Effective Removal of Hexavalent Chromium from Aqueous Solutions Using Quaternary Ammonium-Functionalized Magnetic Graphene Oxide Composites

: Novel quaternary ammonium/magnetic graphene oxide composites (M-PAS-GO) that efﬁciently remove Cr(VI) ions were fabricated through the introduction of the (3-aminopropyl) triethoxysilane and Fe 3 O 4 nanoparticles on the surface of GO, and then modiﬁed with n-butyl bromide. The fabricated M-PAS-GO was comprehensively characterized by SEM, TEM, EDX, XRD, Raman spectroscopy and FTIR, and the results manifest that the quaternary ammonium group was introduced onto the surface of GO. Under the reaction conditions of pH 3.20, temperature of 25 ◦ C and M-PAS-GO dosage of 0.01 g/50 mL, 90% of 10 mg/L Cr(VI) ions were removed from the solution within 20 min. The kinetics study indicates that the adsorption process followed the pseudo-second-order model and was surface reaction-controlled. The thermodynamic parameters calculated from temperature-dependent adsorption isotherms suggest that the adsorption process was an exothermic and spontaneous process. The maximum adsorption capacities of Cr(VI) ions on M-PAS-GO composites calculated by the Langmuir model were 46.48 mg/g. Moreover, the reusability and stability of M-PAS-GO demonstrates its economic sustainability. This study suggests that M-PAS-GO is a potential candidate adsorbent for the separation of Cr(VI) from wastewater.


Introduction
Environmental pollution by heavy metal ions is spreading globally along with industrial progress; in particular, mercury, lead, copper, cadmium and chromium are detected in wastewater from chemical manufacturing, painting and coating, mining, extractive metallurgy, nuclear and other industries, which can accumulate in organisms and poses a serious threat to ecosystems and human health through the food chain [1,2]. Among these heavy metal ions, Cr(VI) is of great concern due to its high toxicity, carcinogenicity, mutagenicity and wide application in industry [3]. Hence, it is critical to remove Cr(VI) from wastewater before it is discharged into water bodies. To date, a variety of methods have been developed to remove Cr(VI) from aqueous media, including adsorption, coagulation, membrane filtration, chemical oxidation, photodegradation and electrodialysis [4][5][6][7][8]. Among these methods, the adsorption process has been most widely employed because of its high availability, easy operability, low cost and high efficiency. Various materials, such as biomaterials, metal oxides [9], nanomaterials [10], activated carbon [11] and fibrous [12] and mesoporous inorganic sorbents [13][14][15] have been applied to remove Cr(VI) from purification. All solutions were prepared with ultrapure water purified by a Millipore-Q system (18.2 MΩ cm).

Fabrication of Fe 3 O 4 Nanoparticles
Fe 3 O 4 nanoparticles were synthesized through the coprecipitation of Fe 2+ and Fe 3+ ions in ammonia solution and treatment under hydrothermal conditions [32,33]. Typically, 2.0 g of Fe 3 O 4 ·7H 2 O in 100 mL double-distilled water and 4.2 g of FeCl 3 ·6H 2 O also in 100 mL ultrapure water were thoroughly mixed, and then added to 8 M NH 4 OH under continuous stirring at 30 • C for 2 h. The precipitates were heated at 80 • C for 30 min, and the pH was maintained at 10 by adding NH 4 OH. Impurity ions (i.e., chlorides and sulphates) were removed by washing with hot distilled water. The obtained Fe 3 O 4 nanoparticles were sequentially washed with distilled water and ethanol several times, and then finally dried in a vacuum oven at 70 • C.

Synthesis of Quaternary Ammonium-Functionalized Magnetic Graphene Oxide Composite (M-PAS-GO)
The graphene oxide (GO) was obtained by the Hummers method through the oxidation of graphite powder [34]. In detail, 1.5 g of graphite powder was added to a concentrated H 2 SO 4 (80 mL) solution at 0 • C. Meanwhile, 4.0 g of KMnO 4 was slowly added to the mixture under vigorous stirring, and the temperature was carefully controlled at 20 • C. Then, the mixture was stirred at 35 • C for 24 h, and diluted with distilled water. After that, 14 mL of 30% H 2 O 2 solution was slowly added to the mixture until the color of the mixture changed to yellow. The mixture was purified by rinsing and centrifugation (10,000 rpm for 20 min) sequentially with 0.2 M HCl and distilled water several times. Finally, the solid GO was obtained after filtration and vacuum drying at 30 • C.
The amine-functionalized GO (APTES-GO) was synthesized by dispersing 100 mg of GO into 100 mL of toluene, and then adding 20 mL of APTES (2%) and magnetically stirring at 110 • C for 16 h. After centrifugation, the obtained APTES-GO was washed with toluene and acetone and then dried at 60 • C for 12 h.
The magnetic graphene oxide composite (M-APTES-GO) was synthesized according to the method reported in the literature [35]. Specifically, 100 mg of APTES-GO was added to 5 mL of buffer A (0.003 M phosphate, pH 6, 0.1 M NaCl), to which was then added 1.0 mL of carbodiimide solution, followed by sonication for 10 min (0.025 g/mL in buffer A). Then, 100 mg of Fe 3 O 4 was dispersed into the mixture solution and then sonicated for 1 h in an ice bath. Finally, the obtained composites were washed thoroughly with ethanol and collected by using an external permanent magnet, and then dried in an oven at 60 • C.
Quaternary ammonium-functionalized magnetic graphene oxide composite (M-PAS-GO) was fabricated by dispersing 100 mg of M-APTES-GO into 100 mL of ethanol, and then adding 20 mL of n-butyl bromide under continuous stirring for 4 h at 90 • C. The composite materials were then collected magnetically from the suspension formed and washed with ethanol, and finally dried at 40 • C (see Scheme 1).

Analytical Methods
Transmission electron microscopy (TEM, H7650, Japan), scanning electron microscopy (SEM, Hitachi S-4800, Tokyo, Japan) and X-ray diffraction (XRD, Smartlab 9, Rigaku, Tokyo, Japan) were used to analyze the surface morphology and crystal structures of Fe 3 O 4 , GO and M-PAS-GO, respectively. A spectrophotometer (Nicolet 6700, Madison, WI, USA) was used to record the Fourier transform infrared (FT-IR) spectra of Fe 3 O 4 , GO and M-PAS-GO between 400 and 4000 cm −1 with a resolution of 2 cm −1 . A vibrating sample magnetometer (VSM, LakeShore7307, Quantum Design, San Diego, CA, USA) was used to determine the magnetic properties of Fe 3 O 4 and M-PAS-GO at room temperature. The zeta potentials for a suspension of 0.5 g/L GO and M-PAS-GO in 0.1 M KNO 3 solution were determined using a Zetasizer Nano ZS90. Raman spectra were recorded with a Renishew Raman spectrometer. Brunauer-Emmett-Teller (BET) surface area was measured using the Micromeritics Separations 2023, 10, 463 4 of 17 ASAP 2010 at 77 K by N 2 adsorption-desorption isotherms. Thermogravimetric analysis (TGA) was recorded using a STA409PC DTA/TGA instrument (Netzsch, Bavaria, Germany) at a nitrogen flow rate of 50 mL/min in the temperature range of 25 • C to 800 • C at a rate of 10 • C/min. The concentrations of Cr(VI) were measured by ICP-OES (Perkin Elmer OptimaTM 2100 DV, PerkinElmer, Inc., Shelton, CT, USA).

Analytical Methods
Transmission electron microscopy (TEM, H7650, Japan), scanning electron microscopy (SEM, Hitachi S-4800, Tokyo, Japan) and X-ray diffraction (XRD, Smartlab 9, Rigaku, Tokyo, Japan) were used to analyze the surface morphology and crystal structures of Fe3O4, GO and M-PAS-GO, respectively. A spectrophotometer (Nicolet 6700, Madison, WI, USA) was used to record the Fourier transform infrared (FT-IR) spectra of Fe3O4, GO and M-PAS-GO between 400 and 4000 cm −1 with a resolution of 2 cm −1 . A vibrating sample magnetometer (VSM, LakeShore7307, Quantum Design, San Diego, CA, USA) was used to determine the magnetic properties of Fe3O4 and M-PAS-GO at room temperature. The zeta potentials for a suspension of 0.5 g/L GO and M-PAS-GO in 0.1 M KNO3 solution were determined using a Zetasizer Nano ZS90. Raman spectra were recorded with a Renishew Raman spectrometer. Brunauer-Emmett-Teller (BET) surface area was measured using the Micromeritics ASAP 2010 at 77 K by N2 adsorption-desorption isotherms. Thermogravimetric analysis (TGA) was recorded using a STA409PC DTA/TGA instrument (Netzsch, Bavaria, Germany) at a nitrogen flow rate of 50 mL/min in the temperature range of 25 °C to 800 °C at a rate of 10 °C/min. The concentrations of Cr(VI) were measured by ICP-OES (Perkin Elmer OptimaTM 2100 DV, PerkinElmer, Inc., Shelton, CT, USA).

Batch Sorption Experiment
K2Cr2O7 was used to prepare a Cr(VI) stock solution. All the sorption experiments were carried out according to the batch technique in a series of 50 mL conical bottles under continuous stirring of 200 r/min in a shaking water bath. The reaction solution was collected at 0 to 120 min during the adsorption process. Adsorption kinetics analysis and isotherm determination of Cr(VI) on M-PAS-GO composite were carried out at pH 3.20, and the initial concentrations of Cr(VI) ranged from 2.0 to 80.0 mg/L. The solution pH was adjusted with 0.1 M HCl and 0.1 M NaOH. After the suspensions were shaken for 24 h, the adsorbents were separated from the solution using a magnet. The concentrations of Cr(VI) in the filtrate were measured by ICP-OES. The amount of Cr(VI) adsorbed onto M-PAS-GO was calculated using Equation (1), and is expressed as the average ± standard

Batch Sorption Experiment
K 2 Cr 2 O 7 was used to prepare a Cr(VI) stock solution. All the sorption experiments were carried out according to the batch technique in a series of 50 mL conical bottles under continuous stirring of 200 r/min in a shaking water bath. The reaction solution was collected at 0 to 120 min during the adsorption process. Adsorption kinetics analysis and isotherm determination of Cr(VI) on M-PAS-GO composite were carried out at pH 3.20, and the initial concentrations of Cr(VI) ranged from 2.0 to 80.0 mg/L. The solution pH was adjusted with 0.1 M HCl and 0.1 M NaOH. After the suspensions were shaken for 24 h, the adsorbents were separated from the solution using a magnet. The concentrations of Cr(VI) in the filtrate were measured by ICP-OES. The amount of Cr(VI) adsorbed onto M-PAS-GO was calculated using Equation (1), and is expressed as the average ± standard deviation of three parallel experiments.
where C 0 is the initial concentration (mg/L), C e is the equilibrium concentration (mg/L), Q e is the amount of Cr(VI) sorbed onto M-PAS-GO at the equilibrium time (mg/g), V is the solution volume (L) and m is the mass of the sorbent (g).

Characterization of the M-PAS-GO Composite
The surface morphologies of Fe 3 O 4 , GO and M-PAS-GO were observed by SEM and TEM. As shown in A1 and A2 of Figure 1, the Fe 3 O 4 nanoparticles appear in spherical shape, with an average diameter of 20-30 mm. B1 and B2 of Figure 1 illustrate that the surface of the prepared GO is in the form of wrinkled fabric. C1 and C2 of Figure 1 clearly show that Fe 3 O 4 particles were successfully integrated with GO, and were well dispersed on the surface of the graphene nanosheets.

Characterization of the M-PAS-GO Composite
The surface morphologies of Fe3O4, GO and M-PAS-GO were observed by SEM and TEM. As shown in A1 and A2 of Figure 1, the Fe3O4 nanoparticles appear in spherical shape, with an average diameter of 20-30 mm. B1 and B2 of Figure 1 illustrate that the surface of the prepared GO is in the form of wrinkled fabric. C1 and C2 of Figure 1 clearly show that Fe3O4 particles were successfully integrated with GO, and were well dispersed on the surface of the graphene nanosheets. The XRD patterns of Fe3O4, GO and M-PAS-GO are shown in Figure 2a. The original graphite shows a sharp characteristic diffraction peak at 26.5°, as reported in previous work [36], corresponding to the (002) crystal plane. After the oxidation treatment, the (002) diffraction peak for graphite shifted to a lower angle of 10.5°, which is the graphene oxide  Figure 2a. The original graphite shows a sharp characteristic diffraction peak at 26.5 • , as reported in previous work [36], corresponding to the (002) crystal plane. After the oxidation treatment, the (002) diffraction peak for graphite shifted to a lower angle of 10.5 • , which is the graphene oxide (001) diffraction peak [37]. The main peaks at 30.  [38]. It is interesting that no carbon peaks are observed in the XRD pattern of M-PAS-GO. This phenomenon may be attributed to the fact that the existence of Fe 3 O 4 reduces the aggregation of the GO sheets, which results in more GO with fewer layers, and thus leads to weak peaks of carbon, or the strong signals of the iron oxides overwhelm the weak carbon peaks. 57.4° (511) and 62.7° (440) represent the iron oxides in Fe3O4 and M-PAS-GO, respectively. These results are consistent with the standard profile of a cubic magnetite structure (JCPDS 65-3107) presented in a previous study [38]. It is interesting that no carbon peaks are observed in the XRD pattern of M-PAS-GO. This phenomenon may be attributed to the fact that the existence of Fe3O4 reduces the aggregation of the GO sheets, which results in more GO with fewer layers, and thus leads to weak peaks of carbon, or the strong signals of the iron oxides overwhelm the weak carbon peaks. Raman spectroscopy was employed to analyze the chemical bonding and structural changes in the carbon framework of the M-PAS-GO composite and GO, and the results are shown in Figure 2b. For both samples, the Raman spectra show two characteristic peaks between 1300 and 1600 cm −1 , which correspond to the D and G bands, respectively. According to a previous study [36], the D band corresponds to the disorder of GO originating from defects associated with vacancies, grain boundaries and amorphous carbon species, whereas the G band is ascribed to the first-order scattering of the E2g mode observed for sp 2   Raman spectroscopy was employed to analyze the chemical bonding and structural changes in the carbon framework of the M-PAS-GO composite and GO, and the results are shown in Figure 2b. For both samples, the Raman spectra show two characteristic peaks between 1300 and 1600 cm −1 , which correspond to the D and G bands, respectively. According to a previous study [36], the D band corresponds to the disorder of GO originating from defects associated with vacancies, grain boundaries and amorphous carbon species, whereas the G band is ascribed to the first-order scattering of the E2g mode observed for sp 2 Figure 2c. The peaks at 1730, 1625 and 1100 cm −1 were observed in the spectrum of GO, which correspond to stretching of the C=O bond of carboxyl groups, the vibrations of skeletal C=C in GO and alkoxy C-O, respectively [39]. The new peak at 575 cm −1 was observed in the FTIR spectrum of M-PAS-GO, which differs from that of GO and is consistent with that of Fe 3 O 4 , thus can be related to the vibration of Fe-O functional groups [40]. Moreover, peaks at 1625 cm −1 and 1147 cm −1 which related to the C=O and C-N group, respectively, was also observed in this study. These findings indicates that the Fe 3 O 4 and quaternary ammonium group were successfully modified onto the surface of GO. In addition, the characteristic peaks in 2900 cm −1 in the spectra of Fe 3 O 4 and M-PAS-GO suggest the presence of unsaturated groups, which bring the prepared material more binding sites on its surface and thus enhance their adsorption capacities [41].
The magnetization hysteresis loops of Fe 3 O 4 and M-PAS-GO at room temperature are illustrated in Figure 2d. It can be seen that the saturation magnetization of the obtained Fe 3 O 4 and M-PAS-GO is about 62.4 emu/g and 18.1 emu/g, respectively. This result indicates that the M-PAS-GO composite possessed excellent magnetic properties. As can be seen in the insert photograph of Figure 2d, without an external magnetic field, a black homogeneous dispersion emerged. This phenomenon further confirms that M-PAS-GO is magnetic and can be used as a magnetic adsorbent to enrich contaminants adsorbed from bulk aqueous solutions.
The zeta potentials of the M-PAS-GO and GO magnetic nanoparticles in aqueous solutions with different pH were measured, as shown in Figure 3a. It is clear that the zeta potentials of M-PAS-GO and GO decreased with the increase in pH from 1 to 8. The zeta potential of GO was negative at the entire pH range, whereas the pH zpc (zero point charge) of M-PAS-GO magnetic composites is about 4.0. These results indicate that when pH is lower than 4, M-PAS-GO is more favorable for adsorption of surface negatively charged pollutants compared to GO.
trum of M-PAS-GO, which differs from that of GO and is consistent with that of Fe3O4, thus can be related to the vibration of Fe-O functional groups [40]. Moreover, peaks at 1625 cm −1 and 1147 cm −1 which related to the C=O and C-N group, respectively, was also observed in this study. These findings indicates that the Fe3O4 and quaternary ammonium group were successfully modified onto the surface of GO. In addition, the characteristic peaks in 2900 cm −1 in the spectra of Fe3O4 and M-PAS-GO suggest the presence of unsaturated groups, which bring the prepared material more binding sites on its surface and thus enhance their adsorption capacities [41].
The magnetization hysteresis loops of Fe3O4 and M-PAS-GO at room temperature are illustrated in Figure 2d. It can be seen that the saturation magnetization of the obtained Fe3O4 and M-PAS-GO is about 62.4 emu/g and 18.1 emu/g, respectively. This result indicates that the M-PAS-GO composite possessed excellent magnetic properties. As can be seen in the insert photograph of Figure 2d, without an external magnetic field, a black homogeneous dispersion emerged. This phenomenon further confirms that M-PAS-GO is magnetic and can be used as a magnetic adsorbent to enrich contaminants adsorbed from bulk aqueous solutions.
The zeta potentials of the M-PAS-GO and GO magnetic nanoparticles in aqueous solutions with different pH were measured, as shown in Figure 3a. It is clear that the zeta potentials of M-PAS-GO and GO decreased with the increase in pH from 1 to 8. The zeta potential of GO was negative at the entire pH range, whereas the pHzpc (zero point charge) of M-PAS-GO magnetic composites is about 4.0. These results indicate that when pH is lower than 4, M-PAS-GO is more favorable for adsorption of surface negatively charged pollutants compared to GO.   The dramatic weight loss of 65.73 % in GO was observed when the temperature increased from 30 to 800 • C, which can be attributed to the thermal decomposition of oxygen containing groups (such as -OH, -COOH, -O-and C=O) [42]. Meanwhile, the thermal degradation of M-APTES-GO and M-PAS-GO composites was observed and the loss was 18.618% and 28.495% when the temperature increased to 800 • C, respectively, which is caused by the reduction in APTES, quaternary ammonium group and Fe 3 O 4 nanoparticles. Compared with M-APTES-GO, an additional weight loss of 9.877% was found in M-PAS-GO, which proves that there were 9.877% of the quaternary ammonium group in M-APTES-GO composites, and further proves that the preparation of quaternary ammonium group functionalized magnetic composites is successful. The BET pattern of GO, M-APTES-GO and M-PAS-GO composites as shown in Figure 3c. According to the N 2 adsorption analysis, the BET surface areas of GO and M-APTES-GO are 62.28 cm 2 /g and 78.44 cm 2 /g, respectively. A significant increase in BET surface area to 102.24 cm 2 /g was found in M-PAS-GO, which may be beneficial for its pollutant adsorption performance.
Combining with the characterizations and adsorption experiments, the EDX patterns of GO and M-PAS-GO before and after adsorption experiments are illustrated in Figure 4. The results show that the surface of M-PAS-GO exhibits high content of Fe (4.28%), N (4.05%) and Si (4.32%), indicating that Fe 3 O 4 nanoparticles and quaternary ammonium groups were successfully introduced into graphene oxide. After adsorption experiment, the content of Cr on the surface of M-PAS-GO was 1.02%, and that on GO was negligible (0.002%), which shows the excellent adsorption ability of M-PAS-GO.
radation of M-APTES-GO and M-PAS-GO composites was observed and the loss was 18.618% and 28.495% when the temperature increased to 800 °C, respectively, which is caused by the reduction in APTES, quaternary ammonium group and Fe3O4 nanoparticles. Compared with M-APTES-GO, an additional weight loss of 9.877% was found in M-PAS-GO, which proves that there were 9.877% of the quaternary ammonium group in M-APTES-GO composites, and further proves that the preparation of quaternary ammonium group functionalized magnetic composites is successful.
The BET pattern of GO, M-APTES-GO and M-PAS-GO composites as shown in Figure 3c. According to the N2 adsorption analysis, the BET surface areas of GO and M-APTES-GO are 62.28 cm 2 /g and 78.44 cm 2 /g, respectively. A significant increase in BET surface area to 102.24 cm 2 /g was found in M-PAS-GO, which may be beneficial for its pollutant adsorption performance.
Combining with the characterizations and adsorption experiments, the EDX patterns of GO and M-PAS-GO before and after adsorption experiments are illustrated in Figure 4. The results show that the surface of M-PAS-GO exhibits high content of Fe (4.28%), N (4.05%) and Si (4.32%), indicating that Fe3O4 nanoparticles and quaternary ammonium groups were successfully introduced into graphene oxide. After adsorption experiment, the content of Cr on the surface of M-PAS-GO was 1.02%, and that on GO was negligible (0.002%), which shows the excellent adsorption ability of M-PAS-GO.

Effect of pH on Cr(VI) Adsorption by M-PAS-GO Composite
The pH of the solution can affect the ionization and the speciation of metal ions, and the surface properties of quaternary ammonium composite. At low pH condition (pH 1-6), HCrO4 − and Cr2O7 2− are the main species in solution, while the main specie transforms to CrO4 2− at the pH 6-10 [23]. The effect of solution pH on Cr(VI) ions adsorption was investigated, and the results are shown in Figure 5a, and the change in pH value before

Effect of pH on Cr(VI) Adsorption by M-PAS-GO Composite
The pH of the solution can affect the ionization and the speciation of metal ions, and the surface properties of quaternary ammonium composite. At low pH condition (pH 1-6), HCrO 4 − and Cr 2 O 7 2− are the main species in solution, while the main specie transforms to CrO 4 2− at the pH 6-10 [23]. The effect of solution pH on Cr(VI) ions adsorption was investigated, and the results are shown in Figure 5a, and the change in pH value before and after adsorption is shown in Figure 5b formation of hydroxyl complexes of chromium may also inhibit the adsorption of Cr(VI) ions. Therefore, the appropriate pH was selected as 3.20.
forms and the surface properties of the M-PAS-GO composite. As analyzed in Figure 3, the pHzpc of M-PAS-GO is 4.00, which makes the surface of M-PAS-GO positively charged at pH < 4.00 and negatively charged at pH > 4.00. Thus, at low pH conditions (pH < 4.00), HCrO4 − and Cr2O7 2− were adsorbed onto positive sites via electrostatic binding; thus, the high adsorption level of the M-PAS-GO composite was achieved. When the pH > 4.0, the main specie in solution is CrO4 2− and the surface of M-PAS-GO becomes negatively charged, which leads to the enhancement of the repulsion between the M-PAS-GO and CrO4 2− . In addition, the competitive adsorption between OH − and CrO4 2− and the formation of hydroxyl complexes of chromium may also inhibit the adsorption of Cr(VI) ions. Therefore, the appropriate pH was selected as 3.20.

Effect of M-PAS-GO Dosage on Cr(VI) Adsorption
The effect of M-PAS-GO dosage on Cr(VI) (i.e., HCrO4 − and Cr2O7 2− ) adsorption was investigated under the conditions of pH 3.20, temperature of 25 °C and initial Cr(VI) concentration of 10 mg/L, and the results are shown in Figure 6. It can be seen that the removal efficiency of Cr(VI) ions increases sharply from 17% to 86% when the adsorbent dosage increased from 0.002 to 0.01 g/50 mL, then a slight decrease occurred, and then a negligible increase in Cr(VI) ion removal was observed when the adsorbent dosage further increased to 0.012 g/50 mL and 0.016 g/50 mL, respectively. Meanwhile, the adsorption level of the M-PAS-GO composite decreased from 46.4 to 22.5 mg/g with the increase in its dosage. This may be because an adsorbent with a higher dosage can increase the number of active adsorption sites. However, aggregation occurs due to the magnetic properties of the M-PAS-GO composite, leading to a decrease in its specific surface area, which in turn leads to a decrease in the adsorption sites and then a decrease in the adsorption ability; additionally, an increase in the mass of aggregated M-PAS-GO results in its precipitation, which is also not conducive to the adsorption of pollutants [43]. Considering the removal efficiency for Cr(VI) ions and the adsorption level of M-PAS-GO, the optimal adsorbent dosage was selected as 0.01 g/50 mL for the subsequent experiments.

Effect of M-PAS-GO Dosage on Cr(VI) Adsorption
The effect of M-PAS-GO dosage on Cr(VI) (i.e., HCrO 4 − and Cr 2 O 7 2− ) adsorption was investigated under the conditions of pH 3.20, temperature of 25 • C and initial Cr(VI) concentration of 10 mg/L, and the results are shown in Figure 6. It can be seen that the removal efficiency of Cr(VI) ions increases sharply from 17% to 86% when the adsorbent dosage increased from 0.002 to 0.01 g/50 mL, then a slight decrease occurred, and then a negligible increase in Cr(VI) ion removal was observed when the adsorbent dosage further increased to 0.012 g/50 mL and 0.016 g/50 mL, respectively. Meanwhile, the adsorption level of the M-PAS-GO composite decreased from 46.4 to 22.5 mg/g with the increase in its dosage. This may be because an adsorbent with a higher dosage can increase the number of active adsorption sites. However, aggregation occurs due to the magnetic properties of the M-PAS-GO composite, leading to a decrease in its specific surface area, which in turn leads to a decrease in the adsorption sites and then a decrease in the adsorption ability; additionally, an increase in the mass of aggregated M-PAS-GO results in its precipitation, which is also not conducive to the adsorption of pollutants [43]. Considering the removal efficiency for Cr(VI) ions and the adsorption level of M-PAS-GO, the optimal adsorbent dosage was selected as 0.01 g/50 mL for the subsequent experiments.

Effect of Contact Time on Cr(VI) Adsorption by M-PAS-GO
To study the absorption kinetics, the adsorption levels of M-PAS-GO for Cr(VI) (i.e., HCrO4 − and Cr2O7 2− ) at different contact times were calculated. The reaction conditions

Effect of Contact Time on Cr(VI) Adsorption by M-PAS-GO
To study the absorption kinetics, the adsorption levels of M-PAS-GO for Cr(VI) (i.e., HCrO 4 − and Cr 2 O 7 2− ) at different contact times were calculated. The reaction conditions were as follows: Cr(VI) concentration of 10 mg/L, pH of 3.20, adsorbent dosage of 0.01 g/50 mL and temperature of 25 • C. As described in Figure 7a, a sharp increase in Cr(VI) adsorption occurred in the first 15 min, and stabilized in the subsequent times. In the study of [44], the removal efficiency of 10 mg/L Cr(VI) by phytogenic zero-valent iron nanoparticles was only~20% in 20 min. Liu et al. [45] used a magnetic zeolite/chitosan composite adsorbent and achieved less than 70% removal of 10 mg/L Cr(VI) within 30 min. It is noteworthy that in this study, 90% of the 10 mg/L Cr(VI) ions were adsorbed within 20 min, and saturation was achieved in 30 min, with the result of 46.48 mg/g of Cr(VI) ions being removed by M-PAS-GO, which is higher than the removal achieved by the other materials listed in Table 1. These results indicate the excellent adsorption ability of M-PAS-GO, which can be attributed to the large number of quaternary ammonium groups on its surface that can interact with Cr(VI) ions.  The pseudo-first-order and pseudo-second-order models were employed to ana the experimental kinetic data according to the following linear equations.
Pseudo-first-order model: The pseudo-first-order and pseudo-second-order models were employed to analyze the experimental kinetic data according to the following linear equations.
Pseudo-first-order model: Pseudo-second-order model: where k 1 (min −1 ) and k 2 (g/(mg·min)) are the pseudo-first-order rate constant and pseudosecond-order kinetic rate constant, respectively, and Q e and Q t are the adsorption level (mg/g) of the adsorbent for metal ions at equilibrium and at contact time t (min), respectively. The linear fitting and the correlation coefficient of the pseudo-first-order and pseudosecond-order equations for the adsorption of Cr(VI) ions by M-PAS-GO are shown in Figure 7b,c and Table 2. The results indicate that the pseudo-second-order model is more suitable for describing the adsorption behavior of Cr(VI) ions when using GO and M-PAS-GO (R 2 > 0.99), which suggests that the rate-limiting step of adsorption is a chemisorption between the metals ions and binding sites of the M-PAS-GO and GO [13,14]. Therefore, it can be inferred that the adsorption of Cr(VI) on M-PAS-GO is mainly controlled by the chemical interaction between adsorbents and Cr(VI) ions.

Effect of Initial Concentration on Cr(VI) Adsorption by M-PAS-GO
The adsorption isotherms of Cr(VI) (i.e., HCrO 4 − and Cr 2 O 7 2− ) by M-PAS-GO were investigated. The reaction conditions were as follows: pH of 3.20, temperature of 25 • C, adsorbent dosage of 0.01 g/50 mL and contact time of 120 min. The results in Figure 8 illustrate that, initially, the isotherm rises sharply with the increase in the equilibrium concentrations, which may be attributed to the fact that plenty of adsorption sites are available for Cr(VI) ions. Then, the adsorption process gradually reaches saturation with the increase in initial Cr(VI) concentration, and no more available adsorption sites remain. In order to further study the interaction between the Cr(VI) ions and the adsorbents, Langmuir, Freundlich and Temkin adsorption models were used to fit the isothermal adsorption data, and the fitting parameters are given in Table 3. The equations for the isotherm models of Langmuir, Freundlich and Temkin are as follows: where Q e and Q m (mg/g) are the equilibrium and theoretical saturated adsorption capacities of the adsorbent in the monolayer cover layer, respectively; b represents the equilibrium constant of the adsorption reaction (L/mg); C e (mg/L) is the concentration of Cr(VI) at equilibrium; K f and n are the Freundlich constants, which are related to the adsorption capacity of the adsorbent and adsorption intensity; 1/n is a heterogeneous factor that is related to the adsorption strength and favorability; R (8.3145 J/(mol·K)) is the gas constant; T (K) is the temperature; b (J·mol) is the Temkin constant related to the heat of adsorption; A (L/g) is the equilibrium binding constant.
Separations 2023, 10, x FOR PEER REVIEW  As can be seen in Table 3, the R 2 values of the three isotherm models are high 0.9, indicating that they are well-fitted to the adsorption processes. The relatively obtained with the Langmuir model indicates that the adsorption is a monolayer tion process. These results are similar to those of other studies on the adsorption o using quaternary ammonium-modified adsorbents [10,28]. Calculated by Langm sorption model, the saturated adsorption levels of Cr(VI) on M-APTES-GO and GO were 24.65 mg/g and 46.48 mg/g, which are four and eight times as much as GO (5.67 mg/g), respectively. The excellent adsorption ability of M-PAS-GO ca  As can be seen in Table 3, the R 2 values of the three isotherm models are higher than 0.9, indicating that they are well-fitted to the adsorption processes. The relatively high R 2 obtained with the Langmuir model indicates that the adsorption is a monolayer adsorption process. These results are similar to those of other studies on the adsorption of Cr(VI) using quaternary ammonium-modified adsorbents [10,28]. Calculated by Langmuir adsorption model, the saturated adsorption levels of Cr(VI) on M-APTES-GO and M-PAS-GO were 24.65 mg/g and 46.48 mg/g, which are four and eight times as much as that of GO (5.67 mg/g), respectively. The excellent adsorption ability of M-PAS-GO can be attributed to its large and positively charged surface (see Figure 3a,c), which promotes electrostatic interactions between M-PAS-GO and anions (i.e., HCrO 4 − and Cr 2 O 7 2− ) in solution. In the Freundlich isotherm, n values between 1 and 10 are generally favorable for adsorption [3]. As shown in Table 3, the n values lie between 1 and 10 for Cr(IV), suggesting that the use of GO, M-APTES-GO and M-PAS-GO as adsorbents favors the adsorption of Cr(IV) in solution. The adsorption process of Cr(VI) in this study was also well-fitted in the Temkin isotherm model, which assumes that the free energy of adsorption per layer decreases linearly with increasing adsorbate on the adsorbent surface due to adsorbate and adsorbate-adsorbent interactions rather than in a logarithmic form [45].

Effect of Temperature on Cr(VI) Adsorption by GO and M-PAS-GO
The effect of temperature on the adsorption of Cr(VI) (i.e., HCrO 4 − and Cr 2 O 7 2− ) by fabricated adsorbents was also investigated. The adsorption conditions were as follows: different temperature conditions, including 298 K, 308 K, 318 K and 328 K, different initial concentrations, including C 0 = 2, 6, 10, 20 and 30 mg/L, adsorbent dosage of 0.01 g/50 mL, pH of 3.20 and contact time of 120 min. Further, the adsorption thermodynamic behavior was studied via different thermodynamic parameters (i.e., Gibb's free energy (∆G • ), entropy (∆S • ) and enthalpy (∆H • )), which were calculated through the following equation: The standard enthalpy change (∆H • ) and the standard entropy (∆S • ) were then obtained from the linear plot of lnK c versus 1/T for Cr(VI) ion adsorption on the GO and M-PAS-GO composites in the following equation: As illustrated in Figure 9a, when the concentrations of Cr(VI) were 2 and 6 mg/L, no obvious difference was found in the adsorption capacity of M-PAS-GO in the temperature range of 298-328 K. Meanwhile, a decrease in the adsorption capacity of M-PAS-GO was observed with increasing temperature when the Cr(VI) concentrations were 10, 20 and 30 mg/L, meaning that the adsorption process is exothermic. The thermodynamic curves of M-PAS-GO and GO for the adsorption of 20 mg/L Cr(VI) at 298-328 K are shown in Figure 9b,c, and the calculated ∆H • and ∆G • values are given in Table 4. The results show that when using M-PAS-GO, ∆G • is negative and gradually increases with the increase in temperature, indicating that the adsorption process is spontaneous, and that the elevated temperature would reduce the adsorption capacity of the adsorbent for Cr(VI) ions [3], which is further demonstrated by the results in Figure 9a. In contrast, ∆G • is positive when using GO, suggesting that the adsorption process is non-spontaneous, and further explaining the low adsorption efficiency. This phenomenon may be caused by the electrostatic repulsion between the negatively charged GO surface (see Figure 3a) and the anions (i.e., HCrO 4 − and Cr 2 O 7 2− ). The obtained negative value of ∆H • can be explained by the fact that Cr(VI) adsorption is an exothermic process, which is consistent with the results in Figure 9a. The obtained negative value of ∆S • reveals that the adsorption of Cr(VI) ions by M-PAS-GO is a process of transition from randomness in solution to relative order on a solid surface [3].

Regeneration and Reuse of Adsorbents
The M-PAS-GO composite exhibits excellent adsorption performance in te Cr(VI) (i.e., HCrO4 − and Cr2O7 2− ); whether it can keep good stability and reusability cial for its large-scale application. Therefore, a desorption experiment was conduc which 0.05 g of M-PAS-GO, after the adsorption process, was subjected to 25 mL o NaOH and ultrasonicated for 120 min. The results show that there was only a 7.85 of Fe element observed in the filtrate, indicating that the M-PAS-GO composite h acceptable regeneration property. Moreover, five consecutive adsorption-desorpt cles were conducted, the results of which are illustrated in Figure 10. These results d strate that the adsorption capacity of M-PAS-GO decreased from 46.14 to 33.41 m the fifth adsorption-desorption cycle, which may be attributed to the fact that t sorbed Cr(VI) ions were not completely released from the adsorption sites, and th loss of the adsorbent during the regeneration processes. Nevertheless, M-PAS-GO tained a high adsorption capacity, and the Cr(VI) ions in solution were removed pletely. These results indicate that M-PAS-GO has promising reusability and stabili can be a potential adsorbent in practical environmental remediation.

Regeneration and Reuse of Adsorbents
The M-PAS-GO composite exhibits excellent adsorption performance in terms of Cr(VI) (i.e., HCrO 4 − and Cr 2 O 7 2− ); whether it can keep good stability and reusability is crucial for its large-scale application. Therefore, a desorption experiment was conducted, in which 0.05 g of M-PAS-GO, after the adsorption process, was subjected to 25 mL of 0.1 M NaOH and ultrasonicated for 120 min. The results show that there was only a 7.85% loss of Fe element observed in the filtrate, indicating that the M-PAS-GO composite had an acceptable regeneration property. Moreover, five consecutive adsorption-desorption cycles were conducted, the results of which are illustrated in Figure 10. These results demonstrate that the adsorption capacity of M-PAS-GO decreased from 46.14 to 33.41 mg/g at the fifth adsorption-desorption cycle, which may be attributed to the fact that the adsorbed Cr(VI) ions were not completely released from the adsorption sites, and the sight loss of the adsorbent during the regeneration processes. Nevertheless, M-PAS-GO maintained a high adsorption capacity, and the Cr(VI) ions in solution were removed completely. These results indicate that M-PAS-GO has promising reusability and stability, and can be a potential adsorbent in practical environmental remediation.

Conclusions
In summary, a quaternary ammonium/magnetic graphene oxide composite (M-PAS-GO) based on Fe3O4 bonded to the surface of 3-aminopropyltriethoxysilane (APTES) functional graphene oxide, and then modified with n-butyl bromide, has been successfully fabricated. Notably, a high surface potential, a surface area of 102.24 cm 2 /g and an adsorption capacity of 46.48 mg/g for Cr(VI) ions were obtained by the M-PAS-GO composite. The effects of adsorbent dosage, solution pH, contact time, initial concentration of Cr(VI) ions and temperature on the adsorption process were comprehensively investigated. When using M-PAS-GO, a satisfying removal efficiency of Cr(VI) ions was achieved, namely, 90% within 10 min under optimal conditions, which can be attributed to the large number of quaternary ammonium groups on its surface that can interact with Cr(VI) ions. The mechanistic studies demonstrated that the adsorption is surface reaction-controlled, and is an exothermic and spontaneous process. In addition, the M-PAS-GO composite exhibited excellent reusability and stability. This study suggests that M-PAS-GO can be used as a potential adsorbent for the removal of Cr(VI) from wastewater, and provides referential support for further quaternary ammonium-modified adsorbent investigation.

Conclusions
In summary, a quaternary ammonium/magnetic graphene oxide composite (M-PAS-GO) based on Fe 3 O 4 bonded to the surface of 3-aminopropyltriethoxysilane (APTES) functional graphene oxide, and then modified with n-butyl bromide, has been successfully fabricated. Notably, a high surface potential, a surface area of 102.24 cm 2 /g and an adsorption capacity of 46.48 mg/g for Cr(VI) ions were obtained by the M-PAS-GO composite. The effects of adsorbent dosage, solution pH, contact time, initial concentration of Cr(VI) ions and temperature on the adsorption process were comprehensively investigated. When using M-PAS-GO, a satisfying removal efficiency of Cr(VI) ions was achieved, namely, 90% within 10 min under optimal conditions, which can be attributed to the large number of quaternary ammonium groups on its surface that can interact with Cr(VI) ions. The mechanistic studies demonstrated that the adsorption is surface reaction-controlled, and is an exothermic and spontaneous process. In addition, the M-PAS-GO composite exhibited excellent reusability and stability. This study suggests that M-PAS-GO can be used as a potential adsorbent for the removal of Cr(VI) from wastewater, and provides referential support for further quaternary ammonium-modified adsorbent investigation.  Data Availability Statement: All data generated or analyzed during this study are included in this published article.