Microorganisms have been producing nanoparticles for billions of years and by controlling and tuning this productivity they have the potential to provide novel materials using environmentally friendly manufacturing pathways. Metal-reducing bacteria are a particularly fertile source of nanoparticles and their reduction of Fe (III) oxides leads to the formation of ferrite spinel nanoparticles, especially magnetite, Fe 3 O 4 . The high yields produced by extracellular biomineralising processes

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... them commercially attractive, and the production of these bionano ferrite spinels can be tuned by doping the precursor Fe(III) phase with Co, Ni, Zn, Mn and V. The oxidation state of the cations and the sites of substitution are determined by X-ray absorption spectroscopy (XAS), especially by examination of metal L-edge spectra and X-ray magnetic circular dichroism (XMCD). Vanadium substitution in bionano ferrite spinels is revealed for the first time, and substitution in the octahedral site as V(III) confirmed. Bionanomagnetite is shown to be effective in the remediation of azo dyes with the complete breakdown of Remazol Black B to colourless amines and acids. XMCD shows this to involve oxidation of the surface Fe(III) and the potential for regeneration of the nanoparticles.