Fe(III) Complexed Polydopamine Modified Mg/Al Layered Double Hydroxide Enhances the Removal of Cr(VI) from Aqueous Solutions

Herein, we report the preparation of Fe(III) complexed polydopamine modified Mg/Al layered double hydroxides composite material (LDHs@PDA-Fe(III)) and its application to the removal of Cr(VI) in aqueous solution. LDHs@PDA-Fe(III) was characterized and analyzed by field-emission scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM–EDS), Fourier transformed infrared (FTIR), X-ray diffraction (XRD), X-ray photoelectron (XPS). The adsorption performance was studied through a series of adsorption experiments. Investigate the effects of pH, time, temperature, concentration and other factors. When C0 = 800 mg/L, T = 308 k and pH 3, the maximum adsorption capacity obtained in the experiment was 683.4 mg/g. In addition, after 5 adsorption cycles, LDHs@PDA-Fe(III) still shows excellent adsorption capacity and stability. Combining adsorption experiments and characterization analysis, it is inferred that the adsorption of Cr(VI) by LDHs@PDA-Fe(III) is the result of the synergistic effect of multiple adsorption mechanisms. Therefore, the efficient removal capacity and excellent stability make LDHs@PDA-Fe(III) an ideal adsorbent for removing Cr(VI) from aqueous solutions.


Introduction
Hexavalent chromium (Cr(VI)) is a common heavy metal, which might increase environmental and public health concern due to its toxic nature. Compared with trivalent chromate (Cr(III)), Cr(VI) poses a greater threat to the ecosystem due to its teratogenesis and cancerogenic [1,2]. The World Health Organization (WHO) stipulates that the maximum concentration of Cr(VI) in surface water and drinking water cannot exceed 0.1 and 0.05 mg/L, respectively [3]. Up to now, many methods have been developed and applied to separate Cr(VI) from aqueous medium through the physical and chemical processes, such as photocatalytic reduction [4], ion exchange [5], membrane separation [6], and adsorption [7]. Among these methods, adsorption is a promising technique for Cr(VI) remediation because of its low cost and easy operation [8,9].
A variety of adsorbents are used for Cr(VI) removal from aqueous solutions, involving double hydroxide composites [10,11], biochar [12][13][14], metal-organic framework [15][16][17], zeolite [18], and zero-valent iron [19,20]. However, disadvantages such as low efficiency still limit their widely use. Presently, organic-inorganic hybrid materials as adsorbents are of increasing concern due to its powerful adsorption performance and versatility [21,22]. Li et al. [23]. synthesized pomegranate shaped organic-inorganic nano adsorbent composite PHN and it has excellent selective removal ability of Cr(VI), with maximum removal capacity of 454.55 mg/g. Lv et al. [24]. synthesized organic-inorganic Fe@LDH/RGO has excellent removal capability to Cr(VI), with a maximum removal rate of 98.8%. Layered double hydroxides (LDHs) is a kind of layered material with the general molecular formula of M 2+ 1 and M 3+ represented divalent and trivalent metal cations in the positively charged host layers respectively, and A n− represented interlayer anion. LDHs contains a large number of exchangeable interlayer anions, which has an advantage excellent anion removal ability. Therefore, LDHs has been widely used in the repair of wastewater contamination caused by anions [25,26].
Polydopamine (PDA) has been widely used as a modified material because of its ability to oxidize and self-polymerize under alkaline conditions and attach to the surface of various materials [23,27,28]. It is worth noting that PDA contains a large number of hydroxyl and amino functional groups which could act as the cloning sites of Cr(VI) [29,30]. Due to the interaction between covalent and non-covalent dopamine (DA) molecules in the polymerization process, the PDA coating will be adversely affected, resulting in the uneven deposition on the surface of the material [31]. Recent studies have found that PDA can form coordination bonds with transition metal metals, such as copper ions [32], titanium oxides [33], and iron ions [34]. The mussel, for example, improves the rigidity and elongation of its threads by binding iron ions (Fe(III)) tightly to the catecholin-like side chains of the 3,4-dihydroxyphenylalanine (DOPA) [35]. By first forming complexes with DA, Fe(III) can also accelerate the polymerization process of DA to form a more uniform and dense PDA deposit [36]. The Fe(III) chelated in PDA can also provide new binding sites for contamination to enhance their adsorption capacity [37]. Therefore, it is expected that the organic-inorganic hybrid LDHs@PDA-Fe(III) composites could be prepared by coating organic PDA on inorganic Mg/Al LDHs through in situ oxidation and self-polymerization under the promotion of Fe(III) to improve the adsorption capacity of Cr(VI). Few studies have been conducted report on the removal of Cr(VI) by modified materials of PDA complexing Fe(III).
In this work, Fe(III) was chelated into PDA network and quickly deposited on the surface of Mg/Al LDHs under alkaline conditions. A novel nano-adsorbent named LDHs@ PDA-Fe(III) was prepared. The adsorption properties were systematically studied by batch adsorption experiments. Regeneration experiments were also carried out to assess the possibility of sustainable application and the stability of the adsorbent. In addition, the potential adsorption mechanism was studied by different characterization techniques.

Preparation of Mg/Al LDHs
Mg/Al LDHs were synthesized by a simple method [38]. 7.8 mmol Mg(NO 3 ) 2 ·6H 2 O, 2.6 mmol Al(NO 3 ) 3 ·9H 2 O and 9.2 mmol HMT were accurately dissolved in 40 mL deionized water and stirred at high speed for 30 min at room temperature. Then it was transferred into a high-pressure reaction kettle lined with built-in PTFE, and reacted at 140 °C for 12 h. After the sample was cooled to room temperature, it was filtered and separated, rinsed with deionized water for three times and dried at 70 °C for 12 h.

Preparation of LDHs@PDA-Fe(III)
The preparation process of polydopamine doped Fe(III) modified Mg/Al layered dihydroxide is shown in Fig. 1. 0.2 g LDHs was dispersed in 100 mL deionized water. Then 0.121 g trimethylaminomethane (10 mmol/L), 0.4 g dopamine hydrochloride and 0.1425 g FeCl 3 ·60H 2 O were added successively. The pH of the solution was adjusted to 8.5 with 1 mol/L NaOH, and the reaction was stirred at room temperature for 24 h. The products were filtered and collected, and rinsed with deionized water for three times. After drying at 70 °C for 12 h, polydopamine modified Mg/Al layered dihydroxide LDHs@PDA without FeCl 3 ·6H 2 O was prepared by the same method.

Adsorption Experiments
All batch adsorption experiments were performed by adding 20 mg of adsorbent to 50 mL Cr(VI) solution. After an oscillating adsorption at 298 K with 160 rpm for a specified time, the adsorbent and solution were separated by a 0.45 μm filter (nylon) and the remaining Cr(VI) concentration was measured by a visible light spectrophotometer (V-1800) at 540 nm. The effect of pH was evaluated in increments of one pH unit between pH 3.0 to 9.0. The influence of adsorbent dosage was studied by controlling the dosage of adsorbent from 0.2 to 1.2 g/L. The influence of Cr(VI) initial concentration was studied by controlling the initial concentration of Cr(VI) from 80 to 800 mg/L. The adsorption kinetics was experimentally studied in the adsorption time range from 10 to 540 min. The adsorption isotherm experiment was carried out by setting the initial concentration of Cr(VI) from 80 to 800 mg/L at 298, 313 and 328 K. The adsorption capacity and removal rate of Cr(VI) was determined as follows: where C 0 (mg/L), C t (mg/L) are the concentration of Cr(VI) at the initial time and t time; V (L) is the volume of Cr(VI) solution; m (mg) represents the amount of adsorbent added; q t (mg/g) is the adsorption capacity of Cr(VI) at time t. E t (%) is the removal rate of Cr(VI) at time t.
The desorption experiment was carried out as follows. The initial concentration of Cr(VI) was 100 mg/L, the solution pH was 3, and the adsorbent dose was 20 mg. After adsorption at room temperature for 24 h, residual Cr(VI) concentration was determined. After centrifugation, the adsorbent was desorbed with 0.05 M NaOH for 24 h and washed with deionized water for several times. After drying overnight at 70℃, the spent-LDHs@PDA-Fe(III) was used to repeat the desorption-regeneration cycles 5 times under same conditions to assess the reusable performance of LDHs@PDA-Fe(III).

Characterization of Materials
The micro-morphology differences of LDHs, LDHs@ PDA, LDHs@PDA-Fe(III) and LDHs@PDA-Fe(III) after adsorption are shown in Fig. 2. It can be seen from Fig. 2a that Mg/Al LDHs are successfully synthesized with a size of about 1-2 μm. As shown in Fig. 2b, after PDA modification, the surface of LDHs@PDA is more rounded and a large number of PDA particles are present, which may be caused by the uneven and non-dense deposition of PDA. As shown in Fig. 2c, after Fe(III) is added, the rounded and granular feeling of the surface of LDHs@PDA-Fe(III) is significantly reduced. This indicates that the PDA distribution on the surface is more uniform and compact [36], and there is no significant difference in LDHs@PDA-Fe(III) before and after adsorption, indicating that the material is relatively stable in the adsorption process. EDS spectra of LDHs@PDA-Fe(III) before and after adsorption are shown in Fig [38]. The strong absorption at 3446 cm −1 and 1628 cm −1 correspond to the stretching vibration of OH and C=O respectively [39], and the peaks at 1356 cm −1 are related to CO 3 2− in LDHs. After modified by PDA [40], new band of LDHs@PDA appeared at 1483 cm −1 , 1420 cm −1 and 1281 cm −1 , which were derived from the stretching vibration of aromatic ring in PDA, the shear vibration of N-H in PDA layer and the stretching vibration peak of C-O in C-O-H, respectively. In addition, the new band of LDHs@PDA-Fe(III) at 580 cm −1 belongs to the stretching vibration absorption peak of Fe-O [36]. The above results indicate that, compared with LDHS, the PDA-modified LDHs@PDA has a large number of additional functional groups that can be used as Cr(VI) adsorption sites. In addition, the infrared results of LDHs@PDA-Fe(III) indicate that the doped Fe(III) may be fixed in the form of Fe-O [36,41].
In order to study the existence forms of crystals in LDHs, LDHs@PDA and LDHs@PDA-Fe(III), XRD was used for characterization analysis. The results are shown in Fig. 4b. The strong diffraction peak of three materials  [26]. In addition, the XRD peaks of the three materials are not significantly different. The diffraction peaks of other impurities are not detected, indicating that Mg/Al LDHs is the only crystal in the material, and has excellent stability and high purity, and the diffraction peaks of the three materials are all sharp and symmetrical, wich indicates that the highly crystallinity of Mg/Al LDHs [38].

Effect of pH
The pH value of the system can not only affect the existing form of Cr(VI) in solution, but also affect the surface charge property of the adsorbent material, which could affect the electrostatic force between the adsorbent and Cr(VI). The effects of pH are demonstrated in Fig. 5a. Under the same conditions, the adsorption capacity of LDHs, LDHs@PDA and LDHs@PDA-Fe(III) decreases gradually with the increase of pH. The adsorption capacity of LDHs@PDA-Fe(III) is better than that of LDHs@PDA and LDHs in the whole pH range. The adsorption capacity of these three materials were 210.2 mg/g, 109.4 mg/g and 61.1 mg/g, respectively, when pH of solution reaches 3.0. When pH 9, compared with the adsorption data at pH 3, the adsorption capacity of Cr(VI) by these three materials decreased by 78.3%, 81.8% and 96%, respectively. In combination with the Zeta potential data of LDHs@PDA-Fe(III) (Fig. 5b), it is speculated that when the pH < 5.6, the LDHs@PDA-Fe(III) is positively charged due to the protonation of surface functional groups [42]. At the same time Cr(VI) in the solution exists in the form of anions.
The electrostatic force between LDHs@PDA-Fe(III) and Cr(VI) is electrostatic attraction. Wich increases with the decrease of pH. On the contrary, when the system pH > 5.6, LDHs@PDA-Fe(III) is negatively charged due to the deprotonation of surface functional groups. And the electrostatic repulsive force could impede the adsorption process of Cr(VI) by LDHs@PDA-Fe(III). In addition, there may be competing adsorption between OH − and Cr(VI) in the system [43]. With the increase of pH, the content of OH − in the system gradually increases, which intensifies the competitive adsorption behavior between OH − and Cr(VI) [44], thus reducing the adsorption amount of Cr(VI).

Effect of Dosage
The experimental results of adsorbent dosage are shown in Fig. 6. As the increase of adsorbent dosage, the removal rate of Cr(VI) increased. When the dosage was 1.2 g/L, the removal rate of Cr(VI) reached 100%, which may be related to the increase of adsorbent surface area and adsorption site. However, with the increase of the amount of adsorbent, the adsorption capacity of Cr(VI) decreases gradually, which may be related to the aggregation of adsorption sites and difficult diffusion of Cr(VI) [45].

Adsorption Kinetics
Pseudo-first-order and pseudo-second-order kinetic models were used to fit the adsorption data of LDHs@PDA-Fe(III) and LDHs@PDA adsorption, respectively. Their model expressions were as follows: (3) Pseudo-first-order ∶ q t = q e (1 − e −k 1 t ), where t (min) is adsorption time; q t (mg/g) and q e (mg/g) represent the adsorption capacity of Cr(VI) at time t and the adsorption capacity at equilibrium respectively; k 1 (min −1 ) and k 2 (g·mg −1 ·min −1 ) represent the kinetic rate constant. The results are shown in Fig. 7, Tables 1 and 2. These two materials showed similar adsorption behavior for Cr(VI), and the adsorption rate decreased rapidly with the increase of time. At 60 min, the adsorption capacity of LDHs@PDA-Fe(III) and LDHs@PDA reached 79.1% and 85.6% of their corresponding saturated adsorption capacity, respectively. This may be due to the existence of a large number of effective adsorption sites on the surface of LDHs@PDA-Fe(III) and LDHs@PDA at the initial stage of adsorption, coupled with the large concentration gradient of Cr(VI) on the surface of solution and material at this time. The strong driving force of mass transfer, and the fast diffusion rate, thus leading to the fast adsorption rate [23]. With the further progress of adsorption, the effective adsorption sites on LDHs@ PDA-Fe(III) and LDHs@PDA surface are occupied, and the Cr(VI) concentration gradient on the surface of the solution and the material rapidly decreases, leading to the gradual decrease of the adsorption rate until the equilibrium state is reached. The equilibrium time of LDHs@PDA-Fe(III) and LDHs@PDA adsorption of Cr(VI) is 300 min and 420 min, respectively, while the equilibrium adsorption capacity reaches 215.52 mg/g and 123.25 mg/g, respectively. It is   suggested that the doping of Fe(III) can not only improve the equilibrium adsorption capacity, but also accelerate the adsorption rate. As displayed in Fig. 7, the experimental results are more consistent with the pseudo-second-order kinetic model (R 2 = 0.9882 and 0.9689). The equilibrium adsorption capacity q e (220.26 mg/g, 124.65 mg/g) obtained by fitting is closer to the actual equilibrium adsorption capacity q e (215.52 mg/g, 123.25 mg/g) obtained by experiment. It indicated that the rate control step of the adsorption process was chemisorption. It is mainly through the chemical reaction between Cr(VI) and LDHs@PDA-Fe(III) and the surface active sites of the bulk material, which indicates that the process is more dependent on the effective adsorption sites on the surface of LDHs@PDA-Fe(III) and LDHs@ PDA rather than the residual concentration of Cr(VI) in the solution [46].

Adsorption Isotherm and Thermodynamics
The adsorption isotherm reflects the relationship between the concentration of adsorbents in solid phase and liquid phase at equilibrium. It is easy to judge the nonlinear adsorption or linear adsorption, and the maximum adsorption capacity of the adsorbent can be obtained by fitting. In order to further explore the isothermal adsorption characteristics of Cr(VI) adsorption process of LDHs@PDA-Fe(III), Langmuir and Freundlich isothermal adsorption models were used to fit the Cr(VI) adsorption process of LDHs@PDA-Fe(III). Relevant equations of isothermal adsorption model are as follows: where q e , q max are adsorption capacity (mg/g) and maximum adsorption capacity at equilibrium, respectively; K L (L/g) represents the Langmuir model constant; K F (mg 1−n ·g −1 ·L n ) represents Freundlich model constant; n is the adsorption force parameters of Freundlich model. The fitting results and parameters of the isothermal adsorption model are shown in Fig. 8 and Table 3. The adsorption capacity of Cr(VI) at the three temperatures all increased significantly with the increase of the initial concentration of Cr(VI). When C 0 = 800 mg/L and T = 308 K, LDHs@PDA-Fe(III) has an effect on Cr(VI) The adsorption amount reached the experimental maximum of 683.4 mg/g, which may be due to the increase of the concentration gradient of Cr(VI) on the surface of the material and the liquid phase. The mass transfer driving force and diffusion rate of Cr(VI) diffusion to the surface of LDHs@PDA-Fe(III) continue to increase, which in turn causes the adsorption equilibrium state to continue to move in the direction of increasing adsorption capacity. As shown in Table 4, although it is difficult to directly compare  LDHs@PDA-Fe(III) with other composite materials due to different experimental conditions, it must be emphasized that Cr(VI) removal ability of LDHs@PDA-Fe(III) is greater than that of other adsorbents, indicating that LDHs@PDA-Fe(III) will have greater advantages in practical application. The adsorption process of Cr(VI) in LDHs@PDA-Fe(III) is more consistent with the basic assumption of Freundlich isothermal adsorption model, indicating that the adsorption process of Cr(VI) in LDHs@PDA-Fe(III) is a multilayer chemical adsorption process. The adsorption capacity increases significantly with the increase of temperature. This implies that the adsorption process of Cr(VI) by LDHs@ PDA-Fe(III) is endothermic. In addition, n < 1 also indicates that Cr(VI) is favorably adsorbed on the surface of LDHs@ PDA-Fe(III) [49].
In order to further study the thermodynamic characteristics of Cr(VI) adsorption by LDHs@PDA-Fe(III), the thermodynamic parameters of adsorption (ΔH 0 , ΔS 0 and ΔG 0 ) were calculated from the temperature-dependent adsorption isotherm. The relevant calculation formula is as follows: where R (8.314 J·mol −1 ·K −1 ) is ideal gas constant; T (K) is temperature; K 0 is standard adsorption equilibrium constant; The fitting and calculated results are shown in Table 5, where ∆H 0 is positive, indicating that the adsorption process of LDHs@PDA-Fe(III) to Cr(VI) is endothermic. A positive value of ∆S 0 indicates that the degree of chaos increases gradually in the process. A negative value of ∆G 0 indicates that the adsorption process is spontaneous. In summary, the adsorption process of LDHs@PDA-Fe(III) to Cr(VI) is a multi-layer chemical adsorption process with spontaneous heat absorption and entropy increase.

Adsorption Mechanism
The XPS analysis results of LDHs@PDA-Fe(III) are shown in Fig. 9. Figure 9a shows that the main components of LDHs@PDA-Fe(III), such as Mg, Al, C, O, N and Fe are successfully identified, indicating the successful preparation of LDHs@PDA-Fe(III) composite material. A new Cr 2p peak is detected on the surface of LDHs@PDA-Fe(III) after adsorption, indicating that Cr is successfully adsorbed and fixed on the surface of LDHs@PDA-Fe(III). In addition, as shown in Fig. 9b and c, the N 1s spectra changed significantly before and after adsorption. Before adsorption, the peaks at 399.94 eV, 399.61 eV and 398.44 eV correspond to C=N, C-N and N-H, respectively. When the adsorption is in equilibrium, the binding energies of C=N and C-N move to 401.7 eV and 399.86 eV, respectively, while the peak of N-H disappears. It is include that the N-containing functional group strongly interacts with Cr(VI) during the adsorption process [23]. Figure 9d shows the Cr 2p pattern fixed on the surface of LDHs@PDA-Fe(III) after adsorption, which can be divided into four peaks. The peaks at 586.45 eV and 579.83 eV belong to the Cr 2p 1/2 and Cr 2P 3/2 peaks of Cr(III) [46], respectively. The peaks at 588.79 eV and 576.91 eV belong to the Cr 2P 1/2 and Cr 2p 3/2 peaks of Cr(VI), respectively. The results reveal that part of Cr(VI) is reduced to Cr(III) by catechol in PDA [49,50]. The catechol in the corresponding PDA would be oxidized to carboxyl group [51]. The O 1s spectrum of LDHs@PDA-Fe(III) after adsorption is shown in Fig. 9e . Based on the above discussion, we briefly described the interaction mechanism of Cr(VI) removal by LDHs@PDA-Fe(III). Firstly, the protonated functional groups adsorb the anion Cr(VI) by electrostatic attraction, and then part of Cr(VI) is reduced to Cr(III) by catechol, and a small part of Cr(VI) is ion exchange with CO 3 2− in LDHs. The reduced Cr(III) can then be deposited on the surface of the material through hydrolysis to form Cr(OH) 3 .

Desorption and Regeneration Performance
Reusable ability is an important index to evaluate the economy and stability of adsorbents, LDHs@PDA-Fe(III) desorption and resorption results are shown in Fig. 10, with the increase of desorption and resorption cycles, the adsorption capacity of Cr(VI) on LDHs@PDA-Fe(III)  . 9 a XPS wide spectra of LDHs@PDA-Fe(III) before and after Cr(VI) adsorption; N 1s spectra of LDHs@PDA-Fe(III), b before and c after Cr(VI) adsorption, d Cr 2p and e O 1s spectra of LDHs@PDA-Fe(III) after Cr(VI) adsorption decreases somewhat. After 5 regeneration cycles, the adsorption capacity is still 79.3% of the original adsorption capacity. which may be due to the incomplete desorption of Cr(VI) fixed by the material. In addition, the pore structure of the material surface is blocked by Cr(VI) deposits during the adsorption process, which gradually reduces the effective adsorption sites on the surface of LDHs@PDA-Fe(III), resulting in a decline in adsorption performance. In addition, with the continuous oxidation of catechol in PDA to carboxyl group, the reducing power of LDHs@PDA-Fe(III) is also decreased. In short, LDHs@ PDA-Fe(III) has good recovery ability and is an ideal material for removing Cr(VI) in wastewater.

Conclusion
In this study, LDHs@PDA-Fe(III) composite material was prepared and applied to the efficient removal of Cr(VI) in aqueous solution. Notably, LDHs@PDA-Fe(III) showed stronger adsorption efficiency than LDHs@PDA and LDHs, indicating that PDA and Fe(III) were the main binding sites for the capture of Cr(VI) and showed synergistic effects. In addition, based on adsorption data and material characterization data, the adsorption process can be explained by ion exchange, electrostatic attraction and reduction. In summary, LDHs@PDA-Fe(III) has the advantages of high adsorption capacity, fast adsorption rate, reusability and sufficient stability. It can be used as a potential adsorbent for removing Cr(VI). residue from the acidic concentrated lithium bromide treated crop residue as biochar to remove Cr (VI). Bioresour Technol. https: