Functionalized porous nanoscale Fe3O4 particles supported biochar from peanut shell for Pb(II) ions removal from landscape wastewater

The large amounts ofheavy metal from landscape wastewater have become serious problems of environmental pollution and risks for human health. The development of efficient novel adsorbent is a very important for treatment of heavy metal. The functionalized porous nanoscale Fe3O4 particles supported biochar from peanut shell (PS-Fe3O4) for removal of Pb(II) ions from aqueous solution was investigated. The characterization of PS-Fe3O4 composites showed that biochar was successfully coated with porous nanoscale Fe3O4 particles. The pseudo second-order kinetic model and Langmuir model were more fitted for describing the adsorption process of Pb(II) ions in solution. The adsorption process of Pb(II) ions removal by PS-Fe3O4 composites was a spontaneous and endothermic process. The adsorption mechanisms of Pb(II) ions by PS-Fe3O4 composites were mainly controlled by the chemical adsorption process. The maximum adsorption capacity of Pb(II) ions removal in solution by PS-Fe3O4 composites reached 188.68 mg/g. The removal mechanism included Fe–O coordination reaction, co-precipitation, complexation reaction, and ion exchange. PS-Fe3O4 composites were thought as a low-cost, good regeneration performance, and high efficiency adsorption material for removal of Pb(II) ions in solution.


Introduction
At present, the large amounts of heavy metal from landscape wastewater have become serious problems of environmental pollution and risks for human health. It affects the growth of aquatic and leads to the destruction of landscape (Castaneda et al. 2019). In general, they mainly are discharged of untreated wastewater from metallurgical, mines, tanneries, textile, leather, etc. Many critical problems of heavy metal pollution on human health have been widely reported (Liu et al. 2010;Shi et al. 2018). Even at low concentration, these heavy metals in aquatic pollution would lead to occurrence of carcinogen in human according to US National Toxicology Program (Hokkanen et al. 2016). Therefore, it is a serious problem of aquatic pollution. It is necessary to search some desirable methods for solving above problems of heavy metal in aquatic pollution. Lead (Pb) is one of most hazardous, toxic, and non-biodegradable heavy metals (Karunanayake et al. 2018). It can enter the human body through respiratory tract, food chain, and drinking water (Thanh et al. 2018;Sahan 2019). Then, it accumulates in human body. Moreover, it would damage various organs and cause neurological dysfunction, anemia, kidney damage, and so on (Mouni et al. 2011;Ifthikar et al. 2017). According to the report of World Health Organization (WHO) (Naushad et al. 2021), the concentration of Pb(II) ions in drinking water could not exceed about 10 μg/L. Therefore, some conventional separation technologies for Pb(II) removal are reported, such as ion-exchange, filtration membranes, electrolysis deposition, co-precipitation, and coagulation processes (Cui et al. 2015;Huang et al. 2018). However, these separation technologies cannot be widely used in practical application because of expensive equipment, complex operation process, chemical pollution, and so on (Liang et al. 2021;Liu et al. 2021a). However, adsorption is recognized as an efficient technology for removal of pollutants. It is high efficiency, low cost, environment-friendly, and simple operation . The development of efficient novel adsorbent is a very important for adsorption technology (Tang et al. 2018).
Biochar is a carbon-rich by-product from the pyrolysis of biomass, such as wood, crop straw, agricultural products, and so on. It was prepared at a certain temperature under hypoxia conditions (Qambrani et al. 2017;Qiu et al. 2021a). It has been regarded as a low-cost and environment-friendly multifunctional material for the removal of environmental pollutants, such as heavy metals, inorganic pollutants, organic pollutants, radionuclide, and so on Lia et al. 2021;Liu et al. 2021b;Qiu et al. 2021b;Yao et al. 2021). However, the removal capacity of the unmodified biochar was not high. Therefore, in order to improve the adsorption capacity of pollutants by biochar, various methods were adopted to functionalize biochar Sun et al. 2020). Xue et al. (2012) reported that hydrogen peroxide modification enhances the ability of biochar produced from hydrothermal carbonization of peanut hull to remove aqueous heavy metals. The experimental results were desired. Ahmed et al. (2021) found that the adsorption capacity of Pb(II) ions in solution by the modified biochar from watermelon seeds were enhanced. The adsorption capacity reached 60.87 mg/g. However, they still had a shortcoming . That is, it was difficult to separate them from aqueous solution after they adsorbed pollutants (Wang et al. 2019). The iron magnet-based materials could effectively solve this problem Wang et al. 2022).
In recent years, magnetic Fe 3 O 4 nanoparticle was applied extensively into the treatment of heavy metal in solution because of its characteristic of easy separation and high surface area (Shen et al. 2015). However, bare Fe 3 O 4 nanoparticle was very easy to be oxidized and self-aggregated, which restricted its application for metal ion removal in aqueous solution (Alhokbany et al. 2019). Thus, surface protection and modification were very important for the practical application of magnetic Fe 3 O 4 nanoparticle (Xie et al. 2015). The combination of magnetic Fe 3 O 4 nanoparticle with material as multiple composites was effective to improve its utility and stability. Pan et al. (2017) depicted that the Fe 3 O 4 NPs coated with organic acid could prevent the aggregation of the Fe 3 O 4 nanoparticle and enhance the adsorption capacity of the heavy metal in solution. Hou et al. (2021) reported that novel SR-Fe 3 O 4 material was fabricated by a facile method using mesoporous silica (RH-MCM-41). The maximum adsorption capacity of As(III) ions in solution by SR-Fe 3 O 4 was 1002.03 mg/g. It was a good adsorbent material for removal of As(III) ions in aqueous solution. Therefore, it was very interesting to develop the nanoscale Fe 3 O 4 particles supported biochar from biomass for heavy metals removal in solution. It could realize the 'win-win effect' between nanoscale Fe 3 O 4 particles and biochar. The related researches were also reported, but they were few Zhang et al. 2021).
In this work, the low-cost and easily obtained agricultural waste (Peanut Shell) was modified by nanoscale Fe 3 O 4 particles. Then, the functionalized porous nanoscale Fe 3 O 4 particles supported biochar from peanut shell (PS-Fe 3 O 4 ) for removal of Pb(II) ions from aqueous solution was investigated. Characteristics of biochar (PS) and PS-Fe 3 O 4 composites were determined by SEM, TEM, EDS, FT-IR, XRD, and XPS, respectively. The adsorption process and mechanism of Pb(II) removal in aqueous solution by PS-Fe 3 O 4 composites would be explored in details. The experimental results of this research would provide a new idea for the ecological utilization of agricultural waste. The successful application of peanut shells could not only effectively deal with heavy metal pollution but also contribute to the recycling of peanut shells.

Preparation of biochar and PS-Fe 3 O 4 composites
For the preparation of biochar derived from peanut shell (PS), peanut shell sample was cleaned with Milli-Q water and dried at 65 °C for 24 h. Then, the dried peanut shell sample was grounded and sieved into 80 meshes. Next, 5.0 g of peanut shell sample was mixed with 20 mL of Milli-Q water and continuously stirred magnetically for 20 min. The obtained mixture was placed into 50 mL of autoclave reactor and heated at 150 °C for 24 h, and then cooled at room temperature and filtrated with qualitative filter paper. The obtained black precipitate was Milli-Q water for three times and dried at 105 °C for 24 h. The biochar derived from peanut shell was obtained.
For the preparation of the functionalized porous nanoscale Fe 3 O 4 particles supported biochar from peanut shell (PS-Fe 3 O 4 ), it was prepared according to the impregnation method (Maneechakr & Karnjanakom 2021). Briefly, 2.0 g of PS powder was added into a 250-mL Erlenmeyer flask. Then, 25 mL of 0.2 mol/L Fe 3+ and 25 mL of 0.1 mol/L Fe 2+ were added, respectively. They were stirred for 10 min. Next, 50 mL of 0.5 mol/L NaOH solution was dropped into the mixture solution and stirred for 20 min. Finally, PS-Fe 3 O 4 composites were obtained with magnetic field, washed with Milli-Q water for three times, and dried at 105 °C for 24 h. The synthesis pathway for PS-Fe 3 O 4 composites was depicted at Fig. 1.

Characterization
The samples of PS and PS-Fe 3 O 4 were characterized by N 2 -BET adsorption method, scanning electron microscope (SEM), energy dispersive X-ray spectra (EDS), transmission electron microscope (TEM), X-ray diffraction (XRD), infrared spectrometer (FT-IR), and X-ray photoelectron spectroscopy (XPS), respectively. The details of those techniques were obtained in the Supporting Information.

Adsorption experiments
To evaluate the adsorption process of Pb(II) ions in solution by PS-Fe 3 O 4 composites, adsorption experiments were carried out under constant stirring. The details of adsorption experiments were provided in the Supporting Information. Briefly, the certain amount of PS-Fe 3 O 4 composites was added into a 250-mL Erlenmeyer flask. Then, 100 mL initial concentration of Pb(II) ions also was added into the Erlenmeyer flask. The flask was sealed by bottle cap and placed in the shaker at 200 rpm and constant temperature. The pH in solution was adjusted with the 0.1 mol/L NaOH or 0.1 mol/L HCl solution. Concentration of Pb(II) ions in solution was analyzed by flame atomic adsorption spectrophotometry, respectively (Fatehi et al. 2017). The residual sample was centrifuged at 4000 rpm for 20 min and determined by microscopic technologies. All experiments were carried out for three times, and the experimental data were analyzed by the mean and standard deviation. Additionally, calculation equations of the uptake capacity ( q(mg/g)) (mg/g)) and the removal rate (R(%))were provided in the Supporting Information.

Characterization
The micro-morphologies of the PS and PS-Fe 3 O 4 were characterized by the analytical technologies of SEM and TEM (Fig. 2). As shown from Fig. 2, the surface morphology of PS was smooth, and the surface morphology of PS-Fe 3 O 4 was a craggy and irregular structure. It might be the reason that the PS-Fe 3 O 4 composites were covered by porous nanoscale Fe 3 O 4 particles. The EDS images also could confirm that porous nanoscale Fe 3 O 4 particles appeared and were well distributed on the surface of PS (Fig. 3). Fig. 3a showed that C, O, and Si elements were distributed in the structure of PS. Their contents were 68.47, 31.35, and 0.17%, respectively. It depicted that the elements of C and O were the main component in the material of PS. In addition to the appearance of C, O, and Si elements, the element of Fe also was observed on the structure of PS-Fe 3 O 4 composites (Fig. 3b). The content of Fe reached 20.67%. The contents of C, O, and Si were 42.76, 36.29, and 0.25%, respectively. Compared with PS, the content of C was decreased, and the contents of O and Fe were increased. It could be confirmed that the functionalized porous nanoscale Fe 3 O 4 particles supported by PS (PS-Fe 3 O 4 composites) were successfully prepared.
The surface area and pore size of PS and PS-Fe 3 O 4 composites were calculated by N 2 adsorption-desorption isotherms. Results showed that a porous structure was present for PS-Fe 3 O 4 composites. BET specific surface areas of PS and PS-Fe 3 O 4 composites were 11.35 and 46.13 m 2 /g, respectively. Due to nanoscale Fe 3 O 4 particles support, the BET surface area of nanoscale was increased obviously compared with PS. It also depicted that PS could effectively decrease the aggregation of nanoscale Fe 3 O 4 particles. The adsorption average pore width of nanoscale Fe 3 O 4 particles were 6.94 and 6.31 nm, respectively. The results of FT-IR spectra about PS and PS-Fe 3 O 4 composites were shown in Fig. 4a. The four strong characteristic peaks at 3398 cm −1 , 2330 cm −1 , 1610 cm −1 , and 1377 cm −1 were observed. Some related researches indicated that the characteristic peak at 3398 cm −1 and 2330 cm −1 should be related to the stretching vibration of -OH and -C≡C, respectively (Yuan et al. 2015). The stretching vibration peak at 1610 cm −1 contributed to the C=C, and the characteristic peak at 1377 cm −1 represented the stretching vibration of C-O-C (Li et al. 2020). Compared with PS, the characteristic peak at 582 cm −1 was present on the surface of PS-Fe 3 O 4 composites. The peak at 582 cm −1 corresponded to the stretching vibration of Fe-O. It also indicated that PS was coated with nanoscale Fe 3 O 4 particles successfully. The crystal structures of PS and PS-Fe 3 O 4 composites were determined by XRD patterns (Fig. 4b).
For PS, the peak at 26.58° should be the characteristic peak of biochar. In addition to the peak at 26.58, the characteristic peak at 34.84° also was observed on the surface of PS-Fe 3 O 4 composites. According the related studies (Fatehi et al. 2017), it should be the appearance of Fe 3 O 4 crystal structure on the surface of PS-Fe 3 O 4 composites.
Formation of Fe 3 O 4 particles could be depicted as follows (Badi et al. 2018): According to the results of SEM, TEM, EDS, FT-IR, XRD, and N 2 -BET, it could be concluded that the functionalized porous nanoscale Fe 3 O 4 particles supported PS from peanut shell was obtained. PS was successfully coated with the nanoscale PS-Fe 3 O 4 particles. It was a craggy and irregular structure with a BET specific surface area of 46.13 m 2 /g and an adsorption average pore width of 6.31 nm.

Effect of operation parameters
The influence of operation parameters on the adsorption of Pb (

Adsorption kinetic, adsorption isotherm, and thermodynamic
To evaluate the adsorption process of Pb(II) ions in solution by PS-Fe 3 O 4 composites, adsorption kinetic, adsorption isotherm, and thermodynamic were investigated according to the experimental data of Fig. 5. In this research, pseudo first-order kinetic model, pseudo second-order kinetic model, Langmuir isotherm model, and Freundlich isotherm model were chosen for describing the adsorption process. The details of equations were provided in the Supporting Information. The adsorption kinetics for adsorption of Pb(II) ions in solution by PS-Fe 3 O 4 composites were described in Fig. 6a-b. It could be concluded that the pseudo second-order kinetic model was more fitted for describing the adsorption process of Pb(II) ions in solution (0.9989>0.9641). Therefore, it also could be confirmed that the adsorption process of Pb(II) ions in solution by PS-Fe 3 O 4 composites was mainly controlled by the chemical adsorption process . Fig. 6c-d was the adsorption isotherms for adsorption of Pb(II) ions in solution by PS-Fe 3 O 4 composites. It could be observed that the R 2 of Langmuir model and Freundlich model were 0.9983 and 0.9374, respectively. Therefore, the adsorption process of Pb(II) ions in solution by PS-Fe 3 O 4 composites could be depicted by the Langmuir model. It also could be implied that the adsorption processes were the homogeneous and monolayer adsorption (Zama et al. 2017  were thought as a low-cost and high efficiency adsorption material for removal of Pb(II) ions in solution. According to the experimental data of Fig. 5d, thermodynamic parameters of Pb(II) ions in solution removal by PS-Fe 3 O 4 composites could be calculated (They could be found in the Supporting Information). The negative value of ∆G 0 could conclude that adsorption process of Pb(II) ions removal by PS-Fe3O4 composites was a spontaneous process. When reaction temperature increased, the value of ∆G 0 decreased. It indicated that temperature was beneficial for enhancing the adsorption capacity of Pb(II) ions removal by PS-Fe 3 O 4 composites. Furthermore, ∆H 0 and ∆S 0 both were positive value. It also depicted that the adsorption process of Pb(II) ions removal by PS-Fe3O4 composites also was an endothermic process.

Adsorption mechanism
In order to elaborate the adsorption mechanism of Pb(II) ions removal in solution by PS-Fe 3 O 4 composites, the samples of PS-Fe 3 O 4 composites before and after adsorption of Pb(II) ions also were characterized by XPS spectroscopy. The XPS spectra of PS-Fe 3 O 4 composites before and after adsorption of Pb(II) ions were shown in Fig. 7.
As observed from Fig. 7a, the spectrum of C 1s, O 1s, and Fe 2p at binding energies of 284.91, 530.69, and 710.52 eV appeared in the survey XPS spectrum before adsorption of Pb(II) ions. The elemental atomic compositions of C, O, and Fe were 76.27, 20.28, and 3.45%, respectively. It was implied that the elements on the surface of PS-Fe 3 O 4 composites were C, O, and Fe. After adsorption of Pb(II) ions, the elemental atomic compositions of C, O, and Fe on the surface of PS-Fe 3 O 4 composites were changed. They were 64.69, 24.34, and 3.97%, respectively. Additionally, the new Pb 4f at binding energies of 143.26 eV appeared in the survey XPS spectrum. Therefore, it could be concluded that Pb(II) ions in solution could be successfully adsorbed by PS-Fe 3 O 4 composites.
The spectrum of high resolved Pb 4f had two peaks at the binding energies of 138.41 and 143.41 eV (Fig. 7b). They corresponded to Pb 4f 7/2 and Pb 4f 5/2 , respectively. Additionally, they were in accordance with Pb(II) . It also indicated that Pb(II) ions in solution should be adsorbed into the active sites on the surface of PS-Fe 3 O 4 composites without being oxidized. This result was consistent with the result of the Fe 2p spectra (Fig. 7c). As from Fig. 7c, it could be observed that two characteristic peaks at the binding energies of 710.35 and 724.38 eV were presented on the surface of PS-Fe 3 O 4 composites. They corresponded to Fe(II) and Fe(III), respectively. The intensity of Fe 2p decreased after adsorption of Pb(II) ions in solution. It was indicated that PS-Fe 3 O 4 composites could adsorb Pb(II) ions with Fe-O coordination reaction (Liu et al. 2016). Additionally, this result could also be confirmed from the O 1s spectra (Fig. 7d). The peak at binding energy of 531.35 eV corresponded to Fe-O before adsorption of Pb(II) ions. After adsorption of Pb(II) ions, the intensity of O 1s increased. It was implied that the interaction of PS-Fe 3 O 4 composites and Pb(II) ions happened through Fe-O coordination reaction.

Regeneration experiment
In order to investigate the regeneration of Pb(II) ions removal by PS-Fe 3 O 4 composites, the adsorption or desorption experiment of Pb(II) ions removal by PS-Fe 3 O 4 composites was carried out. After the adsorption experiment of Pb(II) ions removal by PS-Fe 3 O 4 composites reached equilibrium, the solution was centrifuged at 4000 rpm for 20 min, and the adsorbent of PS-Fe 3 O 4 composites was obtained. Next, they were washed for three times with 0.1 M H 2 SO 4 , and dried at 60 °C for 24 h. Then, the dried PS-Fe 3 O 4 composites were used for adsorption experiment again. The experimental results were depicted in Fig. 9. After four regeneration experiments, the adsorption capacity of Pb(II) ions removal by PS-Fe 3 O 4 composites only decreased slightly, and it still retained 71.86%. It also could be implied that PS-Fe 3 O 4 composites were reused for Pb(II) ions in solution removal; the regeneration performance of PS-Fe 3 O 4 composites was good.

Conclusions
The functionalized porous nanoscale Fe 3 O 4 particles supported biochar from peanut shell (PS-Fe 3 O 4 ) for removal of Pb(II) ions from aqueous solution was investigated. PS and PS-Fe 3 O 4 composites were characterized by SEM, TEM, EDS, FT-IR, XRD, and XPS, respectively. It was a craggy and irregular structure with BET specific surface area of 46.13 m 2 /g and the adsorption average pore width of 6.31 nm. The large number of functional groups (such as -OH, -C≡C, C=C, C-O-C, and Fe-O functional groups) appeared on the surface of PS-Fe 3 O 4 composites. The adsorption process of Pb(II) ions in solution by PS-Fe 3 O 4 composites was mainly controlled by the chemical, homogeneous and monolayer, spontaneous, and endothermic adsorption process. The adsorption mechanisms of Pb(II) ions by PS-Fe 3 O 4 composites were Fe-O coordination reaction, co-precipitation, complexation reaction, and ion exchange. PS-Fe 3 O 4 composites were thought as a lowcost, good regeneration performance, and high efficiency adsorption material for removal of Pb(II) ions in solution.

Data availability
The data and materials presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy restrictions.
Author contribution HH and RL designed the experiment, LH and XJ performed the experiment, MQ processed the experimental data and wrote this article, and BH revised this paper.
Funding This work is financially supported by the Natural Science Foundation of Zhejiang Province, China (LGF20C030001 and LGF21C030001). Authors are very grateful for their support.

Declarations
Ethics approval This section is "not applicable" for this study.

Consent to participate Not applicable.
Consent for publication All authors reviewed and approved the manuscript for publication.

Competing interests
The authors declare no competing interests.