Adsorption of Cd(II) Ion on Aragonite Calcium Carbonate Crystals
Calcium carbonate was prepared with urea hydrolytic method, and was characterized by Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffract meter (XRD) and Ratio of surface area porosity analyzer. It is revealed that the Calcium carbonate obtained here is aragonite calcium carbonate crystals. The adsorption of Cd(II) ion onto aragonite calcium carbonate crystals were studied via investigating parameters as effect of contact time, dosage of aragonite calcium carbonate crystals and initial heavy metal concentration using static batch adsorption experiments. The results reveal that the removal rate of Cd(II) ion is 83% under the following conditions: temperature as 25 ℃, contact time as 120 min, initial concentration of the Cd(II) ion as 100 mg/L, and dosage of aragonite calcium carbonate crystals as 0.6 g/L. The adsorption capacity reaches 200 mg/g. Meanwhile the results suggest that the adsorption of the Cd(II) ion by the aragonite calcium carbonate crystals well follows the pseudo-second-order kinetics model, and the Langmuir isotherm model provides a good fit to the experimental data.
KeywordsAragonite calcium carbonate crystals Cd(II) ion Adsorption
With the development of industry, great amount of cadmium-polluted wastewater generated from metallurgy, electroplate and mining etc. poses great threat to environment . Cadmium is of heavy toxicity, non-biodegradable in water, and accumulative in soil and living body, Cadmium is absorbed by the body, selectively accumulate in the liver and kidney, can cause diarrhea, anemia and Itai Itai disease. Consequently leading to severe impacts both to ecosystem and human health. Hence, the discharge of wastewater containing chromium ion has been strictly restricted . Presently, methods commonly used to remove heavy metals from water are adsorption , ion-exchange , chemical precipitation  and reverse osmosis  etc. Among these methods, adsorption has advantages, such as wide application, efficient and avoidance of secondary pollution etc. Recent years, research in removing of heavy metal ions from wastewater with Calcium carbonate is gradually developed. There are three kinds of calcium carbonate crystal contains calcite, aragonite and vaterite, Calcite is one of the most abundant and stable structures in nature, and aragonite and vaterite are mostly synthetic.
Different crystal types of calcium carbonate have been used as adsorbents to remove contaminants in water, The aragonite and vaterite has better adsorption properties, but the adsorption capacity is about 2–150 mg/g [7, 8]. Therefore, the preparation of special morphology of calcium carbonate by synthetic method can improve its adsorption performance. In this study, aragonite calcium carbonate crystals was prepared with urea hydrolytic method, and was used to adsorb Cd(II) ion in water. Effect of factors such as contacting time, dosage of aragonite calcium carbonate crystals and initial heavy metal concentration on adsorption of cadmium were investigated. Pseudo-second-order kinetics model and the Langmuir isotherm model were also studied. This work provides basis on adsorption of Cd(II) ion by bio-mineral aragonite calcium carbonate crystals.
Materials. Starting materials, calcium acetate, and urea, were of analytical reagent grade and obtained from Sinopharm Chemical Reagent Co., Ltd. in China. Cadmium nitrate was provided by Shanghai Jinshan Tingxin Chemical Co. in China. All chemicals were used as received. Filter paper was purchased from Hangzhou Special Paper Co. Ltd. in China.
Preparation of Aragonite Calcium Carbonate Crystals. Weigh 14.0952 g of calcium acetate powder and 28.8288 g of urea accurately and (Calcium acetate and Urea Mole ratio: 1:6) and dissolve them in 60 mL of ultrapure water to form 41.7% solution. Put the aqueous solution into a 200 mL glass test tube and covered by a rubber stopper with three syringe-needle penetration. The tube was then statically placed in a forced convection oven at 90 ℃ or for 24 h. Subsequently, the solution was cooled to ambient temperature. The precipitate was collected by filtration through a 0.2 μm cellulose acetate membrane filter and rinsed with deionized water. The products were dried at 60 ℃ in a vacuum oven and then stored in a desiccator before characterization.
Characterizations. SEM image was obtained with an S-3400 N scanning electron microscope (Hitachi High-Technologies Co., Japan) at an accelerating voltage of 20 kV after sputtering the precipitates with gold. Powder XRD pattern was obtained with an XRD-7000 X-ray diffractometer (Shimadzu Co., Japan) equipped with Cu-Kα radiation (λ = 1.5405 Å) at a scanning rate of 10° per minute. Infrared spectra were collected with a Vertex-70 FTIR spectrometer (Bruker Optics Co., Germany) on KBr pellets.
Adsorption Experiment. The adsorption experiment was carried out with static batch adsorption method. Add accurately a certain quantity of the prepared aragonite calcium carbonate crystals powder into in a certain concentration of cadmium(II) solution containing in an Erlenmeyer flask. After shaking in 150 rpm for fixed time, the supernatant was extracted by centrifugal filtration. The Cd(II) ion concentration in the supernatant was measured by ICP-OES spectrophotometer. The removal rate R (%) and the adsorption capacity Qe (mg/g) were calculated according to Eq. (1) and (2), respectively.
Results and Discussions
- Adsorption of Cd(II) Ion by Aragonite Calcium Carbonate Crystals. The adsorption capacity of the prepared aragonite calcium carbonate crystals for Cd(II) ion with different contact time was shown in Fig. 2 (temperature as 25 ℃, contact time as 10 to 240 min, initial concentration of the Cd(II) ion as 100 mg/L, and dosage of aragonite calcium carbonate crystals as 0.2 g/L). It can be seen that the adsorption capacity of aragonite calcium carbonate crystals for Cd(II) ion increased from 32 to 191 mg/g with increasing contact time from 10 to 120 min. 120 to 240 min the adsorption capacity tends to moderate, adsorption equilibration is reached. When the adsorption time reaches 240 min, the adsorption capacity reached 200 mg/g.
- Adsorption Isotherm of Aragonite Calcium Carbonate Crystals for Cd(II) Ion. The effects of Cd(II) concentrations on the adsorption of aragonite calcium carbonate crystals powder measured at 25 ℃ were shown in Fig. 4. Accordingly, the equilibrium adsorption capacity was calculated according to Langmuir model  and Freundlich model  by Origin software . The fitting result was depicted in Fig. 4, and the fitting parameters were shown in Table 1. The results indicated that the two models all well characterize the adsorption process of aragonite calcium carbonate crystals on cadmium ion, but the relevance presented by Langmuir model was better than Freundlich model being closer to 1, which suggested that the adsorption process of aragonite calcium carbonate crystals on Cd(II) ion can be better described with Langmuir model. Langmuir model is a typical monolayer adsorption model, consequently, adsorption of aragonite calcium carbonate crystals of Cd(II) ion mainly is monolayer adsorption . The range of adsorption equilibrium constant KL falls in 0 to 1, it indicates that the adsorption was favorable adsorption ; The measured adsorption capacity is close to the theoretical value.Table 1
Fitness of isotherm models and corresponding parameters
KF (mg(1−1/n) L1/n/g)
- Adsorption Kinetics of Aragonite Calcium Carbonate Crystals on Cd(II) Ion. To determine the adsorption kinetics of aragonite calcium carbonate crystals on Cd(II) ion, were used the Lagergren pseudo-first-order kinetics and pseudo-second-order kinetics model to fit the experiment data of effect of contact time on Cd(II) ion adsorption respectively. The fitting results are depicted in Fig. 5 and Fig. 6, respectively. It can be seen that fitting result of pseudo-second-order kinetics (R2 = 0.9825) is better than that of pseudo-first-order kinetics (R2 = 0.9316), suggesting that the adsorption happened mainly is chemical adsorption, which is in consistent with conclusion of related researchers .
In this work, the rod-like aragonite calcium carbonate crystals were prepared by urea hydrolytic method. The average aspect ratio is about 2.6 ± 0.2 (defined as the ratio of length to diameter) while the Mole ratio of Calcium acetate and Urea is 1:6. The aragonite calcium carbonate crystals exhibit the highest adsorption capacity of 191 mg/g under Cd(II) ions concentration of 100 mg/L and dosage of 0.2 mg/L for 2 h, and removal rate of 83% under Cd(II) ions concentration of 100 mg/L and dosage of 0.6 mg/L for 2 h. The theoretical adsorption value is more than 225 mg/g. The adsorption kinetics and isotherms indicate that the adsorption process is very consistent with the pseudo-second-order and Langmuir models. The aragonite prepared by this method can be used to adsorb Cd(II) ion with large adsorption capacity, which is simple and easily operated. It is considered to be a good application prospect as an adsorbent for water treatment.
This work was supported by the National Natural Science Foundation of China (21476005, 21176005), and the Fund from Beijing Municipal Selected Excellent Overseas Scholars Project.
- 3.D. Enshirah, Adsorption of heavy metals on functionalized-mesoporous silica: a review. Microporous Mesoporous Mater. 145, 145–157 (2017)Google Scholar
- 5.Y. Guo, Z. Fang, J. Hu et al., Research development of treating wastewater containing heavy metals by chemical precipitation process. Ind. Water. Treat. 31, 9–13 (2012)Google Scholar
- 9.Faculty of Mineral and Petrology, Department of Geology, Nanjing University, Powder crystal X ray phase analysis (The Geological Publishing House, Beijing, 1980). (in Chinese)Google Scholar
- 13.H.M.F. Freundlich, Over the adsorption in solution. J. Phys. Chem. A 57, 358–471 (1906)Google Scholar
- 16.Z. Ma, W. Di, S. Wang et al., Thermodynamic process of Zn2+ adsorbed on humic acid extracted from albic soil fitted by the non-linear curve from the Origin 7.0 software. Chin. Agr. Sci. Bull. 30, 159–164 (2014)Google Scholar