Highly effective K-Merlinoite adsorbent for removal of Cs+ and Sr2+ in aqueous solution

Abstract

K-Merlinoite or “K-MER” was successfully synthesized and firstly employed as an adsorbent for the removal of Cs⁺ and Sr²⁺ cations in aqueous solution by batch operation. The optimum conditions to synthesize K-MER were hydrothermal temperature of 250 °C and a hydrothermal time of 8 h. As revealed by XPS, TG, NMR, and ICP analyses, the composition of K-MER synthesized under optimum conditions was “K_10.9Al_11.2Si_20.9O_61.6·17.8H₂O”, which corresponded with the typical formula of Merlinoite zeolite. ²⁷Al MAS NMR indicated that no octahedrally coordinated aluminum existed suggesting very high purity of K-MER. The calculated theoretical ion-exchange capacity was 4.2 mmol g⁻¹, assuming exchange of the total amount of potassium. In aqueous solution, K-MER exhibited an excellent Cs⁺ (≥90%) and Sr²⁺ (≥65%) removal performance with the maximum exchange capacity of 3.08 meq. g⁻¹ for Cs⁺ and 2.01 meq. g⁻¹ for Sr²⁺. K-MER showed higher adsorption for Cs⁺ than Sr²⁺ when performed using the same adsorption conditions. Coexisting monovalent cations, Na⁺ and K⁺, significantly influenced Cs⁺ removal more than divalent cations, Ca²⁺ and Mg²⁺, and they were in the order of K⁺ > Na⁺ > Ca²⁺ > Mg²⁺. The most important factor for competitive adsorption between Cs⁺ and the coexisting cation was the hydrated ionic size. Cs⁺ became more difficult to adsorb when Cs⁺ was interfered by a coexisting cation of close size. Although the efficiency of Cs⁺ removal by K-MER decreased around 50% in artificial seawater compared with that in the aqueous solution, K-MER was a promising material for Cs⁺ and Sr²⁺ removal.