Incorporation of Mo⁶⁺ in Ferrihydrite, Goethite, and Hematite

Abstract

Among all iron oxides, hematite (α-Fe2O3), goethite (α-FeOOH), and ferrihydrite (FeOOH⋅nH2O) are the most common mineral species. While immobilization of Mo6+ by surface adsorption on ferric oxides has been studied extensively, the mechanisms of incorporation in their structure have been researched little. The objective of this study was to investigate the relation between Mo content and its structural incorporation in hematite, goethite, and six-line ferrihydrite by a combination of X-ray absorption spectroscopy (XAS), powder X-ray diffraction (pXRD), and inductively-coupled plasma optical emission spectrometry (ICP-OES). Synthesized in the presence of Mo, the hematite, goethite, and six-line ferrihydrite phases incorporated up to 8.52, 0.03, and 17.49 wt. % Mo, respectively. For hematite and goethite, pXRD analyses did not indicate the presence of separate Mo phases. Refined unit-cell parameters correlated with increasing Mo concentration in hematite and goethite. The unit-cell parameters indicated an increase in structural disorder within both phases and, therefore, supported the structural incorporation of Mo in hematite and goethite. Analysis of pXRD measurements of Mo-bearing six-line ferrihydrites revealed small amounts of coprecipitated akaganéite. X-ray absorption near edge structure (XANES) measurements at the Mo L3-edge indicated a strong distortion of the MoO6 octahedra in all three phases. Fitting of extended X-ray absorption fine structure (EXAFS) spectra of the Mo K-edge supported the presence of such distorted octahedra in a coordination environment similar to the Fe position in the investigated specimen. Incorporation of Mo6+ at the Fe3+-position for both hematite and goethite resulted in the formation of one Fe vacancy in close proximity to the newly incorporated Mo6+ and, therefore, charge balance within the hematite and goethite structures.

Molybdenum(VI) Sequestration Mechanisms During Iron(II)-Induced Ferrihydrite Transformation

“…69 The presence of Mo−Fe 2 corner-sharing paths in all reacted solids indicate incorporation of Mo(VI) into neoformed Fe(III) minerals. 51 The presence of Mo−Fe 2 corner sharing, Mo−Mo bonding, and MoO 6 octahedra is consistent with bamfordite precipitation, 44,45 which was confirmed by TEM-SAED analysis of reacted solids from both pH 0 6.5 Cop-Mo experiments. Although EXAFS spectra for reacted solids from the pH 0 5.0 Cop-Mo experiments exhibit minor scattering paths consistent with bamfordite, the apparent absence of this phase during XRD and TEM-SAED analysis indicates it did not strongly contribute to Mo(VI) retention in these experiments.…”

“…33 Although inclusion of these Mo− into Fe(III) (oxyhydr)oxides. 51,52 Bamfordite exhibits Mo−O bonding at interatomic distances of 1.75 and 2.2 Å, plus an Mo− Mo bond at 3.38 Å and an Mo−Fe bond at 3.73 Å. 45,53 The reacted Cop-Mo solids exhibited two Mo−O paths at 1.73 and ∼2.2 Å, an Mo−Fe 2 path at ∼3.1 Å, and an Mo−Mo path at ∼3.3 Å. Additionally, reacted Cop-Mo solids from the pH 0 6.5 experiments exhibited an Mo−Fe 3 path at ∼3.8 Å that was not apparent in reacted Cop-Mo solids from the pH 0 5.0 experiments.…”

“…45 The deviation of corresponding CNs from the ideal bamfordite crystal structure and the presence of an Mo− Fe 2 path at ∼3.0−3.1 Å are consistent with multiple phases contributing to Mo retention. 51 Furthermore, relatively large errors for the fitting parameters of the Mo−O 2 , Mo−Mo, and Mo−Fe paths indicate that although these paths were necessary for the fit, their contributions were relatively minor in the bulk material due to multiple contributors to the EXAFS signal. TEM-SAED.…”

Coagulation removal of nickel (II) ions by ferric chloride: Efficiency and mechanism

“…Experimental data indicating the occurrence of chemisorption during the coprecipitation of various heavy metal ions with iron (III) hydroxide precipitate were obtained in works (Esmadi & Simm, 1995; Görn et al, 2021; Huang et al, 2021; Yang et al, 2019). Doping of particles of two‐line ferrihydrite, similar to the considered compounds, with cobalt ions during coprecipitation of cobalt ions upon alkalinization of iron (III) chloride solution was found with the use of Mössbauer spectroscopy (Stoliar et al, 2017).…”

Surface Coverage Simulation and 3D Plotting of Main Process Parameters for Molybdenum and Vanadium Adsorption onto Ferrihydrite

“…Of the anions that may constitute pollutants if their concentration in surface waters exceeds the limits imposed by environmental standards [ 3 , 4 ], the interactions between molybdenum (Mo) and vanadium (V) and ferrihydrite have so far been little studied [ 5 , 6 , 7 , 8 , 9 , 10 , 11 ]. Besides our previous studies [ 5 , 6 , 12 ], which present high resolution qualitative and quantitative experimental investigations of molybdenum and vanadium interactions with ferrihydrite, the literature contains mostly modelling/computational studies that can be used to simulate the transport and bioavailability of these anions in the environment [ 7 , 9 , 10 , 13 ].…”