1 Introduction

The water crisis has tremendously increased due to the rapid development in agricultural activities, industrialization, and climate change, threatening the sustainability of ecological systems worldwide [1, 2]. According to the United Nations’ report, 2.3 billion people inhabit water-stressed countries [3], which is estimated to substantially upgrade by 2050 [4, 5]. Approximately 4 billion people experience water scarcity for at least 1 month per year, and 2 billion people will suffer from acute water scarcity by 2050 due to water shortage [6, 7]. Therefore, practical approaches are required to deal with water contamination and fulfill sustainable development goals [8,9,10].

The water quality has deteriorated as a result of heavy metals especially mercury (Hg), which is a priority pollutant due to hindering the enzyme binding sites and disturbing protein synthesis based on the WHO [11] and the US EPA reports [12, 13]. Hg is a neurotoxin and its low vapor pressure intensifies human poisoning [14, 15]. It is ranked second toxic waste by the US Agency for Toxic Substances and Disease Registry [16, 17], and a limit of 2 µg/L Hg2+ in drinking water has been established by the US Environmental Protection Agency (USEPA) [15, 18]. Therefore, Hg elimination from water resources and wastewater should receive more attention to avoid its adverse effects on ecosystems and human health.

Despite several accessible techniques for heavy metal removal including ion exchange, reverse osmosis, chemical precipitation, bioremediation, and coagulation, their implementation suffers from byproduct and sludge generation, required time, and cost of operation [19,20,21,22]. In the last few decades, researchers have focused on advanced porous materials regarding the exponential increase of nanomaterial demand [23, 24]. Although more than 99% heavy metal removal efficiency can be achieved through the adsorption process [15, 25], the adsorbent properties have fundamental roles in the removal potential. Magnetic nanoparticles have already exhibited special features including large surface area, high reactivity, selectivity, and catalytic potential. This can be associated with large oxygenated functional groups such as carboxylic, hydroxide, and amide on their surface that facilitate the metal ion bonding through surface complexation and ligand exchange [26]. However, some nanomaterials including CoFe2O4, MnFe2O4, and Fe3O4 have inherent poor adsorption capacity for heavy metals due to the lack of surface-active functional groups. Zhao et al. [27] believed that their characteristics can be improved using a coating layer of SiO2 under the outer layer of CoFe2O4 particles functionalized with polypyrrole. The CoFe2O4-based core–shell carriers with the grafted group of -NH2 is another proposed ideal adsorbent for Hg with 149.3 mg/g adsorption capacity [28]. Bao et al. [29] also synthesized mercaptoamine-functionalised silica-coated magnetic nanoparticles for the efficient removal of Hg and Pb from wastewater with maximum adsorption capacities of 355 and 292 mg/g, respectively. A novel magnetic diatomite-based material of DMT/CoFe2O4-p-ATP with an adsorption capability of 213.2 mg/g was also recommended for Hg removal [30]. Kaolin with a high specific surface area and negative charge is appropriate for heavy metal adsorption but easy agglomeration limits its further usage. Therefore, polypyrrole-functionalized magnetic Kaolin was proposed for Hg removal with substantial adsorption capacity of 317.1 mg/g [31].

Although numerous synthesized magnetic nanomaterials for Hg elimination have been proposed, a cost-effective and practical adsorbent with a simple design is indispensable for removing Hg from wastewater. Therefore, the synthesis, characterization, and adsorption efficiency of cocopeat-based magnetic activated carbon (CPAC-Fe3O4) nanocomposite were considered, and the following goals were determined: (1) preparation of magnetic nanocomposite using cocopeat; (2) characterization of CPAC-Fe3O4 nanocomposite structure and morphology; (3) optimization of Hg removal from aqueous solutions considering the impacts of pH, temperature, contact time, Hg concentration, and adsorbent dosage; and (4) interpretation of adsorption mechanism using isotherm, kinetics, and thermodynamic modeling. Summing up, the results of this study can provide a framework to elaborate cost-effective and promising nanocomposite for industrial wastewater treatment.

2 Materials and methods

2.1 Chemicals

Analytical-grade chemicals were utilized in experimental research. Hg chloride (HgCl2) was used for the preparation of Hg solutions. Iron (II) chloride (FeCl2. 4H2O) and iron (III) chloride (FeCl3. 6H2O) were applied for the magnetization of the adsorbent. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were also utilized to adjust the pH of the solutions. All chemicals were supplied by the Merck Company.

2.2 Synthesis of CPAC-Fe 3 O 4 nanocomposite

Cocopeat samples were purchased from a local plant nursery and collected from the fiber separation of coconut husk. A total of 20 g of cocopeat samples was added to deionized water in a 1000-mL flask and heated to 65 °C for 20 min. Then, FeCl2. 4H2O and FeCl3. 6H2O with a molar ratio of 2 to 1 were added to the mixture and stirred for 30 min to ensure the diffusion of iron cations to the sample. Afterward, the sample was dehydrated and then transferred to 1 M NaOH solution and heated to 60 °C for 30 min to complete the synthesis of cocopeat-based magnetic nanoparticles. The color of the mixture was changed to black, indicating the successful synthesis of magnetic nanoparticles. The as-synthesized sample was collected and rinsed with deionized water two times to remove impurities and adjust the pH around neutral conditions. Afterward, the samples were dried under sunlight for 48 h, thus powdered and sieved to the size of 60 mesh no. (0.25 mm). The cocopeat samples were further subjected to heating at a rate of 5 °C per minute in a muffle furnace at 600 °C under nitrogen gas with a pressure of 100 mL/min for 2 h.

2.3 Characterization

The elemental analysis was applied by an elemental analyzer (Flash EA 1112, USA), which showed 42% C, 47% O, 6.5% H, 1.5% N, and 3% S contents of cocopeat. The morphology of the CPAC-Fe3O4 nanocomposite was analyzed with field emission scanning electron microscopy (FESEM, FEI NOVA NanoSEM 450, Japan). The crystal structure of the CPAC-Fe3O4 nanocomposite was determined by X-ray diffractometer (XRD, Ultima, Japan). Fourier-transform infrared spectroscopy (FTIR, Tensor 27, Bruker, Germany) was performed to assess the surface functional groups of the CPAC-Fe3O4 nanocomposite. The Brunauer–Emmett–Teller (BET, Belsorp mini II, Microtrac Bel Corp, Japan) method was employed to determine the specific surface area of the synthesized CPAC-Fe3O4 nanocomposite. The magnetic properties were measured using the vibrating sample magnetometer (VSM, LBKFB, Kashan Kavir Magnetic Co. Iran).

2.4 Batch adsorption experiment

The adsorption behavior of the CPAC-Fe3O4 nanocomposite was evaluated for Hg removal considering the impact of pH, adsorbent dosage, contact time, initial Hg concentration, and temperature. A 1000 mg/L Hg stock solution was prepared by dissolving the appropriate volume of HgCl2 in deionized water. To determine the optimum condition for maximum Hg removal, the pH of the solution was adjusted on 3, 4, 5, 6, 7, and 8 using 0.1 M HCl and NaOH, while other parameters were constant (0.01 g adsorbent dose, 20 mg/L Hg concentration, and contact time of 60 min at 25 °C). The mixture was shaken at 100 rpm and allowed to react with the magnetic nanocomposite. Then, the CPAC-Fe3O4 nanocomposite was removed from the solution by the magnetic field, and the mixture solution was centrifuged (HERMLE Z300, USA) at a rate of 4000 rpm for 5 min and then filtered. A total of 1 mL HNO3 was further added to the mixture to avoid Hg ions precipitation. The remained Hg concentration in the solution was measured by cold vapor technique with atomic absorption spectroscopy (AAS, Uniam 919). The limit of detection (LOD) for Hg was 0.01 μg/L. The potential of CPAC-Fe3O4 nanocomposite was also apprised at Hg concentrations of 5, 10, 20, 50, 70, and 100 mg/L at pH of 6 and 0.01 g adsorbent for 60 min at 25 °C. The same procedure was followed to optimize the contact time at 15, 30, 45, 60, 75, and 90 min at a fixed pH of 6, Hg concentration of 20 mg/L, and 0.01 g adsorbent at 25 °C. Adsorbent doses of 0.01, 0.02, 0.03, 0.04, 0.05, and 0.1 g were considered at a fixed pH of 6 and Hg concentration of 20 mg/L for 60 min at 25 °C. The impact of temperature (15, 20, 25, 30, 35, and 40 °C) was also evaluated at a fixed pH of 6, Hg concentration of 20 mg/L, and 0.01 g adsorbent for 60 min. All experiments were carried out in three replicates.

3 Results and discussion

3.1 Characterization of CPAC-Fe 3 O 4 nanocomposite

3.1.1 FESEM analysis

The morphology of CPAC-Fe3O4 nanocomposite was determined with SEM at 25 keV as depicted in Fig. 1. The activation with NaOH creates cavities with spherical structure, relatively smooth surface, and diameter size of < 100 nm. Several micropores and mesopores are formed, resulting in a high surface area as shown in Fig. 1b and c. Furthermore, the exterior pores facilitate the efficient transport of Hg ions from the aqueous solution to the inner micropores of the CPAC-Fe3O4 nanocomposite, increasing the adsorption performance [32]. As can be seen, the CPAC-Fe3O4 nanocomposite manifests various pore sizes with relatively uniform distribution. The large size of pores in the raw cocopeat sample and small pores in the cocopeat conversion to activated carbon were also reported by Varghese et al. [33]. The SEM image after Hg adsorption shows the vaccine sites on the surface of the CPAC-Fe3O4 nanocomposite, indicating a high potential of adsorbent to multiple adsorb Hg ions in a batch system (Fig. 1d).

Fig. 1
figure 1

FESEM images of cocopeat fiber (a) and CPAC-Fe3O4 nanocomposite before (b, c) and after (d) Hg removal

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3.1.2 FTIR analysis

The synthesized CPAC-Fe3O4 nanocomposite was analyzed by FTIR spectroscopy to determine the effective functional groups in the adsorption process. Figure 2 manifests the FTIR spectra for synthesized CPAC-Fe3O4 nanocomposite before and after Hg removal. A broad peak at 3434.21 cm−1 is attributed to the stretching vibrations of the O–H band [26, 34, 35]. A peak at 2922.56 cm−1 corresponds to C-H SP3 stretching, which verifies the presence of alkanes in the nanocomposite. Strong peaks at 1733.21 cm−1 and 1639.45 cm−1 can be ascribed to stretching of C = O [36]. Moreover, the absorption at 1200–1600 cm−1 is due to CH2 and CH3 stretching. A peak at 841.06 cm−1 can be associated with the stretching vibration of C-O. Absorption peaks at 618.62 cm−1 and 522.94 cm−1 correspond to the Fe–O band [36, 37], evidence of the formation of magnetic nanocomposite (Fig. 2a). Therefore, the presence of hydroxyl, carboxylic acid, and lactones functional groups on the surface of CPAC-Fe3O4 nanocomposite plays the main role in Hg ion adsorption. The changes in the vibrational spectra at 681.71, 747.42, 804.10, and 1077.28 cm−1 confirmed the adsorption of Hg ions by the functional groups (Fig. 2b).

Fig. 2
figure 2

FTIR spectra of CPAC-Fe3O4 nanocomposite before (a) and after (b) Hg removal

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3.1.3 XRD analysis

X-ray diffraction is a practical technique for investigating the characteristics of the crystal structure, such as the qualitative features of unknown materials, lattice geometry, crystal phase, crystal size, single crystal orientation, and lattice defects [38]. To determine the crystalline phase of the CPAC-Fe3O4 nanocomposite, XRD analysis was performed in the range of angle 2θ = 0–80° and temperature 25 °C. Regarding Fig. 3, the peaks at 2θ = 30.12° and 35.36° reveal the crystalline phase, with 100% purity and maximum intensity of 141 at 36.36°. Based on the obtained spectrum, 6 peaks at 2θ = 35.4°, 39.36°, 43.16°, 53.6°, 56.84°, 64.48° in inverse cubic spinel confirm the presence of Fe3O4 nanoparticles on the CPAC-Fe3O4 nanocomposite [39]. In addition, the obtained peaks are compatible with the standard peak (No JCPDS. 19-0629) and prove the existence of magnetic activated carbon and Fe3O4 particles.

Fig. 3
figure 3

XRD profile of the CPAC-Fe3O4 nanocomposite

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3.1.4 BET analysis

The BET theory is based on a simplified model of physisorption and determines a specific surface area of adsorbent by nitrogen sorption–desorption measurement at a constant temperature of liquid nitrogen (77 K) [40]. The pore size distribution was achieved by the Barrett-Joyner-Halenda (BJH) equation during the desorption phase. The BJH method attributes the relative pressure of nitrogen in equilibrium with the porous adsorbent to the size of the pores, considering the Kelvin equation [41]. The pore size radii which are covered by the BJH calculations ranged from 1.7 to 300 nm [42]. The results of the BET analysis of the CPAC-Fe3O4 nanocomposite revealed that the size of the nanopores was around 2.27 nm, and the total volume was 0.067 cm3/g of the CPAC-Fe3O4 nanocomposite (Table 1). The specific surface area of the prepared nanocomposite was also equal to 118.25 m2/g. The volume of required gas to create a single layer on the CPAC-Fe3O4 nanocomposite was estimated as 27.168 cm3/g. Notably, the high surface area of the nanocomposite indicates a suitable substrate for bonding with Fe3O4 nanoparticles. The shape of the adsorption/desorption isotherms might be included in type IV based on the International Union of Pure and Applied Chemistry (IUPAC), with a hysteresis loop, which showed the porosity of activated carbon (Fig. 4). A lower p/p0 ratio depicted narrow and micropores, raising the agglomeration of Hg ions and adsorption efficiency [43, 44].

Table 1 Physical characteristics of the CPAC-Fe3O4 nanocomposite
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Fig. 4
figure 4

N2 adsorption/desorption isotherms (a) and BJH pore size distribution and MP plot (b)

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3.1.5 VSM analysis

Figure 5 illustrates the magnetization curves of Fe3O4 and the CPAC-Fe3O4 nanocomposite at room temperature and − 10,000 to 10,000 oersted (Oe) field. The saturation values for Fe3O4 particles and the CPAC-Fe3O4 nanocomposite are 63.77 (Fig. 5a) and 2.5 (Fig. 5b) emu/g, respectively, and have an S-like form. The decrease of magnetization properties in the CPAC-Fe3O4 nanocomposite was attributed to the existence of nonmagnetic particles. Moreover, the CPAC-Fe3O4 nanocomposite follows the behavior pattern of superparamagnetic materials and has no hysteresis loop [45]. Therefore, the magnetization curve passes through the origin, implying no coercive field and remanent magnetization. The curve primarily moves away from the center with an almost constant slope. It indicates that the relationship between magnetization and the magnetic field is linear. Then, the slope of the curve gradually begins to reduce and the acceleration of magnetization decreases to reach saturation magnetization. After that, increasing the applied external magnetic field density does not affect enhancing the magnetization, and the magnetic material is completely saturated.

Fig. 5
figure 5

VSM magnetization curve of Fe3O4 (a) and CPAC-Fe3O4 nanocomposite (b)

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3.2 Effect of pH

Figure 6a illustrates the adsorption efficiency of Hg ions on the CPAC-Fe3O4 nanocomposite. As the pH of the solution was increased from 3 to 6, the Hg adsorption enhanced from 101.8 to 182.4 mg/g with a maximum removal rate of 91.2%. Whereas, a further increase in pH to 8 led to a 14.97% reduction in Hg removal. The functional groups such as carboxyl and amino groups are protonated at low pH causing the electrostatic repulsion between the functional groups and positively charged Hg ions and subsequently hindering the adsorption [35, 46]. However, the increase in pH deprotonates the functional groups and the Hg species (Hg (II), Hg(OH)+, and Hg(OH)2) can chelate with the neutral functional groups, leading to high adsorption [35, 46]. Einollahipeer and Okati [47] also referred to a strict competition between Hg2+ and H3O+ at low pH accompanied by HgOH formation at pH higher than 7 with a lower affinity towards amine grafted magnetic graphene oxide (m-GO-NH2), which could reduce the Hg adsorption [48]. The formation of a colloidal precipitate at higher pH was also attributed to lower adsorption efficiency [35]. Ge et al. [49] highlighted the increasing trend of Hg removal with a pH increase to 6, using poly(itaconic acid)-grafted crosslinked chitosan nanoadsorbent. Similar results were reported in the literature [50, 51].

Fig. 6
figure 6

The impacts of operational parameters on Hg removal. pH (a), Hg concentration (b), adsorbent dose (c), contact time (d), and temperature (e)

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3.3 Effect of Hg concentration

The effect of Hg concentration in the range of 5 to 100 mg/L at pH 6 is exhibited in Fig. 6b. The maximum removal efficiency (98.3%) was observed at 5 mg/L, which was decreased to 91.2% at a concentration of 20 mg/L. A further increase to 94.8% was detected at a concentration of 50 mg/L, while reduced to 90.21% at 100 mg/L concentration. A reduction in Hg adsorption at high Hg concentration can be related to the repulsive force between adsorbed Hg ions and the remaining ones in the solution due to limited available sites on the adsorbent [52, 53]. In contrast, at low Hg concentrations, the adsorption occurs on high-energy sites on the adsorbent [15]. The adsorption capacity was also increased from 49.15 to 902.15 mg/g in parallel with the increase in initial Hg concentration. This can be ascribed to the interaction between adsorption sites and Hg ions, which can diffuse the adsorption sites rapidly until all are fully occupied and equilibrium occurs [54]. The obtained adsorption capacity in this study was higher than that for Pistachio-nut/licorice residues (147.1 mg/g) [55], curcumin-based biocomposite (144.9 mg/g) [56], and carboxymethyl CS-sewage sludge (594 mg/g) [57].

3.4 Effect of adsorbent dose

The adsorbent dose as a critical and effective parameter in the adsorption process was investigated. As shown in Fig. 6c, the common pattern is that the increase in adsorbent dose resulted in more Hg ion removal and then decreased when the adsorbent dose exceeded a threshold. Notably, even in the lowest adsorbent dose (0.01 g), substantial removal efficiency was achieved (91.2%). The increase in nanocomposite dose from 0.01 to 0.03 g revealed a 4.3% enhancement in adsorption efficiency. This can be justified by more available active binding sites on the surface of the adsorbent [58]. A further reduction to 88.3% was attained at 0.1 g adsorbent dose. The adsorption capacity was decreased from 182.4 (0.01 g dosage) to 17.66 mg/g (0.1 g dosage), which was because of more vacant sites at higher doses of adsorbent [59].

3.5 Effect of contact time

Adsorption is a time-dependent process and prediction of the removal rate of Hg ions is indispensable to optimize the process in the full-scale operation. Figure 6d illustrates the increase in adsorption efficiency and capacity when the contact time was upgraded from 15 (71.7%) to 90 min (96.07%). It is interesting to mention that a rapid increase in Hg removal at the initial time is relevant to a greater number of available active sites, which gradually are occupied with time increasing [60, 61]. Moreover, the adsorption is controlled through the diffusion process from the solution to the adsorbent surface, which is slowed down due to fewer available adsorption sites [62]. Regarding literature [47], the capability of m-GO-NH2 to adsorb Hg ions displayed a rise from 79.02 to 95.54%, while the contact time was increased from 15 to 90 min. Another study revealed 60% Hg removal by Fe3O4@SiO-NH-COOH in 5 min and reached 75% after 1 h [63]. The highest adsorption capacity in the present study was 192.15 mg/g at 90 min, higher than reported values as 157.9 mg/g in 60 min [64] and 118.55 mg/g in 180 min [65], highlighting the potential of the CPAC-Fe3O4 nanocomposite in Hg ion adsorption.

3.6 Effect of temperature

The increase in the temperature of a solution from 15 to 40 °C enhanced the removal efficiency in the range of 82.07 to 92.67% alongside the adsorption capacity from 164.15 to 185.35 mg/g (Fig. 6e). The temperature enhancement decreases the solution viscosity and accelerates the diffusion of Hg ions into the adsorbent, rapidly occupying the surface of the adsorbent in a shortened time [66, 67]. It should be noticed that the excessive mobility of the metal ions at high temperatures could lessen their accessibility and lower removal efficiency [15]. Although the most common optimal temperature for the adsorption process is an ambient temperature (25 °C), however, various temperature values were also reported. The adsorption capacity of the FeCu-based biochar was as high as 3901 ng/g at an optimal adsorption temperature of 200 °C [68]. The highest adsorption capacity of coconut pith-based char [69] was also achieved at 50 °C (2395.98 µg/g) for an initial Hg concentration of 200 µg/m3. Another study referred to a temperature of 150 °C for maximum Hg removal (120 µg/g) using sunflower husk-based char [70].

3.7 Adsorption isotherms and kinetics

The degree of pollutant adsorption onto the surface of the adsorbent commonly depends on the equilibrium concentration in the aqueous solution and temperature, while at a constant temperature, the adsorption capacity (qe) mainly relates to the final pollutant concentration (Ce) [2, 71]. Hence, Langmuir and Freundlich isotherm models were assessed to determine the Hg adsorption mechanism of the CPAC-Fe3O4 nanocomposite. Table 2 demonstrates the isotherm equations accompanied by the constant values and maximum adsorption capacity. The fitting results derived from the isotherm modeling are depicted in Fig. 7. As can be seen, the Freundlich isotherm model gave the highest R2 (0.9749) with a KF value of 434.85 mg/g (l/mg)1/n, fitting well with the empirical data. The isotherm results confirmed the multilayer adsorption of Hg ions and the heterogeneous surface of the CPAC-Fe3O4 nanocomposite [34, 52]. The value of 1/n in the Freundlich isotherm model was less than 1, indicating the desirable adsorption of Hg ions [47]. Bhatnagar et al. [72] modified pectin and cellulose composites with cocopeat biochar and SDS and highlighted the best fit with Freundlich isotherm, which showed multilayer chemisorption via complexation. The pseudo-first-order and pseudo-second-order kinetic models were employed to further analyze the adsorption kinetic (Table 2). The determination coefficient values demonstrated a better fit of empirical data with the pseudo-second-order kinetic model (R2 = 9997), manifesting that the adsorption of Hg ions through the CPAC-Fe3O4 nanocomposite followed a chemisorption process [73] as depicted in Fig. 7d. The qe value was 204.08 mg/g, higher than that reported in the literature [35, 56]. Sireesha and Sreedhar [32] noted that the chromium adsorption mechanism of H3PO4 surface-modified cocopeat biochar (PSMCB) was chemisorption and followed a pseudo-second-order kinetic model.

Table 2 Isotherm, kinetics, and thermodynamic study of the adsorption process and comparison with other adsorbents for Hg removal
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Fig. 7
figure 7

Langmuir (a) and Freundlich (b) isotherm models and pseudo-first-order (c) and pseudo-second-order (d) kinetic models

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3.8 Thermodynamic studies

The CPAC-Fe3O4 nanocomposite performance in Hg removal was investigated at different temperatures (288, 293, 298, 303, 308, and 313 K). The enthalpy (∆H) and entropy (∆S) were estimated from the slope and intercept of 1/T versus lnKc. Gibbs free energy (∆G) was also calculated using the equation in Table 2. The negative values of ∆G presented the spontaneous nature of Hg adsorption [35]. Moreover, the increase in temperature from 288 to 313 K decreased the ∆G value from − 3.643 kJ/mol to − 6.604 kJ/mol, implying the endothermic adsorption of Hg ions. The positive and high value of ∆H verified the endothermic adsorption process and strong interaction between Hg ions and adsorbent [65]. The positive value of ∆S also presented the randomness adsorption of Hg ions due to the interference of water molecules in the solution [52, 74].

3.9 Comparison with other adsorbents

Table 3 compares the CPAC-Fe3O4 nanocomposite efficiency with other synthesized adsorbents for Hg removal. As can be seen in Table 3, more than 94% Hg removal was achieved for the various applied adsorbents, whereas a significant distinction was observed in adsorption capacity. Arshadi et al. [76] manifested the maximum adsorption efficiency of 3232 mg/L for Hg removal by a novel heterogeneous nanodendrimer. They increased the removal capacity through several pretreatment processes and heterogenized l-cysteine methyl ester dendrimer on the surface. A lower adsorption capacity was also reported for Hg removal due to differences in adsorbent characteristics and operational conditions [71, 77]. However, it is worth mentioning that the CPAC-Fe3O4 nanocomposite could remove 71.7% of Hg ions within 15 min, indicating the significant potential of the synthesized adsorbent. On the other hand, the maximum adsorption capacity was substantially higher than that of other adsorbents. Furthermore, the CPAC-Fe3O4 nanocomposite underlined high stability after 13 cycles with more than 80% Hg removal (Fig. 8). Therefore, the CPAC-Fe3O4 nanocomposite is recommended to be applied for Hg removal due to its simplicity, high potential, and low cost.

Fig. 8
figure 8

Reusability of CPAC-Fe3O4 nanocomposite

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Table 3 Comparison of Hg adsorption potential of synthesized adsorbents
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3.10 Adsorption mechanism

Various forms of Hg such as Hg2+, HgOH+, HgCl+, and Hg(OH)2 are present in an aqueous solution under pH < 3. The formation of Hg2+ is observed with pH enhancement and further dissolved Hg(OH)2 at pH higher than 6 [27]. When the pH of the solution increases more than 4, H3O+ ions decrease and more ionized functional groups are formed [79]. Oxygen functional groups such as carboxyl and carbonyl groups significantly contribute to mercury adsorption [80]. The reaction of Hg ions and these functional groups leads to the formation of surface complexes on the adsorbent surface. The chemical reactions are as follows [55]:

$${C}_{x}{OH}^{2+}+ {Hg}^{2+}\to {C}_{x}{OHg}^{2+}+ {2H}^{+}$$
(1)
$$2{C}_{x}{OH}^{2+}+ {Hg}^{2+}\to {{(C}_{x}{O)}_{2}Hg}^{2+}+ {2H}^{+}$$
(2)

The role of the surface chemistry of CPAC-Fe3O4 nanocomposite on the adsorption mechanism was assessed using FTIR. The peaks at 1639.45 and 1733.21 cm−1 ascribed to the C = O group changed after the Hg adsorption and a new peak at 1629 cm−1 appeared, which was associated with O-Hg. A sharp peak at 841.62 cm−1 also decreased after adsorption, indicating chemisorbed C-O group contribution [81].

4 Conclusion

The application of low-cost and eco-friendly CPAC-Fe3O4 nanocomposite derived from cocopeat, a by-product of coconut husk processing, was apprised for Hg removal from aqueous solution. The CPAC-Fe3O4 nanocomposite illustrated spherical morphology with an average pore diameter of 2.27 nm. The saturation magnetization value was 2.5 emu/g, confirming the appropriate magnetic properties of the synthesized nanocomposite. The equilibrium adsorption capacity of the CPAC-Fe3O4 nanocomposite was 204.08 mg/g with a substantial Hg removal efficiency of 98%. The empirical data was well fitted with pseudo-second-order kinetic and the Freundlich isotherm models. In addition, Hg adsorption was endothermic and spontaneous based on thermodynamic study. It can be concluded that the CPAC-Fe3O4 nanocomposite can be an eco-friendly candidate with a noticeable capability for Hg removal.