Arsenic Incorporation in Synthetic Struvite (NH<sub>4</sub>MgPO<sub>4</sub>·6H<sub>2</sub>O): A Synchrotron XAS and Single-Crystal EPR Study
Bibliographic record
Abstract
Struvite, a common biomineral and increasingly important fertilizer recovered from wastewater treatment plants, is capable of sequestering a wide range of heavy metals and metalloids, including arsenic. Inductively coupled plasma mass spectrometric (ICPMS) analyses and microbeam synchrotron X-ray fluororescence (μ-SXRF) mapping show that struvite formed under ambient conditions contains up to 547 ± 15 ppm As and that the uptake of As is controlled by pH. Synchrotron As K-edge XANES spectra measured at 20 K show that As(5+) is the predominant oxidation state in struvite, irrespective of Na2HAsO4·7H2O or NaAsO2 as the source for As. Modeling of As K-edge EXAFS data suggest that local structural distortion associated with the substitution of As(5+) for P(5+) in struvite reaches up to 3.75 Å. Single-crystal electron paramagnetic resonance (EPR) spectra of gamma-ray-irradiated struvite disclose five [AsO3](2-) radicals and one [AsO4](2-) radical. These arsenic-centered oxyradicals are all readily attributed to form from diamagnetic [AsO4](3-) precursors during irradiation, providing further support for exclusive incorporation and local structural expansion beyond the first shell of As(5+) at the P site in struvite.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.001 | 0.002 |
| Science and technology studies | 0.001 | 0.004 |
| Scholarly communication | 0.000 | 0.002 |
| Open science | 0.001 | 0.003 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.000 | 0.001 |
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 teacher head, 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".