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

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 a 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 (CSTR) at atmospheric pressure and 32 °C. In four experiments, elevating the pH from an initial value of 5.5 or 6.0 to a final value of 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 the 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.