Oxygen-containing groups in cellulose and lignin biochar: their roles in U(VI) adsorption

The adsorption behaviors of cellulose and lignin biochar depend on the evolution of their oxygen-containing groups to some extent. In this study, cellulose-rich pakchoi and lignin-rich corncob were selected to prepare the pyrolytic biochar at variable temperatures, named PBC and CBC, respectively. Their structure–function relationships were in-depth studied via the combination of the adsorption experiments of U(VI) and comprehensive spectral analyses. The maximal adsorption capacity of PBC 300, obtained at 300 °C, was measured as 46.62 mg g−1 for U(VI), which was ⁓1.3 times higher than 35.60 mg g−1 of CBC 300. U(VI) adsorption on PBC and CBC were predominantly ascribed to the coordination interaction between oxygen-containing groups and U(VI). Interestingly, the main complexation groups were distinct in both biochars due to the different inherent evolutions of cellulose and lignin. Volatile d-glucose chains in cellulose were apt to degrade rapidly, and the formed carboxyls acted as the most important sites in PBC. However, the stable aromatic network in lignin led to a slow degradation, and more hydroxyls thus remained in CBC, which controlled U(VI) adsorption. In this study, we obtained greatly cost-effective adsorbents of U(VI) and provided some essential insights into understanding the structural evolution–function relationship of cellulose and lignin biochar.


Introduction
Uranium is one of the most important radionuclides (Abdel-Karim et al. 2016;Liu et al. 2017;Wang et al. 2020a) in the whole nuclear industry (Wen et al. 2016;Wang et al. 2020a). Once discharged into the environment, U(VI) would result in intense threat for ecological safety and human health owing to its horrible radioactive and chemical toxicities (Li et al. 2019a, b). Therefore, the efficient removal of U(VI) from the environment is urgent and desirable. Numerous physical and chemical techniques have been employed for U(VI) removal, such as adsorption (Li et al. 2019b;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. Especially, the adsorption has been identified as the most promising and reliable approach owing to its cost-effective advantage and convenient operation (Ahmed et al. 2021a(Ahmed et al. , 2021d. To date, extensive adsorbents for U(VI) have been designed reported, including zeolites (Wu et al. 2019), minerals (Liu et al. 2017), and artificial materials. By contrast, biochar is undoubtedly an optimal choice because of its low-cost, huge surface area, abundant functional groups, 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. 2019c;Ahmed et al. 2021b). The in-depth researches suggest that the adsorption performance of biochar depends on their Responsible Editor: Zhihong Xu * Jianjun Liang liangjj@lzb.ac.cn 1 chemical compositions, microstructures, and preparation methods (Enders et al. 2012;Suliman et al. 2016;Yoon et al. 2019). As the major composition of biomasses, cellulose and lignin decide the microstructures 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 properties of alkyl alcohol and ether (Rey-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 consisted of aliphatic and aromatic polymer, in which 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 temperature 29 . During 300-400 °C, 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 . 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 °C) (Sharma et al. 2004). Above 400 °C, plenty of CO 2 , CO, CH 4 , furan, methanol, and large-molecule volatiles are released due to the breakage of the C-O bond, C = O, -OCH 3 , and -CH 2 (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. Rutherford et al. (2012) have found that the degradation products of cellulose in case of low temperature charring would likely dominate the properties of biochar due to its more oxidized groups than lignin biochar. Cellulose biochar is superior in Cd(II) enrichment 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, rich in cellulose, may provide excellent removal efficiency for heavy metals due to a large quantity of oxygen-containing groups and thus negative-charged surface (Komnitsas et al. 2016). In comparison, Pb(II) immobilization on lignin biochar demonstrated a competitive method owning to its fast mineral precipitation and surface complexation . For anionic pollutants, lignin biochar can also provide efficient hydrogen bond sites and beneficial surficial electrostatic attraction (Yoon et al. 2019). Biochar from various precursors has been extensively explored to elucidate their adsorption behaviors. However, current studies focus more on the adsorption capacity of biochar and miss the in-depth understanding of the correlation between their structural differences and immobilization capacities, especially for cellulose and lignin biochar. 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). According to Klason method (Benouadah et al. 2019), the contents of cellulose and lignin were measured as 47.40% and 15.64% in pakchoi, as well as 20.46% and 44.86% in corncob, respectively. A series of PBC and CBC were prepared via charing at 300, 400, and 500 °C, 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 the adsorption kinetics and durability of PBC and CBC. The corresponding mechanisms were revealed via the comprehensive spectroscopic analyses. These findings are important for the large-scale practical application of biochar in the sequestration of U(VI).

Materials
Uranium nitrate (UO 2 (NO 3 ) 2 ⋅6H 2 O) was purchased from Jieke Chemapol and was dissolved in the Milli-Q water (18.2 MΩ) to prepare U(VI) stock solution. The pH values were measured by Mettler-Toledo composite electrode. All chemicals in the subsequent experiments were of analytical grade and used without further purification. The preparation method, characterization, and determination of cellulose and lignin content of biochar were described in Appendix A Text S1 ~ S3.

Adsorption experiments
The biochar adsorbent was added into the tubes, making the mass-to-volume ratio (m/v) equal to 0.3 g L −1 . The effect of pH was studied at constant uranium concentration of 10 −5 mol L −1 and NaCl background electrolyte of 0.01 mol L −1 . Milli-Q water was injected to maintain the total volume of 8.0 mL. The pH values of the suspension were adjusted by dropping a negligible volume of HCl and/or NaOH solution and then was shaken at 150 rpm. At pre-set time intervals, the dispersion was partially collected and filtered with a 0.22 μm membrane. The concentration of U(VI) in the filtrates was measured using spectrophotometry at 652 nm with arsenazo III as chromogenic reagent (Li et al. 2019a). The corresponding calculations were stated in Appendix A Text S4, including the adsorption rate of U(VI) and the simulations of the kinetics.
Regeneration and recycling experiments were conducted to investigate the stability of PBC and CBC in the environment. Forty mL Na 2 CO 3 solutions (0.1 mol L −1 ), acted as eluent, were injected to adsorption-equilibrium PBC and CBC (12 mg) to desorb the loading U(VI). The sample was shaken for an hour and then filtered. The U(VI) concentration was measured with the above method. The processes repeated six cycles to evaluate the stability of PBC and CBC.

Characterization
The microscopic morphologies have been recorded for PBC and CBC prepared at the variable temperatures, respectively. As shown in Fig. 1a and c, PBC displayed multilevel fibrous porous that spread in the whole framework regularly. PBC 300 showed a rough surface. Compared with PBC 300, the regular and loose pores were further developed in the PBC 400 and PBC 500 due to the pyrolysis of cellulose. For CBC, irregular folds constructed large quantities of disordered channels, and small carbon sphere particles were observed on their surface ( Fig. 1d and f). The blurred pores and broken fragments were observed in CBC 500 owing to the deterioration of the carbon skeleton at the high pyrolysis temperature. These characteristic morphologies were consistent with previous findings (Ahmed et al. 2021d).
There were obvious differences in the surficial oxygen-containing functional groups of cellulose-rich PBC and lignin-rich CBC ( Fig. 2a and b). PBC 300 showed a quite strong band at ~ 1593 cm −1 with a broad halfwidth, which could be assigned to a combined band of v(C = O) and v(C = C) ). Meanwhile, a weak vibration was observed at 1031 cm −1 ascribed to v(C-O-C) (Li et al. 2019d). These bands indicated a high content of carbonyls and carboxyls but much less ether groups in PBC 300. In fact, the difference in oxygen-containing groups could be ascribed to unstable D-glucose chains in cellulose. The complicated chemical reactions during the hydrolyzation possibly caused the crack of β-1,4 glycosidic connections resulting in a decreasing ether content. Simultaneously, a large number of hydroxyls were oxidized into carbonyls and carboxyls (Meng et al. 2021). This hypothesis could be proved by the analysis of XPS. In the O 1s spectrum of PBC 300 (Fig. 2c), two peaks could be observed including a strong peak at 531.2 eV and another tiny one at 533.3 eV, which corresponded to C = O and C-O moieties, respectively (Xu et al. 2020). The great difference in the integral areas indicated that PBC 300 had abundant carbonyls and carboxyls but a small number of hydroxyls and ether. In the case of CBC 300, an ultra-strong peak was observed at 1057 cm −1 , and three comparable strong ones were also found at 1049, 1261, and 1698 cm −1 , which were, respectively, relative to v (C-O-C), v(C-OH), and v(C = O) (Ahmed et al. 2021b). It seemed to indicate a large number of ether groups, as well as plentiful carboxyls and hydroxyls in CBC 300. Correspondingly, the O 1s spectrum of CBC 300 determined an integral area ratio of 0.38:1 for C = O:C-O, and hydroxyls and ether groups thus proved to be dominant in CBC 300 (Fig. 2d). The abundant C-O moieties in CBC 300 could be explained by the unique phenyl ether units in lignin. The aromatic majority in lignin led to a relatively stable structure and slow degradation of the corncob biochar at 300 ℃. Therefore, large quantities of ether bonds remained in CBC 300.
Structural transformations of PBC and CBC were aggravated with the increasing pyrolysis temperatures. Interestingly, no significant differences were observed in the FT-IR patterns of PBC 300 and PBC 400. The similar spectra indicated a relatively stable structure in cellulose-rich PBC in the range of 300 to 400 °C, which could be also demonstrated by their element analyses and XPS patterns. As listed in (Appendix A Table S1), a series of comparable data were observed for C, N, H, and O elements in PBC 300 and PBC 400, suggesting close elemental compositions and relatively stable structures. Moreover, similar outlines were found in the survey XPS spectra of PBC 300 and PBC 400 (Fig. 2e), except for a slight decline of O 1s in PBC 400. The O 1s fine spectra also exhibited similar plots for the both biochar, indicating the stable structures. The relative stability was broken at 500 °C. After charring, some significant changes were observed in the FT-IR spectrum of PBC 500, in particular, in the fingerprint region concerning the aromatic C-H groups (Li et al. 2019a;Zhao et al. 2019). It indicated an obvious aromatization procedure in PBC 500, which could be attributed to the varying skeleton of cellulose at high temperatures. Meanwhile, a sharp increase was found in the carbon percentage of PBC 500, followed by a corresponding decrease of oxygen content (Table S1), which also indicated intense aromatization and deoxidation reactions. Due to the drastic decomposition, some characteristic bands of metal and silicon appeared in the survey XPS analysis of PBC 500 (Fig. 2e). By contrast, CBC revealed a regular decomposition due to the stable aromatic skeleton in lignin.

Effect of pH on U(VI) adsorption
Zeta potentials of PBC and CBC were investigated to further understand the surface charge densities and adsorption abilities. As shown in Fig. 3a and b, the surface charge densities of PBC and CBC were decreased with the increasing pH value. It was associated with the deprotonation of oxygenous groups retained on the biochar surface. At pH ~ 3.0, the zeta potential of PBC 300 was − 10.70 mV while − 3.25 mV for CBC 300. Obviously, more carboxyls occurred in PBC 300 in comparison with CBC 300. It was the deprotonation reaction of partial carboxyls that led to the higher negative charge in PBC 300. As pH value increased to 5.0, more carboxyls and small amounts of hydroxyls lost their protons, causing an obvious rise of surficial negative charges. Zeta potentials of PBC 300 and CBC 300 thus grew up to − 26.50 and − 6.61 mV, respectively. At pH ~ 7.0, all carboxyls lost the protons and so did most of the hydroxyls. Zeta potentials thus increased to − 34.70 mV for PBC 300 and − 31.70 mV for CBC 300. Faster growth was observed in the zeta potentials of CBC 300 in comparison with that of PBC 300, as pH values increased from 5.0 to 7.0. It could be attributed to more hydroxyls in CBC 300 than PBC 300. Similar fast growths were also observed in zeta potentials of CBC 400 and CBC 500, indicating that plentiful hydroxyls remained in their skeletons. For CBC 400 and CBC 500, positive potentials were even determined at pH ~ 5.0, suggesting their low contents of carboxyl groups. These conclusions were consistent with the previous discussion of FT-IR and XPS analyses.
To understand the effect of pH, the adsorption edges were performed for U(VI) adsorption on biochar. The pH values were maintained in the range of 2.0-10.0, and the contents of biochar and the initial U(VI) were 0.3 g L −1 and 10 −5 mol L −1 , respectively. As depicted in Fig. 3c and d, the adsorption of U(VI) on biochar initially increased in the pH range from 2.0 to 6.0 and then decreased gradually until pH ~ 10.0. Such phenomenon was mainly related to the changes of U(VI) species in aqueous phase and surface charge density of biochar. U(VI) existed mainly in the form of divalent cations under the acidic environment (Ahmed et al. 2021b), i.e., UO 2 2+ . In the cases of PBC 300 and CBC 300, the electrostatic attraction was gradually strengthened between U(VI) and negative-charged biochar as pH values increased from 2.0 to 6.0. The electrostatic adsorption of U(VI) on the biochar surface led to an increasing surficial concentration, which was also beneficial to the next coordination of U(VI), and thus resulted in increasing immobilization capacity. Above pH ~ 6.0, negatively charged species of U(VI) became more and more, and strong electrostatic repulsion thus inhibited U(VI) adsorption on biochar.
Notably, close capacities were observed for U(VI) adsorption on PBC 400 and PBC 500 at pH ~ 5.0 despite the various surficial potentials. A similar phenomenon was found on three kinds of CBC in the whole pH range. These phenomena could be ascribed to the fact that the coordination interaction, rather than electrostatic attraction, dominated the surficial adsorption of U(VI).

Effect of initial U(VI) concentration
U(VI) adsorption on PBC and CBC were investigated at variable initial concentrations of U(VI) from 0.0 to 10 −4 mol L −1 . In the batch experiments, the contents of biochar were maintained to be 0.3 g·L −1 , and the pH value was set as 5.0. As shown in Fig. 4a and b, the adsorption capacity of U(VI) rapidly rose with the increasing initial concentration of U(VI). When U(VI) concentration exceeded 1.0 x 10 −4 mol L −1 , the adsorption on biochar reached equilibrium because of the saturated active sites. Maximum adsorption capacities were determined at 46.62 mg g −1 for PBC 300, 37.55 mg g −1 for PBC 400, and 24.59 mg g −1 for PBC 500, respectively. The declined sequences in the adsorption capacities of PBC were well consistent with the decay in their surficial potentials. Under the same tested conditions, PBC showed better performances than corresponding CBC. Interestingly, CBC 300 and CBC 400, with the distinct surficial potentials, suggested similar adsorption capacities (35.60 mg g −1 and 33.01 mg g −1 , respectively). It could be attributed to the increased micropores in CBC 400 due to the enhanced decomposition. For CBC 500, adsorption property of 21.67 mg g −1 was determined, which was ⁓1.64 times lower than that of CBC300. Furthermore, previous studies about biochar adsorbents were listed in Appendix A Table S3 to compare with our work. Results suggest that PBC300 and CBC300 showed the moderate efficiencies for U(VI) removal among these adsorbents, regardless of various carbon sources. As the unmodified biochar, the performances of PBC300 and CBC300 were only inferior to some carbonaceous nanofibers or modified biochar (Sun et al. 2016;Liatsou et al. 2017). Taking into account of their simple preparations and readily available sources, PBC300 and CBC300 can be regarded as excellent adsorbents of U(VI).

Effect of contact time
The adsorption kinetics of the biochar were also investigated. As shown in Fig. 4c and d, the immobilization of U(VI) was remarkably fast for all the biochar within the initial 4 h. Then, slow equilibriums were achieved at approximately 8 h. The rapid adsorption could be due to the abundant available sites on the biochar surface, i.e., carboxyls in PBC and hydroxyls in CBC (Sun et al. 2012;Lan et al. 2017;Xie et al. 2019). The latter plateau demonstrated the gradual saturation of adsorption sites on external surface (Gu et al. 2018).
The related simulation parameters of the two kinetics models were listed in Appendix A Table S2, and the fitting curves were also depicted in Fig. 4e and f. The higher R-squared value was observed for the pseudo-second-order model, indicating the better fitting degree. Moreover, the calculated q e values from the pseudo-second-order model were quite close to the experimental data. This demonstrated that the adsorption of U(VI) on the PBC and CBC was primarily attributed to the chemisorption (Sun et al. 2016). The rapid adsorption rate might be due to the strong surface complexation for U(VI) from the oxygen-containing groups, rather than physical adsorption (Sun et al. 2012).

Reusable performance of PBC and CBC for U(VI) adsorption
Cycle experiments were conducted to investigate the recycling of PBC 300 and CBC 300 in the natural environment. A 0.1 mol L −1 of Na 2 CO 3 solution was chosen as the eluent because of the stronger complexation competition of CO 3 2− for U(VI) than the oxygen-containing groups on biochar surface. As presented in Appendix A Fig. S1, after six cycles of adsorption and regeneration, PBC 300 and CBC 300 maintained an approximately 88% activities with respect to initial adsorption of U(VI), which suggest quite stable structures during the adsorption and regeneration processes. The wide application should be expected in the practical treatment of wastewater containing U(VI). The slight decrease of adsorption ability for PBC 300 and CBC 300 could be due to the soft extractant, i.e., Na 2 CO 3 , which was hard to extract the stable complexes of U(VI) on biochar. Therefore, the occupied adsorption sites of biochar lost the activities in the next cycles (Yoon et al. 2019;Ahmed et al. 2021b).

Adsorption mechanism of U(VI) on biochar
FT-IR and XPS analyses were performed to reveal the adsorption mechanism of U(VI) on biochar. As shown in Fig. 5a, the stretching vibration of C = O redshifted from 1696.4 to 1636.8 cm −1 in PBC 300, while v(C-OH) in carboxyl blue-shifted from 1383.4 to 1404.7 cm −1 . This phenomenon verified that carboxyls coordinated with U(VI) (Han et al. 2017). Moreover, a significant red-shift from 1456.4 to 1413.2 cm −1 was observed for v(C-OH) of hydroxyls in CBC 300 (Fig. 5b), indicating the strong coordination between hydroxyls and U(VI). After being captured, U 4f 5/2 and U 4f 7/2 occurred at 392.85 eV and 382.10 eV in PBC 300 while 393.18 eV and 382.40 eV in CBC 300 ( Fig. 5c  and d). Notably, the different binding energies of U 4f 5/2 in U(VI) species on PBC 300 and CBC 300 could be roughly ascribed to their different main coordination groups, i.e., carboxyls in PBC and hydroxyls in CBC.
O 1s fine spectra were also measured to explore the adsorption mechanism of U(VI). A new band appeared at 531.10 eV in O 1s spectra of PBC 300 and CBC 300 after the adsorption equilibrium of U(VI), which could be assigned to O = U = O fraction in UO 2 2+ (Schindler et al. 2009) (Fig. 5e and f). In addition, a significant shift from 531.39 to 531.15 eV occurred in the C = O moiety of PBC 300, indicating the coordination between carboxyls and U(VI). Similar red-shifts were also observed in C-O bands in CBC 300, indicating the coordination between hydroxyls and U(VI). This phenomenon could be explained that oxygen atoms in carbonyls or hydroxyls attracted partial elections of U(VI) due to the higher electronegativity. The deviated outer electrons resulted in an increased electrostatic repulsion and consequently reduced the binding property of the nucleus for the inner electrons (Hüfner 2003).

Conclusions
In this work, a series of biochar was prepared from celluloserich pakchoi and lignin-rich corncob for U(VI) adsorption. The comprehensive characterizations were performed to elucidate the decomposition and evolution of biomasses at different temperatures. The adsorption equilibrium of U(VI) could be reached on all of the biochar within 8 h. The best adsorption capacity (q e ) of PBC300 was determined to be 45.62 mg g −1 , ⁓1.28 times higher than that of CBC300 (35.60 mg g −1 ), which benefitted from more carboxyls in PBC in comparison with CBC. U(VI) adsorption on PBC and CBC was mainly ascribed to the coordination interaction of oxygen-containing groups. Carboxyls in PBC and hydroxyls in CBC, respectively, acted as the main active sites. The significant differences in adsorption behaviors of PBC and CBC could be ascribed to the distinct evolution of cellulose and lignin. Our study proves that the readily available pakchoi biochar can act as an effective absorbent for U(VI) removal. These findings provide essential insights into understanding the oxygen-containing structural evolution and the structure-function relationship of cellulose-and lignin-rich biochar.