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Article

An Efficient Strategy for Enhancing the Adsorption Capabilities of Biochar via Sequential KMnO4-Promoted Oxidative Pyrolysis and H2O2 Oxidation

by
Siyao Bian
1,
Shuang Xu
1,
Zhibing Yin
1,
Sen Liu
1,
Jihui Li
1,2,*,
Shuying Xu
1,* and
Yucang Zhang
1
1
Key Laboratory of Ministry of Education for Advanced Materials in Tropical Island Resources, School of Chemical Engineering and Technology, Hainan University, Haikou 570228, China
2
School of Science, Hainan University, Haikou 570228, China
*
Authors to whom correspondence should be addressed.
Sustainability 2021, 13(5), 2641; https://doi.org/10.3390/su13052641
Submission received: 8 January 2021 / Revised: 6 February 2021 / Accepted: 19 February 2021 / Published: 2 March 2021
(This article belongs to the Section Sustainable Materials)
Browse Figures
Versions Notes

Abstract

:
In this study, sequential KMnO4-promoted oxidative pyrolysis and H2O2 oxidation were employed to upgrade the adsorption capacities of durian shell biochar for methylene blue (MB) and tetracycline (TC) in an aqueous solution. It was found that the KMnO4/H2O2 co-modification was greatly influenced by pyrolysis temperature and the optimal temperature was 300 °C. Moreover, a low concentration of H2O2 enabled the improvement of the adsorption capabilities greatly with the catalysis of pre-impregnated manganese oxides, addressing the shortcoming of single H2O2 modification. The co-modified biochar exhibited high adsorption capabilities for MB and TC, remarkably surpassed KMnO4- and H2O2- modified biochars as well as pristine biochar. The increase of adsorption capabilities could be mainly contributed to the incorporation of MnOx and carboxyl by KMnO4-promoted oxidative decomposition and Mn-catalyzed H2O2 oxidation. This would provide a novel and efficient method for preparing highly adsorptive biochar using sequential KMnO4-promoted oxidative pyrolysis and H2O2 oxidation.

1. Introduction

Chemical modification has facilitated the wide application of biochar in the environmental remediation field as a versatile technology for improving its performance [1,2]. In the past decades, different strategies, such as grafting, metal-doping and oxidation, were developed to enhance the adsorption capability of biochar for eliminating contaminants. Particularly, oxidation provides a robust tool for the preparation of diversified biochars by incorporating oxygen-containing groups as well as modulating carbon matrix structure and mineral composition [3,4,5].
Compared to other oxidants, H2O2 shows a unique advantage for improving the adsorption capability of biochar as a low-cost and environmentally friendly modifier. Usually, H2O2 oxidation can tune the physicochemical properties of biochar through regulating surface structure [6,7] besides introducing oxygen-containing groups [8,9]. In the past decade, it has been applied to engineering different biochars for the removal of heavy metals, such as Cu2+, Pd2+, Cd2+ and Ni2+ in aqueous solution [8,10,11]. In addition, it was employed to modify Prosopis juliflora biochar for removal of organic dyes, such as MB and Remazol brilliant blue, but only resulted in slight enhancement of adsorption capability even assisted by microwave activation [7,9]. As highly active HO·species are generated slowly [12], noncatalytic H2O2 oxidation suffers from very limited oxidizing ability. This makes the efficiency of a single H2O2 modification is far from satisfactory even using concentrated H2O2 solution.
As well as surface functional groups, impregnated minerals play an important role in the adsorption of biochar [13,14]. Manganese oxides (MnOx) have a good affinity to heavy metals [15,16,17]; thereby, MnOx-doped biochars were made for removal of Pb(II), Cu(II) and Cd(II) in an aqueous solution using KMnO4 as a precursor in recent years [18,19]. Similarly, a recyclable 3D MnO2-doped biochar-based hydrogel was prepared using KMnO4 and Mn(OAc)2 precursors for adsorption Pb(II) and Cd(II), establishing a method for solid–liquid separation of biochar [20]. A MnOx/biochar composite was also manufactured through co-pyrolysis of pristine biochar with KMnO4 for remedying As(III)-polluted soil, revealing MnOx and oxygen-containing groups acted as oxidant and active adsorption sites for As (III), respectively [21]. In addition, KMnO4 and KOH modifications were combined to engineer peanut shell biochar for removal of Ni(II) recently, where increased specific surface area, surface hydroxyl and amino groups played an important role [22]. Despite such an impressive advance, less effort has been devoted to the KMnO4 modification of biochar for the adsorption of organic pollutants by now.
Being superior to single oxidation, sequential oxidations can offer greater opportunity for regulating physiochemical properties of biochar multi-directionally. Especially, the combination of metal and H2O2 oxidative modifications can gain a synergistic effect for upgrading the adsorption capability of biochar. Metal oxides can be generated in a biochar matrix in small size [23] and acted as stable and active catalysts for H2O2 oxidation [4]. Therefore, subsequent H2O2 oxidation can produce highly active HO· [24] and metal oxide intermediate species [25,26,27] to oxidize various groups into oxygen-containing groups, overcoming the above-mentioned drawback of single H2O2 oxidation.
As KMnO4 can promote oxidative degradation of biomass and introduce oxygen-containing groups and MnOx onto the biochar matrix. Additionally, pre-doped manganese oxides can accelerate H2O2 oxidation for generating additional oxygen-containing groups as an efficient catalyst. Accordingly, a synergistic effect might be achieved through subsequent KMnO4-promoted pyrolysis and H2O2 oxidation for improving the adsorption capability of biochar. Herein, KMnO4-promoted oxidative pyrolysis and H2O2 oxidation were combined to engineer durian shell biochar for removal of MB and TC, which are common dye and antibiotic contaminants in water. The adsorption behaviors of KMnO4/H2O2 co-modified biochar were investigated in an aqueous solution. A comparative study was also conducted using the biochars modified by independent KMnO4-promoted oxidative pyrolysis and H2O2 oxidation. This work was aimed to establish an efficient method for improving the adsorption capability of biochar.

2. Materials and Methods

2.1. Materials

Durian shell was gained from a fruit market in Hainan University in the Hainan province of China. The chemicals, including MB, TC, KMnO4, H2O2, acetic acid, ethanol, NaOH, HCl and oxalic acid, were analytical reagent grades purchased from Aladdin and Macklin Biochemical Technology Co., Ltd. (Shanghai, China). MB and TC solutions were prepared in ultra-pure water, and their pH s were regulated by HCl or NaOH solution.

2.2. Biochar Preparation

Dried durian shell was smashed to powder (60 mesh). Pristine biochar (PB) was prepared from the biomass powder through pyrolysis under N2 at setting the temperature for 1 h in a vacuum tube furnace with 5 °C/min heating speed. H2O2-modified biochar (HB) was manufactured via oxidation of PB by H2O2 for 2 h at room temperature. The KMnO4-modified biochar (KB) was prepared as follows: 10 g biomass powder and 1 g KMnO4 were mixed in 80 mL ultra-pure water and sonicated for 2 h. Then, the mixture was dried at 60 °C, milled and pyrolyzed with the same preparation parameters as for PB. The KMnO4/H2O2 co-modified biochar (KHB) was made through oxidation of unwashed KB by H2O2 in a 1/20 (g/mL) ratio for 2 h at room temperature. The PB, HB, KB and KHB prepared at 300 °C were marked as PB300, HB300, KB300 and KHB300, respectively. Additionally, the HB300 and KHB300 modified with x% H2O2 were labeled as HB300-x and KHB300-x, correspondingly.

2.3. Biochar Characterization

The durian shell biochars prepared at 300 °C (PB300, HB300-30, KB300 and KHB300-30) were characterized in detail. Organic elements, such as C, O, N and H, were determined by an organic element analyzer (Thermo Scientific Flash 2000 CHNS/O, Waltham, MA, USA). An inductively coupled plasma mass spectrometer (Agilent ICPMS 7700, Santa Clara, CA, USA) was used to measured inorganic elements K and Mn. N2 adsorption–desorption isotherms were performed on an ASAP2460 analyzer (Micromeritics, Norcross, GA, USA) at 77 K, where the biochar was degassed in a vacuum at 317 K before the test. The Brunauer–Emmett–Teller (BET) and Barret–Joyner–Halenda (BJH) methods were applied for calculating specific surface area and pore parameters, respectively. A Zetasizer nano ZS instrument (Malvern, London, UK) was used to measure surface zeta potential over a pH range from 3.0 to 9.0. The Fourier-transform infrared (FTIR) spectroscopy was performed on an FTIR spectrometer (Bruker Tensor 27, Ettlingen, Germany) over a range of 400 to 4000 cm−1 using 2 mg biochar and 1 g KBr. X-ray diffraction spectrum (XRD) was conducted on an X-ray powder diffractometer (Brucker D8 Advance, Peoria, IL, Germany) using Cu Kα radiation with 2θ angle ranged from 10° to 100°. A scanning electron microscope with energy-dispersive X-ray analysis (SEM, Phenom ProX, South Holland, The Netherlands) was employed to determine the surface structure and element composition. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific Escalab 250Xi spectrometer (USA).

2.4. Batch Adsorption Experiments

All adsorption experiments were performed in an incubator shaker (180 rpm) for desired time using 1/2 ratio (g/L) of biochar to pollutant (MB and TC) solution. After adsorption, a certain amount of suspension was filtered by a nylon membrane filter for determining the pollutant concentration. The concentrations of MB and TC were measured by a UV-visible spectrophotometer (MAPADA UV-3300PC, Shanghai, China) at 665 nm and a high-performance liquid chromatography (HPLC) system (Waters e2695, Milford, MA, USA) with a UV detector at 357 nm, respectively. A mixture of methanol, acetonitrile and 0.01 M aqueous oxalic acid solution (8/20/72 volume ratio) was used for the determination of TC as mobile phase. The adsorption capability of biochar was calculated by the difference between the initial and final concentrations of the pollutant. The effect of common ions, including Na+, Mg2+, Ca2+, NO3−, CO32−, SO42−, PO43− on MB adsorption was tested in 0.5 mM NaCl, MgCl2, CaCl2, NaNO3, Na2CO3, Na2SO4, Na3PO4 aqueous solutions, respectively. The recycle of biochar was conducted in a mixture solvent of acetic acid and ethanol (1/9 volume ratio) with a 1/2 (mg/mL) ratio. Total organic carbon (TOC) was measured by a total organic carbon analyzer (TOC-L, Shimadzu, Kyoto, Japan).

3. Results and Discussion

3.1. Biochar Characterization

As depicted in Table 1, the O/C ratio of biochar was in the order of PB300 < HB300-30 < KB300 < KHB300-30, indicating KMnO4 and H2O2 oxidations synergistically produced oxygen-containing groups. A catalytic amount of Mn was embedded into KB300 (4.15 w%) and KHB300-30 (3.39 w%). Obviously, Mn decreased after H2O2 oxidation as it participated in an acceleration of H2O2 oxidation and was released from the carbon matrix. As previous biochars prepared at low temperature [28,29], these biochars had inferior specific surface areas (SSAs) with mesopore structure (Table 1).
The KMnO4 and KMnO4/H2O2 modifications resulted in a significant decline of surface pH PZC (Figure 1a), probably attributing to the introduction of easily dissociated groups, such as -OK and -COOK. All biochar surfaces were highly negatively charged over a pH range of 5.0–9.0, benefiting from the adsorption of cation by electrostatic interaction. The FTIR spectra (Figure 1b) showed that the biochars were consisted of OH (3434.0 cm−1), C-H (2926.1 cm−1), COO (1699.0 cm−1), C = O (1625.2 cm−1), C = C (1625.2 cm−1, 1446.6 cm−1) and C-O (1276.3 cm−1, 1113.2 cm−1) groups [30]. It was found that COO at about 1699.0 cm−1 increased with oxidation in the order of H2O2 < KMnO4 < KMnO4/H2O2. This suggested that the combination of KMnO4-promoted pyrolysis and H2O2 oxidation had a synergistic effect on the oxidation of biochar as preformed MnOx accelerated H2O2 oxidation.
Surface functional groups were determined by XPS (Figure 2). The deconvolution of high-resolution C 1s XPS spectra revealed that C–C/C=C (284.7 eV), C-O (286.0 eV), C=O (287.1 eV) and COO (288.6 eV) groups [31] were generated on four biochars (Figure 2a–d). Comparing to PB300, HB300-30 and KB300 possessed more C-O and COO with less C=O. With further oxidation of KB300 by H2O2, the C-O group remarkably decreased, and both C=O and COO increased. This indicated that C-O groups were drastically oxidized into C=O and COO by Mn-catalyzed H2O2 oxidation as highly active HO· and MnIV=O oxidant species [24,25,32] produced. Manganese oxides, including MnO, Mn2O3 and MnO2 [33], were observed on KHB300-30 as KB300 (Figure 2e,f), demonstrating that MnOx is effectively recycled during H2O2 oxidation. XRD spectra revealed that the biochar matrixes consisted of amorphous carbon with crystalline cellulose II (2θ = 19.5°) [33] (Figure S1). Additionally, MnOx were doped into KB300 in an amorphous state (Figure S1c), and H2O2 oxidation did not lead to the obvious change in their crystal structure (Figure S1d).
As shown in Figure S2a,b, irregular grooves and small holes were generated on PB300. HB300-30 had a relatively smooth surface due to H2O2 oxidation (Figure S2c,d). KMnO4-promoted oxidative pyrolysis also led to the formation of groove and porous structure (Figure S2e,f). Subsequent oxidation of KB300 produced new mesopores (Figure S2g,h) as Mn-catalyzed H2O2 decomposition drastically occurred. SEM-EDS spectra indicated that MnOx was uniformly distributed on the surface of KB300 (Figure S2i,j). This made them be not only active catalysts for H2O2 oxidation but also adsorption sites for contaminants. Similarly, MnOx was also well distributed on the surface of KHB300-30 without aggregation (Figure S2k,l).

3.2. Adsorption Capabilities of Durian Shell Biochar

3.2.1. Effect of Preparation and Adsorption Parameters on the Adsorption Capability for MB

As shown in Figure 3a, the adsorption capabilities of PB and KB decreased with the rise of pyrolysis temperature as surface functional groups reduced [34]. Similarly, lower adsorption capabilities were obtained for H2O2 oxidative biochars (HB-30 and KHB-30) pyrolyzed at a higher temperature. The aromatic structure was formed with fewer oxygen-containing groups at high-temperature [35,36], which was not conducive to H2O2.oxidation for the introduction of oxygen-containing groups. This was the main reason why the adsorption capability was reduced for HB-30 and HKB-30 as rising pyrolysis temperature. While the biochar was prepared at 300 °C, KMnO4-catalyzed oxidative pyrolysis enabled to significantly improve the adsorption capability and subsequent H2O2 oxidation further upgraded it. In contrast, single H2O2 oxidation caused a decrease of adsorption capability for PB300. The effect of H2O2 concentration was also evaluated (Figure 3b). 5–30 w% H2O2 were all suitable for KMnO4/H2O2 co-modification to enhance the adsorption capability greatly, but almost inefficient for single H2O2 oxidation. This clearly demonstrated that Mn-catalyzed H2O2 oxidation was superior to noncatalytic H2O2 oxidation for improving the adsorption capability of biochar and low concentration of H2O2 still exhibited high-efficiency. In short, KMnO4/H2O2 co-modification surpassed independent KMnO4 and H2O2 modifications for improving the adsorption performance of biochar toward MB, even using a low concentration of H2O2. Considering the greatest gap of adsorption capability between HB300 and KHB300 oxidized with 30 w% H2O2 (21.56 mg/g vs. 194. 98 mg/g), they were selected to study the adsorption behavior of MB in the following experiment.
The adsorption capabilities of KB300 and KHB300-30 gradually increased as rising pH from 3.0 to 9.0 (Figure 3c), which could be ascribed to the reduction of competitive adsorption between H+ and MB cation. For PB300 and HB300-30, the increase of adsorption capability was limited while enhancing pH (Figure 3c). The adsorption capability of KHB300-30 greatly exceeded that of KB300, HB300-30 and PB300 over a wide pH range, further demonstrating a great advantage of a combination of KMnO4 and H2O2 modifications. The adsorption of KHB300-30, KB300 and PB300 resulted in the pH rise from 4 to around 6 (Figure 3d), indicating they have relatively high cation exchange capability.

3.2.2. Adsorption Kinetics and Isotherms for MB

It seemed that KHB300-30 achieved adsorption equilibrium within a shorter time than PB300, HB300-30 and KB300 when using 50 mg/L initial concentration (Figure 4a–d), suggesting KMnO4/H2O2 co-modification resulted in enhancement of adsorption speed. The adsorptions were well described by pseudo-second-order model (Table S1), thereby were chemically controlled processes.
All the adsorptions were endothermic as the adsorption capabilities rose with adsorption temperature (Figure 4e–h). Their isotherms were perfectly described by the Langmuir isotherm model (Table S1). The maximum adsorption capability followed the order of KHB300-30 > KB300 > PB300 > HB300-30, confirming synergistic effect was achieved via sequential KMnO4-promoted pyrolysis and H2O2 oxidation. This could be ascribed to (1) incorporation of oxygen-containing group (C-O) (Table 1, Figure 2a,b) and manganese oxides (Figure 2) into carbon matrix through KMnO4-promoted pyrolysis, (2) new oxygen-containing groups (C = O and COO) (Figure 2d) and mesopores (Figure S2g,h) were generated by subsequent H2O2 oxidation with the catalysis of small-sized manganese oxides in the carbon matrix. The adsorption capability was greatly enhanced from 80.97 mg/g to 299.40 mg/g for MB at 45 °C with KMnO4/H2O2 co-modification. This biochar outperformed most of the modified biochars in the literature (Table 2). It was worth noting that total organic carbon (TOC) of 100 mg/L MB solution decreased from 62.88 mg/L to 21.26 mg/L after adsorption of KHB300-30.
The effect of coexisted ions was also investigated (Figure 5). Pleasantly, common ions, including cations (Na+, Mg2+, Ca2+) and anions (NO3−, CO32−, SO42−, PO43−), hardly had a negative effect on adsorption of all biochars for MB. Remarkably, the presence of CO32− and PO43− brought an increase in adsorption of MB, as they could be adsorbed onto the surface by hydrogen bonding and facilitated adsorption of MB by electrostatic interaction. Consequently, KMnO4/H2O2 co-modified biochar would be a promising adsorbent for treating MB-polluted wastewater with high adsorption capability.
The regeneration of biochar was briefly examined in the mixed solvent of acetic acid/ethanol (1/9). The adsorption capability gradually decreased after each recycles for KB300 and KHB300-30 (Figure 6a). This may be mainly ascribed to incomplete desorption of MB as demonstrated by the FTIR spectra after desorption (Figure S3b). It was also found that new -COOH (1699.0 cm−1) having a good affinity toward MB [48] were generated during desorption and could contribute to the adsorption of MB.

3.2.3. Adsorption of TC

These biochars were also tested for removing a common antibiotic contaminant TC in an aqueous solution. A brief investigation showed that KHB prepared at 300 °C exhibited much higher adsorption capability than corresponding PB, KB and HB (Figure 6b). This may be ascribed to the introduction of additional oxygen-containing groups and MnOx, which had a strong affinity to TC via hydrogen bonding and complexation [49], respectively. Notably, 5% concentration of H2O2 solution was superior to higher concentration, and 50.24 mg/g adsorption capability was achieved. This further confirmed that a low concentration of H2O2 was suitable for efficiently improving adsorption capability using KMnO4/H2O2 co-modification. The adsorption capability was at a moderate level in comparison with others in the literature (Table 2).

3.2.4. Adsorption Mechanism of MB and TC

As the biochar surfaces were highly negatively charged under neutral conditions (Figure 1a), MB cation could be adsorbed by electrostatic interaction. The adsorptions of biochars led to an increase in the pH of MB solution, suggesting MB cation could exchange with basic cations of the biochars. The C-N peaks of MB at 1398 cm−1 and 1341 cm−1 shifted to 1386 cm−1 and 1325 cm−1 after adsorption, respectively (Figure S3a), indicating hydrogen bonding of C-N bond with COOH and OH might be involved in the adsorption of MB.
To gain insight into the improvement of adsorption capability for MB with KMnO4/H2O2 co-modification, the biochars were acidified with 0.1 M aqueous HCl solution and analyzed by FTIR. The COOH groups (stretching vibration: 1703 cm−1, asymmetric bending vibration: 1386 cm−1) [50,51] increased (Figure S4a) due to the protonation of carboxylates and hydrolysis of lactonic acids [52]. Obviously, the COO peak intensity was in the order of KHB300-30 > KB300 > HB300-30 > PB300 after acidification, substantially proving that KMnO4/H2O2 co-oxidation outperformed independent KMnO4 and H2O2 oxidations for generating carboxyl groups, which can adsorb MB by cation exchange, hydrogen bonding and electrostatic interaction. Moreover, MnOx was well distributed on the surface of KMnO4/H2O2 co-modified biochar (Figure S2i–l), providing active adsorption sites for adsorbing MB through coordination [53] and electrostatic interaction [40]. Thus, the improvement of the adsorption capability could be mainly attributed to the incorporation of additional COO groups and MnOx onto KHB300-30.
As containing various oxygen- and nitrogen-containing groups, TC should be adsorbed by oxygen-containing groups and MnOx on these biochars through hydrogen bonding and hydrogen bonding/coordination, respectively. KHB300-5 was also demonstrated to be much richer in COO groups, which had stronger hydrogen bonding than other oxygen-containing groups for TC by the FTIR spectra before and after acidification (Figure S4b). SEM-EDS spectrum showed that MnOx (8.93 w% Mn) were homogeneously coated on the surface of KHB300-5 (Figure S5). Thereby, the introduction of new COO groups and MnOx also brought a great increase in adsorption capability for TC.

3.2.5. Expansion of the KMnO4/H2O2 Co-Modification to Sugarcane Bagasse Biochar

The developed KMnO4/H2O2 co-modification was applied to engineer sugarcane bagasse biochar to assess its applicability (Figure 7). KMnO4 modification led to a great increase in adsorption capabilities for both MB and TC. Subsequent H2O2 oxidation further enhanced adsorption performance remarkably for MB and TC with 20% and 10% concentrations, respectively. In contrast, H2O2 oxidation showed extremely low-efficiency for improving the adsorption capabilities. It is worth noting that the KMnO4/H2O2 co-modification achieved more than 11 times enhancement in adsorption capability of the biochar for MB (181.20 mg/g vs. 14.46 mg/g). These suggested that the KMnO4/H2O2 co-modification was an efficient and general strategy for improving the adsorption capabilities of different biochars by adjusting H2O2 concentration.

4. Conclusions

In summary, KMnO4-promoted oxidative pyrolysis succeeded in enhancing the adsorption performances of durian shell biochar prepared at 300 °C for MB and TC, and subsequent H2O2 oxidation further upgraded them with the catalysis of predoped MnOx. The KMnO4/H2O2 co-modified biochar showed significantly enhanced adsorption capability, outperformed independent KMnO4- and H2O2-engineered biochars. Especially, KMnO4/H2O2 co-modification brought a great increase in the adsorption capability of durian shell biochar from 72.94 to 302.11 mg/g for MB. In addition, the adsorption capability of the co-modified biochar was less influenced by common cations and anions. Element analysis, FTIR, XPS and SEM-EDS characterizations revealed that COO and MnOx were incorporated into the carbon matrix and contributed greatly to adsorption of MB and TC by hydrogen bonding/electrostatic interaction/cation exchange/coordination and hydrogen bonding/coordination, respectively. Consequently, a novel and efficient approach was established for improving the adsorption capability of biochar through KMnO4-promoted oxidative pyrolysis followed by H2O2 oxidation.

Supplementary Materials

The following are available online at https://www.mdpi.com/2071-1050/13/5/2641/s1, Figure S1: XRD spectra of durian shell biochar (a–d), Figure S2: SEM images (a–h) and SEM-EDS spectra (i–l) of durian shell biochar., Figure S3: The FTIR spectra of durian shell biochars after absorption (a) and desorption (b) of MB, Figure S4: The FTIR spectra of durian shell biochars before and after HCl treatment (a: 1 mg biochar + 1 g KBr; b: 2 mg biochar + 1 g KBr), Figure S5: SEM-EDS spectrum of durian shell biochar KHB300-5, Table S1: The fitted parameters of adsorption kinetics and isotherms.

Author Contributions

Conceptualization, J.L. and S.X. (Shuang Xu); methodology, J.L.; software, S.B.; validation, S.B. and J.L.; formal analysis, J.L.; investigation, S.B. and S.X. (Shuang Xu); resources, J.L. and S.X. (Shuying Xu); data curation, Z.Y. and S.X. (Shuang Xu); writing—original draft preparation, S.B.; writing—review and editing, J.L.; visualization, S.B. and S.L.; supervision, J.L., S.X. (Shuying Xu) and Y.Z.; project administration, J.L. and S.X. (Shuying Xu); funding acquisition, J.L. and S.X. (Shuying Xu). All authors have read and agreed to the published version of the manuscript.

Funding

The research was funded by Hainan Provincial Natural Science Foundation of China (519QN175), National Natural Science Foundation of China (21801053) and the Key Research and Development Program of Hainan Province of China (ZDYF2019192), and the APC was funded by the Key Research and Development Program of Hainan Province of China (ZDYF2019192).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Acknowledgments

Financial support from Hainan Provincial Natural Science Foundation of China (519QN175), National Natural Science Foundation of China (21801053) and the Key Research and Development Program of Hainan Province of China (ZDYF2019192) are gratefully acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Zeta potential (a) and FTIR spectra (b) of durian shell biochar.
Figure 1. Zeta potential (a) and FTIR spectra (b) of durian shell biochar.
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Figure 2. The C 1s (a–d) and Mn 2p (e,f) XPS spectra of durian shell biochar.
Figure 2. The C 1s (a–d) and Mn 2p (e,f) XPS spectra of durian shell biochar.
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Figure 3. Effect of pyrolysis temperature (a), H2O2 concentration (b) and initial pH (c) on adsorption capacity of durian shell for methylene blue (MB), and the pH change (d) before and after adsorption of MB (200 mg/L, 25 °C, pH 7, 24 h); the experiments were repeated thrice.
Figure 3. Effect of pyrolysis temperature (a), H2O2 concentration (b) and initial pH (c) on adsorption capacity of durian shell for methylene blue (MB), and the pH change (d) before and after adsorption of MB (200 mg/L, 25 °C, pH 7, 24 h); the experiments were repeated thrice.
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Figure 4. The adsorption kinetics (a–d: 25 °C, pH 7) and isotherms (e–h: pH 7, 72 h) of durian shell biochar for MB; these adsorption experiments were repeated twice, and average values are reported.
Figure 4. The adsorption kinetics (a–d: 25 °C, pH 7) and isotherms (e–h: pH 7, 72 h) of durian shell biochar for MB; these adsorption experiments were repeated twice, and average values are reported.
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Figure 5. Effect of coexisted ions on adsorption capability of durian shell biochar for MB (200 mg/L, pH 7, 25 °C, 24 h); the adsorption experiments were repeated thrice.
Figure 5. Effect of coexisted ions on adsorption capability of durian shell biochar for MB (200 mg/L, pH 7, 25 °C, 24 h); the adsorption experiments were repeated thrice.
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Figure 6. The adsorption capability of regenerated durian shell biochars for MB. (a) 200 mg/L, 25 °C, pH 7, 24 h) and durian shell biochar for TC; (b) 100 mg/L, pH 5, 25 °C, 48 h); the adsorption experiments were repeated thrice.
Figure 6. The adsorption capability of regenerated durian shell biochars for MB. (a) 200 mg/L, 25 °C, pH 7, 24 h) and durian shell biochar for TC; (b) 100 mg/L, pH 5, 25 °C, 48 h); the adsorption experiments were repeated thrice.
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Figure 7. The adsorption capabilities of sugarcane bagasse biochar for MB. (a) 200 mg/L, pH 7, 25 °C, 48 h) and TC; (b) 200 mg/L, pH 5, 25 °C, 48 h); the adsorption experiments were repeated thrice.
Figure 7. The adsorption capabilities of sugarcane bagasse biochar for MB. (a) 200 mg/L, pH 7, 25 °C, 48 h) and TC; (b) 200 mg/L, pH 5, 25 °C, 48 h); the adsorption experiments were repeated thrice.
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Table
Table 1. The characteristics of the durian shell.
Table 1. The characteristics of the durian shell.
BiocharElement Composition (W%)
ICP (w%)		Surface Properties
CONHMnKO/CSSA (m2/g)PD (nm)PV (cm3/g)
PB30064.0825.751.494.710.010.390.402.1910.250.0051
HB300-3064.8126.811.485.030.010.0340.411.9511.000.0049
KB300-3062.6826.101.405.194.150.010.422.1111.890.0058
KHB300-3059.9628.931.185.193.390.0120.483.0111.500.0081
Table
Table 2. Adsorption capability of modified biochar reported previously.
Table 2. Adsorption capability of modified biochar reported previously.
Modified BiocharConditionsQm (mg/g)Ref.
Hydroxyapatite/biochar compositepH 8, 298 K17.5 (MB)[37]
Biochar/AlOOH nanocompositeNatural pH, 295 K85.04 (MB)[38]
Phosphomolybdic acid-modified biocharpH 7, 298 K122.70 (MB)[5]
Municipal sludge-modified biocharNatural pH, 298 K12.58 (MB)[39]
Biochar/iron oxide compositepH 6.1, 313 K862 (MB)[40]
Citric acid-modified biocharpH 11, 303 K384.87 (MB)[41]
Silica-composited biocharNeutral, 298 K17.51 (TC)[42]
Fe/Mn oxides-doped biocharpH 11, 298 K14.24 (TC)[43]
Iron and zinc-doped sawdust biocharpH 6, 298 K102.0 (TC)[44]
Grapefruit peel extracts-modified biocharNatural pH, 298 K34.58 (TC)[45]
H3PO4-modified biocharpH 7, 298 K174.0 (TC)[46]
NaOH-activated biocharpH 5, 293 K302.37 (TC)[47]
Magnetic crayfish shell biocharMB: pH 7, 318 K; TC: pH 5, 298 K299.40 (MB), 50.24 (TC) aThis work
a The adsorption capability at 100 mg/L of initial concentration and 24 h.
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Bian, S.; Xu, S.; Yin, Z.; Liu, S.; Li, J.; Xu, S.; Zhang, Y. An Efficient Strategy for Enhancing the Adsorption Capabilities of Biochar via Sequential KMnO4-Promoted Oxidative Pyrolysis and H2O2 Oxidation. Sustainability 2021, 13, 2641. https://doi.org/10.3390/su13052641

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Bian S, Xu S, Yin Z, Liu S, Li J, Xu S, Zhang Y. An Efficient Strategy for Enhancing the Adsorption Capabilities of Biochar via Sequential KMnO4-Promoted Oxidative Pyrolysis and H2O2 Oxidation. Sustainability. 2021; 13(5):2641. https://doi.org/10.3390/su13052641

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Bian, Siyao, Shuang Xu, Zhibing Yin, Sen Liu, Jihui Li, Shuying Xu, and Yucang Zhang. 2021. "An Efficient Strategy for Enhancing the Adsorption Capabilities of Biochar via Sequential KMnO4-Promoted Oxidative Pyrolysis and H2O2 Oxidation" Sustainability 13, no. 5: 2641. https://doi.org/10.3390/su13052641

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