We studied the impacts of aqueous Mn(II) (1mM) on the sorption of Ni(II) (200μM) by hexagonal birnessite (0.1gL−1) at pH6.5 and 7.5 with batch experiments and XRD, ATR-FTIR and Ni K-edge EXAFS analyses. In the absence of Mn(II)aq, sorbed Ni(II) was coordinated predominantly as triple corner-sharing complexes at layer vacancies at both pH values. Introduction of Mn(II)aq into Ni(II)-birnessite suspensions at pH6.5 caused Ni(II) desorption and led to the formation of edge-sharing Ni(II) complexes. This was attributed to competitive displacement of Ni(II) from layer vacancies by either Mn(II) or by Mn(III) formed through interfacial Mn(II)-Mn(IV) comproportionation, and/or incorporation of Ni(II) into the birnessite lattice promoted by Mn(II)-catalyzed recrystallization of the sorbent. Similar to Mn(II)aq, the presence of HEPES or MES caused the formation of edge-sharing Ni(II) sorption complexes in Ni(II)-birnessite suspensions, which was attributed to partial reduction of the sorbent by the buffers. At pH7.5, interaction with aqueous Mn(II) caused reductive transformation of birnessite into secondary feitknechtite that incorporated Ni(II), enhancing removal of Ni(II) from solution. These results demonstrate that reductive alteration of phyllomanganates may significantly affect the speciation and solubility of Ni(II) in anoxic and suboxic environments.

We used a combination of batch studies and spectroscopic analyses to assess the impacts of aqueous Mn(II) on the solubility and speciation of Zn(II) in anoxic suspensions of hexagonal birnessite at pH 6.5 and 7.5. Introduction of aqueous Mn(II) into pre-equilibrated Zn(II)-birnessite suspensions leads to desorption of Zn(II) at pH 6.5, but enhances Zn(II) sorption at pH 7.5. XAS results show that Zn(II) adsorbs as tetrahedral and octahedral triple-corner-sharing complexes at layer vacancy sites when reacted with birnessite in the absence of Mn(II). Addition of aqueous Mn(II) causes no discernible change in Zn(II) surface speciation at pH 6.5, but triggers conversion of adsorbed Zn(II) into spinel Zn(II)(1-x)Mn(II)(x)Mn(III)(2)O-4 precipitates at pH 7.5. This conversion is driven by electron transfer from adsorbed Mn(II) to structural Mn(IV) generating Mn(III) surface species that coprecipitate with Zn(II) and Mn(II). Our results demonstrate substantial production of these reactive Mn(III) surface species within 30 min of contact of the birnessite substrate with aqueous Mn(II). Their importance as a control on the sorption and redox reactivity of Mn-oxides toward Zn(II) and other trace metal(loid)s in environments undergoing biogeochemical manganese redox cycling requires further study.