Preparation of biochar derived from waste cotton woven by low-dosage Fe(NO3)3 activation: characterization, pore development, and adsorption

Currently, researchers are looking for efficient and sustainable methods to synthesize biochar for the adsorption of pollutants. In this study, biochar with high specific surface area, tunable pore structure, and abundant functional groups were prepared from waste cotton woven (WCW) using low-dosage Fe(NO3)3 activation at 400–900 °C. The biochar obtained at 800 °C possessed the excellent specific surface area of 1167.37 m2/g with a unique micro-mesoporous structure. XRD analysis showed that the Fe species changed from Fe2O3 to Fe3O4 and then Fe0 with the increase of pyrolysis temperature. TEM images further confirmed the template effect of iron oxides for mesoporous formation. The effect of Fe(NO3)3 on the pyrolysis pathway of waste cotton woven was systematically investigated by TG and XPS analyses to explore the pore development of biochar. The results indicated that Fe(NO3)3 could enhance the dehydration, decarbonylation and dehydroxylation of WCW components, thereby reducing the temperature required for WCW pyrolysis. Moreover, the synergistic effect of Fe and N species improved the development of microporous and mesoporous structure through carbon structure corrosion and reorganization, and volatile release. Additionally, satisfactory adsorption capacity for Eriochrome Black T (456.01 mg/g) of the prepared biochar was obtained at 25 °C. This study demonstrated that low-dosage Fe(NO3)3 activation of waste cotton woven could be used as a facile method to prepare promising inexpensive biochar for contaminants removal.


Introduction
Biochar (BC) is a promising adsorbent characterized by large specific surface area, highly developed porosity, tunable pore size, modifiable surface functional groups, and adjustable surface morphology. Generally, to maximize pore development, the preparation of BC mainly includes physical and chemical activation (Okman et al. 2014). In the case of physical activation, precursors should be first carbonized and then activated by activators like air, CO 2 , H 2 O, and O 2 under high thermal temperature for a long time (Román et al. 2013). In contrast, chemical activation has been widely studied due to its convenient one-step synthesis, low thermal temperature, and short activation time (González-García 2018). The most common activators include ZnCl 2 , H 3 PO 4 , KOH, and K 2 CO 3 ; however, the above traditional activators are toxic and highly corrosive (Prahas et al. 2008;Shu et al. 2013;Zhang et al. 2021). Besides, a large amount of traditional activator needs to be added to ensure the quality Responsible Editor: Zhihong Xu of the BC, which undoubtedly increases the production cost. For example, Li et al. (2020) used extremely high mass ratio (ZnCl 2 /biomass) of 4/1 to prepare biomass-based BC. Shao et al. (Shao et al. 2021) prepared walnut shell-based BCs using KOH as the chemical activator at a mass ratio (KOH/ walnut shell) of 2/1. Liu et al. (2012) prepared BCs from rice husk ash via K 2 CO 3 activation at optimum mass ratio (K 2 CO 3 /rice husk ash) of 1.5/1. Thus, searching for more efficient and environmentally friendly activators remains a research topic for further development.
Recently, iron salt activators are receiving increasing attention because of the advantages of non-toxicity, costeffective, and conferring catalytic and magnetic properties to the resulting BC (Gómez-Avilés et al. 2021). Commonly used iron salt activators include ferric chloride (FeCl 3 ) (Zhu et al. 2014, ferric nitrate (Fe(NO 3 ) 3 ) (Dong et al. 2012), ferric citrate (FeC 6 H 5 O 7 ) (Fu et al. 2014;Qian et al. 2016), and ferrous oxalate (FeC 2 O 4 ) (Fu et al. 2014;Qian et al. 2016). In general, the process of preparing BC by iron salt activation mainly includes two unique aspects. On the one hand, Fe 3+ can promote the dehydration process of biomass materials and restrict the formation of tars for micropores formation (Xu et al. 2019a). On the other hand, the iron (hydro)oxides remaining on BC surface after pickling endow the BC with magnetic and catalytic properties (Bedia et al. 2017;Dong et al. 2012). Although the iron salt activators have achieved desirable results, the current research commonly only focuses on the important role of Fe species, ignoring the potential impact of other components in the activator. In fact, the activation effect of different iron salts is indeed significantly different, which is probably caused by the acid radical ions or anions in the iron salts.
Currently, some studies have also proposed the auxiliary effect of the acid radical ions or anions in iron salts on the activation process from different perspectives. For instance, Fu et al. (2014) reported that the acid radical ions in FeC 6 H 5 O 7 and FeC 2 O 4 produced lots of CO, CO 2 , and H 2 O gases during the decomposition processes, fabricating the formation of mesopores and macropores. This phenomenon may be a common feature of organic iron salt activators. In addition, the role of chloride ions (Cl − ) in FeCl 3 in the activation process was deeply explored in our previous study (Xu et al. 2020). The results demonstrated that the synergistic effect of Fe 3+ and Cl − was beneficial to the cross-linking reaction, which enhanced the development of intricate microporous structure and facilitated the formation of carbonaceous materials. Inspired by this, we expected the auxiliary activation ability of anion in another inorganic iron salt, Fe(NO 3 ) 3 . It has been reported that nitrate can decompose to gases containing a variety of nitrogen oxides at temperature around 500 °C (Chang et al. 2013;Lee et al. 2010). The release of the nitrogen oxides generated from NO 3 − may contribute to the pore formation of BC, and these nascent N species may also participate in the complex carbon matrix decomposition and reorganization process, thereby potentially modifying the resulting BC. Moreover, the synergistic activation of multi-components will help to improve the activation efficiency of activator, promising to prepare high-quality biochar with lower dosage of activator. For these reasons, it is of great significance to investigate the feasibility of preparing BC by low-dose Fe(NO 3 ) 3 activation and to elucidate the multifaceted activation role of Fe(NO 3 ) 3 during carbonization. For many years, researchers have been devoted to finding sustainable and low-cost raw materials to replace traditional fossil fuels, peat, bituminous coal and anthracite as carbon precursors. The most common alternatives are agricultural residues or waste biomass like corn stalk (Zubrik et al. 2017), coconut shell (Vilella et al. 2017), rice straw (Sangon et al. 2018), and cassava stem (Sulaiman et al. 2018). Among them, WCW was also a potential candidate for carbon precursor, which has been produced in an annual volume of 2 × 10 7 t in China. Compared with using traditional landfill or incineration to treat WCW, it is a more environmentally friendly disposal method to utilize WCW to prepare BC. Moreover, as a carbon precursor, WCW has inherent advantages of high carbon content, easy availability and low cost.
Our previous studies have also demonstrated that it is feasible to prepare BC with considerable pore structure and adsorption properties from WCW (Xu et al. 2019a(Xu et al. , 2018b(Xu et al. , 2020. In this work, Fe(NO 3 ) 3 is used as an activator to fabricate BCs from WCW for Eriochrome Black T (EBT, a typical anionic dyes, which is widely used in dyeing industries and has serious threat to environment and human health) removal. The three main aims of this study are: (I) exploring the effects and internal laws of different pyrolysis temperatures on the yield, chemical composition, and pore structure of BCs; (II) using XRD, TEM, FTIR, TG, and XPS to explore the pore-forming mechanism of BCs; (III) elucidating the synergistic activation effect of Fe and N species during pyrolysis.

Preparation of NBCs
WCW was obtained from WUXI No. 1 Cotton Mill Textile Group in Jiangsu Province, People's Republic of China, and its components mainly include cellulose (~ 84.76%), hemicellulose (~ 5.16%), and lignin (~ 6.42%). WCW was chopped into small pieces of 0.5-1.0 cm in length. For one-step chemical activation, WCW was impregnated in Fe(NO 3 ) 3 ·9H 2 O solution at a mass ratio of Fe(NO 3 ) 3 / WCW = 0.5/1 for 24 h at room temperature. The mixture was then dried overnight at 60 °C in a dryer. The dried mixture was placed into a tubular oven and pyrolyzed at a given temperature (400-900 °C) for 1.0 h under continuous N 2 (> 99.99%) flow. The resulted samples were cooled to room temperature and then immersed in 0.1 mol/L boiling HCl solution for 10 min. After rinsing with deionized water until the pH of the effluent was near neutral, the resulted Fe(NO 3 ) 3 -activated BCs (NBCs) were dried in the dryer at 105 °C until constant weight and stored for further study. The samples prepared at different temperatures were denoted as NBC-400, NBC-500, NBC-600, NBC-700, NBC-800, and NBC-900, respectively.

Characterization
Elemental analyses were performed using a Vario MACRO Cube (Elementar, Germany) analyzer to quantify the C, N, H, S and O contents of the samples. The contents of volatile matter, fixed carbon, and ash were determined by the Chinese National Standards (GB/T 212-2008). The surface area and pore characteristics of the samples were determined by N 2 adsorption desorption isotherm using a surface area analyzer (Quantachrome/autosorb-IQ, USA). The specific surface areas of all samples were determined using the multipoint Brunauer-Emmett-Teller (BET) isotherm equations and pore size distributions were calculated by the Barrett-Joyner-Halenda (BJH) method. X-ray powder diffraction (XRD) study was carried out using a D8 Advance diffractometer (Bruker, Germany), working with Cu-Kα radiation at a scan rate of 3° min −1 and 2θ = 5-80°. The morphology of the samples was analyzed with a transmission electron microscopy (TEM, FEI/HELIOS NanoLab 600i, USA). The surface functional groups of the samples were analyzed using a Fourier transform infrared spectrometer (FTIR, Bruker/Tensor 27, Germany), scanned over a region of 4000 to 400 cm −1 using samples prepared as pellets with KBr. The thermogravimetric (TG) analysis of samples was implemented using a thermogravimetric analyzer (PerkinElmer/ STA 8000, USA). X-ray photoelectron spectra (XPS) were obtained using an ESCALAB-250XI (Thermo Scientific, USA) spectrometer with a monochromatic X-ray source of Al-Kα (1486.7 eV) with emission current of 10 mA.

Adsorption kinetics
The prepared NBC-800 was tested as adsorbent towards to EBT from aqueous solution. Standard solutions of EBT (pH = 2.0) were prepared at the concentrations of 50, 200, and 400 mg/L. Batch experiments were carried out using equal concentration of NBC-800 at 0.5 g/L. The suspension was stirred at 150 rpm at 25 °C. Aliquots were withdrawn at specified intervals of time (10, 20, 30, 60, 120, 240, 480, and 720 min) and filtered by a 0.45 μm filter. The EBT concentration in the filtrate was measured in a UV-vis spectrophotometer (λ = 534 nm). The adsorbed amount of EBT at time t, q t (mg/g), was calculated as follows: where C 0 and C t (mg/L) are the initial and equilibrium concentrations of EBT, respectively, V (L) is the volume of the solution, and m (g) is the dry weight of NBC-800.
The pseudo-first-order (Eq. 2) and pseudo-second-order models (Eq. 3) were used, and the nonlinear forms were expressed as: where q e and q t (mg/g) are the adsorptive capabilities at equilibrium and time t, respectively; and k 1 (min −1 ) and k 2 (g/mg min) are the rate constants of the pseudo-first-and second-order models, respectively.

Adsorption isotherms
For the adsorption isotherm experiments, standard solutions of EBT (pH = 2.0) were prepared at the concentrations of 100, 200, 300, 400, 500, and 600 mg/L. Batch experiments were carried out using equal concentration of NBC-800 at 0.5 g/L. The suspension was stirred at 150 rpm for 24 h at 25, 35, and 45 °C. The supernatants were withdrawn after 24-h adsorption and filtered by a 0.45 μm filter. The EBT concentration in the filtrate was measured in a UV-vis spectrophotometer (λ = 534 nm). The equilibrium amount of EBT adsorbed onto the NBC-800, q e (mg/g), was determined as follows: where C 0 and C e (mg/L) are the initial and equilibrium concentrations of EBT, respectively, V (L) is the volume of the solution, and m (g) is the dry weight of NBC-800.
The Langmuir model (Eq. 5) and Freundlich model (Eq. 6) were used to fit the adsorption data [44]: where C e (mg/g) is the equilibrium concentration of EBT, q e (mg/g) is the amount of EBT adsorbed at equilibrium, q m (mg/g) is the maximum adsorption capacity, K L (L/mg) is the Langmuir affinity constant, and n and K F ((mg g −1 )·(L (1) mg −1 ) 1/n ) are the Freundlich constants corresponding to adsorption intensity and capability, respectively.

Ultimate analysis and proximate analysis
The elemental compositions and proximate analysis parameters of WCW and NBCs are displayed in Table 1. Undoubtedly, compared with WCW, the C and fix carbon content of NBCs increased pronouncedly after the addition of Fe(NO 3 ) 3 , while the contents of H, O, and volatile decreased significantly. These indicated that a stronger carbonization reaction occurred during the pyrolysis process under the activation of Fe(NO 3 ) 3 (Cheng et al. 2022). And the increase in N content was attributed to the conversion of NO 3 − during carbonization, which might lead to a series of more complex chemical reactions and changes in the physical and chemical properties of NBCs (Dizbay-Onat et al. 2017). The increasing pyrolysis temperatures had a positive effect on the C content of NBCs. As the pyrolysis temperature increased from 400 to 900 °C, the C content of NBCs increased from 64.83 to 93.57%, indicating that the higher the pyrolysis temperature, the more volatile substances were released (Foo and Hameed 2012;Liew et al. 2018). It could also be observed that the contents of H and O decreased continuously, exhibiting completely opposite trends. Moreover, as the pyrolysis temperature increased from 400 to 800 °C, the N content increased slightly from 0.84 to 1.03%. It could be inferred that the increase of pyrolysis temperature was beneficial to the reaction between NO 3 − and WCW components. However, the N content decreased pronouncedly at 900 °C, indicating that excessive temperature (> 800 °C) sharpened the reaction, resulting in the reduction and loss of nitrogenous functional groups (Chernyak et al. 2019;Kundu et al. 2010;Xiao et al. 2005).

Specific surface area and pore structure
The N 2 adsorption-desorption isotherms of NBCs prepared under different pyrolysis temperatures are shown in Fig. 1a. Apparently, all NBCs exhibited analogous type VI isotherm with H3 hysteresis loop in a P/P 0 range from 0.4 to 1.0. The hysteresis loops of NBCs prepared within 600 to 800 °C were smaller than that of others, revealing that NBC-600, NBC-700, and NBC-800 possessed relatively less mesoporous structure than other NBCs (Kang et al. 2019;Kruk and Jaroniec 2001). As could be seen from Fig. 1b, only the NBCs prepared within 600 to 800 °C exhibited
In order to further explore the variation rules of pore structure of NBCs, as shown in Table 2, the pore structural parameters of various NBCs prepared at different pyrolysis temperatures was calculated by BET and t-plot method. As the pyrolysis temperature was within 400-500 °C, the pore of NBC-400 and NBC-500 were composed of mesopores with specific surface area (S BET ) of 38.27 and 551.74 cm 2 /g, respectively. With the pyrolysis temperature further rising to 800 °C, NBCs exhibited microporous structure. The volume of micropore in NBC-600, NBC-700, and NBC-800 accounted for 78.63, 80.35, and 78.14% of the total pore volume, respectively. Notably, the S BET of NBC-800 has been dramatically improved, reaching 1167.37 cm 2 /g. However, when the pyrolysis temperature increased to 900 °C, the S BET and pore volume of NBC-900 decreased severely, and the microporous structure vanished. These results suggested that the pyrolysis temperature could regulate the pore size of NBCs. In the range of 400-500 °C, the iron oxides formed during the pyrolysis were embedded into NBCs, which promoted the formation of mesoporous structure. Nevertheless, due to the limitation of temperature, the carbon structure might not undergo sufficient pyrolysis cracking and crosslinking reaction. As the pyrolysis temperature reached 600-800 °C, more heat energy was provided to make some specific reaction between Fe(NO 3 ) 3 and carbon skeleton more sufficient, so as to form the unique microporous structure . When the pyrolysis temperature rose to 900 °C, the pore of NBC collapsed due to excessive reaction, resulting in a sharp decrease in specific surface area (Lua et al. 2006).
On the basis of these results, it could be concluded that the pyrolysis temperature was a particularly important factor among the various conditions of preparing BC from WCW in the presence of Fe(NO 3 ) 3 , which could fine-tune the pore structure of NBCs with a gradual process: (I) NBCs exhibited mesoporous structure in the range from 400 to 500 °C. (II) Within 600 to 800 °C, NBCs began to display microporous structure, and the S BET increased significantly, reaching a maximum of 1167.37 cm 2 /g at 800 °C. (III) With the further increase of pyrolysis temperature, the pores of NBCs began to collapse, accompanied by the decrease of S BET . Furthermore, the reactions and pore-forming mechanisms between WCW and Fe(NO 3 ) 3 should be explored in-depth.

XRD
According to the above research results, Fe(NO 3 ) 3 participated in the carbonization of WCW and played specific roles during the pore-forming process. In order to explore the role of iron species during the pore-forming process, the XRD patterns of NBCs prepared under different pyrolysis temperatures before and after pickling were obtained (Fig. 2). As shown in Fig. 2a, the evolution of iron species could be divided into three categories, which could be explained by And the carbon structure of NBCs was further changed and converted, forming some reducing components such as CO and amorphous carbon (Eqs. 9 and 10).
The XRD patterns of NBCs after pickling are shown in Fig. 1b. Obviously, all NBCs exhibited two broad peaks at around 25° and 43° assigned to (002) and (100) interlayer reflection of amorphous carbon (Xu et al. 2018a), proving that using Fe(NO 3 ) 3 activation to prepare BC from WCW was feasible. Additionally, the Fe 3 O 4 formed under the pyrolysis temperature from 600 to 800 °C exhibited high stability and was not removed by pickling, imparting magnetic properties to NBCs. This feature was beneficial for its practical application in pollutant adsorption.
It should be noted that the evolution of Fe(NO 3 ) 3 during pyrolysis was closely related to precursor carbonization and pore development. Under the pyrolysis temperature from 400 to 500 °C, the generated Fe 2 O 3 was removed during the pickling, and the remaining vacancies were favorable for the formation of mesopores. When the pyrolysis temperature increased to the range of 600-800 °C, Fe 3 O 4 was stable and difficult to remove by pickling. Therefore, the proportion of mesopores in NBC is relatively small. While the carbon precursor was violently carbonized under the catalytic activation of Fe species to generate a large amount of volatile (CO 2 and CO), and the release process of these volatile formed a rich microporous structure (Yang et al. 2016). However, when the temperature increased to 900 °C, excessive pyrolysis and volatile release processes led to the collapse of micropores, which corroborated the phenomenon that the S BET and pore volume of NBC-900 plummeted.

TEM
NBC-800 was selected to study the microscopic morphology and iron oxide formation of NBCs. As shown in Fig. 3a, particles of iron species were clearly observed on the NBC-800 before pickling, with particle diameters around 50 nm. Combined with the results of XRD, these particles were identified as Fe 3 O 4 and Fe 0 . Besides, according to Scherrer's equation with the main diffraction peak (220), the average particle size of Fe 3 O 4 was estimated to be 45 nm, which was consistent with the TEM results. In addition, the Fe species could also be observed with good dispersibility, which might be due to the low impregnation mass ratio of Fe(NO 3 ) 3 and WCW (0.5:1), so that the Fe species generated during the activation could be dispersed more uniformly.
After pickling, as shown in Fig. 3b, most of the particles on the NBC-800 were removed, leaving only a small number of particles, which were determined to be Fe 3 O 4 according to the results of XRD. The size of Fe species crystals was similar to the mesoporous pore size of NBCs. It is speculated Fig. 3 TEM images of NBC-800 before (a) and after (b) pickling that part of the mesopores were formed from the residual vacancies after the pickling of Fe species, which further suggested that Fe(NO 3 ) 3 activator could be used as a template to promote the formation of mesopores.

FTIR
The changes in the organic structure of the NBCs were monitored by FTIR analysis (Fig. 4). Concretely, the broad peak at around 3400 cm −1 was attributed to O-H and N-H stretching vibrations (Sulaiman et al. 2018;Zubrik et al. 2017), which might be interacting with EBT though intermolecular or intramolecular H-bonding (Alamzeb et al. 2022). The appearance of N-H might be due to the reduction of amorphous carbon at high temperature, so that nitrogen was doped into the carbon matrix in the form of -NH 2 . The infrared absorption peak located at 1631 and 620 cm −1 represented aromatic C = C stretching vibrations and outof-plane bending vibrations of aromatic C-H (Xu et al. 2019a), the intensity of which remained stable when the temperature was raised to 500 °C, indicating that the aromatization process of carbon matrix was basically completed under this condition. The intensity of aromatic C-O stretching vibrations (located at 1400 cm −1 ) gradually decreased with increasing temperature, showing the decomposition of -COOH and the release of volatiles. The unique characteristic peak located at 1101 cm −1 of NBC-400 represented the stretching vibration of C-O on cellulose (Shen and Gu 2009), suggesting that the glycosidic linkages of cellulose in WCW could be completely decomposed under high pyrolysis temperature (> 500 °C). When the pyrolysis temperature was greater than 500 °C, infrared absorption peak attributed to the C-N stretching vibration in nitrogen heterocycles was found at 900-1100 cm −1 , indicating that N element was doped into the carbon matrix during the pyrolysis. The formation of nitrogen heterocycle in the carbon structure was beneficial to the carbonization of NBCs. FTIR results of NBCs indicated the abundant presence of different surface functional groups, which might be expected provide a better interaction with EBT molecules and high removal efficiency.

TG
As shown in Fig. 5, thermogravimetric (TG) and differential thermogravimetric (DTG) analysis of WCW and Fe(NO 3 ) 3impregnated WCW (Fe(NO 3 ) 3 /WCW) were performed from room temperature to 800 °C and kept at 800 °C for 1 h. Figure 5 a shows that the mass loss process of WCW could be divided into three stages: (I) room temperature-200 °C, (II) 200-500 °C, and (III) 500-800 °C. The first slight mass loss stage was attributed to the release of physically adsorbed water in WCW. The main mass loss was in the second stage, showing the maximum mass loss of 70% at around 348 °C. In this stage, cellulose, hemicellulose, and lignin in WCW suffered violent dehydration reactions, intermolecular and intramolecular hydrogen bonds were broken, and further occurred a series of processes such as decarbonylation, benzene ring opening, and aromatization, resulting in the elimination of groups such as glycosidic bonds, pyran rings, and hydroxyl groups, accompanied by the release of a lot of various gases like CO, CO 2 , and H 2 O (Shen and Gu 2009, Xu et al. 2019a. In the third stage, the mass loss was mainly unchanged, referring that carbonization of WCW was basically completed, and the obtained carbon possessed good thermal stability.
As shown in Fig. 5b, the mixture of Fe(NO 3 ) 3 /WCW showed different mass loss process compared with WCW, which could be divided into four stages: (I) room temperature-200 °C; (II) 200-400 °C, (III) 400-500 °C, and (IV) 500-800 °C. In the first stage, apart from the release of molecules in WCW, part of the crystal water in Fe species was also eliminated, resulting in the mass loss occurring around 150 °C. Thereafter, cellulose, hemicellulose, and lignin in WCW were violently pyrolyzed in the second stage, and a distinct mass loss peak at 330 °C was observed due to the release of a large amount of volatile during this process. Note that the temperature of the mass loss peak of Fe(NO 3 ) 3 /WCW was lower than that of WCW, indicating that Fe(NO 3 ) 3 can activate the dehydration, decarbonylation, and dehydroxylation of WCW components, thereby reducing the temperature required for WCW pyrolysis. In the third stage, there was a small mass loss peak appeared at 425 °C, which could be attributed to the reduction reaction between Fe(NO 3 ) 3 and amorphous carbon, releasing NO and CO 2 (Eq. 7). When the pyrolysis temperature reached around 500 °C, the Fe(NO 3 ) 3 was  (Shen et al. 2006a(Shen et al. , 2006b. It could be concluded that before 500 °C, lots of gases were produced via the cracking of WCW, the reorganization of carbon structure and the gasification of nitrate, and their violent eruption expanded the pore size to form mesopores of NBC-400 and NBC-500. In the fourth stage, interestingly, a sharp mass loss peak was observed when the temperature reached 664 °C. Another study observed a similar phenomenon and simply attributed it to the conversion of iron oxides to Fe 0 . However, according to the XRD analysis in this study, the formation of Fe 0 was not observed in NBC-700, suggesting that this mass loss at 664 °C might be related to other processes. Under the premise of excluding the evolution of Fe species itself, it is reasonable to speculate that under ambient pressure, part of the nascent nitrogen oxides (NO and NO 2 ) might be trapped by carbon structure and made them participate in the activation of WCW and play an important role. On the one hand, N can be doped into the carbon matrix during the reorganization of the carbon structure to form nitrogen-containing heterocycles ). On the other hand, it is well known that nitriding in iron could be achieved at around 650 °C (Shen et al. 2006a), therefore, nitrogen oxides are able to connect with Fe species through Fe-N bonds at 664 °C. Finally, in the subsequent heating process, Fe 3 O 4 was gradually reduced to Fe 0 according to Eq. 10, accompanied by the generation of CO and CO 2 . Based on the thermogravimetric analysis of the fourth stage, it could (11) 3Fe(NO 3 ) 3 + 4C → Fe 3 O 4 + 3NO + 6NO 2 + 4CO 2 be concluded that the dual activation of Fe species and N element introduced abundant micropores for NBC-600, NBC-700 and NBC-800.

XPS
To further elucidate the activation of WCW by Fe species and N elements during pyrolysis, XPS analysis was employed to identify the composition and chemical state of NBC-500 and NBC-800. In the light of XPS survey spectrum (Fig. 6a), the element proportions of Fe and N in NBC-800 were relatively higher than those in NBC-500. According to the analysis results of XRD, the higher Fe content was due to the residual Fe 3 O 4 in NBC-800. The higher N element supported our speculation about the combination of the released NO and NO 2 with carbon matrix and Fe species, further indicating that N element could play an important role in the activation process.
The XPS spectra of Fe2p (Fig. 6b) showed that the proportion of Fe(II) was relatively higher in NBC-800 than that in NBC-500, verifying the evolution process of the gradual reduction of Fe species from Fe 2 O 3 to Fe 3 O 4 to Fe 0 . Moreover, the evolution of Fe species was closely related to the pyrolysis carbonization of WCW and the pore development of NBCs. Fe(III) and Fe(II) could promote the decomposition of cellulose, hemicellulose and lignin in WCW and the subsequent processes of cyclization, aromatization, and carbonization. They were also involved in the subsequent reorganization of the carbon structure while evolving, and eventually became firmly embedded in the carbon matrix. Figure 6 c exhibits the N1s XPS spectra of NBC-500 and NBC-800. Obviously, the content of N element of NBC-800 was much higher than that of NBC-500, indicating that at around 500 °C, the nitrogen oxides trapped by the carbon structure were unstable, while the main reason for doping N into the NBCs was a series of reactions that occurred at 664 °C. As shown in Fig. 6c, three main structures of N-H, Fe-N, and N-C located at 399.0, 399.6, and 402.0 eV were found. Three aspects of N activation could be reasonably proposed. First, under reducing conditions in the presence of N 2 and carbon, NO and NO 2 could be reduced to form -NH 2 .
Second, the C atom on the aromatic ring were replaced by N atom to form nitrogen-containing heterocycles, which might facilitate the adsorption of pollutants. So, N-C bond was probably derived from pyrrolic N or pyridinic N. Third, at high pyrolysis temperature, chemical bonds between N atoms on pyrrole or pyridine and Fe atoms in iron oxides were formed. In summary, the mechanisms of the synergistic activation and pore formation with temperature change are proposed in Fig. 7. When the pyrolysis temperature was lower than 500 °C, the evolution of iron oxides promoted the decomposition of cellulose, hemicellulose and lignin in WCW and the subsequent processes of cyclization, aromatization and carbonization, accompanied by the generation of CO and CO 2 . Meanwhile, the reduction of NO 3 − also released NO and NO 2 . Mesopores were formed by the pickling of Fe species and a large amount of gas spewing out to expand the pore size. When the temperature was higher than 500 °C, the carbonization of WCW was basically completed, and the Fe species participated in the subsequent reorganization of the carbon structure while evolving. N atoms could replace C atoms on aromatic rings to form nitrogen-containing heterocycles, and at the same time, Fe-N bonds are formed under the high temperature driving, so that the embedded iron oxides were more stable and not easily removed by pickling. Note that the release of gases in this process was relatively mild, which was conducive to the formation of microporous structure.

Adsorption kinetics
The adsorption property of NBC-800 towards to EBT was tested. According to the effect of pH variation on EBT adsorption efficiency, NBCs exhibited the highest adsorption capacity for EBT at pH 2, because the lower the pH value, the more positive charges were distributed on the surface of NBC-800, showing stronger electrostatic attraction to the anionic dye EBT. Hence, the adsorption kinetics and adsorption isotherms experiments were performed at pH 2.0. Figure 8 shows the kinetic curves of EBT adsorption onto NBC-800 at different initial EBT concentrations. The NBC-800 showed extraordinary adsorption rate for EBT in the first 30 min. The data are fitted by the pseudo-first-order and pseudo-second-order models, and the calculated parameters are summarized in Table 3. According to the values of R 2 , the pseudo-second-order model provided a better fitting than the pseudo-first-order model, indicating that the adsorption of EBT onto NBC-800 surface could be largely controlled by chemical adsorption process . In addition to electrostatic attraction, the binding between EBT and functional groups on the biochar surface was also an important adsorption pathway, which probably contained hydrogen bonding between protonated O-H/N-H and anionic EBT molecules, and chemical interaction between C-O/C = O in carboxyl groups of biochar and EBT molecules (Zubair et al. 2021(Zubair et al. , 2022. Based on the adsorption rate constant k 2 , the adsorption rate of EBT decreased with increasing initial EBT concentration, because the competition of EBT molecules for adsorption sites on NBC-800 surface was more intense at high concentrations (Xu et al. 2019a).

Adsorption isotherms
As presented in Fig. 9, the isotherms have been evaluated according to the Langmuir and Freundlich models. The fitting parameters are also listed in Table 4, and the Langmuir model (R 2 > 0.98) provided a better fitting than Freundlich model (R 2 > 0.89), indicating that the EBT adsorption onto the surface of NBC-800 was mainly monolayer adsorption and homogeneously distributed (Qiu et al. 2014;Zhu et al. 2011). The maximum adsorption capacities of NBC-800 obtained at 25, 35, and 45 °C were 456.01, 508.18, and 549.51 mg/g. Obviously, this phenomenon reflected an endothermic nature of adsorption, and higher temperature provided stronger molecular diffusion, which was beneficial to the adsorption of EBT by NBC-800. A comparison of the EBT maximum adsorption capacity of NBC-800 obtained in this study with BCs reported by other published literature is shown in Table 5. It is notable that the maximum adsorption capacity of the NBC-800 was highest among these BCs, clearly demonstrating the superiority of the synergistic activation of Fe and N species. Moreover, the mass ratio of activator to raw material used in this study was the smallest, which indicated that this synergistic activation method was also beneficial to improve the activation efficiency, thereby reducing the preparation cost of BCs.

Conclusion
In this study, the NBCs were successfully produced from WCW using low-dosage Fe(NO 3 ) 3 activation at different pyrolysis temperatures. The prepared NBCs possessed excellent specific surface area of 1167.37 m 2 /g, temperature-controlled porous structure, as well as abundant surface functional groups. According to the results of characterizations, the synergistic activation of Fe and N species in Fe(NO 3 ) 3 and the development of microporous and mesoporous structure were proposed in detail. In the lower temperature range (< 500 °C), the templating effect of Fe 2 O 3 and the violent release of a large quantity of gases promoted the formation of mesopores. The volatiles mainly include CO, CO 2 derived from the rapid carbonization of WCW under Fe catalysis, and NO and NO 2 generated from the reduction of NO 3 − . While at higher temperature (> 500 °C), the development of micropores was because of the mild release of gases during the reorganization of carbon structures, which was mainly induced by the erosion of Fe species and the nascent N structures (including N-H, N-C, and Fe-N). Moreover, excellent EBT adsorption capacity (456.01 mg/g) of NBC-800 was also obtained at 25 °C.
This study has demonstrated that producing satisfactory BCs from WCW is feasible by using low-dosage Fe(NO 3 ) 3 activation. The synergistic activation effect of Fe and N  (Gu et al. 2021) species during pyrolysis is probably the main reason for the low-dosage chemical activation. The in-depth study of multi-component synergistic chemical activation mechanism in this work provides a basic understanding for the development of low-dose chemical activation technology in the future, and helps to further reduce the preparation cost of biochar.
Funding This study was supported by the National Natural Science Foundation of China (21707090), the Postdoctoral Science Foundation of China (2017M611590) and the Natural Science Foundation of Shanghai (14ZR1428900).

Data availability
The datasets used in the current study are available from the corresponding author on reasonable request.

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Competing interests
The authors declare no competing interests.