Coprecipitation of Fe(II) and Fe(III) cations in sulphated aqueous medium and formation of hydroxysulphate green rust

Abstract

Fe(II)–Fe(III) solutions having a variable Fe(III) molar fraction were titrated with a NaOH solution. Three plateau regions separated by two equivalent points characterise the pH titration curves. The nature and relative abundance of the phases that form during the first and second plateaux are predicted by using the positions of equivalent points in an iron-compound mass-balance diagram. A slowing down of the pH increase is observed at a point of titration curve called E∗ for solutions that have a Fe(III) molar ratio x Fe(III)=[ n Fe(III)/( n Fe(II)+ n Fe(III))] lower than ∼0.27=(2/7). It corresponds to the addition of ∼7±0.5 moles of OH − per mole of Fe(III). Mössbauer spectroscopy shows that the complete formation of green rust is achieved when exactly 7 moles of OH − per mole of Fe(III) are added. This quantity would be larger than expected to form [Fe II 4Fe III 2(OH) 12] 2+·[SO 4· mH 2O] 2−, if the {Fe(II)/Fe(III)} initial ratio is conserved. This excess is attributed to the adsorption of {[Fe II 4(OH) 8]SO 4} 2− layers on the ferric oxyhydroxide surface that dissolves it and thus forms the hydroxysulphate; this replaces Misawa's ad hoc green complex {Fe II m Fe III n O p [OH] q } (2 m+3 n−2 p− q) prior precipitation.