Adsorption and Desorption of Molybdenum on Rock and Soil Media

The release of radioactive ⁹⁹Mo into the environment may lead to adverse effects on radiation safety and environmental security. Studying the migration behavior of molybdenum in environmental media is of great importance for the healthy development of the nuclear industry and environmental protection. To the best of our knowledge, rare studies focused on radiation safety by studying the molybdenum adsorption in aspecific area. In this study, rock, soil, and groundwater samples were collected from the vicinity of a proposed radioactive pharmaceutical research facility in China. Adsorption-desorption experiments of molybdenum in environmental media were conducted to investigate the adsorption kinetics and adsorption-desorption isotherms of molybdenum in silty clay, coarse sand, strongly weathered silty shale, moderately weathered silty shale, strongly weathered carbonaceous shale, and moderately weathered carbonaceous shale. The mineral compositions of the six kinds of rock and soil samples are similar, with quartz, illite, and chlorite as the primary components and their main chemical components of the six rock-soil samples are SiO₂, Al₂O₃, Fe₂O₃, MgO, K₂O, and FeO. The cation exchange capacity of the six kinds of rock-soil samples ranges from 1.92 to 20.20 cmol⁺/kg and the organic matter content ranges from 9.5 to 60.0 g/kg. The adsorption process of molybdenum in the six types of rock-soil media conforms to pseudo-first-order and pseudo-second-order kinetic models. Both the Langmuir and Freundlich models effectively describe the adsorption-desorption process of molybdenum in rock-soil media. The adsorption K_d values of molybdenum in silty clay, coarse sand, strongly weathered silty shale, moderately weathered silty shale, strongly weathered carbonaceous shale, and moderately weathered carbonaceous shale are 0.064, 0.021, 0.042, 0.028, 0.290 and 0.209 L/g, respectively. The desorption K'_\mathrmd values of molybdenum in these six rock-soil samples are 0.076, 0.027, 0.049, 0.035, 0.410 and 0.278 L/g, respectively. The desorption K'_\mathrmd values are 1.17-1.41 times the adsorption distribution coefficient values. The distribution coefficient values exhibit a trend of strongly weathered carbonaceous shale＞moderately weathered carbonaceous shale＞silty clay＞strongly weathered silty shale＞moderately weathered silty shale＞coarse sand. The organic matter content of rock and soil media is the most critical factor influencing the distribution coefficient values. A linear relationship exists between the organic matter content of rock and soil media and the distribution coefficient. Based on the organic matter content, the distribution coefficient of Mo in rock and soil media can be predicted rapidly. This research provides fundamental data on the migration of molybdenum in rock-soil media and offers a scientific basis for establishing relevant safety standards.