Efficient removal of volatile organic compound by ball-milled biochars from different preparing conditions

Highlights

• Wood biochar has better adsorption capacity for VOCs.

• Ball milling enhanced specific surface area and oxygen functional group of biochar.

• Ball milling improves the adsorption capacity of biochar for VOCs.

• Oxygen functional group contributed more on the adsorption of polar VOCs.

Abstract

Adsorption is an important technology to deal with volatile organic compounds (VOCs), and biochar has attracted much attention as a new type of adsorbent for VOCs. In this study, rice husk, corn stover and pine wood sawdust biochars from different pyrolysis temperatures (300 °C, 500 °C and 700 °C) were synthesized and treated by ball milling. The pristine and ball-milled biochars were used as adsorbents for acetone and toluene removal. Results showed that wood biochar had higher adsorption capacity for VOCs. After ball milling, the BET specific surface area and the oxygen functional group content of biochars increased. With these changes, all the ball-milled biochars showed higher adsorption rate than the pristine biochars. The ball-milled biochars under pyrolysis temperature of 300 °C showed the best adsorption performance for acetone (304 mg g⁻¹), which was 1.7-fold greater than that of pristine biochar. Increasing the surface area by ball milling is conducive to the diffusion of hydrophobic VOCs molecules such as toluene to the adsorption sites in the biochar. However, for hydrophilic VOCs such as acetone, higher oxygen functional groups were the main reason for the enhanced adsorption by ball milling. Therefore, ball-milled biochar can be used as a potential adsorbent material in VOCs treatment.

Introduction

Volatile organic compounds contribute to a high ratio in air pollutants, which are defined with a boiling point in the range of 50–260 ℃ under normal atmospheric pressure by the World Health Organization. On one hand, they can cause toxicity to the nervous system and organs, directly harm the digestive and blood systems, reduce the body's immunity, and cause disorders of the endocrine system and metabolic defects (He et al., 2012). On the other hand, under the sunlight, when nitrogen oxides and VOCs present in the air at the same time, it is quite prone to produce complex photochemical reactions and cause secondary pollution to the environment (Blommaerts et al., 2018, Xu et al., 2017). In recent years, the emissions of VOCs are increasing and the environmental problems caused by them have become increasingly serious (Gałęzowska et al., 2016). Many countries have established relevant standards and laws and regulations to control and restrict VOCs emissions. Meanwhile, Various efficient VOCs control technologies were been developed and widely used, such as adsorption (Luengas et al., 2015, Yu et al., 2018), condensation (Belaissaoui et al., 2016, Hariz et al., 2017), catalytic oxidation (Mei et al., 2016, Zhang et al., 2016) and biological methods (Cheng et al., 2016).

Among different VOCs treatment technologies, adsorption has broad application prospects due to its high processing efficiency and recyclability (Anfruns et al., 2011, Wang et al., 2014). There are significant differences in the adsorption performance of different adsorbents. A variety of carbon materials such as activated carbon, activated carbon fiber, carbon nanotubes, graphene and ordered mesoporous carbons have shown good VOCs adsorption performance (Higashikawa et al., 2016, Zhang et al., 2017a). Among these carbon materials, biochar has received attention in the field of environmental remediation due to its low production cost and excellent adsorption performance (Alhashimi and Aktas, 2017, Tang et al., 2013, Wang and Wang, 2019). Recently, biochar has been widely used in the removal of organic and inorganic pollutants in water or soil remediation, such as Pb (Wang et al., 2015) and antibiotics (Manjunath et al., 2020, Zhou et al., 2014), however, little research has been done on the adsorption of VOCs in air condition by biochar. Adsorption capacity of VOCs by biochar in recent studies was compared in Table S1. Jayawardhana et al. (2019) used biochar produced from pyrolysis of municipal solid waste to evaluate the adsorption capacity of VOCs in landfill leachate, and found that the produced municipal solid waste biochar (MSW-BC) is an economical adsorbent with a strong ability to remove VOCs, and the adsorption capacity of toluene and m-xylene are up to 850 ug g⁻¹ and 550 ug g⁻¹, respectively. The adsorption performance of biochar largely depends on physical properties (for example, surface area and pore structure) and chemical properties (for example, surface functional groups), which are influenced in turn by their pyrolytic condition and feedstock type (Wang et al., 2020). The raw materials of biochar play a key role in the adsorption performance. According to the literatures (Colantoni et al., 2016, Rajapaksha et al., 2016, Zhang et al., 2017b), at the same cracking temperature of 300 ℃, due to the differences in physical and chemical properties of different materials, the adsorption capacity of sugarcane bagasse and walnut biochar on acetone is 91.2 mg g⁻¹ and 5.58 mg g⁻¹, respectively. Wood biochar has a high specific surface area due to its high cellulose, hemicellulose and lignin content (Shaheen et al., 2018). The specific surface area of biochar increases with the increase of pyrolysis temperature. Conversely, the pore size decreases as the pyrolysis temperature increases. In addition, high-temperature pyrolysis helps to remove oxygen-containing groups and increase the aromaticity of biochar, thereby promoting the adsorption of hydrophobic VOCs (Chen et al., 2008). These changes inevitably affect the VOCs adsorption performance of biochar. Also, researchers have found that physical or chemical modification methods of biochar could improve its adsorption performance in environmental remediation (Higashikawa et al., 2016).

Ball milling mainly uses the effect of external mechanical force, that is, through the alternating collision of the grinding ball, the grinding tank and the particles, the hard ball, the grinding medium, the particles are repeatedly squeezed during the ball milling process, deformed, and changed into smaller particles (Cao et al., 2019, Soares et al., 2015). Ball milling has already been applied to produce advanced novel engineered biochar for eco-friendly applications (Kumar et al., 2020). Lyu et al., (2018) used a PQ-N₂ planetary ball miller to grind biochars from different raw materials and prepared under different pyrolysis temperatures to produce new adsorbents. The study found that the ball-milled bagasse biomass (BMBG450) pyrolyzed at 450 °C has the highest methylene blue adsorption capacity. Compared with unmilled bagasse biochar (BG450), BMBG450 has a larger specific surface area and larger pores volume, smaller hydrodynamic radius, stronger negative potential (about 1.6 times increase), and more oxygen-containing functional groups. This shows that the ball milling method can improve the properties of biochar materials effectively, which can enhance their adsorption capacity in environmental applications. Therefore, the ball milling method, as a simple method for improving the adsorption of biochar material, has the advantages of environmental friendly and low cost, and has broad application prospects in the field of VOCs adsorption.

Although some studies have found that biochar has good adsorption capacity for volatile organic compounds, there are few studies on the adsorption performance of ball milled biochar prepared with different raw materials and pyrolysis temperatures on the adsorption of volatile organic pollutants (Zhang et al., 2017a). In this work, three different biomasses were used as raw materials, and the pristine biochars with different characteristics were prepared at different pyrolysis temperatures and ball milling was applied as modification method. A series of laboratory experiments were performed to characterize the physicochemical properties of the synthesized biochar. At the same time, acetone and toluene were used as typical volatile organic pollutants for adsorption experiments to compare the adsorption performance of biochar before and after ball milling, and relative mechanism was illustrated.