The comprehensive investigation on removal mechanism of Cr(VI) by humic acid-Fe(II) system structured on V, Ti-bearing magnetite surface.

• Characteristics of Cr(VI) adsorption on the VTM-HA were well investigated. • The vast majority of Cr(VI) was reduced to Cr(III) by HA-Fe(II) system. • The Cr(VI) was adsorbed on the Ti atom of VTM-HA via ionic bonds. • The reduced Cr(III) was deposited on the VTM-HA surface via complexation. • VTM-HA is a promising complex adsorbent for Cr(VI) pollution treatment. The Cr(VI) could be adsorbed and reduced by the humic acid (HA)-Fe(II) system structured on the V, Ti-magnetite (VTM) surface. The Cr(VI) removal process included adsorption and reduction stages. First, the Cr(VI) was adsorbed on the VTM-HA surface via the ionic bonds between the Ti atoms of VTM core and the O atoms of the HCrO 4 −. The adsorption of Cr(VI) is uniform, monolayer, and controlled by Cr(VI) diffusion. Subsequently, the adsorbed Cr(VI) was reduced by the HA-Fe(II) system on the VTM-HA surface. During the Cr(VI) reduction process, the HA and Fe(II) have a synergistic effect. The Cr(VI) was reduced to the Cr(III) by the HA and Fe(II). Meanwhile, the HA could also reduce Fe(III) to Fe(II), making Fe(II) continue to participate in the Cr(VI) reduction. The olefin, hydroxyl, and aldehyde groups of HA were the primary electron donors during the Cr(VI) reduction. The Fe(II) acted as an electron bridge, transferring the electron from HA to Cr(VI). The reduced Cr(III) was deposited on the VTM-HA surface via the complexation with the carboxyl and hydroxyl groups of HA. The results demonstrated that the Cr(VI) could be adsorbed, reduced and complexed by the HA-Fe(II) system on the VTM-HA surface synchronously. [ABSTRACT FROM AUTHOR]