The sorption of thorium (IV) and uranium (VI) to hematite in the presence of natural organic matter

In most aquatic systems, natural organic acids constitute an important pool of trace-metal binding ligands, The effect of natural organic matter (NOM) on metal ion sorption to mineral surfaces is a complex function of the env ironment in which the mineral and ions reside, as well as the source, molec ular weight distribution and composition of the NOM. In spite of the well-a cknowledged importance of NOM in affecting metal-ion speciation and the wid ely-established use of surface complexation models to predict the behavior of metal ions in heterogeneous systems, little work has been reported on ef forts to incorporate NOM into surface complexation models. In this paper we present experimental results for two such ternary (metal ion/metal oxide/N OM) systems, detailing: (1) U(VI)/Suwannee river humic acid (HA)/hematite; and (2) Th(IV)/marine colloidal material (COM)/hematite. Our objective is t o describe overall ternary system behavior through the construction of a mo del composed of 'binary' subsystems (e.g. HA/hematite). In all cases, howev er, the postulation of ternary surface complexes is required to satisfactor ily simulate ternary system behavior. We also describe the simulation of HA and COM as a suite of monoprotic acids with fixed pK(a) values. This appro ach allows for the simultaneous simulation of both solution and surface rea ctions of NOM within a consistent chemical framework. (C) 1999 Elsevier Sci ence B.V. All rights reserved.