Effects Of Co And Ni On The Transformation Of Phyllo-into Tecto-manganate And Their Geochemical Behavior During Such Process

Mn oxides are in terrestrial and aquatic environments,such as soils,deserts,or sediments of oceans,lakes or rivers.In particular,Mn oxides can represent 40-50%of in deep-sea nodules.Natural Mn oxides are usually rich in transition metals,such as Co and Ni,and/or rare earth and radioactive elements that are present as sorbed or incorporated species.Reactivity of Mn oxides towards these trace metal elements arises from their occurrence as finely divided particles,their high layer charge deficits,and the coexistence of heterovalent Mn cations within their crystal structure.These oxides form two main families:layered and tunnel Mn oxides,the former（e.g.,birnessite,buserite,and vernadite）likely converting to the latter[e.g.,todorokite（3×3）,hollandite（2×2）and nsutite（both 1×2 and 1×1）]during soil formation or sediment diagenesis process.Co and Ni are two bioessential transition metals whose transport and sequestration are strongly mediated by natural Mn oxides.The mobility and fate,and the effect of these two metals during/on the phyllo-to tecto-manganate transformation process remain largely unknown,however.The present thesis thus investigated the transformation processes of synthetic layered Mn oxides containing Co and Ni,and of phyllomanganates present in natural Mn nodules,to tunnel Mn oxides.These transformation processes involved cation exchange(with a variety of cations:Mg²⁺,Ni²⁺,Ba²⁺,K⁺,and H⁺)and hydrothermal treatment,to mimic the naturally occurring conversion process.Both layered precursors and transformation products were characterized using X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FTIR）,high resolution transmission electron microscopy（HRTEM）,thermogravimetric analysis（TGA）,nitric acid treatment,X-ray photoelectron spectroscopy（XPS）,extended X-ray absorption fine structure（EXAFS）spectroscopy,and atomic pair distributed function（PDF）to reveal the mineralogy,elemental composition,morphology,and metals’local environment in samples.The significant results are the following:1)A series of Co-containing birnessite samples with up to 16.9%Co/（Co+Mn）molar ratios was synthesized.In these samples,most of Co（～80%）was structurally incorporated as the result of the isomorphic substitution of Mn in Mn O₂ octahedral layers.Specifically,introduction of Co into MnO₂ layers reduces the numbers of Jahn-Teller distorted Mn（Ⅲ）O₆octahedra,which is a key factor for the conversion of layered structures to tunnel ones.As a consequence,transformation of Co-rich birnessite to todorokite is hampered during reflux treatment,leading to the coexistence of a-disordered,or non-ideal 3×n,todorokite-like tectomanganates and of 9.6?asbolane-like phyllomanganate in the reflux products.These reflux products exhibit plate-like morphology,corresponding to non-ideal 3×n todorokite and asbolane,in addition to fibrous habits,that correspond to ideal todorokite.Overall,the isomorphic substitution of non Jahn-Teller distorted cations like Co（Ⅲ）into layered Mn oxides might enhance their stability and account for the persistence of phyllomanganates in ferromanganese deposits,and for the frequent prevalence of phyllomanganates over tectomanganates in soils and sediments.The present work also indicates that Co（Ⅲ） initially incorporated in birnessite layers is retained in the solid phase during the treatment process,thus resulting in a low Co（Ⅲ） mobility.EXAFS data indicate similar local environment for Co in both layered precursor and reflux product,Co being mainly located in the octahedral layers of 9.6?phyllomanganate and at non-edge sites in non-ideal todorokite.The transformation from Co-containing birnessite to non-ideal todorokite and asbolane-like layered structures preserves Co sequestration in Mn oxides.2)Co-containing vernadite was converted to hollandite after Ba²⁺-exchange and reflux treatment,whereas nsutite was formed if the reflux process was performed in acidic medium.The Mn average oxidation state（AOS）is noticeably higher in newly formed nsutite and,to a lower extent,hollandite than in the layered precursors.This evolution of Mn AOS contrasts with the Mn AOS stability observed during birnessite transformation into todorokite.Mn（Ⅲ） disproportionation occurring under low pH conditions thus appears to favor the formation of nsutite,whereas high contents of Mn（Ⅲ） are required for the formation of hollandite and todorokite.Vernadite transformation to hollandite is not significantly affected by the presence of Co in birnessite layers.During the transformation,Co is retained in the solid phase by substituting Mn site,and Ba²⁺cations are stabilized in the tunnels through their bonding to O^2- forming tunnel walls.Although the Co/Mn ratio is about constant during birnessite conversion to nsutite,a significant proportion（～19%）of Co was excluded from the structure during the transformation and sorbed at the surface of mineral particles.The results suggest that structural incorporation of Co favors the stability and formation of phyllomanganates and of tectomanganates with large tunnel sizes compared to tectomanganates with small tunnel sizes.3)At high Ni loading,Ni is mostly present as weakly bound Ni species,hydrated Ni（Ⅱ） and Ni（Ⅱ）（hydr）oxides,whereas Ni sorbed at layer vacancy sites or structurally incorporated are not detectable in layered precursors.In addition,the phyllomanganate to tectomanganate transformation is hampered,although the content of Mn（Ⅲ） is sufficient（～1/3）for this transformation to occur,owing to the kinetically favored polymerization of Ni（OH）₂ in the interlayers.With increasing Ni content the formation of asbolane,a 9.6 ?phyllomanganate with islands of metal hydroxides in the interlayers,is favored over that of todorokite.A nitric acid treatment,aiming at the dissolution of the island-like interlayer Ni（OH）₂ layer,was developed to allow an easy and unambiguous differentiation between asbolane and todorokite,which is unaffected by this treatment.Both compounds exhibit indeed similar periodicities and can be mistaken when using X-ray diffraction,despite contrasting intensity ratios of low-angle reflection（9.6,4.8,and 3.2?）.Ni（OH）₂ polymerization hampers the formation of todorokite-like tectomanganates and likely contributes to prevalence of phyllomanganates over tectomanganates in natural environments.Most Ni is retained during the reflux process,without obvious release,part of Ni（～20%）being structurally incorporated in the reflux products,enhancing the sequestration of Ni in Mn oxides.4)When exchanged only with Ni（Ⅱ）,Na-buserite does not convert to todorokite,whereas this transformation occurs when Na-buserite is contacted with a mixed solution of Ni（Ⅱ）and Mg（Ⅱ）with a proper molar ratio（Ni:Mg≤2:10）.In this case,the resulting todorokite contains much more 3×3 ideal tunnels compared to the product obtained from the Ni-free counterpart.In contrast,addition of Ni in the reflux medium of Mg-buserite leads to the formation of fibrous crystals,a morphology indicative of ideal todorokite structure,as for Ni-free experiments.Although the presence of Ni in the reflux medium hampers a complete transformation of the layered precursors,that are partly preserved during the reflux process,it also favors the formation of ideal todorokite crystals.Enrichment of Ni in the reflux products may result from the presence of Ni in the layered precursors or during the transformation process to tunnel structures.5)Phyllomanganates from Mn nodules do not convert to tectomanganates whatever the experimental conditions:heating,ions exchange,hydrothermal treatments,and rather preserve their initial layered structure.It thus seems that phyllomanganates’structural features（layer symmetry and content of Mn（Ⅲ）,isomorphic substitutions by foreign cations,nature of interlayer species）are not responsible for the prevalence/persistence of layered Mn oxides in the Mn nodules.Environmental conditions,and more especially temperature,have possibly enhanced this stability kinetically favoring the hydrolysis of interlayer cations over the migration of Mn（Ⅲ） from the octahedral layers.Intimate association of Mn oxides with clay minerals,iron oxides,organic matter,and free radicals present in oceanic environments may also have delayed/hampered tectomanganate formation,and held phyllomanganate stability for millions of years,leading to phyllomanganate prevalence in natural environments.