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Environ Eng Res > Volume 29(5); 2024 > Article
Gour, Kumar, Arland, Roy, and Rahman: Green synthesis of DL-homocysteine decorated magnetic nanoparticles for selective and efficient mercury remediation from simulated wastewater: Kinetics, isotherm, and mechanism studies

Abstract

The present study focuses on the facile green synthesis of magnetic nanoparticles (MNPs) using onion waste of Allium Cepa L. (MNP@OW) for effective removal of noxious mercury from simulated wastewater. Photogenically synthesized MNPs were functionalized with DL-homocysteine (HC@SiO2@MNP@OW) for selective mercury adsorption. Various characterization techniques were employed to confirm their physical properties. Vibrating sample magnetometer (VSM) studies indicated MNP@OW’s superparamagnetic nature with a saturation magnetization (Ms) of 48.35 emu/g, while HC@SiO2@MNP@OW had a reduced Ms of 4.52 emu/g due to a coating of non-magnetic silica and DL-homocysteine. Both adsorbents showed optimal adsorption at 80°C and pH 8. However, it is explicitly mentioned that HC@SiO2@MNP@OW demonstrated efficient mercury removal at a lower dosage and shorter contact time compared to MNP@OW. Fast separation times of 6 and 26 seconds for MNP@OW and HC@SiO2@MNP@OW, respectively, confirm their ease of separation from simulated wastewater. The Freundlich isotherm model fit the data well and kinetic analysis supported a pseudo-second-order model, revealing a chemisorption mechanism. Moreover, HC@SiO2@MNP@OW demonstrated high selectivity, even in the presence of co-existing ions. Green synthesized MNP@OW and HC@SiO2@MNP@OW exhibited promising potential as low-cost sorbents for efficient mercury removal from simulated wastewater, making them feasible for wastewater treatment in low-economic countries.

1. Introduction

The increasing global water crisis and shortage pose a direct threat to human development and survival. One of the major contributors to the water crisis is water pollution, mainly due to anthropogenic activities. Industrial production, such as mining, metallurgy, and pesticide synthesis, generates a significant amount of metal-contaminated wastewater containing hazardous heavy metals like Hg2+, Pb2+, Cu2+, and Cd2+ [14]. These metals are discharged into the environment and accumulate in the human body through the food chain, leading to various diseases and disorders. Mercury is a notorious heavy metal, causing gene mutations, abortions, harm to the nervous system, kidneys, immune system, etc., and overall posing a severe risk to human health [2]. The World Health Organization has set the maximum acceptable concentration of Hg2+ in drinking water as low as 1 ppb. Therefore, the removal of hazardous heavy metals from wastewater is of utmost importance. Adsorption technology is still considered an efficient technique due to its cost-effectiveness, simplicity, high performance, and low energy requirements. Several adsorbents have been developed for the removal of heavy metals, but they often suffer from limitations such as slow kinetics, low adsorptive capacity, and high preparation costs. Metal oxide nanoparticles (NPs) have emerged as potential adsorbents due to their magnetic properties, ease of separation, and low cost of manufacturing. However, the conventional synthesis routes for metal oxide NPs involve high pressure, temperature, and the use of toxic chemicals. Additionally, aggregation of these NPs can reduce their reactivity and surface area. To overcome these challenges, green nanotechnology is introduced as a strategy to fabricate metal oxide NPs using plant biomolecules as reducing and capping agents [5, 6]. This approach offers an environmentally friendly and cost-effective alternative (Table S1, S2 and Fig. S1). In recent years, magnetic nanoparticles (MNPs), have gained significant attention as an adsorbent for heavy metal removal from water due to their fast kinetics, high removal efficiency, super-paramagnetism behavior, eco-friendly nature, ease in separation, recovery, recyclability, surface modification, etc. (Fig. S2). However, stability of magnetic colloidal dispersion in solutions is the major issue, as MNPs tend to aggregate due to their magnetic attractive forces and the long-range van der Waals attractive forces [7, 8]. To address this problem, MNPs are further tailored for surface modification with other compounds or covered by an active compound. By doing this, the adsorption capacity of MNPs can also be greatly enhanced because large numbers of active sites are introduced [9]. Several research investigations were focused on developing functionalized magnetic nanoparticles characterized by substantial surface areas.
The goal has been to enhance the efficacy and selectivity of adsorbents towards specific heavy metals. DL-homocysteine (HC), an environmentally friendly, nonessential, water-soluble, sulphur-containing amino acid containing three active functional groups (-SH, -NH2, -COOH), is reported to have a strong tendency to coordinate with inorganic cations and metals [1016]. The sulphur in the -SH is divalent and endows HC with an additional function, acting as an antioxidant. In addition, the existence of the -NH2 group and -COOH in the HC molecules will enable the HC-capped magnetic particles to have good biocompatibility and good solubility in aqueous solutions. Notably, the -NH2 group and -COOH group demonstrated outstanding ability to remove a wide variety of heavy metal ions such as Cu (II), Co (II), Ni (II), Zn (II), Pb (II), Cr (VI), and Cd (II) from aqueous solutions owing to the strong metal complexing capability of amino groups [2, 3] and carboxylic groups [4] on the surface of MNPs [8, 17]. In this research work, Hg(II) was selected as a model heavy metal ion to evaluate the removal performance of HC@SiO2@MNP@OW under laboratory conditions.
In this context, the objective of present study is to synthesize novel phytogenic magnetic nanoparticles (PMNPs) by exploiting onion waste (OW) derived from Allium Cepa L. as both a reducing agent and capping agent in the preparation process (MNP@OW). It is apparent from the literature that this is the first time paper reporting the usage of onion waste for the green synthesis of functionalized magnetic nanoparticles to remove mercury from simulated wastewater. Further MNP@OW were coated with silica being a predominant compound utilized for the surface coating of MNPs to mitigate their toxicity, to enhance their stability in water and providing protection in acidic environments [18, 19]. Broadly silica coating enhances particle size, modifies the magnetic properties, facilitates binding with ligands, and forms a protective layer, contributing to colloidal stability with a controllable coating process [1821]. Further silica coated MNP@OW are then functionalized with DL-homocysteine (HC@SiO2@MNP@OW) to enhance their adsorption properties for the removal of hazardous mercury ions from simulated industrial wastewater. The hypothesis and proposed mechanism outlining this research course is depicted in Fig. 1. The fabrication and functionalization of the PMNPs are confirmed using various characterization techniques. The study further investigates the adsorptive performance of MNP@OW and HC@SiO2@MNP@OW, including sorption capacity for mercury ions. The influence of different parameters such as adsorbent dose, adsorbent contact time, solution pH, temperature, and co-existing ions on the adsorption process is examined. Various isotherms, kinetics and thermodynamic equations were employed to understand the removal mechanisms. Additionally, the potential reusability of the adsorbent is also evaluated. Overall, the study aims to provide insights into the adsorptive removal of toxic mercury using green-synthesized and functionalized magnetic nanoparticles, offering a cost-effective and environmentally friendly approach for wastewater treatment in low-economy countries [22, 23].

2. Materials and Methods

2.1. Reagents and Instruments

Mercury(II) chloride (HgCl2), lead chloride (PbCl2), nickel nitrate Ni(NO3)2, Iron(III) chloride (FeCl3), Iron(II) sulfate heptahydrate (FeSO4·7H2O), 25% liquid ammonia, tetraethyl orthosilicate (TEOS), isopropyl alcohol, tetrahydrofuran (THF), and DL-homocysteine (C4H7NO2S), ethylenediaminetetraacetic acid (EDTA) were procured from M/S Merck India Ltd. Double distilled water was used for the solution preparation. All chemicals were used as such without any further purification. The collected sample was dried and ground in mixer (Prestige Elegant V2 Mixer 750 Watt). Magnetic nanoparticle prepared was dried in vacuum oven (SSI LAB. VT 6025). The surface morphology, shape of MNP@OW and HC@SiO2@MNP@OW were observed by a field emission Scanning Electron microscope (FESEM) integrated with an energy dispersive X-ray system (EDX) (Hitachi S 3500) and a transmittance electron microscope (TEM) of Jeol JEM 2100 plus 200Kv). The surface characteristics of the material prepared were assessed employing the BET method (BEL Master ver.7.3.1.0) at a temperature of 77 K with nitrogen gas). Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectrometer (Shimadzu IRAffinity-1S) was used to record the functional group. X-ray diffraction (XRD) measurements (Bruker D8 Advance ECO) were taken to investigate the crystal structures of MNP@OW and HC@SiO2@MNP@OW. The magnetization of the products was measured with a vibrating sample magnetometer (VSM, model-7410S, Lakeshore, USA). The adsorption of Hg(II) was analyzed by Inductive coupled plasma–optical emission spectroscopy (ICP-OES, Perkin Elmer from USA, Mode: AVIO 220 Max). Origin 9.0 software was used for all statistical analyses.

2.2. Sample Preparation

2.2.1. Preparation of magnetic nanoparticles from waste of Allium Cepa L. (MNP@OW)

To prepare the MNP@OW, discarded onion waste was collected from local canteen. They were washed several times with distilled water to remove all unwanted wastes and dried under Sun for 10 to 12 days. Dried onion waste was grinded in the mixer and sieved through 150 micro mesh and stored in labelled bottle. By using the above prepared powdered Allium Cepa L., MNP@OW were prepared by co-precipitation method [17]. 50 mL deionized water was taken in a conical flask and boiled for half an hour. 12 g of ferric chloride and 6 g of ferrous sulphate (2:1 ratio) were added in the boiled water, solution turns brown in color. Further 3 g of powdered onion waste of Allium Cepa L. was added to the above solution mixture and heated at 50°C for 1 h with constant stirring. Then 1.5 M of NaOH solution was added in the above reaction mixture (by maintaining pH 8) until the suspension turns dark black. Further black suspension was stirred for 20 min and cooled at room temperature. Sequentially washed with deionized water and ethanol (with each three times). Prepared MNP@OW separated from the black suspension using a powerful neodymium magnet and dried at 65°C in a hot air oven for 10 h.

2.2.2. Preparation of silica coated magnetic nanoparticles from waste of Allium Cepa L. (SiO2@MNP@OW)

Before the chemical grafting of DL-homocysteine onto MNP@OW, they were coated with silica, as the surface of silica provides compatible binding sites for the soft Lewis acids such as Hg(II). This is done by the hydrolysis TEOS as per the St ber sol-gel process [17, 22, 24]. In brief, 0.3 g of MNP@OW was added to a solution of 50 mL of isopropyl alcohol and 4 mL of deionized water. The solution mixture was ultrasonicated for 20 min, and then 5 mL of 25% aqueous ammonia and 2 mL of TEOS were added successively. The reaction mixture was constantly stirred for 24 h. Finally, the SiO2@MNP@OW nanoparticles were washed thoroughly with deionized water under magnetic separation.

2.2.3. Preparation of homo-cysteine functionalised silica coated magnetic nanoparticles from waste of Allium Cepa L. (HC@SiO2@MNP@OW)

Further functionalization of DL-homocysteine on the SiO2@MNP@OW was done by means of an improved form of the protocol earlier utilized in the functionalization of MNPs for the chemical grafting of DL-homocysteine on the silica coated MNP@OW [5, 6]. 0.5 g of DL-homocysteine was dissolved in 50 mL of THF. 0.2 g of SiO2@MNP@OW was added to the above solution. The mixture was refluxed for 5 hours at 67°C. The resulting grey colored HC@SiO2@MNP@OW were magnetically separated and cleansed by washing with methanol and deionized water (each three times). The HC@SiO2@MNP@OW particles were vacuum dried at 65°C for 10 h and stored in labelled vials [25, 26].

2.3. Determination of Point of Zero Charge (PZC) of HC@SiO2@MNP@OW

Functionalized adsorbent (HC@SiO2@MNP@OW) (30 mg) was taken separately in several beakers and suspended in 50 mL of 0.005 M NaCl solution prepared in double distilled water. The pH of different solutions was adjusted using a pH meter, to an initial value of 2.0 to 10.0 by adding HCl (0.1 M) or NaOH (0.1 M) dropwise. All prepared beakers were kept for constant stirring on magnetic stirrer for 48 h at room temperature. The PZC of the HC@SiO2@MNP@OW was estimated by plotting pHfinal - pHinitial (ΔpH) vs. pH initial for each solution [27].

2.4. Determination of Bruauer-Emmet-Teller (BET) of MNP@OW and HC@SiO2@MNP@OW

BET analysis was conducted to examine the pore size of the MNP@OW and HC@SiO2@MNP@OW. Utilizing the Barrett–Joyner–Halenda (BJH) model, the average pore radius was calculated and the total pore volume, determined using the single-point adsorption value at P/Po = 0.9895. Overall, the as-prepared material exhibited a mesoporous nature conducive to the diffusion of pollutants [28].

2.5. Adsorption Studies

For the adsorption of Hg(II) using MNP@OW and HC@SiO2@MNP@OW as biosorbents from the simulated industrial wastewater, several experiments were performed. The batch adsorption study was done to assess the effect of various parameters like adsorbent dose (10–60 mg), and adsorbent-adsorbate contact time (15–75 min). Concerning the thermodynamics of adsorption, the experimental temperatures varied within the range of 40–120°C. To assess the impact of pH on adsorption, the initial pH values of the Hg solution were systematically adjusted within the range of pH 2–12. The metal ions (mercury, nickel, and lead) adsorption capacity of both the prepared bio-adsorbents MNP@OW and HC@SiO2@MNP@OW were examined by following the standard procedure with slight modifications [9, 28]. For that, in 100 mL of a 5-ppm metal ion solution (mercury, nickel and lead), suitable amount of sorbent was dispersed. The resultant suspension was stirred using a magnetic stirrer at 450–500 rpm for 30 min and filtered by vacuum filtration. An aliquot (obtained after vacuum filtration) was investigated for the remaining concentration of Hg(II) using ICP-OES. The uptake capacity of the magnetic nanoparticle adsorbent for Hg(II) ions was calculated using the given formula of Eq. (1):
(1)
q=C0-Ctm*V
where q is the amount of Hg(II) ions adsorbed, C0 and Ct are the initial and final concentrations of Hg(II) ions, V is the volume of the solution, and m is the amount of adsorbent dose. After the adsorption studies, MNP@OW and HC@SiO2@MNP@OW with adsorbed mercury ions were separated from the solution mixture using an external, powerful magnetic field.

2.6. Kinetic Study

For the kinetic investigations, 0.03 0.500 g L1 of powdered HC@SiO2@MNP@OW was consistently introduced into an Erlenmeyer flask containing 100 mL of metal ion solutions with concentrations ranging from 5, 10, 30, 50, 70 and 100 mg/L. This mixture was then placed in a thermostat water bath under atmospheric pressure and at room temperature (25 ± 2°C), with the shaking speed set at 100 rpm. The final metal ion concentration was measured at intervals of (15–75 min). All kinetic experiments were conducted in triplicate, and the average values were utilized in Eq. (1) to calculate the removal efficiency. Four different kinetics models (Table 1) were employed for the sorption kinetics analysis [28].

2.7. Adsorption Isotherm Study

For the sorption isotherm study, the concentration of the metal ion solution was adjusted from (5–100 mg/L), and a fixed amount of 0.03 g/L of powdered adsorbents (MNP@OW and HC@SiO2@MNP@OW both) were added to 100 mL of the metal ion solution. After each experiment, spent adsorbents were separated from the final effluents using a hand-held magnet, and the effluents were vacuum-filtered through a 0.45 mm filter paper. Following that, the Hg(II) concentration was assessed utilizing ICP-OES. All experiments were performed in triplicate, and the average values were used to obtain the final result. To investigate the sorption isotherm, four equilibrium models (Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich) were applied to elucidate the interaction between mercury ions and prepared adsorbents: MNP@OW and HC@SiO2@MNP@OW (equations provided in Table 2) [29].

2.8. Thermodynamic Study

In the thermodynamic studies, 0.03 g L1 of powdered HC@SiO2@MNP@OW was added to 100 mL of a metal ion solution (25 mg/L), and the adsorption performance was evaluated at different temperatures (40, 60, 80, 100 and 120°C). Thermodynamic parameters, including entropy (ΔS°), change in free energy (ΔG°), and enthalpy (ΔH°), were determined using equations outlined in Table 3 [29].

2.9. Influence of Co-existing Ions

The occurrence of heavy metal cationic ions, specifically Ni2+ and Pb2+, in industrial effluents is frequently reported. These cationic ions can exhibit selectivity in the adsorptive removal of mercury metal. The sorption of the mercury onto HC@SiO2@MNP@OW was investigated by taking 5ppm of the co-existing cationic ions, In this study, 0.03 g/L of powdered HC@SiO2@MNP@OW was mixed into a 100 mL binary solution containing the heavy metal cationic ions (Ni2+ and Pb2+) [30].

2.10. Sorbent Reusability

In the desorption experiment, 30 mg of HC@SiO2@MNP@OW, loaded with Hg(II) from a 5 mg/L mercury ions solution, was subjected to elution using 100 mL of saturated EDTA solution. The process involved shaking the flask at 300°C for 120 min. Subsequently, the concentration of Hg(II) was determined using ICP-OES. To assess the adsorbent’s reusability, this adsorption-desorption cycle was replicated four times, employing the same affinity adsorption method [31].

3. Result and Discussions

3.1. Sample Characterization

3.1.1. FTIR analysis

To ascertain the presence of DL-homocysteine on the SiO2@MNP@OW surface, the FTIR spectra of pristine HC@SiO2@MNP@OW, pure DL-Homocysteine, SiO2@MNP@OW and MNP@OW were recorded (Fig. 2). As illustrated in Fig. 2a characteristic peak attributed to the Fe-O were observed at 546.6 cm−1 representing the stretching vibration of the Fe–O bond of the Fe3O4 [10, 11]. It is quite noticeable that less prominent peak at similar region is also visible in SiO2@MNP@OW and HC@SiO2@MNP@OW (Fig. 2b and 2d). In the FTIR image of SiO2@MNP@OW (Fig. 2b) peak at 1064.2 cm−1 corresponds to silica and peak at 455 cm−1 corresponds to Si-O-Si vibration respectively. Further peaks at 1583.9 cm−1 and 2561.3 cm−1 are evident to N-H bending and S-H stretching in HC@SiO2@MNP@OW (Fig. 2d). The small signal can be attributed to weak force (hydrogen bonding and van der Waals) interactions between the thiol SH and the surface of SiO2@MNP@OW [12]. These results confirm the grafting of DL-homocysteine to the particle surface, which most likely occurs via condensation between surface hydroxyl groups and the carboxylic acid moiety in DL-homocysteine. This indicates the role of oxygen-containing functional groups in the removal of mercury(II). In addition, the band at 2561.3 cm1 corresponds to -SH stretching is not visible in the FTIR images of SiO2@MNP@OW and MNP@OW implying that the HC molecule was bound onto the surface of SiO2@MNP@OW via the formation of the covalent bond Fe-S. In addition, the bands of pristine DL-homocysteine at 1583.9 cm-1 and 2561.3 cm−1 are almost disappeared in FTIR spectrum of HC@SiO2@MNP@OW as compared to Fig. 2c and 2d, implying that the DL-homocysteine molecule was bound onto the surface of MNP@OW due to the formation of the covalent bonds via amino, hydroxyl and thionyl groups. Whereas slight shift in 2919.3 cm−1 peak may indicate non-covalent interactions (Van der Waals interactions, hydrogen bonds [1315] in FTIR spectrum of HC@SiO2@MNP@OW as compared to Fig. 2c and 2d, implying that the DL-homocysteine molecule was bound onto the surface of MNP@OW due to the formation of the covalent bonds via amino, hydroxyl and thio groups [31].

3.1.2. XRD analysis

Phase and crystalline structure of the magnetic nano particle was analysed from XRD and the crystalline size was calculated using Scherrer’s equation, expressed as D = 0.89 λ/β · Cos θ, where D is the average particle size, λ is the wavelength of the CuKα irradiation, β is the full width at half maximum intensity of the diffraction peak and θ is the diffraction angle. In Fig. 2e MNP@OW graph the Fe3O4 peaks are reflecting at 30.6° (220), 35.9° (311), 43.2° (400), 53.5° (422), 57.2°(511) and 62.7° (440) matched properly with the standard XRD pattern of Fe3O4 magnetite (JCPDS:00-019-0629). The characteristic peaks of Fe3O4 are appearing in the same 2θ position along with broadening observed in the 2θ of 15.39°–28.81°. This broadening is because of the composite formation with silica (Fig. 2f). Similar observation of silica coated magnetic nanoparticle was reported by Baskar Thangaraj [16]. In the region of 16.08°–28.12° some broad peaks were observed which corresponds to DL-homocysteine. The diffraction pattern of our fabricated magnetic nanoparticles closely aligns with the established standard [22] (Fig. 2g).

3.1.3. FE-SEM and energy dispersive x-ray (EDX/S) analysis

The SEM images shown in Fig. 2h shows that the MNP@OW had a homogenous granular morphology, with spherical-shaped structure. Similarly, HC@SiO2@MNP@OW showed spherical and homogeneous morphology (Fig. 2i). This behaviour can be attributed to the co-precipitation method used for the synthesis of the magnetic nanoparticles which does not offer a way to control the size distribution [14, 15]. Elemental mapping of HC@SiO2@MNP@OW reveals uniform distributions of Si, C, S, and Fe within the adsorbent (Fig. 3a). In addition, EDS/X analysis provided the qualitative and quantitative status of the elements, which may have affected the fabrication of MNPs. From the EDX spectra (Fig. 2j and 2k). it can be observed that only in the MNP@OW sample, metals such as calcium, sodium, phosphorus, aluminium and manganese are present in appreciable amount. This can be attributed to the organic composition (phytochemicals) of the onion waste of Allium Cepa L. The carbon peaks in both the nanoparticles (HC@SiO2@MNP@OW and MNP@OW) are also due to the carbon-containing biomolecules, present in the onion waste of Allium Cepa L. Moreover, the samples HC@SiO2@MNP@OW exhibited the presence of silicon which is attained due to the implementation of the organic molecule TEOS during the functionalization process [16] (Table S3). The diameter of phytogenic MNP@OW and HC@SiO2@MNP@OW particles are found to be in the average size of 20–25 nm and 600–700 nm respectively (Fig. 2l and 2m). Elemental mapping of HC@SiO2@MNP@OW reveals uniform distributions of Si, C, S, and Fe within the adsorbent (Fig. 3a).

3.1.4. VSM analysis

Magnetic nature of prepared material mainly depends on morphology, shape and size. To obtain the hysteresis loop of prepared phytogenic magnetic nanoparticles VSM was employed at 300 K and magnetic field of −15 to +15 kOe. Results depicted in Fig. 3b show that the saturation magnetization is 4.52 emu/g and 48.35 emu/g for MNP@OW and HC@SiO2@MNP@OW respectively (Table S4). The remanence magnetization and coercivity was zero resultant no magnetic hysteresis loop was observed and it signifies that both the adsorbents MNP@OW and HC@SiO2@MNP@OW are superparamagnetic in nature. The decrease value of saturation magnetization of HC@SiO2@MNP@OW attributed to the decreasing fraction of magnetic material in these particles relative to diamagnetic coating materials (silica and homo cysteine) [23, 24]. VSM results verified that both the adsorbents MNP@OW and HC@SiO2@MNP@OW are superparamagnetic and possessed outstanding magnetic property. The inset in Fig. 3b shows the recovery procedure of MNP@OW and HC@SiO2@MNP@OW. It can be seen that the MNP@OW and HC@SiO2@MNP@OW were well dispersed in water to form a black dispersion and remain suspended in aqueous solution. When an external magnetic field was applied, the MNP@OW and HC@SiO2@MNP@OW shifted rapidly toward the walls of the vial and the fluid became clarifying within less than 6 sec and 26 sec respectively. This confirms that under the presence of an applied magnetic field, it is easy to separate the MNP@OW and HC@SiO2@MNP@OW from an aqueous medium [25, 26]. The distinctive property makes MNP@OW and HC@SiO2@MNP@OW possible to be an ideal material for adsorbent and carrier [32].

3.1.5. TEM analysis

The size, shape, and morphological characteristics of both the MNPs (MNP@OW and HC@SiO2@MNP@OW) were elucidated by TEM analysis (Fig. 3c and 3e). TEM results were recorded at different scale bars, results clearly illustrate the formation of MNP@OW with monodisperse, irregular fine and compact shapes (Fig. 3c). The average diameter of the MNP@OW ranged from 20–25 nm. The particles in MNP@OW were agglomerated due to the existence of the hydroxyl groups in the plant material. After the surface functionalization of MNP@OW with DL-homocysteine (HC@SiO2@MNP@OW), TEM results illustrate the spherical morphology with diameter ranging from 700–750 nm (Fig. 3e). The average diameter of both the MNPs are in well agreement with FESEM analysis [33]. In Fig. 3d and 3f, the selected area electron diffraction (SAED) patterns are illustrated for the MNP@OW and HC@MNP@OW nanocomposites, respectively. The existence of cross-fringes suggests the coherent stacking of inter-network layers and confirms the presence of the Fe3O4 crystalline core in all nanoparticles.

3.1.6. X-ray photoelectron spectroscopy analysis

The adsorption mechanism involves heteroatoms (N, S, and O), which are considered to play a key role in the performance of the sorbent for Hg2+ adsorption [34, 35]. Therefore, for HC@SiO2@MNP@OW (homocysteine-functionalized low-cost magnetic nanoparticles), thiol, amino, carboxyl, and other oxygen-containing functional groups are expected to be the major adsorption sites. The X-ray photoelectron spectroscopy (XPS) spectra of the sorbent before and after Hg2+ adsorption are shown in Fig. 4. These spectra provide valuable information about the chemical composition and surface states of the sorbent, shedding light on the specific interactions responsible for Hg2+ adsorption onto the functionalized magnetic nanoparticles. In the XPS spectra of the sorbent after adsorption (Fig. 4a), two new Hg 5f peaks were deconvoluted, with the 5f7/2 peak appearing at 103.90 eV and the 5f5/2 peak appearing at 105.85 eV. These peaks confirm the presence of mercury capping on the surface of HC@SiO2@MNP@OW, providing direct evidence of the successful adsorption of Hg2+ ions onto the functionalized magnetic nanoparticles [36]. The appearance of these distinct Hg peaks in the XPS spectra further supports the adsorption mechanism involving the active sites on the sorbent, such as thiol, amino, carboxyl, and oxygen-containing functional groups, which interact with and capture the Hg2+ ions on the surface of the nanoparticles.
The high-resolution C 1s spectra of HC@SiO2@MNP@OW (Fig. 4b) were deconvoluted into three subpeaks with binding energies (BEs) of 285.02 and 288.13 before adsorption and 287.28 eV after mercury adsorption. These subpeaks were assigned to C–C (285.02), C–O, C–N and C–S (287.28), and O=C–O (288.13), respectively. After Hg2+ adsorption, the relative contents of C–O (C–N and C–S) and O=C–O decreased at different extents. The disappearance of the subpeaks of HC@SiO2@MNP@OW at 288.13 might be due to the interaction of Hg(II) ions with C–OH. The formation of metal complex C–O–Hg+ made the electrons transfer to carbon atom via oxygen atom, which led to their corresponding BE decreased [37].
Similar results were observed in the O 1s spectrum (Fig. 4c), where the relative contents of O=C at 532.93 eV decreased after the adsorption and new peak at 535.11 eV resembles to H2O [38]. The increasing BE in the O 1s spectrum after Hg2+ adsorption suggest that Hg2+ ions formed coordinate bonds with the oxygen atoms, resulting in a reduction in electron density.
The N 1s spectra of HC@SiO2@MNP@OW before and after adsorption of mercury are depicted in Fig. 4d. Before adsorption, the N 1s spectrum could be fitted into two subpeaks. One sub peak centred at 399.52 eV corresponds to the N atom in the –NH2 groups, while the other subpeak located at 401.14 eV is attributed to the protonated species (–NH3+) [39, 40]. However, after Hg2+ adsorption, the subpeak corresponding to –NH3+ in the N 1s spectrum disappeared, indicating that Hg2+ ions were adsorbed onto the sorbent surface through a chemical interaction with the N atom. Furthermore, a new subpeak with a higher binding energy (402.56 eV) emerged after adsorption. This new subpeak is attributed to the fact that the lone pair of electrons on the N atom was donated to the shared bond between N and Hg2+ when a new type of coordination bond formed between Hg2+ ions and the N atom in the –NH2 groups. Similar observations were reported in the adsorption of Ni2+ using amino-functionalized sorbents by [41]. This confirms the involvement of the amino groups in the adsorption mechanism of Hg2+ ions onto HC@SiO2@MNP@OW and highlights the important role of N atoms in forming coordination bonds with Hg2+ ions during the adsorption process. The N 1s spectrum confirmed the complexation between –NH2 and Hg2+ as one of the adsorption mechanisms in HC@SiO2@MNP@OW.
The S 2p spectra of MNP@OW and HC@SiO2@MNP@OW before and after the adsorption of mercury are illustrated in Fig. 4e. Generally, thiols exhibit two subpeaks, namely, the S 2p3/2 peak at around 163.4 eV and the S 2p1/2 peak at about 164.2 eV which are distinctly visible in Fig. 4e. HC@SiO2@MNP@OW also displayed these two peaks at 163.47 and 164.20 eV before mercury adsorption (Fig. 4e). After mercury adsorption, four subpeaks were observed in the binding energy (BE) region of 160–166 eV. The energy peaks at 165.92 and 164.51 eV corresponded to the free thiol group, consistent with the spectrum before adsorption. Additionally, two new peaks at BEs of 162.10 and 163.51 eV emerged in the S 2p spectrum, assigned to S 2p3/2 and S 2p1/2, respectively, indicating the formation of the complex R–S/Hg. Similar results were observed for the form of R–S/Cu [42]. Therefore, the coordination of Hg2+ with the S atom of thiol groups in the sorbent represents another adsorption mechanism. In conclusion, the N 1s spectrum verifies the complexation between –NH2 and Hg2+, while the S 2p spectrum provides evidence for the coordination of Hg2+ with the S atom of thiol groups as adsorption mechanisms in HC@SiO2@MNP@OW for Hg2+ removal. In Fig. 4f, Fe 2p spectrum is shown, with two major peaks at 711.52 and 725.02 ev in the binding energy range of 700–740 eV, which can be correlated to Fe 2p3/2 and Fe 2p1/2 respectively, proving the phase purity of Fe3O4 [43, 44].

3.1.7. PZC determination of HC@SiO2@MNP@OW

The initial pH at which the ΔpH was zero (PZC) for HC@SiO2@MNP@OW was found to be 7.24. At low pH, adsorbent carries a positive charge (Fig. 4g). When the pH is below the PZC, both amino and carboxyl groups undergo protonation, resulting in electrostatic repulsion with metal positive ions (Hg2+) and reduces the adsorption process. When PZC value is higher than pH, which is 7.24, the entire adsorbent takes on a negative charge primarily due to the predominant contribution of deprotonated carboxyl groups (COO). This results in electrostatic attraction between the adsorbent and mercury ions [45, 46].

3.1.8. BET analysis of MNP@OW and HC@SiO2@MNP@OW

The N2 adsorption-desorption analysis was employed to examine the pore size distribution of both the magnetic nanoparticles (MNP@OW and HC@SiO2@MNP@OW). Fig. 4h and 4i (inset) display the results, indicating an average pore radius of MNP@OW is 3.354 nm and for HC@SiO2@MNP@OW it is 5.042 nm. The BET total pore volume was determined to be 0.1557 cm3g−1, and 0.0666 cm3g−1, respectively. The observed mesoporosity is attributed to interparticle volume, resulting in a notable adsorption capacity for Hg ions [28].

3.2. Effect of Parameters on the Adsorption Capacity of Developed Green Adsorbent

To explore the influences of individual parameters and to determine the optimum levels on the mercury adsorption using fabricated adsorbent a considerable number of variable experiments were carried while maintaining the levels of other operational factors constant [16, 26]. In the present work, the initial optimal values for adsorbent dose (10–60 mg), contact time (15–75 min), pH (2–10), and temperature (40–120°C) were chosen.

3.2.1. Effect of adsorbent dose

To optimize the optimal dosage required to reduce the Hg(II) level to the tolerance limit, varying adsorbent doses ranging from 10 mg to 60 mg were tested. The percentage removal of Hg(II) with different adsorbent dosages of both MNP@OW and HC@SiO2@MNP@OW is shown in Fig. 5a. As depicted in Fig. 5a, the adsorption of mercury ions significantly increased with an increase in sorbent dosages (MNP@OW and HC@SiO2@MNP@OW), which can be attributed to the rise in the number of active sites with increasing adsorbent dosage [28, 47]. However, as time progresses, there is no noticeable augmentation in sorption as these sites become progressively occupied by the adsorbate, leading to heightened mass transfer resistance and the prohibition of adsorbate molecule diffusion [48, 49]. Mercury adsorption efficiency was found to be 84% with 30 mg of HC@SiO2@MNP@OW and 55% with 40 mg of bare MNP@OW. Interestingly, the usage of a lower adsorbent dose (HC@SiO2@MNP@OW) resulted in higher mercury adsorption compared to bare MNP@OW. This observation highlights the strong metal complexing capability of the amino groups and carboxylic groups of DL-homocysteine present on the surface of MNP@OW [50]. Based on these results, subsequent experiments were conducted using 30 mg of HC@SiO2@MNP@OW and 40 mg of bare MNP@OW as the optimized adsorbent dosage.

3.2.2. Effect of contact time

The adsorbent dose was fixed at 40 mg for MNP@OW and 30 mg for HC@SiO2@MNP@OW, and the contact time was varied from 15 to 75 min. In the case of MNP@OW, a high rate of Hg adsorption was observed during the first 45 min with an adsorption efficiency of 41.5% for 30 mg. For HC@SiO2@MNP@OW, the highest adsorption efficiency (84%) was achieved at 30 min, and thereafter both adsorbents reached equilibrium. The superior mercury ion adsorption efficiency (84%) of HC@SiO2@MNP@OW can be attributed to the metal-chelating ability of the -NH2 and -SH functional groups present in the appended DL-homocysteine, despite having a lower surface area compared to MNP@OW as shown in Fig. 5b [29, 51, 52].

3.2.3. Effect of pH

To mitigate the significant influence of pH on the adsorption performance, various approaches were applied to MNP@OW and HC@SiO2@MNP@OW nanoparticles. The effect of pH on the adsorption of Hg species by MNP@OW and HC@SiO2@MNP@OW was analysed by adjusting the pH values, thereby manipulating the surface charge of both MNPs for adsorption at specific pH levels. The adsorbent dose was kept constant at 40 mg with a contact time of 45 min for MNP@OW and at 30 mg with a contact time of 30 min for HC@SiO2@MNP@OW. The effect of pH values ranging from 2.0 to 10.0 on the adsorption of Hg(II) onto MNP@OW and HC@SiO2@MNP@OW nanoparticles was studied. Fig. 5c clearly demonstrates that the removal capacity is highly dependent on the pH. The presence of charged Hg(II) species (Hg2+, HgOH+) and neutral Hg(OH)2 in aqueous medium at different pH values influences the adsorption process due to the high affinity of mercury for sulfur [53].
At low pH, DL-homocysteine is positively charged, and free Hg(II) ions (Hg2+) predominate [45, 54]. This leads to electrostatic repulsion between the positively charged adsorbent and the Hg(II) ions, hindering the removal of mercury(II). The protonation of amino and carboxyl groups further contributes to electrostatic repulsion with positive Hg2+ ions, and the formation of positively charged complexes (S-Hg+) may occur. The presence of a large number of such complexes on the sorbent surface can inhibit further binding due to electrostatic repulsion, resulting in reduced adsorption efficiency.
At slightly higher alkaline pH (pHPZC 7.21), the adsorbent surface presents a negative charge, mainly due to the contribution of deprotonated carboxyl groups (COO), which benefits the adsorption of Hg2+ and Hg(OH)2 through electrostatic attraction between them. Consequently, mercuric hydroxide species (HgOH+ and Hg (OH)2) become more prevalent and can bind to homo-cysteine thiol groups forming neutral complexes (-S-Hg-OH). At pH values above 8, the adsorption capacity of the magnetic nanoparticles gradually slows down, which can be attributed to the deposition of mercury ions as Hg(OH)2, leading to the formation of insoluble metal hydroxides and a decrease in overall adsorption efficiency [42]. Therefore, no adsorption experiments were performed at pH > 10, and the optimal pH for selective removal of Hg was determined to be 8. These finding aligns with the results shown in Fig. 5c.
The overall adsorption mechanism involves electrostatic interactions and complexation with DL-homocysteine groups [12, 55]. The excellent ability of HC@SiO2@MNP@OW to chelate mercury suggests its potential for the adsorption of other metallic species in future applications.

3.2.4. Effect of temperature

To investigate the effect of temperature on Hg(II) adsorption using both MNPs, a series of adsorption tests were conducted. Each test utilized 30 mg of HC@SiO2@MNP@OW for 30 min of contact time at pH 8, and 40 mg of MNP@OW for 45 min at pH 8, in 100 mL of a 5 ppm Hg(II) solution, at different temperatures (40, 60, 80, 100, and 120°C). The temperature effect, as shown in Fig. 5d, revealed that the adsorption capacity using HC@SiO2@MNP@OW and MNP@OW increased as the temperature was raised, indicating a favourable adsorption performance at higher temperatures. However, the adsorption capacity of HC@SiO2@MNP@OW showed only a slight enhancement as the temperature increased from 40 to 80°C. This can be explained as follows: “As the temperature increases, the kinetic energy of the system rises, thereby accelerating the movement rate of metal ions towards the adsorbent’s surface and increasing its adsorption capacity. Simultaneously, the elevated temperature heightened the activities of amino, sulfhydryl, and oxygen-containing groups on the adsorbent’s surface. Resulting enhanced activities improved the contact probability of mercury ions with these functional groups, further increasing the adsorption capacity. As the temperature rises 80°C, the attractive forces between the adsorbent and adsorbate weaken, leading to a decrease in sorption. However, at higher temperatures, the thickness of the boundary layer decreases because of the increased tendency of metal ions to escape from the adsorbent surface into the solution phase. This, in turn, results in a reduction in adsorption as the temperature increases [56, 57]. At 80°C, the adsorption efficiency was found to be 65% for MNP@OW and 88.5% for HC@SiO2@MNP@OW, respectively.

3.3. Adsorption Kinetic

The kinetic model helps to determine the rate and mechanism of adsorption. To examine the adsorption kinetics of both MNP@OW and HC@SiO2@MNP@OW adsorbents, the adsorption studies were applied to four kinetic models: pseudo-first order, pseudo-second order, Elovich equation and intra-particle diffusion (Table 1). The values of the kinetics parameters obtained from the respective liner plots were tabulated in Table 1. The experimental data was found to be best fitted to a pseudo-second order kinetic model, with determination coefficients (R2) of 0.99997 for MNP@OW and 0.99992 for HC@SiO2@MNP@OW (Fig. 5e). However, for the Lagergren pseudo-first order kinetic model, poor linearity plots were observed for Hg(II) ions (Fig. S3), indicating that the pseudo-second order kinetic model provides a better fit to the experimental data for both MNP@OW and HC@SiO2@MNP@OW [5860].
Elovich’s kinetic model was applied to study both adsorption and desorption processes (Fig. S4). The values of α were determined as the ratio of qe/t, while β was obtained from the slope of the graph plotting lnt against qt. A comparison of the values (Table 1) revealed that the α values were higher than the β values, indicating that the adsorption rate of both the adsorbents (MNP@OW and HC@SiO2@MNP@OW) was higher than its desorption rate. However, the regression coefficient values (Table 1) suggested that this model might not be entirely applicable for the observed data [34]. The intraparticle diffusion model examines the uptake of adsorbate within the adsorbent particles and the diffusion through pores during the adsorption process. In this model, the transient uptake of the solute is nearly proportional to the square root of time (t) for a majority of adsorption processes, reflecting the thickness of the boundary layer [61, 62]. The linearized form of this model is expressed as given in Table 1. A kinetic curve for the intra-particle diffusion model for MNP@OW and HC@SiO2@MNP@OW was generated, plotting qt against t1/2 (Fig. S5), yielding a value of xi (6.63_ MNP@OW and 10.094_ HC@SiO2@MNP@OW at 298 K). As per the model’s findings, an increase in the value of x corresponds to an amplification of the boundary layer effect.

3.4. Adsorption Isotherm

The equilibrium isotherms for Hg(II) ion adsorption on prepared magnetic nano-particle was attained at 80°C and pH 8 are presented in Fig. 5f. The active interaction between both the adsorbents (MNP@OW and HC@SiO2@MNP@OW) and Hg ions is evident in the minimum value of 1/n (0.82703_ MNP@OW and 0.801_ HC@SiO2@MNP@OW) and the maximum value of n (1.209_ MNP@OW and 1.248_ HC@SiO2@MNP@OW). A 1/n value below unity indicates substantial adsorption at low concentrations and that the adsorption process can be appropriately applied under optimal conditions [63, 64]. The values of Kf and KL are listed in Table 2. Despite the fact that the graph’s straight line did not intersect with the origin, a slight deviation is observed, likely attributed to the swift mixing during the batch adsorption study. The values of Freundlich constants (KF) for the HC@SiO2@MNP@OW adsorbent namely 5.3856 mgL−1, align closely with the experimental results [34]. KL represents the Langmuir constant associated with sorption energy, indicating the affinity of the adsorbent for the adsorbate (Table 2, Fig. S6). Higher KL values signify a robust binding, suggesting a strong interaction between the adsorbate and adsorbent. In the present study a relatively smaller KL value (0.0674_ HC@SiO2@MNP@OW) indicates a weaker interaction between adsorbate and adsorbent [65]. The determination coefficients (R2) ≥0.97 also indicate a good correlation between the experimental and modelled data. The determined isotherm constants for the adsorption of mercury ions onto HC@SiO2@MNP@OW are presented in Table 2. The linear regression correlation coefficient (R2) was calculated to assess the fitting of various models. Based on the results, the Freundlich fittings resulted in higher R2 values (0.96663) for HC@SiO2@MNP@OW, which implies lower uncertainties and standard errors for the regression with the respective parameters. This confirms that the Freundlich isotherm model predicts the experimental data with higher accuracy compared to the lower R2 values of other isotherm models (Refer Table 2, Fig. S7 and Fig. S8 for Temkin and D-R isotherm model) [62, 66].

3.5. Adsorption Thermodynamics

Thermodynamic parameters were calculated to investigate the nature of adsorption and the influence of reaction temperature on the removal of Hg(II) by the prepared magnetic nanoparticles from waste of Allium Cepa L., as shown in Fig. 5g. The estimated values of standard Gibbs free energy (ΔG°) at various temperatures are presented in Table 3. All these values were negative at the respective reaction temperatures, indicating a thermodynamically viable and spontaneous nature of the metal cation adsorption on the magnetic nanoparticles prepared from waste of Allium Cepa L. Furthermore, the ΔG° values increased with an increase in reaction temperature, suggesting a better possibility of adsorption at higher temperatures (Table 3). The negative value of enthalpy (ΔH°= −1.2750 and −0.8449 of MNP@OW and HC@SiO2@MNP@OW) indicates the exothermic nature of the Hg(II) ions adsorption. Moreover, the positive value of entropy (ΔS° = 4.2038 and 2.8716 of MNP@OW and HC@SiO2@MNP@OW) indicates an increased randomness at the solid-solution interface during the fixation of adsorbates on the active sites of the adsorbent. Overall, the thermodynamic analysis supports the favourable and spontaneous nature of Hg(II) adsorption by the magnetic nanoparticles prepared from waste of Allium Cepa L. and highlights the significance of temperature in enhancing the adsorption efficiency [67, 68].

3.6. Sorbent Reusability

The reusability of an adsorbent is often regarded as a crucial aspect in enhancing the technical viability and economic feasibility of adsorption technology for commercial and industrial applications. To assess the removal of mercury ions and the long-term reusability potential of HC@SiO2@MNP@OW, consecutive cycles of sorption and desorption were investigated. Fig. 5h illustrates the adsorption efficacy of HC@SiO2@MNP@OW for Hg2+ ions over more than four progressive adsorption-desorption cycles. The results obtained indicate that the adsorption efficacy for Hg2+ ions gradually diminishes with an increasing number of adsorption-desorption cycles. At the fourth and fifth cycle, the adsorption efficacy was found to be 75.5% for Hg2+ ions. However, after the fourth cycle, the adsorption efficacy became constant, indicating that the pores of HC@SiO2@MNP@OW were effectively cleared by the desorbing agent. Although the desorbing agent successfully regenerated HC@SiO2@MNP@OW by removing Hg2+ ions, the adsorption capacity of the regenerated material was relatively lower compared to fresh HC@SiO2@MNP@OW. This phenomenon might arise from the saturation of binding sites caused by the enhanced chelating property of EDTA. This saturation of binding sites makes it challenging to reverse adsorption after continuous cycles as well as there is a probality of the re-adsorption of Hg2+ ions onto HC@SiO2@MNP@OW after the desorption process [69]. Nonetheless, despite the gradual decrease in adsorption efficacy over multiple cycles, HC@SiO2@MNP@OW proved to be a promising eco-friendly adsorbent that can effectively remove Hg2+ ions from simulated wastewater [70, 71].

3.7. Selectivity of HC@SiO2@MNP@OW Sorbent

In real water samples, a multitude of metal ions are present concurrently, creating a significant challenge concerning their competitive binding to sorbents, particularly influencing sorbent effectiveness in capturing Hg(II). This study focuses on assessing the competitive adsorption behaviour of Hg(II) against two contaminant metal ions Ni(II), and Pb(II) commonly detected in industrial wastewater by employing HC@SiO2@MNP@OW adsorbent. The findings presented in Table S5 demonstrate that these ions exert minimal influence on the adsorption of Hg(II). This indicates that the material HC@SiO2@MNP@OW demonstrated a selective adsorptive capacity, exhibiting high reusability for mercury over four consecutive treatment cycles. This observation can be elucidated by applying the principles of the Hard–Soft Acid–Base (HSAB) theory. According to this theory, Hg(II) is categorized as a soft metal, characterized by its relatively larger ionic size, low electronegativity, and high polarizability [72]. Sulphur, being a soft base, forms a covalent bond with the soft acid Hg [60, 73, 74]. Similarly, DL-homocysteine, acting as a soft base, allows for the formation of a stable complex with thiol groups. As a consequence, Hg(II) displays a higher affinity for soft donor atoms, like sulfur, in contrast to the comparatively harder metal ions (Pb(II) and Ni(II)) [6976]. Therefore, HC@SiO2@MNP@OW exhibits a tendency to bind Hg(II) more strongly than other metal ions, achieving this in a more robust and stable manner. A comparable phenomenon was also noted in other instances where sulphur functionalized materials were employed for the adsorption of Hg(II) [74, 76].

3.8. Adsorption Mechanism

The FT-IR analysis revealed that oxygen-containing functional groups played a pivotal role in mercury(II) removal. The adsorption experiments and Freundlich modeling demonstrated the remarkable mercury(II) adsorption capacity of the surface-functionalized magnetic nanoparticles. Kinetics studies suggested a favorable three-stage process for the removal of mercury(II), involving initial exterior surface diffusion, intra-particle diffusion, and a plateau equilibrium adsorption phase, with some chemisorption involved. Furthermore, thermodynamic analysis confirmed exothermic and spontaneous mercury(II) adsorption, highlighting strong chemical interactions with the magnetic HC@SiO2@MNP@OW material. XPS data verified the primary adsorption mechanism as complexation between Hg2+ and the –NH2, –SH, and –C–O– groups of HC@SiO2@MNP@OW. Additionally, ion exchange of carboxyl groups and electrostatic adsorption played significant roles in the mercury adsorption process. The plausible mechanisms considered were the hydrogen bonding with oxygen-containing functional groups, formation of ionic pairs, interaction with different mercury species (HgOH+, Hg2+, Hg (OH)3+, HgCl+, OHgO and HOHgO), electrostatic attraction and ion exchange. Additionally, these mercury species exhibited more favourable size and greater mobility than Hg, leading to increased mercury(II) adsorption capacity. The interactions between developed magnetic nanocomposite and various mercury(II) species can be described as follows in Eq. (2) to (16):
(2)
M-OHM-O-+H+(Major,de-protonation and dissociation)
(3)
2M-O-+Hg2+M-O-Hg-O-M (more major,ionic-pair formation)
(4)
M-O-+HgOH+M-O---HgOH (Major,electrostatic attraction)
(5)
M-O-+HgCl+M-O---HgCl (Major,electrostatic attraction)
(6)
M-O-+Hg(OH)3+M-O---Hg(OH)3(Major,electrostatic attraction)
(7)
M-COOHM-COO-+H+(Major,de-protonation and dissociation)
(8)
2M-COO-+Hg2+M-COO-Hg-OOC-M (more major,ionic-pair formation)
(9)
M-COO-+HgOH+M-COO---HgOH (Major,electrostatic attraction)
(10)
M-COO-+HgClH+M-COO---HgCl (Major,electrostatic attraction)
(11)
M-COO-+Hg(OH)3+M-COO---Hg(OH)3(Major,electrostatic attraction)
(12)
M-O-+M-COO-+Hg2+M-O-Hg-OOC-M (more major,ionic-pair formation)
(13)
2M-O-H+O-Hg-O-M-O-H---O-Hg-O---H-O-M (Minor,hydrogen bond)
(14)
M-O-H+H-O-Hg-O-M-O-H---O-Hg-OH (Minor,hydrogen bond)
(15)
2M-COOH+O--Hg-O-M-COO-H---O-Hg-O---H-OOC-M (Minor,hydrogen bond)
(16)
M-COOH+HO-Hg-O-M-COO-H---O-Hg-OH (Minor,hydrogen bond)
Note: M represents developed magnetic nanocomposite.

4. Conclusion

Homo-cysteine, with its –SH and –NH2 functional groups, was incorporated into MNP@OW via simple co-precipitation method to enhance its adsorption efficacy for mercury ion in simulated wastewater. This led to a significant improvement in the adsorption capacity of HC@SiO2@MNP@OW for mercury, achieving a maximum adsorption capacity of 88.5%, which surpassed that of MNP@OW (65%). The high adsorption capacity of HC@SiO2@MNP@OW at a lower dosage and with rapid adsorption rates compared to MNP@OW, is due to the metal-chelating ability of the –SH and -NH2 functional groups attached to homo-cysteine. This confirms that the functional groups present on the HC@SiO2@MNP@OW, such as carboxyl, thiol, amino, and oxygen-containing groups, play a crucial role in Hg2+ adsorption, forming strong coordination bonds with the adsorbate and contributing to the efficient removal of Hg2+ ions from the solution.
Furthermore, HC@SiO2@MNP@OW exhibited excellent selectivity for Hg2+ even in the presence of competing metal ions at high concentrations, with a selectivity order of Hg2+> Ni2+ >Pb2+ being observed, highlighting its potential as a promising and selective adsorbent for mercury removal from wastewater matrices. The experimental results revealed that both MNP@OW and HC@SiO2@MNP@OW exhibited optimal adsorption performance at 80°C and pH 8. However, HC@SiO2@MNP@OW demonstrated superior efficiency in mercury removal at a lower dosage and shorter contact time compared to MNP@OW. The fast separation times of 6 and 26 seconds for MNP@OW and HC@SiO2@MNP@OW, respectively, further confirmed their ease of separation from the simulated wastewater. Both MNPs showed multilayer adsorption of mercury ions based on the Freundlich isotherm model. The kinetic analysis indicated a pseudo-second-order model, suggesting chemisorption. Thermodynamic analysis revealed exothermic and spontaneous adsorption, indicating spontaneous and heat-releasing process. XPS results confirmed that the adsorption mechanism mainly involved the complexation of Hg2+ with the –NH2, –SH, and –C–O– groups of HC@SiO2@MNP@OW. Additionally, ion exchange of carboxyl groups and electrostatic adsorption played significant roles in the mercury adsorption process. In conclusion, HC@SiO2@MNP@OW demonstrates great promise as a cost-effective and efficient material for the selective removal of mercury ions from polluted water, making it a valuable tool for water purification and environmental remediation. Its operational efficiency and potential for multiple treatment cycles and metal ion recovery further enhance its significance for water and wastewater treatment processes.

Supplementary Information

Acknowledgements

The authors express their thankfulness to the Office of Research and Innovation, RUAS for providing monetary assistance to PG as JRF under grant number (Ref. No.-RUAS/RD/2023/015). We sincerely acknowledge the financial support provided to JK by RUAS, Bangalore, India, through the Seed Money Scheme - 2023 (Reference No. RUAS/Dean, OoRI/2023/022). The authors gratefully acknowledge Centre for Incubation, Innovation, Research and Consultancy (CIIRC) and Essen and Co., Bangalore, India, for providing instrument facilities.

Notes

Conflicts-of-interest Statement

There are no conflicts to declare

Author Contributions

J.K. (Professor): Conceptualization, Designing, Visualization, Methodology, Data curation, Data analysis/interpretation, Supervision, Funding acquisition and Writing-original draft, Critical revision of manuscript; P.G (PhD Student): Experimental, Data acquisition and curation, Statistical analysis, and Funding acquisition; S.E.A. (PhD Student): Validation and Writing; L.D.R. (PhD Student): Validation, Revision of manuscript and Writing; N.R. (PhD Student): Validation and Writing.

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Fig. 1
Synthesis of functionalised MNP@OW and hypothetical adsorption mechanism for Hg(II).
/upload/thumbnails/eer-2023-584f1.gif
Fig. 2
FTIR spectra (a) MNP@OW, (b) SiO2@MNP@OW, (c) Pure DL-homocysteine and (d) HC@SiO2@MNP@OW respectively, XRD Spectra (e) MNP@OW, (f) SiO2@MNP@OW, and (g) HC@SiO2@MNP@OW respectively, FE-SEM image (h) MNP@OW (i) of HC@SiO2@MNP@OW, EDX/S spectra (j) MNP@OW (k) HC@SiO2@MNP@OW Particle size distribution histogram (l) MNP@OW and (m) HC@SiO2@MNP@OW.
/upload/thumbnails/eer-2023-584f2.gif
Fig. 3
Elemental mapping (a) HC@SiO2@MNP@OW, VSM magnetization curves (b) MNP@OW and HC@SiO2@MNP@OW respectively, High-resolution TEM images, and SAED pattern of MNP@OW (c–d), HC@SiO2@MNP@OW (e–f).
/upload/thumbnails/eer-2023-584f3.gif
Fig. 4
XPS survey spectrum (a) Hg 5f spectra, (b) C 1s spectra, (c) O 1s spectra and (d) N 1s spectra, (e) S 2s spectra and (f) Fe 2p spectra of MNP@OW and HC@SiO2@MNP@OW-Hg respectively, Pzc determination curve (g) HC@SiO2@MNP@OW. N2 adsorption-desorption isotherm and pore size distribution plot (inset) of (h) MNP@OW and (i) HC@SiO2@MNP@OW.
/upload/thumbnails/eer-2023-584f4.gif
Fig. 5
Effect of (a) adsorbent dose, (b) contact time, (c) pH (d) temperature on the adsorption of Hg(II) ions from simulated wastewater (e) pseudo-second order kinetic model graph, (f) Freundlich isotherm model graph, (g) thermodynamic graph and (h) reusability of HC@SiO2@MNP@OW for successive sorption desorption cycles.
/upload/thumbnails/eer-2023-584f5.gif
Table 1
Adsorption kinetics parameters for the adsorption of mercury on MNP@OW and HC@SiO2@MNP@OW
Kinetic Models Parameters Hg2+

MNP@OW HC@SiO2@MNP@OW

qe (Calculated) (mg/g) 7.2225 10.67
Pseudo-first order log(qe-qt)=logqe-k1t2.303 qe (experiment) (mg/g) −0.4875 0.31166
K1 −0.0051 −0.0007
R2 0.9034 0.27373

Pseudo-second order tqt=tqe+qe2k2 qe (mg/g) 7.20 10.65
K2 0.15889 0.23977
R2 0.99997 0.99992

Elovich qt=1βlnβ+1βlnt α/mg/g/min 0.1605 0.3556
β/g/mg 0.1521 0.2087
R2 0.95189 0.62744

Intraparticle diffusion
qt = kipdt0.5
Kipd/mg/g. min−0.5 0.06035 0.06479
I 6.635 10.094
R2 0.9075 0.48137

Note: qe (exp.): Experimental qe (mg/g); qe (cal.): Calculated qe (mg/g); I: Intercept; k1& k2: equilibrium rate constants for Pseudo-first and Pseudo second order kinetic models (min−1); R2: Regression coefficient; α = adsorption rate constant; β= desorption rate constant; kipd = intraparticle diffusion rate constant (mgg−1·min−0.5)

Table 2
Parameters of adsorption isotherm models for the adsorption of mercury on MNP@OW and HC@SiO2@MNP@OW
Langmuir Isotherm modela Freundlich Isotherm Modelb Temkin Isotherm Modelc Dubinin-Radushk evich Isotherm Modeld
Ceqe=1KLqmax+Ceqmax logqe=logKF1nlogCe qt = kit1/2 + C lnqe=−βɛ2
where, ɛ=RTln (1+1/Ce) and E=1/√2β
Qm,cal (mg/g) =91.743 KF=5.3856 BT=39.614 Qm=1859.1
KL=0.0674 n=0.80196 KT=0.36911 E=0.27389
R2=0.87301 R2=0.96663 R2=0.781 R2=0.94808

Note:

Ce is the equilibrium concentration (mg/L), qe is the amount of adsorbate adsorbed per unit mass of adsorbent (mg/g), and Qmax and Kl are Langmuir constant.

where k (mg/g) and 1/n are the constant characteristics of the system.

where ki (mg/g min1/2) is the diffusion rate constant and C is a constant (mg/g).

where qm (mg/g) is the D-R monolayer adsorption capacity, β (mol2/J2) is a constant related to adsorption energy, and ɛ is the Polanyi potential, R is the gas constant (8.314 J/mol K) and T (K) is the absolute temperature. The adsorption energy E (kJ/mol), based upon constant β.

Table 3
Thermodynamic parameter value for the adsorption of Hg(II) by prepared magnetic nano-particle and its functionalized magnetic nano-particle at different temperatures (K)
Sr. No. Thermodynamic equation Adsorbents Temperature (K) KL ΔG° (KJ mol−1) ΔH° (KJ mol−1) ΔS° (KJ mol−1) R2
1 ΔG° = -RTln KL
where
KL at equilibrium is
MNP@OW 313 2.7099 −2.5942 −1.2750 4.2038 0.98461
333 2.6204 −2.6670
353 2.564 −2.7633
2 KL=qeCe HC@SiO2@MNP@OW 313 1.9538 −1.7429 −0.8449 2.8716 0.99828
333 1.9178 −1.8028
353 1.8831 −1.8576
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