Biomass activated carbon derived from pine sawdust with steam bursting pretreatment; perfluorooctanoic acid and methylene blue adsorption

Highlights

• Steam bursting was used to pretreat sawdust to prepare activated carbon.

• The carbon had specific surface area of 962 m²/g, micropore volume of 0.327 cm³/g.

• The surface area and micropore volume decrease with the increase of pressure.

• This carbon achieved a high adsorption capacity of 223 mg/g PFOA and 332 mg/g MB.

Abstract

Using waste biomass to prepare various products by environmentally benign processes is a good way to practice green and sustainable development. In this paper, high porosity and surface area biomass activated carbon was obtained by pyrolysis of pine sawdust without using any chemicals after steam bursting pretreatment. Under hydrothermal conditions at 160 ℃, the differences of steam bursting at 300, 500, or 700 psi pressures on the structure and surface chemical groups of the final activated carbons product were compared. The characterization showed that the specific surface areas and micropore volumes decreased with the increase of pressure, while the relative content of oxygen-containing functional groups changed slightly. The sample obtained following 300 psi pretreatment (HPB300) offered the highest BET surface area and pore volume, 962 m²/g and 0.526 cm³/g respectively, and which also achieved the highest adsorption amounts for both methylene blue (MB) and perfluorooctanoic acid (PFOA).

Introduction

Activated carbon is an amorphous carbon-based material with high specific surface area and high porosity structure (Danish and Ahmad, 2018). Because of its good adsorption performance and porous structure, activated carbon is widely used in the adsorption of pollutants in water and air, and as materials for energy storage, catalysis and electrodes (Jain et al., 2015). Nowadays, activated carbon still has great market needs and broad development prospects (Rashidi and Yusup, 2017).

Currently, commercial activated carbon is commonly produced with coal as a precursor, so it is limited by the non-renewable nature of coal, and its price is still high (Abioye and Ani, 2015). Therefore, people have focused their research and development on biomass activated carbon. Numerous studies have appraised biomass activated carbon, but most of these have focused on the selection of raw materials, such as various sources of plants, as well as plant roots, stems, leaves, seeds, and nut shells, etc. (Yumak, 2021, Mao et al., 2015). However, activators such as KOH, H₃PO₄, ZnCl₂, CO₂, H₂O, or other additional equipment are still required in the preparation process.

Steam bursting (also known as “steam explosion” in other literature) is a technology widely used in biomass pretreatment (He et al., 2020). For this process, biomass is placed in a high pressure and steam condition; and the explosion phenomenon will occur when the pressure is released instantaneously, which is accompanied by the changes of physical and chemical properties of biomass (Jacquet et al., 2015, de Brito et al., 2020). In the process of steam bursting, the large biomass blocks are broken down into smaller pieces and the plant cell walls are damaged and cracked (Zhu et al., 2015, Muzamal et al., 2017). Meanwhile, under this hydrothermal and high-temperature condition, hemicellulose will be hydrolyzed into monosaccharides and oligosaccharides, the chemical bond between lignin and hemicellulose will be broken, the crystallinity of cellulose will be improved, and the rigidity of biomass materials will be reduced (Boonterm et al., 2016, Chen et al., 2019, Martin-Sampedro et al., 2011). These macroscopic and microscopic changes in biomass structure have changed the surface morphology of materials and improved the permeability of materials (Marques et al., 2020, Dai et al., 2021), which may lead to changes in the structure and properties of subsequent activated carbon product. This is the interest and innovation of this study.

Perfluorooctanoic acid (PFOA) and methylene blue (MB) as typical organic pollutants were selected to examine the adsorption properties of prepared activated carbon variants. PFOA is a long-chain perfluorinated compound with negative electronegativity in near-neutral natural water (Table 1), and its pKa = -0.5 (Goss, 2008), which with water and oil resistance properties. (Banzhaf et al., 2017, Arias Espana et al., 2015). It is widely used in packaging coatings, non-viscous cookware, firefighting foam, etc. (Goodrow et al., 2020, Schulz et al., 2020). PFOA is soluble in water, chemically stable, difficult to be degraded, and is bio accumulative, so it can persist in the environment, which seriously threatens human health and ecological environmental safety (Liu et al., 2020a, Liu et al., 2020b, Yuan et al., 2020).

MB as a kind of cationic dye, which has been widely used in textile, printing, dyeing, chemical and pharmaceutical industries, etc. (Dao et al., 2021). If a waste water that contains MB is discharged into natural water, it will not only damage the aquatic ecosystem, but also enter the human body through the food chain, causing skin diseases, hypertension, jaundice, mutation, or cancer. These all pose a serious threat to human health (Yu et al., 2021, Alshabib et al., 2021). MB is recalcitrant to natural degradation, because of its stable chemical structure and high solubility (Ma et al., 2021). At present, there are several removal methods of PFOA and MB, such as activated carbon, ion exchange, advanced oxidation, photocatalytic degradation, reverse osmosis membrane filtration, etc. Among these, activated carbon adsorption is generally considered to be a simple, cheap, fast and efficient removal method (Othmani et al., 2021, Rahman et al., 2014, Yu et al., 2021).

In this study, pine sawdust was selected as the precursor of biomass activated carbon. Pine sawdust was pretreated by steam bursting under different initial pressures. Following this pretreatment, one-step pyrolysis protocol was employed so as to yield a biomass activated carbon that hosted high pore volume, specific surface area, and PFOA and MB adsorption capacities. The effect of pressure variation on the pore structure of prepared activated carbon have been analyzed and discussed herein. The adsorption mechanism of PFOA and MB on prepared activated carbon was investigated by adsorption kinetics, pore diffusion, and isotherm models.