Ultra-efficient copper ions adsorption of Chitosan- 1 montmorillonite composite aerogel for wastewater treatment

13 The modified montmorillonite(MMT) has a two-dimensional stable and ordered lamellar 14 structure. The addition of chitosan(CS) cross-links the two-dimensional sheets to build a three- 15 dimensional network structure with a high specific surface area. We have prepared the best MMT- 16 based water treatment materials that have been reported. This new type of aerogel can efficiently 17 adsorb heavy metal ions in wastewater. The structure and performance of the composite material 18 were characterized in this article. Besides, the adsorption kinetics, adsorption thermodynamics, pH 19 influence, and recycling performance are all focused on. The adsorption equilibrium time of CS- 20 MMT2 is 50 min. The removal rate of Cu 2+ is as high as 98.21%. The maximum adsorption capacity 21 is 86.95 mg/g. The adsorption process of Cu 2+ by CS-MMT composite aerogel conforms to the 22 quasi-second-order kinetic model and the Langrangian adsorption isotherm. After three cycles, the 23 removal rate of Cu 2+ by CS-MMT2 remained above 80%. This article also involves the discussion 24 of the material's adsorption mechanism for Cu 2+ . This is a kind of environmentally friendly material 25 that can be mass-produced, cheap, efficient, and excellent, which is of great significance to the 26 development of environmental protection.

3 techniques, and reverse osmosis (Moja et al. 2020). Compared with others, adsorption is a simple, 45 easy-to-operate, and inexpensive way to treat heavy metal ions (El-Kousy et al. 2020). Recent 46 research has focused on relatively low-cost adsorbents such as zeolite (Bailey et al. 1999 Especially for low-concentration heavy metal ion wastewater, the development of nano-adsorption 51 materials has a long way to go (Liu et al. 2020b). Therefore, it is necessary to obtain a new adsorbent 52 with high adsorption capacity, high specific surface area, rich pore structure, abundant sources, low 53 cost, good stability, easy recovery, and regeneration. 54 In recent years, aerogel has been considered a promising insulation material. (Baetens et al. 55 2011) At the same time, high porosity and large specific surface area show superior adsorption 56 capacity (Cui et al. 2018). It can be widely used in sewage treatment, air purification, nuclear waste 57 treatment, and other environmental protection fields (Pierre and Pajonk 2002). 58 Modified clay, such as montmorillonite, is regarded as one of the best inorganic adsorbents 59 because of its high utilization rate, friendly environment, and high negative charge, which can 60 absorb positively charged metal ions (Mahouachi et al. 2020). Chitosan is hydrophilic, 61 biocompatible, biodegradable, non-toxic, has high mechanical strength and film-forming properties, 62 as well as antibacterial properties (Li et al. 2016). The amino (-NH2) and hydroxyl (-OH) groups 63 inherent in the chemical structure of chitosan are the main functional groups that adsorb various 64 heavy metals in water. By modifying the structure of chitosan with -COOH group, its solubility at 65 pH=7 can be increased without affecting the above properties (Wang et al. 2017 material is expected to achieve a major breakthrough in the treatment of heavy metal ion wastewater. 74 Our team combined chitosan aerogel and montmorillonite designed a new high-efficiency 75 adsorbent. Montmorillonite can be used as ideal templates for preparing nanomaterials and 76 supporting chitosan hydrogels due to their high surface area and stable structure. We use the 77 LiOH/urea dissolving system to process chitosan raw materials and the chemical precipitation 78 method to compound Montmorillonite. By adjusting the concentration of the initial reactants, the 79 size, pore size, and mechanical properties of the synthesized CS-MMT aerogel can be determined. 80 CS-MMT aerogel nanocomposite can be used as an environmentally friendly adsorbent for the 81 removal of Cu 2+ from wastewater. With its outstanding performance, it will occupy a place in the 82 field of wastewater treatment. The adsorption kinetics and adsorption thermodynamics were used 83 to systematically study the maximum adsorption capacity. The separation and reusability of CS-84 MMT aerogel were also studied in detail. 85

Fabrication of CS-MMT aerogel 87
CS-MMT aerogel was prepared by sol-gel method (SI Fig. 1), which could be broken and 88 5 regenerated at low temperature(SI Section 1). According to the absorbance value of Cu 2+ in the 89 supernatant, the concentration, adsorption capacity, and removal efficiency of Cu 2+ in the 90 remaining solution were calculated using equations ((1)-(4)). 91 = 0.0191 + 0.0124 (1) 92 (3) 94 Where C is the content of Cu 2+ in the residual solution after adsorption, which can be calculated 96 by different absorbance values(μg). C0 before for adsorption of Cu 2+ solution concentration 97 (mg/mL). Ce for the adsorption of Cu 2+ solution concentration (mg/mL). m is obtained from the 98 standard curve of Cu 2+ dosage (μg). m1 as adsorbent dose (mg). V1 for copper water volume (mL). 99 Qe for equilibrium adsorption Cu (II) the amount of (mg/g). V2 to join Cu 2+ the volume of a solution 100 (mL), S for Cu 2+ removal efficiency. To further analyze the interaction between MMT and CS matrix, X-ray photoelectron 107 spectroscopy is used to analyze the surface chemical state of different components of CS-MMTs 108 ( Fig. 1(b~f)). The C spectrum shows that the CS-MMTs are rich in C-C, C=O, and C-O/C-N groups. 109 The peak at 283.9 eV corresponds to the binding energy of the C-N bond, and the peaks of C=O and components, the C1 s of CS-MMT2 is significantly enhanced, and the intensity of the C-N peak is 112 also significantly increased, proving that MMT was successfully modified by CS. The O1s (531.8 113 eV) peak is attributed to hydroxyl oxygen, indicating that siloxane was successfully converted to The crystal phase and phase composition of CS-MMT composite aerogel were analyzed by 119 XRD ( Fig. 1 (g)). According to extensive studies, MMT is a clay mineral in the form of octahedral 120 crystals. The characteristic spikes show high crystallinity of nano-MMT at 5. 9°, 19°, and 26°. 121 According to the Scherer equation, we can easily calculate that the interlayer spacing of nano-122 montmorillonite is 1.593nm. In contrast, the XRD patterns of the chitosan-modified 123 montmorillonite composite aerogels have obvious changes, indicating that the crystal structure of 124 the composite aerogel has changed significantly. By comparing and analyzing the standard spectra, 125 we can infer that the crystal structure of CS-MMT is roughly the same as that of square quartz. 126 Besides, we can notice that there is a broad peak at 29°. As the ratio of nano-montmorillonite 127 increases, the peak intensity gradually decreases. We attribute this to the cation exchange between 128 the protonated amino groups of chitosan and the cations in the montmorillonite layer, which results 129 in such formation of disordered exfoliation structures.   Fig.3(a~f), when the relative pressure P/P0 was 0-0.6, the 158 N2 adsorption increased slowly, which proved that there were a few micropores in the samples. In 159 this stage, nitrogen molecules are gradually adsorbed from monolayer to multilayer into the porous 160 structure. Subsequently, when the relative pressure was between 0.6-0.99, the nitrogen adsorption 161 capacity at the high-pressure end increased sharply, indicating the presence of mesopore and 162 macropore in the material. According to the physical property parameters in SI Table S1, the specific 163 surface area of CS-MMT2 is relatively high, which is 14.133m 2 /g, and the pore diameter distribution 164 was uniform. Combined with SEM images (Fig.1(b)), it was found that there was a stable three- According to detailed characterization data, we can get the following material structure model 178 (Fig. 4). The montmorillonite layer is modified by organic quaternary ammonium salt ions. 179 Combined with the XRD diffraction pattern, it can be seen that the interlayer spacing of 180 montmorillonite increases from 1.236 to 1.593 nm. XRD, FT-IR, and XPS are used for detailed 181 elements and surface state analysis. We speculate that strong alkali treatment and ball milling break (6) 208 Where Qe is the adsorption amount when the adsorption process reaches adsorption equilibrium 209 (mg/g), Qt is the adsorption amount corresponding to t at a certain moment in the adsorption process 210 (mg/g), k1and k2 are pseudo-first-order (min -1 ) and pseudo-second-order rate constants (g/ (mg·min -1 )), 211 respectively. 212 213

Fig. 5 Adsorption efficiency of CS-MMT composite aerogels for Cu 2+ at different time 214
The parameters of the two models are shown in SI Table S2. By comparing the correlation 215 coefficient R 2 of the two models, it is obvious that the pseudo-second-order kinetic equation has a good 216 correlation with the adsorption process. This indicates that the adsorption rate is more related to the 217 number of unoccupied active sites. As the layered space of montmorillonite increases, the porosity 218 increases. This will directly increase the number of active sites and increase the adsorption capacity 219 13 of Cu 2+ . Based on the quasi-second-order kinetic equation model, the quasi-second-order rate equation 220 of CS-MMT was shown in Fig. 6. According to the fitting curve of CS-MMTs, the corresponding 221 correlation coefficient R 2 was calculated. The R 2 of CS-MMTs indicates that the adsorption of Cu 2+ in 222 this system follows a pseudo-second-order kinetic model. CS-MMT2 has the best fit, which can reach 223 0.99937, while Qe is 9.906 and K2 is 0.04942. 224 225 Fig. 6 Pseudo-second-order model for adsorption of Cu 2+ on CS-MMT composite aerogels 226

Adsorption isotherms and adsorption thermodynamics 227
As can be seen from Fig. 7, the equilibrium adsorption capacity of the three adsorbents to copper 228 ions increased with the increase of the initial concentration of Cu 2+ solution until the adsorption was 229 completed. When Cu 2+ solution is at a low concentration, the adsorption curve rises very fast. When the 230 initial concentration increases to a certain concentration, the increasing rate of adsorbent adsorption 231 gradually slows down, and the curve flattens out to near saturation adsorption. 232 14 In order to obtain the thermodynamic mechanism of adsorption of Cu 2+ by CS-MMT composite 233 aerogels at different initial concentrations, the Langmuir equation and Freundlich equation were used 234 to fit the experimental data. The Langmuir isotherm model (Eq. 7) and the Freundlich isotherm model 235 (Eq. 8) are as follows: 236 Where Qe is the adsorption amount of Cu 2+ per unit mass of adsorbent at the adsorption equilibrium 239 (mg/g) Qm represents the maximum adsorption capacity of Cu 2+ (mg/g), and Ce represents the equilibrium 240 concentration of Cu 2+ after adsorption (mg/L), where KL and KF are the adsorption equilibrium constants 241 of Langmuir isotherm equation and Freundlich isotherm equation, and 1/n is the heterogeneity factor. 242 As can be clearly seen in Fig.10, the equilibrium adsorption capacity of the five adsorbents on Cu 2+ 243 showed a different increasing trend with the increase of the initial concentration of Cu 2+ solution. When 244 Cu 2+ is at a low concentration, the adsorption curve rises very fast. When the initial concentration 245 increases to a certain concentration, the increasing rate of adsorbent adsorption volume gradually slows 246 down, and the curve flattens out until it approaches saturation adsorption.   Besides, CS is easy to dissolve in an acidic environment, which is the main reason for the limited 273 adsorption capacity. When the pH value is higher than 4, the adsorption capacity of Cu 2+ increases 274 significantly. As the pH of the solution increases, the number of hydrogen ions decreases, and protonation 275 increases. There will be more adsorption sites on the surface of CS-MMT2, and the adsorption capacity 276 will also show an increasing trend. At pH = 6, the adsorption capacity reached its peak and then began 277 to decline. Cu 2+ will form Cu(OH)2 precipitation at higher pH values. This shows from the side that the 278 adsorption capacity of CS-MMT2 for Cu 2+ decreases. Therefore, pH = 6 is the best service condition for

MMTs after adsorption 298
To explore the principle of adsorption, we characterized the adsorbed samples. As shown in 299 Fig. 11(a), the C 1s spectra of CS-MMT2 were fitted with three components centered at 284.8, 285.9, 300 286.6.6, 287.7, and 288.4 eV, which can be assigned to C-C, C-N, C-O, C=O, and O=C-O, 301 respectively. The emergence of a new characteristic peak at 934.4 eV, corresponding to the presence 302 of Cu2p orbital. At the same time, the metal oxidation peak area was changed. Cu 2+ reacted with the 303 adsorbent to combine part of O. It is the oxygen-containing functional group that participates in the 304 adsorption process. The Cu2p spin-orbit peaks showed that Cu 2+ replaced the original hydrogen ion 305 to form the corresponding complex. During the adsorption process, C-N, C-O, O=C-O content 306 increased significantly, indicating that Cu 2+ was successfully adsorbed on CS-MMT2. Therefore, it 307 can be considered that the high adsorption capacity of CS-MMT2 to Cu 2+ is due to chelating and 308 stacking, and electrostatic attraction. As shown in Fig. 11(c), the intensity of the characteristic peak 309 of the infrared spectrum changed to different degrees after copper ions were adsorbed, which 310 indicats that amino, carboxyl, siloxane, and hydroxyl groups in CS-MMTS were involved in the 311