A series of novel magnetic nanoparticles (MNPs) with various sizes for forward osmosis (FO) applications were synthesized via a one-pot reaction under mild conditions. The newly developed MNPs are coated with three different hydroacids (citric acid, oxalic acid and EDTA) carrying abundant carboxylic groups which could make the hydroacid-MNPs highly dispersible and ionizable in water. These features enable the aqueous solutions of hydroacid-MNPs to produce osmotic pressures and FO water fluxes comparable to the traditional draw solutions yet with a negligible reverse flux. The experimental results indicate that better FO performance was achieved when the hydroacid-MNPs with smaller sizes were applied to the FO process. With the model seawater of 3.5 wt% NaCl as the feed solution, the citric acid coated MNPs produced a water flux of 8.5 LMH, which is much higher than those reported for MNPs draw solutes applied in FO for seawater desalination. Additionally, the hydroacid-MNPs were readily regenerated from their diluted solutions after FO via an integrated system including both magnetic and membrane separations. This study significantly extends the potential applicability of FO and provides an economical approach for efficient seawater desalination via hydroacid-MNPs promoted FO processes. (C) 2016 Elsevier B.V. All rights reserved.