Fabrication of green magnetized ferrite biochar nanocomposites from orange peels/MnFe₂O₄, peanut shells/CuFe₂O₄, tree twigs/Ni Fe₂O₄, and wood/CoFe₂O₄: characterization and application as adsorbents for adsorption/desorption/stability of basic Blue-XGRRL dye with possible mechanisms

Water is the most plentiful natural resource, responsible for the existence of life on Earth, but only a small percentage (almost 1%) of this reserve is accessible to man for usage (Anjum et al. 2019). Wastewater from industries is a chief cause of pollution (Almasian et al. 2015a). Dye is a contaminant that has been used in a number of industries, involving food, paper, plastics, pharmaceuticals, cosmetics, paper processing, and textiles, as well as petroleum additives (Zaman et al. 2023). Dye types generally fall into three categories: cationic, anionic, and non-ionic. Blue-XGRRL dye is a cationic dye that is used widely in textile industry. According to several studies, cationic dyes which are more readily able to attach to the cytoplasm of cells are more hazardous than other colors used in industry (Safarzadeh et al. 2022). The robustness and resistance of many dyes to microbiological and optical degradation pose a significant challenge for their efficient removal through conventional methods. Hence, the imperative need to eliminate color molecules from wastewater (Khan et al. 2023a).

Many techniques comprising biodegradation, oxidation, and adsorption have been established. The most supreme strategies are sedimentation, screening, precipitation, crystallization, flotation, flocculation, reverse osmosis, electrolysis, and biological treatments, etc. These methods have certain drawbacks, including reduced efficiency, intricate operational environments, an inability to fully eliminate pollutants, a requirement for high energy input, increased costs and complexity, as well as the generation of noxious sludge that leads to secondary pollution (Munir et al. 2023). In the present era, there is a significant demand for a cost-effective, environmentally friendly, and sustainable technology to treat wastewater. This is crucial for expanding water resources and addressing the issue of water pollution (Asefi et al. 2009; Khan et al. 2023d).

Among these options, adsorption has emerged as an economical, environmentally friendly, effective, selective, and highly convenient approach. This is attributed to its low cost, superior efficiency, higher adsorption limit, ease of operation, and the ability to regenerate sorbents. These advantages make adsorption not only suitable for water treatment but also for analytical purposes (Almasian et al. 2015b). Green technology is economical, effective, and ecologically benign when combined with other methods, and it may complement other water management systems (Arami et al. 2007; Yang et al. 2021; Khan et al. 2022b). There are several studies available that show spinel ferrite-based composites are superior adsorbents than many other traditional adsorbents (Das et al. 2020, 2022; Deb et al. 2021; Das & Debnath 2022). Ferrites and biochars (BCs) have been commonly used for electro-deposition. To give BC materials magnetic properties, a variety of magnetic particles were used. Because of its variable iron valence, the spinel ferrite general formula is AFe₂O₄ (A = Mn, Zn, Ni, Co, etc.), has efficient magnetic properties and structural stability. When these ferrites are combined with BC, the resulting composite should have good magnetic properties as well as efficiency (Beiyuan et al. 2023).

The application of ferrites BC composites for the adsorption of basic dyes has been studied because of the interaction between its negatively charged backbone and positively charged dye ions, which has a larger capacity to adsorb all basic dyes because of its intrinsic cationic character. But there isn't much study on using this ferrite BC combination to get rid of simple dyes. In order to create ferrite BC composite material by incorporating BC, the current research was focused on this issue. In comparison to previous methods, the suggested process is now being considered as a promising solution for the eradication of basic dyes from waste water. Some ferrite-BC composite is presented in Table S1, Supplementary material.

The peculiarity of this process in this instance was that all the biomass-based BC was made in an extremely fine environment, the finished product has a sizable surface area and is repeatable in terms of degree of activation (Khan et al. 2023d). As biomass-based BCs along with ferrite don't generate any additional or toxic material, using them to prepare adsorbent is a very eco-friendly and practical procedure. Orange peels (OP) BC, peanut shells (PS) BC, tree twigs (TT) BC, and wood (W or wood) BC were utilized in the study for BC composites because of their distinctive and alluring behavior in this field, while many biomasses are used for this purpose that were not synthesized before in studies for adsorption of dye, to the best of our knowledge.

In the current experiment, composites of manganese ferrite and orange peels (OP-MnFe₂O₄ or Mn FBC), copper ferrite and peanut shells (PS-CuFe₂O₄ or Cu FBC), nickel ferrite and tree branches (TT-NiFe₂O₄ or Ni FBC), and cobalt ferrite and wood (W-CoFe₂O₄ or Co FBC) were all bio-synthesized and characterized that was not done before. For the Blue-XGRRL dye's adsorption, nanoferrites were used due to limited study on these. It was thoroughly investigated how dye removal regulating factors, such as pH, timings, adsorbent dosage, and initial dye concentrations, affected the process. Through kinetic and isotherm modeling studies, the adsorption experimental data were also used to investigate the nature of adsorption and its process. The main benefits of this approach were that all the products produced in this study were cost-effective because of the availability of biomass and, most critically, had excellent sorption capacity relative to their mass. The sole challenge in this research was separating the adsorbent from the dye. All of this creates a fantastic adsorbent that can be used for other green technologies, such as microbial fuel cells, to get even better outcomes.

The chemical store at the University of Agriculture, Faisalabad provided all the analytical-grade chemicals and reagents that were utilized. All chemicals were brought from Sigma-Aldrich. Distilled water, sodium hydroxide (NaOH pellets, 99.99%), ethanol (CH₃CH₂OH, ≥ 99.5%), iron(II) sulfate heptahydrate (FeSO₄·7H₂O, ≥ 99%), anhydrous copper chloride (CuCl₂, ≥ 99.995%), nickel(II) chloride hexahydrate (NiCl₂·6H₂O, ≥ 99.9%), potassium permanganate (KMnO₄, ≥ 99%), cobalt(II) chloride hexahydrate (CoCl₂·6H₂O, 98%), cetrimonium bromide (C₁₉H₄₂BrN (CTAB), 99.9%), sodium dodecyl sulfate (C₁₂H₂₅NaO₄S (SDS), ≥99.0%).

Collection of different BC materials (OP, PS, TT, and W dust) was carried out through different regions of Punjab in Pakistan. For the purpose of removing debris and other impurities, BC materials were repeatedly washed with distilled water. BCs that had been cleaned were dried in the sunshine followed by oven drying at a temperature of 60 °C overnight. The dried-up BC materials were crushed to powdered form with the help of a flour mill/grinder.

Different types of ferrite BC composites such as manganese ferrite/OP BC composites, copper ferrite/PS BC composites, nickel ferrite/TT BC composites, and cobalt ferrite/W BC composites were synthesized to remove selected dyes up to the maximum extent.

The process of creating MnFe₂O₄ BC composites involves a carefully executed two-stage procedure. In the initial phase, the BC is meticulously prepared using OP sourced from the local market in Faisalabad. This raw material is chosen for its abundance and suitability for BC production. To ensure the purity of the BC, the OP undergoes a thorough cleaning process involving rinsing with distilled water to remove any contaminants, including dirt and sand particles. Following the cleaning, the peels are left to sun-dry for a period of 3 days. This extended drying duration not only eliminates moisture but also contributes to the stabilization of the peels. Once adequately dried, the OP are divided and crushed to achieve an average particle size of about 0.5 mm. The next crucial step involves subjecting these processed peels to pyrolysis in a furnace set at 800 °C for a duration of 3 h. During this thermal treatment, the organic components of the OP decompose, leaving behind a stable carbonaceous material known as BC. In the second stage of the process, the BC undergoes further enhancement through the incorporation of manganese ferrite (MnFe₂O₄). This compound is selected for its unique magnetic and catalytic properties, which can impart specific functionalities to the resulting BC composites. The synthesis of the MnFe₂O₄ BC composites involves a meticulous mixing process to ensure the uniform distribution of manganese ferrite throughout the BC matrix. This careful amalgamation promotes homogeneity and synergistic effects in the final composites (Amin et al. 2019). After pyrolysis, the powdered BCs were sieved to 300-μm mesh size and stored in an air tight bag for the further experiment. In the second stage, the manganese ferrite BC composites were prepared through the co-precipitation method. For this, 100 mL of 0.3 M solution of FeSO₄·7H₂O was prepared and marked as suspension A. Then, 100 mL of 0.1 M solution of potassium permanganate were also prepared and added into suspension A. After that the stirring of the resulting mixture was done for 5 h at 200 rpm. To keep the solution's pH at about 10, 1 M solution of NaOH was applied drop by drop to the mixture. It makes use of it to enhance their crystallinity, controllable nanoscale, and magnetic characteristics. This is a prerequisite for the nanoparticles to precipitate. The solution exhibited a dark brown color initially. Subsequently, the precipitate was washed using distilled water until a colorless solution was achieved. This process aimed to eliminate side products and any additional BC material associated with the precipitate. The material obtained at the end was nominated as MnFe₂O₃-BC composites.

Batch adsorption studies were employed to assess the effectiveness of OP/MnFe₂O₄, PS/CuFe₂O₄, TT/Ni Fe₂O₄, and W/CoFe₂O₄. The necessary parameters, including solution pH, adsorbent dose, dye contact duration, temperature, and initial dye concentration, were optimized for the removal of Blue-XGRRL dye using conventional techniques. Erlenmeyer flasks, each containing 50 mL of dye solution with a defined pH and a specific quantity of adsorbent, were placed in an orbital shaker (PA250/25H) at a shaking rate of 120 rpm. The pH was adjusted using 0.1 M HCl and NaOH solutions within the range of 2–12. The sorbent dose varied from 0.05 to 0.4 g, the contact period ranged from 3 to 90 min, the initial dye concentration spanned from 10 to 400 mg/L, and the temperature was maintained between 30 and 65 °C.

In this context, represents the initial dye concentration in mg/L, while denotes the equilibrium dye concentration in mg/L. The volume of the solution in liters is represented by V, and W signifies the amount of adsorbent in grams.

The specific surface area was determined using a Micromeritics ASAP-2010C automated analyzer and the Brunauer–Emmett–Teller (BET) method at 77 K. Nano adsorbents were observed through scanning electron microscopy (SEM) analysis using an S-3400N HITACHI Japan instrument. X-ray diffraction (XRD) experiments were carried out on a D/max-3B diffractometer with Cu K irradiation and a scan rate of 0.02° 2θs⁻¹ to identify existing phases and determine crystallite sizes. Thermo Nicolet's Fourier Transform Infrared (FTIR) Spectroscopy was utilized to identify functional groups. Energy-dispersive X-ray (EDX) was employed for elemental identification.

The peaks at 1,392–1,121 cm⁻¹ showed that stretching vibrations of the –NH and –SO₃ as well as the –CO contributed. Peaks due to C–O–C vibrations arise between 1,000 and 1,091 cm⁻¹. Bands that form in the finger print zone, which is lower than 1,000 cm⁻¹, are challenging to identify owing to the intricacy of vibrations. Spectra bands at 3,112–3,427 cm⁻¹ emerged with much increased intensity in dye and effluent-loaded samples, indicating active engagement of hydroxyl and amine groups when associating with the sorbent surface. Bands between 2,829 and 2,930 cm⁻¹ that appeared in loaded spectra were slightly shifted, indicating the participation of functional groups involved in dye binding. The insertion of secondary amines and the C = O group responsible for the binding of dye cations caused the bands at 1,501–1,575 cm⁻¹ and 1,603–1,794 cm⁻¹ to shift. In this approach, FTIR spectra aided the identification of functional groups that were active and caused dye cations to adsorb (Bai et al. 2015; Chen et al. 2019).

N₂ physisorption measurement is shown in Figure 5(b); the N₂ physisorption isotherm can be classified as type IV. It was done on the composite to find out about the surface area, typical pore size, and pore volume of the adsorption pores that had formed on the adsorbent surface. The graph representing the composite's N₂ adsorption-desorption isotherm was constructed in Figure 5(b) and tested under ambient circumstances. In terms of surface area, total pore size, and pore volume, the W/CoFe₂O₄ composite was found to have values of 45.93 m²/g, 4.22 nm, and 0.3683 cm³/g, respectively. Tree TT/NiFe₂O₄ has surface area, total pore size, and pore volume of 70 m²/g, 2.65 nm, and 0.125 cm³/g, respectively, while PS/CuFe₂O₄, has 75.0 m²/g, 2.44 nm, and 0.182 cm³/g, respectively. The composite clearly exhibits a high surface area, larger pore size, and high pore volumes based on the aforementioned observations. Composites made of OP/ MnFe₂O₄ have BET surface areas and total pore volumes of 181.2 m²/g and 0.2957 cm³/g, respectively. The average pore size of OP/ MnFe₂O₄ composites was proven to be 6.21 nm, which is a part of the mesoporous structure (2–50 nm), as measured by the desorption branch (Chen et al. 2021).

The point of zero charge (pH_pzc) serves as a crucial metric for evaluating the adsorption capacity of a surface and determining the nature of surface binding centers. It represents the pH level at which the charge on the adsorbent surface reaches zero. This point is instrumental in elucidating the electrokinetic characteristics of surfaces. It's important to note that the pH_pzc value specifically pertains to media where the determination of potential is influenced by H+ and OH– ions (Yagub et al. 2014). The adsorption of cationic species is more favorable at pH values higher than the pH_pzc, while the adsorption of anionic species is favored at pH values lower than the pH_pzc. The pH_pzc values for the composite materials composed of OP, PS, TT, and W were determined using the solid addition technique (Mall et al. 2006).

Furthermore, it was observed that the adsorption of basic dyes is favorable at pH values greater than pH_pzc. The adsorption behavior of basic dye was explained based on the pH_pzc of the adsorbent. For PANI-TGSd, the pH_pzc value was determined to be 5. Consequently, at pH values less than 5, the adsorbent surface becomes cationic due to protonation. Conversely, at pH greater than 5, the surface turns negatively charged due to deprotonation, resulting from a higher concentration of OH̄ ions. Therefore, the favorable medium for the adsorption of Crystal Violet (CV) dye on the material surface is at pH greater than 5, as the material surface acquires a negative charge that facilitates the adsorption of basic dye (CV) (Mashkoor & Nasar 2019).

The pH value provides information about the amount of electrostatic charges on the surfaces of both adsorbing materials and dyes (Salleh et al. 2011). Besides affecting charge on the surface of the sorbent, the pH of the solution also influences the solubility of dyes (Subbaiah & Kim 2016). As a result, the rate of adsorption exhibits variability with changes in the pH of the dye solution. The impact of pH on the adsorption process of a basic dye, utilizing different ferrite BC composites, was investigated over a pH range from 2 to 12, while keeping other conditions constant (0.05 g/50 mL, shaker speed 120 rpm, temperature 30 °C, and time 60 min). The findings indicated that a basic pH was more favorable for the removal of Blue-XGRRL dye. The optimized values of pH for achieving the maximum adsorption capacity of OP-MnFe₂O₄, PS-CuFe₂O₄, TT-NiFe₂O₄ and W-CoFe₂O₄ for the removal of Blue-XGRRL dye were determined to be 11 (43.5 mg/g), 10 (37.8 mg/g), 10 (31.9 mg/g), and 10 (14.9 mg/g), respectively (Figure 9(b)).

As shown in Figure 9(b), dye absorption rose as the pH of the solution climbed and reached its maximum at pH 11 10, then reduced when the solution's pH level increased to a higher level. The pH points zero charger (pH_pzc) can be used to explain this phenomenon. The pH_pzc values of OP/MnFe₂O₄, PS/CuFe₂O₄, TT/NiFe₂O₄, W/CoFe₂O₄ are determined to be 6, 8, and 7, respectively, as stated above. Once the solution's pH exceeds pH_pzc, deprotonation of functional groups on the surface that are exhibiting electrostatic attraction for dye cations causes the net surface charge on the solid surface for the adsorbent to turn negative. Due to the presence of negatively charged sites on the adsorbent surface, a proper removal of dye above pH_pzc may be seen. Furthermore, even if the surface has a negative charge, it is still possible to produce a dye absorption in the alkaline values. This behavior is explained by the severe rivalry between the dye and hydrogen ions for binding to active sites.

From the aforementioned results, it is evident that the pH of the dye solution plays a crucial role in influencing the adsorptive removal efficiency of basic dyes when using different ferrite BC composite materials. The optimized values of pH for achieving the maximum removal efficiency of basic dye were found to be in a basic environment. This observation aligns with the findings of Rajabi and colleagues, who similarly reported a higher pH value for the adsorption of basic dye and a lower pH value for the adsorptive removal of anionic dye (Rajabi et al. 2015, 2016). The elimination degree of methylene blue by BiFeO₃/BC magnetic beads that are connected was influenced by pH. Adsorption amount and the rate of removal both improved significantly when the pH of the mixture was raised from 6.0 to 3.0. With such a pH of 3.0 and a large concentration of H⁺ cations, the mixture is highly corrosive. As a result, the BiFeO₃/BC using the linked magnetic materials is efficient in obtaining methylene blue in basic and moderate conditions (Cai et al. 2020).

The quantity of adsorbent is crucial for the removal of adsorptive dye as it reflects the adsorbent's capacity for a specific concentration of adsorbate in a stable reaction environment (Mahmoodi et al. 2010; Sadaf & Bhatti 2014). The impact of adsorbent dose (0.050.40 g) on the adsorption of basic dye was investigated for various ferrite BC composites, while maintaining constant reaction conditions (optimized pH, shaker speed 120 rpm, temperature 30 °C, and duration 60 min). The results revealed a decrease in the adsorption capacity of the adsorbent materials (OP-MnFe₂O₄, PS-CuFe₂O₄, TT-NiFe₂O₄ and W-CoFe₂O₄) from 44.2 to 5.6 mg/g, 38.9 to 4.1 mg/g, 32.6 to 3.1 mg/g, and 16 to 1.6 mg/g, respectively, with an increase in dosage from 0.05 to 0.40 g/50 mL at their optimum pH levels (11, 10, 10, and 10). The optimal dosage for achieving the highest removal of Blue-XGRRL dye for all ferrite composite materials was identified as 0.05 g/50 mL (Figure 9(c)).

According to all of the data above, adding more adsorbent doses caused a decrease in the adsorption of basic dyes. All ferrite BC composite materials were used in the least amount necessary to achieve the highest adsorption of basic dyes (0.05 g/50 mL dosage). The decline in adsorption potential at higher doses can be attributed to the overlapping of binding active sites. This overlap reduces the total surface area available for binding with dye cations and extends the diffusion path, causing a lack of dye molecules to adequately cover all the binding functional active sites on the exterior of the adsorbent. Consequently, this leads to lower solute uptake. Another contributing factor to the reduced adsorption potential of adsorbents at greater material dosages is the incomplete coverage of binding functional active sites by dye molecules (Sadaf et al. 2014; Foroutan et al. 2018). Furthermore, it was noted that an elevated quantity of adsorbent in the dye solution reduces the spacing between adsorbent particles, resulting in numerous active sites becoming vacant. A higher dose also induces aggregation, leading to a reduction in the available surface area for adsorption (Mashkoor & Nasar 2019).

The impact of dye contact time between the adsorbing material and adsorbate on the adsorptive removal of Blue-XGRRL dye using various adsorbents was investigated through a batch adsorption experiment with variable times ranging from 3 to 90 min. The results are presented in Figure 9(d), revealing a swift removal of Blue-XGRRL dye within the initial 40 min, reaching equilibrium by the 60th minute. The maximal adsorption capacity (45.7, 39.9, 33.9, and 18.9 mg/g) using various adsorbents (OP-MnFe₂O₄, PS-CuFe₂O₄, TT-NiFe₂O₄, and W-CoFe₂O₄) were obtained at 60 min of contact time. After that, adsorption capacity did not change considerably with increasing contact period. Hence, the optimum contact times for the blue-XGRRL dye were 60 and 45 min.

The graphs obtained depict that the adsorption process exhibited a rapid initial increase, followed by a gradual slowdown, and eventually reached a constant level after establishing equilibrium. After that, adsorption capacity did not change considerably with increasing contact period. Such adsorption behavior occurs because there are initially more active functional binding locations outside of the adsorbent that subsequently get engaged with adsorbate molecules. Subsequently, when dye molecules start to diffuse into the adsorbent bulk, the adsorption rate decreases. Electrostatic attraction between positively charged dye molecules that have been adsorbed or other positively charged sorbate molecules that are still present in the reaction medium may also contribute to a lower adsorption potential with an increase in reaction time.

A batch study was conducted to assess the impact of varying initial dye concentration (50 − 400 mg L⁻¹) on the adsorption potential of various BC materials for specific basic dyes at their respective optimal pH, with a dosage of 0.05 g/50 mL adsorbent, temperature set at 30 °C, equilibrium time, and shaking speed of 120 rpm. The highest adsorption capacities of OP-MnFe₂O₄, PS-CuFe₂O₄, TT-NiFe₂O₄, and W-CoFe₂O₄ for Blue-XGRRL at initial dye concentrations of 100, 200, and 75 mg/L were determined to be 46.8, 41.4, 37, and 21.1 mg/g, respectively (Figure 9(e)).

All findings indicate a significant increase in the adsorption efficiency of all ferrite BC composites with an increase in the initial concentration of the dye, possibly due to reaching the saturation point of the adsorbing material. This trend may be attributed to the presence of a substantial driving force resulting from the initial concentration of the dye, which enhances the diffusion rate of the dye toward the adsorbing material. Additionally, the results reveal a similar trend indicating incomplete adsorption of dye at higher concentrations, suggesting the existence of a saturation limit for each hybrid material beyond which it does not adsorb any more dye. At higher dye concentrations, lower adsorption capacity is observed because the adsorption sites become saturated. In contrast, at lower dye concentrations, all dye molecules already present in the aqueous media are adsorbed on all available binding sites on the adsorbent surface, resulting in a larger capacity for adsorption (Almasian et al. 2015a).

Temperature plays a significant role in the adsorptive behavior of dyes, making it a crucial parameter to consider when designing an adsorption system applicable to actual effluents. Under constant reaction conditions, the impact of temperature variations between 30 and 65 °C on the capacity of various adsorbents to remove Blue-XGRRL dye was examined. The adsorption potential of all types of adsorbent materials (OP-MnFe₂O₄, PS-CuFe₂O₄, TT-NiFe₂O₄ and W-CoFe₂O₄) for removing Blue-XGRRL dye from aqueous solutions considerably decreased from 46 to 28.1, 40.8 to 21.8, 35.3 to 17.2, and 24.8 to 8.8 mg/g, respectively, with an increase in temperature in the range of 30 − 65 °C. All the results indicated a significant decrease in the adsorption potential of all adsorbents for removing Blue-XGRRL dye, suggesting the exothermic nature of the adsorption processes. The maximum adsorption capacity for all adsorbent materials was achieved at 30 °C for the removal of the basic dye (Figure 9(f)).

All the data pointed toward a significant reduction in the capability of all adsorbents to remove basic dyes, indicating the exothermic nature of all adsorption pathways. The maximum adsorption capacity for removing all basic dyes using all adsorbent materials was observed at 30 °C. The decline in the adsorption of basic dyes at higher temperatures may be attributed to the weakening of the sorption forces responsible for attaching the dye ions to the adsorbent surface. It also investigated a substantial impact of rising temperature on basic dye elimination utilizing chitosan aniline composite. A higher dye adsorption was achieved at lower temperatures, and the dye removal decreased as the temperature was increased.

The observed decrease in adsorption potential when adding the cationic surfactant (CTAB) may be attributed to the development of greater electrostatic repulsive forces between CTAB molecules and dye cations. Similar investigations on the influence of surfactants on the adsorption of dyes (MB and ARG) have shown that an increase in the concentration of CTAB or SDBS leads to a reduction in the adsorption of dyes. This reduction in adsorption can be explained by the competitive adsorption of surfactant molecules with dyes, where surfactants may occupy and deplete active sites on the adsorbent surface. Furthermore, the formation of micelles by dye molecules with the aid of surfactants can lead to an enlargement of dye molecules, making them less adsorbent. The combination of these factors contributes to the overall decrease in adsorption capacity (Hosseini et al. 2016; Abid et al. 2023). Similarly, studies have described a similar impact of the presence of surfactants on the adsorption of a reactive dye using sugarcane bagasse as the adsorbent. The introduction of SDS in the dye solution led to a significant decrease in the adsorption potential of the adsorbents for the reactive dye. This observation further supports the notion that certain surfactants can adversely affect the adsorption capacity of adsorbents for various dyes.

In the textile industry, a considerable amount of salts is employed during the dyeing of fibers. The quantity of electrolyte in textile effluents plays a crucial role in regulating both electrostatic and non-electrostatic interactions between dye molecules and the surface of the adsorbent. The study investigated the impact of various electrolyte levels ranging from 0 to 0.5 M, including AlCl₃·6H₂O, on the adsorption capacity of various adsorbent materials for the removal of their corresponding basic dye, Blue-XGRRL, under standard operational conditions. The outcomes are presented in Figure 10(e) and 10(f). The findings from the current study suggest that the adsorption capacity of all the utilized adsorbents for removing their specific basic dyes decreased with an increase in electrolyte concentration. This decrease is attributed to the screening of electrostatic attractive forces between the adsorbent and dye molecules caused by the presence of electrolyte ions (Gong et al. 2005).

Similarly, studies have explored the influence of the ionic concentration of sodium sulfate (Na₂SO₄) on the adsorption capacity of ferrites. The findings indicated that an increase in ionic concentration decreases the electrostatic repulsive forces among adsorbate molecules, thereby enhancing the adsorption amount. Furthermore, in the study of the impact of increasing amounts of Na₂SO₄ on Polypyrrole (PPy)/TiO₂, it was observed that the rise in salt concentration led to the contraction of the adsorbent and a reduction in pore size, causing a decrease in electrostatic attractive forces between the adsorbent and adsorbate. These factors collectively contributed to a decrease in adsorption capacity. Similarly, the adsorption of a basic dye onto a starch composite of polyacrylonitrile (PAN) was found to decrease in the presence of NaCl, CaCl₂, KNO₃ and MgSO₄. Additionally, the presence of NaNO₃ salt (0.04 − 0.2) decreased the adsorption ability of Pyracantha coccinea for the removal of acid red 44 dye by 15%. This behavior was attributed to nitrate anions competing with sulfonate groups belonging to the dye for binding sites on the adsorbent surface (Akar et al. 2010).

The adsorbent's regeneration ability (Figure 11(c)) was examined. The nano adsorbent was repeatedly cleaned with distilled water after adsorption, and it was then briefly dried at 25 ± 2 °C before being used again for adsorption. For the first two cycles, the removal efficiencies were constant and then marginally declined. The elimination efficiency was further decreased to 40.01–19% after the second cycles. The decline in adsorbent absorption efficiency may be caused by the nanocomposite's loss of surface shape after repeated washings (Ajibade & Nnadozie 2022). Magnetized ferrite BC composites' potential for leaching was investigated. However, as can be seen from the Figure 11(d), the concentrations of magnetized ferrite BC composites in the leachate were determined to be extremely low. This could be because the behavior of magnetized ferrite BC composites during their dissolution and precipitation varies depending on the pH of the solution (Devi & Saroha 2015). The highest % desorption of dye was achieved with 0.5 M concentration that was due to greater electrostatic repulsive force among dye ions as well as adsorbent functional groups causing detachment of dye molecules from adsorbent surface.

The kinetic parameters were determined using pseudo-first-order kinetics, pseudo-second-order kinetics, intraparticle diffusion kinetics, and Elovich model. Detailed information is available in Supplementary data (Text S1). The results suggest that both pseudo-first-order and pseudo-second-order kinetics are applicable to the kinetic data obtained from batch adsorption experiments for all adsorbents. The selection of the pseudo-second-order model as a better fit was based on higher R² values (0.99) and the closer agreement between theoretical and experimental q_e values. The lower R² values (0.90–0.92) compared to the other two models indicated a lesser fit of the intraparticle diffusion model to the adsorption data of all adsorbents for the removal of Blue-XGRRL. The pseudo-second-order kinetic model, assuming chemical sorption or chemisorption as the rate-limiting phase, is deemed predictive across the entire adsorption range (Table 1).

Important parameters related to Langmuir, Freundlich, Temkin, Harkins-jura and D-R biosorption isotherms such as R², theoretical and practical q_e values for used basic dye Blue-XGRRL, are recorded in Table 2. Detailed text is present in Supplementary data (Text S2). The applicability of mentioned sorption isothermal models on sorption mechanism of chosen basic dyes utilizing their corresponding adsorbents was analyzed. Better fitness to sorption isotherms was decided on behalf of higher value of correlation coefficient (R²) and closeness among q_m calculated and q_m experimental.

The Langmuir sorption isotherm was found to fit the equilibrium sorption data of Blue-XGRRL dye better for the respective adsorbents OP-MnFe₂O₄, PS-CuFe₂O₄, TT-NiFe₂O₄, W-CoFe₂O₄, yielding high values of R² (0.94 − 0.99) and a close agreement between theoretical and experimentally determined values of q_m. In contrast, the Temkin and Harkins-Jura sorption isotherms showed poor fits to the equilibrium sorption data, while the D − R sorption isotherm demonstrated good fitness only for PS-Cu Fe₂O₄ in the removal of Blue-XGRRL. The results indicate that the Langmuir isotherm model has a greater potential to describe the isotherm behavior of the adsorption process, as the correlation coefficient (R²) calculated using the Langmuir isotherm model is closer to 1 compared to the Freundlich model (0.57–0.74, respectively). Additionally, the value of n is greater than 1, indicating that the adsorption process is physical in nature (Abshirini et al. 2019).

Thermodynamic analysis is crucial for comprehending the nature of the adsorption process (Mahmoodi et al. 2011). Adsorption thermal data for the removal of Blue-XGRRL dye using their respective adsorbent materials was utilized to determine various thermodynamic parameters, such as the change in Gibbs free energy (ΔG), entropy (ΔS), and enthalpy (ΔH) (refer to Supplementary data (Text S3)). Numerical values for each thermodynamically significant criterion are documented in Table 3. The negative values obtained for ΔH in relation to basic dye indicate that all sorption processes for removing basic dyes are exothermic in nature. Simultaneously, the negative values of ΔS suggest a reduction in disorder at the solid and liquid interface for all sorption processes involved in removing basic dyes using their corresponding adsorbent materials. The positive values of ΔG imply a non-spontaneous nature of the sorption process.

Table 4 outlines a comparison of the dye removal efficiency between golden marble and its nanocomposites in relation to other adsorbents. The results indicate that the composite materials exhibited a greater capacity for dye removal compared to the previously documented adsorbent.

OP/MnFe₂O₄, PS/CuFe₂O₄, TT/Ni Fe₂O₄, and W/CoFe₂O₄ were used for the adsorption of Blue-XGRRL dye in batch studies. It was characterized by XRD, SEM, EDX, FTIR spectroscopy, and BET analysis. The optimum pH for OP/MnFe₂O₄, PS/CuFe₂O₄, TT/Ni Fe₂O₄, and W/CoFe₂O₄ were found to be at 11, 10, 10 and 10 respectively, contact time of 60 min, the temperature of 30 °C, the initial chromium ion concentration of 100 mg/L and the adsorbent dose of 0.05 g/mL, and under these conditions, the removal efficiency of dye by the adsorbent with maximum adsorption of 46.90, 41.89, 35.33, and 25.09 mg/g. The pseudo-second-order and Langmuir sorption isotherm were fitted well to the dye sorption data with maximum sorption capacities. All sorption processes to remove basic dyes are exothermic in nature. Furthermore, the use of green adsorbents, such as OP, PS, TT, and W, highlights the environmentally friendly nature of our approach. These readily available and sustainable materials not only demonstrate effective dye removal but also contribute to the promotion of green and eco-friendly solutions for wastewater treatment. The incorporation of BC composites as green adsorbents opens avenues for developing innovative, cost-effective, and environmentally conscious strategies for addressing water pollution challenges. The primary challenge encountered in this research pertained to the separation of the adsorbent from the dye. Future perspectives include exploring wider applications of these adsorbents and further research on their long-term efficacy in diverse environmental conditions, emphasizing their potential significance in sustainable wastewater treatment strategies.

The authors would like to extend their sincere appreciation to the acknowledgment and research supporting project (RSP-2024/95, King Saud University, Riyadh, Saudi Arabia).

All authors have read and approved this manuscript.

H.A. was involved in methodology, conceptualization, writing – original draft, collected resources, S.N. was involved in supervision, conceptualization, formal analysis, writing – reviewing and editing, R.M. was involved in Conceptualization, formal analysis, writing – reviewing and editing, G.A. was involved in resource collection, writing-review and editing, A.M. was involved in statistical analysis, writing-review and editing, M.S. was involved in execution, writing-review and editing, F.Y. was involved in conceptualization, interpretation, A.M. was involved in execution, writing-review and editing.

Research does not involve human participants or/and animals.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.