Evaluation of biochar properties exposing to solar radiation: A promotion on surface activities

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Evaluation of biochar properties exposing to solar radiation: A promotion on surface activities Neng Lia,d, Fei Raoa,c, Lili Heb, , Shengmao Yangb, Yongjie Baoa,c, Chengjian Huanga,d, ⁎ Minzhen Baoa, Yuhe Chena,d, ⁎

a

China National Bamboo Research Center, Hangzhou 310012, China Institute of Environment, Resource, Soil and Fertilizer, Zhejiang Academy of Agricultural Sciences, 198 Shiqiao Road, Hangzhou 310021, China c Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing 10000, China d Key Laboratory of High Efficient Processing of Bamboo of Zhejiang Province, Hangzhou 310012, China b

HIGHLIGHTS

GRAPHICAL ABSTRACT

exhibited a strong influence • Exposure on biochar properties. underwent a series of photo• Biochar oxidation reaction during exposing. average growth rate of O2 of BBs • The was 86.63% after accelerated weath-

• •

ering. The mechanism of bamboo biochar exposed to solar radiation was explored. Photodegradation improves biochar for sewage treatment and soil amelioration.

ARTICLE INFO

ABSTRACT

Keywords: Pyrolytic carbon Oxidation Photodegradation

Biochar is an important bio-material used to mitigate environmental and agronomic problems. The complex outdoor environment, which includes exposure to solar radiation, greatly influences biochar structure, which in turn impacts its functions when it is used in outdoors. The objective of this study is to understand the dynamics associated with the physical and chemical changes that occur in bamboo biochar (BB) prepared at different final temperatures during exposure to damaging radiation. The basic physical and chemical properties of fresh and accelerated weathered BBs were investigated by measuring the chemical element, ash content, BET surface area, color changes. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetry analysis and differential scanning calorimetry were used to determine the changes in functional groups, the variation in the surface element species distribution and the differences in thermochemical property of BBs. The experimental results showed that clear changes in BBs were obtained after 400 h of accelerated weathering. The C and N contents of BBs declined, and its O content increased. The average changes in color and decrease in residue yield of BBs were 1.02 and 1.58%, respectively. The average growth rates of O/C ratio (determined by elemental analysis), C4, and O2 of BBs were 17.04%, 13.88% and 86.63%, respectively. These results illustrate that the BBs underwent a series of photooxidation reactions when exposed to UV radiation. The increase of surface oxygen-containing functional groups may improve the surface activities of BB with respect to soilamelioration applications, pollutant-degradation efficiency and the adsorbability toward aqueous contaminants.

⁎

Corresponding authors at: Shiqiao Road 198, Hangzhou 310021, China (L. He); Wenyi Road 310, Hangzhou 310012, China (Y. Chen). E-mail addresses: [email protected] (L. He), [email protected] (Y. Chen).

https://doi.org/10.1016/j.cej.2019.123353 Received 24 July 2019; Received in revised form 29 October 2019; Accepted 1 November 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Neng Li, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123353

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1. Introduction

compositions, ash contents, BET surface areas, and colors. Changes in the functional groups of fresh and weathered BBs during accelerated weathering were examined using Fourier transform infrared spectroscopy (FT-IR), the distributions of surface elemental species of BBs were examined by X-ray photoelectron spectroscopy (XPS), and thermochemical property of BBs were characterized by thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC).

Biochar is composed of carbon-rich solid particles produced from biomass upon pyrolysis under oxygen-limited conditions at a specific temperature (≤700 °C) [1,2]. High-temperature treatment endows biochar with a variety of advantages that typically include well-developed pore structures, large surface areas, high stabilities, and exceptional adsorption properties [3]. Biochar has also been widely advocated for the mitigation of multiple environmental and agronomic problems [4,5], such as climate change [6], soil degradation [7], atmospheric contamination [8], low agronomic yield, and the eutrophication of surface and groundwater [9]. The condensed aromatic structure of biochar is highly stable, which is important for carbon sequestration and emissionsreduction functionalities, which is the key basis for determining its longterm agricultural and environmental benefits. The porous structure and large surface area of biochar provides places for microorganisms to grow, and for nutrient retention and greenhouse-gas reduction; it is also a good adsorber of pollutants [10,11]. Over the past 30 years, the biochar-research field has expanded broadly, from soil science and atmospheric sciences to oceanography and even anthropology [8,12]. Biochar introduced into the natural environment rapidly experience a range of natural aging and weathering processes that are partly governed by solar-radiation, moisture, temperature, oxygen, plant roots, and pH conditions, as well as the presence of other chemical substances, microorganisms, and soil and water fauna [13] that may affect biochar stability and its environmental benefit. The initial weathering of biochar even occurs during storage, which influenced by the contact with moist air and other environment factors [14]. Once applied to soil or water, biochar continues to age at a rate that is partly influenced by temperature, as well as oxygen, soil, and water conditions. Under the influence of moisture and oxygen in the soil, biochar surfaces oxidize to generate more carboxylic and phenolic groups; hence they become more hydrophilic with time, which increase their capacity to retain (cat)ions [15]. Furthermore, the mean annual temperature (MAT) has a major influence factor on the natural oxidation of biochar; higher MAT favored greater biochar oxidation [15]. Plant roots present an important redox interface that may result in the aging of biochar appears as the transformations of C, N, and S in biochar [14,16]. Biochar surfaces are initially either acidic or basic; hence the pH of the environment also ages biochar. Biochar is also sensitive to chemical substances [17], and is typically oxidized by chemical oxidizing agents, such as H2SO4, HNO3 and (NH4)S2O8 [18]. With the aid of microorganisms, biochar in soil can be aged at an accelerated rate; the aliphatic carbon (C) in biochar is mineralized by soil microorganisms as evidenced from the reduction of aliphatic C with aging [19,20]. Solar radiation, especially ultraviolet (UV) radiation with high energy, plays an important role in the degradation of biomass present in outdoor environments [21]. Photodegradation has a significant impact on the terrestrial C cycle, the biomass exposed to solar radiation for 124 days increased the CO2 emission by 9.3% compared with unexposed samples [22], and photodegradation also can accelerate biodegradation [23]. However, biochar as a kind of pyrolyzed biomass, systematic research on its photodegradation remains scarce, and little is known about aspects such as the influence of potential photooxidation on biochar application [20]. The objective of this study was to understand of dynamics associated with the physical and chemical changes that occur in biochar when exposed to damaging radiation. We hypothesized that the physical and chemical structure of biochar pyrolyzed by different temperatures would change when exposed to damaging radiation, especially in the photooxidation of biochar surfaces. This may promote biochar surface activities for its future application in soil-amelioration, pollutant-degradation and the adsorbability toward aqueous contaminants. Bamboo, which is regarded as an ecological material, with its advantages of fast growth and high strength, was used as the feedstock for the preparation of bamboo biochar (BB). The basic physical and chemical properties of fresh and weathered BBs were investigated by determining their elemental

2. Materials and methods 2.1. Preparation of bamboo biochar Four-year-old moso bamboo (Phyllostachys pubescens Mazel) culms were harvested at a plantation in Zhejiang province in Eastern China. The inner and outer skins of the bamboo were scraped to obtain only the middle layer of the bamboo. Raw samples of bamboo biomass, 100 × 20 × 5 mm in size, were prepared for biochar production. The feedstock was dried in an oven at 103 °C for 24 h and stored in a controlled-atmosphere room until further use. The biomass raw materials were pyrolyzed in a programmable tube furnace (Lantian, China) connected to a vacuum pump at a heating rate of 25 °C min−1 to a final temperature of 300, 500, or 700 °C for a 2 h residence time. The bamboo biochar produced in this manner are referred to as “300-BB”, “500-BB”, or “700-BB”, respectively. The obtained biochar was cooled to indoor temperature, and was stored in airtight plastic containers. 2.2. Accelerated weathering Accelerated weathering experiments were performed in a xenon lamp weathering test box (XE-3HS, H.J.Unkel, USA) according to ASTM G155 [24]. Samples were cyclically exposed to UV and visible radiation (λ = 340 nm, simulated outdoor solar radiation) at the black panel temperature of 63 °C and a relative humidity of 50% for 8 h. Furthermore, as the spray system of the test box was inapplicable for powder samples, an artificial water spray system using a watering can was used. The UV-radiation intensity, at a wavelength of 340 nm, was 0.35 W·(m2·nm)−1 and the amount of sprayed water was 0.35 ± 0.09 g·g−1 (the mass ratio of water to the sample). The treatment methods used for the fresh and weathered samples are summarized in Table 1. 2.3. Elemental analysis The C, hydrogen (H), nitrogen (N), and sulfur (S) contents of the samples were analyzed with an elemental analyzer (Vario EL cube, Elementar Analysensysteme GmbH, Hanau, Germany). The ash contents of the BBs were determined from the residual weights of the samples after treatment at 500 °C for 30 min, and at 800 °C for 3 h. The oxygen (O) content was calculated based on the mass difference [25]. Table 1 Final pyrolyzed temperatures and weathering times of the samples (bamboo biochar) in this study.

2

Samples

Temperature (°C)

Time of weathering (h)

300-BB-0 300-BB-200 300-BB-400 500-BB-0 500-BB-200 500-BB-400 700-BB-0 700-BB-200 700-BB-400

300 300 300 500 500 500 700 700 700

0 200 400 0 200 400 0 200 400

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2.4. BET analysis

dehydration occurred during treatment at high temperature [25]. The N and S contents tended to rise with increasing pyrolysis temperature, which can possibly be attributed to the higher stabilities of N and S than those of C, H, and O under pyrolysis conditions. The mass loss during biochar preparation was approximately 40–70%, with the loss rates of C, H, and O being higher than those of N and S during pyrolysis. Weathering was observed to have a significant influence on the contents of the various chemical elements. The C and N contents of BBs declined over 400 h of exposure to light; conversely, the O content of BBs increased over the same period of weathering. These results suggest that weathering degrades BBs through the photooxidation of C and N; some of the C and N are converted into gases, such as CO2, CO, NO2, and NO, which escape from the BBs. The levels of ash in all BBs were in the 2.61–3.51% range. Prolonged radiation times were observed to favor higher ash contents. The H/C, O/C, and C/N molar ratio decreased from 0.86 to 0.27, 0.30 to 0.06, and 279.24 to 183.35, respectively, as the final pyrolysis temperature increased. This trend is consistent with that reported previously [25]. The H/C ratio has been considered as an indicator of aromaticity and biochar stability when attacked by microorganism [29]. The H/C trend reveals increasing aromaticity and stability with increasing pyrolysis temperature. The decrease in the O/C ratio suggests a reduction in polarity and the number of oxygen-containing functional groups [25]. The C/N ratio is an important indicator in environmental management [9]; a decrease in the C/N ratio confirms the stability of N upon heating. The lower-temperature treated 300-BB sample, which contains many functional groups, was susceptible to irradiation with light; lower H/C and C/N ratios were observed after 400 h of exposure to UV radiation. The O/C ratio of 300-BB exhibited the opposite trend. The observed decrease in the H/C ratio indicates that 300-BB was more stable and aromatic following exposure to light. The decrease in the C/N ratio is probably due to a faster loss of C, as compared with the loss of N, in the oxygen-rich 300-BB. The increase in the O/C ratio demonstrates that the carbon on the biochar surface was oxidized, which may be attributed to the formation of functional groups on biochar surface. These functional groups were predominantly carboxyl, hydroxyl, and carbonyl [21]. The H/C ratio of 500-BB increased over 400 h of exposure, which is attributable to an increase in its H content and a decrease in its C content. The H/C ratio of 700-BB exhibited no obvious changes upon irradiation, which suggests that the aromatic structure of 700-BB is stable when exposed to light. Similar trends in the O/C ratios were observed for 500BB and 700-BB as that seen for 300-BB, which indicates that carbon was commonly oxidized in 300-BB, 500-BB, and 700-BB under exposure to UV radiation. The C/N ratios of 500-BB and 700-BB exhibited increasing trends over 400 h of exposure. The carbon in the aromatized and graphitized 500-BB and 700-BB is more stable than that in 300-BB, which resulted in the observed increase in the C/N ratio upon irradiation. The SSAs and TPVs showed increasing trends with increasing final pyrolysis temperature, while a decreasing trend was observed for the MPS values. Obvious decreases in SSA and TPV were observed for 500BB and 700-BB after 400 h of exposure to damaging radiation, while small increases were observed for those of 300-BB. Correspondingly, 500-BB and 700-BB exhibited marked increases in MPS, while a small decrease was observed for that of 300-BB. The results suggest that the numerous micropores in 500-BB and 700-BB are destroyed during accelerated weathering, thereby increasing MPS and decreasing SSAs and TPVs. However, carbonization also occurred during irradiation, which produced more pore structures in BB; this further carbonization dominates the aging progress of 300-BB, which results in decreasing MPS and increasing SSA and TPV values. Weathering degrades the chemical components of the BBs which affect its appearance; this degradation manifests itself in the form of color change. As shown in Table 2, the values of chromaticity parameters L*, a* and b* of BBs are distributed in the range of 34.97–38.70, 0.33–1.13 and 0.47–1.57, respectively. The L* value of samples increased with increasing final pyrolysis temperature. The smallest values

The specific surface areas (SSAs), total pore volumes (TPVs), and mean pore sizes (MPSs) of the BBs were determined by the BET adsorption method (TriStar II 3020, Micromeritics, USA). The SSA was measured with N2 adsorption, and the BET equation was used to calculate the surface area of the BBs samples [26]. 2.5. Color measurement In order to investigate the color change of BBs after weathering, a piece of 1-mm-thick quartz glass was placed over the sample surface. Sample colors were determined through the glass in a dark room using a Konica Minolta CR-10 instrument (Japan). Color parameters L*, a*, b* of the CIELAB color system were measured according to ASTM E1347 [27]. L*, a*, b* represent the indexes of lightness, red and green, yellow and blue, respectively. The overall color modulus difference (ΔE), which represents the change in color, was calculated according to Eq. (1) [28]:

E=

(L1

Lo ) 2 + (a1

ao ) 2 + (b1

bo ) 2

(1)

where L0*, a0*, and b0*, and L1*, a1*, and b1* are the color parameters of BBs before and after weathering, respectively. 2.6. Fourier-transform infrared spectroscopy analysis FT-IR spectra were recorded using an FT-IR spectrophotometer (Nicolet iS10, USA) in the absorbance mode over frequencies of 4000–400 cm−1, a KBr pellet was employed during the process [28]. BBs were mixed with KBr in a weight ratio of approximately 1: 100. Pellets were formed using the same pressure at the same treatment times. The resolution was set at 4 cm−1, and 64 scans were recorded for each analysis. 2.7. X-ray photoelectron spectroscopy analysis XPS spectra of fresh and weathered BBs were acquired at a vacuumchamber pressure of 8 × 10−10 Pa on an ESCALAB 250Xi spectrometer spectra (Thermo Fisher, USA) with K X-ray radiation, with the following instrumental parameters: photon energy, 1253.6 eV; operating voltage, 12.5 kV; filament current, 16 mA. The core level binding energies (BEs) were aligned with C1s peak binding energy (BE) peak (284.60 eV). Data were analyzed using Peakfit software. Atomic C/O ratios were calculated from C1s and O1s peak areas. 2.8. Thermogravimetry and differential scanning calorimetry measurement TGA and DSC were conducted for BBs using a thermogravimetric analyzer (TGA/DSC 1, STARe System, METTLER TOLEDO, USA), which allows for the simultaneous detection of mass changes and heat effects of decomposition. All samples were performed under a dry and pure nitrogen atmosphere with a flow rate of 50 mL·min−1, and at the temperature range of 25–800 °C and heating rate of 10 °C·min−1. 3. Results and discussion 3.1. Physicochemical properties of fresh and weathered bamboo biochar The mass fraction of chemical elements and ash, elemental molar ratios, SSAs, TPVs, MPSs, color parameters, and ΔE of fresh and weathered BBs prepared at 300, 500, and 700 °C has shown in Table 2. The C contents of fresh BBs without accelerated weathering increased from 66.09% to 86.94% as the pyrolysis temperature was increased from 300 to 700 °C, whereas the H and O contents decreased rapidly, from 4.77% to 1.95%, and from 26.19% to 7.42%, respectively. These results suggest the reaction of decarboxylation, decarbonylation, and 3

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Table 2 Changes in the basic physical and chemical properties of fresh and weathered BBs* prepared at different final pyrolysis temperatures. Samples

300-BB-0 500-BB-0 700-BB-0 300-BB-200 500-BB-200 700-BB-200 300-BB-400 500-BB-400 700-BB-400

Mass fraction of C, H, O, N, S (%)

Ash (%)

C

H

O

N

S

66.09 83.63 86.94 63.02 82.86 85.24 61.74 82.69 85.77

4.77 3.06 1.95 4.52 3.05 1.93 4.38 3.13 1.92

26.19 9.74 7.42 29.45 10.32 8.80 30.39 10.42 8.17

0.28 0.49 0.55 0.27 0.46 0.55 0.27 0.43 0.53

0.06 0.12 0.15 0.07 0.09 0.15 0.08 0.10 0.10

2.61 2.96 2.99 2.67 3.21 3.33 3.14 3.23 3.51

Molar ratio

BET

Color parameters

ΔE

H/C

O/C

C/N

SSA* (m2·g−1)

TPV* (cm3·g−1)

MPS* (nm)

L*

a*

b*

0.86 0.44 0.27 0.86 0.44 0.27 0.85 0.45 0.27

0.30 0.09 0.06 0.35 0.09 0.08 0.37 0.10 0.07

279.24 200.68 183.35 271.19 208.25 179.75 271.71 223.73 188.37

5.53 19.14 31.39

0.009 0.027 0.039

6.72 5.70 4.94

34.97 ± 0.15 36.73 ± 0.40 38.70 ± 0.26

1.13 ± 0.06 0.33 ± 0.06 0.53 ± 0.06

1.53 ± 0.06 0.47 ± 0.06 1.07 ± 0.15

5.79 11.81 10.99

0.010 0.019 0.017

6.54 6.42 6.31

33.93 ± 0.23 35.90 ± 0.20 37.77 ± 0.15

0.80 ± 0.00 0.30 ± 0.10 0.63 ± 0.12

0.83 ± 0.06 0.47 ± 0.12 1.10 ± 0.10

1.29 0.83 0.94

*SSA, specific surface area; TPV, total pore volume; MPS, mean pore size.

of a* (0.33) and b* (0.47) in fresh BB were obtained for 500-BB, followed by 700-BB. The L* values of BBs show decreasing tendency after 400 h of exposure, this indicates that all samples darken during UV radiation. However, only minor changes in ΔE values (0.83–1.29) were observed for all BBs after 400 h weathering, which may be attributed to the small a* and b* values of BBs. These results illustrate that the carbonization treatment improves the color stability of bamboo during exposure to damaging radiation.

Progressive decreases in the relative intensities of the absorption peaks at 3384, 2932, 1510, 1454, 1110, and 823 cm−1 were observed with increasing light-exposure time in the spectrum of 300-BBs. In detail, the decrease in intensity of the peak at 3384 cm−1 indicates a gradual decline in the number of hydroxyl groups, while the decline in the intensity of the absorption peak at 2932 cm−1, which is assigned to CeH stretching vibrations, indicates that the methyl and methylene groups in lignin and polysaccharide structure have been degraded. The changes observed at 1510 and 1454 cm−1 reveal a decline in the number of C]C bonds, while the decrease in the intensity of the peak at 1110 cm−1 is characteristic of CeO instability when exposed to sunlight. The change observed for the 823 cm−1 peak is consistent with CeH degradation at positions 2, 5, and 6 of the guaiacyl units. As shown by the relative intensities of the FT-IR peaks for 500-BBs (Table 4), the absorption peak at 1693 cm−1, which corresponds to α, βunsaturated C]O, its relative intensity becomes more larger with increasing irradiation time, which is mainly due to photooxidation of 500BBs during exposing. It should be mentioned that C]O groups often appear at 1740–1715 cm−1 in the FT-IR spectra of the BB raw material (i.e., bamboo). However, the C]O groups in 300-BBs and 500-BBs are observed to resonate at 1697–1693 cm−1, which is attributable to the development of unsaturation near the carbonyl units upon high-temperature treatment. A rapid decrease in the I1404/I1575 ratio was observed upon exposure to sunlight. The absorption peak at 1404 cm−1 was almost absent in the spectra of 500-BB-200 and 500-BB-400; these results are related to the sensitivity of the OeH group (1404 cm−1, which corresponds to deformation vibrations) to UV radiation. The peaks observed at 873, 816 and 751 cm−1, which are attributed to CeH stretching in aromatic rings [32], exhibited a decreasing trend as aging time increasing, implying that the photochemical reactions involve the aromatic CeH groups. The FT-IR spectra of 700-BBs irradiated at different times are compared in Fig. 1. The disappearance of most raw materials peaks indicates that the chemical groups of bamboo, including the aromatic structures, cannot endure a temperature as high as 700 °C; the only absorption peak observed in the spectra, at 1541 cm−1, which probably corresponds to βdiketone and α, β-unsaturated alkyl-β-hydroxy-ketone, increases in intensity with the increasing weathering time. Fresh 700-BB showed a 1541 cm−1 peak with the lowest intensity (0.04), while this peak was more intense after 200 h of exposure (700-BB-200, 0.05), with the highest intensity (0.07) observed for the 700-BB-400. These results suggest that 700-BB, which is mainly composed of graphitic structures, is also affected by solar radiation, oxygen, and water, with the degree of photooxidation proportional to irradiation time. In conclusion, there were considerable differences in the FT-IR spectra of BBs prepared at different final temperatures, with a positive correlation between the degree of BB pyrolysis and treatment temperature. The BBs were susceptible to solar radiation, oxygen, and water. All BBs exhibited photooxidation, with the degree increasing with increasing irradiation duration. Clearly photodegradation was

3.2. Fourier-transform infrared spectroscopy analysis In order to further understand the dynamic changes undergone by the BBs functional groups during exposure to damaging radiation, FT-IR spectroscopy was used to investigate the photooxidation of biochar and the pyrolysis products of bamboo, as well as changes that occur when bamboo is pyrolyzed at different final temperatures. The FT-IR spectra of BBs before and after weathering are illustrated in Fig. 1. The treatment of fresh BBs at different temperatures resulted in significant changes in the observed FT-IR absorbance peaks. Raw bamboo is a type of wooden material, composed of cellulose, hemicellulose, and lignin. The absorption peak at 898 cm−1, which is characteristic of bamboo cellulose, was not observed in the spectrum of 300-BB; this indicates that the bamboo cellulose underwent large-scale pyrolysis when treated at 300 °C due to the lower bond energy of cellulose compared with lignin. Jiang et al. [30] reported a similar result; they found that the crystallinity of bamboo cellulose treated at 315 °C decreased rapidly from 28.28% (without pyrolysis treatment) to 3.88%. The curves of 300-BBs exhibit an abundance of absorption peaks that were probably caused by the high stability of lignin under treatment at 300 °C. Many of these absorption peaks, including those at 3384, 2932, 1213, and 1110 cm−1 [31], disappear when the treatment temperature is increased to 500 °C, which indicates the breakage of chemical bonds in the lignin structural unit and polysaccharide, oxidation of the lignin side chains, and the degradation of parts of the aromatic rings. Almost all of the original absorption peaks except the peak at 1541 cm−1, were absent in the FT-IR spectrum of 700BB. This suggests that the vast majority of the chemical substances were transformed into graphitic structures. The relative intensities of the absorbance peaks of 300-BBs and 500BBs are presented in Table 3 and Table 4. The peaks at 1598 cm−1 (300BB) and 1575 cm−1 (500-BB) are due to the aromatic skeletal stretching of lignin; their strengths were largely retained upon exposure to solar radiation due to the greater binding energies of aromatic rings compared to other units. Therefore, the absorption peaks at 1598 and 1575 cm−1 were used as internal references. The relative intensity of the peak at 1697 cm−1 relative to that at 1598 cm−1 (I1697/I1598) in the spectrum of 300-BB increased markedly with increasing exposure time, which indicates that unconjugated ketones or aldehydes gradually became more abundant; it also reveals that 300-BB undergoes significant photooxidation during weathering. 4

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Fig. 1. FT-IR spectra of fresh and weathered BBs (aged for 200 h and 400 h). Table 3 Relative intensities of the FT-IR absorption peaks of 300-BBs. Samples

300-BB-0 300-BB-200 300-BB-400

Relative intensity values I3384/I1598

I2932/I1598

I1697/I1598

I1598/I1598

I1510/I1598

I1454/I1598

I1213/I1598

I1110/I1598

I823/I1598

1.10 1.04 0.90

0.32 0.31 0.26

0.74 0.78 0.90

1.00 1.00 1.00

0.51 0.37 0.34

0.54 0.48 0.47

0.51 0.47 0.49

0.33 0.31 0.28

0.19 0.12 0.12

Table 4 Relative intensities of the FT-IR absorption peaks of 500-BBs. Samples

500-BB-0 500-BB-200 500-BB-400

Relative intensity values I1693/I1575

I1575/I1575

I1404/I1575

I873/I1575

I816/I1575

I751/I1575

0.17 0.18 0.20

1.00 1.00 1.00

0.16 0.04 0.02

0.36 0.26 0.26

0.21 0.15 0.14

0.26 0.19 0.19

5

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9.60 4.89 14.49 48.24 21.30 8.47 7.51 85.51 0.13 100.00 10.81 2.11 12.92 50.11 23.00 7.73 6.24 87.08 0.11 100.00 7.59 7.50 15.09 58.54 13.27 10.84 2.25 84.91 0.13 100.00

3.3. X-Ray photoelectron spectroscopy analysis XPS analysis was performed to obtain the information about the surface element species distribution of fresh and aged biochar. Results of XPS analysis of the biochar surface are presented in Table 5. The C contents of 300-BB-0, 500-BB-0, 700-BB-0 were 79.59, 85.53 and 87.08, respectively. High final BB-preparation temperatures enhance the C content. The reverse trend was observed for the O content, which was observed to decrease from 20.41 (300-BB-0) to 12.92 (700-BB-0). After 400 h of exposure to radiation, higher O/C ratios were observed on the BBs surfaces. The O/C ratio of 300-BB only increased 1.11% after weathering, this ratio increased 5.05% and 14.21% for 500-BB and 700BB, respectively. An obviously different result was observed in the O/C ratio values determined by the XPS and elemental analysis, which is probably due to the poor penetration of UV radiation, which would result in photodegradation occurring mainly on the surface of BBs, and oxygen that is distributed differently in the outer and inner parts of the BBs as a result of differences in pyrolysis conditions. On the other hand, the inner functional groups of 300-BB were oxidized by a combination of temperature, water, and oxygen when exposed to light. Fig. 2 displays the C1s spectra of fresh biochar and that weathered for 400 h, fitted into four peaks (C1, C2, C3, and C4); their energies and abundances are presented in Table 5. C1 is ascribed to CeC/CeH/C]C, with binding energies of 284.50–284.68 eV, while C2 is related to phenol, alcohol, or other CeO, with binding energies of 285.88–286.27 eV. C3 is attributed to carbonyl C]O (287.66–288.12 eV) and C4 is assigned to carboxyl or ester (O]CeO), with binding energies of 288.70–289.43 eV [14,36]. The proportions of C1 in BB treated at difference temperatures declined during weathering. Conversely, the proportion of C4 in the spectra of BB-300, BB-500, and BB-700 increased after 400 h of irradiation. These results confirm that BBs underwent a series of photooxidation reaction. A similar observation was reported by Singh et al. [33] for biochar aged in soil, the biochar oxidation described in this report was due to the soil environment, such as moisture, oxygen, microorganism, temperature and chemical conditions in the soil environment. Those results also confirmed that biochar is thermodynamically unstable under the oxidative conditions after high temperature pyrolysis [37]. The lowest decrease in C1 content (Table 5) was observed for 300-BB (0.89%), while 700-BB exhibited the largest decrease (1.87%). This trend is similar to that observed for the O/C ratios based on XPS data, and indicates that the surface of 700-BB, with few functional groups, is also sensitive to damaging radiation. The C2 proportion of 300-BB-400 (23.78%) was higher than that of 300-BB-0 (21.71%), whereas C3 was presented in a greater proportion in 300-BB-0 (6.19%); the reverse trends were observed for 500-BB and 700-BB, and the C2 values decreased from 15.94% to 15.63%, from 26.41% to 24.91%; the C3 values rose from 11.85% to 12.77%, from 8.88% to 9.91%, respectively. The results of 300-BB indicate that more new C2 (CeO) formed from C1 (CeC/ CeH) compared to C2 (CeO) transformations to C3 (C]O) and C4 (OeC] O) during the process of photooxidation, and the transformation of C3 play a leading role in the process of C3 formation and transformation. The results of 500-BB and 700-BB illustrate that the dominant is transformation of C2 and formation of C3. The reverse in the trends between 300-BB and 500-BB or 700-BB are probably due to the aromatization and graphitization of 500BB and 700-BB, which resulted in significant differences of chemical groups between 300-BB and 500-BB or 700-BB.

11.91 8.68 20.59 53.53 18.88 3.60 3.39 79.41 0.20 100.00 CeC/CeH CeO C]O OeC]O

284.61 286.27 288.00 289.06

284.50 286.07 288.12 288.70

284.59 285.88 287.91 289.43

284.60 285.94 288.02 289.43

284.68 285.90 287.66 289.48

284.57 285.90 287.82 289.39

16.37 4.04 20.41 54.42 17.28 4.93 2.97 79.59 0.19 100.00 532.79 531.02 532.81 531.28 533.01 531.26 533.00 531.33 532.90 531.52 532.94 531.43

C1s

O1 O2 Subtotal O1s C1 C2 C3 C4 Subtotal C1s O/C Total O1s

OeC/Chemisorbed oxygen O]C/O]N

700-BB-0 500-BB-400 300-BB-400

500-BB-0 Assigned structure Components Transition

Table 5 XPS data of fresh and weathered BBs.

seen in 300-BBs and 500-BBs, especially in their lignin components. These results were consistent with the results of elemental analysis and the O/C ratio, which also demonstrated the photooxidation of BBs. The oxidation of the functional groups on the surface of biochar increases the reactivities of the latter, which may contribute to its cation-exchange capacity in soil [33], enhance its pollutant-degradation efficiency, improve its adsorbability toward aqueous contaminants [34,35], such as the functional groups developed on the surface of biochar can potentially increase interactions between biochar-derived organic materials with soil minerals, nutrients, and contaminants [20].

7.85 6.62 14.47 59.66 13.63 10.14 2.10 85.53 0.13 100.00

700-BB-0 300-BB-0 300-BB-0

700-BB-400

Abundance (%) Peak energy (eV)

300-BB-400

500-BB-0

500-BB-400

700-BB-400

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Fig. 2. C1s XPS spectra of fresh and weathered BBs.

The deconvolution of the O1s spectra of fresh and weathered BBs have shown in Fig. 3. The O1s XPS spectra was fitted into two peaks (O1 and O2). The O1s peaks observed for BB, at 532.79–533.01 eV (O1) and 531.02–531.52 (O2) eV are ascribed to OeC/chemisorbed oxygen and O]C/O]N, respectively. The dominant O functional groups in the 300-BB-0 and 700-BB-0 samples were found to be OeC and chemisorbed oxygen, with O]C and O]N groups making up less than 20% of the total. However, the proportion of O2 (45.76%) was only slightly less than O1 (54.24%) in 500-BB-0, which is consistent with the C1s result for 500-BB-0, in which the proportion of C3 + C4 (14.31%) was close to that of C2 (15.94%). The abundance of O2 affects the chromophoric groups (e.g., O]C), which may be responsible for the low a*, b* and ΔE values observed. A rapid increase in the O2 proportion was observed during 400 h of exposure, especially for 300-BB and 700-BB, which reveals that BBs are clearly oxidized upon exposure to light. These results are similar to those obtained by FT-IR spectroscopy. Although a small increase in the surface O content of 300-BB was observed after weathering (20.41 to 20.59%, a relative increase of approximately 0.88%), the O2 content of 300-BB (Table 5) increased dramatically, from 4.04% to 8.68% (a relative increase of 114.85%), clearly illustrating that photooxidation occurs on the surface of 300-BB. The O2 content of 700-BB increased sharply, by 131.75% (from 2.11 to 4.89) after exposure; this was similar to the increase in 300-BB. The O2

proportion (Fig. 3) and contents of 500-BB (Table 5) increased 8.65% and 13.29%, respectively, after exposure. The lowest increase in the O2 content occurred because of the high proportion O2 in 500-BB-0. This may also be a reasonable explanation for the lowest increase O content of 500-BB (6.98%, determined by elemental analysis; Table 2) after 400 h of weathering, whereas the values of 300-BB and 700-BB increased by 16.04% and 10.01%. The high proportion of O2 limited a further increase in O content due to the production and release of CO2 gas during the continuous photooxidation of O2. 3.4. Thermal analysis TGA was performed to evaluate thermal decomposition and stability characteristics of biochar with quantitative measurement of weight loss over a specific temperature range [38]. The mass loss curves and mass loss rate curves (i.e., TG and DTG) of fresh and accelerated BBs given the increasing operation temperatures are presented in Fig. 4. The mass loss of BBs at the temperature range of 25–180 °C was about 3.24–6.70% due to the removal of absorbed water from the samples. The main pyrolysis temperature range of 300-BBs was 300–600 °C, and their average mass loss during this period was 35.63%. The mass losses of 500-BBs and 700-BBs were moderate at the range of 180–550 °C, and the accelerated thermal decomposition of 500-BBs and 700-BBs were 7

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Fig. 3. O1s XPS spectra of fresh and weathered BBs.

Fig. 4. TG curves of fresh and weathered BBs under N2 at a heating rate of 10 °C·min−1. Fig. 5. DSC curves of fresh and weathered BBs under N2 at a heating rate of 10 °C·min−1.

observed from 550 °C and 750 °C, respectively. The maximum decomposition rate of 300-BBs, 500-BBs and 700-BBs appeared at 343–346 °C, 576–582 °C and 771–772 °C, respectively. The residue yields of 500-BB400 (82.70%) and 700-BB-400 (85.94%) that were exposed to accelerated weathering were obviously lower than the fresh samples (84.49% for 500-BB-0 and 88.67% for 700-BB-0), and there was only a slight decrease in this value for 300-BB-400 (55.92%) compared to 300BB-0 (56.15%). This may be due to increased gasification of weathered

BBs during the process of pyrolysis, which is attributed to the increase in oxygen content of weathered BBs as a result of photooxidation. DSC analysis is convenient, reproducible, and is a useful for characterizing the interaction between biomass components and modification of their chemical structure upon heat treatment [39]. The DSC curves in 8

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Fig. 5 represent the variations in the heat release rate of fresh and 400 h accelerated weathering BBs with the increasing temperature. As shown in Fig. 5, solar radiation has an obviously significant impact on the DSC curves of BBs. A small endothermic peak at temperature of 51 °C was observed in the curve of 300-BB-400 with values lower than that of 300BB-0. This result was due to the greater water evaporation (drying) of 300-BB-400 compared with 300-BB-0; their water contents were 4.20% and 3.24%, according to the mass loss of BBs at the temperature range of 25–180 °C (TGA) [40]. From 101 °C, the curve of 300-BB-400 was above that of 300-BB-0. It may result from the higher oxygen content in 300BB-400 compared to 300-BB-0, meanwhile, the hydrogen is rich in 300BBs. The combination reaction of multiple hydrogen and oxygen functional groups generated more heat, leading to the greater exotherm of 300-BB-400. Similar reasons resulted in the higher exotherm of 500-BB after accelerated weathering at the temperature range of 25–535 °C. However, the DSC values of 500-BB-400 were lower than those of 500BB-0 after the temperature rose to 536 °C. A reasonable explanation for this result is that as more 500-BB-400 decomposed due to the higher oxygen content, this could also be deduced from the TG curves (the residue yield of 500-BB-400 was lower than that of 500-BB-0). The exothermal combination reaction decreased with the depletion of hydrogen functional groups and increasing of temperature, and the endothermal decomposition that gasified CO2, CO, and even NO2 dominated the period of 536–800 °C. The curve of 700-BB-400 was completely under the curve of 700-BB-0. The hydrogen content of 700-BB-0 was only 1.95% according to elemental analysis; this is far less than that of 300BB-0 (4.77%) and 500-BB-0 (3.06%). The major hydrogen in 700-BBs exists as CeH in benzene ring; therefore, it results in less of a combination reaction in 700-BBs as fewer hydrogen and oxygen functional groups are present. Therefore, the oxygen-rich 700-BB-400 decomposed more easily and absorbed more heat than 700-BB-0, which is similar to the reason for the result in 500-BBs at 536–800 °C.

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4. Conclusions BBs obtained by pyrolysis at different final temperatures were used to investigate the effects of solar radiation on the physical and chemical properties of biochar. A higher final pyrolysis temperature led to a greater degree of aromatization and graphitization, for example, the small C/H ratio (0.27), only one absorption peaks left in FT-IR spectrum, and high residue yield in the TGA curve (88.67%) were obtained in 700-BB-0. Solar radiation is an important factor that influences physical and chemical properties. After 400 h of accelerated weathering, the C and N contents of BBs declined, and its O content increased; the average growth rate of the O/C ratio of BBs (as determined by elemental analysis) was 17.04%; the average ΔE was 1.02; the number of oxygen-containing functional groups increased clearly; the average growth rate of C4 and O2 were 13.88% and 86.63%, respectively; and the average decrease in residue yield was 1.58%. These obtained results revealed the photooxidation of BBs during the exposure to damaging radiation. Thus, it can be concluded that the increase in surface oxygencontaining functional groups may lead to improvements in the surface activities of BB for applications including soil-amelioration, efficient pollutant degradation and adsorbing aqueous contaminants. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Acknowledgements This work was funded by National Natural Science Foundation of China (41807088), the Promotion of Project of Forestry Science and Technology of the Chinese Forestry and Grassland Administration (2019-36) and 9

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