Geochemical behavior of cesium and iodine with black carbon materials controlled by different temperature and decomposition conditions

Plenty of radioactive cesium and iodine is volatilized to atmosphere at early stage of severe nuclear accidents. They enter adjacent subsurface environments through wet and/or dry deposition processes. Although black carbons (BC) are ubiquitous materials in soil and groundwater system, their geochemical uptake behavior for radionuclides were rarely studied. The goal of this study is to determine the effects of formation temperature and decomposition condition of BC materials on the cesium and iodine uptake behavior.

The BC materials were produced through pyrolysis in a laboratory using a xylem of oak trees with different ramp rate and formation temperature. After the BCs were characterized by elemental analysis, BET, SEM, NMR, FTIR and XPS, they were used for sorption batch experiments with cesium and iodine and decomposed in a distilled water at 80°C for 1 month. Atomic ratio (H/C and O/C) and BET increased with rise of final temperature regardless of ramp rate. Cesium was sorbed strongly to low temp-produced BC, while the iodine sorption reaction was remarkable with high-temp produced BC. The sorption distribution coefficients reached approximately 400 and 1,000 kg L-1 at low dissolved concentration for cesium and iodine, respectively (Ciw=10 μg L-1). These sorption properties are caused by different sorption mechanisms between cesium and iodine. The FTIR and XPS results indicated that cesium was predominantly reacted with oxygen-containing functionalities on the BC surface. In contrast, iodine uptake occurs through covalent bonding with carbon atom in aromatic structures of BC. The chemical structure and functionalities could be significantly controlled by final temperature of BC production rather than temperature-rise conditions. The decomposed BC materials exhibited the decrease of sorption efficiency for cesium, especially at low dissolved concentrations.