The desorption of radium isotopes in river sediments in Qinzhou Bay

Abstract

Radium isotopes are one of the useful tracers to study the submarine groundwater discharge (SGD) processes. It is indispensable to estimate the desorption flux of the river sediment when estimate SGD flux. Thus, it is necessary to study the desorption behavior of the radium isotopes in river sediment/suspended particles, especially for the smaller size range of sediment. In the present work, the effects of grain sizes of sediments and salinities of sea water from the Qinzhou Bay on the desorption of radium isotopes were investigated by laboratory experiments. The results show that, within the grain size range of 0.9-136.0 μm, the desorption amounts of radium isotopes from the sediment to seawater (salinity 33.9) decrease with the grain size increase, and the desorption amounts keep almost constant when the grain size is larger than 43.7 μm. Within the salinity range of 4.9-33.9, the desorption amounts of radium isotopes from sediments gradually increase with the salinity increasing until the salinity reaches 24.9. By establishing creatively the radium desorption model using the sediment surface fractal structure theory, the maximum exchangeable activities of 224Ra, 226Ra and 228Ra from the river sediment of Qinzhou Bay are estimated to be 1.13 dpm/g, 0.17 dpm/g and 0.85 dpm/g, respectively; and the concerned maximum desorption ratios are 30%, 7% and 18%. Compared to those from other estuarine/coastal regions, the maximum exchangeable activities of 224Ra and 226Ra from the river sediments in Qinzhou Bay are in the middle or low ranges, while the maximum desorption ratios are in the higher or lower levels, respectively. The results of this study is expected to be useful to better understand the desorption behavior of radium isotopes and estimate accurately SGD flux.