Improved removal capacity of magnetite for Cr(VI) by electrochemical reduction

Aqueous hexavalent chromium (Cr(VI)) poses serious threats to ecological environments. Magnetite is a potential adsorbent for Cr(VI). However, its adsorption capacity is limited due to the formation of Fe(III) oxide coating on magnetite surface. Herein, constant potential reduction was conducted to improve the Cr(VI) removal capacity of magnetite, and the influence of pH, potential, and supporting electrolytes including KNO₃, KCl, and K₂SO₄ on the adsorption capacity was also investigated. The results showed that the highest Cr(VI) reduction percentage reached 93.7% with a total Cr removal capacity of 514.7 mg g^-1 at optimized pH 2 and -0.2 V (vs. SCE) in supporting electrolyte of KNO₃. Cr(VI) was reduced to Cr(III) on the surface of magnetite due to the direct electrochemical reduction at low potentials and reduction by Fe²⁺_aq electrochemically generated from magnetite. The Cr(III) was subsequently removed and easily separated due to the formation of Cr(OH)₃ precipitate on magnetite surface when KNO₃ and KCl were used as supporting electrolyte; however, when K₂SO₄ was used instead, Cr(OH)₃ precipitate was not observed. The decrease in pH and electrical potential was found to facilitate the reduction and removal of Cr(VI). This work proposes a facile method to enhance Cr(VI) removal by iron oxides.