A carbonate-based mesoporous magnetic adsorbent IO@CaCO3 was synthesized via a hydrothermal method. The IO@CaCO3 adsorbent removed both anionic (As(V) and Cr(VI)) and cationic (Pb(II)) heavy metal ions at a rate orders of magnitude faster than any conventional adsorbents reported to date; due to synergistic effect of needle-like IO and CaCO3, it completely removed (99.99%) the toxic heavy metal ions from wastewater in only 9 min. In addition, it exhibited excellent As(V), Cr(VI), and Pb(II) removal capacities of 184.1, 251.6, and 1041.9 mg g(-1), respectively, over a wide pH range. The heavy metal ion adsorption mechanism involves ion-exchange reactions between the cationic/anionic heavy metal ions and the positively/negatively charged groups on the adsorbent. Along with the batch adsorption tests, heavy metal ion removal by IO@CaCO3 was also performed by column filtration, demonstrating the potential of IO@CaCO3 for use in practical applications. Moreover, the excellent removal efficiencies of the adsorbent for the heavy metal ions up to drinking water level and its easy separation by an external magnetic field enabled it to be effectively reused. (C) 2016 Elsevier B.V. All rights reserved.