Preparation and adsorption properties of magnetic chitosan/sludge biochar composites for removal of Cu2+ ions

The magnetic chitosan/sludge biochar composite adsorbent was prepared using chitosan, Fe3O4, and sludge biochar as raw materials. The composite adsorbent was able to achieve rapid solid–liquid separation under an applied magnetic field. The morphology and microstructure of the composite adsorbent were characterized by FTIR, XRD, SEM, VSM, and BET analysis. The adsorption performance of the composite adsorbent on Cu2+ was investigated through static adsorption experiments, and the effects of adsorbent dosage, initial concentration of Cu2+, initial pH of the solution, and adsorption temperature on the adsorption efficiency of Cu2+ were discussed. The results showed that chitosan and Fe3O4 were successfully loaded on sludge biochar. When the initial concentration of Cu2+ was 30 mg/L, the dosage of the magnetic chitosan/sludge biochar composite material was 0.05 g, the adsorption time was 180 min, pH was 5, and the temperature was room temperature, the maximum removal rate of Cu2+ reached 99.77%, and the maximum adsorption capacity was 55.16 mg/g. The adsorption kinetics and adsorption isotherm data fitted well with the pseudo-second-order kinetic model and Langmuir adsorption isotherm model, indicating that the adsorption process was chemisorption with monolayer coverage.


Reagents and materials
All chemical reagents were of analytical grade, and the solutions used in this study were prepared with deionized water.Chitosan was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd; Iron oxide was purchased from Beijing Nan Shang Le Chemical Factory; Glutaraldehyde 50%, sodium hydroxide and hydrochloric acid were purchased from Tianjin Fuchen Chemical Reagent Factory; Acetic acid and ammonia solution were purchased from Tianjin Beilian Fine Chemical Development Co., Ltd; 1,3-dicyclohexyl-2-methylidenecarboxamide was purchased from Xi'an Chemical Reagent Factory; Anhydrous ethanol and anhydrous copper sulfate were purchased from Tianjin Damao Chemical Reagent Factory; Triethylenetetramine citrate was purchased from Shanghai Sihewi Chemical Co., Ltd.

Preparation method of adsorbent
Preparation of Sewage Sludge Biochar (SBC) Sludge biochar (SBC) was prepared from the remaining sludge of a sewage treatment plant in Yining City.The sludge was first placed under ventilated conditions to dry naturally drying.It was then thoroughly ground and screened through an 80-mesh sieve size for later use.Next, an appropriate amount of dried sludge was removed from the pretreated sludge and placed in a crucible.The crucible was sealed with tinfoil and placed in a muffle furnace for pyrolysis by thermal cracking.The pyrolysis process consisted of seven steps: 20 °C to 200 °C, heating time 1 h, holding time 1 h at 200 °C; 200 °C to 300 °C, heating time 1 h, holding time 1 h at 300 °C; 300 °C to 400 °C heating time 1 h, holding time 1 h at 400 °C; 400 °C to 500 °C, heating time 1 h, holding time 1 h at 500 °C; 500 °C to 300 °C, cooling time half an hour; 300 °C to 200 °C, cooling time half an hour; and 200 °C to 25 °C, cooling time half an hour.This process produced the desired sewage sludge biochar.In order to remove the excess inorganic substances and ash content, the sewage sludge biochar was treated by acid washing with dilute hydrochloric acid (biochar: dilute hydrochloric acid) in the ratio of 1:10.After stirring on a magnetic stirrer for 1 h, the mixture was filtered and washed with water until neutral.Finally, the biochar was dried and reserved for further use.
Preparation of magnetic chitosan (Fe 3 O 4 @CTS/SBC) 0.5 g of chitosan powder was weighed using an analytical balance and added to 20 mL of acetic acid at 2% volume fraction.The mixture was stirred thoroughly at room temperature with a magnetic stirrer until chitosan was completely dissolved to obtain chitosan acetic acid solution.Added 0.2 g Fe 3 O 4 and stirred well, ultrasonicated for 30 min and dispersed evenly.Added 3 mL of 25% glutaraldehyde and placed the reaction mixture in a 50 °C water bath for cross-linking reaction for 1 h.After completion of the cross-linking reaction, the pH was adjusted to 8 by gradually dropping NaOH solution.Further cross-linking reaction was carried out by continuing heating in a water bath at 65 °C for one hour.Finally, it was washed with distilled water until the pH reached 7. The precipitate was collected and transferred to a vacuum drying oven at 65 °C until reached a constant weight.This will yield magnetic chitosan.
Preparation of magnetic chitosan/sludge biochar composite material 0.5 g of chitosan was weighed and dissolved in 250 mL of aqueous acetic acid solution with a volume fraction of 1%.The solution was stirred at room temperature until completely dissolved.The pH was adjusted to 6 with a NaOH solution of 20% by mass.Slowly poured the chitosan solution into 0.5 g of sludge biochar and placed it in a water bath at 60 °C with stirring for 4 h.After 4 h, slowly added 0.5 g of Fe 3 O 4 magnetic fluid (prepared by dispersing 0.5 g of Fe 3 O 4 particles in 100 mL of distilled water) to the mixture.The reaction was continued for 1 h.The resulting product was washed with distilled water until the supernatant was neutral.The product was placed in a vacuum drying oven at 65 °C and dried thoroughly.The dried product was thoroughly ground and stored for future use.

Determination of Cu 2+ ions
This method is an improved double-ring ketone oxime (BCO) spectrophotometric method based on the dual aldehyde carbonyl oxime spectrophotometric method of China's "Standard Test Method for Drinking Water" (GB/T5750-2006) 11,12 .In this method, aliquots of 0.0 mL, 0.1 mL, 0.2 mL, 0.4 mL, 0.8 mL, 1.2 mL, and 2.0 mL of the copper standard solution were transferred into seven 10 mL colorimetric tubes using a pipette.The color reagent was added sequentially according to the dual acetylacetone oxime (BCO) spectrophotometric method and then distilled water was added at the calibration line.The mixture was thoroughly mixed and allowed to stand for 15 min.The absorbance was measured using a visible spectrophotometer at a wavelength of 600 nm and a 5 cm cuvette with distilled water as reference.The Cu 2+ standard curve was obtained by taking the measured absorbance as the vertical coordinate and the concentration of Cu 2+ standard solution as the horizontal coordinate, from which the concentration of Cu 2+ in the solution could be calculated.

Characterization of SBC and Fe 3 O 4 @CTS/SBC
Powdered sludge biochar (SBC) and magnetic chitosan-modified sludge biochar (Fe 3 O 4 @CTS/SBC) composites were characterized by X-ray diffractometer.The scanning angles for both samples were in the range of 10°-80° with a step size of 0.02° and a scanning speed of 10°/min.Fourier transform infrared (FTIR) spectroscopy was carried out on the samples of SBC, Fe 3 O 4 @CTS, Fe 3 O 4 @CTS/SBC in the range of wave numbers from500 cm −1 to 4000 cm −1 .The morphology and structure of the samples were observed using a scanning electron microscope after dispersing the samples in ethanol solution at an accelerating voltage of 3 kV.The magnetic properties of the composite adsorbent were investigated by measuring the hysteresis return line of Fe 3 O 4 @CTS/SBC using a vibrating sample magnetometer (VSM).The specific surface area and pore size analysis of the samples were performed under normal temperature and pressure conditions.The samples weighed 0.3 g and were subjected to N 2 gas adsorption-desorption at a desorption temperature of 200 °C for approximately 6 h.The p/p 0 values were in the range of 0.05 and 0.99, and BET calculations were used to determine the specific surface area.The pore size distribution of mesoporous materials was analyzed using the BJH model.

Single factor influence experiment
In this study, the effects of composite adsorbent dosage, initial concentration of Cu 2+ , pH and adsorption temperature on the adsorption effect were investigated.The fixed wastewater volume was 30 mL, the temperature of the shaking table was 25 °C, the rotational speed was 180r/min, and the adsorption time was 180 min, and www.nature.com/scientificreports/ the adsorption time was static.The supernatant was filtered with a 0.45um filter tip, and 2 mL of the filtrate was placed in a 10 mL colorimetric tube, and equal amounts of tri-ammonium citrate, pH 9 buffer, and BCO solution were added with the production standard.The absorbance was measured at 600 nm by visible spectrophotometer to determine the optimum conditions for adsorption of Cu 2+ .

Analysis and calculation
The adsorption performance of the composites was evaluated using modified chitosan/sludge biochar as adsorbent.The effects of initial concentration of Cr (VI) ions, pH, temperature and adsorbent dosage on the adsorption performance of heavy metal ions were determined by batch tests.The adsorption rate (R) and adsorption capacity (q) were calculated using Eqs.(1) and (2).
where R is the adsorption rate, expressed as a percentage (%); Q is the adsorption capacity in mg/g; C 0 represents the initial mass concentration of Cu 2+ ions in the simulated wastewater in mg/L; C e is the mass concentration of the equilibrium state of the Cu 2+ ions after adsorption in mg/L; V represents the volume of simulated wastewater in mL; and m represents the mass of the adsorbent in grams (g).

Adsorption kinetics research
Adsorption kinetics models are used to investigate the mechanism of adsorption processes at the solid-liquid interface 13 .Commonly used kinetic models include pseudo-first-order adsorption rate equation 14 , pseudosecond-order adsorption rate equation 15 , and Weber-Morris intra-particle diffusion model 16 , which are used to explore the interaction mechanism between the adsorbent and the adsorbate.Pseudo-first-order kinetic model equation: where q e and q t represent the adsorption amounts at equilibrium and at time t, respectively, in mg/g; q e,1 is the theoretical equilibrium adsorbed amount calculated from the pseudo-first-order kinetic rate equation in mg/g; k 1 is the rate constant of the pseudo-first-order adsorption in min −1 ; plotting ln (q e − q t ) against t, the slope and intercept can be used to calculate k 1 and q e,1 .Pseudo-second-order kinetic model equation: where q t represents the adsorption amount at time t in mg/g; q e,2 represents the theoretical equilibrium adsorption amount calculated from the pseudo-second-order kinetic rate equation in mg/g; k 2 is the rate constant of the pseudo-second-order adsorption in g/(mg•min).Plotting t on the x-axis and t/q t on the y-axis, the slope and intercept can be used to calculate q e,2 and k 2 .Weber-Morris intra-particle diffusion model: where q t represents the adsorption amount at time t in mg/g; k p represents the intra-particle diffusion constant in mg/(g•min 1/2) ; C represents a constant related to the boundary layer thickness.Plotting t 1/2 on the x-axis and qt on the y-axis, the slope and intercept can be used to calculate k p and C. If the fitted line passes through the origin, it indicates that the adsorption rate is solely affected by intra-particle diffusion.If the line does not pass through the origin, it suggests that intra-particle diffusion is not the only rate-controlling step in the adsorption process.Boyd first-order dynamics model.For intraparticle diffusion, Boyd 17 derived the intraparticle diffusion model (Boyd pseudo first model).Boyd's intraparticle diffusion model is based on Fick's second law of diffusion, which describes the change in particle concentration during diffusion.The model involves factors such as diffusion coefficients, diffusion rates and boundary conditions.According to different assumptions and boundary conditions, different forms of intraparticle diffusion models can be obtained.Boyd's intraparticle diffusion model provides a quantitative analytical tool for studying the diffusion behavior of particles.It can help us understand and predict the concentration distribution, rate, and interaction with the environment and boundary conditions of particles during internal diffusion.The intraparticle diffusion model can be derived by solving the diffusion partial differential equation and making assumptions according to different situations, and its linearization formula is as follows: where k i represents the apparent adsorption rate parameter. (1) + t/q e (5) www.nature.com/scientificreports/

Adsorption isotherm model
The adsorption equilibrium relationship between the adsorbent and the adsorbate was described using the Langmuir isotherm model and the Freundlich model 18 .The Langmuir isotherm model is suitable for the monolayer adsorption with uniformly distributed energy adsorption sites, and the adsorption model is as follows 19 : where q e represents the equilibrium adsorption capacity in mg/g; q m represents the maximum adsorption capacity in mg/g; c e represents the equilibrium concentration in mg/L; k L represents the adsorption rate constant in L/mg.The Freundlich isotherm model is suitable for the multilayer adsorption on adsorbents with heterogeneous physicochemical properties.The expression for the Freundlich isotherm model is as follows 20 : where q e represents the equilibrium adsorption capacity in mg/g; n is a constant related to the adsorption intensity; c e represents the equilibrium concentration in mg/L; k F represents the Freundlich adsorption rate constant in mg/g•(L/mg) (−1/n) .
The Temkin isotherm is used to describe the chemisorption process based on electrostatic force of positive and negative charges, and its expression is as follows: where q e represents the equilibrium adsorption capacity in mg/g; c e represents the equilibrium concentration in mg/L; kt is Temkin's constant; b is the heat-related adsorption constant.

Adsorption thermodynamics study
Adsorption thermodynamics focuses on the conversion relationship between macroscopic thermal phenomena and other forms of energy 21 .The spontaneity of the adsorption process can be determined by the Gibbs free energy change (ΔG).When ΔG < 0, the adsorption process occurs spontaneously.When ΔG > 0, it is a nonspontaneous.Whether the adsorption process is exothermic or endothermic can be determined by the enthalpy change (ΔH).When ΔH < 0, the adsorption is exothermic in nature.On the contrary, it is heat-absorbing in nature.The Gibbs free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS) are calculated by Eqs. ( 10) and (11).
where R represents the gas constant, 8.314 J/mol/k; T is the Kelvin temperature, K.

Characterization of Adsorbent Materials
XRD analysis XRD, also known as X-ray diffraction, is a technique used to analyze the crystal structure of different chemical substances.A unique x-ray diffraction pattern is produced by x-ray diffraction of the sample.In this study, SBC, Fe 3 O 4 @CTS and Fe 3 O 4 @CTS/SBC were analyzed by x-ray diffraction.The results are shown in Fig. 1.

FT-IR analysis
FT-IR spectroscopy is used to study the structure of a sample and to determine the type of functional groups present.When a sample is exposed to infrared radiation of different wavelengths, it absorbs specific wavelengths, resulting in an infrared absorption spectrum of the substance 22 .The FT-IR spectra of SBC, Fe 3 O 4 @CTS, and Fe 3 O 4 @CTS/SBC composite adsorbents are shown in Fig. 2.
Sample SBC showed peaks at 874 cm −1 and 1456 cm −1 , corresponding to the aromatic C-H and -COOH groups, respectively, which demonstrated that sludge biochar contains abundant oxygenated functional groups 23 .Moreover, sample Fe 3 O 4 @CTS/SBC also showed a peak at 887 cm −1 , which indicated that the sludge biochar was formed by successful complexation with magnetic chitosan.Samples Fe 3 O 4 @CTS and Fe 3 O 4 @CTS/SBC showed peaks at 1382 cm −1 , which could be attributed to the C-H deformation vibrations of chitosan's -CH 3 group.The bending vibration absorption peak of -NH 2 usually occurs in the range of 1650-1560 cm −1 , and this functional group appeared at 1582 cm −1 in the infrared spectra of Fe 3 O 4 @CTS and Fe 3 O 4 @CTS/SBC, which indicated the presence of -NH 2 bending vibration absorption peak in magnetic chitosan 24 , which confirmed the existence of chitosan in the composites.Sample Fe 3 O 4 @CTS exhibited a characteristic absorption peak of Schiff base (-C=N-) at 1653 cm −1 , which indicated the participation of glutaraldehyde in cross-linking reaction.Both sample Fe 3 O 4 @ CTS (1653 cm −1 ) and sample Fe 3 O 4 @CTS/SBC (1657 cm −1 ) showed characteristic absorption peaks corresponding to the stretching vibrations of the amide C=O group in Fe 3 O 4 @CTS, which indicated the presence of chitosan in the composites 25 .Peaks associated with the stretching vibration absorption of Fe-O appeared at 658 cm −1 in the spectra of Fe 3 O 4 @CTS and Fe 3 O 4 @CTS/SBC 26 , which indicated the successful formation of Fe 3 O 4 on chitosan and the magnetic properties of Fe 3 O 4 @CTS/SBC.

SEM analysis
The Scanning Electron Microscope (SEM) is commonly used for observing the surface morphology of materials.In order to gain a more intuitive and clear understanding of the microstructure of SBC and Fe 3 O 4 @CTS/SBC composite adsorbents, SEM analysis was performed on each sample.Figure 3a,b respectively showed the SEM images of SBC magnified 10,000 times and 20,000 times, while Fig. 4a,b respectively showed the SEM images of Fe 3 O 4 @CTS/SBC composite adsorbent magnified 10,000 times and 20,000 times.

Specific surface area and pore size analysis
There are six common types of adsorption isotherms, as shown in Fig. 5. Type I is the most representative Langmuir isotherm, which is often observed in microporous materials.Type II is an S-shaped isotherm, mainly occurring on non-porous or macroporous solid surfaces.Type III is convex downward throughout the entire  pressure range, with no inflection point.Type IV is similar to Type II, but in the low P/P 0 region, the curve is convex upward.Unlike Type III, Type V is characterized by a convex shape towards the relative pressure axis, and the presence of an inflection point at higher relative pressures.Type VI is well known for its stepped adsorption process, where the stepped shape is attributed to the sequential multilayer adsorption on non-porous surfaces.
The N 2 adsorption-desorption isotherms provide further insight into the properties of the adsorbent materials.In this study, the N 2 adsorption-desorption isotherms of SBC and Fe 3 O 4 @ CTS/SBC composite adsorbents were shown in Fig. 6.Table 1 showed the specific surface area and pore volume of Fe 3 O 4 @CTS/SBC and SBC.www.nature.com/scientificreports/ The pore size distribution of SBC and Fe 3 O 4 @CTS/SBC composite adsorbents could be observed from Fig. 7, which mainly lied between 2 to 50 nm, indicating that they are mesoporous materials.The figure also showed that the pore volume of the composite adsorbent is larger than that of sludge biochar.

Magnetic performance analysis
The magnetization intensity is an important factor for the solid-liquid separation after the adsorbent has completed the adsorption process.To study the magnetic properties of Fe 3 O 4 @CTS/SBC composites, the magnetic hysteresis line of Fe 3 O 4 @CTS/SBC was measured at room temperature, and the result were shown in Fig. 8a: The magnetic hysteresis line of Fe 3 O 4 @CTS/SBC composites showed an "S"-shaped curve.During the magnetization process, the magnetization intensity of the composites increased with the increased of the applied magnetic field   strength and ultimately reached saturation.Fe 3 O 4 @CTS/SBC composites exhibited magnetism with a saturation magnetization intensity of 1.71 emu/g.Figure 8b showed that the composites had large coercivity of 25 Oe, which indicated strong ferromagnetism.The composites could be separated from the aqueous solution by applying an external magnetic field.

Single factor impact experiment
(1) The effect of Fe 3 O 4 @CTS/SBC composites dosage on Cu 2+ adsorption efficiency: The experimental results were shown in Fig. 9.
According to Fig. 9, the adsorption efficiency of Fe 3 O 4 @CTS/SBC composites increased with the increased in the dosage of Fe 3 O 4 @CTS/SBC composites.However, when the dosage exceeded 0.03 g, the increased in adsorption efficiency became slower.
(2) The effect of initial Cu 2+ concentration on the adsorption performance: The experimental results were shown in Fig. 10.
From Fig. 10, the adsorption capacity of Fe 3 O 4 @CTS/SBC composites for Cu 2+ increased with the increased of Cu 2+ initial concentration.When the initial concentration of Cu 2+ was lower than 30 mg/L, the adsorption efficiency showed an increasing trend and remained above 90%.The maximum value was reached when the initial concentration of Cu 2+ was 30 mg/L.
(3) The effect of solution pH on adsorption efficiency of Cu 2+ ions: The experimental results were shown in Fig. 11.
The pH of the solution can affect the surface charge of the adsorbent and the existence state of Cu 2+ ions, thereby influencing the adsorption of Cu 2+27 .From the figure, it could be observed that the adsorption rate of Fe 3 O 4 @CTS/SBC composites Cu 2+ in solution increased with the increased pH in the solution.The adsorption rate and adsorption capacity reached the maximum value at pH 5 and then began to decline.
(4) The effect of temperature on the adsorption efficiency of Cu 2+ ions: The experimental results were shown in Fig. 12.  www.nature.com/scientificreports/From the figure, the adsorption efficiency of magnetic chitosan/sludge biochar was stable at about 90% with increasing temperature.The adsorption capacity also remained relatively stable.This indicated that the variation in temperature has little effect on the adsorption efficiency of the composites.

Adsorption kinetics research
When the adsorption conditions were T = 298 K, pH = 5, Cu 2+ concentration of 30 mg/L in 30 mL solution, and 0.5 g of Fe 3 O 4 @CTS/SBC composites, the experimental data were fitted by using pseudo-first-order, pseudosecond-order kinetic models, intraparticle diffusion model and Boyd first-order dynamics model.The specific results were shown in Fig. 13a-d.
For a copper sulfate solution with an initial concentration of 30 mg/L, the adsorption rate of Cu 2+ was very fast within 100 min and reached adsorption equilibrium around 200 min.Each gram of Fe 3 O 4 @CTS/SBC composites could adsorb 17.9713 mg of Cu 2+ , at which point the equilibrium concentration of Cu 2+ in solution was 0.17 mg/L.The adsorption kinetics of Cu 2+ were shown in Fig. 13, and the fitting results were shown in Table 2. From Fig. 13 and Table 2, it could be observed that Qt/T showed a good linear relationship with T, and the R 2 value of 0.9982.The saturated adsorption capacity was like to the experimental results, and the pseudo-secondorder kinetics better was more in line with the adsorption of Cu 2+ by the Fe 3 O 4 @CTS/SBC composite adsorbent.
The intraparticle diffusion model was fitted with t 0.5 as the x-coordinate and Q t as the y-coordinate.The fitting results were shown in Fig. 13c.The line in the figure did not pass through the origin, indicating that intra-particle diffusion was not the only step controlling the adsorption rate.Therefore, the adsorption of Cu 2+ on Fe 3 O 4 @ CTS/SBC composite adsorbent could be divided into three stages.According to the fitting data in Table 2, the adsorption rate slowed down with increasing adsorption time.
The adsorption capacity of copper ions by 0.5 g composites combined with the linearized adsorption kinetics model (Eq.( 6)), linearized fitted adsorption kinetic model could be obtained respectively (Fig. 13d, and the relevant parameters of the fitted line were shown in Table 2.

Adsorption isotherm research
The adsorption isotherms of Fe 3 O 4 @CTS/SBC composite adsorbent were investigated for Cu 2+ adsorption at constant temperature.The experiments were carried out at 25 °C, 35 °C, and 45 °C, respectively.The Langmuir model, Freundlich model and Temkin model were used to fit the adsorption process.The initial concentration of Cu 2+ was chosen to be in the range of 10-170 mg/L.Other adsorption conditions included pH 5, 0.5 g Fe 3 O 4 @ CTS/SBC composite adsorbent.The adsorption isotherms of Cu 2+ were shown in Fig. 14a-c, and the fitted results were presented in Table 3.
The results showed that although the equilibrium concentration of Cu 2+ (C e ) increased with the initial concentration, the equilibrium adsorption capacity (Q e ) of the adsorbent also increased.Linear regression analyses were performed on the isotherm data, and Fig. 14a,b showed the linear relationships between C e /Q e and C e , as well as lnQ e and lnC e .In Fig. 14a, Q m and K L could be calculated, and the correlation coefficients R 2 at different temperatures were 0.8288, 0.9735, and 0.9719, respectively, which were all closer to 1 than the Freundlich model.In this study, the Langmuir model was more suitable for the adsorption of Cu 2+ by the Fe 3 O 4 @ CTS/SBC composite adsorbent.The average saturation adsorption capacities at the three temperatures were 65.89 mg/g, 70.60 mg/g, and 79.49 mg/g, which were very close to the calculated maximum adsorption capacities of 55.16 mg/g, 61.65 mg/g, and 71.07 mg/g, respectively.In addition, the Freundlich isotherm was fitted with n greater than 1, indicating that the adsorption process was easy to carry out.The Temkin isotherm model was well fitted, which indicated that the presence of electrostatic force for the adsorption of Cu 2+ by Fe 3 O 4 @CTS/SBC.www.nature.com/scientificreports/

Adsorption thermodynamics research
The slope and intercept of the straight line obtained by lnk plotting 1/T gives ΔH and ΔS, and then ΔG.The results were shown in Table 4.
As shown in Table 4, ΔG < 0, ΔH > 0 of Fe 3 O 4 @CTS/SBC adsorbent for Cu 2+ indicated that the adsorption process was a spontaneous endothermic reaction.ΔS was greater than 0, which indicated that the adsorption of Cu 2+ by the adsorbent was a process of entropy increased, and the chaos of the solid-liquid interface increased.

Instrumental analysis
In the XRD analysis of this study, the six diffraction peaks of Fe 3 O 4 @CTS corresponding to (2θ = 30.1°,35.5°, 43.1°, 53.5°, 57.0°, and 62.6°) by comparing them with the PDF standard card of Fe 3 O 4 represent (220), (311), (400), ( 422), (511), and (440) respectively of the crystal surface 28,29 .This indicated that magnetite has been successfully loaded onto the chitosan surface and magnetic chitosan has been successfully prepared.From the XRD spectrum of Fe 3 O 4 @CTS/SBC, it could be seen that the Fe 3 O 4 @CTS/SBC composites also exhibited the same diffraction peaks as Fe 3 O 4 @CTS material.Additionally, the characteristic peaks of Fe 3 O 4 remained at 37.1° and 66.9°, which indicated that Fe 3 O 4 has been loaded onto the SBC.Furthermore, weak peaks were observed at 25.9° and 28.2°, which correspond to the diffraction peaks of chitosan (CTS) on the standard card, which indicated that CTS has been loaded onto SBC.This demonstrated the successful preparation of the Fe 3 O 4 @CTS/SBC composites.The XRD spectrum of the Fe 3 O 4 @CTS /SBC composites retained the main diffraction peaks of SBC, but the intensity was changed.This was due to the loading of CTS and Fe 3 O 4 onto the surface of sludge biochar.
From the SEM analysis of this study, from Fig. 3, it could be observed that sludge biochar (SBC) had a rough wrinkled surface appearance and an abundant pore structure.This is due to the pore structure formed during the pyrolysis process in the tubular furnace, which gave SBC a certain adsorption capacity. Figure 4 showed that the morphology of the composites changed significantly compared to SBC.The Fe 3 O 4 @CTS/SBC had a rough and porous surface with fewer macropores and a predominantly mesoporous structure.Visible white particles of magnetic nanoparticles were attached to the surface of SBC, which were adhered to the surface of SBC through chitosan.This indicated that Fe 3 O 4 had been successfully loaded onto the surface of the biochar.These particles did not appear to be significantly aggregated, which indicated that the Fe 3 O 4 particles could be uniformly dispersed on the biochar surface.The biochar could serve as an effective carrier, which was more conducive to adsorption.
The analysis of the adsorption-desorption isotherms using inert N 2 gas on SBC and Fe 3 O 4 @CTS/SBC adsorbents indicated that both exhibited Type IV adsorption isotherms, which were typical for mesoporous materials.Additionally, a H 2 -type hysteresis return lines were observed.When P/P 0 < 0.4, SBC showed monolayer adsorption of N 2 , and the inflection point was followed by the end of monolayer adsorption and the beginning of multilayer adsorption.It could also be observed from the figure that the adsorption capacity of SBC on N 2 was low, while the adsorption capacity of Fe 3 O 4 @CTS/SBC composite adsorbent was high.This indicated that the specific surface area of the composite adsorbent had increased.
According to Table 1, the specific surface area of Fe 3 O 4 @CTS/SBC was 3.52 times larger than that of SBC, and the pore volume was increased by 2.17 times.The improvement in specific surface area was highly beneficial for enhancing the adsorption capacity of Fe 3 O 4 @CTS/SBC.Furthermore, the introduction of chitosan introduced active functional groups such as -NH 2 and -OH on the surface of the composite adsorbent, which further enhanced the interaction between the adsorbent and heavy metal ions.

Effect of parameters
From Fig. 9, with the increase in the dosage of the composites, the number of active groups bound to Cu 2+ also increased, leading to an increase in the adsorption rate.However, when the dosage exceeded 0.03 g, the concentration of Cu 2+ remained constant and the number of active groups involved in binding Cu 2+ reached a critical value, resulting in a less significant increase in the adsorption efficiency 30 .At the same time, it could be observed from the figure that the adsorption capacity of Cu 2+ gradually decreased with the increased in the dosage of the material.This was because when the dosage of the adsorbent was low, the adsorption capacity of Fe 3 O 4 @CTS/SBC composites for Cu 2+ quickly reached saturation.However, continuing to increase the dosage leaded to more vacant binding sites on the surface of the adsorbent due to the fixed amount of adsorbate, which leaded to a decrease in the adsorption capacity.Based on the above analysis, 0.05 g was selected as the optimal dosage in this study.
As can be seen from Fig. 10, the adsorption capacity of Fe 3 O 4 @CTS/SBC composites for Cu 2+ increased with the increased in the initial concentration of Cu 2+ .This was because as the initial concentration of Cu 2+ increased, there were more adsorbates, resulting in a larger concentration difference between the solid and liquid phases, which provided a driving force for adsorption.In addition, with the increased of adsorbate quantity, the driving force increased, leading to an increase in the attraction between the adsorbate and adsorbent, which increased the adsorption capacity 31 .The adsorption efficiency showed an increasing trend when the Cu 2+ initial concentration was lower than 30 mg/L, and it remained above 90%.The maximum point was reached at the initial concentration of Cu 2+ of 30 mg/L.However, when the initial concentration of Cu 2+ exceeded 30 mg/L, the adsorption efficiency of the material gradually decreased.This was because at lower Cu 2+ concentrations, Fe 3 O 4 @CTS/SBC composites provided more adsorption sites, which resulted in better adsorption efficiency.However, with the increased of Cu 2+ content in the solution, the adsorption sites on the surface of Fe 3 O 4 @CTS/SBC composites gradually decreased during the adsorption process at a certain adsorbent addition amount.When the adsorption sites of the composites reached saturation, it no longer adsorbed Cu 2+ , but the initial concentration of Cu 2+ continued to increase.Therefore, the adsorption efficiency of the magnetic chitosan/sludge biochar composites decreased with the increased of the initial concentration of Cu 2+ .In summary, the initial concentration of Cu 2+ of 30 mg/L should be selected for subsequent experiments.From Fig. 11, under acidic conditions, the amino groups of chitosan in the Fe 3 O 4 @CTS/SBC composites mainly existed in the form of -NH 3 + , which repelled the positively charged divalent Cu 2+ ions 32 .Meanwhile, under acidic conditions, the concentration of H + in water significantly increased.H + competed for the adsorption sites of the adsorbent with Cu 2+ , which weakened the complexing ability of the composites towards Cu 2+ ions 33 .Therefore, under acidic conditions, the concentration of H + in water gradually decreased with the increase of pH, which leaded to the increase of adsorption efficiency.When the pH was greater than 6, Cu 2+ ions in the copper solution easily formed precipitates in the form of Cu (OH) 2 , which leaded to a decrease in the adsorption efficiency.When the pH approached 7, chitosan was prone to coagulation, making it difficult to adsorb copper ions and resulting in a decrease in adsorption efficiency 34 .Comparative experiments showed that when the pH of the solution was less than 6, Cu 2+ was evenly distributed in the solution, and the magnetic composite material had a good adsorption performance for copper ions.When the pH was greater than 7, Cu (OH) 2 precipitation would be formed, which interfered with the adsorption process and the adsorption performance began to decline.Therefore, based on the experimental data, the optimum pH for the adsorption of Cu 2+ by the magnetic chitosan/ sludge biochar composites was 5.
Comparative experiments showed that under the optimum conditions (initial concentration of Cu 2+ = 30 mg/L, dosage of adsorbent = 0.05 g, solution pH 5, temperature = 25 °C), the removal of Cu 2+ by Fe 3 O 4 @CTS/SBC was 99.77%.The maximum adsorption capacity was 55.16 mg/g, while the removal rates of Fe 3 O 4 @CTS and SBC for Cu 2+ were 80.63% and 77.54%, respectively, which was much lower than the adsorption performance of the composites for copper ions, which indicated that the materials prepared in this experiment had better performance.

Adsorption kinetic analysis
The adsorption kinetics analysis indicated that the steps controlling the adsorption rate may be limited by chemisorption, which involved the sharing or exchanging electrons between the adsorbent and the adsorbate 35 .The driving force of this process depended on the square of the adsorption sites 31 , which could explain the rapid adsorption rate at the initial stage of adsorption.However, in the later stage of adsorption, the reduction of adsorption sites on the Fe 3 O 4 @CTS/SBC composite adsorbent and the decreased in Cu 2+ concentration leaded to a decrease in the chemical driving force, which resulted in a slower adsorption rate 36 .
The slopes of the three segments in the broken line were the particle diffusion rate constants k p,1 > k p,2 > k p,3 .The adsorption process of Cu 2+ on Fe 3 O 4 @CTS/SBC composite adsorbent could be described as follows: in the first stage of adsorption, Cu 2+ combines with the adsorptive active sites of the functional groups on the surface of the Fe 3 O 4 @CTS/SBC composite adsorbent, and the initial adsorption rate was fast 37 .When the number of surface-active sites gradually decreased, it entered the second stage of adsorption.The adsorption sites on the surface of the Fe 3 O 4 @CTS/SBC composite adsorbent were mainly occupied by Cu 2+ , which began to enter the micropores of the composite adsorbent and was adsorbed by the micropores.At the third stage of adsorption, the adsorption sites on the surface and inside the pores of the Fe 3 O 4 @CTS/SBC composite adsorbent were basically saturated, and the adsorption reached an equilibrium state 38 .Based on the particle diffusion model, it could be concluded that the adsorption of Cu 2+ by the Fe 3 O 4 @CTS/SBC composite adsorbent was not only a chemical chelation, but also an intraparticle diffusion process.
The Boyd model is based on intraparticle diffusion rate control 39 .The R 2 of Table 2 was 0.6106, which indicated that the model was poorly correlated.The intraparticle diffusion was not the only rate control step in the adsorption process of the Fe 3 O 4 @CTS/SBC on copper ions, which was consistent with the findings of the intraparticle diffusion model.The Boyd model did not describe the adsorption of the Fe 3 O 4 @CTS/SBC on copper ions well.In conclusion, the intraparticle diffusion model and the Boyd model can be useful to identify reaction pathways and adsorption mechanisms as well as to predict the rate-controlling step.

Adsorption isotherm analysis
The Freundlich model is more suitable for adsorption on highly heterogeneous surfaces lacking a saturated adsorption platform, in which case inhomogeneous processes occurred on the adsorbed surfaces and the adsorption process involved adsorption in multiple molecular layers 7 .The adsorption process in this study followed the Langmuir model.The Langmuir model assumes a monolayer adsorption effect, thus the Fe 3 O 4 @CTS/SBC composite adsorbent exhibited monolayer adsorption, where only one molecule could be adsorbed at each active site, and all the adsorption sites had the same priority to form a homogeneous monolayer adsorption film 40 .

Figure 11 .
Figure 11.Effect of solution pH on adsorption of Cu 2+ .

Table 1 .
Surface structure parameters of adsorbent.

Table 2 .
Comparison of kinetic models for adsorption of Cu 2+ by Fe 3 O 4 @CTS/SBC complex adsorbent.

Table 5 .
Comparison of Cu 2+ adsorption by Fe 3 O 4 @CTS/SBC complex adsorbent and other adsorbents.