SYNERGISTIC ROLE OF FUNCTIONALIZED CHITOSAN NANOPARTICLES AND ALIQUAT 336 IN POLY(VINYLIDENE FLUORIDE-CO-HEXAFLUOROPROPYLENE) (PVDF-HFP) POLYMER INCLUSION MEMBRANES FOR HIGH-EFFICIENCY GOLD(III) TRANSPORT
LimCeeKee1,2
NoorFazliani1Emailfazliani.s@umk.edu.my
A
BintiShoparwe1,2✉LimKeanChong1,3
NorZafirah1
BintiNor’azmind1,2
MuhammadAmideeBinAbdullah1,2
VimalanAnbalagan1,2
1
A
A
A
A
Gold, Rare Earth and Material Technopreneurship CentreUniversiti Malaysia Kelantan (UMK)17600 JeliKelantanMalaysia2Faculty of Bioengineering and TechnologyUniversiti Malaysia Kelantan (UMK)17600 Jeli KelantanMalaysia
3Faculty of Chemical Engineering and TechnologyUniversity Malaysia Perlis (UniMAP)02600Arau, PerlisMalaysia
Lim Cee Kee1,2, Noor Fazliani Binti Shoparwe1,2*, Lim Kean Chong1, 3, Nor Zafirah Binti Nor’azmind1,2, Muhammad Amidee Bin Abdullah1,2 and Vimalan Anbalagan1,2
1Gold, Rare Earth and Material Technopreneurship Centre, Universiti Malaysia Kelantan (UMK), 17600 Jeli, Kelantan, Malaysia.
2Faculty of Bioengineering and Technology, Universiti Malaysia Kelantan (UMK), 17600 Jeli Kelantan, Malaysia.
3Faculty of Chemical Engineering and Technology, University Malaysia Perlis (UniMAP), Arau, Perlis 02600, Malaysia.
*Corresponding Author: fazliani.s@umk.edu.my
ABSTRACT
The efficient recovery of gold from acidic aqueous solutions is vital for sustainable resource management and high-value metal recycling. In this work, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymer inclusion membranes (PIMs) incorporating chitosan (CS) nanoparticles (0–2.0 wt.%) and Aliquat 336 as the ionic carrier were fabricated via solvent casting and comprehensively characterized. FTIR analysis confirmed intermolecular interactions between CS and PVDF-HFP through hydrogen bonding, while SEM revealed that moderate CS loading (1.5 wt.%) enhanced surface roughness and pore uniformity, whereas excessive loading (2.0 wt.%) induced nanoparticle aggregation. Contact angle and water uptake measurements showed progressively improved hydrophilicity with CS incorporation, correlating with increased aqueous phase accessibility. Thermogravimetric analysis and Coats–Redfern modelling indicated a slight reduction in thermal stability and activation energy with increasing CS content, attributed to thermally unstable CS functional groups and disruption of PVDF-HFP crystallinity. Gold adsorption experiments analyzed by ICP-OES demonstrated consistently high removal efficiencies for all membranes, with a maximum of 99.92% at 2.0 wt.% CS. Kinetic analysis revealed excellent fitting of both pseudo-first-order (PFO) and pseudo-second-order (PSO) models (R2 ≥ 0.981). The highest adsorption rate constants were observed at 1.5 wt.% CS, confirming rapid uptake dominated by a chemisorption mechanism. Overall, moderate CS incorporation (1.5 wt.%) provided the optimal balance between enhanced hydrophilicity, ion-binding capacity, and structural integrity, offering valuable insights for the design of sustainable, bio-based PIMs for efficient precious metal recovery.
Keywords:
polymer inclusion membrane
poly(vinylidene fluoride-co-hexafluoropropylene)
chitosan
gold adsorption
1. INTRODUCTION
The recovery of precious metals such as gold from aqueous solutions is of increasing industrial and environmental importance due to the depletion of natural reserves, growing market demand, and the high economic value of recovered resources. Conventional recovery methods, including precipitation, solvent extraction, and adsorption, often face limitations such as high chemical consumption, secondary waste generation, and reduced selectivity in complex matrices. In this context, polymer inclusion membranes (PIMs) have emerged as a promising separation technology, offering high selectivity, operational simplicity, and reusability in liquid–liquid extraction processes. In PIM systems, an extractant or carrier is immobilised within a polymer matrix, enabling controlled and selective transport of target metal ions across the membrane–solution interface.
Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) is increasingly recognised as an effective PIM materials due to its excellent chemical resistance, mechanical strength, and good film-forming properties. Unlike conventional cellulose triacetate (CTA) and poly (vinyl chloride) (PVC), PVDF-HFP membranes exhibit superior durability in acidic and oxidising environments, making them suitable for precious metal recovery applications. However, their inherently hydrophobic nature limits wettability and interfacial mass transfer between the aqueous phase and the membrane, which can reduce extraction efficiency. Modifying PVDF-HFP with hydrophilic additives or fillers has proven to be an effective strategy to improve water permeability, increase interfacial contact, and enhance ion transport properties.
Although PVDF-HFP is widely recognised as a durable and effective PIM materials (Gunasegaran, Ravi, & Shoparwe, 2020; Hoque et al., 2021), and Aliquat-type carriers are well known for their strong ability to transport Au(III) (Gherasim et al., 2011; Ngah & Fatinathan, 2010), most previous studies have concentrated on adjusting carrier–plasticizer ratios or testing other ionic liquid or phosphonium carriers (Hernández-Fernández et al., 2023; Sgarlata et al., 2008). Much less attention has been given to adding hydrophilic biopolymeric nanoparticles to PVDF-HFP for precious-metal recovery. Chitosan nanoparticles, with their large surface area and plentiful –NH2 and –OH groups, can improve membrane wettability and create extra binding sites for Au(III), which can boost mass transfer and adsorption (Mourya, Inamdara, & Ashutosh Tiwari, 2010; Shariatinia & Fazli, 2015). However, too much chitosan can lead to particle clumping, blocked pores, reduced carrier mobility, and lower thermal stability (Liu et al., 2022). In addition, there are very few systematic studies that connect chitosan loading levels with changes in particle dispersion, membrane surface properties, thermal stability (e.g., Coats–Redfern activation energy), and extraction behaviour (PFO and PSO models) for Au(III) recovery. Filling this knowledge gap is important for developing nanoparticle-modified PIMs that can deliver high-efficiency and sustainable gold recovery.
The use of chitosan as a membrane additive is particularly promising because of its natural abundance, biodegradability, and non-toxicity, as well as its functional amino and hydroxyl groups, which have a high affinity for metal ions through electrostatic attraction and chelation. Several studies have reported improved hydrophilicity, porosity, and adsorption capacity when chitosan or other hydrophilic nanomaterials are incorporated into hydrophobic membranes (Arthanareeswaran et al., 2007; Ngah & Fatinathan, 2010). Nevertheless, achieving uniform nanoparticle dispersion within a hydrophobic PVDF-HFP matrix is challenging, and excessive loadings can cause agglomeration, reduce effective surface area, and disrupt membrane structure (Lim, Mahamud, & Badrolhisham, 2024). This trade-off underscores the need for optimising filler content to balance the beneficial effects on wettability and ion-binding capacity against potential drawbacks in structural and thermal stability.
In this work, PVDF-HFP-based PIMs incorporating different loadings of chitosan nanoparticles (0–2.0 wt.%) were fabricated via solvent casting and comprehensively characterised using FTIR spectroscopy, scanning electron microscopy (SEM), water contact angle measurement, water uptake analysis, and thermogravimetric analysis (TGA) coupled with Coats–Redfern kinetic modelling. Aliquat 336 was selected as the ionic carrier due to its established effectiveness in facilitating the transport of anionic AuCl4− species from acidic aqueous media. The performance of the membranes in Au(III) extraction was evaluated using inductively coupled plasma optical emission spectrometry (ICP-OES), and the extraction kinetics were analysed using both pseudo-first-order (PFO) and pseudo-second-order (PSO) models to identify the controlling mechanisms. This integrated approach enables a clear correlation between membrane composition, structure, and extraction performance, providing valuable guidelines for the rational design of high-performance, bio-based nanocomposite PIMs for selective precious-metal recovery.
2. EXPERIMENTAL
2.1 Materials
Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) in the form of pellets purchased from Sigma Aldrich was used as polymer matrix. Aliquat 336 (A336) is tricaprylmethylammonium cation combined with a chloride anion also purchased from Sigma Aldrich, served as the ionic carrier. The Dioctyl Phthalate (DOP) with a purity of > 99.5%, was supplied by Sigma Aldrich was employed as a plasticizer to enhance membrane flexibility, thermal stability, and durability. Chitosan (CS) nanoparticles in powder form was purchased from Sigma Aldrich and used as an adsorbent additive. Artificial gold sample was prepared by diluting gold standard solution (Au) 1000 ppm in ca.M hydrochloric acid (Fisher Scientific) into 100 ppm. The receiver solution was prepared by dissolving sodium sulfite (Na2SO3) pellets, supplied by Merck, in distilled water to obtain a 0.100 M solution.
2.2 Preparation of CS nanoparticles
Chitosan (CS) nanoparticles were synthesized via the ionic gelation method, as described by Ghaemi et al. (2018), with slight modifications. Briefly, 0.3 g of chitosan was dissolved in 1 wt.% acetic acid solution and stirred continuously for 8 hours at ambient temperature to ensure complete dissolution. The resulting solution was then filtered using filter paper to remove any undissolved particles or impurities.
To initiate nanoparticle formation, 95 mL of magnesium sulfate (MgSO4) solution was added dropwise to the chitosan solution under constant stirring using a magnetic stirrer. The resulting suspension was left undisturbed for 24 hours to allow for nanoparticle formation. Afterward, the clear liquid above the nanoparticles was carefully discarded, and the precipitated nanoparticles were collected by centrifugation at 6000 rpm for 50 minutes. The nanoparticles was washed by adding 15 mL of distilled water, followed by a second centrifugation at 6000 rpm for 30 minutes. The final product was stored for further use.
2.3 Preparation of PVDF-HFP/CS Membrane
Poly(vinylidene fluoride-co-hexafluoropropylene)/chitosan (PVDF-HFP/CS) polymer inclusion membranes were fabricated following the procedure described by Gunasegaran et al. (2020), with modifications to incorporate CS nanoparticles. To prepare the membrane casting solution, 5 g of PVDF-HFP pellets was dissolved in 50 mL of tetrahydrofuran (THF) under stirring. Once fully dissolved, 20 mL of Aliquat 336 (carrier), 5 mL of dioctyl phthalate (plasticizer), and 0.05 mL of the prepared CS nanoparticle suspension were added to the solution. The mixture was stirred continuously for 4 hours to ensure homogeneity.
After mixing, 25 mL of the solution was poured into a membrane casting machine, ensuring a thickness of 2.0 µm and evenly spread onto a clean glass plate to form a uniform film. The cast membrane was left to dry for 24 hours in a fume hood at room temperature, allowing the THF to evaporate gradually. Once dried, the resulting membrane was peeled off and washed several times with distilled water to remove any residual solvent or unbound components. Table 1 shows the membrane formulation with different chitosan content.
PVDF-HFP (wt.%) | A366 (wt.%) | DOP (wt.%) | CS (wt.%) | Acronyms |
|---|---|---|---|---|
50 | 40 | 10 | - | PVDF-HFP |
50 | 40 | 10 | 0.5 | PVDF-HFP/CS0.5 |
50 | 40 | 10 | 1.0 | PVDF-HFP/CS1.0 |
50 | 40 | 10 | 1.5 | PVDF-HFP/CS1.5 |
50 | 40 | 10 | 2.0 | PVDF-HFP/CS2.0 |
2.4 Testing and Characterisations
Fourier-Transform Infrared Spectroscopy (FTIR) Analysis
FTIR absorption spectra of the fabricated PIMs and reference samples were obtained using a Thermo Fisher Scientific iZ10 FTIR spectrometer (Thermo Fisher Scientific, USA) in transmission mode. The spectra were analysed to identify characteristic functional groups by comparing the results with standard reference peaks for alkane (-CH and -CH₂ bonds), alkyl halide (-C–F, -CF2, and –CF3 bonds), aromatic groups, ester, and alcohol (-P–OH bond) (Guo et al., 2007; Jakriya et al., 2018; Singh & Singh, 2015). Spectral data were collected in the range of 400–4000 cm− 1 with a resolution of 4 cm− 1, using 16 scans per sample. Data acquisition and analysis were carried out using OMNIC software (Zamri & Masri, 2018).
Scanning Electron Microscopy (SEM)
The surface morphology of the PVDF-HFP/chitosan polymer inclusion membranes were determined using scanning electron microscopy (SEM). The membrane was cut into 5 mm × 5 mm in size and coated with gold. The surface morphology of the membrane was then analysed under 4000× magnification with 10 kV acceleration voltage (Ling & Suah, 2017).
Contact Angle (CA)
The hydrophobicity of the membrane was determined using a contact angle goniometer (Model: OCA15plus, DataPhysics). The contact angle was measured by placing a 1 µL droplet of distilled water on the membrane surface through a needle tip attached to the goniometer. A magnified image of the water droplet was captured using a digital camera, and the contact angle readings were recorded 10 s after deposition on the dried membrane surface. For each membrane, measurements were taken at five different spots, and the mean values were calculated.
Water uptake
The membranes were cut into 2 cm × 2 cm in size. Then, the weight of each membrane was determined by using an electronic balance and the membrane were immersed in distilled water for 30 minutes. Subsequently, the membranes were removed from the distilled water and the excess liquid on the membrane was gently dapped by a tissue towel. The membrane was then weighed for the second time in order to determine the increment of the membrane weight after the absorption of the distilled water. The water uptake of the membrane was calculated by using the following Eq. (1) which retrieved from Klaysom et al. (2011).
1
where Wwet is the weight of the wet membrane after the absorption of distilled water, while Wdry is the weight of the dry membrane before immersing the membrane into the distilled water.
Thermogravimetric analysis (TGA)
Thermogravimetric analysis (TGA) of PIMs PVDF-HFP membranes with varying chitosan (CS) loadings was conducted using a TA Instruments Discovery TGA 550 (New Castle, DE, USA) in accordance with ASTM E1131. Approximately 10 mg of sample was heated from 25 to 900°C at 10°C min⁻¹ under a nitrogen atmosphere. The onset (T0) and peak (Tp) degradation temperatures were determined from the TGA and derivative thermogravimetric (DTG) curves, respectively.
Thermal Degradation Kinetics by using Coat Redfern Model
The activation energy, Ea of TGA thermal decomposition was measured using Coats and Redfern model (Dhar et al., 2014; Ebrahimi-Kahrizsangi & Abbasi, 2008). According to this model, a graph of log [-log (1-α)/T2)] against 1/T (K-1) was plotted, where T is the temperature and α is the degree of conversion which can be found by following Eq. (2):
2
where, wi and wf is the initial and final weight of samples and wt is the weight at temperature (T). A straight line from the graph is obtained and the activation energy (kJ mol-1) was calculated using Eq. (3).
Ea = Slope x 2.303 R | (3) |
where, R is the universal gas constant (8.314 J mol-1 K-1).
Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)
The performance of the fabricated PVDF-HFP/CS PIMs in extracting gold ions (Au³⁺) from acidic solution was evaluated by measuring the initial and final concentrations of gold using inductively coupled plasma optical emission spectrometry (ICP-OES) model Optima 8300 (PerkinElmer, Yokohama, Japan). The membrane was immersed in 50 mL of 100 ppm gold solution (adjusted with ~ 1 M HCl) and stirred at room temperature for a predetermined time interval. After extraction, the residual solution was analyzed by ICP-OES to determine the concentration of unextracted gold ions. Eq. (4) and Eq. (5) were used to calculate the removal efficiency and adsorption capacity, respectively:
4
5
Where C0 is the initial concentration of gold (mg/L) and Ct is the concentration after extraction (mg/L), while V is the volume of solution (L) and m is the dry mass of membrane used (g).
Kinetic Studies
The simplest way to describe the mechanism of the gold adsorption is to perform kinetic studies such as pseudo-first-order (PFO) and pseudo-second-order (PSO). These kinetic models describe the rate of retention or release of a solute from an aqueous environment to solid-phase interface that depends on parameters such as flow rate, pH, ionic strength, temperature, concentration of solute, sorbent dose, texture of adsorbents and etc. The adsorption uptake, qt for this PIMs system following the Eq. (6) shown as below:
6
where Vf, Vr are volumes of feed and receiving phases, Cf,0 are initial feed concentration, Cf,t and Cr,t are concentrations at time, t in feed and receiving, respectively. m is mass of membrane (or adsorbent mass equivalent in your analysis). Units for C in mg·L− 1, V in L, m in g and qt in mg·g− 1.
Furthermore, the models are able to identify whether the mechanism is physisorption (van der Waals force of attraction) or chemisorption (ionic bond) (Kajjumba et al., 2018). Thus, PFO and PSO are introduced (Moussout et al., 2018). The general form of the Eq. (7) is shown as below:
7
where qe and qt (mg·g− 1) are the adsorbate amounts uptake per mass of adsorbent at equilibrium and at any time, t (min), respectively; while. kn is the rate constant of the pseudo-n-th order kinetic model per minute.
2.4.8.1 Pseudo-First-Order (PFO) Kinetic
PFO describe the adsorption of solute onto absorbent by following the first order mechanism (Elhadj et al., 2019). The expression of the model is shown as Eq. (8) below:
8
The equation can be written in the non-linear form as Eq. (9) below:
9
The equation can also be written in the linear form as Eq. (10) below:
10
where k1 values (unit: min− 1) are obtained from the slopes of linear plots ln (qe - qt) versus the time, t (Balarak & Mahdavi, 2016). The unit for qe is mg·g− 1.
2.4.8.2 Pseudo-Second-Order (PSO) Kinetic
Pseudo-second order model is derived on the basis of the sorption capacity of the solid phase (Febrianto et al., 2009). The expression of the model is shown as Eqs. (11), (12) and (13) below:
Differential form:
11
Non-linear form:
12
Linear form:
13
Initial adsorption rate, h:
14
where the values of k2 (unit: g·mg− 1·min− 1) is the rate constant for PSO and qe (mg·g− 1) can be determined by the slope and intercept of the plotted graph, “t/qt against t”. h value (unit: mg·g− 1·min− 1) is the initial adsorption rate for PSO. Overall, the R2 value calculated from experimental data for each models is the most important determinants for the best fitted model with high correlation coefficient of R2 = 0.999.
3 RESULTS AND DISCUSSION
3.1 Fourier Transform Infrared Spectroscopy (FTIR) Analysis
Fourier Transform Infrared (FTIR) spectroscopy was employed to investigate the functional groups and molecular interactions within the PVDF-HFP-based membranes incorporating varying concentrations of chitosan (CS) nanoparticles. The FTIR spectra are presented in Fig. 1.
The control membrane (0 wt.% CS) exhibited mainly characteristic peaks corresponding to PVDF-HFP, Aliquat 336 (A336), and dioctyl phthalate (DOP). A sharp absorption band observed at approximately 1404 cm− 1 was attributed to the C–F stretching vibration of PVDF-HFP. Additional peaks at around 1275 cm− 1 and 1170 cm− 1 were assigned to the C–O–C and C–H stretching vibrations of DOP, respectively (Lim et al., 2025). The spectral region between 500–850 cm− 1 displayed multiple peaks corresponding to the crystalline α- and β-phases of PVDF-HFP. These results establish a spectral baseline for evaluating the impact of CS incorporation.
Upon incorporation of 0.5 wt.% CS nanoparticles, subtle spectral changes were observed. A new peak appeared near 1072 cm− 1, corresponding to the C–O stretching of hydroxyl groups, indicative of hydrogen bonding between chitosan and the PVDF-HFP matrix. Minor intensity variations within the 800–900 cm− 1 range suggest a possible influence of CS on the crystalline domain of PVDF .
At 1.0 wt.% CS loading, the interactions became more pronounced. The absorption band near 1724 cm− 1, associated with the C = O stretching of DOP, exhibited a slight shift and increased intensity, implying enhanced interaction between DOP and CS nanoparticles. The broadening and intensified peak at ~ 2925 cm− 1 (–CH stretching) further suggested improved compatibility and dispersion of CS within the polymer matrix.
The membrane containing 1.5 wt.% CS displayed additional peak shifts and intensity changes, particularly at 1404 cm− 1 and 1275 cm− 1, reinforcing the presence of strong intermolecular interactions between CS and PVDF-HFP. Notably, the peaks in the 840–880 cm− 1 region became sharper, suggesting modifications in PVDF’s crystalline arrangement. However, reduced intensity in the 500–600 cm− 1 region may reflect a partial disruption of the β-phase crystallinity due to nanoparticle interference .
At the highest CS concentration (2.0 wt.%), the most significant spectral changes were observed. The broad band at ~ 3400 cm− 1 (–OH and –NH stretching) intensified considerably, indicating a higher presence of CS. However, peak broadening and overlapping within the 800–1200 cm− 1 range suggest nanoparticle aggregation, which could negatively impact dispersion uniformity and the structural coherence of the membrane.
Fig. 1
FTIR spectra of PVDF-HFP with different CS loading
A clear trend of increasing molecular interaction between chitosan nanoparticles and the PVDF-HFP matrix was observed with increasing CS concentration from 0 to 2.0 wt.%. At lower loadings (0.5–1.0 wt.%), the introduction of CS facilitated favorable interactions, evidenced by the appearance of new absorption bands and shifts in existing peaks, indicating enhanced compatibility and dispersion within the membrane. In contrast, higher CS concentrations (1.5–2.0 wt.%) resulted in intensified and broadened peaks, particularly in the 800–1200 cm− 1 region, suggesting possible nanoparticle agglomeration. These changes reflect the dual role of CS in modifying both chemical interactions and the structural characteristics of the membrane system, with optimal performance likely achieved at moderate loading levels.
3.2 Surface Morphology Analysis via Scanning Electron Microscopy (SEM)
The surface morphology of PVDF-HFP-based membranes incorporating varying concentrations of chitosan (CS) nanoparticles was examined using scanning electron microscopy (SEM). This analysis is crucial for understanding how CS addition influences the membrane’s microstructure, pore formation, and potential performance in separation or ion transport applications. Figure 2 show the SEM micrograph of PVDF-HFP with different loading of CS nanoparticles.
The SEM image of the control membrane (0 wt.% CS) shows a relatively smooth and homogeneous surface with a porous microstructure. The pores observed are primarily attributed to the solvent evaporation-induced phase separation during the casting process, where THF, a highly volatile solvent, rapidly evaporates, leaving behind voids in the polymer matrix. The use of DOP as a plasticizer also plays a role in softening the matrix and modulating pore size by altering polymer–solvent interactions during solidification. The baseline morphology provides a reference for evaluating the structural effects of nanoparticle incorporation.
Upon introducing 0.5 wt.% CS, the membrane surface becomes slightly rougher, indicating the initial influence of chitosan on the membrane structure. The nanoparticles appear to be fairly well-dispersed, though minor localized agglomerations are observed. The roughened texture and increased surface area suggest improved hydrophilicity, potentially beneficial for ion transport performance, as supported by previous studies on CS-enhanced membrane systems (Juang & Shiau, 2000).
Fig. 2
SEM micrograph of (a) pristine PVDF-HFP, (b) PVDF-HFP/CS0.5, (c) PVDF-HFP/CS1.0, (d) PVDF-HFP/CS1.5 and (e) PVDF-HFP/CS2.0
At 1.0 wt.% CS loading, the surface features become more prominent. SEM micrographs reveal a more uniform pore distribution and finer integration of CS particles into the matrix. The improved compatibility between CS and PVDF-HFP, likely due to hydrogen bonding between the hydroxyl and amine groups of CS and the fluorinated backbone of PVDF-HFP, enhances nanoparticle dispersion. This optimized microstructure may contribute to better mechanical stability and functional efficiency of the membrane (Kang & Cao, 2012).
With a further increase to 1.5 wt.% CS, the membrane exhibits increased roughness and more visible nanoparticle domains. Although dispersion remains largely consistent, some micro-aggregates become apparent. The CS-rich regions may enhance membrane hydrophilicity and ion exchange potential due to CS’s inherent polar nature. However, such aggregation could begin to affect pore uniformity and compromise homogeneity at the micro-scale.
At 2.0 wt.% CS, aggregation becomes significantly pronounced. The SEM images show clustered nanoparticles disrupting the membrane’s uniform morphology. The surface appears increasingly irregular, and some pore structures are partially obstructed, suggesting that excess CS hinders even distribution within the polymer matrix. This overloading effect may negatively impact membrane permeability and long-term mechanical integrity (Feng & Johnson, 2015).
As the CS content increases from 0 to 2.0 wt.%, the surface topology undergoes progressive transformation from smooth and homogeneous to rough and structurally complex. At moderate concentrations (0.5–1.0 wt.%), the presence of CS improves dispersion and supports the development of a porous, well-integrated matrix. In contrast, higher loadings (1.5–2.0 wt.%) promote aggregation, which may reduce uniformity and impair key membrane properties. These observations underscore the importance of optimizing nanoparticle concentration to tailor membrane morphology and enhance performance.
3.3 Contact Angle (CA) Analysis
Figure 3 illustrates the static water contact angle measurements for PVDF-HFP-based polymer inclusion membranes (PIMs) incorporated with different loadings of chitosan nanoparticles (0–2.0 wt.%). The contact angle values decrease progressively from 97.15° (0 wt.% CS) to 88.53° (2.0 wt.% CS), indicating a consistent enhancement in surface hydrophilicity with increasing CS content.
The pristine PVDF-HFP membrane exhibits a relatively high contact angle of 97.15°, which reflects its inherent hydrophobic nature due to the presence of strong –CF2– groups within the polymer backbone (Lee et al., 2019). Upon incorporating chitosan nanoparticles, a clear decreasing trend in contact angle is observed. This change is primarily attributed to the hydrophilic functional groups present in chitosan, such as –OH and –NH2, which can migrate toward the membrane surface during film formation, enhancing surface polarity and wettability (Darwish et al., 2019; Datta et al., 2025; Spoială et al., 2021).
Fig. 3
Contact angle measurement of PVDF-HFP membrane with different CS loading
At 0.5 wt.% and 1.0 wt.% CS, the slight but significant reduction in contact angle suggests effective dispersion of chitosan within the polymer matrix, providing moderate surface enrichment of polar groups. At 1.5 wt.%, the contact angle drops to 90.40°, indicating near-hydrophilic behavior. The further decrease to 88.53° at 2.0 wt.% confirms that higher chitosan loadings intensify hydrophilic interactions on the membrane surface. However, it is worth noting that excessive CS loading beyond this threshold may potentially lead to nanoparticle agglomeration, which could reduce surface uniformity or alter roughness, possibly influencing future mechanical and transport properties (Lv et al., 2018).
Enhanced hydrophilicity is advantageous in aqueous extraction or separation applications, such as gold ion transport, as it promotes higher membrane–aqueous phase interfacial contact and facilitates ionic interaction at the membrane surface (Chen et al., 2021). This result aligns with previous findings where the addition of hydrophilic fillers (e.g., cellulose nanocrystals, graphene oxide, or chitosan) significantly improved wettability and performance of polymer membranes (Goswami et al., 2021; Sun et al., 2022).
3.4 Water Uptake Analysis
Figure 4 presents the water uptake behavior of PVDF-HFP-based polymer inclusion membranes (PIMs) with increasing concentrations of chitosan nanoparticles (CS) from 0 to 2.0 wt.%. A distinct increasing trend in water absorption is observed: the pristine PVDF-HFP membrane shows the lowest uptake at 17.24%, while the PVDF-HFP/CS2.0 membrane exhibits the highest value at 59.62%. This progressive increase reflects the strong influence of chitosan incorporation on the membrane's hydrophilic character and water-holding capacity.
The pristine PVDF-HFP membrane demonstrates limited water uptake due to the hydrophobic nature of PVDF-HFP, which originates from its highly electronegative –CF2– groups that repel water molecules and restrict swelling. As hydrophilic chitosan nanoparticles are introduced at 0.5 wt.%, 1.0 wt.%, 1.5 wt.%, and 2.0 wt.% loading levels, the membranes (PVDF-HFP/CS0.5 to PVDF-HFP/CS2.0) become progressively more hydrophilic. This is because chitosan contains abundant hydroxyl (–OH) and amine (–NH2) functional groups, which are capable of forming strong hydrogen bonds with water molecules, thereby increasing the membrane’s affinity for water (Datta et al., 2025; Lee et al., 2019).
The enhanced water uptake with higher chitosan loading is in direct correlation with the decreasing contact angle values, as previously shown in Fig. 3. As the chitosan content increases, the surface contact angle decreases from 97.15° for pristine PVDF-HFP to 88.53° for PVDF-HFP/CS2.0, confirming that the surface becomes more hydrophilic. A lower contact angle signifies increased wettability, allowing water to spread and penetrate more effectively, which is consistent with the observed increase in bulk water absorption (Lv et al., 2018). This supports the conclusion that surface wettability and water uptake are closely linked, both controlled by the membrane’s surface chemistry and functional group density.
The use of dioctyl phthalate (DOP) as a plasticizer also contributes to membrane morphology by enhancing flexibility and generating free volume, which facilitates water molecule diffusion. However, since the DOP content remains constant in this study, the differences in water uptake can be primarily attributed to the chitosan nanoparticle loading rather than plasticizer effects.
In terms of functional relevance, membranes with higher water uptake, such as PVDF-HFP/CS2.0, are expected to exhibit improved ion transport capabilities due to better hydration and ionic mobility within the polymer matrix. This is particularly beneficial in gold ion extraction systems where aqueous-phase diffusion is critical. The water retained in the membrane acts as a medium for hydrated ion complexes, thereby enhancing permeability and transport efficiency (Arthanareeswaran et al., 2007; Mulder, 2012).
Fig. 4
Water uptake percent of PVDF-HFP membrane with different CS loading
3.5 Thermal Degradation Behavior by using Thermogravimetric analysis (TGA)
The thermal degradation behavior of PVDF-HFP-based membranes incorporated with varying concentrations of chitosan nanoparticles was assessed via thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG), as illustrated in Fig. 5 and Fig. 6, with detailed thermal data presented in Table 2. All membranes exhibited a typical two-step degradation pattern. The first weight loss event, occurring between approximately 160–210°C, corresponds to the evaporation and decomposition of low molecular weight additives such as the plasticizer dioctyl phthalate (DOP), as well as potential residual tetrahydrofuran (THF) solvent used during membrane casting. The neat PVDF-HFP membrane exhibited the highest onset temperature (Tonset1) and maximum decomposition temperature (Tdmax1) at 189.28°C and 205.46°C, respectively. However, both Tonset1 and Tdmax1 decreased progressively with increasing chitosan content, reaching 166.17°C and 190.48°C in the PVDF-HFP/CS2.0 membrane. This trend suggests that the incorporation of chitosan, which is known to degrade at lower temperatures (~ 200–300°C), reduces the thermal stability of the membranes in the initial degradation stage. Additionally, the presence of chitosan introduces hydrophilic –OH and –NH2 groups, which are more thermally labile and may facilitate interactions with water molecules or DOP, thereby accelerating early degradation (Bahrami et al., 2022; Mourya et al., 2010).
Fig. 5
TGA curve of PVDF-HFP membrane with different CS loading
The second major degradation stage, attributed to the decomposition of the PVDF-HFP polymer backbone via dehydrofluorination and chain scission, occurred in the temperature range of 425–440°C. Similar to the first stage, both Tonset2 and Tdmax2 exhibited a slight reduction with increasing chitosan loading. For instance, the Tdmax2 for the neat PVDF-HFP was 440.38°C, which decreased to 433.59°C in the PVDF-HFP/CS2.0 membrane. This slight decrease may be attributed to structural disruptions in the PVDF-HFP matrix caused by the incorporation of chitosan nanoparticles, which could hinder crystallinity and enhance amorphous regions, making the polymer more prone to thermal decomposition (Dias et al., 2021).
Notably, the residual mass after complete degradation increased marginally from 0.45% in neat PVDF-HFP to 0.69% in the PVDF-HFP/CS2.0 membrane. This increment is consistent with the known behavior of chitosan to form thermally stable char residues upon pyrolysis, contributing to a higher fixed carbon content in the final residue (Muzzarelli, 2009).
Fig. 6
DTG curve of PVDF-HFP membrane with different CS loading
These thermal trends are in good agreement with the previously reported water uptake and contact angle data. Membranes with higher chitosan content showed increased water uptake and decreased contact angles, indicating enhanced hydrophilicity. This increased affinity for water may lead to moisture retention within the membrane structure, which in turn could facilitate earlier thermal decomposition, particularly during the first degradation stage. Hence, although the incorporation of chitosan improves hydrophilic characteristics and potentially enhances ion transport properties, it simultaneously compromises the thermal robustness of the membranes to a limited extent. This trade-off between hydrophilicity and thermal stability must be carefully optimized depending on the intended operating conditions of the membrane in metal ion separation processes.
Type of Sample | First step mass loss | Second step mass loss | Residue (%) | ||||
|---|---|---|---|---|---|---|---|
TOnset 1 (°C) | Td max1 (°C) | Weight Loss (%) | TOnset 2 (°C) | Td max2 (°C) | Weight Loss (%) | ||
PVDF-HFP | 189.28 | 205.46 | 54.63 | 440.48 | 440.38 | 44.92 | 0.45 |
PVDF-HFP/CS0.5 | 189.12 | 205.48 | 54.29 | 440.45 | 440.46 | 45.20 | 0.51 |
PVDF-HFP/CS1.0 | 186.38 | 203.78 | 54.14 | 437.59 | 437.00 | 45.32 | 0.54 |
PVDF-HFP/CS1.5 | 170.71 | 202.59 | 53.60 | 429.48 | 437.59 | 45.74 | 0.66 |
PVDF-HFP/CS2.0 | 166.17 | 190.48 | 53.06 | 425.48 | 433.59 | 46.25 | 0.69 |
3.6 Thermal Degradation Kinetics by using Coat Redfern Model
The thermal degradation kinetics of PVDF-HFP-based membranes with varying chitosan nanoparticle loadings were further investigated using the Coats-Redfern model, using a first-order reaction mechanism. The corresponding Coats-Redfern plots are shown in Fig. 7, and the calculated activation energy (Ea) values are summarized in Table 3. All samples exhibited high coefficients of determination (R2 ≥ 0.978), confirming a good fit to the first-order reaction model and validating the use of Coats-Redfern for kinetic evaluation.
The activation energy of the pristine PVDF-HFP membrane was calculated to be 2.36 kJ/mol, which is consistent with literature values for the thermal degradation of fluorinated polymers (Dias et al., 2021). Upon incorporating chitosan nanoparticles, a general decreasing trend in Ea was observed, with the lowest value of 2.12 kJ/mol recorded for the PVDF-HFP/CS1.5 membrane. This reduction in Ea indicates a facilitation of the degradation process, implying that the presence of chitosan lowers the energy barrier required for thermal decomposition. This can be attributed to the thermally labile nature of chitosan, which contains amino and hydroxyl groups that readily undergo degradation at lower temperatures (Shariatinia & Fazli, 2015). The disruption of the PVDF-HFP polymer matrix by the introduction of chitosan could also reduce the degree of crystallinity and promote amorphous regions, as previously implied by the decreased TGA onset and DTG peak temperatures. These amorphous domains are more susceptible to thermal degradation and consequently require lower activation energy (Begum et al., 2021).
Fig. 7
Coats-Redfern plots using first order reaction for PVDF-HFP membrane with different CS loading
Interestingly, while the Ea values decrease with increasing chitosan content up to 1.5 wt.%, a slight rebound is observed at 2.0 wt.% (Ea = 2.21 kJ/mol). This behavior may be due to particle agglomeration at higher loadings, which can reduce the uniform distribution of chitosan and thus alter the structural integrity of the membrane (Liu et al., 2022). The local densification caused by agglomerates may form more thermally resistant zones within the membrane, offsetting the otherwise decreasing trend in activation energy.
These kinetic results align with the earlier findings on water uptake and contact angle. Membranes with higher chitosan content were more hydrophilic and exhibited greater water absorption, as demonstrated by the decreasing contact angle and increasing water uptake values. The presence of absorbed moisture and the increased number of polar functional groups not only enhances ion transport but also facilitates thermal degradation by promoting the hydrolysis of unstable bonds. Therefore, the reduced activation energy observed with increasing chitosan content can also be partially attributed to the enhanced interaction with water molecules, which destabilizes the polymer chains under thermal stress.
The Coats-Redfern kinetic analysis confirms that incorporating chitosan into PVDF-HFP membranes not only modifies thermal stability but also alters the fundamental degradation kinetics by lowering the activation energy barrier. This reinforces the need to balance functional performance (e.g., hydrophilicity and ion transport) with thermal durability when designing polymer inclusion membranes for practical applications in metal ion recovery or separation technologies.
Samples | Activation Energy, Ea (kJ/mol) | Fitted Equation | Slope, m | Intercept | R2 |
|---|---|---|---|---|---|
PVDF-HFP | 2.36 × 102 | y = -12373x + 11.73 | 12373 | 11.73 | 0.982 |
PVDF-HFP/CS0.5 | 2.26 × 102 | y = -11815x + 10.55 | 11815 | 10.55 | 0.978 |
PVDF-HFP/CS1.0 | 2.32 × 102 | y = -12134x + 11.50 | 12134 | 11.50 | 0.990 |
PVDF-HFP/CS1.5 | 2.12 × 102 | y = -11079x + 9.50 | 11079 | 9.50 | 0.998 |
PVDF-HFP/CS2.0 | 2.21 × 102 | y = -11555x + 10.63 | 11555 | 10.63 | 0.992 |
3.7 Gold Removal Efficiency by using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)
The gold removal efficiencies of PVDF-HFP-based polymer inclusion membranes (PIMs) with varying chitosan nanoparticle (CS) contents are shown in Fig. 8. A clear trend of increasing extraction efficiency was observed as the CS loading increased from 0 wt.% to 2.0 wt.%. The pristine PVDF-HFP membrane shows a gold removal efficiency of 98.28%, which was further improved with the incorporation of CS nanoparticles. Membranes containing 0.5, 1.0, 1.5, and 2.0 wt.% CS achieved removal efficiencies of 99.06%, 99.18%, 99.35%, and 99.92%, respectively. The incremental enhancement indicates that chitosan plays a significant role in facilitating Au(III) ion extraction from the acidic aqueous phase.
Fig. 8
Removal efficiency of PVDF-HFP membrane with different CS loading
Chitosan is a biopolymer well-known for its metal ion binding capability, attributed primarily to the presence of amino (-NH2) and hydroxyl (-OH) functional groups, which serve as active coordination sites for Au(III) complexation via electrostatic attraction and chelation mechanisms (Muzzarelli, 2009; Ngah & Fatinathan, 2010). The increasing trend in gold removal efficiency is consistent with the increase in hydrophilicity (as shown by the decreasing contact angle values) and water uptake of the membranes with rising CS content. These changes improve aqueous-phase ion diffusion into the membrane, facilitating interactions with both the carrier and chitosan functional groups. Incorporation of chitosan nanoparticles further creates hydrated micro-environments within the PIM matrix, promoting ion mobility and reducing diffusion resistance. As shown in Fig. 9, the hydrophobic PVDF-HFP/aliquat system restricts ion exchange to the membrane surface, whereas chitosan establishes hydrated corridors and functional groups (-NH₃⁺, -OH) that enhance ion exchange and gold transport across the membrane.
Additionally, the improved performance can be partially attributed to increased surface area and porosity, as previously supported by SEM micrographs. Chitosan nanoparticles, when well-dispersed within the membrane matrix, contribute to a more porous structure, which can enhance mass transport of gold ions through the membrane (Ngah, Teong, & Hanafiah, 2011). This observation is aligned with prior studies, where the incorporation of hydrophilic nanomaterials was shown to significantly boost the transport and adsorption capacity of polymeric membranes for heavy metals (Crini & Badot, 2008).
Fig. 9
Schematic representation of the transport mechanism of AuCl₄⁻ across PVDF-HFP polymer inclusion membranes (PIMs): (left) PIM without chitosan; (right) PIM with chitosan nanoparticles
The high efficiency even in the absence of chitosan (98.28%) suggests that Aliquat 336, a quaternary ammonium salt acting as the carrier, already facilitates efficient phase transfer of Au(III) across the membrane by forming a complex with the anionic AuCl4− species in acidic medium (Bonggotgetsakul, Cattrall, & Kolev, 2015; Suhaimi, Shoparwe, & Yusoff, 2024). However, with CS addition, the availability of additional sorption sites further enhances gold uptake, likely through a synergistic effect between Aliquat 336-mediated transport and CS-assisted binding.
Interestingly, the highest removal (99.92%) was achieved at 2.0 wt.% CS loading, which also coincides with the lowest contact angle (88.53°) and highest water uptake (~ 59.62%). These properties point toward a more hydrated, permeable membrane structure, which supports better ion diffusion and more efficient interaction with binding sites. However, this enhancement must be balanced with structural stability, as observed in TGA and DTG results, where higher CS content led to a slight decline in thermal stability.
The results suggest that the incorporation of chitosan nanoparticles into PVDF-HFP PIMs significantly enhances gold ion removal efficiency, primarily by increasing the number of active sites, improving hydrophilicity, and promoting aqueous ion diffusion. The data reinforce the suitability of CS-modified PIMs for high-efficiency recovery of gold from acidic solutions.
3.8 Adsorption Kinetics
3.8.1 Pseudo-First-Order (PFO) Model
The pseudo-first-order (PFO) kinetic modeling results are presented in Table 4. Figure 10 shows the adsorption behavior of PVDF-HFP membranes as a function of chitosan loading. The rate constant (k1) increases from 0.0125 min− 1 for pristine PVDF-HFP to 0.0248 min− 1 for PVDF-HFP/CS1.5, indicating that chitosan incorporation enhances the adsorption rate. This improvement is attributed to the presence of additional functional groups (–NH2 and –OH) from chitosan, which provide more active sites for interaction with the target species via hydrogen bonding and electrostatic interactions (Kaczorowska, 2022; Malathi et al., 2014). The higher k1 values reflect faster removal rates, allowing membranes with higher chitosan loading to achieve equilibrium more rapidly.
The equilibrium adsorption capacity (qe) also increases with chitosan content, from 5.294 mg/g for pristine PVDF-HFP to 6.606 mg·g− 1for PVDF-HFP/CS2.0. Similarly, the experimental adsorption capacity (qt) reaches 6.359 mg·g− 1 for PVDF-HFP/CS2.0. This enhancement in adsorption capacity corresponds well with the removal efficiency data, which improves from 97.77% for pristine PVDF-HFP to 99.93% for PVDF-HFP/CS2.0. The progressive increase in removal efficiency demonstrates that chitosan incorporation not only accelerates the adsorption process but also improves the overall uptake of solute molecules. This can be explained by the synergistic effect of higher active site density and improved wettability, which enhance solute accessibility and interaction at the membrane surface (Keskin, Zeytuncu-Gökoğlu, & Koyuncu, 2021).
The model fitting quality, as reflected by R2 values above 0.960 for all membranes, confirms the suitability of the PFO model in describing the adsorption kinetics. Although PVDF-HFP/CS1.5 and CS2.0 show slightly lower R2 (0.960) compared to pristine PVDF-HFP (0.987), this deviation may be due to structural rearrangements or partial pore blocking at higher chitosan loadings, leading to non-ideal adsorption behavior. In contrast, the PVDF-HFP/CS0.5 and CS1.0 membranes show the highest fitting accuracy (R2 ≈ 0.99), suggesting that moderate chitosan incorporation results in a more uniform distribution of adsorption sites and stable adsorption pathways.
Fig. 10
Fitting of the experimental data to the PFO model describing adsorption process over time with different chitosan loading
The correlation between hydrophilicity and adsorption performance is noteworthy. As chitosan loading increases, the reduced contact angle improves water uptake and solute accessibility to active sites, which simultaneously increases qt and qe, enhances removal efficiency, and accelerates removal rate (higher k1). For example, the PVDF-HFP/CS2.0 membrane not only achieved the highest qe (6.606 mg·g− 1) but also demonstrated nearly complete solute removal (99.93%) with rapid adsorption kinetics. Such behavior is consistent with previous studies reporting that hydrophilic modifications of polymeric membranes substantially improve both adsorption capacity and removal performance (Arthanareeswaran et al., 2007; Chen et al., 2021). Therefore, the PFO analysis demonstrates that chitosan functionalization enhances both the kinetics (faster adsorption rate) and thermodynamics (higher removal efficiency) of the adsorption process through synergistic effects of active site enrichment and improved wettability.
Membrane | Predicted qt | Rate Constant K1 | Experimental qe | R2 |
|---|---|---|---|---|
PVDF-HFP | 5.030 | 0.0125 | 5.294 | 0.987 |
PVDF-HFP/CS0.5 | 5.854 | 0.0217 | 5.886 | 0.997 |
PVDF-HFP/CS1.0 | 6.043 | 0.0229 | 6.068 | 0.986 |
PVDF-HFP/CS1.5 | 6.219 | 0.0248 | 6.235 | 0.960 |
PVDF-HFP/CS2.0 | 6.359 | 0.0137 | 6.606 | 0.960 |
3.8.2 Pseudo-Second-Order (PSO) Model
The pseudo-second-order (PSO) kinetic model was applied to describe the adsorption of gold ions onto PVDF-HFP-based membranes with varying chitosan (CS) loadings, and the results are summarized in Table 5 and illustrated in Fig. 11. The model showed an excellent fit to the experimental data, with high correlation coefficients (R2 ≥ 0.99) and low variance across all membrane formulations. Such strong agreement indicates that the adsorption process is predominantly governed by chemisorption, where valence forces or electron sharing between the active sites of the membrane and the gold ions play a key role. This finding is consistent with previous studies on heavy metal adsorption onto functionalized polymer matrices (Gherasim et al., 2011; Hoque et al., 2021).
The calculated rate constants (k2) increased from 0.0017 for pristine PVDF-HFP to 0.0173 for PVDF-HFP/CS1.5, demonstrating that moderate chitosan incorporation significantly accelerates the adsorption kinetics. This improvement can be attributed to the additional amino (–NH₂) and hydroxyl (–OH) functional groups introduced by chitosan, which act as coordination and hydrogen bonding sites for gold ions (Ngah & Fatinathan, 2010). The enhanced hydrophilicity and wettability of CS-modified membranes, as evidenced by contact angle analysis, also promoted solute accessibility by facilitating water penetration into the membrane matrix and increasing the effective interaction between gold ions and active sites.
The initial adsorption rate (h), derived from the PSO model, further supports this trend. For pristine PVDF-HFP, h was relatively low (0.227 mg·g− 1·min− 1), but increased almost twelve-fold to 2.722 mg·g− 1·min− 1 at 1.5 wt.% CS loading. This indicates that the presence of chitosan not only improves equilibrium adsorption capacity but also accelerates the rate at which adsorption equilibrium is achieved. Such rapid adsorption at moderate chitosan content suggests highly favorable kinetics due to the synergistic effects of increased functional groups and improved surface hydrophilicity.
Interestingly, the highest k2 and h values were observed at 1.5 wt.% CS, which also corresponded to the highest predicted qt (12.30 mg·g− 1) and excellent agreement with the experimental qe (12.47 mg·g− 1). This strongly suggests that 1.5 wt.% CS represents the optimum composition, where the balance between functional group density and membrane structure provides the most efficient adsorption performance. Similar trends have been reported by Hernández-Fernández et al. (2023), who found that optimal hydrophilic modifier levels maximized adsorption kinetics before structural limitations became evident.
Fig. 11
Fitting of the experimental data to the PSO model describing adsorption process over time with different chitosan loading
However, further increasing CS loading to 2.0 wt.% resulted in a sharp decrease in k2 (0.0013 g·mg− 1·min− 1) and h (0.264 mg·g− 1·min− 1), despite a higher experimental qe value (13.21 mg·g− 1). This decline in adsorption rate and model agreement (R2 = 0.990) can be attributed to structural drawbacks at excessive CS incorporation, including higher viscosity during membrane casting, poor miscibility between components, and partial pore blockage (Sgarlata et al., 2008). These effects likely restricted mass transfer and reduced the accessibility of some active sites, leading to slower adsorption despite higher overall capacity. This observation aligns with the SEM analysis, where denser and less uniform pore structures were observed at high CS content (≥ 2.0 wt.%).
This PSO kinetic analysis demonstrates that adsorption of gold ions onto PVDF-HFP/CS membranes is chemisorption-controlled. Moderate chitosan incorporation (around 1.5 wt.%) provides the best balance of adsorption rate, initial uptake, and capacity, whereas excessive loading leads to structural limitations that hinder performance. These findings reinforce the earlier contact angle and morphology results, confirming that chitosan functionalization enhances both adsorption efficiency and kinetics up to an optimum level.
Membrane | Predicted qt (mg·g− 1) | Rate Constant K2 | Experimental qe (mg·g− 1) | R2 | h (mg·g− 1·min− 1) |
|---|---|---|---|---|---|
PVDF-HFP | 9.5800 | 0.0017 | 10.587 | 0.995 | 0.227 |
PVDF-HFP/CS0.5 | 11.8772 | 0.0041 | 11.771 | 0.999 | 0.677 |
PVDF-HFP/CS1.0 | 11.5490 | 0.0050 | 12.135 | 1.000 | 0.759 |
PVDF-HFP/CS1.5 | 12.3042 | 0.0173 | 12.469 | 1.000 | 2.722 |
PVDF-HFP/CS2.0 | 11.7450 | 0.0013 | 13.211 | 0.990 | 0.264 |
| h: Initial Adsorption Rate, | |||||
4 CONCLUSIONS
Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymer inclusion membranes (PIMs) incorporating chitosan (CS) nanoparticles were successfully fabricated via a solvent-casting method, and their physicochemical, thermal, and adsorption performances were systematically evaluated. FTIR analysis confirmed hydrogen bonding and other intermolecular interactions between PVDF-HFP and CS, with moderate CS loadings (0.5–1.5 wt.%) promoting good compatibility and dispersion, whereas excessive loading (2.0 wt.%) caused nanoparticle agglomeration. SEM observations revealed that CS addition progressively increased surface roughness and porosity up to 1.5 wt.%, beyond which morphological uniformity declined. Contact angle and water uptake measurements demonstrated a clear enhancement in hydrophilicity with increasing CS content, correlating with improved aqueous phase penetration. Thermogravimetric analysis indicated a slight decrease in thermal stability with higher CS loading, attributable to the thermally unstable –OH and –NH₂ functionalities in chitosan and the disruption of PVDF-HFP crystallinity. Coats–Redfern kinetic modelling further showed that CS incorporation reduced the activation energy for thermal degradation, with the lowest value observed at 1.5 wt.%, confirming facilitated decomposition due to increased amorphous domains.
Gold adsorption tests analyzed by ICP-OES demonstrated that CS-modified PIMs achieved higher removal efficiencies (up to 99.92%) compared to pristine PVDF-HFP (98.28%), due to the synergistic effect of Aliquat 336-mediated ion exchange and the additional chelation sites provided by CS. Kinetic analysis revealed excellent fitting with both pseudo-first-order (PFO) and pseudo-second-order (PSO) models (R2 ≥ 0.981). The highest adsorption rates were obtained at 1.5 wt.% CS, with k1 = 0.0248 min− 1 (PFO) and an initial adsorption rate, h = 2.722 mg·g− 1·min− 1 (PSO), confirming a chemisorption-dominated mechanism. This optimum composition provided the best balance between hydrophilicity, active site density, membrane morphology, and thermal stability.
In summary, this study demonstrates that incorporating a moderate amount of chitosan nanoparticles into PVDF-HFP PIMs is a highly effective strategy for enhancing hydrophilicity, adsorption kinetics, and gold ion removal efficiency while maintaining structural and thermal integrity. These findings provide valuable design guidelines for developing sustainable, bio-based nanocomposite membranes for selective recovery of precious metals from acidic aqueous media.
5 DATA AVAILABILITY STATEMENT
All data that support the findings of this study are included within the article (and any supplementary files).
6 AUTHOR’S CONTRIBUTION STATEMENT
C. K. Lim: Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing. N. F. Shoparwe: Methodology, Validation, Formal analysis, Investigation, Writing – review & editing. K. C. Lim: Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing. N. Z. Nor’azmind: Validation, Investigation, Writing – review. M. A. Abdullah: Methodology, Validation, Investigation, Writing – review. V. Anbalagan: Formal analysis, Investigation, Writing – original draft.
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DECLARATION OF COMPETING INTERESTThe 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.
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FUNDING
DECLARATION
This work was supported by the Fundamental Research Grant Scheme (FRGS) under the grant number of FRGS/1/2023/TK05/UMK/02 from the Ministry of Higher Education Malaysia.
ACKNOWLEDGEMENT
The lab facilities were provided by the Gold Rare Earth and Material Technopreneurship Centre, Faculty of Bioengineering and Technology, Universiti Malaysia Kelantan. Special thanks to those who contributed to this project directly or indirectly.
Additional Files
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