Comparison of radiotracer Ksub(D) values from batch equilibration experiments with in-situ determinations in the deep-sea using the MANOP lander: the importance of geochemical mechanisms in controlling ion uptake and migration

The following mechanisms and controlling parameters of radiotracer uptake were identified: Ksub(D) values of 134Cs are related to sediment clay content, those of 133Ba and 125Sb to the B.E.T. surface area, those of 203Hg to organic carbon content, and those of 54Mn, 60Co, 59Fe, 141Ce and 113Sn to the manganese content of the sediments. None of these Ksub(D) values ions was related to the cation exchange capacity. Uptake rates for 137Cs, 133Ba, 125Sb and 65Zn reflect reversible sorption on surfaces of filtrable particles, while those of the hydrolyzable ions of 59Fe, 113Sn and 141Ce are controlled by colloid aggregation rate. The slow uptake mechanism for 54Mn and 60Co appears to be autocatalytic oxidation on manganese oxide surfaces. In-situ uptake and migration in oxic surface sediments for 134Cs and 133Ba, whose uptake onto natural sediments is controlled by ion exchange, could only be predicted. The authors conclude that applications of Ksub(D) values determined in laboratory batch experiments to in-situ conditions is likely to be inappropriate for easily hydrolyzed radiotracer ions, for ions which are sensitive to slight variations of the redox potential, and ions whose uptake is strongly controlled by surface catalyzed oxidation reactions. (author)