Sorption selectivity of birnessite particle edges: a d-PDF analysis of Cd(ii) and Pb(ii) sorption by δ-MnO₂and ferrihydrite

Abstract

Birnessite minerals (layer-type MnO2), which bear both internal (cation vacancies) and external (particle edges) metal sorption sites, are important sinks of contaminants in soils and sediments. Although the particle edges of birnessite minerals often dominate the total reactive surface area, especially in the case of nanoscale crystallites, the metal sorption reactivity of birnessite particle edges remains elusive. In this study, we investigated the sorption selectivity of birnessite particle edges by combining Cd(ii) and Pb(ii) adsorption isotherms at pH 5.5 with surface structural characterization by differential pair distribution function (d-PDF) analysis. We compared the sorption reactivity of δ-MnO2 to that of the nanomineral, 2-line ferrihydrite, which exhibits only external surface sites. Our results show that, whereas Cd(ii) and Pb(ii) both bind to birnessite layer vacancies, only Pb(ii) binds extensively to birnessite particle edges. For ferrihydrite, significant Pb(ii) adsorption to external sites was observed (roughly 20 mol%), whereas Cd(ii) sorption was negligible. These results are supported by bond valence calculations that show comparable degrees of saturation of oxygen atoms on birnessite and ferrihydrite particle edges. Therefore, we propose that the sorption selectivity of birnessite edges follows the same order of that reported previously for ferrihydrite: Ca(ii) < Cd(ii) < Ni(ii) < Zn(ii) < Cu(ii) < Pb(ii).

Crystal growth and aggregation in suspensions of δ-MnO2nanoparticles: implications for surface reactivity

“…This effect should be especially pronounced at the edge sites, for which the PZC value has been proposed to lie between pH 6 and 7. 12,68,69 The similar Na : Mn ratios across the B (pH 8) and C (pH 11) samples are consistent with the deprotonation of edge sites in these samples. Thus, the data suggest that the negative charge at the edge sites of δ-MnO 2 is mainly balanced by Na + under alkaline conditions.…”

“…Future studies to decouple the effects of nanoparticle aggregation or agglomeration, which may lower the reactivity of edge surface sites, and of solution pH, which enhances the adsorption of cations but diminishes the adsorption of oxyanions, are needed to elucidate the controls on solute-accessible surface area. 12,14,69 Cryo-TEM or HR-TEM techniques, which have been applied successfully to investigate the oriented aggregation of Fe oxide nanoparticles, 85,87,88 may provide valuable mechanistic information on the dynamics of δ-MnO 2 nanoparticle aggregation and agglomeration. Furthermore, variations in the vacancy content of the different δ-MnO 2 batches could be probed by collecting and modelling EXAFS data from trace metal-sorbed δ-MnO 2 in order to determine the local coordination environment of the sorbate.…”

Phosphate Sorption Speciation and Precipitation Mechanisms on Amorphous Aluminum Hydroxide

“…The d-PDF analysis has been demonstrated to be an effective approach to determine element binding geometries on poorly-crystalline minerals [15,16,39,40]. Li et al [39] used d-PDF analysis for the first time in anion (arsenate) adsorption on an Al oxide to characterize structures of sorption complexes.…”

Isolating the Electrocatalytic Activity of a Confined NiFe Motif within Zirconium Phosphate

“…One complication with understanding the intrinsic activity of confined electrocatalysts is the existence of several possible catalytic sites on the edges, surface, and within vacancies. [ 41,43–48 ] Therefore, isolating and disentangling the role of distinct active species found in intercalated catalysts is incredibly important in determining the origins of confinement‐based enhancement.…”