Aqueous ferrous iron (Fe(II)) accelerates the transformation of ferrihydrite into secondary, more crystalline minerals however the factors controlling the rate and, indeed, the underlying mechanism of this transformation process remain unclear. Here, we present the first detailed study of the kinetics of the Fe(II)-accelerated transformation of ferrihydrite to goethite, via lepidocrocite, for a range of pH and Fe(II) concentrations and, from the results obtained, provide insight into the factors controlling the transformation rate and the processes responsible for transformation. A reaction scheme for the Fe(II)-accelerated secondary mineralization of ferrihydrite is developed in which an Fe(II) atom attaches to the ferrihydrite surface where it is immediately oxidized to Fe(III) with the resultant electron transferred, sequentially, to other iron oxyhydroxide Fe(III) atoms before release to solution as Fe(II). This freshly precipitated Fe(III) forms the nuclei for the formation of secondary minerals and also facilitates the ongoing uptake of Fe(II) from solution by creation of fresh surface sites. The concentration of solid-associated Fe(II) and the rate of transport of Fe(II) to the oxyhydroxide surface appear to determine which particular secondary minerals form and their rates of formation. Lepidocrocite growth is enhanced at lower solid-associated Fe(II) concentrations while conditions leading to more rapid uptake of Fe(II) from solution lead to higher goethite growth rates.
"Researchers are increasingly considering benefit transfer approaches that allow welfare measures to be adjusted for characteristics of the policy context. The validity and reliability of such adjustments, however, depends on the presence of systematic variation in underlying WTP. This paper describes a meta-analysis conducted to identify systematic components of WTP for aquatic resource improvements. Model results reveal systematic patterns in WTP unapparent from stated preference models considered in isolation, and suggest that observable attributes account for a substantial proportion of the variance in WTP estimates across studies. The analysis also exposes challenges faced in development, estimation, and interpretation of meta-models for benefit transfer and welfare guidance. These challenges remain salient even in cases where the statistical performance of meta-models is satisfactory." Copyright 2005 Canadian Agricultural Economics Society.
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