Preparative parameters, magnesium effects, and anion effects in the crystallization of birnessites

Na-birnessites (Na4Mn14O27. 9H(2)O) have been prepared by aging MnOx gels, whose composition can be represented as MnA.wMgA.xKMnO(4).yNaOH.zH(2)O (A is any of the anions of chloride, nitrate, sulfate, and acetate). The effects of aging time, aging temperature, basicity, the amount of water, the addition of Mg2+, the ratios of MnO4-/Mn2+, and types of anions on the crystallization of birnessite, the phase transformations of intermediate phases, the morphology, the ion-exchange properties, and the thermal stability have been investigated. XRD (X-ray diffraction), SEM (scanning electronic microscope), EDX (energy dispersive X-ray), IR (Infrared), DSC (differential scanning calorimetry), and TGA (thermogravimetric analysis) techniques have been used. The preparative parameters such as composition and temperature have been optimized. Three stages are observed in the crystallization process of birnessite on increasing the crystallization time: an induction period, a fast crystallization period, and a steady-state period, which are accompanied by phase transformations of prephase I (a phase related to hausmannite, (gamma-Mn3O4) and feitknechtite (beta-MnOOH) to birnessite. When the aging temperature is raised, the induction period is shortened, with both crystallization and phase transformations accelerated. However, high temperature can somehow fix the exchangeable ions in birnessite. Increasing the temperature to 85 degrees C or higher leads to nearly a complete loss of ion-exchange capacity under the experimental conditions used. Increasing the basicity either via increasing the amount of NaOH or via decreasing the amount of water has almost the same effect as increasing the aging temperature. Birnessites synthesized at higher temperatures and/or higher basicities exhibit better layer ordering and/or larger crystal size. The addition of Mg2+ can facilitate the oxidation of low-valent MnOx (as intermediates in the synthesis) and inhibit the transformation from birnessite to manganite (gamma-MnOOH). Mg2+ addition can help retain interlayer hydrates of birnessite but has almost no enhancement of framework thermal stability. However, a birnessite synthesized with Mg2+ added ic the initial stage is intolerant to acid, readily dissolvable in very dilute acid ([H+] < 10(-4) mol/L). A Mg2+-free birnessite is stable to acid of over 2 mol/L H+, forming H-birnessite. The molar ratio of MnO4-Mn2+ not only has an important influence on the induction period, the crystallization rate, and the phase transformations but also determines the phases initially presented and the purity of the final product. Fast crystallization rates of birnessite are achieved when the ratio is between 0.28 and 0.36. Four kinds of anions (nitrate, chloride, sulfate, and acetate) have been used in this study. All anions lead to formation of birnessite and the ion-exchange of birnessite to buserite, which is further converted to todorokite. The sequence of crystallization rate of birnessite for different anions is Ac- > Cl- > NO3- > SO42-. The anions are not present in the resultant birnessites or todorokites and show almost no influence on ion-exchange properties, morphology, and thermal stability.