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Copyrolysis of Biomass, Bentonite, and Nutrients as a New Strategy for the Synthesis of Improved Biochar-Based Slow-Release Fertilizers

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This is the author's final version of the contribution published as:

[

Xiongfang An, Zhansheng Wu, Junzhi Yu, Giancarlo Cravotto, Xiaochen Liu, Qing Li, and Bing Yu. Copyrolysis of Biomass, Bentonite, and Nutrients as a New Strategy for the Synthesis of Improved Biochar-Based Slow-Release Fertilizers. ACS Sustainable Chem. Eng. 2020, 8, 3181−3190, https://dx.doi.org/10.1021/acssuschemeng.9b06483]

The publisher's version is available at:

[https://pubs.acs.org/doi/abs/10.1021/acssuschemeng.9b06483]

When citing, please refer to the published version.

Link to this full text:

[http://hdl.handle.net/2318/1735245]

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Copyrolysis of Biomass, Bentonite, and Nutrients as a New Strategy for the Synthesis of Improved Biochar-Based Slow-Release Fertilizers

Xiongfang An1, Zhansheng Wu2*, Junzhi Yu3, Giancarlo Cravotto4, Xiaochen Liu5, Qing Li6, and

Bing Yu*7

1 School of Chemistry and Chemical Engineering, Shihezi University, Shihezi 832003, PR China 2* School of Chemistry and Chemical Engineering, Shihezi University, Shihezi 832003, PR China;

School of Environmental and Chemical Engineering, Xi’an Polytechnic University, Xi’an 710048, PR China; orcid.org/0000-0001-8207-9151; Phone: +86 02962779279; Email:

wuzhans@126.com; Fax: +86 029-62779281

3 Junzhi Yu − School of Chemistry and Chemical Engineering, Shihezi University, Shihezi 832003,

PR China

4 Department of Drug Science and Technology, University of Turin, Turin 10125, Italy; Institute of

Translational Medicine and Biotechnology, First Moscow State Medical University (Sechenov), Moscow 109807, Russia

5 School of Environmental and Chemical Engineering, Xi’an Polytechnic University, Xi’an 710048,

PR China

6 School of Environmental and Chemical Engineering, Xi’an Polytechnic University, Xi’an 710048,

PR China

7* Fujian Key Laboratory of Pollution Control and Resource Reuse, School of Environmental

Science and Engineering, Fujian Normal University, Fuzhou 350007, China; orcid.org/0000-0002-3139-0246; Phone: +86 18850405140; Email: bing.yu@uon.edu.au

ABSTRACT: Biochar is gaining increasing attention in the field of agriculture, since it can not only be used as a soil amendment for improving soil quality but also as a promising carrier for slow-release fertilizers. Currently, developing cost-effective, environmentally friendly, and high-performance biochar-based slow-release fertilizers (BSRFs) is still challenging. In this study, we propose a new strategy for the synthesis of improved BSRFs by copyrolysis of biomass (cotton straw), nutrients (K3PO4), and bentonite under microwave irradiation. The results show that the

presence of bentonite during the copyrolysis process is beneficial for improving the slow-release performance of BSRFs. The mechanistic study based on SEM, TEM, XRD, and XPS

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characterizations integrated with the release kinetic study reveal that the presence of bentonite in the pyrolysis process facilitates the formation of a desirable structure within BSRFs to improve the slow-release performance, and the formation of P-related chemical bonds during the bentonite participated pyrolysis process also contributes to the improved slow-release performance for P. Moreover, the slow-release performance of BSRFs agrees very well with their positive effects on the growth of pepper seedlings in pot experiments. The economic assessment suggests that the as-synthesized BSRFs should have the advantage of low production costs. We hope this proposed strategy can bring some inspirations for the development of more promising BSRFs in the future.

KEYWORDS: Leaching, Pot experiment, Higuchi model, Release mechanism

1. INTRODUCTION

Biochar is a light and highly porous carbonaceous material which is derived from the thermochemical decomposition of agricultural residues under limited oxygen conditions.1 Due to

the diversity of biochar regarding to its properties and structures, biochar has the great potential for multipurpose applications, which has gained broad interest from researchers in the fields of agriculture,2−5 carbon sequestration,6−8 environmental remediation,9−12 and carbon-based

materials.13,14 In addition, biochar is low-cost, environmentallyfriendly, and renewable, and these

desirable properties allow it more opportunities for industrial-scale applications. As for its application in agriculture, biochar is a carbon-rich, porous substance, with multiple functional groups, which can be used for the retention of nutrients and water in soil.15−18 To be specific, biochar

can serve as a superior carrier of chemicalfertilizers, which can achieve the slow release of nutrients in soil and thus increase the fertilizer utilization efficiency.19−22 To date, several strategies have been

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Gao and colleagues used the method of sorbing nutrients from a solution to prepare the engineered biocharbased slow-release P fertilizer.23 The effectiveness of this strategy is also validated by Zhou

and his colleagues, and they found that the montmorillonite-biochar composites have the adsorption and slow-release characteristics for NH4+ and PO43− in aqueous solutions.24 Li and his colleagues

developed Mg/Ca-modified biochars which can be used for adsorbing phosphorus from the acid extract of incinerated sewage sludge ash and then applied as a slow-release fertilizer.29 However,

the combination of biochar and nutrients via this method is mainly controlled by the weak van der Waals force, which is unfavorable for the slow-release performance. As for the nutrient-loaded biochar fertilizers prepared by adsorption in aqueous solutions, to further improve their slow-release performance, we previously proposed a strategy using the introduction of superabsorbent polymers for wrapping the nutrient-loaded biochar.25 In this reported work, we found that the superabsorbent

polymers wrapped biochar-based fertilizer has a better slow-release behavior compared to that of the unwrapped one. In addition, we also reported a semi- interpenetrating polymer network slow-release nitrogen fertilizer synthesized via microwave irradiation, in which the addition of polyvinylpyrrolidone can facilitate the formation of intensive hydrogen bonds between polyvinylpyrrolidone and the loaded urea, thus improving its slow-release performance.26 However,

the introduction of polymers into the BSRFs may increase their production cost and give rise to serious environmental issues. Kappler and colleagues proposed a co- composting strategy for preparing BSRFs, which consists of blending biochar with manure or other nutrient-rich feedstock before the erobic composting process.27 Pan and colleagues developed a compounding strategy

for preparing BSRFs via direct mixing chemical fertilizers, bentonite and biochar, and the mixture is left to stabilize for several weeks before the field applications.28 There is no doubt that the

co-composting and compounding strategies are simple, and they should have the advantage of low cost. However, given that co-composting is a biological process and that co-composting needs to go through a stabilization process, these two strategies are theoretically time-consuming. Moreover, both of these approaches proceed under ambient conditions without the high-temperature pyrolysis, so the binding force between biochar and nutrients in obtained BSRFs is not as strong as those BSRFs derived from pyrolysis. Evidently, a promising BSRF should have the following properties: (i) low cost for production, (ii) environmentally friendliness, and (iii) good slow-release performance for nutrients. Therefore, to further advance the development of BSRFs toward the practical application, finding a promising strategy for the synthesis of BSRFs which meets these requirements is still needed.

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BSRFs, in which biomass and P fertilizers are copyrolyzed to obtain composite biochars.30 It was

found that the composite biochars display a slower P- release behavior compared to the pristine biochars, and the authors attribute this to the formation of stable C−O−P or C− P bonds during the copyrolysis process. In theory, the high temperature is beneficial to the formation of strong interactions between nutrients and biochar within the produced BSRFs, but it should result in the great energy consumption for the production of BSRFs and thus increase the BSRFs’ production cost. Zhou and his colleagues reported the copyrolysis of bamboo and montmorillonite to derive montmorillonite−biochar composites used as BSRFs, and they found that montmorillonite can act as a solid acidic catalyst for catalyzing the pyrolysis of bamboo powder to biochar which lowers the required pyrolysis temperature. In addition, Zhou and his colleagues demonstrated that montmorillonite is even able to promote the conversion of cellulose to biochar under hydrothermal conditions.24 As a montmorillonite containing clay, bentonite has

been confirmed to be able to promote the pyrolysis of biomass to biochar under microwave irradiation.31 In our previous work, we found that the presence of bentonite in BSRFs can facilitate

the improvement of the physical and chemical properties of soil.25 Therefore, we envisioned the

introduction of bentonite to the copyrolysis system of biomass and fertilizers for the synthesis of BSRFs may bring several benefits: (1) lowering the production cost of BSRFs, (2) enhancing the slow-release performance of BSRFs, and (3) improving the soil quality when BSRFs are applied. For these reasons, we propose a new strategy for preparing BSRFs by the copyrolysis of biomass, fertilizers, and bentonite.

Herein, we synthesize BSRFs by the copyrolysis of cotton straw (CS), K3PO4, and bentonite under microwave irradiation. The as-prepared BSRFs exhibit a better slow- release performance compared to those BSRFs derived from the pyrolysis in the absence of bentonite. The kinetic study integrated with the material characterization reveal that the slow release for P and K from BSRFs should be dominated by the diffusion mechanism, and the improved slow-release performance for K is attributed to the formation of the desirable structure within BSRFs during the bentonite- participating copyrolysis process, while the excellent slow- release performance of P is attributed to not only the desirable structural properties of BSRFs but also the formation of P related chemical bonds. In addition, the as-prepared BSRFs have a more positive effect on the growth of pepper seedlings, and the cost for the production of BSRFs via the copyrolysis of biomass, nutrients, and bentonite is even lower when compared with that of the BSRFs derived from the pyrolysis in the absence of bentonite. Taken together, this study successfully demonstrated that the technique feasibility for the synthesis of BSRFs with improved slow-release

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performance via this new strategy, but the evaluation of the economic viability of BSRFs synthesized by this strategy is still insufficient due to the lack of scaled-up experimental data.

2. MATERIALS AND METHODS 2.1 Materials.

CS was collected from an agricultural test station in Shihezi University, China (85°94′E and 44°27′′N). Bentonite samples were obtained from Xiazijie deposits in Xinjiang, China. Phosphoric potassium (K3PO4) was purchased from Tianjin Fuchen Chemical Reagent Co., Ltd. (China). All reagents were of analytical grade and used without further purification. The physicochemical properties of bentonite were presented in Table S1, and its determination method was based on our previous work.25,26

2.2 Synthesis of BSRFs.

CS was cut into 2 cm portions, washed with distilled water, and dried in an oven at 70 °C for 24 h. The dried CS was then crushed in a pulverizer and sieved to obtain fine powder samples (<80 mesh). Five CS composites were prepared: 100 wt % CS (CS); 90 wt % CS and 10 wt % bentonite (B10CS); 90 wt % CS and 10 wt % K3PO4 (P10CS); 80 wt % CS, 10 wt % K3PO4, and 10 wt % bentonite (P10B10CS); and 60 wt % CS, 10 wt % K3PO4, and 30 wt % bentonite (P10B30CS). These samples (40 g) were completely dispersed and dissolved in 400 mL of deionized water solutions by mechanical stirring at a speed of 750 rpm for 45 min. The above solutions were dried in a vacuum oven at 90 °C, and then the obtained samples were further crushed and sieved (<80 mesh). CS composites (5 g each) were transferred into a 250 mL quartz tube reactor and then placed in a microwave oven (MM823LA6-NS, Midea, China). Pyrolysis was performed using microwave irradiation at 700 W for 15 min under an oxygen-limited atmosphere (vacuum 0.08 MPa). The exhaust gas was purified using a suction filter bottle, and the reactor was then naturally cooled to the room temperature. Finally, the obtained BSRFs were sieved to a uniform size fraction of 0.5−1.0 mm and stored for later use, which were named as BF, B10BF, P10BF, P10B10BF, and P10B30BF, respectively.

2.3 Characterization of BSRFs.

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electron microscopy (SEM, JSM-6700F, Hitachi, Japan), and the bentonite wrapped P and K particles were examined via transmission electron microscopy (TEM) using a JEOL 2100 fitted with an Oxford EDS system. The C 1s, O 1s P 2p, and K 2p peak spectra were analyzed and observed by X-ray photoelectron spectroscopy (XPS; Escalab 250 Xi, USA) with a resolution below 0.5 eV and XPS peak 4.1 Software. Fourier transform infrared spectra (FTIR; Nicolet Avatar 360, USA) was recorded in the wavenumber range of 4000−400 cm−1 with a resolution of 4 cm−1.

The X-ray diffraction results were recorded on an X-ray diffractometer (XRD; D8 Advance, Bruker, Germany) using Cu Kα (λ = 1.54056) radiation and operated at 40 kV and 40 mA. Mineral and metal element analysis for Ca, Fe, K, Mg, and Zn were performed by using inductive coupled plasma optical emission spectroscopy (ICPOES; Varian 720, USA).

Figure 1. (a) Schematic illustration of the synthesis process of BSRFs. (b) Heating behaviors of CS, B10CS, P10CS,

P10B10CS, and P10B30CS under microwave irradiation. (c) SEM image of P10B30BF at 3000× associated with (d) EDX analysis of the formed needlelike crystals. (e) Elemental mapping images for K and P within the as-prepared

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P10B30BF. (f) HRTEM image of P10B30BF. (g) Contrastive analysis for the yields of BSRFs and volatilisation rates of CS in BSRFs.

The yields of BSRFs (Y) and the volatilization rate of CS (V) in BSRFs were calculated using the following equations:

Y = M1/M2 x 100% (1)

V = 1 –[(M1-M3)/M4] x 100% (2)

where M1, M2, M3, and M4 represent the weights of BFs, CS composites, bentonite, and K3PO4, and

CS, respectively.

2.4 Slow Release Behaviours of BSRFs in Soil.

Samples (1 g) of each of the different BSRFs were placed into 400 mesh nylon bags, buried 5 cm beneath the surface of the soil in a plastic pot containing 200 g of soil, making sure that biochar was in full contact with the soil surface. PK fertilizer in biochar can be transported to the soil with water molecules, and the whole experiment was incubated for 20 days at room temperature. The moisture content of the soil was maintained at about 40% by weighing and adding distilled water. The mesh nylon bags with the BSRFs (1 g) were taken out regularly for the measurement of the residual amounts of P and K in BSRFs. Each sample was repeated three times, with an error of 1−3%. The quantitative analysis of P and K in BSRFs was performed by automated molybdenum blue colorimetry and plasma emission spectroscopy, respectively. The release kinetic analysis of the slow release behavior of P and K in BSRFs was performed by data fitting based on the following equations:32,33

First-order release kinetics:

ln(1-Mt/M∞) = -k1t (3) Higuchi model: Mt/M∞ = k2t1/2 (4) Hixson−Crowell model: (1-Mt/M∞)1/3 = 1 – k3t (5) Baker−Lonsdale model. 3/2[1-(1-Mt/M∞)2/3]-Mt/M∞=k4t (6)

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where Mt/M∞ is the release percentage of nutrients in different time intervals and k0, k1, k2, k3, and k4

are the release rate constants which were calculated by the five models.

2.5 Pot Experiments.

There were four group BSRFs samples; each group had four parallel tests. 200 g soil sample was weighed and placed each pot (40% humidity), and then, 3 g samples of different P10BF, P10B10BF, and P10B30BF were added into the soil. Subsequently, 10 pepper seeds were sown in each pot, and seeds that germinated successfully were observed and counted. After 50 days of seedling growth, the pepper plants were carefully removed from each pot. The plant height, root length, fresh weight, and dry weight of the plants were measured. The leaching loss and utilization efficiency of P and K were evaluated, based on the following method. First, the available P and K contents in 200 g of soil and 3 g of BSRFs were estimated. Then, different pepper plants were dried, pulverized, and digested, and their P and K contents determined. The leaching method was used to wash out the remaining P and K in the pot. Finally, the utilization efficiency and leaching loss of P and K were calculated in terms of the percentage. The physicochemical properties of used soil were presented in Table S2.

2.6 Statistical Analysis.

One-way ANOVA, followed by a Student’s t-test at the 95% confidence level was used to determine the significant differences among different groups.

3. RESULTS AND DISCUSSION

3.1 BSRFs Synthesis and Characterization.

Figure 1a shows the schematic diagram of the synthesis process of BSRFs, which mainly consists of two steps. First, CS, nutrients, and bentonite were well-mixed in a water solution via dipping. Then, the obtained mixture was transferred to a microwave oven for copyrolysis. The heating behaviors of CS and CS composites are illustrated in Figure 1b, and it was found that the temperature for CS was always the lowest among all tested samples under the same power of microwave irradiation. For example, the maximum pyrolysis temperature for CS was only 277 °C, while it was about 315 °C for the other tested samples.

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In addition, the rate of temperature increase for CS was obviously lower than that of the other tested samples. Compared to CS, a superior heating behavior was achieved by B10CS, which can be explained by the fact that bentonite can serve as a heat carrier due to its higher thermal conductivity (1.15 W/m K)34 compared to that of CS (0.059−0.082 W/m K),35 which can provide B10CS with a

more uniform and higher heating rate. By comparison with CS and P10CS, it was found that the addition of K3PO4can significantly improve the heating behavior as well. This is because K3PO4 is

an ionic compound which has good microwave absorption ability.34 As P10B10CS exhibited a

slightly better heating behavior than B10CS and P10CS, a plausible explanation is that the microwave heating of biomass can be divided into two steps: First, the microwave radiation is absorbed by microwave-absorbing materials, such as K3PO4in this work, and then the absorbed heat

is further transferred to biomass through highly thermally conductive materials such as bentonite here.36 Therefore, the presence of bentonite and K

3PO4 in the pyrolysis of CS results better heating

behavior for P10B10CS. In addition, the heating behaviors for P10B10CS and P10B30CS were quite close, and this can be explained by the fact that the microwave heating rate mainly depends on the microwave absorption rate rather than thermal conductivity.34

In fact, a better heating behavior means that less time is needed to reach the target pyrolysis temperature and higher heating efficiency, which facilitates the reduction of the required energy consumption for the production of BSRFs. The morphology and microstructure of P10B30BF was first examined by SEM shown in Figure 1c, which shows a representative SEM image of biochar with intensive pore structures. Moreover, the SEM images for BF, P10BF, B10BF, and P10B10BF also display the representative biochar SEM images (Figure S1). In addition, from the SEM images of P10BF, P10B10BF, and P10B30BF, it was found that some needle like crystals were formed and distributed on the surfaces and pores of these BSRFs as well as the pore structure obvious become smaller. However, the needle like crystals were absent in the SEM images of BF and B10BF, and the pore structures became obviously bigger. Thus, it can be inferred that P and K related crystals were formed after the copyrolysis of CS, bentonite, and K3PO4. The results from the EDX analysis

suggested that the P, O, and K were the dominant elements within these crystals, which further confirmed that P and K fertilizers were successfully loaded on the BSRFs by this copyrolysis process (Figures 1d and S2). Moreover, the corresponding EDX mapping images show the homogeneous distribution of P and K over the entire structure of the obtained P10B30BF (Figure 1e). The TEM image of the asprepared P10B30BF is shown in Figure 1f, and it also exhibited the typical structural characteristics of biochar where carbon atoms were arranged irregularly in a wormlike structure.37 Additionally, the pore volumes and surface areas of P10BF, B10BF,

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P10B10BF, and P30B10BF were measured by BET, as shown in Table S3. It was found that the pore volume and surface area for P10B30BF and P10B10BF were significantly larger than those of P10BF and B10BF, which indicated that the formation of improved porous structure in P10B30BF and P10B10BF was attributed to the synergistic effect of bentonite and K3PO4 during the

copyrolysis process.

Figure 1g shows the yield and volatilization ratio of various BSRFs under the microwave-irradiation-based pyrolysis. The yield ratios for all BSRFs followed an order of P10B30BF ≈ P10B10BF > P10BF > B10BF > BF. The lowest yield ratio was achieved by BF, and this is likely due to the fact that some aliphatic compounds such as alkanes, alcohols, and so on are easily converted into grease and gas at high temperature.38 In fact, the formation of substantial grease and

smoke during the pyrolysis of CS alone was also observed in our experiment.

With the addition of bentonite into CS for copyrolysis, the yield ratio was increased by 14 wt %, and the volatilization ratio was decreased by 6 wt %. The positive effect of bentonite here can be explained by two reasons: (1) Bentonite contains a small amount of alkali and alkaline earth metals (Table S1), which may suppress the endothermic self-gasification reaction between the organic carbon of biochar and CO2.31

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Figure 2. (a) FTIR spectra of CS, BF, P10BF, B10BF, P10B10BF and P10B30BF. (b) XRD spectra of P10BF,

P10B10BF, and P10B30BF. XPS spectra: (c) P 2p P10BF, (d) P 2p P10B10BF, (e) P 2p P10B30BF, (f) K 2p P10BF, (g) K 2p P10B10BF, and (h) K 2p P10B30BF.

In fact, Rawal et al.39 found that the presence of bentonite during the pyrolysis of bamboo biomass

can promote the formation of condensed carbon structure and enhance the cross-linking of the volatile aliphatic fraction into condensed aromatic species at the mild pyrolysis temperature of around 350 °C. Reynolds et al.40 reported that the catalytic surface afforded by bentonite can play a

role in altering this charring behavior resulting in higher O/C. (2) Bentonite is a viscous and inert clay mineral that can adhere to CS and thus block the CS volatilization during pyrolysis.Comparing BF and P10BF, it was found that the presence of K3PO4 can also increase the yield ratio of P10BF.

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coating on the biochar surface, which can promote the carbonization of organic matters (e.g., cellulose and hemicellulose), the dehydration of hydroxyl groups, and the reduction of the formation of bio-oils and volatiles during the copyrolysis process.30 Compared with B10BF and

P10BF, a higher yield ratio and a lower volatilization ratio were achieved by P30B10BF and P10B10BF, which suggests that the coexistence of K3PO4 and bentonite during the copyrolysis

process has a positive e ect toward the production of BSRFs. To be specific, the mixture of Kff 3PO4

and bentonite has the potential to combine the above-mentioned advantages of individual K3PO4 or

bentonite alone for promoting the yield of BSRFs.

FTIR was employed to characterize the functional groups of various as-synthesized BSRFs. As shown in Figure 2a, several strong peaks corresponding to oxygen-containing surface groups were identified in CS sample, which included aliphatic groups (e.g., the stretching vibration of C−O−C at 1153 cm−1) and aldehyde groups such as the stretching vibration of C=O at 1735 cm−1.41,42 However,

these characteristic peaks for oxygen-containing functional groups were significantly weakened or disappeared in all BSRFs, which indicated that the decomposition of biomass occurred during the pyrolysis process. Moreover, the H/C and O/C atomic ratios for each BSRF were decreased compared to that of its precursor, shown in Table S4. For example, when P10B10CS was converted into P10B10BF, the H/C and O/C atomic ratios were dramatically decreased from 0.91 to 0.38 and from 0.10 to 0.05, respectively. It was noted that the smallest reduction amounts for H/C and O/C atomic ratios were observed when CS was converted into BF, which suggested that the presence of K3PO4 or bentonite can promote the conversion of cellulose and lignin into aromatic carbonaceous

BSRFs. This result agreed very well with the yield ratios for various BSRFs shown in Figure 1g, which further demonstrated that both K3PO4 and bentonite can assist the pyrolysis of biomass to

biochar. In addition, the peak at around 1120 cm−1 representing the vibrations of PO43− and the

characteristic peak of asymmetrical P−O−P bending vibrations at around 910 cm−1 were detected in

the spectra of P10BF, P10B10BF, and P10B30BF,43 but they were absent in the FTIR spectra of BF

and B10BF. The results further confirmed that the phosphate related fertilizers were successfully loaded on the BSRFs XRD was employed to understand the structure and composition of various BSRFs. As shown in Figure S3, after CS pyrolysis, the characteristic peaks of cellulose at 22.53° disappeared, indicating that the crystalline cellulose and hemicellulose in CS were decomposed into amorphous biochar.38 In addition, the XRD pattern of P10BF showed characteristic diffraction

peaks of Mg3(PO4)2 (Figure. 2b), suggesting that the Mg element in biochar can play the role in the

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Figure 3. Release ratios of (a) P and (b) K from BSRFs in soil and leaching loss of (c) P and (d) K from BSRFs. XPS

spectra of (e) P and (f) K in P10B30BF before and after 30 day’s leaching experiment.

In fact, a similar result had been reported by Yao et al., and they found that the Mg released from the engineered biochar can facilitate the immobilization of P in the forms of MgHPO4 and

Mg(H2PO4)2.23 The characteristic diffraction peaks of Mg3(PO4)2 were also observed in the XRD

patterns of P10B10BF and P10B30BF, and the intensities of these peaks were much greater than those in P10BF, suggesting that the Mg element in bentonite was also beneficial for the stabilization of P in the obtained BSRFs. Moreover, Ca-(H2PO4)2 was identified by XRD analysis for P10B10BF

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and P10B30BF, which indicated that H2PO4− was a form of P-related speciation existed in the

as-synthesized P10B10BF and P10B30BF.

XPS analysis of BSRFs was performed to further investigate the P and K related speciation after the copyrolysis process. Figure 2c-e shows the XPS spectra of the P 2p for P10BF, P10B10BF, and P10B30BF. Three forms of P related speciation were identified such as HPO42−, PO43−, and H2PO4−.

Compared to P10BF, the molar percentages for PO43− in P10B10BF and P10B30BF were

significantly reduced, but the molar percentage for HPO42− and H2PO4− was increased, suggesting

that the presence of bentonite in the copyrolysis process can alter the distribution of P related speciation. In the above XRD analysis, it was found that the Ca element in bentonite resulted in the formation of Ca(H2PO4)2 during the copyrolysis process. Therefore, a higher molar percentage of

H2PO4− was observed in the BSRFs derived from the bentonite-participating copyrolysis. Moreover,

the XPS analysis further revealed that the other P related speciation such as CaHPO4 or MgHPO4

should also exist in P10B10BF and P10B30BF. Figure 2f-h shows the XPS spectra of the K 2p for P10BF, P10B10BF, and P10B30BF, and only diffluent K+ was identified.

3.2. Slow-Release Behaviors of BSRFs in Soil and Slow Release Kinetics.

Figure 3a shows the results of P release performance from P10BF, P10B10BF, and P10B30BF. It was found that the release ratio of P from P10BF within 12 days reached 100.0%, which was much higher than that of P10B10BF (53.5%). In addition, the release of P from P10B30BF was found to be even slower than that of P10B10BF. To be specific, the release ratio of P from P10B30BF within 20 days was about 82.3%, which was less than that of P10B4310BF (95.6%) within the same time period. The similar experimental result was also observed for K release performance from P10BF, P10B10BF, and P10B30BF, as shown in Figure 3b. To further examine the slow-release performance of P and K from P10BF, P10B10BF, and P10B30BF, leaching experiments were performed as well. As shown in Figure 3c,d, the general trend for the leaching loss of P and K was consistent with their slow-release performance in soil shown in Figure 3a,b.

Although the slow-release performance of P10BF was not as good as that of the other two BSRFs, but it still can be regarded as a slow-release fertilizers when compared with pristine fertilizers. For example, Zhao et al. found that the release of P from TSP- and BM-composite biochars was much slower than from TSP and BM themselves.30 Kim et al. also found that the biochar-embedded

fertilizers exhibited slow K and P release performance, and this was attributed to the nutrient-loading caused by small pore sizes of biochars.44

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Figure 4. (a) Germination ratio, (b) plant height, (c) root length, (d) fresh weight and dry weight for pepper seedlings

fertilized with P10BF, P10B10BF, and P10B30BF. (e) P and (f) K leaching loss and their utilization efficiency by plants.

However, with the addition of the bentonite for copyrolysis, the obtained P10B10BF and P10B30BF exhibited a much better slow-release performance, which suggested that the presence of bentonite was beneficial to the improvement of slow-release performance of BSRFs. In fact, in our above characterization results of BSRFs, we confirmed that the presence of bentonite in copyrolysis can make the porous structure of the formed BSRFs more regular and the size of formed pores and channels in BSRFs become smaller, which was one possible way to explain the slow-release behavior of P10B10BF and P10B30BF. Rawal et al.39 found that the copyrolysis of biomass and

clay can result in the incorporation of nanostructured mineral phases within the porous structure of the biochar. Another reason is that the chemical compositions of P10B10BF and P10B30BF were

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quite different from that of P10BF. As confirmed in the above spectrum analysis by FT-IR, XRD and XPS, the formation of stable P-related chemical bonds (e.g., Mg3(PO4)2) in P10B10BF and

P10B30BF should also contribute to their superior slow-release performance of P when compared with release from P10BF. As for related speciation in the assynthesized BSRFs, the diffluent K-related substances (e.g., KCl or KNO3) were always dominant as confirmed by the above XPS

analysis. Therefore, it is reasonable to infer that the better K slow-release performance of P10B10BF and P10B30BF was attributed to their superior structural properties when compared to P10BF. In addition, Reynolds et al.40 found that the hydrophobic compounds were distributed in the

biochars derived from the copyrolysis of bentonite and biomass, and these hydrophobic compounds are all hydrocarbons (branched, straight-chain alkene/alkane compounds, and aromatics). Therefore, the hydrophobic surface of the asprepared P10B10BF and P10B30BF may also contribute to their improved slow-release performance for P and K.

To further explore the slow-release kinetics and mechanism of P10B10BF and P10B30BF in soil, several mathematical models including first-order kinetics, the Higuchi model, the Hixson−Crowell model, the Baker-Lonsdale model, and the Ritger-Peppas model were applied in this study. As shown in Table S5, the Higuchi model fit the data of P and K release better than other models, suggesting that the release of P and K from P10B10BF and P10B30BF should be dominated by the diffusion mechanism.45 In fact, Borges et al. also found that the Higuchi model better described the

P release in slow-release fertilizers prepared by milling montmorillonite or talc with K2HPO4 when compared with pseudo-first-order and pseudosecond-order.46 In addition, the good fit to the Higuchi

model here can reveal the homogeneous distribution of P and K over the entire structure of P10B10BF and P10B30BF, because the Higuchi model is only suitable for a matrix system in which the solute is homogeneously dispersed in the matrix.47 This agreed very well the above EDX

mapping results shown in Figure 1e, which had demonstrated that the P and K were uniformly distributed in P10B30BF.

The XPS analysis of P10B30BF after the 30 day leaching experiment was performed to gain more insights into the slow release behaviors of BSRFs. As shown in Figure 3e, the majority of P-related speciation in P10B30BF was released. For example, the PO43− was completely released, and the

minor residual of P was attributed to the existence of HPO42- - and H2PO4−-related extremely stable

substances in P10B30BF. Figure 3f shows the XPS spectra of K in P10B30BF, and it was found that no residual K-related stable substances was detected after the leaching experiment. This further confirmed that the slow-release behavior of K in the BSRFs derived from bentonite-participating

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copyrolysis was independent of the chemical interactions, and it should be mainly determined by the structural properties of BSRFs.

3.3. Application of BSRFs for the Growth of Plants.

Pot experiments were performed to further validate the positive effect of the as-prepared BSRFs on plant growth. As shown in Figure 4a-d, at 50 days after sowing, the germination ratio, plant height, root length, fresh weight, and dry weight for pepper seedlings fertilized with P10B30BF or P10B10BF were obviously larger than those fertilized with P10BF. The growth of peppers agreed very well with the slow-release performance of used BSRFs. For example, P10B30BF had the best slow release performance of P and K among all BSRFs in this study, and the pepper seedlings fertilized with P10B30BF achieved the most desirable growth. The superior slow-release performance of P10B30BF can ensure a more adequate supply of P and K for the growth of peppers, as compared with P10BF. In fact, similar pot experimental results have been reported by Yao et al.,23 and they also found that a P-laden BSRF can improve the germination of grass seeds.

In addition, compared to the result from the control experiment without the addition of P-laden BSRF, the length of the grasses with the addition of the Pladen BSRF was found to be much longer, and the leaves of the grasses were greener and stronger.

The utilization efficiencies of P and K for pepper seedlings fertilized by P10BF, P10B10BF, and P10B10BF were evaluated, as shown in Figure 4e,f. The utilization efficiencies of P and K adhered to the following order: P10B30BF > P10B10BF > P10BF. This trend was consistent with their leaching loss performance and also agreed very well with their slow-release performance shown in Figure 3. It can be explained by the fact that a better slow-release performance for a BSRF can lead to a better synchronization with the uptake of pepper plants in this study. As for the practical application of BSRFs for the growth of plants, the coordination of the release ratio of nutrients from BSRFs with the uptake rate of plants is vital. In this study, a slight difference for the slow-release performance between P10B30BF and P10B10BF was observed, suggesting that the slow-release performance of a BSRF can be further tuned by controlling the amount of bentonite.

Finally, the economic assessment for the production of P10B30BF, P10B10BF, and P10BF was also performed, as shown in Figure S4 and Table S6. Compared to the production cost for P10BF derived from the copyrolysis of CS and K3PO4, the costs for P10B30BF and P10B10BF via the

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4. CONCLUSIONS

In summary, we have developed a new strategy for the synthesis of improved BSRFs via the copyrolysis of CS, bentonite, and K3PO4 under microwave irradiation. The assynthesized BSRFs

exhibited a better P and K slow-release performance compared to the BSRF derived from the copyrolysis of CS and K3PO4. As revealed by kinetic analysis, the slow release of P and K from the

as-synthesized BSRFs was dominated by the diffusion mechanism. In addition, the spectroscopic studies integrated with the slow-release tests suggested that formation of the improved porous structure within BSRFs during the bentonite-participating copyrolysis process was mainly responsible for the improved slow-release performance of K. The excellent slow-release performance of P was not only due to the formation of desirable porous structure but also the formation of P-related chemical bonds during the bentonite participated pyrolysis process. Moreover, the assynthesized BSRFs can promote the growth of pepper seedlings better than the BSRF derived from the copyrolysis of CS and K3PO4, and they should have the advantage of low

production costs.

ASSOCIATED CONTENT

Supporting Information

Detailed information on the composition and physicochemical properties of the used bentonite, SEM-EDX images and XRD spectra of various BSRFs, kinetic study for the slow release of P and K from BSRFs and the economic evaluation for production of BSRFs.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This study was financially supported by the National Natural Science Foundation of China (U1803332) and the “Double First Class” Science and Technology Project of Shihezi University (SHYL-GH201801).

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