Characterization of magnetic biochar amended with silicon dioxide prepared at high temperature calcination

Kans grass (Saccharum spontaneum) was collected from a wetland in Hangzhou city of China, as described earlier [7]. The sample was placed in a tightly covered stainless steel trays and pyrolyzed in a muffle furnace (SX₂ Shanghai Rong Feng Scientific Instrument, Inc.) under oxygen limited environment. The pyrolysis temperature was raised at a heating rate of 26 °C min⁻¹ until it reached 500 °C and held for 4 h. After cooling, the biochar was taken out and meshed (∽0.154 mm) for further applications. The dried kans grass biochar (KGB) was fabricated with Fe³⁺/Fe²⁺ by chemical coprecipitation [15] to obtain biochar/Fe₃O₄ composite, which has been referred as magnetic biochar (MB). To prepare magnetic biochar-SiO₂ (MBS) composite, supernatants from MB aliquot were removed and appropriate amounts of ethanol, ammonia and DI were successively added. Then, the solution was sonicated for 1 h. In addition, a calculated amount of TEOS (tatraethoxysilane) was added and the reaction was held for 2 h. The solution was kept at 60 °C for overnight after the addition of 50 mL ethanol. Afterward, the aliquot was washed using de-oxygenated DI water for several times to remove all the impurities. MBS materials were separated from the solution using external magnetic field and dried under vacuum at 60 °C for 24 h. The dried MBS was further processed by calcination to prepare a calcined product. Briefly, a proportional amount of MBS was added into the muffle furnace at 1000 °C under nitrogen atmosphere for 2 h to obtain another product, referred as magnetic biochar amended with SiO₂-calcined (MBSC).

Fourier transform infrared spectroscopy (FT-IR) (IR Prestige-21, Japan) was performed using KBr disc method and measured as wavenumber (cm⁻¹). X-ray diffraction patterns (XRD) of the samples were recorded on X’Pert PRO analytical B.V., Netherlands, at 2-theta range (10° to 90°). Scanning electron micrograph (SEM) (obtained with Hitachi S-3000N, Japan) was used to investigate the morphological compositions of the samples. Surface area was determined using BET surface analyzer (ASAP 2050, Micrometrics, Beijing, China). Quantum design PPMS-9 magnetometer was used to measure the magnetic properties in the magnetic field ranging from −20000 Oe to 20000 Oe at room temperature. X-ray energy dispersive spectrometer (EDS, model: ESM-5800, GEOL, Japan) was used to measure the elemental compositions.

Fig. 1 presents the XRD patterns of the prepared KGB, MB, MBC, MBS and MBSC samples. The diffraction peak indexed as (0 0 2) in the KGB sample could be assigned the noncrystalline hemicellulose [16], which still remained after the pyrolysis of the kans grass. Magnetite and quartz were found to be the major constituents in MB and MBS samples, as revealed in Fig. 1b and 1d. Magnetite diffraction peaks appeared at 2θ = 30.0°, 35.3°, 56.8°, and 62.3°, which were assigned to (2 2 0), (3 1 1), (5 1 1) and (4 4 0) planes, respectively. However, after high temperature calcination under nitrogen most of the magnetite peaks were either reduced or converted into other constituents, which is in agreement with the findings of our previous studies [12, 17]. However, in this study, the compositions of the new phases were different, as compared to the findings from our earlier studies and the reason could be the differences in precursor material compositions. The major peaks appeared at 2θ = 44.7°, 64.9°, 82.4° which could be assigned to (1 1 0), (2 0 0) and (2 1 1) planes iron (Fe), as shown in Fig. 1e. In addition, few major diffraction peaks at 2θ = 45.2°, 66.0° and 87.7° in MBSC samples could be assigned to Fe₃Si, which was formed at high temperature calcination under nitrogen. An increase in elemental compositions, particularly of silicon and iron contents was observed in EDX analysis, as given in Table 1. But the oxygen content decreased drastically in the calcined samples (Table 1). Furthermore, the quartz peaks ((1 0 0) and (0 1 1)) and iron oxide ((1 2 1), (⁻¹ 1 0) and (1 2 0)) with the assigned planes, respectively also disappeared in the samples calcined under nitrogen environment.

Fig. 1

Fig. 2 presents the FT-IR spectra of KGB, MB, MBC, MBS and MBSC samples. The presence of various bands in FT-IR spectra indicates various functional groups present in the composite samples [5]. The band appearing at 475 cm⁻¹ could be attributed to Fe–O vibration, which was found to become stronger in the sample calcined at higher temperature (Fig. 2e). Visible bands appearing around 580 cm⁻¹, 800 cm⁻¹ and 898 cm⁻¹ correspond to the available organic residues, such as aromatic C–C bonding in lignin, polysaccharide and polymeric CH2, which were found to be eliminated in both the samples (MBC and MBSC) after calcination at higher temperature. Similarly, higher temperature calcination eliminated H–O–H bonding at around 1550 cm⁻¹ to 1597 cm⁻¹ in water, as shown in Fig. 2c and Fig. 2e, but in other samples it was assumed to be responsible for a wider band [17].

Fig. 2

The strong band appearing at 1091 cm⁻¹ in all the samples could be assigned to Si–O–Si stretching mode, which was found to become stronger in the samples amended with SiO₂ (i.e. MBS and MBSC). Interestingly, high temperature calcination did not affect this band and it remained in all the samples. The band appearing around 3400 cm⁻¹ could correspond to the stretching of –OH group in water molecule, which is consistent with the studies reported earlier [7, 18]. Thus, the FT-IR study revealed the complexity of all the samples and variations of some constituents during magnetization and SiO₂ amendment.

Fig. 3 presents the SEM images of the samples (KGB, MB, MBC, MBS and MBSC). The presence of internal pores and cracks (micropores and mesopores) in KGB contributed to its surface area and pore volume, as shown in Fig. 3a. A good dispersion of Fe₃O₄ was confirmed in the biochar, as shown in Fig. 3b, which helped to keep the morphological structure of the biochar. Similarly, a recent study [19] revealed the successful dispersion of nanosized -Fe2O3 in the biochar matrix. Calcination could cause coarsening of the particles due to thermal treatment process [17]. In this study, calcination further decomposed the sample which appeared to be composed of polyhedral and heterogeneous particles of different sizes, as shown in Fig. 3. Morphological and mineralogical differences were also observed in the samples calcined under different environments [12, 17]. Similarly, both the calcined products (Fig. 3c and Fig. 3e) were observed with variable compositions, as given in Table 1.

Fig. 3

Calcination was considered to decrease the surface area of the samples, which tended to sintering and crystallizing [20]. But in this study, the surface areas of the calcined products were dramatically increased as compared to the uncalcined samples, as shown in Table 2. Interestingly, the surface area of MB increased from 45 m²·g⁻¹ to 336 m²·g⁻¹ after higher temperature calcination and the carbon content remained the same as in KGB (Table 1). Similarly, the surface area of MBSC increased from 45 m²·g⁻¹ to 125 m²·g⁻¹ after calcination, as given in Table 2. The reason could be the decomposition of organics which also affected the magnetic properties of the prepared samples. Thus, calcination caused significant variations in BET surface area, mean pore diameter as well as in pore volume of the samples.

The saturation magnetizations of MB, MBC, MBS and MBSC samples are presented in Fig. 4. Interestingly, the saturation magnetization of both the composite materials was found to increase after higher temperature calcination. Similarly, in our previous study [12], we found that nitrogen environment calcination increased the saturation magnetization of magnetic honeycomb cinders. Saturation magnetization of MB and MBS increased from 7.8 Am²·kg⁻¹ and 12.8 Am²·kg⁻¹ to 74.6 Am²·kg⁻¹ and 85.6 Am²·kg⁻¹, respectively, which was quite comparable to the pure Fe3O4 saturation magnetization [21]. The reason of higher saturation magnetization in calcined samples could be the presence of more superparamagnetic constituents. However, the presence of non-magnetic phase (i.e. iron-oxide) in MB and MBS could cause the lower saturation magnetization behavior. Materials with superparamagnetic behavior can be easily collected by external magnetic field and have wide application potential in environmental remediation measures [7, 12, 17].

Fig. 4

Fig. 5 (a to c) shows the TGA and DSC results of MB, MBC, MBS and MBSC samples. Generally, there are three stages of thermal degradation of samples, as described in details [7, 19, 22]. The TGA analysis (Fig. 5a to Fig. 5c) of both calcined and uncalcined samples showed a stable weight loss at temperature below 120 °C and similar to Fe₃O₄@SiO₂-MWCNTs distribution patterns [23]. In this study, comparison of the TGA curves showed that both the calcined samples preserved higher residual mass (>95 %) and turned out to be thermally more stable as compared to the uncalcined ones. It revealed that all the samples including calcined and uncalcined were characterized by higher thermal stability, as compared to the other materials studied earlier [24, 25]. DSC values of the samples also displayed the same distribution patterns, which was attributed to the loss of surface and bound water during higher temperature calcination. Thus, TGA-DSC analyses confirmed the presence of higher residual mass in the calcined samples, which were thermally more stable and can potentially be used in environmental remediation measures.

Fig. 5