Coprecipitation of Fe(II–III) hydroxycarbonate green rust stabilised by phosphate adsorption

Abstract

[Fe II 4Fe III 2(OH) 12] 2+•[CO 3, nH 2O] 2− green rust one, GR1(CO 3 2−), was synthesised by coprecipitation in the presence of phosphate by dissolving a Na 2HPO 4 salt. In its absence, the resulting product is a mixture of Fe(II–III) hydroxysalt, magnetite Fe 3O 4 and ferrous hydroxide Fe(OH) 2. In the presence of phosphate as low as {[PO 4] min 0/[Fe]}∼1%, the resulting product is GR1(CO 3 2−) alone. No P was detected by TEM-EDX when the basal (0001) crystal faces were analysed, showing that no P was inserted in the GR. In contrast, very low quantities of P were detected when the analyses were performed on the lateral faces of the GR crystals. Analysis of P in solution by ICP-AES evaluates the quantity of phosphate adsorbed species. A model representing the precipitate and its surface showed that the quantity of adsorbed phosphate has the same order of magnitude than the number of Fe atoms present on the lateral faces. The preference of phosphate species for lateral faces is due to the fact that the {10 1 ̄ 0} faces contain mono- and di-coordinated OH −surface groups. PO 4 3− adsorbed on the lateral faces may act as a barrier that slows down the release in solution of the CO 3 2− species preventing GR1(CO 3 2−) to transform into the mixture of Fe 3O 4 and Fe(OH) 2.