Sorption of cobalt(II) on soil: effects of birnessite and humic acid Je Keun Chon • Kun-Jai Lee • Jong-Il Yun Received: 17 January 2012 / Published online: 12 May 2012 Ó Akade ´miai Kiado ´, Budapest, Hungary 2012 Abstract 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 tem- perature. The soil samples collected from location near radioactive waste repository in Korea were used and bir- nessite 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. Keywords Sorption Á Soil Á Birnessite Á Manganese oxide Á Humic acid Á Cobalt Introduction An adsorption of radionuclides to the natural materials is largely responsible for the retardation of radionuclides in the subsurface environment [1]. Adsorption data are critical to the radionuclide retardation calculations for performance assessment for the site selection of low and intermediate level radioactive waste repositories [2]. As the soil can play an important role as a natural barrier to radioactive waste disposal facility, the sorption studies of radionuclides on soils have been considered important. The interaction of nuclides with soil and soil components such as Al, Fe, Mn oxide, and humic substances, directly affects the migration of radionuclides [3–5]. Many other studies have been also devoted to research the sorption processes and mechanism of various radionuclides on natural soils and clay minerals [6–10]. Among the soil component, birnessite, which is the most common stable layered manganese oxide mineral, can play an important role to absorb the heavy metals, particularly Co [11, 12]. Birnessite is a precursor to todorokite and is known to have a unique surface charge, cation exchange and redox properties [11, 13]. For the sorption character- istics of birnessite, the sorption of trace 137 Cs, 89 Sr, and 57 Co on Na- and K-exchanged birnessite was studied across the wide pH range. Na–birnessite was found highly effective for the removal of Co trace as pH increased [13]. As an important sorbent among soil components, humic substances are known as complex, heterogeneous, and refractory organic compounds which may be protonated– deprotonated depending on pH and exist in all soil and water environments. Several investigations have been reported on adsorption behavior of heavy metal–humic acid on adsorbents such as activated carbon [14, 15], clays [16, 17], and goethite [18]. The presence of humic J. K. Chon (&) Korea Institute of Nuclear Safety, 19 Kuseong, Yuseong, Taejon 305-335, Korea e-mail: k393cjk@kins.re.kr K.-J. Lee Á J.-I. Yun Korea Advanced Institute of Science and Technology, 373-1 Kuseong, Yuseong, Taejon 305-701, Korea 123 J Radioanal Nucl Chem (2012) 293:511–517 DOI 10.1007/s10967-012-1806-y