Iron electrocoagulation (EC) is a low-cost arsenic (As) treatment method that is able to produce Fe(II,III) phases that bind As effectively, such as green rust (GR) and magnetite (Mag). We compared the As removal performance of coprecipitated GR, Mag and lepidocrocite (Lp) produced by Fe(0) EC. The As(III) removal efficiency for these phases was >60%, increasing in order of GR < Mag ≤ Lp, and all three Fe phases removed >95% of the initial As(V) (0.5 to 11 mg L−1). Spectroscopic data of EC samples revealed that As was able to substitute for tetrahedral Fe sites in Mag, associating with multiple Fe atoms, while As formed binuclear corner sharing geometries on GR and Lp surfaces. The formation of multinuclear As complexes during Mag production by Fe(0) EC can explain the low fraction of As remobilized from the solids during extractions with 2.5 mM PO3− 4 or 0.01 M NaOH. Taken together, our results show that Mag is the optimum phase for As removal in Fe(0) EC field treatment.