Syntheses, Transformations And Surface Chemistry Characteristics Of The Common Manganese Oxide Minerals In Soils

As essential source of Mn element for the nutrition of animals and plants, one of important adsorbents, redox agents, catalysts and carriers for environmental information in soils and aquatic system, Mn oxide minerals have been attracting more and more concerns of researchers on their resource and environmental properties. Investigations on the syntheses, transformations and properties of them could shed light on geochemistry behaviors of Mn, soil progress, and relations between soil quality and environments, they are also of great significance to promote exploration and utilization of Mn oxide minerals.Base on a review of research development on the common Mn oxide minerals in soils, this dissertation dealt with syntheses conditions, origination and transformation, and surface chemistry characteristics of them, using techniques of XRD, TEM/ED, HRTEM, IR, LR, TGA and surface acidity analysis. Structure identifications, formation mechanisms and their relations to the environments in soils and sediments were also discussed.1 .Fluxion velocity of reactive suspension and the rate of O2 flow significantly influenced the synthesis of birnessite. Vigorous stirring made the synthesis facile to produce pure birnessite. However the pretreatment of the reacting solutions by N2 and the reaction temperature had little effect on the synthesis. Increasing the reaction temperature led to a larger crystal size, better crystallinity and lower surface area. The adopted conditions for synthesis of pure birnessite were: NaOH to Mn molar ratio of 13.7, the O2 flow rate of 2 L/min, and oxidation for 5 h during vigorous stirring at normal temperature. The average composition of the synthesized pure birnessite was Nao 2sMnO2.o7'0.66H2O, and surface area of 38 m2/g.2. At 25 , the formation process of the birnessite by oxidation of Mn(OH)2 with 62 in alkali medium could be divided into four stages: 1) hausmannite and feitknechtite formation period, 2) transformation of hausmannite and feitknechtite into buserite period, 3) buserite crystal growing period, 4) transformation of buserite into birnessite period. The diffusion of O2 was the key step on the product during the course of the oxidation The pathways of the birnessite formation in this study might be:3 x Hydrothermal temperature and treatment time significantly influenced the synthesis of todorokite. Variation of system pressure caused by changing filling ratio of the autoclave had little effect on the synthesis. The crystal of the synthetic todorokite consisted of fibers, grew at 120?angles to form trilling patterns, which morphology and growth characteristics were the same to those of naturally occurring todorokite. Its average composition was Mgo i6MnC>2.o7'0.82H2O, and surface area of 35.5 m2/g. The formation mechanisms of todorikite may be similar under hydrothermal and surficial conditions, it takes longer time to form todorokite in surficial environment than that at a relative high temperature in hydrothermal environment.4> Single phase and well-crystallized todorokite was synthesized by heating and refluxing process from birnessite as a precursor. The average chemical composition of the synthesized todorokites by refluxing for 8 h and for 24 h was Mgo.i9MnO2.n(H2O)i 15 and MgonMnCh io(H2O)o88, respectively. Their surface area was 103.9 and 98.5 m2/g. The crystallinity of the todorokite increased and no other phase was produced with increasing refluxing period. The synthesized todorokites had the same morphologies and the similar structural characteristics with the natural todorokites and hydrothermally synthesized samples. The chemical compositions of the synthetic tordorokites by refluxing process were close to those of todorokites synthesized by hydrothermal process, except a higher average oxidation state of Mn for the former. The synthetic todorokite had strong surface Lewis acidity and weak Bronsted acidity. It could keep thermally stable up to 400 癈, above the temperature, it would gradually decompose, release O2 and transformed to spine phase eventually.5 > D...