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 environment in which the mineral and ions reside, as well as the source, molecular weight distribution and composition of the NOM. In spite of the well-acknowledged importance of NOM in affecting metal-ion speciation and the widely-established use of surface complexation models to predict the behavior of metal ions in heterogeneous systems, little work has been reported on efforts to incorporate NOM into surface complexation models. In this paper we present experimental results for two such ternary (metal ion/metal oxide/NOM) systems, detailing: (1) U(VI)/ Suwannee river humic acid (HA)/hematite; and (2) Th(IV)/marine colloidal material (COM)/hematite. Our objective is to describe overall ternary system behavior through the construction of a model composed of ‘binary’ subsystems (e.g. HA/hematite). In all cases, however, the postulation of ternary surface complexes is required to satisfactorily simulate ternary system behavior. We also describe the simulation of HA and COM as a suite of monoprotic acids with fixed pKa values. This approach allows for the simultaneous simulation of both solution and surface reactions of NOM within a consistent chemical framework. © 1999 Elsevier Science B.V. All rights reserved.