Morphological Features and Sorption Performance of Materials Based on Birnessite Exposed to Various Reductive Conditions

Arseniy Portnyagin,Nina Didenko,Alexey Golikov,Eduard Tokar,Vitaliy Mayorov,Andrey Egorin,Dmitry Mashtalyar,Valentine Avramenko,Tatiana Sokol’Nitskaya

doi:10.3390/colloids2040070

Abstract

The article is devoted to the evolution of structural, morphological, and sorption characteristics of layered manganese oxide (birnessite) under various conditions close to the real operating regime of the sorbents for radioactive waste processing. To identify the phase composition in the birnessites, we implemented XRD analysis, while SEM and temperature-programmed reduction (TPR) were used to study morphological and redox features of the materials, respectively. Structural changes after various kinds of treatment of birnessites were tracked using low temperature nitrogen sorption. Sorption characteristics were assessed under static and in dynamic conditions on the efficiency of Sr2+ removal from simulated seawater. TPR combined with kinetic analysis revealed the decrease of particle sizes in the birnessites after repeated use in sorption-regeneration cycle and reduction with hydrazine. Despite the fact that the porous structure of the materials remains preserved, the surface morphology of birnessite changes drastically depending on the reducing agent. Hydrazine treatment increases the sorption performance of the birnessite followed by degradation of mechanical properties, thus, preventing such sorbent from repeated use. Kinetic analysis of TPR allows quantifying differences in morphology and porous structure of manganese oxide materials. The specific surface area, amorphous surface structure, and accessibility of Mn+3 sites are the most important factors for birnessite sorption performance.

Highlights

Manganese oxide materials have attracted the researchers’ interest for a long time due to their exceptional functional properties
The structure, morphology, reduction, and sorption properties of birnessite-type sorbents exposed to real operating conditions as well as to reductive treatment with hydrazine were investigated
Consistent activation energies of birnessite reduction have been found for all oxide forms corresponding to three reduction stages (77.0, 86.3 and 113.6 kJ mol−1 for MnO2, Mn2O3 and Mn3O4, respectively), which makes the applied approach to kinetic analysis advantageous over the conventional Friedman method

Summary

Introduction

Manganese oxide materials have attracted the researchers’ interest for a long time due to their exceptional functional properties. The wide spectrum of functional properties the materials are characterized with arises from a number of oxidation states manganese can exist in. Manganese can be involved in redox interactions, in particular, it can reversibly change the oxidation state. In addition to versatile chemical properties, manganese oxides form plenty of structural variations [11,12,13]. A specific feature of birnessite-based systems consists of the reversible sorption of alkali and alkali-earth metals proceeding in the interlayer region [15,16,17]. The sorption properties of birnessite are mainly governed by its redox characteristics as well as structure and morphology

Methods

Discussion

Conclusion