1 Introduction

Torrefaction is a practical approach for upgrading biomass, which is conducive to improving the fuel properties of torrefied biochar, including hydrophobicity, grindability, and granulation characteristics (Devaraja et al. 2022; Zhang et al. 2021c). The torrefaction process is a mild pyrolysis procedure, which is mainly conducted at temperatures of 200–300 °C with a duration in the range of 15–60 min (Chen et al. 2021). As a result, torrefaction operation is an energy-efficient treatment for biochar production and solid biofuel accumulation (Zhang et al. 2021a). In view of this, torrefaction operation is a promising method for forestry and agricultural residue conversion and solid biofuel production. Biomass torrefaction technology possesses broad application prospects and is regarded as an effective method to realize the efficient utilization of biomass (Akbari et al. 2020; Bach et al. 2017).

Owing to the excellent fuel properties obtained via torrefaction operation, it is widely used in biochar fuel production areas. Even though the properties of long duration for torrefied biochar storage and transportation are improved (Batidzirai et al. 2013; Sukiran et al. 2017), a better technique for improving hydrophobicity is desired to further enhance the storage and transportation characteristics of torrefied biochar (Chen et al. 2015; Xiaorui et al. 2021). Several methods have been adopted for torrefied biochar storage and transportation characteristics improvement. Among them, applying modifiers appears to be a good choice for establishing superhydrophobicity properties (Arpia et al. 2021). The biochar surface’s superhydrophobic property is determined by surface roughness and surface free energy. Superhydrophobic biochar can be achieved by improving the surface roughness and reducing the surface free energy (Wang et al. 2021b).

So far, the research concerning the preparation of superhydrophobic biochar is considerable. Many researchers are devoted to exploring the prepared material’s superhydrophobic performance and properties for practical application. Meanwhile, the modified superhydrophobic biochar possesses great superlipophilic properties simultaneously, which is beneficial for treating oil leakage. Shi et al. (2021) selected spent coffee grounds as the feedstock and prepared holocellulose nanofiber foam using trichloro(octadecy)silane for superhydrophobic modification. The obtained results indicated that holocellulose nanofiber foam held great potential for waterproof properties upgrading. Peng et al. (2021) chose corn stalk pith as the raw material for adsorbent preparation and found that grafting octadecylamine onto corn stalk pith is an effective method to realize superhydrophobic and superlipophilic properties. Duan et al. (2021) applied biochar loaded on the melamine foam and modified by octadecylamine for oil adsorption and concluded that such a method provides an effective way for low-cost and large-scale production of superhydrophobic adsorbent. Overall, these reviewed studies reveal that superhydrophobic biochar material is a promising substance for keeping storage properties and leaking oil adsorption with low cost and environmental impact. Thus, applying modified biochar produced through a two-stage treatment operation is a good choice for environmental protection and calorific value improvement.

Among the modifiers for establishing superhydrophobic and superlipophilic surfaces, hexadecyltrimethoxysilane (HDTMS) is a good option, owing to its stability and availability. HDTMS has been widely used for the preparation of self-healing superhydrophobic coating with self-cleaning properties (Wang et al. 2021c), including the superhydrophobicity and corrosion resistance of stainless steel (Zhang et al. 2020), and the modification of diatomite powder to separate oil and water for oil adsorption (Peng et al. 2022). The obtained results indicate that HDTMS is an efficient modifier for material surface evolution. Applying HDTMS is conducive to achieving better hydrophobicity and lipophilicity. Furthermore, the property for fuel properties upgrading of torrefied biochar with great moisture resistance and oil adsorption capacity is achieved. For this research, exploring the evolution of storage and transportation characteristics by two-stage treatment of torrefaction and modification for fuel properties upgrading is crucial. This study is the first one to employ the leaking oil adsorption and hydrophobicity modification of microalgal biochar.

In this study, HDTMS was adopted to construct the superhydrophobic and superlipophilic surface of torrefied biochar, which possesses high stability and durability. By introducing HDTMS, the storage and transportation characteristics of torrefied biochar would be further enhanced, thus leading to a longer duration for biochar quality assurance. The two-stage pretreatment of torrefaction and modification for fuel properties upgrading was implemented. As a result, the moisture resistance was improved, which gave rise to better fuel properties. In addition, utilizing the hydrophobically modified biochar for oil adsorption was conducive to cleaning up the pollution caused by oil products and further promotes the HHV of biochar.

This research is meaningful in using biomass wastes for functional material preparation and solid biofuel upgrading, thus achieving a sustainable route for fuel properties upgrading and in line with the concept of green development. By means of modification for hydrophobicity improvement and superlipophilicity enhancement, the obtained biochar possessed the merits of fuel development and environmental protection. It thus leads to a greener approach to waste utilization, which was never reported before. This study was the first to propose torrefaction operation combined with HDTMS modification for biochar waterproof performance improvement and oil adsorption capacity enhancement. This method is cheaper and greener than previous research owing to its lower temperature for energy saving and persistent superlipophilicity for oil adsorption. A microalga was adopted as the feedstock for biochar production owing to its fast growth rate, high CO2 fixation efficiency, and high biomass accumulation capacity, thus leading to a green approach to environmental sustainability. Moreover, the torrefaction process was adopted as the thermal treatment operation due to its low cost and environmental impact. In the industrial area, around 23% of waste heat is at a temperature range of 200–300 °C and is suitable for the torrefaction process, thus achieving energy utilization and saving (Chen et al. 2022). Besides, HDTMS is a stable and cheap chemical for biochar hydrophobic modification, suitable for improving waterproof performance. Several characterizations were adopted for biochar properties evaluation. They included scanning electron microscope (SEM) observation, energy dispersive spectroscopy (EDS) characterization, particle size distribution, Fourier transform infrared spectrum (FT-IR) observation, X-ray photoelectron spectroscopy (XPS) characterization, thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) analysis, and calorific value test (Ong et al. 2020). The obtained results are useful in industry and academia to provide a new view on biomass waste utilization and biochar modification (Kwoczynski and Čmelík 2021).

2 Materials and methodology

2.1 Materials and torrefaction operation

In this study, raw microalga (Nannochloropsis Oceanica) was selected as the feedstock for biochar production, which was produced in the wastewater treatment process and possessed great fuel properties. After being obtained, the microalga was washed and dried in an oven at 105 °C for 24 h. 10 g of the dried microalga was put into the tube furnace for torrefaction operation. Although the microalga was obtained from the ocean in this study, it was washed for desalination before torrefaction. Water washing is an efficient route to remove inorganic salts from biomass, which has been proved by a past study on upgrading rice husk fuel (Zhang et al. 2016). Three torrefaction temperatures of 200, 250, and 300 °C along with the torrefaction duration of 1 h were adopted. The carrier gas (N2) flow rate in the tube furnace was controlled at 200 mL min− 1. The torrefaction system diagram and torrefaction operation details can be found in a previous study (Zhang et al. 2018).

2.2 Biochar modification for superhydrophobicity

At the beginning of the experiment, 100 mL of cyclohexane was poured into a beaker. Then, 6–8 g of torrefied biochar was added to the beaker and dispersed with ultrasound for 10 min to form a uniform suspension which was stirred continuously. After that, 3 g of HDTMS (CH3(CH2)15Si(OCH3)3) and 1 g of glacial acetic acid were dropped into the mixture sequentially. The resulting mixture was stirred at room temperature for 12 h and then centrifuged and dried in an oven. After the aforementioned procedure, the modified torrefied biochar with superhydrophobic characteristics was obtained (Wang et al. 2021a). The schematic of microalgal biochar preparation, modification, and superlipophilicity and superhydrophobicity characterization are shown in Fig. 1.

Fig. 1
figure 1

Schematic of microalgal biochar preparation, modification, and superlipophilicity and superhydrophobicity characterization

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Gasoline, diesel oil, soybean oil, and octadecane were tested in oil adsorption experiments to evaluate the oil adsorption capacities of biochars from different torrefaction conditions. In each run, around 0.2 g of torrefied biochar powder was fixed on the filter membrane (oleophobic) by the suction filtration process, and the weight of the biochar and filter membrane was recorded. For the suction filtration process, the modified biochar was laid on the surface of the filter membrane, and the suction filter was connected under the filter membrane with a suction pressure of 2–3 MPa. The holding time was set as 1–2 min. After that, the biochar was fixed on the membrane. Then the filter membrane with biochar was put in the oil for 10 s, and it was taken out to weigh the oil adsorption capacity of torrefied biochar. In this study, four different oils were adopted as the tests of oil removal by biochar: gasoline, diesel oil, soybean oil, and octadecane. For the oil-water emulsion process, equal volumes of oil and water were mixed up and loaded in a beaker for the oil-water separation experiment. Different names were provided to distinguish the biochars produced at different treatment processes. In the names, T represents the torrefaction operation, and TM refers to the torrefaction and modification operation. For example, 300 °C-T biochar stands for the biochar produced from 300 °C torrefaction, while 300 °C-TM biochar denotes the biochar made from 300 °C torrefaction followed by HDTMS modification.

2.3 Characterization of biochar properties

To obtain the micromorphology of the modified biochar, SEM (TM3030 Plus, Hitachi, Japan) was employed to investigate the microstructures of the biochar surfaces. A particle size distribution analyzer (Mastersizer 2000, Malvern, UK) was adopted to analyze the particle size distribution of the modified biochar. FT-IR (Nicolet is50, ThermoFisher, USA) with the mode of attenuated total reflection (ATR) was employed for the surface functional group detection of the modified biochar. To analyze the chemical transformation and microstructure of the modified biochar, XPS (ESCALAB 250Xi, Thermo Scientific, USA) was adopted to explore the substance conversion. The OCA20 contact angle measurement system (Data-Physics, Germany) was used to analyze the contact angle and investigate the lipophilicity and hydrophobicity of the modified biochar. In addition, an incubator was employed to analyze the moisture adsorption capacity of torrefied and modified biochars. The temperature was controlled at 30 °C, and the humidity was set at 80–90%. A bomb calorimeter (ZDHW-9000 C, Huanuo, China) measured the higher heating value (HHV) of modified biochar after oil adsorption (oilchar). A thermogravimetric analyzer (STA449F5, NETZSCH, Germany) was applied for the TGA and DTG analysis to study the pyrolysis and combustion characteristics of the torrefied biochar. The TGA and DTG curves could further explore the initial and terminated pyrolysis temperatures as well as the ignition and burnout temperatures of raw and torrefied microalga. The initial and terminated pyrolysis temperatures were obtained by making a tangent on the start and final point of the TGA curve (Zhang et al. 2021b).

3 Results and discussion

3.1 Basic property analysis of modified biochar

The basic property analyses of 300 °C-TM microalgal biochar are displayed in Fig. 2, including SEM observations and EDS characterization, particle size distribution, FT-IR observation, and XPS characterization. As shown in Fig. 2a and b, the SEM observations depict the micromorphology of torrefied microalgal biochar as a tiny folded sphere with holes. Such a phenomenon illustrates that the microalgal biomass was damaged during the torrefaction process thus leading to a more extensive surface area and higher bulk density (Chen et al. 2014).

Fig. 2
figure 2

a–c SEM observations and EDS characterization, d particle size distribution, e FT-IR observation, and f–i XPS characterization of 300 °C-TM biochar

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The EDS characterization in Fig. 2c shows that the 300 °C-TM biochar was mainly composed of elemental C, N, O, and Si. Since HDTMS is silanes, the existence of Si implies that HDTMS was successfully grafted on biochar, thus leading to superhydrophobic characteristics. According to the particle size distribution analysis in Fig. 2d, the size of the microalgal biochar was mainly distributed in the range of 10–100 μm, with a small part distributed between 1 and 10 μm. In Fig. 2e, the FT-IR result suggests that the modified microalgal biochar was detected with fewer functional groups than the torrefied biochar. After the torrefaction operation, the functional groups on the biochar surface were mainly O–H, C–H, C–O, C=O, and N–H, which are hydrophilic. When HDTMS, a kind of silane, is introduced, it can efficiently bind with surface hydrophilic functional groups for improving hydrophobicity. As a result, the functional groups of the modified biochar were covered by HDTMS, displaying fewer functional groups. The main differences were reflected in the O–H, C–H, and C–O, implying that the modified microalgal biochar is more hydrophobic (Chen et al. 2012). The XPS results in Fig. 2f and i reveal that the 300 °C-TM biochar possessed three main elemental signals of C1s, O1s, and Si2p. Among them, the signal of C1s was the strongest, followed by O1s and Si2p. The existence of Si2p illustrates that HDTMS was grafted on the surface of biochar, thus possessing a hydrophobic characteristic (Yang et al. 2021).

3.2 Superlipophilicity and superhydrophobicity of modified biochar

Figure 3 shows the hydrophobicity and oil adsorption capacity of raw microalga and unmodified microalgal biochar. The contact angle of the raw microalga was 51.1° (Fig. 3a), showing its hydrophilic nature and not being suitable for storage and transportation. After torrefaction from 200 to 300 °C, the contact angle increased from 88.1° to 112.4°, indicating the improvement of hydrophobicity (Zhang et al. 2021a). Torrefaction is conducive to enhancing the hydrophobicity of the torrefied microalga to a certain extent. The raw microalga’s oil adsorption capacities for gasoline, diesel oil, soybean oil, and octadecane were 0.67, 0.73, 0.78, and 0.75 g g− 1, respectively. For 200 °C-T microalgal biochar, the values were 0.98, 1.04, 1.09, and 1.06 g g− 1, and increased to 1.06, 1.12, 1.18, and 1.15 g g− 1 of 250 °C-T. The oil adsorption capacities for 300 °C-T microalgal biochar were 1.21, 1.24, 1.29, and 1.27 g g− 1. The lipophilicity of the biochar was substantially enhanced, thus leading to a better oil adsorption capacity. Overall, torrefaction operation improved hydrophobicity and lipophilicity to obtain better fuel properties. However, the materials can be further modified for upgrading moisture resistance and oil adsorption properties (Zhang et al. 2019).

Fig. 3
figure 3

Hydrophobicity of raw and microalgal biochar (a), and oil adsorption capacity of raw and microalgal biochar (b) under different torrefaction temperatures

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After the modification treatment, the modified microalgal biochar possessed great superlipophilic and superhydrophobic characteristics and has better oil adsorption capacity, which can separate oil from water more efficiently. As displayed in Fig. 4, the superlipophilicity and superhydrophobicity of modified microalgal biochar, oil adsorption capacity of different modified microalgal biochar, and the process of adsorption and separation of oil from water using the modified microalgal biochar are concluded in this study. The contact angle tests are employed to explore the lipophilic and hydrophobic characteristics of the modified microalgal biochar. The results of the contact angle test indicate that 300 °C-TM microalgal biochar possesses superlipophilic and superhydrophobic characteristics, with contact angles of 0° and 151° of oil and water, respectively. As a result, the modified microalgal biochar is suitable for oil adsorption and acts as an adsorbent when an oil leakage occurs. Compared to the previous research, these results imply that the two-stage treatment is more suitable for solving oil leakage (Zhang et al. 2017).

Fig. 4
figure 4

a, b Superlipophilicity and superhydrophobicity of microalgal biochar, c–e oil adsorption capacity of different microalgal biochar, and f process of adsorption and separation of oil from water using the microalgal biochar

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Table 1 shows the oil adsorption capacities of microalgal biochar from the four oils. It can be found that the modified microalgal biochar is a sound absorbent for oil adsorption and suitable for the four oils. For 200 °C-TM microalgal biochar, its adsorption capacities on gasoline, diesel oil, soybean oil, and octadecane were 2.12, 2.63, 2.92, and 2.77 g g− 1, respectively. Accordingly, biochar had the best performance on soybean oil, followed by octadecane, diesel oil, and gasoline. With increasing torrefaction temperature from 200 to 300 °C, the amount of oil adsorption by the biochar also increased, with the values of 2.57, 3.02, 3.42, and 3.03 g g− 1 for gasoline, diesel oil, soybean oil, and octadecane, respectively. Accordingly, the oil adsorption capacity of biochar could be improved by up to 70–80% after torrefaction and modification compared to raw microalga. Herein, biochar’s oil adsorption capacity was the best in soybean oil, which can be attributed to its larger molecular weight and bonding ability. Compared to the relevant research concerning oil adsorption of spent coffee grounds which possess the oil adsorption capacity of 1.02–1.50 g g− 1 [27], the modified microalgal biochar of this study possessed a better adsorption capacity, especially at higher torrefaction temperatures. As shown in Table 2, the oil adsorption capacities of the biochars in this study were suitable and advantageous, with the value of oil adsorption performance mainly in the range of 1.49–3.42 g g− 1. Although the oil adsorption capacity of pyrolyzed biochars ranges from 1.61 to 3.32 g g−1, the energy input and biochar yield are not comparable with this study, owing to their lower pyrolysis temperature and cheaper cost. It thus leads to better oil removal and environmental protection performance. As illustrated in Fig. 4f, the modified microalgal biochar was efficient in adsorbing the oil from the mixture. The modified biochar possessed alkyl and silane functional groups, thus leading to the superlipophilic property. In other words, torrefaction operation and HDTMS modification changed  the surface group characteristics of the biochar. This contributes to a superhydrophobic and superlipophilic surface to improve oil adsorption capacity.

Table 1 Oil adsorption capacities (g g-1) of raw microalga and modified microalgal biochar
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Table 2 Comparison of oil adsorption capacities (g g-1) in this study and previous research
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Owing to the great lipophilic ability of the modified microalgal biochar, the hydrophobic capacity is also a good characteristic. The equilibrium moisture contents of unmodified and modified microalgal biochar were tested and are displayed in Table 2 and plotted in Fig. 5a to evaluate the storage and transportation characteristics of the modified microalgal biochar. The chart shows that the modified microalgal biochar possessed lower moisture content than the unmodified biochar when the torrefaction temperature and duration were the same. In other words, modified biochar possessed better moisture resistance capacity, so it is more suitable to store and transport for a long period than unmodified biochar. The possible mechanism is that the functional groups (mainly hydrophilic) of the torrefied biochar are mostly removed, resulting in a relatively hydrophobic surface. After adding HDTMS, owing to the superhydrophobic characteristics of silicane, the surface of the biochar is further modified with the waterproof property. As a result, the modified microalgal biochar is superhydrophobic.

Fig. 5
figure 5

a Equilibrium moisture content of microalgal biochar, and b–d calorific value of microalgal oilchar

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With the extension of treatment duration of biochar in a humid environment, the moisture content of the unmodified biochar rapidly increased from 3.89% to 4.78% in 0 days, 4.08–5.13% in 10 days, 6.87–7.82% in 20 days, and to 8.58–9.93% in 30 days. These results suggest that unmodified biochar cannot adapt well to the increase of moisture content, especially under long-term conditions (Rasapoor et al. 2021). In addition, lower torrefaction temperature was inefficient for controlling the increase of moisture content. On the contrary, the modified microalgal biochar was competent in controlling the moisture, no matter the storage duration of the torrefied biochar. As shown in Table 2, the moisture content of the modified microalgal biochar was always lower than 3.27%, illustrating the good moisture resistance performance obtained, thus leading to better storage and transportation characteristics.

3.3 Calorific value enhancement of modified biochar after oil adsorption

The calorific value is essential for evaluating modified microalgal biochar’s fuel properties. As a result of the better oil adsorption performance of the modified microalgal biochar, its calorific value would be enhanced to a certain extent when compared to the unmodified biochar. As shown in Table 3, the HHVs of the unmodified and oil adsorbed microalgal biochar (oilchar) were compared, and the HHV of oilchar under different torrefaction temperatures are plotted in Fig. 5. As can be seen from the chart, the HHVs of the unmodified biochar were 22.35, 24.51, and 26.17 MJ kg− 1, respectively, when torrefied at 200, 250, and 300 °C (Table 3). By contrast, the HHV of the oilchar gradually increased. For 200 °C oilchar, the calorific values were increased to 30.03, 25.87, 23.91, and 34.41 MJ kg− 1, respectively, after the adsorption of gasoline, diesel oil, soybean oil, and octadecane. For 250 °C oilchar, the HHVs were 35.08, 28.13, 25.76, and 36.72 MJ kg− 1, respectively. For 300 °C oilchar, the HHVs were 38.56, 32.24, 28.61, and 41.43 MJ kg− 1, respectively. These phenomena indicate that octadecane is the best choice for the improvement of calorific value of the torrefied microalgal biochar, as a result of the highest HHV (Table 4), followed by gasoline, diesel oil, and soybean oil, respectively. In addition, the HHV was related to the torrefaction temperature, owing to the higher carbonization content and oil adsorption capacity (Zhang et al. 2018).

Table 3 Equilibrium moisture content (%) of microalgal biochar
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Table 4 Calorific value (HHV) of microalgal biochar and oilchar
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3.4 Pyrolysis and combustion characteristics of torrefied biochar

The pyrolysis and combustion characteristics of the microalgal biochar are essential to evaluate the thermal stability and explore the fuel properties. Herein, the pyrolysis TGA, DTG, and combustion TGA and DTG of raw microalga and torrefied microalgal biochar are depicted in Fig. 6. As displayed in Fig. 6a, the pyrolysis TGA curves gradually shifted to the right as the torrefaction temperature increased, which can be attributed to the accumulation of ash content and refractory black carbon (Bach and Chen 2017). The pyrolysis curve can be divided into three parts when considering the DTG result in Fig. 6b, namely, initial pyrolysis stage of 100–305 °C, the main pyrolysis stage of 305–453 °C, and the final pyrolysis stage of 453–800 °C. For raw microalga, a small shoulder peak appeared at the temperature of 243 °C, reflecting the thermal degradation of small molecule organics in microalga (Gan et al. 2021). The main pyrolysis peaks were observed at the temperature of 283 and 327 °C, indicating the pyrolysis of carbohydrates and protein, which are the main components of the microalga (Hong et al. 2020). For 200 °C-TM biochar, the main pyrolysis peaks obtained at 291 and 334 °C reflect the thermal degradation of carbohydrates and protein. This results from the light torrefaction severity and the main components of protein and carbohydrates remained. Moreover, a shoulder peak at 447 °C is attributed to the decomposition of the formed char in the torrefaction process (López-González et al. 2015). After the microalga was torrefied at 250 °C, the basic components of carbohydrates and protein  wereconsumed, and the obtained peak in DTG curve is mainly regarded to the thermal degradation of the torrefied char, which was observed at around 374 °C. The obtained only peak indicates that 250 °C torrefied microalgal biochar is in good homogeneity. For 300 °C-TM biochar, a main thermolysis peak at around 400 °C implies the decomposition of the microalgal biochar. A higher pyrolysis temperature of 300 °C microalgal biochar than 250 °C microalgal biochar suggests the severer torrefaction effect and thus leads to a more stable char component.

Fig. 6
figure 6

Pyrolysis (a) TGA and (b) DTG curves as well as combustion (c) TGA and (d) DTG curves of raw and torrefied microalga

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In Fig. 6c, the combustion process of raw and torrefied microalgal biochar can be divided into three stages. The first stage is the usage of carbohydrates in the temperature range of 100–347 °C, and the second stage of 347–523 °C represents the combustion of protein (Bach et al. 2017). The third stage, with temperatures of 523–800 °C, is related to lipid combustion. The combustion DTG curves are shown in Fig. 6d to more accurately reflect the combustion process of the microalga and microalgal biochar. For raw microalga, it is clear that three main peaks are observed, representing the combustion process of carbohydrates, protein, and lipid at the temperature of 224 °C, 297 °C, and 568 °C, respectively (Choi et al. 2019). For microalgal biochar, as a result of the thermal treatment and the char formation process, the combustion DTG curve gradually changed as the torrefaction temperature increased. For 200 °C-TM biochar, two main peaks appeared at temperatures of 302 and 576 °C, indicating the combustion process of residual microalgal component and formed char, respectively (Ferreira et al. 2015). A similar phenomenon was also observed on the DTG curves of 250 °C-TM and 300 °C-TM biochar. The only difference was the peak temperature of the biochar component during the combustion process. For 250 °C-TM biochar, the peaks were obtained at 351 and 594 °C. For 300 °C-TM biochar, the peaks were observed at 450 and 600 °C.

To deeply explore the pyrolysis and combustion characteristics of raw microalga and torrefied microalgal biochar, it is necessary to analyze the initial and terminated pyrolysis temperatures and ignition and burnout temperatures. The results are concluded in Fig. 7. The initial pyrolysis temperature showed an increasing trend (from 93.6 to 195.5 °C) as the torrefaction temperature gradually increased, implying the growing stability of the torrefied biochar (Chen et al. 2016). However, the terminated pyrolysis temperature of the raw microalga and torrefied microalgal biochar showed a declining trend (from 776.6 to 756.5 °C) as the torrefaction temperature increased , which can be attributed to the accumulation of refractory black carbon and the stability of the torrefied biochar. In addition, when concerning the ignition temperature of the raw microalga and torrefied microalgal biochar, an increasing trend was also observed as the torrefaction temperature gradually increased, reflecting the increase of combustion temperature thus leading to a higher heat release. Furthermore, the burnout temperature was also recorded to reflect the combustion characteristics of the raw microalga and torrefied microalgal biochar. The obtained result indicated that the burnout temperature slightly increased from 679.3 to 683.2 °C, illustrating the reactivity of the biochar being lowered to a certain extent, and the safety of the biochar in storage and transportation was improved.

Fig. 7
figure 7

a Initial and terminated pyrolysis temperatures and b ignition and burnout temperatures of raw and torrefied microalga

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3.5 Comprehensive performance analysis and techno-economic analysis of modified biochar

As mentioned above, the modified microalgal biochar possessed great superlipophilicity and superhydrophobicity characteristics, thus leading to excellent storage and transportation properties and enhancing the microalgal biochar preservation time. Herein, several indicators were adopted and calculated to evaluate the comprehensive performance of the modified microalgal biochar to contribute to a better understanding of the actual function and environmental performance (Lee et al. 2022). Five indicators of hydrophobicity, energy efficiency, sustainability, HHV, and oil adsorption capacity were employed and are shown in Fig. 8 to reflect the function and performance of the modified microalgal biochar. Compared to past studies (Chen et al. 2015, 2020; Li et al. 2012), it is clear that the modified microalgal biochar in this study was functionally balanced and dominant. The weakness of the previous research is mainly caused by relatively poor hydrophobicity and lower HHV. In contrast, the modified microalgal biochar was superhydrophobic, and the oilchar from oil adsorption possessed a high calorific value.

Fig. 8
figure 8

The comparison between microalgal biochar and other materials from hydrophobicity, energy efficiency, sustainability, HHV, and oil adsorption

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In conclusion, modified microalgal biochar had good properties and was superior to past studies. The raw microalga is obtained from the photosynthesis process, which achieves CO2 fixation, and torrefaction operation transforms the microalga into solid biofuel. This is carbon neutral and environmentally friendly (Chen et al. 2015). After using the solid biofuel for combustion, the biochar transfers to CO2, thus in the circle of elemental carbon transformation. Overall, the utilization of microalgal biochar is in good condition, in line with the concept of green development (Ahmad et al. 2011), and meets the circular economy and the idea of waste valorization.

The techno-economic analyses of the pyrolysis process and the torrefaction and modification process were implemented to evaluate the economic feasibility of the latter (torrefaction and modification) for oil adsorption. The main focus was paid on the oil adsorption performance and the economic cost of the preparation process. The results are listed in Table 5. Around 10 g of raw biomass was adopted for biochar production in each run. It was clear that the solid yield of the torrefaction process was higher than pyrolysis; hence torrefaction produced more biochar for oil adsorption. Moreover, the torrefaction process was more energy-saving than the pyrolysis process, achieving a greener biochar production approach. Although the oil adsorption capacity of pyrolyzed biochar is better, its solid yield limits the total adsorption amount. The economic cost of pyrolysis is higher than torrefaction. As a result, torrefaction plus modification is a better choice for oil removal than pyrolysis.

Table 5 Techno-economic analysis of pyrolysis process and torrefaction plus modification process for oil adsorption (10 g raw biomass)
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4 Conclusion

Improving the superhydrophobicity and superlipophilicity characteristics of the modified microalgal biochar is an important issue for upgrading fuel properties and leaking oil adsorption. Herein, the research on modified microalgal biochar for hydrophobicity improvement and oil adsorption was deeply explored. The obtained results suggested that the modification operation was conducive to enhancing the hydrophobicity and lipophilicity of the microalgal biochar, thus leading to a better moisture resistance performance to store and transport for a longer duration. Oil adsorption capacity and hydrophobicity of two-stage treated biochar were enhanced to 70–80% and 60–200% than raw microalga, respectively. In addition, the pyrolysis characteristic of the microalgal biochar indicated the decomposition of carbohydrates, protein, and lipid as the torrefaction severity gradually increased. The combustion characteristic suggested that the microalgal components were separately consumed during the burning process. As the torrefaction operation was implemented, the initial pyrolysis temperature and ignition temperature of the torrefied microalgal biochar gradually increased, which indicates the enhancement of the stability of the biochar. In contrast, the declining trend of terminated pyrolysis temperature and burnout temperature corresponds to the accumulation of refractory black carbon and the stability of torrefied biochar. The comprehensive performance analysis results indicated that the modified microalgal biochar possessed great sustainable property and hydrophobicity than previous research and was more energy efficient.