Factors controlling uranium geochemistry in the mixing zone of river- and seawaters

Abstract

When the transformations of the continental runoff in river mouths is studied, much attention is paid to the radioloecological aspect of the problem, in particular, to the identification of factors controlling the migration of radionuclides at the river‐sea geochemical barrier. For trace elements with relatively low biological accumulation coefficients, a significant role is played by sorption‐desorption processes, which are responsible for the redistribution of solute and particulate modes, because zones where riverine and marine waters are mixed are characterized by drastic changes in the concentrations of dissolved components and coupled changes in the composition of the adsorbed complex of riverine suspensions. The behavior of U in river mouths varies: the Baltic Sea [1] and the mouths of the Volga [2], Gironde [3], Clyde, and Tamar rivers in Great Britain [4], Zaire [3, 5], Godavari [6], and Mahanadi [7, 8] are characterized by the strictly conservative behavior of this component, whereas the mouths of the Charente, France [3], and Forth [4] rivers and the Delaware and Chesapeake bays in the United States [9, 10] show the removal of significant amounts of dissolved U. The removal of U from the Forth and Charente river mouths was thought to be caused by sorption on riverine particulate matter, and U immobilization in the salty marshes of Chesapeake and Delaware bays can be related to the coprecipitation of U(IV), which is produced by the reduction of U(VI) by S-bearing compounds and/or organic matter, together with floccules of the latter or autochthonous Fe hydroxide. However, even in the situation when U formally behaves conservatively, it cannot be ruled out that this element participates in sorption‐desorption processes when riverine particulate matter interacts with seawater, a processes that sometimes cannot be identified during natural observations because of the low contents of particulate matter, which are insufficient for analytically significant changes in the U concentrations in the solutions. For example, a new quasiconservative type of distribution was recognized and described for F and B [11], when, in spite of the formal consistence of the results of natural observations with the conservative type of behavior, the interactions of riverine suspensions with seawater results in the sorption-related removal of much of the dissolved trace elements (7‐18 and 67‐80%, respectively, of the total F and B amounts introduced into the ocean with riverine runoff). This conclusion, which was rigorously confirmed in experiments, has notably modified the concept of the factors controlling the geochemical F and B balance in the ocean. In order to elucidate the role of sorption‐desorption processes in the transforms of the dissolved U runoff in the mixing zone of riverine and marine waters, we conducted an experimental simulation of the interactions of waters of various salinity that contain U(VI) with the predominant clay minerals, which represented the particulate matter of the continental runoff.