Kaolinite Enhances the Stability of the Dissolvable and Undissolvable

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Environmental Processes

Kaolinite Enhances the Stability of the Dissolvable and Undissolvable Fractions of Biochar via Different Mechanisms Fan Yang, Zi-bo Xu, Lu Yu, Bin Gao, Xiaoyun Xu, Ling Zhao, and Xinde Cao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00306 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on June 27, 2018

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Environmental Science & Technology

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Kaolinite Enhances the Stability of the Dissolvable and Undissolvable Fractions

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of Biochar via Different Mechanisms

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Fan Yanga, b, Zibo Xua, Lu Yua, Bin Gaoc, a, Xiaoyun Xua, Ling Zhaoa, Xinde Caoa, d, *

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a

School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

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b

Department of Environmental Engineering, University of Shanghai for Science and Technology, Shanghai

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200093, China

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c

Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611, USA

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d

Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China

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* Corresponding author tel: +86 21 54743926, fax: +86 21 54740825, e-mail: [email protected]

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1

ACS Paragon Plus Environment


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ABSTRACT

Input of biomass-derived biochar into soil is recognized as a promising method of carbon sequestration.

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The long-term sequestration effect of biochar depends on the stability of both its dissolvable and

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undissolvable fractions in soil, which could be affected by their interactions with soil minerals. Here, walnut

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shell-derived biochar was divided into dissolvable and undissolvable fractions and then interacted with

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kaolinite. Stability of kaolinite-biochar associations was evaluated by chemical oxidation and biological

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degradation. At low dissolvable biochar concentrations, the association was mainly attributed to ‘Ca2+

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bridging’ and ‘ligand exchange’, whereas ‘van der Waals attraction’ was dominant at high concentrations. For

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the undissolvable biochar, kaolinite raised the activation energy of its surface by 22.1%, causing a reduction in

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biochar reactivity. By chemical oxidation, kaolinite reduced the C loss of total biochar by 42.5%, 33.1%

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resulting from undissolvable biochar and 9.4% from dissolvable biochar. Due to the presence of kaolinite, the

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loss of biodegradable C in total biochar was reduced by 49.4%, 48.2% from undissolvable fraction and 1.2%

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from dissolvable fraction. This study indicates that kaolinite can increase the stability of both dissolvable and

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undissolvable biochar, suggesting that kaolinite-rich soils could be a beneficial environment for biochar for

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long-term carbon sequestration.

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INTRODUCTION

Application of biochar to soil can be an effective method of carbon sequestration.1 Exploring the factors

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affecting biochar degradation is essential to estimate the long-term carbon sequestration ability of biochar.

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After its addition into soil, biochar can react with various soil constituents, such as organic matter,2

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microorganisms,3 fauna,4 plant roots, metal cations and clay minerals.5 Previous studies have proven that

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layered silicates, such as kaolinite, are capable of protecting soil native organic matter via the formation of

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organometallic complexes.6 Biochar might obtain similar benefits, since it can be regarded as part of the soil

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carbon pool after its addition into the soil. In general, biochar has two existing forms in the moist soil

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environment, free dissolvable biochar and undissolvable particles.7 Soil minerals probably affect both existing

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forms, albeit with different mechanisms, thus increasing the stability of the total biochar.

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Previous studies have reported that dissolvable biochar has more abundant polar functional groups, lower

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aromaticity, and smaller fused aromatic clusters than undissolvable biochar.8 Thus, the dissolvable biochar is

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expected to be more susceptible to the biodegradation and abiotic reactions.9, 10 Spokas et al.11 suggested the

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loss of fresh dissolvable biochar could account for approximately 10% of the total decomposition. On the

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other hand, the rich O-containing functional groups of dissolvable biochar may favor its interactions with clay

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minerals via a variety of mechanisms including ligand exchange, cation bridges, hydrogen bonding, anion and

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cation exchange, and van der Waals interactions.12 Among them, the most relevant mechanisms involved in

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the mineral-organic associations in natural environments are ‘ligand exchange’, ‘Ca2+ bridging’, and ‘van der

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Waals attraction’ processes,13 and ultimately decrease the bioavailability of native organic matter.14 We

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assumed that soil clay minerals probably have a similar effect that protects dissolvable biochar and inhibits its

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degradation.

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As for undissolvable biochar, minerals primarily react with its surface, which results in an increased

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resistance to degradation. In soil, the association of clay minerals on biochar surfaces can occur even within a

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few months. Clay minerals such as kaolinite have been reported to possess variable charges and a high affinity

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for binding to the native organic matter.15 Our previous study demonstrated that kaolinite could associate with

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biochar particle surfaces to form organometallic complexes,6 which was potentially ascribed to the inerting

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effect of clay minerals on the biochar chemical bonds. Since the three sorption mechanisms used for

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dissolvable biochar are not suitable for solid undissolvable biochar, thermogravimetric analysis (TGA) is used

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to calculate the activation energy of the reaction between undissolvable biochar and O2, which could gauge

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the effect of kaolinite on the oxidation resistance of undissolvable biochar. This method has been widely used

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to estimate the role of inorganic compounds in retarding coal oxidation.16

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We hypothesize that kaolinite can associate with both the dissolvable and undissolvable fractions of

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biochar to increase their stabilities. To test this hypothesis, a set of experiments were conducted to (1)

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determine the stability of kaolinite-associated dissolvable biochar and the mechanisms underlying the stability;

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(2) determine the effects of kaolinite on the stability of the undissolvable biochar and the mechanisms

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underlying these effects; (3) evaluate the contributions of the undissolvable and dissolvable fractions of

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biochar to the total biochar C loss in the presence of kaolinite.

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MATERIALS AND METHODS

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Biochar and Kaolinite.

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Walnut shell was selected as the biochar feedstock because of its low ash content (2.30%, Table S1),

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which can prevent effects from inherent minerals. Details on the biochar production method and the biochar

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characterization are described in the Supporting Information. To prepare the dissolvable and undissolvable 4


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fractions of biochar, the fresh biochar was immersed in deionized water at a ratio of 1:30 (w/v) and stirred for

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48 h, and the mixture was then divided into dissolvable and undissolvable parts by filtration through 0.45-µm

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membrane filters.8 The undissolvable biochar was air-dried, and the concentration of dissolvable biochar was

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determined using a multi N/C 2100 TOC (Germany) analyzer. The main organic compounds of the dissolvable

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biochar were determined by GC-MS (QP 2010, Shimadzu, Japan). All analyses were conducted in duplicate.

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Kaolinite (Al2O3·2SiO2·2H2O) (Analytical Reagent) was selected as a typical soil clay mineral and bought

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from a local company.

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Association of Dissolvable Fraction of Biochar with Kaolinite.

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Three solutions including CaCl2, NaCl, and NaCl-NaH2PO4 were used as background electrolytes to

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investigate the association mechanisms of dissolvable biochar and kaolinite, which is commonly used to

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obtain semi-quantitative estimates of binding mechanisms and link the sorption properties of organic

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molecules to its stability.14 In the presence of CaCl2, dissolvable biochar could be sorbed to minerals via ‘Ca2+

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bridging’, ‘ligand exchange’, and ‘van der Waals attraction’. The formation of ‘Ca2+ bridging’ was impeded

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when Na+ was used instead of Ca2+ as the background cation. The presence of both Na+ and H2PO4- was

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unfavorable to both ‘Ca2+ bridging’ and ‘ligand exchange’ but promoted the ‘van der Waals attraction’

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mechanism.12 According to this definition, the sorption of dissolvable biochar to kaolinite was tested at a pH

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of 4.0 using 0.01 M CaCl2, 0.01 M NaCl and 0.01 M NaCl-NaH2PO4 as the background electrolytes, and the

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initial dissolvable biochar concentrations ranged from 5 to 100 mg C L-1. Briefly, 0.1 g kaolinite was

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transferred into acid-washed, 50-mL centrifuge tubes, and then 25 mL of dissolvable biochar solution with

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three different background electrolytes was added. The suspensions were shaken horizontally in the dark for

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24 h at 95 rpm and 25 ± 1 °C to establish quasi-equilibrium and centrifuged for 30 min at 2575 g, and then the 5



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supernatant was filtered through 0.45-µm membrane filters. The concentrations of dissolvable biochar in the

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equilibrium solutions were analyzed. The sorbed dissolvable biochar was calculated as the difference between

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the dissolvable biochar in the initial solution and the equilibrium solution. The contributions of the three

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mechanisms to the sorption of dissolvable biochar on kaolinite were calculated by the following equations:

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(1)

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(2)

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(3)

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where f (Ca2+ bridging), f (Ligand Exchange), and f (van der Waals) represent the contributions (%) of

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‘Ca2+ bridging’, ‘ligand exchange’ and ‘van der Waals attraction’, respectively, to the sorption of dissolvable

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biochar on kaolinite; q (CaCl2), q (NaCl) and q (NaCl-NaH2PO4) are sorption rates of dissolvable biochar (mg

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C g-1 kaolinite) in CaCl2, NaCl, and NaCl-NaH2PO4 background electrolytes, respectively.

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Meanwhile, in order to test the potential flocculation effect of Ca2+ on dissolvable biochar to retard the

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disturbance for the calculation of the ‘Ca2+ bridging’ contribution, a flocculation experiment of dissolvable

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biochar in 0.01 M CaCl2 solution without kaolinite was conducted. The details of the flocculation experiment

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are described in the Supporting Information.

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After the sorption experiment, the kaolinite-associated dissolvable biochar (under the CaCl2 treatment

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with all three association mechanisms) was collected and air-dried. A scanning electron microscope (SEM)

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with an energy dispersive spectrometer (EDS) was used to explore the association of kaolinite and dissolvable

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biochar. Prior to the SEM-EDS analysis, a milled cross-section of the kaolinite grain was prepared by a dual

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beam focus ion beam (FIB) (ZEISS Auriga SEM/FIB Crossbeam System, Japan). The FIB-SEM-EDS analysis

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was also conducted for the kaolinite particle before sorption as a comparison.

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Association of Undissolvable Fraction of Biochar with Kaolinite.

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The association of the undissolvable biochar particles and kaolinite was achieved by an incubation

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experiment conducted in glass containers (66 mm in internal diameter and 90 mm in height) with three

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different arrangements: (1) 2.5 g undissolvable biochar only; (2) 2.5 g undissolvable biochar + 50 g kaolinite;

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and (3) CaCl2 solutions mixed with 2.5 g undissolvable biochar + 50 g kaolinite. This ratio (5%) of

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biochar/kaolinite was chosen based on the appropriate additive proportion of biochar and the range of real soil

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minerals.17 The doses of CaCl2 were 5% (w/w) of the kaolinite, on the basis of metal elements, which could

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represent real environmental conditions.17, 18 Three replicates were used for each arrangement. During the

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3-month incubation, all treatments were maintained at 25 ± 1 °C without light, and the moisture content was

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maintained at 100% maximum water holding capacity by adding deionized water to compensate for water loss

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every other day. The 100% maximum water holding capacity has an extensive environmental significance

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since it is usually used in the water-logged cultivation for the paddy soil in the reality,19 besides, a rain event

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or irrigation could result in the soil moisture obtaining 100% maximum water holding capacity over a period

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of time.20 After incubation, the undissolvable biochar particles were carefully separated from the minerals

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using a 0.5-mm sieve, nippers and ultrasonification. The isolated undissolvable biochar particles were then

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washed three times with deionized water, air dried, and stored dry in an airtight container. The association of

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kaolinite onto undissolvable biochar surface was detected by X-ray photoelectron spectroscopy (XPS), and the

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association energy was determined by a thermogravimetric analysis (TGA).21 Details of these two analyses

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were described in the Supporting Information.

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Stability of Dissolvable and Undissolvable Biochar after Association with Kaolinite.

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Two methods were applied to determine biochar stability. One was a simulated long-term stability 7



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method using the K2Cr2O7 oxidation treatment22 to determine the chemical stability of the biochar. The other

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method was a simulated mineralization experiment using microbe-induced decomposition10 to evaluate the

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biological stability of the biochar. The details of stability measurement experiments were described in the

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Supporting Information.

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The amount of CO2 evolution from the dissolvable biochar sorbed on kaolinite or the kaolinite-associated undissolvable biochar was fit to a single-exponential model by the following equation: (4)

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where St is the percentage of mineralized C (%); Smax is the maximum percentage of mineralizable C for

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the dissolvable or undissolvable biochar; k is the apparent first-order mineralization rate constant for the

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dissolvable or undissolvable biochar; and t is the incubation time (d).

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RESULTS AND DISCUSSION

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Dissolvable Biochar Associated with Kaolinite via Different Binding Mechanisms.

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Most of the components in dissolvable biochar were chain or monocyclic compounds with low

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aromaticity (Table S2), similar to the result found by Qu et al.23 The richness of O-containing functional

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groups in the dissolvable biochar showed a strong complexing ability.24 Note that the extraction method using

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ethyl acetate to illuminate the constituents of the dissolvable biochar was limited by the distribution

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coefficients of the extracts. For example, carboxylic acid was less detected in this study, which was

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inconsistent with the observation of the previous study,23 probably because the carboxyl was highly

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hydrophilic and difficult to extract by ethyl acetate.

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The sorption of the dissolvable biochar by kaolinite in the three background electrolyte solutions (CaCl2, NaCl, NaCl-NaH2PO4) was shown in Figure 1. The sorption in the CaCl2 solution was larger than that in the 8


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NaCl solution, probably due to the formation of Ca2+ bridges on the kaolinite surface. The Figure S1 showed

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that in 0.01 M CaCl2, the flocculation extent of the dissolvable biochar was 26.5% at pH 7, whereas it sharply

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decreased to 1.10% at pH 4, which suggested that at pH 4, the Ca2+ induced flocculation of dissolvable

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biochar could be negligible (the removal rate of dissolvable biochar was 36.5%). In other words, the reduction

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of dissolvable biochar in sorption experiment was mainly ascribed to ‘Ca2+ bridging’. This result was similar

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to previous study that 17.9% of the soil native organic matter flocculated upon the addition of CaCl2, whereas

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it decreased to 1.7% at pH 4.14 The visualization by SEM-EDS mapping (Figure S2b) showed that the main

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elements of the layered silicate, such as O, Al, and Si, were concentrated on the kaolinite grain core, and the C

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from the dissolvable biochar had diffused into the grain interior, which could imply the association of

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dissolvable biochar with kaolinite. Further observations showed that the Ca was evenly distributed on the

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cross-section (Figure S2b), which meant that the Ca2+ probably acted as a cation bridge connecting the

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kaolinite and dissolvable biochar. In contrast, the C of the kaolinite particle cross-section before association

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with dissolvable biochar was barely detectable and even less for Ca (Figure S2a). Majzik et al.25 also obtained

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this binding phenomenon for humic acids and montmorillonite in CaCl2 background solution, and ascribed it

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to Ca2+ bridging. Sowers et al.26 found the presence of 10 mM CaCl2 resulted in the sorption of leaf-litter

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extractable dissolvable organic matter to the ferrihydrite increasing by 20%. They also conducted morphology

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analysis for the natural DOM and found the dissolvable organic carbon was highly associated with Ca

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(R2=0.91).27 In the NaCl-NaH2PO4 treatment, the sorption was even less than that in the NaCl solution (Figure

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1). This is probably due to the competition of the H2PO4- with the dissolvable biochar for sorption on kaolinite.

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This result was similar to those of previous studies that showed phosphate suppressed the sorption of

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low-molecular-weight organic compounds28, 29 and humic acids30 to minerals. The above results also

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demonstrated that the use of different background electrolytes to simulate the three binding mechanisms was 9



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able to reveal the formation of kaolinite-dissolvable biochar associations in natural environments.12, 31

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The contributions of the three mechanisms to the sorption of dissolvable biochar on kaolinite were

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affected by the initial concentrations of dissolvable biochar (Figure 2). Overall, the contributions of the ‘Ca2+

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bridging’ and ‘ligand exchange’ mechanisms decreased with the increase in initial concentrations of

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dissolvable biochar; but the opposite trend was observed for the ‘van der Waals attraction’ mechanism. At low

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dissolvable biochar concentrations, ‘Ca2+ bridging’ and ‘ligand exchange’ were the main contributors to the

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association, whereas ‘van der Waals attraction’ was dominant at initial high concentrations. Specifically, at

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lower biochar concentrations (5-20 mg C L-1), ‘Ca2+ bridging’ and ‘ligand exchange’ accounted for 56-75%

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and 24-30% of the association, respectively, while ‘van der Waals attraction’ contributed to less than 5%;

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However, the contributions of ‘Ca2+ bridging’ and ‘ligand exchange’ at higher biochar concentrations (60-100

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mg C L-1) were only 10-25% and 3-6%, respectively, while ‘van der Waals attraction’ accounted for 70-84%.

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At the medium biochar concentrations (30-40 mg C L-1), the three mechanisms accounted for similar

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proportions. At low loadings (lower biochar concentrations), the dissolvable biochar primarily bound to

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‘high-affinity’ sites via ‘Ca2+ bridging’ and ‘ligand exchange’, whereas at high loadings (higher biochar

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concentrations), the dissolvable biochar had to bind to less reactive sites via ‘van der Waals attraction’.32

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Further, Kleber et al.33 proposed a discrete zonal structure for organo-mineral associations in soils consisting

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of, from the inside to the outside, a contact zone, a hydrophobic zone, and a kinetic zone arranged around the

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kaolinite surface. Similar to the soil organic matter, the dissolvable biochar also consisted of a heterogeneous

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mixture of compounds that displayed a range of amphiphilic properties that was likely to self-organize in an

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aqueous solution.8 Therefore, with an increase in the dissolvable biochar concentration, more hydrophobic and

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aromatic molecules would gather in the hydrophobic zone via ‘van der Waals attraction’, although they might

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not attach to the kaolinite surface directly. Lin et al.34 found the hydrophobic fraction of dissolvable biochar 10


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decreased to an undetectable level in sandy loam soil, which was likely due to the strong affinity of

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hydrophobic dissolvable biochar towards clays.35 Meier et al.36 suggested biochar particles with larger

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molecular weights and aromatic moieties were preferentially sorbed to clay minerals such as kaolinite.

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The strength of the above kaolinite-dissolvable biochar associations was affected by the pH and solution

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salinity. In most cases, a lower pH and higher salinity are favorable for the sorption of organic matter on

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minerals,37, 38 which explains why treatment with Ca2+ resulted in the maximum sorption rate and why the

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dissolvable biochar solution in this study was adjusted to pH = 4 before sorption.

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Association with Kaolinite Enhanced the Stability of Dissolvable Biochar.

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The chemical and biological stabilities of biochar were evaluated by the K2CrO7 oxidation and microbial

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incubation methods, respectively.39 After K2CrO7 oxidation, the C loss of the free dissolvable biochar was 98.0%

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(Figure 3a), which demonstrated that the dissolvable biochar was susceptible to abiotic oxidation.10 However,

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under the NaCl-NaH2PO4 treatment, the C loss was reduced to 80.2%; these loss values were even lower

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under the CaCl2 and NaCl treatments (both were approximately 66.0%). The association with kaolinite

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reduced the C loss of the dissolvable biochar by 18.6-33.3%, compared to that of the free dissolvable biochar,

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which meant the dissolvable biochar that associated with kaolinite via the three binding mechanisms had a

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higher resistance to chemical oxidation. The chemical oxidation resistance associated with the three

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mechanisms was in the following order: ‘Ca2+ bridging’ ≈ ‘ligand exchange’ > ‘van der Waals attraction’. It

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seemed that the chemical bonds of the ‘Ca2+ bridging’ and ‘ligand exchange’ mechanisms were much stronger

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than those of the ‘van der Waals attraction’ mechanism. The effect of Ca2+ was also proposed by Clough et

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al.,40 who found that more organic matter was protected in calcareous soils than in non-calcareous soils, and

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the presence of exchangeable Ca reduced the loss of organic material upon photooxidation by approximately

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7.0% due to the effect of ‘Ca2+ bridging’.

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The bio-mineralization kinetics of the kaolinite-associated dissolvable biochar are shown in Figure S3a.

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The results of the mineralization were fitted using the single exponential model (Equation 4) and summarized

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in Table 1. After a rapid increase during the first week, the mineralization of the free dissolvable biochar

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leveled off after 28 days, with approximately 9.0% of the initial dissolvable biochar evolving as CO2 during

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the entire 56-day incubation period (Figure S3a, Table 1). The CaCl2, NaCl, and NaCl-NaH2PO4 treatments

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reduced the mineralization of C by 47.9-85.3% compared to the free dissolvable biochar, although the

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mineralization rate constants increased slightly (Table 1). This result demonstrated that associations with clay

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minerals improved the biodegradation resistance of dissolvable biochar. Previous studies have yielded similar

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conclusions, reporting that the sorptive interaction of organic matter with minerals is an important

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stabilization pathway against the biodegradation of amino acids,41 enzymes,42 and low-molecular-weight

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organic compounds43, 44 in soil environments. In some cases, the sorbed soil organic matter was 76.0% less

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mineralizable than the dissolvable organic matter.14

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It was interesting to note that, according to the maximum percentage of mineralizable C (Smax), the

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resistance of kaolinite-associated dissolvable biochar to biodegradation was in this order: ‘ligand exchange’ ≈

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‘van der Waals attraction’ > ‘Ca2+ bridging’ (Table 1). This trend was contrary to the results of the chemical

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oxidation. It is likely that the biodegradation resistance of dissolvable biochar is not only related to chemical

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bonds but also dependent on sorption environmental factors, such as the desorption behavior, solution acidity,

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mineral self-dissolution and microbial nutrition in the solution; (1) Mikutta et al.14 observed a significantly

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positive correlation between the mineralization rate constant and the fraction of desorbable organic matter,

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which implied that the desorption of organic matter was the rate-limiting step of the mineralization process. 12


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The organic matter sorbed via Ca2+ was more desorbable than that sorbed via Na+. Because the organic

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matters sorbed via Na+ were often aromatic molecules with more functional groups, which could

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simultaneously bind with mineral surface.14 The cumulative effect of multiple bindings was very large, and the

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sorption would become effectively irreversible due to the improbability that all the bonds were broken

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simultaneously.45 The same process could be applied to the dissolvable biochar, i.e., desorption controlled the

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mineralization, and organic matter bound by ‘Ca2+ bridging’ was more desorbable than that bound by ‘ligand

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exchange’; (2) the self-dissolution of the clay materials was affected by the solution conditions.46 The

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presence of Ca2+ probably affected the kaolinite self-dissolution, exposing more soil microbes to the

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dissolvable biochar; (3) it was possible that the Ca2+ in the solution provided nutrients for the microbes and

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catalyzed the breakdown of organic matter.

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In a real soil environment, the mineralization rates of mineral-associated dissolvable biochar could be

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lower than those measured in this experiment. This is because the biochar used here was fresh and not aged in

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the environment, and the binding strength of mineral-dissolvable biochar associations increases with time.47

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Association with Kaolinite Increased Surface Activation Energy of Undissolvable Biochar.

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The XPS analysis (Figure S4) showed a variety of elements including C, O, Si, Al, and Ca concentrated

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on the biochar surface in the undissolvable biochar + kaolinite + CaCl2 treatment, whereas the blank

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undissolvable biochar contained only C and O, implying that the kaolinite and CaCl2 could interact with

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undissolvable biochar surface during 3-month incubation. The possible formation of biochar-minerals

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complexes could lead to certain changes of undissolvable biochar properties, such as the surface activation

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energy.

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The thermo-gravimetric analysis initially showed a slight decline in mass for all treated or untreated

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undissolvable biochar with increasing temperature (Figure 4), mainly due to the removal of moisture (water

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loss stage). Then, a progressive increase in mass followed as a result of O2 chemisorption and the formation of

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solid oxygenated complexes (O2 chemisorption stage). The mass reached a maximum at ~325 °C, and, as the

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temperature increased further, the mass began to decline rapidly (quick decomposition stage). Compared to

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the control, the temperatures of the peak masses under the kaolinite and kaolinite + CaCl2 treatments shifted to

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higher values, from 288 °C to 303 °C and 304 °C, respectively. Similar results were found in studies

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investigating coal oxidation at low temperatures.16,45 For example, Slova´k et al.16 found that the presence of

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CaCl2 caused the starting temperature of the quick decomposition to shift from 276 °C to 278 °C at a heating

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rate of 20 °C min-1. Zhan et al.48 found that the coal mass reached a maximum value at ~265 °C, with a mass

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increase of 3.6%; however, the extent of the mass increase decreased to 2.8% with the addition of Na3PO4.

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Therefore, the effect of minerals on the low-temperature oxidation process of coal depended on the mineral

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species and amounts, which was also observed for the effect of kaolinite on undissolvable biochar. The

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kaolinite associated with biochar may act as a negative catalyst to increase the activation energies for the

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oxidation reactions and modify the reaction steps.48

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To evaluate the potential of undissolvable biochar to react with O2, the data points of the O2

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chemisorption stage were fit by first-order kinetics, and the activation energy was calculated by Equation S1.

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The first-order kinetic curves of the kaolinite-associated undissolvable biochar used to calculate the activation

287

energy are presented in Figure S5. Compared to the control, kaolinite increased the activation energy of the

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undissolvable biochar by 22.1% (Table 2). Similar to the dissolvable biochar scenario, the presence of Ca2+

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seemed to weaken the strengthening effect of kaolinite and only increased the activation energy by 12.8%.

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The effect of Ca2+ on carbon sequestration was inconsistent with the results of previous studies. Sujanti et al.49 14


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conducted a laboratory investigation into the role of 14 additives on the low-temperature oxidation and

292

spontaneous combustion of a Victorian brown coal and found that CaCl2 inhibited spontaneous combustion,

293

whereas CaCO3 and Ca(OH)2 promoted spontaneous combustion. After a biochar soil amendment, Ca2+ could

294

transform into various occurrence forms, such as CaCO3, and attach to the undissolvable biochar surface,6

295

especially in alkaline environments. Therefore, whether calcium has an inhibitory effect on the oxidation of

296

undissolvable biochar depends on the contributions of the calcium species in real soils.

297

The increased activation energy of the undissolvable biochar was likely due to the kaolinite-induced

298

changes in the side-chain functional groups of biochar, leading to a decrease in biochar reactivity. For example,

299

Zhan et al.48 indicated that a metal salt addition could promote hydroxyl to transform into ether, which was the

300

most stable group among the O-containing groups. In addition, minerals might interrupt energy transmission

301

during the oxidation of undissolvable biochar because once the oxidation of active groups that can react

302

exothermically with O2 is interrupted by minerals, no further energy can be generated, and the subsequent

303

oxidation of other groups is slowed down or halted entirely.50, 51 Moreover, minerals might reduce the number

304

of free radicals generated during the oxidation of the alkyl groups,52 leading to the inhibition of further

305

oxidation.

306 307

Association with Kaolinite Enhanced the Stability of Undissolvable Biochar.

308

The association of undissolvable biochar with kaolinite reduced the C loss by 44.2% compared to that of

309

the control during the K2CrO7 oxidation (Figure 3b), and the presence of Ca2+ had no significant effect on this

310

protective effect. Regarding the biodegradation, the association with kaolinite reduced the C loss by 51.6%

311

compared to that of the control, although the single exponential model was likely not suitable for the

312

undissolvable biochar (R2 = 0.183-0.690, Table 1), as Figure S3b shows that the undissolvable biochars 15



313

associated with kaolinite released less C. Note that the presence of Ca2+ seemed to retard the protective effect

314

of kaolinite, regardless of whether the biochar was dissolvable or undissolvable. The protection of

315

undissolvable biochar particles by the association with minerals has also been found in previous studies.

316

Nguyen et al.53 analyzed soils from forests with frequent fires over the last century and found that Si and Al

317

associated with the biochar particle surfaces more quickly in the first 30 years after a fire. Kleber et al.54

318

suggested that, in acid soils, organic matter is preferentially protected by interactions with poorly crystalline

319

minerals. Eusterhues et al.55 found that, in the subsoil, short-range order Al silicates play a role in forming

320

mineral-bound organic carbon. Their results indicate that the interaction dynamics between mineral elements

321

and biochar could be interpreted as evidence of biochar encapsulation, which represents protection by soil

322

minerals.

323

The increasing stability of the undissolvable biochar associated with kaolinite was mainly because the

324

activation energy of the interactions between the undissolvable biochar surface and O2 was elevated by

325

kaolinite (Table 2). The increase in the activation energy resulted in the decrease in the frequency of effective

326

collisions occurring between the undissolvable biochar surface and O2 molecules, based on molecular gas

327

dynamics; thus, the chemisorption rates of O2 by the undissolvable biochar decreased at a macroscale. This

328

result was supported by our previous study, which showed that, after a 3-month incubation with kaolinite and

329

CaCl2, the C-C/C=C/C-H of the biochar particle surface increased from 63.8% to 73.4% and the

330

C-O/C=O/COOH of the biochar particle surface decreased from 36.3% to 26.6% compared to the control.6

331

Moreover, in the previous study the inhibitory effect of the kaolinite treatment without CaCl2 was higher than

332

that of the kaolinite treatment with CaCl2. This could be well explained by the result in this study showing that

333

the presence of Ca2+ inhibited the increase in the activation energy (Table 2).

16


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334


Another possible reason for the increased stability of the undissolvable biochar by kaolinite was that the

335

fine kaolinite particles attached to the undissolvable biochar surface and blocked channels, providing physical

336

protection for the undissolvable biochar and restricting the O2 or microbes to making contact with the biochar

337

surface or entering the inner pores. The physical protection of biochar by minerals is an important factor for

338

the stability of undissolvable biochar in soil. For example, Lehmann et al.56 proposed that physical protection

339

could increase the turnover time of soil organic matter by several decades, which was demonstrated by

340

radiocarbon and 13C NMR analyses. Foster et al.57 suggested that the substrate protection that occurred by

341

isolation in pores involved pore sizes that were too small for micro-organisms to enter. McGill et al.58

342

suggested that pores with a diameter less than 0.48 µm were small enough to exclude soil microorganisms.

343

344

A Roughly Quantified Estimation of C Loss for the Total Biochar.

345

The above results demonstrated that the association with kaolinite could reduce the C loss of undissolvable

346

and dissolvable fractions of biochar in both chemical oxidation and biodegradation conditions. By considering

347

the proportion of dissolvable and undissolvable fractions of the total biochar (Table S1), the contributions of

348

the undissolvable and dissolvable fractions to the total C loss could be roughly estimated (Table S3, Figure

349

S6). For chemical oxidation, kaolinite reduced the C loss of the total biochar by 42.5%, 33.1% resulting from

350

the reduction of undissolvable biochar and 9.4% resulting from the reduction of dissolvable biochar. For

351

biodegradation, kaolinite reduced the C loss of the total biochar by 49.4%, 48.2% resulting from the reduction

352

of undissolvable biochar and 1.2% resulting from the reduction of dissolvable biochar. These results indicate

353

that, although the protection of the total biochar by kaolinite was mainly attributed to the stabilization of the

354

undissolvable biochar, the stabilization of the dissolvable biochar by kaolinite should not be ignored. In

355

addition, kaolinite stabilized the undissolvable biochar mainly by raising its resistance to biodegradation,

17



356

357

358

while it stabilized the dissolvable biochar mainly by raising its resistance to chemical oxidation.

ENVIRONMENTAL SIGNIFICANCE

Carbon sequestration is a primary function of biochar, and the stability of biochar in soil is an important

359

factor affecting the carbon sequestration efficiency. Most previous studies have considered biochar as a whole

360

with a homogeneous structure, while few studies have researched the dissolvable and undissolvable fractions

361

of biochar. In addition, even less work has been done on the interactions of these two biochar fractions with

362

soil components and their subsequent effects on the stability of the total biochar. In this study, the effects of

363

interactions between kaolinite and two fractions of walnut shell-derived biochar on the stability of the total

364

biochar were explored, producing further insights into the comprehensive assessment of the carbon

365

sequestration efficiency of biochar.

366

This study demonstrated that ‘Ca2+ bridging’, ‘ligand exchange’ and ‘van der Waals attraction’

367

mechanisms contributed to the association of dissolvable biochar and kaolinite, which increased the stability

368

of dissolvable biochar. Kaolinite enhanced the stability of undissolvable biochar by raising the activation

369

energy of the interaction between the undissolvable biochar surface and O2, while soil cations such as Ca2+

370

seem to prohibit the effect of kaolinite on enhancing biochar stability. These findings indicate that clayey soils

371

such as Argi-Udic Ferrosols that are rich in kaolinite and have a low Ca2+ content could be a beneficial

372

environment for biochar in terms of long-term carbon sequestration. It should be noted that the prohibitory

373

effect of Ca2+ should be demonstrated in more types of clay minerals and the prohibiting mechanisms should

374

be further investigated.

18


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375

376 377 378 379

380

381 382

383

384 385


AUTHOR INFORMATION

Corresponding Author *Telephone: +86-21-54743926. Fax: +86-21-5474-0825. E-mail: [email protected]. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS

This work was supported in part by the National Natural Science Foundation of China (No. 21537002, 21607099, 21777095) and China Postdoctoral Science Foundation (No. 2016 M600317).

ASSOCIATED CONTENT

Supporting Information Biochar production and characterization. Experiment of Ca2+ induced flocculation. XPS analysis for the

386

association of undissolvable biochar and kaolinite. TGA analysis and activation energy calculation. The

387

stability measurement experiments. Selected properties of the biochar (Table S1). The main components of the

388

dissolvable biochar (Table S2). The decreased C loss of different fractions of biochar under chemical

389

oxidation and biodegradation conditions (Table S3). Extent of flocculation of dissolvable biochar induced by

390

CaCl2 (Figure S1). SEM elemental mapping of the kaolinite particle cross-section before and after association

391

with dissolvable biochar (Figure S2). Accumulated C mineralization of dissolvable biochar sorbed via three

392

binding mechanisms and kaolinite-associated undissolvable biochar (Figure S3). The XPS result of the

393

undissolvable biochar and the kaolinite-associated undissolvable biochar (Figure S4). First-order kinetic

394

curves of the kaolinite-associated undissolvable biochar used to calculate the activation energy (Figure S5).

395

The decreased ratio of C loss affected by kaolinite under chemical oxidation and biodegradation conditions

396

(Figure S6).

397 19



REFERENCE

1. Woolf, D.; Amonette, J. E.; Street-Perrott, F. A.; Lehmann, J.; Joseph, S. Sustainable biochar to mitigate global climate change. Nat. Commun. 2010, 1 (5), 56. 2. Cross, A.; Sohi, S. P. The priming potential of biochar products in relation to labile carbon contents and soil organic matter status. Soil Biol. Biochem. 2011, 43 (10), 2127-2134. 3. Jones, D.; Murphy, D.; Khalid, M.; Ahmad, W.; Edwards-Jones, G.; DeLuca, T. Short-term biochar-induced increase in soil CO2 release is both biotically and abiotically mediated. Soil Biol. Biochem. 2011, 43 (8), 1723-1731. 4. Liesch, A. M.; Weyers, S. L.; Gaskin, J. W.; Das, K. Impact of two different biochars on earthworm growth and survival. Ann. Environ. Sci. 2010, 4 (1), 1. 5. Joseph, S.; Camps-Arbestain, M.; Lin, Y.; Munroe, P.; Chia, C.; Hook, J.; Van Zwieten, L.; Kimber, S.; Cowie, A.; Singh, B. An investigation into the reactions of biochar in soil. Soil Res. 2010, 48 (7), 501-515. 6. Yang, F.; Zhao, L.; Gao, B.; Xu, X.; Cao, X. The Interfacial Behavior between Biochar and Soil Minerals and Its Effect on Biochar Stability. Environ. Sci. Technol. 2016, 50 (5), 2264-2271. 7. Lehmann, J.; Joseph, S. Biochar for environmental management: science and technology; Routledge, 2012. 8. Qu, X. L.; Fu, H. Y.; Mao, J. D.; Ran, Y.; Zhang, D. N.; Zhu, D. Q. Chemical and structural properties of dissolved black carbon released from biochars. Carbon 2016, 96, 759-767. 9. Singh, B. P.; Cowie, A. L.; Smernik, R. J. Biochar Carbon Stability in a Clayey Soil As a Function of Feedstock and Pyrolysis Temperature. Environ. Sci. Technol. 2012, 46 (21), 11770-11778. 10. Zimmerman, A. R. Abiotic and Microbial Oxidation of Laboratory-Produced Black Carbon (Biochar). Environ. Sci. Technol. 2010, 44 (4), 1295-1301. 11. Spokas, K.; Novak, J. M.; Masiello, C. A.; Johnson, M. G.; Colosky, E. C.; Ippolito, J.; Trigo, C. Physical disintegration of biochar: An overlooked process. Environ. Sci. Technol. Lett. 2014, 1 (8), 326-332. 12. Arnarson, T. S.; Keil, R. G. Mechanisms of pore water organic matter adsorption to montmorillonite. Mar. Chem. 2000, 71 (3-4), 309-320. 13. Yu, S.; Liu, J.; Yin, Y.; Shen, M. Interactions between engineered nanoparticles and dissolved organic matter: A review on mechanisms and environmental effects. J. Environ. Sci. 2018, 63 (1), 198-217. 14. Mikutta, R.; Mikutta, C.; Kalbitz, K.; Scheel, T.; Kaiser, K.; Jahn, R. Biodegradation of forest floor organic matter bound to minerals via different binding mechanisms. Geochim. Cosmochim. Acta 2007, 71 (10), 2569-2590. 15. Chen, Y.; Senesi, N.; Schnitzer, M. Information Provided on Humic Substances by E4/E6 Ratios1. Soil Sci. Soc. Am. J. 1977, 41 (2), 352-358. 16. Slovák, V.; Taraba, B. Urea and CaCl2 as inhibitors of coal low-temperature oxidation. J. Therm. Anal. Calorim. 2012, 110 (1), 363-367. 17. Li, F.; Cao, X.; Zhao, L.; Yang, F.; Wang, J.; Wang, S. Short-term effects of raw rice straw and its derived biochar on greenhouse gas emission in five typical soils in China. Soil Sci. Plant Nutr. 2013, 59 (5), 800-811. 18. Nakamura, S.; Hiraoka, M.; Matsumoto, E.; Tamura, K.; Higashi, T. Humus composition of Amazonian dark earths in the middle Amazon, Brazil. Soil Sci. Plant Nutr. 2007, 53 (3), 229-235. 19. Zhang, Y.; Xu, W.; Duan, P.; Cong, Y.; An, T.; Yu, N.; Zou, H.; Dang, X.; An, J.; Fan, Q. Evaluation and simulation of nitrogen mineralization of paddy soils in Mollisols area of Northeast China under waterlogged 20


Page 20 of 29

Page 21 of 29


incubation. Plos One 2017, 12 (2), e0171022. DOI: 10.1371/journal.pone.0171022. 20. Hillel, D. Introduction to Soil Physics; Academic Press: New York, 1982; Vol. 364. 21. Cheng, J.; Wang, X.; Si, T. T.; Zhou, F.; Zhou, J. H.; Cen, K. F. Ignition temperature and activation energy of power coal blends predicted with back-propagation neural network models. Fuel 2016, 173, 230-238. 22. Crombie, K.; Mašek, O.; Sohi, S. P.; Brownsort, P.; Cross, A. The effect of pyrolysis conditions on biochar stability as determined by three methods. GCB Bioenergy 2013, 5 (2), 122-131. 23. Qu, X.; Fu, H.; Mao, J.; Ran, Y.; Zhang, D.; Zhu, D. Chemical and structural properties of dissolved black carbon released from biochars. Carbon 2016, 96, 759-767. 24. Philippe, A.; Schaumann, G. E. Interactions of dissolved organic matter with natural and engineered inorganic colloids: a review. Environ. Sci. Technol. 2014, 48 (16), 8946-62. 25. Majzik, A.; Tombácz, E. Interaction between humic acid and montmorillonite in the presence of calcium ions II. Colloidal interactions: Charge state, dispersing and/or aggregation of particles in suspension. Org. Geochem. 2007, 38 (8), 1319-1329. 26. Sowers, T. D.; Stuckey, J. W.; Sparks, D. L. The synergistic effect of calcium on organic carbon sequestration to ferrihydrite. Geochem. Trans. 2018, 19 (1), 4. 27. Sowers, T.; Adhikari, D.; Wang, J.; Yang, Y.; Sparks, D. L. Spatial associations and chemical composition of organic carbon sequestered in Fe, Ca, and organic carbon ternary systems. Environ. Sci. Technol. 2018. DOI: 10.1021/acs.est.8b01158. 28. Nilsson, N.; Persson, P.; Lövgren, L.; Sjöberg, S. Competitive surface complexation of o-phthalate and phosphate on goethite (α-FeOOH) particles. Geochim. Cosmochim. Acta 1996, 60 (22), 4385-4395. 29. Mikutta, R.; Kleber, M.; Torn, M. S.; Jahn, R. Stabilization of Soil Organic Matter: Association with Minerals or Chemical Recalcitrance? Biogeochemistry 2006, 77 (1), 25-56. 30. Guan, X. H.; Shang, C.; Chen, G. H. Competitive adsorption of organic matter with phosphate on aluminum hydroxide. J. Colloid Interface Sci. 2006, 296 (1), 51-58. 31. Feng, X. J.; Simpson, A. J.; Simpson, M. J. Chemical and mineralogical controls on humic acid sorption to clay mineral surfaces. Org. Geochem. 2005, 36 (11), 1553-1566. 32. Kaiser, K.; Guggenberger, G. Mineral surfaces and soil organic matter. Eur. J. Soil Sci. 2003, 54 (2), 219-236. 33. Kleber, M.; Sollins, P.; Sutton, R. A conceptual model of organo-mineral interactions in soils: self-assembly of organic molecular fragments into zonal structures on mineral surfaces. Biogeochemistry 2007, 85 (1), 9-24. 34. Lin, Y.; Munroe, P.; Joseph, S.; Henderson, R. Migration of dissolved organic carbon in biochars and biochar-mineral complexes. Pesqui. Agropecu. Bras. 2012, 47 (5), 677-686. 35. Specht, C. H.; Kumke, M. U.; Frimmel, F. H. Characterization of NOM adsorption to clay minerals by size exclusion chromatography. Water Res. 2000, 34 (16), 4063-4069. 36. Meier, M.; Namjesnik-Dejanovic, K.; Maurice, P. A.; Chin, Y. P.; Aiken, G. R. Fractionation of aquatic natural organic matter upon sorption to goethite and kaolinite. Chem. Geol. 1999, 157 (3-4), 275-284. 37. Smebye, A.; Ailing, V.; Vogt, R. D.; Gadmar, T. C.; Mulder, J.; Cornelissen, G.; Hale, S. E. Biochar amendment to soil changes dissolved organic matter content and composition. Chemosphere 2016, 142, 100-105. 38. Vermeer, A. W. P.; van Riemsdijk, W. H.; Koopal, L. K. Adsorption of humic acid to mineral particles. 1. Specific and electrostatic interactions. Langmuir 1998, 14 (10), 2810-2819. 39. Li, F.; Cao, X.; Zhao, L.; Wang, J.; Ding, Z. Effects of Mineral Additives on Biochar Formation: Carbon 21



Retention, Stability, and Properties. Environ. Sci. Technol.2014, 48 (19), 11211-11217. 40. Clough, A.; Skjemstad, J. Physical and chemical protection of soil organic carbon in three agricultural soils with different contents of calcium carbonate. Soil Res. 2000, 38 (5), 1005-1016. 41. Gonod, L. V.; Jones, D. L.; Chenu, C. Sorption regulates the fate of the amino acids lysine and leucine in soil aggregates. Eur. J. Soil Sci. 2006, 57 (3), 320-329. 42. Lozzi, I.; Calamai, L.; Fusi, P.; Bosetto, M.; Stotzky, G. Interaction of horseradish peroxidase with montmorillonite homoionic to Na+ and Ca2+: effects on enzymatic activity and microbial degradation. Soil Biol. Biochem. 2001, 33 (7-8), 1021-1028. 43. Jones, D. L.; Edwards, A. C. Influence of sorption on the biological utilization of two simple carbon substrates. Soil Biol. Biochem. 1998, 30 (14), 1895-1902. 44. van Hees, P. A. W.; Vinogradoff, S. I.; Edwards, A. C.; Godbold, D. L.; Jones, D. L. Low molecular weight organic acid adsorption in forest soils: effects on soil solution concentrations and biodegradation rates. Soil Biol. Biochem. 2003, 35 (8), 1015-1026. 45. Kaiser, K.; Guggenberger, G. Sorptive stabilization of organic matter by microporous goethite: sorption into small pores vs. surface complexation. Eur. J. Soil Sci. 2010, 58 (1), 45-59. 46. Reichard, P. U.; Kretzschmar, R.; Kraemer, S. M. Dissolution mechanisms of goethite in the presence of siderophores and organic acids. Geochim. Cosmochim. Acta 2007, 71 (23), 5635-5650. 47. Collins, M. J.; Bishop, A. N.; Farrimond, P. Sorption by mineral surfaces: Rebirth of the classical condensation pathway for kerogen formation? Geochim. Cosmochim. Acta 1995, 59 (11), 2387-2391. 48. Zhan, J.; Wang, H. H.; Song, S. N.; Hu, Y. A.; Li, J. A. Role of an additive in retarding coal oxidation at moderate temperatures. Proc. Combust. Inst. 2011, 33, 2515-2522. 49. Sujant, W.; Zhang, D. K. Investigation into the role of inherent inorganic matter and additives in low-temperature oxidation of a Victorian brown coal. Combust. Sci. Technol. 2000, 152, 99-114. 50. Shi, T.; Wang, X.; Deng, J.; Wen, Z. The mechanism at the initial stage of the room-temperature oxidation of coal. Combust. Flame 2005, 140 (4), 332-345. 51. Wang, H.; Dlugogorski, B. Z.; Kennedy, E. M. Analysis of the mechanism of the low-temperature oxidation of coal. Combust. Flame 2003, 134 (1-2), 107-117. 52. Wang, D. M.; Dou, G. L.; Zhong, X. X.; Xin, H. H.; Qin, B. T. An experimental approach to selecting chemical inhibitors to retard the spontaneous combustion of coal. Fuel 2014, 117, 218-223. 53. Nguyen, B. T.; Lehmann, J.; Kinyangi, J.; Smernik, R.; Riha, S. J.; Engelhard, M. H. Long-term black carbon dynamics in cultivated soil. Biogeochemistry 2009, 92 (1-2), 163-176. 54. Kleber, M.; Mikutta, R.; Torn, M. S.; Jahn, R. Poorly crystalline mineral phases protect organic matter in acid subsoil horizons. Eur. J. Soil Sci. 2005, 56 (6), 717-725. 55. Eusterhues, K.; Rumpel, C.; Kogel-Knabner, I. Organo-mineral associations in sandy acid forest soils: importance of specific surface area, iron oxides and micropores. Eur. J. Soil Sci. 2005, 56 (6), 753-763. 56. Lehmann, J.; Lan, Z.; Hyland, C.; Sato, S.; Solomon, D.; Ketterings, Q. M. Long-term dynamics of phosphorus forms and retention in manure-amended soils. Environ. Sci. Technol. 2005, 39 (17), 6672-6680. 57. Foster, R. Microenvironments of soil microorganisms. Biol. Fertil. Soils 1988, 6, (3) 189-203. 58. McGill, W.; Myers, R. Controls on dynamics of soil and fertilizer nitrogen. Soil fertility and organic matter as critical components of production systems 1987, (19), 73-99.

22


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Table 1. Summary of the C mineralization parameters of the dissolvable and undissolvable biochar from the single exponential model. k

Smax

Decreasing Smax (%)

R2

Undissolvable Biochar

0.672

4.19

0

0.690

Undissolvable Biochar+Kaolinite

0.273

2.03

51.6

0.648

Undissolvable Biochar+Kaolinite+CaCl2

1.340

2.12

49.4

0.183

Dissolvable Biochar

0.254

9.04

0

0.926

CaCl2

0.226

4.71

47.9

0.986

NaCl

0.291

1.82

80.0

0.933

NaCl-NaH2PO4

0.325

1.33

85.3

0.991

23



Page 24 of 29

Table 2. Activation energy of the undissolvable biochar and kaolinite-associated undissolvable biochar analyzed by fitting the O2 chemisorption stage of the TGA curves. Activation

Increasing Activation

Increasing Activation

Energy KJ/mol

Energy KJ/mol

Energy Percentage (%)

Undissolvable Biochar

10.27 ± 0.13 ca

0

0

Undissolvable

12.54 ± 0.38 a

2.27

22.1

11.58 ± 0.24 b

1.31

12.8

Biochar+Kaolinite Undissolvable Biochar+Kaolinite+CaCl2 a

Different characters within a column indicate a significant difference between treatments (P < 0.05).

24


Page 25 of 29


Figure 1. Sorption of the dissolvable biochar by kaolinite under three background electrolytes. Error bars represent the standard error of the mean (n = 3).

25



Figure 2. The relative contributions of three mechanisms to the sorption of dissolvable biochar on kaolinite for a series of initial dissolvable biochar concentrations (calculated by equations 1, 2 and 3).

26


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Figure 3. Mineral content-corrected C loss by the K2CrO7 oxidation of the free dissolvable biochar and dissolvable biochar sorbed via three binding mechanisms (a), undissolvable biochar and kaolinite-associated undissolvable biochar (b). Different characters indicate a significant difference between treatments (P < 0.05).

27



Figure 4. Mineral content-corrected TGA curves of the undissolvable biochar and kaolinite-associated undissolvable biochar.

28


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TOC

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Kaolinite Enhances the Stability of the Dissolvable and Undissolvable

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