Uranium is regarded as one of the most important radionuclides (Karim et al., 2016; Liu et al., 2017; J. Wang et al., 2020a) in the whole nuclear industry (Wen et al., 2016; Wang et al., 2020). Once released into the environment, hexavalent U(VI) would result in a horrible threat to ecological safety and human health due to its high radioactive and chemical toxicities (Li et al., 2019; Li et al., 2019b). Therefore, the efficient removal of U(VI) from the polluted water is urgent and desirable. Numerous techniques have been adapted for U(VI) removal, such as adsorption (Li et al., 2019; Philippou et al., 2019), chemical precipitation (Foster et al., 2019), ion exchange (Cheng et al., 2019), photocatalysis (Wang et al., 2020b), and solvent extraction (Prabhu et al., 2017), and so on. Among these techniques, the adsorption promises the most feasible approach owing to its cost-effective advantage and convenient operation (Ahmed et al., 2021a, 2021d).
To date, a large number of adsorbents have been designed for U(VI) separations, including zeolites (Wu et al., 2019), minerals (Liu et al., 2017), artificial materials, and so on. By comparison, biochar proves a promising application because of its low cost, huge surface area, and superior stability (Inyang et al., 2016; Ahmed et al., 2021c). Various biochar has shown excellent adsorption capacities for U(VI) pollutants (Liu et al., 2017; Li et al., 2019; Ahmed et al., 2021b). The in-depth researches suggest that the adsorption performance of biochar depends on their chemical compositions, microstructures, and preparation methods (Enders et al., 2012; Suliman et al., 2016; Yoon et al., 2019). As the major biomasses, cellulose and lignin control the structures and functions of the resulting biochar to some extent (Li et al., 2014). It is important to illuminate the microstructural evolution of cellulose and lignin during the pyrolysis procedure, in particular, their oxygen-containing group transformations.
Cellulose is mainly composed of regular D-glucose chains with β-1,4 glycosidic connections, and thus shows the chemical characters of alkyl alcohol and ether (Raap et al., 2019; He et al., 2020; Zhao et al., 2019; Cheng et al., 2021). Lignin is usually regarded as a complicated three-dimensional network constructed by both aliphatic and aromatic polymer, where phenylpropane, interconnected by ether bonds, is the major units (Sharma et al., 2004; Kijima et al., 2011; Meng et al., 2021). As heated in nitrogen or inert atmosphere, cellulose and lignin experience a series of complicated deoxidation reactions (Bradbury and Shafizadeh, 1980), as well as polyaromatic crosslinking and stack (Ishimaru et al., 2007). Due to the aliphatic feature, cellulose readily decomposes and transforms at a relatively low temperature29. During 300–400 oC, the pyrolysis of cellulose involved the rearrangement and oxidation of C = C, C = O, COO, paraffinic, aromatic groups, etc. (Shafizadeh and Sekiguchi, 1984). As the main substructure, the glycosyl units decompose rapidly to levoglucosan (Shafizadeh et al., 1979). Meanwhile, the aliphatic chains in cellulose entangle irregularly, resulting in abundant disordered structures (Bradbury et al., 1979; Shafizadeh and Lai, 1975). By contrast, lignin is more stable, and slowly decomposes at a much wider range (160–900 oC) (Sharma et.al., 2004). Above 400 oC, plenty of CO2, CO, CH4, furan, methanol, and large-molecule volatiles are released due to the breakage of the C-O bond, C = O, -OCH3, and -CH2 (Suzuki et al., 2001) in lignin (Cao et al., 2013). Consequently, the formed lignin carbon shows stronger stability than cellulose carbon because of its higher content of aromatic structures (Xie et al., 2009).
These variable structures lead to significant differences in the adsorption behaviors of cellulose- and lignin-biochar. Cellulose-biochar shows stronger Cd(II) enrichment ability than lignin-biochar, because its plentiful mesopores provide effective channels to facilitate the pollutant penetrations, and abundant oxygen-containing groups guarantee large numbers of lone pair electrons to coordinate with Cd(II) ions (Chen et al., 2018). Sawdust biochar, which remains abundant cellulose, can remove heavy metals effectively due to the negatively charged surface and a large quantity of oxygen-containing groups (Komnitsas et al., 2016). In comparison, Pb(II) immobilization on lignin-biochar is proved a competitive method due to its fast mineral precipitation and surface complexation (Wu et al., 2021). For anionic pollutants, lignin-biochar can also provide efficient hydrogen bond sites and beneficial surficial electrostatic attraction (Yoon et al., 2019). To date, biochar from various precursors has been extensively explored to elucidate their surface characteristics and adsorption properties. However, the inherent differences in cellulose- and lignin-biochar are still scarcely discussed, especially, their adsorption sites and immobilization mechanisms. Moreover, U(VI) sequestration on biochar is also rarely reported.
In this study, we chose cellulose-rich pakchoi biochar (PBC) and lignin-rich corncob biochar (CBC) as adsorbents to explore their adsorption behavior for U(VI). A series of PBC and CBC were prepared via charing at 300, 400, and 500 oC, respectively. The obtained biochar was thus donated as PBC 300, PBC 400, PBC 500, CBC 300, CBC 400, and CBC 500, respectively. Batch experiments were performed to evaluate and compare the adsorption kinetics of U(VI) on PBC and CBC. The corresponding mechanisms were revealed via spectroscopic measurements. These findings about cellulose and lignin carbon are important for the practical application of biochar in the sequestration of U(VI).