Comparison of Arsenate and Molybdate Speciation in Hydrogenetic Ferromanganese Nodules

Abstract

Marine ferromanganese oxides contain a large amount of trace elements, such as arsenic (As) and molybdenum (Mo). However, the host phases of tetrahedral AsO 4 3− and MoO 4 2− oxyanions therein have not been clearly identified thus far. In this work, we explored the mineralogical components of hydrogenetic (HG) ferromanganese nodules and compared the distribution behaviors of As and Mo. The X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) analyses showed that the predominant manganese and iron phases were vernadite (δ-MnO 2 ) and ferrihydrite, respectively. According to the sequential extraction examination, both As and Mo were associated with the iron (oxyhydr)oxide phases. However, the XAS analyses indicated that As was retained by the ferrihydrite phase via double cornershared complexation, while Mo was preferentially adsorbed on δ-MnO 2 via edge-shared complexation. The immobilization of As and Mo by HG ferromanganese samples was attributed to specific chemical binding (ΔG chem ) rather than Coulombic interaction (ΔG coul ) as proposed in previous studies. The comparison of Mo XAS spectra before and after extraction revealed the unreliability of the sequential extraction approach to determine the host phase of trace elements as a result of the potential readsorption risk. The different distribution trends of As and Mo were due to their disparate intrinsic properties (e.g., averaged dissociation constants of conjugate acids) and the diverse properties (i.e., surface site densify, adsorption equilibrium constant, and crystalline structure) of ferrihydrite and δ-MnO 2 . These research findings would be significant for evaluating the geochemical behaviors and environmental fate of trace elements in marine systems.

Nature of High- and Low-Affinity Metal Surface Sites on Birnessite Nanosheets

“…Since its discovery in 1956 at Birness, Scotland, the layered manganese oxide birnessite has been recognized as the most abundant manganese oxide in the Earth surface environment and a key reactive mineral that regulates the geochemical cycling of trace metals. , Metal-containing birnessite has been documented in soils, mineral and rock coatings, sediments, and marine ferromanganese crusts and nodules. − Birnessite also features promising electrochemical energy storage performance and catalytic capabilities when fabricated under appropriate conditions, − and its high sorption capacity can be used in waste water treatment. − Birnessite is a phyllomanganate, meaning a layered compound composed of edge-sharing Mn­(IV)­O 6 octahedra, with ideal chemical formula MnO 2 . However, birnessite is never stoichiometric.…”

Density Functional Theory Modeling of the Oxidation Mechanism of Co(II) by Birnessite

“…In 1997, Drits et al , synthesized and determined the structure of well-crystallized hexagonal birnessite (HBi) using electron and X-ray diffraction and chemical analysis. Ever since, nanoparticulate hexagonal birnessite (δ-MnO 2 ) has been identified in soils, lake and marine sediments, and mineral surface coatings and rock varnish and is the main MnO 2 species produced by bacteria and fungi. − It is recognized to be the most abundant MnO 2 species on the Earth’s surface. ,,,,− …”

Chemical Information in the L3 X-ray Absorption Spectra of Molybdenum Compounds by High-Energy-Resolution Detection and Density Functional Theory

“…Molybdenum-based compounds find broad application in many fields of science and technology, including catalysis − and artificial water splitting, − energy storage and conversion devices, − and the field of transistor electronics. − Mo also plays an important role in environmental science − and in biology, being involved in the nitrogen reduction process. , X-ray absorption spectroscopy (XAS) is one of the most informative and versatile techniques used for characterization of the oxidation state, symmetry of the local environment, and interatomic distances and is applied widely to Mo compounds. Currently, spectroscopy at the K-edge dominates the spectroscopic investigations of these compounds because hard X-rays do not require vacuum conditions and allow use of different auxiliary devices of any design for in situ and operando studies.…”

“…Sensitivity of the white line shape to the crystal field, and, therefore, to the symmetry of the local environment, was successfully applied to the analysis of phase composition and local coordination in many Mo-based compounds. ,,− Since EXAFS studies at these edges are impossible due to the vicinity of the 2p 1/2 and 2p 3/2 levels, L 2,3 -edge spectroscopy cannot fully replace the spectroscopic investigations at the K-edge. Nice examples for a complementary K- and L-edge analysis can be found in refs , , , and − .…”