Pyrolysis of biorenewable feedstocks and iron oxides is potentially a greener and more sustainable pathway to producing zerovalent iron (ZVI) for environmental rehabilitation. The resulting biochar-zerovalent iron (BC-ZVI) also shows improved remediation kinetics of trichloroethylene over conventional ZVI. Understanding the transformations of iron to ZVI and the influence of feedstock chemistry on ZVI is critical to the production of BC-ZVI and has not been reported previously. BC-ZVI production was studied by one-step pyrolysis of cellulose, corn stover, dried distillers’ grain, red oak, and switchgrass pretreated with FeCl3. Pyrolysis at 900 °C effectively reduced Fe to ZVI with most feedstocks; however, the association of silicon (Si) and phosphorus (P) with Fe resulted in formation of fayalite and Fe phosphates and phosphides, which limited ZVI production efficiency and/or facilitated corrosion of ZVI. Dispersion of ZVI phases on biochar surfaces and association with Si facilitated oxidation of ZVI due to greater accessibility to oxygen and enhanced corrodibility of ZVI in association with fayalite. Feedstocks low in Si and P such as cellulose and red oak yield BC-ZVI suitable for environmental applications.