How does organic matter constrain the nature, size and availability of Fe nanoparticles for biological reduction?

► New data demonstrating the impact of HS on the formation and reactivity of Fe oxides. ► Humic substances reduce the extent of the Fe oxidation-hydrolysis reaction. ► Humic substance directly impact the size and the nature of formed Fe oxides. ► A fraction of Fe does not contribute to the genesis of nanoparticles. ► Mixed Fe nanoparticles-organic colloids are much more bioavailable than Fe oxides.