The remediation of arsenic-contaminated aquifers is a formidable challenge to achieve, in part because geochemical conditions often do not favor the stabilization of arsenic within the solid phase. A promising remedial approach involves stimulating iron mineral transformations that immobilize arsenic through sorption or precipitation. Despite intense research, the current immobilization methods are still often ineffective, in part because many iron minerals are susceptible to redox gradients common in subsurface environments. We have been conducting a series of studies to illustrate the potential of nanoparticulate magnetite (Fe3O4) to sequester arsenic. Magnetite is stable under most redox conditions in aquifers, and able to co-precipitate and adsorb arsenic. Here, we present results from microcosm and column experiments using sediments and groundwater from U.S. Superfund sites, and from reactive transport modelling. All these results demonstrate that in situ formation of nanoparticulate magnetite can be achieved by the combination of nitrate and ferrous iron, and that it should be feasible to produce an in situ reactive filter by such nitrate-iron(II) co-injection and immobilize arsenic in contaminated aquifers.