Schwertmannite (Sch) is considered as an effective remover of Chromium (Cr) due to its strong affinity for toxic Cr species. Since the instability of Sch, the environmental fate of Cr deserves attention during the transformation of Sch into a more stable crystalline phase. The ubiquitous manganese(II) (Mn(II)) probably affects the transformation of Sch and thus the environmental fate of Cr. Therefore, this study investigated the impact of Mn(II) on the transformation of Cr-absorbed Sch (Cr-Sch) and the associated behavior of SO and Cr. We revealed that the transformation products of Cr-Sch at pH 3.0 and 7.0 were goethite and Sch, respectively. The presence of Mn(II) weakened the crystallinity of the transformation products, and the trend was positively correlated with the concentration of Mn(II). However, Mn(II) changed the transformation products of Cr-Sch from hematite to goethite at pH 10.0. Mn(II) replaced Fe(III) in the mineral structures or formed Mn-O complexes with surface hydroxyl groups (-OH), thereby affecting the transformation pathways of Sch. The presence of Mn(II) enhanced the immobilization of Cr on minerals at pH 3.0 and 7.0. Sch is likely to provide an channel for electron transfer between Mn(II) and Cr(VI), which promotes the reduction of Cr(VI). Meanwhile, Mn(Ⅱ) induced more -OH production on the surface of secondary minerals, which played an important role in increasing the Cr fixation. In addition, part of the Mn(Ⅱ) was oxidized to Mn(Ⅲ)/Mn(Ⅳ) at pH 3.0 and pH 7.0. This study helps to predict the role of Mn(II) in the transformations of Cr-Sch in environments and design remediation strategies for Cr contamination.

中文翻译：

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Mn(II) 对吸附 Cr 的施威特曼石转变的影响：矿物相变和元素命运

施威特曼石 (Sch) 被认为是铬 (Cr) 的有效去除剂，因为它对有毒铬物质具有很强的亲和力。由于 Sch 不稳定，在 Sch 转变为更稳定的晶相过程中 Cr 的环境命运值得关注。普遍存在的锰 (II) (Mn(II)) 可能会影响 Sch 的转化，从而影响 Cr 的环境归宿。因此，本研究研究了 Mn(II) 对 Cr-吸附 Sch (Cr-Sch) 转化的影响以及 SO 和 Cr 的相关行为。我们发现 Cr-Sch 在 pH 3.0 和 7.0 时的转化产物分别是针铁矿和 Sch。 Mn(II)的存在削弱了转变产物的结晶度，且趋势与Mn(II)的浓度呈正相关。然而，在 pH 10.0 时，Mn(II) 将 Cr-Sch 的转化产物从赤铁矿转变为针铁矿。 Mn(II)取代矿物结构中的Fe(III)或与表面羟基(-OH)形成Mn-O配合物，从而影响Sch的转化途径。在 pH 3.0 和 7.0 下，Mn(II) 的存在增强了 Cr 在矿物质上的固定。 Sch可能为Mn(II)和Cr(VI)之间的电子转移提供通道，从而促进Cr(VI)的还原。同时，Mn(Ⅱ)诱导次生矿物表面产生更多的-OH，这对增加Cr的固定起到了重要作用。此外，在pH 3.0和pH 7.0时，部分Mn(Ⅱ)被氧化为Mn(Ⅲ)/Mn(Ⅳ)。这项研究有助于预测 Mn(II) 在环境中 Cr-Sch 转化中的作用，并设计 Cr 污染的修复策略。