Recrystallization of Manganite (γ-MnOOH) and Implications for Trace Element Cycling.

The recrystallization of Mn(III,IV) oxides is catalyzed by aqueous Mn(II) (Mn(II)) during (bio)geochemical Mn redox cycling. It is poorly understood how trace metals associated with Mn oxides (e.g., Ni) are cycled during such recrystallization. Here, we use X-ray absorption spectroscopy (XAS) to examine the speciation of Ni associated with Manganite (γ-Mn(III)OOH) suspensions in the presence or absence of Mn(II) under variable pH conditions (pH 5.5 and 7.5). In a second set of experiments, we used a Ni isotope tracer to quantify the amount of dissolved Ni that exchanges with Ni incorporated in the Manganite crystal structure during reactions in 1 mM Mn(II) and in Mn(II)-free solutions. XAS spectra show that Ni is initially sorbed on the Manganite mineral surface and is progressively incorporated into the mineral structure over time (13% after 51 days) even in the absence of dissolved Mn(II). The amount of Ni incorporation significantly increases to about 40% over a period of 51 days when Mn(II) is present in solution. Similarly, Mn(II) promotes Ni exchange between Ni-substituted Manganite and dissolved Ni(II), with around 30% of Ni exchanged at pH 7.5 over the duration of the experiment. No new mineral phases are detected following recrystallization as determined by X-ray diffraction and XAS. Our results reveal that Mn(II)-catalyzed mineral recrystallization partitions Ni between Mn oxides and aqueous fluids and can therefore affect Ni speciation and mobility in the environment.