1 Introduction

Synthetic dyes are widely used in industrial sectors such as pharmaceuticals, textile, cosmetics, and plastic industries, resulting in the generation of effluents contaminated with dyes [1, 2]. These wastewaters reduce sunlight penetration, interfere with photosynthesis, and increase chemical oxygen demand as well as negatively impact the aquatic environment [3]. Additional health dangers from exposure to organic dyes include increased heart rate, allergic responses, respiratory tract irritation, and nausea [4]. To protect the ecology and promote water reuse, it is necessary to purify water that has become contaminated with these hazardous chemicals. For the removal of dyes from polluted wastewater, a variety of techniques have been explored such as adsorption [5,6,7], ion exchange [8], nanofiltration [9], biological degradation [10], and Fenton oxidation [11]. Among these, the adsorption technique has gained popularity due to the availability of different adsorbents, simple preparation, easy handling, low operational cost, and high efficiency [12].

Chitosan (CHI) is a non-toxic, biodegradable, and linear cationic polymer that is obtained from chitin deacetylation [13]. CHI is an attractive adsorbent for water remediation due to its unique properties [14]. It can interact with a variety of contaminants owing to its amine and hydroxyl functional groups [15]. However, unmodified CHI has some limitations for use in wastewater treatment. It is soluble in acidic environments and has a high swelling index [16]. To overcome these limitations, CHI can be modified physicochemically. This can involve crosslinking, grafting, or other methods to improve its solubility, swelling index, and adsorption capacity [17].

Covalent cross-linking is a common approach for improving the physical and chemical characteristics of CHI [18]. Cross-linking agents are used to create covalent connections with the functional groups of CHI, resulting in stronger and more stable cross-linked CHI [19]. However, common cross-linking agents like formaldehyde, glutaraldehyde, and glyoxal have been found to reduce the adsorption ability of CHI [20]. To overcome this limitation, alternative cross-linking molecules with plenty of chemical groups are desirable. Benzil (BZI), which contains phenyl rings, is one such substitute. The introduction of phenyl rings through the use of BZI leads to the formation of a more intricate network that has an enhanced affinity for organic dyes. This is because BZI contains conjugated systems of phenyl rings that can interact with the aromatic system of organic dyes through π-π stacking interactions [21].

The utilization of CHI composite adsorbents has emerged as a promising approach to enhance adsorption capabilities [22]. These composites offer improved selectivity, regeneration potential, surface area, mechanical strength, and surface chemistry compared to conventional materials [19]. Among the various composite options, chitosan-organoclay biocomposites have garnered considerable interest [23]. The exceptional sorption behavior and large specific surface area of organoclays make them particularly suitable for adsorbing hydrophobic organic compounds and ionic pollutants, further enhancing their appeal as efficient adsorbents in environmental applications [24,25,26]. Thus, by combining the advantageous properties of both organic and inorganic components, these biocomposites exhibit a versatile range of characteristics like environmentally friendly, adsorption ability, and chemical stability [27]. Recently, researchers have started investigating the possibility of employing magnetic adsorbents based on biocomposites for wastewater treatment [28]. This novel technique entails the development of biomagnetic derivatives of CHI, which have distinct benefits in the removal of contaminants from water [29]. These biomagnetic adsorbents are famous for their capacity to be readily recovered and separated from mixtures via an external magnetic field, removing the need for traditional filtering processes [30].

Thus, the primary goal is to develop a multi-functional absorbent of a cross-linked chitosan-benzail/organoclay/Fe3O4 biocomposite (CHI-BZI/OC/Fe3O4). The focus of this biocomposite is to achieve efficient adsorptive removal of RBBR dye from aqueous environments. To achieve this objective, the present study employed the Box-Behnken design (BBD) to statistically optimize the adsorption experimental data. The obtained data were further validated using analysis of variance (ANOVA) to ensure the reliability and significance of the results. The experimental findings from the adsorption process were also fitted with different kinetic and isotherm models. Furthermore, the study aims to propose a comprehensive mechanism for RBBR adsorption using the CHI-BZI/OC/Fe3O4.

2 Materials and methods

2.1 Materials

CHI (deacetylation degree of ≥ 75%) and BZI were obtained from Sigma-Aldrich. Organoclay was obtained from HmbG and Bendosen Laboratory. Iron (II) chloride tetrahydrate (FeCl2.4H2O) and iron (III) chloride hexahydrate (FeCl3.6H2O) were acquired from HmbG and Bendosen Laboratory Chemicals. Acidic RBBR dye (MW of 626.54 g/mol, λmax of 595 nm, and chemical stctrure is given in Fig.S1) was purchased from ACROS, Organics. All the chemical compounds were of analytical quality and utilized without additional purification in the tests.

2.2 Preparation of CHI-BZI/OC/Fe3O4

This paragraph describes the method used to prepare the CHI-BZI/OC/Fe3O4 adsorbent for use in the removal of synthetic textile dye RBBR. Initially, a 50 mL solution of 5% acetic acid was combined with 1 g of CHI flakes and 1 g of OC, and the mixture was agitated at a high RPM for 24 h to allow for complete CHI dissolution. Then, 10 mL of distilled water was combined with FeCl3.6H2O and FeCl2.4H2O, and the resultant mixture was transferred to the CHI/OC solution and agitated for an hour. The viscous solution was injected into a 1000 mL NaOH solution (2 M) with a syringe needle while being gently stirred, and CHI/OC/Fe3O4 beads started to form right away. To eliminate any remaining sodium hydroxide solution, the beads were then rinsed with distilled water. After that, the CHI/OC/Fe3O4 beads were crosslinked by adding 100 mL of 2% BZI and letting them sit in a water bath shaker at 40 °C for two hours. The CHI-BZI/OC/Fe3O4 beads were dried after being rinsed with distilled water to create tiny particles (~ 250 µm). The end product was employed for adsorption experiments to evaluate its efficacy in removing RBBR synthetic textile dye. The details of employed technologies to characterize the CHI-BZI/OC/Fe3O4 were outlined in the supplementary material (Table S1).

2.3 Box-Behnken Design

Box-Behnken Design (BBD) model was chosen for investigating the influence of variables on the adsorption of RBBR onto CHI-BZI/OC/Fe3O4 from an aqueous system. The BBD model considers the impact of individual or mutual interaction of factors. The Design-Expert software (v 13, Stat-Ease) was used to perform adsorption tests and present the results graphically. The process parameters of CHI-BZI/OC/Fe3O4 dose (A), pH (B), and contact time (C) were selected as input variables, while the RBBR removal (%) was the response (Y). The levels range of factors in BBD are given in Table 1. Second-order expression (Eq. 1) was used to explain the link between the response variable and independent factors.

Table 1 Codes and actual variables and their levels in BBD
Full size table
$$Y={\beta }_{0}+\sum{\beta }_{i}{X}_{i}+\sum{\beta }_{ii}{X}_{i}^{2}+\sum {\beta }_{ij}{X}_{i}{X}_{j}$$
(1)

In this equation, Y stands for the response optimization (RBBR removal, %), Xi and Xj are the examined factors, and β0 and βi, βii, and βij, respectively, are constant coefficients, coefficients of linear effects, coefficients of quadratic effects, and coefficients of interaction effects. Table 2 presents the BBD matrix and the corresponding RBBR removal (%) values. To start the experiment, a specific amount of CHI-BZI/OC/Fe3O4 was added to 100 mL of RBBR solution with fixed concentration (50 mg/L) in Erlenmeyer flasks of volume 250 mL. The flasks were then agitated using a water bath shaker at a speed of 85 rpm. Then, CHI-BZI/OC/Fe3O4 was separated from the mixtures using an external magnetic bar, and a 0.45 μm syringe filter was then applied. The spectrophotometer (HACH DR 3800) was used to measure the concentrations of RBBR at λmax of 595 nm. Equation (2) was used to obtain the efficiency of RBBR removal (%).

$$R \%=\frac{\left({C}_{o}-{C}_{e}\right)}{{C}_{o}} \times 100$$
(2)

where the initial RBBR concentration (mg/L) is Co and the equilibrium concentration (mg/L) of RBBR is Ce.

Table 2 Matrix of BBD model and the related response (RBBR removal (%))
Full size table

2.4 Batch Adsorption Experiment

A batch adsorption system was used to investigate the removal of RBBR using a biocomposite-based adsorbent of CHI-BZI/OC/Fe3O4. Table 2 showed that the highest RBBR removal efficiency of 90.9% was achieved in experimental run 2, where the CHI-BZI/OC/Fe3O4 dosage was 0.1 g/100 mL, pH was 4.0, and the contact time was 20 min. With ranges of beginning RBBR concentrations (20–120 mg/L) and contact times (0–360 min), these ideal circumstances were used for the analysis of adsorption isotherms and kinetic experiments. The equilibrium adsorption capacity of CHI-BZI/OC/Fe3O4 towards RBBR at equilibrium time was determined using Eq. (3).

$${q}_{e}=\frac{\left({C}_{o}-{C}_{e}\right)V}{W}$$
(3)

where the volume of RBBR solution is V (L) and the mass of CHI-BZI/OC/Fe3O4 is W (g).

3 Results and discussion

3.1 Characterization

The physicochemical properties of the CHI-BZI/OC/Fe3O4 are detailed in Table 3. The BET surface area of CHI-BZI/OC/Fe3O4 was 1.78 m2/g; whereas, the mean pore diameter was found to be 22.9 nm. According to the established IUPAC classification, this average pore diameter signifies that the structure of CHI-BZI/OC/Fe3O4 falls under the category of mesostructured materials [31]. Furthermore, the nitrogen adsorption/desorption isotherms observed for CHI-BZI/OC/Fe3O4 exhibited a Type-IV isotherm pattern (see Fig. 1). The accompanying figure displayed within Fig. 1 visually depicts the correlation between pore volume and pore sizes. The Type-IV isotherm pattern and the corresponding plot provide substantial evidence that CHI-BZI/OC/Fe3O4 possesses a mesoporous structure that exists in a significant range of pore diameters [32].

Table 3 Physiochemical characteristics of CHI-BZI/OC/Fe3O4
Full size table
Fig. 1
figure 1

The N2 adsorption–desorption isotherms and pore size distribution of CHI-BZI/OC/Fe3O4

Full size image

XRD technique was applied to meticulously investigate the phase composition and crystalline property of the CHI-BZI/OC/Fe3O4. The XRD spectrum of this composite is visually represented in Fig. 2. The sharp peak located at 2θ = 21° is indicative of presence CHI [33]. XRD pattern of CHI-BZI/OC/Fe3O4 presents key peaks at 2θ = 35.6°, 43°, 53°, 57.4°, and 62.4° providing clear evidence of the incorporation of Fe3O4 into the CHI-BZI/OC/Fe3O4 [34].

Fig. 2
figure 2

XRD pattern of CHI-BZI/OC/Fe3O4

Full size image

The FT-IR spectral analysis method was utilized to investigate the CHI-BZI/OC/Fe3O4's synthesis and its interaction with RBBR. The results from the FT-IR data of CHI-BZI/OC/Fe3O4 and CHI-BZI/OC/Fe3O4-RBBR are illustrated in Fig. 3. The FT-IR spectrum of CHI-BZI/OC/Fe3O4 revealed major peaks (3500–3800 cm−1) that signified the stretching vibration generated by the hydroxyl and amino groups [35]. There were also absorption peaks at 2900–2800 cm−1, which were associated with the presence of chitosan-related C-H [36]. In addition, a peak at 1650 cm−1 revealed a stretching vibration driven by the existence of the C = N bond as a result of the BZI grafting process in CHI chains [20]. Peaks at 1370 cm−1 (C‒N) and 1030 cm−1 (C-O) could be assigned to the CHI [20]. The strong band detected at 1030 cm−1 can also be attributed to the Si–O stretching vibration, while the Fe–O bonds were supported by a vibrational band at 570 cm−1 [37, 38]. The FT-IR spectrum of CHI-BZI/OC/Fe3O4-RBBR resembled that of Fig. 3a, with some peaks' positions shifted, especially the OH and -NH2 groups, indicating that RBBR had been adsorbed onto the CHI-BZI/OC/Fe3O4 surface.

Fig. 3
figure 3

FTIR spectra of (a) CHI-BZI/OC/Fe3O4 and (b) CHI-BZI/OC/Fe3O4 after RBBR adsorption

Full size image

The morphological characteristic of the adsorbent (CHI-BZI/OC/Fe3O4) before and after the uptake of the adsorbate (RBBR) was investigated using SEM–EDX analysis as shown in Fig. 4. The SEM image and EDX spectrum of the CHI-BZI/OC/Fe3O4 surface before RBBR adsorption are presented in Fig. 4a and 4b respectively. As can be seen from Fig. 4a, the surface of CHI-BZI/OC/Fe3O4 before adsorption appears as a wavy, irregular, and heterogeneous surface. Moreover, its corresponding EDX spectrum (Fig. 4b) shows the presence of the Fe, C, Fe, N, S, O, and Si in its molecular structure, and the presence of Fe confirms the existence of Fe3O4 in the CHI-BZI/OC/Fe3O4 which is responsible for its magnetic property. On the other hand, the SEM image of CHI-BZI/OC/Fe3O4 after RBBR adsorption (Fig. 4c) presents a more compact surface and less heterogeneous surface which can be attributed to the loading of RBBR dye molecules onto CHI-BZI/OC/Fe3O4 surface. Additionally, the elemental composition of CHI-BZI/OC/Fe3O4 after RBBR adsorption (Fig. 4d) shows similar profile to elemental composition of CHI-BZI/OC/Fe3O4 before RBBR adsorption (Fig. 4b).

Fig. 4
figure 4

(a) and (b)SEM image and EDX spectrum of CHI-BZI/OC/Fe3O4 respectively; (c) and (d) SEM image and EDX spectrum of CHI-BZI/OC/Fe3O4 after RBBR adsorption respectively

Full size image

3.2 BBD Model Fitting

The RBBR removal data obtained from the experiments were analyzed using ANOVA. The statistical data are outlined in Table 4, and the model F-value of 95.06 indicates that the created model (RBBR removal) is significant from a statistical perspective [39]. The correlation coefficient (R2) value of 0.99 implies a good link between the real and theoretical RBBR removal values [40]. When the P-value is less than 0.05, model codes are considered significant terms. Therefore, AB, AC, BC, A, B, C, and C2 were identified as important terms for removing RBBR. The quadratic polynomial model, as shown in Eq. (4), was applied to establish the actual link between RBBR removal and the factors.

Table 4 ANOVA analysis for RBBR removal (%) by CHI-BZI/OC/Fe3O4
Full size table
$$\mathrm{RBBR \;removal }\left(\mathrm{\%}\right)=+53.50+25.26{\text{A}}-4.35{\text{B}}+15.04{\text{C}}-4.20{\text{AB}}+6.88{\text{AC}}-6.95\mathrm{BC }-12.81{{\text{C}}}^{2}$$
(4)

To verify the experimental data, the real versus theoretical plot and the normal probability plot of residuals extracted from the BBD model were analyzed. Figure 5a shows the real versus theoretical plot of RBBR removal (%), where the real points were found to be mostly close to the theoretical points, signifying the strength of the applied model. Figure 5b shows the normal probability plot of residuals, which indicates the independence of the residuals [41]. The residuals may be properly distributed, according to the normal distribution of all the points surrounding the straight line. Using Cook's distance, another statistical validation was carried out, as shown in Fig. 5c. Cook's distance should be less than 1 to be considered acceptable. All observed values in this investigation were less than 1, and the majority of runs were even lower than 0.2, showing significant implications for the model's predictive capacity [42]. The quality and dependability of the experimental results acquired from the BBD model are confirmed by these statistical validations.

Fig. 5
figure 5

(a) plot of the relationship between the predicted and actual values; (b) normal probability plot of residuals; (c) Cook’s distance plot for RBBR adsorption by CHI-BZI/OC/Fe3O4

Full size image

3.3 Effects of Significant Interactions

Figure 6a-b showcases the 3D response surfaces and 2D contour plots, providing an insight into the dual impacts of parameters on RBBR removal (%). Specifically, the key interaction between CHI-BZI/OC/Fe3O4 dose and pH is presented in these plots. Upon analysis, it becomes evident that the RBBR removal gradually increases with a decrease in solution pH, eventually reaching a pH of 4. This result is closely linked to the CHI-BZI/OC/Fe3O4’s surface charge, which was computed by the pHpzc (cf. Fig. S2). The pHpzc value, established as 6.9 for CHI-BZI/OC/Fe3O4, dictates that the biocomposite surface carries a negative charge when the pH is higher than 6.9, and conversely, a positive charge when the pH falls below 6.9. As the solution pH decreases, creating a more acidic environment, the -NH2 groups on the surface of the CHI-BZI/OC/Fe3O4 undergo protonation due to the presence of H+ ions. Consequently, this protonation leads to a positive charge on the biocomposite surface. The outcome of this charge variation is an electrostatic force between the cationic sites on the CHI-BZI/OC/Fe3O4 surface and the RBBR, as outlined in Eq. (5).

Fig. 6:
figure 6

3D and 2D plots AB (a: 3D; b: 2D), AC (c: 3D; d: 2D), and BC (e: 3D; f: 2D) interactions

Full size image
$${\text{CHI}}-{\text{BZI}}/{\text{OC}}/{{\text{Fe}}}_{3}{{\text{O}}}_{4} { }^{+} + {{\text{RBBR}}}^{-} \leftrightarrow \mathrm{ CHI}-{\text{BZI}}/{\text{OC}}/{{\text{Fe}}}_{3}{{\text{O}}}_{4} { }^{+}\dots {}{}^{-}{\text{RBBR}}$$
(5)

Figure 6a-b shows also increasing the adsorbent dose also contributes to higher RBBR removal. With higher doses of the CHI-BZI/OC/Fe3O4, more adsorption sites become available, increasing the overall adsorption capacity. This means that more RBBR dye molecules can interact with the active sites on the adsorbent surface, resulting in a higher percentage of dye removal. The combination of lower pH and higher adsorbent dose creates a synergistic effect, further enhancing the adsorption of RBBR. The positive charge on the adsorbent's surface is intensified at lower pH, increasing the attraction to the negatively charged dye molecules. The analysis of Fig. 6c-f reveals significant interactions between AC (dose × time) and BC (pH × time) in the adsorption of RBBR using the CHI-BZI/OC/Fe3O4. These interactions indicate that both contact time and the combination of adsorbent dose and pH have a substantial impact on RBBR removal (%). The results in Fig. 6c-f show that the removal of RBBR (%) increases rapidly as the contact time is extended from 5 to 35 min. This observation can be explained by considering that more contact time allows RBBR dye molecules sufficient time to diffuse and penetrate into the CHI-BZI/OC/Fe3O4’s pores.

3.4 Adsorption Study

This study was performed to investigate the effect of contact time and initial RBBR concentrations on the adsorption capacity of CHI-BZI/OC/Fe3O4. During the experiments, the CHI-BZI/OC/Fe3O4 dose (0.1), pH (4.0), and temperature (25 °C) were held constant. The results, as depicted in Fig. 7a, obviously show that the adsorption ability of the CHI-BZI/OC/Fe3O4 material for RBBR uptake increased rapidly (from 17.2 to 96.1) initially and then reached a plateau for all studied initial RBBR concentrations. The large increase in adsorption capacity at the beginning can be attributed to the high availability of the specific surface area of the adsorbent active sites of CHI-BZI/OC/Fe3O4. As the contact time increased, more RBBR molecules were able to interact with the adsorbent, leading to higher uptake. However, beyond a certain contact time, the adsorption capacity plateaued, indicating that the adsorption sites on the CHI-BZI/OC/Fe3O4 surface were saturated, and no further adsorption occurred. This plateau suggests that the maximum adsorption capacity of the CHI-BZI/OC/Fe3O4 material for RBBR has been reached under the given experimental conditions. Furthermore, an interesting observation is that the initial RBBR concentration in the solution also influences the adsorption capacity. As the initial concentration of RBBR increased, the driving force for adsorption also increased, resulting in more RBBR molecules being adsorbed by the CHI-BZI/OC/Fe3O4 [43].

Fig. 7
figure 7

(a) Effect of the contact time on RBBR adsorption by CHI-BZI/OC/Fe3O4 at several concentrations and (b) adsorption isotherms (dose = 0.1 g/100 mL, and pH = 4)

Full size image

3.5 Adsorption Kinetic

The adsorption kinetics provides valuable information about how quickly the adsorption process occurs and gain insights into the mechanism of the adsorption process. The kinetics of RBBR adsorption using the CHI-BZI/OC/Fe3O4 were analyzed by employing the pseudo-second order (PSO) kinetic [44] and pseudo-first order (PFO) kinetic [45] as provided in Table S1. Table 5 presents the kinetic findings for the PFO and PSO models. The outcomes indicate that the RBBR adsorption onto the CHI-BZI/OC/Fe3O4 was governed by PSO kinetic model as confirmed by comapable values of the therotratical qe and experimental qe values. Thus, further confirmation was made by using root means square error (RMSE) the sum of squares error (SSE) in Table 5. In fact, the low values of the RMSE and SSE for PSO over PFO indicate the best fit for the PSO to the experimental data, and revealing that both chemisorption and physisorption modes played a crucial role in the interaction between the CHI-BZI/OC/Fe3O4 and RBBR [46].

Table 5 PFO and PSO Kinetics parameters for RBBR adsorption on CHI-BZI/OC/Fe3O4
Full size table

3.6 Adsorption Isotherm

The adsorption isotherm analysis provides valuable insights into the adsorption behavior of RBBR on CHI-BZI/OC/Fe3O4 and confirms its outstanding performance as an adsorbent for the removal of this dye from wastewater. To analyze the adsorption equilibrium data, three isotherm models i.e., Langmuir [47], Freundlich [48], and Temkin [49] were used. Table S1 contains a collection of the non-linear formulas for isotherms. Figure 7b shows the outcomes of the isotherms of RBBR adsorption. Based on the R2 values shown in Table 6, the data from the adsorption equilibrium were considered to have the best fit. The results revealed that in comparison to the Freundlich isotherm (R2 = 0.95), the Langmuir and Temkin models (R2 = 0.99) gave a superior match to the equilibrium adsorption data of the batch experiment. As a result, the homogenous distribution of surface-active sites on the adsorbent surface allowed the Langmuir and Temkin isotherms to accurately estimate the adsorption of RBBR dye onto chitosan composite [50]. Hence, further confirmation for the sutibiltiy of the adsoption isotherm was made by using root means square error (RMSE) the sum of squares error (SSE) as recorded in Table 6. In fact, the low values of the RMSE and SSE for the Langmuir isotherm model over Temkin and Freundlich isotherm models inciate a monolayer and hemgeneuos adsotpion process of RBBR was occurred on the suarfce of CHI-BZI/OC/Fe3O4. Thus, the highest monolayer adsorption capacity calculated using the Langmuir model was 102.9 mg/g, which is greater than the values noted for most adsorbents (see Table 7).

Table 6 The parameters of isotherm models and equilibrium parameters for RBBR adsorption on CHI-BZI/OC/Fe3O4
Full size table
Table 7 Comparison of the adsorption capacity of CHI-BZI/OC/Fe3O4 with several adsorbents
Full size table

3.7 Adsorption Mechanism

Typically, chemisorption agrees with the Langmuir model, where monolayer formation indicates the chemical interactions between acidic RBBR dye molecules and CHI-BZI/OC/Fe3O4 adsorbent. These chemical interactions, facilitated by functional groups present on the surface of the CHI-BZI/OC/Fe3O4 adsorbent, play a fundemantal role in the adsorption process, contributing to the overall effectiveness of the adsorption technique. The interaction of different functional groups present on the biocomposite surface plays a role in the adsorption process of RBBR on the surface of the CHI-BZI/OC/Fe3O4. These interactions are shown in Fig. 8, which further emphasizes the significant contribution of particular active groups to the adsorption of the RBBR dye. The positively charged protonated amino groups (-NH3+), hydroxyl groups (-OH2+), and silanol groups (SiOH2+) are present in the CHI-BZI/OC/Fe3O4. The negatively charged sulfate groups (-SO3−) of the RBBR dye are effectively attracted to and interacted with by these functional groups, causing the dye to bind to the surface of the biocomposite [48]. The adsorption process also heavily relies on interactions between hydrogen bonds. The nitrogen and oxygen atoms of the RBBR dye molecules create hydrogen bonds with the hydrogen atoms in the CHI-BZI/OC/Fe3O4. These hydrogen bonds improve the link between the dye and the biocomposite and increase its overall capacity for adsorption. The non-polar parts (e.g., -CH2- groups) of the RBBR dye molecules can form hydrophobic interactions with the non-polar parts of the CHI-BZI/OC/Fe3O4 chains (e.g., -CH2- groups). Additionally, n-π interactions between the π system in the RBBR and the electron donor system in the CHI-BZI/OC/Fe3O4 play a key role in the adsorption. A crucial part of the adsorption process is also played by the BZI aromatic ring, which is grafted onto the polymeric matrix of CHI-BZI/OC/Fe3O4. The RBBR dye molecules are strongly adsorbed due to interactions between the BZI ring grafted on the biocomposite and the aromatic rings in the dye. In conclusion, the CHI-BZI/OC/Fe3O4 presents itself as a very effective adsorbent for the removal of RBBR from aqueous solutions due to the combination of these many interactions, including protonated amino groups, π-π interactions, hydrogen bonding, and n-π.

Fig. 8
figure 8

Illustration of the possible interaction between CHI-BZI/OC/Fe3O4 and RBBR including electrostatic forces, hydrogen bonding, π-π, and n-π interactions

Full size image

4 Conclusions

CHI-BZI/OC/Fe3O4 was successfully synthesized as a bio-adsorbent for the removal of RBBR from the aquatic system. The experimental results of the BBD model illustrated that the highest RBBR removal of 90.9% was achieved at a CHI-BZI/OC/Fe3O4 dosage of 0.1 g/100 mL, pH of 4.0, and time of 20 min. Both isotherm models namely Langmuir and Temkin provided a good fit to the experimental data, indicating that the adsorption process occurs on a single layer of adsorbate molecules on the CHI-BZI/OC/Fe3O4 surface, and the heat of adsorption decreases linearly with coverage. The experimental data indicated that the uptake of RBBR by CHI-BZI/OC/Fe3O4 is governed by chemisorption process. The qmax of CHI-BZI/OC/Fe3O4 for RBBR was determined to be 102.9 mg/g. The mechanics of RBBR adsorption by CHI-BZI/OC/Fe3O4 include electrostatic forces, π-π, hydrogen bonding, and n-π interactions. These insights gleaned from the present work provide valuable information for the potential applications of this biocomposite material in wastewater treatment and environmental remediation efforts.