Creation of Hollow Calcite Single Crystals with CQDs: Synthesis, Characterization, and Fast and Efficient Decontamination of Cd(II)

In this work, carbon quantum dots were first prepared through one-pot hydrothermal route of the propyl aldehyde and sodium hydroxide via an aldol condensation reaction, and a novel solid-phase extraction adsorbent of hollow calcite single crystals was prepared via the precipitation of metal nitrates by the CO2 diffusion method in the presence of CQDs and further applied for excessive Cd(II) ions removal from water. The spectra and morphologies of the etched calcite were investigated by X-ray diffraction, Fourier transform infrared spectrometry, Scanning electron microscope, and Transmission electron microscopy. The results show that the CQDs etching technique successfully furnish a strategy for manufacturing interface defects onto the calcite crystal. Bath studies were done to evaluate the effects of the major parameters onto Cd(II) adsorption by the etched calcite, such as pH, contact time, and initial Cd(II) concentration. The Cd(II) adsorption onto the new adsorbent could reach a maximum adsorption amount of 66.68 mg/g at 120 min due to the abundant exterior adsorption sites on the adsorbent. The adsorption kinetics and adsorption isotherms of Cd(II) on the etched calcite were also investigated. The experimental datum showed that the adsorption kinetics and isotherms of Cd(II) on the etched calcite were well-fitted by the pseudo-second-order kinetic model and the Freundlich isotherm model respectively. The adsorption mechanisms could be primarily explained as the formation of Cd(OH)2 and CaxCd1−xCO3 solid solution on the adsorbent surface with the help of X-ray photoelectron spectroscopy.

removal of cadmium. Pérez-Garridoet et al. 37 have investigated the interaction between calcite (10ī4) surfaces and Cd-bearing aqueous solutions. The growth of epilayers of the Cd x Ca 1−x CO 3 solid solution with Cd-rich members happened on the original calcite (10ī4) surface. There are many literatures to demonstrate that calcite has a certain ability to adsorb the heavy metal ions [38][39][40] . In consequence, how to increase the adsorption capacity of calcite has become a hot topic for the chemical researchers. Increasing the specific surface area may be an effective measure to increase the adsorption ability of calcite for heavy metals adsorption 41 . At present, the etching technology can effectively form interface lattice defects, thereby greatly increasing the specific surface area to provide more active sites and improve the adsorption capacity of heavy metal ions 42 .
As a new class of nanocarbon materials, carbon quantum dots (CQDs) contain numerous oxygen-containing functional groups 43 . CQDs are non-toxic with little harm to the environment, commonly used in biological imaging, photoelectric device, sensing and other fields. Taking this into account, CQDs are in prospect to turn into an alternative material for the removal of a variety of organic and inorganic pollutants owing to these oxygen-containing functional groups 44,45 . CQDs can modify inorganic metal compounds to improve some surface properties. Rahmanianet et al. 46 combined LDHs with CQDs to fabricate tailored functional composite-based LDHs, so as to enhance the adsorption capacity. Thus, CQDs would provide a potential strategy for etching technology to form interface lattice defects and increase the specific surface area 47 .
In this study, the hollow calcite single crystals were first prepared by an etching technique with CQDs. They were characterized by using FT-IR, XRD, SEM and HR-TEM. The adsorption properties of the hollow calcite single crystals on Cd(II) have been investigated under the different conditions of pH (the pH change for 2, 3,4,5,6,7,8), the contact time (0, 5,15,30,60,90,120,150,180,210,240,270, 300 min), and the initial concentration of Cd(II) solution (1,5,7,8,10,15,20,25 mg/L), respectively. In the adsorption experiment, the volume of Cd(II) solution was 25 mL, the dosage of adsorbent was 10 mg, and the initial pH was detected by pH meter. The determination of cadmium ion concentration was performed by an inductively coupled plasma mass spectrometer (ICP-MS). Several adsorption models were selected to study the mechanism of kinetic adsorption and the adsorption isotherms. FT-IR, XRD, XPS, and HR-TEM were characterized to the interaction of Cd(II) with the hollow calcite single crystals. The results of this paper would provide insights to the mechanisms of the novel calcite single crystals on Cd(II) adsorption and will provide a potential and effective material for treating heavy metal ions in environment.

Results and Discussion
XRD and FT-IR analysis. The preparation process of novel hollow calcite single crystal was shown in Fig. 1.
The XRD patterns for the CQDs, the CQDs/calcite, and the etched calcite were presented in Fig. 2a. In the XRD pattern of the CQDs, a broad peak attributed to amorphous carbon, appeared near 2θ = 18°. In the XRD patterns of the CQDs/calcite and the etched calcite (as plotted in blue line and purple line in Fig. 2a), a typical rhombohedral phase of calcite was observed. It possessed a well-crystallized calcite structure with typical diffraction peaks related to (012), (104), (110), (113), (202), (018), (024), (122), (119), and (300) planes (conformed to JCPDS card 81-2027). (200) and (208) planes were the peak of NaCl (conformed to JCPDS card 75-0306). The reason for this might be that the CQDs would absorb a small amount of Na + and Cl − in the process of preparing CQDs. As can be seen, characteristic peak of (002) for the CQDs near 2θ = 18° had disappeared for the etched calcite. It can be illustrated that CQDs were almost completely erased from the CQDs/calcite. FT-IR spectra of etched calcite were shown in Fig. 2b. There were four adsorption bands, revealing the appearance of peaks at 729, 876, 1432, and 1756 cm −1 48 . The characteristic peaks at 729, 1432, and 1756 cm −1 for C-O stretching vibrations have been shown up in all spectrums. The adsorption bands appearing at 1432 cm −1 and 1756 cm −1 were associated with the C-O stretching vibration 49 band and C-O antisymmetric stretching vibration, respectively. The peak at 876 cm −1 was  (Fig. 3a-c) represented three classical morphologies of the calcite. Other morphologies such as needle-like (aragonite) or spherulitic (amorphous calcium carbonate, ACC) have not been detected, which have a higher solubility than the crystalline phase 51 . As shown in Fig. 3a, it incarnated that the calcite before etching was interlaced growth and exhibited well developed rhombohedral crystals with sharp straight edges 39 . Figure 3b showed the complete and rhombohedral structure of calcite crystals that had been presented. As demonstrated in Fig. 3c, a certain surface defects had emerged on the surface of the calcite, while the defects were extremely inerratic. SEM images (Fig. 3d-f) presented three classical morphologies of the calcite after etching. The morphologies of the calcite after etching were obviously different from that of the calcite before. Figure 3d showed that the calcite after etching had interlaced growth with the rhombohedra crystal structure. Comparing this with Fig. 3a, a distinct hollow structure appeared in the interior of calcite after etching. By analysis, CQDs penetrated into the calcite in the synthesis process of CQDs/calcite and afterwards washed off by the ethanol solution. It could be observed from Fig. 3e that the calcite after etching formed relatively regular hollow in the interior of the calcite crystal. As shown in Fig. 3f, the calcite after etching had more and irregular surface defects. In conclusion, the interface defects of the etched calcite by an etching technique with CQDs may provide a possibility of increasing the specific surface area.  Effect of pH. The pH of the aqueous solution is an important criterion since it may affect the speciation of heavy metal ions through the formation of complexes or ligands, which, in turn influences the binding mechanism 52 . The solution pH plays a significant role in the Cd(II) adsorption process. For this purpose, the adsorption performance of Cd(II) on the etched calcite as a function of solution pH is often checked. In this study, the pH values above 9 were not studied due to the formation of the cadmium hydroxide precipitate (Cd(OH) 2 ). With the etched calcite as adsorbent, the effect of pH on adsorption of Cd(II) ions was shown in Fig. 5a. As can be seen, it is found that the adsorption capacity of the adsorbent increased distinctly till a maximum value at pH 5.0, and then tended to be gentle above pH 5.0. pH could affect Cd(II) adsorption capacity in two ways, by influencing ion exchange and metal deposition reactions or by affecting the electric charge density of the surface to facilitate/hinder electrostatic interactions 53 . In terms of competition between H + and Cd 2+ and the protonation of the active sites of the adsorbent at a lower pH, the adsorption capacity was proved quite low at lower pH. The adsorption capacity raised up when pH arose as a result of weakening competition and waning repulsion 54 . Apparently, pH 5 was set as the optimized pH for subsequent adsorption experiments.
Adsorption kinetics. To fully understand the dynamic interaction of metal cations with adsorbents and also predict the time required for adsorption equilibrium, the kinetics research of adsorption need to be carried out. The adsorption kinetics of Cd(II) on the etched calcite were studied with the results depicted in Fig. 5b. The adsorption capacity of the etched calcite raised up sharply in the first 60 min and followed by a relatively slower process to reach a maximum value after approximately 90 min. The plateau suggested that the adsorption has reached to equilibrium. The fast-initial uptake is frequently interpreted as being the result of chemisorption and the external surface adsorption and could be explained by that sufficient active sites on etched calcite. Whereas, the following slow removal is assumed to represent co-precipitation or surface precipitation [55][56][57] . Conventional models have been tested for fitting experimental profiles transient studies 58,59 . The uptake kinetics have been modeled using the Pseudo-First Order rate equation (PFORE 59 ) and the Pseudo-Second Order rate equation (PSORE 60 ). Besides, in order to estimate the time-dependent intra-particle diffusion rate of Cd(II)from the surface sorption sites into the interior sites of etched calcite, Weber-Morris model 61 was applied to define the adsorption kinetics mechanism.
where, K 1 (min −1 ) is defined the rate constant of Pseudo-First-Order, and K 2 (g/(mg·min)) is the rate constant of Pseudo-Second-Order models. K id is defined the intra-particle diffusion rate constant (g/mg·min 0.5 ). C is the intercept of Weber-Morris model. The Pseudo-Second-Order kinetic model was used to describe the experimental results using Eq. (7). The calculated results of the above three models were presented in Fig. 5c and listed in Table 1 Adsorption isotherms. Adsorption thermodynamics were investigated to evaluate the adsorption performance of the etched calcite towards Cd(II), through which the relationship between absorbent concentration and adsorption capacity was further studied. The adsorption isotherms were presented in Fig. 6a. The adsorption

Pseudo-First-Order
Pseudo-Second-Order Weber-Morris   where, Q m is the maximum sorption capacity (mg/g), and K L is defined a constant related to binding energy of the sorption system (L/mg). K F ((mg/g)·(mg/L) 1/n ) is defined the Freundlich constant linked with the relative capacity. n corresponds with adsorption intensity 70 . The thermodynamic models were used to describe the experimental results using Eqs 8 and 9 with the results plotted in Fig. 6b,c and the model parameters listed in Table 2. The Freundlich model better fitted the experimental results than Langmuir model. It indicated that the adsorption of Cd(II) onto the heterogeneous surface of the etched calcite belonged to the multilayer adsorption. The obtained amount of n was 1.5207 from the slope (1/n) 0.6576, indicating a favorable adsorption process (1 < n < 10) 71 .
Adsorption mechanisms. The XRD patterns for the etched calcite before and after Cd(II) adsorption were presented in Fig. 7. As shown in Fig. 7a, the typical diffraction peaks of the etched calcite before and after Cd(II) adsorption possessed a well-crystallized structure related to the (012), (104), (110), (113), (202), (018), (024), (122), (119), and (300) planes (conformed to JCPDS card 81-2027). This might be because that both calcite and otavite are crystal structures of a trigonal system, and their lattice parameters are basically similar. However, there were some changes in the width or shape of typical diffraction peaks of the etched calcite before and after Cd(II) adsorption in three regions, which had been marked in Fig. 7b. In the regions of S1 and S3, the width of the typical diffraction perks (012) and (104) of the etched calcite after Cd(II) adsorption had dramatically increased compared with that of the etched calcite before Cd(II) adsorption. After adsorption, the Cd(II) ions might be penetrated into the interior of the etched calcite to form the (Ca, Cd)CO 3 solid solution or rhombic cadmium 72 , consequently, the width of peaks of the etched calcite after adsorption of Cd(II) would be increased. In S2 region, some other diffraction peaks had disappeared on the interface of the etched calcite after Cd(II) adsorption, which was a calcite conformed to the JCPDS card 03-0612 with small content in the etched calcite.
In order to study the adsorption behavior of Cd(II) onto the etched calcite, HR-TEM and TEM-mapping were applied to analyze the lattice distribution and element distribution of the etched calcite crystal after Cd(II) adsorption. HR-TEM and TEM-mapping images of the etched calcite in contact with the 10 mg/L solution of Cd(II) for 5 h were presented in Fig. 8. As displayed in Fig. 8a, the etched calcite after Cd(II) adsorption had crystal structure of rhombohedra and still existed in interface defects obtained by the etching technique. As can be observed in Fig. 8b,c, Fig. 8d,e, respectively. The lattices of the etched calcite after Cd (II) adsorption were distributed clearly, which meant that the Cd (II) did not result in the lattice defects of the etched calcite. And, diffraction patterns were distributed regularly. This was owing to that the ionic radius of Ca 2+ (0.99 Å) is very close to that of Cd 2+ (0.97 Å), and the electronic configurations of them formed in a similar way by losing the outermost electron from S atomic orbital 74,75 . The X-ray photoelectron spectroscopy (XPS) wide scan and different elements core-level spectra of the etched calcite after Cd(II) adsorption were employed to further investigate the function group information. The results of the XPS analysis were shown in Fig. 9. It revealed that the etched calcite after Cd(II) adsorption comprised a number of phases, including Cd(OH) 2 , CO 3 2− and CdO. As observed in Fig. 9a, it shows that signal with binding energy of O 1 s Cd 3d, Ca 2p, and C were centered at 531 eV, 405 eV, 347 eV and 284 eV, respectively. this implies that the Cd(II) was successfully adsorbed by the etched calcite. As illustrated in Fig. 9b, two peak components of the Cd 3d core-level spectrum have binding energies at about 404.4 eV and 405 eV, which can be assigned to CdO (64.16 wt. %) and Cd(OH) 2 (35.84 wt. %) species, respectively. The results clearly demonstrated that the deposition and chelation of Cd(II) played a significant role in the adsorption process 35

Conclusion
In this study, hollow calcite single crystals were synthesized by the etching technique with CQDs provide an extremely potential and important environmental protection material for removing cadmium ions from wastewater. By a series of pH experiments, it is found that the adsorption capacity of Cd(II) onto samples could reach the highest adsorption capacity at pH 5. In the kinetic adsorption study, the Cd(II) adsorption could reach adsorption equilibrium at 90 min. The process of Cd(II) adsorption onto the hollow calcite single crystals could be described as two steps. The fast initial uptake is frequently interpreted as being the result of chemisorption, whereas the following slow removal is assumed to represent surface precipitation or co-precipitation. In the thermodynamic adsorption study, the Cd(II) adsorption onto the hollow calcite single crystals could reach maximum adsorption amount at 66.68 mg/g. It manifested that the Cd(II) adsorption onto the etched calcite belonged to the chemical adsorption and multilayer adsorption in the form of CdCO 3 , Cd(OH) 2 , and (Ca, Cd)CO 3 solid solution by the results of XRD, XPS and TEM. The novel hollow calcite single crystals will provide an efficient and environmentally friendly material for application into the removal of Cd(II) from wastewaters.

Materials and Methods
Materials. Nitric acid (AR, 65%), sodium hydroxide (AR, 90%), propyl aldehyde (AR, 99.5%), anhydrous ethanol (AR, 99.7%)and ammonium carbonate (AR, the NH 3 content is not less than 40%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Stock solutions of Ca(II) and Cd(II) of 1000 mg/L were prepared by dissolving a certain amount of calcium chloride anhydrous and cadmium nitrate tetra hydrate into distilled water and subsequently diluting to 1000 mL with deionized water (S < 1.5 × 10 −4 S•m −1 ), respectively, which were further confirmed by the inductively coupled plasma mass spectrometer (ICP-MS). The samples were weighed using a Sartorius BS224S balance with an error of ±0.1 mg. Deionized water (DIW) was used in all experiments.

Synthesis of carbon quantum dots (CQDs).
In a typical procedure, CQDs were prepared as follows; as shown in Fig. 1. 2g of sodium hydroxide was slowly added into10 mL propyl aldehyde solution under the stirring condition at 20 °C for 6 h. The tawny colloidal sol would be obtained and then was laid in a sealed container for five days. After that, the tawny solid was rinsed with 15 mL of 0.5 M nitric acid to form yellow turbid liquid. Finally, the yellow turbid liquid was centrifuged for 15 min with a speed of 4000 rpm and dried at 60 °C for 6 h. The tawny solid prepared (as shown in Fig. 1a) was used for further experiments.
Synthesis of etched calcite. The etched calcite was synthesized by a simple means, (see Fig. 1). In this typical synthesis, the synthetic steps were showed as follows: firstly, 0.4 g CQDs were added to 200 mL of 0.1 M Ca 2+ aqueous solution and done ultrasonic treatment for 2 h with the purpose of making the CQDs completely dispersed into the Ca 2+ aqueous solution; then, the mixed solution was placed in an airtight container with ammonium carbonate to release CO 2 and supply CO 3 2− for precipitate. In the process of CO 2 diffusion, the mixed solution need be kept stirring constantly, so that CQDs and calcite could be combined completely; after 12 h, the CQDs/calcite solid sample obtained was centrifuged for 5 min with a speed of 4000 rpm and dried at 50 °C for 1 h. The flaxen solid sample of CQDs/calcite (as shown in Fig. 1b) was successfully prepared.
The characteristics of CQDs for being soluble in anhydrous ethanol would provide a feasible method for obtaining etched calcite. A certain amount of the flaxen solid sample obtained above was washed for four times with anhydrous ethanol solution. The off-white turbid liquid was then centrifuged for 5 min with a speed of 4000 rpm, and dried at 50 °C for 1 h to acquire the etched calcite. The off-white solid (as shown in Fig. 1c) prepared was used for the next adsorption experiments.

Material characterization.
A pH meter, with Amtast AMT12 (USA) model glass-electrode, was employed for measuring pH values of the aqueous phase. The chemical compositions were analyzed by an X-ray powder diffractometer (XRD) (Bruker D8 Advance, Germany) with Cu Kα radiation at 40 kV and 40 mA in a scanning range of 10°-90° (2θ). The surface functional groups were identified using a Fourier transform infrared spectrometry (FT-IR) spectrophotometer in range of 400-4000 cm −1 with the KBr disk method (Thermo Nicolet 5700, USA). Scanning electron microscope (SEM) images were recorded using a Philips-PEI model Quanta 200 with an accelerating voltage of 100 kV. High resolution transmission electron microscopy (HR-TEM) analysis was performed on a TecnaiG2 F20 S-TWIN for observing surface morphology and identifying of the elements of the samples. X-ray photoelectron spectroscopy (XPS) analysis were performed with an Axis Ultra spectrometer (Kratos Analytical Ltd.) using Al monochromatic X-ray source (Al Ka = 1486.6 eV) at 25 °C in a high vacuum environment (approximately 5 × 10 −9 torr). All the binding energies were calibrated by using containment carbon the C1s (284.8 eV). The detection of cadmium ion was performed on an inductively coupled plasma mass spectrometer (ICP-MS, 5300DV, Perkin-Elmer, USA) by the standard addition method. In kinetic studies, the Cd(II) adsorption amount (Q t ) could be determined by the following Eq. (10): where C 0 and C t (mg/L) refer to cadmium concentration at initial and t (min), respectively. V (L) is volume of Cd(II) solution. m (g) is Cd(II) adsorbent mass.
In thermodynamics studies, the adsorption capacity for cadmium uptake at equilibrium, Q e (mg/g), could be calculated by the following Eq. (11): where C e (mg/L) refer to Cd(II) concentration at equilibrium.