For the first time, nanoscale zero-valent iron (nZVI)-Fe3O4 nanocomposites, prepared by an in situ reduction method, are employed for chromium(VI) removal in aqueous environment. 96.4% Cr(VI) could be removed by these novel materials within 2 h under pH of 8.0 and initial Cr concentration of 20 mg L-1, compared with 48.8% by bare nFe(3)O(4) and 18.8% by bare nZVI. Effects of several factors, including mass composition of nZVI-Fe3O4 nanocomposites, initial pH and Cr(VI) concentration, were evaluated. The Optimal ratio of nFe(3)O(4) to nZVI mass lies at 12:1 with a fixed nZVI concentration of 0.05 g L-1. Low pH and initial Cr(VI) concentration could increase both the Cr(VI) removal efficiency and reaction rate. Corresponding reaction kinetics fitted well with the pseudo second-order adsorption model. Free energy change (Delta G) of this reaction was calculated to be -4.6 kJ mol(-1) by thermodynamic study, which confirmed its spontaneous and endothermic characteristic. The experimental data could be well described by the Langmuir and Freundlich model, and the maximum capacity (q(max)) obtained from the Langmuir model was 100 and 29.43 mg g(-1) at pH 3.0 and 8.0, respectively. The reaction mechanism was discussed in terms of the mutual benefit brought by the electron transfer from Fe-0 to Fe3O4. (C) 2011 Elsevier Inc. All rights reserved.