Geochemical mobility of 137Cs in marine environments based on laboratory and field studies

Abstract

Cesium-137, a unique geochemical tracer, behaves as a particle-reactive species in freshwater that is useful as a geochronometer, and soluble in seawater that is used as a water mass tracer, as evidenced by higher solid-solution partition coefficient, Kd, in freshwater and much lower Kd values in seawater. A suite of sediment cores were collected from the Southern Yellow Sea and coastal regions of Croatia in 2014 and analyzed for vertical profiles of excess 210Pb (210Pbex), 239+240Pu, and 137Cs. A set of laboratory experiments were also conducted to investigate the partitioning of 137Cs between a suite of solid material comprised of commonly occurring minerals in the environment and solution under different contact times, pH, and salinity. The partition coefficient showed that for illite Kd sharply increased during the first two hours and then slowly increased after 4 h. Overall, the salinity significantly affects the 137Cs behavior in aqueous environments. The Kd values of 137Cs were higher (6.5 × 103–1.1 × 104 mL g−1) when the salinity was below 0.5 psu but drastically decreased to 9.0 × 102 mL g−1 when salinity was 3.6 psu and then slowly decreased to 2.7 × 102 mL g−1 with the salinity of 27.5 psu. Combining with the previously published results, it is observed that higher suspended particle (SPM) concentration could enhance the Kd values of 137Cs when SPM was below 12 mg L−1. Our results show that post-depositional mobility of 137Cs in the marine environment, except for river-dominated coastal areas, result in the 137Cs chronology is not suitable to validate the 210Pbex-based chronology. The penetration depths of 137Cs were found to be much deeper than it was expected based on the 137Cs and 239+240Pu peak and 210Pb chronology. Chitin showed a higher affinity for 137Cs than clay particles (e.g., smectite and illite). The desorption experiment showed that the NH4+ could leach 5.6% of sorbed 137Cs, accounting for 79% of the total exchangeable fraction while 2.8% of the total 137Cs was bound to organic matter. These observations support the field observation degradation of organic matter in anoxic marine sediment could enhance the mobility of 137Cs.