The effect of birnessite and humic acid on sorption of cobalt by surface soil

Abstract

To understand the sorption mechanism of Co in the individual and mixed adsorbent systems in the application of the short-term performance assessment of surface disposal facilities and the evaluation of radionuclide deposition/migration in the contaminated surface areas, the sorption behavior of Co(II) on soil and soil components such as birnessite, humic acid (HA) and their mixture was studied. A series of experiment were conducted using the batch equilibrium technique on parameters such as equilibrium time, ionic strength, solution pH, and temperature to characterize the Co sorption behavior on soil and soil components. To help understanding the Co sorption behavior in the aqueous environment and experimental results, Co speciation was investigated on the parameters of ionic strength and HA presence. The speciation calculation shows that both of Co2+ reduction rate and Co(OH)+ production rate are decreased with the increasing ionic strength, which explains the negative impact of competitive ionic strength of Co sorption on adsorbent as the interaction of positively charged Co2+ and Co(OH)+ with the negatively charged adsorbent is decreased. The presence of HA decreases the Co2+ reduction rate and Co(OH)+ production rate. Due to the Co-HA binding species, Co adsorption to HA can be enhanced even at low pH. The soil samples collected from location near radioactive waste repository in Korea were used and birnessite was synthesized using a method by McKenzie for experiment. The experimental results indicate that Co sorption on soil, birnessite and soil with birnessite are strongly affected by the pH of solution. Typical for metal sorption to soils, the fraction of Co adsorbed increased as a function of pH at the experimental conditions. For sorption isotherm, the Freundlich equation provides a good fit for sorption on soil and soil with birnesssite. Desorption of Co on soil in presence of birnessite was decreased as the amount of added binessite was increased. C...