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 di fferent lakes located near Sudbury, Ontario. X-ray diffraction and ele ctron microscopic analyses indicated that the Fe-rich material collect ed was predominantly ferrihydrite and poorly crystallized lepidocrocit e, while the Mn-rich material was a mixture of poorly crystallized Mn oxyhydroxides. Conditional adsorption constants (K-Fe-M and K-Mn-M) we re 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 co nstants of the metals; (2) laboratory-derived intrinsic surface comple xation constants obtained for adsorption of these metals on well-chara cterized Fe and Mn oxyhydroxides, and (3) predicted K-Fe-M and K-Mn-M values determined using the surface complexation model under the geoch emical conditions observed in the lakes. Complexation of these metals with adsorbed natural organic matter was also compared to metal comple xation with dissolved natural organic matter. The results are consiste nt 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 i n the more acidic (pH = 4.8) Clearwater Lake. Alkaline earth metals Ca and Mg bind, presumably as outer-sphere complexes, to the organic coa tings. Our results provide support for the argument that laboratory-de rived adsorption datasets may be useful for predicting metal adsorptio n in the field.