Green synthesis of a nicotinamide-functionalized cobalt ferrite nano-adsorbent for aqueous phase removal of some virulent dyes: evaluation of adsorption isotherms, kinetics, and mechanisms

Abstract

Biogenic processes are increasingly appealing alternatives to standard synthesis approaches. They are environmentally conscious approaches, employing nontoxic and biocompatible substances and yielding materials with exceptional characteristics. Herein, an eco-friendly production of cobalt ferrite (CoFe₂O₄) nanoparticles by employing the fruit extract of orange citrus (Citrus sinensis) via a green synthesis method is reported. The surface modification of super magnetic CoFe₂O₄ nanoparticles with nicotinamide (ex situ approach) produced ecofriendly and novel NAM/CoFe₂O₄ nanoparticles. Following their structural elucidation using XRD, the nanoparticles were further characterized using FTIR spectroscopy; surface analysis was performed using SEM-EDX, TEM, and BET (S_BET: 22 m² g⁻¹ and pore diameter: 2–50 nm), and the point of zero charge (7.0) and magnetic strength (CoFe₂O₄: 158 emu g⁻¹; NAM/CoFe₂O₄: 158 emu g⁻¹) of the NPs were determined using VSM. The produced NAM/CoFe₂O₄ NPs displayed good dye removal efficiencies, with maximum adsorption capacities of 101 and 177 (mg g⁻¹) for Celestine blue (CB) and Bismarck brown (BB), respectively, at 298 K. The operational parameters, including the dose (CB: 0.55 g L⁻¹; BB: 0.7 g L⁻¹), time (CB, BB: 40 min), pH (CB, BB: 8.0) and concentration (CB, BB: 30 mg L⁻¹), were optimized using batch adsorption technique. The equilibrium observations predicted using isotherm models fitted well with the Freundlich and pseudo-second order models for CB and BB. In particular, the adsorption process correlated well with the pseudo-second order kinetic model. The adsorption mechanisms for Celestine Blue and Bismarck brown were principally controlled via intermolecular H-bonding, π–π interactions, and electrostatic interactions. The most economical (560 INR per kg) method for removing CB and BB was found to be NAM/CoFe₂O₄ NPs. The reusability of the spent NPs was consistent for up to five adsorption–desorption cycles.

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1. Introduction

The increasing deterioration in the physical and bio-chemical properties of surface water resources owing to the disposal of untreated or inefficiently treated aqueous effluents containing high levels of organic and inorganic pollutants from different industries has aroused worldwide concern; these effluents also induce significant adverse ecological and toxicological impacts. The abundant use of synthetic dyes (which are often poorly biodegradable because of their complex aromatic structures and resistance to light, temperature and oxidizers)^1,2 and the voluminous discharge of liquid wastes from textile industries (including paper printing and leather) not only leads to the contamination of natural environment but also seriously impairs the aquatic life owing to diminished photosynthetic activity and depleted dissolved oxygen content due to sunlight obstruction,³ ruining soils and causing adverse consequences to the human health. Most of these dyes, including azo-colorants or their reductive-cleaved by-products, such as aromatic amines, are conceivably perilous and are carcinogens and mutagens. Contact dermatitis, permanent visual impairment, regurgitating/retching gastritis, chemosis, rhabdomyolysis, exophthalmos, lacrimation, laryngeal/pharyngeal oedema, and respiratory problems are some human health-related issues they can cause. Some of them are even responsible for splenic-/hepato-carcinomas, bladder malignancy, lung and liver cancers, damage to the central nervous system, endocrine disruption, and interference in the immune system.^4,5 Bismarck brown (BB), a diazo dye used as a cotton, wool, and leather colorant, is reported to be potentially carcinogenic to aquatic organisms and human beings.⁶ Exposure to BB may result in harmful health effects on humans. Upon short-term and long-term contact, this dye can cause eye irritation/conjunctival injury or skin rashes. Throat irritation, coughing and wheezing are observed upon inhalation.⁷ Following ingestion, both Bismarck brown and Celestine blue can cause gastrointestinal discomfort, producing nausea, vomiting, and diarrhoea and may adversely affect the health of a person suffering from liver or kidney morbidity. Ingestion of CB or its metabolites may harm the afflicted person, causing methemoglobinemia through binding to hemoglobin and impeding normal oxygen uptake (anoxia).⁸ At varying levels of blood methemoglobin, cyanosis of the lips/nose, severe headache, weakness, dizziness, ataxia, rapid shallow respiration, drowsiness, nausea, vomiting, and stupor are observed. In an acute instance, a high concentration might cause breathing difficulties, respiratory distress, arrhythmia or bradycardia, and seizures.⁹ Direct contact with the eyes may cause conjunctive redness or serious abrasive damage. Abrasions or lesions may prove harmful when dyes are exposed to blood through skin cuts. Long-term exposure to high CB dust concentrations (<0.5 μm size) may cause pneumoconiosis and worsen the conditions of already impaired respiratory function, airway diseases, emphysema, or chronic bronchitis. Considering the potential toxic effects and health hazards associated with the residual dyes and the larger public health and environmental safety concerns, the treatment of dye-polluted water is highly desirable. During the past few decades, coagulation/flocculation, ion exchange, advanced oxidation processes, catalytic degradation,^10,11 photochemical degradation,¹² separation via membrane,¹³ and sorption^14–16 have been harnessed for the attenuation of undesirable quantities of dye-laden aqueous wastes. Although most of these treatment techniques are acceptable, they have certain deficiencies, such as lower efficiency, time consumption, cost, slower removal rates and environmental unfavourability, and are, therefore, not widely applicable. However, adsorption has been established as an efficacious and low-cost strategy for wastewater remediation due to its facile and convenient operation, relatively simplistic design, easy regeneration of spent adsorbents and high quality of the resulting treated water. In the recent past, affordable and environmentally benign nanostructured materials, including different metal oxides, have incited considerable interest as promising adsorbents owing to their exceptionally higher surface areas and smaller particle sizes which provide plenty of active adsorption sites, thereby demonstrating boosted efficiencies and faster adsorption kinetics.^17–19 However, their wide usage in wastewater decontamination is constrained because of the presence of monotonous functional groups. In recent years, functionalized metal oxide nano-sorbents have been employed as attractive materials for sequestering inorganic and organic pollutants from water.^20,21 Recently, magnetic metal ferrites have received substantial interest as adsorbents for water cleaning due their superior chemical stability, facile magnetic separability, high surface areas and active surface sites, and ease of modification/functionalization.²² Magnetic ZnFe₂O₄,²³ CoFe₂O₄ NPs synthesized by an ethanol-assisted hydrothermal method,²⁴ and MFe₂O₄ (M = Co and Ni) NPs synthesized through a microwave-hydrothermal route²⁵ have shown promising capacities for the adsorption of Evans blue (Q_m = 46 mg g⁻¹), Congo red, Eriochrome Black T (82.6 mg g⁻¹) and Bromophenol blue (25.6 mg g⁻¹). Amongst the metal ferrites, cobalt ferrite nanoparticles are of significant interest due to their cheaper and facile synthetic procedures, moderate saturation magnetization (84 emu g⁻¹), good coercivity (∼5400 Oe), high chemical and thermal stability, rapid separation from solution, and better adsorption capacities.^26–28 Moreover, CoFe₂O₄ has wide applications in magnetic recording.²⁸ The functionalization of magnetic metal oxide nanoparticles with organic moieties facilitates the incorporation of a wide array of functional groups, resulting in an enhancement of their adsorption efficiencies and selectivities. Recently, SDS modified CoFe₂O₄ NPs synthesized via simple combustion routes^29,30 and amine functionalized CoFe₂O₄ NPs have been found to efficiently capture crystal violet, direct red 80, direct green, and acid blue 92 from the aqueous phase.

Recently, CB and BB dyes adsorbed via different adsorbents like SDS modified CoFe₂O₄ NPs, poly(curcumin-citric acid)/MnFe₂O₄ NC, Mt@STB MnO₂, CuCo₂O₄·MgO and Ch-mPC@FM date seed AC W₅O₁₄ respectively.^31–37

Nicotinamide (NAM), an active form of vitamin B₃, is naturally found in small amounts in legumes, nuts, lean meats, fish, eggs, poultry, and yeast and is necessary for good health. NAM (Fig. S1a, ESI†) is biologically active and is used to effectively treat pellagra,³¹ squamous cell carcinoma,³² bullous diseases, high blood cholesterol, type 1 diabetes mellitus, and neurodegenerative diseases.³ It was, therefore, thought worthwhile to attempt the synthesis of a novel NAM-modified bimetallic cobalt–iron oxide and study its application towards the adsorption of some cationic synthetic dyes from simulated wastewater. To the best of our knowledge, there are no reports on the surface modification of CoFe₂O₄ NPs using nicotinamide or any evaluation of its adsorptive performance for pollutant cleansing from aqueous solution. In the current study, the adsorptive uptake behaviors of bimetal oxide (CoFe₂O₄) nanoparticles synthesized through simple co-precipitation in alkaline medium and modified with nicotinamide functionality on the surface towards Celestine blue (CB, Fig. S1b, ESI†) and Bismarck brown R (BB, Fig. S1c, ESI†) were investigated under different operational parameters. The bimetal oxide (CoFe₂O₄) nanoparticles, synthesized through a green method using fruit extract of orange citrus (Citrus sinensis), modified with nicotinamide (NAM/CoFe₂O₄) is an unused precursor and important aspect of novelty. The adsorption equilibrium data were modelled using different isotherm models, and the monolayer saturation capacity of the adsorbent was determined using the Langmuir isotherm equation. The kinetics and mechanism of the adsorption process were evaluated in terms of PFO (pseudo-first order) and PSO (pseudo-second order) rate equations and IP (intraparticle) and IFD (liquid film diffusion) models. Information about the feasibility, spontaneity, and nature of the adsorption phenomena was obtained from various thermodynamic parameters. Additionally, the regeneration and reuse of the absorbent NAM/CoFe₂O₄ NPs were assessed.

2. Materials/instruments and methods

The nicotinamide (Sigma Aldrich, India); Bismarck brown R, a dark brown colored powder (MF = C₂₁H₂₆C_l2N₈, MP = 227, BP = 729 °C, FW = 424.9 g mol⁻¹, λ_max = 464 nm) and Celestine blue, a dark blue colored powder (MF = C₁₇H₁₈ClN₃O₄, MP = 227–230 °C, BP = 100 °C, FW = 363.8 g mol⁻¹, λ_max = 642 nm) (both Merck, India); and HCl, FeCl₃·6H₂O, and CoCl₂·6H₂O (Thermo Fisher Scientific, India) were used without further purification. The orange citrus fruits (Citrus sinensis) were collected from a garden (Jamia Millia Islamia, New Delhi). The characterization of NAM/CoFe₂O₄ NPs was performed using the following analytical methods and techniques (Table S1, ESI†).

2.1. Green preparation of CoFe₂O₄ nanoparticles

The synthesis of CoFe₂O₄ NPs was accomplished following a green method using the fruit extract of orange citrus (Citrus sinensis). The extract was acquired by boiling modestly sized pieces of fruit in 500 mL of DDW and filtering. The extract was stored in a cool place. In this study, FeCl₃·6H₂O (2.86 g) and CoCl₂·6H₂O (1.98 g) were added to 250 mL of fruit extract and stirred vigorously at 90 °C for 5 hours, yielding a dark-green suspension. After allowing the reaction mixture to cool to ambient temperature, the precipitate was separated by centrifugation and calcined at 300 °C for 2 hours. The product (CoFe₂O₄ NPs) was washed several times with a DDW–ethanol mixture, dried at 80–90 °C for 24 h in an electric oven, and calcined for 5 hours at 200 °C.³⁸

2.2. Preparation of NAM/CoFe₂O₄ NPs via ex situ approach

1.0 g of nicotinamide (NAM) and 2.0 g CoFe₂O₄ NPs were combined in 150 mL DDW in a 500 mL beaker and agitated for 2 hours at 95 °C. The resultant NAM/CoFe₂O₄ NPs were washed with DDW and dried for 24 hours at 90 °C, then stored in a vacuum-desiccator. The scheme in Fig. 1 depicts the production of NAM/CoFe₂O₄ NPs.

Fig. 1 Schematic of the production of NAM/CoFe₂O₄ NPs.

2.3 Adsorption studies

The efficiencies of NAM/CoFe₂O₄ NPs for the adsorption of CB and BB molecules from their respective aqueous solutions were studied under different adsorption times (10–60 min), adsorbent dosages (0.25–1.00 g L⁻¹), initial pH of dye solution (4–12), adsorbate concentrations (10 to 60 mg L⁻¹) and temperatures (298–303 K) following a batch methodology. The sorption experiments were performed by agitating 20 mL of the dyes with a known mass of NAM/CoFe₂O₄ NPs in Erlenmeyer flasks (50 mL) at 210 rpm in a constant temperature water bath shaker under varying conditions of different operational parameters. After completion of each set of experiments, the NAM/CoFe₂O₄ NPs were separated from the solutions by centrifugation, and the concentration of the unabsorbed CB or BB in solution was determined by a spectrophotometric method. Each set of investigations was carried out in triplicate, and mean data were employed. The adsorptions (A) of CB and BB from their solutions and the adsorption capacities of NAM/CoFe₂O₄ NPs (Q_e, mg g⁻¹) were determined.

2.4. Reusability NAM/CoFe₂O₄ NPs

An essential factor in ensuring an adsorbent's economic viability is its reusability. Numerous adsorption–desorption cycles were carried out in order to evaluate the degree of reusability of the NAM/CoFe₂O₄ NPs. For the best contact time, NAM/CoFe₂O₄ NPs (1 g L⁻¹) were added to the CB and BB dye solutions (30 mg L⁻¹, 50 mL) and stirred. After being washed and magnetically recovered, the adsorbent was oven-dried overnight at eighty degrees Celsius. A water-based solution of 0.1 M HCl was used for regeneration in the A–D cycles. The desorption efficiency was calculated aswhere the initial, equilibrium, and volumetric concentrations of the CB and BB solutions are denoted by , C_ea, and V_a, respectively, and C_ed and V_d represent the desorption solution's equilibrium concentration and volume, respectively. To maintain its neutrality, the used adsorbent was thoroughly cleaned with double-distilled water before use in the subsequent cycle.

2.5. Determination of the zero-point charge (pH_zpc) of the adsorbent

The pH values of 0.01 M KNO₃ (20 mL each) solutions in ten 50 mL beakers were adjusted to values ranging from 4 to 12 using 0.1 M HCl or NaOH solution and were considered as pH_(initial). The adsorbent was then added (200 mg) to each flask, and the contents were agitated at 200 rpm for 80 minutes in a water-based shaker. The pH of the residual liquid (pH_final) was obtained after 45 hours of equilibration. The pH_zpc value, at which the adsorbent's net surface charge is zero, of NAM/CoFe₂O₄ NPs was derived from the point of crossover of the pH(_final) vs. pH(_initial) curves and was found to be 7. The pH_zpc (zero-point surface charge) of a sorbent is known to play an important role in adsorption phenomena. At solution pH values lower than pH_zpc (pH < pH_zpc), the sorbent surface is positively charged and the adsorbent predominantly adsorbs anionic species. However, when the solution pH is above the pH_zpc (pH > pH_zpc), the adsorbent surface is negatively charged and the adsorption of cationic species is preferred.

3. Results and discussion

3.1. Characterization of NAM/CoFe₂O₄ nanoparticles

The successful incorporation of nicotinamide functionality on the core cobalt ferrite NPs was deduced by comparing the IR spectra of NAM/CoFe₂O₄ NPs and CoFe₂O₄ NPs. The FTIR spectra of CoFe₂O₄ and NAM/CoFe₂O₄ NPs are displayed in Fig. 2(a). The bands observed in the infrared spectrum of CoFe₂O₄ NPs at 590 and 3448 cm⁻¹ were attributed respectively to M–O and M–OH (M = Fe or Co) vibrations.^39,40 In the spectrum of NAM/CoFe₂O₄ NPs, the bands corresponding to the υ(N–H/OH) and υ(C=O) modes due to the NAM moiety appeared at 3485 cm⁻¹ and 1671 cm⁻¹. The band characteristic of the υ(C–N)_asym stretching mode was observed at 1380 cm⁻¹.⁴¹ The C–H_aliphatic and C–H_alkyl stretching vibration (str. vib.) modes were observed at 2916 and 2849 cm⁻¹, respectively.^42,43 The pyridine ring peak at 1592 cm⁻¹ in the free NAM showed a shift to 1470 cm⁻¹ in the NAM/CoFe₂O₄ NPs which was attributed to the coordination of the pyridine nitrogen with the metal center,⁴⁴ corroborating the successful functionalization of CoFe₂O₄ NPs. The FTIR spectra of CB- and BB-loaded NAM/CoFe₂O₄ showed various absorption bands which correspond to different organic functional groups present in the synthetic dyes. The CB-loaded NAM/CoFe₂O₄ NPs showed peaks at 1345–1350 cm⁻¹ (Fig. 2(b)), ascribed respectively to the C–N and C=N str. vib. of the aromatic rings of absorbed the CB.⁴⁵ Similarly, the BB-loaded NAM/CoFe₂O₄ NPs spectrum showed peaks at 1467–1385 cm⁻¹ and a new peak at 1105 cm⁻¹ attributed to the str. vib. of the C=C, C–C, and C–N bonds of the aromatic ring of the absorbed BB.⁴⁶ Several absorption bands, observed at 3485, 2016, 2849, 1570 and 1671 cm⁻¹ in the NAM/CoFe₂O₄ NPs, were shifted to 3438, 2918, 2856, 1550 and 1664 cm⁻¹ and 3448, 2918, 2845, 1560 and 1634 cm⁻¹ after CB and BB loading, respectively,³⁶ which indicated that intermolecular H-bonding and electrostatic interaction could possibly occur between the NAM/CoFe₂O₄ NPs’ surface and the dye molecules, as depicted in Fig. 3.

Fig. 2 FTIR spectra of (a) CoFe₂O₄ and NAM/CoFe₂O₄ NPs and (b) BB- and CB-loaded NAM/CoFe₂O₄ NPs, (c) powder XRD patterns of CoFe₂O₄ NPs and NAM/CoFe₂O₄ NPs and (d) N₂ adsorption–desorption isotherm curves for NAM/CoFe₂O₄ NPs.

Fig. 3 Plausible adsorption mechanism of NAM/CoFe₂O₄ NPs.

XRD investigations validated the surface structure of the NAM/CoFe₂O₄ NPs. The XRD patterns of the NAM/CoFe₂O₄ NPs and CoFe₂O₄ NPs showing the diffraction peaks are given in Fig. 2(c). The XRD pattern of the CoFe₂O₄ NPs displayed peaks at 2θ = 30°, 35.3°, 43°, 47.1°, 56.7° and 62.4° that were matched to the (220), (311), (400), (331), (511) and (440) planes corresponding to the spinel structure.³² The NAM/CoFe₂O₄ NPs displayed well resolved peaks of various planes corresponding to the cubic structure of CoFe₂O₄ consistent with JCPDS card no. 00-022-1086 (Table S2, ESI†). Moreover, the observed broadening of the peaks pointed towards a small particle size and crystalline nature. Employing the Scherrer equation, (D = Kλ/β cos θ), where K (shape factor) = 0.94, λ (X-ray wavelength) = 1.5405 Å, β = full width at half maximum (FWHM = 0.64), and θ = diffraction angle, the crystallite sizes D of the NPs were evaluated. The average crystallite size of NAM/CoFe₂O₄ NPs at the highest intensity peak (35°, (311)) was 13.5 nm. The estimated crystallite size percentages of NAM/CoFe₂O₄ and CoFe₂O₄ were 34% and 50%, respectively (Table S2, ESI†).

The N₂ adsorption–desorption isotherm curve for NAM/CoFe₂O₄ NPs is given in Fig. 2(d). The BET technique was used to find the specific surface area (S), while the BJH (Barrett–Joyner–Halenda) method was used to compute the pore diameter and volume. The S_BET and average pore width and volume of pores were, respectively, 22 m² g⁻¹, 40.841 nm, and 0.2268 cm³ g⁻¹ (Table S3, ESI†). The lower surface area might be due to the presence of organic functionalities on the surface of the material. The pore diameter in the 2–50 nm range suggested that NAM/CoFe₂O₄ NPs are mesoporous in nature.

The surface morphology and the distribution of the chemical elements of the prepared material were studied through scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), as depicted in (Fig. 4(a) and (b)). The SEM micrographs revealed rough, spherical shaped particles with some agglomeration with many pores and grooves of different shapes and sizes which could readily entrap the dye molecules. The roughness of the particles might be ascribed to the presence of nicotinamide functionalities on its surface, while the slight agglomeration might be due to interaction between the magnetic NPs. The SEM-EDX mapping of the NAM/CoFe₂O₄ NPs (Fig. 4(b)) demonstrated the presence of the elements iron (37.47%), cobalt (20.44%), oxygen (31.92%), carbon (7.38%), and nitrogen (1.54%), while CoFe₂O₄ NPs (Fig. 4(a)) showed the presence of iron (45.43%), cobalt (30.62%), carbon (3.40%) and oxygen (20.45%). The existence of an additional EDX peak caused by N and an increase in the wt% of C indicated the presence of NAM. The SEM-EDX mapping of CB- and BB-loaded NAM/CoFe₂O₄ NPs, shown in Fig. 4(b), indicated higher weight percentages of both C and Cl in comparison to those in NAM/CoFe₂O₄ NPs, which might be due to CB or BB functionalities (Fig. 4(c) and (d)).

Fig. 4 SEM images of (a) CoFe₂O₄ NPs and (b) NAM/CoFe₂O₄ NPs and EDX graphs of (c)–(f) CB- and BB-loaded NAM/CoFe₂O₄ NPs.

The TEM images of the NAM/CoFe₂O₄ NPs, displayed in Fig. 5(a), showed that the functionalized Co–Fe oxide NPs are oval and nearly spherical. The image taken at a high resolution of 10 nm showed fringes with spacing of 1 nm.

Fig. 5 (a) TEM image, (b) particle size distribution curve and (c) magnetization curve of NAM/CoFe₂O₄ NPs.

The magnetic strengths of CoFe₂O₄ NPs and NAM/CoFe₂O₄ NPs were determined using a vibrating sample magnetometer, and the hysteresis loops are depicted in Fig. 5(c). For CoFe₂O₄ NPs and NAM/CoFe₂O₄ NPs, magnetic measurement shows magnetic behavior at room temperature with hesteresis activity. The saturation magnetization value (Ms) is 101 and 158 emu g⁻¹ respectively. An increase in magnetization with decrease in size (average size 20 to 30 nm) of the NPs showed that the surface effect was more dominant in the present study.³⁵ The hysteresis curve suggested that the modified adsorbent was magnetically active and that it could be efficiently isolated from the adsorbate solution after adsorption using a strong magnet.³⁶

3.2. Adsorption studies of NAM/CoFe₂O₄ NPs

3.2.1. Change in initial solution pH. The initial pH of the sorbate solution is known to influence the net adsorbent surface charge as well as the speciation or ionization of sorbate molecules, hence influencing the rate of sorption.⁴⁷ The initial pH of the dye solutions varied between 2.0 to 9.0 at the optimal conditions of other process parameters, and the results are presented in Fig. 6(a). The removal percentages of CB and BB were relatively lower in the acidic range, but increased subsequently in the 5.0–9.0 range, with no further adsorption thereafter. The NAM/CoFe₂O₄ NPs surface is negatively charged at pH > pH_zpc (7.0), the dyes solutions were gradually increased the surface acquired increasingly negative charge and electrostatic attraction caused the dye removal to increase. Maximum adsorptions of CB (96%) and BB (94%) were observed to occur at pH 8 (Fig. 6(b)).

Fig. 6 Adsorption efficiencies of NAM/CoFe₂O₄ NPs as a function of initial solution pH (a), pH_zpc (b), NAM/CoFe₂O₄ dosage (c), concentration (d), and contact time (e) for CB and BB removal and the van’t Hoff plot (f).

3.2.2. Variation in mass of NAM/CoFe₂O₄. The effect of NAM/CoFe₂O₄ NP dosage was examined by equilibrating different amounts of nanomaterial ranging from 0.25 to 1 g L⁻¹ with CB or BB solution (30 mg L⁻¹), and the trend in variation of percent adsorption with change in dosage is shown in Fig. 6(c). A rapid increase in the removal rates of dyes was noticed initially, but it subsequently subsided with an increase in the dose. The high rate of adsorption at the lower dosage might be ascribed to a high effective driving force for mass transfer, greater accessibility of the dye molecules per unit adsorbent mass, and availability of more surface area for dyes to be adsorbed. With a gradual increase in adsorbent loading, the increased surface area/larger number of adsorption sites might be responsible for augmentation of the dyes absorbed. However, at higher adsorbent amounts, the resistance to mass transfer and/or availability of a larger number of adsorption sites compared to the number of dye molecules due to lower dye concentration remaining in the solution diminished the adsorption. The maximum adsorption capacities of CB (101 mg g⁻¹) and BB (177 mg g⁻¹) were observed at NAM/CoFe₂O₄ NPs doses of 0.55 g L⁻¹ for CB and 0.7 g L⁻¹ for BB. Similar trends in the sequestration of cationic dyes with a rise in adsorbent dosage have been reported by many researchers.⁴⁸

3.2.3. Change in initial dye concentration. The variation of adsorption efficiency of the adsorbent with change in initial CB or BB concentrations from 10–60 mg L⁻¹ at optimized NAM/CoFe₂O₄ NPs dosage is depicted in Fig. 6(d). The removal percentages of both the dyes showed a decreasing trend from 95% to 86% (CB) and 94% to 89% (BB), and the adsorption capacities changed from 17.3 to 51.6 mg g⁻¹ and 13.5 to 53.8 mg g⁻¹, respectively, with increase in initial concentration. The higher absorption at smaller concentrations might be attributed to a higher number of active locations accessible for dye species/molecule absorption. At constant adsorbent mass, the number of surface binding sites remained fixed, while the number of moles of dye was elevated with the increase in initial dye concentration, causing a decrease in percent adsorption. Similar trends in the removal of BB and CB dyes have been demonstrated by various other researchers.⁴⁹

3.2.4. Contact time. In adsorption investigations, determining the ideal contact time is crucial, since it affects the adsorption rate or kinetics. The adsorbent should remove the adsorbate quickly to be economical. At a constant sorbate concentration (30 mg L⁻¹) and sorbent dose (0.55 g L⁻¹ for CB and 0.7 g L⁻¹ for BB), the dependence of sorptive uptake on the advancement of the sorption process was investigated for 10–60 min at 10 min intervals. Due to the high availability of active sites, effective absorptions of 92.72% BB and 80.95% CB were seen in the initial 40 min, with corresponding adsorption capacities of 40.74 mg BB per g and 44.15 mg CB per g.⁵⁰ But as the active sites were gradually occupied over time, the rate of sorption fell off. This demonstrated that the detoxification procedure took a short time, which should lower the overall process cost (Fig. 6(e)).

3.3. Thermodynamic parameters of NAM/CoFe₂O₄ NPs

The impact of increase in solution temperature on the efficiency of the removal processes of CB and BB from aqueous dye solutions was studied at 298, 303 and 308 K. A decrease in the adsorption capacities from 102 to 92.6 mg g⁻¹ and 169 to 91.7 mg g⁻¹ for CB and BB with the increase in temperature suggested that the nature of the adsorption process was exothermic. The experimental data acquired at different temperatures were utilized to estimate the changes in thermodynamic parameters that occurred throughout the adsorption reaction. Adsorption enthalpy (ΔH°) and entropy (ΔS°) were computed from the slope and intercept of the plot of log(q_e/C_e) vs. 1/T (van’t Hoff plots) (eqn (2)) (Fig. 6(f)), while the Gibbs free energy changes (ΔG°) were calculated by applying eqn (3), and the values are tabulated in Table S4 (ESI†).

ΔG° = ΔH° − TΔS° (3)

The estimated values of ΔG° for CB and BB at 298 to 308 K are −3.9 to −1.47 kJ mol⁻¹, and −5.27 to −1.78 kJ mol⁻¹, respectively. The corresponding values of ΔH° were −76.04 kJ mol⁻¹ and −108.9 kJ mol⁻¹, respectively. The negative values of both ΔG° and ΔH° (Table S4, ESI†) confirmed that the adsorption process was thermodynamically spontaneous, feasible and exothermic. The positive values of (ΔS°) for CB (0.241 kJ mol⁻¹ K⁻¹) and BB (0.347 kJ mol⁻¹ K⁻¹) suggested an increased unpredictability at the solid–solution interface and good affinity of the dye molecules towards NAM/CoFe₂O₄ NPs.

3.4. Modelling of BB and CB adsorption datasets

The most prevalent linear regression approach for modelling isotherms and kinetic relationships in the sorption system usually has intrinsic biases, such as estimation errors and fit distortions due to linearization, when estimating the different model parameters and assessing the best-fit relationship.⁵¹ In contrast, non-linear regression analysis provides results that are nearly correct, dependable, and have a uniform error distribution.⁵² Thus, in this investigation, non-linear regression was performed using the curve-fitting method in SigmaPlot version 14.0. software.

3.5. Adsorption isotherms modelling

The distribution of pollutant species on the surface of NAM/CoFe₂O₄ NPs, those remaining in the solution at a given temperature, the surface characteristics, mode of adsorbent–adsorbate interactions and affinity of adsorbent towards adsorbate can be reasonably explained with the help of different isotherm model parameters. The Langmuir (LA),⁵³ Freundlich (FR),⁵⁴ Temkin (TM),⁵⁵ and Dubinin–Radushkevich (DR)⁵⁶ isotherm models were employed to find the best correlations of the equilibrium data at 298, 303 and 308 K with varying CB or BB concentrations, which might be helpful for designing appropriate adsorption systems.

3.6. Langmuir isotherm

The Langmuir isotherm parameters, which suggest the collaborating relationship of adsorbate molecules to the active positions of adsorbent at equilibrium, were calculated with suitable equilibrium data to the non-linear isotherm equation (Fig. 7(a) and (b)).

Fig. 7 Isotherm curves of CB and BB adsorbed onto NAM/CoFe₂O₄ NPs: Langmuir (a, b), Freundlich (c, d), Temkin (e, f), and D–R isotherms (g, h).

The maximal saturation capacities Q_m, which indicate the adsorbent's adsorptive competitiveness, were calculated using the Langmuir model and was found to be 177 mg BB per g and 101 mg CB per g at 298 K. Also, the higher Q_m revealed that NAM/CoFe₂O₄ NPs were more competitive in terms of adsorption compared to a few other reported adsorbents (Table S6, ESI†). A decrease in Q_m with rise in temperature was indicative of exothermic adsorption. The dimensionless separation factor (R_L),⁵⁷ which is a measure of the favorability of the adsorption process, was calculated using the following relationship (eqn (4)).

R_L = 1/(1 + bC_e) (4)

The calculated Langmuir model parameters, including R_L values, are summarized in Table S5 (ESI†). The magnitude of the correlation coefficient (R²) is generally used to judge the applicability of a given model. The values of b (L mg⁻¹) at 298, 303 and 308 K decreased from 0.13 to 0.06 and 0.23 to 0.12, which pointed out that the extent of binding energy (adsorbate–adsorbent affinity) of CB or BB molecules, respectively, to NAM/CoFe₂O₄ NPs decreased with temperature.

3.7. Freundlich isotherm

Adsorbents with energetically heterogeneous surfaces at which adsorbate molecules are adsorbed physically and form multilayer(s) are explained through the Freundlich isotherm. This isotherm depends on the premise that a solid surface has a variety of adsorption sites, each with a distinct bonding energy, resulting in a heterogeneous surface on which greater binding sites occupy space first. The obtained K_F values [(mg g⁻¹) (L mg⁻¹)^1/n] for CB and BB adsorption were 17.44, 14.09, and 11.52 and 19.34, 18.43, and 13.88, respectively, at 298, 303 and 308 K. The n values varied between 1.69 to 1.39 for CB and 1.29 to 0.96 for BB. Also, the lower than unity values of 1/n (0.59 to 0.71 for CB; 0.72 to 0.94 for BB) suggested the favorable adsorption efficiencies of CB and BB onto NAM/CoFe₂O₄ NPs.

3.8. Temkin isotherm

The TM isotherm examines the interactions of sorbate molecules with various adsorbent surfaces. The isotherm suggests that adsorbate–adsorbent interaction reduces adsorption heat with the adsorbent surface site occupied. The calculated isotherm model parameters are tabulated in Table S5 (ESI†). When the solution temperatures were elevated from 298 to 308 K, there was no significant variation in K_T values, indicating that the binding energy remained constant over the investigated temperature range. However, the values of b_T increased slightly from 1.91 to 5.14 kJ mol⁻¹ and 1.90 to 8.49 kJ mol⁻¹ for CB and BB, respectively, suggesting nearly the same extent of heat of adsorption and probability of bonding at different temperatures.⁵⁸ The binding energy for the ion exchange process is often stated to be 8–16 kJ mol⁻¹. The current study's low results indicated that there was no strong interaction between CB or BB and NAM/CoFe₂O₄ NPs.

3.9. D–R isotherm

The mechanism of an adsorption process can be interpreted by the application of the Dubinin–Radushkevich (D–R) isotherm equation, which is given in Table S5 (ESI†).

The values of q_D–R for CB and BB decreased from 96.83 to 85.11 mg g⁻¹ and 107 to 87.56 mg g⁻¹, respectively, when the ambient temperatures of the solutions were elevated from 298 to 308 K; it emerged that the uptake of the CB and BB on NAM/CoFe₂O₄ NPs were exothermic processes. The obtained E_D–R values (kJ mol⁻¹) for CB and BB removal of between 3.45 to 1.23 kJ mol⁻¹ and 2.83 to 1.73, respectively, were an indication of physical adsorption, because E_D–R values below 8 kJ mol⁻¹ represent physical adsorption.

3.10. Statistical estimation of isotherm validity

The ranges of R² and SEE are typically used to determine which model best fits the data (Table S5, ESI†). The Freundlich isotherm model demonstrated that the equilibrium data denoted multilayer sorption of BB onto the heterogeneous surface of NAM/CoFe₂O₄ NPs with close to unity standards of R² (0.99–0.98 for BB; 0.97–093 for CB) and reduced standards of SEE for BB (1.40–3.56) and CB (4.1–6.9).

3.11. Effect of variation in equilibrium time and evaluation of adsorption kinetics

Fig. 8(a) shows the dependence of the amounts of CB and BB (30 mg L⁻¹) absorbed into 0.55 g L⁻¹ and 0.7 g L⁻¹ of NAM/CoFe₂O₄ NPs, respectively, on different contact times between 10–60 min. Expectedly, the uptake percentages of CB and BB increased with the increase in agitation time. The CB and BB adsorptions increased continuously from 61% to 84% and 81% to 95%, respectively, up to 40 min. The gradual increase in the removal of the dyes during the initial stages of shaking time might be ascribed to the rapid transfer of dye molecules onto the many available surface sites. However, at later stages, the intraparticle diffusion of dye molecules from the surface to the interior pores of the adsorbent probably resulted in slower adsorption rates until equilibrium was achieved. The study of adsorption rates is necessary to develop an appropriate adsorption system for pollutant removal from water. The kinetics and the mechanistic steps required in the adsorption process were explored using the results obtained for the adsorptions of CB and BB at different contact times. Different kinetic models were applied to the experimental data for the determination of various kinetic parameters (Table S7, ESI†).

Fig. 8 Pseudo-first order (a) and (b) and pseudo-second order (c) and (d) kinetic plots and intraparticle diffusion plot (e) and Elovich kinetics plots (f) for CB and BB adsorption.

3.11.1. PFO and PSO kinetic models. In Lagergren's PFO kinetic model,⁵⁹ the rate of change in adsorbate adsorption as a function of time is assumed to be directly correlated with the variation in concentration of saturation and the amount of solid adsorbent and is commonly used over the initial adsorption stage. It has been proposed that the pseudo–first order rate equation is usually followed when adsorption takes place via diffusion through the interface.⁶⁰ The PSO kinetic model⁶¹ is based on the theory that adsorption rate is determined via energetic locations on the solid surface and the adsorbate in an aqueous medium where k₁ and k₂ are pseudo-first order and pseudo-second order rate constants, respectively. The non-linear plot q_tvs. t yielded the values of k₁ and k₂. The close to unity values of R² (0.98 and 0.89) for pseudo-second order kinetic models for CB and BB adsorption implied chemisorption controlled the adsorption of both the dyes on NAM/CoFe₂O₄ NPs.

3.12. Intraparticle diffusion

The overall adsorption rate is governed by surface (or film) diffusion or IPD, either or both of which may be the rate controlling step(s) in the adsorption process. The kinetic data for CB or BB adsorption onto the NAM/CoFe₂O₄ NPs’ surface was fitted into the intraparticle diffusion model, to clarify whether intraparticle diffusion is the rate limiting step. Intraparticle diffusion assumes that adsorbate molecules diffuse into the interstitial spaces of the phase adsorbent particles and binds either physically or chemically with the interior active sites. It is proportional to the square of the difference between the number of unoccupied active sites on an adsorbent surface and the number of engaged sites. The rate limiting step is the chemical connection between adsorbate charges and suitable adsorption sites on absorbent.

The pseudo-first order (PFO) (eqn (5)) and pseudo-second order (PSO) (eqn (6)) equations' rate constants, adsorption phenomena, and associated parameters were calculated using their nonlinear forms:

q_t = q_e(1 − e^−k₁·t) (5)

Intraparticle diffusion assumes that adsorbate molecules diffuse into the interstitial spaces of the adsorbent particles and binds either physically or chemically with the interior active sites. Weber and Morris IPD model is expressed as follows eqn (7).

Q_t = k_ipdt^0.5 + C (7)

where k_ipd (mg g⁻¹ min^−0.5) and C are the IPD rate constant and intercept, respectively. The linear plots between Q_t and t^0.5 (Fig. 8(e)) were used for the calculations of k_ipd and C, and the values are given in Table S7 (ESI†). The values of k_ipd and R² for CB and BB adsorption onto the adsorbent surface were 0.090 mg g⁻¹ min^−0.5 and 0.092 mg g⁻¹ min^−0.5, respectively, and 0.98. IPD is the rate determining step only if the graph passes through the origin. In the cases of CB and BB adsorptions, the graph did not pass through the origin, implying that IPD is not the only rate-controlling step. The values of C (2.55 for CB and 2.60 for BB) suggested a larger role of surface diffusion in the rate-limiting step concurrent with intraparticle diffusion.

3.13. Elovich kinetics model

This model is based on the consideration that no lateral interaction takes place between adsorbate molecules and an energetically heterogeneous adsorbent surface.⁶² The linear equation for this model is expressed as eqn (8),

Q_t = (1/β)ln(αβ) + (1/β)ln t (8)

where the Elovich coefficients α and β depict the initial rates of sorption and desorption. The slope and intercept of the Q_tversus ln t graphs (Fig. 8(f)) were used to calculate the kinetic variables (Table S7, ESI†). The comparatively higher values of α (2.46 mg g⁻¹ min⁻¹ for CB, 0.71 mg g⁻¹ min⁻¹ for BB) than β (0.14 mg g⁻¹ for CB, 0.29 mg g⁻¹ for BB) suggested higher sorption rates than desorption, which indicated the viability of the CB and BB uptake processes onto NAM/CoFe₂O₄ NPs.

3.14. Regeneration and reusability NAM/CoFe₂O₄ NPs

The abilities to regenerate and recycle an adsorbent are essential in terms of overall treatment cost reduction. To investigate the reusability of NAM/CoFe₂O₄ NPs, five sorption–desorption (A–D) cycles (Fig. 9) were performed. NAM/CoFe₂O₄ NPs (1 g L⁻¹) were introduced to CB or BB solution (30 mg L⁻¹, 50 mL) and swirled for optimized contact time. The adsorbent was magnetically retrieved, cleaned, and oven-dried overnight at 80 degrees Celsius. In the A–D cycles, a water-based mixture of 0.1 M HCl was utilized for regeneration. CB and BB percent adsorptions onto spent NAM/CoFe₂O₄ NPs ranged from 93% to 84% and 94% to 83%, respectively, and CB desorption ranged from 88% to 73% and BB desorption ranged from 92% to 71% from the first to fifth cycle. The results suggest that the current adsorbent demonstrated high recycling ability for up to five sessions.

Fig. 9 A–D cycle graph of CB and BB dyes and desorption mechanism of NAM/CoFe₂O₄ NPs.

3.15. Cost analysis (economical aspects) of NAM/CoFe₂O₄ NPs

The current study proposes to assess the costs of both low-cost and industrial adsorbents. Understanding that waste-based NPs are not free, the cost of the raw materials was also assessed and taken into account when calculating the overall cost. The cost of commercial adsorbents, as well as the cost of each step and the total cost for obtaining 1 kg of NPs are displayed in Table S8 (ESI†). Cost analysis is one of the most important factors in assessing any treatment method. The adsorbent used largely determines the cost of adsorption activities. Although adsorbance is a well-known and effective method of removing pollutants, its high cost is a major disadvantage, especially for impoverished countries. As can be seen, commercial adsorbents such as activated carbon (3650 kg⁻¹) and silver NPs (1390 g⁻¹) and nano powder (1800 g⁻¹) are more expensive than the low-cost NAM/CoFe₂O₄ NPs (total cost 560 kg⁻¹). Aside from economic considerations, the environmental effects of an adsorbent should be considered. Economic and environmental analyses should always be undertaken prior to selecting the best NC for a certain application.

3.16. Adsorption from real wastewater spiked with BB and CB

An actual wastewater sample spiked with BB and CB was analyzed in order to evaluate the adsorption efficacy of NAM/CoFe₂O₄ NPs for the elimination of BB and CB in the presence of different pollutants. Genuine effluent from a nearby dyeing facility (1–5 mL) was filtered, diluted with distilled water to 150 mL, and then loaded with a BB and CB mixture (60 mg L⁻¹). Decent amounts of BB (89 mg g⁻¹, 78%) and CB (85 mg g⁻¹, 74%) were successfully extracted from the wastewater sample by the NAM/CoFe₂O₄ NPs (1.2 g) after 60 minutes (Fig. 10(a)). Predictably, the q_e was lower than that of a water-based solution, possibly because other dissolved contaminants interfered with the actual water sample.

Fig. 10 (a) Dye removal from real wastewater spiked with CB and BB and (b) influence of interfering ions on the adsorption of CB and BB on NAM/CoFe₂O₄ NPs.

3.17. Effect of interfering substances in the adsorption of CB, BB

The dye removal effectiveness of NAM/CoFe₂O₄ NPs was investigated in relation to coexisting ions in wastewater by mixing dye solutions with 10 mL solutions of 0.1 M Cl⁻, SO₄²⁻, PO₄³⁻, Na⁺, and Mg²⁺ ions. Because of the electrostatic repulsion between the negatively charged NAM/CoFe₂O₄ NPs surface and anions (Cl⁻, SO₄²⁻, and PO₄³⁻), the batch tests to validate the dye eradication efficiency of NAM/CoFe₂O₄ NPs showed that cations alone compete with cationic CB and BB dyes and increase the dye elimination efficiency of NAM/CoFe₂O₄ NCs. As seen in Fig. 10(b), the adsorbent's ability to remove dye is unaffected by the presence of anions in wastewater.

4. Conclusions

Novel nicotinamide-functionalized CoFe₂O₄ NPs (NAM/CoFe₂O₄ NPs) were created through an environmentally friendly approach using orange citrus (Citrus sinensis) extract as a reducing agent. Analyses employing XRD, SEM, BET and TEM verified the synthesis of spherical NAM/CoFe₂O₄ NPs that were crystalline and had an average size of 13.5 nm. The NAM/CoFe₂O₄ NPs pore volume (0.22 cm³ g⁻¹), pore width (40.84 nm), and S_BET (22.21 m² g⁻¹) were obtained. Furthermore, VSM measurements confirmed the super magnetic character of the generated NAM/CoFe₂O₄ NPs (158 emu g⁻¹). The NAM/CoFe₂O₄ NPs were studied for adsorptive removal of CB and BB from aqueous solution in terms of different operating conditions. The results showed that 0.055 g L⁻¹ and 0.07 g L⁻¹ of NAM/CoFe₂O₄ NPs were sufficient doses to decontaminate CB and BB (30 mg L⁻¹) solutions, respectively, at pH 8. The sorption data were well matched by the FR isotherm model, with maximum LA sorption capacities of 101 mg g⁻¹ for CB and 177 mg g⁻¹ for BB. The processes of sorption followed PSO kinetics, and the rates of sorption were regulated by both IPD and liquid film diffusion stages. Thermodynamics parameters revealed the adsorption process to be spontaneous, feasible and exothermic. Thus, NAM/CoFe₂O₄ NPs could be utilized as a highly effective and low-cost adsorbent for the recovery of these synthetic colorants from water/wastewater. The exothermic aspect of the adsorption process was demonstrated through the findings of ΔH° for CB (−47.48 kJ mol⁻¹) and BB (−12.77 kJ mol⁻¹) and ΔG° for CB (−7.9 to −6.6 kJ mol⁻¹) and BB (−7.2 to −7.0 kJ mol⁻¹). Essentially, the adsorbent surface interacted with the CB and BB molecules via electrostatic interactions, H-bonds, and π–π interactions. To evaluate the adsorbents' economic feasibility, a thorough cost analysis was conducted. The most economical (560 INR per kg) method for removing CB and BB was found to be NAM/CoFe₂O₄ NPs. The used NAM/CoFe₂O₄ NPs could be regenerated and reused for up to five adsorption–desorption cycles without substantial reduction in their efficacy. Thus, NAM/CoFe₂O₄ NPs could be utilized as a highly effective and low-cost adsorbent for the recovery of synthetic colorants from water/wastewater.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data supporting this article have been included as part of the ESI.†

Acknowledgements

One of the authors (Z. S.) is thankful to the University Grants Commission, New Delhi, India, for providing the non-NET fellowship.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5nj01991e

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