Modelling Metal-Humic Substances/Surface Systems: Reasons for Success, Failure and Possible Routes for Peace of Mind

Pascal E. Reiller (1)

1 LANIE - Laboratoire de développement Analytique Nucléaire Isotopique et Elémentaire

Iron oxides and oxy-hydroxides are commonly of considerable importance in the sorption of ions onto rocks, soils and sediments. They can be the controlling sorption phase even if they are present in relatively small quantities. In common with other oxides and clay minerals, the sorption pH-edge of metals is directly linked to their hydrolysis: the higher the residual charge on the metal ion, the lower the pH-edge. Modelling of this process has been successfully carried out using different microscopic or macroscopic definitions of the interface (e.g. surface complexation or ion exchange models that may or may not include mineralogical descriptions). The influence of organics on the sorption of many metals is significant. This organic material includes simple organic molecules and more complex exopolymeric substances (e.g., humic substances) produced by the decay of natural organic matter. Sorption of these organics materials to mineral surfaces has also been the subject of a large body of work. The various types of organics do not share the same affinities for minerals in general, and for iron oxides and oxy-hydroxides in particular. In those cases in which successful models of the component binary systems (i.e., metal/surface, metal/organic, organic/surface) have been developed, the formation of mixed surface complexes, the evolution of the surface itself, the addition order in laboratory systems, and the evolution of natural organic matter fractions during sorption, have often precluded a satisfying description of the metal/surface/organic ternary system over a sufficiently wide ranges of parameter values (i.e. pH, ionic strength, concentration of humic substances). This manuscript describes the reasons for some successes and failures in the modelling of the ternary systems. Promising recent advances and possible methods of providing more complete description of these intricate systems are also discussed.

Mots clés: Sorption modelling; metal ions; radionuclides; humic acids; humic substances

Domaines: Géochimie; Milieux et Changements globaux

Pascal E. Reiller. Modelling Metal-Humic Substances/Surface Systems: Reasons for Success, Failure and Possible Routes for Peace of Mind. Mineralogical Magazine, 2012, 76 (7), pp.2643-2658. ⟨10.1180/minmag.2012.076.7.02⟩. ⟨cea-00766089⟩