The U.S. EPA recommends adding of 5-25mg/L ferric salt to remove arsenate (V) at pH 5 to 8 in water treatment plants. Citrate has been proven to inhibit ferric hydroxide floc formation and corresponding arsenic adsorption. Although most research has been conducted at high molar ratios of citrate to iron, low molar ratios were used in this work and the inhibition of floc formation remained constant above a molar ratio of 0.05. Nucleation kinetics of ferric hydroxides and arsenic removal was investigated in the presence of citrate at low molar ratios (citrate/Fe ranging from 0 to 0.28). At concentrations found in natural waters (4.5-25muM), citrate effectively inhibited ferric hydroxide nucleation and precipitation by forming a non-crystalline macromolecular complex (FeO(Fe3O12H3)Cit) detected in aqueous phase by electrospray ionization mass spectrometry (ESI-MS). Arsenate removal by nano magnetite was also retarded by a low concentration of citrate. The effectiveness of citrate was more significant at higher pH values. Possible mechanisms of citrate inhibition at low molar ratios were compared and discussed. This research demonstrated that the citrate impact upon arsenate removal should be considered when using the iron coagulation and precipitation method in water treatment plants.