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Scientific Reports Volume 11, Article Number: 22328 (2021) Cite this article

Today, bagasse (SB) is used in bioethanol and biodiesel production, energy production, and adsorbent synthesis. The goal of the project is to determine the optimal conditions for the production of adsorbents from bagasse using hydrothermal carbonization (HTC) and KOH activation. In order to optimize the process parameters such as reaction temperature, residence time, ZnCl2/SB mixing ratio and water/SB mixing ratio, the response surface method was used. The results showed that the best modified adsorption occurred at 180 °C, 11.5 h, the ratio of water to biomass (5:1) and the ratio of ZnCl2 to precursor (3.5:1). The physical and chemical properties of the best activated carbon and batch adsorption experiments were studied. It is found that the pseudo-second-order kinetic model and the Langmuir isotherm model are consistent with the experimental results in the batch adsorption study [\({q}_{max}=90.1\) (mg/g)]. Thermodynamic experiments further confirmed the spontaneous and exothermic adsorption mechanism.

Agricultural biomass waste is composed of organic matter produced by humans in agricultural activities. Due to its abundance and availability1,2, these wastes can be used as raw materials for the manufacture of valuable products such as fuels, biogas, and adsorbents. Bagasse (SB) is a kind of agricultural biomass waste that is rich in cellulose, hemicellulose and lignin. In the past, SB was used for power generation, but now it is an important substrate for environmentally friendly methods such as bioethanol, energy power generation, biodiesel and adsorbent synthesis. SB can be a suitable candidate for adsorbent production and removal of pollutants such as lead in the environment.

Lead is one of the most common and dangerous metals in wastewater from smelting, battery and paint industries. Studies have shown that Pb2 can enter the human body through the skin, digestive tract and respiratory tract, causing harm to humans (brain, mental, kidney and liver diseases, anemia and other diseases), and will accumulate in mammals. Chemical precipitation, ion exchange, liquid membrane, electrochemistry and adsorption procedures are one of the various methods currently used to repair lead-contaminated wastewater. Adsorption is a good alternative to heavy metal treatment because it has basic equipment, easy operation, low cost, and acceptable efficiency4. Today, hydrothermal carbonization is used as a green technology for biomass waste management to produce valuable materials such as adsorbents.

Hydrothermal carbonization (HTC) is a thermochemical conversion process with mainly exothermic heat, which can convert waste biomass into useful materials under low temperature (180–250 °C) and autogenous pressure. HTC's products include Hydrochar (HC), a carbonaceous coal-like solid material with rich oxygen functional groups, process water (a mixture of bio-oil and water) and a small amount of gas5,6. The type of waste biomass used, as well as process parameters, such as temperature, residence time, water to biomass ratio, number of catalysts, and activation methods, all affect the characteristics of the HC produced7. Because the physical and chemical properties (ionization strength and dielectric constant) of water change at high temperatures, water is used as a solvent medium, reactant and catalyst in the HTC process. It contributes to the hydrolysis and cracking of waste biomass7,8.

The raw materials have undergone many important reactions during the HTC process. The initial step of each reaction is hydrolysis. After that, a series of reactions will take place, including dehydration, decarboxylation, condensation and polymerization9,10. HC can be used as an ideal material for fuel, soil improvement and adsorption materials. Compared with raw materials, HC has a lower oxygen-to-hydrogen ratio and a hydrogen-to-carbon ratio (O/H and H/C) and a higher oxygen-containing functional group (OFG), so it can be used as a suitable production activated carbon and adsorbent Candidate 11, 12.

Activated carbons play an important role in wastewater treatment because they are easy to handle, have low treatment costs, and have higher adsorption efficiency than other pollutant removal methods13,14. The HTC of lignocellulosic biomass for removing organic and inorganic pollutants in wastewater is a promising technology because of the low cost of raw materials, the use of renewable and diversified material sources, and ecologically beneficial processes.

In recent years, activated carbon has been produced from HTC’s biomass, such as cotton stalks 15, avocado seeds 16, flax shavings and oat husks 17, mango peels 18, palm leaves 19, cassava residues 1, beer lees 20, corn cobs 21, and straw 22. Straw bananas studied fruit bunch 24, hickory wood and peanut shell 25, bamboo shoot 26, teak sawdust 27, bamboo sawdust 28, American agave fiber and mimosa tannin 29, prolifera-green-tide 30.

As far as the author knows, no studies have investigated the complete influence of process factors (temperature, residence time, ratio of biomass to water, and ratio of biomass to catalyst) on the adsorption capacity of water and carbon derived from waste biomass. This experiment investigated the influence of technological factors on the alkalized bagasse HTC, and found the best adsorption conditions. Then, the properties of activated carbon were studied. The main purpose of this study is to study the HTC optimization process parameters of BG based on adsorption capacity and water coke yield for the first time.

The exact goals of the research are:

Evaluate the potential of SB's HTC for lead adsorption.

Using the central composite design-response surface method (CCD-RSM) technology to study the influence of process variables (temperature, residence time, water-to-biomass ratio, and catalyst dosage) on changes in adsorption capacity.

Perform batch adsorption studies to study isotherms, kinetic and thermodynamic studies, and characterization of optimal adsorbents.

Table 1 lists the independent factors and response values ​​for each experiment. The CCD design is scheduled for 30 runs, with 4 variables and 5 levels, 16 full factorial design matches, 8 axial trials and 6 repetitions at the center position. The modified adsorption capacity (MAC) of activated hydrocarbons (AHC) increased from 6.70 ± 0.42 to 53.56 ± 1.98 (mg/g), indicating that HTC parameters have a significant impact on adsorption capacity and yield.

Hypothesis test (p-value) and F test (Fisher test) are used to evaluate important factors and model fit. Table S1 (Supplementary Information) shows the results of the analysis of variance (ANOVA) for the independent variables temperature (A), water to biomass ratio (B), ZnCl2 to biomass ratio (C), and residence time (D). The linear terms A, B, C, and D, as well as the quadratic terms A2, C2, D2, and the synergistic term AD, are all noteworthy variables. Equation (1) describes the second-order regression equation adjusted to the response (MAC) in the form of coded values ​​after removing non-important variables.

According to the fitting equation and F test (Table S1), temperature and residence time are more significant than other factors. The most important variable to change the adsorption capacity is temperature (the F value is equal to 83.80). The interaction between temperature and residence time is the most important of the various interactions between variables (Table S1).

Table S2 shows the analysis of variance for the response model. The response model has a reasonable F-value of 27.04 and a p-value of 0.0001 (less than 0.05), and a lack-of-fit p-value of 0.37 (above 0.05) 31.

Figure 1 depicts the scattering of the residue and the relationship between actual (experimental) and predicted values ​​(Figure 1a, b). As can be seen from Figure 1a, the closeness of these points to the predefined line indicates that the actual value of the regression model is very close to the expected value. As shown in Figure 1b, the model has a random distribution and low residual values, with substantial 32,33.

The relationship between actual (experimental) and predicted values ​​(a) and the relationship between residuals and experimental order (b).

The effect of two-factor interaction (other variables are fixed to zero code value) on MAC is studied, and the results are shown in Figure 2. As shown in Table S1, for the F value, temperature is the most important variable, and the interaction between temperature and other factors is more relevant than other imaginable interactions. In addition, temperature and time play a crucial role in adsorption performance. Therefore, the influence of time and temperature (as fixed parameters) on the two-factor interaction is shown in Figure S1. The number of oxygen-containing functional groups (OFG) in hydrogenated carbon-based adsorbents affects its adsorption capacity. Previous studies have shown that temperature and residence time affect the amount of OFG in HC7. The response of the surface to the synergy items (AB, AC, ..., CD) is shown in Figure 2. In addition, other variables are fixed to zero code value, and the influence of temperature and residence time on the synergy coefficient is shown in Figure 2. Figure S1.

The three-dimensional response surface of MAC: water to biomass ratio and temperature (a), temperature and ZnCl2 to biomass ratio (b), temperature to time ratio (c), water to biomass ratio and ZnCl2 to biomass ratio (d) , Water-biomass ratio to time (e) and time ZnCl2 to biomass ratio (f).

The interaction between temperature and water-to-biomass ratio (the other two variables are fixed at a zero code value) is shown in Figure 2a. At any ratio of water to biomass, MAC increases and then decreases with increasing temperature. Oxygen-containing functional groups may increase as the temperature increases, and then decrease at higher temperatures 34,35. As shown in Figure S1, the interactions between temperature and water-to-biomass ratio at different times are similar, but at lower residence times, the maximum MAC appears at higher temperatures. The relationship between temperature and the ratio of ZnCl2 to biomass (the other two variables are fixed at a zero code value) are shown in Figure 2b. At any ratio of ZnCl2 to biomass, as the temperature increases, the modified adsorption capacity increases, reaches a maximum value, and then decreases, similar to the previous interaction. As shown in Figure S1, the interaction between temperature and the ratio of ZnCl2 to biomass is similar at different times, but at higher residence times, the maximum MAC appears at lower temperatures. The interaction between temperature and residence time (the other two variables are fixed to zero code values) is represented by Figure 2c. Similar to the previous interaction, the maximum adsorption is obtained at any time as the temperature increases, and then decreases. Unlike earlier interactions, the time dependence of the response decreases with increasing temperature, so time becomes more important at lower temperatures. This can be explained by the dependence of OFG on temperature and time. At a certain temperature, as time increases, OFGs reach a maximum value and then decrease. At higher temperatures, the reaction rate in the HTC process increases, and the time for maximum OFG formation and reaching equilibrium OFG decreases7. The relationship between the ratio of water to biomass and the ratio of ZnCl2 to biomass (the other two variables are fixed at zero code values) are shown in Figure 2d. Compared with other interactions, this interaction has fewer consequences. As the ratio of ZnCl2 to biomass increases and the ratio of water to biomass decreases, the adsorption capacity increases slightly. The interaction between the water-biomass ratio and the ZnCl2 biomass ratio at different times and temperatures is shown in Figure S1. Regardless of time and temperature, the trend of MAC with water biomass ratio and ZnCl2 biomass ratio is similar. At higher temperatures, the dependence of MAC on temperature decreases, but at lower temperatures, as time increases, MAC increases. The interaction between time and water-to-biomass ratio (the other two variables are fixed at zero code value) is shown in Figure 2e. As the water content increases, the adsorption decreases, but at any water-to-biomass ratio, the reaction reaches a maximum, and then decreases slightly over time. The interaction between the ratio of water to biomass and the time at different temperatures is shown in Figure S1. The change trend of MAC with water-to-biomass ratio and time is similar, but as the temperature increases, the maximum value of MAC appears at a lower residence time. The interaction between time and the ratio of ZnCl2 to biomass (the other two variables are fixed at a zero code value) are shown in Figure 2f. As time increases, the adsorption capacity of any ZnCl2 ratio increases, and then slightly decreases. The interaction between time and the ratio of ZnCl2 to biomass at different temperatures is shown in Figure S1. As the temperature increases, the maximum MAC appears at a lower residence time. At any ratio of ZnCl2 to biomass, OFG forms and reaches its maximum value as time increases. Then they are reduced due to excessive dehydration/carbonation reaction or decomposition of OFG into gaseous products which convert OFG into stable oxygen surface groups.

The software provides 100 optimized conditions in a series of experimental studies. Economic considerations indicate that, among the various optimization conditions for maximizing the response (MAC), a lower temperature is preferable to a higher temperature.

The optimum temperature is 180 °C, the ratio of water to biomass is 5 (w/w), the ratio of ZnCl2 to biomass is 3.5 (w/w), and the residence time is 11.5 h. The actual value and response value of (MAC) (by using a quadratic regression model) under this condition are 53.56 ± 1.98 and 54.45 (mg/g) respectively (relative error equals 1.7%). AHCop is the name of activated water carbon produced under ideal conditions, and its characteristics have been studied.

Table 2 shows the approximate analysis of BG and HC under optimal conditions (HCop). As the reaction progresses, the dehydration and condensation reactions in HTC cause the hydrogen and oxygen content of HC instead of biomass to decrease. As a result, it was found that the atomic ratio of H/C and O/C of HC was lower than that of biomass7. Table 2 shows the BET surface area of ​​BG, HCop and AHCop. Other experiments have confirmed that the surface area of ​​HC is limited 37,38.

Figures 3a-f show the SEM images of BG, HCop and AHCop, respectively.

(a, b) SEM images of bagasse, (c, d), HCop and AHCop (e, f).

Due to the low temperature (180 °C) in HTC, the structure of HCop and AHCop is equivalent to that of BG. It should be noted that HTC produced spongy structures with disordered cracks and canals in both HCop and AHCop. Figure 3 shows that the surface of HCop is rougher than that of BG, and the surface of AHCop is rougher than that of HCop. It may be that KOH activation resulted in the removal of contaminants from partially closed pores, which affected the growth of surface cracks39. The removal of contaminants from the pores and the growth of surface cracks facilitate the diffusion of the adsorbent and increase the adsorption capacity.

Figure S2 shows the EDX diagram of the sample. It can be seen from Figure S2 (a, b) that when bagasse is activated with KOH, the O and C peaks rise and the Si peaks decrease. The presence of the Pb2 peak in Figure S2d,f confirms the Pb(II) adsorption. Since the Pb(II) peak of AHCop is higher than that of HCop, alkali activation increases the adsorption capacity. Regarding Figure S2, after the adsorption of Pb(II), the K peak disappeared, and then the Pb(II) peak appeared. This means that Pb(II) ions have replaced K ions.

The Si peak in HCop and the C peak in AHCop were significantly reduced, followed by a Pb(II) peak, confirming the ion exchange mechanism.

In HTC, XRD is used to study the crystallization changes of BG. Figure S3 shows the XRD patterns of BG, HCop and AHCop. The peaks around 16 degrees and 22 degrees confirm the crystal structure of BG, as shown in Figure S3a40. The narrow peaks of HCop and AHCop in Figure S3b and ​​c are about 21°, indicating that cellulose decomposition is incomplete during HTC. The peaks in HCop and AHCop match the (0 0 2) graphite structure and show a graphene-like layer at about 26°20. In Figure S3b and ​​c, you can see the reduction of the cellulose peak in AHCop instead of HCop. KOH activation reduces this peak by causing partial cellulose decomposition. A small peak can also be seen at about 43° in HCop, which corresponds to (1 0 0/1 0 1) and points to the diffraction of the graphite layer, but due to KOH activation, this intensity is destroyed in AHCop.

The functional groups of the materials were studied using FTIR technology. Figure 4 shows the FTIR spectra of BG, ABG, HCop and AHCop. All representatives showed adsorption peaks near 3350, 2920, 1603 and 1513 cm-1. The broadband peak detected near 3350 cm-1 is related to the OH stretching vibration of the carboxyl or hydroxyl group. Raw materials containing a large amount of cellulose or hemicellulose can improve the conformation of OH groups on the surface of HC 41. The band peak located near 2920 cm-1 is attributed to the C=C stretching vibration of the aromatic structure and the peaks found at 1603 cm-1 and 1513 cm-1 from the methyl, methylene and methine groups. related. Due to dehydration, the peak increases and the aromatization reaction is completed during the HTC process 43, 44.

FTIR spectra of BG, ABG, HCop and AHCop.

The peaks found in HCop and AHCop at 1693 cm-1 are related to the C=O stretching vibration of the carbon group. The band peaks detected by AHCop and HCop near 1452 cm-1 are related to CH deformation in lignin and carbohydrates11. The small bands seen in AHCop and HCop in the range of 1300-1000 cm-1 are related to the OH and CO stretching vibrations of carboxylic acid, lactone, ether and alcohol groups. These bonds do not appear in BG and ABG and will be produced as a result of the HTC process, so that HTC can produce abundant oxidized groups on the surface of HC46,47. It must be mentioned that the CO, C=C and OH groups have a considerable influence by adsorbing Pb2 on the surface of the adsorbent. Due to surface complexation, carboxyl (-COOH) and/or hydroxyl (-OH) functional groups can be linked to Pb2. Heterocyclic compounds (C=C) have electron donating ability. Therefore, these compounds can generate electrostatic interactions in the adsorption of Pb248. This band appears near 1161 cm-1 and is related to COC cellulose and hemicellulose; after HTC, this group is the weekend 41.

Figure 5 shows the TG and DTG curves of thermal degradation of BG and HCop at a heating rate of (10 °C/min). Table 3 shows their pyrolysis parameters, including Tv, Tf, Tm, DTGm, Rm and the pyrolysis temperature range. In the pyrolysis process of BG, three main decomposition stages have been identified, as shown in Figure 5. The evaporation of inherent water occurs at temperatures below 210 °C. The decomposition of hemicellulose is mainly related to the first stage, which occurs in the temperature range of 219.9 to 308.1 °C, resulting in a weight loss of 28.26I.

TG and DTG graphs of bagasse and HCop.

The second stage took place at a temperature of 308-370 °C, resulting in a weight loss of 49.67%, seeing most of the cellulose and some lignin degradation50. The third stage occurs at a temperature of 370.9-504.5 °C, resulting in a weight loss of 22.78% due to the decomposition of lignin. Compared with BG, HCop's TGA and DTG graphs show significant differences. Compared with BG, the weight loss of HCop decreases at temperatures between 250 and 360 °C and increases at temperatures between 360 and 500 °C. Therefore, during the HTC period, it may be seen as a decrease in volatile matter and an increase in fixed carbon 51,52.

Compared with BG, the first decomposition temperature of HCop increased from 239.7 to 247.8°C, as shown in Table 3; this may be due to the reduction of volatile materials during HTC53. HTC also increased the final pyrolysis temperature from 509.2°C to 583.7°C. DTGm decreased from 17.54 (percent/minute) of BG to 5.49 (percent/minute) of HCop. In addition, due to increased ash content and reduced volatile content, HCop exhibited greater Tm and Rm than BG54.

The equilibrium adsorption capacities of BG, ABG, HCop and AHCop are shown in Table 4. Obviously, KOH activation increased the adsorption capacity of HC from 34.59±2.27 to 92.24±3.41 (mg/g) (about a three-fold increase). In addition, the BG adsorption capacity of HTC was increased by about five times, followed by KOH activation.

Due to the low surface area of ​​the adsorbent and the poor porosity of HCop and AHCop, the mechanism of Pb2 adsorption can be explained by surface complexation and electrostatic interaction. At low pH values, H is more conducive to electrostatic interactions than lead and cannot form OFG complexes with Pb ions48,55. In Figure 6a, the effect of initial pH on adsorption is depicted. At different pH values, AHCop and HCop show similar adsorption modes. As shown in Figure 6a, as the pH value increases (decrease of H ions in the solution), the complexation reaction between the surface and Pb(II) ions can be completed. In addition, the number of H ions is reduced, and Pb(II) ions can be absorbed by the active sites occupied by H ions at lower pH values. At a pH of 6 ± 0.1, the maximum adsorption of AHCop and HCop are 92.24 ± 3.41 and 34.59 ± 2.27 (mg/g), respectively. Pb(II) ions compete with H for adsorption on the surface of HC (electrostatic attraction). At lower pH values, H adsorption is more favorable than Pb(II) adsorption. In addition, it is difficult to form complexes of OFG and Pb(II) ions at higher H concentrations. As the pH value increases, the functional group undergoes deprotonation, Pb(II) ions replace H, and complex formation occurs, and finally Pb adsorption increases56.

The effect of pH on the adsorption of Pb2 by AHCop and HCop (a); the zero charge point of AHCop (b). Error bars represent the standard deviation of three replicate experiments.

The pHpzc of AHCop is approximately 6.1 ± 0.1, as shown in Figure 6b. Because the surface of the adsorbent is positively charged, it is easier to remove anions when the pH is lower than pHpzc. The adsorption of cations is more advantageous at higher pH values ​​1,57. Since the maximum adsorption capacity occurs at a pH value of 5.7 ± 0.1, the repulsive electrostatic force greatly affects the adsorption of Pb2 on the AHCop surface. In addition, the adsorption capacity is affected by the initial pH4,57.

Figure 7 depicts the kinetics of Pb2 adsorption by AHCop. In the first 20 minutes, a large amount of Pb2 adsorption occurred immediately, and then gradually increased to an equilibrium value. Regarding Figure 7, the adsorption capacity changes steadily after 90 minutes and remains constant at around 90 (mg/g), indicating that equilibrium has occurred.

The kinetic data from the adsorption of Pb2 onto AHCop (T = 25 °C). Error bars represent the standard deviation of three replicate experiments.

The large number of adsorption sites on AHCop is related to the higher Pb2 ​​adsorption capacity in the first 20 minutes. The number of free adsorption sites decreases over time, while the adsorption capacity remains stable58. Because AHCop has suitable adsorption kinetics and acceptable adsorption capacity, it is a suitable adsorbent for Pb2 ​​removal. Table 5 compares the Pb2 adsorption capacity of other HC-based adsorbents from other studies and this study 4,55,59,60,61,62. Table 6 shows the calculated kinetic coefficients and adsorption capacity of the PFO, PSO and Elovich kinetic models. The regression coefficient (R2) and adjusted R-squared (R2Adj) of the PSO model are much higher than those of the PFO and Elovich models. In addition, the residual sum of squares (RSS) and reduced chi-square (χ2red) of the PSO model are lower than those of the PFO and Elovich models, as shown in Table 6. Therefore, the PSO model is more suitable for experimental data, indicating that chemical adsorption is the main process63.

The adsorption isotherm of Pb2 is shown in Figure 8. Figure 8a shows the Langmuir model fitting of experimental data at different temperatures. At different temperatures, the equilibrium adsorption capacity is significantly enlarged at low equilibrium concentrations, and then the adsorption capacity gradually increases. The adsorption rate of Pb2 is related to the driving force (the concentration gradient between Pb2 ions in the solution and the adsorbent) and the mass transfer coefficient. After the adsorption process, filling of active sites occurs. Therefore, if the driving force for mass transfer decreases, the mass transfer rate (adsorption rate) will decrease. Finally, all active sites are filled, and the balance reaches 64. The convex climbing angle of the nonlinear adsorption isotherm supports a promising adsorption map. The adsorption capacity decreases with increasing temperature due to the exothermic nature of adsorption.

Equilibrium data from adsorption of Pb2 to AHCop. Error bars represent the standard deviation of three replicate experiments.

Langmuir, Freundlich and Simps isotherm models are used to study the adsorption mechanism of Pb2 on AHCop at 25 °C, as shown in Figure 8b. Table 7 shows the parameters of the adsorption isotherm. In terms of the correlation constant (R2), the Freundlich model performs poorly compared to the Langmuir and Sips models.

As shown in Table 7, compared with the Sips and Freundlich models, the Langmuir isotherm model has a better correlation coefficient (R2 = 0.99) and a lower RSS value. In addition, the reduced chi-square value of the Langmuir model is higher than that of the Sips model (1.76 times). This higher χ2red value is related to the difference between the predicted adsorption capacity (using the Langmuir model) and the actual adsorption capacity at the lower concentration. The Langmuir and Sips models show an acceptable fit with the experimental data. The Langmuir and Sips models describe better experimental data at higher and lower concentrations, respectively. The maximum adsorption capacity (qmax) calculated by the Langmuir model at 25 °C is 90.1 (mg/g). The Sips model simplifies the Freundlich and Langmuir models at low and high adsorbate concentrations, as shown in Figure 8b. The heterogeneity coefficient (1/n) of the Sips model in this study is 1.04, which confirms the better match of the Langmuir model65.

The thermodynamic properties of Pb2 on AHCop at three different temperatures (25, 35, and 45 °C) were calculated using Vanter Hoff's equation 66, and the results are shown in Table 8. The adsorption amount of Pb2 on AHCop decreases with increasing temperature, as shown in Figure 8b, indicating that the adsorption of Pb2 on AHCop is essentially exothermic, as confirmed by estimated values ​​(Table 8). The negative value of \(\Delta {G}^{0}\) indicates that the adsorption of Pb2 on AHCop is a spontaneous process. In addition, the positive value of \(\Delta {S}^{0}\) means that the degree of randomness at the solid solution interface increases.

The optimal process conditions for Pb2 ​​adsorption on AHC were found (temperature is 180 °C, water-biomass ratio is 5 (w/w), ZnCl2-biomass ratio is 3.5 (w/w), and residence time is 11.5 h). Under the optimal conditions, the equilibrium adsorption capacity is found to be 92.24 ± 3.41 (mg/g). According to the isotherm survey, the Langmuir and Sips models fit the adsorption isotherm well. The adsorption kinetics of Pb2 was changed to fit the PSO model. The thermodynamic test showed that the adsorption of Pb2 on AHCop is a process of simultaneous exothermic heat. FTIR research revealed the generation of oxidized functional groups after HTC and KOH activation.

The important process of pb2 adsorption is electrostatic interaction and complex formation. The structural changes caused by the HTC process are cleared by SEM and XRD inspection of the sample.

The bagasse is provided by Karen Sugar, located in Shoshtar, Ahwaz State, Iran. The bagasse (BG) was gently rinsed with water several times, and then dried in an oven at 105 °C for 24 hours. After that, BG was crushed and sieved to a particle size of less than 35 mesh.

All compounds and substances used in this experiment are of analytical grade. Dissolve an appropriate amount of Pb(NO3)2 in water and dilute to an appropriate concentration to prepare a pb2 stock solution (2000 mg/L).

The hydrothermal carbonization (HTC) study was carried out in a stainless steel autoclave reactor using a 200 ml polytetrafluoroethylene vessel. First, 75 mL of distilled water was introduced into the reactor along with various amounts of biomass and ZnCl2. The reactor was then exposed to different times and temperatures at a heating rate of 10°C/min to determine the influence of hydrothermal conditions on the characteristics of water charcoal. The reactor was cooled at room temperature, the solid product was separated and washed several times with distilled water, and then dried at 105°C for 24 hours and labeled as HC.

The HC was chemically modified by mixing 2 g of HC with 200 mL of 2 M KOH solution for 1 hour at ambient temperature (25 ± 0.5 °C). Next, the modified water charcoal (AHC) is separated, washed with deionized water, and neutralized with 0.1 M HNO3/KOH solution. After that, the AHC was dried in an oven at 105 °C for 24 hours.

Temperature, residence time, the ratio of water to biomass, and catalyst to biomass all affect the yield and adsorption capacity of HC produced by HTC. Therefore, temperature, residence time, ratio of water to biomass, and catalyst to biomass are selected as independent variables. The corrected adsorption capacity is used as the response value, because the adsorption capacity of Pb2 per unit amount of BG is affected by the adsorption capacity and yield of HC. Using traditional experimental methods, multiple experiments are required to study these factors. Therefore, the response surface method (RSM) is combined with the central composite design (CCD) to optimize the HTC process. Design-expert (version 8.0.6) used 30 experiments at 5 levels. The independent variables are temperature (× 1), residence time (× 2), ratio of water to biomass (× 3) and ratio of ZnCl2 to biomass (× 4), and the response is the modified adsorption capacity (MAC). The variable range is determined with respect to other studies. In addition, the low water-to-biomass ratio limits its use to technical feasibility (biomass is completely immersed in the water environment). Table 9 shows the variable range. MAC is calculated by formula. (2):

Among them, \({Q}_{e}\) is the equilibrium adsorption capacity of AHCs, and \({Y}_{HC}\) is the yield of HCs under different conditions.

Design-expert (Version 8.0.6) is used for randomization testing, divided into five stages and 30 experiments. Use the following equation to adjust the response of the variable to a quadratic regression model:

In the equation. (3) \({Y}_{i}\) is the predicted response, \({x}_{i}\) and \({x}_{j}\) are the independent variables. \({A}_{0}\), \({A}_{i}\), \({A}_{ii}\) and \({A}_{ij}\) are respectively Average response, linear, quadratic and interaction coefficient effects, \({\varepsilon }_{i}\) is a random error 67. Analysis of variance (ANOVA) is used to investigate the significance of the model and independent variables.

The ash content in the product is calculated according to the guidelines of the American Society for Testing and Materials (ASTM E1755-01). Use an element analyzer (Vario EL III, Elementar) to determine the chemical composition (C, H, N, S). The Micromeritics Quantachrome device was used to determine the BET surface area at 77 K. In order to evaluate the effects of carbonization and activation processes on surface chemistry, a Cary 630 FT-IR spectrometer was used to obtain FTIR spectra from 1 to 400 cm-1 in the 4000 cm-wavenumber range. Hitachi S-2700 Scanning Electron Microscope (SEM) is used to check the morphology of the sample (SEM). In order to study the characteristics of thermal decomposition, TG-DTG (METTLER TOLEDO, TGA2) analysis was performed on HC. HC and BG are burned from room temperature to 800 °C (10 °C/min, 50 mL/min) with ambient air. On an X-ray diffractometer (D/MAX2200, Rigaku, Japan), Ni filtered Cu K radiation (= 1.54) was used to observe the X-ray diffraction (XRD) pattern of the sample. Measure the XRD value of the sample in the range of 5° to 80°. To determine the point of zero charge (pHpzc), 0.1 g of adsorbent was added to 50 mL of KNO3 solution and stirred at 250 rpm for 24 hours. In the range of 2 to 10, use KNO3/KOH (0.01 mol) to adjust the initial pH of the solution. The suspension is then filtered, the final pH is measured, and the difference between the initial and final pH and the initial pH is plotted.

In order to find the best adsorbent, a 1 g/L AHC dose was added to a 250 mL Erlenmeyer flask containing 100 mL of a standard Pb2 solution (200 mg/L) (initial pH value of 6 ± 0.1). Place all samples on the shaker and shake at ambient temperature (25 ± 0.1 °C) for 24 hours. The effect of pH on adsorption was checked when the pH was changed from 2.0 to 7.0. Experiment with batch kinetics (initial pH value of 6 ± 0.1) under different contact time (5 to 240 minutes), contact with 1 g/L of optimal AHC (AHCop) and 100 mL of Pb2 solution (200 mg/ L). The Pb2 solution was diluted with distilled water from the stock solution. For adsorption isotherm studies, 100 mL of a lead solution with an initial concentration of (20–200 mg/L) and an initial pH of (6 ± 0.1) and 1 g/L AHCop were shaken for 6 hours (200 rpm) under different conditions . Temperature (25, 35 and 45 °C). The concentration of Pb2 solution was measured with an inductively coupled plasma emission spectrometer (ICP-OES 730-ES, Varian, USA). All Pb2 solutions are serially diluted from the stock solution (200 mg/L).

The amount of Pb2 adsorbed at different times \({q}_{t}\) is determined by the equation in mg/g. (4)

Where \(V\) is the volume of the solution (mL), \(m\) is the amount of adsorbent used (mg), \({C}_{0}\) and \({C}_{t }\ ) Are respectively the main concentration and equilibrium concentration after t (\(\frac{mg}{L}\)) time. Under equilibrium conditions, the equilibrium adsorption capacity \({q}_{eq}\) is calculated by the equation. (4).

The Elovich model 68, the pseudo first-order model 69 (PFO) and the pseudo-second-order model 70 (PSO) are used for dynamic studies as follows:

Where t is the time (min), k1 (1/min) and k2 (\(\frac{{\text{g}}}{{\text{mg}} {{\text{min}}}^{- 1}}\)) are the rate coefficients of PFO and PSO respectively. b (\(\frac{{\text{g}}}{{\text{mg}}\)) and a (\(\frac{{\text{mg}}}{{\text{g} } {\text{min}}}\)) are the constants of the Elovich model.

Langmuir71, Freundlich72 and Sips73 models are used for isotherm research as follows:

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The author thanks the Iranian Vice President of Tehran Technology for financial support.

Department of Chemical Engineering, Amir Kabir University of Technology (Tehran Institute of Technology), 424 Hafez Avenue, 1591634311, Tehran, Iran

Mohamad Ebrahim Malool & Mostafa Keshavarz Moraveji

Department of Chemistry and Petroleum Engineering, Sharif University of Technology, Tehran, Iran

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MEM conducted the entire research and wrote the article. MKM supervised the entire project and designed the research. JS came up with this idea and edited this article.

Correspondence with Mustafa Kshawaz Moraviji.

The author declares no competing interests.

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Malool, ME, Keshavarz Moraveji, M. & Shayegan, J. Optimized production, lead(II) adsorption and characterization of bagasse alkali-modified hydrogenated carbon. Scientific Report 11, 22328 (2021). https://doi.org/10.1038/s41598-021-01825-y

DOI: https://doi.org/10.1038/s41598-021-01825-y

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