Rate laws of steady-state and non-steady-state ligand-controlled dissolution of goethite

Rapid changes of chemical conditions are rather common in natural systems including soils and aquatic environments. Non-steady-state conditions can be induced by biological activity, precipitation events, and other transient processes causing energy or concentration gradients. The quantitative description of biogeochemical processes under such non -steady-state conditions requires kinetic modeling, particularly if slow processes such as mineral dissolution are involved. One example for such a process is plant iron acquisition by diurnal exudation of siderophores into the rhizosphere, as observed for many grasses, leading to enhanced mobilization of iron by dissolution of iron oxides. In this study, we investigated ligand-controlled dissolution of goethite under steady-state and non-steady-state conditions and developed rate laws for these processes. The ligands used in this work included plant and microbial siderophores and the low molecular weight organic acid oxalate. Non-steady-state conditions were experimentally induced by pulse-additions of siderophores to goethite suspensions in the presence of oxalate to mimic the diurnal siderophore exudation pattern observed for grasses. We presume that before the addition of the siderophore, a slow oxalate-promoted surface reaction created a pool of kinetically labile iron species at the mineral surface. The subsequent addition of a siderophore increased the iron solubility and triggered a fast dissolution reaction until the labile iron pool was depleted. We concluded that the rate of accumulation of kinetically labile iron is controlled by the same rate determining step as ligand-controlled dissolution under steady-state conditions and that it can be described with the same rate laws. We parameterized the rate laws and successfully modeled goethite dissolution under steady-state and non-steady-state conditions. Thus, we show that general rate laws describe ligand-controlled iron oxide dissolution under both steady-state and non-steady-state conditions. The capability to model such processes has important implications for understanding of weathering processes and nutrient uptake in natural systems. (C) 2007 Elsevier B.V. All rights reserved.