Oxide-water interface transport

James RO, Healy TW (1972) The adsorption of hydrolyzable metal ions at the oxide-water interface. III. A thermodynamic model of adsorption. J Colloid Interface Sci 40 65-81 Jardine PM, Fendorf SE, Mayes MA, Larsen IL, Brooks SC, Bailey WB (1999) Fate and transport of hexavalent chromium in undisturbed heterogeneous soil. Environ Sci Technol 33 2939-2944 Jayanetti S, Mayanovic RA, Anderson AJ, Bassett WA, Chou I-M (2001) Analysis of radiation-induced small Cu particle cluster formation in aqueous CuCl2. J Chem Phys 115 954-962. 

Mam heterogeneous processes such as dissolution of minerals, formation of he solid phase (precipitation, nucleation, crystal growth, and biomineraliza-r.on. redox processes at the solid-water interface (including light-induced reactions), and reductive and oxidative dissolutions are rate-controlled at the surface (and not by transport) (10). Because surfaces can adsorb oxidants and reductants and modify redox intensity, the solid-solution interface can catalyze rumv redox reactions. Surfaces can accelerate many organic reactions such as ester hvdrolysis (11). 

Tewari, P.H. and Campbell, A.B., The surface charge of oxides and its role in deposition and transport of radioactivity in water-cooled nuclear reactors, in Proceedings of the Symposium on Oxide-Electrolyte Interfaces, Alwitt, R.S., ed.. The Electrochemical Society, Princeton, 1973, p. 102. 

Diffusive Fluxes of Mn(II). If Mn is not transported to the 5-m layer of bottom water by lateral turbulent diffusion or advection, we should observe maximum values of the diffusive fluxes similar to the oxidation rate across the sediment-water interface (up to 3 mmol/m2 per day). Profiles of Mn concentrations in the pore water are shown in Figure 6a. The steep concentration gradients near the sediment-water interface are not at steady state the gradients are at maximum in summer and decrease to a minimum in spring. Sharp peak profiles were observed in June and July 1990. The diffusive flux of Mn(II), Fm in millimoles per square meter per day, was estimated from pore water profiles by using Fick s law with a correction for the porosity, c )... 

In the last decade, however, in-situ techniques have been developed to overcome these problems. Profiling lander systems were deployed to record the pore water microprofiles of oxygen, pH and pCOj, and Ca whereas benthic chambers were deployed to measure solute fluxes across the sediment-water interface directly. Very often, reactive-transport models are used to explain the interrelation between measured microprofiles, to predict overall calcite dissolution rates by defining the dissolution rate constants, and to distinguish between dissolution driven by organic matter oxidation and by the undersaturation of the bottom water. 

The removal of Fe(ii), S( — ii), and Mn(ii) when they are transported toward the sediment-water interface is predicted by the pe scale shown in Figure 2. The oxidation of Fe " " (the reverse of the reaction in Table 2) could be coupled to the reduction of Mn02,... 

Figure 5 illustrates these oxidative removal processes through loss of dissolved components of pore waters. Equally important is the upward mixing of solid phase Fe sulfides to zones of O2 or Mn oxide reduction, followed by their oxidation. Particle mixing by bioturbation is an important mechanism for transport of reduced phases toward the sediment-water interface, particularly in warm-weather (and high productivity) months. 

It is clear that chemical processes do not involve just alteration of particles that fall to the sediment-water interface. Rather, these systems are dominated by cycling between oxidized and reduced chemical forms within the sediment column. Heterotrophic respiration leads to the oxidation of organic mattei releasing C, N, and P into solution, and the reduction of O2, Fe, Mn, and S. The latter three elements are then subject to transport, both in dissolved and solid forms, back toward the sediment-water interface, where they may be reoxidized either abiotically or by lithotrophic bacteria. The burial of reduced Fe, Mn, and S is a slow leak from these rapid internal cycles (Figure 6). 

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