Eco-Friendly Green Approach to the Biosorption of Hazardous Dyes from Aqueous Solution on Ragweed (Ambrosia artemisiifolia) Biomass

Abstract

The presented research includes the preparation, characterization, and implementation of magnetic biosorbent (Fe₃O₄/RWB), obtained from ragweed (Ambrosia artemisiifolia) biomass. Fe₃O₄/RWB was examined for the removal of a hazardous dye, malachite green (MG), from an aqueous solution in a batch system. The effects of the experimental parameters—initial dye concentration (10–300 mg/L), contact time (0–120 min), biosorbent dose (1–5 g/L), initial pH (2–10), ionic strength (0–1 mol/L), and temperature (298–318 K) on dye biosorption—were studied. The results showed that increases in biosorbent dose, contact time, and initial pH led to an increase in biosorption efficiency, while the increase in initial dye concentration, the ionic strength, and temperature had the opposite effect. The biosorption kinetics for MG on Fe₃O₄/RWB were analyzed with pseudo-first-order, pseudo-second-order, and Elovich kinetic models, while the Langmuir, Freundlich and Temkin isotherm models were used for equilibrium data analysis. It was observed that the MG biosorption followed the pseudo-second-order kinetic model, whereas the Langmuir model was the best fit for the equilibrium biosorption data of MG, with a Q_max of 34.1 mg/g. the desorption of MG from Fe₃O₄/RWB indicated reusability in five adsorption/desorption cycles, good performance, and potential in practical applications.

1. Introduction

Industrialization is associated with the disposal of large amount of toxic pollutants, such as synthetic dyes, which can have negative effects on the environment and human health [1]. Around the world, about 7 × 10⁵ tons of synthetic dyes are produced every year, and 2/3 of this amount are used in the textile industry. During the textile-dyeing process, about 30% of synthetic dyes are wasted through washing and exhaustion procedures and up to 2 × 10⁵ tons of these chemicals are disposed as wastewaters in the environment [2]. Environmental pollution with synthetic dyes has become a focus in recent decades because dyes can have negative ecotoxicological effects, with a tendency to bioaccumulate. Many of these compounds are very toxic and can have negative effects on public health, especially when present in water [3]. Malachite green (MG) is an organic and toxic triphenylmethane compound [4]. It is used in aquaculture, textile, pharmaceutical, and food processing industries, among others. Because of its solubility and water stability, MG is highly resistant to microbial degradation [5]. Due to its application in many industrial areas, MG can enter the environment relatively easily, mostly affecting aquatic ecosystems. Human exposure to MG could result in mutagenesis, carcinogenesis, respiratory diseases, chromosomal fracture, etc. In many developed countries the usage of MG has been banned, or is heavily regulated [4,6]. Conventional methods for removing synthetic dyes from wastewater include adsorption, ion exchange, photocatalysis, membrane filtration, precipitation, and oxidation [7,8]. These methods require the excessive use of chemicals and other resources, and are often expensive. The effectiveness of these methods often depends on many factors, including the secondary treatment of pollutants not removed by the primary treatment [9]. Therefore, it would be desirable to develop an efficient and economical method for the removal of synthetic dyes in order to prevent environmental pollution and health risks.

Many researchers have studied the use of biomass in wastewater treatments. Biomass contains cellulose, lignin, hemicellulose, lipids, and proteins, with functional groups (alcohol, carboxylic, amino, phenol groups, etc.), making biosorbents attractive materials for dye removal [10]. The use of biosorption has rapidly expanded because of its advantages, such as its availability, sustainability, reusability, eco-friendliness, and low cost [11]. Various biosorbents have been applied in the biosorption of dyes, including microorganisms, forestry, wood wastes, etc. [12,13,14].

Ambrosia artemisiifolia is one of the most important invasive plants, and has spread widely in America, Asia, and Europe [15]. In Europe, approximately 13 million people suffer from allergies caused by Ambrosia, with medical treatments costing about EUR seven billion annually [16]. This plant is also widespread in the Republic of Serbia, especially in its northern regions [17]. According to the currently available data, the maximum concentration of Ambrosia pollen in the air in Serbia was 12,356 grain/m³, making it dominant in comparison with the other allergens [18]. Aside from being allergenic, A. artemisiifolia is, as already mentioned, also known as an invasive plant, causing significant yield losses in many crops [15]. Although invasive, the use of A. artemisiifolia as a biosorbent for hazardous chemicals has been a subject of interest by several researchers [19,20,21,22]. Still, there is a deficiency of literature data, leaving open many possibilities for testing the potential of this plant as a biosorbent. In this study, a magnetic biosorbent (Fe₃O₄/RWB) obtained from a cheap and eco-friendly A. artemisiifolia biomass was tested as an MG biosorbent in an aqueous solution in a batch system (Figure 1).

The characterization of the obtained magnetic biosorbent (Fe₃O₄/RWB) was carried out using FT-IR, SEM-EDS, XRD, and the determination of pH_pzc. The effects of different parameters on the biosorption process’s efficiency, including the initial dye concentration, contact time, biosorbent dose, initial pH, ionic strength, and temperature, were assessed. The biosorption kinetics were determined using the pseudo-first-order (PFO), pseudo-second-order (PSO), and Elovich models. Three isotherm models, Langmuir, Freundlich, and Temkin, were used to examine equilibrium sorption data. Thermodynamic studies were also performed. Finally, the desorption of MG and reusability of the magnetic biosorbent (Fe₃O₄/RWB) were also analyzed. The ability of a magnetic biosorbent to regenerate and reuse was tested in several cycles of the adsorption/desorption process. This low-cost approach, with the minimal use of chemicals, could lead to significant savings, does not involve the formation of secondary waste, and is in line with the principles of green chemistry. Progress in this research area could be a driving force for further investigations, with an impact on industry (new businesses), economy (circular economy), and society (environmental and health sustainability).

2. Materials and Methods

2.1. Materials and Instrumentation

For the purposes of this research, the following chemicals were used: iron (II) chloride (FeCl₂∙4H₂O, >98%), iron (III) chloride (FeCl₃∙6H₂O, ≥97%), Malachite Green (MG, C₂₃H₂₅ClN₂), sodium hydroxide (NaOH), hydrochloric acid (HCl), and sodium chloride (NaCl). All chemicals were purchased from Sigma Aldrich (Saint Louis, MO, USA). Fe₃O₄/RWB was characterized via Nicolet SUMMIT FT-IR Spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), in ATR mode, in the range 4000–400 cm⁻¹, scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (SEM-EDS) (JEOL JSM-6390 LV, JEOL Ltd., Tokyo, Japan), and an EDS detector (Oxford instruments X-MaxN, High Wycombe, UK). The X-ray diffraction (XRD) analysis (Rigaku Ultima IV diffractometer, Tokyo, Japan) of Fe₃O₄/RWB was also performed in the range of 10–80° 2θ. The Point of Zero Charge (pH_pzc) of the Fe₃O₄/RWB was determined according to the pH drift method. The pH values were measured using a pH meter (Hanna HI 2210, Hanna Instruments, Navi Mumbai, India). UV–Vis spectroscopy (NOVEL-102S, COLOLab Experts, Polje ob Sotli, Slovenia) was used for determination of the dye concentrations. During the experiments, the samples were mixed in an orbital shaker (Orb-Pro Labbox Labware, Barcelona, Spain). All measurements in this research were performed in triplicate.

2.2. Sample Collection and Pretreatment of A. artemisiifolia Biomass

A. artemisiifolia samples were obtained from the outskirts of Belgrade (capital of the Republic of Serbia), by cutting off plant’s parts above the soil surface. After sampling, the plant material was washed with tap water and dried in sunlight for seven days. After drying, the raw plant material was cut into small pieces and ground in an electric mill. The resulting particles were sieved to obtain particles with the size < 300 µm. They were washed with distilled water to remove probable impurities and dried in an oven at 323 K.

2.3. Preparation of Magnetic Biosorbent (Fe₃O₄/RWB)

The magnetic biosorbent was prepared using the modified coprecipitation method proposed by Medina-Zazueta et al. [23]. The method consists of mixing 3.9 g of FeCl₃∙6H₂O and 1.7 g of FeCl₂∙4H₂O in 190 mL of deionized water under a stream of N₂, at 353 K, with stirring (450 rpm, 20 min). After that, a few drops of 10 mol/L NaOH were added to the reaction mixture for magnetite precipitation [24], and stirring was continued under the same conditions (for 30 min). A total of 2 g of biomass (RWB) was added to the reaction mixture and stirring was continued (for 30 min). Finally, the prepared Fe₃O₄/RWB was filtered and washed with deionized water until the pH was neutral and dried in an oven at 323 K.

2.4. Biosorption Studies

The effects of biosorbent dose (1–5 g/L), contact time (0–120 min), initial dye concentration (10–300 mg/L), initial pH (2–10), ionic strength (0–1 mol/L), and temperature (298–318 K) on MG biosorption on Fe₃O₄/RWB were analyzed in a batch system. During the typical biosorption process, 5 g/L of the Fe₃O₄/RWB was added to a vial flask with 20 mL of a 50 mg/L MG solution. The samples were shaken for 60 min at 400 rpm. After biosorption, Fe₃O₄/RWB from the samples was separated using an external magnet, and after decanting, the remaining MG concentration was measured using a UV–Vis spectrophotometer at λ_max = 618 nm. MG removal efficiency (R%) and the biosorption capacity (Q_t) were calculated according to Equations (1) and (2) [25,26]:

$$R\left(\%\right)={\displaystyle \frac{C_0-C_e}{C_0}}\times 100$$

(1)

$$Q_{t }={\displaystyle \frac{V(C_0-C_t)}{m}}$$

(2)

where C₀, C_e, and C_t (mg/L) are MG concentrations in the solution at the initial, equilibrium, and any time t, respectively. V (L) is the solution volume and m (g) is the biosorbent mass.

For the regeneration/reusability analysis, Fe₃O₄/RWB (5 g/L) was added to the MG dye solution (C₀ = 50 mg/L; contact time = 60 min; shaking speed = 400 rpm). After MG biosorption, the MG-loaded Fe₃O₄/RWB was separated using an external magnet. Desorption agents (20 mL), comprising absolute ethanol [27], 0.1 mol/L HCl [28], and 0.1 mol/L NaOH [29], were added to the MG-loaded Fe₃O₄/RWB and shaken for 60 min at 400 rpm. The samples were separated using an external magnet and the concentrations of MG were measured using a UV-Vis spectrophotometer at λ_max = 618 nm. The desorption percentage (D%) of MG was determined following Equation (3) [30]. The best desorbing agent after the first cycle was absolute ethanol, and this was used for the remaining regeneration/reusability analysis.

$$D\left(\%\right)={\displaystyle \frac{C_{des}}{C_{ads}}}\times 100$$

(3)

where C_des and C_ads (mg/L) are the desorbed and adsorbed MG concentration, respectively.

3. Results and Discussion

3.1. Characterization of Fe₃O₄/RWB

The FT-IR spectra of Fe₃O₄/RWB before and after MG biosorption are shown in Figure 2. For biosorbent materials, adsorption peaks from 3500 to 3200 cm⁻¹, representing the stretching vibrations of the –NH₂ groups, are typical. In the FTIR spectrum of Fe₃O₄/RWB, the peak from 3100 cm⁻¹ to 3400 cm⁻¹ is also related to –OH stretching vibrations from macromolecules of cellulose, hemicellulose, and lignin [3,31,32]. The peaks at 1641.5 cm⁻¹ and 1419.6 cm⁻¹ could be attributed to (C = C) vibrations in aromatic rings [33]. The peak at 1026.5 cm⁻¹ is distinctive for vibrations of –COH [34,35]. The peak at 579.9 cm⁻¹ refers to magnetite, which is incorporated into the structure of Fe₃O₄/RWB [36]. Distinctive variations were observed in the FT-IR spectrum of Fe₃O₄/RWB–MG. In addition to the changes in existing peaks, new specific peaks were also detected in the fingerprint region (500–1600 cm⁻¹) after MG biosorption. It was found that the peak at 3400 cm⁻¹, which corresponds to the –OH stretching vibration, was drastically decreased. The vibration at 1580.1 cm⁻¹ could be a consequence of the C = C stretching of benzene rings in MG [37]. The peak at 1355.2 cm⁻¹ for C–N stretching vibrations corresponds to the aromatic tertiary amine group [38], while the peak at 1162.6 cm⁻¹ is assigned to the C–O-stretching vibrations from –COOH groups [39]. The FT-IR spectra of Fe₃O₄/RWB indicate that the functional groups on the biosorbent surface, such as –COOH, –OH, and –NH₂, could be potential binding sites for MG biosorption.

In order to obtain morphological information, the Fe₃O₄/RWB surface was observed using an SEM under 100, 10, and 5 µm magnification; the results are displayed in Figure 3. The SEM analysis indicated that the Fe₃O₄/RWB surface appears rough, which is typical for plant biosorbents [40]. The observed porous structure of Fe₃O₄/RWB might be able to provide additional active sites for MG biosorption.

EDS analysis revealed carbon (46.6%) and oxygen (31.2%) to be the main elements of Fe₃O₄/RWB, which is typical for plant materials [41]. The presence of nitrogen, sulfur, and phosphorus was negligible, while the presence of iron (22.1%) suggests Fe₃O₄/RWB magnetization (Figure 4).

The XRD pattern of Fe₃O₄/RWB, presented in Figure 5, contains reflections at ~30°, ~36°, ~43°, ~54°, ~57°, and ~63° 2θ, corresponding to the (220), (311), (400), (422), (511), and (440) planes of magnetite [42]. This confirms the presence of magnetite in the biosorbent structure. Also, according to the XRD analysis, Fe₃O₄/RWB is composed of cellulose. The presence of distinct reflections at ~17°, ~23°, and ~35° 2θ corresponds to the (110), (200), and (004) planes of cellulose I [43].

The point of zero charge (pH_pzc) of Fe₃O₄/RWB was determined by adapting a pH drift method previously reported by Lemos et al. [44]. The pH_pzc of the biosorbent allows for the optimal pH conditions under which the best biosorption performances can be obtained to be predicted [45]. The Fe₃O₄/RWB (0.05 g) was added in glass vials with 20 mL of 0.1 mol/L NaCl, in a pH range from 2 to 10. The pH of the suspensions was measured after 24 h, and the pH_pzc determined for Fe₃O₄/RWB was 4.8 (Figure 6). Based on this result, it can be concluded that, at pH values < 4.8, a positive charge will be dominant, while a negative charge will be dominant at pH values > 4.8. Since MG is a cationic dye, it is expected that more dye molecules will bind to the Fe₃O₄/RWB when the dye molecules are positively charged and the biosorbent surface is negatively charged.

3.2. Batch Biosorption Study

3.2.1. Effect of pH

The removal of MG from aqueous solutions using the Fe₃O₄/RWB biosorbent was assessed at different pH values (2, 4, 6, 8, and 10), keeping the other parameters constant (C₀ = 50 mg/L; biosorbent dose = 5 g/L; T = 298 K; t = 60 min; shaking speed = 400 rpm). As shown in Figure 7a, the MG removal percentage increased from 63.8% (pH 2) to 86.6% (pH 8). When initial pH values were lower than the pH_pzc (4.8), electrostatic repulsions between the positively charged surface of the Fe₃O₄/RWB biosorbent and the positively charged functional groups of the MG dye hindered biosorption. The efficiency of the removal of MG by the Fe₃O₄/RWB is attributed to other interactions, such as hydrogen bonding and π-π interactions (Scheme 1). On the other hand, when the initial pH is higher than 4.8, electrostatic attractions between the negatively charged functional groups (–O⁻, –COO⁻) on the Fe₃O₄/RWB surface and positively charged MG become dominant, promoting MG’s removal from the solution. At pH > 8.0, the decolorization of MG is intensive due to a chemical reaction between the excess of OH⁻ ions in the solution and the MG molecules. The OH⁻ ions attack the central carbon atom of the MG molecules, quenching the resonance and forming a carbinol base, which is colorless. This could provide an explanation of the seeming increase in removal efficiency at pH > 8, while the colorless dye remains in the solution [46]. Therefore, pH 8 was selected as the optimal pH value for the removal of MG using Fe₃O₄/RWB.

3.2.2. Effect of Sorbent Dose

The influence of different Fe₃O₄/RWB doses (1–5 g/L) on the removal efficiency of MG, with the other experimental variables being kept constant (C₀ = 50 mg/L; pH 8; T = 298 K; t = 60 min; shaking speed = 400 rpm), is represented in Figure 7b. The results show an increase in the MG removal efficiency from 24.6% to 62.2% in the range of 1–5 g/L of Fe₃O₄/RWB. Based on the obtained results, the biosorption efficiency of MG increases with an increase in the sorbent dose. Therefore, the optimal dose of biosorbent was 5 g/L.

3.2.3. Effect of Ionic Strength

The influence of ionic strength (Figure 7c) on MG biosorption was examined using different concentrations of NaCl (0.1–1 mol/L), while the other experimental variables were kept constant (C₀ = 50 mg/L; biosorbent dose = 5 g/L, pH 8; T = 298 K; t = 60 min; shaking speed = 400 rpm). With the increase in NaCl concentration from 0 to 1 mol/L, the removal efficiency decreased from 62.2% to 4%. With an increase in NaCl concentration, the adverse effect of ionic strength on the MG removal efficiency was higher. A possible explanation for this phenomenon lies in the competition between Na⁺ ions and MG molecules for the binding sites on the Fe₃O₄/RWB. The Na⁺ ions had an advantage compared to the MG molecules, probably due to their increased concentration and smaller size. This influence was most noticeable with the increase in the concentration of NaCl from 0.8 to 1 mol/L, where the efficiency of the MG removal was drastically decreased from 44.8% to 4%. At the same time, Cl⁻ ions reduce the electropositivity of the –N(CH₃)₂⁺ group in MG, negatively affecting the electrostatic attraction between negatively charged active sites on the biosorbent and positively charged MG [47].

3.2.4. Biosorption Thermodynamics

The thermodynamic parameters, Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) for MG biosorption on Fe₃O₄/RWB were analyzed at three temperatures (298, 308, and 318 K), while the other experimental variables were kept constant (C₀ = 50 mg/L; biosorbent dose = 5 g/L, pH 8; t = 60 min; shaking speed = 400 rpm). To evaluate the spontaneity of the biosorption, the Gibbs free energy was determined (

Table 1. Thermodynamic parameters for the biosorption of MG on Fe₃O₄/RWB.

Temperature (K)	ΔG° (J/mol)	ΔH° (J/mol)	ΔS° (J/mol·K)
298	−2010	−31,097	−98
308	−691
318	−60

). The decrease in ΔG° and removal efficiency (Figure 7d) with increasing temperature indicates that the biosorption of MG on Fe₃O₄/RWB is a spontaneous and thermodynamically stable process. The negative values for ΔH° and ΔS° indicate that biosorption is an exothermic process with a reduction in randomness at the interface between the solution phase and the biosorbent. Thermodynamic parameters were determined using the following Van’t Hoff Equations (4)–(7) [48,49]:

$$∆G°=-RT ln K$$

(4)

$$K_c={\displaystyle \frac{C_a}{C_e}}$$

(5)

$$ln K={\displaystyle \frac{∆S°}{R}}-{\displaystyle \frac{∆H°}{RT}}$$

(6)

$$∆G°=∆H°-T∆S°$$

(7)

where R is the ideal gas constant (8.314 J/mol K), T (K) is the absolute temperature, K is the thermodynamic equilibrium constant, C_a (mg/L) is the amount of dye adsorbed at equilibrium, and C_e (mg/L) is the dye concentration in the solution at equilibrium.

3.2.5. Biosorption Kinetic

The effect of contact time on the removal of MG is shown in Figure 8. As can be seen, the uptake of MG by Fe₃O₄/RWB reached 91.8% after 50 min of biosorption, indicating relatively high affinity. The obtained results clearly show that biosorption was faster at the beginning, and then the removal efficiency slowly increased until the biosorption reached equilibrium. The fast elimination of MG from the solution reveals that the examined biosorbent could be a good candidate for wastewater treatment. The biosorption mechanism of MG on Fe₃O₄/RWB was evaluated using the linearized form of pseudo-first-order (PFO), pseudo-second-order (PSO), and Elovich kinetic models (Table S1). The higher R² value of PSO (0.999) indicates that this model is suitable for describing the biosorption kinetics of MG (

Table 2. Kinetics parameters for the biosorption of MG (C₀ = 50 mg/L; MB dose = 5 g/L; T = 298 K; pH = 8; shaking speed = 400 rpm).

Parameter	Values
Qeexp, mg/g	9.3
(PFO)
Qecal, mg/g	4.24
k1, min−1	0.054
R2	0.960
(PSO)
Qecal, mg/g	9.59
k2, g/mg min	0.034
R2	0.999
Elovich model
αE, mg/g	72.02
βE, g/mg	0.81
R2	0.960

; Figure 9), suggesting that chemisorption limits the rate of the biosorption process. Also, the values of the equilibrium biosorption capacity of the PSO (9.59 mg/g) and the experimentally obtained biosorption capacity (9.3 mg/g) were matched.

3.2.6. Biosorption Isotherms

Figure 10 represents the biosorption of MG at different initial concentrations of the dye solution (10–300 mg/L) with a constant biosorbent dose of 5 g/L as a function of contact time (0–120 min) for a stirring speed of 400 rpm, at 298 K and pH = 8. The obtained results indicate that an increase in initial dye concentration leads to an increase in the biosorption capacity of Fe₃O₄/RWB. It also can be concluded that the biosorption capacity remains constant at MG concentrations higher than 150 mg/L due to the fact that the biosorbent reaches its maximum capacity at this concentration. The Langmuir, Freundlich, and Temkin isotherm models (Table S2) were used to evaluate the biosorption on Fe₃O₄/RWB and to predict the maximum efficiency (

Table 3. Parameters of the adsorption isotherms of MG biosorption on Fe₃O₄/RWB.

Parameter	Value
Langmuir model
Qm, mg/g	34.1
KL, L/mg	0.085
R2	0.997
Freundlich model
nF	2.25
KF, L/g	4.54
R2	0.937
Temkin model
AT, L/mg	10.31
R2	0.9187

; Figure 11). The Langmuir isotherm model assumes a monolayer and homogeneous biosorption, in which each molecule possesses constant enthalpies and a constant biosorption activation energy. A Freundlich model can be applied to multilayer biosorption, with a heterogeneous distribution over the biosorbent surface [50]. The Temkin model assumes that the heat of biosorption would linearly decrease with the increase in the coverage of the biosorbent, and this has been applied to describe biosorption on heterogeneous surfaces [51]. The coefficient of determination value, R² = 0.9736, indicates that biosorption probably follows the Langmuir isotherm model. Another confirmation of this fact is that the theoretical maximum sorption capacity, calculated from the Langmuir adsorption isotherm model, was 34.1 mg/g, which is very close to the experimentally determined sorption capacity of 31.5 mg/g. The maximum sorption capacity of MG on the pristine biosorbent (RWB) was 33.4 mg/g, and a minimal difference was observed compared with the Fe₃O₄/RWB.

3.2.7. Comparation Studies

A comparison of the maximum biosorption capacity of MG with other biosorbents reported in the literature is shown in

Table 4. Comparison of MG removal using different biosorbents.

Biosorbent	Concetration (mg/L)	pH	Biosorbent Dosage (g/L)	Time (min)	Temperature (K)	Qm (mg/g)	Ref.
Fe3O4/RWB	50	8.0	5.00	120	298	34.1	This study
Mag. Cortaderia selloana flower spikes	25–350	6.0	4.00	45	298	56.5	[52]
Catha edulis stem	10–50	10.0	10.00	60	/	5.6	[53]
Red algae Corallina ofcinalis	20	6.0	1.00	120	300	101.3	[54]
Mag. Bauhinia variagata fruits	150	8.0	0.20	60	298	30.1	[55]
Fir (Abies nordmanniana) cones	110	3.3	50.00	8760	294	11.0	[56]
Paracentrotus lividus shells	10	initial	4.00	180	294	7.2	[57]
Coastal biowaste (Zostera marina)	15	4	0.01	360	298	97.6	[58]

. The calculated capacities indicate that Fe₃O₄/RWB has significant potential for dye removal from aqueous solutions and can be considered suitable for the treatment of polluted waters. Compared to other biosorbents used for wastewater treatments, Fe₃O₄/RWB showed relatively good efficiency, with a biosorption capacity of 34.1 mg/g. However, it is important to note that the studies mentioned in Table 4 were performed under different experimental conditions, and for a more precise evaluation it will be necessary to perform a set of studies under uniform conditions.

3.3. Reusability of Fe₃O₄/RWB

From the environmental and economic perspectives, it is important that biosorbents have the possibility of regeneration and reuse. A regeneration/reusability analysis of Fe₃O₄/RWB was carried out with three different desorption agents (0.1 mol/L HCl, 0.1 mol/L NaOH, and absolute ethanol), and the obtained results are presented in Figure 12. Based on the presented results, the best desorbing agent after the first adsorption/desorption cycle was absolute ethanol; as such, this was used for the rest of the regeneration/reusability analyses. As shown in Figure 13, after the first adsorption/desorption cycle, the efficiency of MG removal decreased significantly, from 79% to 49%. This can be explained by the fact that all binding sites of the Fe₃O₄/RWB were free during the first adsorption/desorption cycle, while during the second cycle, despite the desorption that occurred after the first cycle, a certain portion of the binding sites on the biosorbent remained occupied by MG molecules. After the second cycle, the trend regarding reuse of the Fe₃O₄/RWB did not change significantly, suggesting that it has significant potential for future commercial use in the treatment of wastewater, and that it is probably usable over a larger number of cycles.

4. Conclusions

This investigation revealed the MG biosorption potential of Fe₃O₄/RWB (from Ambrosia artemisiifolia biomass) in a batch mode. The results showed that the PSO model was appropriate for modeling MG biosorption using Fe₃O₄/RWB, indicating that chemisorption limits the biosorption process. An analysis of the adsorption isotherms shows that the Langmuir model matched the experimental data well, which assumes that a monolayer is present, as well as the homogeneous biosorption of MG. Under optimal experimental conditions (C₀ = 50 mg/L; Fe₃O₄/RWB dose = 5 g/L; T = 298 K; t = 60 min; pH = 8) the maximum adsorption capacity of MG was 34.1 mg/g. Based on the obtained results, it can be concluded that Fe₃O₄/RWB has the potential to eliminate MG from wastewater. This approach offers a cheap, eco-friendly, and effective technological alternative to conventional methods for the removal of MG. Further research directions will focus on the applicability of Fe₃O₄/RWB in the biosorption of wide range of different contaminants (other synthetic dyes, pesticides, drugs, etc.). Transference of the lab-scale results to industrial-scale applications is also one of the future research perspectives regarding this approach. That will involve determining the technological feasibility, economy, and ecological performance of the application of biosorption at a large scale.