La pollution par les colorants est un problème majeur de l’environnement dans de nombreux pays, et c’est pourquoi, cette thèse avait pour objectif principal de tester les potentialités de la birnessite pour dégrader des colorants organiques. Dans un premier temps, nous avons déterminé les conditions opératoires adéquates pour obtenir des couches minces de birnessite Mn7O13 pure, avec une bonne reproductibilité de synthèse. La bonne réactivité spontanée de ces couches pour la dégradation et la minéralisation des colorants phénothiaziniques (Bleu de Méthylène, Azure A, Azure B, Azure C, Thionine), dans des conditions douces, a été démontrée. Cette dégradation se fait via une réaction de N-déméthylation suivie par une étape d’adsorption de la thionine qui est ensuite minéralisée en ions ammonium et nitrate. Leur réactivité spontanée est plus efficace pour la dégradation et la minéralisation de l’Indigo Carmine. Sans aucune étape d’adsorption de composé intermédiaire, il est possible de réutiliser les échantillons sans apport d'énergie. L’activation électrochimique a amélioré considérablement leur efficacité pour la dégradation des colorants phénothiaziniques, par l’absence de l’étape d’adsorption de la thionine, par rapport à la réactivité spontanée. En plus de leurs faibles besoins énergétiques, les échantillons de birnessite apparaissent très prometteurs au vu de leur efficacité prouvée lors des interactions réalisées dans une cellule à circulation de solution en flux continu. La birnessite en couche mince est un matériau très intéressant pour le développement d'une méthode simple et écologique appliquée au traitement des eaux usées chargées en colorants. / Pollution of dyes is a great problem of the environment in many countries that is why the main objective of this thesis was to test the potential of birnessite to degrade organic dyes. Initially, we determined the adequate experimental conditions to obtain thin layers of pure birnessite Mn7O13, with good reproducibility of synthesis. The good spontaneous reactivity of these layers to degrade and mineralize phenothiazine dyes (Methylene Blue, Azure A, Azure B, Azure C, Thionine), under mild conditions, was demonstrated. This degradation is via a reaction of N-demethylation followed by an adsorption step of Thionine, which is then mineralized into ammonium and nitrate ions. Their spontaneous reactivity is more effective for the degradation and mineralization of Indigo Carmine. Without the adsorption step of an intermediate compound, it is possible to reuse these samples without energy supply. The electrochemical activation of the layers of birnessite has significantly improved their efficiency for the degradation of phenothiazine dyes, by the absence of the adsorption step of the Thionine, compared to their spontaneous reactivity. In addition to their low energy requirement, the thin layers of birnessite appear very promising because their proven efficacy in interactions carried out in a cell with a continuous flow of solution. The birnessite thin film is a very interesting material for the development of a simple and ecological method applied to the treatment of waste water loaded with dyes.

Ces travaux s’inscrivent dans le cadre du contrat d’objectifs AQUAL dont le but est la lutte contre les pollutions diffuses en milieu rural. Ils portent sur la compréhension des mécanismes de rétention du cuivre dans les sols viticoles. Afin de s’affranchir de la complexité des sols, sept constituants ont été sélectionnés (quartz,calcite, kaolinite, matière organique, goethite, ferrihydrite et birnessite). Le comportement du cuivre a été étudié sur les constituants seuls puis dans des mélanges de complexité croissante, afin d’évaluer l’implication de chacun d’entre eux, en fonction du pH et de la concentration en cuivre. L’influence du vieillissement et de la concentration sur la répartition géochimique du cuivre a ensuite été évaluée dans des sols synthétiques et naturels contaminés. L’ensemble des résultats a été obtenu grâce à la combinaison de plusieurs techniques permettant d’obtenir des informations complémentaires aux échelles macroscopique, microscopique (STEM-EDX et μ-XRF) et moléculaire (XANES).Les résultats montrent d’une part, que les principaux constituants responsables de la rétention du cuivre sont la matière organique et les oxydes de fer et de manganèse et d’autre part, que l’existence d’interactions organo-minérales influe significativement sur la rétention du cuivre. Lacombinaison d’expériences de sorption et d’extractions séquentielles a permis de mettre en évidence que les processus de rétention du cuivre se produisant dans le sol naturel peuvent être mimés dans un sol synthétique, ce dernier pouvant ainsi être utilisé comme modèle. / This work is a part of the AQUAL research program which aims to strive against diffuse pollution in rural environment. It deals with the understanding of copper retention mechanisms in vineyard soils.To overcome the soil complexity, seven constituents were selected (quartz, calcite, kaolinite,organic matter, goethite, ferrihydrite and birnessite). The copper behavior was firstly studied on the single constituents, then on increasing complexity constituents mixtures in order to assess their implication as a function of pH and copper concentration. The influence of aging time and copper concentration was evaluated on the copper geochemical partitioning in synthetic and natural soils.Conclusion could be drawn by combination of several techniques providing complementary information at macroscopic, microscopic (STEM-EDX, μ-XRF) and molecular (XANES) scales.The results showed on the one hand, that organic matter and iron and manganese oxides are the main constituents involved in copper retention and on the other hand, that the presence of organo-mineral interactions also significantly influences the copper retention. Sorption experiments combined with sequential extractions emphasized that copper retention processes occurring in a natural soil can be mimicked in a synthetic soil which could then be used as a model.

Manganese dioxide compounds are preferred curing agents for Polysulphide resins used as sealants in industry. These are required to have consistent setting characteristics and the investigation was initiated to characterise a number of proffered compounds of this type an to establish criteria by which an informed choice could be made of an optimum curing ages for a specific set of conditions. Several different chemical and physical properties were examined and critical parameters were established. A compound - sodium birnessite- was identified as a significant agent in the determination of curing properties. It was synthesised and its curing properties alone and in combination with other manganese dioxide compounds was evaluated. In an effort to find a specific reaction which might be used to characterise manganese dioxide curing agents it was decided to examine the classical reaction between these compounds and potassium chlorate. A literature search revealed major contradictions in the reported conditions under which potassium chlorate undergoes thermal decomposition as result of which it was decided to study the decomposition of potassium chlorate alone and in the presence of manganese dioxide and other catalysts. During this investigation a hitherto unreported high temperature structural change in potassium chlorate at 341° C was identified. The existence of this reversible change was confirmed by Powder Diffraction X-Ray analysis and an orthorhombic (near tetragonal) more open structure was assigned to it. It is suggested that the rapid decomposition of potassium chlorate in the solid state presence of catalysts is related to this change to a more open structure.

The goal of this project is the design of heterogeneous catalysts to facilitate the oxygen evolution reaction (OER). Considering the industrial feasibility for this reaction, first-row transition-metal-based materials are good candidates since they are cheap, abundant and possess variable oxidation states. However, most of them give only moderate catalytic activities, compared with noble-metal-based materials. To achieve efficient catalysts while maintaining low cost, it is important to discover and modify new systems based on the study of existing materials.In chapter 3 we present a study of the effect of surface reduction of birnessite on catalytic activity. A sample of birnessite was reduced by stirring with sodium dithionite, in which case the oxidation states of surface Mn decreased faster than those of inside Mn. We characterized the difference between the oxidation states of Mn of surface and inside (ΔAOS) and further investigate the effect of ΔAOS on catalysis. The catalytic activity was examined by reaction of birnessite samples with ceric ammonium nitrate, and O2 evolution was monitored using a dissolved oxygen probe with respect to time. The most reduced samples with ΔAOS of 0.15 was found to possess a turnover number (TON) of 36 mmol O2 per mol Mn, a value 10-fold higher than the unmodified sample. This result suggests oxidation state differential across layers aids the catalysis. In chapter 4, a more rigorous study is conducted by the examination of few-layer catalysts constructed by manganese oxide sheets with different oxidation states. We stacked low-AOS manganese oxide sheets with high-AOS manganese oxide sheets in various ordered combinations to obtain few-layer birnessite samples with non-uniform distribution of Mn(III). We found samples with more variation in AOS had a lower overpotential (~510 mV) in electrochemical OER catalysis than uniform stacks of the parent manganese oxide sheets (~750 mV for low-AOS sheets, >1000 mV for high-AOS sheets. The result indicates that the distribution of Mn(III) in stacking direction was the dominant factor for OER catalysis in birnessite and is more important than the overall Mn(III) content. We also found the band structures via scanning tunneling microscopy (STM) and provide an electronic-structure-based explanation of the observed activity. In chapter 5 an analogous strategy to that used in chapter 4 is applied to optimize lithium cobalt oxide (LCO) and lithium nickel oxide (LNO) layered catalysts. LCO and LNO contains various oxidation states (or spin states) of cobalt and nickel atoms. With alternatively stacking a high-AOS and a low-AOS cobalt (or nickel) oxide sheets one by one, the electrochemical OER catalytic activity of the obtained few layer LCO (or LNO) sample was enhanced. The results indicated that the structural feature of the alternating distribution of oxidation states affected not only the birnessite catalysts but also both cobalt and nickel oxide materials. In chapter 6 we incorporated both cobalt and nickel oxide sheets into layered heterostructured catalysts. We present findings that mixed transition metal oxide material K-CoxNiyO2 with alternating distribution of cobalt and nickel oxide layers showed enhanced activity mixed Ni-Co metal oxides with homogeneously distributed transition metals. The overpotential of the sample K-Co0.5Ni0.5O2 with alternating distribution of Co and Ni is 460 mV, 190 mV smaller than that of the sample with homogeneously distributed Co and Ni, even though they had a similar elemental composition. / Chemistry

The unifying goal of this work is the design of a heterogeneous catalyst that can facilitate the energy intensive oxygen evolution reaction (OER) in water splitting, considered one of the ‘holy grails’ in catalytic science. In order for this process to be industrially feasible, an efficient catalyst composed of first row transition metal based materials must be used. To accomplish this, existing systems must be studied in order to determine which properties are important and subsequent creation and modification of new systems based on lessons learned must be employed. Birnessite, a 2D layered manganese dioxide, comprises the majority of the effort. In the studies leading to this work, this material was primarily studied by mineralogists with the majority focusing on structural characterization. However, the material’s moderate activity toward performing the OER has revived interest. In this work, we look to determine important species, the role dopants play in activity, and the function of the interlayer and surface chemistry. From these findings, an enhanced, earth abundant OER catalyst will be designed. We determine that Mn3+ in the system plays and important role in producing a catalytic species with large oxygen production capabilities. By increasing the amount of Mn3+ in the system via a simple comproportionation reaction by exposing the Mn4+ to Mn2+ ion, we increase the total turnover of birnessite 50-fold. Additionally, the addition of dopants to the system , both within and between the sheets, has a positive effect on the activity of birnessite. In particular, incorporation of cobalt into the lattice of birnessite brings the activity level on par to that of precious metal oxide catalysts due to the cobalt offering a deeper electron acceptor than in birnessite alone. In conjunction with these studies, the role of the interlayer species and catalyst confinement has demonstrated the ability to greatly enhance a catalyst’s ability to perform the OER by ordering and orienting the water around the active confined catalyst. Combining confinement effects with the cobalt-doped birnessite sheets resulted in further enhancement in the material’s OER capabilities. This system mimics that of an enzyme where the cobalt-doped birnessite sheets facilitate greater electron-hole transfer to the interlayer active site, where the confinement effects enhance electron transfer kinetics and water organization for O-O bond formation. Additionally, metal chalcogenide OER catalysts were explored with mattagamite phase cobalt pertelluride. Through the work, we determine the formation of a Te-Co-O heterostructure as the catalytically active phase, where the metallic nature of the cobalt pertelluride facilitates charge mobility between the electrode and catalyst’s cobalt oxide surface functioning as the active OER species. / Chemistry

Dans les sédiments marins, la conversion de l'azote en diazote est traditionnellement supposée avoir lieu via le processus de nitrification-dénitrification bactérienne ou par l'oxydation anaérobie de l'ammonium (anammox). Mais, les faibles concentrations d'ammonium observées dans les sédiments riches en oxyde de manganèse suggèrent l'existence d'une voie alternative qui pourrait avoir lieu via un processus chimique. Parmi les oxydes de manganèse, la birnessite est la forme la plus existante dans les sols et les sédiments. C'est un oxyde lamellaire contenant à la fois du Mn (III) et du Mn (IV) dans sa structure et caractérisé par sa haute réactivité. Ce travail vise à mettre en évidence la nature de la réactivité de la Na-birnessite triclinique synthétisée par une méthode redox avec l'ammonium. Cette interaction était soupçonnée, dans des études antérieures, d'impliquer un mécanisme d'échange cationique seulement. La première partie de ce manuscrit est consacrée à la synthèse d'une birnessite pure via un processus redox. Le rapport Mn (VII): Mn (II) de 0,33 a été choisi et trois méthodes ont été utilisées consistant en un mélange rapide sous agitation vigoureuse de deux des trois réactifs et ensuite en l’ajout goutte à goutte du troisième. L'étude de paramètres réactionnels (ordre de mélange des réactifs, temps d'addition du troisième réactif, méthode de vieillissement, nature du cation utilisé et présence ou absence de l'oxygène dissous) a mis en évidence le rôle clé de l'oxygène dans la formation de la birnessite pure et un mécanisme de formation a été proposé. La deuxième partie a porté sur l'interaction de la Na-birnessite avec l'ammonium. L'étude de nombreux paramètres réactionnels (concentration initiale en ammonium, temps de contact, pH et température du milieu), en suivant à la fois les changements solides et liquides, a permis de démontrer que la birnessite n'agit pas seulement comme échangeuse cationique envers NH4+. En effet, l'analyse de surface réalisée par XPS a montré que les spectres N1s sont caractérisés par l'existence de deux environnements différents : un attribuable à NH4+ dans le domaine interfoliaire et le second à une espèce N chimisorbée. Des transformations structurales et chimiques ont été observées sur la birnessite avec un déficit de bilan massique d'azote. L’analyse des espèces en solution : NH4+, Na+, Mn2+, NO3- et NO2- et solides (état d'oxydation moyen de Mn, capacité d'échange cationique, teneur en azote solide et évolution de symétrie identifiée par DRX et FTIR) indique sans ambiguïté que NH4+ réagit chimiquement avec la birnessite / In marine and sediments, the conversion of combined nitrogen to dinitrogen is traditionally assumed to take place via the coupled bacterial nitrification-denitrification process or through the anaerobic ammonium oxidation (anammox). But, the low ammonium concentrations observed in the Mn-oxide rich sediments suggested the existence of an alternative pathway that might take place via a chemical process. Among manganese oxide, birnessite was found to be the most existing form in soils and sediments. It is a lamellar oxide containing both Mn(III) and Mn(IV) in its structure and characterized by its high reactivity. This work aims to highlight the nature of the reactivity of the triclinic Na-birnessite synthesized via a redox method with ammonium. This interaction was suspected, in previous studies, to involve a cationic exchange mechanism only. The first part of this manuscript is dedicated to the synthesis of a pure phase birnessite via a redox process. The Mn(VII):Mn(II) ratio of 0.33 was chosen and three methods were used consisting in a quick mixing under vigorous stirring of two of the three reagents and then on the dropwise addition of the third one. The study of numerous reaction parameters (the order of reagent mixing, the addition time of the third reagent, the aging method, the nature of the cation used and the dissolved oxygen effect) highlighted the key role played by oxygen in the formation of a pure birnessite and a mechanism of formation has been proposed. The second part focused on the interaction of Na-birnessite with ammonium. The study of numerous reaction parameters (the initial ammonium concentration, the contact time, the pH and the temperature of the medium) by monitoring both solid and liquid changes allowed us to demonstrate that birnessite is not only acting as a cationic exchanger toward NH4+. Indeed, the surface analysis performed by XPS showed that N1s spectra are characterized by the existence of two different environments: one assignable to an interlayer NH4+ and the second to a chemisorbed N-species. Structural and chemical transformations were observed on birnessite with nitrogen mass balance deficit. The monitoring of NH4+, Na+, Mn2+, NO3- and NO2- and solid changes (average oxidation state of Mn, cation exchange capacity, solid nitrogen content and symmetry evolution identified by XRD and FTIR) indicate unambiguously that NH4+ reacts chemically with the birnessite

Cette thèse a pour but de caractériser le fonctionnement de supercondensateurs asymétriques composés de dioxyde de manganèse de structure birnessite et de carbone activé dans différents électrolytes. Les électrolytes aqueux neutres à base de sels inorganiques montrent les meilleures performances électrochimiques. La nature et la structure des cations et des anions du sel semblent impacter les performances électrochimiques et la stabilité de la structure du matériau d’oxyde de manganèse. Lors de cyclage en milieu aqueux avec de large de fenêtre de tension de fonctionnement appliquée, un mécanisme de dégradation du dispositif a été avancé tenant compte de la nature des anions ou des cations des sels utilisés. Quelques voies de modification du matériau MnO2, afin d’améliorer ces performances électrochimiques, ont été étudiés. Des électrolytes non aqueux originaux ont été également caractérisés et plus particulièrement, les solvants « Deep Eutectic » à base de N-méthylacétamide et de sels de Lithium. Ces derniers semblent prometteurs comme électrolytes pour des applications en température sur carbone activé ou matériaux d’insertion tels que le ferrophosphate de lithium. Cependant ils semblent non adaptés aux oxydes de manganèse, mais donnent de bons résultats en cyclage avec le carbone activé. / The aim of this thesis was to investigate the performances of asymmetric supercapacitors based on manganese dioxide (birnessite) and activated carbon electrode materials using various electrolytes. From this work, it appears that neutral aqueous electrolytes containing inorganic salts have the best electrochemical performances. Furthermore, the nature and the structure of both ions (cations and anions) in solution seem to impact strongly the electrochemical performances of the supercapacitors, as well as, the MnO2’s structure stability and affinity. In the case of aqueous-based electrolyte, a device degradation mechanism has been proposed as a function of salt ions structure and nature to further understand the supercapacitor’s life-cycling when a large potential window is applied. Some novel synthesis ways and/or modifications were investigated to further improve the electrochemical properties of MnO2 material. Additionaly, original non-aqueous electrolytes has been also formulated and then characterized, particularly the ‘Deep Eutectic’ Solvents, based on the N-methylacetamide mixed with a lithium salt. However, these electrolytes don’t have a good affinity with manganese oxide-based materials. Interestingly, these Deep Eutectic Solvents show good cycling results with activated carbon. In fact, these electrolytes seem to be promising for high temperature energy storage applications, especially using activated carbon or insertion electrode material like the lithium ferrophosphate.

Many countries intend to achieve the safe management of their radioactive wastes through geological disposal. In addition, radioactively contaminated land is of global concern. To address both of these technical challenges it is imperative to understand the behaviour and subsequent migration of radionuclides in the subsurface. This thesis addresses uncertainties in the behaviour of the long-lived, risk-driving radionuclides U and Np in their most mobile and environmentally relevant oxidation states, U(VI) and Np(V). The formation the U(VI) colloidal nanoparticles is identified under the high pH, low carbonate conditions expected within the near field of a cementitious Geological Disposal Facility (GDF). XAS, SAXS, and TEM have been used to characterise these U(VI) colloids as 60-80 nm clusters of 1-2 nm clarkeite-like (Na uranate) nanoparticles, which are stable in cement leachate for a period of at least 5 years. The reactivity of these U(VI) colloids towards a range of mineral phases was investigated. In the presence of the common rock-forming minerals biotite, orthoclase, and quartz, only limited reactivity was observed with > 80 % of the U(VI) remaining in the filtered fraction after up to 5 years of reaction. In contact with cement, > 97 % of the U(VI) was removed from solution within 1 month. Reversibility studies, luminescence spectroscopy, and XAS suggest that a large portion of the cement associated U(VI) is in a uranophane-like coordination environment, likely incorporated into the C-S-H interlayers or as a stable surface precipitate. Together, this suggests that while U(VI) colloids could form in high pH (> 13) cement leachate, providing an additional pathway for migration, many of them are likely to be removed from suspension by the presence of solid cement, although 2.4 % (1.0 IμM) U(VI) remained in the filtered fraction even after 21 months of reaction. The interaction of aqueous U(VI) and Np(V) with a range of environmentally relevant Mn minerals has also been studied under circumneutral to alkaline conditions. Here, extensive (up to 99 %) uptake of U(VI) and Np(V) was observed in systems containing δ-Mn(IV)O2, triclinic (Na)-birnessite [Na0.5Mn(IV/III)2O4 · 1.5H2O], hausmannite [Mn(III/II)3O4], and rhodochrosite [Mn(II)CO3]. The uptake of U(VI) by δ-MnO2 and hausmannite was found to be partially irreversible, suggesting that these minerals could be particularly important in determining radionuclide migration. XAS indicated that both U(VI) and Np(V) formed edge-sharing bidentate adsorption complexes on the surface of δ-MnO2 and hausmannite, implying that these complexes are responsible for the observed reversibility. These complexes were also identified on triclinic (Na)-birnessite; however, after 1 month of reaction U(VI) was found to have migrated into the triclinic (Na)-birnessite interlayer, replacing Na+. Reaction with all three investigated Mn oxide phases was rapid, with equilibrium being reached within at least 2 weeks. However, whilst U(VI) and Np(V) were both extensively removed from solution in systems containing rhodochrosite, these reactions were much slower, with equilibrium taking up to 4 months to be established. XAS suggested that this was due to the formation of a U(VI) or Np(V) containing precipitate on the rhodochrosite surface.

Microbes exert extensive control on redox element cycles. They participate directly orindirectly in the concentration and fractionation of elements by influencing the partitioningbetween soluble and insoluble species. Putative microbially mediated manganese (Mn) oxidesof the birnessite-type, enriched in rare earth elements (REE) + yttrium (Y) were recentlyfound in the Ytterby mine, Sweden. A poorly crystalline birnessite-type phyllomanganate isregarded as the predominant initial phase formed during microbial Mn oxidation. Owing to ahigher specific surface area, this biomineral also enhances the known sorption property of Mnoxides with respect to heavy metals (e.g. REE) and therefore has considerable environmentalimpact.The concentration of REE + Y (2±0.5% of total mass, excluding oxygen, carbon and silicon)in the Ytterby Mn oxide deposit is among the highest ever observed in secondary precipitateswith Mn and/or iron. Sequential extraction provides evidence of a mineral structure where theREE+Y are firmly included, even at pH as low as 1.5. Concentration ratios of Mn oxideprecipitates to fracture water indicate a strong preference for the trivalent REE+Y overdivalent and monovalent metals. A culture independent molecular phylogenetic approach wasadopted as a first step to analyze the processes that microbes mediate in this environment andspecifically how the microbial communities interact with the Mn oxides. Plausible players inthe formation of the investigated birnessite-type Mn oxides are mainly found within theferromanganese genera Hyphomicrobium and Pedomicrobium and a newly identified YtterbyBacteroidetes cluster most closely related to the Terrimonas. Data also indicate that thedetected microorganisms are related to the environmental constraints of the site including lowconstant temperature (8°C), absence of light, high metal content and possibly proximity to theformer storage of petroleum products.

Characterization of a material’s surface, structural and physical properties is essential to understand its chemical reactivity. Control over these properties helps tailor a material to a particular application of interest. The research presented in this dissertation focuses on characterizing a synthetic method for carbon nanomaterials and the determination of structural properties of manganese oxides that contribute to its reactivity for environmental chemistry. In particular, one research effort was focused on the tuning of synthetic parameters towards the formation of carbon nanomaterials from gaseous methane and gaseous mixtures containing various mixtures of methane, argon and hydrogen. In a second research effort, photochemical and water oxidation chemistry were performed on the manganese oxide, birnessite, to aid in the remediation of arsenic from the environment and provide more options for alternative energy catalysts, respectively. With regard to the synthesis of novel carbonaceous materials, the irradiation of gaseous methane with ultrashort pulse laser irradiation showed the production of carbon nanospheres. Products were characterized with transmission electron microscopy (TEM), scanning electron microscopy (SEM), ultraviolet (UV) Raman spectroscopy, and infrared spectroscopy. Increasing the pressure of methane from 6.7 to 133.3 kPa showed an increase in the median diameter of the spheres from ~500 nm to 85 nm. Particles with non-spherical morphologies were observed by TEM at pressures of 101.3 kPa and higher. UV Raman spectroscopy revealed that the nanospheres were composed of sp2 and sp3 hybridized carbon atoms, based on the presence of the carbon D and T peaks. A 30% hydrogen content was determined from the red shift of the G peak and the presence of a high fluorescence background. Upon extending this work to mixtures of methane, argon, and hydrogen it was found that carbon nanomaterials with varying composition and morphology could be obtained. Upon mixing methane with other gases, the yield significantly dropped, causing flow conditions to be investigated as a method to increase product yield. Raman spectra of the product resulting from the irradiation of methane and argon indicated that increasing the argon content above 97% produced nanomaterial composed of hydrogenated amorphous carbon. In a second research effort, the effect of simulated solar radiation on the oxidation of arsenite [As(III)] to arsenate [As(V)] on the layered manganese oxide, birnessite, was investigated. Experiments were conducted where birnessite suspensions, under both anoxic and oxic conditions, were irradiated with simulated solar radiation in the presence of As(III) at pH 5, 7, and 9. The oxidation of As(III) in the presence of birnessite under simulated solar light irradiation occurred at a rate that was faster than in the absence of light at pH 5. At pH 7 and 9, As(V) production was significantly less than at pH 5 and the amount of As(V) production for a given reaction time was the same under dark and light conditions. The first order rate constant (kobs) for As(III) oxidation in the presence of light and in the dark at pH 5 were determined to be 0.07 and 0.04 h−1 , respectively. The As(V) product was released into solution along with Mn(II), with the latter product resulting from the reduction of Mn(IV) and/or Mn(III) during the As(III) oxidation process. Experimental results also showed no evidence that reactive oxygen species played a role in the As(III) oxidation process. Further research on the triclinic form of birnessite focused on its activation for water oxidation. Experiments were performed by converting triclinic birnessite to hexagonal birnessite in pH 3, 5, and 7 DI water with stirring for 18 hrs. Once the conversion was complete, the solid samples were characterized with TEM and x-ray photoelectron spectroscopy (XPS). The resulting hexagonal birnessites from experiment at pH 3, 5, and 7 possessed the same particle morphology and average surface oxidation states within 1% of each other. This observation supported the claim that upon transformation, Mn(III) within the sheet of triclinic birnessite migrated into the interlayer region of the resulting hexagonal birnessite. Furthermore, the migration of Mn(III) into the interlayer and formation of the hexagonal birnessite led to an increased chemical reactivity for water oxidation compared to the bulk. Electrochemical studies showed that the overpotential for water oxidation associated with the pH 3, 5, and 7 samples was 490, 510, and 570 mV, respectively. In another set of experiments, ceric ammonium nitrate was used to test birnessite for water oxidation reactivity. These experiments showed that the pH 3 birnessite produced the most O2 of all the samples, 8.5 mmol O2/mol Mn, which was ~6 times more than hexagonal birnessite which did not undergo post-synthesis exposure to low pH conditions. / Chemistry