Metal sorption to diagenetic iron and manganese oxyhydroxides and associated organic matter: Narrowing the gap between field and laboratory measurements

Diagenetic Fe and Mn oxyhydroxides were isolated in situ by vertically inserting inert collectors into the sediments of two geochemically different lakes located near Sudbury, Ontario. X-ray diffraction and electron microscopic analyses indicated that the Fe-rich material collected was predominantly ferrihydrite and poorly crystallized lepidocrocite, while the Mn-rich material was a mixture of poorly crystallized Mn oxyhydroxides. Conditional adsorption constants (K-Fe-M and K-Mn-M) were calculated using the concentrations of metals (Ca, Cd, Cu, Mg, Ni, Pb, Zn) associated with the Fe- and Mn-rich material and the measured dissolved concentrations of these metals. Comparison of these in situ derived K-Fe-M and K-Mn-M values were made with: (1) the hydrolysis constants of the metals; (2) laboratory-derived intrinsic surface complexation constants obtained for adsorption of these metals on well-characterized Fe and Mn oxyhydroxides, and (3) predicted K-Fe-M and K-Mn-M values determined using the surface complexation model under the geochemical conditions observed in the lakes. Complexation of these metals with adsorbed natural organic matter was also compared to metal complexation with dissolved natural organic matter. The results are consistent with the scenario that trace metals bind directly to the OH groups of the Fe and Mn oxyhydroxides in circumneutral McFarlane Lake and to the functional groups of organic matter adsorbed on Fe oxyhydroxides in the more acidic (pH = 4.8) Clearwater Lake. Alkaline earth metals Ca and Mg bind, presumably as outer-sphere complexes, to the organic coatings. Our results provide support for the argument that laboratory-derived adsorption datasets may be useful for predicting metal adsorption in the field.