Computer Science
Scientific paper
Apr 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000gecoa..64.1237p&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 64, Issue 7, pp.1237-1247
Computer Science
8
Scientific paper
The reduction of Mn-oxide by Fe 2+ was studied in column experiments, using a column filled with natural Mn-oxide coated sand. Analysis of the Mn-oxide indicated the presence of both Mn(III) and Mn(IV) in the Mn-oxide. The initial exchange capacity of the column was determined by displacement of adsorbed Ca 2+ with Mg 2+ . Subsequently a FeCl 2 solution was injected into the column causing the reduction of the Mn-oxide and the precipitation of Fe(OH) 3 . Finally the exchange capacity of the column containing newly formed Fe(OH) 3 was determined by injection of a KBr solution. During injection of the FeCl 2 solution into the column, an ion distribution pattern was observed in the effluent that suggests the formation of separate reaction fronts for Mn(III)-oxide and Mn(IV)-oxide travelling at different velocities through the column. At the proximal reaction front, Fe 2+ reacts with MnO 2 producing Fe(OH) 3 , Mn 2+ and H + . The protons are transported downstream and cause the disproportionation of MnOOH at a separate reaction front. Between the two Mn reaction fronts, the dissolution and precipitation of Fe(OH) 3 and Al(OH) 3 act as proton buffers. Reactive transport modeling, using the code PHREEQC 2.0, was done to quantify and analyze the reaction controls and the coupling between transport and chemical processes. A model containing only mineral equilibria constraints for birnessite, manganite, gibbsite, and ferrihydrite, was able to explain the overall reaction pattern with the sequential appearance of Mn 2+ , Al 3+ , Fe 3+ , and Fe 2+ in the column outlet solution. However, the initial breakthrough of a peak of Ca 2+ and the observed pH buffering indicated that exchange processes were of importance as well. The amount of potential exchangers, such as birnessite and ferrihydrite, did vary in the course of the experiment. A model containing surface complexation coupled to varying concentrations of birnessite and ferrihydrite and a constant charge exchanger in addition to mineral equilibria provided a satisfactory description of the distribution of all solutes in time and space. However, the observed concentration profiles are more gradual than indicated by the equilibrium model. Reaction kinetics for the dissolution of MnO 2 and MnOOH and dissolution of Al(OH) 3 were incorporated in the model, which explained the shape of the breakthrough curves satisfactorily. The results of this study emphasize the importance of understanding the interplay between chemical reactions and transport in addition to interactions between redox, proton buffering, and adsorption processes when dealing with natural sediments. Reactive transport modeling is a powerful tool to analyze and quantify such interactions.
Appelo A. J. C.
Postma Dieke
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