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Dopamine functionalized tannic-acid-templated mesoporous silica nanoparticles as a new sorbent for the efficient removal of Cu2+ from aqueous solution

A simple, environmentally friendly and cost-effective nonsurfactant template method was used to synthesize tannic-acid-templated mesoporous silica nanoparticles (TMSNs), and then dopamine functionalized TMSNs (Dop-TMSNs) which was synthesized by a facile and biomimetic coating strategy, was developed as a new sorbent for the removal of Cu2+ from aqueous solution. The Dop-TMSNs were thoroughly characterized by SEM, TEM, BET, FT-IR and TGA, and the effects of contact time, initial pH, K+ and Na+ concentrations, co-existing polyvalent metal ions and adsorption-desorption cycle times on the sorption capacity of Dop-TMSNs were studied. It was demonstrated that the maximum adsorption capacity of Cu2+ by Dop-TMSNs was 58.7 mg/g at pH 5.5, and the sorption reached equilibrium within 180 min. Moreover, the K+ and Na+ concentrations had a very slight influence on the sorption process and the adsorption capacity of the Dop-TMSNs still remained 89.2% after recycling for four times. All the results indicated that the Dop-TMSNs could be utilized as an excellent sorbent for the sequestration of Cu2+.

studied as an effective material for the removal of Cu 2+ from aqueous solution [12][13][14] . Liu et al. reported that amino-functionalized SBA-15 showed exceptional binding ability with Cu 2+ in waste water 15 . Mureseanu et al. developed an effective sorbent for the removal of Cu 2+ from aqueous solution with N-propylsalicylaldimine functionalized SBA-15 (SA-SBA- 15) 16 . Da'na et al. grafted 3-aminopropyltrimethoxy-silane on the pore walls of SBA-15 for the removal of Cu 2+ ions 17 .
Presently, there have been many strategies to prepare MSNPs such as Stöber method, co-structure-directing route and using surfactant as templating agents 18,19 . The utilization of templated synthesis includes the following steps: (1) the preparation of template; (2) template-directed synthesis of target materials; (3) template removal 20 . However, this synthetic method of MSNPs has several disadvantages as following: (1) the surfactants are expensive and toxic; (2) the calcinations used to remove the surfactants may lead to the reduction of the amount of silanol groups on the surface of MSNPs because of the high-temperature. Hence, a non-toxic, low-cost and especially nonsurfactant templating method that utilized to synthesize MSNPs with interconnected pores is highly required 21 .
Currently, tannic acid (TA), which can be used as a porogen, has attracted great research attention for the reason that it is not only cheap, environmentally friendly and nontoxic but also a nonsurfactant template. Gao and Zharov utilized TA as the template in the preparation of mesoporous silica materials with tunable mesopore sizes (from 6 to 13 nm) [9][10][11] . Jiang et al. synthesized TA-MSNs with TA as the template, and then the TA-MSNs were used as the supports for BCL immobilization 21 .
In addition, the traditional surface modification strategies used to functionalize MSNPs still have several limitation such as low adsorption capacity, complexity of experimental procedure and specific equipments 22 . The utilization of dopamine as a surface modification reagent have attracted great research attention, because it is inexpensive, adhesive, and simple to deposit onto substrates without the need of surface pretreatment 23 24 . However, to the best of our knowledge, dopamine modified tannic-acid-templated mesoporous silica nanoparticles has never been used as sorbent for the removal of Cu 2+ from aqueous solution.
In this work, the mesoporous silica nanoparticles (TMSNs) were prepared utilizing the tannic acid as a nonsurfactant template, and then the dopamine was grafted on the surface of TMSNs to develop a new sorbent of Dop-TMSNs for the removal of Cu 2+ from aqueous solution. Several experiments were conducted to systematically investigate the influences of contact time, initial pH, K + and Na + concentrations, co-existing polyvalent metal ions and adsorption-desorption cycles on the sorption process. The results demonstrated that Dop-TMSNs exhibited improved performance for the removal of Cu 2+ , and it has great potential for practical applications.

Materials and Methods
Materials. Tannic  Preparation of dopamine-functionalized TMSNs (Dop-TMSNs). The TMSNs were synthesized as described by Jiang et al. 21 . TMSNs were functionalized with dopamine using the post-grafting method. Specifically, 0.4 g of TMSNs were added into 100 mL of 1.0 g/L dopamine solution, which was freshly prepared in phosphate buffer (pH 8.5), and the suspension was stirred for 3 h. Then, the slurry was centrifuged, washed three times with distilled water, and dried at 40 °C in vacuum for 24 h. The as-prepared solid product was denoted as Dop-TMSNs.

Batch adsorption experiments.
In the batch adsorption experiments, an exact amount of Dop-TMSNs were added into 50 mL Cu(NO 3 ) 2 ·3H 2 O solution in a conical flask, and the mixture was agitated using a mechanical shaker at 200 rev/min at 298 K. The initial pH of solutions was adjusted with 0.1 M HNO 3 or 0.1 M NaOH. In the equilibrium study, the initial pH of influent solutions was adjusted to 5.5. When the sorption equilibrium was reached, the solid was filtered, and the concentration of Cu 2+ in the filtrate was measured by a atomic absorption spectrophotometer (

Results and Discussion
Characterization of Dop-TMSNs. The SEM and TEM images of Dop-TMSNs are shown in Fig. 1 and  (Fig. 2) showed that the Dop-TMSNs possessed porous structure with disordered pore arrangement, which was in agreement with the previous reports 11, 21 .
The nitrogen adsorption/desorption isotherms of TMSNs and Dop-TMSNs are shown in Fig. 3. According to the surface area measurements based on the BET method, the surface area of TMSNs was 454 m 2 /g, and the calculated BJH pore size and pore volume were 7.3 nm and 0.72 cm 3 /g, respectively. After modification with dopamine, the surface area, pore size and pore volume of Dop-TMSNs were 396 m 2 /g, 6.9 nm and 0.63 cm 3 /g,  respectively. The large pore volume and pore size were beneficial for the Cu 2+ to enter into the internal structure of Dop-TMSNs, which could enhance the sorption capacity 25 . Compared with the pristine TMSNs, the Dop-TMSNs showed an apparent decrease in the surface area, pore size and pore volume, which indicated that the dopamine was successfully grafted on the surface of TMSNs. Figure 4 shows the FT-IR spectra of TMSNs and Dop-TMSNs. The peaks at 3415 cm −1 and 1633 cm −1 were assigned to the stretching and bending vibration of hydroxyl and water, respectively 26 . In all spectra, the typical peaks at 804 cm −1 and 468 cm −1 corresponded to the symmetric stretching and bending vibration of Si-O-Si, respectively, and the peaks at 1089 cm −1 and 968 cm −1 were assigned to the asymmetric stretching of Si-O-Si and symmetric stretching of Si-OH, respectively, indicating that the structure of TMSNs was well preserved in the Dop-TMSNs 27 . For the Dop-TMSNs, the peak at 1502 cm −1 was belong to the stretching of aromatic rings in the polydopamine, which demonstrated that the dopamine was successfully grafted on the surface of TMSNs 24 . Figure 5 shows the TGA profile for the Dop-TMSNs. It is clear that the weight loss of Dop-TMSNs with the increase of temperature showed three stages. In the first stage, about 0.5% of weight loss occurred from ambient temperature to 190 °C, which was attributed to the volatilization of absorbed water in the pores of Dop-TMSNs. In the second stage, from 190 °C to around 480 °C a weight loss of about 7.8% was ascribed to the decomposition of the polydopamine grafted on the outside surface of the Dop-TMSNs. In the third stage, the weight loss of 2.1% between 480 °C and 690 °C was associated with the pyrolysis of the grafted functional groups of polydopamine inside the pores. According to the above results, the total amount of the grafted functional groups in the Dop-TMSNs was about 9.9%, which further confirmed the successful modification of TMSNs with dopamine.
Sorption kinetics. The sorption kinetics were determined with an initial Cu 2+ concentration of 100 mg/L at 298 K and pH 5.5, and the dosage of Dop-TMSNs was 1.0 g/L. The effect of contact time on the sorption of Cu 2+ by Dop-TMSNs is shown in Fig. 6. The amount of Cu 2+ adsorbed on Dop-TMSNs increased with the increase of contact time. It could be observed that the adsorption occurred rapidly within the fist 120 min, and gradually reached equilibrium at 180 min. In the first stage of 0-120 min, the faster adsorption rate could be ascribed to the larger quantity of binding sites on the surface of Dop-TMSNs and the higher Cu 2+ concentration in the solution.
In the second stage of 120-180 min, the amount of Cu 2+ adsorbed on Dop-TMSNs increased slowly, which could  be attributed to that the Cu 2+ penetrated into the inside pores of Dop-TMSNs needed more energy. Actually, approximate 76.1% of the equilibrium adsorption quantity was obtained within 60 min. To ensure the completeness of adsorption, a contact time of 210 min was used in the subsequent experiments.
The pseudofirst order and pseudosecond order kinetics models, which were shown in Eqs (1) and (2) 28 , were applied to study the specific kinetic parameters of Cu 2+ adsorpted on Dop-TMSNs.
where q e (mg/g) is the adsorption capacity at equilibrium; k 1 (min −1 ) and k 2 (g/(mg min)) represent the rate constants of the pseudofirst order adsorption and pseudosecond order adsorption, respectively. The values of k 1 and k 2 could be determined by the slope and intercept of the lines in Figs 7 and 8, respectively, and the results were summarized in Table 1. According to the results, the coefficient (R 2 ) of the pseudosecond order kinetic equation (R 2 = 0.995) was higher than that of the pseudofirst order kinetic equation (R 2 = 0.827). Furthermore, the q e value calculated from the pseudosecond order kinetic model, which was 45.4 mg/g, was close to the experimental value of 42.8 mg/g. Hence, the pseudosecond order kinetics model was more suitable for the description of the adsorption kinetics of Cu 2+ on Dop-TMSNs.
The effect of initial pH. The initial pH values of the Cu 2+ solutions have important influence on the adsorption of copper ions 29,30 . According to the previous research reports, it was unsuitable for adsorption experiments to be carried out when the pH value of the Cu 2+ solution was greater than 5.5, for the reason that Cu(OH) 2 precipitation will be observed at pH above 5.5 12,31 . Thus, the adsorption of Cu 2+ by Dop-TMSNs was investigated by varying the initial pH values of the Cu 2+ solutions from 2 to 5.5 at 298 K, and the initial Cu 2+ concentration and Dop-TMSNs dosage were 100 mg/L and 1.0 g/L, respectively. Figure 9 shows the effect of initial pH values on the sorption of Cu 2+ with Dop-TMSNs. The results indicated that the adsorption capacity of Dop-TMSNs increased      The sorption isotherm. The sorption isotherm of Cu 2+ on Dop-TMSNs was measured by increasing the Cu 2+ concentration from 20 mg/L to 300 mg/L at 298 K and pH 5.5, and the Dop-TMSNs dosage was 1.0 g/L. As shown in Fig. 10, with the increase of equilibrium Cu 2+ concentration, the adsorption capacity of Dop-TMSNs for Cu 2+ increased and the maximum adsorption capacity was 58.7 mg/g. The superior adsorption capacity was mainly attributed to that the Dop-TMSNs had large surface areas and the functional groups on the surface of Dop-TMSNs had excellent Cu 2+ chelation ability. Additionally, the Cu 2+ sorption capacity of Dop-TMSNs was compared with other adsorbents, and the results are shown in Table 2. Although the adsorption capacity of Dop-TMSNs was not the largest one, the synthetic method of Dop-TMSNs used in this work was simple, economic, environmentally friendly and nontoxic. Thus, the Dop-TMSNs had great potential for widespread practical applications.
To study the adsorption behaviour between the Cu 2+ and Dop-TMSNs, Langmuir and Freundlich isotherms were used to analyze the equilibrium adsorption data [27][28][29][30][31][32] . Linear equations of Langmuir and Freundlich models are shown in Eqs (3) and (4) 33,34 : where q m (mg/g) and K L are the maximum adsorption capacity for fitting and a constant related to the free energy of adsorption, respectively. K F and n are the constants for Freudlich model. In agreement with Eqs. (3) and (4), two straight lines were shown in Fig. 11 and 12, respectively. The regression coefficient (R 2 ) in Table 3 indicated that the sorption data of Cu 2+ on Dop-TMSNs fitted the Langmuir model (R 2 = 0.994) better than the Freundlich model (R 2 = 0.948). Additionally, the calculated Langmuir adsorption capacity (58.7 mg/g) was close to the experimental adsorption capacity (55.6 mg/g). Therefore, the Langmuir  model could properly describe the observed adsorption process, which indicated that the adsorption of Cu 2+ might take place at homogeneous binding sites on the surface of Dop-TMSNs and formed a monolayer 1,35 . The phenolic groups in polydopamine had chelation ability with a variety of metals 36 , therefore, bidentate chelating bonding in which two oxygen atoms bound to a copper might be one of the mechanisms of polydopamine on the surface of Dop-TMSNs interaction with Cu 2+ ions, as shown in Fig. 13. Additionally, Cu 2+ is one of the borderline metals with ambivalent properties that possesses favourable affinity with amine groups 37 , thus the amino ligands in the polydopamine could coordinate with the Cu 2+ ions, as shown in Fig. 13.
The effect of K + and Na + concentrations. K + and Na + concentrations are significant factors influencing the sorption of Cu 2+ . To study the effect of K + and Na + concentrations on the Cu 2+ sorption by Dop-TMSNs, the experiments were carried out in the presence of KNO 3 or NaNO 3 with concentrations varying from 0.01 to 0.2 mol/L at 298 K and pH 5.5, and the initial Cu 2+ concentration and Dop-TMSNs dosage were 50 mg/L and 0.6 g/L, respectively. The results are shown in Fig. 14. It could be seen that the presence of K + or Na + had a very slight influence on the adsorption capacity of Dop-TMSNs for Cu 2+ , which might be attributed to that the surface functional groups on Dop-TMSNs had stronger chelation capability with Cu 2+ than with K + or Na + . Furthermore, the possible explanation for the decline of Cu 2+ sorption capacity of Dop-TMSNs might be that the increase of ionic strength resulted in the decrease of the activity of Cu 2+ ions, and the K + or Na + ions also had competition with Cu 2+ ions for the active sites on Dop-TMSNs 25 .     3 with concentrations varying from 0.1 to 1 mmol/L, respectively, and the initial Cu 2+ concentration and Dop-TMSNs dosage were 1 mmol/L and 0.6 g/L, respectively. As illustrated in Fig. 15, it was obvious that the presence of polyvalent metal ions had a very slight influence on the adsorption capacity as the concentration of the metal ions was below 0.2 mmol/L. With increasing the concentrations of the metal ions, the adsorption capacity decreased, which might be ascribed to the competition between the polyvalent metal ions and Cu 2+ ions for the active adsorption sites 38 . However, the co-existing polyvalent metal ions had insignificant influence on the adsorption of Cu 2+ ions on Dop-TMSNs. Additionally, among the polyvalent metal ions, the Cr 3+ ions had the greatest influence on the adsorption capacity of Dop-TMSNs for Cu 2+ , which might be attributed to that the Cr 3+ ions had stronger affinity with the adsorption sites than the other three metal ions.  Desorption and reusability. The regeneration and reuse of Dop-TMSNs can make the adsorption process cost-effective in the large-scale application. To test the reusability of Dop-TMSNs, the adsorption-desorption experiments were carried out with the Dop-TMSNs dosage of 0.6 g/L and the initial Cu 2+ concentration of 50 mg/L at 298 K and pH 5.5. As illustrated in Fig. 16, the Dop-TMSNs could still remain 89.2% of its initial adsorption capacity after four adsorption-desorption recycle experiments. The decrease of adsorption capacity of Dop-TMSNs might be ascribed to the loss of the sorbent or the irreversible occupation of part adsorption sites 25 . These results suggested that the reusability of Dop-TMSNs was superior, and it could be used as a recyclable and efficient adsorbent for the removal of Cu 2+ .

Conclusions.
In conclusion, a dopamine functionalized tannic-acid-templated mesoporous silica nanoparticles (Dop-TMSNs) for the removal of Cu 2+ was synthesized by a facile, simple, environmentally friendly and cost-effective method. The maximum adsorption capacity of Dop-TMSNs was 58.7 mg/g at 298 K and pH 5.5. The adsorption equilibrium was reached with 180 min, and the adsorption kinetics could be described well by the pseudosecond order kinetics model. The sorption isotherm parameters were fited well with the Langmuir model, and the adsorption capacity of Dop-TMSNs could still remain 89.2% after recycling for four times. Additionally, the Cu 2+ adsorption by Dop-TMSNs was pH dependent, and the influence of K + and Na + concentrations was very weak. Taken together, the Dop-TMSNs had great potential to be utilized as Cu 2+ adsorbent in practical applications.