Fabrication of attapulgite/magnetic aminated chitosan composite as efficient and reusable adsorbent for Cr (VI) ions

An efficient composite was constructed based on aminated chitosan (NH2Cs), attapulgite (ATP) clay and magnetic Fe3O4 for adsorptive removal of Cr(VI) ions. The as-fabricated ATP@Fe3O4-NH2Cs composite was characterized by Fourier Transform Infrared Spectroscopy (FTIR), Thermal Gravimetric Analyzer (TGA), Scanning Electron Microscope (SEM), Zeta potential (ZP), Vibrating Sample Magnetometer (VSM), Brunauer–Emmett–Teller method (BET) and X-ray photoelectron spectroscope (XPS). A significant improve in the adsorption profile was established at pH 2 in the order of ATP@Fe3O4-NH2Cs(1:3) > ATP@Fe3O4-NH2Cs(1:1) > ATP@Fe3O4-NH2Cs(3:1) > Fe3O4-NH2Cs > ATP. The maximum removal (%) of Cr(VI) exceeded 94% within a short equilibrium time of 60 min. The adsorption process obeyed the pseudo 2nd order and followed the Langmuir isotherm model with a maximum monolayer adsorption capacity of 294.12 mg/g. In addition, thermodynamics studies elucidated that the adsorption process was spontaneous, randomness and endothermic process. Interestingly, the developed adsorbent retained respectable adsorption properties with acceptable removal efficiency exceeded 58% after ten sequential cycles of reuse. Besides, the results hypothesize that the adsorption process occurs via electrostatic interactions, reduction of Cr(VI) to Cr(III) and ion-exchanging. These findings substantiate that the ATP@Fe3O4-NH2Cs composite could be effectively applied as a reusable adsorbent for removing of Cr(VI) ions from aqueous solutions.

Synthesis of aminated chitosan (NH 2 Cs ) derivative. NH 2 Cs was synthesized according to the authors preceding work with a slight modification 42 .Transformation of chitin to NH 2 Cs was achieved via three main steps. The first step involves the activation of -OH − groups of chitin in which 8 g of chitin was soaked into PBQ solution (6.9 mM; pH 10) which acts as an activator agent. The reaction mixture was conducted under continuous stirring for 6 h at 60 °C. The resultant activated chitin was washed with distilled H 2 O to remove the excess of PBQ molecules. The second step includes the formation of amino-chitin, since, and followed by dispersion in EDA solution (6.9 mM) for 6 h under constant stirring at 60 °C. The obtained aminated chitin was separated and washed several times using distilled H 2 O to remove the unreacted EDA molecules. Finally; the third step involves deacetylation of aminated chitin which was achieved by immersing it in NaOH (50%) solution for 22 h under magnetic stirring at 140 °C. The gotten aminated chitosan (NH 2 Cs) was filtrated, washed with distilled H 2 O and dried at 60 °C. Characterization. To investigate the surface morphologies of developed ATP@Fe 3 O 4 -NH 2 Cs composite as well as NH 2 Cs and ATP clay a Scanning Electron Microscope (SEM; Joel Jsm 6360LA, Japan) was employed under a voltage potential of 20 kV. The examined samples were placed on aluminum stumps and coated with a thin layer of gold via a sputter coating system. The thermal stability was examined under nitrogen atmosphere by Thermal Gravimetric Analyzer (TGA; Shimadzu-50, Japan), while the temperature was raised from 10 to 700 °C at constant heating rate of 20 °C/min and flow rate of 40 mL/min. In addition, the chemical composition of ATP@Fe 3 O 4 -NH 2 Cs composite was explored by Fourier Transform Infrared Spectroscopy (FTIR; Shimadzu-8400 S, Japan), while the absorbance was scanned in the wavenumber range 500-4000 cm - After each experiment, the magnetic adsorbent was separated by an external magnet and the remaining concentration of Cr (VI) was detected via a spectrophotometer at λ max = 540 nm. The adsorption capacity (q) and the removal percent (R%) were calculated from Eqs. 1 and 2, respectively.

Fabrication of ATP@Fe 3 O 4 -NH 2 Cs composite.
where, C o and C t , are the Cr(VI) initial concentration and its concentration at time t, respectively. While, V and W are the volume of Cr(VI) and the weight of ATP@Fe 3 O 4 -NH 2 Cs composite, respectively.

Reusability test.
From the economical point of view, the selection of an efficient adsorbent strongly depends on its recycling characteristic quality. Therefore, recyclability test was executed to assess the reuse aptitude for the ATP@Fe 3 O 4 -NH 2 Cs magnetic composite. In brief, the magnetic ATP@Fe 3 O 4 -NH 2 Cs composite was collected after completion the adsorption process by an exterior magnet, and followed by immersing in 25 mL of the desorption medium comprising of Methanol/NaCl solution mixture under stirring for 1 h. After complete the desorption process, ATP@Fe 3 O 4 -NH 2 Cs composite was separated magnetically for reuse for ten consecutive cycles. All experiments were conducted in triplicate, and the results obtained were represented as the means corrected by standard deviation (± S.D.).   48 . Moreover, the high-resolution spectrum of O1s (Fig. 3C) shows a peak at BE of 530.63 eV which is ascribed to the oxygen atoms in Fe 3 O 4 lattice. Furthermore, the peak at BE of 532.18 eV is related to OH groups, while the peak at BE of 532.59 is due to Si-O-Si of ATP clay 49 . Besides, the high-resolution of N1s TGA . Thermal stability of the fabricated samples was scrutinized using TGA analysis at the temperature range from 45 to 700 °C (Fig. 5A BET. Figure 5C depicts the N 2 adsorption/desorption hysteresis loop and the pore size distribution of ATP@ Fe 3 O 4 -NH 2 Cs composite. The hysteresis loop reveals microporous structure of ATP@Fe 3 O 4 -NH 2 Cs composite at which the relatively low pressure (P/P o < 0.05 atm) significantly increased. Besides, the BET isotherm represents type IV with H4 hysteresis loop, indicating the existence of mesoporous. Moreover, the specific surface area and the total pore diameter were 164.24 m 2 /g and was 1.50 nm. www.nature.com/scientificreports/  www.nature.com/scientificreports/ Zeta potential. Figure 5D displays that the point of zero charges of ATP@Fe 3 O 4 -NH 2 Cs composite is 6.9. Thence at pH < 6.9; ATP@Fe 3 O 4 -NH 2 Cs surface is positively charged due to the protonation of NH 2 groups, providing columbic interactions between the positive charges on the ATP@Fe 3 O 4 -NH 2 Cs composite surface and the negatively charged Cr(VI) ions. Contrariwise, beyond pH 6.9 ATP@Fe 3 O 4 -NH 2 Cs composite displays negative charges, causing electrostatic repulsion forces with the anionic Cr(VI).

Removal of Cr (VI) by ATP@Fe 3 O 4 -NH 2 Cs composite.
A comparative test was executed to compare the efficacy of the different synthesized composites to determine which ratio has the finest adsorption capacity towards the Cr(VI) ions under the same adsorption conditions. Moreover, the same test was conducted for the pristine materials to trace the improvement in their adsorption behaviors after combination. Figure 6A showed that the adsorption capacity value increased in the order of    57 . In general, at high acidic medium NH 2 groups protonate to NH 3 + , that charges ATP@ Fe 3 O 4 -NH 2 Cs composite surface with a positive charge. It is apparent from Fig. 6B that the increase in pH from 1 to 2 results in an increase in the removal percentage from 65.53 to 90.81% and the adsorption capacity from 72.03 to 96.54 mg/g. This anticipated behavior can be assigned to the existence of Cr(VI) at pH = 1 in a neutral form (H 2 CrO 4 ) which leads to a decrease in the columbic interactions between the cationic groups of ATP@ Fe 3 O 4 -NH 2 Cs composite and the neutral H 2 CrO 4 molecules 7 . However, at pH 2, there are resilient electrostatic interactions between the protonated NH 3 + positive groups on the adsorbent surface and the negative charges of Cr(VI) species. One the other hand, beyond pH 2 there is an anticipated decrease in the number of protonated amine groups on the ATP@Fe 3 O 4 -NH 2 Cs surface. Consequently, the electrostatic interactions between ATP@ Fe 3 O 4 -NH 2 Cs composite and Cr(VI) decreases, so the removal percentage and the adsorption capacity directly dwindle from 90.81% and 96.54 mg/g to 38.57% and 39.85 mg/g, respectively 58 . Based on these results, pH 2 was selected as an optimum pH value for the following adsorption studies. Figure 6C denotes the effect of adsorbent dosage on the adsorption profile. It is evident that increasing the ATP@Fe 3 O 4 -NH 2 Cs composite dosage from 0.005 to 0.025 g leads directly to a dramatically reduction in the adsorbed quantity of Cr(VI) from 136.07 to 41.34 mg/g, respectively, which may be attributed to the aggregation of ATP@Fe 3 O 4 -NH 2 Cs particles. Contrariwise, the Cr(VI) removal % was gradually increased from 63.17 to 97.17% with increasing the composite dosage as a result of increasing the adsorption active sites on the composite surface 59 .

Effect of ATP@Fe 3 O 4 -NH 2 Cs dosage.
Effect of initial concentration. Figure 6D points out that the adsorption capacity value significantly increases from 99.99 to 270.68 mg/g with increasing Cr(VI) concentration from 50 to 200 mg/L. These findings are expected due to increasing the driving forces that overcomes the mass transfer resistance of Cr(VI) ions from bulk to the ATP@Fe 3 O 4 -NH 2 Cs surface with increasing the initial Cr(VI) concentration. On the contrary, Figure (S1) shows a decline in the removal (%) value from 94.24 to 64.93% with rising the Cr(VI) concentration, which could be explained by the shortage of the adsorption active sites at constant adsorbent dosage 12 .

Adsorption isotherms. To deduce the interaction sort between Cr(VI) and ATP@Fe 3 O 4 -NH 2 Cs compos-
ite, the obtained equilibrium data were scrutinized by bountiful isotherm models like Langmuir, Freundlich, Temkin and Dubinin-Radushkevich (D-R). The linearized isotherm equations are listed in Table S1 60,61 .
The plots of the applied isotherm models are illustrated in Fig. 7A-D. It was inferred from R 2 values ( Table 2) that the inspected Cr(VI) adsorption process obeys Langmuir (0.999) and Temkin (0.998) more than Freundlich Adsorption kinetics. The Cr(VI) adsorption mechanism of onto ATP@Fe 3 O 4 -NH 2 Cs composite was identified utilizing Pseudo 1st order, Pseudo 2nd order and Elovich (Fig. 8A-C). The linearized kinetic equations were summarized in Table S2.
It was concluded that the Cr(VI) adsorption process onto ATP@Fe 3 O 4 -NH 2 Cs best fits pseudo 2nd order based on the R 2 values (Table 3). Also, the computed q values from pseudo 2nd order seem to resemble the experimental values, evincing the suitability of pseudo 2nd order to represent the studied adsorption process. In addition, it was noticed a decline in the k 2 values with the rising in the Cr(VI) initial concentration, suggesting the chemical adsorption process of Cr(VI) onto ATP@Fe 3 O 4 -NH 2 Cs composite 59 . Moreover, the computed Elovich coefficients elucidate that the rate of Cr(VI) adsorption is vaster than the desorption since the α values exceed the β values 63 . Thermodynamics. To assess the impact of change the reaction temperature on the nature of Cr(VI) adsorption process onto ATP@Fe 3 O 4 -NH 2 Cs composite, the thermodynamics parameter such as, change in entropy (ΔSº), change in enthalpy (ΔHº) and change in free energy (ΔGº) were reckoned from Eqs. 3 and 4.
where, K e = C Ae C e is the thermodynamic equilibrium constant; C e and C Ae are the Cr(VI) concentration in the solution and onto ATP@Fe 3 O 4 -NH 2 Cs surface at equilibrium, respectively. R and T are gas constant and adsorption temperature, respectively.
The computed thermodynamics parameters demonstrate that the Cr(VI) adsorption onto ATP@Fe 3 O 4 -NH 2 Cs is randomness and endothermic process owing to the positive values of both ∆S o and ∆H o that have been

Reusability.
To evaluate the ability of ATP@Fe 3 O 4 -NH 2 Cs magnetic composite to reuse for several adsorption cycles, ten successive adsorption-desorption processes were executed. Figure 8D points out that the devel-  www.nature.com/scientificreports/   Fig. 9D shows the peaks at BE of 398.38 and 400.16 eV which are ascribed to -NH 2 and NH groups, respectively. Whereas, the spectrum of N1s after the adsorption of Cr(VI) infers the protonation of the amine group at low pH, since the distinctive peak of NH 3 + appeared at BE of 402 eV, suggesting the possibility of electrostatic interaction mechanism between the anionic Cr(VI) ions and NH 3 + on the surface of the composite. Furthermore, the wide-spectrum of O1s after the adsorption process (Fig. 9E) clearly revealed a decrease in the intensity of OH and Si-O-Si peaks which could be attributed to exchange of OH and Si with Cr(VI) and Cr(III) ions 68 . In addition, it was found a slight shift around 0.3 eV in the binding energy of C1s after the adsorption of Cr(VI) (Fig. 9F) which may be ascribed to the reaction of Cr(VI) ions with oxygen and nitrogen function groups, agreeing with FTIR results 69 . To sum, FTIR and XPS results suppose that mechanism of Cr(VI) adsorption onto ATP@Fe 3 O 4 -NH 2 Cs composite involve the electrostatic interactions, reduction of Cr(VI) to Cr(III) and ionexchanging (Fig. 10).

Conclusion
In this study, ATP@Fe 3 O 4 -NH 2 Cs composite was formulated with different proportions for efficient adsorption for Cr(VI) ions from their aqueous solutions. The utilized characterization tools elucidated the good thermal and magnetic characteristics of the as-fabricated ATP@Fe 3 O 4 -NH 2 Cs composite in addition to its higher surface area. Furthermore, batch adsorption experiments clarified that the best adsorption capacity values were attained at pH 2 and achieved by ATP@Fe 3 O 4 -NH 2 Cs(1:3). Moreover, isotherms studies revealed an analogy between the calculated maximum adsorption capacity under Langmuir isotherm model (294.12 mg/g) and the experimental one (270.68 mg/g). In addition, kinetics studies validated that the adsorption process follows the pseudo 2nd order kinetic model, while the thermodynamic parameters recognized the process to be endothermic, spontaneous www.nature.com/scientificreports/ and randomness. Furthermore, the results assumed that the adsorption process of Cr(VI) ions occurred via the electrostatic interaction between opposite charges, reduction of Cr(VI) to Cr(III) and ion-exchanging mechanisms. Finally, reusability test proved also the excellent potential of ATP@Fe 3 O 4 -NH 2 Cs adsorbent composite to be reuse for several times, which is a beneficial for its application for removing of Cr(VI) ions from contaminated water. It can be concluded that the as-fabricated ATP@Fe 3 O 4 -NH 2 Cs composite could be applied as sustainable and reusable adsorbent for removing Cr(VI) ions from wastewater.