Comparison study on transformation of iron oxyhydroxides: Based on theoretical and experimental data

Ferrihydrite transformation impacted by coprecipitation of lignin: Inhibition or facilitation?

Lignin is a common soil organic matter that is present in soils, but its effect on the transformation of ferrihydrite (Fh) remains unclear. Organic matter is generally assumed to inhibit Fh transformation. However, lignin can reduce Fh to Fe(II), in which Fe(II)-catalyzed Fh transformation occurs. Herein, the effects of lignin on Fh transformation were investigated at 75°C as a function of the lignin/Fh mass ratio (0-0.2), pH (4-8) and aging time (0-96 hr). The results of Fh-lignin samples (mass ratios = 0.1) aged at different pH values showed that for Fh-lignin the time of Fh transformation into secondary crystalline minerals was significantly shortened at pH 6 when compared with pure Fh, and the Fe(II)-accelerated transformation of Fh was strongly dependent on pH. Under pH 6, at low lignin/Fh mass ratios (0.05-0.1), the time of secondary mineral formation decreased with increasing lignin content. For high lignosulfonate-content material (lignin:Fh = 0.2), Fh did not transform into secondary minerals, indicating that lignin content plays a major role in Fh transformation. In addition, lignin affected the pathway of Fh transformation by inhibiting goethite formation and facilitating hematite formation. The effect of coprecipitation of lignin on Fh transformation should be useful in understanding the complex iron and carbon cycles in a soil environment.

Application of community data to surface complexation modeling framework development: Iron oxide protolysis

This study presents a comprehensive community data-driven surface complexation modeling framework for simulating potentiometric titration of mineral surfaces. Compiled community data for ferrihydrite, goethite, hematite, and magnetite are fit to produce representative protolysis constants that can reproduce potentiometric titration data collected from multiple literature sources. Using this framework, the impact of surface complexation model type and surface site density (SSD) on the fit quality and protolysis constants can be readily evaluated. For example, the non-electrostatic model yielded a poor data fit compared to diffuse double layer model and constant capacitance models due to the absence of known surface charge effects. Regardless of the choice of iron oxide mineral, pK_a1 decreased with increasing SSD while the opposite tendency was observed for pK_a2. This newly developed framework demonstrates a method to reconcile community data-wide potentiometric titration data using Findable, Accessible, Interoperable, Reusable data principles to produce mineral protolysis constants that improve robustness of surface complexation models for applications in metal sorption and reactive transport modeling. The framework is readily expandable (as community data increase) and extensible (as the number of minerals increase). The framework provides a path forward for developing self-consistent, comprehensive, and updateable surface complexation databases for surface complexation and reactive transport modeling.

New Fe₂O₃-Clay@C Nanocomposite Anodes for Li-Ion Batteries Obtained by Facile Hydrothermal Processes

New iron-oxide-based anodes are prepared by an environmentally-friendly and low-cost route. The analysis of the composition, structure, and microstructure of the samples reveals the presence of a major hematite phase, which is accompanied by a certain concentration of an oxyhydroxide phase, which can act as a "lithium-reservoir". By using sodium alginate as a binder, the synthesized anodes display superior electrochemical response, i.e., high specific capacity values and high stability, not only versus Li but also versus a high voltage cathode in a full cell. From these bare materials, clay-supported anodes are further obtained using sepiolite and bentonite natural silicates. The electrochemical performance of such composites is improved, especially for the sepiolite-containing one treated at 400 °C. The thermal treatment at this temperature provides the optimal conditions for a synergic nano-architecture to develop between the clay and the hematite nanoparticles. High capacity values of ~2500 mA h g-1 after 30 cycles at 1 A g-1 and retentions close to 92% are obtained. Moreover, after 450 cycles at 2 A g-1 current rate, this composite electrode displays values as high as ~700 mA h g-1. These results are interpreted taking into account the interactions between the iron oxide nanoparticles and the sepiolite surface through hydrogen bonds. The electrochemical performance is not only dependent on the oxidation state and particle morphology, but the composition is revealed as a key feature.