Formation of a vitreous phase at the surface of some commercial diatomaceous earth prevents the onset of oxidative stress effects.
To understand the effect of the commercial processing of diatomaceous earths (DEs) on their ultimate surface structure and potential toxicity, we investigated the influence of the industrial processing and the nature of the deposit. Two flux calcined specimens from different deposits, DE/1-FC and DE/2-FC, and the simply calcined sample DE/1-C, from the same deposit as DE/1-FC, were compared in both their bulk and their surface properties. X-ray diffraction (XRD) analysis in a heating chamber revealed the presence of cristobalite in all samples, more abundant on the flux calcined ones. The crystal lattice is probably imperfect, as the alpha-beta transition, visible by XRD in DE/1-FC and DE/2-FC, is not detected by differential scanning calorimetry. Progressive etching with HF solutions suggests that most of the crystalline phase is at the core and not at the outer region of the samples. The combined use of spectroscopic (UV-vis and IR) and calorimetric techniques (heat of adsorption of water as a measure of hydrophilicity) reveals that DE/1-FC and DE/2-FC particles have an external layer of glass, absent in DE/1-C, where iron impurities act as network-forming and sodium ions as modifier species, with few patches of a hydrophobic phase, the latter relatable to a heated pure silica phase. When tested on a macrophage cell line (MH-S) in comparison with appropriate positive and negative controls (an active and an inactive quartz dust, respectively), only DE/1-C exhibited a cell damage and activation similar to that of active quartz (measured by lactate dehydrogenase release, peroxidation of membrane lipids and synthesis of NO). It is likely that the presence of a vitreous phase mitigates or even eliminates the cellular responses of silica in DE.
Ghiazza
Mara
M
Interdepartmental Center G. Scansetti for Studies on Asbestos and Other Toxic Particulates, Universita degli Studi di Torino, 10125 Torino, Italy.
Gazzano
Elena
E
Bonelli
Barbara
B
Fenoglio
Ivana
I
Polimeni
Manuela
M
Ghigo
Dario
D
Garrone
Edoardo
E
Fubini
Bice
B
D000818:Animals; D002151:Calorimetry; D007692:Diatomaceous Earth / Q000633:toxicity*; D007770:L-Lactate Dehydrogenase / Q000378:metabolism; D015227:Lipid Peroxidation; D008264:Macrophages / Q000378:metabolism; D051379:Mice; D019001:Nitric Oxide Synthase / Q000378:metabolism; D018384:Oxidative Stress; D013056:Spectrophotometry, Ultraviolet; D013499:Surface Properties; D014874:Water Pollutants, Chemical / Q000633:toxicity*; D014961:X-Ray Diffraction
D014874:Water Pollutants, Chemical; D007692:Diatomaceous Earth; D007770:L-Lactate Dehydrogenase; D019001:Nitric Oxide Synthase