Synthesis of goethite from Fe(OH)2 precipitates:: Influence of Fe(II) concentration and stirring speed

Magnetic methods are efficient tools in soil and environmental science. But in such natural environments, several magnetic minerals are generally present. So, synthetic standard samples are necessary for calibration of laboratory techniques. The aim of this study was to synthesise goethite free of magnetic impurities (concentration < similar to 1 mu g kg(-1)) with different crystal sizes. Goethite was prepared by oxidation of aqueous suspensions of Fe(OH)(2) precipitates. Final products were characterised by X-ray diffraction, infrared spectroscopy, scanning and transmission electron microscopy and magnetic methods. Goethite could be obtained in the absence of any trace of strong magnetic minerals using FeSO4 center dot 7H(2)O and NaOH as reactants with the following experimental conditions: temperature = 45 degrees C, [FeSO4 center dot 7H(2)O] = 0.50mol L-1, [NaOH] = 0.20mol L-1, stirring speed = 760rpm. The Fe(II) concentration and the stirring speed were varied. It proved possible to modify the size of the goethite crystals by varying the Fe(II) concentration and the stirring speed, but important changes of these parameters induced the formation of other phases, lepidocrocite when the oxidation reaction was drastically accelerated and Fe3O4 when the reaction was slowed down. In any case, for weak magnetic fields, a low-coercivity magnetic mineral saturating at weak magnetic fields was observed. It may correspond to traces of delta-FeOOH or to domains structurally similar to delta-FeOOH inside the multidomainic crystals of delta-FeOOH. (c) 2008 Elsevier Ltd. All rights reserved.