Citrate assisted fabrication of magnetite for hexavalent chromium removal: Implication from thermodynamic aspect

Magnetite is a fascinating platform for remediating Cr(VI) via adsorption and (or) reduction. Herein, magnetite was fabricated via co precipitation with the assistance of citrate and tested for Cr(VI) removal. The removal mechanism was meticulously inspected from thermodynamic aspect. Specifically, redox reaction between magnetite and Cr(VI) in solution and at solid-liquid interface was scrutinized. Firstly, species distribution function of Cr(VI), Cr(III), Fe(III) and Fe(II) was deduced. Secondly, redox reaction was determined according to the species distribution function. Thirdly, spontaneity of redox reaction was estimated using the Van 't Hoff equation. Result presents, when pH = 0-5, redox reaction both in solution and at solid-liquid interface is spontaneous. When pH = 6-7, redox reaction in solution is non spontaneous, while that at solid-liquid interface is spontaneous. When pH = 8-14, redox reaction in solution is spontaneous, while that at solid-liquid interface is non spontaneous. In the application scenario, Cr(III) must be incorporated when evaluating Cr(VI) removal efficiency, and magnetism decay of magnetite occurs at pH = 0-7. Above thermodynamic explorations were validated by experimental inspection. This work illuminates the chemistry of Cr(VI) removal using magnetite.