1 Introduction

Paper mill sludge is a by-product of the paper industry, containing various pathogenic microorganisms, organic matter, nitrogen, and phosphorus. This composition poses a risk of environmental pollution and public health issues [1, 2]. The China Paper Association reports that China’s paper production in 2022 reached approximately 283.91 Mt, representing a 1.32% increase compared to the previous year [3]. It is estimated that for every ton of paper produced, an average of 1 m3 of paper sludge is generated [4]. Currently, the annual output of paper mill sludge is substantial, and commonly used disposal methods include discarding, landfilling, and incineration. However, these methods fail to achieve sludge reduction or prevent the occurrence of secondary pollution [5]. Similarly, dye wastewater (such as methyl orange) is a common pollutant in the dye processing industry [6]. It is characterized by its complex tissue composition, high chroma, and poor biodegradation performance. The global usage of dye exceeds 10,000 t/year, with approximately 100 t entering the environment through water supply, posing risks to both humans and the environment [7]. Given the negative impacts of paper mill sludge and dye wastewater, it is crucial to explore environmentally friendly technologies to address these issues [8]. Consequently, the utilization of sludge-based activated carbon for resource recovery and environmental protection has been proposed. With its excellent porosity, sludge-based activated carbon can serve as an adsorbent carrier for dye wastewater.

Compared with biochar, sludge-based activated carbon has lower carbon content and a less developed specific surface, which limits its industrial application [9, 10]. To overcome this limitation, researchers have explored the addition of substances with high carbon content, such as lignocellulose, to sludge precursors [8, 11]. For instance, Wang et al. [12] incorporated biomass into the sludge and observed an improvement in the pore structure, with a maximum BET surface area of 1342 m2/g after chemical treatment, leading to enhanced adsorption performance. Similarly, Liang et al. [13] demonstrated that the addition of biomass to sludge improved the physicochemical properties, particularly in the field of adsorbents. In another study, Li et al. added 25 wt.% corn stalk to the sludge and observed an increase in the specific surface area of sludge-based activated carbon from 475 to 769 m2/g, which was advantageous for adsorption [8]. These findings highlight the significant improvement in adsorption capacity achieved by using lignocellulosic materials, such as rice husk, coconut shell, and peanut shell, as the carburizer in sludge-based activated carbon.

The physicochemical properties and adsorption performances of sludge-based activated carbon are influenced by various factors, including the proportion of sludge and carburizer, activation method, and activation conditions such as heating rate, temperature, time, and activator. Nielsen et al. [14] found that adding fish waste to the sludge increased the pore volume and specific surface area of the activated carbon by 1.5 times and 3 times, respectively, resulting in enhanced adsorption of dyes. Dos Reis et al. [15] determined that the optimal specific surface area (SBET) was achieved at a pyrolysis temperature of 500 ℃, impregnation ratio of 0.5 (ZnCl2-to-sludge), and holding time of 15 min. Tay et al. [16] investigated the preparation process of sludge-derived adsorbents using ZnCl2 as an activator, considering factors such as concentration, activation temperature, and residence time. Their results showed that a low concentration of ZnCl2 increased the proportion of micropores in the activated carbon. Activation temperature and residence time also affected the specific surface area and pore size distribution of the activated carbon. It is important to note that the specific surface area and adsorption capacity of activated carbon can vary significantly depending on the raw materials and preparation methods used. Furthermore, many activated carbon experiments lack a comprehensive consideration of different preparation conditions. Therefore, it is crucial to conduct further research and make meaningful attempts to synthesize low-cost and high-performance activated carbon from waste.

In this study, sludge and peanut shells (as a carburizer) were chosen as the raw materials for producing activated carbon. The optimal activated carbon process was determined through an orthogonal experiment involving three activation modes: physical, chemical, and microwave activation. The resulting activated carbon was then characterized and analyzed. Subsequently, the optimal activated carbon was used for adsorbing dye wastewater (specifically, methyl orange), and the adsorption kinetics were calculated to investigate the underlying adsorption mechanism. These endeavors are crucial for advancing the utilization of sludge resources and mitigating the adverse impacts of dye wastewater.

2 Materials and methods

2.1 Materials and equipment

Paper mill sludge, purchased from Shandong Chenming Paper Co., Ltd., and peanut shells, produced in Ningling County, Shangqiu City, Henan Province, were used as raw materials. The raw materials were first purified and then dried naturally in a cool, dark place. The dried samples were crushed using a grinder and sieved into 80 mesh (particle size ~ 0.2 mm). Subsequently, the samples were further dried in a 105 °C oven for 24 h and stored in sealed bags. Zinc chloride (analytical pure) and methyl orange (analytical pure) were obtained from Nanjing Chemical Reagent Co., Ltd. for the experiment.

The test equipment included a horizontal tube furnace (YG-120810, Shanghai Yuzhi Co., Ltd.), UV–visible spectrophotometer (UV1800, Shanghai Jinghua Technology Instrument Co., Ltd.), and microwave chemical reactor (WBFY-205, Zhengzhou Yiheng Instrument Equipment Co., Ltd.).

Based on physical activation, chemical activation, and microwave activation methods, the corresponding optimal activated carbons were determined using orthogonal experiments and named AC-Pmax, AC-Cmax, and AC-Mmax, respectively. To compare samples without physical, chemical, and microwave activation, activated carbon was prepared by directly carbonizing raw materials (sludge 70 wt.% and peanut shell 30 wt.%) under a N2 atmosphere with a nitrogen flow rate of 300 mL/min, heating rate of 10 °C/min, carbonization temperature of 500 °C, and time of 60 min. This comparison group was named AC-Raw. AC stands for activated carbon, and ACs include AC-Raw, AC-Pmax, AC-Cmax, and AC-Mmax.

2.2 Preparation of activated carbon

This study focused on three main methods for preparing activated carbon: physical activation, chemical activation, and microwave activation. The paper mill sludge-peanut shell mixture (hereafter referred to as mixture) was obtained by adding peanut shells into paper mill sludge at proportions of 10%, 20%, and 30%.

  1. (1)

    Physical activation. Physical activation consists of two stages: pyrolysis and activation. In the pyrolysis stage, a mixture with different mass fractions of 5 g was weighed into the tube furnace. Nitrogen gas (purity, 99.999%) was used as the protective gas at a flow rate of 300 mL/min. The mixture was then heated to 600 °C at a rate of 10 °C/min and carbonized for 60 min. In the activation stage, the carbonized sample was heated to the activation temperature (700, 800, 900 °C) using different heating rates (10, 20, 30 °C/min). Once the activation temperature was reached, the nitrogen gas was switched to high-purity CO2 (flow rate 300 mL/min) for activation, and a specific activation time (30, 60, 90 min) was maintained. The effects of heating rate, peanut shell content, activation temperature, and activation time on the properties of activated carbon were investigated.

  2. (2)

    Chemical activation. A 5 g mixture consisting of different mass fractions was weighed and then mixed with a specific amount of zinc chloride impregnation. The mixture was subsequently dried in an oven at 105 °C. Next, the mixture was placed in a tube furnace under a nitrogen atmosphere with a flow rate of 300 mL/min. The heating rate was set at 10 °C/min, and the mixture was heated to different activation temperatures (500, 550, 600 °C) for 30, 60, or 90 min. The resulting activated carbon was thoroughly washed with distilled water to eliminate the activator and other inorganic ash. Finally, the activated carbon was dried in the oven at 105 °C and sealed for future use.

  3. (3)

    Microwave activation. A 5 g mixture with varying mass fractions was weighed, impregnated, and mixed with a specific mass of zinc chloride. The mixture was stirred thoroughly and left to sit for 12 h before being dried and utilized. Next, the mixture was placed into a microwave chemical reactor and subjected to microwave irradiation at power levels of 300, 500, and 700 W for durations of 10, 20, and 30 min, respectively. The resulting activated carbon was then washed multiple times with distilled water to remove any remaining activator and inorganic ash. Finally, the activated carbon was dried and sealed.

2.3 Orthogonal experimental design

The specific surface area and adsorption properties of activated carbon were influenced by heating rate, microwave power, activation temperature, and impregnation ratio [17, 18]. To account for the numerous influencing factors, an orthogonal experimental design was employed, which allowed for the equivalent of a large number of comprehensive tests while maintaining high efficiency, speed, and cost-effectiveness [19]. For each of the three activation modes (physical, chemical, and microwave activation), four factors and three different levels were selected. The L9(34) orthogonal table design test was used. The design factors and levels for physical, chemical, and microwave activation are listed in Tables 1, 2, and 3, respectively. By synthesizing various factors and levels based on the data obtained from the orthogonal test table of the three activation modes (see supplementary data), the optimal preparation conditions for activated carbon could be determined. The orthogonal experiment design focused on analyzing the adsorption properties of activated carbon at different levels and factors, primarily using iodine and methylene blue adsorption values. The analysis methods employed in the orthogonal experiment design included intuitive analysis and variance analysis. Intuitive analysis encompassed range analysis, primary and secondary analysis, and the selection of the optimal scheme. ANOVA was used to determine the significance of each influencing factor. Range analysis involved calculating the average of the levels of each test factor, with range representing the difference between the maximum and minimum averages. The primary and secondary order was determined based on the results of the range analysis, with higher average values indicating better test levels.

Table 1 Levels of physical activation and design factors
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Table 2 Levels of chemical activation and design factors
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Table 3 Levels of microwave activation and design factors
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2.4 Material characterization and adsorption performance

Through an orthogonal test, the char characteristics of optimum activated carbon obtained by physical, chemical, and microwave activation were characterized. In order to obtain the specific surface area and other parameters, the activated carbon was determined by an automatic gas adsorption analyzer (Autosorb-IQ, Quantachrome, USA). The surface morphology of activated carbon was observed by scanning electron microscope (Quanta 200, FEI Corporation, USA). The crystal structure of activated carbon was studied using a combined multifunctional horizontal X-ray diffractometer (Ultima IV, Rigaku, Japan). Infrared spectroscopy (Nicolet iS10, Thermo Fisher, USA) was used to characterize the functional group changes on the surface of activated carbon.

In the orthogonal experimental design, the adsorption properties of activated carbon were primarily evaluated based on the adsorption values of iodine and methylene blue. The iodine adsorption value of activated carbon was determined according to the China national standard GB/T12496.8–2015. The methylene blue adsorption capacity of activated carbon was determined according to the China national standard GB/T 12496.10–1999. The literature [20] provided specific procedures for the adsorption of iodine and methylene blue by activated carbon, which could be referred to.

3 Results and discussion

3.1 Activated carbon adsorption experiment

The results of the orthogonal adsorption of iodine and methylene blue on activated carbon, prepared using three different activation methods (physical, chemical, and microwave activation), are presented in Table S1-S6 in the supplementary data. When compared to AC-Raw (with iodine and methylene blue adsorption capacities of 310 mg/g and 50 mg/g, respectively), the activated carbon prepared by physical activation exhibited average adsorption capacities of 577.2 mg/g for iodine and 107 mg/g for methylene blue. For activated carbon prepared by chemical activation, the average adsorption capacities were 682.5 mg/g for iodine and 121.5 mg/g for methylene blue. Activated carbon prepared by microwave activation showed average adsorption capacities of 503.1 mg/g for iodine and 96.5 mg/g for methylene blue. These results indicated that the adsorption performance of activated carbon prepared by the chemical activation was the highest, followed by the physical activation and finally the microwave activation. Although microwave activation had the advantage of significantly shortening production time, improving production efficiency, and reducing environmental pollution [21], it was not as effective as the physical and chemical activation in improving the adsorption performance of activated carbon. This was likely because the chemical activation was more conducive to the formation of micropores and mesopores. Once inside the micropores, iodine molecules underwent strong physical adsorption with the micropore walls and also underwent substitution reactions with carbonyl or hydroxyl groups on the surface of activated carbon [22]. The presence of mesopores provided a pathway for methylene blue to enter, further enhancing the adsorption performance [23]. The physical activation method used CO2 reaming to obtain the main mesoporous and macropores [24], resulting in the largest Vtotal, as shown in Table 4. Moreover, Table 4 indicates that compared to physical and microwave activation, the activated carbon produced through chemical activation had a higher specific surface area of 803.48 m2/g, which promoted better adsorption of iodine and methylene blue. Additionally, the total pore volume of AC-Pmax, AC-Cmax, and AC-Mmax was 2.2–3.1 times higher than that of AC-Raw, and the average pore size was also increased by 1.6–1.9 times.

Table 4 Specific surface area and pore structure parameters of activated carbon
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Figure 1 illustrates the orthogonal range analysis of the adsorption of iodine and methylene blue by activated carbon under three activation modes (physical, chemical, and microwave activation) by different factors (A, B, C, and D). The values presented in Fig. 1 correspond to the R values in the orthogonal test table. Specifically, Fig. 1(a), (c), and (e) depicts the iodine adsorption capacity of activated carbon prepared through physical activation, chemical activation, and microwave activation, respectively. Similarly, Fig. 1(b), (d), and (f) demonstrates the methylene blue adsorption capacity of activated carbon prepared through physical activation, chemical activation, and microwave activation, respectively. In Fig. 1(a) to (f), A, B, C, and D corresponded to the factors listed in Tables 1, 2, and 3. The results shown in Fig. 1(a) and (b) indicate that the peanut shell content (factor B) was the primary factor influencing the adsorption capacity in the physical activation method. In the chemical activation method (Fig. 1(c) and (d)), the activation temperature (factor C) played a crucial role in the iodine adsorption capacity, while the peanut shell content (factor B) had the greatest impact on methylene blue adsorption. In the microwave activation method (Fig. 1(e) and (f)), the microwave power (factor B) was found to be the main factor affecting the adsorption capacity. The peanut shell content was found to be significant in both physical and chemical activation methods. This was because the addition of peanut shells to the sludge improved the porosity of the activated carbon and increased the number of oxygen-containing functional groups, which facilitated the preparation of high-performance carbon materials. This finding aligned with the conclusion of Liang et al. regarding the co-pyrolysis of coconut shells and sludge for activated carbon production [13]. In terms of the impact of activation time, Zhao et al. discovered, through orthogonal experiment results of coal-based activated carbon, that activation time had the greatest influence on the iodine value of activated carbon [25]. This was primarily due to the fact that a longer activation time led to a higher specific surface area. Similarly, higher microwave power resulted in increased energy input to the activated sample, leading to the formation of more active sites and pores, thus enhancing the adsorption capacity [26]. However, it is important to note that excessive microwave power could cause the activated carbon to burn and damage the pore structure [27].

Fig. 1
figure 1

Orthogonal range analysis of adsorption of iodine and methylene blue by activated carbon with different factors (A, B, C, and D) in three activation modes (physical activation, chemical activation, and microwave activation)

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Through orthogonal test analysis (see supplementary data for specific comparison process), the optimal process parameters and adsorption values for iodine and methylene blue were obtained for three different methods: (1) physical activation method: the heating rate was 20 ℃/min, the peanut shell content was 30%, the activation temperature was 800 ℃, and the activation time was 30 min, resulting in an iodine adsorption value of 686.68 mg/g and a methylene blue adsorption capacity of 138 mg/g. (2) Chemical activation method: the impregnation ratio was 1:2, peanut shell content was 30%, activation temperature was 500 °C, and activation time was 30 min, resulting in an iodine adsorption value of 913.86 mg/g and a methylene blue adsorption capacity of 153 mg/g. (3) Microwave activation method: the peanut shell content was 30%, microwave power was 700 W, impregnation ratio was 1:2.5, and activation time was 10 min, resulting in an iodine adsorption value of 607.67 mg/g and a methylene blue adsorption capacity of 118.5 mg/g. In summary, the chemical activation method showed significant advantages over physical activation and microwave activation in terms of preparing activated carbon with high adsorption properties.

3.2 Characterization of activated carbon

Based on the orthogonal test results, BET, SEM, XRD, and FTIR tests were carried out on the activated carbon samples (AC-Pmax, AC-Cmax, and AC-Mmax) prepared under optimized conditions by physical activation, chemical activation, and microwave activation, respectively. In order to understand the char characteristics of the optimal activated carbon, activated carbon (AC-Raw) was selected for comparison.

Figure 2a shows the nitrogen adsorption–desorption isotherm of ACs. The results of Fig. 2a demonstrate that the adsorption–desorption curves of ACs followed typical type IV isotherms at a relative pressure of 0.4–0.9, with the exception of the chemical activation method not being clearly evident. This was attributed to the presence of mesoporous pores, which resulted in capillary condensation during the mesoporous adsorption process at high pressure, leading to the formation of a hysteresis ring [28, 29]. The pore size distribution of ACs is illustrated in Fig. S1, which clearly shows that the primary pore structure of ACs consisted of mesopores, and the pore size distribution was broad, aligning with the average pore size results presented in Table 4.

Fig. 2
figure 2

Characterization of ACs: a N2 adsorption–desorption isotherms; b SEM images; c XRD pattern; d FTIR spectra analysis

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The surface microscopic appearance of ACs is shown in Fig. 2b. It was evident that the surface morphology of AC-Raw was rough, with pores mainly formed due to volatile precipitation during the pyrolysis process. This suggested that the activation achieved through the direct carbonization method was insufficient. On the other hand, AC-Pmax exhibited CO2-etched pores on its surface after activation, indicating that the use of high-purity CO2 as a gas activator had a positive impact on the activated carbon. Furthermore, after chemical activation, AC-Cmax displayed an abundant pore structure on its surface. This could be attributed to the significant infiltration of ZnCl2 into the feedstock, which acted as a de-hydroxylation and dehydration agent during the activation process. Pimentel et al. found that ZnCl2, a Lewis acid, promoted the production of large-ring aromatic compounds, leading to the formation of uniform micropores [30]. ZnCl2 molecules acted as a support within the carbon structure and evaporated at high temperatures, contributing to the pore size. Washing ZnCl2 resulted in the formation of a porous structure, while some ZnCl2 on the surface eroded the surface, creating pore structures during activation. Xia et al. also observed irregular cracks on the surface of cotton textile waste after ZnCl2 activation, which was attributed to ZnCl2 evaporation during pyrolysis [31]. Microwave activation resulted in minimal pore and crack formation on the surface of AC-Mmax, indicating a limited activation effect.

Figure 2c presents the XRD pattern of the ACs, revealing a diffraction peak at a 2θ value of 36.5°, which was attributed to the characteristic diffraction peak of the quartz phase. Referring to the standard card, it could be concluded that this characteristic peak corresponded to the crystal surface (111), suggesting the presence of Si (in the form of quartz phase) in the carbon-based structure of the carbonization and activation products obtained from the feedstocks. Moreover, the number and intensity of diffraction peaks in the spectra of AC-Pmax, AC-Cmax, and AC-Mmax were higher than AC-Raw, indicating an enhanced degree of graphitization in the activated carbon after activation.

The infrared spectrum of ACs is presented in Fig. 2d. The infrared spectra of activated carbon prepared using different activation methods exhibited variations. The absorption peak at 3501 cm−1 in AC-Cmax was attributed to the presence of –OH and Si–OH groups in the organic matter [32]. AC-Raw, AC-Cmax, and AC-Mmax showed distinct peaks around 1600 cm−1, primarily associated with the stretching vibration of –CN. All ACs displayed minor absorption peaks around 2300 cm−1, which were caused by the stretching vibration of alkynes. The absorption peak near 670 cm−1 indicated the presence of Si–O–C, Si–O–Si, or C–O–C structures, which were primarily influenced by the significant amount of silicon present in the sludge [33, 34].

3.3 Methyl orange adsorption by optimum activated carbon (AC-Cmax)

The study used chemically activated optimum activated carbon (AC-Cmax) with a higher specific surface area, developed pore size structure, and stable physicochemical properties to investigate its adsorption effect on methyl orange. The effects of adsorption time, activated carbon dose, initial concentration of methyl orange, and pH value of the solution on the adsorption were investigated.

3.3.1 Adsorption property

The adsorption properties were determined by the adsorption capacity (Q) and removal efficiency of methyl orange solution on activated carbon. As shown in the following formula:

$$Q = \frac{{(C_{0} - C_{e} )V}}{M}$$
(1)
$$R = \frac{{(C_{0} - C_{e} )}}{{C_{0} }} \times 100\%$$
(2)

where Q is the adsorption capacity of activated carbon (mg/g), R is the removal efficiency of methyl orange by activated carbon (%), C0 is the initial concentration of methyl orange solution (mg/L), Ce is the concentration of methyl orange solution after adsorption (mg/L), V is the volume of solution (L), and M is the mass of activated carbon (g).

A 25 mL solution of methyl orange with an initial concentration of 500 mg/L was prepared. The pH value of the solution was adjusted to 7, and 0.05 g of AC-Cmax was added. The effect of activated carbon on the adsorption of methyl orange was studied at 25 ℃ for different adsorption times (5, 15, 30, 45, 60, 90, 120, 240, and 480 min), and the results are presented in Fig. 3a. The adsorption capacity and removal efficiency of methyl orange solution remained consistent as the adsorption time increased. Initially, the adsorption capacity and removal efficiency of activated carbon increased rapidly within the first 90 min and then gradually stabilized. After 120 min, the adsorption process reached equilibrium. Therefore, the entire adsorption process could be divided into three stages: fast, slow, and dynamic equilibrium. The adsorption equilibrium time for subsequent tests was determined to be 120 min. Initially, the sludge-based activated carbon had numerous vacant adsorption points on its surface. As time progressed and the adsorption reaction took place, methyl orange molecules occupied a significant number of adsorption sites on the activated carbon surface and gradually penetrated into its interior [35]. This resulted in a decrease in the mass transfer rate, affecting the adsorption rate and amount. When dynamic equilibrium was reached, the maximum adsorption capacity of activated carbon for methyl orange solution was found to be 192 mg/g, with a maximum removal rate of 76.8%.

Fig. 3
figure 3

The effect of optimal activated carbon on the adsorption capacity (Q) and removal efficiency of methyl orange under different factors: a adsorption time; b activated carbon dose; c initial concentration; d pH value

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Figure 3b illustrates the impact of adding different amounts of activated carbon (1 g/L, 2 g/L, 3 g/L, 4 g/L, and 5 g/L) on the adsorption performance of methyl orange solution at 25 ℃ for 120 min. As the dose of activated carbon increased, the removal efficiency of methyl orange solution noticeably improved, while the adsorption capacity exhibited a downward trend. The increase in activated carbon dose led to an increase in the surface area and surface active groups, which enhanced the removal efficiency of methyl orange [36]. However, the amount of methyl orange adsorbed by activated carbon gradually decreased during this process. This could be attributed to the competition for adsorption sites between methyl orange molecules in the solution when the amount of carbon added was lower than the concentration of methyl orange solution, resulting in a large adsorption amount. As the activated carbon dose increased, the relative concentration of carbon in the solution increased, causing particle condensation and reducing the surface area available for effective carbon adsorption. This, in turn, decreased the number of effective adsorption sites and increased the diffusion distance of methyl orange molecules on the surface and inside of the carbon, ultimately leading to a decrease in the adsorption amount [37]. Considering the adsorption capacity, removal efficiency, and economy, a dose of 2 g/L of activated carbon was chosen for subsequent tests.

When the activated carbon was 0.05 g and the adsorption time was 2 h at 25 ℃, the effects of different initial concentrations of methyl orange solution on the adsorption properties of activated carbon were investigated (Fig. 3c). The initial concentrations tested were 200 mg/L, 300 mg/L, 400 mg/L, 500 mg/L, 600 mg/L, 800 mg/L, and 1000 mg/L. It was observed that the adsorption capacity of methyl orange by activated carbon increased rapidly when the initial concentration of the solution was in the range of 200 to 500 mg/L. This could be attributed to the low concentration of methyl orange solution, which was not sufficient to occupy all the adsorption sites on activated carbon. As a result, there was a significant collision contact between methyl orange molecules and activated carbon, allowing for full adsorption. Additionally, the concentration difference between the solid and liquid phases increased the propagation rate of methyl orange molecules, leading to a reduction in transfer resistance. However, with the increase in solution concentration, the number of available adsorption sites on activated carbon gradually decreased. Liu et al. also suggested that the increase in initial concentration provided a driving force to overcome the transfer resistance, thereby enhancing the adsorption capacity of activated carbon [38]. Nevertheless, as the active sites on activated carbon approached saturation at higher concentrations, the adsorption capacity tended to stabilize. Consequently, the adsorption capacity of methyl orange by activated carbon initially increased rapidly and then slowed down steadily, while the removal efficiency showed a downward trend. When the solution concentration reached 500 mg/L, the adsorption capacity of activated carbon gradually reached saturation.

The pH value of the solution had an impact on the surface charge of activated carbon and the form of methyl orange, which in turn affected the adsorption performance of activated carbon. In the experiment, when 0.05 g of activated carbon was used, the adsorption time was 2 h at 25 ℃, and the concentration of the methyl orange solution was 500 mg/L. Figure 3d illustrates the influence of different pH values (3, 4, 5, 6, 7, 8, 9, 10) on the adsorption of methyl orange solution by activated carbon. Under acidic conditions, the presence of a large amount of H+ in the solution could neutralize the negatively charged active sites on the slightly alkaline surface of methyl orange, thereby hindering the cross-linking between the active sites of methyl orange and activated carbon. As the pH value of the solution continued to rise, the degree of neutralization decreased, allowing the pore structure of activated carbon to continuously adsorb the active sites of methyl orange. Consequently, the removal efficiency and adsorption capacity of methyl orange continued to increase. The highest removal efficiency and adsorption capacity of methyl orange, reaching 86.37% and 215.93 mg/g respectively, were achieved at a pH value of 7. As the pH value of the solution further increased, the surface of activated carbon became negatively charged, leading to electrostatic repulsion with the negatively charged methyl orange under alkaline conditions. Consequently, the adsorption capacity and removal efficiency of activated carbon on methyl orange decreased significantly.

3.3.2 Adsorption kinetics

Adsorption kinetics refers to the change in reaction rate over time during a chemical reaction. In the study of adsorption kinetics of methyl orange, both quasi-first-order and quasi-second-order kinetic models were employed. The quasi-first-order kinetic model describes the adsorption rate based on the adsorption amount. According to this model, the un-adsorbed point is directly proportional to the adsorption rate, and the adsorption process is controlled by the diffusion step. The quasi-first-order kinetic equation for adsorption can be expressed as follows:

$$q_{t} = q_{e1} (1 - e^{{ - k_{1} t}} )$$
(3)

The quasi-second-order kinetic model assumes that the rate at which adsorbents occupy active sites on activated carbon is proportional to the square of the number of unoccupied active sites. The quasi-second-order adsorption kinetic equation is expressed as follows:

$$\frac{t}{{q_{t} }} = \frac{1}{{k_{2} q_{e2}^{2} }} + \frac{1}{{q_{e2} }}t$$
(4)

where qt is the adsorption capacity of methyl orange by activated carbon at time t (mg/g), qe1 and qe2 are theoretical equilibrium adsorption capacity (mg/g), k1 is the quasi-first-order adsorption kinetic constant, and k2 is the quasi-second-order adsorption kinetic constant.

According to Eqs. (3) and (4), the data were fitted and calculated, and the results are presented in Fig. 4a and b. The parameters of the calculated quasi-first-order equation and quasi-second-order equation are summarized in Table 5. It was found that Lagergren’s quasi-second-order kinetic model provided a better description of the adsorption behavior of methyl orange on activated carbon compared to the quasi-first-order kinetic model. The correlation coefficient R2 was greater than 0.999, indicating a strong relationship. Furthermore, the equilibrium adsorption amount calculated by the quasi-second-order adsorption kinetics model closely matched the actual value, suggesting the occurrence of chemical bond formation during the chemical adsorption of methyl orange by the optimal activated carbon [39].

Fig. 4
figure 4

Lagergren dynamics fitting: a quasi-first-order equation; b quasi-second-order equation

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Table 5 Adsorption kinetics model fitting results and parameter values
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The adsorption process of adsorbates on the adsorbent is generally more complex and can be divided into two stages: the first stage involves the transfer of adsorbates to the surface of the adsorbent particles, known as external diffusion, and the second stage involves the transfer of adsorbates from the surface of the adsorbent to the pores inside the particles, known as internal diffusion. In order to provide a better understanding of the adsorption process, researchers have developed intraparticle diffusion models [40]. The formula is as follows:

$$q_{t} = k_{p} t^{0.5} + C$$
(5)

where kp is the intraparticle diffusion constant (mg/g·min−1), C is a constant related to the thickness of the boundary layer, and qt is the amount of methyl orange adsorbed by activated carbon at time t (mg/g).

Table 6 shows that the adsorption of methyl orange by activated carbon has a strong fitting effect in the external diffusion stage, with R2 > 0.99. However, the fitting effect in the internal diffusion stage was poor, and the linear relationship was not observed. These results suggested that the adsorption rate of methyl orange was not solely determined by the intraparticle diffusion process. The overall adsorption process was also influenced by factors such as the concentration of adsorbates, affinity, pore size, and distribution of the adsorbent [41]. Furthermore, comparing the kp values of the two diffusion stages revealed that the movement speed of adsorbates from the external diffusion stage to the particle surface was higher than that from the adsorbent surface to the particle pores in the internal diffusion stage.

Table 6 Model parameters of intraparticle diffusion of methyl orange by activated carbon
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4 Conclusions

In this study, an orthogonal test was conducted to screen activated carbon treated by physical activation, chemical activation, and microwave activation, in order to obtain the optimal activated carbon. The surface morphology and physicochemical properties of the optimal activated carbon were analyzed, and its application in the adsorption of methyl orange was investigated. Based on the above discussion, the following conclusions can be drawn:

    • Through the orthogonal test, it was found that AC-Cmax prepared by chemical activation outperformed the activated carbon prepared by physical and microwave activation. The iodine adsorption value of AC-Cmax was measured to be 913.86 mg/g, while the methylene blue adsorption value was found to be 153 mg/g. The optimal chemical activation conditions for AC-Cmax were determined to be an impregnation ratio of 1:2, peanut shell content of 30%, activation temperature of 500 °C, and activation time of 30 min.

    • According to BET and SEM analysis, AC-Cmax exhibited a higher specific surface area and a more developed pore structure. XRD analysis indicated that Si was present in the carbon-based structure as a quartz phase in both the carbonization and activation products of feedstocks, with the activated samples showing a higher degree of graphitization. FTIR analysis revealed an increased presence of oxygen-containing functional groups on the surface of AC-Cmax.

    • The adsorption equilibrium time of AC-Cmax for methyl orange solution was found to be 120 min. As the activated carbon dose increased, the removal efficiency of methyl orange noticeably improved, while the adsorption capacity decreased. Similarly, the adsorption capacity of AC-Cmax on methyl orange increased with an increase in the initial concentration. However, it was observed that the adsorption capacity gradually reached saturation when the initial concentration reached 500 mg/L. At a pH value of 7, the highest removal efficiency and adsorption capacity of methyl orange were achieved, with values of 86.37% and 215.93 mg/g, respectively.

    • The adsorption kinetics of methyl orange by AC-Cmax was better described by the quasi-second-order kinetic equation, with R2 > 0.999. The fit of AC-Cmax adsorption of methyl orange in the external diffusion stage was highly accurate, but the fit in the internal diffusion stage is poor, suggesting that internal diffusion did not play a significant role in controlling the adsorption process.