BackgroundTodorokite, a 3 × 3 tectomanganate, is one of three main manganese oxide minerals in marine nodules and can be used as an active MnO6 octahedral molecular sieve. The formation of todorokite is closely associated with the poorly crystalline phyllomanganates in nature. However, the effect of the preparative parameters on the transformation of “c-disordered” H+-birnessites, analogue to natural phyllomanganates, into todorokite has not yet been explored.ResultsSynthesis 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, using “c-disordered” H+-birnessites pre-exchanged with Na+ or K+ or not before exchange with Mg2+, was conducted under reflux conditions to investigate the effects of Mn AOS and interlayer cations. The results show that all of these “c-disordered” H+-birnessites exhibit hexagonal layer symmetry and can be transformed into todorokite to different extents. “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. 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. 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, indicating that a key process is Mn(III) species migration from layer into interlayer to form the tunnel structure during todorokite formation.ConclusionsStructural Mn(III), 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. This provides further explanation for the common occurrence of todorokite in the hydrothermal ocean environment, where is usually enriched in large metal ions such as Mg, Ca, Ni, Co and etc. These results have significant implications for exploring the origin and formation process of todorokite in various geochemical settings and promoting the practical application of todorokite in many fields.Graphical abstractXRD patterns of Mg2+-exchanged and reflux treatment products for the synthetic “c-disordered” H+-birnessites.
The
ability to predict coupled kinetic reactions of arsenic (As)
that occur at the manganese oxide–aqueous solution interface
is crucial for management and remediation of As-contaminated sites
worldwide. A quantitative understanding of the coupling between As
oxidation and adsorption/desorption kinetics will enable us to more
accurately predict the kinetic behavior of As. In this study, we developed
a novel quantitative model for the coupled kinetics of As adsorption/desorption/oxidation
on δ-MnO2. The kinetics of As(III) adsorption and
oxidation on δ-MnO2 was studied using a stirred-flow
method under varying influent As(III) concentrations. The model was
able to account for the variations in As concentration, different
binding behavior of As(III) and As(V), and flow rates. Our model quantitatively
shows that the coupled As adsorption/desorption/oxidation processes
are highly dependent on As mass loading rates on δ-MnO2. Our model suggests that the As(III) adsorption rate is much higher
than its oxidation rate. Both As(III) oxidation and subsequent As(V)
desorption may proceed with similar reaction rates and control the
overall reaction rates, although As(V) desorption rates may be affected
by As(V) re-adsorption. Our kinetic model contributes to prediction
of the dynamic behavior of As when manganese oxides are present.
The Mn average oxidation state (Mn AOS) of Mn oxides has a significant impact on their reactivity towards trace metals and organic contaminants via sorption, catalysis and oxidation processes.
Amorphous calcium sulfate (ACS) is intimately involved in the crystallization of calcium sulfate minerals, a major contributor to sulfur sequestration and a technological hindrance of scale formation, but its structural characteristics remain largely undiscovered. Here, we studied the structure of ACS nanoparticles captured under low CaSO 4 concentration conditions. The ACS nanoparticles have a proto-gypsum property ranging from shortrange to medium-range order, and water molecules are key structural constituents that are in a disordered state and 8-fold coordinated with Ca in the first coordination shell. More importantly, on a molecular scale, we give evidence concerning the energetic role of water molecules in promoting ions transport, molecule rearrangement, and development of a local coordination environment that facilitates structural evolution of ACS. This work reveals some important structural characteristics of ACS and has significance in providing a better understanding of the crystallization and biomineralization of water-bearing minerals.
■ MATERIALS AND METHODSSynthesis of Calcium Sulfate. 3.00 M CaCl 2 and 50.00 mM Na 2 SO 4 precursor solutions were prepared by dissolving
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