Magnetite synthesis from ferrous iron solution at pH 6.8 in a continuous stirred tank reactor

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Abstract
Partial oxidation of defined Fe2+ solutions is a well-known method for magnetite synthesis in batch systems. The partial oxidation method could serve as basis for an iron removal process in drinking water production, yielding magnetite (Fe3O4) as compact and valuable product. As a first step toward such a process, a series of experiments was carried out, in which magnetite was synthesized from an Fe2+ solution in a 2-L continuous stirred tank reactor at atmospheric pressure and 32 °C. In four experiments, elevating the pH from a start value of 5.5 or 6.0 to a final pH between 6.8, 7.0 or 7.5 caused green rust to form, eventually leading to magnetite. Formation of NH4+ in the reactor indicated that NO3− and subsequently NO2− served as oxidant. However, mass flow analysis revealed an influx of O2 to the reactor. In a subsequent experiment, magnetite formation was achieved in the absence of added nitrate. In another experiment, seeding with magnetite particles led to additional magnetite precipitation without the need for a pH elevation step. Our results show, for the first time, that continuous magnetite formation from an Fe2+ solution is possible under mild conditions, without the need for extensive addition of chemicals.

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