Gallic acid (GA), with a reducing and in situ coordinating capability towards to reducible metal ions, was successfully conjugated onto the amine-silane coated magnetite nanoparticles by stable amide bond through 1-ethyl-3-(3-dimethylaminepropyl) carbodiimide hydrochloride/N-hydroxysuccinimide (EDC/NHS) chemistry, with the aim of engineering a magnetically-separable Fe3O4@TEOS@AMEO@GA applicable for Cr(VI) concurrent reduction and removal, together with explicating reduction-adsorption mechanism. The structure, composition, and morphology of the nanocomposite were systematically characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscope (AFM), low-temperature nitrogen adsorption/desorption experiment, X-ray spectroscope (EDS), X-ray photoelectronic spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), total organic carbon (TOC), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and fluorescence spectra, showing a core-shell, cubic inverse spinel structure of the nanocomposite with superparamagnetism roughly containing 7.8% (w/w) GA on its surface. Cr(VI) reduction-adsorption kinetics, equilibrium, and thermodynamics studies were carried out under solution pH of 6.0, where partially deprotonated phenolic hydroxyl dominated the nanoadsorbent surface. Results showed that pseudo-first order kinetic model and Freundlich isotherm model described the Cr(VI) reduction-adsorption well. Thermodynamics studies indicated an endothermic, spontaneous, and feasible process of such Cr(VI) reduction-adsorption. Furthermore, Cr(VI) reduction-adsorption mechanism was explicated by EDS and XPS technology, finding that phenolic hydroxyls of those conjugated gallates were only functional and active groups. Based on these, we intend to offer a predictable, and hence controllable approach for Cr(VI) reductive immobilization that may be a potential direction for those poisonous and reducible metal ions thorough detoxification and in situ removal based on magnetite-supported materials in practical application.