1 Introduction

Biomass is the most abundant and sustainable feedstock on the earth (Antar et al. 2021). It is highly desirable to develop high-efficient materials from these renewable and sustainable resources. Biochar is the solid carbon-rich product from the thermochemical conversion of biomass, including torrefaction (Li et al. 2019b), hydrothermal carbonization (Dai et al. 2017), pyrolysis (Dai et al. 2018) and gasification (Liu et al. 2021c). Attributed to its various merits, e.g., renewable and sustainable source, carbon-negative potential, and easy production, biochar has attracted considerable attention as a platform material for various applications (Jin et al. 2021; Li et al. 2020b). Nevertheless, direct carbonization of biomass into biochar still suffers from some undesirable limitations, e.g., low porosity (typically < 150 m2/g for wood biochar and < 50 m2/g for crop straw biochar) and poor surface functionality (Liu et al. 2015). The performances of biochar-based materials involve the physical and chemical interactions at the interface and surface. Specifically, surface oxygenated functional groups (OFGs) are the dominant and most important surface groups on biochar-based materials (Dai et al. 2021). They affect various surface properties of biochar-based materials, e.g., polarity, acidity, wettability, catalytic activity, electroactivity, and chemical activities (Dai et al. 2021), while the porosity can limit the kinetics and rate of the physical and chemical interactions. Therefore, the porosity and surface functionality play critical roles in the selective application performance of biochar.

Biochar-based materials, in the form of hydrochar, pyrochar, engineered biochar, biomass-derived porous carbon (activated biochar), or biochar-based composites, have been extensively developed for various advanced applications (Borghei et al. 2018; Clurman et al. 2020; Guo et al. 2018; Wan et al. 2019; Wen et al. 2018). Booming advances have been achieved in the surface engineering of biochar-based materials (Ibrahim et al. 2021; Jin et al. 2021; Li et al. 2020b; Liu et al. 2019; Wang et al. 2018c; Wareing et al. 2021; Yang et al. 2019; Zhou et al. 2021). Generally, there are three kinds of surface engineering routes, including pore development (i.e., activation) (Chen et al. 2015), enrichment of surface organic functional groups (e.g., functional groups containing oxygen, nitrogen, sulphur or other elements) (Jin et al. 2020), and introduction of inorganic component (e.g., transition metal and its (hydr)oxides, clay minerals, and carbon materials) (Wan et al. 2020). Among these surface engineering strategies, air oxidation has several advantages, such as its low cost and easy access to air, green processes without the need of extra chemicals and the production wastewater, and multi-efficacy in pore development and surface oxygenation (Dai et al. 2021). Consequently, air oxidation has attracted increasing attention in surface engineering of biochar-based materials.

Air oxidation of carbon is one of the most important chemical reactions. In the energy and combustion area, air oxidation of fuels and soots has been extensively researched (Frenklach et al. 2018; Rotavera & Taatjes 2021; Wang et al. 2017c, 2019a; Xi et al. 2021). In addition, controlled partial oxidation of carbon material with air/O2 is critical for its surface functionality (Jiang et al. 2022; Molavi et al. 2018). Air oxidation is an O2 chemisorption process on the free active sites of a carbon material (Batchu et al. 2021; Frenklach et al. 2018; Xu & Ye 2014), resulting in the etching of carbon matrix as CO or CO2 and introduction of OFGs on the carbon surface. Therefore, air oxidation is efficient for porosity development and/or surface oxygenation of biochar-based materials. However, there is still a lack of in-depth understanding of air oxidation in the surface engineering of biochar-based materials.

Numerous review articles have summarized the progress of biochar-based materials for various advanced applications (Dai et al. 2021; Jin et al. 2021; Li et al. 2020b; Liu et al. 2019; Wareing et al. 2021; Zhou et al. 2021). Few of them focused on knowledge of air oxidation in biochar preparation and modification. Recently, Dai et al. (2021) reviewed the strategies for enriching surface OFGs on biochar for water pollution control, and Xiao (2022) reviewed the progresses of direct in-situ air oxidation during pyrolysis and the post air oxidation of biochar after pyrolysis in enhancing the performances of resultant biochars in adsorption of various pollutants. These reviews overlooked some important aspects about the application of air oxidation in the preparation and modification of various biochar-based materials, e.g., hydrochar, pyrochar, activated biochar and biochar composites for various applications. Actually, besides the application of air oxidation in aerobic pyrolysis or as a post-modification step, air oxidation can be directly applied in torrefaction (aerobic torrefaction), and as a pretreatment step before pyrolysis, activation, or doping heteroatoms and metal species. Besides the application as an adsorbent, the resultant biochar can be applied in catalysis and energy storage. Moreover, the evolution of pore structure and surface chemistry during air oxidation, and the mechanisms and controlling factors for air oxidation have not been well analyzed. Herein, this review analyzed the mechanisms of air oxidation, summarized the application of air oxidation under conventional heating conditions in the preparation and modification of biochar-based materials, investigated the impacts of controlling factors (including operation parameters and intrinsic properties of biochar) on pore development and surface oxygenation during air oxidation, and discussed the enhanced performances of air oxidation-engineered biochar-based materials in various applications. This review also highlighted the challenges of air oxidation in surface engineering of biochar-based materials. Finally, this review might inspire new discoveries to promote the application of air oxidation in surface engineering of biochar-based materials for various advanced applications.

2 Mechanisms of air oxidation

2.1 Air oxidation processes on biochar surface

The interactions of O2 with various carbon-based materials and fuels, e.g., graphite (Kane et al. 2017; Rodriguez et al. 2021), graphene (Cao et al. 2022; Wang et al. 2017a), carbon nanotube (Arshad 2020; Huang et al. 2016; Lavagna et al. 2021), soot (Frenklach et al. 2018), and coal (Vallejos-Burgos et al. 2016; Xi et al. 2021), and fuels (Rotavera & Taatjes 2021; Wang et al. 2017c, 2019a) have been well documented by experiments and theoretical calculations. Compared to these carbon-based materials and fuels, little is known about the interaction between O2 and biochar. As shown in Fig. S1, in favor of theoretical calculation (e.g., density functional theory (DFT), molecular dynamic simulation (MDS)), various aromatic clusters and hydrocarbons have been applied as model molecules to investigate the air oxidation chemistry of carbon-based materials and fuels (Fig. S1). Biochar contains both aromatic and aliphatic hydrocarbons. Thus these results are available for shedding light on the air oxidation chemistry of biochar.

Air oxidation produces gaseous CO and CO2, and temporarily stable carbon–oxygen surface complex, C(O) or C(O2) (Fig. 1a). These carbon–oxygen surface complexes as intermediate species include ether, ketone, hydroxyl, carbonyl, anhydride, peroxide, dioxyranyl, carboxyl, quinone, phenol, lactone and oxypinyloxy (Raj et al. 2012). Thus air oxidation is efficient in etching carbon matrix and enriching surface OFGs. The mechanistic interaction of O2 with the typical graphene cluster (molecule) was proposed by Radovic et al. (2011) (Fig. 1a), and shown as follows:

$$\begin{array}{llr} &{\mathrm C}_{\mathrm{zz}}+{\mathrm O}_2\left(\mathrm g\right)\ \longrightarrow \ {\mathrm C}_{\mathrm e}\left({\mathrm O}_2\right)\;\mathrm{Molecular}\;\mathrm{oxygen}\;\mathrm{chemisorpotion}\;\left(\mathrm{non}-\mathrm{dissociative}\right) & (\textrm{a}1)\\ &2{\mathrm C}_{\mathrm{zz}}+{\mathrm O}_2\left(\mathrm g\right)\ \longrightarrow \ 2{\mathrm C}_{\mathrm e}\left(\mathrm O\right)\;\mathrm{Dissociative}\;\mathrm{oxgen}\;\mathrm{chemisorption} & (\textrm{a}2)\\ &2{\mathrm C}_{\mathrm{ac}}+{\mathrm O}_2\left(\mathrm g\right)\ \longrightarrow \ 2{\mathrm C}_{\mathrm e}\left(\mathrm O\right)\;\mathrm{Dissociative}\;\mathrm{oxgen}\;\mathrm{chemisorption} & (\textrm{a}3)\\ &{\mathrm C}_{\mathrm e}\left({\mathrm O}_2\right)+{\mathrm C}_{\mathrm b}\ \longrightarrow \ {\mathrm C}_{\mathrm e}\left(\mathrm O\right)+{\mathrm C}_{\mathrm b}\left(\mathrm O\right)\;\mathrm{Dissociation},\;\mathrm{spillover}\;\mathrm{to}\;\mathrm{basal}\;\mathrm{plane} & (\textrm{b}1)\\ &{\mathrm C}_{\mathrm b1}\left(\mathrm O\right)+{\mathrm C}_{\mathrm b2} \ \longleftrightarrow \ {\mathrm C}_{\mathrm b1}+{\mathrm C}_{\mathrm b2}\left(\mathrm O\right)\;\mathrm{Surface}\;\mathrm{diffusion} & (\textrm{b}2)\\ &{2\mathrm C}_{\mathrm e}+{\mathrm O}_2\left(\mathrm g\right)\ \longrightarrow \ {2\mathrm C}_{\mathrm e}\left(\mathrm O\right)\;\mathrm{Dissociative}\;\mathrm{oxygen}\;\mathrm{chemisorption} & (\textrm{b}3)\\ &{\mathrm C}_{\mathrm e}\left(\mathrm O\right)+{2\mathrm C}_{\mathrm b}\ \longrightarrow \ \mathrm{CO}\left(\mathrm g\right)+2\mathrm C_{{\mathrm{e}}^{\mathrm{^{\prime}}}}\;\mathrm{Direct}\;\mathrm{desorption}\;\mathrm{as}\;\mathrm{CO} & (\textrm{c}1)\\ &{\mathrm C}_{\mathrm e}\left({\mathrm O}_2\right)+{2\mathrm C}_{\mathrm b}\ \longrightarrow \ {\mathrm{CO}}_2\left(\mathrm g\right)+2\mathrm C_{{\mathrm{e}}^{\mathrm{^{\prime}}}}\;\mathrm{Direct}\;\mathrm{desorption}\;\mathrm{as}\;{\mathrm{CO}}_2 & (\textrm{c}2)\\ &{\mathrm{C}}_{\mathrm{e}}\left(\mathrm{O}\right)+{\mathrm{C}}_{\mathrm{b}}\left(\mathrm{O}\right)+{\mathrm{C}}_{\mathrm{b}} \ \longleftrightarrow \ {\mathrm{CO}}_{2}\left(\mathrm{g}\right){2\mathrm{C}}_{{\mathrm{e}}^{\mathrm{^{\prime}}}}\;\mathrm{Indirect}\;\mathrm{desorption}\;\mathrm{as}\;{\mathrm{CO}}_{2} & (\textrm{c}3)\\ &{2\mathrm{C}}_{{\mathrm{e}}^{\mathrm{^{\prime}}}} \ \longleftrightarrow \ {2\mathrm{C}}_{5\mathrm{e}}\;\mathrm{Deactivation}\;\mathrm{to}\;\mathrm{5}\;\mathrm{membered}\;\mathrm{rings} & (\textrm{c}4) \end{array}$$

where Czz represents the carbene-like zig-zag carbon, Cac represents the carbine-like armchair carbon, Ce represents the edge carbon, Cb represents the basal plane carbon, Ce’ represents the nascent edge site with different coordination and reactivity with Ce, and C5e represents the newly formed 5 membered rings after the desorption of CO or CO2. The reactions (a1) and (a2) represent the chemisorption of O2 on carbon edge site, specifically on activated sites such as zig-zag carbene sites and armchair carbyne sites (Fig. 1a). The direct chemisorption of O2 onto the basal plane of carbon is not thermodynamically favorable (Fig. 1a) (Silva-Tapia et al. 2012; Yamada et al. 2014, 2018), which might be attributed to the fact that the chemisorption of O2 on defect-free basal plane of graphene layer is an endothermic process, while on the active sites, it’s an exothermic process. The chemisorption of O2 by the various active sites is a barrierless process, which can be achieved under ambient conditions (Li et al. 2020a; Silva-Tapia et al. 2012). For example, chemisorption of O2 on the armchair sites of the model nanotube molecules (i.e., C48H14 and C46H18) is a barrierless process with a quite low activation energy and forming two quinone groups as the final adsorption product (Silva-Tapia et al. 2012). Thus Li et al. (2020a) observed that the storage of biochar in ambient air condition for 6 months decreased its electron donating capacity, while increased its electron accepting capacity. In addition, biochar from pyrolysis was rich in oxygen-/carbon-centered PFRs, which would react with O2 under ambient condition. Thus the remarkable decline (~ 75%) of PFR concentration in biochar after the storage for 6 months was also observed (Li et al. 2020a).

Fig. 1
figure 1

a The schematic illustration of the interaction between aromatic active site and O2 (modified from Kane et al. (Kane et al. 2017)). b The interaction between aliphatic active site and O2

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Attributed to the rich content of aromatic and aliphatic carbon atoms in biochar, besides the classic chemisorption of O2 on the aromatic active sites, the chemisorption of O2 by the aliphatic carbon should also be considered (Rotavera & Taatjes 2021). The interaction between O2 and aliphatic carbon molecules (e.g., alkane, alcohol) has a different reaction route (Hansen et al. 2021; Jalan et al. 2013; Savee et al. 2015; Tang et al. 2020; Wang et al. 2019a; Xi et al. 2021). Air oxidation of alkane forms carboxyl and ketone groups through the classical Korcek decomposition of ketohydroperoxide (KHP) (Fig. 1b) (Hansen et al. 2021; Jalan et al. 2013; Savee et al. 2015; Tang et al. 2020; Wang et al. 2019a; Xi et al. 2021). The pathway starts from the chemisorption of O2 by alkane radical R• forming ROO• (Hansen et al. 2021; Savee et al. 2015). The isomerization of ROO• into a carbon-centered radical (hydroperoxyalkyl radical, typically denoted as QOOH) is then achieved via intramolecular hydrogen abstraction (Bhagde et al. 2022; Hansen et al. 2021; Jalan et al. 2013; Savee et al. 2015). The further reaction of QOOH with O2 forms OOQOOH intermediate which is decomposed into KHP and •OH (Hansen et al. 2021; Jalan et al. 2013; Savee et al. 2015). The KHP will be decomposed to acids and ketones (Hansen et al. 2021; Jalan et al. 2013; Savee et al. 2015). Thus previous results showed that low aromatic biochars under the air oxidation conditions showed remarkable increases in carboxyl groups after air oxidation (Chen et al. 2011; Dai et al. 2020; Wang et al. 2018b).

It’s worth noting that, besides the dominant free edges on a carbon material, the OFGs on carbon surface are also responsible for the chemisorption of O2 (Wang et al. 2013b). The DFT calculation results by Wang et al. (2013b) showed that OFGs on the surface of graphene have lower adsorption energies than the aromatic carbon site for the chemisorption of O2, suggesting that the original OFGs on carbon surface are more thermally favorable for the chemisorption of O2. Consequently, the original and newly formed OFGs on carbon surface might be oxidatively removed during air oxidation.

2.2 Pore development versus surface oxygenation during air oxidation

Air oxidation has been extensively applied to modify the pore structure and/or surface functionality of various biochar-based materials from various sources and various carbonization routes. Some studies focused on pore development by air oxidation (Chen et al. 2015; Xiao et al. 2020), while some studies focused on the surface oxygenation by air oxidation (Chen et al. 2011; Clurman et al. 2020; Han et al. 2018). In addition, most of the studies investigated the evolution of both pore structure and surface functionality (Dai et al. 2020). These results suggested that, although air oxidation is efficient in pore development and surface oxygenation, the evolution of pore structure is not consistent with the evolution of surface functionality. For example, the specific surface area of cuttlebone biochar was increased from 535 to 1489 m2/g after air oxidation at 300 °C for 3 h, then declined to 846 m2/g after air oxidation at 300 °C for 8 h, while the bulk and surface oxygen contents on the biochar were gradually increased with the progressing of air oxidation (Guo et al. 2018).

The pore development process is an etching process of biochar carbon matrix by carbon–oxygen reactions. With the progressing of the oxidation, new pores are formed. However, the pore formation process might also destruct the wall between pores, resulting in the loss of structure for holding different pores and subsequent collapse of pore network. So over-activation is always observed during the preparation of activated biochar.

The differed evolution processes of pore development and surface oxygenation are presumably ascribed to the fact that some oxygen is not distributed on the edge of surface. The oxygen is initially adsorbed at the edge sites, while with the progressing of oxidation, migration of oxygen to the basal plane occurred (Batchu et al. 2021; Kane et al. 2013). The migration of oxygen to basal plane has been experimentally and theoretically elucidated (Batchu et al. 2021; Kane et al. 2013). Therefore, with the progressing of oxidation to a certain point, although the surface area is declined, oxygen is not only accumulated on the edge sites, but also on the basal planes, resulting in a higher oxygen content after the oxidation.

3 Surface engineering via air oxidation

Figure 2 shows the schematic illustration of air oxidation in surface engineering of biochar-based materials. Air oxidation can be applied in the carbonization processes (e.g., torrefaction and pyrolysis in the presence of air). It has also been extensively applied as a post modification process to prepare porous and/or oxygenated biochars. In addition, it can be integrated as a pretreatment process into other thermochemical or chemical processes (e.g., activation, metal doping, NH3 annealing). Thus the application of air oxidation process for the preparation of biochar-based materials is quite flexible and diverse.

Fig. 2
figure 2

Schematic illustration of the process of air oxidation in surface engineering of biochar

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3.1 Air oxidation during the carbonization process

3.1.1 Air oxidation in torrefaction (oxidative torrefaction)

Torrefaction of biomass is conducted at a low temperature (i.e., 200 – 300 °C) (Chen et al. 2021). Traditionally, torrefaction is conducted in an anaerobic condition. Recently, attention is payed on the torrefaction of biomass in air atmosphere for the preparation of oxygenated biochar for various applications. For example, Kobayashi et al. (2016) and Li et al. (2019b) observed that direct torrefaction of biomass in air atmosphere facilitated the formation of carboxyl and/or ketone groups on biochar surface (Fig. 3). Specifically, the biochar from oxidative torrefaction of Eucalyptus powder in air at 300 °C for 1 h showed 2.1 mmol/g carboxyl groups (Kobayashi et al. 2016).

Fig. 3
figure 3

a The FTIR spectra of corn stover, biochars from oxidative and anoxic torrefaction. Reprinted with permission from Li et al. (2019b). Copyright 2019 Elsevier Ltd. b The NMR spectra of wood powder (blue dashed line), biochar from oxidative torrefaction of wood powder (red bold solid line), the biochar from oxidative pyrolysis of the wood powder residue after hydrolysis (orange narrow solid line), and the biochar from anoxic torrefaction for wood powder (green dotted line). Reprinted with the permission from Kobayashi et al. (2016). Copyright 2016 RSC

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In addition, compared to torrefaction in nitrogen atmosphere, air oxidation under torrefaction facilitates the aromatization of pristine biomass (Kobayashi et al. 2016; Li et al. 2019b). As shown in Fig. 3a, the biochar prepared from oxidative torrefaction at 250 °C had a weaker intensity of the peak at around 2920 cm−1 than the biochar from torrefaction in N2, suggesting that oxidative torrefaction facilitated the removal of aliphatic carbon in the resultant biochar. In Fig. 3b, a dominant peak at 125 pm ascribed to the non-oxygenated aromatic carbons was observed, which was not observed for the resultant biochar prepared at the same temperature in N2 atmosphere (Kobayashi et al. 2016). Low temperature torrefaction in N2 showed little impact on the chemical structure of pristine biomass (Fig. 3b). Consequently, the resultant biochar from air torrefaction showed a black color (Kobayashi et al. 2016). The enhanced aromatization of oxidative pyrolysis might be ascribed to the ring closure reactions in unsaturated alkylperoxyl radicals during air oxidation (Barber et al. 2021; Vereecken et al. 2021). Therefore, oxidative torrefaction facilitates the preparation of biochar with a higher carbonization degree and richer carboxyl groups than the biochar from conventional torrefaction in inert atmosphere.

3.1.2 Air oxidation in pyrolysis (oxidative pyrolysis)

Conventionally, pyrolysis is carried out under inert atmosphere. Different from the pyrolysis at inert atmosphere, oxidative pyrolysis with restricted supply of air will provide the heat through the exothermic char-oxygen and/or volatile-oxygen reactions, initiating the primary thermal degradation of biomass and the subsequent secondary reactions (Fig. 4a) (Huang et al. 2020c). Besides the impact of oxidative pyrolysis on fuel properties of biogas, biofuel and biochar, the impact of oxidative pyrolysis on the biochar surface properties is also intriguing (Bakshi et al. 2021). Previous results have suggested that the pore structure and OFGs could be modified under oxidative pyrolysis conditions without significant changes in char yield (Li et al. 2019a; Liu et al. 2018; Yang et al. 2021). Generally, the surface area and porosity of biochar from oxidative pyrolysis are greatly increased with the presence of low amount oxygen at a mild temperature (Fig. 4b) (Liu et al. 2018; Plaza et al. 2014). The temperature and oxygen concentration during oxidative pyrolysis are controlling parameters controlling the pore structure of biochar from oxidative pyrolysis (Fig. 4b) (Kim et al. 2014a, 2014b; Liu et al. 2021a). For example, Kim et al. (2014a) reported that the surface area of red oak biochar from oxidative pyrolysis was increased from 2.4 m2/g under inert atmosphere to 93.1 m2/g with 4.2% oxygen, then decreased to 77.6 m2/g with a higher oxygen concentration. The decline of porosity under higher oxygen concentration might be ascribed to the collapse of pore structure induced by the over-oxidation of the biochar matrix. Thus an optimal O2 concentration is required to promote the development of biochar porosity under oxidative pyrolysis condition.

Fig. 4
figure 4

a The schematic illustration of oxidative pyrolysis. b The N2 adsorption–desorption isotherms for the biochars from oxidative pyrolysis of pinewood at various conditions. Reprinted with permission from Li et al. (2019a). Copyright 2018 Elsevier Ltd. c The FTIR spectra of biochars from oxidative pyrolysis of red oak in various O2 concentrations c. Reprinted with permission from Kim et al. (2014a). Copyright 2014 Elsevier Ltd

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Besides the development of porosity, oxidative pyrolysis facilitates the oxygenation of biochar surface (Kim et al. 2014a; Li et al. 2019a, 2017; Liu et al. 2021a). Generally, at a certain temperature, the increase of O2 concentration is favorable for the increase of bulk and surface oxygen contents of biochar from oxidative pyrolysis (Kim et al. 2014a; Li et al. 2019a; Liu et al. 2018). Li et al. (2019a) observed that, at 800 °C pyrolysis temperature, the bulk oxygen content of pine wood biochar was increased from 1.29% under inert atmosphere to 1.44% under 1/10 air atmosphere, then to 1.74% under 1/5 air atmosphere. Kim et al. (2014a) observed that the band intensity at 1712 cm−1 in FTIR spectra (ascribed to aromatic –COOH groups) for the red oak biochar from oxidative pyrolysis at 500 °C was enhanced with the increase of O2 concentration, suggesting the enhanced carboxylation of biochar surface with the increase of O2 concentration (Fig. 4c). However, some studies observed that the bulk oxygen content and surface acidic OFGs were decreased with the increase of O2 concentration (Liu et al. 2021a). For example, Liu et al. (2021a) found that, under oxidative pyrolysis at 450 °C, the oxygen content of corn stover biochar was decreased from 15.36% in 0% O2 to 12.81% in 6% O2. Thus the oxygenation of biochar under oxidative pyrolysis condition is a complex process, which might be a combined effect of O2 concentration and temperature.

3.2 Air oxidation as a post-modification process

Biochar-based materials always suffer from low porosity and/or poor surface functionality. For example, pyrochar has a low porosity and a poor content of OFGs attributed to the high temperature-induced deoxygenation during pyrolysis; although activated biochar is high in porosity, it is poor in surface functionality due to the fact that high temperature activation process of activation is not favorable for the preservation of OFGs. Thus biochar-based materials need further surface engineering to improve their porosity and/or surface functionality.

Recently, air oxidation has been extensively applied as a post-modification process to improve the porosity and/or surface functionality of various biochar-based materials (Tables S1—S2, Fig. 5a), including pyrochar (Clurman et al. 2020; Dai et al. 2020; Xiao & Pignatello 2016; Yang et al. 2021), hydrochar (Han et al. 2018; Huang et al. 2020a; Yan et al. 2018), activated biochar (Chairunnisa et al. 2020; Lawtae & Tangsathitkulchai 2021; Wang et al. 2013a) and biochar-based composites (Bakshi et al. 2021; Liu et al. 2021b; Wang et al. 2017b, 2018a). Tables S1 and S2 summarize the efficacy of air oxidation on the pore structure and surface chemistry of various biochar-based materials, respectively. Generally, air oxidation can simultaneously increase specific surface area and porosity, and enrich surface OFGs of biochar-based materials (Fig. 5a - c) (Clurman et al. 2020; Dai et al. 2020; Ducousso et al. 2015; Xiao & Pignatello 2016; Yang et al. 2021). For example, the specific surface area for the bamboo biochar from gasification was increased from 388 to 554 m2/g after air oxidation at 300 °C for 30 min (Fig. 5b) (Dai et al. 2020). The increase of specific surface area is dependent on the structure of biochar. Dai et al. (2020) noticed that air oxidation was more efficient in increasing the specific surface area of biochars from gasification and/or with poor ash contents.

Fig. 5
figure 5

a The schematic illustration of air oxidation on pore development and surface oxygenation of biochar. b The change of specific surface areas of various biochars after air oxidation, c the FTIR spectra of rice straw biochar before and after air oxidation. Reprinted with permission from Dai et al. (2020). Copyright 2019 Elsevier Ltd. d The XPS spectra of cuttlebone biochar before and after air oxidation for various times, and e the release of various gases with the change of temperature in air atmosphere. Reprinted with permission from Guo et al. (2018). Copyright 2018 American Chemical Society

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Attention should be paid on the air oxidation of activated biochar (activated carbon) (Table S1). Both increase and remarkable decrease of specific surface area were observed. For example, the specific surface area of steam activated wood biochar was decreased from 1025 to 906 m2/g after oxidation in air at 200 °C for 1 h (Bardestani & Kaliaguine 2018), while that of the steam activated biochar from nutshell was increased from 780 to 1025 m2/g after oxidation in air at 300 °C for 2 h (Chairunnisa et al. 2020).

It’s worth noting that when nitrogen-rich biochar is modified by air oxidation, oxygen and nitrogen contents of biochar could be simultaneously increased (Fig. 5d). For example, oxygen and nitrogen contents of the nitrogen-rich cuttlebone biochar were increased from 7.6% and 7.2%, to 13.1% and 8.4%, respectively, after air oxidation at 300 °C for 3 h, then remarkably increased to 16.3% and 12.9%, respectively, after air oxidation for 8 h (Fig. 5d) (Guo et al. 2018). This might be attributed to the relative thermal stability of nitrogen-containing groups (Fig. 5e), which were preserved during air oxidation, while the carbon was consumed during air oxidation. Thus besides the impacts on pore structure and surface OFGs, air oxidation as the post modification process also has an impact on the composition of heteroatoms on biochar surface.

3.3 Air oxidation as a pretreatment process

Air oxidation as a pretreatment process has been applied to pretreat biomass before pyrolysis or chemical activation, or to pretreat biochar before physical activation, metal doping, or NH3 annealing. In favor of air oxidation, various surface engineered biochar materials were developed.

3.3.1 Air pre-oxidation before pyrolysis

In 1999, Toles et al. (1999) observed that the activated biochar prepared by air pre-oxidation of H3PO4-impregnated nut shell in air at 170 °C for 0.5 – 1 h before pyrolysis at 400 °C in N2 for 1 h was more efficient than the activated biochar prepared by direct pyrolysis for lead (Pb) removal. Recently, air pre-oxidation before pyrolysis has been applied to prepare spherical biochar from potato peel waste (Yang et al. 2018), and oxygen/nitrogen/phosphorus co-doped biochars from ammonium phosphate-pretreated corn stover (Tang et al. 2021) and stillage (Jin et al. 2020). Commonly, air pre-oxidation also facilitates pore development and/or oxygenation (Jin et al. 2020; Tang et al. 2021; Yang et al. 2018). Yang et al. (2018) further observed that air pre-oxidation facilitated the preparation of spherical biochar from waste starch, while direct pyrolysis would result in the collapse of pristine morphology of starch. Therefore, air pre-oxidation before pyrolysis is favorable for the preparation of engineered biochar.

3.3.2 Air pre-oxidation before activation

Air pre-oxidation of carbon precursor is efficient in promoting pore development during the activation process (Shang et al. 2018; Tam & Antal 1999). Shang et al. (2018) reported that air pre-oxidation of wood-derived phosphoric acid-impregnated microsphere at 220 – 310 °C in air for 1 h before activation at 800 °C in N2 for 1 h was favorable for the development of microporosity. The BET surface area and microporous surface area were increased from 1229 and 982 m2/g to 1473 and 1209 m2/g, respectively, by air pre-oxidation at 250 °C. However, mesoporosity was not efficiently affected by air pre-oxidation. These results suggested that the chemically adsorbed oxygen on the biomass precursor might contribute to the formation of micropores in the resultant activated biochar (activated carbon).

While Lawtae and Tangsathitkulchai (2021) observed that air pre-oxidation of the microporous activated carbon (prepared by CO2 activation of longan seed biochar at 850 and 900 °C) at 230 °C for 12 h before reactivation by CO2 facilitated the development of mesoporosity and total specific surface area in the resultant activated carbon. Specifically, a volume of 0.474 cm3/g (accounted for 44.1% for the total pore volume) and a BET surface area of 1773 m2/g were achieved with the favor of air pre-oxidation of microporous activated carbon before reactivation (Lawtae & Tangsathitkulchai 2021). These results suggested that the decomposition of OFGs on the micropores might enlarge the micropores to mesopores.

3.3.3 Air pre-oxidation before NH 3 annealing

Surface chemistry significantly affects the performance of biochar-based materials. The surface chemistry is controlled by the heteroatoms (e.g., oxygen, nitrogen) on biochar surface in the form of various surface functional groups. Among the surface functional groups, OFGs and nitrogen-containing functional groups are the most important groups for the performance of biochar-based materials. Recently, NH3 annealing has been developed for nitrogen doping onto biochar (Lian et al. 2016; Mian et al. 2018). However, NH3 treatment is not as effective as air oxidation in developing porosity of biochar. Wang et al. (2021) proposed that air pre-oxidation before NH3 annealing was favorable for the preparation porous nitrogen-doped biochar (Fig. 6a). By this strategy, the BET surface area of the resultant biochar reached 724.4 m2/g (Wang et al. 2021), which was much higher than the nitrogen-doped biochars from direct NH3 annealing (Lian et al. 2016; Mian et al. 2018). Under NH3 annealing condition, the surface OFGs formed during air pre-oxidation would be consumed by NH3, forming amine-containing sites on biochar surface (Wang et al. 2021). Thus the consumption of OFGs during NH3 annealing facilitated both pore development and amine group formation.

Fig. 6
figure 6

a Schematic illustration of the air pre-oxidation followed by NH3 annealing for the preparation of porous biochar with basic surface chemistry. Reprinted with permission from Want et al. ( 2021). Copyright 2020 Elsevier Ltd. b The elemental mapping images of nano ZVI doped biochar b. Reprinted with permission from Mortazavian et al. (2019). Copyright 2019 The Korean Society of Industrial and Engineering Chemistry

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3.3.4 Air pre-oxidation for metal species doping

The OFGs, e.g., ketone, lactone and carboxyl groups, on the surface of a carbon material can complex with metal ions, favoring the doping of transition metal species on biochar surface for advanced applications (Bardestani et al. 2020; Saha et al. 2020; Yan et al. 2015). So surface oxygenation is an important route to create anchor sites for doping of metal species on carbon surface (Bardestani et al. 2020; Saha et al. 2020; Yan et al. 2015). Recently, chemical oxidation is a conventional strategy to create anchor sites for doping metal species. For example, Saha et al. (2020) have reported that chemical oxidation of biochar facilitated the doping of NiFe layered double hydroxide (NiFeLDH) by enhancing the coordination between OFGs and metal species.

Compared to the chemical oxidation as a pretreatment step for doping metal species, air pre-oxidation before metal species doping received less attention. Recently, Mortazavian et al. (2019) investigated the efficacy of air pre-oxidation in promoting the doping of zero valent iron (ZVI) nanoparticles on a biochar surface, which showed that air pre-oxidation increased the iron content and dispersity on biochar surface, and the iron content was increased with the air oxidation time of the biochar (Fig. 6b) (Mortazavian et al. 2019). Bardestani et al. (2020) further found that air pre-oxidation of biochar at 230 °C for 1 h enhanced the doping of ruthenium oxide on biochar surface. Thus air oxidation can be applied as a pretreatment step before the doping of metal species on biochar surface.

3.4 Combined use of air pre-oxidation and post-oxidation

Liu et al. (2021b) developed a combined use of air pre-oxidation and post-oxidation for surface engineering of MnOx/biochar composite. In this route, the raw biochar was initially oxidized in air at 400 °C for 2 h before impregnation of biochar with manganese nitrate at room temperature for 12 h, then the resultant biochar was re-oxidized at 250 °C for 2 h in air. The resultant MnOx/biochar composites had a higher porosity and a richer content of surface OFGs than the biochar composites without air pre-oxidation and post-oxidation, favoring the adsorption affinity to NH3. Attributed to the low temperature in air post-oxidation, this route avoided the reduction of high valance manganese (Mn), favoring the NH3 selective catalytic reduction (NH3-SCR) by high valence Mn. This study demonstrated that the combined use of air oxidation in the pretreatment and post-treatment step facilitated the preparation of MnOx/biochar composites with desired properties.

4 Controlling factors for air oxidation

Although air oxidation favors pore development and/or surface oxygenation of biochar-based materials, attention should be paid on the specific conditions of air oxidation for pore development and/or surface oxygenation. The efficacy of air oxidation is highly dependent on the operating parameters (including O2 condition, temperature, and time) and biochar intrinsic structure (including original biomass component, ash content, particle size, and carbonization degree) (Fig. 7). In addition, biochar after pyrolysis is always moisturized with water during its milling process or storage. However, no information is available on the impact of humidity during air oxidation process. Under some conditions, the porosity will be reduced, and the surface OFGs will be decomposed, leading to undesirable properties. Actually, no universal condition is technically available for guiding air oxidation in surface engineering of biochar-based materials. For example, under oxidative pyrolysis conditions, the evolution of pore structures of almond shell biochar with the change of temperature and O2 condition is not in line with that olive stone biochar (Plaza et al. 2014). Considering these, this review discussed the controlling factors of air oxidation for surface engineering (Fig. 7) to further promote its application in surface engineering of biochar-based materials.

Fig. 7
figure 7

Controlling factors for air oxidation

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4.1 Impacts of O 2 condition

Oxygen for surface engineering of biochar-based materials has several advantages, such as inexpensive source and easy availability, green process without extra chemicals, and energy saving ascribed to exothermic nature of carbon–oxygen reaction. However, the exothermic nature makes the reaction difficult to control, which is the main reason for its infrequent use as the physical activation agent than CO2 and H2O. Generally, two strategies have been developed to conduct the controlled air oxidation: reducing the temperature or lowering the O2 concentration (Huang et al. 2020a; Plaza et al. 2014; Zhu et al. 2018). Therefore, high concentration O2 (air) is directly used at a low temperature, while low concentration O2 is applied at a high temperature.

4.1.1 Pore development at various O 2 conditions

Generally, for air oxidation at a specific temperature for a certain time, higher O2 concentration facilitates pore development. Zhu et al. (2018) found that, for air oxidation at 700 °C for 3 h, the specific surface area was increased from 363 to 586 m2/g with the increase of air/N2 flux ratio from 0/200 mL/min to 50/200 mL/min. Then the further increase of air concentration showed little increase in porosity. However, for oxidative pyrolysis, both high and low O2 concentrations were observed to be favorable for pore development. For example, under oxidative pyrolysis condition, Li et al. (2019a) observed that when the temperatures were 700 and 800 °C, 1/5 air was more efficient than 1/10 air in promoting the porosity and surface area of wood biochar, while Zhao et al. (2014) observed that pine wood biochar from 21% O2 condition had a lower surface area than the biochar from 5% O2 condition.

In addition, O2 condition also has an impact on the optimal temperature and time for pore development. For example, Plaza et al. (2014) found that, at 500 °C, a shorter time (24 min) was favorable for pore development of olive stone-derived biochar at a high O2 concentration (21%), while a longer time (115 min) was favorable for pore development at a low O2 concentration (5%). Under low oxygen concentration (0.5% O2) for 3 h, a moderate temperature of 500 °C was more efficient in enhancing the porosity of hydrochar than the high temperature (700 °C) (Huang et al. 2020a). Thus the pore development of biochar is not only dependent on O2 condition, but also dependent on the biochar property, and the oxidation temperature and time.

4.1.2 Surface oxygenation at various O 2 conditions

Under oxidative pyrolysis conditions, both low and high O2 concentrations were observed to be efficient in surface oxygenation. For example, Li et al. (2019a) observed that, when the temperatures were 700 and 800 °C, 1/5 air was more efficient than 1/10 air in increasing the oxygen content and O/C ratio of the biochar, while Liu et al. (2021a) found that 2% O2 was more efficient than 4% and 6% O2 in increasing the acidic OFGs on the biochars by oxidative pyrolysis of corn stover and rice husk at 450 °C. Thus the impact of O2 condition on biochar surface OFGs during oxidative pyrolysis might be specific to the biomass feedstock, and pyrolysis temperature.

Air oxidation as post modification route for biochar surface oxygenation showed a different trend compared to oxidative pyrolysis. Under this condition, higher O2 concentration is more favorable for surface oxygenation of biochar. For example, Yang et al. (2021) observed that, in the temperature range of 250 – 400 °C, the increase of O2 concentration from 1 to 21% favored the increase of various acidic OFGs (i.e., carboxyl, lactone and phenol groups) on maple wood biochar. These observations suggested that the role of O2 condition was more dominant than other parameters when air oxidation was applied as a post modification route.

4.2 Impacts of temperature

Temperature is one of the leading factors that control the pore development and surface oxygenation of carbonaceous materials. The air oxidation temperature ranged from 100 to 850 °C in previous reports. For example, Chen et al. (2011) conducted air oxidation of glucose hydrochar at the temperature ranging from 100 to 350 °C, while Zhu et al. (2018) conducted air oxidation of wood biochar at the temperature ranging from 600 to 800 °C, and Zhao et al. (2019) conducted air oxidation at 850 °C in static air atmosphere to prepare porous carbon from hydrochar. Typically, the temperatures for air oxidation of biochar were in the range of 250 to 400 °C to balance the pore development and surface oxygenation (Zhao et al. 2019).

4.2.1 Pore development at different temperatures

Some studies reported that the porosity or surface area of biochar was gradually increased in a specific temperature range, while some studies reported that the porosity or surface area of biochar was initially increased with the increase of air oxidation temperature, followed by a decline with the further increase of temperature. For example, Zhu et al. (2018) observed that, at the air/N2 ratio of 50/200 mL/min, the specific surface area of a wood biochar (prepared at 400 °C) was increased gradually with the increase of air oxidation temperature from 600 to 800 °C for 3 h, and Chen et al. (2015) observed that the specific surface area of wood biochar (prepared at 800 °C) was gradually increased with the increase of air oxidation temperature from 300 to 370 °C for 5 h. While Xiao et al. (2020) observed that the specific surface area of the coconut shell biochar (prepared at 500 °C) was initially increased with the increase of air oxidation temperature from 400 °C, peaked at 500 °C, then sharply declined with the further increase of temperature. Therefore, no universal air oxidation temperature is available for pore development. The pore development of biochar by air oxidation might be a combined effect of temperature, time and O2 condition.

4.2.2 Surface oxygenation at different temperatures

High temperature is not favorable for the preservation of OFGs on carbon materials (Cerciello et al. 2021a; Yang et al. 2021), especially for carboxyl and lactone groups (Han et al. 2019). Experimental and theoretical results have shown that ether and ketone groups have higher thermal stabilities than carboxyl and lactone, while phenol groups have a moderate thermal stability (Cerciello et al. 2021b; Han et al. 2019). The formation and decomposition of OFGs compete with each other during air oxidation. Higher temperature is more thermodynamically favorable for air oxidation. However, high temperature facilitates the decomposition of OFGs. Therefore, the surface oxygenation efficiency is always initially increased with the increase of temperature, then declined. For example, Chen et al. (2015) observed that the surface O/C ratio and OFGs of wood biochar (prepared at 800 °C) were gradually increased with the increase of air oxidation temperature from 330 to 360 °C for 5 h oC. Thus for a wider temperature range, Yang et al. (2021) observed that acidic OFGs on maple wood biochar were initially increased with the increase of air oxidation temperature from 250 °C, peaked at 350 °C, then declined at 400 °C.

Moreover, different OFGs will show different evolution processes with the change of air oxidation temperature, presumably ascribed to their different thermal stability and reactivity in air oxidation. For example, Chen et al. (2011) found that the amount of carboxyl groups on glucose hydrochar was increased with the increase of temperature from 100 to 350 °C, while the amount of phenol groups was initially increased, then gradually decreased with the increase of temperature. More efforts are needed to investigate the evolution of different OFGs during air oxidation process at different temperatures.

4.3 Impacts of time

A wide range of air oxidation time from 10 min to 24 h is applied for air oxidation. Typically, air oxidation time is < 2 h, dependenting on the processing temperature. For example, Dai et al. (2020) conducted air oxidation at 300 °C for 30 min, and the resultant biochar showed a much higher performance.

4.3.1 Pore development at different time periods

Like the impact of temperature, within a certain time range, the gradual increase of porosity and the initial increase followed by a decline in porosity were reported. Lopez et al. (2003) observed that the specific surface area of the olive stone-derived granular activated carbon was gradually increased with the increase of air oxidation time from 4 to 8 h at 350 °C, and Chen et al. (2015) also noticed that the specific surface area of wood biochar (prepared at 800 °C) was gradually increased with the increase of air oxidation time from 1 to 7 h at 350 °C. However, at a higher temperature (400 °C), the specific surface areas of four nutshell biochars (prepared at 500 °C) (Xiao et al. 2020) and one maple wood biochar (prepared at 400 °C) (Xiao & Pignatello 2016) were initially increased with the increase of air oxidation time, peaked at time range of 30 – 60 min, then sharply decreased with the increase of air oxidation time.

In addition, mesopore and micropore also have different evolution processes with the change of air oxidation time. An extended air oxidation time might widen micropores attributed to the deconstruction of walls between adjacent pores, resulting in enlarged micropores and increased mesoporosity (Xiao et al. 2020). There is an exception from this presumption (Guo et al. 2018). Guo et al. (2018) observed that the mesopores in cuttlebone-derived carbonaceous sheets became fewer and smaller with the increase of air oxidation time from 3 to 8 h at 300 °C. Therefore, the evolution of pore structure might be a combined effect of various factors, e.g., biochar feedstock or property.

4.3.2 Surface oxygenation at different time periods

The surface oxygenation of biochar is not dependent on its pore development during air oxidation. Although some studies reported that the specific surface area of biochar is declined with the increase of time, the oxygenation was gradually enhanced by the increase of air oxidation time (Guo et al. 2018; Xiao & Pignatello 2016). Generally, the extension of air oxidation time has a positive impact on surface oxygenation. For example, although the specific surface area of cuttlebone-derived carbonaceous sheets was sharply decreased from 1489 to 846 m2/g when the air oxidation time was extended from 3 to 8 h, while the bulk and surface oxygen contents were remarkably increased with the extension of air oxidation time (Guo et al. 2018). However, a longer processing time will result in a greater energy consumption and a lower product yield, which is not economically favorable. There should be a tradeoff between the performance and the oxygenation efficacy during air oxidation of biochar.

4.4 Impacts of original lignocellulosic composition

Lignin and cellulose are the main components in lignocellulosic biomass, e.g., wood, and nutshell. These components will be converted to amorphous carbon during the carbonization process. Chen et al. (2015) and Gan et al. (2021) observed that lignin-derived carbon in biochar favored the pore development during air oxidation. They found that lignin carbon had a lower graphitization degree than cellulose carbon, resulting in the lower resistance of lignin carbon to air oxidation (Chen et al. 2015; Gan et al. 2021). In favor of low resistance of lignin carbon to air oxidation, Chen et al. (2015) prepared mesoporous carbon with a specific surface area of 805 m2/g and an average pore diameter of 4.53 nm, from wood biochar (carbonized at 800 °C in N2 for 2 h) by air oxidation at 370 °C for 5 h. Gan et al. (2021) also obtained mesoporous carbon with a specific surface area of 555 m2/g and an average pore diameter of 2.3 nm, from high-lignin walnut shell-derived biochar by air oxidation the biochar at 320 °C for 4 h. However, Xiao et al. (2020) assumed that the pore development of biochar from lignin-rich biomass during air oxidation was attributed to the higher thermal stability of lignin-carbon, which was not consistent with the assumption of Chen et al. (2015) and Gan et al. (2021). Besides the impacts on pore development, little information is available for highlighting the impact of original lignocellulosic composition on surface oxygenation of biochar during air oxidation.

4.5 Impacts of ash minerals in biochar

The catalytic functions of ash during various thermochemical processes have been well documented (Huang et al. 2020b, 2022). During the interaction between O2 and char, minerals can act as the oxygen shuttle, improving the reaction rates, facilitating the transport of oxygen to the char surface, and enhancing the formation of CO2 during oxidation (Peterson & Brown 2020). However, there is still a lack of the investigation into the pore development and surface oxygenation of biochar before and after ash removal, which can be a direct proof for the impact of ash in pore development of biochar during air oxidation.

4.5.1 Pore development with the presence of minerals

Ash minerals might clog pores in biochar, resulting in a poor porosity. Air oxidation has been observed to be in efficient in enhancing the porosity of ash-rich biochar (Atienza-Martinez et al. 2021; Dai et al. 2020) and mineral-doped biochar (Wang et al. 2018a). For example, Dai et al. (2020) found that the specific surface area of the ash-rich corn stover biochar (49% ash) was decreased from 362 to 360 m2/g, while that for ash-poor bamboo biochar (13% ash) showed a remarkable increase of 166 m2/g to 554 m2/g after air oxidation at 300 °C for 30 min. Atienza-Martinez et al. (2021) also observed that the air oxidation was not efficient in increasing the ash-rich sewage sludge biochar. For the Fe2O3 composited biochar, Wang et al. (2018a) found that air oxidation also decreased its specific surface area and porosity. Therefore, the ash might be not favorable for pore development, presumably attributed to the block of pores by ash minerals.

4.5.2 Surface oxygenation with the presence of minerals

Although the catalysis of biomass combustion with the presence of ash minerals in biomass has been widely observed (Cai et al. 2017b), there is still a lack of the observation about the role of intrinsic ash minerals in biochar on surface oxygenation during air oxidation of biochar. In this review, the results about extra doping of minerals were applied as analogies. The results from Bakshi et al. (2021) showed that the iron doped biochar (prepared by oxidative pyrolysis of FeSO4 impregnated corn stover) had a higher increase in O/C ratio after air oxidation at 400 °C for 15 min than the undoped biochar. The results from Liu et al. (2021b) showed that the manganese nitrate impregnated coconut shell biochar showed a stronger intensity for the band at 1713 cm−1, representing a higher content of carboxyl C = O groups. Therefore, intrinsic ash minerals might promote the surface oxygenation of biochar.

4.6 Impacts of particle size of biochar

Particle size is important for mass transfer during a thermochemical process. Little is known about the effects of particle size on the pore development and surface oxygenation during air oxidation of biochar. Xiao et al. (2020) compared the pore evolution behaviors of granular and powdered biochars, and found that small particle size was more favorable for enhancing specific surface during air oxidation. Specifically, the specific surface area of the powdered biochar (0.18 – 0.23 mm) reached about 450 m2/g after air oxidation at 400 °C for 40 min, while that for the granular biochar only reached about 350 m2/g (Xiao et al. 2020). More efforts are needed to elucidate the role of particle size on air oxidation of biochar.

4.7 Impacts of initial carbonization degree of biochar

4.7.1 Pore development

The carbonization degree of biochar also has an impact on the pore development during air oxidation. Suliman et al. (2016) and Xiao et al. (2018) found that air oxidation was more efficient in increasing specific surface area of high temperature wood biochars. For example, the specific surface areas for the hybrid poplar wood biochars prepared at 350 – 550 °C were decreased after oxidation at 250 °C for 30 min, while that for the biochar prepared at 600 °C was remarkably increased by 140 m2/g after the oxidation (Suliman et al. 2016). This difference in pore development of biochars with different carbonization degrees might be ascribed to the observation that biochar from a lower temperature had a greater decrease of sp3-rich structures (e.g., alkyl-aryl C–C structures) after air oxidation (Wang et al. 2018b). The sp3-rich structures are important in cross-linking of aromatic clusters in biochar matrix. Consequently, the greater decrease of sp3-rich structures resulted in a higher loss of structural stability, which was not favorable for the preservation of pores formed during air oxidation.

4.7.2 Surface oxygenation

The carbonization degree of biochar also affects its surface oxygenation during air oxidation (Suliman et al. 2016; Wang et al. 2018b; Xiao et al. 2018, 2020). The results from Suliman et al. (2016) showed that a higher increase of O/C ratio was observed for the wood biochar prepared at a temperature ≤ 500 °C. When the temperature was > 500 °C, the increase of O/C ratio for the biochar was slight by air oxidation at 250 °C for 30 min (Suliman et al. 2016). More importantly, acidic OFGs (i.e., carboxyl, lactone and phenol groups) are more prone to be formed on the biochar prepared at a lower temperature (Suliman et al. 2016; Xiao et al. 2018). For example, Suliman et al. (2016) found that more acidic groups (i.e., carboxyl, lactone and phenol groups) were formed on the low temperature biochars (< 500 °C) after air oxidation at 250 °C for 30 min. The gasification biochar prepared at 600 °C with a lower graphitization degree showed a higher increase in the intensity of the band at around 1700 cm−1 (assigned to carboxyl C = O) than the biochar from 900 °C (Wang et al. 2018b). These observations are attributed to the different air oxidation chemistries for aromatic and aliphatic sites on biochar. Specifically, as analyzed in Sect. 2.1, the interaction between aliphatic carbon and O2 results in the formation of carboxyl and ketone groups. Thus the biochar prepared at a lower temperature is richer in aliphatic carbon, which is more favorable for the formation of carboxyl groups.

5 Application performances

The applications of biochar-based materials modified by air oxidation include environmental pollution control, biomass catalytic conversion, and energy storage. Recently, more efforts have been paid on the environmental application of biochar-based materials modified by air oxidation. There is a need to develop more advanced applications.

5.1 Environmental applications

5.1.1 Nutrient adsorption

Biochar-based materials are not high-efficient in ammonium adsorption (Weldon et al. 2022). Specifically, the reported medium ammonium adsorption capacity was 4.2 mg NH4-N/g by various biochar-based materials (Weldon et al. 2022). Oxidation enriches carboxyl, lactone, ketone and/or phenol groups on biochar (Dai et al. 2021). These OFGs can directly be complexed with ammonium ions and/or form H-bond with ammonium or phosphate ions (Dai et al. 2021). Thus sewage sludge biochar after air oxidation showed a higher ammonium adsorption capacity (Atienza-Martinez et al. 2021). However, the reported ammonium adsorption capacity of biochar after air oxidation was still not high enough to act as an efficient ammonium adsorbent. More efforts should be made to enhance the ammonium adsorption performance of biochar-based materials.

In addition, biochar-based phosphate adsorbent always requires the doping of bi-/tri-valent metal minerals to enhance the phosphate adsorption affinity. Besides the enhanced interaction between OFGs and phosphate through H-bond, air oxidation could also enhance the relative amount of minerals in mineral-doped biochar ascribed to the decomposition of the organic phase. Therefore, phosphate adsorption capacity for Fe-doped biochar was remarkably improved after air oxidation, attributed to the combined effects of enriched OFGs and minerals after air oxidation (Bakshi et al. 2021).

5.1.2 Heavy metal adsorption or reduction

Surface carboxyl, lactone, ketone and phenol groups are very active in the adsorption of cationic heavy metals through surface complexation and electrostatic attraction (Dai et al. 2021). Oxygenated heavy metals (cationic uranyl and anionic dichromate ions) can be adsorbed as an H-bond acceptor through H-bond with the H-bond donor OFGs (e.g., protonated carboxyl and phenol groups). Therefore, biochar with richer surface OFGs is more active in heavy metal adsorption. As shown in Table 1, various air oxidation-engineered biochars, e.g., biochar from air torrefaction (Li et al. 2019b), biochar from oxidative pyrolysis (Liu et al. 2021a), air oxidized hydrochar (Han et al. 2018), air oxidized pyrochar (Dai et al. 2020) and air oxidized biochar composites (Wang et al. 2018a), were developed for the high-efficient adsorption of various heavy metals. The targeted heavy metals removal through adsorption included lead (Chen et al. 2011), copper (Tang et al. 2021), zinc (Dinh et al. 2022), uranium (U(VI)) (Dai et al. 2020), mercury (Wang et al. 2018a), nickel (Dinh et al. 2022), cadmium (Qian et al. 2015), and cesium (Khandaker et al. 2018) in previous studies. For example, the maximum U(VI) adsorption capacity for corn cob biochar was increased from 68.82 to 163.18 mg/g after air oxidation at 300 °C for 30 min (Dai et al. 2020), and the magnetic hydrochar after air oxidation at 400 °C for 1 h was 5 times higher than the magnetic hydrochar after thermal treatment in N2 at the same temperature for the same time period in mercury adsorption (Wang et al. 2018a). In addition, reducing OFGs, i.e., phenol groups, are active sites for reducing dichromate to tri-valent chromium. Thus the air oxidized biochar was found to have a high reducing capacity for chromium reduction (Yan et al. 2018). However, although extensive results were obtained for heavy metal adsorption or reduction by the air oxidation engineered biochars, the structure-performance relationship is not well elucidated.

Table 1 Heavy metal adsorption performances of air oxidation-engineered biochars
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5.1.3 Organic pollutant adsorption

Generally, surface oxygenation might be a double sword for organic pollutant adsorption. The introduction of polar OFGs (e.g., carboxyl, lactone groups) can enhance H-bonding between carbon surface and some organics, and simultaneously enhance the competition between non-/low-polar organics and water molecules on the material surface. However, air oxidation-engineered biochars have shown enhanced adsorption performances towards both neutral and ionizable organic pollutants (Xiao & Pignatello 2016; Yang et al. 2021). These enhanced performances towards various organic pollutants might be ascribed to combined effects from the pore development-enhanced π-π interaction and surface oxygenation-enhanced H-bonding (Xiao & Pignatello 2016; Yang et al. 2021). Air oxidation can improve porosity, resulting in more available aromatic surface for π-π interaction between biochar surface and organics containing aromatic rings. In addition, OFGs on biochar surface can act as H-bond acceptor/donor, favoring the formation of H-bond between biochar surface OFGs and organic molecules containing H-bond acceptor/donor (i.e., N-/O-containing groups). Table 2 shows the enhanced organic pollutant adsorption performances of biochars with air oxidation. For example, the tetracycline adsorption capacity of poplar hydrochar was increased from 6.29 to 96.23 mg/g after air oxidation in 0.5% O2 at 500 °C for 3 h (Huang et al. 2020a). The congo red adsorption capacity for illite composited hydrochar was also increased from 18 to 238 mg/g with an equilibrium time of < 10 min after air oxidation at 300 °C for 5 h (Wang et al. 2017b).

Table 2 Organic pollutant adsorption performances of air oxidation-engineered biochars
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5.1.4 Gas adsorption

Biochar-based materials engineered by air oxidation were also applied for gas adsorptions. These biochar-based materials include microporous biochars prepared from oxidative pyrolysis of almond shells and olive stones for CO2 adsorption (Plaza et al. 2014), nitrogen-doped porous biochar prepared by the coupling of air pre-oxidation and pyrolysis of ammonium phosphate impregnated stillage for toluene adsorption (Jin et al. 2020), and porous biochar prepared by consecutive air pre-oxidation and NH3 treatment of poplar sawdust biochar for toluene adsorption (Wang et al. 2021). Specifically, the microporous biochars from oxidative pyrolysis of olive stones and almond shells under a low O2 concentration and a high temperature are efficient for CO2 adsorption at a low partial pressure with high CO2 selectivity to N2 (Plaza et al. 2014). The toluene adsorption capacity of the biochar prepared by direct pyrolysis of ammonium phosphate impregnated stillage was 296 mg/g, which was remarkably increased to 438 mg/g prepared by air pre-oxidation at 350 °C for 2 h before pyrolysis (Jin et al. 2020). These results suggested that, when air oxidation was applied in oxidative pyrolysis or as a post-modification strategy, it would modify the pore structure and enhance the surface polarity of biochar, favoring the adsorption of polar molecules, e.g., CO2 (Plaza et al. 2014); while air pre-oxidation before the pyrolysis of nitrogen chemical-pretreated biomass favored the pore development and decrease of hydrophilicity, favoring the adsorption of low-polar molecules, e.g., toluene (Jin et al. 2020).

5.1.5 Catalytic removal of NO x 

The NH3-SCR is the most commonly used technology for the treatment of NOx attributed to its high efficiency, low cost and good selectivity. Carbon material is a good carrier for transition metals for low temperature NH3-SCR. The acidic OFGs, e.g., carboxyl and phenol groups, on carbon-based catalyst facilitate the removal of NO ascribed to the enhanced affinity of carbon-based catalyst to NH3 by these OFGs (Guo et al. 2015). Liu et al. (2021b) found air oxidation remarkably enhanced the performance of MnOx-doped biochar in NH3-SCR of NO. The denitration results demonstrated that the conversion efficiency of NO by the resultant biochar-base catalyst reached 97% at 150 °C, which was much higher than that by the biochar without air oxidation (62%) (Liu et al. 2021b).

5.2 Biomass catalytic conversion

Various carbon-based catalysts for biomass conversion have been developed from various sources and by various functionalization routes (Gabe et al. 2021; Mahajan & Gupta 2020; Shrotri et al. 2017; Xiong et al. 2021). Among these strategies, oxidative torrefaction has been proved to be efficient in directly preparing biochar-based solid acid catalyst for biomass hydrolysis (Kobayashi et al. 2016). Kobayashi et al. (2016) directly applied oxidative torrefaction of wood powder at 300 °C for 1 h, and the resultant biochar-based solid acid catalyst showed rich carboxyl groups (2.1 mmol/g) and had a high sugar yield (i.e., 77% for glucose and 91% for xylose) from catalytic hydrolysis wood biomass within 1 h in trace HCl solution (Fig. 8a). Furthermore, after the catalytic hydrolysis reaction, the solid residue containing the catalyst and insoluble wood biomass was efficiently converted to the solid acid catalyst again by the oxidative torrefaction (air oxidation) at the same condition (Fig. 8a) (Kobayashi et al. 2016).

Fig. 8
figure 8

a The schematic illustration of air oxidation in preparation of solid acid catalyst from wood powder for biomass catalytic hydrolysis. Reprinted with permission from Kobayashi et al. (2016). Copyright 2016 RSC. b The illustration of the air oxidation in surface engineering of cuttlebone biochar for NICs. Reprinted with permission from Guo et al. (2018). Copyright 2018 American Chemical Society

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In addition, ruthenium particle doped biochar, prepared by air pre-oxidation of biochar at 230 °C for 1 h followed by surface complexation on biochar surface between OFGs and ruthenium ions, has been developed for hydrogenation of furfural to furfuryl alcohol (Bardestani et al. 2020). The resultant catalyst showed 93% furfuryl alcohol selectivity at 53% furfural conversion (Bardestani et al. 2020). This result suggested that oxidized biochar by air oxidation might be a superior carrier for catalyst preparation in biomass catalytic conversion.

5.3 Energy storage

5.3.1 Sodium-ion capacitors (NICs)

Heteroatoms on carbon material surface significantly affect their electrochemical properties. Guo et al. (2018) found that, besides the development of porosity, air oxidation could simultaneously increase the oxygen and nitrogen contents of cuttlebone biochar. Consequently, the resultant biochar materials achieved superior sodium capacity-cycling retention-rate capability combinations (Fig. 8b). Specifically, a high Na+ storage capacity of 640 mAh/g was obtained (Fig. 8b), which was as high as those of lithium ion storages in carbon materials (Guo et al. 2018). The resultant material also showed a high integrated energy-power density and a good cycling stability (Guo et al. 2018). These results suggested that air oxidation can be applied to further promote the performances of heteroatom-doped biochars.

5.3.2 Adsorption thermal energy storage

Adsorption thermal energy storage (ATES) is a green and economical technology for the use of low-grade thermal energy. It depends on the interaction between the surface of a solid material and the fluid. Activated carbon is the commonly used material in ATES, attributed to its low cost and high porosity to adsorb water. However, activated carbon always suffers from low hydrophilicity, resulting in a low water adsorption capacity. Air oxidation has been applied to enhance the water vapor adsorption capacity of activated biochar for ATES. For example, Chairunnisa et al. (2020) applied air oxidation to enhance the ATES performance of nutshell activated biochar, and Wang et al. (2013a) applied air oxidation to enhance the ATES performance of bamboo-derived activated biochar. Results showed that the isosteric heat released from the water vapor adsorption onto the nutshell-derived activated biochar was between 2400 to 2500 kJ/kg (Chairunnisa et al. 2020), and the working pair could be driven at a temperature < 50 °C. These results suggested that the surface chemistry of activated biochar controlled its performance in ATES, and air oxidation is an alternative method for enhancing its performance in ATES.

6 Conclusions and outlooks

The applications of air oxidation in surface engineering of biochar-based materials have demonstrated promising results in diverse applications. This is attributed to the ability of air oxidation in pore development and/or surface oxygenation. Air oxidation can be conducted in oxidative torrefaction/pyrolysis, or applied as a post-modification process. In addition, air oxidation can also be integrated with other surface engineering processes, e.g., air pre-oxidation before metal doping. The air oxidation process is controlled by the operation parameters (i.e., O2 concentration, temperature, time), and intrinsic nature of raw biochar (i.e., original biomass composition, ash content, particle size and carbonization degree). The air oxidation-engineered biochar-based materials have been efficiently applied in environmental pollution control (e.g., nutrient, heavy metal, organics, and gaseous pollutant), biomass catalytic conversion (e.g., cellulose hydrolysis as solid acid catalyst), and energy storage (e.g., NICs, ATES). During air oxidation, the evolution of surface functionality is independent to pore development. A tailored design of pore development and surface oxygenation during air oxidation should be considered for promoting the specific application performance of biochar. Furthermore, there are still some challenges to be addressed in future works. These challenges are outlined below.

The thermochemistry of biochar air oxidation is still unclear

The first issue to be addressed in biochar air oxidation is the thermochemistry of the chemical reactions during air oxidation. Thermochemistry concerns the energy change of a chemical reaction. It describes the map of the reactions, showing possible reactants, intermediate and product states, and energies of these states. Understanding and controlling the process of biochar air oxidation, e.g., chemisorption of oxygen on an active site, isomerization of carbon–oxygen complex, migration of oxygen, and the decomposition of carbon–oxygen complex, rely on the knowledge of thermochemistry. Recently, air oxidation has been extensively applied in the surface engineering of biochar. These studies focused on the surface structure and subsequent application performance. However, the thermochemistry of biochar air oxidation is still unclear. Experiments and theoretical calculations are needed to elucidate the thermochemistry of biochar air oxidation to promote the understanding of the chemical reactions in air oxidation.

The impacts of controlling factors for air oxidation are not well understood

Biochar air oxidation process is controlled by various operation parameters and biochar intrinsic properties (Fig. 7). Recently, attention is paid on the impact of operation parameters, e.g., O2 condition, temperature and time. However, the intrinsic properties of biochar also have critical impacts on the pore development and surface oxygenation efficacy. For example, air oxidation is more efficient in increasing the surface area of the biochar with a higher aromaticity or from a higher temperature (Dai et al. 2020). Much less is known about the impacts of the intrinsic properties of biochar on its air oxidation process. Moreover, among the intrinsic properties of biochar, the moisture content is overlooked. Commercial biochar is always moisturized during its milling or storage. Commercial biochar has a moisture of around 10%. H2O also can be chemically adsorbed on the active sites of carbonaceous materials (Li et al. 2018; Wang et al. 2019b). When air oxidation is applied as a post-modification process, the moisture might affect the air oxidation process (Li et al. 2018; Wang et al. 2019b). Therefore, efforts are needed to shed light on the impacts of both extrinsic operation parameters and intrinsic properties on the pore development and surface oxygenation of biochar during air oxidation. These efforts facilitate the achievement of controllable and predictable surface engineering of biochar.

The evolution of biochar surface functionality during air oxidation is not well unveiled

Recent characterization results were dominantly from N2 adsorption–desorption isotherms for pore structure analysis, spectroscopic analyses for surface functionality analyses, and chemical titration for acidic and basic functional group quantification. Among the spectroscopic analyses, Fourier transform infrared spectroscopy (FTIR), solid state 13C nuclear magnetic resonance spectroscopy (NMR), and X-ray photoelectron spectroscopy (XPS) are the commonly used strategies for exploring the evolution of biochar structure during air oxidation. There is a lack of precise and quantitative characterization of the evolution of biochar surface functionality during air oxidation. Considering these, temperature-programmed oxidation and desorption (TPO and TPD) (Cerciello et al. 2021a, 2021b) and other more insightful spectroscopic analyses (e.g., two-dimensional perturbation correlation infrared spectroscopy (2D-PCIS) (Harvey et al. 2012), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS))(Singh et al. 2014) should be applied to profoundly elucidate the evolution of biochar surface functionality during air oxidation. For example, the combined use of TPO and TPD can elucidate the oxygen chemisorption process and subsequent process of formation and preservation of carbon–oxygen complex (i.e., various OFGs) on biochar.

In addition, attention on air oxidation of biochar focuses on the pore structure and surface functionality. Efforts should also be made to examine other properties of biochar, e.g., redox capacity and conductivity. These surface properties also have substantial impacts on its application performance (Chacon et al. 2020). Specifically, these properties are critical for its application in advanced oxidation or electrochemical energy storage.

More tailored advanced applications should be developed

The interfacial behavior of biochar is controlled by its surface structure and functionality. Recently, attributed to the improved porosity and/or enriched surface OFGs of biochar after air oxidation, the application of air oxidation-engineered biochar focuses on environmental application as an adsorbent for pollutant adsorption (Tables 1 and 2). The redox property of biochar can also be adjusted by the surface redox OFGs, such as the electron donor –OH and electron acceptor C = O. The enrichment of these redox OFGs might be favorable for its application as a carbocatalyst in advanced oxidation. Furthermore, the porous biochar modified by air oxidation with improved surface functionality can also be potentially applied as an electrode material for electrochemical energy storage.

Air oxidation can be integrated with other modification process

Air oxidation has been found to be efficient in the preservation of nitrogen-containing functional groups on biochar, attributed to the relatively high thermal stability of nitrogen-containing functional groups (Guo et al. 2018). Moreover, under air atmosphere, boron- and phosphorus-doped graphene oxide and reduced graphene oxide showed higher thermal stabilities than the pristine graphene oxide (Yuan et al. 2016) and reduced graphene oxide (Feng et al. 2019), suggesting that boron- and phosphorus-containing functional groups have high thermal stabilities during air oxidation. Therefore, air oxidation can be applied as a post modification route to further enrich heteroatom amounts in heteroatom-doped biochar. In addition, air oxidation can be applied as a pretreatment process for enriching anchor sites for metal species before metal doping. Recently, the improvements in ZVI (Mortazavian et al. 2019) and ruthenium oxide (Bardestani et al. 2020) doping on biochars were achieved by air oxidation before metal doping. By enriching anchor sites by air pre-oxidation before metal doping, enhanced biochar composites with various metal species can be obtained.