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iron oxide composite for methylene blue removal
Journal of Hazardous Materials 384 (2020) 121286
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A green biochar/iron oxide composite for methylene blue removal a,b
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Ping Zhang , David O’Connor , Yinan Wang , Lin Jiang , Tianxiang Xia , Liuwei Wang , ⁎ Daniel C.W. Tsangc, Yong Sik Okd, Deyi Houa,
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School of Environment, Tsinghua University, Beijing 100084, China National Engineering Research Centre of Urban Environmental Pollution Control, Beijing Key Laboratory for Risk Modeling and Remediation of Contaminated Sites, Beijing Municipal Research Institute of Environmental Protection, Beijing 100037, China c Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China d Korea Biochar Research Center & Division of Environmental Science and Ecological Engineering, Korea University, Seoul 02841, Republic of Korea b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Editor: R. Teresa
Adsorbents that effectively remove dye substances from industrial effluents are needed for the protection of human health and the natural environment. However, adsorbent manufacture is associated with secondary environmental impacts. In this study, a green biochar/iron oxide composite was produced using a facile approach involving banana peel extract and FeSO4. The modified biochar’s capacity to adsorb methylene blue (MB) was considerably enhanced (Langmuir Qmax of 862 mg/g for MB when C0 = 500 mg/L, pH = 6.1, T =313 K) compared to the unmodified banana peel biochar, and exhibited good performance for a wide range of pH values (pH 2.05–9.21). The Langmuir isotherm model and pseudo second-order kinetic model accurately describe the adsorption process. The material properties and corresponding adsorption mechanisms were investigated by various experimental techniques. Enhanced MB adsorption by the biochar/iron oxide composite is attributed to increased electronic attraction to MB molecules, as evidenced by XPS analysis. High adsorption capacity was retained after 5 regeneration cycles. This study suggests that biochar can be modified by a green synthesis approach to produce biochar/iron oxide composite with good MB removal capacity.
Keywords: Green synthesis Modified biochar Engineered biochar Methylene blue Dye adsorption
1. Introduction In recent years, there have been increasing calls for sustainable
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solutions to environmental issues. Biochar is attracting significant interest (Yu et al., 2019; Shen et al., 2019) as a low-cost material that complements the greener paradigm (O’Connor et al., 2018a; Yoder
Corresponding author. E-mail address: [email protected] (D. Hou).
https://doi.org/10.1016/j.jhazmat.2019.121286 Received 25 July 2019; Received in revised form 17 September 2019; Accepted 21 September 2019 Available online 24 September 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
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et al., 2011; Wang et al., 2019a) for a number of applications, including carbon sequestration (Woolf et al., 2010), soil amendment (Atkinson, 2018; Baiga, 2017) and wastewater treatment. Inadequately treated wastewater effluents, from industries such as textiles, leather, and pharmaceutical and personal care products (PCPPs) (de Luna et al., 2014; Forgacs et al., 2004), risk harm to human health and the natural environment (Cheuyglintase et al., 2018). As a wastewater adsorbent, biochar seen as an alternative to activated carbon (AC) for the removal of various contaminants, including inorganic anions (NO3−,PO43-), metal cations (Pb(II),As(III) and Cd(II)), organic dyes (crystal violet, cationic red X-GRL), etc. (Ahmed et al., 2018a; Ahmad et al., 2014). However, biochar’s relatively low surface area and the influence of abiotic and/or biotic processes can limit its effectiveness in certain applications (Premarathna et al., 2019a). The emergence of engineered biochars has taken its performance to the next level (Wang et al., 2019b; Rajapaksha et al., 2016; Ok et al., 2015). Various modifications have been attempted, including chemical, physical, mineral impregnation, and magnetic modifications (Cho et al., 2019; Sun et al., 2019a; Premarathna et al., 2019b; Sahin et al., 2017). Biochar/metal composites are particularly promising. For example, aluminum species in biochar/aluminum composites increase catalytic activity (Yu et al., 2019). Biochar/iron composites have multi-functionality for adsorption, reduction, and complexation (Sun et al., 2019a). Chen et al. (2011) reported that biochar/iron oxide composites have significantly enhanced As sorption capability. A number of lowcost metal oxides have been added to composite adsorbents for the treatment of various contaminants (Chen et al., 2016; Chaukura et al., 2017). In biochar/metal oxide composites, the biochar acts as a porous carbon body that is surface modified by the metal oxide. This has the benefit of increasing the adsorbent surface area (Premarathna et al., 2019a). In the treatment of cationic dyes, enhanced sorption can also be ascribed to stronger electrostatic interactions (Fig. 1). However, existing synthesis techniques for producing biochar/metal oxide composite usually involve soaking biochar in large volumes of modifying chemicals and/or the use of potentially toxic chemicals (Premarathna et al., 2019a), which undermines sustainability. As compared to conventional synthesis, green synthesis seeks to avoid secondary impacts, by either (i) using green materials, or (ii) they consume less energy or natural resources in synthesis process, aspiring for ambient synthesis reaction conditions (Wang et al., 2019c; O’Connor et al., 2018b, c). For example, plant extracts have proved useful for the manufacture of metal oxide adsorbents (Mohammadinejad et al., 2016) with active biomolecules functioning as effective reducing and capping agents (Shen et al., 2017; Shankar et al., 2003; Weng et al., 2017). In our previous work, we presented a green pathway for the production of size-controllable bimetallic Fe-Cu oxide adsorbents, which have exceptional performance for malachite green dye removal (Zhang et al.,
2018). The use of plant extracts for the green synthesis of biochar/metal oxide composites, however, remains an underexplored research topic. In this study it was hypothesized that a biochar/iron oxide (FexOy) composite could be successfully produced by a green synthesis method (Fig. 1), which would possess good MB removal capacity. To demonstrate this, banana peel biochar was modified using banana peel extract and FeSO4 under ultrasonic energy at room temperature. The synthesized material was characterized and the adsorption behavior investigated. The adsorption mechanism was investigated by kinetic and thermodynamic modelling, and by XPS observations. The reusability of the material was also assessed. 2. Materials and methods 2.1. Reagents All reagents, including ferrous sulfate (FeSO4·7H2O), were supplied by Beijing Chemical Reagents Company, China. 2.2. Preparation methods 2.2.1. Preparation of banana peel extract and biochar Banana peel was rinsed with deionized water to remove surface dirt. The peel was then air dried (to constant mass) and cut into small pieces. The prepared banana peel was added to deionized water (60 g/L) for 90 min at 353 K in order to produce an extract (Weng et al., 2017; Wang et al., 2014). The liquid extract and banana peel were separated by vacuum-filtration. The banana peel was then placed in a N2-filled muffle oven at 873 K for 1 h to produce biochar, which was cooled to room temperature and ground. 2.2.2. Biochar modification The banana peel biochar was modified by adding 1.2 g to 200 mL of banana peel extract under ultrasonic energy at room temperature. After sonicating for 15 min, FeSO4 (200 mL, 0.1 mol/L) was added dropwise under continuous sonication for a further 1.5 h. A change of color revealed the production of the composite material. The solid was obtained then by centrifugation at 4000 r.p.m. for 10 min and dispersed in deionized water. The product was oven-dried at 333 K for 12 h. A conceptual illustration of the potential formation mechanism is presented in Fig. 1. Firstly, banana peel biochar and FeSO4 react with reductive biomolecules in banana peel extract to form biochar/zero-valent iron composite. Then, under the action of dissolved oxygen in water, zero-valent iron is oxidized and hydrolyzed to form biochar/ FexOy composite.
Fig. 1. Conceptual schematic illustrating a plausible mechanism for the green synthesis of biochar/FexOy composite. 2
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2.3. Characterization Scanning electron microscopy (SEM) was undertaken to observe structural morphology (JSM-6360, Jeol, USA). X-ray diffraction (XRD) data were acquired using a diffractometer (D/Max-RB, Rigaku, Japan) with Cu/Kα emission (k = 0.15406 nm, 35 kV, 40 mA). Fourier transform infrared spectroscopy (FT-IR) was undertaken using KBr in the range 400-4000 cm−1 (Nicolet IS10, Thermo Fisher, USA). X-ray photoelectron spectroscopy (XPS) was undertaken with spectra recorded using Al Kα excitation (Escalab 250Xi, Thermo Fisher, USA). 2.4. Adsorption experiments Batch experiments were conducted to investigate adsorption performance. The effects of various initial MB concentrations (25 mg/L to 500 mg/L), system temperatures (293 K, 303 K, and 313 K), pH levels (2.05–9.21), and biochar/FexOy dosages (0.25 and 5 g/L) were investigated. Each of these variables was tested individually, using the following standard levels for the other variables: MB concentration = 50 mg/L, system temperatures = 293 K, pH level unadjusted, and biochar/FexOy dosage = 0.5 g/L. Batches were shaken at 180 r.p.m. for 12 h before being subjected to centrifugation at 4000 r.p.m. for 5 min. Residual MB concentrations in solution were determined by UV–vis spectrometry (UV-1800, Shimadzu, Japan) at a wavelength of 664 nm. The equilibrium adsorption capacity, Qe (mg/g), and the removal efficiency were calculated using the following formulae:
Fig. 3. XRD patterns for biochar and biochar/FexOy composite. Identified peaks include: Q: SiO2, C: CaCO3, K: KCl, and P: MgO.
(EDS) of the biochar/FexOy composite surface (Fig. S1) revealed the presence of evenly distributed Fe and O, implying successful synthesis of a biochar/iron oxide composite material. 3.1.2. XRD Characteristic peaks in the XRD pattern (Fig. 3) for unmodified banana peel biochar indicate the existence of SiO2, CaCO3, KCl, and MgO, whereas the XRD pattern for biochar/FexOy composite displays no identifiable peaks (Fig. 3). This finding may indicate that the FexOy component is a non-crystalline amorphous phase (Richter et al., 1999; Liang et al., 2017) and that other stable components, such as SiO2, were concealed by the FexOy which surface coated the biochar body (Liang et al., 2017; Escande et al., 2015).
Qe = (C0-Ce)V/m Removal efficiency (%) = 100((C0-Ce))/C0) Where, C0 and Ce are MB concentrations (mg/L) in solution at t0 and te. V (L) is the solution volume, and m (g) is the adsorbent mass. 2.5. Desorption and regeneration
3.1.3. FT-IR Some oxygen-containing functional groups were identified on the surface of the unmodified banana peel biochar (Fig. 4). A broad FT-IR band at 3368-3429 cm−1 is ascribed to eOH stretching vibration. Peaks at 2927 and 1049 cm−1 are attributed to CH2 distortion and CeO tensile vibration of polysaccharides, respectively (Liang et al., 2017; Song et al., 2014; Yao et al., 2011). C]C, carboxyl C]O stretching vibrations, and C–H aromatic group peaks were identified at 1620, 1386, and 826 cm−1 (Liang et al., 2017; Yao et al., 2011). Analysis of the biochar/FexOy composite revealed that the peaks identified for unmodified biochar were repressed, suggesting that the surface modification concealed these functionalities. An additional peak at 586 cm−1 corresponds to Fe-O vibration (Fig. 4 inset), suggesting successful surface modification with FexOy.
The recyclability of the adsorbents was tested through five cycles. In each operation, the MB-loaded adsorbent was washed in anhydrous ethanol to desorb MB gained during the previous cycle (Ahmad et al., 2013). Regenerated samples were oven dried at 343 K to remove moisture, and stored for the subsequent cycles. 3. Results and discussion 3.1. Characterization 3.1.1. SEM The biochar/FexOy composite and unmodified banana peel biochar were found to be irregular layered particles (Fig. 2). Elemental mapping
Fig. 2. SEM images of (A) biochar/FexOy composite and (B) unmodified biochar. 3
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reported for other adsorbents (Zhang et al., 2017; Liu et al., 2019). This is because at low pH, cationic MB+ ions compete with H+ ions for limited sorption sites, thus reducing adsorption efficiency (Liu et al., 2019; Weng et al., 2018). 3.2.3. Adsorbent dose The amount of MB removed from solution is related to the adsorbent dose up to a value of ∼2 g/L (Fig. 8). This finding is attributed to an increase in the number of total active sites available to an optimal level (Zhang et al., 2017). On the other hand, the MB adsorption performance decreased with increasing dosage, this may be related to a relative decrease in the number of MB molecules per unit of adsorbent, or the decrease of the active adsorption sites caused by adsorbent aggregation (Zhang et al., 2018; Santhosh et al., 2017). 3.2.4. Initial MB concentration and temperature The adsorption capacity of the biochar/FexOy composite was found to increase with increasing system temperature, which indicates that the adsorption process is endothermic (Fig. 9). The observed adsorption data were fitted to isotherm models (Langmuir, Freundlich, and Dubinin-Redushckevich (D–R) models) (Fig.S3), with the relevant isotherm parameters presented in Table 2. The correlation coefficient (R2) was highest for the Langmuir model, indicating that adsorption process is best described by this model. The prepared biochar/FexOy composite sample showed a high maximum adsorption capacity (Qmax) of 862 mg/ g for MB (C0 = 500 mg/L, pH = 6.1, T =313 K). In the Freundlich model, the 1/n values garnered were less than 1, which conventionally represents good adsorption (Amini Tapouk et al., 2019; Pamphile et al., 2019; Sun et al., 2019b). In comparison with other recently reported adsorbents (Table S1), the biochar/FexOy composite reported here displays high Langmuir adsorption capacity Qmax at low dosage, while also benefitting from improved sustainability associated with green synthesis. The D–R model shows that the free energy of adsorption (E = (2k)− 0.5 ) is < 8 kJ/mol. Thus, it can be assumed that physisorption plays a key role in the adsorption process. Adsorption processes can be examined by the dimensionless adsorption factor RL, which can be used to judge whether adsorption is favorable or not, which is defined as follows:
Fig. 4. FT-IR spectra of biochar/FexOy composite and unmodified biochar (Fe-O peak shown inset).
3.1.4. XPS XPS analysis of the biochar/FexOy before MB adsorption (Fig. 5A-C) shows C 1s spectrum peaks at 284.8 eV, 286.1 eV, and 288.3 eV. These are attributed to CeC/C]C, CeO, OeCeO, respectively (Ho et al., 2017). The binding energies of O 1s at 533.3 and 531.62 eV show that oxygen is mostly bound to organic carbon (Ahmed et al., 2018b), peaks at 531.6 eV suggest the presence of lattice oxide oxygen in iron oxides (Zhang et al., 2018; Wang et al., 2017). The Fe 2p3/2 and Fe 2p1/2 core levels are fitted at binding energies of 711.3 and 725.0 eV, respectively. This is attributed to the presence of Fe-O (Sarmah and Pratihar, 2017). Please note that the interpretation of biochar/FexOy after MB adsorption (Fig. 5D–F) is discussed in Section 3.3 3.2. Effects of operating conditions on MB adsorption 3.2.1. Contact time Adsorption experiments revealed a fast initial phase and a later slower phase that tended toward horizontal asymptote (Fig. 6). This represents typical progression of an adsorption process, as available sites decrease with time and adsorption capacity is attained (Ai et al., 2011; Kim et al., 2017). It is also evident from Fig. 6 that the biochar/ FexOy composite has much greater adsorption capability than that of the unmodified biochar. Adsorption kinetic modelling was used to describe dye transfer from solution to the solid adsorbent (Xiao et al., 2018), with the observed data fitted to pseudo first-order and pseudo second-order kinetic models. The two models are expressed as follows: ln(qe qt ) = lnqe k1 t (pseudo first-order) t/ qt = 1/(k2 qe2) + t / qe (pseudo second-order) where qt and qe (mg/g) are the amount of MB adsorbed at time t and at equilibrium, respectively, and k1 (per min) and k2 (g/mg per min) are the pseudo-first-order and pseudo-second-order model rate constants, respectively. The relevant kinetic parameters are listed in Table 1, and plots shown in Figs. S2A and S2B. The pseudo second-order model is associated with a higher R2 value, suggesting that it better describes the adsorption process than the pseudo first-order model (Rajapaksha et al., 2014).
RL = 1/(1 + KLC0) Where C0 is MB concentration in the beginning and KL (L/mg) is a constant of Langmuir. The RL values were calculated to be < 1, which indicates that the capture of MB molecules by biochar/FexOy is favorable (Zhang et al., 2018; Yang et al., 2018). Free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS) were determined in order to estimate inherent energy changes and mechanisms of interaction. The relationships between ΔG, ΔH, and ΔS are as follows (Chen et al., 2014; Ai et al., 2019):
Kd =
qe Ce
G0 = H0 lnK d =
T S0
H0/(RT ) + S0/ R
Where, Kd is the equilibrium constant (mL/g), qe is equilibrium adsorption capacity, and Ce is the concentration of MB at equilibrium (mg/L). R is the universal gas constant (8.314 J/(mol·K)), and T is the system temperature (K). The thermodynamic parameters calculated are shown in Fig. 10 and Table 3. The positive ΔH values indicate an endothermic adsorption process, while the ΔS values indicate that the randomness of the solidliquid interface increases during adsorption (Ai et al., 2019). Negative ΔG values (-5.286 to -8.843 kJ/mol) show that MB adsorption is feasible and spontaneous, and suggests a physisorption process (Ai et al., 2019).
3.2.2. Solution pH Overall, the biochar/FexOy sample exhibited good adsorption performance over a wide range of pH values, indicating its application potential in different environments (Fig. 7). The amount of MB adsorption by the sample increased from 2.82 to 118.27 mg/g as the solution pH increased from 2.05 to 9.21. Similar trends have been 4
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Fig. 5. XPS spectra of biochar/FexOy before methylene blue (MB) adsorption (A: C 1s, B: O 1s, C: Fe 2p) and after MB adsorption (D: C 1s, E: O 1s, F: N 1s).
3.3. Suggested adsorption mechanism
C 1s spectrum (CeC/C]C (284.8 eV), CeO (286.1 eV), and OeC]O (288.3 eV)) and O 1s spectrum (Fe-O, organic CeO (531.6 eV)) of biochar/FexOy-MB decreased. This relates to electron transfer between functional groups and MB molecules (Fu et al., 2016; Zhang and Xu, 2014). Moreover, for the N 1s XPS spectrum of biochar/FexOy-MB, peaks at 399.6 and 402.0 eV are attributed to N atoms in the MB,
The thermodynamic parameters (Section 3.2.4) suggest that physisorption is the main mechanism by which MB adsorbs to biochar/FexOy. This was further investigated using XPS techniques. Fig. 5 D–F shows XPS spectra of biochar/FexOy after MB adsorption. The intensities of the
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Fig. 8. The effect of biochar/FexOy composite dosage (0.25–5 g/L) on methylene blue (MB) removal. unadjusted pH (∼6.1), T =293 K, MB C0 = 50 mg/L, contact time = 12 h.
Fig. 6. Methylene blue (MB) removal by biochar/FexOy composite and unmodified biochar at various contact times. 20 mL MB solution (50 mg/L), unadjusted pH (∼6.1), T =293 K, 0.01 g adsorbent, total contact time = 12 h. Table 1 Kinetic parameters obtained by various models for the adsoprtion of MB on biochar/FexOy. Sample
biochar/FexOy biochar
Qe,exp (mg/ g)
Pseudo first order
Pseudo second order
Qe,1 (mg/ g)
K1×102 (per min)
R2
Qe,2 (mg/ g)
K2×104 (g/mg per min)
R2
83.56 40.19
8.38 46.99
3.40 8.25
0.975 0.933
86.29 42.92
4.85 22.43
0.993 0.985
Fig. 9. Biochar/FexOy composite methylene blue (MB) adsorption isotherms. Unadjusted pH (∼6.1); T = 293,303, and 313 K, dosage =0.01 g/20 mL, MB C0 = 25–500 mg/L.
but the performance remained within an acceptable range throughout. Therefore, the biochar/FexOy is relatively stable and has reuse potential. 4. Conclusions Banana peel biochar/FexOy composite was prepared via a rapid green synthesis procedure at room temperature. FT-IR, XPS, and EDS analysis revealed successful surface modification, evidenced by: (1) a FT-IR spectra peak at 586 cm−1 corresponds to FeeO vibration; (2) XPS spectra binding energies of O 1s suggest the presence of lattice oxide oxygen. Fe 2p3/2 and Fe 2p1/2 core levels can be attributed to the presence of FeeO; and (3) EDS mapping results show iron and oxygen to be uniformly distributed on the surface. The modified adsorbent exhibited enhanced adsorption capability for MB dye. Langmuir adsorption isotherm and pseudo second order kinetic models accurately describe the adsorption process. Moreover, the adsorbent performed well under successive adsorption cycling, indicating its reusability. These results demonstrate that this green synthesis method is an effective way for improving the properties of biochar, indicating potential practical applications for environmental remediation. It is recommended that further studies are undertaken to optimize the green synthesis approach and minimize total raw material cost (Jareonkitpoolpol et al., 2018).
Fig. 7. Effect of pH on methylene blue (MB) adsorption by biochar/FexOy composite. 20 mL MB solution (50 mg/L), pH = 2.05–9.21, T =293 K, 0.01 g adsorbent, contact time = 12 h.
suggesting that the adsorption involves N interaction (Zhang and Xu, 2014). In summary, the adsorption of MB on biochar/FexOy can be described as an electron transfer process driven by electrostatic attraction involving N. 3.4. Reusability From the perspective of practical application and economic efficiency, the regeneration and reuse of adsorbents is important. The biochar/FexOy composite showed a slight decrease in adsorption performance after five adsorption-desorption cycle experiments (Fig.11),
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Table 2 Isotherm model data. T (K)
293 303 313
Langmuir isotherm
Freundlich isotherm 2
Qm (mg /g)
KL (L/mg)
R
562 730 862
0.025 0.032 0.051
0.991 0.992 0.991
D-R isotherm 2
KF (L/g)
N
R
33 42 64
1.916 1.791 1.795
0.971 0.963 0.964
Qm (mg/g)
E (kJ/mol)
R2
316 376 417
0.451 0.583 0.977
0.767 0.756 0.727
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.121286. References Yu, I.K.M., Xiong, X., Tsang, D.C.W., Wang, L., Hunt, A.J., Song, H., Shang, J., Ok, Y.S., Poon, C.S., 2019. Aluminium-biochar composites as sustainable heterogeneous catalysts for glucose isomerisation in a biorefinery. Green Chem. 21, 1267–1281. Shen, Z., Fan, X., Hou, D., Jin, F., O’Connor, D., Tsang, D.C.W., Ok, Y.S., Alessi, D.S., 2019. Risk evaluation of biochars produced from Cd-contaminated rice straw and optimization of its production for Cd removal. Chemosphere 233, 149–156. O’Connor, D., Peng, T., Zhang, J., Tsang, D.C.W., Alessi, D.S., Shen, Z., Bolan, N.S., Hou, D., 2018a. Biochar application for the remediation of heavy metal polluted land: a review of in situ field trials. Sci. Total Environ. 619-620, 815–826. Yoder, J., Galinato, S., Granatstein, D., Garcia-Pérez, M., 2011. Economic tradeoff between biochar and bio-oil production via pyrolysis. Biomass Bioenergy 35, 1851–1862. Wang, L., Chen, L., Tsang, D.C.W., Kua, H.W., Yang, J., Ok, Y.S., Ding, S., Hou, D., Poon, C.S., 2019a. The roles of biochar as green admixture for sediment-based construction products. Cem. Concr. Compos. 104. Woolf, D., Amonette, J.E., Street-Perrott, F.A., Lehmann, J., Joseph, S., 2010. Sustainable biochar to mitigate global climate change. Nat. Commun. 1. Atkinson, C.J., 2018. How good is the evidence that soil-applied biochar improves waterholding capacity? Soil Use Manage. 34, 177–186. Baiga, R., 2017. B.K. Rajashekhar Rao, effects of biochar, urea and their co-application on nitrogen mineralization in soil and growth of Chinese cabbage. Soil Use Manage. 33, 54–61. de Luna, L.A.V., da Silva, T.H.G., Nogueira, R.F.P., Kummrow, F., Umbuzeiro, G.A., 2014. Aquatic toxicity of dyes before and after photo-Fenton treatment. J. Hazard. Mater. 276, 332–338. Forgacs, E., Cserháti, T., Oros, G., 2004. Removal of synthetic dyes from wastewaters: a review. Environ. Int. 30, 953–971. Cheuyglintase, S., Hanly, J.A., Horne, D.J., 2018. Assessing the agronomic effectiveness of wastewater-treated Allophanic soil as a phosphorus source for plant growth. Soil Use Manage. 34, 472–478. Ahmed, M.B., Zhou, J.L., Ngo, H.H., Johir, M.A.H., Sun, L., Asadullah, M., Belhaj, D., 2018a. Sorption of hydrophobic organic contaminants on functionalized biochar: protagonist role of pi-pi electron-donor-acceptor interactions and hydrogen bonds. J. Hazard. Mater. 360, 270–278. Ahmad, M., Rajapaksha, A.U., Lim, J.E., Zhang, M., Bolan, N., Mohan, D., Vithanage, M., Lee, S.S., Ok, Y.S., 2014. Biochar as a sorbent for contaminant management in soil and water: a review. Chemosphere 99, 19–33. Premarathna, K.S.D., Rajapaksha, A.U., Sarkar, B., Kwon, E.E., Bhatnagar, A., Ok, Y.S., Vithanage, M., 2019a. Biochar-based engineered composites for sorptive decontamination of water: a review. Chem. Eng. J. 372, 536–550. Wang, S., Zhao, M., Zhou, M., Li, Y.C., Wang, J., Gao, B., Sato, S., Feng, K., Yin, W., Igalavithana, A.D., Oleszczuk, P., Wang, X., Ok, Y.S., 2019b. Biochar-supported nZVI (nZVI/BC) for contaminant removal from soil and water: a critical review. J. Hazard. Mater. 373, 820–834. Rajapaksha, A.U., Chen, S.S., Tsang, D.C., Zhang, M., Vithanage, M., Mandal, S., Gao, B., Bolan, N.S., Ok, Y.S., 2016. Engineered/designer biochar for contaminant removal/ immobilization from soil and water: potential and implication of biochar modification. Chemosphere 148, 276–291. Ok, Y.S., Chang, S.X., Gao, B., Chung, H.-J., 2015. SMART biochar technology—a shifting paradigm towards advanced materials and healthcare research. Environ. Technol. Innov. 4, 206–209. Cho, D.W., Yoon, K., Ahn, Y., Sun, Y., Tsang, D.C.W., Hou, D., Ok, Y.S., Song, H., 2019. Fabrication and environmental applications of multifunctional mixed metal-biochar composites (MMBC) from red mud and lignin wastes. J. Hazard. Mater. 374, 412–419. Sun, Y., Yu, I.K.M., Tsang, D.C.W., Cao, X., Lin, D., Wang, L., Graham, N.J.D., Alessi, D.S., Komárek, M., Ok, Y.S., Feng, Y., Li, X.-D., 2019a. Multifunctional iron-biochar composites for the removal of potentially toxic elements, inherent cations, and hetero-chloride from hydraulic fracturing wastewater. Environ. Int. 124, 521–532.
Fig. 10. Biochar/FexOy composite methylene blue (MB) adsorption at 293, 303, and 313 K. Unadjusted pH, adsorbent dosage =0.01 g/20 mL. Table 3 Thermodynamics parameters for MB adsorption on biochar/FexOy. Temp (K)
ΔG0 (kJ/mol)
ΔH0 (kJ/mol)
ΔS0 (J/mol per K)
293 303 313
−5.286 −7.064 −8.843
n/a 46.833 n/a
n/a 177.886 n/a
Fig. 11. Biochar/FexOy composite adsorption capacity for methylene blue (MB) after various adsorption-regeneration cycles. Unadjusted pH; T =293 K, adsorbent dosage =0.01 g/20 mL, MB C0 = 50 mg/L.
Acknowledgements This work was supported by China’s National Water Pollution Control and Treatment Science and Technology Major Project (Grant No. 2018ZX07109-003), and the National Key Research and Development Program of China (Grant No. 2018YFC1801300).
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Premarathna, K.S.D., Rajapaksha, A.U., Adassoriya, N., Sarkar, B., Sirimuthu, N.M.S., Cooray, A., Ok, Y.S., Vithanage, M., 2019b. Clay-biochar composites for sorptive removal of tetracycline antibiotic in aqueous media. J. Environ. Manage. 238, 315–322. Sahin, O., Taskin, M.B., Kaya, E.C., Atakol, O., Emir, E., Inal, A., Gunes, A., 2017. Effect of acid modification of biochar on nutrient availability and maize growth in a calcareous soil. Soil Use Manage. 33, 447–456. Chen, B., Chen, Z., Lv, S., 2011. A novel magnetic biochar efficiently sorbs organic pollutants and phosphate. Bioresour. Technol. 102, 716–723. Chen, J., Feng, J., Yan, W., 2016. Influence of metal oxides on the adsorption characteristics of PPy/metal oxides for Methylene Blue. J. Colloid Interface Sci. 475, 26–35. Chaukura, N., Murimba, E.C., Gwenzi, W., 2017. Synthesis, characterisation and methyl orange adsorption capacity of ferric oxide–biochar nano-composites derived from pulp and paper sludge. Appl. Water Sci. 7, 2175–2186. Wang, Y., O’Connor, D., Shen, Z., Lo, I.M.C., Tsang, D.C.W., Pehkonen, S., Pu, S., Hou, D., 2019c. Green synthesis of nanoparticles for the remediation of contaminated waters and soils: constituents, synthesizing methods, and influencing factors. J. Clean. Prod. 226, 540–549. O’Connor, D., Hou, D., Ok, Y.S., Song, Y., Sarmah, A.K., Li, X., Tack, F.M.G., 2018b. Sustainable in situ remediation of recalcitrant organic pollutants in groundwater with controlled release materials: a review. J. Control. Release 283, 200–213. O’Connor, D., Peng, T., Li, G., Wang, S., Duan, L., Mulder, J., Cornelissen, G., Cheng, Z., Yang, S., Hou, D., 2018c. Sulfur-modified rice husk biochar: a green method for the remediation of mercury contaminated soil. Sci. Total Environ. 621, 819–826. Mohammadinejad, R., Karimi, S., Iravani, S., Varma, R.S., 2016. Plant-derived nanostructures: types and applications. Green Chem. 18, 20–52. Shen, W., Qu, Y., Pei, X., Li, S., You, S., Wang, J., Zhang, Z., Zhou, J., 2017. Catalytic reduction of 4-nitrophenol using gold nanoparticles biosynthesized by cell-free extracts of Aspergillus sp. WL-Au, Journal of Hazardous Materials 321, 299–306. Shankar, S.S., Ahmad, A., Pasricha, R., Sastry, M., 2003. Bioreduction of chloroaurate ions by geranium leaves and its endophytic fungus yields gold nanoparticles of different shapes. J. Mater. Chem. 13, 1822. Weng, X., Guo, M., Luo, F., Chen, Z., 2017. One-step green synthesis of bimetallic Fe/Ni nanoparticles by eucalyptus leaf extract: biomolecules identification, characterization and catalytic activity. Chem. Eng. J. 308, 904–911. Zhang, P., Hou, D., Li, X., Pehkonen, S., Varma, R.S., Wang, X., 2018. Greener and sizespecific synthesis of stable Fe-Cu oxides as earth-abundant adsorbents for malachite green. ACS Sustain. Chem. Eng. 6, 9229–9236. Wang, T., Jin, X., Chen, Z., Megharaj, M., Naidu, R., 2014. Green synthesis of Fe nanoparticles using eucalyptus leaf extracts for treatment of eutrophic wastewater. Sci. Total Environ. 466-467, 210–213. Ahmad, M., Lee, S.S., Rajapaksha, A.U., Vithanage, M., Zhang, M., Cho, J.S., Lee, S.E., Ok, Y.S., 2013. Trichloroethylene adsorption by pine needle biochars produced at various pyrolysis temperatures. Bioresour. Technol. 143, 615–622. Richter, M., Berndt, H., Eckelt, R., Schneider, M., Fricke, R., 1999. Zeolite-mediated removal of NOx by NH3 from exhaust streams at low temperatures. Catal. Today 54, 531–545. Liang, J., Li, X., Yu, Z., Zeng, G., Luo, Y., Jiang, L., Yang, Z., Qian, Y., Wu, H., 2017. Amorphous MnO2 modified biochar derived from aerobically composted swine manure for adsorption of Pb(II) and Cd(II). ACS Sustain. Chem. Eng. 5, 5049–5058. Escande, V., Petit, E., Garoux, L., Boulanger, C., Grison, C., 2015. Switchable alkene epoxidation/oxidative cleavage with H2O2/NaHCO3: efficient heterogeneous catalysis derived from biosourced Eco-Mn. ACS Sustain. Chem. Eng. 3, 2704–2715. Song, Z., Lian, F., Yu, Z., Zhu, L., Xing, B., Qiu, W., 2014. Synthesis and characterization of a novel MnOx-loaded biochar and its adsorption properties for Cu2+ in aqueous solution. Chem. Eng. J. 242, 36–42. Yao, Y., Gao, B., Inyang, M., Zimmerman, A.R., Cao, X., Pullammanappallil, P., Yang, L., 2011. Biochar derived from anaerobically digested sugar beet tailings: characterization and phosphate removal potential. Bioresour. Technol. 102, 6273–6278. Ho, S.-H., Zhu, S., Chang, J.-S., 2017. Recent advances in nanoscale-metal assisted biochar derived from waste biomass used for heavy metals removal. Bioresour. Technol. 246, 123–134. Ahmed, M.B., Zhou, J.L., Ngo, H.H., Johir, M.A.H., Sornalingam, K., 2018b. Sorptive
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
A green biochar/iron oxide composite for methylene blue removal a,b
a
a
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a
Ping Zhang , David O’Connor , Yinan Wang , Lin Jiang , Tianxiang Xia , Liuwei Wang , ⁎ Daniel C.W. Tsangc, Yong Sik Okd, Deyi Houa,
T
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School of Environment, Tsinghua University, Beijing 100084, China National Engineering Research Centre of Urban Environmental Pollution Control, Beijing Key Laboratory for Risk Modeling and Remediation of Contaminated Sites, Beijing Municipal Research Institute of Environmental Protection, Beijing 100037, China c Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China d Korea Biochar Research Center & Division of Environmental Science and Ecological Engineering, Korea University, Seoul 02841, Republic of Korea b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Editor: R. Teresa
Adsorbents that effectively remove dye substances from industrial effluents are needed for the protection of human health and the natural environment. However, adsorbent manufacture is associated with secondary environmental impacts. In this study, a green biochar/iron oxide composite was produced using a facile approach involving banana peel extract and FeSO4. The modified biochar’s capacity to adsorb methylene blue (MB) was considerably enhanced (Langmuir Qmax of 862 mg/g for MB when C0 = 500 mg/L, pH = 6.1, T =313 K) compared to the unmodified banana peel biochar, and exhibited good performance for a wide range of pH values (pH 2.05–9.21). The Langmuir isotherm model and pseudo second-order kinetic model accurately describe the adsorption process. The material properties and corresponding adsorption mechanisms were investigated by various experimental techniques. Enhanced MB adsorption by the biochar/iron oxide composite is attributed to increased electronic attraction to MB molecules, as evidenced by XPS analysis. High adsorption capacity was retained after 5 regeneration cycles. This study suggests that biochar can be modified by a green synthesis approach to produce biochar/iron oxide composite with good MB removal capacity.
Keywords: Green synthesis Modified biochar Engineered biochar Methylene blue Dye adsorption
1. Introduction In recent years, there have been increasing calls for sustainable
⁎
solutions to environmental issues. Biochar is attracting significant interest (Yu et al., 2019; Shen et al., 2019) as a low-cost material that complements the greener paradigm (O’Connor et al., 2018a; Yoder
Corresponding author. E-mail address: [email protected] (D. Hou).
https://doi.org/10.1016/j.jhazmat.2019.121286 Received 25 July 2019; Received in revised form 17 September 2019; Accepted 21 September 2019 Available online 24 September 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
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et al., 2011; Wang et al., 2019a) for a number of applications, including carbon sequestration (Woolf et al., 2010), soil amendment (Atkinson, 2018; Baiga, 2017) and wastewater treatment. Inadequately treated wastewater effluents, from industries such as textiles, leather, and pharmaceutical and personal care products (PCPPs) (de Luna et al., 2014; Forgacs et al., 2004), risk harm to human health and the natural environment (Cheuyglintase et al., 2018). As a wastewater adsorbent, biochar seen as an alternative to activated carbon (AC) for the removal of various contaminants, including inorganic anions (NO3−,PO43-), metal cations (Pb(II),As(III) and Cd(II)), organic dyes (crystal violet, cationic red X-GRL), etc. (Ahmed et al., 2018a; Ahmad et al., 2014). However, biochar’s relatively low surface area and the influence of abiotic and/or biotic processes can limit its effectiveness in certain applications (Premarathna et al., 2019a). The emergence of engineered biochars has taken its performance to the next level (Wang et al., 2019b; Rajapaksha et al., 2016; Ok et al., 2015). Various modifications have been attempted, including chemical, physical, mineral impregnation, and magnetic modifications (Cho et al., 2019; Sun et al., 2019a; Premarathna et al., 2019b; Sahin et al., 2017). Biochar/metal composites are particularly promising. For example, aluminum species in biochar/aluminum composites increase catalytic activity (Yu et al., 2019). Biochar/iron composites have multi-functionality for adsorption, reduction, and complexation (Sun et al., 2019a). Chen et al. (2011) reported that biochar/iron oxide composites have significantly enhanced As sorption capability. A number of lowcost metal oxides have been added to composite adsorbents for the treatment of various contaminants (Chen et al., 2016; Chaukura et al., 2017). In biochar/metal oxide composites, the biochar acts as a porous carbon body that is surface modified by the metal oxide. This has the benefit of increasing the adsorbent surface area (Premarathna et al., 2019a). In the treatment of cationic dyes, enhanced sorption can also be ascribed to stronger electrostatic interactions (Fig. 1). However, existing synthesis techniques for producing biochar/metal oxide composite usually involve soaking biochar in large volumes of modifying chemicals and/or the use of potentially toxic chemicals (Premarathna et al., 2019a), which undermines sustainability. As compared to conventional synthesis, green synthesis seeks to avoid secondary impacts, by either (i) using green materials, or (ii) they consume less energy or natural resources in synthesis process, aspiring for ambient synthesis reaction conditions (Wang et al., 2019c; O’Connor et al., 2018b, c). For example, plant extracts have proved useful for the manufacture of metal oxide adsorbents (Mohammadinejad et al., 2016) with active biomolecules functioning as effective reducing and capping agents (Shen et al., 2017; Shankar et al., 2003; Weng et al., 2017). In our previous work, we presented a green pathway for the production of size-controllable bimetallic Fe-Cu oxide adsorbents, which have exceptional performance for malachite green dye removal (Zhang et al.,
2018). The use of plant extracts for the green synthesis of biochar/metal oxide composites, however, remains an underexplored research topic. In this study it was hypothesized that a biochar/iron oxide (FexOy) composite could be successfully produced by a green synthesis method (Fig. 1), which would possess good MB removal capacity. To demonstrate this, banana peel biochar was modified using banana peel extract and FeSO4 under ultrasonic energy at room temperature. The synthesized material was characterized and the adsorption behavior investigated. The adsorption mechanism was investigated by kinetic and thermodynamic modelling, and by XPS observations. The reusability of the material was also assessed. 2. Materials and methods 2.1. Reagents All reagents, including ferrous sulfate (FeSO4·7H2O), were supplied by Beijing Chemical Reagents Company, China. 2.2. Preparation methods 2.2.1. Preparation of banana peel extract and biochar Banana peel was rinsed with deionized water to remove surface dirt. The peel was then air dried (to constant mass) and cut into small pieces. The prepared banana peel was added to deionized water (60 g/L) for 90 min at 353 K in order to produce an extract (Weng et al., 2017; Wang et al., 2014). The liquid extract and banana peel were separated by vacuum-filtration. The banana peel was then placed in a N2-filled muffle oven at 873 K for 1 h to produce biochar, which was cooled to room temperature and ground. 2.2.2. Biochar modification The banana peel biochar was modified by adding 1.2 g to 200 mL of banana peel extract under ultrasonic energy at room temperature. After sonicating for 15 min, FeSO4 (200 mL, 0.1 mol/L) was added dropwise under continuous sonication for a further 1.5 h. A change of color revealed the production of the composite material. The solid was obtained then by centrifugation at 4000 r.p.m. for 10 min and dispersed in deionized water. The product was oven-dried at 333 K for 12 h. A conceptual illustration of the potential formation mechanism is presented in Fig. 1. Firstly, banana peel biochar and FeSO4 react with reductive biomolecules in banana peel extract to form biochar/zero-valent iron composite. Then, under the action of dissolved oxygen in water, zero-valent iron is oxidized and hydrolyzed to form biochar/ FexOy composite.
Fig. 1. Conceptual schematic illustrating a plausible mechanism for the green synthesis of biochar/FexOy composite. 2
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2.3. Characterization Scanning electron microscopy (SEM) was undertaken to observe structural morphology (JSM-6360, Jeol, USA). X-ray diffraction (XRD) data were acquired using a diffractometer (D/Max-RB, Rigaku, Japan) with Cu/Kα emission (k = 0.15406 nm, 35 kV, 40 mA). Fourier transform infrared spectroscopy (FT-IR) was undertaken using KBr in the range 400-4000 cm−1 (Nicolet IS10, Thermo Fisher, USA). X-ray photoelectron spectroscopy (XPS) was undertaken with spectra recorded using Al Kα excitation (Escalab 250Xi, Thermo Fisher, USA). 2.4. Adsorption experiments Batch experiments were conducted to investigate adsorption performance. The effects of various initial MB concentrations (25 mg/L to 500 mg/L), system temperatures (293 K, 303 K, and 313 K), pH levels (2.05–9.21), and biochar/FexOy dosages (0.25 and 5 g/L) were investigated. Each of these variables was tested individually, using the following standard levels for the other variables: MB concentration = 50 mg/L, system temperatures = 293 K, pH level unadjusted, and biochar/FexOy dosage = 0.5 g/L. Batches were shaken at 180 r.p.m. for 12 h before being subjected to centrifugation at 4000 r.p.m. for 5 min. Residual MB concentrations in solution were determined by UV–vis spectrometry (UV-1800, Shimadzu, Japan) at a wavelength of 664 nm. The equilibrium adsorption capacity, Qe (mg/g), and the removal efficiency were calculated using the following formulae:
Fig. 3. XRD patterns for biochar and biochar/FexOy composite. Identified peaks include: Q: SiO2, C: CaCO3, K: KCl, and P: MgO.
(EDS) of the biochar/FexOy composite surface (Fig. S1) revealed the presence of evenly distributed Fe and O, implying successful synthesis of a biochar/iron oxide composite material. 3.1.2. XRD Characteristic peaks in the XRD pattern (Fig. 3) for unmodified banana peel biochar indicate the existence of SiO2, CaCO3, KCl, and MgO, whereas the XRD pattern for biochar/FexOy composite displays no identifiable peaks (Fig. 3). This finding may indicate that the FexOy component is a non-crystalline amorphous phase (Richter et al., 1999; Liang et al., 2017) and that other stable components, such as SiO2, were concealed by the FexOy which surface coated the biochar body (Liang et al., 2017; Escande et al., 2015).
Qe = (C0-Ce)V/m Removal efficiency (%) = 100((C0-Ce))/C0) Where, C0 and Ce are MB concentrations (mg/L) in solution at t0 and te. V (L) is the solution volume, and m (g) is the adsorbent mass. 2.5. Desorption and regeneration
3.1.3. FT-IR Some oxygen-containing functional groups were identified on the surface of the unmodified banana peel biochar (Fig. 4). A broad FT-IR band at 3368-3429 cm−1 is ascribed to eOH stretching vibration. Peaks at 2927 and 1049 cm−1 are attributed to CH2 distortion and CeO tensile vibration of polysaccharides, respectively (Liang et al., 2017; Song et al., 2014; Yao et al., 2011). C]C, carboxyl C]O stretching vibrations, and C–H aromatic group peaks were identified at 1620, 1386, and 826 cm−1 (Liang et al., 2017; Yao et al., 2011). Analysis of the biochar/FexOy composite revealed that the peaks identified for unmodified biochar were repressed, suggesting that the surface modification concealed these functionalities. An additional peak at 586 cm−1 corresponds to Fe-O vibration (Fig. 4 inset), suggesting successful surface modification with FexOy.
The recyclability of the adsorbents was tested through five cycles. In each operation, the MB-loaded adsorbent was washed in anhydrous ethanol to desorb MB gained during the previous cycle (Ahmad et al., 2013). Regenerated samples were oven dried at 343 K to remove moisture, and stored for the subsequent cycles. 3. Results and discussion 3.1. Characterization 3.1.1. SEM The biochar/FexOy composite and unmodified banana peel biochar were found to be irregular layered particles (Fig. 2). Elemental mapping
Fig. 2. SEM images of (A) biochar/FexOy composite and (B) unmodified biochar. 3
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reported for other adsorbents (Zhang et al., 2017; Liu et al., 2019). This is because at low pH, cationic MB+ ions compete with H+ ions for limited sorption sites, thus reducing adsorption efficiency (Liu et al., 2019; Weng et al., 2018). 3.2.3. Adsorbent dose The amount of MB removed from solution is related to the adsorbent dose up to a value of ∼2 g/L (Fig. 8). This finding is attributed to an increase in the number of total active sites available to an optimal level (Zhang et al., 2017). On the other hand, the MB adsorption performance decreased with increasing dosage, this may be related to a relative decrease in the number of MB molecules per unit of adsorbent, or the decrease of the active adsorption sites caused by adsorbent aggregation (Zhang et al., 2018; Santhosh et al., 2017). 3.2.4. Initial MB concentration and temperature The adsorption capacity of the biochar/FexOy composite was found to increase with increasing system temperature, which indicates that the adsorption process is endothermic (Fig. 9). The observed adsorption data were fitted to isotherm models (Langmuir, Freundlich, and Dubinin-Redushckevich (D–R) models) (Fig.S3), with the relevant isotherm parameters presented in Table 2. The correlation coefficient (R2) was highest for the Langmuir model, indicating that adsorption process is best described by this model. The prepared biochar/FexOy composite sample showed a high maximum adsorption capacity (Qmax) of 862 mg/ g for MB (C0 = 500 mg/L, pH = 6.1, T =313 K). In the Freundlich model, the 1/n values garnered were less than 1, which conventionally represents good adsorption (Amini Tapouk et al., 2019; Pamphile et al., 2019; Sun et al., 2019b). In comparison with other recently reported adsorbents (Table S1), the biochar/FexOy composite reported here displays high Langmuir adsorption capacity Qmax at low dosage, while also benefitting from improved sustainability associated with green synthesis. The D–R model shows that the free energy of adsorption (E = (2k)− 0.5 ) is < 8 kJ/mol. Thus, it can be assumed that physisorption plays a key role in the adsorption process. Adsorption processes can be examined by the dimensionless adsorption factor RL, which can be used to judge whether adsorption is favorable or not, which is defined as follows:
Fig. 4. FT-IR spectra of biochar/FexOy composite and unmodified biochar (Fe-O peak shown inset).
3.1.4. XPS XPS analysis of the biochar/FexOy before MB adsorption (Fig. 5A-C) shows C 1s spectrum peaks at 284.8 eV, 286.1 eV, and 288.3 eV. These are attributed to CeC/C]C, CeO, OeCeO, respectively (Ho et al., 2017). The binding energies of O 1s at 533.3 and 531.62 eV show that oxygen is mostly bound to organic carbon (Ahmed et al., 2018b), peaks at 531.6 eV suggest the presence of lattice oxide oxygen in iron oxides (Zhang et al., 2018; Wang et al., 2017). The Fe 2p3/2 and Fe 2p1/2 core levels are fitted at binding energies of 711.3 and 725.0 eV, respectively. This is attributed to the presence of Fe-O (Sarmah and Pratihar, 2017). Please note that the interpretation of biochar/FexOy after MB adsorption (Fig. 5D–F) is discussed in Section 3.3 3.2. Effects of operating conditions on MB adsorption 3.2.1. Contact time Adsorption experiments revealed a fast initial phase and a later slower phase that tended toward horizontal asymptote (Fig. 6). This represents typical progression of an adsorption process, as available sites decrease with time and adsorption capacity is attained (Ai et al., 2011; Kim et al., 2017). It is also evident from Fig. 6 that the biochar/ FexOy composite has much greater adsorption capability than that of the unmodified biochar. Adsorption kinetic modelling was used to describe dye transfer from solution to the solid adsorbent (Xiao et al., 2018), with the observed data fitted to pseudo first-order and pseudo second-order kinetic models. The two models are expressed as follows: ln(qe qt ) = lnqe k1 t (pseudo first-order) t/ qt = 1/(k2 qe2) + t / qe (pseudo second-order) where qt and qe (mg/g) are the amount of MB adsorbed at time t and at equilibrium, respectively, and k1 (per min) and k2 (g/mg per min) are the pseudo-first-order and pseudo-second-order model rate constants, respectively. The relevant kinetic parameters are listed in Table 1, and plots shown in Figs. S2A and S2B. The pseudo second-order model is associated with a higher R2 value, suggesting that it better describes the adsorption process than the pseudo first-order model (Rajapaksha et al., 2014).
RL = 1/(1 + KLC0) Where C0 is MB concentration in the beginning and KL (L/mg) is a constant of Langmuir. The RL values were calculated to be < 1, which indicates that the capture of MB molecules by biochar/FexOy is favorable (Zhang et al., 2018; Yang et al., 2018). Free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS) were determined in order to estimate inherent energy changes and mechanisms of interaction. The relationships between ΔG, ΔH, and ΔS are as follows (Chen et al., 2014; Ai et al., 2019):
Kd =
qe Ce
G0 = H0 lnK d =
T S0
H0/(RT ) + S0/ R
Where, Kd is the equilibrium constant (mL/g), qe is equilibrium adsorption capacity, and Ce is the concentration of MB at equilibrium (mg/L). R is the universal gas constant (8.314 J/(mol·K)), and T is the system temperature (K). The thermodynamic parameters calculated are shown in Fig. 10 and Table 3. The positive ΔH values indicate an endothermic adsorption process, while the ΔS values indicate that the randomness of the solidliquid interface increases during adsorption (Ai et al., 2019). Negative ΔG values (-5.286 to -8.843 kJ/mol) show that MB adsorption is feasible and spontaneous, and suggests a physisorption process (Ai et al., 2019).
3.2.2. Solution pH Overall, the biochar/FexOy sample exhibited good adsorption performance over a wide range of pH values, indicating its application potential in different environments (Fig. 7). The amount of MB adsorption by the sample increased from 2.82 to 118.27 mg/g as the solution pH increased from 2.05 to 9.21. Similar trends have been 4
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Fig. 5. XPS spectra of biochar/FexOy before methylene blue (MB) adsorption (A: C 1s, B: O 1s, C: Fe 2p) and after MB adsorption (D: C 1s, E: O 1s, F: N 1s).
3.3. Suggested adsorption mechanism
C 1s spectrum (CeC/C]C (284.8 eV), CeO (286.1 eV), and OeC]O (288.3 eV)) and O 1s spectrum (Fe-O, organic CeO (531.6 eV)) of biochar/FexOy-MB decreased. This relates to electron transfer between functional groups and MB molecules (Fu et al., 2016; Zhang and Xu, 2014). Moreover, for the N 1s XPS spectrum of biochar/FexOy-MB, peaks at 399.6 and 402.0 eV are attributed to N atoms in the MB,
The thermodynamic parameters (Section 3.2.4) suggest that physisorption is the main mechanism by which MB adsorbs to biochar/FexOy. This was further investigated using XPS techniques. Fig. 5 D–F shows XPS spectra of biochar/FexOy after MB adsorption. The intensities of the
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Fig. 8. The effect of biochar/FexOy composite dosage (0.25–5 g/L) on methylene blue (MB) removal. unadjusted pH (∼6.1), T =293 K, MB C0 = 50 mg/L, contact time = 12 h.
Fig. 6. Methylene blue (MB) removal by biochar/FexOy composite and unmodified biochar at various contact times. 20 mL MB solution (50 mg/L), unadjusted pH (∼6.1), T =293 K, 0.01 g adsorbent, total contact time = 12 h. Table 1 Kinetic parameters obtained by various models for the adsoprtion of MB on biochar/FexOy. Sample
biochar/FexOy biochar
Qe,exp (mg/ g)
Pseudo first order
Pseudo second order
Qe,1 (mg/ g)
K1×102 (per min)
R2
Qe,2 (mg/ g)
K2×104 (g/mg per min)
R2
83.56 40.19
8.38 46.99
3.40 8.25
0.975 0.933
86.29 42.92
4.85 22.43
0.993 0.985
Fig. 9. Biochar/FexOy composite methylene blue (MB) adsorption isotherms. Unadjusted pH (∼6.1); T = 293,303, and 313 K, dosage =0.01 g/20 mL, MB C0 = 25–500 mg/L.
but the performance remained within an acceptable range throughout. Therefore, the biochar/FexOy is relatively stable and has reuse potential. 4. Conclusions Banana peel biochar/FexOy composite was prepared via a rapid green synthesis procedure at room temperature. FT-IR, XPS, and EDS analysis revealed successful surface modification, evidenced by: (1) a FT-IR spectra peak at 586 cm−1 corresponds to FeeO vibration; (2) XPS spectra binding energies of O 1s suggest the presence of lattice oxide oxygen. Fe 2p3/2 and Fe 2p1/2 core levels can be attributed to the presence of FeeO; and (3) EDS mapping results show iron and oxygen to be uniformly distributed on the surface. The modified adsorbent exhibited enhanced adsorption capability for MB dye. Langmuir adsorption isotherm and pseudo second order kinetic models accurately describe the adsorption process. Moreover, the adsorbent performed well under successive adsorption cycling, indicating its reusability. These results demonstrate that this green synthesis method is an effective way for improving the properties of biochar, indicating potential practical applications for environmental remediation. It is recommended that further studies are undertaken to optimize the green synthesis approach and minimize total raw material cost (Jareonkitpoolpol et al., 2018).
Fig. 7. Effect of pH on methylene blue (MB) adsorption by biochar/FexOy composite. 20 mL MB solution (50 mg/L), pH = 2.05–9.21, T =293 K, 0.01 g adsorbent, contact time = 12 h.
suggesting that the adsorption involves N interaction (Zhang and Xu, 2014). In summary, the adsorption of MB on biochar/FexOy can be described as an electron transfer process driven by electrostatic attraction involving N. 3.4. Reusability From the perspective of practical application and economic efficiency, the regeneration and reuse of adsorbents is important. The biochar/FexOy composite showed a slight decrease in adsorption performance after five adsorption-desorption cycle experiments (Fig.11),
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Table 2 Isotherm model data. T (K)
293 303 313
Langmuir isotherm
Freundlich isotherm 2
Qm (mg /g)
KL (L/mg)
R
562 730 862
0.025 0.032 0.051
0.991 0.992 0.991
D-R isotherm 2
KF (L/g)
N
R
33 42 64
1.916 1.791 1.795
0.971 0.963 0.964
Qm (mg/g)
E (kJ/mol)
R2
316 376 417
0.451 0.583 0.977
0.767 0.756 0.727
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Fig. 10. Biochar/FexOy composite methylene blue (MB) adsorption at 293, 303, and 313 K. Unadjusted pH, adsorbent dosage =0.01 g/20 mL. Table 3 Thermodynamics parameters for MB adsorption on biochar/FexOy. Temp (K)
ΔG0 (kJ/mol)
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ΔS0 (J/mol per K)
293 303 313
−5.286 −7.064 −8.843
n/a 46.833 n/a
n/a 177.886 n/a
Fig. 11. Biochar/FexOy composite adsorption capacity for methylene blue (MB) after various adsorption-regeneration cycles. Unadjusted pH; T =293 K, adsorbent dosage =0.01 g/20 mL, MB C0 = 50 mg/L.
Acknowledgements This work was supported by China’s National Water Pollution Control and Treatment Science and Technology Major Project (Grant No. 2018ZX07109-003), and the National Key Research and Development Program of China (Grant No. 2018YFC1801300).
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