A novel core-shell bio-adsorbent was fabricated by using biological materials for removing methyl orange (MO) from aqueous solution. The structure characteristics results of scanning electron microscopy (SEM), Fourier transform infrared spectrometry (FT-IR), thermo-gravimetric analysis (TGA), vibrating sample magnetometer (VSM), and Brunauer–Emmett–Teller (BET) shows that Fe3O4-CS-L has been successfully prepared. The effects of contact time, pH, temperature and initial concentration were explored. The results suggested pH was a negligible factor in adsorption progress. Kinetic studies showed that the experiment data followed pseudo-second-order model. Boyd mode suggested that external mass transfer showed a rather weak rate control for MO adsorption onto Fe3O4-CS-L. Equilibrium studies showed that isotherm data were the best described by Langmuir model. The maximum adsorption capacity of MO estimated to be 338.98 mg/g at 298 K. Moreover, the adsorption capacity of Fe3O4-CS-L can keep about 74% in the fifth adsorption–regeneration cycle. Thus, the Fe3O4-CS-L could be a kind of promising material for removing MO from wastewater.

In recent years, water pollution has caught social and academic attention. Dye and heavy metals pollution are the two most common pollution scenarios in water pollution. More than 7 × 105 tones commercial dyes are produced per year for coloring product (Sen et al. 2010). Among them, the consumption of dyes are more than 10,000 t/year in the textile industry and over 100 t/year of dyes is discharged into environment (Yagub et al. 2012). It is very difficult to degradation for most of dyes, due to complex composition, stable chemical properties and high chemical oxygen consumption. Hence, the treatment of dye wastewater are attracted more and more attention.

Various of techniques have been used to deal with dye wastewater such as membrane filtration (Alventosa et al. 2012), aerobic or anaerobic treatment (Hosseini et al. 2011), electrochemical treatment (Körbahti et al. 2011), coagulation–flocculation (García et al. 2008) and adsorption methods (Tang et al. 2013; Zhang et al. 2014). Among of them, more and more attention is given to adsorption because of its cost-effective and easy to operate. Conventionally, inorganic nanoparticle (Muthukumaran et al. 2016; Wang et al. 2016), zeolites (Faki et al. 2008), bentonite (Weng & Pan 2007; Lian et al. 2009) and active carbon (Jiang et al. 2015) were reported to deal with dye wastewater. As one coin has two sides, these adsorbents also have their own shortcomings, such as separation inconvenience, high cost of raw materials and low adsorption capacity. Hence, it is the design aim of adsorbent which own the advantages of high adsorption capacity, low cost of raw materials and facile separation.

Chitosan, as a biopolymer, can be extracted from chitin and has abundant sources (Wu et al. 2016). Due to abundant amino groups and hydroxy groups, which were located on the carbon skeleton of chitosan, chitosan have potential for removing dye from wastewater by adsorption method. Recently, magnetic adsorbents have been widely used to remove dyes and heavy metals from aqueous solution, because it can enhance the separation rate (Asfaram et al. 2015; Habila et al. 2016). Chitosan was often used to combine with magnetic materials to prepare magnetic core-shell nanoparticles which consist of an iron oxide core and chitosan shell. Li has investigated that 3-chloro-2-hydroxypropyl trimethyl ammonium modified chitosan magnetic composite was used to remove methyl orange (MO) from aqueous solution (Li et al. 2016). Elwakeel also discussed magnetic chitosan for the removal of reactive black 5 from wastewater (Elwakeel 2009). However, chitosan is not stable and soluble in acidic solution and its ability to adsorb dye is still insufficient, which limits its future application in the treatment of dye wastewater. In modifying the chitosan shells by chemical modification, it not only can solve the instability and acid resistance, but also can achieve a specific function, such as large adsorption capacity and bioseparation (Reddy & Lee 2013). Various dyes could be adsorbed by some specific interactions such as hydrogen bond and electrostatic attraction (Sakkayawong et al. 2005). Various functional groups could be flexibly grafted onto chitosan for effective adsorption of dyes (Li et al. 2016). L-arginine with amino groups and carboxyl groups has good water solubility. It could improve stability and adsorption capacity of chitosan, if chitosan was modified by L-arginine (Arash et al. 2016).

In this paper, a novel magnetic core-shell adsorbent was prepared by grafting chitosan and L-arginine on the surface of Fe3O4. This magnetic adsorbent was developed for the adsorption of MO from aqueous solution. Combining functionalities of magnetism and biopolymer, magnetic core-shell chitosan nanoparticles show fast separation properties and high adsorption capacity.

Materials

Potassium iodide, FeSO4·7H2O, L-arginine, sodium hydroxide, sodium thiosulfate, acetone, NaOH, HNO3 and ethanol were purchased from Sinopharm Chemical Reagents Corp. (Shanghai, China). Chitosan, glutaraldehyde and epoxy chloropropane were Aladdin products (Shanghai, China). In this study, the water is deionized water and all reagents were of analytical grade.

Synthesis of magnetic composite

Fe3O4 is prepared by traditional hydrothermal synthesis method (Guo et al. 2017).

Chitosan (1.0 g) was dissolved in acetic solution (3%, 100 mL), then Fe3O4 (1.0 g) were added in the chitosan solution. Glutaraldehyde (8.0 mL) was dropwise added into the mixed solution under stirring at 333 K for 2 h (Fan et al. 2012). The residues were washed with deionized water and ethanol until pH was about 7. Afterward, the residues were dissolved in acetone (100 mL) and stirred to form a suspension. Epichlorohydrin (5.0 mL) was added dropwise to suspensions. This suspension was stirred for 24 h at 303 K (Kuang et al. 2013).

L-arginine (6.0 g) was dissolved in dimethyl formamide (50 mL) and then slowly dropped into the reaction, K2CO3 (3.0 g) which have roasted under high temperature and KI (0.05 g) were added in solution and the mixture was reflux at 338 K for 12 h. The residues were washed with distilled water, ethanol and acetone in turn. The residue was dried at 333 K under vacuum condition. The structure diagram of Fe3O4-CS-L is shown in Figure 1.

Figure 1

The structure diagram of Fe3O4-CS-L.

Figure 1

The structure diagram of Fe3O4-CS-L.

Characterization

X-ray powder diffraction (XRD) results were obtained on Rigaku IV (Rigaku, Japan). Scanning electron microscope (SEM) images were obtained on IGMA (Carl Zeiss, Germany). The hysteresis loops were tested by a vibrating sample magnetometer (LDJ 9600, USA). Fourier transform infrared (FT-IR) spectra were gained by infrared spectrometer (Thermo Nicolet 380, USA). The thermogravimetric analysis (TGA) result was obtained by thermogravimetric analyzer (SDTA851, Swit). The concentration of MO was determined with double beam ultraviolet (UV)–vis spectrophotometer (TU-1901, China).

Batch adsorption experiments

Batch adsorption experiments were shocked on water bath equipment (Bote Ltd, China) with a shock speed of 300 rpm, Fe3O4-CS-L (0.1 g) was added in 100 mL different concentrations of MO solution. NaOH(0.1 mol/L) or HNO3(0.1 mol/L) solution was used to adjust the pH of the aqueous solutions. The adsorption capacity of Fe3O4-CS-L at any time and equilibrium are calculated according to the formula:
formula
(1)
formula
(2)
The removal percentage (%) was calculated as follows:
formula
(3)
where C0 (mg/L), Ct (mg/L) and Ce (mg/L) represent the initial MO concentration, at time t and the equilibrium time, respectively; V on behalf of the volume of the MO solution (mL) and W is the amount of adsorbent (mg).

Characteristics of the prepared Fe3O4-CS-L

Figure 2 shows the microtopography of Fe3O4 and Fe3O4-CS-L. From Figure 2, we found that Fe3O4 and Fe3O4-CS-L showed irregular polyhedron morphologies. Moreover, the radius of Fe3O4-CS-L was larger than that of Fe3O4. The agglomeration of Fe3O4 was more serious than Fe3O4-CS-L.

Figure 2

SEM morphology of Fe3O4 (a) and Fe3O4-CS-L (b).

Figure 2

SEM morphology of Fe3O4 (a) and Fe3O4-CS-L (b).

Figure 3 shows FT-IR spectra of Fe3O4, Fe3O4-CS and Fe3O4-CS-L. As for Fe3O4, Fe3O4-CS and Fe3O4-CS-L, the absorption peak in 568 cm−1 appears which related to Fe-O stretching vibrations. As for magnetic chitosan, a broad weak absorption peak in the range of 2,600–3,300 cm−1 appears which corresponds to N-H and O-H stretching vibrations, the new peaks at 2,030 cm−1 are ascribed to the ammonium salt feature peaks, the peaks at 1,210–1,010 cm−1 are related to the C-O and C-N stretching vibrations. It proved that chitosan has been grafted on surface of magnetic Fe3O4 with the help of glutaraldehyde. When L-arginine is grafted on magnetic chitosan, a new band at 1,750 cm−1 and 1,690–1,640 cm−1 due to C = O and C = N stretching vibration in L-arginine. In addition, the obviously strengthened peak at 1,073 cm−1 in Fe3O4-CS-L, which is related to the reaction between acyl chloride (in Fe3O4-CS) and amino (in L-arginine). The FT-IR spectra results proved that Fe3O4-CS-L has been prepared successfully.

Figure 3

The IR spectra of Fe3O4, Fe3O4-CS and Fe3O4-CS-L.

Figure 3

The IR spectra of Fe3O4, Fe3O4-CS and Fe3O4-CS-L.

Thermogravimetric results of Fe3O4, Fe3O4-CS and Fe3O4-CS-L are shown in Figure 4. From Figure 4, it can be found that there were two decomposed quantities of 3.67% from 55 °C to 122 °C, and of 3.27% from 601 °C to 708 °C in magnetic Fe3O4 thermogravimetric curve, corresponding to adsorbed water and to condensation of the iron oxide hydroxyl. For Fe3O4-CS, there is an additional mass loss step of 44.1% from 135 °C to 715 °C attributed to the chitosan grafted on surface of magnetic Fe3O4. As for Fe3O4-CS-L, it showed four decomposed quantities of 4.15% from 55 °C to 120 °C, of 31.46% from 122 °C to 352 °C, of 7.19% from 352 °C to 419 °C, and of 15% from 419 °C to 516 °C, which are related to adsorbed water, amino, carboxyl and hydroxyl in chitosan carbon skeleton thermal decomposition. The TGA results showed that the grafted chitosan and L-arginine were calculated to be approximately 58.41% for the total mass of Fe3O4-CS-L.

Figure 4

Thermogravimetric curve of magnetic Fe3O4, Fe3O4-CS and Fe3O4-CS-L.

Figure 4

Thermogravimetric curve of magnetic Fe3O4, Fe3O4-CS and Fe3O4-CS-L.

Figure S1 (available with the online version of this paper) shows Brunauer–Emmett–Teller (BET) results of Fe3O4-CS-L. Adsorption measurements were performed under the same conditions, followed by desorption measurements. According to International Union of Pure and Applied Chemistry (IUPAC) classification, the isotherm is a type III and the BET surface area is 3.63 m2/g.

The magnetization of Fe3O4 and Fe3O4-CS-L were investigated with a vibrating sample magnetometer (VSM). From Figure S2 (available online), the saturation magnetization (Ms) value of Fe3O4 and Fe3O4-CS-L were 57.6 and 14.1 emu/g, respectively. Although magnetization strength decreased after grafting chitosan and L-arginine, it could be concluded that the separation of Fe3O4-CS-L in aqueous solution could be controlled by electromagnet with convertible magnetic field.

Adsorption experiments

Effect of initial MO concentration

Figure 5 shows the effect of initial MO concentration on adsorption capacity. As shown in Figure 5, the equilibrium adsorption capacity increases while the removal efficiencies drop rapidly with increases of initial MO concentrations from 100 mg/L to 500 mg/L. At range of 500–700 mg/L, the equilibrium adsorption capacity keeps in a stable value (240 mg/g). This is due to the fact that the functional groups which were located on the surface of Fe3O4-CS-L tend to saturate with increases of MO concentration and it reached saturation at a concentration of 500 mg/L. So, the equilibrium adsorption capacity remains constant in the range of 500–700 mg/L. Thus, the optimal MO concentration was confirmed at 500 mg/L for the following experiments.

Figure 5

Effect of initial concentration of MO on adsorption removal efficiency and adsorption capacity (Experiment condition: pH = 3, T = 298 K, contact time is 50 min).

Figure 5

Effect of initial concentration of MO on adsorption removal efficiency and adsorption capacity (Experiment condition: pH = 3, T = 298 K, contact time is 50 min).

Effect of pH

As we all know, there are two main structures of MO at different pH values. It is present as a quinone type at a lower pH (under acidic conditions), while it is rearranged into azo structure at a higher pH (Zhu et al. 2010). The rearrangement of MO is shown below:

The adsorption capacity was certainly influenced by the change of structure of MO, so it is essential to explore the effect of pH changes on adsorption capacity. Figure 6 shows that the change of adsorption capacity was investigated at varying pH values 2–11. As shown in Figure 6, the adsorption capacity increased from 231 mg/g to 252 mg/g with increases of pH from 2 to 3. The maximum value of adsorption capacity was reach at pH = 3, and the adsorption capacity decreases at range of 3–11.

Figure 6

Effect of pH on the adsorption capacity (Experiment condition: C0 = 500 mg/L, T = 298 K, contact time is 50 min).

Figure 6

Effect of pH on the adsorption capacity (Experiment condition: C0 = 500 mg/L, T = 298 K, contact time is 50 min).

The dissociation constant of MO is 3.46, it means the structure of MO is presented as quinone at the pH<3.0, the two main active sites of quinone structure are sulfinyl and amino groups. The –S = O group can be served as the electron pair acceptor and form regular hydrogen bonding with the amino group (–NH2) of Fe3O4-CS-L surface, while the –NH group can be served as the electron pair donor and form regular hydrogen bonding with the carboxyl group (–COOH) in Fe3O4-CS-L composite. The hydrogen bonding between MO and Fe3O4-CS-L may result in the presence of large adsorption capacity at lower pH. With increases of pH from 3 to 11, the adsorption capacities of MO drop gradually. When the pH is increased, quinone structure of MO is gradually transformed into its azo structure. Azo structure of MO only has one reaction site –S = O can form regular hydrogen bonding and the hydrogen bonding between MO and Fe3O4-CS-L is reduced, which leads to the decrease of adsorption capacity (Liu et al. 2015). In addition, there is competition repulsion existence between MO anions and OH, which lead to the decrease in MO adsorption (Subbaiah & Kim 2016).

Effects of contact time and temperature

Contact time and temperature are two of the most important factors which can affect adsorption progress. As shown in Figure 7, the effect of temperature and contact time on adsorption MO. From Figure 7, we can find that the adsorption capacity is increasing with increases of contact time within 50 min and then the adsorption capacity keep a stable value beyond 50 min. In addition, the adsorption capacity is increasing with increases of temperature from 298 K to 313 K, it also proved that the adsorption progress is endothermic in nature. According to the above results, 50 min was considered to equilibrium time in batch adsorption experiments. There is no significant change of adsorption capacity from 298 K to 313 K, so the temperature is determined as 298 K in the following experiment.

Figure 7

Effect of contact time and temperature on the adsorption removal efficiency of MO (Experiment condition: pH = 3, C0 = 500 mg/L, T = 298 K).

Figure 7

Effect of contact time and temperature on the adsorption removal efficiency of MO (Experiment condition: pH = 3, C0 = 500 mg/L, T = 298 K).

Adsorption kinetic

The mechanism of adsorption progress can be investigated by fitting kinetic model. Considerable research efforts have been devoted to three kinetic models (pseudo-first-order kinetic model, pseudo-second-order kinetic model and intraparticle diffusion model) to describe adsorption progress mechanism.

The equation of pseudo-first-order model and pseudo-second-order are represented as follows:
formula
(4)
formula
(5)
where qt and qe (mg/g) are the adsorption capacity for MO adsorbed at any time t and at equilibrium time. K1 (min−1) is the rate constant of the pseudo-first-order and K2 (min−1) is the rate constant of the pseudo-second-order. t is contact time. The data of the fitted models by use of these equations are shown in Table 1 and Figure S3 (available online).
Table 1

Dynamic parameters for the adsorption of MO onto Fe3O4-CS-L

T(K)Prsudo-first-order
Prsudo-second-order
K1qeR2K2qeR2
298 0.075 219.28 0.991 3.60 × 10−4 292.40 0.9991 
303 0.081 233.35 0.988 3.66 × 10−4 297.62 0.9992 
308 0.092 259.42 0.971 3.74 × 10−4 302.11 0.9990 
313 0.097 272.27 0.966 3.94 × 10−4 302.75 0.9991 
T(K)Prsudo-first-order
Prsudo-second-order
K1qeR2K2qeR2
298 0.075 219.28 0.991 3.60 × 10−4 292.40 0.9991 
303 0.081 233.35 0.988 3.66 × 10−4 297.62 0.9992 
308 0.092 259.42 0.971 3.74 × 10−4 302.11 0.9990 
313 0.097 272.27 0.966 3.94 × 10−4 302.75 0.9991 

The R2 values of the pseudo-second-order model are higher than the pseudo-first-order model. In others word, the pseudo-second-order model is more suitable for describing the adsorption behavior of MO on Fe3O4-CS-L, indicating the sort of adsorption is chemical adsorption.

As we all know, the adsorption process can be divided into three stages: (1) MO was moved from the bulk solution to the boundary layer; (2) diffuse through the boundary layer to the surface of adsorbent; (3) adsorb onto an active site. Intraparticle diffusion model was often used to describe the contribution of intraparticle diffusion to rate control step. The intraparticle diffusion model can be described as follows:
formula
(6)
where C is a constant and Ki is the rate constant. The data of the fitted models by using of these equations are shown in Table 2 and Figure S4 (available online). It is obvious that the qtt0.5 polts can be divided into three distinct regions. The external mass transfer is related to the first linear portion. The intraparticle diffusion is related to the second linear portion. The adsorption–desorption equilibrium is related to the third linear portion. It is important to highlight that the time of intraparticle diffusion is decreasing with increases of temperature, indicating temperature would influence the diffusion rate in intraparticle diffusion stage. Table 2 data show that the second linear portion does not get through the origin point from 293 to 308 K, meaning that intraparticle diffusion is not the only rate-limiting step.
Table 2

Parameters of the Intraparticle diffusion model for the adsorption of MO onto Fe3O4-CS-L

T(K)First linear portion
Second linear portion
Kd1C1R12Kd2C2R22
298 58.923 −33.230 0.9995 19.951 110.036 0.9840 
303 61.543 −34.377 0.9990 21.513 106.308 0.9652 
308 60.776 −29.622 0.9999 27.335 83.003 0.9864 
313 60.512 −25.688 0.9977 27.904 86.055 0.9818 
T(K)First linear portion
Second linear portion
Kd1C1R12Kd2C2R22
298 58.923 −33.230 0.9995 19.951 110.036 0.9840 
303 61.543 −34.377 0.9990 21.513 106.308 0.9652 
308 60.776 −29.622 0.9999 27.335 83.003 0.9864 
313 60.512 −25.688 0.9977 27.904 86.055 0.9818 
Boyd mode was used to distinguish the rate control step in adsorption between intraparticle diffusion and mass transfer. Boyd mode can been described as follows:
formula
(7)
where qt and qe are the adsorption capacity of MO adsorbed on the adsorbent (mg/g) at any time t (min) and at equilibrium time (min), respectively. As shown in Figure S5 (available online), the fitted linear (298–313 K) do not pass through the origin point, confirming the existence of external mass transfer in the whole adsorption process. What is more, it is clear that there are two parts in the fitting line, implying that external mass transfer shows a rather weak rate control for MO adsorption onto Fe3O4-CS-L materials (Ma et al. 2012).

Adsorption isotherms

Adsorption isotherms can be expressed as functional equations which describes the amount of metal ions adsorbed per unit mass of the adsorbent and the concentration of metal ions in bulk solution under equilibrium conditions. The three most common adsorption isotherm for describing solid–liquid sorption systems are Langmuir, Freundlich and Temkin isotherms The equation for the Langmuir (8), Freundlich (9) and Temkin (10) isotherm are as follows:
formula
(8)
formula
(9)
formula
(10)
where Ce and qe are the equilibrium concentration of the metal ions (mg/L) and the adsorption capacity (mg/g), respectively. The maximum adsorption capacity of adsorbents (mg/g) and the adsorption constant (L/mg) in Langmuir mode are represented by qm and KL. KF is related to the adsorption capacity. n is the Freundlich constants which positive relate to the adsorption strength. b and A are the constant related to the adsorption heat (J/mol) and isothermal adsorption constant of Temkin, respectively. T is the temperature (K). R is the ideal gas constant (8.314 J/mol K).

The fitted data for MO adsorption on Fe3O4-CS-L were obtained as shown in Figure S6 (available online) and Table 3. As shown in Table 3, R2 of Langmuir isotherm is greater than the Freundlich and Temkin model, it means that the Langmuir isotherm model is more suitable for the adsorption process. In other words, the adsorption process is a single adsorption and active sites evenly distributed on the surface of Fe3O4-CS-L.

Table 3

Langmuir, Freundlich and Temkin isotherm model parameters for MO adsorption on Fe3O4-CS-L

T(K)Langmuir
Freundlich
Temkin
qmKLRLR2KFnR2AbR2
298 338.98 0.0031 0.315–0.762 0.997 6.668 1.816 0.979 0.026 31.09 0.994 
303 353.36 0.0030 0.323–0.769 0.999 6.486 1.790 0.977 0.025 30.53 0.992 
308 363.64 0.0029 0.330–0.775 0.997 6.389 1.772 0.982 0.025 30.25 0.991 
313 374.53 0.0027 0.346–0.787 0.998 6.310 1.757 0.980 0.024 29.99 0.995 
T(K)Langmuir
Freundlich
Temkin
qmKLRLR2KFnR2AbR2
298 338.98 0.0031 0.315–0.762 0.997 6.668 1.816 0.979 0.026 31.09 0.994 
303 353.36 0.0030 0.323–0.769 0.999 6.486 1.790 0.977 0.025 30.53 0.992 
308 363.64 0.0029 0.330–0.775 0.997 6.389 1.772 0.982 0.025 30.25 0.991 
313 374.53 0.0027 0.346–0.787 0.998 6.310 1.757 0.980 0.024 29.99 0.995 
In Langmuir mode, the degree of appropriate of adsorbent towards MO is predicted from the values of separation factor constant (RL), which is shown as follows:
formula
(11)
where KL is the Langmuir isotherm constant (L/mg). C0 is the initial MO concentration (mg/L). The value of RL shows that the adsorption MO on Fe3O4-CS-L is favorable (0 < RL < 1) or irreversible (RL = 0), linear (RL = 1), unfavorable (RL > 1) (Ren et al. 2013). As shown in Table 3, the values of RL were between 0.3 and 0.7, suggesting favorable adsorption of the MO on Fe3O4-CS-L.

Possible adsorption mechanism of MO by Fe3O4-CS-L

Figure 8 shows the conceptual model for the MO removal mechanism by Fe3O4-CS-L. MO is represented as the quinone structure in the solution when the pH is lower than 3.46. The secondary amino groups and sulfinyl on the surface of the quinone structure of MO can form hydrogen bonds with the carboxyl and amino groups on the surface of Fe3O4-CS-L, respectively. In addition, the electrostatic force is a factor that cannot be ignored in the adsorption process. There is electrostatic repulsion that exists between N+ group in quinone type structure of MO and –NH3+ group of Fe3O4-CS-L surface. With increasing the value of pH, the quinone structure of MO was gradually transformed into azo structure, the hydrogen bond between –NH and –COOH becomes gradually weakened and the electrostatic repulsion between N+ group in quinone type structure of MO and –NH3+ group of Fe3O4-CS-L surface also become gradually weakened. As shown in Figure 8, the adsorption capacity gradually decreased with the increase of pH value, suggesting that the influence of hydrogen bond is much larger than electrostatic interaction on MO adsorption progress. What is more, the Van der Waals force enhances as the unit surface charge density increase, which suppress the adsorption capacity of MO in Fe3O4-CS-L.

Figure 8

The conceptual model for the MO removal mechanism.

Figure 8

The conceptual model for the MO removal mechanism.

Removal of MO in real wastewater samples

The adsorption progress was often influenced by various kinds of factors such as ion strength and coexistence organic molecules. In order to confirm the adsorption capacity of Fe3O4-CS-L under real conditions, removal of MO in real wastewater samples were investigated. The sample of wastewater was obtained from Ruibei printing and dyeing mill (Jiangsu, China). Nylon membrane(0.45 μm) was used to removal contamination particles of samples. Table 4 shows the main components of the sample. Figure S7 (available online) shows that the effect of contact time on MO adsorption under real conditions. From Figure S7, it can be found that the adsorption equilibrium contact time was 60 min, which was larger than the adsorption time in the simulated solution. Moreover, the adsorption capacity of Fe3O4-CS-L is 145 mg/g, which is reduced by 23% comparing to the adsorption capacity in the simulated solution at pH = 8.3. It can be explained that there are a variety of co-existing ions present in the actual wastewater. Azo structure of MO could be combined with organic groups by hydrogen bonding, resulting in a larger molecular structure and increased resistance to diffusion. In addition, azo structure with negative charged can interact with the coexistence of cations by electrostatic attraction, thereby reducing the interaction with Fe3O4-CS-L, which also results in slower adsorption rates and lower adsorption capacity. Moreover, the coexistence of cations can also interact with the functional groups on Fe3O4-CS-L, resulting in a decrease in hydrogen bonding between Fe3O4-CS-L and MO. This also leads to a decrease in adsorption capacity.

Table 4

The main components of the water samples

Main componentspHMO (mg/L)Amine base class (mg/L)Salts contain (%)
Concentration 8.3 431 76 19.2 
Main componentspHMO (mg/L)Amine base class (mg/L)Salts contain (%)
Concentration 8.3 431 76 19.2 

Regeneration of Fe3O4-CS-L

It is a very important feature for adsorbent in industrial applications. Figure S8 (available online) shows desorption and reusability studies of Fe3O4-CS-L in 1 mol/L HNO3. The adsorption capacity of Fe3O4-CS-L can keep about 74% in the fifth adsorption-regeneration cycle, suggesting that this material has the prospect of industrial applications.

Comparison with other adsorbents

Table 5 shows the maximum adsorption capacities of different types of adsorbent. It was found that the adsorption capacity of Fe3O4-CS-L was greater than other adsorbent for MO adsorption, suggesting that Fe3O4-CS-L can be used as an efficient adsorbent of MO.

Table 5

Comparison of MO adsorption capacity among different adsorbents

Adsorbentqm(mg/g)References
γ-Fe2O3/2C nanocomposite 42.34 Istratie et al. (2016)  
Mesoporous NiO microspheres 137.00 Jia et al. (2014)  
CuO/NaA zeolite 79.49 Mekatel et al. (2015)  
Activated carbon/NiFe2O4 182.82 Jiang et al. (2015)  
Chitosan biomass 29.00 Allouche et al. (2015)  
Maghemite/chitosannanocomposite films 29.41 Jiang et al. (2012)  
Immobilized PANI/glass 93.0 Haitham et al. (2015)  
Activated clay 16.78 Ma et al. (2013)  
Fe3O4-CS-L 338.98 In this work 
Adsorbentqm(mg/g)References
γ-Fe2O3/2C nanocomposite 42.34 Istratie et al. (2016)  
Mesoporous NiO microspheres 137.00 Jia et al. (2014)  
CuO/NaA zeolite 79.49 Mekatel et al. (2015)  
Activated carbon/NiFe2O4 182.82 Jiang et al. (2015)  
Chitosan biomass 29.00 Allouche et al. (2015)  
Maghemite/chitosannanocomposite films 29.41 Jiang et al. (2012)  
Immobilized PANI/glass 93.0 Haitham et al. (2015)  
Activated clay 16.78 Ma et al. (2013)  
Fe3O4-CS-L 338.98 In this work 

In summary, a novel magnetic bio-adsorbent was prepared by use of chitosan and L-arginine. The SEM and FT-IR results show that chitosan and L-arginine have been grafted on surface of magnetic Fe3O4 and TGA results future confirms that chitosan and L-arginine account for 58.4% of total mass. pH plays an important role in adsorption progress and the optimum pH for adsorption is 3. The adsorption capacity of Fe3O4-CS-L has reached saturated at a concentration of 500 mg/L. Kinetic studies show that the experiment data can be better described by pseudo-second-order model, not pseudo-second-order model. Boyd mode shows that external mass transfer shows a rather weak rate control for MO adsorption onto Fe3O4-CS-L. Equilibrium studies show that isotherm data were best described by Langmuir model. The maximum adsorption capacity of MO estimated to be 338.98 mg/g at 298 K and pH 3.0. The adsorption capacity of Fe3O4-CS-L can keep about 74% in the fifth adsorption-regeneration cycle, indicating that it has potential for industrial applications.

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Supplementary data