Multi-method characterization of the ferrihydrite to goethite transformation

Abstract The reactivity of soils and sediments is often described by parameters such as the specific surface area (SSA) and the cation exchange capacity (CEC). Both, however, are bulk values controlled by the presence and crystallinity of different phases, e.g. clay minerals and iron oxides. Amongst the latter, ferrihydrite and goethite are the most abundant minerals. Goethite gradually forms from a ferrihydrite precursor, a process involving a decrease in surface area. Today the BET gas adsorption technique is the most common method to measure SSA. In this study, BET surface areas of synthetic and natural ferrihydrite and goethite of variable crystallinity were compared to (a) mineral-specific measurements, including infrared analysis (IR), differential thermal analysis (DTA), and (b) bulk methods, such as cation exchange capacity and oxalate-soluble iron. Correlations showed that the SSA can be indirectly inferred from all methods. There is no general correlation between IR and BET measurements due to the variability of adsorbed water, which depends on drying and ambient conditions. DTA, particularly using the dehydroxylation peak at 300°C, allows quantification of the goethite content, and hence the SSA, because adsorbed water does not affect this peak. The data set of the present study enabled us to estimate either SSA and/or the content of iron oxides, even in natural mixtures, e.g. those in which smectites determine the SSA.