Adsorption of heavy metal ions using hierarchical CaCO3–maltose meso/macroporous hybrid materials: Adsorption isotherms and kinetic studies

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Abstract

Highly ordered hierarchical calcium carbonate is an important phase and has technological interest in the development of functional materials. The work describes hierarchical CaCO3–maltose meso/macroporous hybrid materials were synthesized using a simple gas-diffusion method. The uniform hexagonal-shaped CaCO3–maltose hybrid materials are formed by the hierarchical assembly of nanoparticles. The pore structure analysis indicates that the sample possesses the macroporous structure of mesoporous framework. The distinguishing features of the hierarchical CaCO3–maltose materials in water treatment involve not only high removal capacities, but also decontamination of trace metal ions. Langmuir model fitted the equilibrium data better than the Freundlich isotherm. The maximum removal capacity of the CaCO3–maltose hybrid materials for Pb2+, Cd2+, Cu2+, Co2+, Mn2+ and Ni2+ ions was 3242.48, 487.80, 628.93, 393.70, 558.66 and 769.23 mg/g, respectively. Adsorption data were modeled using the pseudo-first-order, pseudo-second-order and intra-particle diffusion kinetics equations. The results indicate that pseudo-second-order kinetic equation and intra-particle diffusion model can better describe the adsorption kinetics. The adsorption and precipitation transformation mechanism can be considered due to hierarchical meso/macroporous structure, rich organic ligands of the CaCO3–maltose hybrid materials and the larger solubility product of CaCO3.

Highlights

► Hierarchical CaCO3–maltose meso/macroporous materials are firstly synthesized. ► The preparation method is simple and friendly environment. ► Hierarchical CaCO3–maltose materials have high removal capacities of metal ions. ► Hierarchical CaCO3–maltose materials could decontaminate of trace metal ions.

Introduction

The contamination of freshwater has become one of the main threats to the normal life of human [1]. Heavy metals, such as lead and cadmium, because of toxicity, are harmful to living organism [2], [3], [4]. To decontaminate the water polluted by heavy metals, various treatment technologies have been tried and used, including chemical precipitation, solvent extraction, adsorption, etc. In these technologies, adsorption seems to be the most convenient method and is often the method of choice. However, economy and efficiency have largely hampered the commercial applications of some adsorbents. Therefore, the high efficiency and inexpensive adsorptive materials are still demanded.

The porous materials, as adsorbents, have attracted tremendous research interest and have been put into practice [5], [6], [7], [8]. Especially, the chemically designed inorganic–organic hybrid materials with porous structures have attracted considerable attention because of the combination of properties with respect to the inorganic and the organic components, which allows to tailor density, chemical reactivity, and thermal stability [9], [10], [11], [12], [13], [14]. Compared with single-sized porous materials, the materials with hierarchical meso/macroporous structure as adsorbents can enhance properties due to increased mass transport through the large pore channels of the materials and maintenance of a specific surface area on the level of fine pore systems [15], [16], [17], [18], [19], [20]. However, the fabrication of inorganic–organic hybrid materials with hierarchical meso/macroporous structure, which have more advantages on adsorption, is till a challenge as regards their application potential.

Carbonate minerals are effective in removing heavy metals and the mechanism of adsorption was suggested to be ion-exchange and precipitation on the carbonate surface [21], [22], [23]. It is well known that calcium carbonate is one of the cheapest materials in nature and harmless to humans. Furthermore, the larger solubility product of CaCO3 could be lead to precipitation transformation happened between CaCO3 and some carbonates, which could increase greatly the adsorption capacity of the metal ions. Although calcium carbonate has been studied for removal of heavy metal ions [22], [24], [25], [26], the low efficiency and further treatment necessity for the sludge significantly limit the practical applications in water treatment. Otherwise, maltose is one of the predominant saccharide and has many hydroxyl groups, which could interact with heavy metal ions. The interaction makes it possible to remove the heavy metal ions from the waste water.

Herein, our aim is to facilely synthesize the inorganic–organic CaCO3–maltose hybrid materials with hierarchical meso/macroporous structure, which could be the high efficiency and inexpensive adsorbents in removal of heavy metal ions from waste water. The maximum removal capacities for Pb2+, Cd2+, Cu2+, Co2+, Mn2+ and Ni2+ ions were higher than the reported previously. The adsorption and precipitation transformation mechanism can be considered due to rich organic ligands of the CaCO3–maltose hybrid materials and the larger solubility product of CaCO3. From these results, the CaCO3–maltose hybrid materials with hierarchical meso/macroporous structure could be an alternative and efficiency absorbent for removal heavy metal ions.

Section snippets

Materials

All chemicals used in the study are of analytical grade and were used without further purification. The corresponding metal nitrates were employed as the metal ions sources. Anhydrous calcium chloride and ammonium bicarbonate were purchased from Chemical Reagent Company of Tianjin. Maltose was purchased from Xiamen Sanland Chemicals Company Limited, China. Double distilled water was also used in our experiment. All glassware were soaked in a mixing K2CrO7/H2SO4 solution, then rinsed with double

Material characterizations

Fig. 1 shows the representative SEM and TEM images of the samples. It indicates that the hexagonal plates-shaped CaCO3 (10–15 μm in diameter) is composed of the small granular micro-aggregates (see Fig. 1a and b). As a result of the aggregation of these micro-aggregates, the disordered wormhole-like macropores with diameter distribution of 200–500 nm are formed in the hybrid materials (Fig. 1c). As Fig. 1d shown, it reveals that the granular micro-aggregates consist of the smaller nanoparticles

Conclusion

The CaCO3–maltose hybrid materials with hierarchical meso/macroporous structure have been successfully prepared in the presence of maltose under a simple evaporation diffusion method. The CaCO3–maltose hybrid materials is found to be effective and alternative adsorbent for the removal of heavy metal ions from wastewater as a result of hierarchical meso/macroporous structure, rich organic ligands of the CaCO3–maltose hybrid materials and the larger solubility product of CaCO3.

Acknowledgments

This work was financially supported by the National Science Foundation of China (Grant Nos. 21171051 and 20871042), the Innovation Scientists and Technicians Troop Construction Projects of Henan Province (Grant No. 114200510004) and Program for Chang jiang Scholars and Innovative Research Team in University (Grant No. IRT 1061).

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