Magnetic nanocomposites prepared from red mud and durian husk as an effective bio-adsorbent for methylene blue adsorption

Organic chemicals, especially organic dyes, are over-controlled and discharged from a variety of industrial effluents such as plastics, textiles, food, and paper which have considerably contributed to environmental pollution in recent years (Aksu et al. 2010; Zhou et al. 2014). Many toxic dye components such as aromatic compounds can resist biodegradation (Shahi et al. 2017) and threaten aquatic living and humans (Aksu 2005; El-Bindary et al. 2015). Among them, methylene blue (MB) is a common cationic dye that has received many applications including cosmetics, textiles, papers, and hygienic and health care sections. Gradual exposure to MB over a long duration of time would be harmful to human health and cause certain side effects such as difficulty in breathing, hypertension nausea, and cancer (Aksu et al. 2010). Therefore, it is essential to remove MB dye from contaminated water/wastewater before being discharged into the environment. So far, adsorption technology has gained great attention for MB remediation because the process is inexpensive, easy to design and operate, and has almost no formation of secondary pollutants (Rafatullah et al. 2010; Shitu & Ibrahim 2014; Wang et al. 2016; Ayalew & Aragaw 2020; Hurairah et al. 2020; Radoor et al. 2022).

The choice of adsorbent is key to determining the removal efficiency of the process. In consideration of the available diversity of adsorbents, nanomaterials have shown fascinating properties such as extremely high specific area and abundant reactive sites, which can satisfy the demand for fast kinetic adsorption and high adsorption efficiency (Alver et al. 2020; Bui et al. 2021; Ghereghlou et al. 2022). However, nanoparticles (NPs) tend to aggregate and reduce the surface area and active sites for the adsorption which is resulted from van der Waals forces or other interactions (Kamiya et al. 2018). Surface functionalization/incorporation with suitable chemicals or surfactants could allow for nanomaterials to stabilize in a dispersing medium, with sufficient repulsive force against the van der Waals forces. Although nanomaterials exhibit exclusive properties over conventional materials, efforts have been made to separate them during/after the adsorption process. Recently, magnetic metal oxide NPs having outstanding magnetic response and electronic structure have been extensively studied for various applications (Bardajee & Hooshyar 2014; Mohebbi et al. 2018) due to their advantages of a short-time reaction, easy manipulation, and feasibility to separate from solution by applying a magnetic field.

Based on the above discussions, the study aims to develop a magnetic biocomposite of Fe₃O₄ NPs recovered from red mud incorporated with biopolymer extracted from a durian husk, called bp-Fe₃O₄, as a low-cost bio-adsorbent for the removal of MB dye from water. The material is expected to exhibit a high adsorption capacity toward MB owing to NP size and the ease of separation from the aqueous medium by using an external magnetic field (Kuai et al. 2013). Red mud is known as a huge source of solid waste discarded from the production of alumina from bauxite ores via the Bayer process (Brunori et al. 2005), estimated up to 1.5 tons of red mud per each ton of alumina product. Annually, approximately 70 Mt of red mud is generated globally due to the increasing demand for alumina in industrial activities (Vangelatos et al. 2009; Liu & Zhang 2011), and about 5.0 Mt is estimated in Vietnam particularly (Hai et al. 2014). It causes a huge challenge to utilize this massive volume of red mud waste and reduce the environmental impacts of its disposal (Nguyen et al. 2022). Red mud contains a rich source of inorganic minerals including, remaining aluminum, iron, silica, and calcium (Rivas Mercury et al. 2011). Of those, iron accounts for the largest percentage reaching up to 50% in oxide forms (Paramguru et al. 2004; Zhang et al. 2011). Thus, efforts have extensively focused on red mud as an iron-bearing resource for iron recovery (Piga et al. 1993). Regarding agricultural byproducts, durian husk is a waste produced from processing durian fruit (Durio zibethinus), a highly valued and desired tropical fruit indigenous to Southeast Asia and Vietnam particularly (Hokputsa et al. 2004). It is noted that the husk usually represents more than half of the total fruit mass and contains a high amount of cellulose, hemicellulose, lignin, and pectin (Hokputsa et al. 2004; Payus et al. 2021). Thus, durian husk has recently been utilized as a valuable source for environmental applications such as preparing a low-cost adsorbent for removing water hardness and total dissolved solids (Payus et al. 2021), lead (Ngabura et al. 2019), and MB dye (Sudrajat et al. 2021). The utilization of those agricultural and industrial byproducts for environmental reclamation could reduce their negative impact on the environment and increase value-added opportunities for agricultural/industrial products.

In this study, Fe₂O₃ NPs recovered from red mud were converted to magnetic nanostructured Fe₃O₄ using a facile and one-pot method via carbon combustion. This synthesis process can overcome the drawbacks of high-temperature conditions or long processing times as previously reported (Daou et al. 2006; Ni et al. 2009; Wu et al. 2011). The Fe₃O₄ NPs were then incorporated with biopolymers extracted from durian husk to form the bp-Fe₃O₄ composite. A series of instrumental analyses were used to characterize the morphologic, structural, and magnetic properties of the prepared materials. The resultant biocomposite was used to investigate the applicable adsorption of MB dye from water. The research outcome would provide a low-cost magnetic-based composite for the removal of MB dye in an eco-friendly way and productively.

Ethylenediaminetetraacetic acid (EDTA) and MB (>82%) were purchased from Merck (Merck Millipore, Darmstadt, Germany). Granular activated carbon (CAS 64365-11-3) was purchased from Zhongju (Henan, China). Other chemicals e.g., ammonium hydroxide (NH₄OH, 25–28%), ethanol (C₂H₅OH, 99.7%), hydrochloric acid (HCl, >36%), and sodium hydroxide (NaOH, 96%) were bought from Xilong Chemicals (Guangdong, China).

Durian RI-6 fruits were bought from a local market in Ho Chi Minh City, Vietnam, and taken off to collect only the white part of the husk. Then, the white husk was cut into small species, dried at about 60–70 °C, and ground to powder form. Red mud was collected from Alumin Tan Rai factory (Lam Dong province, Vietnam).

Red mud (30 g) was mixed with 700 mL deionized (DI) water in a 1 L beaker and the suspended solution was continuously stirred for 15 min. The supernatant was decanted and repelled water at about 100 °C to collect a red-orange residue of Fe₂O₃ particles. The dry residue was mixed with activated carbon at a mass ratio of 5:1 and conducted an air-limited calcination process at 700 °C for 2 h. The as-obtained product was washed with plenty of DI water and oven-dried at 100 °C to obtain magnetic Fe₃O₄ NPs. Subsequently, the Fe₃O₄ NPs were treated with a 2.5% w/v NH₃ solution (containing 50% ethanol) at 60 °C for 24 h. The final product of Fe₃O₄ hydroxylated (Fe₃O₄–(OH)_n) NPs was separated from the solution using an external magnet, followed by washing with DI water and ethanol and dried in ambient air.

Biopolymers were extracted from the treated durian husk followed by a previous report (Khan et al. 2014) with some modifications. Firstly, an amount of durian husk was mixed with a diluted HCl solution (pH 3) at a ratio of 1/12 (w/v) in a 500 mL two-necked flask. The mixture was then mechanically stirred at 80 °C for 4 h. After the reaction, the solution was hot-filtered to remove the residue. Subsequently, the filtered solution was crystalized by adding a suitable ratio of ethanol, and the residues were washed and stored with ethanol for further use.

The as-obtained biopolymers were re-dissolved in 100 mL of DI water (1%, w/v), followed by adding 1 g of hydroxylated-Fe₃O₄ NPs. The mixture was dispersed using ultrasound-assisted and continuous stirring (mechanically) at 90 °C for 30 min. The resultant residue was recovered from the solution followed by a similar route to section 2.2.1. The formed powder was collected as called bp-Fe₃O₄.

Batch adsorption experiments were typically carried out in a 50 mL glass bottle containing 20 mL of MB (50 mg/L) and a known amount of each material at pH 7, then the mixtures were continuously shaken at air-ambient temperature for 2 h to reach an equilibrium state. The supernatant solution was then collected from the mixture by using an external magnet and stored at 4 °C for the analysis of MB concentration. Duplicate experiments were done.

The effect of the adsorbent dose was investigated in a range of 1–5 g/L, while other parameters were kept constant. The effect of solution pH on MB adsorption of the nanocomposite was conducted at a pH range of 2 and 8. A suitable diluted HCl or NaOH solution was used for pH adjusting. The kinetic adsorption was conducted with varying adsorption times up to 3 h. The experiments were performed using a series of adsorption solutions at the same experimental parameters with varying predetermined reaction times.

The reusability test of the bp-Fe₃O₄ was performed for seven successive cycles of the MB adsorption–desorption process. The adsorption test was performed as described above. After the adsorption was completed, the MB-laden materials were collected using an external magnet and washed with DI water. The desorption of MB from the material was then performed with 50 mL of 0.1 M EDTA solution for 4 h, followed by subsequent washing with DI water, and ethanol and air-dried, which was ready for the next adsorption test. Note that the magnetic separation was conducted to separate and collect the bp-Fe₃O₄ after each adsorption or desorption process.

The prepared materials were characterized using a scanning electron microscopy (SEM; S-4800, Hitachi, Japan), a transmission electron microscopy (TEM; JEM1010, JEOL, Japan), a Fourier transform infrared spectrometer (FTIR; PerkinElmer, Inc., USA), an X-ray diffraction spectrometer (XRD) with monochromatic Cu Kα radiation (D8-Advance; Bruker, Germany), dynamic light scattering (DLS; Horiba SZ-100, Japan), and a vibrating sample magnetometer (VSM; MicroSence EZ9, USA).

The concentration of MB dye in aqueous solutions was determined using a colorimetric method. The absorbance of MB was recorded at a maximum wavelength of 660 nm using a UV/Vis spectrophotometer (Genesys 20, Thermo Scientific, USA).

Figure 3(d) shows the FTIR spectrum of the bp-Fe₃O₄ compared with those from the biopolymers extracted from the durian husk (Figure 3(c)) and original Fe₃O₄ NPs prepared from red mud (Figure 3(a) and 3(b)). As shown, the bp-Fe₃O₄ appeared to combine the characters of the original biopolymer and Fe₃O₄ NPs. Indeed, the peaks at 2,982.6 cm⁻¹ (C–H stretching) and 1,633.6 cm⁻¹ (COO– asymmetric stretching) were inherited from the biopolymers, while the assigned peak at 597.81 cm⁻¹ was from Fe₃O₄ NPs (Seenuvasan et al. 2013; Silva et al. 2013; Biswal et al. 2016; Ghibaudo et al. 2019; Alterary & AlKhamees 2021; Khashei Siuki et al. 2022; Zhang et al. 2022). The details of peak characteristics are summarized in Table 1. The results proved the successful incorporation of the Fe₃O₄ and biopolymers to form the bp-Fe₃O₄.

Figure 6(b) shows the MB adsorption on the bp-Fe₃O₄ against time function at pH 7. The removal of MB appeared very fast and reached an equilibrium state within 60 min of the reaction, indicating the fast kinetic adsorption of the bp-Fe₃O₄. When the reaction occurred, the number of active sites on the adsorbent was high enough to ease the adsorption of all MB molecules, resulting in a rapid adsorption rate initially. Further increasing time, the intraparticle diffusion steps into limited adsorption sites made the removal rate for dye molecules become slow and reached an equilibrium state. Overall, the fast kinetic adsorption of MB on the bp-Fe₃O₄ would benefit further process development for practical applications.

The dose of adsorbent is a crucial parameter to evaluate the minimum amount of adsorbent required for effective removal of the dye, and is useful for establishing the experimental conditions to meet the performance goals. Figure 6(c) plots the change of the MB adsorption percentage versus the adsorbent dose ranging between 1 and 5 g/L at the equilibrium condition. The adsorption efficiency increased with increasing the adsorbent load and the adsorbent capacity was in a versus trend (Sharma et al. 2011). Indeed, the removal efficiency of MB increased with an increase of the adsorbent dose to 3 g/L. When a dose reached > 4 g/L, the removal efficiency became constant. Generally, an increase in the adsorbent amount resulted in excessive adsorption sites for adsorbing MB dyes and thus, increasing the adsorbed amount of MB dye, while unsaturation of adsorption sites through the adsorption process contributed to the decrease in equilibrium uptake with increasing adsorbent dose (Pooresmaeil et al. 2018). Table 2 shows the comparison of equilibrium adsorption capacity between the prepared composite and other similar adsorbents for the adsorption of MB dye.

This study reports the facile preparation of the low-cost magnetic bp-Fe₃O₄ for environmental applications from industrial/agricultural waste sources. The bp-Fe₃O₄ NPs exhibited the combined physicochemical properties of each constituent component and retained the outstanding magnetic properties of the pristine Fe₃O₄ NPs, allowing for easy separation using a suitable magnet. The MB dye adsorption was a fast kinetic reaction and showed effective removal (>90%) at an optimal pH of 7.0 for most natural water sources. Additionally, the material exhibited excellent reusability (by <2% of efficiency decrease) and retained its magnetic property after six regeneration cycles. Further study on the mechanisms of MB adsorption on the bp-Fe₃O₄ as well as its application to real wastewater treatment is necessary.

N.T.B.: Conception and design, experiments and data acquisition and interpretation, Writing – original draft. P.L.N.N.: Data acquisition. Q.-A.T.: Data acquisition. N.T.M.L.: Acquisition of data. T.T.H.: Data acquisition and analysis, Writing – original draft. T.H.B.: Conception and design, data acquisition and interpretation, Writing – review and editing. All authors approved the version of the manuscript to be published.

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

The authors declare there is no conflict.