Removal of Copper(II) from Aqueous Environment Using Silk Sericin-Derived Carbon

Sericin is a by-product of the silk industry. Its recycling contributes to environmental protection and the sustainable development of the cocoon silk industry. In this paper, on the basis of realizing sericin enrichment in solution, the Cu(II) adsorption capacities of sericin-derived carbon (SC), prepared at different pyrolysis temperatures, were studied. SC was characterized using scanning electron microscopy (SEM) and the zeta potential. The effects of the initial concentration of Cu(II), pH, adsorption temperature, and contact time on the adsorption process were evaluated, followed by an investigation of the mechanism of Cu(II) adsorption by SC. The results showed that SC has a porous structure that provides sites for Cu(II) adsorption. The maximum adsorption capacity of Cu(II) onto SC1050, 17.97 mg/g, was obtained at an adsorption temperature of 35 °C and a pH of 5.5. In addition, the pseudo-second-order kinetic model and Langmuir isotherm model correctly described the adsorption process of Cu(II) onto SC1050. Therefore, SC can act as a potential adsorbent for removing Cu(II) from water. This study helps promote the effective use of cocoon silk resources.


Introduction
Industries such as the electronics, brass manufacturing, fertilizer, pesticide, and textile industries are major sources of copper pollution in water [1][2][3]. Due to the nonbiodegradability, persistence, extreme toxicity, and accumulation effect of Cu(II) [4], drainage of wastewaters without effective treatment can cause not only serious contamination of water resources, but also severe damage to the health of aquatic life, and from there to human beings. Relevant studies show that Cu(II) accumulation limits the growth and development of plants and causes a reduction in the fecundity of aquatic life [5,6]. Moreover, an enrichment of trace copper(II) in organisms can be achieved through the food web, leading to excessive intake of copper by people, which can evoke diarrhea, hepatic and renal dysfunctions, and severe mental and neurological illnesses [5,[7][8][9]. Numerous approaches and techniques, including adsorption [10], membrane separation [11], electrocoagulation [12], chemical precipitation [13], and microbial treatment [14], have been used to remove copper ions from water. Compared with other technologies, adsorption has the advantages of simple operation, high efficiency, and relatively low cost, with a wide range of adsorbents [2,[15][16][17][18][19].
Silk sericin (SS) is a highly hydrophilic macromolecular protein ranging from 20 and 400 kDa in molecular weight and composed of 18 amino acids [20]. It is a glue-like protein that coats silk fibroin fibers in raw silk offering protective and adhesive effects to the silk fibroin, and constituting 25-30% of the cocoon silk fibers [21]. To improve the luster, softness, and dyeability of silk fabrics, SS is traditionally discarded by a degumming Figure 1 shows the SEM images of freeze-dried hydrogel and the derived carbon pyrolyzed at different temperatures. As shown in Figure 1a, the SS/CMCS hydrogel surface exhibited an irregular, reticular, and porous structure, possibly due to intermolecular crosslinking among the CMCS and SS molecules [25]. Figure 1b-e shows that a higher pyrolysis temperature could lead to a more developed pore structure. As the pyrolysis temperature increased from 450 • C to 1050 • C, the pore size on the surface of SC450, SC650, SC850, and SC1050 gradually shrank and the pore structure became densely arranged, which was consistent with a previous report that the carbon framework continued to shrink as the temperature of heat treatment gradually increased in [26]. Table 1 shows that the BET surface area of each sample gradually increased with the carbonization temperature, providing a larger contact area and more adsorption sites, thus improving the adsorption capacity of SC [27]. process, resulting in much protein loss, which can further cause water contamination an eutrophication [22].
Several studies have worked on using discarded SS to remove heavy metal ions fro water. Dusi et al. [23] reported the use of sericin-alginate particles to remove Cu(II) io from aqueous solutions, and their maximum adsorption capacity was 87.27 mg/g. Sing et al. [24] showed that prepared chitosan-sericin conjugates could effectively remo Cr(VI) ions from water, with a maximum adsorption capacity of 139 mg/g. However, t above works did not discuss enriching SS from the aqueous environment. Furthermo because of its hydrophilicity, SS from prepared composites can re-enter the water durin the adsorption process.
Our previous work [25] reported that SS can be cross-linked with carboxymeth chitosan (CMCS) through hydrogen bonding and then electrodeposited with CMCS on anode to form composite hydrogel under low voltage. We proved that SS was the ma component of SS/CMCS hydrogel, accounting for 77.12% of the hydrogel total mass.
Here, a prepared SS/CMCS hydrogel was pyrolyzed at 450 °C, 650 °C, 850 °C, an 1050 °C. Then, the fabricated hydrogel was applied to Cu(II) adsorption in water. T pyrolysis process effectively avoided the re-entry of SS into water during the adsorptio Figure 1 shows the SEM images of freeze-dried hydrogel and the derived carb pyrolyzed at different temperatures. As shown in Figure 1a, the SS/CMCS hydrog surface exhibited an irregular, reticular, and porous structure, possibly due intermolecular crosslinking among the CMCS and SS molecules [25]. Figure 1b-e show that a higher pyrolysis temperature could lead to a more developed pore structure. As t pyrolysis temperature increased from 450 °C to 1050 °C, the pore size on the surface SC450, SC650, SC850, and SC1050 gradually shrank and the pore structure becam densely arranged, which was consistent with a previous report that the carbon framewo continued to shrink as the temperature of heat treatment gradually increased in [26]. Tab 1 shows that the BET surface area of each sample gradually increased with t carbonization temperature, providing a larger contact area and more adsorption site thus improving the adsorption capacity of SC [27].   The influence of Cu(II) initial concentration on the adsorption capacity is shown in Figure 2. Increasing Cu(II) initial concentration enhanced the adsorption capacity of Cu(II) on all SC samples. With the Cu(II) initial concentration increasing from 5 mg/L to 40 mg/L, the adsorption capacity of Cu(II) on SC450, SC650, SC850, and SC1050 increased gradually. This result was due to the increase in Cu(II) initial concentration enhancing the driving force of adsorption processes, thus enhancing the removal of Cu(II) ions from the solution [28,29].

Effect of Initial Concentration
The influence of Cu(II) initial concentration on the adsorption capacity is show Figure 2. Increasing Cu(II) initial concentration enhanced the adsorption capacity of C on all SC samples. With the Cu(II) initial concentration increasing from 5 mg/L to 40 m the adsorption capacity of Cu(II) on SC450, SC650, SC850, and SC1050 increa gradually. This result was due to the increase in Cu(II) initial concentration enhancing driving force of adsorption processes, thus enhancing the removal of Cu(II) ions from solution [28,29].
On the other hand, SC had a higher absorption capacity when pyrolyzed at 105 instead of 450 °C, 650 °C, and 850 °C. At a Cu(II) solution concentration of 40 mg/L equilibrium adsorption amount of SC450, SC650, and SC850 for Cu(II) was 11.71 m 13.80 mg/g, and 15.06 mg/g, respectively. Increasing the pyrolysis temperature to 105 increased the adsorption amount of Cu(II) on SC1050 to 20.08 mg/g. SEM and BET res showed that the SC prepared at 1050 °C had a denser pore structure, which led to a la SC1050 surface area, provided more adsorption sites, and thus increased the adsorp capacity of Cu(II) on SC1050 [30]. Therefore, SC1050 was used as the adsorben subsequent experiments.

Effect of pH
The initial solution pH is crucial for the metal ion adsorption capacity of an adsor because it can affect not only the surface charge and ionization degree of the adsor but also the aqueous solution chemistry of metal ions [31]. Since turbidity (suspension On the other hand, SC had a higher absorption capacity when pyrolyzed at 1050 • C instead of 450 • C, 650 • C, and 850 • C. At a Cu(II) solution concentration of 40 mg/L, the equilibrium adsorption amount of SC450, SC650, and SC850 for Cu(II) was 11.71 mg/g, 13.80 mg/g, and 15.06 mg/g, respectively. Increasing the pyrolysis temperature to 1050 • C increased the adsorption amount of Cu(II) on SC1050 to 20.08 mg/g. SEM and BET results showed that the SC prepared at 1050 • C had a denser pore structure, which led to a larger SC1050 surface area, provided more adsorption sites, and thus increased the adsorption capacity of Cu(II) on SC1050 [30]. Therefore, SC1050 was used as the adsorbent in subsequent experiments.

Effect of pH
The initial solution pH is crucial for the metal ion adsorption capacity of an adsorbent because it can affect not only the surface charge and ionization degree of the adsorbent but also the aqueous solution chemistry of metal ions [31]. Since turbidity (suspensions of copper hydroxide) could be clearly observed in Cu(II) solution when the solution pH was adjusted to 6 [32], a pH range of 4.5-5.5 was selected to explore the influence of pH on Cu(II) adsorption. Figure 3 shows that SC1050 exhibited the lowest Cu(II) ion adsorption capacities when the solution pH was 4.5. This result could be attributed to a large amount of H + ions in the solution competing with Cu 2+ ions for the active adsorption sites, resulting in low adsorption capacity of Cu(II) ions [33].
copper hydroxide) could be clearly observed in Cu(II) solution when the solution pH adjusted to 6 [32], a pH range of 4.5-5.5 was selected to explore the influence of pH Cu(II) adsorption. Figure 3 shows that SC1050 exhibited the lowest Cu(II) ion adsorp capacities when the solution pH was 4.5. This result could be attributed to a large am of H + ions in the solution competing with Cu 2+ ions for the active adsorption sites, resu in low adsorption capacity of Cu(II) ions [33].
In addition, the zeta potential of SC1050 at pH values of 4.5, 5.0, and 5.5 measured to explore its adsorption mechanism. Figure 3 shows that a solution pH o made the surface charge of SC1050 more negative, increasing the ionic strength betw SC1050 and the positively charged Cu(II) ions, which made the adsorption more effec Therefore, a pH of 5.5 was selected for subsequent adsorption experiments.

Effect of Temperature on the Sorption of Cu(II) Ions and a Thermodynamic Stu
The influence of temperature on the sorption of Cu(II) is shown in Figure 4 increase in temperature was found to enhance the sorption of Cu(II) onto SC1050. enhancement might have been due to the higher temperature providing sufficient en for copper ions to bind to the surface and interior of SC1050 [2].
The thermodynamic fitting curve is shown in Figure 4b, and the thermodyn parameters are reported in Table 2. According to Table 2, the negative ΔG value incre with increasing temperature, showing that the adsorption process was more favorab higher temperatures and implying that the adsorption process was chemisorption [9 addition, the positive ΔH value indicated that adsorption was endothermic. Moreove the value of ΔS was positive, the probability of a successful collision between Cu(II) SC1050 might increase, which could strengthen the Cu(II) adsorption [33]. In addition, the zeta potential of SC1050 at pH values of 4.5, 5.0, and 5.5 was measured to explore its adsorption mechanism. Figure 3 shows that a solution pH of 5.5 made the surface charge of SC1050 more negative, increasing the ionic strength between SC1050 and the positively charged Cu(II) ions, which made the adsorption more effective. Therefore, a pH of 5.5 was selected for subsequent adsorption experiments.

Effect of Temperature on the Sorption of Cu(II) Ions and a Thermodynamic Study
The influence of temperature on the sorption of Cu(II) is shown in Figure 4. An increase in temperature was found to enhance the sorption of Cu(II) onto SC1050. This enhancement might have been due to the higher temperature providing sufficient energy for copper ions to bind to the surface and interior of SC1050 [2].
The thermodynamic fitting curve is shown in Figure 4b, and the thermodynamic parameters are reported in Table 2. According to Table 2, the negative ∆G value increased with increasing temperature, showing that the adsorption process was more favorable at higher temperatures and implying that the adsorption process was chemisorption [9]. In addition, the positive ∆H value indicated that adsorption was endothermic. Moreover, as the value of ∆S was positive, the probability of a successful collision between Cu(II) and SC1050 might increase, which could strengthen the Cu(II) adsorption [33].  As shown in Figure 5, the adsorption amount of Cu(II) on SC1050 varied with contact time and could be divided into three stages: the adsorption rate was fast within the initial 30 min, it gradually slowed down from 30 min to 6 h, and the adsorption reached equilibrium at 6 h. The explanation was that in the early stage of the adsorption process, many adsorption sites on the SC1050 surface were rapidly occupied by Cu(II) ions, resulting in a fast adsorption rate [34]. However, the adsorption sites progressively became saturated over time and reached equilibrium. To explore the adsorption process kinetics, pseudo-first-order and pseudo-secondorder kinetic models were used to fit the experimental results. The fitting curve and the calculated parameters are reported in Figure 5 and Table 3, respectively. The adsorption appeared to fit better to a pseudo-second-order kinetic model, indicating that the

Effect of Contact Time on the Adsorption Process and the Adsorption Kinetics
As shown in Figure 5, the adsorption amount of Cu(II) on SC1050 varied with contact time and could be divided into three stages: the adsorption rate was fast within the initial 30 min, it gradually slowed down from 30 min to 6 h, and the adsorption reached equilibrium at 6 h. The explanation was that in the early stage of the adsorption process, many adsorption sites on the SC1050 surface were rapidly occupied by Cu(II) ions, resulting in a fast adsorption rate [34]. However, the adsorption sites progressively became saturated over time and reached equilibrium.   As shown in Figure 5, the adsorption amount of Cu(II) on SC1050 varied with contact time and could be divided into three stages: the adsorption rate was fast within the initial 30 min, it gradually slowed down from 30 min to 6 h, and the adsorption reached equilibrium at 6 h. The explanation was that in the early stage of the adsorption process, many adsorption sites on the SC1050 surface were rapidly occupied by Cu(II) ions, resulting in a fast adsorption rate [34]. However, the adsorption sites progressively became saturated over time and reached equilibrium. To explore the adsorption process kinetics, pseudo-first-order and pseudo-secondorder kinetic models were used to fit the experimental results. The fitting curve and the calculated parameters are reported in Figure 5 and Table 3, respectively. The adsorption appeared to fit better to a pseudo-second-order kinetic model, indicating that the To explore the adsorption process kinetics, pseudo-first-order and pseudo-secondorder kinetic models were used to fit the experimental results. The fitting curve and the calculated parameters are reported in Figure 5 and Table 3, respectively. The adsorption appeared to fit better to a pseudo-second-order kinetic model, indicating that the adsorption process of SC1050 was mainly controlled by chemisorption, which involved the sharing or exchange of electrons between Cu(II) and SC1050 [35]. Table 3. Pseudo-first-order, pseudo-second-order, and IPD models of Cu(II) adsorption onto SC1050. Furthermore, the kinetics and rate-limiting step of the adsorption were elucidated using the intra-particle diffusion (IPD) model. The IPD fitting curve of Cu(II) adsorption and the values of the rate constant are reported in Figure 6 and Table 4, respectively. Figure 6 shows that the plot exhibited a multi-linearity, in accordance with two stages: the first linear portion was due to the diffusion of Cu(II) through the solution to the external surface of SC1050 and the second linear portion described intra-particle diffusion [36]. The rate constant of the first stage was greater than that of the second stage (Table 4), indicating the initial adsorption was rapid surface adsorption controlled by film diffusion, and intra-particle diffusion controlled the adsorption process [35]. Additionally, the fitting plot did not intersect the origin, indicating that intra-particle diffusion was not the only rate-limiting step [33]. adsorption process of SC1050 was mainly controlled by chemisorption, which involved the sharing or exchange of electrons between Cu(II) and SC1050 [35]. Furthermore, the kinetics and rate-limiting step of the adsorption were elucidated using the intra-particle diffusion (IPD) model. The IPD fitting curve of Cu(II) adsorption and the values of the rate constant are reported in Figure 6 and Table 4, respectively Figure 6 shows that the plot exhibited a multi-linearity, in accordance with two stages: the first linear portion was due to the diffusion of Cu(II) through the solution to the external surface of SC1050 and the second linear portion described intra-particle diffusion [36]. The rate constant of the first stage was greater than that of the second stage (Table 4) indicating the initial adsorption was rapid surface adsorption controlled by film diffusion and intra-particle diffusion controlled the adsorption process [35]. Additionally, the fitting plot did not intersect the origin, indicating that intra-particle diffusion was not the only rate-limiting step [33].  Adsorption isotherms are used to describe the relationship between the adsorbate and adsorbent at equilibrium [37]. The fitting plots and fitting parameters of Cu(II) adsorption onto SC1050 are shown in Figure 7 and Table 5, respectively, which reveal that the Langmuir model could better describe the process of Cu(II) adsorption onto SC1050 at 35 °C, indicating monolayer adsorption of Cu(II) ions onto SC1050. Furthermore, Figure  7d shows that the RL values calculated by the Langmuir model were greater than 0 but

Adsorption Isotherm
Adsorption isotherms are used to describe the relationship between the adsorbate and adsorbent at equilibrium [37]. The fitting plots and fitting parameters of Cu(II) adsorption onto SC1050 are shown in Figure 7 and Table 5, respectively, which reveal that the Langmuir model could better describe the process of Cu(II) adsorption onto SC1050 at 35 • C, indicating monolayer adsorption of Cu(II) ions onto SC1050. Furthermore, Figure 7d shows that the R L values calculated by the Langmuir model were greater than 0 but less than 1 for all initial concentrations, meaning that the adsorption process of Cu(II) onto SC1050 was favorable [1].

Determination of Cu on SC Using ICP-OES
SC1050 was placed in 15 mg/L of Cu(II) solution for 24 h (experimental group). Then, ICP-OES was used to determine the Cu on SC1050. According to Table 6, an enrichment of Cu on SC1050 was observed, compared with the control group (no immersion in Cu solution).

Determination of Cu on SC Using ICP-OES
SC1050 was placed in 15 mg/L of Cu(II) solution for 24 h (experimental group). Then, ICP-OES was used to determine the Cu on SC1050. According to Table 6, an enrichment of Cu on SC1050 was observed, compared with the control group (no immersion in Cu solution).

Fabrication of the SS/CMCS Hydrogel
A CMCS solution with a concentration of 1% (w/v) was prepared and adjusted to pH = 12 with 5 M NaOH to ensure that the CMCS could be completely dissolved [38]. Then, SS powder was dissolved in deionized water to prepare an SS solution with a concentration of 8% (w/v). The SS and CMCS solutions were mixed in equal volumes at room temperature, and the resulting solution was adjusted to pH = 7.5 with 0.1 M NaOH. After magnetic stirring for 4 h, the SS/CMCS solution was stored at 4 • C overnight for later use.
A three-electrode assembly was immersed in 20 mL of the SS/CMCS mixture containing 5 mg of NaCl as the electrolyte, and the SS/CMCS hydrogel was fabricated using an electrochemical workstation (CHI760E, Shanghai Chenhua Instruments Co., Ltd., Shanghai, China), according to the method described in our previous work [25]. The hydrogel generated on the working electrode was transferred to deionized water and rinsed for 15 min to remove any residue. This rinsing process was repeated three times.

Preparation of Sericin-Based Carbon
The prepared SS/CMCS hydrogel was placed in a −20 • C refrigerator for 4 h and then transferred to a −80 • C refrigerator for preservation. After 8 h, the frozen SS/CMCS hydrogel was freeze-dried using a lyophilizer (LGJ-10, YuMing Instrument Co., Ltd., Shanghai, China), and then the freeze-dried hydrogel was pyrolyzed. Adopting a carbonization procedure, similar to that of our previously reported work [25], we placed the sample in a tube furnace (OTF-1200X-S, Hefei KeJing Materials Technology Co., Ltd., Hefei, China) and maintained it for 120 min after the temperature reached 450 • C, 650 • C, 850 • C, and 1050 • C, then cooled it to ambient temperature. Subsequently, the sample was rinsed with deionized water at 60 • C every 30 min until the pH of the rinse water was neutral, and, then, the sample was freeze-dried to remove water. Finally, the SC was obtained. For simplicity, SCs obtained at different pyrolysis temperatures were labeled as SC450, SC650, SC850, and SC1050, respectively, and stored in desiccators for later use.

SC Characterization
The relationships between the mass and temperature change in the freeze-dried SS solution, CMCS hydrogel, and SS/CMCS hydrogel were investigated using a thermogravimetric analyzer (STA 449 F3 Jupiter, Netzsch, Germany). The samples were heated from 30 • C to 800 • C at a heating rate of 10 • C/min under a constant N 2 flow. The micromorphology of the SS/CMCS hydrogel and its derived carbon prepared at different temperatures was observed using scanning electron microscopy (SEM, TM4000Plus, Tokyo, Japan). The specific surface area was estimated by the Brunauer-Emmett-Teller (BET) method through a N 2 adsorption-desorption test at 77 K (ASAP 2460, Micromeritics, Norcross, GA, USA). The zeta potential of SC1050 under different pH values was measured by a Zetasizer Nano ZS90 (Malvern Instruments, Worcestershire, UK).

Batch Adsorption Experiment
The effects of Cu(II) initial concentration, solution pH, adsorption temperature, and contact time on Cu(II) removal were investigated. A certain mass of copper nitrate was dissolved in deionized water to obtain a Cu(II) stock solution of 1000 mg/L, which was then diluted to the corresponding concentration in subsequent experimental use.

Effect of Cu(II) Initial Concentration
The 1000 mg/L Cu(II) stock solution was diluted into 6 groups with initial concentrations of 5, 10, 15, 20, 30, and 40 mg/L, and the initial solution pH was adjusted to 5.0. The ratio of the SC sample mass to the Cu(II) solution volume in the weighing bottle was 0.5 g/L. The weighing bottles were placed in a shaking incubator (LYGZ-2102C, JTLIANGYOU Instrument Co., Ltd., Jiangsu, China) at a speed of 100 rpm at 25 • C for 24 h. The filtrate was obtained after filtration through a 0.22 µm filter membrane (Jiangsu Green Union Science Instrument Co., Ltd., Jiangsu, China). The concentration of Cu(II) in the filtrate after adsorption was determined according to Chinese Standard HJ 486-2009 (water quality determination of copper-2,9-dimethyl-1,10-phenanthroline spectrophotometric method) [39]. Briefly, the Cu(II) ion was reduced by hydroxylamine hydrochloride to its cuprous form, which, in turn, was reacted with 2,9-dimethyl-1,10-phenanthroline to form a yellow complex. This complex was then measured via an ultraviolet-visible (UV-Vis) spectrophotometer (UV1900, Shanghai Jinghua Technology Instrument Co., Ltd., Shanghai, China) at 457 nm. To ensure experimental accuracy, all experiments were performed in triplicate.
The equilibrium adsorption amount q e (mg/g) and the percentage removal R (%) were calculated by Equations (1) and (2), respectively [40]: where C 0 and C e are the concentrations (mg/L) of Cu(II) in solution at the initial and equilibrium states, M is the SC weight (g), and V is the volume of Cu(II) solution (mL).

Effect of Initial pH
Cu(II) solution with a concentration of 15 mg/L was selected, and the initial solution pH was adjusted to 4.5, 5.0, and 5.5. The mass-to-volume ratio of the SC sample and Cu(II) solution in the weighing bottles was maintained at 0.5 g/L. The weighing bottles were placed in a shaking incubator at a speed of 100 rpm at 25 • C for 24 h. After filtration, the Cu(II) solution concentrations were determined according to Chinese Standard HJ 486-2009 [39].

Effect of Adsorption Temperature and a Thermodynamic Study
Cu(II) solution with a concentration of 15 mg/L was selected, and the initial solution pH was adjusted to 5.5. The mass-to-volume ratio of the SC sample and Cu(II) solutions in the weighing bottle was maintained at 0.5 g/L. The weighing bottles were placed in a shaking incubator with a rotational speed of 100 rpm for 24 h at different temperatures (15 • C, 25 • C, and 35 • C). After filtration, the Cu(II) solution concentrations were determined according to Chinese Standard HJ 486-2009 [39].
The Gibbs free energy change (∆G, kJ/mol), the enthalpy change (∆H, kJ/mol), and the entropy change (∆S, J/mol·K) were calculated by the van't Hoff equation to investigate the thermodynamic behavior of Cu(II) adsorption onto SC1050. The equations are shown in (3)-(5) [9]: where T is the absolute temperature (K), and R is the gas constant (8.314 J/mol·K).

Effect of Contact Time and Adsorption Kinetics
Cu(II) solution with a concentration of 15 mg/L was selected, and the initial solution pH was adjusted to 5.5. The mass-to-volume ratio of the SC sample and Cu(II) solution in the weighing bottle was maintained at 0.5 g/L. The weighing bottles were placed in a shaking incubator at 35 • C with a rotational speed of 100 rpm to oscillate for 5 min, 10 min, 30 min, 60 min, 90 min, 120 min, 240 min, 360 min, 720 min, and 1440 min. After filtration, the Cu(II) solution concentrations were determined according to Chinese Standard HJ 486-2009 [39].
The amount of Cu(II) adsorbed at time t, q t (mg/g), can be calculated through Equation (6): To explore the kinetics of the adsorption process of Cu(II) onto SC1050, pseudo-firstorder kinetics, pseudo-second-order kinetics, and the intra-particle diffusion model were selected to fit the experimental data The pseudo-first-order kinetic model is shown in Equation (7) [27]: where k 1 is the pseudo-first-order adsorption rate constant (L/min), and t is the adsorption time (min). The pseudo-second-order kinetic model is shown in Equation (8) [35]: where k 2 is the pseudo-second-order rate constant (g·mg −1 ·min −1 ) and t is the adsorption time (min). The intra-particle diffusion (IPD) model is shown in Equation (9) [35]: where k i is the diffusion coefficient of the particle (mg·g −1 ·min −1/2 ), and C is a constant. A larger C indicates a stronger effect of the boundary layer on the adsorption process [33].

Adsorption Isotherm
To investigate the maximum adsorption capacity of the adsorbent, Cu(II) stock solution of 1000 mg/L was diluted to 5, 10, 15, 20, 30, and 40 mg/L. The initial solution pH was then adjusted to 5.5. The mass-to-volume ratio of the SC sample and Cu(II) solution in the weighing bottle was maintained at 0.5 g/L. The weighing bottles were then placed in a shaking incubator operating at a rotational speed of 100 rpm at 35 • C for 24 h. After filtration, the Cu(II) solution concentrations were determined according to Chinese Standard HJ 486-2009 [39].
Langmuir and Freundlich isotherm models were applied to modeling the adsorption isotherms. The Langmuir isotherm model for monolayer adsorption on homogeneous sites is described by Equation (10) [2]: where q m is the maximum adsorption amount at equilibrium (mg/g), and k L is the Langmuir isotherm constant associated with the adsorption free energy (L/mg). The dimensionless separation factor R L is the basic characteristic of a Langmuir isotherm and can be used to determine the favorability of an adsorption process. It is calculated by Equation (11) [1]: where k L is the Langmuir isotherm constant. The adsorption is favorable when 0 < R L < 1, unfavorable when R L > 1, and irreversible when R L = 0. The Freundlich isotherm model for multilayer adsorption on heterogeneous sites is expressed as Equation (12) [2]: where n is the Freundlich constant related to surface heterogeneity.

Adsorption of Cu(II) Determined by Inductively Coupled Plasma Optical Emission Spectrometry
The sample was carefully weighed and then mixed with 5 mL of HNO 3 , 1 mL of HF, 1 mL of H 2 O 2 , and 1 mL of HCl. The mixture was placed in an autoclave at 180 • C for 8 h. Then, the reactor was cooled to room temperature. Finally, the mixture inside was transferred into 25 mL volumetric flasks and made up to the volume for inductively coupled plasma optical emission spectrometry (ICP-OES; Agilent 730, Agilent Technologies, Santa Clara, CA, USA) analysis.

Data Analysis
All adsorption experiments were repeated 3 times. Microsoft Excel 2019 (Redmond, WA, USA) and Origin software (Origin Pro 8.0, OriginLab Corp., Northampton, MA, USA) were used for data analysis and graphing.

Conclusions
In this study, the adsorption capacity of carbon derived from SS for Cu(II) was investigated. An SS/CMCS hydrogel with porosity was fabricated by electrodeposition. The SC samples were further fabricated by pyrolysis at 450 • C, 650 • C, 850 • C, and 1050 • C. SEM results indicated that SC1050 had a more developed pore structure, which increased the surface area and provided more adsorption sites for Cu(II) adsorption.
The results of batch adsorption experiments showed that SC1050 had the highest adsorption capacity and removal rate of Cu(II), and the optimum condition for Cu(II) adsorption by SC1050 was T = 35 • C and pH = 5.5. The adsorption of Cu(II) by SC1050 was a spontaneous, endothermic process. The fitted Cu(II) adsorption results agreed with the pseudo-second-order kinetic model and the Langmuir isotherm. The maximum adsorption amount of Cu(II) on SC1050 was 17.97 mg/g. Compared with pistachio green hull-derived carbon (19.84 mg/g) [41], apple tree branch-derived carbon (11.41 mg/g) [33], and Chaenomeles Sinensis seed-derived carbon (15.78 mg/g) [27], SC1050 showed a better Cu(II) adsorption capacity. Therefore, SC1050 can act as an effective adsorbent for removing Cu(II) from aqueous solution.