanding. Five identical membranes were prepared for each variation using the same manufacturing protocol to ensure reproducibility. In

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Overview illustration of research flow for this work: (a) PV waste leachate preparation, (b) Cu/Ag MOF generation from the PV waste leachate, (c) as-synthesized Cu/Ag MOF from the PV waste leachate, (d) TFN membrane fabrication, (e) Ag recovery from the PV waste leachate via nanofiltration.</figcaption></figure>
addition, each characterization assay was performed in triplicate to ensure statistical significance and minimize error.

## 2.5. Characterization

The detailed characterization of both the Cu/Ag MOFs and the TFN membrane, including XRD, FESEM, FTIR, TGA, zeta potential analysis, nitrogen adsorption (BET surface area), water adsorption, contact angle measurements, elemental mapping, and AFM surface roughness analysis are explained in the supporting document (S2.4 Characterization) for comprehensive understanding.

## 2.6. Nanofiltration experiment

### 2.6.1. Metal ion separation

The metal ion separation and Ag recovery experiments were carried out using a high pressure stirred cell apparatus. The details and schematic system of the high pressure stirred cell are given in section S4 and shown in Fig. S8. Typically, a Cu/Ag MOF based TFN membrane was placed at the bottom of the cell with a stainless-steel support. The feed solution was stored in the reservoir and poured into the cell body, while the entire apparatus was tightly sealed. Dry nitrogen (N₂) gas (99.999 %) was then supplied from the top of the reservoir at a pressure of 2.5-17.5 bar. Figure S9 shows the Ag recovery experiment setup for clarity. The feed solution was then pushed through the TFN membrane, pumped up through the permeate tube and collected in a container as the permeate solution (Fig. 1(e)). Stirring is applied to ensure uniform mixing of the feed solution and to minimize concentration polarization on the TFN membrane surface. During the experiment, the permeate-containing vessel (Fig. S9, No.8) was placed on the balance and the change in overall weight was recorded as a function of time using the built-in weight-time analysis software. In addition, the metal concentrations in the final permeate solution was quantified by ICP-OES analysis (PerkinElmer Avio 200).

### 2.6.2. Regeneration and antifouling investigation

The regeneration test is performed in the high pressure stirred cell where the permeability of the membrane is monitored as the feed solution continuously passes through it. The permeability is evaluated at every 250 mL of feed solution as it is the maximum loading capacity of the stirred cell body. Within each nanofiltration cycle, 1 L of feed solution is used, after which regeneration occurs and then the next nanofiltration continues. After each nanofiltration experiment, the used TFN membrane was immersed in excess deionized (DI) water and agitated at 300 rpm under ambient conditions to facilitate the solubilization of any contaminants that may have adhered to the TFN membrane surface or pores. Water flux and permeability of the TFN membrane were compared before and after each regeneration cycle at 7.5 bar. The fouling behavior of the TFN membrane is influenced by several factors such as hydrodynamic operating conditions (e.g., cross-flow velocity), TFN membrane properties (e.g. surface roughness, charge and wettability) and feedwater composition (e.g. concentration/ type of foulant). In this study, the fouling behavior of the newly fabricated TFN membrane was evaluated by exposing it to a multi-ion solution (same feed solution used for metal ion separation experiments) containing BSA (bovine serum albumin) at a concentration of 200 mg/L under a constant pressure of 10 bar for a period of 4 h. The contaminated TFN membrane was then subjected to several water rinsing cycles of 1 h each.

### 2.6.3. Ageing test

The detachment tendency of nanoparticles was tested - using a modified quantification method as reported by Ng et al. (2020) - by immersion of the TFN membrane into 100 mL DI water and stirred for 16 h at room temperature. The main reason for selecting M15 for the aging test is that it has the largest amount of Cu/Ag MOF on the surface

and theoretically, has the highest tendency for the nanoparticles to detach under the same hydrodynamic shear stress agitation. If the interaction between the MOF-based NPs and the TFN membrane is weak, the hydrodynamic shear stress generated by stirring may cause the nanoparticles to detach from the TFN membrane. The detachment percentage ($D_{etach}$%) of the MOF can be calculated using the formula:

$$D_{etach}\% = \frac{C_{Cu} \cdot V_{H_2O}}{Cu\% \cdot M_{MOF}} \times 100\% \quad (1)$$

where $C_{Cu}$ represents the concentration of copper ions in the solution, which was measured hourly by ICP-OES. Cu% is the weight percentage of copper in Cu-BTC MOF (C₁₈H₆Cu₃O₁₂, copper benzene-1,3,5-tricarboxylate) with a value of 31.52 %. In addition, $M_{MOF}$ denotes the total mass of Cu-BTC MOF based NPs on the M15 TFN membrane, which was 15 mg,

## 2.7. TFN membrane performance assessment

Separation factor (Selectivity, $S_{M₁M₂}$)

$$S_{M_1 M_2} = \frac{c_{M_1 p} / c_{M_2 p}}{c_{M_1 f} / c_{M_2 f}} \quad (2)$$

where $c_{M_1 p}$ and $c_{M_1 f}$ represent the concentration of metal ion $M_1$ in the permeate and feed solution, respectively. $c_{M_2 p}$ and $c_{M_2 f}$ stand for the concentration of metal ion $M_2$ in the permeate and feed solution, respectively. Therefore, $S_{M_1 M_2} = 1$ means that the TFN membrane has no separation effect for $M_1$ and $M_2$ ions. On the other hand, $S_{M_1 M_2} > 1$ indicates that $M_1$ ions penetrate the TFN membrane preferentially over $M_2$ ions.

Water permeability (A, L/(m²·h·bar))

$$A = V / (A_m \cdot t \cdot \Delta P) \quad (3)$$

where V represents the volume of permeate solution (L), $A_m$ indicates the effective surface area of the TFN membrane (0.0016 m² in this work), t stands for the permeation time (s) and $\Delta P$ is the system pressure (bar).

Water flux ($J_w$, L/(m²·h))

$$J_w = \frac{\Delta V}{A_m \cdot \Delta t} \quad (4)$$

where $\Delta V$ indicates the changing volume of permeate solution (L), and $\Delta t$ is the total permeation time (s).

### 2.7.1. Flux recovery rate (FRR, %)

$$FRR = \left( \frac{J_{w2}}{J_{w1}} \right) \times 100 \quad (5)$$

where $J_{w1}$ represents the initial water flux of fresh TFN membrane before fouling, and $J_{w2}$ stands for the subsequent water flux of the TFN membrane after fouling investigation.

# 3. Results and discussion

## 3.1. Characterization of Cu-BTC and Cu/Ag-BTC MOFs

PXRD peaks can be indexed and matched well with the recorded peaks of the pristine Cu-MOF reported previously in the literature (Yang et al., 2019), which is indicative of a high purity and crystallinity of the as-synthesized sample. In addition, the well-matched XRD patterns between the pristine Cu-MOF and the leachate-synthesized Cu/Ag-MOFs suggest negligible impact of the varying Cu:Ag concentration in the PV waste leachates on the crystal structures of the obtained Cu/Ag-MOFs (Fig. 2(a)).

Similarly to XRD results, peak positions are consistent with those
<figure><img src="image_5.png" /><figcaption>Fig. 2. (a) PXRD patterns with peak indexations and (b) FTIR profiles of the pristine Cu-MOF and leachate-synthesized Cu/Ag-MOFs.</figcaption></figure>

reported in other work (Sun et al., 2019), and the spectra are similar between the pristine Cu-MOF and the leachate-synthesized bimetallic Cu/Ag-MOFs (Fig. 2(b)). The signal recorded at 728 cm⁻¹ is attributed to the Cu-O vibration present in the MOF structure. The bands recorded near 1448/1370 cm⁻¹ and 1647/1549 cm⁻¹ are related to the vibrations of carboxylate (-C(O)O) groups. The broad band observed at the range of 3600-3200 cm⁻¹ in all the samples, is attributed to the weak bond (-OH) between water and the MOF structure. These FTIR observations indicate that the chemical bonding integrity of the Cu/Ag MOFs is maintained during the synthesis process using the PV waste leachate as the metal ion source.

The thermal stabilities of the synthesized bimetallic Cu/Ag MOFs were carried out and are illustrated in Fig. 3(a). As the temperature increases from 30 °C to 600 °C, a three-stage weight loss pattern is observed for all samples. An initial weight loss occurs between 30 °C and 85 °C for the pristine Cu-MOF corresponds to the evaporation of water molecules from the outer cage of the sample (step I in Fig. 3(a)). Subsequently, further dehydration of water vapor from the micropores leads to a more significant weight loss as the temperature rises to 350 °C.

During this phase, the MOF framework collapses, and decomposition occurs around 325–328 °C. Finally, carbonization takes place at higher temperatures (~600 °C), resulting in a weight residue (Step III in Fig. 3 (a)). Similarly, the leachate-synthesized Cu/Ag MOFs follow similar weight loss profiles, with comparable weight loss ratios and decomposition temperatures. The TGA derivative curves of all samples are shown in Fig. S10 for full disclosure and local peak decomposition temperature determinations. These results demonstrate the similarities in thermal stability behavior of the leached MOF NPs.

Zeta potential analysis shows that the leachate-synthesized MOFs are positively charged under acidic conditions (1 ≤ pH ≤ 5), and they exhibit stronger positivity than the pristine Cu-MOF at pH = 1 (which is also the pH of the PV waste leachate in this work) (Fig. 3(b)). According to the solubility-diffusivity theory (Zhao et al., 2020), pressure-driven ion transfer across TFN membranes occurs in two consecutive steps: (i) adsorption and dissolution of ions at the TFN membrane surface, and (ii) their subsequent diffusion through the TFN membrane channels. The electrostatic repulsion between the ions and the TFN membrane is crucial for the selective separation of metal ions as the surface charge of

<figure><img src="image_6.png" /><figcaption>Fig. 3. (a) TGA results and (b) Zeta-potential analysis of the pristine Cu-MOF and leachate-synthesized Cu/Ag-MOFs.</figcaption></figure>
the TFN membrane influences the retention and transport of the metal ions. Theoretically, a TFN membrane surface with a higher positive charge is expected to exert greater rejection of positively charged ions, particularly divalent ions such as Cu²⁺ (when compared to monovalent ions such as Ag⁺), due to electrostatic repulsion. By loading these NPs onto the TFN membrane, the surface positivity of the TFN membrane is increased, which would enhance the selectivity of Ag over Cu.

The hydrophilicity and water adsorption behavior of Cu/Ag MOFs over a wide pressure range (0< P/Ps < 1) at 30 °C is illustrated in Fig. 4 (a). As P/Ps increases, three distinct steps in water uptake are observed for the pristine Cu-MOF: (i) initially, the overall behavior resembles a Type I isotherm, with a rapid increase in water uptake up to 0.172 g/g near the Henry's region (0 < P/Ps < 0.1); (ii) subsequently, the water uptake rate gradually increases within a narrow pressure range (0.1 < P/Ps < 0.2), reaching a maximum capacity of 0.325 g/g at P/Ps = 0.2; (iii) finally, the water uptake continues to increase steadily at higher pressure levels, with a maximum capacity of 0.469 g/g observed at P/Ps = 0.9. This result demonstrates the hydrophilic and water adsorption performance of pristine Cu-MOF in aqueous environment, which is in good agreement with the reported literature (Han et al., 2024h). Similar water adsorption isotherm profiles are observed for all three leachate-synthesized Cu/Ag MOFs; however, the maximum capacities of these NPs are found to be reduced (<0.35 g/g) even at high relative pressure. This finding suggests that the leachate-synthesized samples have similar porosity, although with smaller pore volume compared to the pristine Cu-MOFs, as can be observed on the quantitative analysis of porosity for all Cu-MOFs shown in Table 1.

This property is further validated by N₂ adsorption experiments (Fig. 4(b)), and the detailed porosity information (such as BET surface area and pore volume) is given in Table 1. The BET surface area of the pristine Cu-MOF was found to be high (1962.0 m²/g), providing a large surface availability for metal ions to interact during filtration. It also exhibits a remarkable total pore volume of 0.91 cm³/g, suggesting a large capacity for metal ion adsorption and storage. A high abundance of micropores is found in the pristine Cu-MOF, where the micropore occupancy (Vm/Vt) is determined to be 0.768. Conversely and in coherence with previous observation, the MOFs generated from the PV waste leachate show reduced BET surface areas (Cu/Ag MOF(1–3): 1211.2 m²/g, 906.8 m²/g and 714.4 m²/g, respectively) and narrower pores (Vm for Cu/Ag MOF(1–3): 0.34 cm³/g, 0.25 cm³/g and 0.16 cm³/g, respectively). These differences are mainly attributed to the fact that the pore formation of the MOF during the synthesis process (deprotonation-

<table><caption>**Table 1**
Porosity (BET surface area, pore volume and pore size) of the pristine Cu-MOF and leachate-synthesized Cu/Ag-MOFs.</caption><thead><tr><th>Samples</th><th>Average pore diameter (nm)</th><th>BET surface area (m²/g)</th><th>Micropore volume (V<sub>m</sub>, cm³/g)</th><th>Total pore volume (V<sub>t</sub>, cm³/g)</th><th>V<sub>m</sub>/V<sub>t</sub></th></tr></thead><tbody><tr><td>Pristine Cu-MOF</td><td>1.86</td><td>1962.0</td><td>0.70</td><td>0.91</td><td>0.77</td></tr><tr><td>Cu/Ag-MOF (1)</td><td>2.29</td><td>1211.2</td><td>0.34</td><td>0.69</td><td>0.49</td></tr><tr><td>Cu/Ag-MOF (2)</td><td>2.45</td><td>906.8</td><td>0.25</td><td>0.56</td><td>0.46</td></tr><tr><td>Cu/Ag-MOF (3)</td><td>2.67</td><td>714.4</td><td>0.16</td><td>0.36</td><td>0.43</td></tr></tbody></table>

complexation-termination) is strongly related to the metal-ligand bonding strength (Abdelkareem et al., 2022). In the process of creating nanoscale MOFs, complexation and termination occur rapidly, reducing the concentration of metal ions relative to the linkers (Marshall et al., 2019). To produce such nanostructures, it may be necessary to incorporate an excess of linkers or acid modulators during synthesis (Marshall et al., 2019). The Cu²⁺ concentration in the leachates decreases from No.1 to No.3 (Table S1). Assuming that the acid modulator does not interfere with linker deprotonation, in reduced metal concentration scenario, both linkers and modulators can effectively encapsulate nano-MOF structures. The pore size distribution (PSD) of all the MOFs is shown in Fig. S11 for a comprehensive understanding. Furthermore, there is pH difference between the synthetic precursors (Cu(NO₃)₂·3H₂O solution: pH ~3 vs PV waste leachate: pH ~1 respective) which can influence the protonation states, stability and coordination chemistry of the metal ions and ligands (Stock et al., 2011), hence (Stock et al., 2011) affecting the formation of MOF crystals (Jiang et al., 2015). At low pH, the kinetics of nucleation and crystal growth can vary and in turn, affecting the size and shape of the resulting MOF crystals (Van Vleet et al., 2018). Therefore, all the leachate-synthesized MOFs show promising micropore occupancy (Vm/Vt), indicating the densely packed micropores. NPs with narrow pore tunnels would enhance the sieving effect during the nanofiltration and bring favorable properties for the metal ion separation between Cu²⁺ and Ag⁺.

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** (a) Water adsorption isotherms and (b) N₂ adsorption isotherms of the pristine Cu-MOF and leachate-synthesized Cu/Ag-MOFs.</figcaption></figure>
The SEM images show that the pristine Cu-MOF is analogous to an octahedral shape with a relatively smooth surface (Fig. 5(a)). Comparable microstructure with similar particle size are observed for all three leach-synthesized MOFs (Fig. 5(b)–(d)). Some crystals with irregular angles or broken surfaces are detected, which may be attributed to defects formation during synthesis. In addition, aggregation behaviour is observed for all Cu/Ag MOFs based NPs. These results demonstrate that the morphological integrity and structural stability of the Cu/Ag MOFs are maintained by the generation process in the PV waste leachate.

In summary, the characterizations corroborate the structural properties of the Cu/Ag MOFs are insensitive to the compositional variations of the PV waste leachate. Reproducibility of Cu/Ag MOFs with consistent and comparable properties could be consistently achieved from different batches of PV waste leachate. As this represents the first demonstration of Cu/Ag MOF synthesis directly from PV waste leachate, the work focusses on validating the feasibility and effectiveness of this approach. Future work will involve more extensive, systematic investigation of additional experimental parameters (such as reaction temperature, time, and reactant ratios) to further refine and optimize the material properties for improved performance.

## 3.2. Characterization of TFN membrane

### 3.2.1. Chemical structure

XRD and ATR-FTIR techniques are employed to investigate the chemical structure of the PVDF substrate and the Cu/Ag MOFs TFN membrane skin layer. The presence of additional XRD peaks indicates that the MOFs have been successfully integrated into the TFN membrane structure of the pure PVDF substrate (Fig. 6(a)). In the case of a PVDF substrate, the peaks observed at 836 and 875 cm⁻¹ can be attributed to the stretching vibrations of C-F and the asymmetric stretching vibrations of C-C-C (Fig. 6(b)). The presence of an adsorption peak at about 1173 cm⁻¹ is associated with the C-C band. An adsorption peak corresponding to the bending vibrations of CH₂ is detected at around 1402 cm⁻¹ (Han et al., 2024g; Lim et al., 2020). In addition, a strong peak around 1650 cm⁻¹ is observed, corresponding to C=O stretching vibrations in the

amide linkage of the polyamide selective layer. The bands observed at 1730 cm⁻¹ and 1828 cm⁻¹ correspond to the stretching vibration of the C=C bond. The bands at 2982 cm⁻¹ and 3024 cm⁻¹ are due to the symmetric and asymmetric vibration of the CH₂ groups in the PVDF substrate (Han et al., 2024g; Lim et al., 2020). The additional peaks observed in the Cu/Ag MOFs based TFN membrane are mainly caused by the interfacial polymerization process and the presence of MOFs within the polyamide selective layer (Fig. 6(b)). The XRF spectra corresponding to the PVDF substrate and TFN membranes are shown in Fig. 6(c). The peaks recorded at 8.05 and 8.91 keV in the TFN membranes support the presence of Cu-based MOF NPs on the TFN membrane surface.

### 3.2.2. Thermal stability

To access the thermal stability of the as-prepared membrane, two experiments were performed. Firstly, the TFN membrane is heated in a drying oven at different temperatures (25/50/75/100 °C) for 1 h. No delamination or mechanical failure observed at the higher temperature condition (Fig. S6). Secondly, the TFN membrane is tested in the TGA apparatus using the same method as for MOF. The decomposition temperature of the TFN membrane is determined to be 478 °C (Fig. S7). These results indicate that the membrane has sufficient thermal robustness for practical applications under moderate temperature conditions.

### 3.2.3. Surface morphology and thickness

Fig. 7(a.1 to a.3) shows the surface morphology of the TFN membranes (M5, M10 and M15), where similar wrinkled structures are observed in all three of them. The distinctive "ridge and valley" morphologies of the TFN membrane's surface result from interfacial polymerization. Indeed, it has been reported that the presence of hydrophilic NPs promotes the miscibility of the organic (solution A) and aqueous (solution B) phases during IP process (Lind et al., 2009). Contact between the hydrophilic Cu-based MOFs in the aqueous solution and TMC from the organic phase leads to exothermic particle hydration (Cu²⁺(MOF) + xH₂O → [Cu(H₂O)ₕ]²⁺). This localized heating effect increases the mobility of PIP and enhances its diffusion to the interface,

<figure><img src="image_8.png" /><figcaption>Fig. 5. SEM images of the (a) pristine Cu-MOF and leachate-synthesized Cu/Ag-MOFs ((b) 1, (c) 2 and (d) 3).</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** (a) XRD curves, (b) ATR-FTIR profiles, and (c) XRF spectrum of pure PVDF substrate, leachate-synthesized Cu/Ag-MOFs based TFN membrane.</figcaption></figure>

where it can more effectively react with TMC and hence facilitates polymerization ((NH)₂-CH₂-CH₂ + (COCl)₃-Ph → PA layer + HCl) (Zhang et al., 2020). In Fig. 7, a relatively uniform distribution of nanoparticles on the TFN membrane surface is observed for M5 (Fig. 7 (a.1)) and M10 (Fig. 7(a.2)), with no pinhole, gap or crack, indicating a good compatibility between the Cu/Ag MOFs and the PA matrix. However, increasing the MOF loading results in thicker and more hydrophilic MOF layers, which may prevent contact between the two solutions during the IP reaction process. Therefore, increasing the MOF

incorporation in M15 resulted in observable NPs aggregation (Fig. 7 (a.3)). NPs aggregation leads to uneven dispersion of MOFs within the PA selective layer (hence creating voids), which are detrimental to the TFN membrane's metal ion separation performance.

The cross-sectional view of the TFN membranes M5/M10/M15 are shown in Fig. 7(b.1-b.3), respectively, together with the EDX mapping of F (mainly detected for the PVDF substrate) and Cu (mainly present for the polyamide selective layer). These SEM images allow to measure the thicknesses of the selective layer + PVDF substrate combined structure of the TFN membranes with different MOF loading levels.

As observed in Fig. 7(b.1), the PVDF substrate has a thickness of 55.83 µm, while the selective layer thickness for M5 is approximately 53.33 µm. When the MOF encapsulation amount increases, the selective layer thickness increases to 98.96 µm and 125.00 µm (Fig. 7(b.2 and b.3) for M10 and M15) respectively. As expected, this confirms an increase in the thickness of the MOF NPs layer with increasing MOF loading. Notably, the presence of MOF at different loading levels may affect the efficiency of the IP process between PIP and TMC. It should be highlighted that the PA layer thickness of the TFN membrane does not typically increase linearly with the NP loading. At low to moderate concentrations (i.e., M5 and M10), MOF NPs can be relatively well dispersed, potentially resulting in a slightly thicker and still evenly-uniform PA layer due to increased interaction sites between the monomers and nanoparticles. Therefore, the thickness of the PA layer increases from 55.83 µm to 98.96 µm (doubling the thickness means doubling the amount of MOF NPs). However, as more NPs are incorporated into the IP process, they can interfere with the interfacial polymerization process by physically blocking or altering the distribution of monomers at the interface (Wang et al., 2017; Abadikhah et al., 2018). This can result in uneven or discontinuous growth of the PA layer. In addition, high NP concentrations can lead to agglomeration, causing defects, voids or localised variations in the thickness of the PA layer (Wang et al., 2017; Abadikhah et al., 2018). If NPs disrupt the smooth diffusion of monomers, this can result in an irregular PA layer despite increased NP loading. Furthermore, the thickness of the PVDF substrate decreases continuously with increasing MOF loading, which may be due to the compression effect during the TFN membrane fabrication process (vacuum filtration). Due to the shorter diffusion pathways, thinner open TFN membranes have relatively high permeability. They also require less energy to operate as there is less material to push fluid through (Tang et al., 2023). However, as the thickness of the TFN membrane increases, thicker TFN membranes would provide more physical barriers against foulants and mechanical failure (Kadhom, 2023).

### 3.2.4. Surface roughness

The 3D rendering AFM images of the pure PVDF substrate and the TFN membranes (M5, M10 and M15) are shown in Fig. 8. Typical "ridge and valley" structures are observed for the PA selective layer of the TFN membrane, which corresponds to the "leaf-like" morphology obtained by FESEM. It is also observed that higher ridges are formed when higher amounts of Cu/Ag MOFs based nanoparticles are incorporated into the TFN membrane. This is confirmed by the measurement of the surface roughness analysis that are summarized in Table 2. A lower roughness value indicates a more uniform dispersion of MOF NPs over the PVDF substrate. However, as the Cu/Ag MOF encapsulation increases, the significant aggregation of the MOF particles becomes apparent. The higher surface roughness leads to a greater increase in surface area, which in turn promotes interaction between water molecules and the TFN membrane, potentially improving water permeability (He et al., 2020). It can also create complex microenvironments on the surface, which improves the selective separation efficiency. Conversely, the TFN membrane with a rougher surface is likely to show increased foulant adhesion and poor antifouling performances (Han et al., 2024g).
<figure><img src="image_10.png" /><figcaption>Fig. 7. (a.1-a.3) Surface morphologies of TFN membrane M5, M10 and M15, respectively. (b.1-b.3) Cross-sectional SEM images with EDS elemental mapping analysis of TFN membrane M5, M10 and M15, respectively.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. AFM images of (a) pure PVDF surface, and (b–d) leachate-synthesized Cu/Ag-MOFs based TFN membrane surface: M5, M10 and M15, respectively.</figcaption></figure>

### 3.2.5. Wettability

The wettability of the TFN membranes is examined by the water contact angle (Fig. 9). Smaller contact angles (i.e., more hydrophilic) of the TFN membranes are observed as compared with that of the PVDF

substrate (Table 3). With the addition of MOFs particles on the PVDF substrate, the contact angle is reduced from about 89° to 29°, 25°, and 22° respectively with the increasing amount of MOF NPs (Fig. 9(a–d)). The improved wettability of the TFN membrane is mainly attributed to
<caption>**Table 2**</caption>

TFN surfaces' root mean square roughness ($R_{ms}$), average roughness ($R_a$) and maximum vertical spacing ($R_z$) for pure PVDF substrate and leachate-synthesized Cu/Ag-MOFs based TFN membranes.

<table><thead><tr><th>Membrane</th><th>R<sub>ms</sub> (μm)</th><th>R<sub>a</sub> (μm)</th><th>R<sub>z</sub> (μm)</th></tr></thead><tbody><tr><td>Pure PVDF substrate</td><td>0.355</td><td>0.281</td><td>2.713</td></tr><tr><td>TFN M5</td><td>0.517</td><td>0.421</td><td>2.655</td></tr><tr><td>TFN M10</td><td>0.520</td><td>0.430</td><td>3.350</td></tr><tr><td>TFN M15</td><td>0.580</td><td>0.463</td><td>3.505</td></tr></tbody></table>

(i) the hydrophilic nature of the Cu/Ag MOFs additives results in higher surface hydrophilicity of the TFN membrane; (ii) the surface free energy and polarity of the TFN membrane surface is enhanced by the loading of Cu/Ag MOF NPs, thus increasing the surface hydrophilicity for TFN membranes and (iii) the pores of the hydrophilic nanoparticles promote the water transport via capillary effect (Ghanbari et al., 2015). Theoretically, higher surface hydrophilicity suggests higher water affinity, permeability and fouling resistance. Based on the Wenzel's Law $cos θ^* = r·cos θ$ (where r represents roughness factor, $θ$ and $θ^*$ refer to the Young's angle and measured contact angle respectively), the surface hydrophilicity of the TFN membrane is also related to the surface roughness. It is known that r is greater than 1 for hydrophilic and rough surface, the $θ^*$ is negatively correlated with r. In other words, the rougher the TFN membrane surfaces, the smaller of the contact angle and the more hydrophilic of the TFN membrane (Yang et al., 2020). According to the surface roughness and wettability analysis conducted, the TFN membrane has a rougher and more hydrophilic surface as compared to that of TFC, which will favor permeability.

## 4. Metal ions separation via nanofiltration

### 4.1. Silver recovery performance

Fig. 10(a) shows the metal ion separation mechanism of TFN membranes, demonstrating the electrostatic repulsion (Fig. 10(a.2)) and pore sieving effect (Fig. 11(a.1)). In addition, Fig. 11(b) shows the metal ion

separation performance of TFN membranes with insufficient MOF loading (Fig. 10(b.1)), adequate MOF loading (Fig. 10(b.2)), and MOF overloading (Fig. 10(b.3)). The effect of MOF loading of the TFN on the $Ag^+$ selectivity over $Cu^{2+}$ (denoted as $S_{Ag^+, Cu^{2+}}$) was investigated at the operating pressure of 7.5 bar and results are shown in Fig. 11(c). As a blank test, pure PVDF substrate was also tested and found to have no discernible separation effect ($S_{Ag^+, Cu^{2+}} = 0.978$). All the TFN membranes show $Ag^+$ selectivity ($S_{Ag^+, Cu^{2+}}$ for M5, M10 and M15 is 4.42, 7.99 and 5.72, respectively) which is mainly attributed to the enhanced surface sinuosity of the MOFs and the polymer additives. Furthermore, the metal ion separation efficiency can be influenced by the surface charges on the TFN membrane. Due to the synergistic effects of molecular sieving (Fig. 10(a.1)) and Donnan repulsion (Fig. 10(a.2)), a positively charged TFN membrane tends to reject more divalent cations (such as $Cu^{2+}$) than monovalent anions (such as $Ag^+$). As the NPs loading increases, the $Ag^+$ selectivity first increased (from M5 to M10) then decreased (from M10 to M15). Initially, more NPs improve the penetration barrier for the larger divalent metal ions (such as $Cu^{2+}$). In addition, MOF additives with intrinsic three-dimensional structure will heighten the pore tortuosity of TFN membrane surface and reduce the $Cu^{2+}$ passage, further improving the $Ag^+$ recovery performance. However, the $Ag^+$ selectivity gradually declines with further increase of Cu/Ag MOFs based NPs loading, which is mainly attributed to the random aggregation of MOF NPs on the TFN membrane (Fig. 10(b.3)) surface. Therefore, uncovered and/or unaligned nanochannels could also act as a metal ion transport channel, resulting in poor metal ion separation. Furthermore, it is reported that the salt rejection by the polyamide

<caption>**Table 3**</caption>

Water contact angle (CA) of pure PVDF substrate and TFN membranes (with different MOF loading amount).

<table><thead><tr><th>Membrane</th><th>CA left (°)</th><th>CA right (°)</th></tr></thead><tbody><tr><td>Pure PVDF substrate</td><td>88.9</td><td>89.1</td></tr><tr><td>TFN M5</td><td>30.1</td><td>28.9</td></tr><tr><td>TFN M10</td><td>25.0</td><td>25.1</td></tr><tr><td>TFN M15</td><td>22.1</td><td>22.4</td></tr></tbody></table>

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Water contact angle screenshots of (a) pure PVDF surface, and (b–d) leachate-synthesized Cu-BTC MOF based TFN membrane surface: M5, M10 and M15, respectively.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** (a) Working mechanism of TFN membrane, including pore sieving (a.1), and electrostatic repulsion (a.2) effect, (b) Schematic diagram of TFN membrane with insufficient (b.1), appropriate (b.2) and excess (b.3) MOF loading, (c) Effect of the various MOF loadings on Ag⁺ selectivity (S<sub>Ag⁺, Cu2⁺</sub>) at the operating pressure of 7.5 bar, (d) Ag⁺ and Cu2⁺ concentration of permeate solution after each separation cycle for TFN membrane M10.</figcaption></figure>

selective matrix is assumed to be unchanged (Ben-David et al., 2006). However, the salt rejection by the void space between the polyamide matrix and the nanoparticles is reduced due to the increased channel size as more nanoparticles are loaded (Ghanbari et al., 2015). As a result, more Cu²⁺ passes through the TFN membrane and Ag⁺ selectivity decreases. It should be noted that M10 is used for all nanofiltration performance tests, including selectivity, water flux, permeability, regeneration, and antifouling test. Especially for the selectivity analysis, the operating pressure is 7.5 bar.

A series of multi-filtration experiments was performed to evaluate the Ag⁺ recovery capability of the TFN membrane. As a start, the PV waste leachate was used as the feed solution for the first filtration cycle. The resulting permeate solution was then reintroduced as the feed solution for the second filtration cycle. This process was repeated for five filtration cycles. The concentration of Ag⁺ gradually increased during the successive filtration cycles (Fig. 10(d)). During the first four filtration cycles, the Ag⁺ concentration in the permeate solution increased from 44.75 mg/L to 158.34 mg/L. The Ag⁺ recovery rate has reached 93.8 % and 99.1 % by the fourth and fifth cycles (with respect to the initial feed solution), respectively. In addition, during the cyclic nanofiltration, the Cu²⁺ concentration in the permeate solution was reduced to 1557.25 mg/L, which corresponds to a 67.8 % reduction with respect to the initial PV waste leachate (feed solution). In other words, the molar ratio of Ag⁺/Cu²⁺ is enhanced by three-fold from 1:28.6 to 1:9.2.

Fig. 11 shows the water flux and permeability of the TFN membranes. All TFN membranes show a promising water flux, which is mainly attributed to the additional water channels introduced by the hydrophilic MOF nanoparticles. The water flux is approximately linearly proportional to the transmembrane pressure (Fig. 11(a)) (with a linear regression coefficient R² = 0.999, 0.996 and 0.997 for M5, M10 and M15, respectively). For example, from 2.5 bar to 15 bar pressure, the water flux of M10 increases from 146.5 LMH to 551.5 LMH. In addition, the Cu/Ag MOFs based additives enhance the surface hydrophilicity/water affinity of the TFN membrane, which facilitates the diffusion of

water molecules through the TFN membrane, resulting in a promising permeability for TFN membranes. It is also worth highlighting that the permeability of TFN membranes is not only related to the hydrophilic property of nanoparticles, but also to other factors such as porosity, surface charge and roughness. Furthermore, a non-monotonic behavior of water permeability is observed with increasing MOF loading. As the MOF loading increases from M5 and M10 and to M15, the water permeability of the TFN membrane initially increases due to the increased hydrophilicity. However, when the MOF is over-loaded (i.e., M15), the water permeability starts to decrease significantly due to the higher degree of particle aggregation and blocking effect from the excess MOF layer. This finding also implies that the optimum MOF loading is M10 as it has the highest Ag⁺ selectivity and water permeability. Therefore, TFN M10 was utilised for further studies on regeneration and antifouling tests.

In addition, the results show a linear increase in water flux with increasing operating pressure, which is consistent with the behavior of nanofiltration membranes governed by Darcy's law (Nassehi, 1998; Zhang et al., 2018). This indicates that the membrane maintains a relatively stable permeability without significant compaction effects, confirming its mechanical robustness. At lower pressures (2–4 bar), the water flux increases steadily, suggesting that the driving force is sufficient to overcome the intrinsic resistance of the selective layer while maintaining efficient transport through the membrane pores. As the pressure increases above 6 bar, a more gradual increase in flux is observed, which could be attributed to the increasing effect of concentration polarization and the intrinsic resistance of the polyamide layer. However, the membrane does not show any signs of flux decrease, indicating that the Cu/Ag MOF incorporation does not lead to severe pore blockage at higher pressure conditions.

On the other hand, Fig. 11(b) shows the permeability trends of the Cu/Ag MOF-based TFN membrane under different operating pressures, providing important insights into its filtration performance. As shown in Fig. 11(b), the water permeability of the TFN membrane decreases
<figure><img src="image_14.png" /><figcaption>Fig. 11. (a) Water flux; (b) Permeability assessments of Cu/Ag-MOFs based TFN membranes.</figcaption></figure>

progressively with increasing applied pressure. This behavior can be attributed to the enhanced concentration polarization effect and compaction of the fouling (cake) layer formed during filtration. At higher pressures, the increased permeate drag promotes solute accumulation at the membrane surface, forming a concentration boundary layer with elevated osmotic pressure. This reduces the effective trans-membrane driving force for water transport. Simultaneously, the higher pressure compresses the fouling layer, making it denser and less permeable, further impeding water flux. Therefore, the interplay between intensified concentration polarization and pressure-induced fouling compaction leads to the observed reduction in permeability at elevated pressures. The permeability of the M10 TFN membrane remains relatively stable over the pressure range (5–15 bar), reaching a maximum of 58.6 L/(m²·h·bar) at 2.5 bar, demonstrating the balance between enhanced water transport and selectivity. Therefore, the presence of Cu/Ag MOFs facilitates additional water transport pathways, increasing overall permeability without compromising rejection efficiency.

## 4.2. Antiaging, antifouling and regeneration performances

One of the primary failure modes of TFN membranes is the detachment of nanoparticles from the PA selective layer. This detachment can occur due to weak interfacial bonding between the nanoparticles and the polymer matrix, leading to nanoparticle leaching under high shear forces or prolonged operating conditions such as high pressure and

turbulent flow. The loss of nanoparticles not only compromises the structural integrity of the membrane, but also deteriorate its performance, resulting in reduced permeability, selectivity and antifouling properties. In addition, detachment can create defects or voids in the PA layer, creating non-selective pathways that reduce the rejection efficiency of the membrane. Fig. 12(a) shows an indirect assessment of detachment tendency of MOF NPs from the TFN membrane by monitoring the Cu²⁺ concentration. It is observed that the concentration of copper ions increases linearly with time (with a linear regression coefficient R² = 0.9383), starting from 0.198 mg/L in the second hour and reaching 0.222 mg/L (remaining below 0.25 mg/L) in the sixteenth hour. The *D*<sub>etach</sub>% of the MOF is as low as 0.51 % (calculated by Eq. (1)) over the 16-h period, highlighting the stable adhesion between the substrate, the PA matrix and the MOF NPs.

TFN membrane fouling must then be considered in this flow through geometry as the rejected components do accumulate on the TFN membrane's top surface (Antony et al., 2011). TFN membrane fouling is synthetically influenced by the TFN membrane-contaminant interaction, such as van der Waals interaction, electrostatic repulsion, hydrogen bonding and hydrophobic interaction (Yan et al., 2016). The flux reduction of the TFN membrane for antifouling investigation is mainly caused by the "cake enhanced osmotic pressure" effect, where the osmotic pressure increase is caused at the salt accumulated foulant-based gel layer (Ghanbari et al., 2015). The flux recovery rate (FRR) consistently decreases with increasing transmembrane pressure (Fig. 12(b)). In particular, the Cu/Ag MOFs based TFN membrane exhibits superior antifouling properties (with FRR ranging from 86.7 % to 92.8 %). The promising antifouling performance of the Cu/Ag MOFs incorporated TFN membrane is mainly attributed to two reasons: (i) the Cu/Ag MOFs additives enhance the surface hydrophilicity of the TFN membrane, which consequently weakens the interaction between the foulant and the polyamide dense layer (Saraswathi et al., 2017); and (ii) the pH of the BSA foulant solution under the working conditions of this work is around 6 and is negatively charged. Based on the Zeta potential analysis, the Cu/Ag MOFs-based NPs also exhibit negative charge under this pH level. Therefore, the enhanced electrostatic repulsion force would have a favorable effect on TFN membrane fouling control (Childress et al., 1996). For a comprehensive understanding, the water flux of the TFN membrane before and after fouling are illustrated in Fig. 12(b). It is found that at lower pressures (e.g., 2–4 bar), the fouling layer formed on the membrane surface tends to be loosely bound and consists primarily of reversible fouling components such as weakly adsorbed metal ions and organic residues. As a result, the FRR remains relatively high, indicating effective flux recovery after cleaning. However, as the pressure increases (above 6 bar), the compaction of the fouling layer becomes more pronounced, resulting in greater adhesion of foulants to the membrane surface. This results in a gradual decrease in FRR values as higher pressure operation promotes the formation of dense cake layers and potentially induces pore blockage, making it more difficult to completely remove the fouling during the regeneration process. In addition, the surface morphology of the TFN membrane before fouling, after fouling and after regeneration is shown in Fig. S12.

Fig. 12(c) shows the regeneration performance of the TFN membranes. It can be observed that as the metal ion removal test progresses, within each nanofiltration cycle (without regeneration), the permeability gradually decreases with the flow of the feed solution up to 1 L. Subsequently, regeneration procedures were carried out, which resulted in the permeability being restored to its initial level. Furthermore, Fig. 12(c) shows the view and surface morphology images of the TFN membranes over nanofiltration cycles, from which the conditions of the TFN membrane surface could be visualized. A promising regeneration effect on the TFN membrane surface morphology is observed both before Ag⁺ recovery experiment (Fig. 12(d.1)), after Ag⁺ recovery experiment (Fig. 12(d.2-d.3)) and after regeneration (Fig. 12(d.4)). These results collectively demonstrate the effectiveness of the leachate-synthesized Cu/Ag MOFs based TFN membrane in recovering Ag⁺ from PV waste
<figure><img src="image_15.png" /><figcaption>**Fig. 12.** (a) Evaluation of detachment tendency of MOF-based nanoparticles from TFN membrane; (b) Antifouling performance; (c) Regeneration performance; and surface morphology of the TFN membrane (d.1) before nanofiltration, (d.2-d.3) after nanofiltration, and (d.4) after regeneration.</figcaption></figure>

<table><caption>**Table 4**
Examples of MOFs based TFN membranes for water purification and solute separation purpose.</caption><thead><tr><th>Work</th><th>NP Characteristics</th><th>Methodology remarks</th><th>Performances</th></tr></thead><tbody><tr><td>This work</td><td>Cu/Ag MOF synthesized from the PV waste</td><td>A case study on closing the loop of photovoltaic waste utilization</td><td>S<sub>Ag</sub>, Cu<sub>2</sub><sup>+</sup> = 7.99 Antifouling efficiency: FRR ~92.8 %<br>Anti-ageing: 0.5 % detachment of Cu/Ag MOF Permeability: 58.6 L/(m<sup>2</sup>·h-bar))</td></tr><tr><td>Jia et al. (2024)</td><td>NH<sub>2</sub>-MIL-101 (Cr) NPs enhanced permeability and facilitated rapid transport pathways for Li<sup>+</sup>.</td><td>The PEI interlayer modulated the structure of the PA layer and augmented the positive charge characteristics of NF membranes.</td><td>High water flux:<br>17 Lm<sup>-2</sup>·h<sup>-1</sup>·bar<sup>-1</sup><br>Li<sup>+</sup>/Mg<sup>2+</sup> separation factor: 102</td></tr><tr><td>Yin et al. (2024)</td><td>UiO-66-NH<sub>2</sub> was crucial in facilitating the formation of supplementary water channels for Li<sup>+</sup></td><td>A new technology integrated TFN with the flow-electrode capacitive deionization device (FCDI) for the selective separation of Li<sup>+</sup>/Co<sup>2+</sup><br>The modified FCDI equipment includes a concentration chamber to streamline the recovery process</td><td>Acceptable rejection of CoCl<sub>2</sub> (>95 %)<br>Effective separation of Li<sup>+</sup>/Co<sup>2+</sup>: 27.44</td></tr><tr><td>Ren et al. (2024)</td><td>Cellulose triacetate (CTA) membrane: large pore size and high porosity<br>Poly-dopamine (PDA)/graphene oxide (GO)-Fe<sup>3+</sup>-tannic acid (TA) interlayer</td><td>Spray-modified non-solvent induced phase separation (s-NIPS)</td><td>Water flux: 34.4 Lm<sup>-2</sup>·h<sup>-1</sup> with the draw solution of 1 M NaCl</td></tr><tr><td>Mi et al. (2024)</td><td>Zwitterionic polymeric nanoparticles (ZNPs) were produced using inverse miniemulsion polymerization</td><td>The microstructure and surface characteristics of TFN-ZNPs can be concurrently modified by varying NaCl concentrations.</td><td>The salt-responsive characteristics of TFN-ZNPs concurrently increased water permeance: 13.2 Lm<sup>-2</sup>·h<sup>-1</sup>·bar<sup>-1</sup><br>NaCl/dye selectivity: ~104 dye/dye selectivity: ~127<br>better antifouling behavior: from 85.6 % to 96.5 %</td></tr><tr><td>Usman et al. (2024)</td><td>MXene 2D nanosheet<br>Thin-layer nanoflakes-like structure</td><td>IP reaction for MXene-based sulfonated TFN (STFN) membrane</td><td>Higher rejection of >93 % for MgSO<sub>4</sub> and Na<sub>2</sub>SO<sub>4</sub></td></tr><tr><td>Hu et al. (2024)</td><td>ZIF-8 nanocrystals were grown in situ on the dendrimer layer</td><td>Asymmetric two-layered TFN structures (a-TFN)<br>The porous dendrimer layer serves as a gutter layer</td><td>Water permeance:<br>8.67 Lm<sup>-2</sup>·h<sup>-1</sup>·bar<sup>-1</sup><br>NaCl rejection: 98.1 %</td></tr><tr><td>Lu et al. (2024)</td><td>Boron nitride nanosheets (BNNS) sheet-like structures with a lateral size of hundreds of nanometers</td><td>PA layer thickness: ~39.53 nm<br>The fouling resistance test was evaluated by using BSA as contaminant</td><td>Water flux: 96.29 Lm<sup>-2</sup>·h<sup>-1</sup><br>Na<sub>2</sub>SO<sub>4</sub> rejection: 95.05 %</td></tr></tbody></table>
leachate while maintaining TFN membrane integrity.

### 4.3. Comparative study

This section provides comparative evaluation of other reported TFN membranes, that used different approaches to improve separation performance, antifouling properties and durability under challenging conditions. The key information of these works is summarized in Table 4 for comprehensive understanding. The performance of the hereby reported TFN membrane, designed for Ag recovery from PV waste leachate, is characterized by high selectivity ($S_{Ag^+} \cdot Cu^{2+} = 7.99$) and silver recovery efficiency (99.1 %) over multiple filtration cycles, together with excellent permeability and antifouling properties (FRR of 92.8 %). In comparison, the positively charged TFN membrane based on a polyethyleneimine (PEI) interlayer (Jia et al., 2024) achieved effective $Li^+/Mg^{2+}$ separation (separation factor: 102) but lacked the specific metal ion selectivity and high permeability observed in our study. The use of the PEI interlayer improved ion selectivity due to electrostatic interactions, but the flux performance remains lower than that of our Ag recovery membrane.

The UiO-66-NH₂ modified TFN membrane showed improved selectivity for $Li^+/Co^{2+}$ separation in a flow electrode capacitive deionization system, exploiting the affinity of the functional MOF for metal ions (Yin et al., 2024). Although the selective separation of metal ions is comparable, the study lacks detailed data on anti-aging and anti-fouling performance, which are critical for long-term applications such as Ag recovery. In addition, the incorporation of PDA/GO-Fe³⁺-TA interlayers in forward osmosis membranes improved mechanical robustness and water flux (Ren et al., 2024). However, the selectivity for specific metal ions such as Ag⁺ and Cu²⁺ was not investigated.

The introduction of zwitterionic polymeric nanoparticles into TFN membranes achieved high permeability and dye desalination efficiency due to improved hydrophilicity and antifouling performance (Mi et al., 2024). This is similar to the antifouling efficiency demonstrated by the TFN membrane in the current study, but the lack of metal ion selectivity data limits direct comparison. In the context of improved water flux and permeability, the MXene-enhanced TFN membranes offer significant advances in desalination, but their metal ion recovery capabilities, particularly for Ag, are not reported (Usman et al., 2024).

Both the ultra-permeable TFN membrane with an asymmetric structure (Hu et al., 2024) and the boron nitride nanosheet-embedded nanofiltration membrane (Lu et al., 2024) demonstrate high permeability and fouling resistance, comparable to the anti-fouling performance of our Ag recovery membrane. However, their primary focus was on water purification and desalination, without addressing metal ion selectivity and recovery, hence the interest of the present contribution.

## 5. Conclusions

In this study, we have reported a comprehensive approach to the management of EOL PV through the synthesis of Cu/Ag MOFs and TFN membrane technology. The results emphasize the insensitivity of the leachate-synthesized MOFs to the compositional variations of the precursor solutions, a key advantage when dealing with lixiviates from wastes. The successful syntheses of Cu/Ag MOFs from PV waste leachate validate the potential of industrial waste as a sustainable precursor material. Furthermore, Cu and Ag are commonly found together in many other wasted electronic components, it underlines our approach suitability for various applications beyond our study, such as the metal recovery from printed circuit boards (Charpentier et al., 2023, 2024; Xia et al., 2024). In addition, the fabrication of Cu/Ag MOFs into TFN membranes for nanofiltration applications demonstrated a new effective strategy for the recovery of Ag⁺ present in PV wastes.

Overall, this research demonstrates the synergistic integration of material synthesis and membrane technology for the sustainable management of PV waste leachate. By highlighting the utilization of PV

waste leachate, the promising performance of Cu/Ag MOFs and the high efficiency of Ag recovery with the TFN membrane, our study contributes to the advancement of sustainable waste management practices and resource recovery technologies in the fields of environmental engineering and materials science. This paves the way for future research to further optimize Cu/Ag MOFs synthesis, TFN membrane fabrication and process conditions to improve the efficiency and scalability of metal ion recovery from PV waste leachate.

### CRediT authorship contribution statement

**Bo Han:** Writing – original draft, Validation, Methodology, Formal analysis, Data curation, Conceptualization. **Ying Sim:** Writing – review & editing, Methodology, Investigation. **Qingyu Yan:** Writing – review & editing, Supervision, Funding acquisition. **Nripan Mathews:** Writing – review & editing, Supervision, Project administration, Funding acquisition. **Jean-Christophe P. Gabriel:** Writing – review & editing, Validation, Supervision, Project administration, Funding acquisition, Formal analysis, Conceptualization.

### Claim

During the preparation of this work the authors used "DeepL Write" to double-check for English spelling or grammatical errors or typos. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication.

### Declaration of competing interest

The authors 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, and National Environment Agency 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 B. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2025.145528.

### Data availability

Data will be made available on request.

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