Thermal inertization of amphibole asbestos modulates Fe topochemistry and surface reactivity

Abstract

This study discloses the morphological and chemical-structural modifications that occur during thermal degradation of amphibole asbestos. Low-iron tremolite and iron-rich crocidolite were heated at temperatures ranging from r.t. to 1200°C. Heating promoted a complex sequence of iron oxidation, migration and/or clustering and, finally, the formation of brittle fibrous pseudomorphs consisting of newly formed minerals and amorphous nanophases. The effects of the thermal modifications on toxicologically relevant asbestos reactivity were evaluated by quantifying carbon-and oxygen-centred, namely hydroxyl ( % OH), radicals. Heating did not alter carbon radicals, but largely affected oxygencentred radical yields. At low temperature, reactivity of both amphiboles decreased. At 1200°C, tremolite structural breakdown was achieved and the reactivity was further reduced by migration of reactive iron ions into the more stable TO 4 tetrahedra of the newly formed pyroxene(s). Differently, crocidolite breakdown at 1000°C induced the formation of hematite, Fe-rich pyroxene, cristobalite, and abundant amorphous material and restored radical reactivity. Our finding suggests that thermally treated asbestos and its breakdown products still share some toxicologically relevant properties with pristine fibre. Asbestos inertization studies should consider morphology and surface reactivity, beyond crystallinity, when proving that a thermally inactivated asbestos-containing material is safe.

Towards a detailed comprehension of the inertisation processes of amphibole asbestos:in situhigh-temperature behaviour of fibrous tremolite

“…Heating run 1 K. The only exception is represented by the absence of cristobalite among the reaction products of our experiment. We may hypothesize that, in the present experiment, excess SiO 2 occurs under the form of silica-rich amorphous material, which has been, in effect, observed by Pacella et al (2020) in the TEM images of the same sample thermally treated at 1473 K (see fig. 2 of Pacella et al, 2020).…”

“…Thermally induced inertisation is considered as the most promising approach for asbestos containing materials (ACMs) (Paolini et al, 2019), and this process has been recently proposed by the European Parliament as a preferable way to landfilling (European Parliament resolution of March 14, 2013). For a safe use of the products of thermal treatments of ACMs it is of obvious interest the detailed knowledge of the structural modification occurring in asbestos during heating as well as its correlation with their surface reactivity (Pacella et al, 2020). Under such aspect, the extensive knowledge of Fe topochemistry has a relevant importance from the toxicological point of view.…”

“…It has been proposed that not only the presence of Fe on the fibre surface, but also its structural coordination, represent the most relevant factors of asbestos toxicity (Fubini et al, 1995), together with other parameters as high aspect ratio (e.g., Stanton et al, 1981) and biopersistence (Van Oss et al, 1999). As a general scheme, thermal treatments induce in asbestos two main modifications: a) surface oxidation and partial de-hydroxylation occurs at lower T, often accompanied by cation migration, while preserving the crystalline phases of the pristine minerals; and b) at higher T a dramatic loss of crystallinity is followed by the appearance of newly formed mineral phase (Bloise et al, 2016;Pacella et al, 2020).…”

“…0.6 × 10 -5 K -1 ). In a recent paper (Pacella et al 2020) the products of riebeckite and tremolite fibres heated at several temperatures, even above those of the breakdown of their structures, have been analysed ex situ by XRPD. In the case of riebeckite, the results were fully consistent with those reported by Oberti et al (2018) for a prismatic sample confirming, once more, the potentiality of XRPD for the structural investigation of fibrous amphiboles (Ballirano et al, 2017).…”

“…In the present work we focus on the analysis in-situ of the structure modifications occurring in tremolite fibres upon heating using the same well characterized sample from Maryland analysed by Pacella et al (2010) and Pacella et al (2020). This sample, owing to its relatively high iron content (ca.…”