Rapid removal of anionic organic dye from contaminated water using a poly(3-aminobenzoic acid/graphene oxide/cobalt ferrite) nanocomposite low-cost adsorbent via adsorption techniques

3.1.1 Fourier-transform infrared spectroscopy (FTIR) analysis

The functional groups of CoFe₂O₄, GO, P3ABA, and the P3ABA/GO/CoFe₂O₄ composite (before and after adsorption of the dye) were investigated via the FTIR technique, the spectra for which are shown in Fig. 3. FTIR spectra of CoFe₂O₄ magnetic (Fig. 3a, green line) show two broad bands, the first at 3370 cm⁻¹ and the second between 410 and 680 cm⁻¹, which are assigned to the O—H stretching vibration of the hydroxyl group attached to the cobalt ferrites (due to adsorbed water molecules) and the stretching vibrations for the tetrahedral and octahedral coordination in ferrite metal, respectively (Belhaine et al., 2022). The O—H transmittance bands in and out of the plane were observed at 1335 cm⁻¹ to 1646 cm⁻¹ (Margabandhu et al., 2017). For the GO sample (Fig. 3a, black line), the broad absorption peak at 3394 cm⁻¹ was attributed to the O—H stretching vibration while the peak at 1704 cm⁻¹ corresponds to the carbonyl (C⚌O) stretch. The other peaks in the GO layers are located at 1561 cm⁻¹, 1156 cm⁻¹, and 1004 cm⁻¹, which correspond to the C⚌C and the C—O of the epoxide group, the O—H deformation of the C–OH group, and an alkoxy C—O stretch, respectively (Ismail et al., 2022a, 2022b).

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Fig. 3

FTIR of a) CoFe₂O₄ and GO, and b) P3ABA and P3ABA/GO/CoFe₂O₄ nanocomposite (before and after adsorption of dyes).

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In the FTIR spectrum of P3ABA (Fig. 3b, blue line), it can be seen that the broad bands located at around 3215 to 2999 cm⁻¹ are related to carboxylic (COO-H) and amine (N — H) groups, respectively, while the band at 2554 cm⁻¹ was due to the C — H of the aromatic ring. The bands at 1690 cm⁻¹, 1565 cm⁻¹, and 1502 cm⁻¹ are attributed to the C⚌O, quinoid C⚌C stretching, and benzenoid ring C⚌C stretching vibrations, respectively (Zare et al., 2018a, 2018b). The peak at 1435 cm⁻¹ is attributed to the –COO symmetric stretch of the of carboxylic acid, whereas the S⚌O vibration observed at 1034 cm⁻¹ was due to the use of ammonium persulfate acidic solution. In addition, other bands were observed at 1282 cm⁻¹ and 656 cm⁻¹ due to the in-plane C—H and C—C bending vibrations, respectively. The two absorption peaks at 746 and 823 cm⁻¹ can be ascribed to C—H out-of-plane bending vibrations (Ramohlola et al., 2017).

All of the FTIR peaks of both GO and CoFe₂O₄ are present in the P₃ABA/GO/CoFe₂O₄ composite spectrum, confirming that P3ABA chains were incorporated at the edges of GO sheets and CoFe₂O₄. The peak observed at 576 cm⁻¹ (Co − O − Fe) and the broad band between 3433 and 2755 cm⁻¹ (Fe − O—C, or Co − O—C) indicates the presence CoFe₂O₄ and GO sheet (O—H) in the polymer composite (associated with the N—H of P3ABA). The interaction between the P3ABA structure and functional groups on the surface of GO (–COOH, –OH, and epoxy group) and CoFe₂O₄ can be achieved through electrostatic, π-π, and hydrogen-bonding interactions, as illustrated in Fig. 2. After adsorption, the peaks of CR (Fig. 3b, red line) are observed in the P₃ABA/GO/CoFe₂O₄ composite (Fig. 3b, brown line), and due to electrostatic, hydrogen-bonding, and π-π interactions the majority of peaks are shifted to higher wavenumber in all nanocomposites compared to those seen in the P₃ABA/GO/CoFe₂O₄ composite alone, indicating the occurrence of physiochemical interactions between the CR dye and polymer nanocomposites.

3.1.2 X-ray diffraction analysis

The crystalline structure was investigated via X-ray diffraction (XRD), as depicted in Fig. 4. It can be seen that graphene oxide (black line) shows broad and weak peaks located at 23.7° and 43.2°, which are attributed to the (0 0 2) and (1 0 0) reflections, respectively, and which are in agreement with those obtained in previous studies (Sun et al., 2020, Kooti et al., 2021). For the CoFe₂O₄ sample, the diffraction peaks located around at 2θ = 30.45°, 35.88°, 38.00°, 43.56°, 54.17°, 57.59°, 63.16°, and 75.1° are ascribed to the (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0), and (5 3 3) crystalline planes, respectively, suggesting strong crystallization in the structure of CoFe₂O₄, as reported in the literature (Kim et al., 2016, Kooti et al., 2021). The XRD of P3ABA pattern exhibits an amorphous structure, showing two diffraction peaks at 2θ = 18.18° and 26.1° (blue line) attributed to the (0 2 0) and (2 0 0) crystal planes, which is similar to the XRD spectra of polyaniline in previous work. Comparing the functional groups of GO, CoFe₂O₄, and P3ABA samples with the sample of P3ABA/GO/CoFe₂O₄ composite, all XRD peaks associated with GO, CoFe₂O₄, and P3ABA were present in the P3ABA/GO/CoFe₂O₄ composite sample. Furthermore, in the P3ABA/GO/CoFe₂O₄ composite, the diffraction peak for the P3ABA become shaper at 2θ = 18.52° while the peaks associated GO and CoF₂O₄ interacted to produce a high intensity peak at 43.44°, suggesting an increased crystallinity in the P3ABA/GO/CoFe₂O₄ composite. Thus, these indicated that P3ABA had successfully interacted with GO, CoFe₂O₄, and the structure of the P3ABA/GO/CoFe₂O₄ composite was preserved without changes in the crystal planes except for slight shifts in positions and differences in the intensities of the associated XRD peaks compared to those found in the individual GO, CoFe₂O₄, and P3ABA samples. In summary, these results confirm that GO and CoFe₂O₄ was successfully coated with P3ABA, which is in accordance with the FTIR results.

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Fig. 4

XRD spectra of GO, CoF₂O₄, P3ABA, and the P3ABA/GO/CoFe₂O₄ composite. Each color indicates a typical sample (see graph keys).

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3.1.3 Thermogravimetric analysis (TGA)

Fig. 5 illustrates TGA and DTG (derivative of TGA) curves for the P3ABA and P3ABA/GO/CoFe₂O₄ composite samples. From the TGA curves in this figure, we can see that there are three regions where weight loss occurs. This observation is similar to the findings reported in the literature (Abd Ali et al., 2021), where P3ABA showed an initial weight loss of 5.8 % at 83 °C, while the P3ABA/GO/CoFe₂O₄ composite showed a 4.9 % weight loss at 89 °C. This can be attributed to the removal of moisture from the surfaces of the polymeric materials. The second phase of weight loss for P3ABA occurred at 222 °C with a considerable weight loss of around 12.3 %; however, the weight loss for the P3ABA/GO/CoFe₂O₄ composite at this stage was found to be 13.8 % at 276 °C. The weight loss at this stage is related to the loss of the dopant anion in both cases. The final weight loss for P3ABA occurred at 503 °C with a loss of 37.1 %, while for the P3ABA/GO/CoFe₂O₄ composite a loss of 19.1 % at 522 °C was observed, which can be attributed in each case to the degradation of the polymer backbone. The small peak (Fig. 5b) at 768 °C for P3ABA/GO/CoFe₂O₄ composite could be related to metal oxides present in the polymer nanocomposite. The relative total weight losses up to 1000 °C at the end of experiment stage were found to be 66 % and 51.5 % for the pure P3ABA and P3ABA/GO/CoFe₂O₄ composite, respectively. These data indicate that the thermal stability of the P3ABA/GO/CoFe₂O₄ composite has been improved due to the interactions between the functional groups (amine, carboxyl, hydroxyl, epoxy group, and CoFe₂O₄) present in the structure of the P3ABA/GO/CoFe₂O₄ nanocomposite.

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Fig. 5

a) TGA and b) DTG curves for P3ABA and the P3ABA/GO/Fe₂O₄ composite. Each color indicates a typical sample (see graph keys).

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3.1.4 Pore size distribution BET analysis

The specific surface areas and porosity characteristics of samples were determined from nitrogen adsorption–desorption isotherms using the BET technique and the BJH method, as depicted in Fig. 6. The surface characterization for pure P3ABA, CoFe₂O₄, and the P3ABA/GO/Fe₂O₄ nanocomposite is shown in Table 2. As shown in Fig. 6a, the hysteresis loops for P3ABA and P3ABA/GO/Fe₂O₄ at relative pressures (P/P₀) were around 0.75 and 0.81, respectively, suggesting the mesoporosity of the structures of both samples. The specific surface area, total pore volume, and mean pore diameters for pure P3ABA were found to be 22.50 m²/g, 0.026 cm³ g⁻¹, and 18.14 nm, respectively. However, for the P3ABA/GO/Fe₂O₄ nanocomposite, the specific surface area, total pore volume, and mean pore diameters were 51.61 m²/g, 0.163 cm³ g⁻¹, and 30.25 nm, respectively, indicating that the adsorbents exhibit mesoporous behavior (note that a material containing pores with diameters of less than 2 nm is referred to as having macropores, between 2 and 50 nm as having mesopores, while>50 nm as having macropores (Kim et al., 2016). According to the BJH plot (Fig. 6b), the pore size distribution for the polymer nanocomposite was in the range of 1 to 100 nm, indicating to there are two groups of pores as follows: (i) from 2 to 50 nm is attributed to the mesopore range (most pores fall within this range), and (ii) from 50 to 100 nm is related to the macropore range. It is apparent that the fabrication of P3ABA with GO and CoFe₂O₄ led to a decrease to mesoporous characteristics, somewhat owing to pore blockage. However, the coating of P3ABA on the surface of GO and CoFe₂O₄ led to an increase in surface area and total pore volume, making a significant improvement to the porosity and better adsorption performance compared to the P3ABA homopolymer, as shown in Table 1, where a large pore size facilitates the binding of dye molecules onto the surface of adsorbent. This could be attributed to the change in the growth orientation and morphology of the surface of the P3ABA/GO/Fe₂O₄ nanocomposite, which is agreement with the TEM data (Fig. 9). It can be concluded that the P3ABA/GO/Fe₂O₄ nanocomposite plays a significant role in the extraction of the dyes due to its enhanced porosity and surface area characteristics, as well as the presence of a large number of functional groups.

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Fig. 6

a) Nitrogen adsorption–desorption isotherms, and b) pore size distributions for the P3ABA and P3ABA/GO/CoFe₂O₄ nanocomposite samples.

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3.1.5 Morphology and microstructural analysis

The surface morphologies of GO, CoF₂O₄, P3ABA, and the P3ABA/GO/CoFe₂O₄ nanocomposite were studied using scanning electron microscopy (SEM), as shown in Fig. 7. Fig. 7a shows the SEM image of GO sheets, which illustrate a homogeneous surface with thin layers and crumpled edges, while the SEM image of CoFe₂O₄ in Fig. 7b exhibits two irregular regions related to identified elements of Fe and Co, aggregated magnetic sphere-shaped regions with grain sizes in the nanometer range (∼15 nm). The typical morphology of pure P3ABA (Fig. 7c) was observed to be a homogeneous and network-shaped with porous surfaces and a chain thickness of about 20 nm. However, the surface morphology of the P3ABA/GO/CoFe₂O₄ nanocomposite is completely changed when P3AB is incorporated with GO and CoF₂O₄ nanoparticles, as observed in Fig. 7d. In addition, the surface of the nanocomposite sample clearly indicates irregular porosity with a rough surface showing the grafting of CoF₂O₄, and P3ABA onto the surface of the GO, resulting in aggregation and the formation of a network structure. This change in the morphology of P3ABA/GO/CoFe₂O₄ occurs due to the electrostatic, π-π, and hydrogen-bonding interactions between the CoF₂O₄, GO sheets, and P3ABA/GO/CoFe₂O₄, which are the result of the presence of large active functional groups in the structure of the composite such as hydroxyls, epoxies, amines, and carboxyls, in addition to electrostatic forces. The morphology of the P3ABA/GO/CoFe₂O₄ nanostructure could be one of significant parameters for improving adsorption of dye, which is consistent with pore features such as specific surface area.

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Fig. 7

SEM images of a) GO, b) CoFe₂O₄, c) P3ABA, and d) the P3ABA/GO/CoFe₂O₄ nanocomposite.

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EDX was also used to assess the chemical content of the P3ABA/GO/CoFe₂O₄ nanostructure, as depicted in Fig. 8. The chemical analysis of P3ABA/GO/CoFe₂O₄ nano-adsorbent was investigated using mapping and peak lines, the results of which indicate an atomic composition of 47 % C, 6 % N, 15 % O, 22 % Fe, and 10 % Co, indicating that the prepared P3ABA/GO/CoFe₂O₄ had good homogeneity. This result is consistent with the FTIR and XRD results, confirming the successful synthesis of P3ABA/GO/CoFe₂O₄.

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Fig. 8

EDX images for a) elemental mapping, and b) peak line analysis of the P3ABA/GO/CoFe₂O₄ nanocomposite.

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The transmission electron microscopy (TEM) images of GO, CoFe₂O₄, P3ABA, and the P3ABA/GO/CoFe₂O₄ nanocomposite were studied, as per the data displayed in Fig. 9. The TEM image of GO (Fig. 9a) shows a thin and transparent layer due to the occurrence of GO exfoliation during its chemical fabrication from graphite. Fig. 9b exhibits clear uniform oval-shaped squashed-sphere nanoparticles of CoFe₂O₄ with a few aggregation particles and an average diameter of about 15 nm. In the case of the P3ABA image, the TEM result suggests an agglomerated nanoparticle morphology, as can be seen in Fig. 9c. The TEM image of P3ABA/GO/CoFe₂O₄, depicted in Fig. 9d-f, reveals that the P3ABA and CoFe₂O₄ nanoparticles deposited on the surface of GO sheets had different sizes and aggregation distributions due to the magnetic property, confirming the occurrence of the interaction between the components (GO with P3ABA and CoFe₂O₄ nanoparticles).

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Fig. 9

TEM images for a) GO sheet, b) CoFe₂O₄ nanoparticles, c) P3ABA, and (d-f) the P3ABA/GO/CoFe₂O₄ nanocomposite at different magnifications.

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3.1.6 Electromagnetic shielding performance

The characterization of magnetic behavior for pure CoFe₂O₄ and the P3ABA/GO/CoFe₂O₄ nanocomposite was investigated using a vibrating sample magnetometer (VSM) from −15000 to 15,000 Oe at 298 K. Fig. 10a shows the magnetization curve for pure CoFe₂O₄ and the P3ABA/GO/CoFe₂O₄ nanocomposite. Both samples exhibit the magnetic properties in their behaviors throughout typical hysteresis loops. The saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) for CoFe₂O₄ are 34.39 emu/g, 14.49 emu/g, and 353.51 Oe, respectively, whereas for the P3ABA/GO/CoFe₂O₄ nanocomposite these are 13.62 emu/g, 1.21 emu/g, and 17.13 Oe, respectively. These data are also consistent with previous studies (Hafeez et al., 2019, Chandel et al., 2020). The decrease in the magnetic properties of the P3ABA/GO/CoFe₂O₄ nanocomposite correspond to the strong interfacial interaction between GO and P3ABA with CoFe₂O₄, which indicates that the properties of magnetic CoFe₂O₄ were present in the P3ABA/GO/CoFe₂O₄ nanocomposite to some extent. In addition, and as shown in Fig. 10b, the P3ABA/GO/CoFe₂O₄ nanocomposite adsorbent can easily attracted by an external magnetic field to allow the separation of the adsorbent from the suspended system after adsorption of dye.

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Fig. 10

a) VSM magnetization curve of CoFe₂O₄ (green line) nanomagnetic and the adsorbent of the P3ABA/GO/CoFe₂O₄ nanocomposite (blue line). b) shows the dye solution before and after the adsorption process and the separation of the P3ABA/GO/CoFe₂O₄ nanocomposite using an external magnet.

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3.2.1 Impact of initial solution pH

The initial solution pH is an important factor in the elimination mechanism of dye from the liquid solution. Also, the pH of the solution influences the charge of any pollutants, and the activity of any functional groups in the adsorbent and thus the adsorbent’s capacity. The protonation of the adsorbed surface groups differs at acidic and basic pH, which may relate to the anionic structure of the CR dye. It is well known that, firstly, with increasing pH, the elimination rate (E %) in CR improved, reaching its maximum at pH 5, which is 98.3 % (Fig. 11a). Then, as the pH increases (pH > 5.0), the elimination rate declines considerably, which could be attributable to the deprotonation of the adsorbent surface, as shown in Fig. 1a. It may be possible that an anionic CR dye (neutrally charged in the basic medium, as shown in Fig. 1b) and neutrally charged adsorbent experience a decreased attraction to each other electrostatically, limiting adsorption between them in comparison to other interactions such as π-π and hydrogen bonding, and therefore the elimination rate steadily declined with increasing pH > 5. Finally, the minimum elimination rate was reached at pH 10 (E % = 48.22 %). At a pH of around 5 to 6.45 (pH < pH_ZPC), the CR dye experiences its highest removal efficiency at pH 5. This is because of imine and amine groups on the surface of P3ABA/GO/CoFe₂O₄ becoming protonated, which leading to an increase in electrostatic attraction between the CR dye (negatively charged) and the positively charged sites of the P3ABA/GO/CoFe₂O₄ adsorbent, in addition to other interactions such as π-π and hydrogen bonding. However, at the lowest pH of around 4 to 2 (pH less than 5), the large amount of Cl^- produced under these pH conditions (Cl^- released from addition of the required amount of 0.1 N HCl to adjust pH) might compete with the anionic dye (CR) for the adsorption sites on imine and amine groups presented on the adsorbent surface, leading to an associated decline in CR adsorption. Therefore, the π–π interaction between CR and P3ABA/GO/CoFe₂O₄ is of most significance in the adsorption process (Wang et al., 2018, Karim et al., 2022). The pH that results in a zero net charge is termed the zero-point charge (ZPC). The ZPC is a significant factor frequently employed to describe the adsorption properties of materials (Koohi et al., 2021). In this test, the pH_ZPC is 6.45 (Fig. 10b). Therefore, for this reason the surface charge of the P3ABA/GO/CoFe₂O₄ nanocomposite would be positive below pH_ZPC, and neutrally charged above it (pH > 6.45), as shown in Fig. 1a. Therefore, when pH < pH_ZPC, the adsorbent would be favorable for CR anionic dye adsorption due to the dye being negatively charged (Fig. 1b); as pH increases (pH > pH_ZPC) the adsorption of CR dyes from the solution will accordingly decline.

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Fig. 11

a) The impact of initial pH of the solution (2–10), b) the study of the pH_ZPC of the P3ABA/GO/CoFe₂O₄ nanocomposite adsorbent.

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The complicated mechanisms involved in the adsorption of CR dye molecules, as well as the P3ABA/GO/CoFe₂O₄ nanocomposite adsorbent, can involve mechanisms such as π-π interactions and hydrogen bonding, in addition to electrostatic interaction. To understand the adsorption mechanism of the P3ABA/GO/CoFe₂O₄ nanocomposite for the CR dye, a plausible diagram for the P3ABA/GO/CoFe₂O₄ nanocomposite with the CR dye can be constructed, as depicted in Fig. 12. Certainly, the interpretation of these tendencies can be basically dependent on the value of pH_ZPC, the nature of the chemical structure of the adsorbate, the surface chemistry of the adsorbent, and the adsorption conditions (Doan et al., 2021), where CR dyes are negatively charged and the P3ABA in P3ABA/GO/CoFe₂O₄ nanocomposite is positively charged when pH < pH_ZPC. Therefore, reasonable mechanisms for CR adsorption onto the surface of P3ABA/GO/CoFe₂O₄ nanocomposite are as follows: (i) a π-π interaction between the aromatic rings of the CR molecules and π electrons in the P3ABA/GO/CoFe₂O₄ nanocomposite construction, containing charge transfer, scattering force, and polar electrostatic, (ii) an electrostatic interaction between the net positive charge of the P3ABA/GO/CoFe₂O₄ nanocomposite adsorbent at a pH less than pH_ZPC and anionic construction of CR dye in liquid solutions, and (iii) other functional groups in the P3ABA/GO/CoFe₂O₄ nanocomposite containing oxygen (–OH, –COOH, –CO) that could form hydrogen bonding interactions with nitrogen and oxygen present in the structure of CR dye, which play a crucial role in the adsorption mechanism of dye onto the adsorbent surface (P3ABA/GO/CoFe₂O₄).

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Fig. 12

Reaction scheme for the P3ABA/GO/CoFe₂O₄ nanocomposite with the CR dye during the adsorption process.

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3.2.2 Impact of nanocomposite adsorbent dosage

In the adsorption method, the adsorbent dosage is a significant practical factor that has important effects on the elimination rate, and thus shows whether the batch process is cost-effective or otherwise. The impact of nanocomposite adsorbent dosage was made apparent via an adsorbent dosage of 10–70 mg by keeping other affecting influences constant (conditions: initial pH 5; V = 100 mL; initial C_CR = 50 mg/L; T = 298 K, t = 60 min). The details of the adsorbent dosage variants in the elimination rate of the P3ABA/GO/CoFe₂O₄ nanocomposite are presented in Fig. 13a.

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Fig. 13

Effects of a) adsorbent dosage (10 to 70 mg), b) initial CR dye concentration (10–100 mg/L), c) reaction temperature (298–310 K), d) addition of salt (0–500 mg/L) on the elimination rate of CR dye.

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The data obtained showed that on increasing the nanocomposite adsorbent dosage, the elimination rate improved from 47.89 % to 98.42 % on increasing the amount of nanocomposite adsorbent from 10 mg to 50 mg, which indicates that the number of adsorbent particles improved with increasing nanocomposite adsorbent dose, further indicating that the surface area and number of active sites increased during dye adsorption (Yu et al., 2021), which then reaches a maximum at the adsorption equilibrium. However, the elimination rate slightly decreased after this point (i.e., after 50 mg) due to the accumulation of adsorbents (Fig. 13a). In the other words, it means aggregation of adsorbents in excess to the amount of the adsorbent leads to a decrease in the adsorption capacity, which could result from a decrease in the total surface area of the adsorbent.

3.2.3 Impact of initial concentrations of CR dye

Initial concentration variations are one of the key factors in equilibrium studies and for achieving optimized studies to improve the relevant values between pollutants in aqueous solution as well as adsorbents. Fig. 13b explains why the dye elimination rate decreased from 98.4 to 75.9 % with an increase in initial solution concentration from 10 to 100 mg/L (other conditions: initial pH 5; V = 100 mL; dosage = 50 mg; T = 298 K, t = 60 min). By increasing the concentration of the solution, the ion potential transfer to the adsorbent surface increases as a result of the high abundance of the functional groups on the adsorbent surface and the increase in driving forces or a high diffusion of CR dye molecules from solution onto the surface of adsorbent. However, at high dye concentrations, the competition between dye molecules for the reduced abundance of active site could be the main reason for a decline in the percentage removal of dye. It can be concluded that by increasing the initial concentration of CR dye in solution, the elimination rate will increase until finally reaching a maximum, which can decrease the number of active adsorption sites and their saturation with increasing solute concentration (Zhou et al., 2019).

3.2.4 Impact of reaction temperature

In general, one can predict that with the increase in temperature associated with the increase in velocity, as well as the diffusion of dye ions from the solution to the surface of the adsorbent, that a significant increase in the rate of the elimination reaction will be observed. The temperature of a reaction is a significant factor that can produce a change in the removal system. The temperature of the reaction was used with a dose of 50 mg of adsorbent in 50 mg/L of solution containing CR dye at an initial pH 5 for a time of 60 min. Fig. 13c shows the impact of temperature on elimination of CR dye via the P3ABA/GO/CoFe₂O₄ nanocomposite. It can be determined that with an increase in temperature from 298 to 310 K, the elimination rate (E %) increased slightly from 98.32 to 99.06 %, and thus the method used in this study is endothermic in nature (Koohi et al., 2021). The maximum elimination rate was found at 310 K, at 99.06 %.

3.2.5 Impact of salt addition

The impact of salt addition on the rate of elimination of CR dye via the P3ABA/GO/CoFe₂O₄ nanocomposite was investigated under optimal conditions (pH 5, dosage = 50 mg, T = 298 K, and 100 mL of 50 mg/L CR solution). NaCl concentration was adjusted from 0 to 500 mg/L in steps of 50 mg/L. As illustrated in Fig. 13d, the elimination rate reduced from 98.32 % to 64.18 % as the salt concentration was increased from 0 to 500 mg/L. These data may attribute to the competition between Cl^– (from NaCl) and CR anions for the active sorption sites (Wu et al., 2020). It can be concluded that the rate of elimination of CR dye is significantly affected by the ionic strength of the saline aqueous liquor due to the reduction of the electrostatic interaction between CR and the adsorbent.

3.2.6 Impact of reaction time

The equilibrium reaction time of the adsorption method, in addition to the rate at which equilibrium is reached, are vital to characterizing the adsorption mechanism. Not only should an adsorbent quickly reach equilibrium, but the equilibrium itself should be as heavily biased towards adsorption as possible. In order to realize the reaction time for an increased CR dye adsorption process, the test was carried out via an adsorbent dosage of 50 mg with different initial concentrations of CR dye (10, 40, and 80 mg/L solution) at pH 5, The impact of reaction time on the rate of elimination of CR dye by the P3ABA/GO/CoFe₂O₄ nanocomposite is shown in Fig. 14a. When equilibrium was achieved, the concentration of CR dye remaining in the solution was determined. As can be seen, due to the availability of large functional sites, the adsorption rate was very fast in the first 5–50 min of the reaction. After 60 min, only a small further change in the adsorption capacity was detected, as the process moved towards equilibrium.

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Fig. 14

a) Impact of reaction time; fitting the adsorption kinetics plots for CR dye for P3ABA/GO/CoFe₂O₄ nanocomposite adsorbent: b) pseudo-first order model; c) pseudo-second order model; and d) Elovich model (conditions: initial pH 5; dosage = 50 mg; V = 100 mL; initial C_CR = 10, 40, and 80 mg/L; T = 298 K).

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After equilibrium has been achieved, increasing the reaction time will have no effect on adsorption. Past equilibrium, the sites on the surface of the adsorbents will have been filled, and the diffusion of the remaining ions in the solution, as well as the bonding with the adsorbent, becomes increasingly difficult. The illustrations demonstrated that the rapid reaction was essentially completed within 50 min, and that equilibrium had been reached after 60 min (dual step) (Abdul Rahim et al., 2021). The optimal time for proper adsorption for the P3ABA/GO/CoFe₂O₄ nanocomposite is thus considered to be 60 min.

3.4.1 Kinetics modelling study

To observe the adsorption process of the CR dye kinetically, three different initial concentration of CR dyes (10, 40 and 80 mg/L) were used and the linear form of the pseudo-first order (PFO) model (Lagergren 1898) was applied:

From Equation (5), q_e and q_t refer to dye adsorbed per unit weight of P3ABA/GO/CoFe₂O₄ nanocomposite at equilibrium (mg/g) and at time t (min), respectively. k₁ (min^{− 1}) defines the rate constants of adsorption for the PFO model, where the magnitude of k₁ can be identified from the linear plots between ln(q_e−q_t) against t (Fig. 14b), from the results observed in Table 3; obviously, lower correlation coefficients (R²) were found, so the experimental results of the adsorption method are not obeyed entirely by the PFO model. The results were then validated by a pseudo-second order (PSO) kinetics model (McKay et al., 1999).

From Equation (6), k₂ (mg/g. min) expresses the rate constants for PSO kinetic models at 298 K. Both k₂ and q_e can be determined from the intercept as well as the gradient of the plot of t/q_t against t, as shown in Fig. 14c. From the results summarized in Table 3, the correlation coefficient for PSO (R² > 0.992) for the three initial CR dye concentrations showed that the kinetic experimental adsorption results were a better fit to the theoretical results obtained from the PSO model; furthermore, the results shown from the PSO are in agreement with the adsorption kinetic results through comparing the experimentally determined adsorption capacity q_m (exp.) (mg/g) and the calculated adsorption capacity q_e cal. (mg/g). In addition, the low statistical errors according to the χ² and SSE tests revealed that the PSO model is a better fit for the adsorption of CR dye onto the P3ABA/GO/CoFe₂O₄ nanocomposite adsorbent.

Moreover, the Elovich model (Equation (7))(Elovich and Zhabrova 1939) is one of the most useful for describing the chemical adsorption process, where adsorption sites can evolve exponentially, as well as α and β being the two coefficients of the equation that can be computed from the gradient and intercepts of the linear plots between q_t vs ln t, as presented in Fig. 13d. Further, the Elovich model describes multilayer adsorption. Table 3 reports the factors calculated for the Elovich model for CR dye at three different initial concentrations (10, 40, and 80 mg/L).

3.4.2 Isothermal modelling study

The experimental results for CR dye adsorption, in addition to the homogeneous and heterogeneous nature of the adsorbent, were predicted using three different theoretical models: the Langmuir (Langmuir 1916), Freundlich (Freundlich 1907), and the Temkin (Tempkin and Pyzhev 1940). Isotherms adsorption studies were performed at initial CR dye concentrations in the range of 10 to 100 mg/L at three separate temperatures 298, 305, and 310 K (Fig. 15a). The Langmuir model is assumed to be the structure of adsorbate molecules on a homogeneous surface of the adsorbent. The linear mathematical expression for the Langmuir model (Equation (8)) can be given as:

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Fig. 15

a) Adsorption isotherms of CR dye onto the P3ABA/GO/CoFe₂O₄ nanocomposite adsorbent (conditions: initial pH 5; dosage = 50 mg; V = 100 mL; reaction time = 60 min; T = 298, 305, and 310 K); fitting the adsorption isotherm plots for CR dye on the P3ABA/GO/CoFe₂O₄ nanocomposite adsorbent: (b) Langmuir isotherm model; (c) Freundlich isotherm model; and (d) Temkin isotherm model.

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C_e refers to the concentration (in mg/L) of CR dye uptake by the P3ABA/GO/CoFe₂O₄ nanocomposite adsorbent at equilibrium, q_e is the equilibrium capacity of CR dye (mg/g), k_L (L/mg) is the Langmuir isotherm constant associated with the binding energy between the adsorbate molecules (CR dye) and the P3ABA/GO/CoFe₂O₄ nanocomposite adsorbent, and q_max is the maximum adsorption capacity of the adsorbent (mg/g). A straight line can be acquired when C_e/q_e is plotted against C_e, as illustrated in Fig. 15b. q_max and k_L were determined from the gradient and intercepts of this graph, and the parameters so obtained are summarized in Table 4. The Langmuir separation factor, R_L (R_L = 1/1 + K_LC_i), was utilized to estimate the suitability of the adsorption process isothermally, where the computed value of R_L was in the range 0 < RL less than 1 (summarized in Table 4), which shows that adsorption of CR dye via the P3ABA/GO/CoFe₂O₄ nanocomposite adsorbent is a favorable process. The linear mathematical expression of the Freundlich model (Equation (9)) can be given as:

where K_f (mg/g (L/mg)^1/n) is the Freundlich coefficient, C_e characterizes the adsorbate concentration, q_e is the amount of CR dye adsorbed at equilibrium, while n refers to Freundlich heterogeneity constant. In Fig. 15c, both K_f and n can be determined from a plot of ln q_e against ln C_e. The value of the constant n is frequently applied to indicate the suitability of the adsorption process. The calculated values of n are in the range from 1 to 10 (Table 4), which indicates a favorable adsorption of CR dye on the P3ABA/GO/CoFe₂O₄ nanocomposite. This agrees well with the results for R_L (see Table 4).

The Temkin isotherm model (Equation (10)) considers the relationships between the adsorbent and the adsorbate as well as managing a linear decline in the adsorption energy across the surface coverage of the adsorbent. The linear mathematical expression of the Temkin model (Equation (10)) can be given as:

where A refers to the Temkin equilibrium binding constant (L/g), and B is the heat of adsorption constant in J/mol. Both constants A and B can be determined from a plot of q_e against ln C_e (Fig. 15d). The isotherm factors for the adsorption of CR dye are reported in Table 4. Based on the values of R² at three different temperatures (Table 4), it can be assumed that the results of the Langmuir adsorption model were in best agreement with the experimental results. Furthermore, it was recognized from the χ² and SSE tests that the Langmuir model characterized the best fitting for adsorption CR dye on the P3ABA/GO/CoFe₂O₄ nanocomposite adsorbent (Table 4). In addition, the monolayer maximum adsorption capacities of the CR dye calculated from the Langmuir adsorption isotherm q_max were very close to the actual experimental values. Furthermore, in this work, non-linear least-squares (NLLS) adjustments were applied for the kinetic and isothermal study, the results of which are shown in Figures S3-S4.