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. 2022 Nov 14;7(47):42854–42864. doi: 10.1021/acsomega.2c04882

One-Step Pyrolysis Fabrication of Magnetic Bagasse Biochar Composites with Excellent Lead Adsorption Performance

Jinming Chang ‡, Sheng Yu ‡, Yunwen Liao ‡, Xiaoyu Guan †, Hejun Gao ‡,*, Yulong Li §
PMCID: PMC9713865  PMID: 36467949

Abstract

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In the present study, a magnetically separable adsorbent, manganese ferrite (MnFe2O4)/sugarcane bagasse biochar magnetic composites (MFSCBB-MCs), was fabricated through a one-step pyrolysis method. The characterization of the prepared adsorbents indicated that MnFe2O4 nanoparticles were successfully embedded into the biochar matrix, offering magnetic separability and increasing the negative charges on the surface relative to the pristine biochar. Batch adsorption tests indicated that the adsorption of lead on MFSCBB-MCs was pH- and dose-dependent. The experimental results were effectively fitted using the pseudo-second-order kinetic model (R2 > 0.99) and the Langmuir isotherm equation (R2 > 0.99), indicating the main chemisorption pathway and monolayer coverage process. Meanwhile, lead adsorption was found to be spontaneous and endothermic, as shown by the study of thermodynamic parameters. The maximum capacity, qm, calculated from the Langmuir model was 155.21 mg·g–1 at 25 °C, demonstrating excellent adsorption capability compared with several previously reported bagasse adsorbents. Based on adsorption mechanism analysis, physical adsorption, electrostatic attraction, and complexation were all involved in the lead(II) adsorption process on MFSCBB-MCs. Furthermore, the adsorbent was easily regenerated as indicated by the high magnetic separation and chemical desorption potential after five cycles, so it is a cost-effective and environmentally favorable adsorbent for wastewater lead removal.

1. Introduction

Heavy-metal-contaminated water derived from industrial effluents and municipal wastes, along with growing urbanization and industrialization, presents serious threats to public health and ecosystems.1,2 Lead has been widely explored owing to its known toxic nature and constant bioaccumulation throughout the food web. The emission of lead in drinking water has been limited to less than 0.01 mg·L–1 through strict environmental safety regulations.3 Studies should explore an effective approach to alleviating lead-associated water pollution.

Physical or chemical methods have been previously used for the removal of lead from water, such as adsorption, membrane separation, ion exchange, and chemical precipitation methods.4 Adsorption is one of the most prospective alternatives owing to its flexibility, time-saving, convenient operation, and cost-effectiveness. Several adsorbents have been successfully developed and applied in the purification of lead-contaminated water, such as activated carbon,5 zeolites,6 activated alumina,7 clay,8 and sugarcane bagasse (SCB).9,10 SCB is an economical and renewable biomaterial with abundant polar groups (carbonyl, carboxyl, and hydroxyl groups) and a strong coordinating affinity to heavy metals irons. However, raw SCB cannot be directly employed as an adsorbent due to its unstable structure, poor regeneration, low porosity, and limited metal adsorption capacity.11,12 To address these limitations, raw SCB is modified and functionalized by loading additional polymers/inorganic material and carbonization to increase its applicability in environmental remediation including lead sorption. Currently, the most attractive technique for improving SCB is hydrothermal carbonization, which imparts a large specific surface area, microporous structure, and stable chemical characteristics to the resultant biochar, thus offering a higher adsorption capacity.13,14 El-Banna et al. developed oxidized SCB biochar for lead sorption and reported that chemical oxidation improved the physicochemical properties of biochar.15 Doumer et al. reported that SCB-derived biochar had a high sorption capacity for Cd(II), Cu(II), lead, and Zn(II), making it suitable for the treatment of heavy-metal-contaminated water.16 The findings of these investigations typically demonstrated higher effectiveness of metal removal from aqueous solution. However, these SCB-produced biochar sorbents were difficult to separate and recycle. The residual sorbent species are potential secondary contamination risks. Therefore, SCB biochar sorbents with “recovery–reuse” capability for lead removal from water should be developed.

Limited studies report the fabrication and application of magnetic SCB biochar sorbents. Rattanachueskul et al. used Fe3O4/SCB biochar composites to remove undesirable tetracycline from water.14 Dong et al. removed 17β-estradiol from an aqueous solution using magnetic SCB biochar nanoparticles.17 Yi et al. prepared SCB magnetic biochar using steel pickling waste liquor, which was eventually used for Cr(VI) removal.18 Notably, the approaches mainly magnetized the final SCB biochar by introducing iron oxides. In addition, most of the fabrication methods involve multistep and complicated procedures, such as initial co-precipitation, followed by pyrolysis. Moreover, the incorporation of common magnetite (Fe3O4) endows the biochar with magnetic separation ability; however, it does not always improve heavy metal removal efficiency.19,20 Previous studies report that the predominant lead species in solution at a maximum pH of about 9.0 are Pb2+ and [Pb (OH)]+. The zero-point charge (pHzpc) of iron oxides is in the neutral pH range, implying that loading Fe3O4 increases the surface positive charge in an acidic environment, which can restrict adsorption to cationic lead species due to electrostatic repulsion.21,22

Manganese oxides are more suitable for adsorbent preparation compared with iron oxides, particularly for the elimination of cationic contaminants. Manganese oxides exhibit a porous structure, and their surface charge is generally negative throughout a wide pH range. As a result, several studies currently explore the incorporation of Mn into iron oxides to form manganese iron oxides (MnFe2O4). MnFe2O4 is insoluble in water and preserves its magnetic properties as well as the effective sites (carboxyl and hydroxyl groups) accessible for pollutant adsorption. Significant adsorptive removal of heavy metals (Ni, Hg, Cd, Zn, Pb, Cu, and Cr) and arsenic have been achieved with the aid of MnFe2O4 nanopowders or MnFe2O4-loaded composites.23,24 These findings imply that loading MnFe2O4 onto the pristine SCB biochar can improve lead adsorption ability and promote effective separation through an external magnetic field.

Therefore, MnFe2O4/SCB biochar magnetic composite (MFSCBB-MCs) was prepared in the current study through a one-step pyrolysis method. The structure of MFSCBB-MCs, its potential for lead removal from solutions, and magnetic separation performance were systematically characterized and evaluated. A series of batch experiments with varied pH, adsorbent dose, contact time, initial lead concentration, and temperature were performed to explore the effects of operational parameters on the lead removal capability of MFSCBB-MCs. The adsorption mechanism was determined using X-ray photoelectron spectroscopy (XPS) and different mathematical models, for instance, kinetics, isotherms, and thermodynamics. Moreover, the feasibility of regenerating exhausted adsorbents through chemical desorption and their recyclability were explored. The findings of this study provide a recyclable adsorbent with enhanced lead remediation ability and effective magnetic separation property.

2. Materials and Methods

The materials used in this study and characterization of MFSCBB-MCs are available in the Supporting Information.

2.1. Preparation of MFSCBB-MCs

MFSCBB-MCs were synthesized through a one-step pyrolysis procedure. SCB was cleaned by thoroughly washing it with distilled water to remove any adhering dust and sugar. The clean SBC was then dried for 48 h at 80 °C, pulverized, and finally sieved using a 200-mesh screen to obtain a powder of uniform size. One gram of SCB powder was homogeneously dispersed into a 100 mL solution containing 0.01 mol·L–1 KMnO4 and 0.01 mol·L–1 FeCl3 under ultrasonic oscillation. The mixture was heated in a beaker at 80 °C for 24 h to obtain a brown solid precursor. The precursor was pyrolyzed in a digital temperature control tube furnace (GSL-1500X, Hefei Kejing Material Technology Co., Ltd., China) at 600 °C for 1 h under a nitrogen atmosphere and a heating rate of 5 °C·min–1. The residues obtained were cooled under a nitrogen atmosphere. The residues were severally washed with ultrapure water (18.2 MΩ·cm, 25 °C) and dried under a vacuum at 80 °C overnight to obtain MFSCBB-MCs. The synthesis process and structure of MFSCBB-MCs are presented in Scheme 1. Pristine SCB biochar (only pyrolyzed, donated as SCBB) was also prepared through the same procedure for comparison.

Scheme 1. Schematic Illustration of the Synthesis Procedure and Structure of MFSCBB-MCs (photograph courtesy of ‘Sheng Yu’. Copyright 2022).

Scheme 1

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2.2. Batch Experiments

Effects of operational parameters, such as adsorbent dose, pH, temperature, and initial lead concentration on lead adsorption performance of MFSCBB-MCs as well as the adsorption mechanism were investigated. A standard 1000 mg·L–1 lead solution was prepared by adding lead nitrate into a volumetric flask and diluting it to obtain the desired concentrations. A predetermined amount of MFSCBB-MCs was dispersed in an aqueous solution of lead under constant shaking using a thermostatic shaker (SYC-C, Shanghai Shengke Instrument Equipment Co., Ltd., China) at 200 rpm. The magnetic separation of MFSCBB-MCs was performed and a 0.2 μm syringe filter was used to filter the solution. The concentration of the remaining lead solution was determined by atomic absorption spectroscopy (WYS2200, AnHui Wanyi Science and Technology Co., Ltd., China). The removal rate, adsorption capacity, adsorption isotherms, and kinetics were determined using various equations (Table S1, Supporting information). The effect of pH on adsorption was investigated over a wide pH range (pH 2–8) through adjustment using HCl and NaOH solution. The effect of ionic strength (KNO3, Ca (NO3)2) on adsorption was also evaluated.

2.3. Recyclability Tests

Reusability is an important economic parameter of adsorbents. Adsorption–desorption experiment was carried out to explore the recovery of lead adsorbed to MFSCBB-MCs during the adsorption process. The process was carried out as follows: 0.2 g of MFSCBB-MCs was added to 100 mL of lead-contaminated aqueous solution (10 mg·L–1). After achieving adsorption equilibrium and magnetic separation, the separated adsorbents were mixed with 100 mL of sodium acetate solution (1.0 M). The mixture was oscillated continuously in a shaker incubator at 200 rpm for 12 h at 25 °C. The absorbent was collected, washed with ultrapure water, and dried before use in the next adsorption–desorption cycle. Five cycles were conducted under the same conditions.

3. Results and Discussion

3.1. Physicochemical Properties of MFSCBB-MCs

A facile and convenient pyrosis method was used to simultaneously complete the biochar transformation and crystallization of metals for a one-step fabrication of magnetic biochar. Surface morphologies and chemical composition of MFSCBB-MCs were explored using scanning electron microscopy (SEM), SEM energy-dispersive X-ray (SEM-EDX), and transmission electron microscopy (TEM) (Figure 1). The SEM investigation revealed that the carbon layer of MFSCBB-MCs was rough, wrinkled, and uneven, with numerous crystal-like tiny particles (Figure 1a,b). Notably, TEM analysis showed consistent results (Figure 1e). Image J software (U.S. National Institutes of Health) was used to determine the particle size distribution. The results showed that the particles exhibited a monodispersed polygon shape with an average size of about 400 ± 32 nm. These small particles can provide a lot of extra sites for lead adsorption. Elemental mapping for Figure 1b showed that the particles were mainly composed of Mn, Fe, and O (Figure 1c,d), whereas only C and O were found in the biochar beyond these particles (see Supporting Information Figure S1). The even distribution of C was attributed to bagasse biochar, and O distribution was ascribed to the particles and the oxygen-containing functional group of bagasse biochar. The microstructure of MFSCBB-MCs was further explored by HRTEM and selected area electron diffraction (SAED). HRTEM findings showed relatively clear crystal lattice fringes, indicating the good crystallization property of particles (Figure 1f). The calculated lattice spacing of 0.29 and 0.23 nm was indexed to the (220) and (222) planes of the MnFe2O4 crystal, respectively. SAED pattern in Figure S2a showed several evident diffraction rings, indicating the formation of MnFe2O4 crystals. The lattice planes of (111), (220), (311), (400), (440), and (511) were marked and were in accordance with the standard JCPDS card no. 10-0319. The amorphous layer observed around the crystals was attributed to the presence of biochar.25,26

Figure 1.

Figure 1

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SEM images (a), element micrograph region (b), energy-dispersive system (EDS) mapping patterns of Mn (c), Fe (d), TEM image (e), and HRTEM image (f) of MFSCBB-MCs. The insets (a, b) represent the appearances and the diameter distribution histogram of MnFe2O4 particles, respectively.

The crystallographic structure of MFSCBB-MCs was characterized by X-ray powder diffraction measurement in the range of 2θ (10–80°) (Figure S2a). The diffraction peaks at 2θ = 18.3, 29.8, 35.8, 36.6, 42.8, 52.8, 56.5, and 62.0° accurately matched with the (111), (220), (311), (222), (400), (422), (511), and (440) planes from the JCPDS card (10-0319) for cubic spinel-type MnFe2O4. Notably, a slight broad peak was observed at about 21°. The amorphous component was attributed to the disordered carbon in the sample.27 This result was compatible with HRTEM and SAED findings, indicating that MFSCBB-MCs were successfully synthesized.

FTIR was performed to explore the surface functional groups of MFSCBB-MCs, and the resultant spectrum is shown in Figure S2b. The broad band observed at about 3440 cm–1 was ascribed to the −OH stretching vibration. The absorption bands peaking at 2973, 2919, and 2850 cm–1 corresponded to C–H stretching in the aliphatic structures. Peaks at 1635 and 1382 cm–1 represented the C=O asymmetric and symmetric stretchings of surface oxygen-containing functional groups. The C–O stretching vibration that occurred at 1078 cm–1 was also attributed to oxygen-containing functional groups.28,29 Two characteristic peaks at 670 and 443 cm–1 corresponding to Fe–O and Mn–O vibrations were observed, indicating the presence of MnFe2O4.30

The XPS survey spectrum and high-resolution XPS spectra were used to further explore the chemical composition of MFSCBB-MCs (Figure 2). The findings showed the presence of photoelectron peaks of C 1s, O 1s, Fe 2p, and Mn 2p in MFSCBB-MCs (Figure 2a). The high-resolution C 1s spectrum was deconvolved into three components centered at 288.2 (C=O), 285.4 (C–O), and 284.5 (C–C or C–H) eV. In addition, the O 1s spectrum comprised three components centered at 531.1, 533.3, and 532.6 eV, which were attributed to the hydroxyl bonded to metal (M–OH), O=C–O species, and C–OH, respectively.31 These findings indicate that MFSCBB-MCs are rich in oxygen-containing functional groups, including hydroxyl groups. The presence of MnFe2O4 was demonstrated by the simultaneous occurrence of Fe 2p and Mn 2p signals. The Fe 2p signal was represented by three Gaussian functions, which corresponded to different bonding states with binding energy centered at 711.8 (Fe 2p3/2) and 725.1 (Fe 2p1/2), respectively. Analysis of the Mn 2p signal revealed two components with binding energy centered at 641.9 (Mn 2p3/2) and 654.0 (Mn 2p1/2) eV, respectively.32,33

Figure 2.

Figure 2

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(a) XPS survey spectrum and high-resolution XPS spectra of (b) C 1s, (c) O 1, (d) Fe 2p, and (e) Mn 2p for MFSCBB-MCs.

The specific surface area and pore size distributions of SCB and MFSCBB-MCs are presented in Figure 3. The results showed that SCB and MFSCBB-MCs exhibited a typical type I curve with a sharp increase in a 0–0.1 range of relative pressure (Figure 3a). This indicates the predominant microporous characteristics. The surface area of SCB was approximately 373.32 m2·g–1. The surface area slightly decreased to 305.41 m2·g–1 after loading MnFe2O4 to form MFSCBB-MCs. This was attributed to a few adsorption sites on the external surface of SCB, which were occupied by deposited MnFe2O4 nanoparticles. The findings on the pore size distribution (Figure 3b) showed that SCB and MFSCBB-MCs had microporous (pore size < 2 nm) and mesoporous (2 nm < pore size < 50 nm) structures, respectively. Mesopores with sizes ranging from 3 to 8 nm were blocked and new mesopores with sizes of approximately 13.5 nm occurred during the formation of MnFe2O4 nanoparticles. The most important micropores (pore size < 2 nm) to the overall pore volumes were preserved. The total pore volume of MFSCBB-MCs (0.10 cm3·g–1) was therefore not significantly different from that of SCB (0.13 cm3·g–1). These results indicated that hierarchical porous MFSCBB-MCs were successfully fabricated. This characteristic structural feature of the composite provides more adsorption sites for lead removal.

Figure 3.

Figure 3

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N2 adsorption and desorption isotherms determined at 76 K (a) and pore size distributions (b) of SCB and MFSCBB-MCs.

A vibrating sample magnetometer was used to explore magnetic field-dependent characteristics of MFSCBB-MCs at room temperature. The essential magnetic parameters were obtained from the hysteresis loops presented in Figure 4. The saturation magnetization (Ms) was 4.5 emu·g–1. The remnant magnetization (Mr) and coercivity (Hc) were approximately 0.52 emu·g–1 and 97 Oe, respectively. The results showed that MFSCBB-MCs dispersed in an aqueous medium were separated within 5 min after the application of an external magnetic field, exhibiting outstanding magnetic separation characteristics. The dispersion became translucent from dark-black color after the completion of the recovery. These findings indicate that MFSCBB-MCs had excellent magnetic properties, which were favorable for adsorption separation and desorption regeneration.

Figure 4.

Figure 4

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Room-temperature magnetic hysteresis loops of MFSCBB-MCs. The inset figure represents 0.03 mg·mL–1 MFSCBB-MCs dispersed in a solution containing 0.03 mg·mL–1 Pb, and the behaviors in response to magnetic fields (photograph courtesy of ‘Sheng Yu’. Copyright 2022).

3.2. Effect of Adsorbent Dosage on Adsorption

Adsorbent dosage is a significant parameter as it determines the metal-adsorbent equilibrium within the adsorption system. The effect of biochar dose on lead adsorption for MFSCBB-MCs is shown in Figure S3. The lead removal efficiency increased from 34.1 to 71.8 % as the biochar dose increased, whereas the adsorption efficiency (qt) decreased from 66.82 to 24.23 mg·g–1. The increase in lead removal percentage can be attributed to the availability of more adsorption sites in the solution. An increase in the biochar dose above a particular dose showed a plateau in the lead removal percentage. This phenomenon can be attributed to competition among the active sites as well as a decrease in the metal ion concentration gradient. Moreover, a reduction in the surface area and an increase in the diffusional path length associated with aggregation/agglomeration of sorbent particles at higher doses can result in a decrease in adsorption capability (qe). These effects as well as previous findings that additional adsorption sites are available at high pollutant amounts,34 indicate that the qt value can be used to approximate the adsorption-saturation capacities under low doses.

3.3. Effect of Initial pH on Adsorption

The pH of the initial solution can affect the ionic form of the contaminant and the adsorbent’s functional surface. pH-related protonation or deprotonation of the functional groups can change the adsorbent surface charge distribution. Precipitation of lead hydroxide occurs when the aqueous solution pH reaches 8.0, so the experiment of the influence of pH on lead adsorption performance on MFSCBB-MCs was manifested in the pH range of 2.0–8.0 (Figure 5b). The results showed that the lead removal ability of the prepared magnetic adsorbent was strongly pH-dependent. The adsorption capacity increased significantly by increasing the pH of the solution from 2 to 7, whereas the increase was not significant when the pH was increased from 7 to 8. Accordingly, it can be easily concluded that the optimum pH for lead adsorption is around 7.0. The ζ potentials of the samples (Figure 5a) showed that the pHzpc of MFSCBB-MCs was about 2.0, which was lower than that of SCBB, owing to the incorporation of MnFe2O4. The oxygen-containing functional groups, derived from both biochar and MnFe2O4 particles, were continually deprotonated when the pH was increased to 7.0, resulting in a more negatively charged surface. Notably, the predominant lead species at pH ≤ 7.0 was Pb2+.35 Therefore, an anticipated strong intersection occurred between the negative MFSCBB-MCs surface and the cationic Pb2+ species, although particular chemical adsorption cannot be excluded as a significant contributing force in metal ion adsorption.

Figure 5.

Figure 5

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(a) ζ-potential of SCBB and MFSCBB-MCs. (b) Effect of pH on adsorption of lead by MFSCBB-MCs (dosage = 0.1 g·L–1, contact time = 2 h, the initial Clead = 20 mg·L–1).

3.4. Adsorption Kinetics

Adsorption kinetics were explored at different initial concentrations of lead(II). The effect of contact time on the adsorption of lead onto MFSCBB-MCs is presented in Figure 6a. Within the first 60 min, lead(II) was quickly adsorbed onto MFSCBB-MCs. The adsorption rate then gradually decreased, whereas the adsorption efficiency increased to >85% and equilibrium was attained within 180 min. Notably, limited additional adsorption was observed after 180 min. Adsorption gradually increased with a further increase in C0.

Figure 6.

Figure 6

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Effect of contact time on lead adsorption capacity (a) and intraparticle diffusion plots for lead adsorption (b) (dosage = 0.1 g·L–1, pH = 6.8).

Pseudo-first-order, pseudo-second-order, and Elovich models are used to fit the experimental kinetic data to further understand the kinetic mechanism of adsorption. The fitting results, as well as the values of the characteristic parameters, are tabulated in Table 1. The values of k1 and k2 for the first-order and second-order kinetic models were obtained from their respective linearized plots as shown in Figure S4. The results of the pseudo-first-order model showed that the value of the calculated qe was less than that of the experimental qe, indicating that the pseudo-first-order model did not fit this adsorption kinetic. The high correlation coefficient (R2 > 0.99) based on the pseudo-second-order model offered a better explanation of lead adsorption on MFSCBB-MCs compared with the pseudo-first-ordination model. Furthermore, the calculated qe values of the pseudo-second-order model were close to the experimental values. These results implied that the rate-limiting step was chemisorption involving valence forces through electron sharing or exchange between the adsorbate and adsorbent.24 The plot of qt versus ln t derived from the Elovich equation, which effectively describes the predominant chemical absorption on highly heterogeneous sorbents, offered an inferior correlation coefficient by linear fitting. However, the significantly higher α value relative to the β value indicated that lead cations were adsorbed on MFSCBB-MCs mainly through the formation of coordination bonds.36

Table 1. Parameters for the Adsorption of Lead on MFSCBB-MCs Obtained from Different Kinetic Models.

    initial conc. (mg·L–1)
kinetic model parameter 40 100 200
  qe exp (mg·g–1) 105.65 150.38 202.53
pseudo-first-order qe cal (mg·g–1) 42.88 60.89 60.75
k1 (h–1) × 10–2 2.12 1.88 1.70
R2 0.9727 0.8929 0.8821
pseudo-second-order qe cal (mg·g–1) 106.61 148.59 199.20
k2 (g·mg–1 min–1) × 10–2 0.94 0.67 0.50
R2 0.9979 0.9955 0.9970
Elovich α (mg·g–1.min–1) 320.36 1760.99 115 176.18
β (g·mg–1) × 10–1 1.07 0.90 0.87
R2 0.8868 0.9249 0.9507
intraparticle diffusion Kid1 (mg·g–1·min–1/2) 5.47 7.57 7.348
C1 (mg·g–1) 53.49 81.39 132.30
R2 0.96 0.89 0.93
Kid2 (mg·g–1·min–1/2) 0.99 3.42 3.40
C2 (mg·g–1) 90.29 100.96 152.03
R2 0.92 0.96 0.89
Kid3 (mg·g–1·min–1/2) 0.06 0.05 0.10
C3 (mg·g–1) 104.13 148.98 199.16
R2 0.955 0.92 0.89
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To further explore the actual adsorption rate-controlling step, the kinetic adsorption data were fitted using the Weber–Morris model. The fitted results are depicted in Figure 6b and Table 2. Intraparticle diffusion plots showed a multilinear feature, indicating the presence of two or more steps in the sorption process. The intraparticle diffusion rate constant determined from the slope was Kid1 > Kid2 > Kid3. The large slope in the first stage implied that instantaneous adsorption and external diffusion were achieved within a short time. The external surface diffusion slowed down and the adsorption mainly occurred in microscopic pores in the second step, resulting in a sharp decrease in the slope. The slope decreased toward zero thereafter, and the adsorption gradually changed to equilibrium. Notably, linear fitting of the Weber–Morris model exhibited a nonzero intercept, showing that the adsorption process depended on intraparticle diffusion and extra diffusive resistance was involved.37

Table 2. Langmuir and Freundlich Isotherm Parameters for Lead Desorption on MFSCBB-MCs.

    temperature (K)
isotherm model parameter 298 308 318
  qe exp (mg·g–1) 150.87 230.07 243.52
Langmuir qmax (mg·g–1) 155.21 232.93 240.83
KL (L·mg–1) 7.00 × 10–2 8.20 × 10–2 3.83 × 10–1
R2 0.9935 0.9996 0.9998
RL 0.525–0.058 0.529–0.053 0.368–0.011
Freundlich KF (mg1 –n·Ln·g–1) 52.61 102.95 176.78
n 5.06 6.31 16.84
R2 0.8782 0.9123 0.8000
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3.5. Adsorption Isotherms

Adsorption isotherms at three different temperatures (298, 308, and 318 K) were obtained by changing the lead concentration from 0 to 240 mg. The Langmuir and Freundlich models were then used to fit equilibrium data. Adsorption isotherms of lead on MFSCBB-MCs under the three different temperatures, including the fitting of the experimental data, are shown in Figure 7. The fitted values of the characteristic parameters and the coefficients (R2) are presented in Table 3. The Langmuir model showed a higher correlation of the experimental isothermal data concerning the high determination coefficients compared with the Freundlich model. This indicates that adsorption was a uniform and monolayer coverage process.38 The dimensionless separation factor (RL) for all of the adsorption processes in the Langmuir model was in the range of 0–1, indicating that the adsorption process for this model was favorable.39 The theoretical maximum adsorption capacity (qmax) at room temperature was calculated to be 155.21 mg·g–1. The results showed that MFSCBB-MCs exhibited a high lead adsorption capacity when compared to the qmax of other modified bagasse adsorbents previously proposed for lead removal, as shown in Table S2.

Figure 7.

Figure 7

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Adsorption isotherm of lead on MFSCBB-MCs at three different temperatures (dosage = 0.1 g·L–1; pH = 6.8; contact time = 3 h).

Table 3. Thermodynamic Parameters of Lead Adsorption over the MFSCBB-MCs Adsorbent at Three Temperatures.

T (K) ΔG° (kJ·mol–1) ΔH° (kJ·mol–1) ΔS° (J·mol–1·K–1)
308 –4.07 74.08 261.41
323 –5.90
338 –9.33
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3.6. Adsorption Thermodynamics

Thermodynamic parameters were calculated to further investigate if lead adsorption on MFSCBB-MCs occurs spontaneously. The parameters including ΔG (Gibbs free energy), ΔH (enthalpy), and ΔS (entropy) were calculated using the following equations

3.6. 1
3.6. 2

where R is the general gas constant (8.314 J·mol–1·K–1), T is the absolute temperature (K), and Kc is the thermodynamic equilibrium constant (L·g–1). Based on the experimental data provided in Figure 7, a linear plot of ln Kc versus 1/T was obtained. ΔH and ΔS of the adsorption process were calculated according to eq 2, using the slope and intercept of the plot, respectively. The difference between the adsorption enthalpy and adsorption entropy derived from eq 1, multiplied by the temperature, gives Gibbs free energy. The calculated values of ΔG, ΔH, and ΔS are presented in Table 3. The positive value of ΔH indicated that the lead adsorption process was an endothermic process, and the positive value of ΔS implied that the adsorbent/liquid interface became more disordered. The results showed that all of the values of ΔG were negative, and their absolute values increased as the temperature increased, indicating spontaneous lead adsorption upon MFSCBB-MCs at the three temperatures.40

3.7. Sorption Mechanism

The hierarchically porous MFSCBB-MCs with a large specific surface area were beneficial for the capture of lead(II) by physical adsorption. Meanwhile, the surface electronegativity of MFSCBB-MCs caused electrostatic attraction as well as electrostatic surface complexation with cationic lead(II) species. According to the kinetic study, chemisorption was more important during lead(II) adsorption on MFSCBB-MCs. To further investigate the removal mechanism of lead(II) by MFSCBB-MCs, XPS analysis on the Pb-MFSCBB-MCs sample (after the sorption of Pb2+) was performed (Figure S5). The significant peak corresponding to Pb 4f was observed on the MFSCBB-MCs surface in Figure S5a, indicating the successful adsorption of Pb2+ by MFSCBB-MCs. The Pb 4f spectrum comprised two components centered at 139.0 and 143.9 eV, which were assigned to Pb 4f7/2 and Pb 4f5/2, respectively (Figure S5b). The energy separation of 4.9 eV between the Pb 4f5/2 and 4f7/2 confirmed the coordination interaction between Pb2+ and MFSCBB-MCs.41 The Pb 4f7/2 peak, which shifted to lower binding energy than that of lead nitrate (139.5 eV), was ascribed to the binding (Pb–O) formed with the hydroxyl groups, while the Pb 4f5/2 peak was attributed to the binding (Pb–OOC) formed with the carboxyl groups on MnFe2O4 or the biochar surface of MFSCBB-MCs. The high-resolution Fe 2p, Mn 2p, and O 1s spectra also corroborated it. After lead(II) adsorption, Fe 2p (Figure S5d) and Mn 2p (Figure S5e) showed a slight shift in the binding energy. This may be explained by the transformation from Fe/Mn–OH to (Fe/Mn–O)2Pb or Fe/Mn–COOH to (Fe/Mn–COO)2Pb.42 Three components of the O 1s spectrum located at 531.1, 533.3, and 532.6 eV (Figure 2c) shifted to 530.4, 533.5, and 532.1 eV (Figure S5c), which were attributed to the formation of Mn/Fe–O–Pb, C–O–Pb, and Mn/Fe–COO–Pb or COO–Pb on the surface of MFSCBB-MCs.43 These results suggest that the oxygen-containing functional groups on MFSCBB-MCs are crucial for the high lead(II) adsorption ability. The lead(II) adsorption process on MFSCBB-MCs involves physical adsorption, electrostatic attraction, and complexation.

3.8. Effect of Coexisting Ions

The results from the ζ-potential experiments showed that the introduction of MnFe2O4 resulted in a highly negatively charged surface of the composites (MFSCBB-MCs) at a pH ≥3. Previous studies reported that the binding of cations to a negatively charged surface neutralizes the surface charge.44 Gradient concentrations of KNO3 and Ca(NO3)2 ranging from 0.02 to 0.1 M were used to explore the effect of coexisting cation ions, K+ and Ca2+, on lead adsorption. The cations in solution increase the electrostatic repulsion between cations and lead or can compete with lead for the active sorption sites. Lead adsorption capacity significantly decreased with an increase in ionic strength (Figure 8a). Ca2+ showed a significant effect on lead adsorption compared with the effect of K+. This can be partially attributed to the higher positive charge of Ca2+. Thus, they effectively shield more negative charges on MFSCBB-MCs as well as occupy more active sites.

Figure 8.

Figure 8

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(a) Effect of various co-ions on lead removal by MFSCBB-MCs. (b) Reusability of MFSCBB-MCs during five cycles.

3.9. Regeneration and Reusability

The capacity of the adsorbent to regenerate for practical application is critical for enhancing the cost efficiency of the adsorption process. In the present study, lead desorption was performed by dispersing exhausted MFSCBB-MCs into 1 M CH3COONa aqueous solution at room temperature. The regenerated adsorbent was then used to remove lead from water under the same conditions. The results showed that the lead removal percentage was maintained above 85% after five sorption–desorption cycles (Figure 8b). This indicates that MFSCBB-MCs can be repeatedly and effectively used as an adsorbent for lead removal, thus reducing the cost of the process. Notably, CH3COONa treatment promotes the desorption of adsorbed lead from MFSCBB-MCs.

4. Conclusions

In the present study, MnFe2O4/SCB biochar magnetic composites (MFSCBB-MCs) were successfully prepared through a one-step pyrolysis method and used as a recoverable absorbent for lead removal from water, with the benefits of bagasse resource utilization and facile separation. The incorporation of MnFe2O4 resulted in a more negatively charged surface, which was more effective for cation capture. Adsorption kinetics and adsorption thermodynamics studies indicated that lead adsorption on MFSCBB-MCs was spontaneous chemisorption and endothermic in nature. The sorption of lead on MFSCBB-MCs included physical adsorption, electrostatic attraction, and complexation, and they showed higher lead adsorption capability compared with previously reported bagasse adsorbents for lead removal. The exhausted adsorbent was regenerated and recycled through chemical desorption with the aid of magnetic MnFe2O4 using a magnetic field, implying that MFSCBB-MCs have a high potential for environmental application and are relatively affordable.

Acknowledgments

The authors gratefully acknowledge the financial support from the Science and Technology Planning Project of Sichuan Province (No. 2022NSFSC0350), the Opening Project of Key Laboratory of Green Chemistry of Sichuan Institutes of Higher Education (No. LZJ1902), the Open Project of Shaanxi Collaborative Innovation Center of Industrial Auxiliary Chemistry & Technology (No. XTKF-2020-02), the Opening Project of Key Laboratory of Leather Chemistry and Engineering (Sichuan University, 2020), the Ministry of Education, the Ph.D. Start-up Fund (No. 18Q025), and the Basic Scientific Research Operating Expense Project (No. 416981) of China West Normal University.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c04882.

  • Materials used in this study and characterization of MFSCBB-MCs. Equations and parameters of qt, removal rate, kinetic and isotherm models, the carbon and oxygen element mapping, elemental analysis, FTIR spectrum, SAED pattern, XRD pattern of MFSCBB-MCs, effect of adsorbent dose and linear fits of pseudo-first-order, pseudo-second-order, and Elovich equations for Pb adsorption on MFSCBB-MCs. Adsorption capacities of different bagasse sorbents for lead (PDF)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ao2c04882_si_001.pdf (603.1KB, pdf)

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