| The interfacial structure between the (001) surface of muscovite mica and fulvic acid (FA) solutions with, and without, dissolved heavy metals was investigated using in-situ X-ray reflectivity. Resonant anomalous X-ray reflectivity (RAXR) was applied to determine the distributions of heavy metals at the interface. Derived electron-density profiles revealed atomic-scale characteristics of metal ions (Ba2+, Sr2+, Hg2+, Cu 2+, Zn2+, and Pb2+) and FA sorbed on the mica surface. The results showed that sorbed FA forms an organic layer that varies in structure as a function of phi and reaction time. Incorporation of heavy metals within the layer also changed the properties of the sorbed FA. The FA layer enhanced metal uptake on mica especially for metal ions that have a high affinity for organic matter. The opposite occurred for metal ions with a low affinity for organic matter. The effect of sorbed FA on metal uptake on mica also depends on the hydration strength of the metal ions. When metal ions have weak hydration strength, they can partially dehydrate and adsorb on the mica surface as inner-sphere complexes. In this case, some metal ions may physically bridge the FA molecules and the mica surface implying a chemical bridging mechanism as well. When metal ions hold their hydration shell more strongly, FA that is directly-sorbed on the surface hinders inner-sphere complexation and metal ions adsorb on the mica surface mostly as outer-sphere complexes. These results show that how heavy metals will sorb on mineral surfaces largely depends on their molecular-scale interactions with natural organic matter. X-ray reflectivity combined with RAXR together provide a unique approach to see such changes in-situ at an atomic-scale. |
