Transformation, Morphology, and Dissolution of Silicon and Carbon in

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Transformation, Morphology, and Dissolution of Silicon and Carbon in Rice Straw-Derived Biochars under Different Pyrolytic Temperatures Xin Xiao,†,‡ Baoliang Chen,*,†,‡ and Lizhong Zhu†,‡ †

Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310058, China Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou, Zhejiang 310058, China

‡

S Supporting Information *

ABSTRACT: Biochars are increasingly recognized as environmentally friendly and cheap remediation agents for soil pollution. The roles of silicon in biochars and interactions between silicon and carbon have been neglected in the literature to date, while the transformation, morphology, and dissolution of silicon in Si-rich biochars remain largely unaddressed. In this study, Si-rich biochars derived from rice straw were prepared under 150−700 °C (named RS150-RS700). The transformation and morphology of carbon and silicon in biochar particles were monitored by FTIR, XRD, and SEMEDX. With increasing pyrolytic temperature, silicon accumulated, and its speciation changed from amorphous to crystalline matter, while the organic matter evolved from aliphatic to aromatic. For rice straw biomass containing amorphous carbon and amorphous silicon, dehydration ( RS500 > RS > RS350 > RS150 ≈ RS_ash > RS250, which is in accordance with the dissolved silicon quantities shown in Figure S-3a. Based on Figure 3, the long-term release of silicon from the tested samples was divided into three groups. For RS, RS150, and RS250, the released silicon linearly increased with the reaction time, and their time-dependent release curves almost overlapped. For higher temperature biochars (RS350, RS500, and RS700), the released silicon nonlinearly and sharply increased with the dissolution time; silicon dissolution kinetics at 1−30 d were similar, while at 30−60 d, different silicon dissolution behaviors were observed. For the RS_ash sample, the released silicon was at a minimum, and silicon release was unchanged with reaction time. Therefore, silicon release behavior is controlled by the silicon speciation and content as well as by the interaction of silicon and carbon. The corresponding pH values of the dissolution kinetics in Figure S-5 were a little variable over the first ten days, and then relatively stable pH values appeared, increasing with pyrolytic temperature. The solution pH may be another important factor that influences silicon dissolution from biochars. To eliminate the influence of pH on silicon release, repeated extraction by fresh solution was conducted to monitor the silicon dissolution kinetics. The extraction solution pH value

°C), the biochar yields decreased from 93.9% to 37.0%, ash contents increased from 13.2% to 49.2%, and total silicon contents increased from 4.90% to 18.29%, indicating the accumulation of silicon components. To date, silicon transformation and silicon speciation in biochar under different pyrolytic temperatures have not been illustrated. In the current study, the different silicon speciation forms in biochars were initially detected by different extraction solutions and are shown in Figure S-3. The final pH value of the extraction solution for dissolved silicon is shown in Figure S-4. The silicon speciation and contents in rice straw-derived biochars were significantly affected by the pyrolytic temperatures. For a given biochar, the speciation of silicon content (24-h extraction) follows the order of amorphous silicon ≫ dissolved silicon > active silicon > available silicon. The available silicon and active silicon in biochars tend to increase with the carbonization temperature (except for active silicon in RS500). As for dissolved silicon, RS has a siliconreleasing capacity of 3.69 ± 0.10 mg g−1, which rapidly decreased with carbonization temperature (0 → 150 °C → 250 °C) to 0.52 ± 0.06 mg g−1 for RS250. However, as the pyrolytic temperature increased further (250 °C → 700 °C), the dissolved silicon content increased to its highest value at RS700 (7.11 ± 0.32 mg g−1). Obviously, the dissolved silicon content in biochar is 1−2 orders of magnitude higher than that of typical soil (e.g., 0.0745−0.980 mg g−1 for the soil of Shaanxi province and the recommended silicon content of 0.095 mg g−1 in China38,39). Although the RS_ash sample contained extremely high total silicon content, the dissolved silicon in the sample was very low. The amorphous silicon content increased with the carbonization temperature. After normalization by the total silicon (Table 1), interestingly, all the amorphous silicon percentages were approximately 100% except for those for RS700 (77.64%) and RS_ash (14.08%) (Table 1), suggesting that the silicon in the biochars (RS150RS500) and RS biomass were amorphous silicon. The crystal silicon contents in RS700 and RS_ash were approximately 22.36% and 85.92%, respectively. The silicon transformation in biochars was supported by FTIR (Figure 1) and XRD (Figure 2). FTIR spectra displayed

Figure 2. XRD spectra of derived biochars and ash residues before and after water washing. RS250, RS350, RS500, and RS700 refer to biochars; w-RS250, w-RS350, w-RS500, and w-RS700 refer to washed biochars. RS_ash and w-RS_ash refer to ashes before and after washing. (Sy: sylvite, Go: gonnardite, Q: quartz, Cr: cristobalite, Tr: tridymite, Ca: calcite). 3414

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Figure 3. Silicon-dissolving kinetics of RS, derived biochars (RS150, RS250, RS350, RS500, and RS700) and ash residues (RS_ash). The solid to liquid ratio is 50 mg/50 mL. The corresponding pH values in extraction solution are shown in Figure S-5.

was kept at 6.58 ± 0.12 after the first five extractions (Figure S6). The cumulative silicon dissolution amount is displayed in Figure 4. The daily dissolution amount can roughly be regarded

still depended on the pyrolysis temperature. RS showed a slow silicon dissolving rate for the first 10 days but then showed a quadratic increase of cumulative silicon dissolution. A similar trend was found in RS150 but with a shorter period of slow silicon dissolution. RS250 showed a linear curve for cumulative silicon dissolution. RS350, RS500, and RS700 all showed a quadratic increase. Figure 4 displays the changed trend of the final cumulative silicon dissolution order after the 28-day extraction (red arrows).This order would be magnified if adjusted by the total silicon content in biochars (data not shown). Based in Figure 4, the decrease in silicon release from RS to RS250 is possibly caused by the gradual elevation of the compacted structure of carbon components, which has also been suspected as the reason for the slow sorption rate.26 The drastic change in the silicon release from RS250 to RS350 was attributed to the cracking of organic components, causing the exposure of the silicon components. The decreased silicon dissolution from RS350 to RS700 may be associated with the amorphous silicon content within biochars because the final cumulative released silicon quantity at 28 days reflects the potential capacity of silicon release and amorphous silicon is more soluble than crystal silicon. As shown in Table 1, the amorphous silicon percentage for RS350, RS500, and RS700 decreases with the pyrolysis temperature. In other words, the silicon dissolving rate and quantity are tightly connected to the pyrolysis temperature, which leads to variations in pH, structure, silicon form, and other properties. The results of kinetic and continuous extraction experiments suggest that silicon dissolution is affected by many factors such as pH value, silicon form, biochar structure, and others. To determine the other factors that may influence silicon dissolution, the biochars before and after washing were monitored by XRD (Figure 2) and SEM-EDX (Figure 5). These two silicon dissolution experiments show large and longterm silicon dissolution from RS biochars, implying a large

Figure 4. Cumulative silicon dissolution of RS, derived biochars (RS150, RS250, RS350, RS500, and RS700) and ash residues (RS_ash). The solid to liquid ratio is 50 mg/50 mL. Because the pH is stable at 6.58 ± 0.12 after 5 days (Figure S-6), the pH effect on dissolution is negligible.

as the dissolving rate. Hence, the first day’s silicon dissolving amount represents the initial dissolving rate, which is in line with the kinetics results of the batch continuous release experiments. The initial dissolution rate of silicon was in the order of RS700 > RS500 > RS > RS350 > RS150 > RS250, suggesting that the initial dissolving rate decreased at first and then rose with pyrolysis temperature. This phenomenon may be partly explained by the difference in pH values in the extraction solution. Higher pH leads to a higher dissolving rate. As the pyrolysis temperature increases, the pH also increases (Figure S-6), thus resulting in an increased silicon dissolving rate after 250 °C. However, the low silicon release for RS, RS150, and RS250 may be attributed to other reasons (such as structure and silicon speciation). Although the pH effect was ruled out after five days, the cumulative silicon dissolving curves 3415

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Figure 5. SEM-EDX images of RS and biochars (RS250, RS350, RS500, and RS700) before and after washing. The bold red arrow shows the linescan path.

Figure 6. Schematic illustration of the carbon−silicon structural interaction, along with the variations in carbon and silicon form caused by the biochar preparation temperature. The numbers after “RS” represent the carbonization temperatures.

relative intensity of different elements does not directly indicate the elemental content, we set RS as the contrast material. By comparing with RS, the qualitative carbon, silicon, and oxygen contents on the surface of biochar could be analyzed. For RS, the main elements detected on the surface were carbon, silicon, and oxygen, and their signals overlapped. The oxygen line-scan signal decreased with the pyrolysis temperature (RS → RS250 → RS350), reaching almost zero for RS500 and RS700, which is in accordance with the charring processes described with TG-

application potential in agriculture in order to improve the available silicon content in soils. The Carbon−Silicon Interaction within Biochars and Its Effect on Dissolution. The morphologies of carbon and silicon colocated in RS biochars under different temperatures were monitored by SEM-EDX (Figure 5). The line-scan was used to analyze the superficial elemental composition. Compared to mapping, line-scans give a visual picture that can reflect the relative elemental content. Noting that the 3416

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silicon dissolution. As the carbon chars, the silicon crystallizes, which results in a slightly decreased total silicon dissolution. These two processes reflect the protection of carbon by silicon. Overall, the protective interaction between carbon and silicon is the comprehensive result of the structures and morphologies of the carbon and silicon components. The protective interaction between carbon and silicon within biochar particles strongly influences silicon dissolution, further determining the application of biochars as silicon fertilizers in agriculture. On the other hand, this interaction may also dominate carbon stability within biochars, including its bioavailability, sorption, and dissolution. Environmental Implications. Increasing attention has been focused on the use of biochars as soil additives in agriculture. Biochars have proved to be valuable in a variety of processes that use them as the N, P, and K nutrient sources.44 Furthermore, biochars impact the biogeochemical cycling of carbon, nitrogen, and phosphorus.45−47 However, the silicon content in silicon-rich biochars, which is generally neglected, has not raised concerns. The testing of silicon dissolution and the discovery of carbon−silicon protective interactions in the current study indicate the importance of silicon in biochars. Therefore, more studies are needed to examine the siliconfertilizing potential of biochar and its effect on silicon transport from the land ecosystem to the sea ecosystem; large amounts of arable land (including those in China48) are actually silicon depleted, and Si-rich biochar may create a potentially large pool of reactive biogenic silica49 in the silicon geochemical cycle. On the other hand, biochar is considered to be a carbonsequestering material; the protective interaction between carbon and silicon is essential to its stability, and silicon release is a part of the aging process of biochars. Moreover, the dissolution of silicon from biochar in the soil may influence the sequestration of atmospheric carbon dioxide.50 Further studies should focus on inorganic matter in biochars, especially components such as silicon whose morphology may change during biochar preparation.

DTG (Figure S-1). The carbon line-scan signal relative to silicon seemed to increase for RS250 and RS350 due to the decreased oxygen during the pyrolysis period and then decreased for RS500 and RS700 due to the loss of carbon. Taking the washing process into account, we found a siliconpredominant surface for w-RS250 and a carbon-predominant surface for w-RS700. The surfaces of w-RS350 and w-RS500 seem to represent a transitional state between w-RS250 and wRS700. It is easy to deduce that the carbon (or silicon) in the surface was swept away during washing by comparing w-RS250 with RS250 (or w-RS700 with RS700). As proposed by Ma and Takahashi,41 Si-containing compounds are deposited in different tissues of rice, and the special cuticle wax silica layer is located in the rice leaf.42,43 Therefore, we put forward a C−Si structural model for rice straw biochars, which is presented in Figure 6. In this model, for RS, the outermost layer (“C1”) is a cuticle layer that mainly contains carbon, the inner layer (“Si”) is a silicon layer, and the innermost (“C2”) is carbon again. The color of the “C1” and “C2” layers changes from orange to dark with increasing preparation temperature, indicating the change in carbon form. The biochar changes identified with SEM-EDX strongly support the carbon−silicon structural interaction model. To be specific, the silicon-predominant surface for w-RS250 was caused by water that flushed out the cracking “C1” layer and thus exposed the “Si” layer. As for w-RS700, both the“C1” and “Si” layers were swept away by water, and thus the innermost “C2” layer was exposed. Using the carbon−silicon interaction model (Figure 6), the distinct silicon dissolution characteristics in different biochars may easily be illustrated. Silicon dissolution is controlled by the cointeraction of the silicon location, content, and form. In RS, the free water inside dissolves silicic acid and the “C1” layer is loose, which results in a high quantity of dissolved silicon (Figure S-3a) and a relatively fast dissolution rate (Figure 3). The Si dissolution rate increases after an eight-day slow period (Figure 4) because the dissolution of organic matter from the “C1” layer promotes silicon dissolution. RS150 is similar to RS, but dehydration makes RS150 more compact and free from silicic acid polymerization, which leads to a low quantity of dissolved silicon in comparison with RS (Figure 4). The structural difference between RS250 and RS150 is that RS250 starts to pyrolyze (Figure S-1), but the more compact structure and the polymerization of silicic acid in RS250 results in the lowest silicon dissolution quantity (Figure 4). Interestingly, in RS350, carbon cracking makes the “C1” layer tatter, and then the “Si” layer is exposed (Figure 6), bringing about the largest silicon dissolution among the tested biochars (Figure 4). As the pyrolysis temperature increases to 500 °C, the exposed silicon (Figure 5) and the initial dissolving rate in RS500 become greater. However, due to the formation of silicon crystal, the final cumulative silicon dissolution quantity is less than RS350. In RS700, the charring process increases the exposure of silicon to water, which is shown in Figure 5, but the crystal process limits the final cumulative silicon dissolution quantity (Figure 4). The variations in carbon and silicon form in biochars result in the mutual protection between carbon and silicon under different preparation temperatures (Figure 6). In brief, dehydration results in a more compact structure so that the silicon in biochars becomes difficult to dissolve, reflecting the protection of silicon by carbon. The pyrolysis of organic matter exposes the silicon to the solution, leading to fast and extensive

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ASSOCIATED CONTENT

S Supporting Information *

The measurements of different silicon forms in biochar are presented here. The TG-DTG curves of rice straw biomass are presented in Figure S-1, the dissolved carbon quantity of biochars is presented in Figure S-2, the silicon forms content within biochars is presented in Figure S-3, the final pH in extraction solution is presented in Figure S-4, the pH variation corresponding with silicon dissolution kinetics is presented in Figure S-5, and the daily pH of the continuous extraction experiments is presented in Figure S-6. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 0086-571-88982587. Fax: 0086-571-88982587. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Basic Research Program of China (Grants 2014CB441106), the National Natural Science Foundation of China (Grants 21277120, 3417

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21137003, 41071210), and the Doctoral Fund of Ministry of Education China (Grant J20130039).

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