Analysis of arsenic speciation in ultramafic rocks by sequential chemical extraction
Bibliographic record
Abstract
Arsenic-enriched ultramafic rocks from northern Vermont were studied using a sequential extraction process to determine arsenic speciation. Attention was brought to high arsenic levels in these rocks when groundwater with elevated arsenic (above the EPA MCL of 10 ppb) was discovered in bedrock wells in north-central Vermont. Previous work suggests that the arsenic source is metasomatized ultramafic rocks, specifically serpentinites and talc-carbonates found within Rowe-Hawley Belt, a tectonic assemblage of thrust slices in the suture zone of the Taconian Orogeny. Arsenic levels in these rocks are highly elevated, with concentrations up to 450 ppm in serpentinites and up to 1104 ppm in talc-carbonates. Arsenides, sulfides and other minerals frequently associated with arsenic were not detected in these samples, indicating that other minerals host the arsenic. Mineral sites identified as likely arsenic hosts included tetrahedral sites of antigorite or talc as As(V) substituted for Si(IV), octahedral sites of antigorite or talc as As(III) substituted for Mg(II), magnetite octahedral sites as As(III) substituted for Fe(III), magnesite crystallographic sites, and adsorption onto mineral surfaces. A 3-step sequential extraction was applied to determine arsenic speciation, using 1 M ammonium nitrate (pH = 4.7) to extract exchangeable ions, 0.11 M acetic acid (pH = 2.6) to dissolve carbonates, 35% nitric acid, if necessary, to dissolve remaining magnesite, and aqua regia to dissolve antigorite and talc. At each extraction step, ICP-MS, XRD and FTIR were used to determine geochemistry, mineral content, and bond configurations, respectively, in remaining pulps and extracted solutions. Additional data collected by collaborators at the University of Ottawa include XANES analysis of arsenic oxidation state and electron microprobe assessment of arsenic content in individual mineral grains. Results indicate that the majority of arsenic in these rocks is securely bound through crystallographic substitution. Sequential extraction results show that surface sorption does not play a significant role in arsenic content, except in one sample. Arsenic is instead bound within crystal structures of antigorite, talc, magnetite, and magnesite, and was not released in extraction until the host minerals dissolved. FTIR and XRD data indicate disorder in antigorite crystallographic structures; this may be the result of arsenic substitution in crystallographic sites sites. XANES results for two Vermont anitgorite samples indicate that arsenic is As(III), indicating that it is sited in octahedral sites of antigorite and/or magnetite. Electron microprobe data provided confirmation of arsenicenriched antigorite and magnetite in the same two samples. Future work using XANES and an electron microprobe on remaining samples is recommended to provide data on arsenic oxidation state and trace-mineral arsenic content.
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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 | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 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) | 0.001 | 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".