The surface area of iron oxyhydroxides is a key factor when removing As from water. However, research related to this matter shows that this issue has not been explored in detail. The use of capping agents is a viable method to synthesize ferric oxyhydroxide nanoparticles; however, this method to our knowledge has not been applied for the anchorage of iron oxyhydroxide nanoparticles on activated carbon (AC). In the present work, the addition of PO(4) (as a capping agent) in forced hydrolysis of FeCl(3) in AC was investigated. Results revealed that the surface area of modified materials reached a maximum of about 900 m(2)/g with a molar ratio PO(4)/Fe of 0.1. Moreover, microscopy studies indicate a size range of iron nanoparticles from 2 to 300 nm, where the smallest particles are attained with the highest concentration of PO(4). The surface charge distribution of modified samples became less positive; however, the As removal increased, indicating that electrostatic interaction is not the controlling sorption mechanism. Modified samples showed a 40% increase on As(V) adsorption capacity when using a molar ratio PO(4)/Fe of 1.5. The proposed method allowed anchoring of iron oxyhydroxides nanoparticles on AC, which have a high As(V) adsorption capacity (5 mg/g).