Hydrothermal transformation of geopolymers to bulk zeolite structures for efficient hazardous elements adsorption

This paper reports a facile route to prepare bulk zeolites with tunable phase compositions and microstructures by combining hydrothermal treatment and geopolymer precursor technique. Amorphous Na-based geopolymer (NaGP) is transformed into crystalline analcime following hydrothermal treatments. By systematically investigating the effects of hydrothermal conditions on the phase compositions and microstructures of the products, the optimal hydrothermal procedure is screened as treating NaGP in 1 M NaOH solution at 160 °C for 6 h. Furthermore, we achieve control over phase compositions of the resulting bulk zeolites by tailoring the initial Na/K ratio of geopolymer precursors. For instance, treating the geopolymer precursor with a Na/K ratio of 9: 1 under the optimal hydrothermal procedure leads to the formation of zeolite consisting of analcime and zeolite-P. The as-prepared adsorbents exhibit outstanding adsorption performance for the hazardous elements, among which analcime-zeolite-P shows an adsorption efficiency of 93.3% for Cs⁺, and NaGP exhibits an adsorption efficiency of 99.6% for Sr²⁺. Moreover, we reveal the mechanisms underlying the adsorption of Cs⁺ and Sr²⁺ in the adsorbents to be chemisorption. Meanwhile, ion exchanges also occur in NaGP and analcime-zeolite-P during Cs⁺ adsorption. These results render geopolymers and their derived bulk zeolites promising for hazardous elements adsorption.