Novel Layered Double Hydroxides@carboxymethyl Cellulose Composite Aerogel Towards Co(II) Absorption

Cobalt (Co) ion is a common heavy metal ion and the main metal ion in nuclear wastewater. However, the study on the absorption for Co(II) is not sufficient. Herein, a novel ES-layered double hydroxide (LDH) is prepared by intercalated ethylenediaminetetraacetic acid disodium salt (EDTA) and sodium dodecylbenzene sulfonate (SDBS) in a magnesium, iron pristine LDH, which is subsequently used for ES-LDH@carboxymethyl cellulose (CMC) composite aerogel. The results of Fourier transform infrared spectroscopy, X-ray diffraction spectra and thermogravimetric analysis shows successfully preparation of intercalated LDH. The pore morphology, specific surface area and pore size distribution of the composite aerogels are characterized by scanning electron microscope and Nitrogen-BET curves. The significantly improved adsorption capacity, adsorption kinetics and isotherms the absorbents on Co(II) are systematically revealed. This study thus provides an important understanding and path for dealing with the threat of Co(II).


Introduction
With the global efforts towards Carbon Neutrality, it will be a developing trend to promote application of clean energy, including safe and stable use of nuclear energy. The isotope of Cobalt (Co), i.e. 60 Co, are one of the main pollution sources of nuclear wastewater. Furthermore, Co is a typical heavy metals with density greater than 4.5 g/cm 3 . Heavy metals are widely used in the manufacture of electronic components, antibacterial materials, batteries and photosensitive materials. Attention must be paid to the post-treatment of the heavy metal ions (HMIs). In case of improper treatment, HMIs will cause serious environmental and health crisis. Excessive HMIs in a body can lead to a variety of diseases, and the limit is actually extremely low [1,2]. The main reason is that HMIs can inactivate human protein, and the organism can not metabolize HMIs, which aggravate through the food chain [3]. Various methods have been applied to eliminate HMIs pollution, such as precipitation, 24 h. The product was centrifuged at low-speed, washed and freeze-dried to obtain pristine Mg(II), Fe(III)-CO 3 2− -LDH (abbr. MgFe-LDH).

Preparation of ES-LDH
The pristine MgFe-LDH powder was calcined at 500℃ in muffle furnace for 4 h. During this process, the interlayer anions, bound water and some hydroxyls were removed to obtain double oxide (LDO). The LDO was put in 200 mL solution of 0.02 mol EDTA and/or 0.02 mol SDBS, and stirred for 24 h. Again, the product was centrifuged, washed and freeze-dried to obtain EDTA and SDBS intercalated MgFe-EDTA-LDH, MgFe-SDBS-LDH and MgFe-LDH (ES-LDH) (Scheme 1).

Preparation of ES-LDH@CMC composite aerogel
ES-LDH (0.5 g) was first dispersed in 95 mL water, after which CMC (4.5 g) was added in it until the system completely swelled to a homogeneous jelly-like intermediate product. After that, sulfuric acid was added to make the pH value about 6. Then 10 mL glutaraldehyde (aq) was added into it for crosslinking 24 h. The obtained product was ES-LDH@CMC composite hydrogel. Unreacted small molecules were removed by dialysis. The hydrogel was further freeze casted and freeze dried to obtain ES-LDH@CMC-10 composite aerogel, in which the number '10' indicates 10% of ES-LDH in dry matter.

Absorption experiments of Co(II)
Composite aerogel sample (0.1 g) was put in a gauze and immersed in the Co(Ac) 2 (100 mg/L) solution for some time. After the absorption, the liquid was taken out and centrifuged at 10,000 rpm to obtain the supernatant, which was diluted 4 times. The solution was put in a polytetrafluoroethylene beaker and added aqua regia (4 mL) and hydrofluoric acid (0.05 mL). This solution was left for 15 min at RT and then heated to 150℃ for digesting organic molecules on until the solution was clear. After cooling down, the solution composites [15] and humic acid and LDH composites [16] for cadmium ion removal. Composite materials of nanoiron(0) fixed LDH [17], ZIF-8 fixed CoAl-LDH carbon fiber [18], electrospinning fiber membrane with EDTA intercalated LDH [19], and silanized citric acid intercalated LDHpolyaniline [20] have excellent adsorption and separation of a variety of HMIs. Nevertheless, there are still some points should be noted, such as high cost of raw materials, secondary pollution in post-treatment and complicated separation process, etc. Therefore, it is of great value to prepare LDH based adsorption materials with no secondary pollution and easy separation from waste water by a simple method.
Carboxymethyl cellulose (CMC) is a widely used modified anionic polysaccharide with biocompatibility and biodegradability, which can be obtained by one-step reaction from cellulose [14,18,19]. In recent years, the CMC based hydrogels develops rapidly [12,13,17]. These researches prove that CMC is a competent component for HMIs removal. In this work, a novel intercalated magnesium (Mg) and iron (Fe)-based LDH is prepared, which is fabricated with CMC to obtain a composite aerogel. This aerogel is tested as an adsorbent to remove Co(II), and the adsorption behaviors and mechanism are revealed. It is believed that this study provides an important basis for dealing with threat of Co(II) and design of novel absorbents.

Preparation of LDHs
Prepartion of pristine MgFe-LDH Na 2 CO 3 (0.06 mol) and NaOH (0.1 mol) were dissolved in a 500 mL three-necked flask with 100 mL water and heated in a 80℃ oil bath. FeCl 3 ·6H 2 O (0.06 mol) and MgCl 2 ·6H 2 O (0.02 mol) were dissolved in 100 mL water and then transferred to a constant pressure dropping funnel. This solution was slowly dropped into the flask and the pH value was controlled at about 10. After a reaction of 30 min, the solution was transferred to a beaker and aged in a oven at 65℃ for Scheme 1 Structure of ES-LDH  (5) in which α [mg/(g·min)] is the initial adsorption rate and β (g/mg) is the desorption constant related to the extent of the surface coverage and activation energy for chemical absorption. Elovich equation could confirm a chemisorption. However, it could not predict any definite mechanism. The adsorption equilibrium is usually described by an isotherm equation, in which the parameters express the surface property of the absorbent and affinity between the absorbent and the absorbate [25][26][27]. For the newly developed composite aerogels in this study, different isotherm models are essentially required to discuss the absorbing behavior, which include.
• (1) Langmuir isotherm. This model has been widely applied to many metal ions absorption process, which takes the following form: where q m is the quantity of adsorbate required to form a single monolayer on unit mass of adsorbent (mg/g). q e is the amount adsorbed on unit mass of the adsorbent (mg/g) when the equilibrium concentration is c e (mg/L). k L (L/mg) is Langmuir constant that is related to the apparent energy of adsorption.
• (2) Freundlich isotherm. This model relates the concentration of an absorbate on the surface of an adsorbent. The heat of adsorption decreases in magnitude with increasing the extent of adsorption. This isotherm is commonly expressed as: where k F [(mg 1-1/n L 1/n )/g] and n (g/L) are the deduced constants of the system, indicating the relative adsorption capacity of the adsorbent related to the bonding energy and the adsorption intensity, respectively.
• (3) Tempkin isotherm. In this mode, the uniformly distributed adsorbates repulse each other, and the heat of adsorption decreases linearly. This equation is given by Eq. (8): was diluted in a 10 mL-volumetric flask. The concentration of the rest Co(II) was determined by ICP-OES. According to the same method, kinetic analysis was carried out at T = 20℃ and pH = 7.0, solution sample was taken out every 1 h to determine the adsorption capacity. Furthermore, Co(Ac) 2 solutions with different concentrations of 50, 65, 80, 105, 150 and 200 mg/L were prepared and the isotherms under the condition of T = 20℃, pH = 7.0 and adsorption time of 6 h were recorded.

Absorption kinetics and isotherms
The calculation method of adsorption capacity (q e , mg/g) at some time is: where c 0 is the initial Co(II) concentration (mg/L), c t is the Co(II) concentration at the tested time, V is the volume of the suspension (L), and m is the mass of the adsorbent (g).
• (1) Pseudo-first-order kinetic. The integrated rate law form of the pseudo-first-order equation is described by Eq. (2): • in which the deduced coefficient k 1 is considered as an indicator for mass transfer coefficient. The value q t is the adsorption capability at time t. • (2) Pseudo-second-order kinetic. This model has been widely applied to a number of metal/absorbent systems. The integrated form of the second-order kinetics equation is described in the following form: (3) Intra-particle diffusion equation. Intra-particle diffusion model is commonly used for discussing the adsorption mechanism. According to this model, for most adsorption processes, the uptake varies almost proportionately with t 0.5 rather than with the contact time: where t 0.5 is the square root of the time and k i [mg/(g·min 0.5 )] is the rate constant.
Micromeritics, Ltd. The degassing was operated at 77 K for 8 h.

Characterization of ES-LDH
First, the pristine MgFe-LDH matrix and it derived ES-LDH are analyzed by FTIR. In Fig. 1, the large absorption peak near 3400 cm − 1 corresponds to the stretching vibration of -OH of interlayer of LDH plate and interlayer bound water. This wide and large peak not only indicates a lot of hydroxyls, but also corresponds to the association effect of hydrogen bonds. The peak at 1637 cm − 1 corresponds to the C = O stretching vibration of CO (4) Dubinin-Radushkevich (D-R) isotherm. This model is deduced from the mean free energy of adsorption (E). If the value of 1 ≤ E ≤ 16 kJ/mol, then physical adsorption prevails, and if E > 16 kJ/mol, then chemisorption prevails. D-R equation is represented as Eq. (7): where B (mol 2 /kJ 2 ) is a constant related to mean adsorption energy and ε is the Polanyi potential, which can be further calculated from Eq. (8): The the mean free energy of adsorption can be deduced from:

Characterization
• The Fourier transform infrared spectroscopy (FTIR) of LDH and ES-LDH were determined by Nicolet 380 spectrometer, Thermo Fisher Scientific, Ltd. • The X-ray diffraction spectra of Mg(OH) 2 , Fe(OH) 3 , LDH and ES-LDH were recorded by Bruker AXS D8 advanced X-ray diffractometer. The testing conditions were tube current 40 mA, tube voltage 40 kV, copper test target, diffraction angles were 5-50° the scanning speed at 10°/min and the scanning wavelength at 0.154 nm. • The thermogravimetric curve of ES-EDH was recorded by TGA 550 thermogravimetric analyzer, TA instruments, Ltd. The conditions were N 2 atmosphere, gas flow rate of 30 mL/min and heating rate of 10℃/min. • The high resolution transmission electron microscope (HRTEM, FEI Talos F200x) and/or the scanning electron microscope (SEM, Zeiss Merlin Compact) were used for the morphology of the products. • The specific surface area and pore size distribution of the products were characterized by ASAP 2460 N 2 -BET full automatic surface area and porosity analyzer,   surface area means an effective intercalation, which also implies an increase in adsorption capacity.
• The TG and DTG curves of ES-LDH are shown in Fig. 5.
From which, ES-LDH begins to decompose at 100℃ with a weight loss of 2.2%, mainly due to the thermal desorption of adsorbed water between LDH layers, but the layered structure does not damaged. From 210 to 570℃, the weight loss is 40.4%, which is the main loss range for ES-LDH. According to the DTG curve, the hydroxyl decomposition dehydration reaction starts at 200-300℃, with the highest weight loss at 400-450℃, which corresponds to the decomposition and release of interlayer SDBS and EDTA, collapse the layered structure and formation of LDO [35,36]. Therefore, these Fe(OH) 3 . Furthermore, all of the characteristic 2θ values shift to lower values ( Table 1). Because the 2θ value is inversely proportional to the spacing between the planes, shifting to lower values of these characteristic planes for ES-LDH is typical manifestation of the entry of substances larger than the original molecules between the planes in LDH. The main reason for this result is that SDBS and EDTA molecules are larger than CO 3 2− . Once SDBS and EDTA, especially SDBS, enter the interlayers of LDH, the interlayer spacing increases. In addition, there are many burr peaks in the diffraction spectrum of ES-LDH, indicating that the crystallinity of ES-LDH decreases. This phenomenon could be attributed to that the intercalation of SDBS and EDTA increases spacing of the interlayer too much and decreases the crystallinity.
• Figure 3 shows the TEM images of MgFe-LDH and ES-LDH, which confirm the formation of sheet-like particles with the size of 50-100 nm. In addition, the aggregation of particles can also be observed.
• The N 2 isotherm absorption and linear fitted multi-point BET plots of MgFe-LDH and ES-LDH are carried out for analyzing their specific surface area (Fig. 4). Based on these data, the specific surface areas of MgFe-LDH and ES-LDH are deduced as 32.721 m 2 /g and 177.246 m 2 /g, respectively. The obvious change of specific

Adsorption capacity
To understand the absorption behavior of the material, the absorption capacities of kinds of LDHs@CMC composite aerogels for Co(II) are first recorded (Fig. 8). The adsorption capacity of pristine MgFe-LDH@CMC composite aerogel to Co(II) is relatively low, which is ca. 22 mg/g. As discussed above, the layer spacing of LDH is widened by SDBS intercalation, which also could form Co(DBS) 2 compounds. However, this modification only slightly improves the adsorption capacity. In comparison, the adsorption of Co(II) by LDH with only EDTA@CMC composite aerogel increases to 30 mg/g, which exhibits the significant role of complexation reaction. Finally, when SDBS and EDTA are used together as intercalators, i.e. ES-LDH@CMC aerogel, TG-DTG curves are consistent with the typical thermogravimetric curves of LDH.

Characterization of ES-LDH@CMC
• Figure 6 shows the SEM figure of a representative composite aerogel of ES-LDH@CMC-30. From which, it can be clearly seen that the internal micro-structure of the composite aerogel is lamellar. There are two reasons for this morphology. First, CMC molecular chains entangle with lamellar ES-LDHs to form these layered structures [37][38][39]. Second, the composite aerogel is prepared by the classic freeze-casting and then freezedrying method, the resulting pores are the routes left by the ice crystals [40,41]. However, because of the relatively rigid molecule structure of CMC and intermolecular crosslinking, ice crystals can not grow freely in the gel network, and therefore make pore structure basically in disorder. In general, composite aerogels have porous structure in the microscale, which is the basis of high specific surface area.
The adsorption-desorption curves and corresponding pore size distribution of ES-LDH@CMC-30 composite aerogel are determined by BET method [42][43][44], and the results show in Fig. 7. Based on the N 2 adsorption-desorption curves, the adsorption isotherm hysteresis loop is complex H3 type. Due to the nano flake morphology of ES-LDH particles, the pore structure is slit, crack and wedge-shaped, and there is no saturated adsorption under high pressure. From the pore size distribution, it can be seen that the highest pore volume in the mesoporous range, indicating that the pore is mainly mesoporous. The specific surface area is deduced as 6.0854 m 2 /g, which could be attributed to the randomly distributed ES-LDH lamellae in the CMC molecular network. The pore capacity of the aerogel is 0.012144 the maximum. The reason will be further discussed with the following absorption kinetics and absorption isotherms analysis.

Absorption kinetics
To analyse the absorption behavior, the absorption kinetics of ES-LDH@CMC-30 for Co(II) are further fitted by pseudo first-order adsorption kinetic model (Fig. 9 A) and pseudo second-order adsorption kinetic model (Fig. 9B). The correlation coefficients (R 2 ) for the pseudo-second equation is close to 1 and obviously larger than that of pseudo firstorder equation. The reason for relative inapplicability of the pseudo first-order model could be attributed to that the the adsorption capacity exceeds 40 mg/g, which shows that the physical expansion and chemical complexation collaboratively work and remarkably promote the adsorption of Co(II).
For the ES-LDH@CMC aerogels, when the ES-LDH content in the aerogel is low, the increase of the adsorption capacity of the composite aerogels is due to the increase of CMC content and the corresponding adsorption sites of hydroxyl groups on the polymers. With high ES-LDH contents, the adsorption behavior of composite aerogels is closely related to the adsorption of ES-LDH, and the intercalation of EDTA and SDBS. When the content of ES-LDH is 30%, CMC polymers and ES-LDH adsorption cooperate with each other, making the total adsorption amount reach and adsorbent, the pore-size distribution in the adsorbent, the diffusion coefficient of the adsorbate and the degree of mixing [47]. In addition, because the clay-based ES-LDH particles could not completely uniformly distributed in the macromolecular network, and the surfaces of ES-LDHs have heterogeneous adsorption energy of Co(II), which fits the description of Elovich model [48]. All the deduced parameters of kinetic models are summarized in Table 2.

Adsorption isotherms
The adsorption isotherms of ES-LDH@CMC-30 for Co(II) are fitted by Langmuir, Freundlich, Temkin and D-R equations (Fig. 10) and the relevant parameters are listed in Table 3. Based on these results, the absorption isotherm of ES-LDH@CMC-30 composite aerogel for Co(II) is more consistent with the descriptions of the first three equations, but less consistent with the description of D-R equation. The reason could be first attributed to the large specific surface area of the composite aerogel. Because of the insertion of SDBS into ES-LDH, the interlayer spacing of LDH expands, the specific surface area and physical adsorption capacity further increase. In addition, due to complexation function of EDTA, the composite aerogels contain a large number of O, N groups, chemical adsorption ability of the materials are further enhanced. Through the chemisorption limited boundary layer controls the absorption process. The second-order reaction of absorption suggests the rate control process may be a chemical adsorption. In fact, the functional groups, such as hydroxyl, carboxyl and amine groups, could contribute to the chemisorption of Co(II) in the composite aerogel [45,46].
Based on the time-adsorption quantity data, intra-particle diffusion model (Fig. 9 C) and Elovich equation (Fig. 9D) are also applied for consideration of mechanism. There are three stages for intra-particle diffusion: (1) external surface adsorption, (2) the gradual adsorption where intra-particle diffusion is rate-controlled and (3) intra-particle diffusion started to slow down because of decreasing concentration of adsorbate. Furthermore, the absorption rate could be controlled by the size of the adsorbate molecule, the concentration of the adsorbate, the affinity between adsorbate   [54]. Third, due to the intercalation of SDBS and EDTA, the functional groups which can contact with Co(II) further increase. The absorption behavior indicates the efficiency and uniform distribution of these functional groups and molecules. The Freundlich adsorption isotherm and the Temkin adsorption isotherm are related to the concentration of the adsorbate and the adsorption potentials of the adsorbent for adsorbates, respectively. The relatively high regression coefficients imply that multilayer adsorption behavior exists. This behavior is reasonable because of intercalated modification by the rich functional molecules.
The D-R model is to estimate the apparent free energy (E) of adsorption. From the high value of the fitting curve, it verifies the strong chemisorption behavior of the composite aerogel [55].

Conclusions
In this study, SDBS and EDTA are intercalated in pristine MgFe-LDH to obtain a novel ES-LDH. Then, ES-LDH@ CMC composite aerogel is fabricated. The intercalation of SDBS and EDTA in the LDH not only expands the interlayer spacing, but also increases the active adsorption groups, which significantly improves the adsorption capacity of the composite aerogel to Co(II) ions. This design solves the disadvantage of the difficulty in separation of LDH from water when it is used for adsorption. Simultaneously, the raw materials of this composite aerogel is easy to obtain, which will not add burden to the environment. The adsorption kinetics analysis show that the pseudo second-order model and Elovich model fit well with the adsorption processes, which reveals that the adsorption mainly depends on chemical complexation. And the adsorption isotherms fit well with the Langmuir, Freundlich and Temkin models. The reason could be attributed to the porous properties of aerogels and the lamellar structure of the LDH, which results in a variety of adsorption behaviors inside the composite aerogels.
sites, there are both monolayer covered adsorption and heterogeneous multilayer adsorption inside the composites, which are in line with the description of Langmuir, Freundlich and Temkin models [46,49,50]. For the D-R model, although the correlation degree is the lowest, which is still greater than 0.9, indicating that the behavior still has the characteristics of pore adsorption [51]. However, because of the relatively large pore size, the absorption behavior could not fully conform to the characteristics of D-R model.

Absorption mechanism
The adsorption of pristine MgFe-LDH on Co(II) is based on its huge specific surface area and abundant oxygen-containing groups on the inner surface. Nevertheless, the adsorption capacity is not high. SDBS has a larger molecular volume. After exchanging with CO 3 2− in the pristine MgFe-LDH, the ES-LDHs have significantly wider layer spacing, more smooth movement space for Co(II), and better complexation conditions of EDTA molecules with Co(II). In addition, SDBS molecule itself can also form complex compound of Co(DBS) 2 with Co(II) to promote the adsorption of Co(II) [52]. As we see, this effect is not significant.
Furthermore, the adsorption behaviors of the composite aerogels can be further analyzed by combining adsorption kinetics and isotherms. The basic assumption of the Langmuir theory is that is all the adsorption sites have equal adsorbate affinity and the adsorption at one point does not affect the surrounding points. Therefore, uptake of adsorbate occurs on a homogenous surface in a monolayer without any inter-action between the adsorbates. Therefore, this model estimates the maximum adsorption capacity in a complete monolayer coverage. Among the four isotherms, the Langmuir model has the highest regression coefficient, suggesting generally uniform adsorption for Co(II) in the material. There are three main functional mechanisms in the composite aerogel for Co(II) uptake. First, after losing two electrons in the 4s orbit, one Co ion has two positive charges, which can easily interact with negatively charged group through electrostatic interaction, such as carboxyl in CMC and sulfo group in SDBS. Second, there are still three unfilled 3d orbits for Co(II), which can receive lone pair electrons and form a more stable complex. There are lots of scattered O and N atoms in the composite aerogel which can offer lone pair electrons [53]. Furthermore, EDTA is an