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Insight into Multiple and Multi-level Structures of Biochars and Their Potential Environmental Applications: A Critical Review Xin Xiao, Baoliang Chen, Zaiming Chen, Lizhong Zhu, and Jerald L. Schnoor Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06487 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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Environmental Science & Technology

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Insight into Multiple and Multi-level Structures of Biochars and

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Their Potential Environmental Applications: A Critical Review

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XIN XIAO1,2; BAOLIANG CHEN1,2*; ZAIMING CHEN3; LIZHONG ZHU1,2;

5

JERALD L. SCHNOOR4

6 7

1. Department of Environmental Science, Zhejiang University, Hangzhou 310058, China;

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2. Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou 310058,

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China

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3. Department of Environmental Engineering, Ningbo University, Ningbo 315211, China;

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4. Department of Civil and Environmental Engineering, University of Iowa, Iowa City, Iowa

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52242, United States

13 14

Corresponding Author Phone & Fax: 86-571-88982587

15

Email: [email protected]

16 17 18

Manuscript including abstract, text, tables and figures contains 12932 words.

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[10232 (Abstract, Text) + 300×9(8 Figures, 1 Table) = 12932 words]

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ACS Paragon Plus Environment


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Table of Contents (TOC)

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25

Abstract

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Biochar is the carbon-rich product of the pyrolysis of biomass under oxygen-limited

27

conditions, and it has received increasing attention due to its multiple functions in the

28

fields of climate change mitigation, sustainable agriculture, environmental control and

29

novel materials. To design a “smart” biochar for environmentally sustainable

30

applications, one must understand recent advances in biochar molecular structures and

31

explore potential applications to generalize upon structure-application relationships. In

32

this review, multiple and multi-level structures of biochars are interpreted based on

33

their elemental compositions, phase components, surface properties, and molecular

34

structures. Applications such as carbon fixators, fertilizers, sorbents and carbon-based

35

materials are highlighted based on the biochar multi-level structures as well as their

36

structure-application relationships. Further studies are suggested for more detailed

37

biochar structural analysis and separation and for the combination of macroscopic and

38

microscopic information to develop a higher-level biochar structural design for

39

selective applications.

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41

1. Introduction

42

Biochar is a solid, carbon-rich product obtained from the thermal treatment of biomass

43

pyrolyzed under an oxygen-limited atmosphere.1 The massive production of

44

agricultural waste in the worldwide provides an abundant source for biochar preparation.

45

It was estimated that both China and the USA have ~1.4 billion tons of annual

46

agricultural biomass waste production which has the potential to produce ~420 Mton

47

biochar per year.2, 3 Other countries, such as Zimbabwe4 and Ghana5 can produce 3.5

48

Mton and 7.2 Mton of biochar annually, respectively. Increasing attention is being

49

devoted to biochar applications in the fields of agriculture, climate change,

50

environmental remediation, and materials science. Biochar, originating 2000 years ago

51

in the Amazon Basin as terra preta black soil, is a long-lasting additive producing rich

52

agricultural soil.6, 7 In 2006, Johannes Lehmann commented that biochar could lock-up

53

carbon from the atmosphere in the soil to mitigate climate change.8, 9 Later on, the

54

sorption behaviors of biochar towards different organic pollutants and heavy metals

55

were investigated for environmental remediation in 200810 and 2009,11 respectively.

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Recently, biochars were developed into functional materials due to their cheap and

57

sustainable properties.12 The diverse functionalities of biochar suggest multipurpose

58

applications, but they require deeper understanding for smart biochar design.

59

Many different biomass materials can be utilized to produce biochars for different

60

purposes. The challenge is how to choose the right precursors and preparation

61

procedures to obtain biochars for specific purposes. The preparation methods for

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biochars could be classified as pyrolysis, gasification, hydrothermal carbonization, and

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ﬂash carbonization.13 Compared to biomass, which is a living, colorful, and water-rich

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material with a low surface area, biochar is a black carbon-rich product with a high

65

surface area and no cellular structure. The transformation of biomass during pyrolysis 4 / 78


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alters the structure in different ways and further influences the properties of biochar.

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Typical properties vary with the pyrolysis temperature and are listed in Figure 1. With

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increasing pyrolysis temperatures (100 ºC~700 ºC), the biomass color changes from its

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original color to black, and the product yield (typically 91%~14% for pine needle-

70

derived biochars10), polarity, hydrogen content (H%) (typically 6.2~1.3% for pine

71

needle-derived biochars10), oxygen content (O%) (typically 42~11% for pine needle-

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derived biochars10), and albedo of the biochars decreased. In contrast, the carbon

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content (C%) (typically 50%~86% for pine needle-derived biochars10), ash percentage

74

(typically 13%~49% for rice straw-derived biochars14), aromaticity, pH, surface area

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(typically 0.65 m2 g-1 ~ 490 m2 g-1 for pine needle-derived biochars10), and zeta potential

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( ζ ) increased with increasing pyrolysis temperature (100 ºC~700 ºC). The

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hydrophobicity of biochar increases and then decreases with pyrolysis temperature, as

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recent studies show that aromatic surfaces are intrinsically mildly hydrophilic, but

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adsorbed hydrocarbon contaminants provide the hydrophobicity of the aromatic

80

surfaces in the mid-temperature range.15 The diversity of biochar with regards to its

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properties, structures, applications and mechanisms inevitably requires a clarification

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of the relationships among variables, especially for structure-application relationships,

83

which are indispensable and of fundamental concern.

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84 85

Figure 1. Biochar property variations with increased pyrolysis temperatures

86 87

Several reviews summarize the applications of biochar.16,

17

For example,

88

researchers reviewed the sorption properties of biochars towards both organic and

89

inorganic contaminants.18,

90

reviewed. In 2015, Liu et al. suggested that biochars be considered as functional carbon

91

platform materials considering their lost-cost and sustainability, thus increasing the

92

applications of biochar into the field of novel materials.12 These reviews made great

93

strides to summarize and characterize biochar applications, which will greatly promote

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research. However, the body of literature regarding the structure of biochar, especially

95

with regards to biochar applications, is inadequate. Despite considerable research and

19

Applications for climate13 and agriculture20 were also

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reviews on biochar, a critical review on the structure and application relationships of

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biochar is urgently needed to understand and develop these novel materials.

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A large number of precursor biomasses and different preparation methods offers

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selective utilization opportunities for biochar, despite the enormous uncertainty and

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heterogeneity of biochar structures. To investigate the aromatic structure of biochar,

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many researchers suggested graphene and/or graphite as an ideal model.1,

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Nonetheless, the biomass will go through aromatization and graphitization processes at

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high pyrolysis temperatures (≥500 ºC), but the heterogeneous nature and structures of

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middle pyrolysis temperature-derived biochar are not fully comprehended. Additionally,

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besides the major element “carbon”, many other elements exist in biochar, which affects

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the corresponding role, function, and application of the material. The elements form

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different phases within biochar, which provides unique physical properties. The surface

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chemistry of biochar thus becomes critical because the interfaces between biochar and

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the aqueous phase provides important sites for sorption and reaction, including surface

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functional groups, surface radicals and surface charge. Heterogeneous biochar can be

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viewed as an aggregation of multiple molecules, containing both skeletal and small

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extracted molecules (Figure 2). To date, a few studies have focused on biochar

113

structures, but a systematic summary detailing the structures of biochars is still lacking.

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In 2017, a quaternary structural model of biochars was proposed from the macroscopic

115

to microscopic level for high pyrolysis temperature-derived biochars.21 In this review,

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we will interpret the multiple and multi-level structures of biochars from their elements,

117

to phases, to surfaces, and to molecular structures (Figure 2). With the knowledge of

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the biochar structures, their relationships to applications is further discussed in four

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different areas: agriculture, fertilizers; climate, carbon sequestration; environment,

120

sorbents; and materials, functionalization. We summarize the advances in biochar 7 / 78


21, 22


121

structures and applications and begin to establish structure-application relationships

122

with the overall goal to design smart biochars for environmentally sustainable

123

applications.

124 125 126

Figure 2. Heterogeneous structures and potential applications of biochar from a

127

consideration of the elemental composition, phase, surface chemistry, and molecular

128

structure.

129 130

2. Multiple and Multi-level Structures of Biochars

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2.1 The Elements

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The elements present are the fundamental building blocks of biochars. During the 8 / 78


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pyrolysis of biomass, elements within the biochar experience different physicochemical

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processes and form different species and products, thus contributing to the diversity of

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biochars. The species and functions of these elements are listed in Table 1. These

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elements are part of global geochemical processing, and they have different mass

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percentages and functions within biochars. Among these elements, C, H, O, and N are

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the most common elements and generally contribute to the major structures of biochar;

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Si, Fe, P, and S show a wide range of mass percentages in specific biochars, while some

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elements (such as Si, N, P, S, and Fe) are nutrient elements for specific plants. In global

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geochemistry cycles, some elements (C, H, O, S, and N) are bidirectional in their

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cycling from the atmosphere to the earth, while other elements (P, and Si) are

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unidirectional due to the lack of atmospheric species. The multiple roles of multiple

144

elements endow biochar with unique structures and functions. Thus, in this section, the

145

chemical state, species, function, applications, and geochemistry cycling involved with

146

biochars are discussed.

147 148

Table 1. The species, functions and applications of selected elements within biochars Elements

Species

Functions a) Key element within biochars;

C

Organic:

(single bond),

(double b) Develop functional groups by linking to other elements;

bond),

(aromatic

carbon),

c) Inorganic carbon contributes to the alkalinity and buffering

(functionalized carbon)

properties of biochars;23 Inorganic: HCO3-, CO32-

d) Carbon sequestration.9 a) Nutrient element;

Si

Inorganic: Silicon acid, polymeric Si, crystal Si. b) Co-precipitator with heavy metals;24

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c) Major element of biochar inorganic phases;14 d) Protect the organic phase.14 a) For association/ dissociation; H b) To form the hydrogen bond;25 (aliphatic H),

(aromatic H),

(hydrogen in functional groups),

c) H/C atomic ratio is the aromaticity index of biochars.22

(hydrogen in hydrogen bond)

O

Organic:

a) To form the functional groups;

(ether),

(hydroxyl),

b) Complexation with metal

(epoxy),

(carboxyl, ester),

ions;26

(acyl, carbonyl)

Inorganic: MxOy, HCO3-, CO32-, SO42-, NO3- etc.

c) O/C atomic ratio is an index of the degree of aging of biochars.27

N

Organic:

(imine-N),

(amine-

a) Nutrient element; b) Improve the thermal stability;

N),

(acylamide-N),

(pyridine-

N),

(pyrrole-N), protein-N

Inorganic: NH4+, NO3-, NO2-

c) Active sites for reaction and modification; d) Nitrogen fixation.28

P

a) Nutrient element; Organic:

(mono- phosphate ester),

(phosphate di-ester)

b)

Precipitator

with

heavy

metals.11

Inorganic: PO43-, P2O74S

a) Increase the solubility of Organic:

(sulfonate), 10 / 78


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(sulfonamide)

carbon materials;29

Inorganic: sulfate

b) Solid acid catalyst;30 c) Nutrient element.

Fe

Fe2O3, Fe3O4, FeO, α-Fe, Fe3C, FeO(OH), Zero

a) Provide magnetism;31

valent iron, other metal complex

b) Increase the sorption ability for anions;31 c) Promote graphitizing during pyrolysis;32 d) Catalysis/activator for organic pollutant degradation.33

Mg

MgO, MgCO3, other metal complex

a) Increase anions sorption;34 b) CO2 capture;35 c) Nutrient element.

Mn

MnOx, Mn(OH)x, other metal complex

a)

Increase

heavy

metal

sorption;36 b) Increase capacity. Ni

Ni(OH)2, NiO, Ni2O3, other metal complex

a) H2 evolution; b) Catalysis the formation of syngas during pyrolysis.37

149 150

Carbon: Carbon is the most important and the most abundant element in biochars.

151

The carbon species within biochars can be classified as carbonate and bicarbonate in

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the inorganic phase and aliphatic carbon, aromatic carbon and functionalized carbon in 11 / 78



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the organic phase (Table 1). It is quite difficult to precisely differentiate these carbon

154

species. During the pyrolysis process, the inorganic carbon may transform from

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hydrated carbonate to bicarbonate, carbonate or even be lost in the form of CO2,

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depending on the type of inorganic crystal.38 As for the organic carbon, the carbon

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transformed from cellulose/hemicellulose/lignin in the biomass becomes aliphatic

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carbon in the middle pyrolysis temperature-derived biochars, and further also results in

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aromatic carbon at high pyrolysis temperatures after dewatering, cracking and

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aromatization reactions.39 These transitions were confirmed by Fourier transform

161

infrared spectroscopy (FT-IR),10,

162

angle spinning (CPMAS), solid 13C nuclear magnetic resonance spectroscopy (NMR)42

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and other characterizations. More recently developed technologies were applied to

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gather more chemical information about the carbon species in biochar, such as two-

165

dimensional

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atomic and alkyl carbons; however, it is still difficult to differentiate the

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functionalization of the carbon. The chemical species of carbon is critical for

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applications in agriculture, ecosystems, and carbon sequestration because the chemical

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species determine the stability and reactivity of the carbon in biochar.

13

40

Raman spectroscopy,41 cross-polarization magic

C NMR.43, 44 These characterization methods can distinguish between

170

Silicon: Although not all of the biomass or agricultural waste that is pyrolyzed into

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biochars contains silicon, silicon-rich biomass such as rice straw, rice husk, and corn

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straw are vital feedstocks for biochar production and essential plants for food

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production.14, 45 In fact, silicon is a required nutrient for silicophilic plants (such as rice,

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maize, wheat, and barley),46 protecting these plants from insects, heavy metals,

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infestation, light and drought.47-49 Moreover, our previous studies found that the silicon

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in biochars contributed to the sorption and retention of heavy metals (such as

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aluminum26, 50 and cadmium24) in aqueous systems. The dissolution of silicon from 12 / 78


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biochars to form monsilicic acid in water25 was found to be important in the biochar

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sorption of metals.

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Our research group first investigated silicon transformation in rice straw derived

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biochars under different pyrolysis temperatures.14 In the biomass, silicon existed as

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phytoliths in the form of silicic acid and polymeric silicon which can easily dissolve.45

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During charring, the silicon becomes dehydrated and polymerized, and upon further

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pyrolysis, and the polymeric silicon was partially crystalized. Rice straw biochars

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pyrolized at high temperature display higher silicon dissolution than low pyrolysis

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temperature-derived biochars. Additionally, Si-rich biochars show much higher silicon

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dissolution tendencies than their corresponding ash or soil, which contain large amounts

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of silicon in the form of silicate.14, 51 Since many arable lands in the world are suffering

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from silicon depletion, the Si-rich biochars with high silicon dissolution capacities may

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provide a slow-release Si-fertilizer for soil.

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Silicon represents a major fraction of the inorganic phase in Si-rich biochars

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(composing ~38% of the mass in rice straw ash)14, and it seems to be separated from

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the organic matter in biochars. However, according to the results of scanning electron

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microscopy-energy dispersive spectroscopy (SEM-EDS), a mutual protection exists

195

between carbon and silicon under different pyrolysis temperatures. The carbon layer

196

protects the silicon from dissolution under low pyrolysis temperatures, and the silicon

197

layer protects the carbon from loss under high pyrolysis temperatures.14 Our study

198

further indicated that the silicon may promote recalcitrance and stability in biochars.52

199

Figure 3 provides a schematic of the coupling cycles of carbon and silicon in

200

biochar and the terrestrial ecosystem. In the environment, the Si-rich biomass takes-up

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CO2 from the atmosphere and silicic acid from the soil solution. However, with

202

increased charring temperature, the interlaced C-Si content starts to separate and form 13 / 78



203

corresponding crystals, which are stable over long periods. With burning and

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dissolution over a long period of time, the carbon and silicate minerals release CO2 and

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silicic acid which becomes available to plants, thus completing the geochemical cycle.

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This type of C-Si coupling illustrates the vital role that biochars play in the geochemical

207

cycling of these essential nutrient elements. It also indicates the need for more research

208

for a better understanding of the role of biochar in global carbon and silicon balances

209

as well as the important agricultural function of silicon in biochar.

210 211

Figure 3. The carbon and silicon cycles along with the carbon-silicon coupling

212

relationship in biochar and the terrestrial ecosystem.

213 214

Hydrogen: There are three species of hydrogen that exist in biochars: aliphatic

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hydrogen, aromatic hydrogen, and active functionalized hydrogen (Table 1). For typical

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biochar, the hydrogen weight percentage in organic matter decreases from ~6% at low

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pyrolysis temperatures to ~1% at high pyrolysis temperatures.10 The low hydrogen

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content makes it more difficult to characterize biochars by FT-IR or H NMR. Therefore,

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the speciation of hydrogen within biochar is qualitatively assessed by elemental

220

analysis and characterizing other elements, such as carbon or oxygen. Since the H/C

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atomic ratio for aliphatic organic matter in biochars is approximately 2.0, and the H/C

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atomic ratio of aromatic organic matter decreases with larger fused aromatic clusters, 14 / 78


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the H/C atomic ratio is widely regarded as an aromaticity index for biochars.22

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Compared to the stable aliphatic and aromatic hydrogen in biochars, more uncertainty

225

exists regarding the active functionalized hydrogen. Active hydrogen atoms in

226

functional groups can be associated/dissociated under proper pH conditions, and this is

227

easily probed by a pH meter. The association/dissociation interaction may lead to

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formation of intra- and intermolecular H-bonds, causing different polarities between

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the bulk biochar and its surface.53 Hydrogen bonds (D–H•••:A) are generated between

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the biochar surface and ionizable molecules by sharing protons.54 Therefore, hydrogen

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acts as an important building block for biochar structures, and it plays a key bridging

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role in biochar sorption towards ionizable molecules.

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Oxygen: In the inorganic phase of biochars, oxygen mainly exists in the form of

234

metallic oxides, hydroxides, or inorganic clusters such as carbonate, bicarbonate, and

235

sulfate (Table 1). Inorganic oxygen is generally stable and contributes to the alkalinity

236

of biochars. In the organic phase of biochars, oxygen atoms are mostly attached to

237

carbon atoms, forming different types of functional groups, including hydroxyl, epoxy,

238

carboxyl, acyl, carbonyl, ether, ester, and sulfonic groups (Table 1). Since the oxidation

239

process in natural environments regularly induces oxygen into the organic structures of

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biochars, the O/C atomic ratio is thus widely considered to be an indicator of the degree

241

of aging/oxidization undergone by biochars.55-58 This oxidation induces the formation

242

of carboxyl groups, and it could somehow benefit the sorption of aluminum59 and

243

lead.60 The release of CO2 from carboxyl groups may be an important reaction in the

244

labile fraction of biochars, although the structural framework to which the carboxyl

245

groups were attached might be considered to be stable carbon. More details are

246

discussed in the section on surface functional groups because oxygen-containing groups

247

contribute significantly to biochar functional groups. 15 / 78



248

Nitrogen: In biomass, nitrogen mainly contributes to the peptide bond in protein.

249

Normally, biochar feedstocks such as plants and wood are nitrogen-poor materials due

250

to their rather low protein content, while materials such as grass or leaves contain

251

relatively higher protein contents and are nitrogen-rich materials. For example, casein,

252

grass, and wood-derived biochars were reported as containing ~15%, ~7%, and 0.3~1

253

wt.% nitrogen, respectively.10, 61 Although many researchers have studied nitrogen-rich

254

biochars, only a few focused on the nitrogen speciation within biochars. Theoretically,

255

the structures related to nitrogen contain pyrrole, pyridine, amine, imine, acylamide,

256

nitroso, and nitro groups (Table 1). Reports indicate that the total nitrogen content

257

within biochars increases and then slightly decreases with increasing pyrolysis

258

temperature.10, 61, 62 Knicker et al. identified a model biochar and suggested that the N

259

forms are mainly pyrrole-N, amide-N, and pyridine-N.63 During pyrolysis, peptide-N

260

bonds were transformed to N-heteroaromatic carbon compounds, while some amide-N

261

within biochars decreased with increasing pyrolysis temperature.61 Pietrzak et al.64, 65

262

studied the reaction of ammonia/urea with coal and found that the nitrogen was

263

introduced at 500-700 ºC in the form of pyridinic-N and pyrrolic-N as well as imine-N,

264

amine-N and amide-N. For the pyridinic-N and pyrrolic-N, many studies on carbon

265

materials (such as graphene66) were conducted with nitrogen-doped carbon materials to

266

modify the electronic band structures, and they acted as the active sites for reaction.

267

This may explain the observations that nitrogen-rich biochars show greater heat

268

resistance but less chemical-oxidation stability than nitrogen-poor biochars.61 After

269

application into soil, the fate of extractable N becomes of research interest. By applying

270

different extraction methods, different species of extractable N, including that from

271

ammonia, amino sugars, amino acids, and total hydrolysable forms, were found to

272

decrease with pyrolysis temperature.67 16 / 78


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In fact, there is uncertainty whether the extracted N comes from organic matter or

274

inorganic matter in biochar. Nitro- and nitrito-N were also found in biochars derived

275

from agricultural biomass (rice husk), forestry waste (sawdust), and wetland plants

276

(Acorus calamus) at 650 °C.68 However, the nitrogen content of biochars is usually too

277

low to be considered a nitrogen fertilizer alone. The biochars with high C/N ratios have

278

limited N availability, and additional nitrogen fertilizer might be needed to improve

279

microbial and plant growth in soils. 69,70, 71 For example, it was reported that 5% biochar

280

combined with mineral fertilizer (containing 0.0134% N) shows remarkably higher

281

promotion of oat growth than biochar alone.72

282

The nitrogen dynamics of biochars applied as additives in soil systems are of

283

considerable interest, including the effects on nitrogen-sorption, nitrogen-fixation,

284

nitrification, and N2O greenhouse gas (GHG) emissions. Multiple studies indicate that

285

biochars show a high affinity for the sorption of ammonium nitrogen (NH4+) by cation-

286

exchange, but no influence on the removal of nitrate-N.73-77 Ammonia sorbed onto

287

biochars is reported to be bioavailable,78 and thus biochars are regarded as a nutrient-

288

retention additive to maintain the nitrogen in soil.73, 79 It is a soil “conditioner,” which

289

improves the tilth, water holding capacity, and nutrient holding capacity of soil. 1

290

Additionally, it stabilizes the carbon and nitrogen, thus reducing the emissions of GHGs

291

(CO2 and N2O). Biochars were reported to increase the biological nitrogen fixation

292

proportion of common beans from 50% without biochar to 72% with 90 g kg-1 biochar

293

added,28 and stimulated both the nitrification and denitrification processes with a result

294

of reducing N2O emissions.80 The reduction order of N2O emissions by giant reed-

295

derived biochars additives in soils differed with pyrolysis temperatures, following the

296

order BC200 ≈ BC600 > BC500 ≈ BC300 ≈ BC350 > BC400 (BC refers to biochar, the

297

suffix number represents the preparation pyrolysis temperature of biochar).81 The 17 / 78



298

“electron shuttle” of biochar may facilitate the transfer of electrons to denitrifying

299

microorganisms as well as the “liming effect” of biochars, which promote the reduction

300

of N2O to N2, leading to a 10~90% decrease in N2O emissions in 14 different

301

agricultural soils.82 The carbon and nitrogen cycles are affected in three ways by biochar.

302

(1) During biochar processing, carbon becomes more recalcitrant and nitrogen becomes

303

less available, thus stabilizing the biochar product.83 (2) Microbial and plant growth in

304

soils with biochar additives are limited by the low nitrogen content, and less microbial

305

growth stabilizes and reduces carbon mineralization.84 (3) The sorption of NH4+ onto

306

the carbon components of biochars improves the nitrogen retention in soil systems.

307

Phosphorus: Unlike carbon, hydrogen, oxygen, and nitrogen which are lost during

308

the pyrolysis process, phosphorus does not volatilize at temperatures below 700 ºC,16

309

leading to an enhancement of phosphorus under typical pyrolysis temperatures. Ngo et

310

al.85 reported an overwhelming dominance of inorganic phosphorus compared to

311

organic phosphorus in bamboo-derived biochar produced at 500-600 ºC. Considering

312

the fact that the phosphorus in biomass is mostly organic (such as mono-phosphate and

313

phosphate di-esters62) (Table 1), the phosphorus species transformation during

314

pyrolysis is dramatic and discussed by Uchimiya & Hiradate.86 Using 31P NMR analysis,

315

they proposed that the phytate is converted to inorganic phosphorus from manure and

316

plant-derived biochars at 350 ºC.86 Regarding inorganic phosphorus, the pyrophosphate

317

(P2O74-) in plant material was persistent in biochars pyrolyzed at temperatures up to 650

318

ºC and the orthophosphate (PO43-) became the sole P-species for manure biochars at

319

pyrolysis temperatures higher than 500 ºC.86

320

Similar to silicon, phosphorus is an important nutrient for plant and microbe

321

growth. The geochemical cycle of phosphorus is also unidirectional from land to sea

322

because there is no gaseous phosphorus species, and it is difficult to transport ocean 18 / 78


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phosphorus back to the land ecosystem except via the slow crustal plate movement of

324

subduction. Thus, making a slow releasing P-additive in the form of biochar was

325

proposed to retain more phosphorus in soil.87 The effects of the pH and other anions

326

and cations on the release of phosphorus were further investigated.88 These results

327

report that high pH soils greatly hinder the release of phosphorus.88 However, other

328

anions could enhance the orthophosphate (PO43-) release due to competing ion

329

exchange reactions.

330

formation of precipitates.89 Furthermore, phosphate in biochars could immobilize some

331

heavy metals by precipitation, such as β-Pb9(PO4)6 precipitates

332

hydroxypyromorphite Pb5(PO4)3(OH)90. In summar, phosphorus in biomass was

333

transformed from organic P to inorganic P via carbonization and can then serve as a

334

nutrient for the soil system or as a sorption site for heavy metals.

89

Competing cations will decrease the release of P due to the

11

or insoluble

335

Other Elements: In addition to the major elements mentioned above, there are

336

other trace elements that could exist in biochar. Under most circumstances, these

337

elements constitute only a small fraction of biochars (typically with the concentration

338

level of mg kg-1 28). Dopants may also be added to provide special chemical properties

339

to biochars. Nutrient elements such as Na, K, Ca, Mg, and Mn88,

340

important fractions of biochars, and other elements designed to functionalize biochars

341

may be added. For example, ~20% Mg (in the form of MgO) was loaded onto biochars

342

and increased CO2 capture from ~0.1 mol to ~5 mol CO2 per kg biochar.35 By inducing

343

sulfuric acid groups onto biochars, a solid acid catalyst can be formed for

344

transesterification,97,

345

Sulfonation is known to increase the solubility of carbon materials in aqueous

346

systems.29 Early in 2011, our research group31 reported on novel magnetic biochars

347

prepared by the co-precipitation of Fe3+/Fe2+ onto orange peels followed by pyrolysis.

98

hydrolysis reactions30,

99

91-96

constitute

and biodiesel production.100,

19 / 78


101


348

These magnetic biochars (with ~13 wt.% Fe) show efficient sorption performance

349

towards organic pollutants and almost thrice the sorption capacity towards phosphate

350

compared to non-magnetic biochars pyrolyzed at 700 ºC. Magnetic biochars can be

351

easily separated from wastewater and thus have great potential for environmental and

352

agricultural applications. Manganese oxide-modified biochars have been reported to

353

benefit the sorption of arsenate, lead and copper.102, 103 Moreover, some elements have

354

a great impact on shaping the biochar structure, such as Zn as a pore-maker and Fe for

355

its promotion of graphitization.32 In addition, some trace metals (such as Cu, and Al)

356

show promise for producing favorable structural properties in biochars, but they must

357

be evaluated for their toxicity prior to being adopted as soil additives.104, 105 There are

358

many other elements that could be bonded to biochars, but we must first understand

359

their toxicity, speciation within biochars during pyrolysis, and influence on biochar

360

carbon stability and structure.

361 362

2.2 The Phases

363

Organic Phase: The bulk of biochar was formed via forming massive chemical

364

bonds between the mentioned elements. Previous researchers, without knowing the

365

exact structure, considered biochar to be a solid phase. Furthermore, this solid biochar

366

could be sub-divided into various different phases. Our knowledge of biochar phases

367

continues to evolve over time (Figure 4). Besides classifying biochars into organic and

368

inorganic phases, we may also consider their aliphatic and aromatic chemical nature.106-

369

108

370

partitioning and a surface adsorption phase, which comprises the total sorption of

371

organic molecules.10,

372

classified into a dissolved phase and an undissolved phase;25, 112-114 and in view of

In terms of their sorption mechanisms, we can differentiate biochar into an organic-

109-111

Furthermore, on the basis of solubility, biochar can be

20 / 78


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Page 21 of 78


373

carbon stability, a labile phase can be defined in which the carbon fraction can be

374

mineralized under natural soil conditions, while the stable phase remains

375

recalcitrant.115-118 With increasing pyrolysis temperature, biochar undergoes

376

dehydration, disruption of the organic components, and successive aromatization. The

377

aliphatic labile portion and dissolved-phase fraction decrease, while the aromatic stable

378

portion and undissolved-phase increase. It was reported that the labile carbon

379

percentage of sugarcane-derived biochars decreased from 1.08% at a 350 ºC pyrolysis

380

temperature to 0.29% at a 550 ºC pyrolysis temperature.118 Additionally, the dissolved

381

carbon percentage decreased from ~14% in 100 ºC-derived dairy manure biochar to

382

~0.2% in 700 ºC-derived dairy manure biochar at a pH of ~12.25

383 384

Figure 4. The knowledge evolution concerning the phases, surface chemistry, and

385

molecular structure of biochars as a function of the inverse biochar scale (size) and

386

research time. Inner figures were cited from ref.

387

(2010,2016,2017) American Chemical Society. 21 / 78


1, 21, 22, 40, 119

Copyright©


388 389

Recently, four different chemical phases and physical states were proposed: 1) a

390

transition phase with the preserved crystalline character of the precursors; 2) an

391

amorphous phase with cracked molecules and aromatic polycondensations; 3) a

392

composite phase with poorly ordered graphene stacks; and 4) a turbostratic phase with

393

disordered graphitic crystallites.40 The aliphatic phase, labile phase, partition phase and

394

dissolved phase are similar but not the same as each other. The aromatic phase, stable

395

phase, adsorption phase, and undissolved phase are also similar but not the same as

396

each other. For example, unlike the other phases where the pH has little effect, the

397

dissolved carbon phase is strongly affected by the pH.25 Part of the dissolved carbon

398

phase can be oxidized to CO2, indicating that the dissolved phase partially contributes

399

to the labile phase.113 The labile carbon fraction is related but not the same as the

400

dissolved carbon fraction.117 A portion of the biochar aromatic phase represents the

401

labile form in biochars but may not be persistent.108 Even though there are various

402

definitions and research on the organic phase of biochar, more investigations are needed,

403

especially regarding the separation methods, structure-function relationships,

404

biological effects and environmental fate of these different phases including their nano-

405

formed particles.

406

Inorganic Phase: Due to the existence of minerals in biomass, biochars are

407

actually a mixture of interlaced inorganic-organic structures. These inorganic phases

408

interact with and influence the organic phases in biochars and thus impact their

409

properties, making the understanding of mineral phase transformations during pyrolysis

410

of great importance.

411

Figure 5 summarizes the possible phase transformations of several typical mineral

412

components generated during biomass pyrolysis. Possible reactions that could occur in 22 / 78


Page 22 of 78

Page 23 of 78


413

the

inorganic

phase

during pyrolysis

include

dehydration,

414

polymerization, decarboxylation, crystallization, and reduction. For example, the silicic

415

acid within the rice straw will polymerize to form polymer silicon and further crystalize

416

to quartz with increasing pyrolysis temperatures.14 Commonly, with increasing

417

pyrolysis temperatures under limited oxygen, the inorganic matter starts as chloritized,

418

hydroxylated, or carbonated metals and then transitions to metallic oxides, some of

419

which can be reduced to pure metals because of the strong reducing atmosphere during

420

pyrolysis (Figure 5). By taking advantage of this transformation process during biochar

421

preparation, researchers are able to create magnetic,31 zerovalent Fe,33 and zerovalent

422

Cu coated biochars.120 Under higher pyrolysis temperatures, calcium carbide (CaC2),

423

an important starting material for the production of many chemicals, could be

424

manufactured by mixing fine biochars with CaO.121 Figure 5 gives the mineral

425

speciation of several elements in biochar. Studies on the inorganic phase of biochars

426

are relatively easier to define than the organic phase due to the higher atomic sensitivity

427

of metal atoms for characterization. Still, there are many other metals, species and

428

mixed crystals that remain unknown and require further research.

429 23 / 78


dechlorination,


430

Figure 5. Possible phase and chemical transformations of several mineral components

431

during biomass pyrolysis under limited oxygen. For each inorganic element, the color

432

illustrates the corresponding transition from one state to another. Cited from the

433

corresponding references: Si,14 Ni,37 Fe,37, 122, 123 and Mg.34, 35

434 435

The functions of inorganic matter in biochars include but not limited to the

436

following: providing magnetism;31 catalyzing the decomposition of organic pollutants

437

and metals-involved catalysis reactions;33,

438

agents,128-130 carbon sequestration agents,35, 131, 132 slow-release nutrients,87, 88, 133-136

439

heavy metal co-precipitators,11, 103, 137-144 and heavy metal stabilizers145. Magnetism

440

emanates from the loading of γ-Fe2O331 and the catalysis is from active zero valent iron

441

or transition metals, 33, 37, 120, 124-127 and pore-forming agents include KOH, ZnCl2, and

442

others.

443

laboratory experiment indicated that 0.5%~8.9% of biochar carbon was mineralized,

444

and manure-derived biochars mineralize faster than plant-derived biochars,131 while co-

445

pyrolysis with kaolin, calcite (CaCO3), or calcium dihydrogen phosphate (Ca(H2PO4)2)

446

decreases 0.3% ~ 38% loss of carbon from biochars during tests with potassium

447

dichromate oxidation.132

128-130

37, 120, 124-127

and providing pore-forming

The effect of carbon fixation depends on the mineral type. A five year

448

By converting the easy-to-dissolve metallic elements to their difficult-to-dissolve

449

species, heavy metals can be stabilized within biochars.145 In summary, mineral species

450

within the biochar are transformed by the pyrolysis temperature and affect the biochar

451

structure and applications in various ways. However, more knowledge is needed on the

452

dissolution behavior, toxicity, carbon stability, nano-effects, and possible catalytic

453

ability of inorganic nanoparticles.

454 455

2.3 The Surfaces 24 / 78


Page 24 of 78

Page 25 of 78


456

The surface of biochar is the main interface where many chemical and biological

457

reactions and interactions occur. Knowledge of the biochar surface chemistry has

458

evolved over time (Figure 4). After categorizing the nature of the biochar surface

459

(aliphatic or aromatic), the functional groups on biochar were elucidated.25 Following

460

that came the discovery of free radicals on the biochar surface.146 In this section, the

461

surface functional groups, charge, and free radicals are discussed.

462

Surface Functional Groups. Characterizing the surface functional groups of

463

biochars can explain their surface chemistry and reactions. Besides aliphatic and

464

aromatic groups, surfaces may include hydroxyl, epoxy, carboxyl, acyl, carbonyl, ether,

465

ester, amido, sulfonic, and azyl groups. Characterization methods have been applied to

466

analyze these functional groups, such as solid-state

467

photoelectron spectroscopy (XPS),149 and C-1s near-edge X-ray absorption fine

468

structure spectroscopy (NEXAFS).149-153 Although there are many different types of

469

functional groups and advanced technology for the analysis of functional groups, the

470

precise characterization to distinguish all functional groups was still difficult.

13

C NMR,147 FT-IR,148 x-ray

471

To simplify this process, some studies have focused on surface phenolic-OH,

472

carboxyl, carbonyl and ester groups as functional groups on biochars.101, 154 These are

473

among the most abundant functional groups at the surface of biochars.149, 154 Another

474

reason is that these functional groups can associate/dissociate with protons in the

475

environment, thus leading to hydrogen bond-induced sorption,59,

476

alkalis,23 pH buffering,158 hydrophilicity/hydrophobicity, surface charge alterations, a

477

cation exchange capacity (CEC)158 and capacitance improvement159. Consequently,

478

defining the dissociation constants (pKa) and the contents of these functional groups

479

become vital to understanding the surface properties of biochars. Boehm titration

480

methods have been widely applied to measure the carboxylic acid fraction, lactonic 25 / 78


143, 155-157

biochar


Page 26 of 78

481

group fraction, and phenolic functional groups for organic materials by NaHCO3,

482

Na2CO3, and NaOH, respectively.160-162 This method offers an easy way to measure

483

different functional groups without knowing their pKa values.

484

Recently, we proposed an integrated method combining FT-IR and a modified

485

acid/base consuming model to quantitatively identify the chemical states, dissociation

486

constants (pKa), and contents of surface functional groups within dairy manure-derived

487

biochars.25 This model offers a method to quantify the contents of different functional

488

groups, but it still requires further investigation to improve the detection accuracy and

489

parameter-fitting results due to the complexity of biochars.

490

Surface functional groups vary greatly with pyrolysis temperature. The original

491

lignocellulose H-bond networks in biomass were removed, and free hydroxyls were

492

transformed into carboxyls during the pyrolysis process.163 FT-IR indicated that

493

oxygen-containing

494

temperatures.10, 164 Two-dimensional (2D)

495

resulted in a dehydroxylation/dehydrogenation and aromatization process, along with

496

cleavage of O-alkylate and aromatic O-C-O groups as well as the formation of aromatic

497

C-O groups.43

groups

decreased

markedly 13

with

increasing

pyrolysis

C NMR data showed that carbonization

498

The functional groups on biochar not only served as sorption sites for metal

499

cations26, 50, 155, 157, 165 and ionizable organic pollutants166, 167 but also provided a perfect

500

and cheap active site for the surface chemistry modification of biochar, including

501

oxidation,60 amination,168 and sulfonation98. For example, the oxidation of biochar can

502

induce more carboxyl groups after soil aging149 or treatment with hydrogen peroxide.60

503

The amino-modification of biochar enhanced the sorption of copper ions.168

504

Additionally, sulfonation increased the stearic acid esterification speed of pine pellet

505

biochar.98 Other studies have reported the introduction of sulfonic acid as benefiting the 26 / 78


Page 27 of 78


506

solubility of carbon materials.29 As a vital part of the biochar surface, functional groups

507

are important interfacial sites, and more studies are required to quantify them, their

508

contents, their chemical states, and their corresponding applications.

509

Surface Charge. We are lacking sufficient knowledge of the surface charge on

510

biochars. The surface charge makes biochar attractive or repulsive to other charged

511

molecules24, 169 and bacteria,170 forming an electrical double layered structure. Thus,

512

biochars display colloidal properties similar to the properties of soil. The origin of the

513

surface charge on biochars is believed to be its aliphatic/aromatic surface and the

514

dissociation of functional groups, such as carboxyl moieties.163 Therefore, the surface

515

charge of biochars is highly correlated to the pH in the aqueous phase. It remains

516

negative over the natural pH range (pH = 4~12), but in strongly acidic systems, the

517

surfaces of biochars are positively charged. Once the pH increases above a pH of 4, the

518

surfaces of biochars become more negatively charged.171 Nonetheless, electrostatic

519

interactions impact the transport of ions, nanoparticles, and biochar particles172. This

520

influence is strongly regulated by the solution pH.

521

Surface Free Radicals. In 2014, Liao et al. published a comprehensive description

522

of the biochar’s surface free radicals of biochars.173 They reported abundant and quite

523

persistent free radicals within biochars derived from corn stalks, rice and wheat straws

524

as observed by electron paramagnetic resonance (EPR).173 The radical species

525

transitioned from oxygen-centered radicals to carbon-centered and oxygen-centered

526

combined radicals with increasing pyrolysis temperatures.173 Free radicals can be

527

generated during the charring process.173-175 Free radical transitions were monitored via

528

in-situ EPR observations.173 Aromatic carbon atoms played an important role in

529

stabilizing the surface free radicals due to their electron-rich aromatic surface at high

530

pyrolysis temperatures. 27 / 78



531

Although it is difficult to know the specific structures of all free radicals, phenoxyl

532

and semiquinone radicals were typically assumed to be the oxygen-centered radicals.173

533

Strong •OH radicals in the aqueous phase were induced by free radicals in biochars.

534

The free radicals in biochars may be a major reason for the inhibition of the germination

535

of plant seeds, root and shoot growth retardation, and damage to the plasma

536

membrane.173 Other studies indicated that abundant free radicals on the biochar surface

537

could react to activate hydrogen peroxide,146 persulfate176 and benefit the degradation

538

of organic contaminants, such as 2-chlorobiphenyl,146 diethyl phthalate177 and

539

polychlorinated biphenyls176. Free radicals within biochars may be the reason for the

540

electron accepting capacity in high temperature pyrolysis-derived biochars, as

541

measured by mediated electrochemical analysis.178 Surface radicals on biochars are

542

essential reactive sites, and more attention to their persistence, toxicity, pollutant

543

degradation ability, and environmental impact should be paid.

544 545

2.4 Molecular Structures

546

Extracted Small Compounds. As one part of biochar molecules, the extracted small

547

compounds have received much attention due to their high mobility and possible

548

toxicity. It was reported that the released fraction from two softwood pellets-derived

549

biochars with high volatile organic compounds (VOCs) almost completely inhibited 94%

550

and 100% of cress seed germination under 5% biochar treatment.179 After excluding the

551

effects of salt and water stress, researchers believed that the mobile organic compounds

552

were responsible for the adverse effects on germination.180 However, it is still unclear

553

which compounds are responsible, although phenolic compounds are the leading

554

suspect.180 Additionally, full identification remains a big challenge due to their low

555

concentration on biochars, lack of viable extraction methods, and theoretical limitation 28 / 78


Page 28 of 78

Page 29 of 78


556

for interpreting mass spectral results. Even if these compounds could be identified,

557

quantification becomes difficult due to the lack of standards and low concentrations. In

558

that case, using a similar compound as a standard or applying relative contents

559

calculated from the peak area could be compromise methods.181, 182

560

The extraction of small molecules from different biochars is reported in Table

561

S1.183-190 Regarding content analysis of the compounds, the depolymerization and C3-

562

side chain shortening occurred at approximately 200-300 ºC. The demethoxylation of

563

syringyl and gualacyl lignins appeared at 300-400 ºC; dehydroxylation and

564

demethoxylation to remove methoxyl groups appeared at 350-400 ºC; complete

565

dehydroxylation of lignin and other biopolymers occurred at 400-500 ºC; condensation

566

into aromatic clusters occurred at 300-500 ºC; and the partial removal of alkyl bridges

567

between aromatic moieties occurred at 450-500 ºC.182 The related results showed that

568

short carbon chain aldehydes, furans and ketones were the major adsorbed VOCs on

569

biochars pyrolyzed below 350 ºC, while adsorbed aromatic compounds and longer

570

carbon chain hydrocarbons dominated in biochars produced under 350 ºC.

571

Regardless of the impact of the pyrolysis temperature, gasified biochars contained

572

higher polycyclic aromatic hydrocarbons (PAHs) and more bioavailable PAHs than

573

produced by fast pyrolysis, which were generally less than biochars derived at slow

574

pyrolysis temperatures.185 Normally, the total PAH content within a biochar was ~60

575

mg kg-1, which also depends on the type of precursors. For example, biochars produced

576

from rice husks contained 64 mg total PAHs/kg, whereas wood biochar produced 9.5

577

mg kg-1 PAH.189 Hydrothermally carbonized plant materials displayed much higher

578

VOC contents (0.2-1.6 wt.%), including benzenic, phenolic and furanic volatiles along

579

with other aldehydes and ketones.183 Mostly, the total PAHs refers to the 16 USEPA

580

PAHs, which are considered toxic and persistent. They generally decrease with 29 / 78



581

increasing pyrolysis time and temperature,185 and some researchers suggested that the

582

solvent-extractable PAH concentrations in biochars reach a peak concentration at a

583

pyrolysis temperature between 400 ºC and 500 ºC.187, 191 However, some researchers

584

argued that the total PAHs extracted from biochars are quite low in most cases, and they

585

are below the existing environmental quality standards (maximum acceptable

586

concentration of 8 mg kg-1 for Norway).184, 185

587

The conversion of sewage sludge containing noticeable PAHs to biochar could

588

greatly reduce the PAH content,190 and the addition of biochar could adsorb the

589

dissolved PAHs from sewage sludge.192 Thermal drying at 100 ºC, 200 ºC and 300 ºC

590

can greatly reduce the PAH concentrations of biochar within 24h at the levels of 33.8–

591

88.1%, >93.3% and >97%, respectively.188 In addition, blending with low VOC biochar

592

(containing 90% low VOC biochar) was effective in reducing the toxicity of small

593

compounds by at least 50% while simple open-air storage was insufficient.179 In short,

594

the extraction of small compounds comprised only a small part of the total biochar mass

595

(generally

A Critical Review - ACS Publications - American Chemical Society

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