Where is iron in erionite? A multidisciplinary study on fibrous erionite-Na from Jersey (Nevada, USA)

Our plain-English summary of Where is iron in erionite? A multidisciplinary study on fibrous erionite-Na from Jersey (Nevada, USA), by A.F. Gualtieri, N.B. Gandolfi, S. Pollastri, et al (Scientific Reports, 2016).

The study

• Authors: A.F. Gualtieri, N.B. Gandolfi, S. Pollastri, K. Pollok, F. Langenhorst
• Journal: Scientific Reports
• Published: 2016;6:37981
• Cited by: 26 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Fibrous erionite is a mineral fibre of great concern but to date mechanisms by which it induces cyto- and geno-toxic damage, and especially the role of iron associated to this zeolite species, remain poorly understood. One of the reasons is that we still don't know exactly where iron is in natural erionite. This work is focused on fibrous erionite-Na from Jersey (Nevada, USA) and attempts to draw a general model of occurrence of iron in erionite and relationship with toxicity mechanisms. It was found that iron is present as 6-fold coordinated Fe3+ not part of the zeolite structure. The heterogeneous nature of the sample was revealed as receptacle of different iron-bearing impurities (amorphous iron-rich nanoparticles, micro-particles of iron oxides/hydroxides, and flakes of nontronite). If iron is not part of the structure, its role should be considered irrelevant for erionite toxicity, and other factors like biopersistence should be invoked. An alternative perspective to the proposed model is that iron rich nano-particles and nontronite dissolve in the intracellular acidic environment, leaving a residue of iron atoms at specific surface sites anchored to the windows of the zeolite channels. These sites may be active later as low nuclearity groups.

In plain English

This laboratory study examined one natural sample of fibrous erionite-Na, a fibrous zeolite mineral, from Jersey, Nevada (USA), to find out where the iron in it is located. The authors used synchrotron X-ray powder diffraction, electron microscopy with chemical analysis, and X-ray photoelectron spectroscopy. They also drew on their earlier spectroscopy work on the same sample, which had found iron only as Fe3+ in six-fold coordination. Analysis of the crystal structure found no evidence of iron inside the erionite structure, either in its framework or in its internal cavities. Instead, iron was found in impurities: iron-rich nanoparticles and larger particles on the fibre surfaces, and flakes of an iron-bearing clay called nontronite. Surface analysis attributed 30% of the surface iron signal to Fe3+ and 70% to Fe3+OOH. The authors concluded that the fibres contain no structural iron, and that the role of iron in erionite toxicity should be reconsidered, with other factors such as biopersistence invoked. They also proposed an alternative: some impurities might dissolve inside cells, leaving iron attached at fibre surfaces. The methods were mineral analyses only, with no tests on cells, animals or people.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory mineralogical characterization of a single mineral sample (synchrotron X-ray powder diffraction with Rietveld structure refinement, TEM, FEG/SEM with EDS, XPS), interpreted together with the authors' earlier Mössbauer and X-ray absorption spectroscopy of the same sample
• Who was studied: No human or animal subjects. One natural sample of fibrous erionite-Na from Jersey, Nevada (USA), containing less than 1 wt% clinoptilolite as an impurity detectable by X-ray diffraction
• Exposure: Not an exposure study. The material analysed was fibrous erionite-Na. Its calculated chemical formula listed iron (Fe3+, 0.29 per formula unit) separately from the other cations because its location was uncertain
• Main results:
  • Structure refinement of the erionite-Na sample, using data collected at and away from the iron absorption edge, found no evidence of iron in the zeolite framework or in the extraframework cavities, despite several refinement strategies
  • TEM showed amorphous iron-oxide/hydroxide nanoparticles on the fibre surfaces, about 10 nm in diameter or larger (around 50 nm)
  • FEG/SEM showed micrometric particle clusters at the fibre surfaces that were enriched in iron compared with the fibre bulk
  • Electron diffraction identified micro- to nano-metric flakes associated with the fibres as nontronite, an iron-bearing clay. The flakes were enriched in iron, and Mg was concentrated in them
  • XPS analysis of surface iron attributed 30% of the Fe 2p3/2 signal to Fe3+ and 70% to Fe3+OOH
  • Attempts to locate magnesium in the erionite structure failed. The authors attributed the minor Mg to clay impurities
  • The authors' earlier X-ray absorption and Mössbauer spectroscopy of the same sample (reported in a separate paper, Pollastri et al.) indicated that iron was present only as Fe3+ in an octahedral (6-fold) environment
• The authors' conclusion: The authors concluded that the Jersey erionite fibres contain no structural Fe3+, and that the iron is associated with iron-rich impurities, mostly at the fibre surface. They stated that, if iron is not part of the structure, its role in toxicity should be reconsidered and other factors such as biopersistence invoked. As an alternative, they proposed that iron-rich nanoparticles and nontronite could dissolve in acidic conditions inside cells, leaving iron atoms anchored at surface sites of the fibre that might become active

Topics

Medical terms in this study, explained in our Lexicon: erionite, Zeolite, biopersistence.

Related studies

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