A new and efficient sorbent for the removal of formaldehyde from water samples was proposed by grafting the external surface of gamma-alumina (gamma-Al(2)O(3)) nanoparticles with functional groups of 2,4-Dinitrophenylhydrazine (DNPH) that have a strong affinity to the formaldehyde. This new modifying strategy greatly improves the accessibility of these sorption sites to adsorbate and thus efficiency of the nano-sorbent. This adsorbent was prepared by the amination of the gamma-Al(2)O(3) surface with a simple method. Nitrogen-containing groups were grafted on the gamma-Al(2)O(3) nanoparticles by reaction of the sodium dodecyl sulfate coated nano-alumina with DNPH solution. The surface chemistry of the adsorbent was characterized by BET, SEM and FT-IR spectroscopy. The produced sorbent has shown very good performance for removing formaldehyde in real samples. Optimal experimental conditions including pH, contact time and adsorbent dosage have been studied and established. Langmuir and Freundlich isotherm models were applied to analyze the experimental data. The retained formaldehyde on modified adsorbent was then determined, after desorption, based on its catalytic effect on the oxidation reaction of thionine with bromate in acidic media. (C) 2011 Elsevier B.V. All rights reserved.