Adsorption and Removal of Cr6+, Cu2+, Pb2+, and Zn2+ from Aqueous Solution by Magnetic Nano-Chitosan

Magnetic nano-chitosan (MNC) was prepared and characterized. The kinetics, thermodynamics, and influencing factors of the adsorption of Cr6+, Cu2+, Pb2+, and Zn2+, as well as their competitive adsorption onto MNC in aqueous solution, were studied. The results showed that the adsorption kinetics and thermodynamics of Cr6+, Cu2+, Pb2+, and Zn2+ were well described by the pseudo-second-order kinetic model and Langmuir isothermal adsorption model, indicating that the adsorption was mainly chemical adsorption and endothermic. Increasing the dosage of MNC, the equilibrium adsorption capacity (qe) of Cr6+, Cu2+, Pb2+, and Zn2+ decreased; their removal rate (η) increased. With the increase in the solution’s pH, the qe and η of Cr6+ first increased and then decreased; the qe and η of Cu2+, Pb2+, and Zn2+ increased. With the increase in the metal ion initial concentration, the qe increased; the η of Cr6+, Cu2+, and Zn2+ decreased, while the η of Pb2+ increased first and then decreased. Temperature had a weak influence on the qe of Cr6+ and Pb2+, while it had a strong influence on Cu2+ and Zn2+, the qe and η were greater when the temperature was higher, and the adsorption was spontaneous and endothermic. The qe and η of Cu2+, Pb2+, and Zn2+ decreased in the presence of co-existing ions. The influences among metal ions existed in a binary and ternary ion system. The current study’s results provide a theoretical support for the simultaneous treatment of harmful metal ions in wastewater by MNC.


Adsorption Thermodynamics Experiments
The solutions (pH = 5) of Cr 6+ with concentration gradients of 5, 10, 20, 50, 60, and 80 mg/L were prepared. Six pre-washed 20 mL headspace vials were prepared. In total, 50 mg of the prepared magnetic nano-chitosan was added in each headspace vial, and 10 mL of the prepared gradient solution of Cr 6+ was added in turn. They were shaken for 120 min at 180 r/min in a constant temperature water bath shaker of 298 K, and were then magnetically separated by a magnet. Each sample was repeated three times. Meanwhile, the adsorption thermodynamic experiment was performed at 308 K and 318 K. In addition, the gradient solutions of 5, 10, 20, 50, 60, and 80 mg/L for Cu 2+ , 10,20,40,50,60, and 80 mg/L for Pb 2+ , and 5, 10, 20, 50, 60, and 80 mg/L for Zn 2+ were prepared, with the natural pH of 5, 6, and 4 for Cu 2+ , Pb 2+ , and Zn 2+ , respectively. Under the temperature conditions of 298 K, 308 K, and 318K, respectively, the adsorption thermodynamics experiment was carried out in a constant temperature water bath oscillator for 120 min, with an oscillation frequency of 180 r/min and magnetic separation, and each sample was repeated three times. The concentrations of heavy metal ions in the solution were analyzed using ICP-AES.

Influencing Factor Experiments
Initial concentrations of metal ions and temperatures: the experimental process was the same as that used for the adsorption thermodynamic experiments.
Magnetic nano-chitosan doses: Nine pre-washed 20 mL headspace vials were first dosed with 10,20,30,40,50,60,80, 100, and 120 mg of the prepared magnetic nanochitosan, respectively, followed by 10 mL of solution (pH = 5) of Cr 6+ at a concentration of 50 mg/L. Nine pre-washed 20 mL headspace vials were first added with 10, 20, 30, 50, 60, 80, 100, 120, and 140 mg of the prepared magnetic nano-chitosan, respectively, followed by 10 mL of solution (pH = 5) of Cu 2+ at a concentration of 30 mg/L. Seven pre-washed 20 mL headspace vials were first dosed with 10,20,40,60,80,120, and 150 mg of the prepared magnetic nano-chitosan, respectively, and then 10 mL of solution (pH = 6) of Pb 2+ at a concentration of 50 mg/L was added. Eight pre-washed 20 mL headspace vials were first added with 10, 20, 30, 50, 60, 80, 100, and 120 mg of the prepared magnetic nano-chitosan, respectively, followed by 10 mL of solution (pH = 4) of Zn 2+ at a concentration of 20 mg/L. They were shaken for 120 min at 180 r/min in a constant temperature water bath shaker of 298 K and were then separated by a magnet. Each sample was repeated three times. The concentrations of heavy metal ions were analyzed using ICP-AES.
Solution pH: 50 mg of the prepared magnetic nano-chitosan was added to 7 prepared 20 mL headspace vials, respectively, and 10 mL of solution of Cr 6+ at a concentration of 50 mg/L with solution pH values of 1, 2, 3, 4, 5, 6, and 7 was added in turn. They were shaken for 120 min at 180 r/min in a constant temperature water bath shaker of 298 K, and then magnetically separated. Each sample was repeated three times. Experiments on the effect of pH on the adsorption of Cu 2+ , Pb 2+ , and Zn 2+ were also carried out. The concentrations of Cu 2+ , Pb 2+ , and Zn 2+ in the solution were 30, 50, and 20 mg/L, respectively, with solution pH values of 1, 2, 3, 4, 5, 6, and 7, respectively. The concentrations of heavy metal ions in the solution were analyzed using ICP-AES.
The competitive adsorption experiments were performed by adding 50 mg of the prepared magnetic nano-chitosan into each headspace vial, followed by adding 10 mL of the above concentration gradient solution of single or mixed ion solution in turn. They were shaken for 120 min at 180 r/min in a constant temperature water bath shaker of 298 K to reach adsorption equilibrium and were then separated using a magnet. Each sample was repeated three times. The concentrations of heavy metal ions in the solution were analyzed by ICP-AES.

Data Analysis
The adsorption capacity at the time t (q t , µmol/g), equilibrium adsorption capacity (q e , µmol/g), and removal efficiency (η, %) of the heavy metal ions at time t or equilibrium were calculated as follows.
where c 0 , c t , and c e (mg/L) are the initial, time t, and equilibrium concentrations of heavy metal ions, respectively; V (mL) is the volume of the solution; m (mg) is the weight of the adsorbent; and M (g/mol) is the molar mass of heavy metal ions. Adsorption kinetic models, such as the pseudo-first-order kinetic model (4), pseudosecond-order kinetic model (5), Elovich model (6), and intraparticle diffusion model (7), were used to fit the adsorption kinetic curves [10,[40][41][42][43].
where t (min) denotes the adsorption time; K 1 (g·µmol −1 ·min −1 ) and K 2 (g·µmol −1 ·min −1 ) are the rate constants for the pseudo-first-order and pseudo-second-order kinetic model, respectively; in the Elovich model, K (g·µmol −1 ·min −1 ) is the adsorption rate constant and a (g·µmol −1 ·min −1 ) is a constant; and K d (µmol·g −1 ·min −0.5 ) is the intraparticle diffusion model rate constant and C is a constant term used to estimate the boundary layer thickness. The Langmuir isotherm adsorption model describes an ideal single-molecule adsorption [43], commonly used in the adsorption of contaminants in liquid solutions, and the model is given in Equation (8); the R L calculated in Equation (9) represents the affinity between the absorbents and adsorbates, and the adsorption is irreversible for R L = 0, favorable for 0 < R L < 1, linear for R L = 1, and unfavorable for R L > 1 [44]. The Freundlich isothermal adsorption model can be applied to a multilayer adsorption with the affinity on non-homogeneous surfaces [45], the heat of the adsorption decreases with an increasing surface coverage due to the inhomogeneity of the solid surface, and the proposed empirical model is given in Equation (10). The Temkin isothermal adsorption model assumes a linear decrease in adsorption heat at all surface locations due to adsorbent and adsorbate interactions [42,46], and the model is given in Equation (11).
where q m is the maximum adsorption capacity, µmol/g; K L is the Langmuir model constant, L/µmol, and the surface adsorption capacity of the adsorbent is generally stronger when K L is larger; K F is the Freundlich model constant, (µmol/g)/(µmol/L) 1/n ; n is the index related to the adsorption strength, and 1/n < 1 indicates normal Freundlich adsorption, 0.1 < 1/n < 0.5 implies that there is an attraction between adsorbents and adsorbates that promotes adsorption, 1/n = 1 illustrates a linear adsorption generally occurring in relatively dilute solutions and on relatively low surface coverage adsorbents, and 1/n > 1 suggests that there is a synergistic adsorption and weak attraction between adsorbents and adsorbents, especially difficult adsorption at 1/n > 2; and A (J/mol) and K T (L/µmol) are the Temkin model constants related to adsorption heat and binding energy, respectively. The adsorption thermodynamics may reflect that the adsorption process is endothermic or exothermic, and temperature is an important factor affecting the adsorption. Thus, the adsorption thermodynamic parameters, i.e., Gibbs free energy (∆G, kJ/mol), the enthalpy of adsorption (∆H, kJ/mol), and the entropy of adsorption (∆S, J/(mol·K)), were analyzed and calculated as follows [45]: where K D is the solid-liquid partition coefficient; V s and V e are the activity coefficients, both taken as 1; T is the absolute temperature, K; and R is the gas constant, 8.314 J/(mol·K). When ∆G < 0, the reaction can proceed spontaneously; when ∆G = 0, the reaction is in equilibrium; and when ∆G > 0, the reaction cannot proceed spontaneously. ∆H > 0 indicates that the reaction is endothermic; ∆H < 0 implies that the reaction is exothermic. The actual reaction is always in the direction of increasing entropy, i.e., ∆S > 0. According to the principle of entropy increase, ∆S = 0 suggests that the reaction has reached equilibrium. Figure 1 shows the characterization results of the prepared magnetic nano-Fe 3 O 4 and magnetic nano-chitosan. From the TEM results in Figure 1a,b, the size of the prepared magnetic nano-Fe 3 O 4 and magnetic nano-chitosan were below 50 nm and spherically arranged in an orderly manner, while the surface was not very smooth and there existed agglomerates. As shown in Figure 1c, the prepared magnetic nano-chitosan had seven characteristic peaks; the 2θ angles of the seven peaks were 30.1 • , 35.5 • , 43.1 • , 57.1 • , 62.5 • , 71.5 • , and 74.5 • , respectively, with the corresponding crystallographic planes of (220), (311), (400), (511), (440), (620), and (533), respectively, which were the same as the crystallographic planes of the prepared magnetic nano-Fe 3 O 4 . There were the same diffraction peaks between the prepared magnetic nano-Fe 3 O 4 and magnetic nano-chitosan, and no new diffraction peaks appeared in the prepared magnetic nano-chitosan, indicating that the synthesis of the magnetic nano-chitosan did not affect the crystal structure of Fe 3 O 4 and did not change the crystallographic phase of Fe 3 O 4 . As shown in Figure 1d, the prepared magnetic nano-Fe 3 O 4 had characteristic absorption peaks belonging to the stretching vibration of Fe-O at 560-600 cm −1 [35,37], indicating that the magnetic nano-Fe 3 O 4 was successfully prepared. The main characteristic adsorption peaks of the prepared magnetic nano-chitosan were around 3433 cm −1 (O-H and N-H stretching vibration peaks), 2875 cm −1 (the stretching vibration peak of -CH), 1601 cm −1 (the bending vibration peak of -NH in -NH 2 ), and 560 cm −1 (Fe-O stretching vibration peak) [35,46,47], indicating that chitosan was successfully loaded onto the magnetic nano-Fe 3 O 4 . As shown in Figure 1e,f, the prepared magnetic nano-Fe 3 O 4 and magnetic nano-chitosan exhibited typical type IV isotherm adsorption characteristics, indicating that there was a relatively strong interaction of nitrogen onto the sample surfaces, the prepared magnetic nano-Fe 3 O 4 was a mesoporous material (pore width: 2-50 nm), and the magnetic nano-chitosan was a microporous material (pore width: 0-2 nm). The specific surface area of the prepared magnetic nano-Fe 3 O 4 based on the BET model was 17.45 m 2 /g; without a microporous surface area, the total pore volume was 0.14 cm 3 /g, and the average adsorption pore width measured by the BJH model was 7.84 nm. The prepared magnetic nano-chitosan had a specific surface area of 1.13 m 2 /g, no mesoporous surface area, a total pore volume of 0.02 cm 3 /g, and a mean adsorption pore width of 9.15 nm. Compared with that of the prepared magnetic nano-Fe 3 O 4 , the specific surface area of the prepared magnetic nano-chitosan reduced, while the pore width became larger.

Characterization Results
Molecules 2023, 28, x FOR PEER REVIEW 7 of 20 the principle of entropy increase, ∆S = 0 suggests that the reaction has reached equilibrium. Figure 1 shows the characterization results of the prepared magnetic nano-Fe3O4 and magnetic nano-chitosan. From the TEM results in Figure 1a,b, the size of the prepared magnetic nano-Fe3O4 and magnetic nano-chitosan were below 50 nm and spherically arranged in an orderly manner, while the surface was not very smooth and there existed agglomerates. As shown in Figure 1c, the prepared magnetic nano-chitosan had seven characteristic peaks; the 2θ angles of the seven peaks were 30.1°, 35.5°, 43.1°, 57.1°, 62.5°, 71.5°, and 74.5°, respectively, with the corresponding crystallographic planes of (220), (311), (400), (511), (440), (620), and (533), respectively, which were the same as the crystallographic planes of the prepared magnetic nano-Fe3O4. There were the same diffraction peaks between the prepared magnetic nano-Fe3O4 and magnetic nano-chitosan, and no new diffraction peaks appeared in the prepared magnetic nano-chitosan, indicating that the synthesis of the magnetic nano-chitosan did not affect the crystal structure of Fe3O4 and did not change the crystallographic phase of Fe3O4. As shown in Figure 1d, the prepared magnetic nano-Fe3O4 had characteristic absorption peaks belonging to the stretching vibration of Fe-O at 560-600 cm −1 [35,37], indicating that the magnetic nano-Fe3O4 was successfully prepared. The main characteristic adsorption peaks of the prepared magnetic nano-chitosan were around 3433 cm −1 (O-H and N-H stretching vibration peaks), 2875 cm −1 (the stretching vibration peak of -CH), 1601 cm −1 (the bending vibration peak of -NH in -NH2), and 560 cm −1 (Fe-O stretching vibration peak) [35,46,47], indicating that chitosan was successfully loaded onto the magnetic nano-Fe3O4. As shown in Figure 1e,f, the prepared magnetic nano-Fe3O4 and magnetic nano-chitosan exhibited typical type IV isotherm adsorption characteristics, indicating that there was a relatively strong interaction of nitrogen onto the sample surfaces, the prepared magnetic nano-Fe3O4 was a mesoporous material (pore width: 2-50 nm), and the magnetic nano-chitosan was a microporous material (pore width: 0-2 nm). The specific surface area of the prepared magnetic nano-Fe3O4 based on the BET model was 17.45 m 2 /g; without a microporous surface area, the total pore volume was 0.14 cm 3 /g, and the average adsorption pore width measured by the BJH model was 7.84 nm. The prepared magnetic nano-chitosan had a specific surface area of 1.13 m 2 /g, no mesoporous surface area, a total pore volume of 0.02 cm 3 /g, and a mean adsorption pore width of 9.15 nm. Compared with that of the prepared magnetic nano-Fe3O4, the specific surface area of the prepared magnetic nano-chitosan reduced, while the pore width became larger.  Figure 2a depicts the adsorption kinetics curves of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic nano-chitosan in aqueous solution, respectively. The adsorption process was divided into two stages, i.e., the adsorption capacity and removal rate of Cr 6+ as well as Cu 2+ , Pb 2+ , and Zn 2+ increased sharply in the first 60 and 120 min, respectively (fast adsorption stage); the adsorption capacity and removal rate changed slowly (slow adsorption stage). In general, when the adsorption involves a surface reaction process, the initial adsorption is relatively rapid due to the large number of available adsorption sites on the adsorbent; then, as the number of available adsorption sites gradually decreases, the adsorption slows down and reaches an equilibrium [47][48][49]. In addition, the experimentally obtained adsorption capacity (qexp) presented the order of Cu 2+ (81.141 mol/g) > Cr 6+ (61.208 mol/g) > Pb 2+ (45.276 mol/g) > Zn 2+ (43.092 mol/g), and the maximum removal rate () followed the order of Pb 2+ (93.72%) > Cu 2+ (88.48%) > Zn 2+ (70.03%) > Cr 6+ (31.83%).  Figure 2a depicts the adsorption kinetics curves of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic nano-chitosan in aqueous solution, respectively. The adsorption process was divided into two stages, i.e., the adsorption capacity and removal rate of Cr 6+ as well as Cu 2+ , Pb 2+ , and Zn 2+ increased sharply in the first 60 and 120 min, respectively (fast adsorption stage); the adsorption capacity and removal rate changed slowly (slow adsorption stage). In general, when the adsorption involves a surface reaction process, the initial adsorption is relatively rapid due to the large number of available adsorption sites on the adsorbent; then, as the number of available adsorption sites gradually decreases, the adsorption slows down and reaches an equilibrium [47][48][49]. In addition, the experimentally obtained adsorption capacity (q exp ) presented the order of Cu 2+ (81.141 µmol/g) > Cr 6+ (61.208 µmol/g) > Pb 2+ (45.276 µmol/g) > Zn 2+ (43.092 µmol/g), and the maximum removal rate (η) followed the order of Pb 2+ (93.72%) > Cu 2+ (88.48%) > Zn 2+ (70.03%) > Cr 6+ (31.83%). Molecules 2023, 28, x FOR PEER REVIEW 9 of 20  Figure 2b shows that the adsorption kinetic curves of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic nano-chitosan in aqueous solution were fitted by using pseudo-first-order, pseudo-second-order, and Elovich kinetic models. As shown in Figure 2b, the adsorption of metal ions onto magnetic nano-chitosan could reach the equilibrium of adsorption at 30 min for Cr 6+ and at 60 min for Cu 2+ , Pb 2+ , and Zn 2+ . Table 1 shows the fitted results of the parameters of pseudo-first-order, pseudo-second-order, and Elovich kinetic models for the adsorption kinetics of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ in aqueous solution by magnetic nano-chitosan. As shown in Table 1, the fitted correlation coefficient (R 2 ) by the pseudosecond-order kinetic model for Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ was larger than that by the pseudo-first-order kinetic model, with R 2 being greater than 0.960, and the equilibrium adsorption capacity (qe) fitted by the pseudo-second-order kinetic model was also closer to the qexp, indicating that the pseudo-second-order kinetic model could well describe the adsorption kinetic process, including liquid film diffusion, surface adsorption, internal diffusion, and chemical bound formation. It can be inferred that the adsorption of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ in aqueous solution by magnetic nano-chitosan was dominated by chemisorption. The Elovich kinetic model also provided good fits to the adsorption kinetic  Figure 2b shows that the adsorption kinetic curves of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic nano-chitosan in aqueous solution were fitted by using pseudo-first-order, pseudo-second-order, and Elovich kinetic models. As shown in Figure 2b, the adsorption of metal ions onto magnetic nano-chitosan could reach the equilibrium of adsorption at 30 min for Cr 6+ and at 60 min for Cu 2+ , Pb 2+ , and Zn 2+ . Table 1 shows the fitted results of the parameters of pseudo-first-order, pseudo-second-order, and Elovich kinetic models for the adsorption kinetics of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ in aqueous solution by magnetic nano-chitosan. As shown in Table 1, the fitted correlation coefficient (R 2 ) by the pseudosecond-order kinetic model for Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ was larger than that by the pseudo-first-order kinetic model, with R 2 being greater than 0.960, and the equilibrium adsorption capacity (q e ) fitted by the pseudo-second-order kinetic model was also closer to the q exp , indicating that the pseudo-second-order kinetic model could well describe the adsorption kinetic process, including liquid film diffusion, surface adsorption, internal diffusion, and chemical bound formation. It can be inferred that the adsorption of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ in aqueous solution by magnetic nano-chitosan was dominated by chemisorption. The Elovich kinetic model also provided good fits to the adsorption kinetic curves, with R 2 > 0.96. The Elovich kinetic model is mainly used to study the nonhomogeneous diffusion process in the combined presence of adsorbent adsorption behavior and adsorbate diffusion, which does not predict any conventional mechanism [50,51] and is suitable for the reaction process with large activation energies. The good fitting indicates that the adsorption process is a non-homogeneous diffusion process regulated by a combination of the reaction rate and diffusion factors. Table 1. Fitted parameter values of pseudo-first-order, pseudo-second-order, and Elovich kinetic models.

Pseudo-First-Order
Pseudo-Second-Order Elovich  Figure 2c shows the fitting results of the intraparticle diffusion model. As shown in Figure 2c, the adsorption of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic nano-chitosan in aqueous solution was a combined existence process of adsorption and diffusion rather than a simple first-order reaction; the whole adsorption process was divided into the dynamic processes of fast surface adsorption, intraparticle diffusion, and adsorption and desorption equilibrium, in which the equilibrium dynamic process of intraparticle diffusion was relatively fast and cannot be regarded as the rate-limiting step. Table 2 shows the fitted parameter values of the intraparticle diffusion model for Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ adsorption in aqueous solutions by magnetic nano-chitosan. From Table 2, Cr 6+ and Cu 2+ followed K 2d > K 1d > K 3d , indicating the intraparticle diffusion rate > surface diffusion rate > equilibrium dynamic rate; Pb 2+ presented K 1d > K 2d > K 3d , showing the surface diffusion rate > intraparticle diffusion rate > the equilibrium dynamic rate; and Zn 2+ exhibited K 3d > K 1d > K 2d , indicating the equilibrium dynamic rate > surface diffusion rate > intraparticle diffusion rate. Meanwhile, the fitted curves of the intraparticle diffusion model did not pass through the origin, q t and t 0.5 were nonlinear relations, and the C value was not zero, indicating that other mechanisms besides intraparticle diffusion might have been involved. The adsorption process was possibly controlled by the boundary layer [52]. The reason is that the boundary layer effect is stronger when the C value (intercept value) is larger [52,53].

Adsorption Thermodynamics
Adsorption isotherms show how adsorbent molecules are distributed between liquid and solid two phases when the adsorption process reaches an equilibrium [54,55], and they help to determine the properties of the adsorbent, such as the pore size, pore volume, and surface area [23]. In this study, the isothermal adsorption models of Langmuir, Freundlich, and Temkin were used to fit the adsorption thermodynamic curves of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic nano-chitosan in aqueous solution at 298 K, 308 K, and 318 K, respectively, and the results are presented in Figure 3 and Table 3. As shown in Figure 3, the q e of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ increased exponentially with the increase in the equilibrium concentration (c e ) of heavy metal ions in the solution, and the q e of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ increased rapidly at the c e of less than 200, 100, 20, and 200 µmol/L, respectively. Among them, temperature had the strongest effect on Cu 2+ adsorption, followed by Pb 2+ and Zn 2+ adsorption, and was relatively weak for the Cr 6+ adsorption influence. At the same c e , the higher the temperature, the greater the q e of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ , indicating that the adsorption process was endothermic and the increase in the temperature was beneficial to the adsorption reaction.
Molecules 2023, 28, x FOR PEER REVIEW 11 of 20 Pb 2+ , and Zn 2+ onto magnetic nano-chitosan in aqueous solution at 298 K, 308 K, and 318 K, respectively, and the results are presented in Figure 3 and Table 3. As shown in Figure  3, the qe of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ increased exponentially with the increase in the equilibrium concentration (ce) of heavy metal ions in the solution, and the qe of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ increased rapidly at the ce of less than 200, 100, 20, and 200 µmol/L, respectively. Among them, temperature had the strongest effect on Cu 2+ adsorption, followed by Pb 2+ and Zn 2+ adsorption, and was relatively weak for the Cr 6+ adsorption influence. At the same ce, the higher the temperature, the greater the qe of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ , indicating that the adsorption process was endothermic and the increase in the temperature was beneficial to the adsorption reaction. As shown in Table 3, the R 2 fitted by the Langmuir isothermal adsorption model for the adsorption thermodynamics of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic nano-chitosan in the solution was slightly larger than that fitted by Freundlich and Temkin isothermal adsorption models, and the maximum adsorption capacity (qm) obtained by the Langmuir isothermal adsorption model for Cu 2+ and Zn 2+ was very close to that of the qexp, indicating that the Langmuir isothermal adsorption model could well describe the adsorption thermodynamics and that the adsorption belonged to monolayer adsorption. Among them, the RL values of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ were in the range of 0 to 1 at different temperatures and concentrations, indicating that the affinity between magnetic nano-chitosan and heavy metal ions was favorable for adsorption. The qm of magnetic nano-chitosan for Cr 6+ , Cu 2+ , Pb 2+ , and Zn2+ at 318 K obtained by fitting the Langmuir adsorption isotherm model were up to 301.057, 198.861, 121.942, and 62.727 µmol/g, respectively. Meanwhile, the qm obtained by fitting the Langmuir adsorption isotherm model shows that the adsorption Figure 3. Fitting of Langmuir, Freundlich, and Temkin isotherm models to the adsorption of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic nano-chitosan in aqueous solution. Table 3. Fitting parameters of Langmuir, Freundlich, and Temkin isothermal models.

Langmuir
Freundlich Temkin K L (L/µmol) q m (µmol/g) R 2 1/n K F (µmol/g)/(µmol/L) 1 As shown in Table 3, the R 2 fitted by the Langmuir isothermal adsorption model for the adsorption thermodynamics of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic nanochitosan in the solution was slightly larger than that fitted by Freundlich and Temkin isothermal adsorption models, and the maximum adsorption capacity (q m ) obtained by the Langmuir isothermal adsorption model for Cu 2+ and Zn 2+ was very close to that of the q exp , indicating that the Langmuir isothermal adsorption model could well describe the adsorption thermodynamics and that the adsorption belonged to monolayer adsorption. Among them, the R L values of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ were in the range of 0 to 1 at different temperatures and concentrations, indicating that the affinity between magnetic nano-chitosan and heavy metal ions was favorable for adsorption. The q m of magnetic nano-chitosan for Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ at 318 K obtained by fitting the Langmuir adsorption isotherm model were up to 301.057, 198.861, 121.942, and 62.727 µmol/g, respectively. Meanwhile, the q m obtained by fitting the Langmuir adsorption isotherm model shows that the adsorption effect of magnetic nano-chitosan for Cr 6+ in aqueous solution was more obvious, followed by Pb 2+ and Cu 2+ , and the adsorption effect of Zn 2+ was relatively low. In addition, the values of 1/n by fitting the Freundlich adsorption isotherm model were all less than 1, indicating the existence of an attraction between the adsorbent surface and adsorbate that promotes adsorption. The fitted constant A of the Temkin isotherm model suggests that the heat of adsorption increased with the increase in temperature, further indicating that the adsorption process was endothermic.
The adsorption thermodynamic parameters, including ∆G, ∆H, and ∆S, are shown in Table 4. As shown in Table 4, the values of ∆G were all below 0 and decreased with the increase in temperature, indicating that the adsorption processes of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic nano-chitosan in aqueous solution were spontaneous. The values of ∆H were all above 0, further indicating that the adsorption processes were endothermic and that the increase in temperature is favorable for adsorption.  Figure 4 shows the effects of the magnetic nano-chitosan dosage and solution pH on the adsorption and removal of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ in aqueous solution. As shown in Figure 4a, the η of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ first showed a linear and rapid increase, followed by a slow increase towards equilibrium; the q e decreased from fast to slow with the dosage increase of magnetic nano-chitosan. When the concentrations of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ in aqueous solution are constant, increasing the dosage of magnetic nano-chitosan means increasing the active adsorption sites where Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ can be adsorbed, so the adsorption removal efficiency gradually increases with the increase in the magnetic nano-chitosan dose; when the amount of magnetic nano-chitosan dosed is too high, the number of adsorption sites is much larger than the amount of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ , then the adsorption capacity per unit adsorbent reduces instead, i.e., the adsorbent utilization rate reduces. As shown in Figure 4b, the q e and η of Cr 6+ first increased and then decreased with the increase in the solution's pH; the q e and η decreased from 92.758 to 40.498 µmol/g and from 95% to 40% at the solution's pH of 2-7. The q e and η of Cu 2+ , Pb 2+ , and Zn 2+ increased as the solution's pH increased, while it basically ceased to change when the solution's pH was above four; the maximum η could reach 100% for Cu 2+ and Pb 2+ and 75% for Zn 2+ . When the pH of the solution is low, the concentration of H + in the solution is high, and the amino groups on the surface of magnetic nano-chitosan are prone to a protonation reaction to form -NH 3 + , which can decrease the number of amino groups that produce the effective complexation of Cu 2+ , Pb 2+ , and Zn 2+ . Therefore, the adsorbent has a low adsorption capacity for Cu 2+ , Pb 2+ , and Zn 2+ under a low pH condition and the q e are also relatively small. As the pH of the solution increases, the concentration of H + in the solution gradually decreases, the competition ability between H + and Cu 2+ , Pb 2+ , and Zn 2+ gradually decreases, the adsorption sites on the surface of magnetic nanochitosan are released, and the adsorbent protonation effect is gradually weakened and the electrostatic repulsion is also reduced. Under this condition, the magnetic nano-chitosan adsorption capacities for Cu 2+ , Pb 2+ , and Zn 2+ gradually increased, leading to an increase in the adsorption capacity of Cu 2+ , Pb 2+ , and Zn 2+ [56]. At the solution of pH < 2, Cr 6+ mainly exists as HCrO 4 − , and the magnetic nano-chitosan surface is positively charged, HCrO 4 − will be adsorbed on the magnetic nano-chitosan, thus the q e of Cr 6+ increases; when the solution's pH increases from two, the concentration of OH − increases, and OH − will compete with CrO 4 2− , so the q e of Cr 6+ decreases [57].

Adsorbent Dosage and Solution pH
too high, the number of adsorption sites is much larger than the amount of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ , then the adsorption capacity per unit adsorbent reduces instead, i.e., the adsorbent utilization rate reduces. As shown in Figure 4b, the qe and η of Cr 6+ first increased and then decreased with the increase in the solution's pH; the qe and η decreased from 92.758 to 40.498 µmol/g and from 95% to 40% at the solution's pH of 2-7. The qe and η of Cu 2+ , Pb 2+ , and Zn 2+ increased as the solution's pH increased, while it basically ceased to change when the solution's pH was above four; the maximum η could reach 100% for Cu 2+ and Pb 2+ and 75% for Zn 2+ . When the pH of the solution is low, the concentration of H + in the solution is high, and the amino groups on the surface of magnetic nano-chitosan are prone to a protonation reaction to form -NH3 + , which can decrease the number of amino groups that produce the effective complexation of Cu 2+ , Pb 2+ , and Zn 2+ . Therefore, the adsorbent has a low adsorption capacity for Cu 2+ , Pb 2+ , and Zn 2+ under a low pH condition and the qe are also relatively small. As the pH of the solution increases, the concentration of H + in the solution gradually decreases, the competition ability between H + and Cu 2+ , Pb 2+ , and Zn 2+ gradually decreases, the adsorption sites on the surface of magnetic nanochitosan are released, and the adsorbent protonation effect is gradually weakened and the electrostatic repulsion is also reduced. Under this condition, the magnetic nano-chitosan adsorption capacities for Cu 2+ , Pb 2+ , and Zn 2+ gradually increased, leading to an increase in the adsorption capacity of Cu 2+ , Pb 2+ , and Zn 2+ [56]. At the solution of pH < 2, Cr 6+ mainly exists as HCrO4 -, and the magnetic nano-chitosan surface is positively charged, HCrO4will be adsorbed on the magnetic nano-chitosan, thus the qe of Cr 6+ increases; when the solution's pH increases from two, the concentration of OHincreases, and OHwill compete with CrO4 2-, so the qe of Cr 6+ decreases [57].  Figure 5 shows the effect of the initial concentration of metal ions and temperature on the adsorption and removal of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ from aqueous solution by magnetic nano-chitosan. As can be seen from Figure 5, the qe of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic nano-chitosan showed an overall linear increase when the initial concentrations of metal ions increased, the temperature had a weak influence on the adsorption of Cr 6+ and Pb 2+ and a strong influence on Cu 2+ and Zn 2+ at the same initial concentration, and the higher the temperature, the greater the qe, i.e., 318 K > 308 K > 298 K; the η of Cr 6+ , Cu 2+ , and Zn 2+ decreased continuously with the increase in the initial concentration of metal ions, and the lower the temperature, the lower the η at the same initial concentration, i.e., 318 K > 308 K > 298 K. With the increase in the initial concentration of metal ions, the η of Pb 2+ increased first and then decreased, and reached a maximum of 95% when the initial concentration of Pb 2+ increased to 289.855 µmol/L. As the initial concentration of Pb 2+  Figure 5 shows the effect of the initial concentration of metal ions and temperature on the adsorption and removal of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ from aqueous solution by magnetic nano-chitosan. As can be seen from Figure 5, the q e of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic nano-chitosan showed an overall linear increase when the initial concentrations of metal ions increased, the temperature had a weak influence on the adsorption of Cr 6+ and Pb 2+ and a strong influence on Cu 2+ and Zn 2+ at the same initial concentration, and the higher the temperature, the greater the q e , i.e., 318 K > 308 K > 298 K; the η of Cr 6+ , Cu 2+ , and Zn 2+ decreased continuously with the increase in the initial concentration of metal ions, and the lower the temperature, the lower the η at the same initial concentration, i.e., 318 K > 308 K > 298 K. With the increase in the initial concentration of metal ions, the η of Pb 2+ increased first and then decreased, and reached a maximum of 95% when the initial concentration of Pb 2+ increased to 289.855 µmol/L. As the initial concentration of Pb 2+ continued to increase, the η of Pb 2+ decreased rapidly. The probable reason for this is that the process of Pb 2+ reaching adsorption equilibrium is relatively slow; the removal rate gradually increases with the increase in its initial concentration in the solution and has started to decrease when its adsorption sites reach saturation. continued to increase, the η of Pb 2+ decreased rapidly. The probable reason for this is that the process of Pb 2+ reaching adsorption equilibrium is relatively slow; the removal rate gradually increases with the increase in its initial concentration in the solution and has started to decrease when its adsorption sites reach saturation. Figure 5. Effects of initial concentration and temperature on adsorption of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic nano-chitosan in aqueous solution.

Competitive Adsorption of Metal Ions
Figure 6a1-c1 shows the fitted results of the Langmuir and Freundlich isothermal adsorption models for the single and competitive adsorption of Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic nano-chitosan in aqueous solution as well as the comparison of the adsorption capacities and removal rates of Cu 2+ , Pb 2+ , and Zn 2+ in single, binary, and ternary ion systems. The qe of Cu 2+ , Pb 2+ , and Zn 2+ in single, binary, and ternary ion systems showed an increasing trend with the increase in the metal ion equilibrium concentration. In binary and ternary iron systems, the qe of Cu 2+ , Pb 2+ , and Zn 2+ decreased compared to the corresponding single system. The experimentally obtained qexp of Cu 2+ , Pb 2+ , and Zn 2+ in the binary ion system of Cu 2+ -Pb 2+ and Cu 2+ -Zn 2+ , Pb 2+ -Cu 2+ and Pb 2+ -Zn 2+ , and Zn 2+ -Cu 2+ and Zn 2+ -Pb 2+ reduced by 16.01% and 5.44%, 32.28% and 29.97%, and 7.12% and 45.01%, respectively, indicating that the influence presented Pb 2+ to Cu 2+ >> Zn 2+ to Cu 2+ , Cu 2+ on Pb 2+ > Zn 2+ on Pb 2+ , and Pb 2+ to Zn 2+ >> Cu 2+ to Zn 2+ . The qe of Cu 2+ , Pb 2+ , and Zn 2+ in the ternary ion system decreased by 18.34%, 43.36%, and 13.02%, respectively, suggesting that mutual effects among the metal ions existed.
The removal efficiency of magnetic nano-chitosan for Cu 2+ , Pb 2+ , and Zn 2+ was also reduced in the presence of coexisting ions. As shown in Figure 6a2, the influence of Pb 2+ to Cu 2+ was stronger than that of Zn 2+ to Cu 2+ . It can be seen from Figure 6b2 that the effect of coexisting ions on Pb 2+ exhibited Cu 2+ and Zn 2+ > Cu 2+ > Zn 2+ . From Figure 6c2, the

Competitive Adsorption of Metal Ions
Figure 6a1-c1 shows the fitted results of the Langmuir and Freundlich isothermal adsorption models for the single and competitive adsorption of Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic nano-chitosan in aqueous solution as well as the comparison of the adsorption capacities and removal rates of Cu 2+ , Pb 2+ , and Zn 2+ in single, binary, and ternary ion systems. The q e of Cu 2+ , Pb 2+ , and Zn 2+ in single, binary, and ternary ion systems showed an increasing trend with the increase in the metal ion equilibrium concentration. In binary and ternary iron systems, the q e of Cu 2+ , Pb 2+ , and Zn 2+ decreased compared to the corresponding single system. The experimentally obtained q exp of Cu 2+ , Pb 2+ , and Zn 2+ in the binary ion system of Cu 2+ -Pb 2+ and Cu 2+ -Zn 2+ , Pb 2+ -Cu 2+ and Pb 2+ -Zn 2+ , and Zn 2+ -Cu 2+ and Zn 2+ -Pb 2+ reduced by 16.01% and 5.44%, 32.28% and 29.97%, and 7.12% and 45.01%, respectively, indicating that the influence presented Pb 2+ to Cu 2+ >> Zn 2+ to Cu 2+ , Cu 2+ on Pb 2+ > Zn 2+ on Pb 2+ , and Pb 2+ to Zn 2+ >> Cu 2+ to Zn 2+ . The q e of Cu 2+ , Pb 2+ , and Zn 2+ in the ternary ion system decreased by 18.34%, 43.36%, and 13.02%, respectively, suggesting that mutual effects among the metal ions existed.   The removal efficiency of magnetic nano-chitosan for Cu 2+ , Pb 2+ , and Zn 2+ was also reduced in the presence of coexisting ions. As shown in Figure 6a2, the influence of Pb 2+ to Cu 2+ was stronger than that of Zn 2+ to Cu 2+ . It can be seen from Figure 6b2 that the effect of coexisting ions on Pb 2+ exhibited Cu 2+ and Zn 2+ > Cu 2+ > Zn 2+ . From Figure 6c2, the influence of coexisting ions on Zn 2+ presented Pb 2+ stronger than Cu 2+ and Pb 2+ stronger than Cu 2+ . Overall, it seems that the mutual competitive adsorption of Cu 2+ , Pb 2+ , and Zn 2+ was obvious, with Pb 2+ being relatively strongly affected by the coexisting ions.
The decrease in the adsorption capacity of Cu 2+ , Pb 2+ , and Zn 2+ in the binary and ternary ion systems compared to that in the single ion system was mainly due to the competitive adsorption effects of coexisting Pb 2+ and Zn 2+ , Zn 2+ and Cu 2+ , and Cu 2+ and Pb 2+ . Table 5 shows the fitted results of the Langmuir and Freundlich isothermal adsorption models for the competitive adsorption of Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic nano-chitosan in aqueous solution. As shown in Table 5, the R 2 fitted by the Langmuir isothermal adsorption model for the competitive adsorption of Pb 2+ -Cu 2+ , Pb 2+ -Zn 2+ , and Zn 2+ -Cu 2+ in aqueous solution onto magnetic nano-chitosan was above 0.930, and the R 2 fitted by the Freundlich isothermal adsorption model was above 0.940, indicating good fitting effects.
In order to clearly determine the specific effects of coexisting metal ions on the adsorption of Cu 2+ , Pb 2+ , and Zn 2+ , the absolute equilibrium adsorption capacity (∆q e = q e -q e competitor ) of the target ions was compared and the equilibrium adsorption capacity of the competitor (q e competitor of Cu 2+ /Zn 2+ /Pb 2+ ) was further conducted. From Figure 7a, when Pb 2+ and Zn 2+ were competitors, with the increase in the initial concentration of target Cu 2+ , the q e of Pb 2+ kept at a constant level, the ∆q e Cu-Pb first increased and then remained at a constant level, indicating no influence between Cu 2+ and Pb 2+ ; the q e of Zn and ∆q e Cu-Zn exhibited a decreasing trend, illustrating the existing mutual inhibition effects between Cu 2+ and Zn 2+ . From Figure 7b, when Cu 2+ and Zn 2+ were competitors, with the increase in the target Pb 2+ initial concentration, the q e of Cu 2+ and Zn 2+ decreased and the ∆q e Pb-Cu and ∆q e Pb-Zn increased, implying an inhibition of Pb 2+ to Cu 2+ and Zn 2+ . From Figure 7c, when Cu 2+ and Pb 2+ were competitors, with the increase in the target Zn 2+ initial concentration, the q e of Cu 2+ and ∆q e Zn-Cu decreased, showing a mutual inhibition between Zn 2+ and Cu 2+ ; the q e of Pb 2+ and ∆q e Zn-Pb were kept a constant level, indicating no effect of Zn 2+ on Pb 2+ as well as the inhibition of Pb 2+ to Zn 2+ . From Figure 7d, in the ternary ion system, with the increase in the metal ion initial concentration (C 0 ), ∆q e Pb 2+ increased linearly, ∆q e Cu 2+ first increased and then tended to stable, and ∆q e Zn 2+ first increased and then decreased. They presented ∆q e Zn 2+ > ∆q e Cu 2+ ≈ ∆q e Pb 2+ at the C 0 of < 200 µmol/L, ∆q e Cu 2+ > ∆q e Zn 2+ or ∆q e Pb 2+ at the C 0 of 200-600 µmol/L, and ∆q e Pb 2+ > ∆q e Cu 2+ > ∆q e Zn 2+ at the C 0 of > 600 µmol/L. These indicate that the inhibition of coexisting ions to Pb 2+ adsorption gradually decreased with the increase in the metal ion initial concentration, and the inhibition of coexisting ions to Zn 2+ and Cu 2+ adsorption first decreased and then tended to be strong or stable, indicating that the three heavy metal ions have mutual effects when they coexist, and the competitive adsorption was obvious.

Conclusions
The adsorption kinetics of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic mano-chitosan in aqueous solution was well described by the pseudo-second kinetic model, being mainly chemisorption. The adsorption thermodynamics was well fitted by the Langmuir isothermal adsorption model, the adsorption was mainly unimolecular layer adsorption, and the qm of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ at 318 K was 301.057, 198.861, 121.9421, and 62.727 µmol/g, respectively. With the dosage increase in magnetic nano-chitosan, the qe of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ decreased from fast to slowly and their η first increased and then slowly changed. With the increase in the solution's pH, the qe and η of Cr 6+ first increased and then de- (b), and Zn 2+ (c) in binary ion system, respectively; ∆q e Cu 2+ , ∆q e Pb 2+ , and ∆q e Zn 2+ represent the absolute equilibrium adsorption capacity of Cu 2+ , Pb 2+ , and Zn 2+ in ternary ion system (d), respectively).
The number of metal ions adsorbed on the surface of magnetic chitosan is not only related to the characteristics of the adsorbent but is also related to other factors, such as the hydration radius, ion-exchange, metal ion complexation, and electrostatic interactions, which were the main governing mechanisms for almost all the chitosan-based materials and usually function together to achieve the adsorption of metal ions from the aqueous solution [58]. The present competitive adsorption results showed that the competitive adsorption order of the three metal ions was Cu 2+ > Pb 2+ > Zn 2+ . Generally, the metal adsorption affinity increases with the increasing hydrolysis constant of the metal ions. Previous studies have shown that the order of hydrolysis constants of the metal ions studied is Pb 2+ (10 −7.71 ) > Cu 2+ (10 −8 ) > Zn 2+ (10 −9 ). The electronegativity of metal ions studied followed Pb 2+ (2.33) > Cu 2+ (1.96) > Zn 2+ (1.65), indicating that Pb 2+ has a greater competitive advantage in adsorption [59,60]. Meanwhile, the hydration radius of Pb 2+ (4.01 Å) is smaller than that of Cu 2+ (4.19 Å) and Zn 2+ (4.30 Å), which is consistent with the metal adsorption capacity of Pb 2+ and Zn 2+ [39,43]. A previous study also demonstrated that metal ions with smaller ionic diameters have higher adsorption rates [60,61]. The adsorption capacity of magnetic chitosan for Cu 2+ was higher than that of Pb 2+ and Zn 2+ , which can be attributed to the formation of Cu· · · NH-complex [62], in which a pair of lone electrons in the nitrogen atom are contributed to the common bond between N and Cu 2+ .

Conclusions
The adsorption kinetics of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic mano-chitosan in aqueous solution was well described by the pseudo-second kinetic model, being mainly chemisorption. The adsorption thermodynamics was well fitted by the Langmuir isothermal adsorption model, the adsorption was mainly unimolecular layer adsorption, and the q m of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ at 318 K was 301.057, 198.861, 121.9421, and 62.727 µmol/g, respectively. With the dosage increase in magnetic nano-chitosan, the q e of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ decreased from fast to slowly and their η first increased and then slowly changed. With the increase in the solution's pH, the q e and η of Cr 6+ first increased and then decreased, being up to their maximum values at pH = 2; the q e and η of Cu 2+ , Pb 2+ , and Zn 2+ increased at the solution pH of < 4, and slowly changed at the solution pH of > 4. With the increase in the initial concentration of metal ions, the q e increased, the temperature was higher, and the q e was larger, i.e., 318 K > 308 K > 298 K; the η of Cr 6+ , Cu 2+ , and Zn 2+ decreased continuously, while the η of Pb 2+ showed a trend of first increasing and then decreasing, and the adsorption of metal ions was a spontaneous and feasible endothermic process. The q e and η in the binary and ternary ion systems decreased compared to those in the single ion system. There was the mutual adsorption influence among metal ions when they co-existed. In the ternary ion system, the q m of Cu 2+ could be up to 78.4616 µmol/g. The current study's results provide theoretical support for the simultaneous treatment of harmful metal ions in wastewater by magnetic mano-chitosan.
In the present study, the single and completive adsorption of Cr 6+ , Cu 2+ , Pb 2+ , and Zn 2+ onto magnetic mano-chitosan in aqueous solution were systematically studied; however, the adsorption capacities were relatively moderate. Therefore, the recycle of magnetic mano-chitosan was not conducted. In future studies, the magnetic nano-Fe 3 O 4 will be first salinized or aminated, and then cross-linked with chitosan by using glutaraldehyde to improve the adsorption capacities of metal ions. Additionally, the recycle of the adsorbents will also be studied.