Cr(VI) Formation from Cr_xFe_{1- x}(OH)₃Induced by Mn(II) Oxidation on the Surface of Cr_xFe_{1- x}(OH)₃

Cr(III)-Fe(III) hydroxide (CrxFe1-x(OH)3) is a common product of Cr(VI) reduction by Fe(II) during remediation and natural processes. However, reoxidation of this immobilized Cr(III) could significantly impair water quality and potentially undermine remediation efforts. Here, we examine the potential for Cr(III) oxidation from CrxFe1-x(OH)3 under oxic and pH-neutral conditions in the presence of Mn(II). Although homogeneous Mn(II) oxidation by dissolved oxygen is slow, CrxFe1-x(OH)3 surfaces catalyze Mn(II) oxidation to rapidly form manganese oxides, followed by a process in which the manganese oxides oxidize CrxFe1-x(OH)3 and get reduced back to Mn(II). The redox cycling of manganese can continuously drive Cr(VI) generation from CrxFe1-x(OH)3 as long as there is a supply of dissolved oxygen. The overall Cr(VI) production rates are controlled by both the rate of Mn(II) oxidation to manganese oxides and the rate of the subsequent oxidation of Cr(III) in CrxFe1-x(OH)3. These rates are affected by pH and the Fe/Cr ratio in CrxFe1-x(OH)3. X-ray diffraction patterns and SEM images indicated that hausmannite (Mn3O4) is the dominant manganese oxide in Mn(II) oxidation only and the presence of Fe or Cr hydroxides facilitates the production of Mn(III) hydroxides (such as feitknechtite and manganite) in this process. Our findings demonstrate that Cr(VI) can be naturally produced from CrxFe1-x(OH)3 within days when catalyzed by oxidized Mn species from Mn(II) oxidation in oxic environments. This process is likely to play an important role in the oxidation and mobilization of Cr in shallow sediments of ponds, estuaries, and rivers.