Molecular Structure of Molybdate Adsorption on Goethite at pH 5–8: A Combined DFT + <i>U</i>, EXAFS, and <i>Ab Initio</i> XANES Study
Bibliographic record
Abstract
The adsorption of molybdate on goethite plays an important role in its reactivity, speciation, mobility, and bioavailability. However, the current understanding of the molybdate adsorption mechanism on the surface of goethite remains controversial. In this contribution, samples of molybdate-adsorbed goethite have been synthesized at various pH values (5–8) and a pH value of 7 with varying Fe/Mo molar ratios (10–100). All molybdate-adsorbed samples have similar X-ray absorption near-edge spectroscopy (XANES) features and have a pre-edge feature that lies in the middle between reference compounds containing tetrahedral MoO 4 (pH 7 Mo(VI) solution, Na 2 MoO 4 ·2H 2 O, and Fe 2 (MoO 4 ) 3 ) and distorted octahedral MoO 6 (pH 4 Mo(VI) solution and MoO 3 ) groups, suggesting the formation of fivefold coordinated molybdate oxyanions on the surface of goethite. This suggestion is further supported by combined density functional theory (DFT) + U calculations and ab initio XANES analyses, which also demonstrate that the corner-sharing monodentate mononuclear complex between molybdate oxyanions and goethite surfaces is energetically more stable than the corner-sharing bidentate binuclear complexation on the surface of goethite. These results shed new light on the mechanistic understanding of molybdate adsorption on goethite and define the nature of the bonding environment of adsorbed molybdate in this mineral structure.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".