A novel synthesis of Fe₂O₃@activated carbon composite and its exploitation for the elimination of carcinogenic textile dye from an aqueous phase

Abstract

The present work reports the synthesis of Fe₂O₃@activated carbon composite by the co-precipitation of iron salts onto activated carbon. The prepared composite was explored for the remediation of a carcinogenic textile dye, Remazol Brilliant Blue R (RBBR) from an aqueous solution. The surface morphology, composition and textural characteristics of the prepared magnetic composite (FAC) were investigated by FTIR, SEM-EDS, P-XRD, TEM, selected area electron diffraction (SAED), VSM, TGA, surface area and pore size measurements. TEM images showed that the prepared composites were wire-shaped. The saturation magnetization value (26.99 emu g⁻¹) was sufficient for magnetic separation in wastewater treatment. The unique micro-/meso-porous structure, high surface area (1199 m² g⁻¹), and pore volume (0.9909 cm³ g⁻¹) further enhance its utilization for the treatment of dye laden wastewater. The pseudo-second-order kinetic model with high correlation coefficients (R² > 0.999) was suitable to described the process of RBBR adsorption onto FAC. The Langmuir model fitted the adsorption isotherm data better than the Freundlich, Temkin and DR model. Values of thermodynamic parameters (namely, ΔG°, ΔH° and ΔS°) indicated that the adsorption process was strongly dependent on the temperature of the aqueous phase, and it was spontaneous and exothermic in nature. Therefore, FAC composite displays main advantages of excellent dispersion, convenient separation and high adsorption capacity, which implies their potential application in environmental clean-up processes.

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Introduction

The rapid industrialization, urbanization, and unplanned activities of human beings have significantly enhanced the environmental pollution. Every year, world-wide, about 50 000 tons of dyes are discharged into the environment¹ causing a serious environmental impact in the ecosystem. Nowadays, government legislation is becoming stricter regarding the removal of carcinogenic and toxic dyes from industrial effluents, especially in the more developed countries.² The industrial effluents are toxic and characterized by high biological oxygen demands (BODs), chemical oxygen demands (CODs), suspended solids and intense color.³ Dyes producing and consuming industries, such as food, paint, plastic, cosmetics, printing and pharmaceutical industries, generate a huge volume of toxic wastewater contaminated with coloured synthetic dyes.⁴ These dyes in aquatic systems interfere with the photosynthesis of aquatic plants/life and hinder the growth of microbes; moreover, the mutagenic and carcinogenic nature of dyes is a threat to the human health.⁵ Due to the synthetic origin and aromatic nature, dyes are biologically non-degradable, and thus it is rather difficult to treat dye containing wastewater.⁶ Remazol Brilliant Blue R dye (RBBR) is one of the most important dyes in the textile industry. It is frequently used as a starting material in the production of polymeric dyes. It is an anthracene derivative and represents an important class of toxic and recalcitrant organo-pollutants.

Activated carbon (AC) is one of the most widely used and a versatile adsorbent material for the removal of organic and inorganic contaminants from wastewater.^7–9 However, AC is difficult to retrieve, separate, and regenerate when it is exhausted. The traditional method for separating the powdered AC can cause blockage of filters and loss of AC. The traditionally discarded sludge process of the spent adsorbent without further treatment leads to generation of secondary pollution.^10,11 The difficulties of powder AC are overcome by magnetic AC composite, an effective and low-cost adsorbent, which is attracting a lot of attention nowadays. Functional nanoporous materials, nowadays, are widely used as efficient adsorbents for environmental cleaning purpose.^12–14 Magnetic filtration is an emerging technology in wastewater treatment processes, which can provide rapid and efficient contaminant removal from aqueous waste streams.¹⁵ Inexpensive adsorbents could be developed that can bind to environmental contaminants, and then be magnetically separated. The advantages of magnetic activated carbon adsorbents over the traditional adsorbents are that it can be easily separated from solution using a magnetic separator even if the solution contains a significant concentration of solids.

The present work addresses the development of eco-friendly, low-cost activated carbon from a biomass source and further magnetized by co-precipitation of iron salts onto activated carbon for the treatment of simulated reactive dye effluent (RBBR). An iron oxide/activated carbon magnetic composite was characterized by FTIR, SEM-EDX, TEM, P-XRD, VSM, surface area and pore size measurements. The adsorption behavior of the composite material was analysed in the batch mode, studying the impact of operational parameters (pH, adsorbent dose, temperature, and interaction time). The experiments have been also carried out to observe the impact of ionic strength, salt, and hardness on the removal of RBBR from an aqueous phase. A simulated mixture was also prepared to assess the effect on the removal of RBBR. The adsorption process was carried out in a batch system with emphasis on kinetics, isotherm modeling and error analysis.

Materials and methods

Materials

The dye used as target molecule for the adsorption evaluation of magnetic carbon was Remazol Brilliant Blue R (acronym: RBBR; MF: C₂₂H₁₆N₂Na₂O₁₁S₃; MW: 626.54 g mol⁻¹; λ_max = 595 nm; purity = 99%), procured from Sigma-Aldrich. All the chemicals were purchased from Merck (India) in analytical purity and used in the experiments directly without further purification. All the solutions were prepared using ultrapure water from a Millipore water purification system (Model: ELIX 3S KIT (IL), France).

Development of activated carbon and synthesis of Fe₂O₃@activated carbon composites

The precursor, Schumannianthus dichotomus (SD) was obtained from a nearby village. It was dried in an oven at 383 K for 24 h and saturated with a sufficient amount of concentrated ortho-phosphoric acid. The impregnation time was 1 h to obtain a uniform mixture. Then, acid impregnated SD was carbonized at 773 K in a muffle furnace (Alfa Instruments) for 1 h at a constant heating rate of 283 K min⁻¹ under an inert atmosphere. The carbonized materials were washed with pure water, dried, grounded and stored in the dessicator until use.

The impregnation of AC with hematite was achieved by the co-precipitation of the two salts of iron (FeCl₃ and FeSO₄) on activated carbon. A freshly prepared FeCl₃ (3.46 g in 250 mL) and FeSO₄ (13.3 g in 100 mL) solution were mixed and vigorously stirred at 333 K for 30 min. The mixed solution of iron salts was successively added into the previously prepared aqueous suspension of activated carbon at 303 K and stirred with a magnetic stirrer for 40 min. The resulting solution was precipitated by adding dropwise 2 M NaOH solution with continuous vigorous stirring. During the reaction process, the pH was maintained at about 10–11. The suspension was further mixed for 1 h and aged at room temperature for 24 h. The suspension was repeatedly washed with distilled water followed by ethanol and then finally dried in a vacuum oven at 353 K. The developed adsorbent was denoted as magnetically activated Schumannianthus dichotomus (FAC).

Analytical techniques for characterization

Spectroscopic characterization (FTIR, SEM-EDX, P-XRD, TEM, SAED). Fourier transform infrared (FT-IR) spectroscopic analysis of the adsorbent sample was performed on a Nicolet MAGNA-550 FTIR spectrophotometer in the range of 400–4000 cm⁻¹ to characterize the functional groups on the surface of the adsorbent.

The texture and morphology of the adsorbent were examined by a scanning electron microscope (FEG-SEM, Model: JSM-7600F, Magnification: 25× to 1 000 000). The sample was first coated with gold using Sputter Coater, Edwards S150, which provides conductivity to the sample, and then the SEM micrographs were obtained.

Powder X-ray diffraction (XRD) patterns were obtained on an X-ray diffractometer (Model: Philips X'pert MPD system) with X'pert software using a 30 mA, 40 kV, Cu Kα radiation (λ = 1.546 Å) at a scan rate of 1° min⁻¹. The diffraction peaks are indexed by comparing with JCPDS database. The interlayer spacing (d_{h k l}) was determined using the Bragg equation given as follows:

where λ is the wavelength of the X-ray used and θ is the scattering angle. The crystallite size is determined from the half width of the diffraction peak using the Scherrer equation as follows:where L is average crystallite size, B is the full width half maxima of the peak, and K is the shape factor.

The TEM images and SAED pattern of magnetic and non-magnetic activated carbons were recorded using PHILIPS, CM200 transmission electron microscope operated at an accelerating voltage: 20–200 kV; resolution: 2.4 Å. Samples were deposited on a carbon coated grid and the surface morphology was studied.

Textural characterization of the carbon samples (surface area and pore size). Micromeritics, ASAP 2010 surface area analyzer was used to determine the textural features of the adsorbent. N₂ adsorption–desorption isotherm at 77.71 K in the relative pressure (P/P₀) range was used to determine the Brunauer–Emmett–Teller (BET) surface area by using the equation of Brunauer et al. BJH (Barrett–Joyner–Halenda) desorption method was used to compute the cumulative volume and average pore diameter of the pores. The particle size analysis of the adsorbent was performed on a Malvern Mastersizer 2000 particle size analyser.

Hysteresis and magnetic moment measurements. The magnetic properties of a ferromagnetic material are represented by plot of magnetization (M) against the field strengths giving the hysteresis loop. The hysteresis measurements were carried out on a Lakeshore VSM 7410 Susceptometer at room temperature (300 K) in a magnetic field that varied from −15 000 Oe to +15 000 Oe.

Experimental procedure and data analysis

The retention of RBBR in a batch system was carried out in a series of 100 mL stoppered Erlenmeyer flasks to examine the effects of influencing parameters, viz., initial pH (pH_o), interaction time (t), adsorbent load (m), and temperature (T). The experiments were conducted in a temperature controlled incubator-cum-shaker (shaking speed: 180 rpm) at various temperatures (303, 313, and 323 K). At the end of the pre-determined time, the samples were withdrawn, centrifuged at 5000 rpm for 2 min and the supernatant was analyzed for the residual concentration of RBBR.

The residual concentration in the supernatant solution was determined using a UV-visible spectrophotometer (Thermo Scientific) at λ_max = 595 nm. The percentage removal and adsorptive uptake of RBBR were estimated using the following mass-balance equations:

where C₀ = initial RBBR concentration (mg L⁻¹), C_e = equilibrium RBBR concentration (mg L⁻¹), V is the volume (L) of RBBR sample taken, and m is the weight (g) of the adsorbent.

Results and discussion

Interpretation of characterization analysis

FTIR spectroscopy. The functional groups of FAC can be better understood from a FTIR (Fourier transform infra-red) study of the adsorbent. The spectra can therefore help in the interpretation of the functional groups accountable for adsorption. The FTIR spectra of AC and FAC are presented in Fig. 1. The AC displays absorption bands at 3420, 2380, 1725 and 1180 cm⁻¹. The FTIR spectrum of AC depicts a relatively broad peak at 3420 cm⁻¹, which is due to H-bonded OH stretch, confirming the presence of a hydroxyl group. The position and shape of the band at 3420 cm⁻¹ are compatible with the involvement of hydrogen-bonded hydroxyl groups. The OH groups of alcohol, phenol and carboxylic acid are located at 3625 cm⁻¹, 3605 cm⁻¹ and 3530 cm⁻¹, respectively. The self-associated OH groups centered at 3400 cm⁻¹ predominate in AC. The band at 3420 cm⁻¹ is slightly broader towards lower wavelengths suggesting that some OH–ether hydrogen bonds are present. The appearance of weak bands at 1300–1000 cm⁻¹ indicate the presence of both hydroxyl and ether type C–O structures in AC.

Fig. 1 FTIR spectra of (a) AC and (b) FAC.

FAC exhibits peaks at 3411, 1580, and 936 cm⁻¹ and two additional overlapped bands at 692 and 587 cm⁻¹, compatible with the presence of iron oxides in the prepared composite. These bands were situated very close in frequency for iron oxide (γ-Fe₂O₃) in the carbon matrix.¹⁶

Energy dispersive X-ray spectroscopy. The SEM-EDX analysis spectra of AC, before and after magnetization are shown in figure (Fig. 2(a) and (b)). The elemental composition of AC is carbon, oxygen, nitrogen, and no peak for iron was observed. Fig. 2(b) confirms the presence of Fe, O and C on the FAC surface. The spectra shows that the atomic percentage of oxygen of FAC is increased (from 10.63% to 24.73%), indicating the formation of iron oxide on the surface. The appearance of additional peaks for phosphorous is probably because of the activating agent, H₃PO₄.

Fig. 2 (a) EDS spectra of AC. (b) EDS spectra of FAC.

Transmission electron microscopy and SAED pattern. Transmission electron micrographs of FAC are shown in Fig. 3(a–d), and the morphology is found to be different from AC. The prepared FAC was observed to be wire-shaped with a typical diameter in the range of ∼57.07–218.36 nm. Fe₂O₃ interacts with the surface of the AC by chemical linking.¹⁷ This may lead to the change in the surface morphology of FAC. However, further study is required to know more about any changes in the morphology. The individual properties of metal oxide have been modified by encapsulating them in carbon material such as activated carbon.¹⁸ The appearance of diffraction rings and bright spots represent the higher degree of crystallinity of the particles, as shown in the SAED pattern of FAC (Fig. 3(c) and (d)). The spots of FAC before magnetization could be indexed as (1 2 0), (0 0 2) and (1 1 0). The additional spots in Fig. 3(d) could be indexed to the reflections from the (0 1 2), (1 1 0), (2 0 2), (0 2 4), (0 1 8), (3 0 0), and (2 1 4) lattice plane, which efficiently matched with the JCPDS values (89-2810) of hematite, iron oxide in the pores of the carbon matrix.

Fig. 3 (a) TEM image of AC. (b) TEM image of Fe₂O₃. (c) TEM image of FAC (low magnification) (d) TEM image of FAC (high magnification) (e) SAED pattern of AC (f) SAED pattern FAC.

Crystalline properties (P-XRD). The powder XRD pattern of AC before magnetization (Fig. 4(a)) portrays broad diffraction peaks at around the 2θ values of 14.86°, 22.94° and 26.47°, which can be ascribed to the hexagonal carbon-like reflections (JCPDS file no. 50-0926). The stack of parallel layers results in the highest intensity peak at 2θ = 26°, signifying the existence of graphitic ordering in the molecular planes, and the interlayer spacing d_{(1 0 3)} = 0.336 nm is calculated with the help of eqn (1). It should be noted that the d value is greater than that of graphite (0.335 nm), which confirms the disordered framework far from graphitization. The average crystallite size computed from the Scherrer equation is 11.18 nm.

Fig. 4 P-XRD of (a) AC and (b) FAC.

The hematite impregnation of activated carbon showed iron content of 26.58% in a magnetic sample. The co-precipitated iron component was examined by X-ray diffraction patterns. The X-ray diffraction pattern for FAC (Fig. 4(b)) displayed a number of prominent Bragg reflections, which are compatible with the presence of Fe(OH)₂ and Fe(OH)₃ (peaks at 2θ = 36.63°, 38.85°, 39.48° and 60.746°) and of Fe₂O₃ (peaks at 2θ = 49.437°, 53.64° and 63.43°). The diffraction peak at 2-theta values of 24.2°, 33.2°, 35.7°, 40.9°, 49.5°, 54.1°, and 63.3°, and the relative intensity are reasonably close to the standard data for rhombohedral centered hematite, iron oxide lattice (JCPD file no. 89-2810). From Fig. 4(b), the appearance of five peaks are indexed as (1 1 0), (0 0 6), (1 1 3), (1 1 6), and (3 0 0). The presence of these peaks efficiently matched with the JCPDS value (89-2810) of Fe₂O₃, corroborating the presence of iron oxide nanoparticles in the pores of carbon.

These iron oxides maybe formed via the following tentative reaction paths. The solubility product constant for Fe(OH)₃ (= 10⁻³⁶) is higher than that of Fe(OH)₂ (10⁻¹⁴ to 10⁻¹⁷). Initially, as the pH increases, Fe(OH)₃ precipitates during the magnetization of AC.

Step-I: ferrous hydroxide may be oxidized by dissolved oxygen.

2Fe(OH)₂ + 1/2O₂ + H₂O → 2Fe(OH)₃

Step-II: during oven-drying by air:

2Fe(OH)₂ + 1/2O₂ → Fe₂O₃ + 2H₂O

Hysteresis and magnetic moment measurement. The suitability of magnetic material for application depends on the characteristics shown by their hysteresis loops,¹⁹ obtained from the plot of magnetization (M) against the field strengths. Magnetization properties were investigated at 300 K by a vibrating sample magnetometer (VSM), which quantified the magnetic behavior. FAC exhibited a paramagnetic behavior, characterized by strong magnetic susceptibility. The saturation magnetization value of FAC was found to be 26.99 emu g⁻¹, which is strong enough for convenient magnetic separation. The coercivity and retentivity of FAC determined from VSM data are 0.303 G and 2.305 emu g⁻¹, respectively. The carbon sample without iron impregnation does not show any magnetic properties at 300 K (Fig. 5).

Fig. 5 Magnetization curves of FAC.

Textural characteristics. Porous and high surface areas are important features of a good adsorbent for wastewater treatment. The RBBR dye molecules can easily adhere on the surface of FAC because the BET surface area was found to be 1199.98 m² g⁻¹. The single point surface area 1164.87 m² g⁻¹ was almost comparable to the BET surface area. According to IUPAC classification, N₂ adsorption–desorption isotherm plot of FAC belongs to type-II. Pore structure and pore volume are important factors for the efficient adsorption of dyes. If the pore diameter of adsorbent is lesser than the pore diameter of the adsorbate molecules, then lesser adsorption would take place due to steric hindrance. The pore size distribution of the FAC computed by the BJH method is shown in Fig. 6. FAC shows very narrow pore size distribution. The fractions of pores open at both ends were found to be nil. At a relatively low pressure of N₂ adsorption–desorption isotherm, a hysteresis loop was observed. The pore distribution shows that pores are in a diameter range of 1–18 nm with an average diameter of 4.2 nm. The targeted dye, RBBR, having a diameter of 2.1 nm can easily penetrate into the pore of the FAC.²⁰ According to the IUPAC classification of pore dimensions, there are three broad classifications grouped as micropore (diameter, d ≤ 2 nm), mesopore (2 < d < 50 nm) and macropore (d ≥ 50 nm).²¹ Most of the pores are mesopores, followed by a small fraction of micropores. The total pore volume (single point adsorption) of FAC is 0.9909 cm³ g⁻¹ and can verily accommodate the dye molecules. The investigation on pore volume and surface area revealed that the prepared composite may be suitable for the adsorption of RBBR molecules from wastewater.

Fig. 6 Pore size distribution of FAC.

Thermogravimetric analysis. The thermal degradation characteristics of pre- and post-activated adsorbent are determined by thermogravimetric analysis. The TGA curves (Fig. 7) of AC, before and after activation exhibited two main weight loss regions. The weight loss (up to 70%) region from 250–350 °C shifted to 500–600 °C after activation. Thus, activation leads to an increased stability of the prepared adsorbent. After activation, a small weight loss was observed below 100 °C, which can be attributed to the dehydration and elimination of oxygen-containing functional groups from the surface. The second rapid weight loss region after activation may be attributed to the decomposition of organic matter. The maximum degradation temperature after activation increases from 500 °C to 700 °C. This indicates that the stability of the prepared adsorbent increased after activation by phosphoric acid.

Fig. 7 TGA curve of AC before and after activation.

Optimization of operational parameters (pH, adsorbent load, interaction time and temperature)

The zeta potential of the adsorbent, degree of ionization, and solubility of the adsorbate are remarkably affected by the pH of the medium. The adsorption system usually depends on the degree of speciation of adsorbate and the dissociation of functional groups of the adsorbent, which varies accordingly at different pH values.²² The percentage removal of RBBR was studied as a function of pH (3–9). The maximum removal percentage of 99% for RBBR was observed at pH of 3–4. In aqueous solution, the reactive dye molecule dissolved and converted into its corresponding dye anions, namely, DSO₃⁻ and Na⁺. These dissociated dye anions migrated from solution to the surface of FAC and adsorption occurred through the electrostatic attraction. When the pH was increased (4–9), the electrostatic interaction between the dye anion and composite became repulsive and the competition between OH⁻ ion and the reactive dye anions had also reduced the adsorption rate of dyes on FAC. This was confirmed by the pH_pzc value (6.8) of the FAC composite. Generally, the adsorption of dye anions is highly favored at pH < pH_pzc.²³ Thus, the limited adsorption of RBBR was observed in neutral and alkaline pH, and it could be due to the combination of other factors such as van der Waal's forces and hydrogen bonding.

The experiments were performed to optimize the load by agitating 25 mL of RBBR solution (100 mg L⁻¹) with varying adsorbent. The uptake of RBBR remarkably enhanced up to 0.8 g L⁻¹, and thereafter a plateau is obtained. The removal efficiency remained unaffected for further increases in the adsorbent load. At the initial stage, the high concentration of RBBR in the bulk solution increases the adsorbing tendency of the RBBR molecules in the unoccupied active site of the FAC. On the contrary, at the later stage, the efficiency of the adsorbent shows marginal increment due to the overcrowding of the particles. This stage indicates the attainment of the equilibrium of RBBR molecules at the surface of FAC. A comparison of % removal of FAC with AC showed that there is no significant increase in the adsorption efficiency of FAC (Fig. 8). However, the presence of Fe₂O₃ in the composite indicates mainly the magnetic separation of dyes from the aqueous phase and that the activated carbon is responsible for the adsorption.^24,25 They can be easily recovered by an external magnetic field and can be reused.

Fig. 8 Comparison of % removal of RBBR on AC and FAC.

The experiments were performed to evaluate the interaction time for the uptake of RBBR onto FAC. The removal of 94.82% was achieved in the initial 10 min of the interaction time followed by slow increment of adsorption with a time lapse of up to 60 min. A saturation condition was achieved thereafter, which is indicated by a plateau-line to a maximum contact time of 120 min. The high concentration gradient of RBBR solution on the surface of the FAC aids a bulk transport of RBBR molecules from the liquid phase to the solid surface of the adsorbent. We have observed that after 80 min of interaction time, the rate of increment is negligible. Therefore, it can be concluded that the quasi-equilibrium state was reached at the interaction time of 60 min. Therefore, the optimum interaction time of 60 min was chosen to execute the subsequent adsorption experiments.

The solution temperature is one of the key parameters to impact the rate of adsorption. The experiments were executed under the reaction conditions varied from 303 K to 323 K, which portrayed that the adsorption capacity of RBBR on FAC decreased with increase in temperature (Fig. 9). This may be due to the fact that the entropy of the RBBR molecules increases on the surface, which leads to the movement of adsorbate from the solid surface to the liquid solution phase. On the basis of adsorption capacity, the optimum reaction temperature was maintained at 303 K to perform subsequent experiments.

Fig. 9 Impact of temperature on the adsorption capacity of RBBR.

Impact of water characteristics (Ca²⁺, Mg²⁺, SO₄²⁻, Cl⁻, and SDS) on the removal of RBBR

The adsorption capacity of FAC for RBBR in the presence of Ca and Mg was compared by performing a blank experiment. The result suggested that the presence of hardness producing salts, such as Ca and Mg, has little effect on the adsorption of RBBR (Fig. 10). The percentage removal is decreased from 99% to 97%. The decrease in efficiency can be attributed to the competitions of Ca and Mg for occupying the active site of the adsorbent, which leads to earlier saturation of the surface in the simultaneous presence of these divalent ions.

Fig. 10 Influence of hardness on the retention of RBBR.

The real dye industry wastewater contains different concentration of salts apart from the target dyes. Therefore, it is very important to investigate the impact of chlorides, sulphates, and surfactant on the adsorption of RBBR. The influences on adsorption were studied by varying the concentration (from 0 to 70 mM), and the results are presented in Fig. 11. The study reveals that the presence of these salts has impact on the adsorption of RBBR on FAC. The percentage removal was reduced with the increase in concentration of anions. The impact on the removal efficiency was maximum in the case of surfactant and followed the order: surfactant > sulphate > salt > chloride. The removal percentage was reduced to 93.56%, 95.3%, 94.49% and 97.36%, for surfactant, sulphate, salt and chlorides, respectively, whereas almost 99% removal was achieved in the absence of these anions. The decline in the efficacy of adsorbent may be due to the fact that the active site of the adsorbent was blocked by these anions. The electrostatic repulsion may also play a role in decreasing the adsorption capacity of the prepared composite.

Fig. 11 Impact of ionic strength in the removal of RBBR.

A simulated mixture was prepared by adding chlorides, sulphates, nitrate, sodium, potassium, calcium, and magnesium. An adsorption test was performed to observe the impact on the removal of RBBR by FAC. The presence of various ions along with the target dye reduced the percentage of adsorption to 96% (Fig. 12). The reason can be attributed to the fact that the relative competition among the ion (Ca²⁺, Na⁺, NO₃⁻, Cl⁻¹, K⁺, and Mg²⁺) species for the active sites of FAC plays an important role in decreasing the rate of adsorption. The increased numbers of cations/anions render the surface of FAC not easily accessible for dye molecules, and thus the rate of adsorption is decreased. When solid adsorbent is in contact with adsorbate molecules in solution, an electrical diffused double layer is surrounding the adsorbent/adsorbate species, and the thickness of the layer is significantly expanded by the presence of electrolytes. Such expansion forbids the approach of the dye molecules towards the adsorbent surface.²⁶

Fig. 12 Impact of the simulated mixture on the removal RBBR.

Thermodynamics feasibility study for the removal of RBBR

The computed thermodynamic parameters revealed that the negative value of standard enthalpy (ΔH° = −13.68 kJ mol⁻¹) confirms the exothermic nature of the adsorption process. The negative value of entropy change (ΔS° = −2.18 J mol⁻¹ K⁻¹) signifies the decrease in the randomness of the adsorption system at the liquid/solid interface. The decrease in Gibbs free energy value (ΔG; from −13.01 to −12.97 kJ mol⁻¹) with increase in temperature signifies the feasibility and spontaneity of the adsorption process. The free energy ranges of chemisorption and physisorption are from −20 to 0 kJ mol⁻¹ and from −80 to −400 kJ mol⁻¹, respectively.²⁷ The reported free energy change indicates that the adsorption of RBBR onto FAC takes place through physisorption.

Adsorption isotherm and kinetic assays

The experimental adsorption data for RBBR removal at different time interval was examined with four kinetic models, viz., pseudo first order, pseudo second order, Elovich equation and liquid film diffusion.²⁸ The result indicates that the rate of adsorption is very fast and reaches equilibrium within 1 h. The relatively high kinetics reflects the good congregation of RBBR molecules in the binding sites of FAC. The pseudo-second order kinetic model shows very high regression coefficient (<0.99) over the other investigated models (Table 1). The experimental, q_e value for the pseudo second order kinetic model agreed well with the calculated values. The least standard deviation value also justified the validity of pseudo second order over the other models. The data of different kinetic models were compared with the experimental kinetic data, as shown in Fig. 13. It has been observed that the pseudo second order model is fully superimposed with the experimental data. The other models shows highly scattered values from the experimental observation, which further validated that the rate of adsorption was mainly governed by the pseudo second order kinetic model.

Table 1 Kinetic model parameters for the adsorption of RBBR on FAC

Kinetic model	Model parameters	Values
Pseudo-first order	qe,exp (mg g^−1)	124.12
	qe,cal (mg g^−1)	10.19
	k1 (g mg^−1 min^−1)	0.0359
	R^2	0.9532
	Δq%	16.85
Pseudo-second order	qe,cal (mg g^−1)	123.45
	k2 (g mg min^−1)	0.0123
	R^2	0.9998
	Δq%	0.241
Elovich equation	a (mg g^−1 min^−0.5)	1.02 × 10^19
	b	0.3841
	R^2	0.8301
Liquid film diffusion	kfd (min^−1)	2.49
	Ifd	0.0358
	R^2	0.9521

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Fig. 13 Validity of the kinetic models with experimental data at 303 K.

Batch mode executed experimental data were fitted to the Langmuir, Freundlich and Temkin and DR isotherms models^29,30 to obtain the best isotherm model. The experimental data was well explained by the Langmuir model (Fig. 13) for the target dye molecules having a linear regression coefficient of 0.9513–0.9871 at all the studied temperatures (303–323 K) (Table 2). A comparison of the experimental adsorption data with the linearized plot of Freundlich, Langmuir, Temkin and Dubinin Radushkevich models is shown in Fig. 14. A high scattering from the experimental values was observed in the case of Temkin and Dubinin Radushkevich model, while Langmuir isotherm curve was almost superimposed by the experimental data followed by Freundlich. On the basis of the highest correlation, low error function values and comparison of data, the fitting of different models followed the order: Langmuir > Freundlich > Temkin > Dubinin Radushkevich. The computed theoretical monolayer adsorption capacity was found to be 211.05 mg g⁻¹.

Table 2 (a) Langmuir isotherm parameters and their respective error functions for the adsorption of RBBR on FAC. (b). Freundlich isotherm parameters and their respective error functions for the adsorption of RBBR on FAC. (c). Temkin isotherm parameters and their respective error functions for the adsorption of RBBR on FAC. (d). Dubinin–Radushkevich isotherm parameters and their respective error functions for the adsorption of RBBR on FAC

(a) T (K)	aL (mg g^−1)	bL (L g^−1)	RL	R^2	Sum of the square of the errors (ERRSQ)	Hybrid fractional error (HYBRID)	Marquardt's percent standard deviation (MPSD)	Average relative error (ARE)	Sum of absolute error (EABS)
303	211.05		0.70	0.9803	22 642.3	1893.63	13.67	10.44	189.32
313	107.20		1.31	0.9513	11 173.13	1058.76	9.35	9.26	199.75
323	76.92		1.62	0.9871	15 994.49	1761.21	16.14	13.63	188.28

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(b) T (K)	nF^−1	bL (mg g^−1)	R^2	Sum of the square of the errors (ERRSQ)	Hybrid fractional error (HYBRID)	Marquardt's percent standard deviation (MPSD)	Average relative error (ARE)	Sum of absolute error (EABS)
303	0.77	134.89	0.9339	27 401.01	3211.36	40.98	23.89	337.25
313	0.60	138.03	0.9635	7290.85	488.06	87.26	30.50	177.24
323	0.45	120.22	0.9205	35 632.76	1824.38	8.15	13.61	317.37

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(c) T (K)	B	KTem (L g^−1)	R^2	Sum of the square of the errors (ERRSQ)	Hybrid fractional error (HYBRID)	Marquardt's percent standard deviation (MPSD)	Average relative error (ARE)	Sum of absolute error (EABS)
303	238.84	1.42	0.9950	35 152.34	3044.30	4.07	15.49	243.14
313	208.74	1.57	0.9872	14 538.19	1194.13	6.16	9.71	182.35
323	164.48	1.78	0.9815	20 874.02	2269.33	20.10	15.28	218.99

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(d) T (K)	ΨD	ΦD	R^2	Sum of the square of the errors (ERRSQ)	Hybrid fractional error (HYBRID)	Marquardt's percent standard deviation (MPSD)	Average relative error (ARE)	Sum of absolute error (EABS)
303	3.36	523.21	0.8168	57 679.64	3297.82	17.83	19.48	446.39
313	2.29	468.71	0.8721	52 437.43	4995.08	45.48	35.53	547.90
323	34.48	572.49	0.8731	23 066.85	2943.25	19.72	20.20	353.85

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Fig. 14 Validity of the adsorption isotherm with experimental data at 303 K.

Mechanism of adsorption

The proposition of the mechanism in the adsorption study is still a foremost challenge. Various factors that play important roles in the establishment of adsorption are the structure of adsorbate and functional groups, textural and surface chemistry of adsorbents, and the specific interaction between adsorbent surface and adsorbate.³¹ It appears that the mechanism of interactions between carbonaceous materials and adsorbate is the π–π stacking: the carbon surface of the adsorbent interacts with the π-electron of the aromatic ring of the adsorbate (RBBR). A similar type of mechanism is also reported in the literature.^31,32 Adsorption on the magnetic material, attached on the carbon surface, also takes place via π–π stacking.³³ The band at 1050 cm⁻¹ in the FTIR spectra can be attributed to phenols or to the formation of hydroxyl species (–FeOH/Fe–OH–Fe). The band indicates that hydroxyl species may be additionally getting involved in the binding of the dye molecules on the surface of the carbon matrix. The proposed mechanism is shown in Fig. 15. However, further study is required to establish the mechanism of the adsorption of RBBR dye molecules on the surface of the prepared composite.

Fig. 15 Proposed mechanisms for RBBR adsorption onto FAC.

Concluding notes

This work demonstrated the development of a paramagnetic adsorbent from waste biomass via the precipitation of a mixture of Fe²⁺ and Fe³⁺ salts by NaOH solution and its application in the removal of a carcinogenic textile dye, RBBR. The TEM micrographs of the prepared adsorbent confirmed the presence of a magnetite wire-shaped composite with an average typical diameter of 57.07–218.36 nm. The P-XRD studies confirmed the formation of hematite iron oxide on the surface of the carbon matrix. The Freundlich and Langmuir isotherm efficiently fitted the experimental data. The adsorption of RBBR on FAC followed the pseudo second-order equation. 99% of MB was absorbed in 10–20 min, and the pseudo second order kinetic model was feasible to describe the RBBR adsorption process in Fe₂O₃/activated carbon composite. The prepared composite showed both good magnetic response and high BET surface area (1099.98 m² g⁻¹). The FAC composite could be easily separated and retrieved by an outer magnet after the removal of contaminants from water. This prepared carbon composite can be easily re-dispersed into the solution after removing the magnetic field. Therefore, from practical point of view, the prepared AC/Fe₂O₃ composite material would be a promising magnetic adsorbent for the removal of dye from wastewater.

Acknowledgements

The authors are thankful to Director NIT Silchar, for providing laboratory facility and financial assistance for the analysis of the developed adsorbents. One of the authors is thankful to the University Grant Commission (UGC), New Delhi for financial assistance under Maulana Azad National Junior Research Fellowship (MANJRF) in completion of the research work.

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