Preparation Of Nano-mineral Ecomaterials And Their Applications In Heavy Metals Treatment

Heavy metals are significant environmental pollutants, and their toxicity is a problem of increasing significance for ecological, evolutionary, nutritional and environmental reasons. They are able to enter into the aqatic and food chains of humans and animals from a variety of anthropogenic sources as well as from the natural geochemical weathering of soil and rocks. The issue of heavy metal pollution is very much concerned in the field of environment, materials and engineering science. The existing methods for the removal of heavy metals from the environment can be grouped in biotic and abiotic. Biotic methods are based on the accumulation of the heavy metal by plants or microorganisms; abiotic methods include physicochemical processes such as precipitation, coprecipitation, ion exchange, solvent extraction, electrolysis, reverse osmosis and adsorption by a suitable adsorbent. Among them, adsorption process is cost-effective, flexible, and easy to design and operate. Currently, nanomaterials and nanotechnology have garnered worldwide attention for their application in environmental remediation and pollution control, owing to the enhanced affinity and sorption capability toward pollutants of nanomaterials. Pollutants at very low concentration, from μg·L-1 to mg·L-1 level, could be successfully sequestered from wastewater by nanoadsorbents In particular, clay and three-dimensional (3D) nanostructures composed of hierarchically assembled subunits demonstrated outstanding properties of low density, high surface area, and excellent surface permeability, and thus attracted much attention. Moreover, microwave-assisted synthetic method has been recognized as much faster, cleaner, and more economical than conventional methods due to its dielectric volumetric heating, and used widely for the synthesis of different inorganic/organic compounds and nanomaterials. In this dissertation, several mineral ecomaterials with nanostructure and/or mesocrystal structure were prepared and their removal abilities to heavy metals were investigated systematically. The important results of this dissertation were summarized as follows:1. Sepiolite is natural nanorod-like clay with a large specific surface area, as well as high chemical and mechanical stability. Sepiolite-supported magnetie nanoparticles (SSMNPs) with good dispersion were prepared via facile microwave irradiations technique. The composite exhibits high removal efficiency low concentration Cr(VI), and much more significant adsorption capacity (33.4 mg/g) than that of the unsupported magnetite nanoparticles (22 mg/g). The removal of Cr(VI) by the SSMNPs involves an electrostatic attraction followed by a reduction process of high toxicity Cr(VI) to less toxicity Cr(Ⅲ), and the subsequent surface precipitation of Cr(Ⅲ) in the forms of Cr(OH)3, meanwhile the Fe2 in magnetite is oxidized into Fe3 In the system of magnetite-sepiolite composite, magnetite plays the main role in the removal and reduction of Cr(VI), while sepiolite as a support matrix, could not only disperse magnetite nanoparticles and prevent them from aggregation, thereby increasing the removal capacity to Cr(VI), but also effectively immobilize Fe3+ in the final solution, reducing the toxic effects of magnetite nanoparticles. Non-linear regression analysis reveals that the adsorption data fit well with the Redlich-Peterson isotherm model. The microwave-assisted synthetic method is efficient, simple, time-saving, and suitable for large-scale preparation. Taking into account of the possibility of magnetic separation of adsorbent with water, the SSMNPs can effectively remove heavy metals from aqueous solutions.2. SiO2-Mg(OH)2 nanocomposite has been successfully prepared derived from sepiolite by the couple-treatment of acid and alkali. The magnesium ions in octahedral structure of sepiolite are leached out during the acid treatment, and the amorphous silica gel was formed. The leached magnesium ions are transformed into Mg(OH)2 with the addition of ammonia, leading to the formation of SiO2-Mg(OH)2 nanocomposites. The composite presents an amazing removal capacity for Gd(III), ca. 4.45 mmol/g, which is about 28 and 67 times of that of raw sepiolite (0.159 mmol/g) and amorphous silica (0.066 mmol/g), due to the synergistic effect of brucite and silica with high specific surface. Moreover, this composite also shows exceptional removal capacity to common heavy metals (6.84 mmol/g for Pb2+,4.88 mmol/g for Cd2"), and the removal capacity for Pb2+ and Cd2+ follows the trend:SiO2-Mg(OH)2 nanocomposites> raw sepiolite> partially acid-activated sepiolite> amorphous silica. The removal capacity of sepiolite to heavy metals is greatly enhanced by the facile acid-alkali modification. Our results of this work could provide an alternative technique for treating heavy metals.3. Iron oxides are common compounds which are widespread in nature and readily synthezised in the laboratory, and have been used widely in wastewater treatment. Hematite with hollow core/shell microstructures have been successfully prepared by a simply calcination of PVP-pyrite hybrid microspherolites in air. The PVP-pyrite microspherolites are obtained by a facile microwave-assisted reflux method with the assistance of PVP. The formation from the precursor to the hollow core/shell hematite microspheres experiences the oxidation and sulfation of pyrite, combustion of occluded PVP, desulfation, aggregation and fusion of nanosized hematite, as well as mass transportation from the interior to exterior of the microspheres. Calcining temperature plays a crucial factor to control the hollow structure formation, and the combustion of PVP makes the hematite nanoparticles have ample space to restructure. The aggregation of hematite nanocrystals and the core shrinking during the oxidation of pyrite should be responsible for the formation of the hollow structures. The hematite microspheres show good adsorption ability to remove samarium ions in water treatment and are expected to be useful in removal rare earth elements (REE) and also provide references for dealing with radioactive nuclides, as the samarium has radionuclides such as Sm-149 and Sm-153. Due to the unique 3D structure of hollow core/shell hematite microspheres, this kind of novel superstructure can be also expected to have potential applications in catalysis, catalyst support, magnetic devices, drug delivery, and other fields.4. Hierarchically nanostructured y-Fe2O3 microspheres with high specific surface have been successfully obtained via thermolysis of iron alkoxide precursor, which was prepared by a fast and facile microwave-assisted reflux method in ethylene glycol (EG) solvent without the assistance of any other organic additives and/or high pressure conditions. The hierarchical microspheres exhibit high sorption performance for Sb(Ⅲ) over a broad pH range, but for Sb(V) at low pH, and the maximum sorption capacity for Sb(Ⅲ) and Sb(V) is 128.2 and 98.7 mg. g-1 at pH 3.0, respectively. Moreover, after the treatment of low level Sb solution by γ-Fe2O3 microspheres, the residual Sb could fit the US EPA limit, and the sorbent exhibits good reusability. The hierarchically nanostructured γ-Fe2O3 microspheres show great promise in wastewater treatment in terms of their low manufacturing cost, simple processing, high removal efficiency of antimony, and convenient handling properties.5. Calcium carbonate (CaCO3), occurring geoglogically as mineral constituents of sedimentary rocks and biogically as inorganic components in the skeletons of many mineralizing organisms, is one of the most abundant minerals. CaCO3 minerals could be a potential cost-effective replacement for activated carbons, due to their omnipresence in nature and high sorption affinity to most heavy metals. Hierarchically nanostructured shuttle-like aragonite mesocrystals have been easily synthesized in a water/EG mixed solvent system by a microwave irradiation route without any other organic additives. Under the current conditions, EG and acetate ions play important roles in the formation of the shuttle-like hierarchical aragonite. The volumetric heating of microwaves provides more uniform conditions for the generation of the hierarchical aragonite mesocrystals. The well-defined mesocrystals are formed via a multistep process including the phase transformation from ACC to aragonite, aggregation of nanoparticles into ellipsoid subunits, assemble of ellipsoid-like subunits, and Ostwald ripening. Moreover, the aragonite mesocrystals show excellent removal ability to La(Ⅲ) via La(Ⅲ) sorption and subsequent precipitation of La2(CO3)3·8H2O on the sorbent surfaces. As the chemical behavior of lanthanum has been considered typical of REEs and some trivalent actinides as well, the hierarchical aragonite mesocrystals are expected to be useful for other lanthanide and actinide elements removal.