Original Article

Environmental Geology

, Volume 52, Issue 7, pp 1365-1374

First online:

Arsenic adsorption on nanocrystalline goethite: the natural example of bolar earths from Mt Amiata (Central Italy)

  • A. ManasseAffiliated withDipartimento di Scienze della Terra, University of Siena
  • , C. VitiAffiliated withDipartimento di Scienze della Terra, University of Siena Email author 

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Bolar earths deposits from Mt Amiata (Central Italy) consist of nanosized pseudo-spherical goethite, with average crystal size of 10–15 nm (as determined by X-ray powder diffraction and transmission electron microscopy observations), possibly associated to amorphous silica and minor sheet silicates, quartz and feldspars. Chemical analyses revealed high As contents (up to 7.4 wt% As2O5), thus indicating the occurrence of a potentially dangerous contaminant. Arsenic doesn’t occur as a specific As phase, but it is strictly associated with goethite nanocrystals. Eh and pH measurements suggest that As occurs as arsenate anions (H2AsO 4 and HAsO 4 2− ), which are easily and strongly adsorbed to goethite surfaces. The high specific surface area, resulting from goethite nanosize, and the absence of competitive anions explain the extremely efficient adsorption of arsenate and the anomalously high As content in bolar earths. Overall physical/chemical data suggest stable arsenate adsorption, with very limited risk for As release to the environment.


Bolar earths Arsenic Adsorption Goethite nanocrystals Mt Amiata