Differential dissolution of hematite and goethite in dithionite and its implication on soil yellowing

Abstract

This paper describes a modification of the deferrification method of Mehra and Jackson based on the parallel utilization at low temperature (20°C and occasionally 4°C) of the citrate-bicarbonate-dithionite (DCB) reagent on the one hand and citrate-bicarbonate (CB), on the other. The differences in amounts of Fe and Al solubilized by these two reagents during different periods of time were used for characterizing the chemical composition of fractions of free iron undergoing reduction. Simultaneously, the mineralogy of the same fractions was monitored by a differential X-ray diffraction method (DXRD) and the changes in colour of the residues of deferrification were recorded. With reddish samples containing the hematite-goethite pair, the application of the reducing reagent led to the preferential dissolution of Al-poor hematites over Al-rich goethites and reproduced in vitro the yellowing process often observed in the environments from which the samples were selected. It is noted, however, that the deferrification may not be perfectly selective for the re-pigmenting hematitic minerals but can affect also that part of the goethitic population made of Al-poor goethites. These observations suggest that, depending on the mineralogy of free iron, two types of yellowing are likely to occur in situ, the first one solubilizing hematite and leaving goethite unaffected and the second one mobilizing both hematite and Al-poor goethite. In both cases, the yellowing must be regarded as a process promoting net iron losses and relative alumina accumulations.