Stable and highly active zero-valent iron-nickel nanofilaments/silica for the hexavalent chromium reduction

•Synthesis of zero-valent iron-nickel nanoparticles stable to oxidation.•Iron-nickel/SiO2 stable for months without using a vacuum, solvent or inert media.•Nanofilaments of iron-nickel promote expressive hexavalent chromium depletion.•Nickel forms a barrier against oxidation in iron-nickel nanofilaments.•Stable nanoparticles can decrease the costs of chromium reduction on large scales.