Magnetic carbon–iron nanoadsorbents fabricated by carbonizing cellulose and reducing Fe₃O₄ nanoparticles or Fe(NO₃)₃ (the products are denoted as MC–O and MC–N, respectively) have demonstrated great Cr(VI) removal. MC–N with a higher proportion of zero-valence iron (ZVI) and bigger specific surface area exhibited better resistance to oxygen and acid than MC–O due to its smaller pore size. The Cr(VI) removal was highly pH-dependent. For example, 4.0 mg L⁻¹ Cr(VI) neutral solution was completely purified by 2.5 g L⁻¹ MC–O and MC–N within 10 min. 1000 mg L⁻¹ Cr(VI) solution at pH 1.0 was completely removed by both nanoadsorbents in 10 min. The MC–O nanoadsorbents had a higher removal percentage (98.1%) than MC–N (93.5%) at pH 7.0, while MC–N had a removal capacity of 327.5 mg g⁻¹, much higher than 293.8 mg g⁻¹ of MC–O at pH 1.0. A chemical adsorption was revealed from the pseudo-second-order kinetic study. Monolayer adsorption of Cr(VI) was revealed by a better fitting of the Langmuir model isotherm, rather than multilayer adsorption for the Freundlich model. These nanoadsorbents could be easily separated from solution by using a permanent magnet after being treated with Cr(VI). Finally, the Cr(VI) removal mechanisms were proposed considering the Cr(VI) reduction and precipitation of Cr(III).