Investigation of Natural Diatomite Sorption Properties for Application in Radioactive Waste Management

Abstract

This study has focused on characterizing diatomite samples from the Kariandusi Mining area, Kenya, for its possible application as a low-cost sorbent for radioactive ions of Ba, Co and Zn. The analysis of the chemical composition, mineral phases, radioactivity and particle size of diatomite was conducted using portable S1 Titan 600 series Bruker x-ray Fluorescence, gamma spectroscopy with the High Purity Germanium Spectrometer, Brunker D2 2 Gen Phaser XRD Machine and the Rotap screen shaker respectively. The sodium acetate method was used to determine the cation exchange capacity of diatomite. The sorption capacity of diatomite was investigated using prepared aqueous solutions containing stable ions of Ba, Zn and Co for the effect of sorbent concentration, initial metallic concentrations, sorption time and pH on the sorption capacity. The chemical and mineral phase analysis of diatomite indicated that the diatomite was mainly composed of silica in the form of cristobalite, in agreement with the results on the typical composition of diatomite. The radiological characterization indicated that the radium equivalent activity of diatomite to be 77.96 ± 13.56 Bq/Kg, which was below the safe limit of 370 Bq/Kg recommended by the Organization for Economic Cooperation and Development (OECD). Particle size analysis show that 10% of particles had diameter less than 171 μm; 50% had a diameter less than 589 μm and while 90% of particles had less than 1523 μm. The particle size distribution may have affected the sorption capacity of diatomite by limiting the diffusion path length inside the sorbent. The diatomite's cation exchange capacity (CEC) was calculated as 7.94 ± 0.83 meq/100g compared to 9.8 meq/100g and 11.3 meq/100g obtained by Belousov et al (2019). The values of CEC vary because diatomite samples originating from different sources vary in pH, nature and quantity of clay minerals present. Analysis of the adsorption efficiency of diatomite for removal of Ba, Co and Zn ions showed that this increased with the diatomite concentration, contact time and pH because of increase in the number of sorption sites, continued interaction of the ions with the diatomite and enhanced physisorption. The sorption capacity was found to increase with the initial concentrations of ions because of the increase in the concentration gradient between the ions and the diatomite sorbent. The highest removal efficiencies achieved in this study were 99.83%, 99.14% and 94.27% for Zn, Ba and Co respectively, showing that with a proper selection of sorption parameters highest removal efficiencies close to 100% can be achieved. This establishes that the diatomite samples from Kariandusi region have excellent sorption properties and can therefore be explored for radioactive waste management.