Preparation of core–shell structured CaCO3 microspheres as rapid and recyclable adsorbent for anionic dyes

Core–shell structured CaCO3 microspheres (MSs) were prepared by a facile, one-pot method at room temperature. The adsorbent dosage and adsorption time of the obtained CaCO3 MSs were investigated. The results suggest that these CaCO3 MSs can rapidly and efficiently remove 99–100% of anionic dyes within the first 2 min. The obtained CaCO3 MSs have a high Brunauer–Emmett–Teller surface area (211.77 m2 g−1). In addition, the maximum adsorption capacity of the obtained CaCO3 MSs towards Congo red was 99.6 mg g−1. We also found that the core–shell structured CaCO3 MSs have a high recycling capability for removing dyes from water. Our results demonstrate that the prepared core–shell structured CaCO3 MSs can be used as an ideal, rapid, efficient and recyclable adsorbent to remove dyes from aqueous solution.


Introduction
In recent years, dyes have been widely used in textiles, plastics, paper, cosmetics, pulp manufacture, tanning, pharmaceuticals and food processing industries [1,2]. However, these dyes will pollute water resources, influence the photosynthesis of underwater plants and the growth of aquatic animals, and may even bring toxicity, carcinogenicity and teratogenicity to human beings [3][4][5][6][7]. The disposal of toxic effluents without proper treatment is the major source of water pollution. Therefore, the removal of dyes from industrial effluents is an urgent need for the protection of water resources [8][9][10][11]. Currently, many treatment techniques have been adopted to remove dyes from waste water,  To investigate the adsorption properties of obtained core-shell structured CaCO 3 MSs, as illustrated in figure 1, we chose Coomassie brilliant blue, Congo red, Alcian blue and methylene blue as model dyes. Calcination is used as a simple and effective desorption method, because calcium carbonate materials can remain stable, but dye molecules will be converted into carbon dioxide during calcination. We believe that such core-shell structured CaCO 3 MSs can be considered as easily separated, high-efficiency and recyclable adsorbent for removal of dyes.

Preparation of CaCO 3 microspheres
The core-shell structured CaCO 3 MSs were synthesized by a facile, one-pot method. Firstly, 0.4% (wt %) Hesp solution was prepared by alkaline aqueous solution (pH = 11.0, adjusted by ammonia and HCl). Then, CaCl 2 (5 ml, 1 mol l −1 ), NaHCO 3 (10 ml, 1 mol l −1 ) and the prepared Hesp solution (5 ml added into a beaker (150 ml), into which 80 ml of water was further added. The mixture was vortexstirred (IKA, Vortex, Genius 3) to obtain a homogeneous solution. After that, the solution was heated to 65°C and kept for 20 min. The beaker and 100 ml of ammonia (in another beaker) were placed in a closed desiccator for 24 h at room temperature. By NH 3 diffusing and dissolving in the solution, the mineralization of CaCO 3 MSs was initiated. The products were collected by centrifugation, and rinsed with deionized water several times. Finally, the obtained products were dried at 70°C for further analysis.

Characterization
The morphologies of the obtained CaCO 3 MSs were investigated by scanning electron microscopy (SEM, Hitachi, S4800, Tokyo, Japan) and transmission electron microscopy (TEM, JEM-1200EX, Tokyo, Japan). The composition of the CaCO 3 MSs was identified by using Fourier transform infrared (FTIR) spectroscopy (Shimadzu, Kyoto, Japan) in the range of 4000 to 400 cm −1 with the KBr disc method. The as-prepared CaCO 3 MSs were examined with powder X-ray diffraction analysis (XRD, Shimadzu, Kyoto, Japan) with Cu Kα radiation to obtain the crystallographic structure of the MSs. N 2 adsorptiondesorption measurements were performed on a ASAP-2460 instrument with the MicroActive software (sample mass: 0.1269 g; equilibration interval: 10 s; sample density: 1.000 g cm −3 ), using Brunauer-Emmett-Teller (BET) calculations for surface area and Barrett-Joyner-Halenda calculations for pore size distribution.
The adsorption experiments of CBB, CR, AB and MB from an aqueous medium on the coreshell structured CaCO 3 MSs were studied by a UV-visible absorption spectrophotometer (PerkinElmer Lambda 605S UV/Vis spectrometer). The dye solution was centrifuged; then, the supernatant was analysed to get absorbance by the UV-visible spectrometer at each maximum absorption wavelength (CBB (585 nm), CR (498 nm), AB (600 nm) and MB (664 nm)).
The effect of the adsorbent dosage (AD) on the adsorption process was investigated by adding different dosages of CaCO 3 MSs, which varied in the ranges of 0-50 mg (CBB), 0-50 mg (CR), 0-200 mg (AB) and 0-300 mg (MB) for a 10 ml sample bottle containing 4 ml of dye solution. After a given adsorption time, the mixture was centrifuged, and then the supernatant was analysed to obtain the absorbance with the UV-visible spectrometer at each maximum absorption wavelength.
According to the experimental results listed in this paper, the adsorption efficiency of CaCO 3 MSs for anionic dyes (CBB and CR) has already reached approximately 100% when the AD increases to 20 mg. Therefore, the adsorbent dosage of 20 mg (for CBB and CR) was used to determine the adsorption time.
To determine the adsorption time of AB and MB, the adsorbent dosage was set as 100 mg and 200 mg , respectively.
CaCO 3 cubic microcrystals were prepared and used as dye adsorbent in the contrast experiments.
To investigate the adsorption efficiency in the above experiments, we adopt a similar calibration curve method based on UV-visible spectroscopy as reported in our previous work [24,25].
The equilibrium adsorption capacity (q e ) was calculated according to the following equation: where C 0 and C e are initial and equilibrium dye concentrations, respectively (mg l −1 ), V is the solution volume (l) and m is the adsorbent dosage of CaCO 3 MSs (g).

Recycling experiment
The recyclability of the adsorbent is very important. After dye adsorption, the CaCO 3 MSs were separated (from the dye solution), dried and burned at 450°C for 2 h to desorb the dyes by using a muffle furnace. Then, those calcinated CaCO 3 MSs were reused to adsorb dyes according to the previous experiment steps. To investigate the recyclability of those core-shell structured CaCO 3 MSs, such an 'adsorption-calcination (desorption)-adsorption' cycle was carried out five times in each group. Owing to the hierarchical core-shell structure, these CaCO 3 MSs are endowed with a large surface area. Thus, BET measurements were carried out to study the porosity and pore size distribution of the MS samples. A BET surface area of 211.77 m 2 g −1 is achieved for CaCO 3 MSs with an average pore size of 7.8 nm (electronic supplementary material, figure S1). These data suggest that the CaCO 3 MSs can offer lots of adsorption sites for organic dye molecules. The absorption peaks (FTIR spectrum, figure 3a) located at 712, 876 cm −1 (ascribed to characteristic absorption peaks of calcite) and 745 cm −1 (ascribed to characteristic absorption peaks of vaterite) indicate that these core-shell structured CaCO 3 MSs are composed of both calcite and vaterite [24]. The XRD pattern in figure 3b further proved the polycrystalline property of the CaCO   (f v ) and calcite in the crystalline phase according to the literature [26,27]: The result indicates that the calcite and vaterite contents in the core-shell structured CaCO 3 microspheres are 76.01% and 23.99%, respectively. The prepared CaCO 3 MSs were rinsed thoroughly. However, Hesp was still found in the obtained CaCO 3 MSs. We used thermogravimetric analysis to determine the content of Hesp in the obtained coreshell structured CaCO 3 MSs. The result in electronic supplementary material, figure S2, confirms that the content of Hesp in CaCO 3 MSs is less than 1 wt%. Therefore, the influence of Hesp on the adsorption performance of CaCO 3 might be negligible. Figure 4 shows the effect of the AD on the adsorption capability of CaCO 3 MSs in the solution of the four dyes (CBB, CR, AB and MB). Figure 4a shows that the increase in the AD of CaCO 3 MSs will decrease the absorbance at 585 nm (maximum absorption peak of CBB) gradually. Moreover, these absorption curves tend to be more flat on increasing the AD to 10 mg. When the AD reaches 20 mg, it can be found that nearly all (99.57 wt%) the CBB dye has successfully been adsorbed by the CaCO 3 MSs. The insets in figure 4a show the images of the CBB solution (500 mg l −1 , left image) and the supernatant after adsorption by 20 mg of CaCO 3 MSs (right image). It is obvious that the blue colour of the CBB solution has faded in the supernatant. Both the disappearance of the absorption peak and the fading of the colour indicate that the core-shell structured CaCO 3 MSs have strong adsorption capability for CBB. From the adsorption curves and insets shown in figure 4b, it can be found that the adsorption of CR by the CaCO 3 MSs is similar to that of CBB in figure 4a. When the AD reaches 20 mg, the added CaCO 3 MSs can strongly adsorb almost all the CR dye (99.60 wt%) from the solution.   The results in figures 4 and 5 prove that the obtained CaCO 3 MSs can efficiently adsorb both CBB and CR as quickly as 2 min. For AB molecules, CaCO 3 MSs can also completely remove them, but the AT is 28 h. For MB, CaCO 3 MSs cannot efficiently remove all dye molecules, even with a given AT of more than half a month. The above difference may be ascribed to the chemical structure of the four dyes. As illustrated in figure 1,   With regard to AB and MB ( figure 1c,d), they are cationic dyes with tertiary ammonium and quaternary ammonium groups. Those tertiary ammonium groups will bind the Ca 2+ of the CaCO 3 MSs. However, positively charged quaternary ammonium groups will repel the Ca 2+ on the CaCO 3 MSs. For AB, it has more tertiary ammonium groups (12) than quaternary ammonium groups (4). Thus, CaCO 3 MSs can efficiently adsorb AB molecules due to the tertiary ammonium groups. The AT is quite long (28 h) because of the exclusion between quaternary ammonium groups and the Ca 2+ ions. With regard to MB, its molecular size is much smaller than AB, so the exclusion between quaternary ammonium groups and the Ca 2+ ions is stronger than that of AB. Therefore, the CaCO 3 MSs only remove about 76.61 wt% MB from water after a quite long AT of 18 days.
The recycling experiments were performed for five cycles (adsorption-desorption) and the results are depicted in figure 6. In the first run, very high removal efficiencies are seen for CBB, CR and AB of up   figure S4, shows that the BET surface area of CaCO 3 MSs after five cycles has no obvious change. Furthermore, it was found that the removal efficiency of dyes has declined slightly to 93.86% for CBB, 95.63% for CR and 92.40% for AB after the 10th cycle. This suggests that the obtained CaCO 3 MSs are stable in the recycling experiments and maintain a high removal efficiency for dyes.
We use calcite microcrystals as adsorbent in the contrast experiment of dye adsorption (figure 7). The SEM image (figure 7a) reveals that the morphology of the calcite materials is that of smooth cubic microcrystals. The adsorption of dyes by CaCO 3 materials could be attributed to the electrostatic interaction between Ca 2+ and organic dye molecules. Compared to the smooth, cubic calcite microcrystals, the core-shell structured CaCO 3 MSs have a much larger surface area which can provide more adsorption sites (Ca 2+ ). Therefore, the removal of dyes by core-shell CaCO 3 MSs from aqueous solution is a more rapid and efficient process (figure 7b-e) than that by using cubic CaCO 3 materials as adsorbents.
A comparison of the adsorption capacity (q e ) for Congo red in this paper and reported results is presented in table 1. It is obvious that the adsorption capacity of Congo red by the presented coreshell structured CaCO 3 MSs is larger (99.6 mg g −1 ) and the adsorption time is remarkably faster (2 min) than for those materials reported previously [28][29][30][31][32]. For instance, Saygılı reported that the adsorption capacity of a biomagnetic composite as adsorbent for the removal of Congo red is 86.96 mg g −1 , and the adsorption time for the equilibrium is 200 min [28]. Yu et al. reported that the adsorption capacity of mesoporous Fe 2 O 3 is 53 mg g −1 , and the adsorption time for the equilibrium is 120 min [29]. Therefore, these core-shell structured CaCO 3 MSs can be used as a rapid and efficient adsorbent to remove anionic dyes from aqueous solution.

Conclusion
In this work, we prepared core-shell structured CaCO 3 MSs by a facile, one-pot method at room temperature. The results suggest that these CaCO 3 MSs can rapidly and efficiently remove 99-100% of CBB and CR dyes within 2 min. We also found that these core-shell structured CaCO 3 MSs have a high recycling capability for removing dyes from water. Our results demonstrate that the obtained core-shell structured CaCO 3 MSs can be used as an ideal, rapid, efficient and recyclable adsorbent to remove dyes from aqueous solution.
Data accessibility. We include all the experimental data in the electronic supplementary material, which are available at http://dx.doi.org/10.5061/dryad.m1b34 [33].
contributed towards fabrication and characterization of materials. Z.C. and X.R. analysed data. Z.C. wrote the paper. All authors discussed the results and commented on the manuscript. Competing interests. We declare we have no competing interests. Funding. This work was supported by Natural Science Foundation of Liaoning Province (no. 20170540386) and National Natural Science Foundation of China (no. 81771987, no. 51202199, no. 81471854 and no. 81671907).