Synthesis and characterization of mesoporous geopolymer solid adsorbents containing nano-crystalline zeolites

Abstract

The geopolymers, a class of alkali-activated materials, are inorganic aluminosilicate binders, and are often considered as amorphous analogues of crystalline zeolites owing to their similar chemical structure. The primary objective of this dissertation is to synthesize mesoporous geopolymers containing nano-crystalline zeolites by hydrothermal conversion of amorphous geopolymeric gel to crystalline zeolites. The scope of this dissertation includes the following: 1) investigation of novel, energy and time efficient hydrothermal methods (two-step and one-step method) for conversion of gel to zeolites, 2) microstructural characterization of synthesized mesoporous geopolymers, and 3) determination of their adsorption potential for heavy metals ($Pb^{2+}$ and $Cu^{2+}$) and radionuclides ($Cs^{+}$ and $Sr^{2+}$).

The mesoporous geopolymers containing crystalline zeolites were successfully synthesized using different synthesis methods. Zeolites Na-P1 and hydroxysodalite were found to be the major crystalline phases in samples activated with high molarity NaOH solution (12 M). However, a decrease in the molarity (8 M) resulted in the formation of pure phase zeolite Na-P1, while an increase in the slag content gave rise to the formation of hydroxylsodalite. Moreover, analcime content slightly increased in samples treated at higher temperature ($150^\circ C$ ). From Q-XRD results, it can be stated that the calcium in slag hardly contributed to the formation of zeolites, but contributed to the formation of C-A-S-H gel, giving strength to geopolymers. Moreover, alkali activator-to-binder ratio, silicate modulus of alkali activator, and characteristics of starting materials played vital role in development of zeolite crystals.

Lastly, samples were foamed with $H_2O_2$ to increase the porosity of samples which assisted in enhanced diffusion of adsorbate ions within the matrix, ultimately resulting in higher adsorption capacity for heavy metals and radionuclides. The maximum adsorption capacity for $Pb^{2+}$, $Cs^{+}$, $Sr^{2+}$, and $Cu^{2+}$ was found to be 55.4, 43.6, 31.7, and 28.2 mg/g, respectively.