Activated Carbon Based on Recycled Epoxy Boards and Their Adsorption toward Methyl Orange

With the swift progress of the electronics industry, discarded circuit boards have become an important source of non-degradable waste. In this work, discarded epoxy resin was collected as a precursor to prepare activated carbon (AC) through stepwise carbonization/activation methods. The rough carbon materials with a certain graphite and amorphous structure reveal the multiple oxygen-containing groups on their surface. In the process of studying the adsorption of methyl orange by activated carbon, it is found that the adsorption is in accordance with the quasi-secondary kinetic model, and equilibrium adsorption amounts can reach 41.051 mg/g. The adsorption isotherm of AC is more in line with the Langmuir model, and the saturation adsorption amount at three different temperatures is 23.137 mg/g, 30.358 mg/g, and 37.202 mg/g, respectively. The enthalpy (ΔH) is 17.30 KJ/mol in the adsorption process, which indicates that is a physical process with heat-absorbing capabilities. This work is of great significance with regard to the recycling of waste to reduce pollution and in terms of gaining economic benefits.


Introduction
With the rapid progress of electronic information and industrial technology, the performance requirements of electronic products are rapidly increasing, and the replacement frequency also has also risen sharply, which has led to the generation of a large amount of waste electrical and electronic equipment (WEEE) [1,2].However, the annual global production of WEEE is growing at an alarming rate of 20 to 25 million tons per year, of which printed circuit boards (PCBs), which are an important part of all electronic equipment, make up 4-7% of the total mass of WEEE and are composed mainly of thermosetting epoxy resins [3,4].Therefore, the recycling of epoxy resins has become a very important issue today.Thermosetting epoxy resins are three-dimensional crosslinked polymers with a complex chemical structure, which usually consists of epoxy groups and polyanhydrides containing aromatic or aliphatic structures, among others.This structure is difficult to be decomposed effectively in the recycling process and its insoluble and immiscible properties make it difficult to be recycled [5,6].Secondly, epoxy resins inherently have excellent properties, such as high heat resistance and chemical resistance, which also make the recycling of epoxy resins difficult during the recycling process.At present, the most important treatment methods are incineration and direct landfill; however, incineration produces polluting harmful gases, causing air pollution and jeopardizing human health [7].Direct landfill is not conducive to soil stabilization because PCBs are extremely difficult to degrade Therefore, residual WPCBs are recycled to prepare activated carbon.This is an effective way of recycling and reusing thermosetting epoxy resin.It not only reduces the deteriorating environmental problems but also expands the raw material sources for the production of activated carbon, bringing significant economic benefits.Herein, the waste thermosetting epoxy resin was used as a raw material to prepare activated carbon materials via a carbonization/activation process to study the adsorption behavior of class-activated carbon on methyl orange.In this experiment, we chose the temperatures of 600 • C and 800 • C for the carbonization and activation of the raw materials because the decomposition temperature of epoxy resin is usually between 300 • C and 500 • C, and because the epoxy resin can be decomposed efficiently and thoroughly at 600 • C and 800 • C.After carbonization, KOH was used as an activator to enable a more suitable activation process for the raw material.

Experimental Section 2.1. The Preparation of Activated Carbon
The waste strengthened epoxy resin boards coming from the garbage station were first pulverized into a powder with diameter less than 0.25 mm (60 mesh) using a pulverizer (XY-6008 Yongzhou Xiaobao Electrical Appliance Company Limited, Yongzhou, China).Then, 15 g of the powder was placed in a tube furnace and heated to a carbonization temperature of 600 • C at a ramp rate of 10 • C/min and calcined for 2.5 h; then, it was naturally cooled to room temperature under the protection of a N 2 atmosphere (0.5 L/min) to obtain the carbide.The carbide was mixed and ground with the chemical activator KOH at a ratio of 1:3 (wt/wt) in a mortar and was then heated to an activation temperature of 800 • C at a heat rate of 25 • C/min, kept at this temperature for 1 h, and naturally cooled to room temperature.Afterward, the solid particles in the crucible were taken out for grinding, making the crude product.
The crude product was dissolved in an appropriate amount of deionized water and titrated to a neutral pH with hydrochloric acid solution.Deionized water and ethanol were used to rinse and centrifuge it for 3 to 5 times, respectively, to remove residual impure ions such as K + and Cl − .The precipitate was then dried at 60 • C for 12 h and ground to obtain the desired activated carbon.
Since the obtained waste circuit boards were pure epoxy resins and different strengthened epoxy resin boards, the activated carbons obtained were named AC-1 AC-2, AC-3, and AC-4, respectively.AC-1 is a pure epoxy resin board, and AC-2, AC-3, and AC-4 are epoxy resin boards strengthened with polyethersulfone with successively increasing content.

Characterization
The morphology of as-prepared activated carbon was observed by a field-emission SEM instrument (Zeiss SIDMA-300, Jena, Germany) after sputter coating with gold.X-ray data were collected on a XRD (Shimadzu XRD-6000, Kyoto, Japan) with a scanning range of 5-60 • and a scanning speed of up to 10 degrees per minute.Fourier transform infrared spectra (FT-IR) were obtained using a Nicolet iS10 infrared spectrometer from Thermo Fisher Scientific, Waltham, MA, USA.The KBr method was used with a wavelength range of 400~4000 cm −1 .
The methyl orange solution was configured as 0, 20, 40, 60, 80, 100, 120, 140, 160 mg/L, and the standard curve was plotted using a Q6 UV-Vis spectrophotometer manufactured by Metash Instruments, Shanghai, China.The concentration was taken as the horizontal axis and absorbance as the vertical axis to plot the standard curve of methyl orange.The standard curve of methyl orange was plotted with the concentration as the horizontal axis and the absorbance as the vertical axis.
The amount of methyl orange adsorbed by the activated carbon sample was calculated with Equation (1): where C t (mg/L) is the concentration of methyl orange at time t, C 0 is the initial concentration of methyl orange (mg/L), m is the mass of activated carbon (g), V 0 is the volume of the solution (L), and q t is the amount of methyl orange adsorbed at time t (mg/g).

Structural Characterization of Activated Carbon
As can be seen in Figure 1, there are several peaks in the FT-IR spectrum.
where C (mg/L) is the concentration of methyl orange at time t, C is the initial co tration of methyl orange (mg/L), m is the mass of activated carbon (g), V is the v of the solution (L), and q is the amount of methyl orange adsorbed at time t (mg/g

Structural Characterization of Activated Carbon
As can be seen in Figure 1, there are several peaks in the FT-IR spectrum.The strongest peak at 3441 cm −1 corresponds to the O-H functional group, and are three other peaks with smaller intensities at 1101 cm −1 , 1384 cm −1 , and 1631 cm −1 , represent the C-O-C functional group of aryl ether, the C-O functional group of c ylate, and the C=O functional group, respectively.Some of these oxygen-containing tional groups are capable of hydrogen bonding or chemically reacting with methyl o dye.This helps to increase the affinity of the material for organic dyes and enhan adsorption.
The four prepared samples were subjected to X-ray diffraction (XRD) analysis alyze the crystal structures, and the results of the XRD tests are shown in Figure 2.  The strongest peak at 3441 cm −1 corresponds to the O-H functional group, and there are three other peaks with smaller intensities at 1101 cm −1 , 1384 cm −1 , and 1631 cm −1 , which represent the C-O-C functional group of aryl ether, the C-O functional group of carboxylate, and the C=O functional group, respectively.Some of these oxygen-containing functional groups are capable of hydrogen bonding or chemically reacting with methyl orange dye.This helps to increase the affinity of the material for organic dyes and enhance its adsorption.
The four prepared samples were subjected to X-ray diffraction (XRD) analysis to analyze the crystal structures, and the results of the XRD tests are shown in Figure 2.
where C (mg/L) is the concentration of methyl orange at time t, C is the initial tration of methyl orange (mg/L), m is the mass of activated carbon (g), V is the of the solution (L), and q is the amount of methyl orange adsorbed at time t (mg

Structural Characterization of Activated Carbon
As can be seen in Figure 1, there are several peaks in the FT-IR spectrum.The strongest peak at 3441 cm −1 corresponds to the O-H functional group, an are three other peaks with smaller intensities at 1101 cm −1 , 1384 cm −1 , and 1631 cm −1 represent the C-O-C functional group of aryl ether, the C-O functional group of ylate, and the C=O functional group, respectively.Some of these oxygen-containin tional groups are capable of hydrogen bonding or chemically reacting with methyl dye.This helps to increase the affinity of the material for organic dyes and enh adsorption.
The four prepared samples were subjected to X-ray diffraction (XRD) analysi alyze the crystal structures, and the results of the XRD tests are shown in Figure 2   As can be seen in Figure 2, two diffraction peaks appear on the XRD spectra of the four samples, including a strong diffraction peak at 2θ from 20 • to 30 • and a broader peak at 2θ = 43 • , which are diffraction peaks from the (002) and (101) crystal planes of the graphite structure, respectively.The (002) crystal plane is mainly due to the interconnection of the lamellar graphite layers and parallel stacking, and the (101) crystal plane shows that the samples contain a hexagonal honeycomb carbon structure.The appearance of the two facets suggests that some graphitic carbon was formed in the preparation process, which contributes to the improvement in the adsorption of the material.In addition, the intensity of the diffraction peaks corresponding to the (002) facet represents the degree of graphitization of the sample.It can be seen that the graphitization degree of AC-1 is the lowest and that the graphitization degrees of AC-2, AC-3, and AC-4 increase, which is because AC-1 has the richest surface pore structure and the most defects on the surface of the carbon atoms and it has the lowest degree of graphitization.Increasing the specific surface area of the material can provide more adsorption active sites and help to improve the adsorption properties of the material.
In the industry, the adsorption capacity of activated carbon on small molecule impurities can be reflected by the adsorption iodine value of activated carbon, and we mainly want to recycle the industrial waste circuit boards and obtain the activated carbon through high-temperature calcination for a secondary utilization.So, we carried out an iodine adsorption test on the prepared activated carbon samples, and the test results shown in Figure 3 indicate that our activated carbon had a better adsorption capacity for small molecule impurities.As can be seen in Figure 2, two diffraction peaks appear on the XRD spectra of the four samples, including a strong diffraction peak at 2θ from 20° to 30° and a broader peak at 2θ = 43°, which are diffraction peaks from the (002) and (101) crystal planes of the graphite structure, respectively.The (002) crystal plane is mainly due to the interconnection of the lamellar graphite layers and parallel stacking, and the (101) crystal plane shows that the samples contain a hexagonal honeycomb carbon structure.The appearance of the two facets suggests that some graphitic carbon was formed in the preparation process, which contributes to the improvement in the adsorption of the material.In addition, the intensity of the diffraction peaks corresponding to the (002) facet represents the degree of graphitization of the sample.It can be seen that the graphitization degree of AC-1 is the lowest and that the graphitization degrees of AC-2, AC-3, and AC-4 increase, which is because AC-1 has the richest surface pore structure and the most defects on the surface of the carbon atoms and it has the lowest degree of graphitization.Increasing the specific surface area of the material can provide more adsorption active sites and help to improve the adsorption properties of the material.
In the industry, the adsorption capacity of activated carbon on small molecule impurities can be reflected by the adsorption iodine value of activated carbon, and we mainly want to recycle the industrial waste circuit boards and obtain the activated carbon through high-temperature calcination for a secondary utilization.So, we carried out an iodine adsorption test on the prepared activated carbon samples, and the test results shown in Figure 3 indicate that our activated carbon had a better adsorption capacity for small molecule impurities.Furthermore, we can obviously find that the iodine adsorption value decreases gradually from sample AC-1 to AC-4, which indicates that the adsorption effect of sample AC-1 on small molecule impurities is better than in the other samples.
The surface morphology of the epoxy resin-based activated carbon was observed using SEM.From Figure 4a-d, it can be observed that samples AC-2, AC-3, and AC-4 have smooth polygonal irregular plate-like solid surfaces, while sample AC-1 has a rough irregular granular surface.Because polyethersulfone has good acid and alkaline resistance, even at high temperatures, the strengthened epoxy resin is difficult to be etched by potassium hydroxide at high temperatures, resulting in a smooth surface of AC-2, AC-3, and AC-4.Compared with the other samples, the rough surface of sample AC-1 increases the pore volume and improves the pore size structure, which is favorable for providing more adsorption sites to improve the adsorption performance of the sample.Furthermore, we can obviously find that the iodine adsorption value decreases gradually from sample AC-1 to AC-4, which indicates that the adsorption effect of sample AC-1 on small molecule impurities is better than in the other samples.
The surface morphology of the epoxy resin-based activated carbon was observed using SEM.From Figure 4a-d, it can be observed that samples AC-2, AC-3, and AC-4 have smooth polygonal irregular plate-like solid surfaces, while sample AC-1 has a rough irregular granular surface.Because polyethersulfone has good acid and alkaline resistance, even at high temperatures, the strengthened epoxy resin is difficult to be etched by potassium hydroxide at high temperatures, resulting in a smooth surface of AC-2, AC-3, and AC-4.Compared with the other samples, the rough surface of sample AC-1 increases the pore volume and improves the pore size structure, which is favorable for providing more adsorption sites to improve the adsorption performance of the sample.

Adsorption Behavior of Methyl Orange on Activated Carbon
The adsorption of methyl orange to activated carbon was explored.It is known tha the characteristic wavelength of the methyl orange solution is 460 nm.Thus, the methy orange solutions with concentrations of 0, 20, 40, 60, 80, 100, 120, 140, and 160 mg/L wer configured.A wavelength of 460 nm was adopted, and the standard curve of the methy orange solution in concentration/absorbance was plotted via linear fitting, as shown in Figure 5a.According to the standard curve, the concentration of methyl orange can b calculated from the absorbance of a solution, and then the adsorption amount can be cal culated.
The initial concentration of 100 mL methyl orange was 80 mg/L, and four 50 mg acti vated carbons were added, respectively, to determine the adsorption capacity of methy orange at different adsorption times, as shown in Figure 5b.At the initial stage of adsorption, the rate of adsorption increases and the absorption curve is steep.During the middle stage of the process of adsorption, the growth rate o adsorption slows down with time and gradually stabilizes during the late stages of th process of adsorption.This is because in the initial stage of adsorption, the adsorbent ha a high concentration, and the large concentration difference of the adsorbate between the methyl orange solution and the adsorbent produces a good transfer impetus, prompting

Adsorption Behavior of Methyl Orange on Activated Carbon
The adsorption of methyl orange to activated carbon was explored.It is known that the characteristic wavelength of the methyl orange solution is 460 nm.Thus, the methyl orange solutions with concentrations of 0, 20, 40, 60, 80, 100, 120, 140, and 160 mg/L were configured.A wavelength of 460 nm was adopted, and the standard curve of the methyl orange solution in concentration/absorbance was plotted via linear fitting, as shown in Figure 5a.According to the standard curve, the concentration of methyl orange can be calculated from the absorbance of a solution, and then the adsorption amount can be calculated.

Adsorption Behavior of Methyl Orange on Activated Carbon
The adsorption of methyl orange to activated carbon was explored.It is known tha the characteristic wavelength of the methyl orange solution is 460 nm.Thus, the methy orange solutions with concentrations of 0, 20, 40, 60, 80, 100, 120, 140, and 160 mg/L were configured.A wavelength of 460 nm was adopted, and the standard curve of the methy orange solution in concentration/absorbance was plotted via linear fitting, as shown in Figure 5a.According to the standard curve, the concentration of methyl orange can be calculated from the absorbance of a solution, and then the adsorption amount can be cal culated.
The initial concentration of 100 mL methyl orange was 80 mg/L, and four 50 mg acti vated carbons were added, respectively, to determine the adsorption capacity of methy orange at different adsorption times, as shown in Figure 5b.At the initial stage of adsorption, the rate of adsorption increases and the absorption curve is steep.During the middle stage of the process of adsorption, the growth rate o adsorption slows down with time and gradually stabilizes during the late stages of the process of adsorption.This is because in the initial stage of adsorption, the adsorbent ha a high concentration, and the large concentration difference of the adsorbate between the methyl orange solution and the adsorbent produces a good transfer impetus, prompting The initial concentration of 100 mL methyl orange was 80 mg/L, and four 50 mg activated carbons were added, respectively, to determine the adsorption capacity of methyl orange at different adsorption times, as shown in Figure 5b.
At the initial stage of adsorption, the rate of adsorption increases and the absorption curve is steep.During the middle stage of the process of adsorption, the growth rate of adsorption slows down with time and gradually stabilizes during the late stages of the process of adsorption.This is because in the initial stage of adsorption, the adsorbent has a Polymers 2024, 16, 1648 7 of 12 high concentration, and the large concentration difference of the adsorbate between the methyl orange solution and the adsorbent produces a good transfer impetus, prompting methyl orange to enter into the adsorption site rapidly.The concentration difference decreases and the continuous possession of the adsorption site by methyl orange results in a decreasing force in the solid-liquid push, with the adsorption rate continuing to decrease.When the adsorption sites on the surface of the activated carbon sample were saturated, the adsorption of methyl orange reached an equilibrium, with methyl orange no longer being adsorbed.
The adsorption behavior of methyl orange was studied using quasi-primary and quasi-secondary kinetic models.The fitted results are shown in Figure 6, and the kinetic parameters are presented in Table 1.
Polymers 2024, 16, x FOR PEER REVIEW 7 of 12 methyl orange to enter into the adsorption site rapidly.The concentration difference decreases and the continuous possession of the adsorption site by methyl orange results in a decreasing force in the solid-liquid push, with the adsorption rate continuing to decrease.When the adsorption sites on the surface of the activated carbon sample were saturated, the adsorption of methyl orange reached an equilibrium, with methyl orange no longer being adsorbed.
The adsorption behavior of methyl orange was studied using quasi-primary and quasi-secondary kinetic models.The fitted results are shown in Figure 6, and the kinetic parameters are presented in Table 1.In the whole adsorption process from 0 to 200 min, the R 2 values all are more than 0.90 (Table 1), which means that the adsorption process conforms to the quasi-primary kinetic model and quasi-secondary kinetic model.We can further find from Figure 7 that in the first 40 min of adsorption, the adsorption behavior is more consistent with the quasiprimary kinetic model, while the adsorption process after 40 min is more consistent with the quasi-secondary kinetic behavior.In the first 40 min of adsorption, there are more adsorption sites and a larger concentration difference of the adsorbate, with the adsorption mainly being affected by the diffusion process and having a faster adsorption process.However, after 40 min, most of the adsorption points on the adsorbent are occupied by the adsorbate, and some electrostatic adsorption process of sharing or transferring electron pairs between the adsorbent and the adsorbate occurs.According to the quasi-secondary kinetic model (Table 1), the equilibrium adsorption amounts (qe) from AC-1 to AC-4 are 41.051 mg/g, 31.556mg/g, 28.121 mg/g, and 23.607 mg/g, respectively.Due to the high-temperature stability as well as acid and alkali resistance of polyethersulfone, the anti-activation of epoxy resin is improved (which makes it difficult to form an effective porous structure in the KOH activation process) and its adsorption capacity is low.Therefore, AC-1, without a toughening agent, exhibits the best adsorption behavior.Table 1.The adsorption kinetic models' parameters for the adsorption process.

Samples Kinetic Model
Quasi-Secondary Kinetic Model R 2 q e mg•g −1 k 1 R 2 q e mg•g −1 k 2 × 10 In the whole adsorption process from 0 to 200 min, the R 2 values all are more than 0.90 (Table 1), which means that the adsorption process conforms to the quasi-primary kinetic model and quasi-secondary kinetic model.We can further find from Figure 7 that in the first 40 min of adsorption, the adsorption behavior is more consistent with the quasi-primary kinetic model, while the adsorption process after 40 min is more consistent with the quasisecondary kinetic behavior.In the first 40 min of adsorption, there are more adsorption sites and a larger concentration difference of the adsorbate, with the adsorption mainly being affected by the diffusion process and having a faster adsorption process.However, after 40 min, most of the adsorption points on the adsorbent are occupied by the adsorbate, and some electrostatic adsorption process of sharing or transferring electron pairs between the adsorbent and the adsorbate occurs.According to the quasi-secondary kinetic model (Table 1), the equilibrium adsorption amounts (q e ) from AC-1 to AC-4 are 41.051 mg/g, 31.556mg/g, 28.121 mg/g, and 23.607 mg/g, respectively.Due to the high-temperature stability as well as acid and alkali resistance of polyethersulfone, the anti-activation of epoxy resin is improved (which makes it difficult to form an effective porous structure in the KOH activation process) and its adsorption capacity is low.Therefore, AC-1, without a toughening agent, exhibits the best adsorption behavior.
on solid surfaces.For AC-1, due to it exhibiting the best adsorption behavior, the adsorption thermodynamic behavior of activated carbon was further studied.A series of 50 mL of methyl orange solution at concentrations of 20 mg/L, 40 mg/L, 60 mg/L, 80 mg/L, and 100 mg/L was put into different flasks, with 30 mg of activated carbon to AC-1 being added at each concentration.At constant temperatures of 30 °C, 40 °C, and 50 °C, the concentration of methyl orange in the solution was measured and the equilibrium adsorption amount was calculated.The adsorption isotherms of the samples are shown in Figure 7.As can be seen from Figure 7, the shape of the adsorption isotherm is an upwardly convex curve.The equilibrium concentration and equilibrium adsorption capacity of the AC-1 adsorption isotherm increased with the increase in the initial concentration of methyl orange in the solution.The larger slope of the adsorption isotherm at low concentrations indicates that adsorption is more likely to occur, which is attributed to the presence of many unoccupied adsorption active sites on the surface of AC-1.The slope of the adsorption isotherm slows down as the concentration of methyl orange increases and eventually tends to reach an equilibrium.Moreover, we also compared the adsorption performance of methyl orange with different materials (Table 2) and found that the activated carbon sample prepared from waste epoxy resin could basically reach the adsorption efficiency of commercial activated carbon in this experiment, and we found that the equilibrium adsorption amount also had a great advantage over other substances.Adsorption thermodynamics can be used to study the adsorption of gases or liquids on solid surfaces.For AC-1, due to it exhibiting the best adsorption behavior, the adsorption thermodynamic behavior of activated carbon was further studied.A series of 50 mL of methyl orange solution at concentrations of 20 mg/L, 40 mg/L, 60 mg/L, 80 mg/L, and 100 mg/L was put into different flasks, with 30 mg of activated carbon to AC-1 being added at each concentration.At constant temperatures of 30 • C, 40 • C, and 50 • C, the concentration of methyl orange in the solution was measured and the equilibrium adsorption amount was calculated.The adsorption isotherms of the samples are shown in Figure 7.
As can be seen from Figure 7, the shape of the adsorption isotherm is an upwardly convex curve.The equilibrium concentration and equilibrium adsorption capacity of the AC-1 adsorption isotherm increased with the increase in the initial concentration of methyl orange in the solution.The larger slope of the adsorption isotherm at low concentrations indicates that adsorption is more likely to occur, which is attributed to the presence of many unoccupied adsorption active sites on the surface of AC-1.The slope of the adsorption isotherm slows down as the concentration of methyl orange increases and eventually tends to reach an equilibrium.The most commonly used Langmuir and Freundlich isothermal adsorption models [30] were fitted for the isothermal adsorption of methyl orange using AC-1 at 30 • C, 40 • C, and 50 • C. The fitted results are shown in Figure 8 and Table 3.The most commonly used Langmuir and Freundlich isothermal adsorption models [30] were fitted for the isothermal adsorption of methyl orange using AC-1 at 30 °C, 40 °C, and 50 °C.The fitted results are shown in Figure 8 and Table 3.It can be seen that the correlation coefficients (R2) of both the Langmuir and Freundlich models are greater than 0.9, which represents that the fitting effects are significant.As shown in Table 3, the maximum adsorption amounts of ( m ) for methyl orange using AC-1 were 23.137 mg/g, 30.358 mg/g, and 37.202 mg/g at 30 °C, 40 °C, and 50 °C, respectively.Therefore, the effect of temperature on the adsorption amount exhibited a positive correlation, which indicates that heating can promote the adsorption process.It may be because the pore diameter of the sample surface increased and because the adsorption activity of the pore increased with the increase in temperature.From the nf of the Freundlich adsorption model, it was found that 1/n was equal to 0.83, 0.82, and 0.80-between 0 and 2-representing that absorption can easily occur.
The allocation factor (RL) is an important property of Langmuir isotherms for describing the affinity of an adsorbate to an adsorbent, and it can be calculated with Equation (2) [31], as shown in Figure 9a.
where C0 is the initial concentration before adsorption.
RL is favorable for adsorption when it is between 0 and 1, unfavorable for adsorption when the partition factor is greater than 1, and unfavorable for linear adsorption when the partition factor is equal to 1 [32].From Figure 9a, it can be seen that L is between 0 and 1, indicating that the adsorption of AC-1 on methyl orange is favorable.
The reversibility and spontaneity of the adsorption process can be evaluated by calculating the thermodynamic parameters, which include the Gibbs free energy (∆G 0 ), enthalpy value (∆H 0 ), and entropy value (∆S 0 ), which can be calculated with Equation (3) [31] and are fitted as shown in Figure 9b.It can be seen that the correlation coefficients (R 2 ) of both the Langmuir and Freundlich models are greater than 0.9, which represents that the fitting effects are significant.As shown in Table 3, the maximum adsorption amounts of (q m ) for methyl orange using AC-1 were 23.137 mg/g, 30.358 mg/g, and 37.202 mg/g at 30 • C, 40 • C, and 50 • C, respectively.Therefore, the effect of temperature on the adsorption amount exhibited a positive correlation, which indicates that heating can promote the adsorption process.It may be because the pore diameter of the sample surface increased and because the adsorption activity of the pore increased with the increase in temperature.From the n f of the Freundlich adsorption model, it was found that 1/n was equal to 0.83, 0.82, and 0.80-between 0 and 2-representing that absorption can easily occur.

T/K Langmuir
The allocation factor (R L ) is an important property of Langmuir isotherms for describing the affinity of an adsorbate to an adsorbent, and it can be calculated with Equation (2) [31], as shown in Figure 9a.R L is favorable for adsorption when it is between 0 and 1, unfavorable for adsorption when the partition factor is greater than 1, and unfavorable for linear adsorption when the partition factor is equal to 1 [32].From Figure 9a, it can be seen that R L is between 0 and 1, indicating that the adsorption of AC-1 on methyl orange is favorable.
The reversibility and spontaneity of the adsorption process can be evaluated by calculating the thermodynamic parameters, which include the Gibbs free energy (∆G 0 ), enthalpy value (∆H 0 ), and entropy value (∆S 0 ), which can be calculated with Equation (3) [31] and are fitted as shown in Figure 9b. Polymers where K L is Langmuir adsorption constant (L/mg), T is the absolute temperature (K), and R is the gas molar constant with a value of 8.314 J/(K•mol).
where C 0 is the initial concentration before adsorption.
18.09 43.54 4.85 4.57 3.98 The increase in randomness of the interface with increasing temperature also indicates that the adsorption process can easily occur.The temperature increases from 303K to 323 K and ∆G 0 decreases from 4.85 KJ/mol to 3.98 KJ/mol, indicating that temperature promotes the adsorption process.∆H 0 > 0 indicates that this adsorption process absorbs heat from the outside.In addition, it is known that the enthalpy of physical adsorption is less than 20 KJ/mol, the enthalpy of electrostatic adsorption is in the range of 20-80 KJ/mol, and the enthalpy of typical chemical adsorption is in the range of 80-450 KJ/mol [33].The enthalpy of the adsorption here is 18.09 KJ/mol, so the adsorption in this experiment is mainly physical adsorption.

Conclusions
In this work, activated carbon with good adsorption properties was prepared from waste epoxy resin boards from used circuit boards with a simple preparation process comprising stepwise carbonization and activation.The prepared activated carbon had rough and porous surface morphology and displayed rich information pertaining its chemical functional groups.The adsorption characteristics and mechanisms of activated carbon on the methyl orange solution were investigated via adsorption thermodynamic and kinetic analyses, which showed that activated carbon had a good adsorption capacity for methyl orange, especially for sample AC-1.The adsorption process is both a physical adsorption and an endothermic process.The preparation of activated carbon with an excellent adsorption effect through the recycling of waste circuit boards not only greatly solves the The curves were plotted with T as the horizontal axis and ∆G 0 as the vertical axis, and they were fitted linearly, as shown in Figure 9b.The slope of the fitted straight line is −∆S 0 , the intercept is ∆H 0 , and the related parameters are shown in Table 4.As shown in Table 4, ∆S 0 is positive, which means that methyl orange is widely distributed on the surface of AC-1.
The increase in randomness of the interface with increasing temperature also indicates that the adsorption process can easily occur.The temperature increases from 303 K to 323 K and ∆G 0 decreases from 4.85 KJ/mol to 3.98 KJ/mol, indicating that temperature promotes the adsorption process.∆H 0 > 0 indicates that this adsorption process absorbs heat from the outside.In addition, it is known that the enthalpy of physical adsorption is less than 20 KJ/mol, the enthalpy of electrostatic adsorption is in the range of 20-80 KJ/mol, and the enthalpy of typical chemical adsorption is in the range of 80-450 KJ/mol [33].The enthalpy of the adsorption here is 18.09 KJ/mol, so the adsorption in this experiment is mainly physical adsorption.

Conclusions
In this work, activated carbon with good adsorption properties was prepared from waste epoxy resin boards from used circuit boards with a simple preparation process comprising stepwise carbonization and activation.The prepared activated carbon had rough and porous surface morphology and displayed rich information pertaining its chemical functional groups.The adsorption characteristics and mechanisms of activated carbon on the methyl orange solution were investigated via adsorption thermodynamic and kinetic analyses, which showed that activated carbon had a good adsorption capacity for methyl orange, especially for sample AC-1.The adsorption process is both a physical

Figure 6 .
Figure 6.(a) Linear fitting using a quasi-primary kinetic model.(b) Linear fitting using a quasisecondary kinetic model of AC-1.

Figure 6 .
Figure 6.(a) Linear fitting using a quasi-primary kinetic model.(b) Linear fitting using a quasisecondary kinetic model of AC-1.

Figure 9 .
Figure 9. (a) Variation of the partition factor with equilibrium concentration.(b) Variation of the Gibbs free energy with temperature.

Figure 9 .
Figure 9. (a) Variation of the partition factor with equilibrium concentration.(b) Variation of the Gibbs free energy with temperature.

Table 1 .
The adsorption kinetic models' parameters for the adsorption process.

Table 2 .
Adsorption parameters for kinetic modeling of different materials' adsorption.

Table 3 .
Fitting parameters of isothermal adsorption model of AC-1.