One-pot fabrication of magnetic biochar by FeCl3-activation of lotus seedpod and its catalytic activity towards degradation of Orange G

An advanced magnetic biochar (MBC) was facilely prepared via one-pot FeCl3-activation of lotus seedpod. Simultaneous carbonization, activation, and magnetization formed magnetic Fe3O4 nanoparticles and nanowires over the biochar base. The specific surface area (SBET) and the total pore volume (Vtotal) of MBC were 349 m2 g−1 and 0.31 cm3 g−1, which were 2.0-fold and 3.9-fold higher than those of biochar, respectively. In addition, the saturation magnetization of MBC reached 6.94 emu g−1, facilitating its magnetic separation and recovery. In heterogeneous Fenton-like catalytic oxidation, 0.40 g l−1 MBC decolorized 100% Orange G and reduced 58% COD by 350 ppm H2O2 within 120 min. The degradation kinetics were calculated with different MBC samples and reactions followed pseudo-first-order kinetics with the highest rate constant of 0.034 min−1. Moreover, the catalytic activity dropped by only 6.4% after four reuse cycles, with negligible iron leaching of 1.31–1.44 mg l−1. Based on these results, MBC could be a low-cost, highly effective, and relatively stable catalyst for treating Orange G in wastewater.


Introduction
Biochar (BC) is an inexpensive material produced from the carbonization of diverse renewable biomass resources [1][2][3]. Due to its favorable physicochemical properties, porous system and diverse functional groups [4][5][6], biochar is widely used for various purposes, such as wastewater treatment, soil remediation, and gas storage and separation [7,8]. Nevertheless, the obstacle of biochar is difficult to separate from suspensions. Conventional sedimentation, filtration, coagulation, and clarification processes are generally costly or inadequate, greatly restricting the use of biochar [9]. Such reports have proposed dispersing magnetic particles like Fe 3 O 4 on biochar to form magnetic biochar (MBC) [9,10]. This combination has the potential to overcome obstacles on both sides. Magnetic particles might aid in the easy recovery and reuse of MBC by magnetic fields [11,12]. In the opposite direction, the carbon base could prevent the magnetic particles from aggregating into larger particles [13,14]. The MBC composite possesses attractive physicochemical properties from not only porous systems and diverse functional groups of carbon base, but also catalytic activity and magnetic recoverability of iron-based particles [5,12].
To prepare MBC, magnetic precursors are typically impregnated onto carbon surfaces [10]. However, this traditional method not only is complicated but also locks existing pores in carbon support [15]. Recently, an innovative strategy for fabricating magnetic biochar using one-step pyrolysis of biomass and iron precursors has been introduced [12,14]. Magnetic precursors like FeCl 3 are dispersed directly into biomass, and the mixtures are then pyrolyzed to produce MBC [16]. Simultaneous carbonization, activation, and magnetization have the potential to remarkably improve MBC properties [9,14]. Bedia et al [17] proved that FeCl 3 −activation of 2. Materials and methods

Materials
The lotus seedpods were obtained from Thap Muoi District, Dong Thap Province, Vietnam. The agricultural residue was first washed with distilled water to remove adhering soil and dust, and then oven-dried at 110°C for 24 h to a constant weight. Next, the dried biomass was crushed, sieved, and preserved in an airtight plastic container for further use. All chemicals were purchased from Xilong Scientific Co., Ltd. They were of analytical grade and were used without any purification.

Preparation of magnetic biochar from lotus seedpod
Magnetic biochar was fabricated through one-pot pyrolysis of FeCl 3 −loaded lotus seedpod. First, 4.00 g dried LSP was added into an aqueous FeCl 3 solution. The mixture was stirred for 3 h at room temperature before being dried at 110°C for 24 h. Pyrolysis was conducted in a tube furnace under a constant nitrogen flow of 250 ml min −1 . The tube was heated to 600°C at a rate of 5°C min −1 and maintained at that temperature for 60 min. The resultant solid was washed with distilled water until the pH of wastewater became neutral to remove all remaining FeCl 3 , and dried at 80°C for 24 h to obtain MBC. The as-prepared samples were labeled as MBC-x, where x indicates the mass ratio of FeCl 3 to LSP. For comparison of the physical characteristics and catalytic activities, biochar (denoted as BC) was prepared by direct pyrolysis of the lotus seedpod without FeCl 3 addition under the same condition.

Characterization of magnetic biochar
The crystalline structures of MBC samples were characterized by powder x-ray diffraction (XRD) using a Bruker AXS D8 diffractometer over the 2θ range of 10°-80°. The Cu-Kα radiation was used as the target (λ=1.5418 Å). A Perkin Elmer AAnalyst 800 atomic absorption spectrophotometer (AAS) was used to determine the quantity of loaded iron in MBC samples that had been dissolved in concentrated hydrochloric acid. Nitrogen adsorption isotherms of BC and MBC samples were analyzed at 77 K on a Micromeritics Gemini VII. All samples were outgassed for 3 h at 200°C. Brunauer-Emmett-Teller equation was used to calculate the specific surface area (S BET ). Total pore volume (V total ) was measured at P/P o =0.99. The average pore size (d average ) was given by 4V total /S BET equation. The magnetic properties of MBC were measured using a vibrating sample magnetometer (VSM) at room temperature. Fourier-transform infrared (FTIR) spectra of BC and MBC were analyzed by using a Tensor 27 spectrometer (Bruker Optics, Germany). The surface morphology of MBC was observed using a scanning electron microscopy with field emission source (FE-SEM, S-4800). Elemental mapping of MBC was analyzed with an energy dispersive x-ray spectroscopy (EDS) instrument, JSM-IT200. Transmission electron microscopy (TEM) images of BC and MBC were obtained from a JEOL JEM-1400 equipment.

Degradation of Orange G using magnetic biochar
The catalytic activity of MBC was investigated through OG degradation at room temperature (30°C). 500 ml OG solution with an initial concentration of 100 ppm was contained in a 1000 ml glass cylinder. Different masses of MBC were dispersed in the solution. Initial pH values of the mixture were adjusted by adding H 2 SO 4 (0.5 M) and NaOH (0.1 M) solutions. After adsorption reached equilibrium in 20 min, H 2 O 2 was rapidly poured into the mixture to initiate catalytic oxidation. At different time intervals, a certain amount of the reaction suspension was taken out. For determination of OG concentrations, each sample was immediately added to a solution of phosphate buffer and Na 2 S 2 O 3 to adjust the pH value to ∼7.0 and remove the excess H 2 O 2 . OG concentrations were then measured using a UV-vis spectrophotometer (Lovibond PC Spectro) at 480 nm.
To assess the stability and reusability of MBC, consecutive experiments were performed with the same sample. The catalyst was recovered by a magnet bar, cleaned by distilled water and ethanol, filtrated and dried at 110°C for 24 h. The dried MBC was weighted to prepare for the next experiment. After each cycle, iron leaching was measured by the aforementioned AAS equipment. Chemical oxidation demand (COD) was measured according to the closed reflux titrimetric method (5220C) [36]. To prevent residual H 2 O 2 from interfering, all samples were added with 20 g l −1 Na 2 CO 3 , sealed to reduce evaporation losses, and heated in a water bath at 90°C for 60 min before COD analyses, according to Wu and Englehardt [37]. When the mass ratio of FeCl 3 to LSP increased from 0.1 to 0.4, the peak intensities of Fe 3 O 4 decreased gradually. At high ratios, a lack of carbon and water released from the carbonization of the LSP biomass might decrease the conversion of FeCl 3 into Fe 3 O 4 . The remaining FeCl 3 was removed from MBC through washing with distilled water. Moreover, the formation of other crystals or amorphous phases might weaken Fe 3 O 4 peaks.

Porous properties and iron content of magnetic biochar
As presented in table 1, increasing the mass ratio of FeCl 3 to LSP from 0.1 to 0.5 resulted in an increase of Fe content in MBC from 3.3 to 10.4 wt%. Similar trends have been reported in literature [40,41]. Increasing the mass ratio of FeCl 3 /biomass yields materials with higher Fe and O contents but lower carbon content.
Furthermore, higher mass ratio of FeCl 3 to LSP could boost the activation process to expand existing pores and create new pores in the carbon framework [15]. In fact, MBC-0.4 sample possesses a specific surface area of 349 m 2 g −1 , a total pore volume of 0.31 cm 3 g −1 , and an average pore size of 3.1 nm, which are 2.0-fold, 3.9-fold and 3.3-fold higher than those of BC sample, respectively. MBC−0.4 sample, generated from a larger mass ratio of FeCl 3 to LSP, has superior porosity characteristics than MBC-0.2 sample due to stronger activation.

Magnetic properties of magnetic biochar
In addition to carbonization and activation, magnetization introduces magnetic Fe 3 O 4 crystals to MBC. All asprepared MBC samples could be attracted effectively by a bar magnet. Their magnetic properties are clarified in figure 2. Generally, the magnetic hysteresis curves with very low coercivity and almost negligible magnetic hysteresis cycles describe superparamagnetic behavior. Therefore, these materials are easily magnetized and demagnetized. When the mass ratio of FeCl 3 to LSP was increased from 0.1 to 0.5, Fe content in MBC increased from 3.3 to 10.4 wt%, leading to an increase in specific saturation magnetization from 1.21 to 6.94 emg g −1 . Similar tendencies have been revealed in previous studies [40,42]. Assuming that all Fe element in MBC samples exists in Fe 3 O 4 form only, the saturation magnetization of 100% Fe 3 O 4 would be between 38-92 emu g −1 , which could be generally comparable with that of Fe 3 O 4 nanoparticles (56 emu g −1 [43], 50-61 emu g −1 [44], 42-49 emu g −1 [45]). As a result, one-pot immobilization of Fe 3 O 4 on the biochar base may not considerably diminish their magnetic property. As reviewed by Bedia et al [15], specific saturation magnetization of MBC from one-pot pyrolysis is comparable to or higher than that from magnetic activated carbon prepared by different synthesis procedures.
With FeCl 3 -activation, MBC introduced a broad spectral feature of 3150 cm −1 derived from O-H stretching vibration. While the peaks at 1570 (C=C), 1138 (C-O-C ) and 1046 (C-C) cm −1 were present in both BC and MBC, the peak at 2410 cm −1 related to the stretching vibration of O-H was found in MBC only [15]. The disappearance of the peak at 1428 cm −1 and the very short peak at 875 cm −1 proved that the activation process carbonized almost all lignin and holocellulose. Generally, FeCl 3 -activation considerably varied functional groups on the carbon surface. The enhancement of oxygen functional groups could increase the polarity of the material surface, making it more favorable for interactions with water-soluble organic matters. Moreover, it is  worth noting that the vibration peak of the Fe-O bond was discovered at 570 cm −1 [42,43], which was attributed to the existence of Fe 3 O 4 , implicitly confirming the successful synthesis of MBC.

SEM images of magnetic biochar
The surface morphology of MBC samples is depicted in figure 4. In general, the materials have a smooth surface and fragments with sharp edges. Crushing, carbonization, and activation may reduce MBC to the microscale. When the mass ratio of FeCl 3 to LSP was increased, no significant change in the surface morphology of MBC was observed. Interestingly, the lack of spheres from obviously aggregated Fe 3 O 4 particles was shown on the surface of MBC samples. This reveals that Fe 3 O 4 components seem to be diffused onto the biochar surface without gathering. Bedia et al [17] reported that nanoscale Fe 3 O 4 particles generated by FeCl 3 -activation of biomass can be entrapped inside the carbon base rather than on its surface. As a result, the particles can maintain their stable performance during long-term operation.
3.1.6. EDS elemental mapping of magnetic biochar EDS analysis was used to determine the type and amounts of constituent elements in the chemical structure of MBC ( figure 5). The results showed that the contents of C, Fe, and O in MBC-0.4 were 80.15, 7.74, and 10.90 wt%, respectively. Interestingly, the surface Fe content determined by EDS was comparable to the bulk Fe content determined by AAS (7.9 wt%). This finding suggests that Fe may be highly dispersed both inside and outside the carbon framework. In addition, the high oxygen content revealed that this element existed in not only Fe 3 O 4 but also oxygen functional groups, as discussed in the FTIR results. According to Chen et al [22], LSP derived-biochar may contain such trace amounts of K, Ca, Mg, N, S, P, and Cl. However, no metal elements were discovered in MBC, indicating that these minerals were removed by cleaning or did not exist in the raw material. In reverse, P and Cl elements were detected at 0.20 and 1.00 wt%, respectively. P could only originate from LSP. However, Cl could partially or entirely come from the addition of FeCl 3 . After multiple cleanings of MBC, the wastewater was analyzed using various techniques (pH, electrical conductivity, NaOH, AgNO 3 ) to ensure that no further leaching of Fe 3+ and Cl − and PO 4 3− ions from MBC occurred. Hence, these minor elements could be trapped tightly within the carbon framework by strong mechanical or chemical bonds. Finally, microscale element mapping images revealed that all existing elements were uniformly distributed on the MBC surface. To clarify the internal structure of MBC, nanoscale TEM analysis was presented in the following section.

TEM images of biochar and magnetic biochar
The internal structure and spatial distribution of BC and MBC were explored by TEM images ( figure 6). Biochar at the nanoscale had relatively regular brightness in a large area. After the pyrolysis process, the carbon framework together with functional groups was formed by covalent bonds, resulting in a relatively homogeneous BC structure. In contrast, irregular brightness in MBC showed the morpophology of Fe 3 O 4 . It seems that not only nanoparticles but also nanowires were formed. Typically, the formation of Fe 3 O 4 nanoparticles in MBC has been described in such reports [39,49,50]. Even though the nanoparticles had a nonuniform shape and were grouped into clusters, their size was small, with some below 20 nm. Notably, the presence of nanowires in MBC was uncommon. The lengths and diameters of these nanowires were generally lower than 10 nm and 100 nm, respectively. The FeCl 3 -activation process could generate and expand mesopores in the carbon framework. Therefore, it seems possible that Fe 3 O 4 nanowires could develop in tube-like mesopores. In regard to the dispersion of ionic Fe 3 O 4 in MBC, these nanostructures were embedded well within the carbon framework by mechanical bonds. Thus, MBC is expected to maintain its stability during long-term operation.   In addition to catalytic iron sites, minor elements present in MBC might affect its catalytic activity. Chloride ions may be detrimental to OG degradation [53,54]. The inhibitive effect of Cl − ions on the decolorization of OG could come from the interaction of Cl − ions with ·OH. Similarly, phosphate ions have been shown to inhibit catalytic oxidation processes [55,56]. However, minor amounts of Cl and P in the carbon framework may not exist in ion form. In addition, they may be difficult to leach into treatment media, as discussed in EDS results.  With the efficient catalytic activity of MBC on OG degradation, the role of these minor elements might be negligible. Figure 8 depicts the effect of MBC-0.4 dosage on OG degradation. Without MBC, H 2 O 2 was not able to process OG, indicating that the presence of catalysts significantly contributed to the OG degradation. As expected, when the MBC dosage was increased from 0.10 g l −1 to 0.40 g l −1 , the degradation rate increased. As mentioned before, high catalyst dosage could enhance the number of active sites for H 2 O 2 decomposition into ·OH and the adsorption of OG. However, the originally generated radicals can be deactivated because of the excess catalyst [57,58]. In the first 30 min, the degradation rate with 0.60 g l −1 MBC-0.4 was faster than that with 0.40 g l −1 , but slowed in subsequent periods. The excess of Fe(II) sites might reduce the number of •OH radicals (equation 9).

Effect of pH on orange G degradation
The pH of the reaction mixture significantly affected OG degradation ( figure 9). The OG concentration almost remained constant at a high pH of 5.0. Wang et al [32] found that under high pH conditions, Fe(OH) 3 figure 11, the removal efficiencies after each cycle were 96.0, 93.0, 92.1 and 89.6%, respectively. It means that MBC lost only 6.4% catalytic activity after four cycles and an average of 1.6% after each cycle, establishing its reusability. The slight decrease in the removal efficiency could be partly caused by Fe leaching. AAS results revealed that only 1.31-1.44 mg l −1 of Fe leaching was detected after each cycle. This leaching corresponded to 4.1%-4.6% Fe in MBC-0.4 sample. Furthermore, these Fe concentrations were lower than the legal limit of the European Union at 2 mg l −1 . As described in the literature, one-pot FeCl 3 activation of biomass could provide MBC catalysts with improved stability and reduced metal leaching [9,15]. The dispersion of nanoscale Fe 3 O 4 within the carbon framework may slow the rate of Fe leaching into the aqueous medium. Moreover, with very low concentrations of Fe leaching in the treatment medium, homogeneous catalysis could be neglected. Thus, MBC proved to be a potential catalyst for the treatment of organic pollutants thanks to its relative stability and reusability.
3.2.8. COD reduction during Orange G degradation catalyzed by magnetic biochar COD is the total quantity of oxygen required to convert organic matter to carbon dioxide and water. To assess the mineralization during the catalytic degradation of OG, COD was monitored. As a result, figure 12 illustrates the decrease in OG and COD as the Fenton-like degradation proceeds. Initially, the adsorption process decreased both OG and COD concentrations. Subsequently, according to the pseudo-first-order kinetics, the concentration of OG decreased continuously. Within the first 30 min of catalytic oxidation, COD also decreased significantly from 86 to 50 mg l −1 . In the subsequent period, however, there was a very slow decline in COD to 40 mg l −1 . The MBC catalyst enabled the removal of 58% COD in 120 min. Even though almost all OG had vanished, mineralization was not yet complete. The degradation of OG molecules may lead to the formation of certain smaller organic intermediates [60,61]. According to Gan et al [62], the mechanism for Fenton-like degradation of OG is complicated. ·OH radicals can attack species nonselectively, producing a variety of intermediates such as aniline, phenol, 7-hydroxy-8-(hydroxyamino) naphthalene-1, 3-disulfonic acid, 7, 8-dihydroxy-naphthalene-1, 3-disulfonic acid, alpha naphthol, and carboxylic acid. Additional treatment time may be required for complete mineralization of OG.

Conclusion
This study proposed an advanced route to facilitate the preparation of magnetic biochar via one-step pyrolysis of lotus seedpod and FeCl 3 mixture. The formation of Fe 3 O 4 nanoparticles and nanowires on the carbon framework was demonstrated by XRD, FTIR, SEM, EDS and TEM results. The presence of the magnetic components contributed to a specific saturation magnetization of 6.94 emu g −1 for MBC. In addition, FeCl 3 activation improved carbon-based pores, resulting in the as-prepared MBC with a high specific surface area of 349 m 2 g −1 and a large total pore volume of 0.31 g cm −3 . The experimental findings showed that MBC was not only a useful adsorbent but also an efficient catalyst for the treatment of Orange G. At pH 3.0, 350 ppm H 2 O 2 , 0.40 g l −1 MBC-0.4, 100% Orange G (100 ppm) and 58% COD were eliminated after 120 min of treatment. The degradation with MBC catalysts approximately followed pseudo-first-order kinetics with the highest rate constant of 0.034 min −1 . In addition, the catalytic activity declined by only 6.4% following four reuse cycles, with minimal iron leaching of 1.31-1.44 mg l −1 . Overall, environmentally friendly, sustainable and low-cost MBC could be a potential catalyst for the treatment of Orange G in wastewater.