Arsenic continues to be a contaminant of concern due to its toxicity to living things and   its wide occurrence in groundwater resources globally from both natural and anthropogenic causes. Materials are urgently needed to be used for arsenic removal from water. Previous studies have shown that nanoscale zerovalent iron (NZVI) and iron oxides are such promising sorbents. However, the removal kinetics and mechanisms are not well understood. Here we report recent batch tests conducted using nanoiron (25-nm), magnetite (20-30 nm), maghemite (20-50 nm), and hematite (20-50 nm) to remove the two predominant forms of arsenic: arsenate [As(V)] and arsenite [As(III)] under anaerobic conditions (nanoiron) and aerobic conditions (iron oxides) as a function of time and pH with or without phosphate and silicate. Both As(V) and As(III) removal increased with increasing time to approach a steady state. There is generally more removal of As(III) than As(V). Complete or near complete removal of As(V) and As(III) was achieved at pH levels less than 10. Competition of phosphate and silicate against As(V) and As(III) was observed at alkaline pHs. New solid phases formed such as parasymplesite in the As(V) system and vivianite in the phosphate system with nanoiron. This study demonstrated that As(V) removal by nanoiron involves both solid precipitation and adsorption; whereas, As(III) removal involves surface adsorption. Surface adsorption is responsible for As removal by the iron oxides.