Treatment of wastewater containing hazardous metal ions using organic-inorganic composite adsorbents

Abstract

Selective removal of aqueous $^{137}$Cs and $^{90}$Sr is the key technology for the volume reduction and safe storage of the nuclear waste. In this work, highly effective adsorbents are synthesized for the purification of waste solution contaminated with radioactive $Cs^{+}$ and $Sr^{2+}$. The as-prepared adsorbents are thoroughly evaluated in terms of adsorption capacity, kinetics, selectivity, and stability. In chapter 2, potassium copper hexacyanoferrate nanoparticles (KCuHCF) are embedded in the poly(vinyl alcohol) hydrogel structure via a two-step method of Cu immobilization, followed by the diffusion of potassium hexacyanoferrate accelerated by acetone evaporation (HPC). The diffusion-derived KCuHCF formation in the preformed hydrogel facilitates the preservation of the 3D-interconnected hydrogel structure and dispersion of the KCuHCF nanoparticles. The HPC shows an excellent $Cs^{+}$ removal efficiency of above 99.5 % with remarkably rapid removal kinetics even in the highly competitive seawater conditions. In chapter 3 and 4, KCuHCF-immobilized magnetic hydrogel film (MHPVA) and bead (HCF-Mbead) are synthesized through a facile freeze/thaw crosslinking method. The citric acid coated $Fe_{3}O_{4}$ is embedded into the hydrogel matrix to enhance the dispersion of nano-sized KCuHCF particles for $Cs^{+}$ removal, followed by the rapid recovery of the composite in a magnetic field. From a practical perspective, the MHPVA and HCF-Mbead exhibit stable and promising selective properties even in real water conditions and the used adsorbents are rapidly recovered leaving a turbidity-free aqueous environment. In chapter 5, dual-cation form of pharmacosiderite type titanosilicate (DTS, $M_{3}HTi_{4}O_{4}(SiO_{4})_{3}$, M = $Na^{+}$ and $K^{+}$) is introduced, and shows the enhanced performance for simultaneous $Cs^{+}$ and $Sr^{2+}$ removal over the conventional mono-cation form of titanosilicate. Remarkably, DTS exhibits a high removal rate for both $Cs^{+}$ and $Sr^{2+}$ during consecutive cycles of repeated adsorption without regeneration, and suggests the potential for use as a membrane for continuous purification of the radionuclides.