Sorption of cobalt(II) on soil: effects of birnessite and humic acid

To study the sorption behavior of Co(II) on soil and soil components such as birnessite, humic acid (HA) and their mixture, a series of experiment were conducted using the batch equilibrium technique on parameters such as equilibrium time, ionic strength, solution pH, and temperature. 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 birnessite. Adsorption of HA on birnessite decreased with increase of pH, with a sharp decrease at pH 5–6. From Co sorption experiment in a ternary system of Co, birnessite, and HA, the presence of HA enhanced Co adsorption at pH below 6.5 and reduced the Co sorption at the intermediate and high pH.