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Frontiers in Membrane Science and Technology ·

Volume 5 - 2026 | https://doi.org/10.3389/frmst.2026.1892716

Primary, secondary, tertiary, and quaternary Alkyl amine-functionalized PVDF membranes for DBSA removal

  • Hadi Rouhi1
  • Steven T. Weinman1*corresponding author
  1. Department of Chemical and Biological Engineering, The University of Alabama, Tuscaloosa, AL, United States
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Abstract

Water remediation has become increasingly critical over recent decades due to the growing discharge of surfactant-containing wastewater into the environment. Dodecylbenzene sulfonic acid (DBSA), a widely used detergent, presents significant challenges for water treatment because of its high solubility and chemical stability. To address this challenge, polyvinylidene difluoride (PVDF) membranes were functionalized with three types of diamines; primary (P), secondary (S), and tertiary (T); followed by quaternization of the tertiary diamine-modified membrane to obtain a quaternary-modified variant (Q). The modification likely occurred via diamine (base)-driven dehydrofluorination creating polyene segments which were partially modified by the diamines via a Michael addition. These membranes were employed to adsorb the negatively charged DBSA molecules, and their adsorption performance was systematically compared. Initial surface characterization using ATR-FTIR, XPS, water contact angle, and SEM indicated successful modification for all membranes. Performance tests of P, S, and T membranes demonstrated DBSA removal efficiencies of 93.1%, 91.4%, and 95.3% from a ∼600 mg/L solution within 3 h of contact time. Q membranes showed a lower DBSA removal of 37.3% at 3 h, which increased to 72.1% at 24 h. The membranes exhibited enhanced adsorption capacities of up to 21.6 mg/g, with adsorption strongly influenced by solution pH due to protonation-driven electrostatic interactions. Kinetic analysis revealed that DBSA adsorption followed pseudo-second-order kinetics for P, S, and Q membrane and pseudo-first-order kinetics for T membrane. The isotherm studies showed the Temkin model provided the best fit, indicating heterogeneous surface interactions and predominantly physisorption-driven adsorption. Thermodynamic and isosteric analyses further confirmed the exothermic nature of DBSA adsorption, with enthalpy values ranging from ∼2.1 to 25.4 kJ/ mol and a decrease in adsorption favorability at elevated temperatures. All the modified membranes showed promising regeneration results, with P, S, T, and Q membranes having 86.6%, 88.8%, 87.4%, and 72.0% removal in first cycle and 69.9%, 68.6%, 70.6%, and 72.0% removal in fifth cycle. This study demonstrates that amine functionality influences surface chemistry and DBSA adsorption behavior and highlights the potential of amine-functionalized PVDF membranes as promising adsorptive materials for the efficient removal of harmful anionic surfactants from contaminated water systems.

Keywords

1 Introduction

Continuing global population increase, economic development, and climate change introduces water pollutant threats in various concentrations in drinking water, groundwater, and surface water (Lu and Astruc, 2020). Around one-third of the global urban population lacks access to a safe drinking water source, with projections of a worsening situation every year (He et al., 2021; Rouhi et al., 2025). Surfactants are organic chemicals that can easily enter and persist in aquatic environments from industrial and household sources (such as laundering practices). These surfactants, often present as unreacted organics in industrial effluents, are difficult and costly to remove and contribute to the contamination of domestic wash waters, adding strain to sewer networks and treatment plants (U.S.E.P. Agency, 2006; Taffarel and Rubio, 2010). These pollutants are recognized as emerging harmful species to the ecosystems due to their toxicity (Arora et al., 2022). As conventional water treatment technology has shown limited efficiency in some cases, there is significant interest in developing advanced materials capable of selectively removing these pollutants from water (Siyal et al., 2020; Rouhi et al., 2024). Dodecylbenzene sulfonic acid (DBSA, C18H30O3S) is an anionic surfactant that is widely released into wastewater and sludge because of its extensive use in detergents and agricultural formulations (Mimanne et al., 2012). DBSA and other derivatives such as sodium dodecylbenzene sulfonate (SDBS) is shown to be toxic and disruptive to humans, animals, and microbial activity at low concentrations (Mimanne et al., 2012; Kim et al., 2018; Zhao et al., 2017; Geng et al., 2020; Parhizgar et al., 2016). U.S. Environmental Protection Agency guidelines (2006) restrict DBSA levels in products that contact food-processing systems to 400 mg/L and limit it to only 5.5 mg/L in dairy equipment.

Polyvinylidene difluoride (PVDF) membranes are widely employed in water treatment technologies due to their strong chemical and mechanical stability (Wang et al., 2023; Kang and Cao, 2014). These membranes have found extensive utilization in filtration techniques such as microfiltration and ultrafiltration (Vitola et al., 2015). PVDF resists most of the chemicals such as halogens, oxidants, and inorganic and organic acid and solvents, except for esters, ketones, and strong base solutions. PVDF has a hydrophobic surface, therefore modification of the membrane to increase the hydrophilicity of the membrane is an effective choice to improve its antifouling properties (Wang et al., 2023).

Surface functionalization strategies are critical for improving membrane performance and fouling resistance in water treatment (Kamaz et al., 2019). PVDF membranes were used for functionalization under different methods of modifications. Surface grafting has been used to introduce different functional groups such as hydroxyl, carboxyl, zwitterionic, or other hydrophilic charged groups (Wang et al., 2022). Surface coating techniques have been used to introduce phenolic, silanol, and other polar groups to PVDF (Zhang et al., 2016). Blending PVDF with a hydrophilic polymer, including polyionic liquids, zwitterionic polymers, and amphiphilic copolymers, or additives during fabrication embeds functional groups into the membrane matrix (Wang et al., 2022). Techniques such as nanoparticle deposition, plasma treatment, and surface imprinting are used to incorporate nanoparticles, hydrophilic groups, and synthetic spacers, respectively (Kamaz et al., 2019; Samree et al., 2020; Olivares Moreno et al., 2024). Combined, these approaches enable PVDF membranes for antifouling ultrafiltration, oil/water emulsion separation, catalytic/photocatalytic degradation of organics, selective adsorption, and enzyme-immobilized biocatalytic membranes (Rouhi et al., 2025; Kamaz et al., 2019; Shen et al., 2017; Kannathasan et al., 2025; Chiao et al., 2020; Khui et al., 2025).

Amine functional groups are widely used in membrane surface functionalization for water treatment due to their ability to become positively charged under acidic to neutral pH conditions, enabling strong electrostatic interactions with negatively charged contaminants such as anionic surfactants, dyes, and organic acids (Rouhi et al., 2024; Byun et al., 2013; Sahu et al., 2023). The structure of amines—primary, secondary, or tertiary—plays a critical role in determining protonation behavior, surface charge density, and stability across varying pH conditions, which in turn affects adsorption efficiency and selectivity (Numpilai et al., 2025). Additionally, quaternary ammonium groups represent a permanently charged form of amine functionalization that does not depend on solution pH for protonation. Unlike tertiary amines, which lose positive charge under alkaline conditions, quaternary ammonium groups maintain a stable positive charge across a wide pH range, leading to stronger and more consistent electrostatic interactions with negatively charged species. Primary and secondary amines typically provide strong interactions due to their high basicity but often suffer from excessive binding strength and reduced regeneration efficiency, whereas tertiary and sterically hindered amines offer a more balanced interaction by maintaining sufficient affinity with improved stability and reversibility (Ko et al., 2011; Wadi et al., 2023). Quaternary ammonium groups fixed ionic nature can lead to stronger, less selective, and more difficult-to-reverse binding. Because of this stability, quaternary amines are widely used in anion-exchange membranes, ion-exchange resins, and adsorptive membranes for the removal of anionic surfactants, dyes, nitrate, chromate, PFAS, and organic acids from water (Kim and Hallinan, 2025; Yin et al., 2022; Yu et al., 2018). Overall, amine and quaternary ammonium functionalization offer tunable surface charge characteristics, enabling targeted membrane design for adsorption-driven water treatment and ion-separation applications.

In this research, the effects of functionalizing PVDF membrane with a primary, secondary, tertiary, and quaternary diamine on DBSA, a negatively charged pollutant, adsorption has been investigated. Prior research indicates that amine-functionalization can improve adsorption of charged pollutants, though structure-function relationships need deeper investigations (Kamaz et al., 2019; Thompson et al., 2024). We hypothesized that if PVDF membranes were modified with primary, secondary, tertiary, and quaternary diamines, then DBSA can be adsorbed and removed (the membrane being regenerated) easier with non-quaternary amines. Unmodified and modified PVDF membranes were tested for static DBSA adsorption to study the effects of pH, temperature, diamine concentrations, and diamine contact time. Chemical and physical structure, hydrophilicity, and XPS results of PVDF membrane were examined before and after modification. The unmodified and modified PVDF membranes were performance tested for 5 cycles of regeneration with n-hexane:1-nonanol mixture. The membranes were tested for dynamic DBSA adsorption and regeneration.

2 Materials and methods

2.1 Materials and chemicals

The BX 250 kDa PVDF ultrafiltration membranes used in this study were sourced from Synder Filtration Inc. 1,3-Propanediamine (98%), N,N-Dimethyl-1,3-propanediamine (99%), 1-nonanol (99%), and iodomethane (99+%) were purchased from Thermo Fisher Scientific company. N-Methyl-1,3-propanediamine (98%) was purchased from Ambeed. Ethyl alcohol (200 Proof), sodium chloride (NaCl, anhydrous, 99+%), and sodium carbonate (Na2CO3, >99.5%) was purchased from Millipore-Sigma. Sodium sulfate (Na2SO4), n-hexane (97%, HPLC grade), and sodium hydroxide (NaOH) beads (reagent grade) were used as received from VWR. All chemicals were used as received without further purification unless specifically mentioned. Aqueous solutions were made with deionized (DI) water from a Millipore Synergy UV water purification system to generate Type 1 DI water with conductivity ≤0.05 μS/cm.

2.2 PVDF membrane modification

PVDF membranes were functionalized using aqueous diamine solutions with concentrations of 0.2, 0.5, 1, 2, and 5 M prepared in deionized water. Each diamine solution was homogenized by stirring for 10 min at room temperature (∼20 °C). Prior to surface modification, PVDF membrane coupons (2-inch diameter) were sequentially washed with DI water and ethanol, rinsed with DI water, and air-dried. Membrane functionalization was conducted on a hot plate using 20 mL of diamine solution at 50 °C. Once the solution reached the target temperature, membrane coupons were immediately immersed in the solution and agitated at 300 rpm using a magnetic stirrer for a predetermined duration (0.25 h–24 h, with 3 h being the most commonly used). A control study was done by exposing a PVDF membrane to a 0.1 M NaOH solution, which has an equivalent pH (approximately 12) as the 0.2 M diamine solution, for 3 h. Throughout the modification process, the membranes were fully submerged in the diamine solution. Following functionalization, the membrane coupons were thoroughly rinsed with DI water multiple times to remove unreacted species and then stored in DI water for subsequent adsorption experiments. Membrane modified with 1,3-Propanediamine (containing two primary amines), N-Methyl-1,3-propanediamine (containing primary and secondary amines), N,N-Dimethyl-1,3-propanediamine (containing primary and tertiary amines) and [N,N-Dimethyl-1,3-propanediamine + iodomethane] (quaternary) are denoted as P, S, T, and Q membranes.

2.3 Membrane characterization

2.3.1 SEM

The surface morphology of all unmodified and modified membranes was examined using a Thermo Fisher Apreo field-emission scanning electron microscope (FE-SEM). All membrane samples were dried under vacuum at 70 °C and −0.95 barg vacuum before measurement. An aluminum stub and carbon tape were used to prepare the sample prior to imaging and samples were gold coated for 90 s using a Cressington 208 HR sputter coater. FE-SEM measurements were conducted at an accelerating voltage of 20 kV, a current voltage of 0.4 nA, a 10 mm working distance, and a magnification of 6500x.

2.3.2 ATR-FTIR

Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was used to determine the chemical bonds and functional groups on the unmodified and modified PVDF membranes. All membrane samples were dried under vacuum at 70 °C and −0.95 barg vacuum before measurement. ATR-FTIR measurements were done using an Anton Paar Lyza 7000 equipped with a diamond ATR crystal in the range of 4000-450 cm-1 and the data were processed by the Lyza 7000 version 1.5.1 software. Each spectrum was collected for 32 scans at a resolution of 4 cm-1 and was ATR and baseline corrected by the software. All spectra were manually normalized to the peak at ∼1179 cm-1 which is the PVDF -CF2- characteristic peak. A background of the ATR crystal was taken before each set of samples was tested to ensure the crystal was clean.

2.3.3 X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) analysis of the unmodified and modified PVDF membranes was performed using a Thermo Fisher Scientific Nexsa G2 Surface Analysis System equipped with a K-Alpha + spectrometer. All membrane samples were dried under vacuum at 70 °C and −0.95 barg vacuum before measurement. The measurements were carried out using a micro-focused, monochromated Al Kα X-ray source. Survey spectra and high-resolution scans for C1s, F1s, and N1s were collected with a 400 µm analysis area and an energy step size of 0.1 eV. Each spectrum was acquired with three repeated scans to improve signal quality. Data processing and peak analysis were conducted using Avantage software (version 6.10.0).

2.3.4 Static contact angle goniometry

The static water contact angles (WCA) of the unmodified and modified membranes were measured using a KRUSS DSA25 goniometry instrument equipped with ADVANCE software version 1.19.2.26301. Prior to measurement, membrane samples were dried under vacuum at 70 °C and −0.95 barg vacuum. All static WCAs were measured using the sessile drop method. A liquid drop of deionized water (∼7 µL) was placed carefully on the sample surface and measurements were taken 15 s after each water droplet was placed on the sample surface for consistency. To determine each contact angle, the KRUSS drop shape analyzer ADVANCE software version 1.19.2.26301 was used. Measurements were done at a minimum of 5 locations on at least three membranes of each sample type to get an average contact angle value with standard deviation (at least 15 data points per membrane sample type).

2.4 Membrane testing

2.4.1 Static adsorption experiments

Typical experimental adsorption studies for DBSA are conducted in the range of 15-400 mg/L (Zhao et al., 2017), however higher concentrations of DBSA or similar surfactants also was observed (Mimanne et al., 2012; Kim et al., 2018). To account for a more challenging, worst-case scenario, a DBSA stock solution with 555–600 mg/L was prepared using Milli-Q water. The use of a higher concentration also enables a clearer and more meaningful comparison between P, S, T, and Q membranes. Adsorption experiments were performed on unmodified and modified PVDF membrane coupons with an average dry mass of 0.275 g at 20 °C to evaluate the effects of solution pH (3.0–11.0), contact time (0.25 h–48 h), initial DBSA concentration (25 mg/L to 588 mg/L), and diamine concentration (0.1 M–5 M) on the adsorption capacity of the membranes. The initial pH of the DBSA solution was three and was adjusted to the desired value using 0.1 M NaOH. For each adsorption test, a circular membrane coupon (∼5.07 in2, 2.54 in diameter) was placed in a sealed jar containing 10 mL of DBSA solution and shaken for the specified duration. After adsorption, the solution was collected and analyzed by UV–Vis spectroscopy (Thermo scientific Genesys 10S UV-Vis, software version 4.006) at 261 nm to determine the residual DBSA concentration. A calibration curve constructed using DBSA concentrations ranging from 10 mg/L to 1000 mg/L is provided in Supplementary Figure S1. The membrane coupons were stored in water for subsequent experiments. The adsorption capacity of each membrane was calculated using Equation 1:Where q is the adsorption capacity (mg/g), Ci is the initial concentration of DBSA (mg/L), Ce is the equilibrium concentration of DBSA (mg/L), W is the membrane weight (g), V is the volume of solution used (L).

q = C i − C e W × V (1)

The removal efficiency, RE%, of each membrane was calculated using Equation 2:

R E % = C i − C e C i × 100 (2)

2.4.2 Isotherm and kinetic models

The equilibrium adsorption characteristics were evaluated using the Langmuir, Freundlich, Langmuir-Freundlich, and Temkin adsorption isotherms. The nonlinear Langmuir isotherm is represented by Equation 3 (Ayub et al., 2020; Wang and Guo, 2020; Jonas Bayuo and Abdullaiabukari, 2018):

q e = q max K L C e 1 + K L C e (3)

The empirical Langmuir isotherm was linearized using Equation 4:Where qmax is the maximum adsorption capacity (mg/g) and KL (L/mg) is the Langmuir’s isotherm constant, reflecting the binding affinity between DBSA and the membrane adsorption sites. The Langmuir isotherm is most applied to gas-solid adsorption, however liquid-solid adsorption is also considered in some cases (Wang and Guo, 2020; Jonas Bayuo and Abdullaiabukari, 2018).

1 q e = 1 K L q max . 1 C e + 1 q max (4)

The nonlinear form of the Freundlich isotherm is represented by Equation 5 (Ayub et al., 2020; Wang and Guo, 2020; Fernandez-Ramos et al., 2014):

q e = K f C e 1 n (5)

The linear form of the Freundlich isotherm is represented by Equation 6:Where Kf (L1/n.mg1-1/n/g) is the Freundlich constant which represents the adsorption capacity and 1/n is the adsorption intensity. The Freundlich model will reduce to the linear model when n = 1. Many publications interpreted this isotherm as a representative of the multi-layer adsorption on heterogamous surfaces (Wang and Guo, 2020).

L o g q e = L o g K f + 1 n Log C e (6)

The Langmuir-Freundlich (Sips) isotherm aims to capture heterogeneous surfaces illustrating the distribution of adsorption on an adsorbent surface. At low adsorbate concentration, this isotherm becomes Freundlich isotherm while at high concentration, it moves toward Langmuir isotherm. The Langmuir-Freundlich isotherm is represented by Equation 7:Where qmax (mg/g) indicates the Langmuir-Freundlich maximum adsorption capacity, KL is the heterogeneous surface’s equilibrium constant (L/mg), n is heterogenous parameter between 0 and 1, and Ce is the equilibrium concentration (mg/L) (Babatunde et al., 2022; Chen et al., 2023).

q e = q max K L C e n 1 + K L C e n (7)

The Temkin isotherm model expects that the adsorption energy decreases directly with the surface inclusion due to adsorbent-adsorbate interactions (Sivaranjanee et al., 2022). This model was used to analyze for a possible multilayered adsorption process and is represented by Equation 8:Where AT (L/mg) denotes the constant of equilibrium binding, bT (J/mol) represents the adsorption heat constant, and T is the temperature (K) (Fernandez-Ramos et al., 2014; Chen et al., 2022; Salama et al., 2015).

q e = R T b T ln ⁡ A T C e (8)

Pseudo first order (PFO) and pseudo second order (PSO) kinetic models were used to analyze the adsorption rate of DBSA. The PFO model is represented by Equation 9 (Ayub et al., 2020; Tran, 2023):where qt is the adsorption capacity (mg/g) at time t (h) and k1 (1/min) is the equilibrium rate constant. The PSO model is represented by Equation 10:where k2 (g/mg/min) is the equilibrium rate constant. The values for the linear coefficient regression (R 2) were used to determine the most suited isotherm and kinetic model for the adsorption process (Ayub et al., 2020; Chen et al., 2022). To statistically discriminate between the two models, the corrected Akaike Information Criterion (AICc) was applied to the nonlinear residuals (Neath and Cavanaugh, 2025).

ln ⁡ q e − q t = l n q e − k 1 t (9)

t q e = 1 k 2 q e 2 + 1 q e (10)

2.4.3 Thermodynamic studies

The effect of temperature on DBSA adsorption was examined by conducting adsorption experiments at 20, 40, and 60 °C using a 588 mg/L DBSA solution at pH ∼3 and a contact time of 3 h. Thermodynamic parameters were determined according to classical thermodynamic relationships using Equation 11:Where ΔG° is the standard Gibbs free energy change (J/mol), ΔH° is the standard enthalpy change (J/mol), ΔS° is the standard entropy change (J/(mol.K)), T is the absolute temperature (K), and R is the universal gas constant (8.314 J/mol/K) (Raghav and Kumar, 2018). KL (from the Langmuir model) was converted to molar-based dimensionless KL° referenced to the standard-state concentration (1 mol/L) using Equation 12 (Tran, 2022):Where MDBSA is molecular weight of DBSA (326.5 g/mol) and 1000 is a unit conversion factor (mg/g).

Δ G ° = − R T l n K L ° = Δ H ° − T Δ S ° (11)

K L ° = K L L m g × M D B S A g m o l × 1000 m g g × 1 m o l L (12)

The isosteric heat of adsorption is the enthalpy change determined from adsorption isotherms at a constant number of adsorbed molecules at two or more temperatures (Builes et al., 2013). This model was also used to further elucidate the energetic characteristics of DBSA adsorption onto the diamine-modified membranes. The isosteric method evaluates the variation of equilibrium concentration with temperature at constant surface coverage and reflects the interaction strength between the adsorbate and the membrane surface. Unlike global thermodynamic parameters derived from Langmuir constants, the isosteric approach accounts for coverage-dependent energetic variations and is particularly useful for assessing surface heterogeneity (Siperstein et al., 2021). The variations of surface loading provide insight into the energetic distribution of adsorption sites and the possible contribution of cooperative interactions such as surface micelle formation. The isosteric isotherm model is represented by Equation 13:Where ΔHst (kJ/mol) is isosteric enthalpy of adsorption (Ruthven, 1984).

ln ⁡ C e = − Δ H s t R T + constant (13)

2.4.4 Regeneration and reusability

The reusability of the membranes was investigated by conducting five successive adsorption-desorption cycles. After performing each 555 mg/L DBSA adsorption test for 3 h, desorption was attempted by immersing the membrane in a 10 mL solution of n-hexane:1-nonanol (1:1 v/v) and shaking on a shaker plate for 6 h. Then, the membranes underwent multiple rinses with DI water and at least 1 h sonication bath to remove excess DBSA and 1-nonanol residues. The membranes were subjected to air drying after that for subsequent usage in the succeeding adsorption cycle. UV measurement of each solution after adsorption were measured after filtering the solution through a VWR PTFE syringe filter (0.22 µm) to remove any possible contaminants and residuals of the membrane caused by the sonication process. The removal efficiency of DBSA was calculated after the completion of each adsorption cycle.

2.4.5 Dynamic adsorption

Dynamic membrane performance was evaluated using a Sterlitech HP4750 dead-end stirred filtration cell (Sterlitech, USA) with a total volume of 200 mL and an effective filtration area of 14.6 cm2. The cell was pressurized with compressed air to 1 bar for both unmodified and modified PVDF membranes. Each membrane was separately challenged with deionized (DI) water and a 545 mg/L DBSA solution. Prior to data collection, membranes were compacted by permeation for 10 min. For pure water permeance measurements, permeation was continued for an additional ∼2 min following compaction, and the mass of permeated water was recorded. For dynamic DBSA adsorption experiments, after the 10 min compaction step, 10 sequential permeate samples, each with the time of 3 min and 30 min of collection time total were collected. The DBSA concentration in each permeate sample was determined using UV–Vis spectroscopy, allowing calculation of the removal efficiency (RE%) for each fraction. Membrane permeance was calculated using Equation 14 and DBSA rejection was calculated using Equation 15. Where Cf is the DBSA concentration in the initial feed solution and Cp is the DBSA concentration in each permeate solution.

Permeance = Volume of Water Permeated Membrane Active Area × Time × Pressure (14)

Rejection % = 1 − C f C p × 100 (15)

3 Results and discussion

3.1 Membrane characterization

3.1.1 SEM

Figure 1 presents the SEM images of the unmodified and modified PVDF membranes. In opposite to surface polymer grafting which include a multilayer attachment, the diamine modification is expected to have a monolayer attachment on the surface and inside of the pores. Consequently, a monolayer modification does not significantly alter the membrane surface morphology, as seen in Figure 1. However, Figure 2 shows clear visual color changes following diamine and NaOH modification. All modified membranes exhibit a brownish coloration, with the intensity increasing progressively from the P membrane to the Q membrane. This color change is likely due to the formation of conjugated polyene segments caused by diamine-induced dehydrofluorination of PVDF (Sharma et al., 2023; Dallaev et al., 2022; Ross et al., 2000). The amines are likely being attached to the membrane via a Michael addition to some of these C=C bonds, rather than direct substitution of the C–F bond (Taguet et al., 2005), which would disrupt the polyene conjugation and is likely why the color change is not as dark for the amine-modified membranes.

SEM images of the unmodified and P, S, T, and Q membranes. All modified membranes were treated with a solution containing 0.2 M diamine for 3 h. The red scale bar represents 5 μm at 6500x magnification.
Figure 1. SEM images of the unmodified and P, S, T, and Q membranes. All modified membranes were treated with a solution containing 0.2 M diamine for 3 h. The red scale bar represents 5 μm at 6500x magnification.
Visual color changes of the PVDF membranes after modification with 0.2 M of diamine concentration or 0.1 M NaOH for 3 h.
Figure 2. Visual color changes of the PVDF membranes after modification with 0.2 M of diamine concentration or 0.1 M NaOH for 3 h.

3.1.2 ATR-FTIR

Figure 3 presents the ATR-FTIR spectra of unmodified and diamine-modified PVDF membranes. The characteristic absorption bands at 2980 cm-1 and 3022 cm-1 correspond to the symmetric and asymmetric stretching vibrations of–CH2– groups in PVDF. Peaks observed at 880 cm-1 and 840 cm-1 are attributed to the β-phase and C–F stretching vibrations of PVDF, respectively. The band at approximately 1178 cm-1 is associated with the–CF2– stretching vibration of the PVDF backbone (Daneshkhah et al., 2017). The NaOH treated membrane showed a similar spectrum to the unmodified PVDF membrane. In contrast, all diamine-modified membranes exhibited additional absorption bands at 1532 cm-1 and 1630 cm-1, which are assigned to N–H bending and C–N stretching vibrations, supporting the presence of amine functionalities on the membrane surface (Fontinha et al., 2013; Ludfiaastu Rinawati et al., 2015). These features support successful grafting of diamines onto the PVDF membranes. However, the relatively weak intensity of the amine-related peaks suggests monolayer attachment of the diamines rather than multilayer deposition. Furthermore, ATR-FTIR spectra of DBSA and the modified membranes after DBSA adsorption were compared and illustrated in Supplementary Figure S11. The observed changes in characteristic absorption bands after adsorption provide additional evidence of successful DBSA uptake by the modified membranes.

ATR-FTIR spectra of the unmodified, P, S, T, Q, and NaOH membranes. The diamine concentration for membrane modification was 0.2 M and the NaOH concentration was 0.1 M with a 3 h modification time.
Figure 3. ATR-FTIR spectra of the unmodified, P, S, T, Q, and NaOH membranes. The diamine concentration for membrane modification was 0.2 M and the NaOH concentration was 0.1 M with a 3 h modification time.

3.1.3 X-ray photoelectron spectroscopy

The XPS spectra of the unmodified and modified PVDF membranes are shown in Figure 4. XPS analysis indicates that, consistent with the expected monolayer grafting of diamines, the N1s signal intensity is relatively weak. Nevertheless, all modified membranes exhibit a distinct N1s peak at 398 eV, confirming the presence of nitrogen-containing functional groups on the membrane surface and providing strong evidence for the successful attachment of diamine molecules onto the PVDF surface. Additional evidence of quaternization is observed through the appearance of three distinct peaks near ∼200, ∼620, and ∼630 eV shown in Supplementary Figure S9, which can be attributed to iodine species originating from the methyl iodide reagent used during the quaternization process (Nazi et al., 2022).

XPS spectra corresponding to N1s binding energy for (Left) unmodified and P membranes, (Middle) unmodified and S membranes, and (Right) unmodified, T, and Q membranes.
Figure 4. XPS spectra corresponding to N1s binding energy for (Left) unmodified and P membranes, (Middle) unmodified and S membranes, and (Right) unmodified, T, and Q membranes.

3.1.4 Static contact angle goniometry

Table 1 shows the static water contact angle data of the unmodified, P, S, T, and Q membranes. The results generally indicate very slight changes in WCA after modification. While the P membrane is significantly different from other modified samples, S, T, and Q membranes are not significantly different (Supplementary Table S1). This very slight WCA change can be attributed to the attachment of hydrophilic amine groups (Kafetzi and Pispas, 2021; Thijs et al., 2007). However, as mentioned before and supported by ATR-FTIR and XPS data, monolayer addition of amine groups to the surface is not able to change the hydrophilicity of the surface significantly.

TABLE 1. The static WCA of the unmodified and modified PVDF membranes. The error represents one standard deviation among at least three tests.

MembraneWCA (degrees)
Unmodified93.2 ± 3.5
P83.0 ± 2.5
S91.3 ± 2.5
T93.4 ± 4.7
Q89.4 ± 3.3

3.2 Membrane performance

3.2.1 Effect of diamine concentrations

The effect of diamine concentration on DBSA adsorption was evaluated under constant modification conditions (3 h contact time and 50 °C). The effect of diamine concentration and a visual indication of modified membranes are shown in Figure 5. Unmodified PVDF membrane exhibited a DBSA removal of 27.6%, likely due to hydrophobic interactions with DBSA. PVDF membranes modified with 0.1 M NaOH also showed poor adsorption capacity of 31.3%, indicating that polyene segments are not responsible for the increase seen in DBSA removal for the membranes exposed to diamines. All diamine-modified membranes showed increased DBSA adsorption compared to the unmodified and NaOH-modified PVDF membranes. Maximum removal efficiencies of 86.7%, 81.7%, and 90.8% were achieved for 0.2 M P, S, and T membranes, respectively. Further increasing the diamine concentration up to 5 M resulted in a decrease in DBSA removal efficiency to 60.0%, 57.3%, and 70.9% for P, S, and T membranes, respectively. This could be attributed to the increased steric hinderance at higher amine concentration, which results in lower adsorption (Abbasi et al., 2019; Bae et al., 2025; Ko et al., 2013). For all three amine types, the initially white, unmodified membrane gradually transitions to darker brown and eventually near-black coloration as the concentration increases reflecting greater number or conjugation of polyene segments from more dehydrofluorination at elevated concentrations. T membranes color development appears more pronounced compared to other membranes, suggesting stronger basicity (Ma et al., 2017).

(Left) Effect of diamine concentration on DBSA removal efficiency. Diamine modification time and temperature were 3 h and 50 °C, respectively. The NaOH modification was done at 0.1 M for 3 h at 50 °C. The adsorption tests were conducted using 600 mg/L DBSA at 20 °C for 24 h. The initial DBSA solution pH was 3. The error bars represent one standard deviation among at least three tests. (Right) Unmodified, P, S, T, and Q membranes visual photos at 0, 0.2, 0.5, 1, 2, and 5 M diamine concentration.
Figure 5. (Left) Effect of diamine concentration on DBSA removal efficiency. Diamine modification time and temperature were 3 h and 50 °C, respectively. The NaOH modification was done at 0.1 M for 3 h at 50 °C. The adsorption tests were conducted using 600 mg/L DBSA at 20 °C for 24 h. The initial DBSA solution pH was 3. The error bars represent one standard deviation among at least three tests. (Right) Unmodified, P, S, T, and Q membranes visual photos at 0, 0.2, 0.5, 1, 2, and 5 M diamine concentration.

3.2.2 Effect of diamine modification time

Figure 6 shows the effect of diamine modification time on DBSA removal efficiency using a 588 mg/L DBSA solution at 20 °C. Adsorption data were evaluated by measuring the residual DBSA concentration at 3 h contact time. Membranes were modified with different contact times including 0.25, 0.5, 1, 1.5, 3, 6, 8, and 24 h. The results indicate a rapid adsorption process, with removal efficiencies of approximately 88.4%, 83.5%, and 89.9% achieved within 15 min modification time for the P, S, and T membranes, respectively. The maximum removal efficiencies reached 93.9%, 92.0%, and 95.5% after prolonged contact with the diamine solution. In all cases, a plateau was observed after 3 h of contact time, indicating saturation of available amine active sites and no significant further increase in DBSA adsorption.

The effect of diamine modification time with unmodified and P, S, and T PVDF membranes using a 588 mg/L DBSA solution at a pH of 3. The diamine concentration for all cases was 0.2 M. The error bars represent one standard deviation among at least three different samples. The dashed lines are to guide the eyes.
Figure 6. The effect of diamine modification time with unmodified and P, S, and T PVDF membranes using a 588 mg/L DBSA solution at a pH of 3. The diamine concentration for all cases was 0.2 M. The error bars represent one standard deviation among at least three different samples. The dashed lines are to guide the eyes.

3.2.3 Effect of DBSA contact time

Figure 7 illustrates the effect of contact time on DBSA adsorption for each modified membrane over a range of 15 min –48 h. Among the membranes, the Q membrane exhibited the slowest adsorption kinetics, whereas the T membrane showed the fastest adsorption rate. Based on the results, the P, S, and T membranes reached equilibrium after approximately 8 h, while the Q membrane followed a different trend, reaching equilibrium only after 24 h. Additionally, the initial DBSA removal efficiencies at 15 min were comparable for the P (31.2%), S (32.8%), and T (33.3%) membranes, whereas the Q membrane showed a significantly lower initial removal efficiency of 4.0%, similar to the unmodified membrane, indicating a delayed adsorption behavior.

Effect of DBSA contact time on removal efficiency for the unmodified, P, S, T, and Q membranes. The error bars represent one standard deviation among at least three different samples. The dashed lines are to guide the eyes.
Figure 7. Effect of DBSA contact time on removal efficiency for the unmodified, P, S, T, and Q membranes. The error bars represent one standard deviation among at least three different samples. The dashed lines are to guide the eyes.

3.2.4 Adsorption kinetics

The adsorption kinetics were analyzed using Figure 7 to fit with PFO and PSO models, and the corresponding results are presented in Supplementary Table S1 and Table 2 (Supplementary Figures S6, S7). The data indicates that the adsorption process aligns very well with the PSO model, as evidenced by the high coefficient of determination (R 2 = 0.99) for the P, S, and Q membranes, while the T membranes were uniquely described by PFO kinetics. AICc statistical analysis provided in Supplementary Table S3 supports these results. Although the linearized PFO model yielded a low R2 (0.60) for the T membrane, this is a known artifact of the linearization procedure - which requires computing ln (qe-qt) and becomes unreliable when (qe–qt) values approach zero due to rapid equilibration - rather than a true indicator of poor model fit. The AICc nonlinear regression statistical analysis supports PFO as the superior model for the T membrane (R 2 = 0.993, ΔAICc = −13.14) and PSO as the superior model from the P (ΔAICc = 6.17), S (ΔAICc = 6.95), and Q (ΔAICc = 6.38) membranes.

TABLE 2. Pseudo-second order adsorption kinetic parameters of the unmodified, P, S, T, and Q membranes. The diamine modification concentration was 0.2 M and time was 3 h.

MembraneParameterValue
Unmodifiedqe,exp (mg/g) at 24 h5.19
qe,cal (mg/g) at 24 h5.13
k2 (g/mg/min)0.126
R 20.99
Pqe,exp (mg/g) at 24 h15.93
qe,cal (mg/g) at 24 h16.03
k2 (g/mg/min)0.069
R 20.99
Sqe,exp (mg/g) at 24 h16.50
qe,cal (mg/g) at 24 h16.48
k2 (g/mg/min)0.074
R 20.99
Tqe,exp (mg/g) at 24 h17.87
qe,cal (mg/g) at 24 h18.14
k2 (g/mg/min)0.145
R 20.99
Qqe,exp (mg/g) at 24 h14.21
qe,cal (mg/g) at 24 h13.99
k2 (g/mg/min)0.020
R 20.99

The PSO rate constant (k2) increases from P to S to T membranes but decreases for the Q membrane. Tertiary amine functionalities provide a higher effective positive charge density and stronger electron-donating capability compared to secondary and primary amines, which intensifies electrostatic attraction toward negatively charged adsorbates (Hall, 1957; Carey and Sundberg, 2008). In addition, tertiary amines are less prone to intramolecular hydrogen bonding, resulting in greater availability of active sites and improved accessibility of the adsorption surface (LibreTexts, 2012; qixuanchemtec, 2025). The lower adsorption capacity and slower kinetics of the Q membrane relative to other modified membranes can be attributed to the steric hindrance introduced by the bulkier trimethylammonium head group (Chin-Pao HUANG and Delaware, 2019), and the strong ordered hydration shell maintained around the permanently charged N+ center partially screens the electrostatic field (Medos et al., 2019). These factors collectively facilitate faster adsorbate–adsorbent interactions, leading to a higher k2 value and a more rapid approach to equilibrium (Ebelegi et al., 2020).

The T membrane exhibited a continuously decreasing adsorption rate over time, which is characteristic of PFO kinetics, where the adsorption rate is proportional to the number of remaining vacant adsorption sites (qe - qt) (Wang and Guo, 2020; Ho and McKay, 1999). This behavior suggests that all available adsorption sites are readily accessible and interact with DBSA molecules through relatively unhindered electrostatic ion-pairing. In contrast, the P and S membranes showed a two-stage adsorption profile. The initial rapid uptake was primarily driven by electrostatic attraction, followed by a slower stage associated with orientation-dependent hydrogen-bond formation. This more complex adsorption mechanism is better described by the PSO model, in which the adsorption rate depends on the square of the remaining available sites (qe - qt)2 (Ho and McKay, 1999). The Q membrane, however, exhibited a sigmoidal adsorption profile, indicating an initial barrier to adsorption likely caused by steric hindrance and the need to partially displace the hydration shell surrounding the permanently charged N+ groups before effective binding could occur (Medos et al., 2019).

3.2.5 Adsorption isotherm

The effect of initial DBSA concentration on the adsorption behavior of P, S, T, and Q membranes was evaluated using DBSA concentrations ranging from 30 mg/L to 875 mg/L, and the corresponding adsorption isotherms are presented in Figure 8. For the unmodified PVDF membranes, the adsorption capacity (qe) changed from 0.13 to 3.92 mg/g as the DBSA concentration increased. These results indicate that the unmodified PVDF membranes exhibit a limited affinity toward DBSA, which can be attributed to weak hydrophobic interactions between the membrane surface and DBSA molecules. In contrast, all modified membranes showed a clear increase in adsorption capacity with increasing DBSA concentration. The qe values increased from 0.94 to 22.2 mg/g for the P membranes, from 0.88 to 21.1 mg/g for the S membranes, from 0.78 to 21.6 mg/g for the T membranes, and from 0.68 to 14.9 mg/g for the Q membranes, highlighting the role of surface amine functionality in enhancing DBSA adsorption compared to the unmodified PVDF membrane (Raghav and Kumar, 2018; Alqadami et al., 2016). The higher initial DBSA concentrations increase the frequency of DBSA–membrane collisions and provide a stronger mass-transfer driving force, resulting in enhanced adsorption performance (Albadarin et al., 2012; Di et al., 2022). The slope of the linear region in Figure 8 represents the affinity of DBSA for the membrane surface in the dilute concentration regime. A higher slope indicates stronger adsorbate–adsorbent interactions and more efficient uptake at low concentrations, where adsorption occurs independently without significant intermolecular interference. All modified membrane showed higher slopes than unmodified membrane (0.005) indicating stronger affinity between DBSA and membrane surface when they are modified. However, S membrane showed highest affinity (slope: 0.381) with DBSA molecule in adsorption process.

Adsorption isotherms for the unmodified, P, S, T, and Q membranes after 24 h contact with DBSA solutions at 20 °C. All membrane were modified with 0.2 M diamine for 3 h at 50 °C. The inserts in the P, S, T, and Q membranes graphs are the linear region of the adsorption equilibrium data. The error bars represent one standard deviation among at least three measurements. The dashed line between data points is included to guide the eyes.
Figure 8. Adsorption isotherms for the unmodified, P, S, T, and Q membranes after 24 h contact with DBSA solutions at 20 °C. All membrane were modified with 0.2 M diamine for 3 h at 50 °C. The inserts in the P, S, T, and Q membranes graphs are the linear region of the adsorption equilibrium data. The error bars represent one standard deviation among at least three measurements. The dashed line between data points is included to guide the eyes.

3.2.6 Adsorption isotherm models

The Langmuir, Freundlich, Temkin, and Sips isotherm models were employed to analyze the P, S, T, and Q membrane adsorption isotherm data from Figure 8, and the corresponding results are summarized in Table 3 and Supplementary Table S2 (Supplementary Figure S2-5). Based on correlation coefficient obtained from isotherm fitting, the Temkin model provided the best description of DBSA adsorption for the P, S, and T membranes. The Sips isotherm model also showed good agreement for these three membranes. Although the Langmuir model provided the highest fitting correlation for the Q membrane, the resulting parameters were not physically realistic due to the excessively large number for qmax and extremely low KL value, suggesting mathematical overfitting of the adsorption system. In contrast, the Freundlich and Sips models provided more reasonable and realistic descriptions of DBSA adsorption behavior for the Q membrane. Additionally, the Sips isotherm model fitting provided the highest fitting correlation for the unmodified membrane, however, the fitting parameters yielded physically unreliable results. This parameter identifiability failure is attributed to the absence of a saturation plateau in the unmodified membrane isotherm, whereby weak non-specific hydrophobic interactions between the PVDF backbone and DBSA produce a nearly linear qe vs. Ce relationship within the experimental concentration range that cannot constrain a finite-capacity adsorption model.

TABLE 3. Temkin and Sips adsorption isotherms parameters of the unmodified, P, S, T, and Q membranes (0.2 M diamine concentration with 3 h modification time).

IsothermMembraneParameterValue
TemkinUnmodifiedbT (J/mol)2410
AT (L/mg)0.0221
R 20.76
PbT (J/mol)476
AT (L/mg)0.1876
R 20.97
SbT (J/mol)475
AT (L/mg)0.1738
R 20.95
TbT (J/mol)423
AT (L/mg)0.1195
R 20.98
QbT (J/mol)497
AT (L/mg)0.0565
R 20.75
SipsUnmodifiedqmax (mg/g)86.0
KL (L/mg)0.000126
n1.28
R 20.97
Pqmax (mg/g)19.4
KL (L/mg)0.0324
n1.73
R 20.94
Sqmax (mg/g)18.5
KL (L/mg)0.0365
n2.06
R 20.95
Tqmax (mg/g)21.3
KL (L/mg)0.0238
n1.85
R 20.99
Qqmax (mg/g)16.5
KL (L/mg)0.0088
n3.09
R 20.88

The Temkin binding constant (AT) decreases from P to Q membrane, reflecting reduced average initial affinity as surface heterogeneity increases with higher diamine substitution. The relatively constant bT in the range of ∼400-500 J/mol for all membranes shows processes predominantly physisorption-driven. High R 2 values for the Temkin model indicate that it accurately describes the adsorption behavior across all P, S, and T membranes (Foo and Hameed, 2010; Abdel-Ghani et al., 2015; Girish, 2025). Furthermore, the observed non-ideal and non-uniform adsorption behavior could be attributed to the chain structure of DBSA and interactions among adsorbed molecules. Miranda et al. reported that when surfactant molecules have sufficiently long hydrocarbon chains (up to C14), their penetration into the surfactant monolayer can eliminate initial gauche defects within the chains due to strong chain–chain interactions, resulting in non-ideal adsorption behavior (Miranda et al., 1998).

The Sips isotherm provides a combined description of the adsorption behavior by accounting for both surface heterogeneity and finite adsorption capacity (Babatunde et al., 2022; de Vargas Brião et al., 2023). The T membrane exhibits the highest adsorption capacity and best model fit, indicating a heterogeneous surface with significant contribution from non-uniform interactions. P and S membranes show higher affinity constants, reflecting stronger initial adsorption driven by electrostatic and hydrogen bonding interactions (Hall, 1957; Trotman-Dickenson, 1949). The Q membrane displays the lowest affinity and capacity but the highest heterogeneity factor (n), suggesting a highly non-ideal adsorption process (Mehrvarz et al., 2016; Tzabar and ter Brake, 2016). This behavior can be attributed to cooperative surfactant aggregation of DBSA on the surface, which deviates from classical monolayer adsorption assumptions (Mehrvarz et al., 2016).

3.2.7 Effect of pH

Figure 9 illustrates the effect of solution pH on DBSA removal efficiency. Adsorption experiments were conducted at pH values of 3.0, 5.0, 7.0, 9.0, and 11.0 using a DBSA concentration of 588 mg/L at 20 °C. The results show that the P, S, T, and Q membranes exhibited the highest removal efficiency under acidic conditions (pH 3.0), with values of 93.4%, 91.3%, 95.1%, and 71.8%. As the solution pH increased toward basic conditions, a gradual decrease in DBSA removal efficiency was observed for all modified membranes, reaching 70.9%, 84.4%, 86.4%, and 37.9% for the P, S, T, and Q membranes at pH 11.0. Under acidic conditions, the amine groups on the modified membrane surfaces are protonated, enhancing electrostatic attraction between the positively charged amine groups and the negatively charged DBSA molecules (pKa of 0.7-1.35) (fiveable, 2025; OECD, 2009; EPA, 2026). In contrast, at higher pH values, deprotonation of the amine groups reduces electrostatic interactions, leading to lower adsorption efficiency. The pKa values of the primary, secondary, and tertiary diamines used in this study are reported to be in the range of 9.9–10.9 (ChemicalBook, 2025a; ChemicalBook, 2025b; PubChem, 2025). Notably, despite the reduced electrostatic contribution at high pH, appreciable DBSA removal (in the range of 70.9%-86.4%) was still observed at pH 11, indicating the involvement of additional adsorption mechanisms such as hydrophobic interactions, Coulombic attraction, dipole interactions, and weak electrostatic forces (Tong et al., 2019).

(Left) Effect of initial solution pH on removal efficiency with a DBSA concentration of 588 mg/L, a temperature of 20 °C, and a DBSA solution contact time of 24 h. The error bars represent one standard deviation among at least three tests. The dashed line between data points is included to guide the eyes. (Right) Grafted amine state at different acidic and basic conditions.
Figure 9. (Left) Effect of initial solution pH on removal efficiency with a DBSA concentration of 588 mg/L, a temperature of 20 °C, and a DBSA solution contact time of 24 h. The error bars represent one standard deviation among at least three tests. The dashed line between data points is included to guide the eyes. (Right) Grafted amine state at different acidic and basic conditions.

3.2.8 Coexistence effect

The effects of coexisting ions on DBSA adsorption were evaluated using NaCl and Na2SO4 as representative background electrolytes. Typical groundwater concentrations of these salts range from approximately 200–300 mg/L (USGS, 2025). Accordingly, 200 mg/L of NaCl and 200 mg/L of Na2SO4 were added to a 588 mg/L DBSA solution, and static adsorption experiments were conducted for the unmodified and P, S, T, and Q membranes. The results indicate that the presence of background ions caused a slight reduction in DBSA removal efficiency for all modified membranes. After 24 h of contact, removal efficiencies reduced to 84.9% ± 0.4%, 84.8% ± 0.4%, 81.2% ± 1.9% and 67.2% ± 0.9% for the P, S, T, and Q membranes, respectively. These values correspond to a decrease of 9.6%, 7.9%, 14.9%, and 6.5% relative to DBSA-only adsorption results for the P, S, T, and Q membranes. These results suggest that the T membrane is more sensitive to the presence of competing ions, whereas the P and S membranes exhibit greater tolerance under ionic strength variations.

3.2.9 Thermodynamics of adsorption

3.2.9.1 Gibbs free energy of change

Thermodynamic parameters were evaluated at 20, 40, and 60 °C to elucidate the nature of DBSA adsorption on the P, S, T, and Q membranes. As shown in Figure 10, for all modified membranes, the equilibrium adsorption capacity slightly decreased with increasing temperature, while the equilibrium constant also declined, indicating that higher temperatures weaken the adsorbate–adsorbent interactions. The calculated Gibbs free energy changes were negative at all temperatures and becomes less favorable with temperature (Shirsath and Shirivastava, 2015). The negative enthalpy changes for all modified membranes (−20.0 kJ mol-1, −16.6 kJ mol-1, −27.5 kJ mol-1, and −2.9 kJ mol-1 for the P, S, T, and Q membranes, respectively) demonstrate that DBSA adsorption on these surfaces was exothermic, whereas the Q membrane shows a smaller magnitude enthalpy change, consistent with weaker but more accessible electrostatic interactions (Tran et al., 2016). The positive entropy changes observed for all membranes indicate increased randomness at the solid–liquid interface during adsorption attributed to the release of structured water molecules from both the DBSA sulfonate head group and the charged membrane surface upon ion-pairing (Mansurov et al., 2016). Overall, these thermodynamic findings demonstrate that DBSA adsorption onto amine-functionalized PVDF membranes is exothermic, physisorption-driven, and less favorable at higher temperatures, in agreement with the Temkin and PSO analyses.

Effect of temperature on RE on the unmodified, P, S, T, and Q membranes. Modified membranes were contacted with a 588 mg/L DBSA solution at the specified temperatures for 3 h with an initial solution pH of 3. The membrane modification parameters were 0.2 M diamine solution for 3 h at 50 °C. The error bars represent one standard deviation among at least three tests.
Figure 10. Effect of temperature on RE on the unmodified, P, S, T, and Q membranes. Modified membranes were contacted with a 588 mg/L DBSA solution at the specified temperatures for 3 h with an initial solution pH of 3. The membrane modification parameters were 0.2 M diamine solution for 3 h at 50 °C. The error bars represent one standard deviation among at least three tests.

3.2.9.2 Isosteric thermodynamic model

The isosteric heat of adsorption derived from the isosteric thermodynamic model represents the differential enthalpy of adsorption at constant surface coverage, capturing coverage-dependent energetic heterogeneity across the membrane surface. The data in Figure 11 shows that all modified membranes have a linear negative trend by increasing the temperature. The magnitude values within the typical physisorption range (<40 kJ/mol) indicates that DBSA adsorption is predominantly governed by electrostatic attraction and weak intermolecular interactions rather than chemical bond formation (fiveable, 2025; Hiden Analytical, 2021). Figure 11 shows the calculated isosteric heats of adsorption for DBSA on the modified membranes range from 2.11 to 25.4 kJ/mol. These values fall within the typical range for physisorption which indicates that adsorption is primarily governed by electrostatic interactions and weak intermolecular forces rather than chemisorption. T membrane exhibits the highest adsorption enthalpy (25.4 kJ/mol), suggesting stronger interactions between DBSA molecules and tertiary amine functional groups. In contrast, the Q membrane shows a significantly lower adsorption enthalpy (2.11 kJ/mol), implying that adsorption on this surface is dominated by weak electrostatic interactions and possible cooperative surfactant aggregation rather than strong surface binding (Rosen, 2004).

Isosteric thermodynamic model for the unmodified, P, S, T, and Q membranes. Note that the Q membrane has a different y-axis range than the P, S, and T membranes. The error bars represent one standard deviation among at least three measurements.
Figure 11. Isosteric thermodynamic model for the unmodified, P, S, T, and Q membranes. Note that the Q membrane has a different y-axis range than the P, S, and T membranes. The error bars represent one standard deviation among at least three measurements.

3.2.9.3 Adsorption mechanism

All the thermodynamic data, the bT parameters from the Temkin model (Table 3), the ΔH° values from the Gibbs Free Energy change (Table 4), and the isosteric heat of adsorption [ΔHST] values (Figure 11), support the adsorption mechanism being physisorption over chemisorption. The conventional threshold is that if these values are below 40 kJ/mol (absolute value) then the adsorption mechanism is physisorption and if they are above 80 kJ/mol (absolute value) then the adsorption mechanism is chemisorption (Saha and Chowdhury, 2011). These values are summarized in Table 5. All three models provide independent evidence that electrostatic attraction and hydrogen bonding physisorption is the adsorption mechanism and not chemisorption. The relative binding strength is also consistent across the thermodynamic models with T > P > S >> Q. Tertiary amines are typically the strongest bases among primary, secondary, and tertiary amines, which provides the strongest interaction with the acidic DBSA head groups. Quaternary amines are permanently protonated, so the Q membrane can ion-pair with the DBSA head groups; which the data supports as a weaker interaction here. While the PSO model, which fits all of our data well, has traditionally been used to support chemisorption, it was not derived from a chemisorption mechanism (Plazinski et al., 2009), and thus should not be used to describe the adsorption mechanism, only the kinetics. The thermodynamic data directly informs bond energies, and thus should be the models used to inform the adsorption mechanism.

TABLE 4. Thermodynamic parameters of DBSA adsorption on the unmodified, P, S, T, and Q membranes.

MembraneThermodynamic parameters20 °C40 °C60 °C
UnmodifiedCe434.2372.7386.1
qe (mg/g)5.67.87.4
KL°4208.96863.06215.9
ΔG° (kJ/mol)−20.3−23.0−24.2
ΔH° (kJ/mol)−8.2
ΔS° (J/(mol.K))45.9
R 20.61
PCe40.683.096.7
qe (mg/g)19.918.417.9
KL°160104.672233.060368.5
ΔG° (kJ/mol)−29.2−29.1−30.5
ΔH° (kJ/mol)−20.0
ΔS° (J/(mol.K))30.6
R 20.90
SCe50.692.1102.7
qe (mg/g)19.618.017.7
KL°126121.163933.156106.5
ΔG° (kJ/mol)−28.6−28.8−30.3
ΔH° (kJ/mol)−16.6
ΔS° (J/(mol.K))40.2
R 20.89
TCe27.680.992.7
qe (mg/g)20.418.518.0
KL°241368.674438.063437.6
ΔG° (kJ/mol)−30.2−29.2−30.6
ΔH° (kJ/mol)−27.5
ΔS° (J/(mol.K))8.0
R 20.86
QCe153.0157.9170.0
qe (mg/g)15.8215.6515.21
KL°33760.932359.429205.6
ΔG° (kJ/mol)−25.4−27.0−28.5
ΔH° (kJ/mol)−2.9
ΔS° (J/(mol.K))76.9
R 20.93

TABLE 5. Thermodynamic parameters of the unmodified and P, S, T, and Q membranes.

MembraneTemkin bT (kJ/mol)ΔH° (kJ/mol)ΔHST (kJ/mol)
Unmodified2.41−8.217.9
P0.48−20.017.9
S0.48−16.614.6
T0.42−27.525.4
Q0.50−2.92.1

3.2.10 Desorption and re-usability

Figure 12 shows the results of desorption and re-usability experiments over the course of 5 cycles. After each adsorption cycle, the membranes were contacted with a n-hexane:1-nonanol (1:1 v/v) to desorb DBSA from the membrane surface by reducing hydrophobic interactions, electrostatic interactions, and disrupting any possible micelle cores formed in the adsorption process. The n-hexane:1-nonanol mixture likely disrupts DBSA adsorption through complementary mechanisms, where 1-nonanol penetrates and disorders the hydrophobic DBSA aggregates or any possible formed hemimicelle-like structures on the membrane surface, weakening hydrophobic interactions and cooperative packing between surfactant molecules. It also indirectly destabilizes electrostatic amine–sulfonate interactions through alteration of the local interfacial environment. Meanwhile, n-hexane acts as a nonpolar extraction medium that solvates the released DBSA hydrocarbon chains and reduces hydrophobic association, facilitating desorption of the surfactant from the membrane surface (Milton and Rosen, 2012; Atkins, 2006; Somasundaran and Huang, 2000). The DBSA concentration in the desorbed solution was measured by UV-Vis. The results in Figure 12A revealed an initial DBSA removal of 88.6% ± 2.8%, 88.8% ± 1.2%, 87.4% ± 0.9%, and 72.0% ± 3.5% using a ∼555 mg/L DBSA solution for P, S, T, and Q membranes, respectively. Subsequent adsorption cycles showed a decrease in DBSA removal efficiency, reaching a final value of 69.9 ± 0.9, 68.6 ± 1.3, 70.6 ± 1.1, and 72.0% ± 0.5% in the fifth cycle. The first desorption process for these membranes illustrated in Figure 12B showed a removal of 344.5 ± 19.2, 348.0 ± 34.5, 329.7 ± 10.1, and 354.5 ± 8.2 mg/L of DBSA from the membrane surface to the solution. This decline in adsorption compared to the first cycle, which is an approximate 19%-23% decline for the P, S, and T membranes and almost no change for the Q membranes, could be attributed to the occupation of adsorption sites on the membrane surface during previous adsorption cycles. The maximum DBSA desorption amount observed was 412 mg/L after the second desorption cycle. It is worth noting that not all of the adsorbed DBSA was desorbed in next desorption cycle, resulting in an accumulation of DBSA on the surface and a decrease in adsorption efficiency in subsequent cycles.

Regeneration of modified membrane in 5 cycles. **(A)** DBSA removal efficiency of each cycle and **(B)** desorbed DBSA in each cycle. The error bars represent one standard deviation among at least three samples.
Figure 12. Regeneration of modified membrane in 5 cycles. (A) DBSA removal efficiency of each cycle and (B) desorbed DBSA in each cycle. The error bars represent one standard deviation among at least three samples.

3.2.11 Dynamic adsorption

The dynamic adsorption results in Supplementary Figure S10 shows an initial removal of DBSA for all modified membranes (P, S, T, and Q), characterized by a sharp increase in DBSA removal in permeate concentration. This indicates rapid adsorption onto the available active sites at the early stage. However, after this initial period, the DBSA concentration quickly approaches the feed concentration, suggesting fast saturation of the adsorption sites and limited long-term adsorption capacity under dynamic conditions. In contrast, the unmodified membrane shows no significant change, maintaining a near zero DBSA removal throughout the experiment, confirming the absence of adsorption capability. This low dynamic performance compared to static performance can be attributed to the limited residence time of the feed solution at the membrane surface under continuous-flow conditions. Future efforts to bridge this gap may focus on increasing the surface density of functional groups, for instance through deposition of a thin polymer interlayer prior to amination, a strategy previously shown to enhance amine group loading and improve dynamic removal efficiency in related membrane systems (Rouhi et al., 2024). The monolayer modification here allows us to study the differences in the chemistry but is not practical for a dynamic adsorption system. Table 6 also presents the pure water permeance of the unmodified and modified membranes. Following surface modification (Rouhi et al., 2024), not a huge decrease in membrane permeance was observed. This behavior is consistent with the formation of a monolayer diamine coating on the PVDF surface, which does not extremely hinder water and DBSA transport through the membrane.

TABLE 6. Permeance of unmodified and P, S, T, and Q membranes.

Performance parameterUnmodifiedPSTQ
Permeance (L.m-2.h-1.bar-1)109011427511604959310542

4 Conclusion

PVDF membranes were functionalized with four different reagents to introduce primary, secondary, tertiary, and quaternary amine groups onto the membrane surface, enabling adsorption of negatively charged DBSA molecules from aqueous solution. Significant color change is observed upon modification. NaOH treatment revealed the color change is likely due to dehydrofluorination, which results in long, conjugated polyene segments. These polyene segments are then partially disrupted via Michael addition of the diamines. The P, S, and T membranes demonstrated promising adsorption performance, achieving over 91% DBSA removal within 3 h, whereas the Q membrane required a longer equilibration time of 24 h and achieved only 72% removal. Surface modification substantially enhanced adsorption capacity, increasing from 3.9 mg/g for unmodified PVDF to a maximum of 22.2 mg/g for the modified membranes. Kinetic analysis showed psuedo-second-order behavior for the P, S, and Q membranes, while the T membrane uniquely followed pseudo-first-order kinetics statistically supported by AICc analysis. Isotherm analyses indicated that DBSA adsorption involved heterogeneous surface interactions with predominantly physisorption-driven behavior. Thermodynamic and isosteric analyses further confirmed the exothermic nature of adsorption, with enthalpy values ranging from 2.1 to 25.4 kJ/mol and decreasing adsorption favorability at elevated temperatures. In addition, all modified membranes exhibited promising regeneration performance, with first-cycle DBSA removal efficiencies of 88.4%, 89.9%, 87.5%, and 74.9% for P, S, T, and Q membranes, respectively, with a 19.3%, 22.7%, 19.2%, and 0.1% decline after five regeneration cycles. Future work will focus on applying this surface chemistry to a polymer-modified system to improve dynamic performance for practical continuous-flow water treatment applications.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/frmst.2026.1892716/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Lloyd Njoka Ndlovu, National University of Science and Technology Faculty of Engineering, Zimbabwe

Reviewed by: Muhammad Yousif, Tianjin Polytechnic University, China

Safoura Bakhodaye Dehghanpour, Arak University, Iran

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Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) declared that financial support was received for this work and/or its publication. H.R. was supported in part by the National Science Foundation under award number CBET-2427919. S.T.W thanks the Alabama Water Institute for funding to purchase the ATR-FTIR and contact angle goniometer used in this study.

Data availability

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author contributions

HR: Conceptualization, Data curation, Formal Analysis, Investigation, Methodology, Validation, Visualization, Writing – original draft, Writing – review and editing. SW: Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Supervision, Writing – review and editing.

Acknowledgements

We thank the Alabama Materials Institute for the use of their SEM and XPS, which is financially supported by The University of Alabama Division for Research. We thank Center for Water Quality at The University of Alabama for the use of their UV-Vis instrument.

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Citation

Rouhi, H. & Weinman, S. T. (2026). Primary, secondary, tertiary, and quaternary Alkyl amine-functionalized PVDF membranes for DBSA removal. Frontiers in Membrane Science and Technology, Volume 5. https://doi.org/
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*Correspondence: Steven T. Weinman

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