One-step hydrothermal treatment of polyacrylonitrile fiber (PANF) with hyperbranched polyethylenimine (HPEI) resulted in zwitterionic PANF-g-HPEI that contained not only the grafted HPEI moieties but also many COOH groups generated in situ. Increasing the weight gain of PANF-g-HPEI from 10% to 90% resulted in the increase of its COOH, amino and amide groups from 0.12 to 1.86 mmol/g, 1.44 to 8.90 mmol/g, and 0.67 to 2.12 mmol/g, respectively. Dye adsorption experiments demonstrated that (1) such PANF-g-HPEls could effectively adsorb anionic Methyl Orange (MO) or cationic Methylene Blue (MB), through the pretreatment with acidic or basic solution, respectively; (2) PANF-g-HPEIs could selectively adsorb the anionic MO or the cationic MB from MO/MB mixture through the pretreatment with solution of pH = 5 or 10, respectively; (3) the cationic or anionic dyes adsorbed by PANF-g-HPEls could be reversibly desorbed by the aqueous solution of pH = I or 10, respectively; (4) PANF-g-HPEI could be recycled efficiently, and its dye adsorption performances did not show pronounced loss even after 10 adsorption/desorption cycles, superior to PANF treated with the low molar-mass polyamines. (C) 2014 Elsevier B.V. All rights reserved.