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Article

Development of Poly(diallyldimethylammonium) Chloride-Modified Activated Carbon for Efficient Adsorption of Methyl Red in Aqueous Systems

by
Simeng Li
and
Madjid Mohseni
*
Department of Chemical and Biological Engineering, University of British Columbia (UBC), Vancouver, BC V6T 1Z3, Canada
*
Author to whom correspondence should be addressed.
Clean Technol. 2025, 7(3), 61; https://doi.org/10.3390/cleantechnol7030061
Submission received: 18 June 2025 / Revised: 9 July 2025 / Accepted: 11 July 2025 / Published: 15 July 2025
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Abstract

A modified activated carbon (AC) was developed by modifying with poly(diallyldimethylammonium) chloride (PDADMAC) to enhance its adsorption performance for water treatment applications. Different PDADMAC concentrations were explored and evaluated using methyl red as a model contaminant, with 8 w/v% PDADMAC yielding the best adsorption performance. The kinetics data were well described by the pseudo-first-order equation and homogeneous surface diffusion model. The Freundlich isotherm fit the equilibrium data well, indicating multilayer adsorption and diverse interaction types. The removal efficiency remained similar across a pH range of 5–9 and in the presence of background inorganic (NaCl)/organic compounds (sodium acetate) at different concentrations. Rapid small-scale column tests were performed to simulate continuous flow conditions, and the PDADMAC-modified AC effectively delayed the breakthrough of the contaminant compared to raw AC. Regeneration experiments showed that 0.1 M NaOH with 70% methanol effectively restored the adsorption capacity, retaining 80% of the initial efficiency after five cycles. Quantum chemical analysis revealed that non-covalent interactions, including electrostatic and Van der Waals forces, governed the adsorption mechanism. Overall, the results of this study prove that PDADMAC-AC shows great potential for enhanced organic contaminant removal in water treatment systems.

Graphical Abstract

1. Introduction

Organic contaminants in natural waters pose a persistent environmental challenge, with far-reaching consequences for aquatic ecosystems, public health, and economic activities [1]. Achieving Sustainable Development Goal 6, which aims to ensure universal access to clean water by 2030, is projected to require an investment of approximately USD 90 billion in water infrastructure [2]. The inadequate treatment of wastewater can exacerbate these issues, leading to ecological degradation and long-term societal costs [3]. For example, textile industries discharge an estimated 20,000 tons of dyes annually into wastewater [4]. Effluents containing reactive dyes contribute to approximately 17–20% of global water pollution. Among various dye types, azo dyes are the most widely used, accounting for about 60% of total dye consumption [5]. Methyl red (MR) is one such azo dye that can cause the sensitization of the eyes and skin and lead to respiratory and digestive tract irritation when inhaled or ingested [6].
Among the various water treatment technologies, adsorption techniques are very attractive due to their simplicity, cost-effectiveness, high efficiency, and ease of operation [6]. In particular, activated carbon (AC) is commonly utilized in water purification due to its high surface area, porous architecture, and excellent adsorption ability [7,8,9,10,11]. Some ongoing research is focused on further enhancing the adsorption performance of AC through various physical, chemical, and biological modification methods [12,13,14]. Among these strategies, the incorporation of quaternary ammonium compounds has received considerable attention [15,16,17,18,19]. It is expected that they could provide excess positive charge to increase the anion exchange capacity of AC. However, the excess number of cationic polymers may also reduce the removal efficiency by blocking the pores, hence reducing the effective surface area for adsorption [20,21,22]. Thus, it is important to optimize the amount of added polymer that can maximize the removal efficiency of a contaminant.
Among quaternary ammonium compounds, poly(diallyldimethylammonium) chloride (PDADMAC) stands out due to its high charge density, water solubility, and environmental compatibility [23,24]. These properties make PDADMAC a promising candidate for AC surface modification. This study aims to synthesize PDADMAC granular activated carbon (GAC) composites and assess their adsorption performance under varying conditions. Methyl red (MR) was chosen as a model contaminant due to its representative chemical structure, featuring both aromatic rings and a negatively charged carboxylate group. These functionalities enable it to participate in key adsorption mechanisms—including electrostatic attraction, hydrophobic interactions, and π–π stacking—commonly involved in the removal of organic pollutants by engineered adsorbents. Thus, MR is a relevant and versatile probe organic contaminant for assessing the adsorption performance of newly developed materials intended for water purification. As far as the authors are aware, this is the first publicly reported study using PDADMAC-GAC composites for MR adsorption. Batch experiments were conducted to study adsorption kinetics and isotherms, along with the influence of solution pH and background inorganic and organic compounds (represented by NaCl and sodium acetate, respectively) on MR removal. To simulate real-world operation performance, rapid small-scale column tests (RSSCTs) were also conducted. Additionally, regeneration tests were carried out to assess the reusability and long-term effectiveness of the adsorbents. A deeper understanding of the adsorption mechanisms is gained through the insights provided by quantum chemical analysis.

2. Materials and Methods

2.1. Materials

Methyl red sodium salt, poly(diallyldimethylammonium) chloride (PDADMAC), sodium chloride, sodium sulfate, and sodium acetate were purchased from Sigma-Aldrich (Oakville, ON, Canada). Sodium hydroxide, sodium nitrate, sodium bicarbonate, and methanol were purchased from Fisher Scientific (Ottawa, ON, Canada). All chemicals used in this study were of analytical grade or higher and were applied without any additional purification. Solutions were prepared using deionized water (DI) produced by a Milli-Q system with a resistivity of 18.2 MΩ·cm. Filtrasorb 400 GAC (0.55–0.75 mm, bituminous coal-based) was purchased from Calgon Carbon (Markham, ON, Canada). The GAC was thoroughly washed with DI several times and then dried overnight at 80 °C prior to use.

2.2. Synthesis of PDADMAC-AC

To synthesize PDADMAC-AC, one gram of GAC was agitated with 100 mL of PDADMAC solution at varying concentrations (0, 2, 4, 8, 10, and 12% w/v) in DI at 150 rpm for 4 h at room temperature. After treatment, the GAC was thoroughly rinsed several times with DI. Subsequently, 50 mg of the dried PDADMAC-AC was mixed with 100 mL of DI at 150 rpm for 24 h. Following the separation of the AC, the total organic carbon (TOC) content in the water was confirmed to be below 0.3 ppm, indicating the complete removal of residual PDADMAC and negligible leaching from the modified GAC.

2.3. Characterization and Analytical Methods

The micromorphology and elemental composition of the adsorbents were examined using a FEI Quanta 650 scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDX) (FEI Company, Hillsboro, OR, USA), operating at an accelerating voltage of 20 kV. The Brunauer–Emmett–Teller (BET) surface area was measured from nitrogen adsorption–desorption isotherms using a Quantachrome Autosorb-1 MP instrument (Anton Paar, Saint-Laurent, QC, Canada). Thermogravimetric analysis (TGA) was performed with Waters Discovery TGA 5500 (Waters, Milford, MA, USA), heating the samples from room temperature to 900 °C at a rate of 10 °C/min under a nitrogen atmosphere. The isoelectric point was measured using the SurPASS 3 electrokinetic analyzer (EKA) (Anton Paar, Saint-Laurent, QC, Canada). MR concentrations were quantified at a 435 nm wavelength using a UV–vis spectrophotometer (Cary 100, Agilent Technologies, Santa Clara, CA, USA) [25]. The TOC was measured using a Sievers M5310C UV/persulfate TOC analyzer with an autosampler (Sievers M5310C with autosampler, SUEZWater Technologies & Solutions, Boulder, CO, USA). Nitrate concentrations were determined via ion chromatography (Dionex ICS-1100, Dionex Corporation, Sunnyvale, CA, USA) equipped with an AS22-Fast analytical column and autosampler.
The anion exchange capacity of the adsorbents was evaluated by quantifying the displacement of nitrate (NO3−) ions by chloride (Cl−) ions [26]. Initially, 1.5 g of adsorbent was mixed with 100 mL of 1 M NaNO3 at 150 rpm for 24 h to convert it to the NO3− form. The adsorbent was then rinsed multiple times with DI until the residual NO3− concentration dropped below 1 ppm. Next, the sample was agitated with 50 mL of 1 M NaCl at 150 rpm for another 24 h. The concentration of NO3− released into the NaCl solution was then measured at 210 nm using the Agilent Cary 100 UV–vis spectrophotometer.

2.4. Batch Adsorption Experiments

To evaluate the effect of the PDADMAC concentration incorporated into the activated carbon, 50 mg of PDADMAC-AC composite prepared with varying PDADMAC concentrations was added to 100 mL of 50 ppm MR solution. The mixtures were agitated at 150 rpm for 23 h at room temperature, with the solution pH maintained at approximately 6.5. Samples were collected at predetermined time intervals and analyzed for MR concentration. The PDADMAC-AC exhibiting the highest adsorption performance was selected for further experiments. The adsorption kinetics of both unmodified AC and the optimal PDADMAC-AC were analyzed using pseudo-first-order and pseudo-second-order models, as well as the homogeneous surface diffusion model (HSDM), using Equations (S1)–(S9) in the Supplementary Materials.
For the adsorption isotherm experiments, 10–30 mg of the adsorbent was added to 100 mL of MR solutions with initial concentrations ranging from 50 to 100 ppm. The mixtures were agitated at 150 rpm for 7 days to ensure equilibrium, with the solution pH maintained at approximately 6.5 throughout the experiment. The equilibrium adsorption data were subsequently analyzed and fitted using both the Langmuir and Freundlich isotherm models to evaluate the adsorption capacity and surface interaction characteristics (Equations (S10) and (S11) in the Supplementary Materials).
To evaluate the effect of pH, 25 mg of PDADMAC-AC was added to 100 mL of 50 ppm MR solution. The pH was adjusted to 4, 5, 6.5, 8, and 9, and the samples were agitated at 150 rpm. The influence of inorganic solutes, represented by NaCl, was assessed by adding 25 mg of PDADMAC-AC to 100 mL of 50 ppm MR solution containing 0–40 ppm NaCl. Similarly, the effect of background organics, represented by sodium acetate, was studied by adding 15 mg of PDADMAC-AC to100 mL of 50 ppm MR solution with 0–40 ppm sodium acetate. The equilibrium data were analyzed to interpret the adsorption behavior.
All adsorption batch experiments were conducted in duplicate. UV–vis absorbance measurements for each sample were performed in triplicate to ensure accuracy.

2.5. Rapid Small-Scale Column Tests (RSSCTs)

Rapid small-scale column tests (RSSCTs) were developed to simulate the performance of full-scale GAC systems using smaller-scale setups [27]. They replicate the hydrodynamic and mass transfer characteristics of full-scale adsorbers by employing miniature columns packed with fine-sized adsorbents [28]. Compared to batch or pilot-scale tests, RSSCTs offer several advantages, including simpler operation under controlled laboratory conditions, faster breakthrough times, reduced water requirements, and the ability to reflect full-scale operational parameters with high accuracy [29]. In this study, RSSCTs were conducted for unmodified AC and PDADMAC-AC. Constant diffusivity (CD) design was used, since CD is a more conservative approach than proportional diffusivity (PD) design for performance prediction [30]. The equation used for CD design is shown below:
E B C T G E B C T U = d G d U 2
where EBCT is the empty bed contact time (min); d is the adsorbent diameter (mm); the subscript G refers to the ground particles; and the subscript U refers to unground particles.
Since large-scale GAC columns are usually operated with a 5–30 min EBCT, a 10 min EBCTU was applied in this test [31]. The average original and ground particle sizes of the adsorbents were 0.65 mm and 0.231 mm, respectively. The grinding resulted in an approximately 2.81-fold reduction in the particle size; thus, the EBCTG was 1.27 min in this test.
RSSCTs were run in the downward flow mode with a 2.5 mL/min flow rate. This flow rate was chosen to meet the minimum flow rate to ignore the dispersion effect [32]. The columns were constructed of borosilicate glass, with a 1.5 cm inner diameter and a 10 cm length. Wall effects were avoided by assuring that the ratio of the column diameter to the particle diameter exceeded the minimum requirement of 10 [33]. Glass beads were packed below and above the adsorbent bed to prevent adsorbent migration. Before pumping influent water into the column, the packed column was flushed with DI for one day to further wash the column and assess any leakage of water. The inlet water contained 10 ppm MR + 10 ppm sodium acetate + 10 ppm NaCl + 10 ppm Na2SO4 + 10 ppm NaNO3 + 10 ppm NaHCO3.

2.6. Regeneration Tests

A total of 25 mg of PDADMAC-AC was added to 100 mL of 50 ppm MR solution and agitated at 150 rpm for 48 h. After adsorption, the spent adsorbents were regenerated by treating them in 50 mL of one of the following solutions: 1 w/v% NaCl, 70 vol% methanol, 1 w/v% NaCl + 70 vol% methanol, 0.1 M NaOH, 0.5 M NaOH, or 0.1 M NaOH + 70 vol% methanol. Regeneration was carried out on a shaker at 150 rpm for 24 h. The regenerated adsorbents were then thoroughly rinsed with DI and reused in the subsequent adsorption cycle. This adsorption–regeneration process was repeated for five cycles to evaluate the adsorbent reusability. A control experiment was also performed, in which the used PDADMAC-AC was exposed to DI alone after the adsorption test, following the same procedure.

2.7. Quantum Chemical Calculations

Quantum chemical calculations are important tools for investigating contaminant adsorption behavior at the atomic and molecular scales [34]. By applying mathematical approximations to solve the Schrödinger equation for electrons and nuclei within a system, quantum chemical methods provide detailed insights into electron distribution and molecular orbital characteristics [35]. These calculations enable the modeling of optimized molecular geometries and the identification of potential adsorption sites. As a result, quantum chemical simulations play a critical role in predicting adsorption behavior and guiding the rational design of more effective adsorbent materials [36,37]. The molecular geometries were first optimized in ORCA 6.0.1 using the B97-D3 functional with the def2-SVPD basis set, incorporating an implicit solvation model in water [38,39,40,41]. The optimized structures were further analyzed, including molecular orbitals and electrostatic potential (ESP) surfaces, using the B97-D3/def2-TZVPD method. Reduced density gradient (RDG) analysis and Fukui function calculations were conducted using Multiwfn 3.8 (dev) [42,43]. Molecular structures and isosurfaces were visualized using Avogadro (Version 1.2.0), VMD (Version 1.9.4), and Gnuplot (Version 6.0.2).

3. Results and Discussion

3.1. Effect of PDADMAC Concentration

To evaluate the effect of PDADMAC loading on the adsorption efficiency of the PDADMAC-AC composite, activated carbon modified with varying concentrations of PDADMAC was exposed to a 50 ppm MR solution for 23 h, as shown in Figure 1. All AC modified with PDADMAC showed a better MR removal performance than raw AC (PDADMAC-AC (0 w/v%)). It is hypothesized that PDADMAC binds to the AC surface through a combination of electrostatic interactions with dissociated carboxyl and hydroxyl groups, along with van der Waals forces [44]. Since only a portion of its quaternary ammonium groups are neutralized by the surface’s negative charge, the adsorption of PDADMAC results in the introduction of additional positive charge, which increases the electrostatic attraction with anionic molecules. Furthermore, because the polymer molecule attaches to the AC surface at multiple points, its desorption back into the solution is highly unlikely under typical conditions due to entropy constraints. Since PDADMAC-AC (8 w/v%) performed better than PDADMAC-AC (2 w/v% and 4 w/v%) and further increasing the PDADMAC concentration did not lead to a higher removal efficiency, PDADMAC-AC (8 w/v%) was used in subsequent experiments. The observed performance plateau beyond 8 w/v% is likely due to excess long-chain polymers blocking the pores of the activated carbon, which may reduce the accessible surface area and offset the benefit of additional adsorption sites. This hypothesis is further examined using BET surface area analysis, as discussed in the following section.

3.2. Characterization of Adsorbents

SEM images of unmodified and PDADMAC-modified AC (Figure 2) show that the surface morphology remains largely unchanged after coating. A thin polymer layer forms on the surface without compromising the AC structure. EDX analysis reveals elemental differences: raw AC contains C, O, Si, Al, and S, whereas PDADMAC-AC also includes N and Cl. The EDX spectrum is shown in Figure S1. The even distribution of Cl and N (Figure 3) confirms uniform PDADMAC coating. The TGA curves of AC and PDADMAC-AC are shown in Figure S2 in the Supplementary Materials. PDADMAC-AC displayed a larger mass loss than AC due to the decomposition of PDADMAC. Based on the mass balance, PDADMAC-AC contains ~8.74 mass% of PDADMAC.
The measured BET surface areas of AC and PDADMAC-AC are 879.780 m2/g and 755.282 m2/g, respectively. The incorporation of PDADMAC slightly reduces the surface area of AC, likely due to partial pore blockage caused by the polymer. The decrease in the surface area following the polymer modification of activated carbon has also been observed in previous studies [15,16]. Although the reduction in the surface area seems to be detrimental for adsorption, its effect could be offset by the introduction of new adsorption sites from PDADMAC, which enhances the overall removal efficiency. The anion exchange capacity of raw AC is determined to be 0.078 meq/g, while the anion exchange capacity of PDADMAC-AC is determined to be 0.200 meq/g. It proves that modification with PDADMAC results in the introduction of additional positive charge, which is favorable for the electrostatic attraction with anionic molecules.

3.3. Batch Adsorption Results

3.3.1. Adsorption Kinetics and Isotherm

The equilibrium data of 10–30 mg of adsorbents in 100 mL of 50–100 ppm MR solution were fitted to Freundlich and Langmuir isotherms as demonstrated in Figure 4 and Table 1. Although both Freundlich and Langmuir isotherms yield similar R2 values, the root mean square error (RMSE) of the Freundlich model is lower than that of the Langmuir model, suggesting that the Freundlich isotherm provides a better fit to describe the MR adsorption equilibrium. This suggests that multilayer and multiple types of interactions, such as electrostatic attraction, hydrophobic interactions, and π–π stacking, are involved in the adsorption process. The n-value greater than 1 suggests a strong affinity between MR molecules and the adsorbent [45]. The values of Kf and n of PDADMAC-AC are larger than those of AC, suggesting that PDADMAC-AC has a larger adsorption capacity and a higher affinity than AC. In addition, the Kf value of this study exceeds that of most activated carbons for MR adsorption reported in the literature, as shown in Table S1 in the Supplementary Materials, highlighting the high adsorption capacity of PDADMAC-AC.
Pseudo-first-order, pseudo-second-order, and HSDM equations have been widely applied for the adsorption kinetics analysis of dyes [46,47,48,49,50]. The fitted results of this study are shown in Figure 5 and Table 2 and Table 3. Based on the regression correlation coefficient (R2) and RMSE, the pseudo-first-order model describes the kinetic data slightly better than the pseudo-second-order model, indicating that chemisorption may not be the dominant mechanism, and that MR molecules likely bind to individual active sites on the adsorbent [51,52]. The fitted adsorption rates of PDADMAC-AC are higher than those of AC, which proves that PDADMAC modification enhances the adsorption rate. For the HSDM, the adsorbent is considered a homogeneous medium, and the concentration gradient within the particle drives the adsorbate’s transport [53]. At the particle surface, the adsorbate concentrations in the liquid and solid phases are assumed to be in equilibrium. Since the Freundlich isotherm is proven to describe the MR adsorption equilibrium well, it is used to describe equilibrium at the particle surface when solving HSDM equations. The fitted Ds values are within the typical range of surface diffusion coefficients found for activated carbons [31]. The lower Ds value of PDADMAC-AC may indicate a larger adsorption strength, since being in the adsorbed state leads to the decreasing mobility of adsorbate [31]. This could indicate more stable adsorption, potentially beneficial for long-term contaminant retention. The Biot number quantifies the ratio between mass transfer across the external liquid film and intraparticle diffusion, providing a criterion to assess whether external mass transfer or surface diffusion is the rate-limiting step [54]. In this study, the calculated Biot numbers are between 1 and 100, suggesting that both external and internal mass transfer processes play a significant role in controlling the overall adsorption rate. The slightly higher Biot number of PDADMAC-AC implies that the internal diffusion of PDADMAC-AC plays a more prominent role than AC. These values suggest that optimizing reactor design should involve strategies to enhance both mass transfer steps—for example, by improving the mixing or flow conditions to reduce boundary layer resistance, and selecting adsorbent particles with appropriate porosity and size to facilitate intraparticle diffusion. Ensuring a balance between these transport mechanisms is essential in maximizing adsorption efficiency in full-scale treatment systems.

3.3.2. Effects of Water Matrix

Figure 6a illustrates the effects of solution pH on MR’s adsorption efficiency. The adsorption capacity remained relatively consistent across a pH range of 5 to 9. This stability may be attributed to the quaternary ammonium groups in PDADMAC, which carry a permanent positive charge and help to maintain the composite’s adsorption performance at higher pH levels. The isoelectric point of PDADMAC-AC was measured to be 10.28 ± 0.48, indicating that its surface remains positively charged throughout the tested pH range. A slight decrease in performance was observed at lower pH (pH = 4). Given that the pKa of MR is approximately 5.0, MR molecules are less negatively charged at pH < 5.0, reducing the electrostatic attraction to the positively charged adsorbent surface and thus decreasing removal efficiency [55]. Similar trends have been reported in other MR adsorption studies [52].
The effects of NaCl and sodium acetate in solution on MR’s adsorption performance are demonstrated in Figure 6b,c. NaCl is selected as the representative of inorganic compounds, as it is widely used in the dyeing and textile industries to improve dyes’ adherence to fabrics [56,57]. Sodium acetate is used as a model background organic solute, since organic matters are likely to be present in the water and many studies use sodium acetate as a source for carbon in preparing synthetic textile wastewaters [58,59,60]. The adsorption performance remained relatively stable at 0–40 ppm NaCl and 0–40 ppm sodium acetate. This indicates that the adsorbent has a relatively high affinity for MR molecules, making it suitable for real-world applications. Wang et al. also reported that chloride and nitrate had negligible effects on phosphate sorption via PDADMAC-modified biochars [44].

3.4. Rapid Small-Scale Column Tests

Figure 7 presents the RSSCT breakthrough curves for ground AC and ground PDADMAC-modified AC with inlet water containing 10 ppm MR + 10 ppm sodium acetate + 10 ppm NaCl + 10 ppm Na2SO4 + 10 ppm NaNO3 + 10 ppm NaHCO3. Both adsorbents exhibited a gradual initial rise in the normalized concentration (C/C0) as the bed volumes (BVs) increased. The C/C0 values reached 0.05 at approximately 6900 BV for unmodified AC and 9000 BV for PDADMAC-AC. This was followed by a sharp increase in C/C0, reaching 0.5 at around 11,400 BV and 15,200 BV, respectively. As the C/C0 ratio approached 0.8, the rate of increase slowed again, indicating that the adsorbents were close to saturation. To evaluate MR’s removal performance, the linearized Thomas model and a material balance approach were applied, as shown in Equations (2) and (3). The Thomas model was selected because it has been widely recognized for accurately fitting breakthrough data in fixed-bed column studies [61,62,63].
l n C 0 C − 1 = k m q 0 Q − E B C T   k C 0 B V
where C0 is the influent MR concentration (mg/L); C is the effluent MR concentration (mg/L); k is the Thomas rate constant (mL (mg min)−1); m is the mass of adsorbent in the column (g); q0 is the equilibrium sorption capacity (mg/g); Q is the water volumetric flow rate (ml/min); and BV is the number of bed volumes fed to the column.
C 0 Q ∫ 0 t 1 − C C 0 d t   = q 0 m + C 0 ε B V R
where ε B is the bed porosity and VR is the adsorber volume (L).
The fitted k values of the Thomas model are 0.029 mL (mg min)−1 for AC and 0.031 mL (mg min)−1 for PDADMAC-AC (Figure 8). The fitted q0 values are 277.4 mg/g and 334.5 mg/g, respectively. The adsorption capacities calculated from the material balance are 296.1 mg/g and 316.8 mg/g for unmodified AC and PDADMAC-AC, respectively, which is consistent with the results from the Thomas model. It proves that PDADMAC-AC has a higher adsorption capacity than AC under the continuous flow condition. The increased adsorption capacity suggests longer operation times before breakthrough, a reduced frequency of media replacement, and lower operational costs for water treatment facilities.

3.5. Regeneration of Adsorbents

AC is typically regenerated via thermal treatment after use, but this process tends to decrease its surface area and increase the presence of acidic groups, which can negatively impact its ability to adsorb anionic contaminants in subsequent cycles [64,65,66]. Modification with PDADMAC enhances the contaminant adsorption capacity of activated carbon by introducing electrostatic interactions, supplementing hydrophobic adsorption and improving regeneration efficiency using brine solutions. To assess the reusability of PDADMAC-modified AC, different regenerants were used between adsorption cycles, as shown in Figure 9 and Figure S3 (in Supplementary Materials). The control test with deionized water demonstrated minimal regeneration effects, with the removal efficiency dropping to 4.1% in the fifth cycle. Regeneration with either pure NaCl or methanol solutions slightly improved efficiency by disrupting electrostatic attraction or hydrophobic attraction, respectively. The combination of NaCl and methanol (1% NaCl + 70% methanol) further improved the regeneration performance, since it interfered in both electrostatic and hydrophobic attractions. NaOH solution exhibited a greater regeneration effect than NaCl solution, as its high pH environment increased the negative charge on the adsorbent surface, thereby enhancing desorption through electrostatic repulsion rather than relying solely on ion exchange—the primary regeneration mechanism of NaCl. Since increasing the NaOH concentration from 0.1 M to 0.5 M did not significantly enhance regeneration performance, a mixture of 0.1 M NaOH + 70% methanol was tested. This combination effectively preserved the adsorbent’s performance, maintaining a 71.3% removal efficiency in the fifth cycle compared to 88.7% in the first cycle. These results indicate that the PDADMAC-AC has good reusability, and suggest that both electrostatic and hydrophobic interactions play a role in the adsorption process. The regeneration procedure used in this study differs from that of conventional industrial practices, which typically involve on-column regeneration. A batch approach was chosen in this study to more accurately assess the effectiveness of each regenerant. Future studies could incorporate on-column regeneration to better simulate real-world industrial applications.

3.6. Quantum Chemical Calculations

The optimized molecular configurations of MR and a PDADMAC consisting of three monomer units are presented in Figure S4 of the Supplementary Materials. Electrostatic potential (ESP) maps were used to visualize the charge distribution across each molecule, where regions of high ESP indicate electron-deficient (positively charged) sites and regions of low ESP correspond to electron-rich (negatively charged) areas. These maps, shown in Figure S5, provide insight into the electrostatic interactions that may influence the adsorption behavior between MR and PDADMAC-functionalized materials. For the MR molecule, regions of negative potential are primarily localized around the nitrogen atoms and the carboxylate group, indicating these as likely sites for electrostatic interaction with positively charged moieties. In contrast, the PDADMAC polymer exhibits predominantly positive ESP surfaces, particularly surrounding the quaternary ammonium groups. These groups carry a permanent positive charge, making them ideal candidates for electrostatic attraction with anionic species such as MR.
Reduced density gradient (RDG) analysis is a graphical method used to characterize non-covalent interactions (NCIs) between molecules by visualizing electron density and its gradient. The RDG isosurfaces and corresponding scatter plots of MR, PDADMAC, and their complex (MR + PDADMAC) are presented in Figure 10. In RDG isosurfaces, blue regions (sign (λ2)ρ < −0.02) indicate strong attractive interactions such as hydrogen bonding and halogen bonding, green regions (−0.02 < sign (λ2)ρ < 0.01) correspond to weak interactions such as Van der Waals forces and electrostatic interactions, and red regions (sign (λ2)ρ > 0.01) represent strong steric repulsion. In the scatter plots, peaks at sign (λ2)ρ < 0 are associated with attractive interactions, while peaks at sign (λ2)ρ > 0 indicate repulsive forces.
For the MR molecule, the RDG isosurface shows several green regions near the nitrogen atoms, suggesting the presence of weak van der Waals interactions. Red patches in the aromatic rings indicate steric hindrance. The absence of blue regions suggests no significant hydrogen bonding or other strong attractive forces within the molecule. For the PDADMAC molecule, the widespread green isosurfaces indicate weak interactions between monomers, while red regions again highlight steric repulsion. The RDG isosurface of the MR + PDADMAC complex reveals intensified green regions at the interface between the two molecules, implying the existence of Van der Waals and electrostatic interactions. The absence of blue isosurfaces suggests a lack of strong directional interactions like hydrogen bonds, but the prominent green regions confirm the role of electrostatic and dispersion interactions in stabilizing the complex. These findings indicate that such non-covalent interactions contribute significantly to the adsorption of MR onto PDADMAC.
The calculated chemical reactivity descriptors for MR, PDADMAC, and MR + PDADMAC are shown in Table 4. For the MR molecule, the relatively small HOMO–LUMO energy gap indicates moderate chemical reactivity and suggests that MR has the potential to participate in charge transfer processes under suitable conditions. Its relatively high electrophilicity index (ω) further reflects a strong tendency to accept electrons. In contrast, PDADMAC exhibits a much larger energy gap, implying higher chemical stability and insulating behavior. The higher ionization energy (I) and lower electron affinity (A) of PDADMAC compared to MR indicate that it is less likely to donate or accept electrons, reinforcing its low reactivity. In addition, PDADMAC has a higher global hardness (η) and lower global softness (σ) than MR, suggesting that it is less polarizable and chemically more robust. MR also displays higher values of electrophilicity (ω) and nucleophilicity index (ε) compared to PDADMAC, indicating its greater capacity to accept electrons and stabilize additional charge. For the MR + PDADMAC complex, its HOMO and LUMO energy levels and the energy gap remain similar to those of the isolated MR molecule. This suggests that the interaction between MR and PDADMAC does not significantly alter the frontier molecular orbitals of MR, implying weak orbital overlap and minimal electronic coupling. As a result, their interaction is predominantly governed by non-covalent forces, such as electrostatic and Van der Waals interactions.
The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) plots for MR, PDADMAC, and MR + PDADMAC are shown in Figures S6–S8. In the MR molecule, both HOMO and LUMO are delocalized across the conjugated π-system. This delocalization enables efficient electronic communication within the molecule and indicates its potential for charge transfer. In contrast, PDADMAC exhibits localized HOMOs and LUMOs along the polymer backbone, indicating limited electronic activity, which is consistent with its large energy gap and chemical stability. For the MR + PDADMAC complex, both HOMO and LUMO are on the MR molecule, which is likely due to the inert electronic nature of PDADMAC. The HOMO and LUMO of MR in the complex remain similar to those of the isolated MR molecule, and there is no HOMO–LUMO overlap between the two species, further suggesting that their interaction is not governed by frontier orbital interactions or charge transfer.
Fukui indices analysis could provide further insight into the local reactivity of MR and its interaction with PDADMAC. The Fukui isosurface plots for MR, PDADMAC, and MR + PDADMAC are shown in Figures S9–S11. For the MR molecule, the highest f+ value—indicating an electrophilic site that is susceptible to nucleophilic attack—is located on the azo nitrogen atom. Conversely, the highest f− value—associated with a nucleophilic site that is prone to electrophilic attack—is located on the nitrogen atom bonded to the methyl groups. In the MR + PDADMAC complex, the spatial distribution and magnitude of both the largest f+ and largest f− remain nearly identical to those observed in the isolated MR molecule, suggesting that the electronic structure of MR is largely unaltered upon adsorption. These findings align with the frontier molecular orbital analysis, confirming the non-disruptive, physisorption-type interactions driven by non-covalent interactions. This non-covalent interaction mechanism suggests reversible binding, which is important for adsorbent regeneration and reuse in water treatment. A schematic representation of the non-covalent interaction is shown in Figure S12.

4. Conclusions

In this study, PDADMAC-modified AC was synthesized and evaluated for its MR adsorption performance in different conditions. The modified AC exhibited a superior adsorption capacity compared to the raw AC, in which the AC modified with 8 w/v% PDADMAC solution provided the greatest adsorption performance. The anion exchange capacity analysis proved that PDADMAC modification introduced additional positive charges to the AC. The adsorption kinetics were well described by the pseudo-first-order model and HSDM, while the Freundlich isotherm accurately captured the equilibrium behavior. The modified AC maintained stable removal efficiency across a broad pH range (5–9) and in the presence of up to 40 ppm of NaCl or sodium acetate, demonstrating its robustness under variable water chemistry. The RSSCT results reinforce its potential for successful scale-up to pilot or industrial systems. Furthermore, regeneration with a 0.1 M NaOH + 70% methanol solution effectively restored its adsorption capacity, indicating strong potential for repeated use and reduced material costs. The quantum chemical analysis suggests that the adsorption of MR on PDADMAC relies mainly on non-covalent interaction, such as electrostatic and Van der Waals forces, which supports easier regeneration and long-term material stability. Overall, these findings demonstrate that PDADMAC modification is a cost-effective and scalable strategy for enhancing the performance of the commercial GAC adsorbent in real-world water treatment applications.
For future studies, the applicability of PDADMAC-AC to a broader range of organic contaminants—such as pharmaceutical and personal care products (PPCPs), endocrine-disrupting compounds (EDCs), per- and polyfluoroalkyl substances (PFASs), and pesticides—could be explored. Additionally, while the current work conducted RSSCTs, future studies could further validate these findings through larger-scale pilot column experiments. From the theoretical perspective, other advanced computational methods like molecular dynamics (MD) and Monte Carlo (MC) simulations could be conducted to offer more insights into the adsorption mechanisms and interactions at the molecular level.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cleantechnol7030061/s1. Figure S1: EDX spectrum of PDADMAC AC synthesized with 8 w/v% polymer; Figure S2: Thermogravimetric analysis of AC and PDADMAC AC; Figure S3: Regeneration tests of PDADMAC AC using different regenerants for 5 cycles. First cycle used new adsorbents; later cycles used regenerated adsorbents. The error bars are calculated from standard deviation of the UV absorbance readings; Figure S4: Optimized configurations of methyl red molecule (left) and PDADMAC molecule (right). Red atom: oxygen; White atom: hydrogen; Gray atom: carbon; Blue atom: nitrogen; Figure S5: Electrostatic potential (ESP) of methyl red molecule (left) and PDADMAC molecule (right). The high-ESP areas are shown in blue, and low-ESP areas are shown in red; Figure S6: The highest occupied molecular orbital (HOMO) (left) and the lowest unoccupied molecular orbital (LUMO) (right) plots for MR molecule; Figure S7: HOMO (left) and LUMO (right) plots for PDADMAC molecule; Figure S8: HOMO (left) and LUMO (right) plots for MR+ PDADMAC; Figure S9: f+ (left) and f− (right) isosurface plots for MR molecule; Figure S10: f+ (left) and f− (right) isosurface plots for PDADMAC molecule; Figure S11: f+ (left) and f− (right) isosurface plots for MR+ PDADMAC; Figure S12: Schematic representation of the non-covalent interaction between ionized MR and PDADMAC; Table S1: Comparison of Freundlich parameters values in the literature for MR adsorption on activated carbons. References [52,67,68,69,70,71,72] are cited in the Supplementary Materials.

Author Contributions

Conceptualization, S.L.; methodology, S.L.; formal analysis, S.L.; investigation, S.L.; writing—original draft preparation, S.L.; writing—review and editing, S.L. and M.M.; funding acquisition, M.M.; supervision, M.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. MR removal efficiency vs. time plot of PDADMAC-AC synthesized with different PDADMAC concentrations. MR initial concentration: 50 ppm; Adsorbent dose: 0.5 g adsorbent/L water.
Figure 1. MR removal efficiency vs. time plot of PDADMAC-AC synthesized with different PDADMAC concentrations. MR initial concentration: 50 ppm; Adsorbent dose: 0.5 g adsorbent/L water.
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Figure 2. SEM images of (a) raw AC and (b) PDADMAC-AC synthesized with 8 w/v% polymer.
Figure 2. SEM images of (a) raw AC and (b) PDADMAC-AC synthesized with 8 w/v% polymer.
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Figure 3. Distribution of elements in PDADMAC-AC synthesized with 8 w/v% polymer.
Figure 3. Distribution of elements in PDADMAC-AC synthesized with 8 w/v% polymer.
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Figure 4. (a) Freundlich isotherm fitting and (b) Langmuir isotherm fitting for MR adsorption on AC and PDADMAC-AC.
Figure 4. (a) Freundlich isotherm fitting and (b) Langmuir isotherm fitting for MR adsorption on AC and PDADMAC-AC.
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Figure 5. Experimental, pseudo-first/second-order fitted and HSDM fitted MR adsorption kinetics of (a) raw AC and (b) PDADMAC-AC. MR initial concentration: 50 ppm; Adsorbent dose: 0.5 g adsorbent/L water.
Figure 5. Experimental, pseudo-first/second-order fitted and HSDM fitted MR adsorption kinetics of (a) raw AC and (b) PDADMAC-AC. MR initial concentration: 50 ppm; Adsorbent dose: 0.5 g adsorbent/L water.
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Figure 6. Equilibrium MR adsorption amount under (a) different pH, (b) different NaCl concentrations, and (c) different sodium acetate concentrations.
Figure 6. Equilibrium MR adsorption amount under (a) different pH, (b) different NaCl concentrations, and (c) different sodium acetate concentrations.
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Figure 7. RSSCT breakthrough curves for ground AC and PDADMAC-AC.
Figure 7. RSSCT breakthrough curves for ground AC and PDADMAC-AC.
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Figure 8. Fitted Thomas model for AC and PDADMAC-AC columns. The data with C/C0 between 0.2 and 0.8 were used for fitting.
Figure 8. Fitted Thomas model for AC and PDADMAC-AC columns. The data with C/C0 between 0.2 and 0.8 were used for fitting.
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Figure 9. Regeneration tests of PDADMAC-AC using 0.1 M NaOH and 0.1 M NaOH + 70% methanol for five cycles. First cycle used new adsorbents; later cycles used regenerated adsorbents.
Figure 9. Regeneration tests of PDADMAC-AC using 0.1 M NaOH and 0.1 M NaOH + 70% methanol for five cycles. First cycle used new adsorbents; later cycles used regenerated adsorbents.
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Figure 10. RDG isosurfaces of (a) MR, (b) PDADMAC, and (c) MR + PDADMAC, and scatter plots of (d) MR, (e) PDADMAC, and (f) MR + PDADMAC.
Figure 10. RDG isosurfaces of (a) MR, (b) PDADMAC, and (c) MR + PDADMAC, and scatter plots of (d) MR, (e) PDADMAC, and (f) MR + PDADMAC.
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Table 1. Fitted Freundlich and Langmuir isotherm parameters for MR adsorption.
Table 1. Fitted Freundlich and Langmuir isotherm parameters for MR adsorption.
IsothermParametersACPDADMAC-AC
FreundlichKf (mg/g) (L/mg)1/n110.3 ± 17.6225.9 ± 4.4
n4.3 ± 0.913.6 ± 1.3
R20.9760.993
RMSE1.702.69
Langmuirqm (mg/g)306.6 ± 17.7308.1 ± 8.5
b (L/mg)0.13 ± 0.030.59 ± 0.16
R20.9810.969
RMSE34.682.50
Table 2. Fitted pseudo-first/second-order adsorption rates of MR adsorption on AC and PDADMAC-AC.
Table 2. Fitted pseudo-first/second-order adsorption rates of MR adsorption on AC and PDADMAC-AC.
Pseudo First OrderPseudo Second Order
k1 (min−1)R2RMSEk2 (g/mg min)R2RMSE
AC0.0027 ± 0.00010.9880.0350.000039 ± 0.0000010.9760.050
PDADMAC-AC0.0039 ± 0.00010.9940.0260.000060 ± 0.0000010.9860.040
Table 3. Fitted parameters using HSDM for MR adsorption on AC and PDADMAC-AC.
Table 3. Fitted parameters using HSDM for MR adsorption on AC and PDADMAC-AC.
Ds (m2/s)kf (m/s)BiR2
AC1.54 × 10−11 ± 3 × 10−130.00084 ± 0.0000410.3 ± 0.30.993
PDADMAC-AC1.02 × 10−11 ± 1 × 10−130.00115 ± 0.0000518.9 ± 0.90.999
Table 4. Chemical reactivity descriptors of MR, PDADMAC, and MR + PDADMAC.
Table 4. Chemical reactivity descriptors of MR, PDADMAC, and MR + PDADMAC.
MRPDADMACMR + PDADMAC
EHOMO (eV)–4.617−7.439−4.698
ELUMO (eV)–2.826−0.419−2.915
Energy gap ∆EGAP (eV)1.7917.0201.783
Ionization energy I (eV)4.6177.4394.698
Electron affinity A (eV)2.8260.4192.915
Global hardness η (eV)0.8963.5100.892
Global softness σ (eV−1)1.1170.2851.122
Chemical potential μ (eV)−3.721−3.929−3.807
Electronegativity χ (eV)3.721 3.9293.807
Electrophilicity index ω (eV)7.7322.1998.128
Nucleophilicity index ε (eV)4.5041.6824.423
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Li, S.; Mohseni, M. Development of Poly(diallyldimethylammonium) Chloride-Modified Activated Carbon for Efficient Adsorption of Methyl Red in Aqueous Systems. Clean Technol. 2025, 7, 61. https://doi.org/10.3390/cleantechnol7030061

AMA Style

Li S, Mohseni M. Development of Poly(diallyldimethylammonium) Chloride-Modified Activated Carbon for Efficient Adsorption of Methyl Red in Aqueous Systems. Clean Technologies. 2025; 7(3):61. https://doi.org/10.3390/cleantechnol7030061

Chicago/Turabian Style

Li, Simeng, and Madjid Mohseni. 2025. "Development of Poly(diallyldimethylammonium) Chloride-Modified Activated Carbon for Efficient Adsorption of Methyl Red in Aqueous Systems" Clean Technologies 7, no. 3: 61. https://doi.org/10.3390/cleantechnol7030061

APA Style

Li, S., & Mohseni, M. (2025). Development of Poly(diallyldimethylammonium) Chloride-Modified Activated Carbon for Efficient Adsorption of Methyl Red in Aqueous Systems. Clean Technologies, 7(3), 61. https://doi.org/10.3390/cleantechnol7030061

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