Mechanisms of Pb(II) Sorption on a Biogenic Manganese Oxide

Abstract

Macroscopic Pb(II) uptake experiments and Pb L3-edge extended X-ray absorption fine structure (EXAFS) spectroscopy were combined to examine the mechanisms of Pb(II) sequestration by a biogenic manganese oxide and its synthetic analogues, all of which are layer-type manganese oxides (phyllomanganates). Relatively fast Pb(II) sorption was observed, as well as extremely high sorption capacities, suggesting Pb incorporation into the structure of the oxides. EXAFS analysis revealed similar uptake mechanisms regardless of the specific nature of the phyllomanganate, electrolyte background, total Pb(II) loading, or equilibration time. One Pb-O and two Pb-Mn shells at distances of 2.30, 3.53, and 3.74 A, respectively, were found, as well as a linear relationship between Brunauer-Emmett-Teller (BET; i.e., external) specific surface area and maximum Pb(II) sorption that also encompassed data from previous work. Both observations support the existence of two bonding mechanisms in Pb(II) sorption: a triple-corner-sharing complex in the interlayers above/ below cationic sheet vacancies (N theoretical = 6), and a double-corner-sharing complex on particle edges at exposed singly coordinated -O(H) bonds (N theoretical = 2). General prevalence of external over internal sorption is predicted, but the two simultaneous sorption mechanisms can account for the widely noted high affinity of manganese oxides for Pb(ll) in natural environments.

Using FlFFF and aTEM to determine trace metal–nanoparticle associations in riverbed sediment

“…While some of the property changes are directly related to the increase of specific surface area, some are emerging from changes in the surface properties and chemistry of these surfaces at or below certain particle sizes. [16][17][18][19][20][21] In other words, surface and particle properties, even when normalised for surface area, often show changes in the nanoscale size range. Characteristics like these are key in understanding metal behaviour in the environment as they alter characteristics of the metal, such as bioavailability and toxicity.…”

Surface complexation of Pb(II) by hexagonal birnessite nanoparticles

“…In nature, birnessite is produced by bacteria and fungi, yielding a poorly-crystalline mineral with a strong scavenging affinity for the priority metal pollutant, Pb (Wilson et al, 2001;Dong et al, 2003;Tebo et al, 2004;Hochella et al, 2005;Beak et al, 2008). This high affinity has been attributed to significant negative structural charge arising from cation vacancy defects and to nanoparticle size, leading to significant surface area at lateral edges (Villalobos et al, 2005;Takahashi et al, 2007).…”

“…This triple corner-sharing (TCS) surface complex ( Fig. 1) has a characteristic Pb-nearest-Mn distance [d )] of 3.74 ± 0.05 Å (Matocha et al, 2001;Morin et al, 2001;Villalobos et al, 2005;Takahashi et al, 2007). At high Pb loading, another interlayer adsorbed species, a triple edge-sharing (TES) surface complex (Fig.…”

Zinc surface complexes on birnessite: A density functional theory study

“…Lastly, doubly-occupied Zn-TCS complexes (i.e., Zn IV -TCS + Zn IV -TCS; Zn VI -TCS + Zn VI -TCS; Zn IV -TCS + Zn VI -TCS) were considered by analogy with the double Zn occupancy found in chalcophanite. Structural distortion of a Mn(IV) vacancy site was also investigated to understand why interatomic distances determined by X-ray absorption spectroscopy often are not compatible with those determined by X-ray diffraction, with reconciliation between the two then requiring ad hoc displacement of the surface O positions around a vacancy (Manceau et al, 2002;Villalobos et al, 2005;Lanson et al, 2008).…”