Controllable Growth Of Alkaline-earth Metal Carbonate Crystals And Their Oxides Derivatives For Adsorption Applications

Alkaline earth metal carbonates are widely used in drug delivery and industrial refractory materials due to their advantages of pH response mechanism,good biocompatibility,high temperature resistance and high strength.A variety of alkaline earth metal carbonates with different morphologies and sizes have been prepared via hydrothermal/solvothermal methods,low-temperature precipitation methods,microwave-assisted hydrothermal methods,etc.Among them,the hydrothermal/solvothermal method is commonly employed to study the growth process of metal carbonates.The crystal growth are determined by various experimental factors such as the source and concentration of precursors,type of solvents,pH of solution,reaction temperature,reaction time and so on.The concentration of carbonate ions produced by the precursors has an important influence on the growth of carbonate crystals.How to adjust the controllable release of carbonate ions under high-temperature hydrothermal conditions is the key to the controllable growth of carbonate single crystals.Moreover,the porous alkaline earth metal oxides prepared by thermal decomposition of alkaline earth metal carbonates also have excellent application prospects.The alkaline property makes them widely studied as sorbents for acid gases capture and as catalysts for organic reactions such as condensation reaction between aldehydes and ketones.Because of the adjustable pore structure and high surface area,they can also be used as supports to load active components such as transition metals for catalytic reactions.Compared with other metal oxides,magnesium oxide exhibits excellent adsorption capacity during the removal of arsenic,but its adsorption mechanism still needs in-depth study,and the cyclic adsorption performance needs to be improved.Calcium oxide,as a chemical adsorbent,has better performance than physical adsorbents in carbon dioxide capture,but there exists a problem of poor cyclic stability.Hence,this thesis mainly focuses on regulation of carbonate ions release during the high-temperature hydrothermal process.A new method for the controllable release of carbonate ions using arginine as the precursor was developed and the growth of alkaline earth metal carbonates under different experimental parameters has been explored.In addition.in view of the unclear adsorption mechanism of magnesium oxide and poor cyclic adsorption performance in arsenic adsorption,as well as the poor cyclic stability of calcium oxide in the carbon dioxide capture,the thesis also demonstrated the synthesis of porous alkaline earth metal oxides for arsenic removal and CO2 capture.First,a variety of micron-sized alkaline earth metal carbonate single crystals were synthesized by using arginine to release carbonate ions slowly and controllably during the hydrothermal process.The carbonate phase transformation and crystal growth under different experimental conditions were explored.Afterwards,the magnesium carbonate single crystals synthesized via the hydrothermal method were calcined to prepare mesoporous magnesium oxide quasi-single crystals and its adsorption performance for different concentrations of arsenite was studied.The arsenite adsorption mechanism was explored by Fourier Transform Infrared Spectroscopy and X-ray Photoelectron Spectroscopy,and the cyclic performance was improved by means of washing and calcination after adsorption.Finally,a series of calcium oxide-based adsorbents with aluminum-based and magnesium-based stabilizers were prepared by the hydrothermal method,and the carbon dioxide uptake performance was studied.The research contents and main results of the thesis are summarized as follows:1.We found that arginine can slowly and continuously release CO32-ions through multi-step hydrolysis under hydrothermal conditions,which was important for subsequent carbonation and growth of MgCO3 rhombohedral single crystals.The growth rate of MgCO3 crystals can be adjusted by varying the type of magnesium salts and amino acids,the concentration of precursor and the pH value of hydrothermal solution.MgCO3 crystals with adjustable sizes(30.0?121.0 ?m)and controllable morphologies were obtained.This controllable slow-release behavior of CO32-was also versatile for the controllable growth of other alkaline earth metal carbonates,including CaCO3,BaCO3 and SrCO3.2.Mesoporous MgO quesi-single crystals were synthesized by the calcination of MgCO3 single crystals.They possess a high specific surface area(?86.2 m2 g-2),uniform mesopores(?11.4 nm),large particle size(?44.0 ?m).They also possess abundant and accessible hydroxyl groups.Enhanced As(?)adsorption performance(230 mg g-1),and improved adsorption kinetics can be achieved.The surface hydroxyl groups are formed firstly after MgO comes into contact with water,and then the chemical adsorption of As(?)is completed in the form of As(?)-O bond between As(?)and Mg-OH.There is a competitive relationship between the As(?)adsorption and the continued reaction of MgO With water to produce Mg(OH)2,but the former is preferential.The advanced treatment can meet the strict drinking water standards set by WHO for arsenic.Calcination step in the regeneration process is critical to the cyclic stability of As(?)adsorption.-65%of the first capacity can be retained after four cycles.3.A series of CaO-based adsorbents were prepared via the one-step hydrothermal method followed by calcination.The phase separation of y-AlOOH and CaCO3 appeared during hydrothermal synthesis,and the cyclic adsorption performance of CO2 was improved with the addition of Al.In the Mg-doped CaO-based adsorbent,single phase(Ca,Mg)CO3 can be synthesized.The addition of anionic surfactant sodium of dodecyl benzene sulfonate in the hydrothermal reaction can lead to small-sized crystals and CaO-based adsorbents.The adsorption capacity and stability in CO2 capture can be correspondingly improved.