Ferrihydrite is one of the dominant phases regulating the dissolved concentration and transport of arsenic. Although extensive research has been conducted on arsenic retention, the extent and strength of retention on iron oxides remains unresolved. Here we examine adsorption/desorption behavior of arsenate and arsenite as affected by pH and As:Fe molar ratios under aerobic, hydrodynamic conditions and desorption under iron reducing (ferrous iron producing) conditions. Specifically, adsorption/desorption maxima and ‘cross-over' pH (the point at which equal loading of two species occur) for arsenite-arsenate were obtained to examine. Under anaerobic conditions, reductive dissolution of ferrihydrite, and other ferric (hydr)oxides, is generally perceived to liberate arsenic from the solid to the solution phase. However, we observed limited arsenic release under rapid ferrihydrite biotransformation. Arsenic appears retained via formation of a ferrous-arsenite solid that results from biological (or abiologically introduced) Fe(II) during ferrihydrite reduction. Thus, despite the current paradigm of arsenic release under anaerobic conditions, biomineralization of ferrihydrite may stabilize arsenic relative to its transport under aerobic conditions.