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)_aq) 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)_aq under variable pH conditions (pH 5.5 and 7.5). In a second set of experiments, we used a ⁶²Ni 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)_aq 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)_aq is present in solution. Similarly, Mn(II)_aq 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.