Fabrication and characterization of novel lignocellulosic biomass tailored Fe₃O₄ nanocomposites: influence of annealing temperature and chlorazol black E sequestration

Abstract

This study discusses the prospect of using a waste biomass material in the development of novel technological materials for sustainable environmental applications. Novel iron oxide nanocomposites (IONCs) were fabricated using a waste lignocellulosic biomass (LB) material (papaya leaves) by employing the chemical precipitation technique. The IONCs were subjected to annealing temperatures of 353, 573, and 773 K, and precursor ratios of 1 : 1 and 2 : 1. The nanocomposites were characterized by FTIR, XRD, TEM, SAED, EDX, CHN(O), N₂ adsorption, and VSM analyses. An increase in annealing temperature of the LB–IONCs resulted in larger particle size, better crystallinity, and improved magnetic properties. LB–IONCs annealed at 773 K were found to be the most effective material (among its counterparts) for the removal of chlorazol black E, with an efficiency of 96.45% and a monolayer sequestration capacity of 40.02 mg g⁻¹. Hydrogen bonding and π–π dispersive interactions governed the sequestration mechanism.

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1. Introduction

Since the inception of synthetic dyes in 1856 to the present time, there has been a significant rise in the use of dyes in the textile, leather, plastic, paint, and other industries. Azo dyes are organic compounds characterized by azo (–N=N–) linkage and they represent around 70% of the total dyes used in those industries.¹ Azo dyes with benzidine [BZ; (C₆H₄NH₂)₂] moieties are known as BZ-based azo dyes. These dyes are classified as carcinogenic and teratogenic by the National Institute for Occupational Safety and Health (NIOSH), the International Agency for Research on Cancer (IARC), and the Environmental Protection Agency (EPA). The carcinogenic and teratogenic effects of these dyes have been confirmed from experiments carried out on rats, dogs, and epidemiological research on employees exposed to BZ-based dyes.^2,3 Nevertheless, BZ-based dyes are extensively used in industry. Due to the incomplete fixation of these dyes with target materials, they are often discarded with industrial wastewater to natural water bodies causing water pollution. Therefore, the removal of these dyes from wastewater and the aquatic setting is a vital concern for the safeguard of living beings and the environment.

Several researchers worldwide have utilized different dye-wastewater treatment techniques; however, adsorption is considered to be one of the best treatment methodologies.^4,5 Although it has many advantages, the fact that adsorption is accompanied by separation issues is still problematic. Magnetic materials are an alternative and attractive solution to the above problem due to their ease of magnetic separation.⁶ With the recent developments in nanoscience and nanotechnology, significant advances have been achieved in this area of research. Recently, researchers have been more interested in the synthesis of nanoscale composite materials by embedding the magnetic nanoparticles (NPs) in organic matter (e.g. carbon matrix). Previously, commercial carbon materials (activated carbon, graphene, carbon nanotubes, chitosan)^7–10 were considered for the purpose but, with the emerging industrial exploitation of the chemical value of waste materials for revenue generation and a cleaner environment, waste materials have become an alternative material for such use.¹¹

Papaya leaves, a lignocellulosic waste biomass material, have been used for the first time in the synthesis of iron oxide nanocomposites (IONCs). The potential use of this waste biomass for the synthesis of IONCs may revamp agricultural wastes as a material for technological applications. Herein, we have focused on the fabrication of lignocellulosic biomass tailored IONCs under varying annealing temperatures, precursor to iron ratios, and their potential applications for the sequestration of BZ-based azo dye–chlorazol black E from the aquatic setting. The controlled synthesis of bare iron oxide nanoparticles (IONPs), their material properties, and dye sequestration efficiency are also evaluated for a better understanding of the properties and performances of the synthesized nanocomposites. Thus, the present research work may enlighten the scientific community about the innovative exploitation of waste materials for the development of technological materials and their utilization to curb water pollution.

2. Experimental

All of the glassware was purchased from Borosil and Merck (India), and the chemicals were used as received without any further purification. Ultrapure water (18.0 MΩ cm) from a Merck Millipore Direct-Q system was used throughout the experiments. FeCl₃ (anhydrous), FeSO₄·7H₂O, and NaOH were procured from Merck (India). Chlorazol black E (CBE; C₃₄H₂₅N₉Na₂O₇S₂) was purchased from Sigma-Aldrich (India) and a simulated solution (1000 mg L⁻¹) was prepared in the laboratory, and diluted to the requisite test concentrations. The biomass (papaya leaves) was collected from NIT Silchar campus.

The precursor material–papaya leaves – are lignocellulosic materials, and are considered to be an agricultural waste. The as-collected leaves were thoroughly washed with hot ultrapure water to remove any dirt or colour. They were then dried overnight in a hot air oven at 353 K, pulverised, and sieved through a 150 micron mesh. The material thus obtained was referred to as lignocellulosic biomass (LB). A simple chemical precipitation technique was employed for the fabrication of the IONCs. In a typical synthesis route, 50 mL of 0.5 M Fe(II) and 50 mL of 0.75 M Fe(III) solutions were mixed along with the LB (Fe : LB = 1 : 1) by mechanical agitation assisted by ultrasonication for 20 min. Subsequently, 20 mL of 8 M NaOH solution was gradually added to the above mixture, under constant agitation at 343 K for 30 min. The resultant black precipitate obtained was separated using a hand-held magnet, repeatedly rinsed with 50% ethanol–ultrapure water (until the solution reached pH ∼ 7), and then dried overnight at 353 K. The nanocomposites thus obtained were pulverised and referred to as lignocellulosic biomass–iron oxide nanocomposites (LB–IONCs). The dried nanocomposites were annealed in a muffle furnace under an air atmosphere at 573 K and 773 K with a heating rate of 10 K min⁻¹, and a holding time of 1 h. The corresponding nanocomposites were labelled as LB–IONC@353, LB–IONC@573, and LB–IONC@773, respectively. The role of the LB in the nanocomposite was studied by conducting a controlled synthesis of bare IONPs as per the aforementioned procedure but without the addition of the LB. Another ratio of iron precursor to LB (2 : 1) was also prepared and its subsequent effects on the shape and size of the IONCs were also studied. The LB (papaya leaves) was used as a template to immobilize the IONPs and was not removed during the annealing process. The IONCs were annealed at different temperatures so as to (a) increase the stability of the nanocomposites for sustainable environmental applications, and (b) improve the development of a carbon matrix with increased mesopores for the enhanced removal of a wide range of pollutants.

The spectroscopic analysis of the functional groups of the LB–IONCs and bare IONPs was carried out on a Fourier transform infrared (FTIR) spectrometer (Nexus-870, Thermo Nicolet Corporation, USA) in the range 4000 to 400 cm⁻¹, with a spectral resolution of 4 cm⁻¹ in the transmittance mode. The microstructures of the nanocomposites and bare IONPs were characterized by X-ray diffraction (XRD; PANalytical X'Pert Pro, The Netherlands, operating voltage = 40 kV, and tube current = 30 mA) using CuKα (λ = 0.154 nm) radiation within a diffraction range of 10–70° and at a continuous scan rate of 2° min⁻¹. The microstructure and composition of the LB–IONCs and bare IONPs were verified with the aid of a high resolution transmission electron microscope (HR-TEM; JEM-2100, JEOL, Japan, acceleration voltage = 200 kV, filament = LaB₆, and lattice resolution = 0.14 nm) equipped with energy dispersive X-ray (EDX) analyzer (Oxford Instruments, UK). The surface morphology of the nanocomposites was studied by scanning electron microscope (SEM; JSM-6360, JEOL, Japan). The selected area electron diffraction (SAED) patterns of the fabricated materials were also recorded using TEM. The average particle size (from TEM images) and the lattice planes (from SAED patterns and HR-TEM images) of the nanocomposites and nanoparticles were measured using Image J (1.48) software. The carbon, hydrogen, nitrogen, and oxygen content of the LB and the LB–IONCs were measured on a CHN(O) Analyzer (Flash EA 1112, Thermo Finnigan, Italy). The Brunauer–Emmett–Teller (BET) surface area and pore size distribution (PSD) of the nanocomposite were characterised by N₂ adsorption at 77.71 K in a Surface Area Analyser (ASAP 2010, Micromeritics, USA). In order to investigate the magnetic properties of the fabricated materials and bare IONPs, a vibrating sample magnetometer (VSM; Lakeshore 7410, USA) was used. The magnetization parameters were measured at room temperature in an applied magnetic field of ±15 kOe. The detection of CBE in aqueous medium (λ_max = 520 nm) and its concentration analysis were performed on a UV-Visible spectrophotometer (GENESYS 10S, Thermo Scientific, USA). All of the graphical data were plotted using Origin Pro (9.0), and Microsoft Office 2013 Professional Plus was used to prepare the manuscript.

To evaluate the sequestration capacity of the LB–IONCs, the sequestration of CBE was carried out in a batch system at 303 K. In a typical sequestration procedure, the nanocomposites (load; m = 0.5–5.0 g L⁻¹) were interacted with 20 mL of the CBE solutions (initial concentration; C_o = 100 mg L⁻¹). The above mixtures were stirred for 5 h (agitation time; t) in a series of 100 mL stoppered Erlenmeyer flasks, kept in an incubator-cum-shaker at a speed of 140 rpm. Thereafter, the CBE-laden nanocomposites were separated from the mixtures using a hand-held magnet and the supernatants were analysed for residual CBE concentrations. A reference dye sequestration was also performed (as per the above-mentioned procedure) with the bare IONPs and the LB to compare their dye sequestration capacities with that of the LB–IONCs. For isotherm studies, the aforementioned procedure was adopted for different CBE concentrations (C_o = 50–400 mg L⁻¹) with an optimum nanocomposite load (m = 5.0 g L⁻¹). The percent removal and sequestration capacity of the LB, bare IONPs, and LB–IONCs were calculated using eqn (S1) and (S2),† respectively.¹² To ensure the accuracy of the experimental data for quality control and quality assurance, all of the measurements were made in triplicate and the average values were reported.

3. Results and discussion

The surface functional groups of the LB–IONCs were investigated by infrared spectroscopy. The FTIR spectra of the nanocomposites annealed at various temperatures are shown in Fig. S1, given in the ESI.† It is observed that the spectra are all very similar to each other. A wide transmittance band at around 3400 cm⁻¹ in the spectra can be ascribed to the O–H stretching mode of hydroxyl groups. The band at around 1600 cm⁻¹ can be assigned to the C–O stretching of carboxyl groups. It is observed that, with the increase in annealing temperature from 353 K to 773 K, the absorption band which corresponds to the carboxyl group decreases from 1614 cm⁻¹ to 1585 cm⁻¹. This may be due to the loss of oxygen containing surface functional groups at higher temperatures. The transmittance bands in the range 750–500 cm⁻¹ are characteristic bands which correspond to Fe–O vibrations. These bands show the presence of iron oxide in the carbon matrix of the fabricated nanocomposites. In the FTIR spectra, the band at around 1100 cm⁻¹ is a typical band for organic matter adsorbed on iron oxide, and this further confirms the precipitation of iron oxide on the carbon matrix of the LB. The presence of bands at around 1400 cm⁻¹ can be attributed to water molecules adsorbed on the nanocomposite surface. FTIR investigation of the bare IONPs (figure not shown) revealed bands corresponding to –OH groups, Fe–O vibrations, and adsorbed water molecules. With increasing annealing temperature of the nanocomposites, a loss or decomposition of oxygenated surface functional groups occurs, and this is observed from the decrease in intensity of these groups in the FTIR spectra. All of the transmittance bands observed were similar to the values reported in the literature.^1,11,13,14

The XRD patterns of the LB–IONCs (shown in Fig. 1) are in agreement with the standard patterns of bare IONPs (Fig. S2(a)†). At 353 K, the iron oxides present in the nanocomposites were magnetite (black in colour) but due to oxidation at 573 K and 773 K, they changed to maghemite (reddish brown in colour).¹⁵ The characteristic peaks with lattice indices (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) observed for 2θ values of 30.2°, 35.6°, 43.3°, 53.7°, 57.2°, and 62.9°, respectively, correspond to magnetite (JCPDS card no. 19-629). In addition to the above indices, the lattice indices of (2 2 2) and (5 3 1) were observed for maghemite (JCPDS card no. 39-1346). A close inspection of the reference patterns for magnetite and maghemite NPs shows a higher diffraction angle (0.3°) for maghemite (considering the difficulty in interpretation of such results).¹⁶ The same trend was observed for the IONCs annealed at higher temperatures (573 K and 773 K). The average crystallite sizes of the IONCs were 10.36, 10.43, and 11.89 nm, as calculated from Scherrer's equation corresponding to the (3 1 1) diffraction peak, for nanocomposites annealed at 353, 573, and 773 K, respectively. The average crystallite size of the bare IONPs was 8.10 nm. The increase in the crystallite sizes of the LB–IONCs compared to bare IONPs may be due to the presence of carbon in the nanocomposites. The low intensity peaks at 26.2° and 45.5° correspond to (0 0 2) and (1 0 0) planes and are generally assigned to microcrystalline graphitic structures present in disordered carbon materials of the LB.¹⁷ The diffraction peaks weakened or disappeared when these graphitic structures were exfoliated with IONPs and the same was observed in the XRD patterns of the LB–IONCs.⁸ Although crystalline iron oxide is produced at all of the selected annealing temperatures, the IONCs synthesized at the highest annealing temperature (773 K) show the highest intensity of the diffraction peaks compared to IONCs annealed at lower temperatures. It is known that, with the increase in crystallinity of a sample, there is an increase in the intensity of the diffraction peaks, which is observed in the case of the IONCs. The intensity of the diffraction peaks increases and becomes sharper for both graphitic carbon and Fe crystallite grains.

Fig. 1 XRD patterns of the LB–IONCs annealed at different temperatures.

The size and shape of the LB–IONCs are illustrated in the TEM images and their structural information is shown in the SAED patterns. It is evident from the TEM images (Fig. 2(a–c)) that the IONPs are embedded in the carbon matrix of the lignocellulosic biomass. The SEM image (Fig. S3(a)†) of the nanocomposite further confirms that the IONPs are embedded in the carbon matrix of the LB. A schematic representation of the same is also illustrated in Fig. S3(b).† The engrained IONPs have distorted spherical shape geometry with average particle sizes of 18.0, 18.25, and 46.21 nm for the LB–IONCs annealed at 353, 573, and 773 K, respectively. Fig. S4(a–c)† demonstrates the particle size distributions of the LB–IONCs annealed at different temperatures. For a better understanding of the size of the nanocomposites, a reference TEM image of the bare IONPs (Fig. S2(b)†) is also presented. The average particle size of the bare IONPs is 10.39 nm. It has been observed that, with the increase in annealing temperature, the particle size of the IONPs increases, which may be due to the growth of grain sizes. A number of parallel fringes observed in the HR-TEM images (Fig. 2(d–f)) demonstrates the lattice planes of the IONPs. The prominent lattice spacing is calculated as 0.252 nm, which can be indexed to the (3 1 1) lattice plane of the IONPs. The appearance of concentric fused diffraction rings with bright spots in the SAED patterns (Fig. S5(a–c)†) demonstrates the polycrystalline nature of the fabricated materials. The lattice spacings of 0.295, 0.252, 0.209, 0.171, 0.161, and 0.148 nm calculated from the SAED patterns correspond to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) diffraction planes of the IONPs.¹⁸ A lattice spacing of 0.342 nm corresponds to the (0 0 2) plane of disordered graphitic carbon of the lignocellulosic biomass.¹⁹ It is evident from the SAED patterns that the crystallinity (greater number of brighter spots) of the nanocomposites increases with annealing temperature. The above findings from the TEM and SAED investigations are in accordance with the reported results of XRD analysis, apart from the differences in particle sizes.

Fig. 2 TEM images (a–c) and HR-TEM images (d–f) of the LB–IONCs annealed at different temperatures.

In this context, it needs to be mentioned that TEM is the primary analytical tool for measuring particle sizes and their distribution,²⁰ whereas XRD, in accordance with Scherrer's equation, measures the average structural diameter of crystallite grains corresponding to the most intense diffraction peaks.²¹ In addition, the difference in particle sizes also indicates that the IONPs are not composed of similar crystallite grain sizes. However, the smaller crystallite grains may crystallize on the seeds of larger crystallite grains to form larger particles which are observed in TEM images. Similar results were reported in the literature for the synthesis of IONPs.²² The magnetic nanoparticles may become agglomerated when dispersed in a liquid and therefore we have also characterized the nanocomposite (LB–IONC@773) by TEM (Fig. S6†), after removing the agglomerated particles from the aqueous dispersion. An increase in particle size (64.55 nm) of the agglomerates was observed compared to their original size of 46.21 nm. Fig. S4(d)† shows the particle size distribution of the LB–IONC@773 (after agglomeration). The available surface area of the agglomerated particles, as estimated by BET surface area analysis, showed a decrease from their original surface area of 75.55 to 57.83 m² g⁻¹. This may be due to agglomeration of the particles, which reduces the available surface area of the nanocomposite. A study of the pore size distribution of the nanocomposite done by the N₂ adsorption method revealed that the Barret–Joyner–Halenda (BJH) adsorption average pore diameter was around 15.62 nm, which is in the mesoporous range (2–50 nm). Fig. S7† demonstrates the pore size distribution of LB–IONC@773.

The effect of the ratio of iron precursor to LB on the shape and size of the particles was also studied. Fig. S8(a–c)† illustrates the TEM images of LB–IONC@353 (Fe : LB = 2 : 1) at different scales. It is worth mentioning that the size of the IONPs in the nanocomposites (annealed at 353 K) decreased from 18.0 nm to 12.01 nm when the ratio of Fe : LB was increased from 1 : 1 to 2 : 1. This may be due to the increase in Fe content and the decrease of C content, which restricts the size of the particles. In regard to the shape of the particles, the one with the increased ratio of Fe precursor (Fe : LB = 2 : 1) had a distorted square geometry compared to the spherical shape of the former (Fe : LB = 1 : 1).

The composition of the IONCs was established by EDX spectroscopy. The presence of predominant peaks of carbon (C), iron (Fe), and oxygen (O) in the EDX spectra shown in Fig. 3(a–c) further demonstrates the formation of the LB–IONCs. The EDX spectrum of the bare IONPs (Fig. S2(c)†) shows the presence of C and O alone. A decrease in the content of Fe in the LB–IONCs compared to the bare IONPs was observed, which may be due to the presence of carbon in the nanocomposites. The presence of carbon in the IONCs is contributed by the lignocellulosic biomass (LB); the Fe and O content are from the IONPs. The O content of the LB also adds to the total oxygen of the IONCs. Thus, in addition to the C and O content of the LB, the Fe and O content of the IONPs is predominantly responsible for the synthesis of the nanocomposites. In regard to the reproducibility of results, the composition of two different batches of LBs were compared by CHN(O) analysis (Table S1†). It is evident from the table that the C and O content of the two batches (batch 1 and batch 2) of papaya leaves was very similar with a varying percentage of 0.61% (for carbon) and 0.40% (for oxygen). Since, from EDX analysis, it was confirmed that only C, O, and Fe are predominantly present in the LB–IONCs, therefore a negligible difference in carbon and oxygen content will not affect the reproducibility of the overall performance of the LB–IONCs. The LB can be held responsible for the development of the carbon matrix as shown in the SEM image of the nanocomposite. With the increase in annealing temperature, iron becomes more ordered and exposed to larger surfaces, hence, there is an incremental change in its atomic weight%. However, due to increasing annealing temperature, there is a loss of oxygen containing surface groups of the nanocomposite, which results in the decrease in atomic weight% of oxygen. Similarly, with increasing annealing temperature, a small decrease in the carbon content is also observed, which may be due to the loss of carbon materials at higher temperature. The weight percentage (wt%) of the elements as predicted by EDX and CHN(O) (Table S1†) showed no large difference in wt% of C and O between the two analyses, which shows the uniformity of the results. A plausible mechanism for the formation of the nanocomposites is established from the interaction of cellulose [(C₆H₁₀O₅)_n] (the main component of lignocellulosic biomass) with the IONPs. Fig. S9(a)† illustrates the mechanism of formation of the LB–IONCs. The hydroxyl functionalities of the cellulose may be incorporated onto the surface of the IONPs during the synthesis process, which results in the formation of the nanocomposites.

Fig. 3 EDX spectra (a–c) of the LB–IONCs annealed at different temperatures.

The magnetic properties of the LB–IONCs were ascertained from the magnetization data. The magnetization curves for the nanocomposites, measured at room temperature, are shown in Fig. S9(b),† and that of the bare IONPs is depicted in Fig. S2(d).† The values of saturation magnetization (M_s), remanent magnetization (M_r), coercivity (H_c), and squareness (M_r/M_s) of the fabricated nanocomposites are included in Table 1. The magnetization data increase with the increase in annealing temperature for the synthesized materials. The fabricated LB–IONCs are suitable materials for magnetic separation by the application of an external magnetic field because of low values of M_r and H_c and high values of M_s.¹ It is evident from the review of the M_s with respect to nanocomposite size that size is not the sole parameter influencing the magnetic properties of a material.²³ The difference in carbon content of the IONCs may be the reason for the changes observed in the M_s values. The M_s values increased from 14.44 emu g⁻¹ to 46.87 emu g⁻¹, as the carbon content of the IONCs decreased from 50.56% to 46.94% (Fig. 3(a–c)) with increasing annealing temperature. The values of M_s for the fabricated LB–IONCs are smaller than that of bare IONPs (56.91 emu g⁻¹) and other reported magnetite and maghemite NPs.^24,25 This is due to the presence of carbon content in the fabricated nanocomposites apart from the IONPs. These findings are in accordance with trends reported in the literature.⁷

Table 1 Magnetic properties of the fabricated nanocomposites

Magnetic parameters	LB–IONC@353	LB–IONC@573	LB–IONC@773
Ms (emu g^−1)	14.44	36.84	46.87
Mr (emu g^−1)	0.04	0.11	0.68
Hc (kOe)	0.00312	0.00337	0.01434
Mr/Ms	0.0027	0.003	0.0145

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The equilibrium sequestration of CBE by LB–IONCs was studied for optimal operating parameters, such as nanocomposite load (m), agitation time (t), initial CBE concentration (C_o), and reaction temperature (T). From preliminary sequestration studies (optimal parameters; m = 5 g L⁻¹, t = 5 h, C_o = 100 mg L⁻¹, and T = 303 K), it was observed that LB–IONCs annealed at 353, 573, and 773 K had removal efficiencies of 10.02%, 29.83%, and 96.45%, respectively. The observed trend indicates a positive effect of annealing temperature on the sequestration of CBE. This can be explained by the fact that a higher annealing temperature resulted in better development of a carbon matrix with increased mesopores (as predicted from pore size distribution), which contribute to the enhanced removal of CBE. It is established in the literature that activated carbon materials show promising results in the sequestration of dyes from wastewater. Therefore, the carbon content of the synthesized IONCs contributes to the sequestration of the dye. The sequestration experiments were also performed with only the bare IONPs and the LB. Low sequestration efficiencies of 7.36% (bare IONPs) and 2.75% (LB) were observed under the above operating parameters. Hence, the experimental isotherm that corresponds to the LB–IONC annealed at 773 K is shown in Fig. 4, and validated with Langmuir and Freundlich isotherms.²⁶ The isotherm profile can be interpreted as a type II isotherm, as classified by the International Union for Pure and Applied Chemistry (IUPAC). It was observed that the experimental isotherm follows the Langmuir isotherm type with lower standard deviation. The monolayer sequestration capacity (q_m) and separation factor (R_L), calculated from the Langmuir isotherm, were 40.02 mg g⁻¹ and 0.06 (R_L < 1 = favourable sequestration), respectively.

Fig. 4 Isotherm validity plot for the sequestration of CBE onto LB–IONC@773.

The interaction of CBE with LB–IONC@773 was interpreted from FTIR spectra of the blank and CBE-laden nanocomposite, as shown in Fig. S10.† The characteristic peaks of LB–IONC@773 were discussed earlier and herein the shifting of the participating functional groups (post CBE sequestration) is demonstrated. The participation of H-bonding can be predicted as the peak at 3396 cm⁻¹ shifted to 3410 cm⁻¹. After CBE sequestration, the bands at 1585 cm⁻¹ shifted to 1600 cm⁻¹ (signifies the involvement of carboxyl groups) and 1420 cm⁻¹ shifted to 1430 cm⁻¹ (justifies the involvement of H-bonded water molecules). The characteristic peaks of Fe–O vibrations (565 and 630 cm⁻¹) remained unaffected post-sequestration, which indicates that this functional group does not take part in the sequestration process. The sequestration of aromatic compounds on carbonaceous materials often undergoes π–π dispersive interaction.²⁷ The π-electrons of the carbon matrix present in the LB can easily interact with the π-electrons of the aromatic benzidine present in CBE, which may result in the uptake of CBE in an aquatic setting. However, elucidation of the complex mechanism involved requires further insight in future to fully understand the sequestration process.

4. Conclusions

We have established that LB, an agricultural waste material, can be tailored for the synthesis of IONCs using a simple chemical precipitation technique. This study represents an advancement in the transformation of papaya leaves (agricultural waste) into a technological material for environmental applications. The synthesized nanocomposites were subjected to different annealing temperatures (353, 573, and 773 K) and precursor ratios (1 : 1 and 2 : 1), and the subsequent effects on material characterization and dye sequestration were studied. The material LB–IONC@773 has been found to be the most effective nanocomposite, among its counterparts, for the abatement of CBE with a removal efficiency of 96.45% and a sequestration capacity of 40.02 mg g⁻¹. The potential of the synthesized IONCs may be explored in future for the sequestration of other toxic pollutants from the aquatic environment. Reusability and disposability studies would further enhance the prospects of using IONCs for wastewater treatment on an industrial scale.

Acknowledgements

Md. Juned K. Ahmed gratefully acknowledges the University Grant Commission (UGC), New Delhi for financial assistance under the Maulana Azad National Senior Research Fellowship (MANSRF). The authors are grateful to the National Institute of Technology Silchar for providing laboratory facilities.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1 FTIR spectra of the LB–IONCs; Fig. S2 (a) XRD pattern, (b) TEM image, (c) EDX spectrum, and (d) room temperature magnetization curve of the bare IONPs; Fig. S3 (a) SEM image of the LB–IONC@773 and (b) schematic representation of the synthesized nanocomposites; Fig. S4 particle size distributions of (a) LB–IONC@353, (b) LB–IONC@573, (c) LB–IONC@773 (before agglomeration), and (d) LB–IONC@773 (after agglomeration); Fig. S5 SAED patterns (a–c) of the LB–IONCs; Fig. S6 TEM image of LB–IONC@773, after removal from the aqueous dispersion; Fig. S7 pore size distribution of LB–IONC@773; Fig. S8 TEM images of LB–IONC@353 (Fe : LB = 2 : 1) at different scales (a) 100 nm, (b) 10 nm, and (c) 2 nm; Fig. S9 (a) plausible interaction of the cellulose of the LB with the IONPs in the formation of the LB–IONCs and (b) room temperature magnetization curves of the LB–IONCs; Fig. S10 FTIR spectra of the blank and CBE-laden LB–IONC@773; Table S1 elemental composition of the LB (papaya leaves) and the LB–IONCs. See DOI: 10.1039/c5ra20605g

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