Anoxic-oxic cycles play a significant role on the forms and migrations of arsenic in groundwater systems, especially in those with high concentrations of Fe(II). Dissolved Fe(II) can absorb onto the surface (basal plane or OH-edge groups) of iron-containing clay minerals to form Fe(II)/Fe(III) redox couples. Researches showed that electron transfer could happen from sorbed Fe(II) to structural Fe(III) in iron-rich clays to impact the concentrations of As in groundwater, which leaves the inverse way [electron transfer from structural Fe(II) to aqueous Fe(III)] unknown. Therefore, we selected partially reduced iron-rich nontronite (NAu-1-PR) to interact with As(III)-Fe(II) solutions through anoxic-oxic cycles to test the above hypothesis. The methodology included clay/Fe(III) interaction under anoxic condition and clay/Fe(II)/As(III) interaction through anoxic-oxic cycles batch experiments, coupled with a series of analytical techniques (e.g., XRD, FT-IR, and XPS). During the anoxic experiments of NAu-1-PR interaction with Fe(III), the electron transfer from structural Fe(II) to aqueous Fe(III) added in the low pH value had been proved by XRD and XPS evidence, as well as change of aqueous and solid iron concentration in the system. In the anoxic-oxic transform experiment with the addition of both Fe(II) and As(III), aqueous arsenic stayed mainly as trivalent oxidation state in the anoxic process. When the system underwent oxic conditions, there was a sharp decline of aqueous As(III) in parallel with arising As(V) concentration. The structural Fe(II) fell off due to electron transfer from structural Fe(II) in clay to aqueous Fe(III), which is responsible for the oxidation of aqueous As(III) instead of the oxidation by Fenton action due to the added OH-scavenger. Therefore, it is concluded that electron transfer could happen from structural Fe(II) in clay lattices to aqueous Fe(III) and it is the driving force for the oxidation of As(III) to As(V) when an anoxic condition switches to oxic one.