Antimony, a pollutant of priority interest by United States Environment Protection Agency (USEPA) and Europe Union (EU), is detected in soil and water with elevated concentration due to its extensive use in a variety of industries. However, antimony purification from liquid system is yet poorly documented. In this study, effect of competing ions on antimony (III/V) removal by ferric chloride (FC) coagulation was comprehensively investigated. At lower FC dose (2x and 4 x 10(-4) mol/L of Fe), the percentage of Sb(V) removal showed a significant and continued decrease with the increasing presence of competing ions, including bicarbonate, sulfate, phosphate, and humic acid (HA). In contrast to Sb(V), Sb(III) removal remained highly effective, and was only slightly affected by the presence of HCO3- and SO42-. Phosphate and HA considerably decreased Sb(III) removal at lower coagulant dose. However, with coagulant dose reaching 6 x 10(-4) mol/L of FC, Sb(III) removal became independent of phosphate and HA effect. The mechanism of antimony (III) adsorption onto hydrous ferric oxide (HFO) formed during FC coagulation, combined with a coprecipitation approach of anomalous incorporation of Sb(III) into HFO nanocrystaline, was proposed to interpret the effective Sb( III) removal under various suboptimal conditions. The predominance of Sb(V) adsorption onto HFO, including both surface and internal adsorption, can satisfactorily explain the Sb(V) removal with high sensitivity to competing ion effect. (C) 2010 Elsevier B.V. All rights reserved.