Stability of Schwertmannite and Goethite submitted to extraction procedures

Goethite is an iron oxide widely found in tropical soils and very stable in oxic environments. On the other hand, schwertmannite is a metastable oxyhydroxysulfate commonly found in acid mine drainage environments. Due to their high specific surface area, iron oxides are known to behave as sinks of trace elements, such as As. However, due to the reductive dissolution that can occur with the fluctuation of redox potentials, their stability needs to be well studied, otherwise, environmental matrices can be contaminated. The mineral stability of Sch and Gt was assessed by selective chemical extractions in synthetic samples in order to evaluate the mobilization of Fe and possible other trace elements. Synthetic samples of Gt were prepared by adjusting the pH of a solution of 0.1 M Fe(NO₃)₃·9H₂O by adding 5M NaOH dropwise. Synthetic samples of Sch were obtained by forced hydrolysis by mixing 0.02 M Fe(NO₃)₃ and 0.02M K₂SO₄ at 60º. Confirmation of Gt and Sch synthesis was assessed by X-Ray diffraction. The zeta potential of the colloids was 9.3 and 7.4 for Gt and Sch, respectively. The chemical extractants were hydroxylamine 0.5M, ascorbic acid 0.25M, ascorbic acid 0.2M + citrate 0.4M and HCl 1M. Selective chemical extractions were conducted using 1:100 solid:solution ratio, with replicates, and shaking for 72h. Suspensions were filtered and then soluble Fe was measured by Atomic Absorption Spectroscopy. The ascorbic acid alone was able to dissolve more than 60% of the Fe in the structure of Gt, while the other extractants dissolved less than 10%. This effect can be ascribed to the dissolution of Fe from the surface and the accelerated transformation of Fe(III)/Fe(II) in solution. For Sch, the mixture of ascorbic acid and citrate and HCl 1M were the most efficient extractant, showing the difference regarding crystallinity towards extraction procedures. This study highlights the importance of using different extraction procedures in pedogenesis studies and sediments analysis in natural systems in order to better assess different iron oxides and in order to better predict possible dissolution process