Water soluble polyacrylamide-grafted carboxymethyl cellulose flocculant: Synthesis, characterization, and flocculation activity evaluation

2.1. Materials

CMC (Degree of substitution = 0.7; M.wt = 250.000), AM (Assay > 98%; Mwt = 71.08), and potassium persulfate (KPS; purity 99%, M.wt=270.322) were obtained from Sigma-Aldrich chemicals Ltd (Germany). Acetone (Assay 99%) and hydrochloric acid (Assay 37%) were acquired from the United Chemical Industry (Egypt). Bentonite clay (Aluminum silicate Hydrate) was obtained from Loba Chemia, India.

2.2. Preparation of grafted copolymer

The fabrication of carboxymethyl cellulose grafted polyacrylamide (CMC-g-PAM) was conducted following a systematic procedure. Initially, a predetermined quantity of CMC of 0.2-1% w/v was accurately quantified and subsequently dispersed in distilled water within a reaction flask. The mixture underwent continuous stirring until complete dissolution was achieved. The flask was then positioned inside a temperature-regulated water bath, maintained at temperatures between 50°C and 80°C.

Following this, a pre-prepared solution containing potassium persulfate (KPS) at concentrations of 0.005% to 0.04% w/v and AM monomer at concentrations of 0.6% to 2% w/v was gradually introduced into the reaction vessel, while maintaining continuous and uniform stirring. The grafting reaction was left to progress for 1 to 4 h. Upon reaction completion, the mixture was permitted to rest overnight. To facilitate the precipitation of the grafted polymer, the reaction mixture was transferred into a beaker encompassing absolute ethanol (300 mL), with gentle stirring continued until precipitation was fully realized. Homopolymers were subsequently removed using acetone, enhancing the product’s purity. The resulting material underwent drying at 60°C until its weight stabilized.

For comparative analysis, PAM was synthesized under identical reaction conditions but without the incorporation of CMC (utilizing 0.2 g of CMC and 0.02 g of KPS at 60°C for 3 h). The dried products were characterized and employed in flocculation studies. The proposed synthesis mechanism has been illustrated in Scheme 1. Various grafting parameters, including GP%, GE%, GC%, and GY%, were calculated to assess the efficiency of the process (Eqs. 1-4) [44].

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Scheme 1.

Proposed mechanism for the grafting of AM onto CMC.

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Figure 1.

FTIR spectrum for CMC, PAM, and CMC-g-PAM.

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Where A, B, and C denote the weights of CMC, grafted polymer, and CMC + AM. D is the weight of AM in the grafted copolymer, which can be calculated as: Weight of M is the weight of feeding AM utilized during the grafting reaction.

2.3. Flocculation test

A synthetic wastewater suspension was prepared by dispersing 0.2% (w/w) bentonite clay in deionized water. The suspension was subjected to vigorous stirring for 5 minutes to ensure complete homogenization. Subsequently, a predetermined volume of the graft copolymer (CMC-g-PAM) solution was added dropwise over a 2-min period under continuous gentle stirring to facilitate uniform dispersion and interaction with the suspended particles.

Following the addition of the flocculant, the treated suspension was transferred into a 100 mL graduated cylinder to evaluate the settling performance. The settling ratio (SR%) was determined by measuring the volume of sediment formed at the bottom of the cylinder after a standing period of 90 min (Eq. 5).

Thereafter, the system was left undisturbed for an additional 30 min to allow residual flocs to settle. The turbidity of the supernatant was then measured 1 cm below the liquid surface using a TurbDirect turbidity meter (Lovibond, Germany). This procedure enabled the assessment of both sedimentation efficiency and clarity of the treated water.

2.4. Characterization

The functional groups present in CMC, PAM, and CMC-g-PAM were identified using Fourier transform infrared (FTIR) spectroscopy, utilizing a Bruker FTIR spectrometer (Invenio S, Germany). The spectra were documented within a wavenumber spectrum of 400-4000 cm^-1 at 4 cm^-1 resolution. Energy dispersive X-ray (EDX) spectroscopy was employed for elemental composition analysis to ascertain the presence of key elements.

The point of zero charge (PZC) was evaluated following established methodologies [45]. Scanning electron microscopy (SEM) was utilized to analyze the samples’ surface morphology. Before imaging, the samples were fixed on a sample stub with carbon tape and coated with a thin gold film to enhance conductivity.

Thermal stability was assessed through thermogravimetric analysis (TGA) accomplished under a nitrogen atmosphere. The analysis was performed with a heating rate of 10°C/min and a nitrogen flow rate of 20 mL/min, across temperatures spanning from 25 to 800°C.

3.1. Fourier transform infrared (FTIR) analysis

Figure 1 presents the FT-IR spectra of CMC, PAM, and the synthesized graft copolymer (CMC-g-PAM), highlighting key functional group vibrations associated with each material.

In the spectrum of CMC, a broad absorption band observed at 3358 cm⁻¹ corresponds to the stretching vibrations of hydroxyl (–OH) groups. The carboxylate (–COO⁻) groups exhibit characteristic asymmetric and symmetric stretching vibrations at 1573 cm⁻¹ and 1415 cm⁻¹, respectively. Aliphatic C–H stretching vibrations appear at 2924 cm⁻¹ and 2871 cm⁻¹, while the band at 1057 cm⁻¹ is attributed to C–O stretching, consistent with polysaccharide backbones.

The FT-IR spectrum of PAM shows distinct peaks at 3445 cm⁻¹ and 3335 cm⁻¹, which are assigned to N–H stretching vibrations of primary amine groups (–NH₂). A shoulder peak near 3162 cm⁻¹ indicates hydrogen bonding interactions involving amine groups. The C–H stretching vibration appears at 2926 cm⁻¹, and the carbonyl (C=O) group from the amide moiety displays a strong band at 1649 cm⁻¹. Additional peaks at 1511 cm⁻¹ and 1457 cm⁻¹ are attributed to N–H bending and C–N stretching, respectively.

The FT-IR spectrum of the grafted copolymer CMC-g-PAM incorporates characteristic features from both CMC and PAM, confirming successful grafting. A broad absorption band between 3200 and 3400 cm⁻¹ corresponds to overlapping O–H and N–H stretching vibrations. The –CH stretching vibration observed at 2924 cm⁻¹ in CMC is shifted to 2953 cm⁻¹ in the copolymer, indicating the formation of new –CH₂ groups. Notably, peaks at 1673 cm⁻¹ and 1614 cm⁻¹ (in PAM) and a band at 2165 cm⁻¹ (in CMC) are shifted to 1679 cm⁻¹ and 1621 cm⁻¹, respectively, in the graft copolymer. These spectral shifts are indicative of alterations in the polymer backbone and provide strong evidence for the successful covalent grafting of PAM chains onto the CMC substrate [46].

3.2. Energy dispersive X-ray (EDX)

The elemental composition of CMC, PAM, and the graft copolymer (CMC-g-PAM) was examined using energy-dispersive X-ray spectroscopy (EDX), as illustrated in Figure 2 and summarized in Table 1. The EDX spectrum of CMC indicates the dominant presence of carbon (52.33%) and oxygen (47.67%), with negligible nitrogen content. This composition is consistent with the polysaccharide nature of CMC. The trace level of nitrogen may be attributed to residual proteins or naturally occurring impurities within the cellulose matrix.

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Figure 2.

EDX analysis for a) CMC, b) PAM, and c) CMC-g-PAM.

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In contrast, PAM exhibits a markedly different elemental profile, characterized by a substantial nitrogen content (42.54%), a reflection of the polymer’s amide functionalities, alongside lower carbon and oxygen levels compared to CMC. Upon grafting PAM chains onto the CMC backbone, the resulting CMC-g-PAM copolymer demonstrates a significant increase in nitrogen content to 41.00%, which is absent in native CMC. This sharp rise in nitrogen percentage strongly confirms the successful incorporation of PAM into the grafted structure. Concurrently, a notable decrease in carbon content from 52.33% in CMC to 30.34% in CMC-g-PAM is observed, further indicating the chemical modification of the CMC matrix.

This elemental shift, particularly the appearance and quantification of nitrogen, provides compelling evidence of effective grafting and serves as a reliable indicator of the modification efficiency [20].

3.3. Scanning electron microscopic (SEM) analysis

The morphological characteristics of CMC, PAM, and the grafted copolymer (CMC-g-PAM) were examined using SEM, as shown in Figure 3. The native CMC surface displays a layered, flaky, and relatively smooth morphology with limited porosity, typical of polysaccharide-based materials. In contrast, PAM exhibits a dense, compact, and film-like surface structure, lacking significant surface texture or porosity.

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Figure 3.

Surface scanning microscope of CMC, PAM, and CMC-g-PAM.

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Upon grafting PAM onto the CMC backbone, the resulting CMC-g-PAM copolymer exhibits notable morphological transformations. The surface becomes significantly more heterogeneous, with a rough, wrinkled, and porous architecture. The formation of interconnected fibrillar networks and irregular folds suggests successful structural reorganization due to grafting. This enhanced surface complexity likely contributes to an increased surface area and improved accessibility of functional groups, thereby facilitating more efficient interactions with suspended particles during flocculation. These changes support the improved performance of CMC-g-PAM in adsorption and bridging mechanisms, consistent with prior findings on modified polysaccharide-based flocculants [47].

3.4. Thermogravimetric analysis (TGA)

The thermal stability of CMC, PAM, and their grafted copolymer (CMC-g-PAM) was investigated using TGA, as illustrated in Figure 4. All samples exhibit an initial weight loss below 150°C, which corresponds to the evaporation of adsorbed and bound moisture. Notably, the grafted sample (CMC-g-PAM) shows a slightly lower moisture loss compared to pure CMC, suggesting a decrease in hydrophilicity due to the introduction of hydrophobic PAM segments.

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Figure 4.

TGA thermograms for CMC, PAM, and CMC-g-PAM.

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CMC undergoes a major degradation event in the temperature range of approximately 220-350°C, which is associated with the decomposition of the polysaccharide backbone, particularly the cleavage of glycosidic linkages within the pyranose rings. A secondary weight loss is observed beyond 400°C, corresponding to the degradation of residual carbonaceous materials and the decarboxylation of carboxylate groups (–COO⁻).

In contrast, PAM shows a more gradual degradation profile, with weight loss beginning after the moisture evaporation phase and continuing steadily through 250-550°C, primarily due to the decomposition of amide groups and evolution of ammonia.

The thermal degradation of CMC-g-PAM displays a two-step pattern that reflects the combined decomposition of both CMC and PAM components. The first significant weight loss occurs between 250-450°C, indicating overlapping degradation of the grafted PAM chains and the CMC backbone. A second degradation phase occurs between 450-600°C, likely associated with the breakdown of crosslinked or cyclized byproducts formed during grafting.

Overall, the TGA profile of CMC-g-PAM demonstrates improved thermal behavior compared to CMC alone, with a broader degradation range and reduced residual weight. This enhancement in thermal stability can be attributed to the structural reinforcement imparted by the covalently grafted PAM chains, which delay thermal decomposition and stabilize the polymer matrix [20].

3.5. Point of zero charge (pHₚzc)

The pHₚzc specifies the pH where a material’s surface maintains charge neutrality, with balanced positive and negative charges. This parameter was determined in accordance with established methodologies. When the solution pH exceeds the pHₚzc, the bentonite suspension is characterized by a mostly positive charge. Conversely, at pH values below the pHₚzc, the surface charge is predominantly negative. In this investigation, 50.0 mg of bentonite clay was dispersed in 20.0 mL of a 0.05 mol/L NaCl solution. The initial pH (pH₀) was systematically maintained between 2 and 10 utilizing 0.1 mol/L NaOH or HCl. The samples were sealed, stirred for 24 h at room temperature, and subsequently filtered. The supernatant’s final pH (pHf) was quantified, and the pHₚzc was ascertained by graphing the difference between pH₀ and pHf against pH₀. As illustrated in Figure 5, the pHₚzc was determined to be 7.5, indicating that the bentonite surface exhibited a positive charge at pH values exceeding 7.5, particularly at pH levels of 8 and 10.

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Figure 5.

Point zero charge (pHpzc) for bentonite clay.

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3.6. Effect of grafting conditions

Numerous studies have investigated various conditions for the graft copolymerization of PAM onto CMC, employing a range of substrates, monomers, initiators, and aqueous reaction environments. The grafting process is markedly influenced by reaction time and temperature, both of which are critical in identifying the characteristics of the resulting copolymer. Nonetheless, the optimal conditions for grafting have exhibited considerable variability across different research efforts [47-50]. In the present study, an orthogonal experimental design was utilized to assess the effects of five key variables: substrate concentration, initiator dosage, reaction temperature, grafting duration, and pH. Furthermore, turbidity was incorporated as an additional evaluation parameter. The influence of these variables on the copolymer characteristics is given in Table 2, which presents data on grafting efficiency (GE%), GP%, graft yield (GY%), grafting content (GC%), and solution turbidity. The findings from the orthogonal experiments indicated that the optimal synthesis conditions comprised an initiator dosage of 0.02 g, a reaction temperature of 60°C, a polymerization duration of 3 h, and a monomer mass of 1 g in a 20 mL solution with 0.2 g of CMC.

As per Table 2, a rise in monomer concentration corresponds to a rise in GP%. This trend can be attributed to the heightened accessibility of monomer molecules near the immobilized cellulose macroradicals, thereby providing a higher density of active grafting sites. The elevated monomer concentration within the reaction medium enhances molecular collisions, consequently promoting polymerization, particularly the formation of homopolymers. However, the observed decline in GE% suggests that at higher monomer concentrations, (a) homopolymerization becomes more dominant than grafting, (b) the medium’s increased viscosity restricts the mobility of free radicals and monomer molecules, and (c) there is an augmented likelihood of chain transfer to monomer molecules.

Furthermore, the results demonstrate that GP% and GE% improve with temperatures up to 60°C, after which a decline is noted, reaching their minimal values at 70°C. This initial increase may be attributed to an enhanced diffusion rate of monomer and initiator molecules, which facilitates the grafting process. Nonetheless, at temperatures exceeding 60°C, a reduction in GE% is observed, primarily because AM becomes more water-soluble and termination reactions accelerate, inducing excessive homopolymer formation. Similar findings have been reported in prior studies on graft copolymerization [51,52]. Additionally, gelation was observed when the reaction temperature was maintained at 70°C, which may be attributed to the excessive generation of radicals during the activation process. This increase in radical concentration intensifies interactions among reactive species, promoting self-polycondensation and ultimately resulting in the gelation of the products [53]. Therefore, precise temperature control is imperative for the optimization of the synthesis process.

As presented in Table 2, increasing the reaction duration up to 3 h resulted in elevated grafting parameters. This enhancement is likely attributable to the augmented availability of grafting sites on the CMC backbone, facilitating the incorporation of more monomer molecules into the developing graft chains. Furthermore, the intrinsic structure of CMC, characterized by multiple potential grafting sites, positively influences GE%. It was also observed that grafting parameters increased with initiator concentration, peaking at 0.01 g. The KPS initiator selectively targets specific functional groups on the CMC backbone, generating free radicals that initiate the grafting process. However, concentrations of more than 0.01 g promoted reduced GE, likely stemming from the premature termination of CMC free radicals before monomer introduction. This reduces the availability of reactive sites for grafting. In addition, higher KPS concentrations could lead to increased homopolymer formation because of a faster termination reaction rate. This phenomenon, leading to diminished GE%, has been corroborated by previous studies [54].

The grafting parameters on the flocculation and settling behavior of synthetic wastewater were also evaluated. The grafted samples demonstrated enhanced flocculation and settling rates across various grafting conditions. This improvement is linked to the formation of longer carbon chains and an increased presence of carboxyl and amide groups, which contribute to an increased charge density. These modifications enhance the copolymer’s ability to adsorb and bridge colloidal particles, thereby improving its efficacy in scavenging and settling sewage colloids. Similar findings have been documented in the existing literature [55,56].

3.7. Flocculation performance

Flocculation occurs when suspended fine particles clump together upon contact with a high-molecular-weight polymer. This polymer is integral to the process, as it adsorbs multiple particles simultaneously, thereby linking them to form larger aggregates known as flocs. Although the mechanism of flocculation via bridging is relatively straightforward, the performance of various flocculants can exhibit significant variability under differing conditions. The efficiency of these flocculants relies on a range of physicochemical factors, including pH, concentration, duration, ion types, and temperature [57]. In this study, two samples, PAM and CMC-g-PAM, were evaluated for their flocculation properties in a bentonite suspension. The grafted copolymer was selected based on previously optimized grafting conditions. This research aimed to assess the graft copolymer influence on the flocculation performance of simulated wastewater across varying environmental conditions.

3.7.1. Effect of pH

As shown in Figure 6, pH plays a pivotal role in governing the flocculation efficiency of both PAM and CMC-g-PAM. The results reveal a distinct pH-dependent trend in turbidity reduction: the supernatant turbidity of the treated suspension initially decreases under acidic conditions, reaches a minimum at low pH values (pH 2), and then gradually increases with rising pH. The lowest turbidity values were achieved at pH 2, where CMC-g-PAM and PAM reduced turbidity to approximately 0.7 NTU and 2.3 NTU, respectively, compared to 1098 NTU in the untreated control.

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Figure 6.

Effect of pH on turbidity removal using PAM and CMC-g-PAM.

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For CMC-g-PAM, maximum flocculation performance was observed around pH 6, whereas PAM reached its peak efficiency near pH 7. This behavior can be attributed to the interplay between the surface charge of the clay particles and the ionization state of functional groups on the flocculant chains. At lower pH, the reduction in negative surface charge on bentonite particles diminishes electrostatic repulsion, allowing for more effective bridging and adsorption by the flocculants. As the pH increases, the clay particles become increasingly negatively charged, intensifying the electrostatic repulsion and stabilizing the suspension [58].

The enhanced performance of CMC-g-PAM at acidic pH reflects the presence of both hydroxyl and amide groups, which contribute to an increased density of cationic adsorption sites. These sites facilitate stronger electrostatic interactions with negatively charged colloidal surfaces, thereby enhancing floc formation and sedimentation. In contrast, the efficiency of both flocculants declines in alkaline conditions due to excessive electrostatic repulsion and reduced polymer-particle affinity. This behavior underscores the critical influence of pH on the structure–property–performance relationship, as the functional group ionization, polymer conformation, and particle surface charge collectively determine the flocculation outcome [59-61].

3.7.2. Effect of aqueous medium, clay concentration, and flocculant dose

To elucidate the role of ionic constituents in flocculation behavior, the turbidity of clay suspensions was assessed in both distilled and tap water, with and without flocculant addition. As depicted in Figure 7, the untreated clay suspensions exhibited higher turbidity in distilled water compared to tap water, across all tested concentrations. This observation can be attributed to the absence of electrolytes in distilled water, which results in a thicker electrical double layer around clay particles, thus promoting electrostatic repulsion and hindering aggregation. In contrast, tap water contains multivalent ions (e.g., Ca²⁺, Mg²⁺) that compress the electrical double layer and reduce the zeta potential, thereby facilitating particle agglomeration through charge neutralization [62,63].

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Figure 7.

Variation in supernatant turbidity based on suspension medium and clay concentration.

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Subsequently, the effect of flocculant dosage on turbidity reduction was evaluated for both CMC-g-PAM and PAM flocculants at a fixed clay concentration (2 g/L). The performance of these flocculants in distilled and tap water is shown in Figures 8(a, b), respectively. In both aqueous systems, turbidity decreased significantly with increasing flocculant dose, reaching a plateau beyond the optimal concentration. Notably, CMC-g-PAM exhibited superior performance at lower dosages compared to PAM, especially in tap water, suggesting improved bridging and charge neutralization mechanisms.

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Figure 8.

Impact of flocculant dosage on turbidity removal using PAM (a) and CMC-g-PAM (b) in distilled water and tap water.

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The enhanced efficacy of the grafted copolymer can be explained by its branched architecture, which facilitates multiple contact points for particle adsorption. In linear PAM, the chains tend to adopt train-loop-tail configurations, which limit their bridging capacity. Conversely, the grafted structure of CMC-g-PAM allows for extended polymer chains that increase interaction sites, promoting effective inter-particle bridging and rapid floc formation [31]. Furthermore, the ionic strength of tap water enhances this process by suppressing repulsive forces between clay particles and enabling closer approach of the flocculant chains.

These findings emphasize the importance of flocculant structure and aqueous medium composition in determining flocculation performance. The presence of background ions in tap water not only reduces the required dosage of CMC-g-PAM but also improves overall turbidity removal efficiency, confirming the synergistic role of electrostatic screening and polymer architecture [64].

As illustrated in Figure 9, increasing the clay concentration in suspension leads to a corresponding rise in the initial turbidity values. This trend is primarily due to the greater number of suspended particles per unit volume, which not only increases light scattering but also intensifies inter-particle repulsive forces. At higher solid loadings, the electrostatic repulsion among negatively charged clay particles becomes more pronounced, resulting in enhanced colloidal stability and reduced propensity for spontaneous aggregation [65].

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Figure 9.

Clay concentration effect on turbidity removal utilizing PAM and CMC-g-PAM.

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The elevated particle concentration also contributes to a more dynamic system, wherein increased Brownian motion hinders efficient floc formation and sedimentation. As a result, the system exhibits higher turbidity values in the absence of sufficient flocculant to overcome the stabilization forces. In this context, the graft copolymer CMC-g-PAM demonstrates superior turbidity reduction performance compared to linear PAM, particularly at elevated clay concentrations. This behavior is attributable to the graft copolymer’s multivalent interaction sites and enhanced bridging capability, which facilitate more effective destabilization and aggregation of the dispersed clay particles.

These findings highlight the critical role of flocculant architecture in addressing system challenges posed by high solid concentrations, affirming that grafted polymers with expanded chain conformations can maintain effective performance across a range of clay loadings due to their structural adaptability and surface affinity.

3.7.3. Effect of temperature

As shown in Figure 10, increasing the temperature from 25°C to 65°C leads to a progressive reduction in residual turbidity for both flocculants (PAM and CMC-g-PAM). This trend reflects enhanced flocculation performance at elevated temperatures and can be attributed to multiple interrelated physicochemical factors.

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Figure 10.

Effect of temperature on turbidity removal using PAM and CMC-g-PAM.

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Higher temperatures increase the kinetic energy of both the flocculant molecules and the suspended particles, leading to an elevated collision frequency and more effective particle–polymer interactions. Additionally, the rise in temperature facilitates polymer chain mobility and diffusion, enabling the flocculant chains to extend and interact more efficiently with dispersed colloids [9].

The improvement in clarity of the supernatant with temperature also suggests that thermal activation enhances bridging and charge neutralization mechanisms, particularly in the case of the graft copolymer CMC-g-PAM, which showed superior performance across the temperature range. This could be attributed to the flexible architecture of the graft copolymer, allowing more effective conformational adaptation and surface coverage of the clay particles compared to linear PAM.

These results are consistent with the findings of other investigations [65], who reported that increasing temperature enhances the sedimentation rate and improves the optical clarity of the treated suspension. Overall, these observations underscore the importance of temperature as a process variable influencing flocculant efficiency through its effect on polymer dynamics and colloidal interactions.

3.7.4. Effect of stirring time

The impact of stirring time on flocculation performance was evaluated by measuring the supernatant turbidity at successive time intervals, as shown in Figure 11. Both PAM and CMC-g-PAM flocculants exhibited a marked reduction in turbidity over time, indicating efficient floc formation and sedimentation dynamics.

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Figure 11.

Stirring time effect on turbidity removal using PAM and CMC-g-PAM.

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Notably, CMC-g-PAM achieved a more rapid decline in turbidity within the first 100 seconds compared to PAM, demonstrating its superior initial flocculation kinetics. This enhanced performance can be attributed to the grafted copolymer’s extended and flexible architecture, which promotes stronger bridging interactions and faster particle capture. The rapid drop in turbidity reflects efficient particle–polymer collisions and aggregation during the early stages of the process.

As time progressed, the rate of turbidity reduction plateaued, suggesting that most particle removal occurred early in the flocculation process and that the formed flocs remained stable without significant re-dispersion. This behavior highlights the strong cohesive forces within the flocs and suggests that both flocculants, particularly CMC-g-PAM, exhibit sustained floc stability under the applied conditions.

These findings underscore the importance of polymer structure in governing time-dependent flocculation performance, with grafted architectures enhancing early-stage interactions and overall process efficiency.

3.7.5. Effect of stirring rate

The influence of agitation rate on residual turbidity is a critical factor in understanding the balance between floc formation and floc disruption during the flocculation process. As presented in Figure 12, turbidity values increased progressively with rising agitation speeds for both flocculants, with CMC-g-PAM consistently exhibiting lower turbidity than PAM at all tested rates.

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Figure 12.

Effect of stirring rate on turbidity removal using PAM and CMC-g-PAM.

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At low agitation rates (≤50 rpm), minimal shear force favors the formation and preservation of flocs, resulting in reduced turbidity due to efficient particle aggregation and sedimentation. However, the limited mixing may restrict the dispersion of flocculant chains and their interaction with suspended particles, thereby limiting overall removal efficiency.

As the agitation rate increases (up to ∼150 rpm), improved dispersion of the flocculant enhances polymer-particle contact, promoting bridging and charge neutralization mechanisms. However, beyond this optimal threshold, excessive shear disrupts the flocculation process. High turbulence (>200 rpm) can lead to floc breakup, preventing effective sedimentation and increasing the number of dispersed fine particles, which in turn raises the turbidity of the supernatant.

The more gradual turbidity increase observed with CMC-g-PAM compared to PAM under high agitation suggests that the grafted copolymer flocs possess superior mechanical stability. This can be attributed to the structural reinforcement provided by the CMC backbone and the entangled PAM side chains, which confer enhanced resistance to shear-induced fragmentation.

These results highlight the importance of optimizing agitation conditions based on flocculant architecture to maximize performance while preserving floc integrity during industrial water treatment operations.

3.8. Mechanism

The flocculation behavior of polymeric flocculants is generally governed by several key mechanisms, including charge neutralization, polymer bridging, and hydrogen bonding interactions. In the current study, the synthesized CMC-g-PAM copolymer possesses functional groups capable of engaging in multiple flocculation pathways.

Under near-neutral conditions, the copolymer exhibits cationic character due to the protonation of amide groups derived from PAM, while bentonite particles bear a net negative surface charge arising from isomorphic substitution within their layered structure. Upon addition of CMC-g-PAM, electrostatic attraction between oppositely charged species initiates charge neutralization, effectively destabilizing the colloidal system and diminishing repulsive forces among suspended particles.

Simultaneously, the grafted copolymer structure enhances polymer bridging, whereby the extended side chains of PAM adsorb onto multiple particle surfaces, physically linking them and promoting the formation of larger aggregates or flocs. The carboxyl and hydroxyl groups from the CMC backbone may also participate in hydrogen bonding with surface hydroxyls of the clay particles, further stabilizing the floc structures.

These cooperative interactions, electrostatic attraction, bridging, and hydrogen bonding—facilitate the formation of compact, sedimentable flocs that can effectively settle from suspension. This multi-modal mechanism is consistent with previous findings on hybrid natural-synthetic polymer flocculants [33,66], and underscores the enhanced flocculation performance of CMC-g-PAM compared to conventional linear polymers.