Enhancing the valorization of pulping black liquors in production effective aerogel–carbon nanostructure as adsorbents toward cationic and ionic dyes

This work deals with promoting the efficiency of removing the cationic and ionic dyes by new aerogel–carbon nanostructures. For cleaner production the rice straw-pulping black liquors, which regards serious environmental risk during routine disposing, is used in preparing the aerogel precursors. These aerogels (AGBs) depend on using pulping black liquor in hybrid with resorcinol and the less carcinogenic formaldehyde butyraldehyde. Black liquors from five pulping processes are used, Elemental, thermogravimetric (TGA and DTG), and FTIR-ATR analyses are used to characterize the carbon precursors. While their adsorption behavior toward cationic and anionic dyes are accessed via iodine-value, adsorption capacity and kinetic models, textural characterization, and SEM. The TGA measurements reveal that AGBs from BLs of neutral sulfite and soda-borohydride pulping reagents have higher activation and degradation energies than other aerogels. In terms of cationic and anionic dyes adsorption as well as textural characterization, the AGB-CNSs surpass that made from BLs. The discarded KOH/NH4OH black liquor is used to synthesize the best aerogel precursor for producing cationic methylene blue dye (MB) adsorbent, where it provides an adsorption capacity 242.1 mg/g. The maximum anionic brilliant blue dye (BB) adsorption capacity, 162.6 mg/g, is noticed by Kraft BL-aerogel-CNSs. These finding data overcome the literature carbon adsorbents based on lignin precursors. All examined CNSs toward MB dye follow the Langmuir adsorption equilibrium; while primarily the Freundlich model for BB dye. The pseudo-second-order kinetic model well fits the adsorption kinetics of investigated AGB-CNSs. The textural characterization and SEM revealed a mixture of mesoporous and micro porous features in the CNSs.


Preparation of Black liquors
Rice straw (RS) was pulped using a variety of reagents, including sodium hydroxide without or with addition (sodium borohydride [BH]), potassium hydroxide/ammonium hydroxide, neutral sulfite, and Kraft pulping conditions.The following operating parameters were used for the pulping processes: 6:1 liquor ratio for 1 h at 120 °C in autoclaves.For NaOH pulping without and with 3% BH], 18.6% Na 2 O is comparable.The black liquor produced by pulping soda was marked with the following codes: BL1 for NaOH pulping and BL2 for NaOH-3%BH pulping.The BLs produced by potassium hydroxide/ammonium hydroxide mixture in ratio 1:5, neutral sulfite pulping (mixture of sodium sulfite and sodium carbonate with mass ratio 4:1 and the Kraft method by using 10% active alkali and 25% sulfidity, were designated BL3, BL4, and BL5, respectively. Based on the Dumas combustion method, the Profilenc Technologies ECS 8020 CHNS-O elemental analyzer (Italy) was used to identify the main elements (C, N, H, S, and O) of the produced BLs.It represents a development of chromatographic separation and sample combustion-based elemental analysis methods.

Synthesis of BL-resorcinol butyraldehyde carbon aerogels (AGBs)
The preparation of organic gel, in absence of black liquors, was depended on replacing of carcinogenic formaldehyde with long chain aldehyde called butyraldehyde to form resorcinol (R)/butyraldehyde (BA) aerogel (AGB).The AGB was synthesized under the following circumstances: the R/BA molar ratio equal to stoichiometric (0.5), resorcinol/carbonate molar ratio (50) and dilution ratio (5.7).The resorcinol and carbonate were dissolved before the combination with BA and heated to 80 °C for 30 min to form the gel.After gel formation, it was left 24 h to form a network, the temperature was increased for 2 h to 60 °C and then maintained for an additional 2 h at 100 °C.The resulting aqueous gels were then frozen and dried for at least 48 h using a freeze dryer (Telstar-LyoQuest).The synthesis of black liquor-based aerogels involved replacing black liquor (BL) with 50% resorcinol, and due to its alkalinity, the polycondensation reaction was carried out without sodium carbonate.The gels synthesized from BL1 to BL5 were labeled AGB1-AGB5.
Phosphoric acid was used to activate the dried BLs or synthesized aerogels in a horizontal tubular furnace at 450 C for 60 min, without the presence of air.The ratio of phosphoric acid to dried precursor was 3:1.The carbon nanostructures prepared from aerogels AGB1 to AGB5 were labeled as AGB1-CNSs to AGB5-CNSs.

Characterization of carbon precursors and nanostructure
Thermo-gravimetric analyses This test was carried out using Setaram LABSYS EVO STA, France.The non-isothermal thermo-gravimetric analysis (TGA) and derivative thermogravimetry (DTG) of the BLs or butyraldehyde carbon aerogels (BAGs) as precursors for CNS production is done.Using a heating scan rate of 10 °C/min and an inert environment of nitrogen gas (30 mL/min), the analysis is carried out throughout a temperature range of 30-600 °C.According to the references 29,30 , the Kinetic parameters of thermal degradation are estimated using the following equation.The thermal activation energy (E a ) has been evaluated by applying Coat and Redfern method for the thermogravimetric analysis.
where α is the fraction of material decomposed at time t, k is the specific rate constant, T is the absolute temperature, Ea is the activation energy, R is real gas constant, z is the frequency factor (s −1 ) and n is the order of reaction.

ATR-FTIR spectra
Using a German-made VERTEX 80v-FTIR spectrophotometer, the prepared black liquors, aerogels, or their carbon nanostructure is exposed to FTIR analysis.To assign the function groups to the prepared samples, spectral data between 400 and 4000 cm −1 were gathered.

Adsorption studies
The prepared CNSs' adsorption to iodine, methylene blue (MB), an example of a cationic dye, and Coomassie brilliant blue R-250, an example of an anionic dye, is evaluated in order to assess the adsorption behavior of the CNSs in aqueous solutions.

Iodine value
At room temperature (25 °C), the evaluation of the iodine value was conducted.The iodine number test is regarded as the most fundamental measure to describe the development of micro-porosity in carbon-based materials.The method defined by ASTM D 4607-94 in 2006 31 was followed to ascertain the results of the iodine value experiment.Briefly, the experiment involved adding 0.05g of CNSs to dry flask, followed by adding 25ml of 0.1N Iodine solution and 2.5ml of 5% HCl.The flask was shaken for 30 s, filtered, and collected.The filtrate was then titrated against standard sodium thiosulphate solution using starch as an indicator.A blank sample was prepared without adding CNSs.

Batch equilibrium studies
Adsorption tests were run in a batch mode.The dyes methylene blue (MB) and Coomassie brilliant blue R-250 (CBB) were combined to create a stock solution.In each experiment, 25 mg of CNSs were combined with 10 mL of a known dye solution.The mixture was stirred continuously for 100 rpm at a temperature of 30 °C.Filtration was used to remove the dye solutions from the adsorbent after 24 and 48 h.A UV-Vis Single Beam spectrophotometer (UV1720, USA) was used to measure the dye content in the filtered solutions.The equilibrium adsorption of the MB and CBB dyes was calculated using the absorbance at 664 nm and 553 nm 32 .The Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) isotherms [33][34][35][36] , were used to compute the MB and BB adsorption capacities at equilibrium (qe, mg/g) as follows: where C o and C e (mg/L) are the liquid-phase concentrations of the dye at initial and equilibrium.
V (L): volume of the dye solution.W (g): weight of CNSs.

Adsorption kinetic studies
The kinetics of the MB and CBB dyes adsorption on CNSs are studied by employing the Lagergren first-order, pseudo-second order, and intraparticle diffusion models at 500 and 50 mg/L, respectively, with time intervals ranging from 1 to 48 h.The rate equations [37][38][39] , that have been most frequently employed for the adsorption of an adsorbate from an aqueous solution are expressed by the equations.

Kinetic model Linear form Plots References
Lagergren first order ln q e − q t = lnq e − K 1 t ln q e − q t versus t 37 pseudo-second order Intraparticle diffusion Where q e and q t are the amount of dye adsorbed per unit mass of the adsorbent (in mg/g) at equilibrium time and time t, respectively, k is the rate constant, and C is the intraparticle diffusion constant.

Effect of pH and temperature
This study was carried out on samples which achieved optimum adsorption, of MB (AGB3-CNSs) and CBB (AGB5-CNSs) starting dye concentrations of 600 mg/L for MB and 200 mg/L for CBB at 30 °C.The impact of pH on the adsorption capacity was investigated by adjusting the pH from 4 to 11.The dye solution's initial pH was adjusted by adding a 0.1M HCl or NaOH solution.The same samples with the same concentration were also subjected to the effects of temperature variations between 20 and 40 °C.

Surface acidity and basicity
In a closed flask, 150 mg of ABG-CNSs were combined with 15 ml of either 0.1M NaOH or 0.1M HCl solution, and the mixture was shaken for 24 h at room temperature to determine the surface acidity and basicity.After filtering the suspension, 0.1M HCl or 0.1M NaOH solution was used to titrate and determine how much NaOH or HCl was left in the solution to measure the acidity and basicity of the surface carbon nanostructure.
For further evaluating the role of pulping agents on the adsorption behavior of BL-CNSs or AGB-CNSs, selected CNSs samples prepared from CNSs of pulping agents NaOH, KOH/NH 4 OH, and Na 2 SO 3 /Na 2 CO 3 were subjected to the following analyses.

Textural characterization
Nitrogen adsorption-desorption isotherms that were performed at 77K using the BELSORP III analyzer series in Japan were used to characterize the texture of CNSs samples made from BL samples.In a 250 °C oven for 24 h, the samples were degassed before being examined with BELMaster version 7.3.1 software.Before the measurement got to 5.267E−5 Pa, the device was set to a vacuum level.With the help of the BET, BJH, and t-plot methods, this test was carried out to determine the surface area and pore volume.

Scanning electron microscope (SEM)
Scanning electron microscopy (SEM) was used to analyze the morphology of the selected CNSs.Using a Quanta 3D 200i-Russia system operating at 30 kV, the samples were evaluated.

Elemental analysis of black liquors
Table 1 displays the elemental analysis of dried black liquors.These components generally match the amounts of lignin and other low-molecular-weight substances present in black liquor.The percentages of hydrogen, oxygen, and carbon in BLs range from 4.05 to 5.11%, from 16.3 to 21.2%, and from 57.85 to 66.69%, respectively.The pulping reagents NaOH, NaOH-BH, and neutral sulfite which are related to BL1, BL2, and BL4, have the largest carbon content; more than 60%.For other pulping agents, including Kraft-BL (BL5), the highest levels of lignin and silica removal are 73.11% and 64.82%, respectively.This resulted in moderately distributed carbon at 58.9% and the highest oxygen concentration at 21.2%.Specific BL samples produced from pulping by sulfur-containing reagents coded BL4 and BL5, as a result of neutral sulfite and Kraft pulping are indicative of their sulfur content 5.50 and 4.95%, respectively.While BL samples produced from using KOH/NH 4 OH pulping reagent; BL3, are indicative of their nitrogen content 12.5%.According to the aforementioned results, BLs with high carbon content moderate amounts of lignin and silica, are useful as carbon precursors or as resorcinol substitution materials in the synthesis of aerogels.The high carbon content of BL1 is significant carbon nanostructure with yield, 55.58%.
With regard to the role of adding BH to the NaOH pulping agent it has a negligible effect on the ratio of hydrogen to carbon element; H/C ratio, of BL, where it changes from 6.2 × 10 −2 to 6.4 × 10 The previous observation is focused on the effects of silica content and delignification degree.

Thermo-gravimetric analyses
The thermal analysis of the studied BL in comparison with AGB samples as carbon precursors is shown in Fig. 1 and Tables 2, 3.According to the TGA thermograms, the evaporation of moisture content and the binding water detected with derivative thermogravimetry (DTG) at temperatures below 125 °C.Low-molecular-weight organic components found in BLs or AGB begin to breakdown between 150 and 350 °C, which indicates to the volatilization stage, while high-molecular-weight components and polymerized molecules begin to break down between 400 and 600 °C, which known carbonization stage.Based on the obtained TGA data, the BL and AGB samples can be activated and carbonized at 450 °C, as candidates for our investigation.Due to BL samples containing low molecular weight components like hemicellulose and lignin, they start to degrade at lower temperatures than pure aerogel or BL-resorcinol butyraldehyde carbon aerogel.The onset temperatures of aerogels are between 175 and 217 °C; while those of BLs range from 140 to 206 °C.As displayed in Fig. 1, BL samples can be degraded in two to three stages with high activation energies (Ea) that range from 400 to 1660 kJ/mol.BL1 and BL3 (NaOH and KOH/NH 4 OH) had lower Ea of 400 and 508 kJ/mol, respectively, than BL2, BL4 and BL5, which correspond to NaOH-BH, neutral sulphite, and kraft pulping agents, respectively.This indicates that adding BH to the NaOH pulping process will increase the black liquor (BL2) thermal  www.nature.com/scientificreports/activation energy.Additionally, the thio-lignin and sulfonyl containing lignin may be the cause of the high Ea of BL4 and BL5.It is noticed that the aerogel in absence of BL, AGB, decomposes in one stage whereas the 2 stages decomposition are noticed for BL-substituted aerogels, AGB1-AGB5.This additional stage is probably related to the breakdown of crosslinked lignin aerogels or unreacted BL components.As seen,the degradation of BL-substituted aerogels begins at a higher temperature, 176-218 °C, than that of pure aerogel,190 °C.The major peak temperature of BL-substituted aerogels, 261-280 °C, is lower than that of pure aerogels; 293 °C.Finally, BL-aerogels have higher activation energy (Ea), ranging from 304 to 376 kJ/mol, as compared to pure aerogels, 232 kJ/mol.

ATR-FTIR spectra
ATR-FTIR analysis of the different pulping BLs in contrast to the unsubstituted and substituted aerogels is shown in Fig. 2a-e.As can be seen from the spectra, the majority of the lignin and silica bands are present in all BL samples derived from the various pulping processes.Broad bands assigned to the OH stretching vibrations of phenolic lignin, uncondensed -CH 2 OH groups, carboxylic acid, or adsorbed water are observed at 3250-3460 cm −1 .Low-intensity bands are attributed to CH asymmetric stretching at 2940 cm −1 .A band at 1650 cm −1 and 1550 cm −1 is used to assign the aromatic skeletal vibration (C=C) of lignin (guaiacyl or syringyl) in BLs.In the range of 800-873 cm −1 , the benzene ring of the lignin group exhibits stretching and bending as well 40 .The C=O stretching (non-conjugated ketones or carbonyl groups), CH deformation in CH 3 or CH 2 , or phenyl propane skeleton vibrational absorption bands, as well as the ether group C-O-C linkage, are assigned by the bands at 1650, 1440, and 1210 cm −1 , respectively.Bands in areas 850-1050 cm −1 for Si-O-Si asymmetric stretching and 400-800 cm −1 for Si-O-Si bending highlight the presence of silica 10,41 .
In contrast to the aerogels produced from resorcinol and butyraldehyde, AGB ,the FTIR spectra of aerogels from BL-resorcinol/butyraldehyde, AGB1-5, the spectra show that the bands associated with OH stretching vibrations of BL-based aerogels are red shifted from 3443 cm −1 to wave number range 3228-3286 cm −1 with degree of change ranging Δν215-157 cm −1 .The substituted aerogels with BL from NaOH-BH and KOH/NH 4 OH pulping, AGB2 and AGB3, exhibit the highest degree of shifting, Δν190 and 215, these values correlating to their highest activation energy which reach 376 and 349 kJ/mol, respectively.The peak intensity at 1000-1100 is increased in comparison to pure aerogel when the resorcinol was replaced with BLs in the aerogel.Additionally, www.nature.com/scientificreports/due to the presence of BLs' silica, strong peaks are seen in all aerogels' fingerprint regions at 600-800 cm −1 , especially in AGB3.For CH asymmetries of 2871, 2930, and 2957 cm −1 all substituted aerogels exhibit three distinct bands.

Iodine value
The iodine adsorption values on the surface of carbon nanostructures produced in absence and presence of BL-aerogels, AGB-and AGB1-5-CNSs, as opposed to black liquors, BL-CNSs, are shown in Fig. 3. Contrarily, the iodine value of CNSs from pure aerogels, substituted aerogels, and black liquor are 595.7,50.7-448.6,and 122.2-728.6 mg/g, respectively.The BL3-CNSs have the greatest capacity to remove iodine from aqueous solution.The substituted aerogel with the same black liquor, AGB3, follows the same pattern and has the greatest iodine value, 448.6 mg/g, when compared to other BLs substituted aerogels.
Our results surpass those reported in many literatures.Saka 42 , investigated the production of activated carbon from acorn shell by chemical activation with zinc chloride and reported the iodine number in the range of 37-1209 mg/g; Mopoung et al. 43 , reported that the iodine adsorption of tamarind seed-based activated carbon with KOH activation ranged from 150 to 300 mg/g; Limet et al. 44 , found the iodine number of oil palm trunk was 500-880 mg/g and Liu et al. 45 , found that the iodine number of H 3 PO 4 -activated hydrochar at 600-1000 °C temperature was 200-830 mg/g.According to the iodine values presented above, black liquors or aerogels made from BL-resorcinol/butyraldehyde are effective precursors for carbon nanostructures that are mostly mesoporous in nature.

Dyes adsorption behavior
Figure 4a,b shows the impact of initial concentrations of methylene blue (MB) and Coomassie brilliant blue R-250 (CBB) dyes on the adsorption capacity of the CNSs to evaluate and compare the contribution of BLs to performance of the produced aerogel or BL itself as effective precursor for development of CNSs.According to the findings, the BL-substituted aerogels' entire carbon nanostructure with a better effect on eliminating MB and CBB than BL-CNSs and pure aerogel-CNSs, where its capacity is 121.9 for MB and 41.6 for CBB mg/g.To produce CNSs with a high adsorption capacity, 242.1 mg/g, the aerogels from BLs of alkaline pulping (KOH/ NH 4 OH) coded AGB3-CNSs recommend as precursors, followed by those made from neutral pulping AGB4-CNSs; 188.32 mg/g.Comparing black liquor CNSs to pure aerogel the BL3 and BL4 have the highest MB adsorption capacities with values 127.06 and 163.93 mg/g, respectively.
Figure 4b shows the CNSs made from BL-aerogel of Kraft pulping, AGB5, precursor has an effective role in the removal of Coomassie Brilliant Blue, CBB, as example to anionic dye, the removal reaches 162.6 mg/g.Using this BL5 as precursor of CNSs also provides the highest adsorption capacity reaches to 70.5 mg/g.The parameters registered in Tables 4 and 5 are related to the MB and CBB adsorption capacity at equilibrium by applying the isotherms of Langmuir, Freundlich, Temkin, and dubinin-radushkevich (D-R).Based on the correlation coefficient (R 2 ) for the elimination of MB dyes, it can be shown that the Langmuir model offers a better match (R 2 ) than other models that have been looked at for all analyzed CNSs.The (R 2 ) value lay between 0.99 and 1.00.Furthermore, the measured R L values between 0 and 1 demonstrated that the produced CNSs were advantageous for MB dye adsorption under the circumstances employed in this study.This indicates that the MB adsorption is valid for the monolayer adsorption on homogenous surface.All the samples fit the Freundlich isotherm, which is valid for heterogeneous surfaces, well for CBB adsorption on the examined CNSs.The adsorption of CBB on carbon-based materials fits well to the Freundlich isotherm, in agreement with 32,46 .The high value of n, 0.74-36.2,points to a robust interaction between the surface of the carbon nanostructure and the CBB dye.With R 2 at 0.98 and R L value 0.027, the Langmuir isotherm model fits BL7-CNSs the best.A    6 and 7 respectively [47][48][49][50][51][52][53][54] .

Batch kinetic studies
The kinetic parameters for the removal of dyes using the models Lagergren first-order, pseudo second order, and intraparticle diffusion are shown in Tables 8 and 9.The pseudo-second order model is more relevant based on the maximum correlation coefficient (R 2 ) and minimum standard error estimate (SEE) values, R 2 0.99-1.00,SEE 0.001-2.440.In comparison to the first order and intraparticle diffusion models, the pseudo-second order model has lower Standard Error of Estimate (SEE) values.The pseudo-second-order kinetics model was the most fit model to the adsorption behavior of both dyes on both CNSs (aerogels and BLs), and the results suggest that chemisorption may be the adsorption mechanism and that the adsorption kinetics is dominated by adsorption onto the active site 55 .

Effect of pH and temperature
The adsorption capacity of MB by ACB3-CNSs was higher in alkaline pH values, as Fig. 5a illustrates.This is because the negatively charged adsorbent surface facilitates the positively charged MB molecules.When the pH rose, the adsorption capacity grew progressively until reaching a maximum at 11. Adoption of anionic dye CBB showed a reversal trend because the negatively charged adsorbent surface increased the anionic dye's repulsion forces and decreased the adsorption capacity at high pH levels.
As the temperature rises from 20 to 40 °C, the adsorption of both dyes (MB and CBB) increases significantly (Fig. 5b).The increase in temperature causes the large dye ions to become more mobile and less swollen, which Additionally, the outcomes showed that MB and CBB adsorption is an endothermic process.

Surface acidity and basicity
Various heteroatoms from the raw material or activating agent primarily influence the surface chemistry of activated carbons.The acidic nature of activated carbon is attributed to heteroatoms, such as carboxyl, carbonyl, lactone, phenol, and other bound surface functional groups.Table 10 presented the surface functional group in ABG-CNSs by the Boehm technique.The total acidity of AGB-CNSs ranges from 6.32 to 8.39 (mmol/g) based on the Boehm titration data (Table 10).It was discovered that the surface of the carbon nanostructure had more total acidity than total base.When H 3 PO 4 is used to produce activated carbon, the carbon's surface may develop acidic surface functional groups associated with the oxygen-containing groups.On comparing the obtained data with other studies, the total acidity of our case is exceeded the other activated carbons (1.27 and 5.60 mmol/g) 56,57 as recorded in Table 10 and as high as the oxidized activated by nitric acid (1.33-14.25 mmol/g) [58][59][60] .

ATR-FTIR spectra of CNSs
Based on the adsorption data and thermal analysis, some CNS samples prepared from BLs and their corresponding aerogels of various pulping agents, namely NaOH, KOH/NH 4 OH, and Na 2 SO 3 /Na 2 CO 3 are chosen for further examination of the impact of BL pulping on functional groups included the resultant CNSs.The adsorption and thermal stabilities of these materials vary widely.The studied FT-IR spectra of CNSs are shown in Fig. 6a-c.Broad bands between 3200 and 3400 cm −1 are attributed to the hydrogen-bonded -OH stretching vibration of water molecules adsorbed on the carbon surface 61 .According to References 61,62 , the peak in the range 1580 and 1705 cm −1 is designed as a C=C stretching vibration or C=O group.O-C-O asymmetric stretching vibration is attributed to the sharp band between 1000 and 1100 cm −163 .Because the pulping process removes silica from RS and results in greater silica content in BLs, the sharpness of the ether band is related to lignin, hemicellulose, or Si-O-Si stretching 64 .The benzene ring stretching and bending at 600-700 cm −1 and the sharp bands at 400-500 cm −1 are attributed to Si-O-Si bending.
The findings demonstrate that the frequency of the OH broad band of AGB-CNSs increases from 3049 cm −1 to 3209-3340 cm −1 in the case of using aerogels AGB1 AGB3 and AGB4.Additionally, the bands of carbonyl  group's intensity at 1700 and 1600 cm −1 as well as the emergence of a band at 470 cm −1 decreased because of the BLs substitution's included silica (Table 1).The spectra of BL-CNSs, also reveal a broadening of the -OH band, with a decrease in the frequency of the OH band.A rise in the frequency of the band associated to the Si-O-Si bending, indicates a larger silica content.Thus, negatively charged carboxyl, hydroxyl, and silicate groups are present on the carbon nanostructure surface of BLs and BL-based aerogels, which motivates the cationic dye adsorption on these active sites.

Textural characterization of CNSs
The nitrogen adsorption isotherms of the prepared AGB-CNSs in comparison with BL-CNSs is illustrated in Fig. 7a-c, while their pore volume distribution and S BET and adsorption capacity relationship are shown in Fig. 7d,e.The nitrogen adsorption-desorption at 77K.N 2 isotherms (Fig. 7a-c) display hybrid Type I-IV isotherms with H4 hysteresis, in accordance with the IUPAC classification 65,66 .These isotherms were distinguished by a significant rise in N 2 absorption at extremely low pressures, up to relative pressures of 0.1, which suggested the presence of microporosity.Indicating the presence of mesopores since the hysteresis loop occurs between P/P0 = 0.50-0.99,this is followed by a constantly increasing N 2 adsorbed quantity (plateau form) in the region of medium to high relative pressures.According to the findings, CNSs are made up of a variety of microspores and mesopores in varying amounts.

Figure 3 .
Figure 3. Iodine value of CNSs from AGBs free and based on BLs versus Black liquors (BLs).

Figure 4 .
Figure 4. Adsorption capacity of (a) MB dye and (b) CBB dye on CNSs from AGBs free and based on BLs (AGB) versus Black liquors (BLs).

Figure 5 .
Figure 5. Adsorption capacity of MB and CBB on AGB-CNSs (a) effect of temperature (b) effect of pH.

Table 1 .
CHNS-O elemental analysis in different black liquors and the removed lignin and silica from RS in BLs.

Table 2 .
Non-isothermal TGA measurements of AGBs free and based on BLs of RS from NaOH, NaOH-BH and KOH-NH 4 OH pulping reagents.

Table 3 .
Non-isothermal TGA measurements of AGBs free and based on BLs of RS from neutral sulphite and Kraft pulping reagents.

Table 6 .
Adsorption capacities of present investigated CNSs to MB versus literature.

Table 9 .
Lagergren first order, pseudo second order and intraparticle diffusion kinetics parameters for CBB dye adsorption onto AGBs free and based on BLs-CNSs versus BL-CNSs.allows the large dye molecule to penetrate and increase adsorption capability of the carbon nanostructure.

Table 10 .
Total acidity and the base amount of AGBs based on BLs (AGB) determined by the Boehm method.