The Effect Mechnisms Of Mn(?) On The Transformation Of Soil Manganese Oxides

Manganese oxides,including oxides and oxyhydroxides,are ubiquitous in soils and sediments.Due to their low point zero charge,high surface reactivity,and high redox potential,they affect the oxidation and/or catalytic oxidation of valence-variable elements?such as As?III?,Cr?III?,Se?IV?,Pu?IV?,Fe????and some organic matter,and further influence the migration,transformation,bioavailability,and toxic of the related elements in soils and natural water systems.The reactivity of different manganese oxides to metal ions and organic matter is significantly different.In this study,high-valence layer structural manganese oxide?birnessite?,high-valence tunnel structural manganese oxide?cryptomelane?,and low-valence layer structural manganese oxide?feitknechtite?were chosen as the research subject.The formation,transformation,and related influence factors of these manganese oxides were investigated by combining elements analysis,X-ray diffraction?XRD?,transmission electron microscope?TEM/SAED?,high resolution transmission electron microscopy?HRTEM?,scanning electron microscope?SEM?,fourier transform infrared?FTIR?,and X-ray absorption spectroscopy?XAS?,and so on.The results provide new insights into understanding the geochemical processes of the abiotic formation and the transformation pathway of low valence Mn oxides and high valence ones in the environment.The main results are listed as following:1.The change of birnessite layer symmetry.Birnessite,the most common type of Mn oxides in soils,is the precursor to many other manganese oxides.?1?Hexagonal layer Birnessite?Hex LayBir??e.g.,d-MnO₂ and acid birnessite?transforms to orthogonal layer Birnessite?OrthLayBir?after reaction with various molar ratios of aqueous Mn???to solid Mn at various pH.The chemical compositions of the20-d products ofd-MnO₂ and acid birnessite are Na_0.21MnO_1.87·1.04H₂O and Na_0.35MnO_1.88·0.23H₂O,respectively.The Mn AOS of the transformation products are3.53 and 3.41,respectively,which are close to those reported previously for Orth LayBir.The Orth LayBir derived fromd-MnO₂ has higher crystallinity compared to that originating from acid birnessite,though the latter was processed at a much higher pH?p H13 vs.pH 9?.The estimated crystallite thickness along the c axis is¹6 nm for the final transformation product ofd-MnO₂ at pH 9,much larger than that?⁸ nm?of the acid birnessite transformation product at p H 13.The linear combination fitting analysis of the transformation products with EXAFS spectra of pured-MnO₂/acid birnessite and triclinic birnessite showed that equivalently about 44.5%ofd-MnO₂ transformed into OrthLayBir and about 89.0%of acid birnessite transformed into OrthLayBir.As the reaction progressed,spherical microcrystallites ofd-MnO₂ grew into an aggregation of larger lamellar crystals,flower-like of acid birnessite also grew into plates.?2?Since the complete transformation ofd-MnO₂ layer symmetry from hexagonal to orthogonal needs a lower pH and Mn???concentration than that of acid birnessite?pH 9vs.pH 13,10%vs.24%[Mn???]/[Mn]?,the transformation ofd-MnO₂ is more facilitated than that of acid birnessite during reaction with Mn???in an alkaline medium.This difference can be ascribed to the smaller size ofd-MnO₂ relative to acid birnessite,which may permit a more efficient growth of OrthLayBir particles.Acid birnessite has much lower SSA thand-Mn O₂.A larger number of stacked layers of acid birnessite may stabilize the hexagonal structure,making it more difficult to transform into OrthLayBir.?3?The results of transformation experiments conducted at[Mn???]/[Mn]of 0%to15%and pHs of 7 to 9,indicate that the concentration of Mn???and pH jointly determine the transformation of birnessite layer symmetry from hexagonal to orthogonal,and their magnitudes affect the extent and rate of the transformation.Whend-MnO₂ treated with 5mM Mn???,with pH increased from 7 to 9,the amount of formed feitknechtite gradually decreased and Orth LayBir appeared,indicative of the promotive effect of higher pH on the formation of OrthLayBir.At pH 8,d-Mn O₂ remained unchanged at the lower Mn???concentration?[Mn???]/[Mn]?2%?.It converted to Orth LayBir when Mn???concentration was increased to[Mn???]/[Mn]=5%.Further increasing Mn???concentration at this pH?[Mn???]/[Mn]=10%?,feitknechtite was obtained.Mn???commonly adsorbs on vacant sites as a tridentate corner-sharing complex.At a low Mn???concentration,each vacant site is probably capped by only one adsorbed Mn???.Each Mn???reacts with adjacent Mn?IV?along the b axis through electron transfer,i.e.,a comproportionation reaction,to produce two Mn?III?,situated in the layer and above the vacant site,respectively.Then the Mn?III?above the vacant site migrates into the vacant site.However,at a high Mn???concentration,two Mn???ions may sorb below and above a vacant site,respectively,with a minor fraction of Mn???adsorbed on the edge sites.The presence of Mn???or the produced Mn?III?cations on both sides of a vacant layer site prevents the migration of the Mn?III?into the octahedral layer because of electrostatic repulsion.This situation may favor the formation of separated Mn?III?phases at the surface in an oxic system or even a complete transformation of birnessite to Mn?III?phases.2.Transformation of“c-disordered”H⁺-birnessite to todorokite Synthesis of“c-disordered”H⁺-birnessites with different average manganese oxidation states?AOS?was performed by controlling the MnO₄^-/Mn²⁺ratio in low-concentrated NaOH or KOH media.Further transformation to todorokite was conducted under reflux conditions.When the Mn AOS values of Na-H-birnessite increase from 3.58 to 3.74,the rate and extent of the transformation sharply decrease.Na⁺pre-exchange,i.e.to form Na-H-birnessite,greatly enhances transformation into todorokite,whereas K⁺pre-exchange,i.e.to form K-H-birnessite,inhibits the transformation.This is because the interlayer K⁺of birnessite cannot be completely exchanged with Mg²⁺,which restrains the formation of tunnel“walls”with 1 nm in length.“c-disordered”H⁺-birnessite without pre-exchange treatment contains lower levels of Na/K and is preferably transformed into ramsdellite with a smaller 1×2 tunnel structure rather than todorokite.Mn AOS,together with the content and type of interlayer metal ions,plays a crucial role in the transformation of“c-disordered”H⁺-birnessites with hexagonal symmetry into todorokite.3.Reductive transformation of cryptomelane by aqueous Mn???to manganite At pH 7 under N₂ atmosphere,Cry-Na with high crystallinity remained unchanged at the lower Mn???concentration.It converted to manganite and a little of groutite when Mn???concentration was increased.Compared to that under anoxic conditions,the transformation of cryptomelane to manganite was substantially compromised under oxic conditions.This may be because that initial a little of Mn???was consumed by dissolved O₂,weakening the reduction of cryptomelane.Under N₂ atmosphere,when Cry-Na with weak crystallinity was treated with Mn???,feitknechtite was appeared.This may be because that a little?-MnO₂ was included in Cry-Na with weak crystallinity,and the?-MnO₂ was reduced to feitknechtite by Mn???.With the reaction time increased,the characteristic peaks of feitknechtite were disappeared,and that of manganite and groutite were appeared.4.Transformation of feitknechtite to birnessite through disproportionation Feitknechtite was synthesized by homogeneous precipitation.Feitknechtite keeps stable under neutral and alkaline conditions?pH?7.5?,but transforms into layer structural birnessite?MnO₂?at pH<6 through the disproportionation reaction 2 MnOOH+2 H⁺?MnO₂+2 H₂O+Mn²⁺.Feitknechtite remains the sheet structure during the transformation process,suggesting a solid phase transformation.The disproportionation rate of feitknechtite increases with decreasing pH?6-2?and accordingly the formation of birnessite favors at lower pH.These results provide new insights into understanding the abiotic formation pathway of the common high valence Mn oxides,such as birnessite,and the transformation relationship between low valence Mn oxides and high valence ones in the environment.