Cr(III)–Fe(III) hydroxide (Cr_xFe_1–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 Cr_xFe_1–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, Cr_xFe_1–x(OH)₃ surfaces catalyze Mn(II) oxidation to rapidly form manganese oxides, followed by a process in which the manganese oxides oxidize Cr_xFe_1–x(OH)₃ and get reduced back to Mn(II). The redox cycling of manganese can continuously drive Cr(VI) generation from Cr_xFe_1–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 Cr_xFe_1–x(OH)₃. These rates are affected by pH and the Fe/Cr ratio in Cr_xFe_1–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 Cr_xFe_1–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.

中文翻译：

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选自Cr的Cr（VI）生成_X的Fe _{1- X}（OH）₃的Mn（II）氧化上Cr的表面诱导_X的Fe _{1- X}（OH）₃

Cr（III）–Fe（III）氢氧化物（Cr _x Fe _{1– x}（OH）₃）是在修复和自然过程中通过Fe（II）还原Cr（VI）的常见产物。但是，这种固定化的Cr（III）的再氧化会严重损害水质，并可能破坏修复工作。在这里，我们研究了在Mn（II）存在下，在有氧和pH中性条件下，Cr（III）从Cr _x Fe _{1– x}（OH）₃氧化的可能性。尽管Mn（II）均被溶解氧氧化很慢，但Cr _x Fe _{1– x}（OH）₃表面催化Mn（II）氧化以快速形成氧化锰，然后进行氧化锰将Cr _x Fe _{1- x}（OH）₃氧化并还原为Mn（II）的过程。只要有溶解氧供应，锰的氧化还原循环就可以连续地驱动Cr _x Fe _{1– x}（OH）₃生成Cr（VI）。Cr（VI）的总生产速率受Mn（II）氧化成锰氧化物的速率以及随后在Cr _x Fe _{1– x}（OH）_3中氧化Cr（III）的速率控制。这些速率受pH和Cr _x中的Fe / Cr比的影响Fe _{1– x}（OH）₃。X射线衍射图谱和SEM图像表明，仅锰锰矿（Mn ₃ O ₄）仅是Mn（II）氧化中的主要锰氧化物，Fe或Cr氢氧化物的存在促进了Mn（III）氢氧化物（如白云母）的生产和锰矿）。我们的发现表明，当在有氧环境中被Mn（II）氧化的氧化Mn物种催化时，Cr（VI）可以在几天之内从Cr _x Fe _{1– x}（OH）₃天然生成。该过程可能在池塘，河口和河流的浅层沉积物中铬的氧化和迁移中起重要作用。