Abstract
This study aimed at investigating both the surface and bulk modifications occurring on fibrous erionite during leaching in a mimicked Gamble’s solution (MGS) at pH of 4.5 and T = 37 °C, up to one month of incubation. Samples were characterized by a multi-analytical approach: field-emission scanning electron microscopy (FE-SEM) was employed to investigate the morphological changes of both pristine and reacted fibres, inductively coupled plasma optical emission spectrometry (ICP-OES) was used to measure the concentration of the released cations; X-ray photoelectron spectroscopy (XPS) was exploited for highlighting possible modifications of surface chemistry; X-ray powder diffraction (XRPD) and high-resolution transmission electron microscopy (HR-TEM) were applied aiming to get information on the structural state of the fibres following the incubation. ICP results integrated with those obtained by both bulk- and surface-chemical characterization highlighted that erionite binds Na especially in the first 24 h of sample incubation in the MGS, following ion exchange with the extra framework cations, in particular Ca. Moreover, our new results show that the Na binding process caused structural modifications with the migration of Na toward the Ca2 site and redistribution of the cations within the erionite cage. TEM investigation pointed out that the interaction between erionite and MGS results in the formation of a new surface amorphous layer with an irregular lobate pattern on an earlier surface weathered layer. However, the silicate framework is not weakened by incubation in the MGS at acidic pH. In addition, on the basis of the Si release normalized to the mineral surface area, fibrous erionite resulted significantly more biodurable than amphibole asbestos. Notably, considering the primary role played by biodurability in inducing pathogenicity, this result certainly supports in vivo observations showing that erionite is much more tumorigenic than asbestos. Moreover, the ions released by erionite when immersed in MGS may trigger biological effects, such as those on lipid packing and membrane permeability. On this basis, we expect a regulatory definition that would provide protection from this carcinogenic fibre.
Highlights
Erionite is a naturally occurring zeolite often showing fibrous morphology, occurring in diagenetically or hydrothermally altered volcanic ash, tuffs, and sediments [1,2,3]
The preferential release of both Ca and K with respect to Mg was already observed by some of the authors during incubation of fibrous erionite samples in FeCl2 solution [64], and this finding is in agreement with their lower bond valences
We investigated the chemical structural modifications occurring on fibrous erionite during incubation in mimicked Gamble’s solution (MGS) at pH of 4.5 and T = 37 ◦C, up to 1 month of incubation
Summary
Erionite is a naturally occurring zeolite often showing fibrous morphology, occurring in diagenetically or hydrothermally altered volcanic ash, tuffs, and sediments [1,2,3]. It is hexagonal, space group P63/mmc [4] and has a general formula K2(Na,Ca0.5)8[Al10Si26O72]·30H2O5. The structure of erionite can be described on the basis of the stacking along the z-axis of layers made of six-membered rings of TO4 tetrahedra (T = Si, Al). Six H2O sites (OW7, OW8, OW9, OW10, OW11, and OW12) are located around the axis of the cage coordinating the cations [12]
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