Rhodochrosite Oxidation by Dissolved Oxygen and the Formation of Mn Oxide Product: The Impact of Goethite as a Foreign Solid Substrate

Rhodochrosite conversion to Mn (oxyhydr)­oxides
significantly affects
the fate and transport of various substances in the environment. We
examined rhodochrosite oxidation by dissolved oxygen and the oxidation
product formation with an emphasis on the effects of goethite substrate.
Without goethite, rhodochrosite oxidation was slow as no detectable
change was observed for 28 d with microscopic and spectroscopic analyses,
except a minor change in X-ray diffraction. Interestingly, by contrast,
it was greatly accelerated and completed in 7 d in the presence of
goethite, resulting in the heteroepitaxial growth of groutite (α-MnOOH)-like
Mn oxides on the goethite (α-FeOOH) tip surfaces. The formation
of this secondary Mn oxide likely induced the acidification of the
microenvironment in the vicinity of rhodochrosite particles and thereby
promoted their dissolution. Subsequently, their oxidative conversion
to the Mn oxide was expedited by the surface catalyzed Mn­(II) oxidation
on the goethite tip. Our results revealed that goethite as a foreign
substrate imparts a decisive control on not only the rate but also
the type of the reaction product of rhodochrosite oxidation. This
study presents a new insight into the geochemical roles of foreign
particles on the dynamics of redox-sensitive solid phases in the environment.

菱锰矿（Rhodochrosite）向锰（氧氢）氧化物（Mn (oxyhydr)oxides）的转化，会显著影响环境中多种物质的归趋与迁移行为。本研究针对溶解氧介导的菱锰矿氧化过程及氧化产物生成规律展开探究，重点考察针铁矿（goethite）基质的调控效应。
在无针铁矿存在的条件下，菱锰矿氧化速率极为缓慢：通过显微与光谱分析，28天内未观测到可检测的显著变化，仅X射线衍射（X-ray diffraction）出现微弱变化。
值得注意的是，与无针铁矿的体系形成鲜明对比，当体系中存在针铁矿时，菱锰矿氧化被大幅加速并可在7天内完成，最终在针铁矿（α-FeOOH）的尖端表面形成类groutite（α-MnOOH）型锰氧化物的异质外延生长结构。
这类次生锰氧化物的形成，大概率会诱发菱锰矿颗粒周边微环境的酸化，进而促进矿物溶解；随后，针铁矿表面催化的Mn(II)氧化过程，进一步加快了菱锰矿向锰氧化物的氧化转化。
本研究结果表明，针铁矿作为外源基质，不仅对菱锰矿氧化的反应速率起到决定性调控作用，同时也决定了反应产物的类型。本研究为理解外源颗粒对环境中氧化还原敏感固相动态过程的地球化学作用提供了全新视角。