A Biomimetic Approach to the Chemical Inactivation of Chrysotile Fibres by Lichen Metabolites

Abstract

Some lichens were recently reported to modify the surface state of asbestos. Here we report some new insight on the physico-chemical modifications induced by natural chelators (lichen metabolites) on two asbestos samples collected in two different locations. A biomimetic approach was followed by reproducing in the laboratory the weathering effect of lichen metabolites. Norstictic, pulvinic and oxalic acid (0.005, 0.5 and 50 mM) were put in contact with chrysotile fibres, either in pure form (A) or intergrown with balangeroite, an iron-rich asbestiform phase (B). Mg and Si removal, measured by inductively coupled plasma atomic emission spectrometry (ICP-AES) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), reveals an incongruent dissolution for pure chrysotile (A), with Mg removal always exceeding that of Si, while chrysotile-balangeroite (B) follows a congruent dissolution pattern in all cases except in the presence of 50 mM oxalic acid. A much larger removal of Mg than Si in the solutions of 0.5 and 50 mM oxalic acid with chrysotile (A) suggests a structural collapse, which in the case of chrysotile-balangeroite (B) only occurs with 50 mM oxalic acid; in these cases both samples are converted into amorphous silica (as detected by X-ray diffraction (XRD)). Subsequent to incubation, some new phases (Fe(2)O(3), CaMg(CO(3))(2), Ca(C(2)O(4)) x H(2)O and Mg(C(2)O(4))2 x H(2)O), similar to those observed in the field, were detected by XRD and micro-Raman spectroscopy. The leaching effect of lichen metabolites also modifies the Fenton activity, a process widely correlated with asbestos pathogenicity: pure chrysotile (A) activity is reduced by 50 mM oxalic acid, while all lichen metabolites reduce the activity of chrysotile-balangeroite (B). The selective removal of poorly coordinated, highly reactive iron ions, evidenced by NO adsorption, accounts for the loss in Fenton activity. Such fibres were chemically close to the ones observed in the field. Chrysotile-rich rocks, colonised by lichens, could be exposed to a natural bioattenuation and considered as a transient environmental hazard.

New Detoxification Processes for Asbestos Fibers in the Environment

“…The U.S. EPA standard for waterborne asbestos fibers is calculated considering only fibers longer than 10 μm. In agreement with previous findings (Favero-Longo et al, 2005;Turci et al, 2007a), the long curled natural fibers of chrysotile ( Figure 2A) leached with Ox for 21 h ( Figure 2B) preserved the fibrous habit and fibers were morphologically unaltered, though some small debris were observed. Following power US for 21 h in pure Milli-Q water ( Figure 2C), the long fibers of chrysotile were strongly reduced in length, but fibrous habit was largely preserved and fiber lengths remained within the micrometric range.…”

“…The relative amount of Mg with respect to Si is a well-established approach to describe depletion of brucitic layer in chrysotile and investigate the congruence/incongruence of mineral dissolution (Favero-Longo et al, 2005;Turci et al, 2007a). X-ray fluorescence spectra of the two untreated chrysotiles and some treated samples were collected and the intensity of integrated Mg peak was normalized by the Si peak ( Figure 5).…”

The Iron‐Related Molecular Toxicity Mechanism of Synthetic Asbestos Nanofibres: A Model Study for High‐Aspect‐Ratio Nanoparticles

“…A linear relationship betweenof the considered form of asbestos. Taking into account the complexity of natural asbestos specimens, even within the same mineral species, [9] high-purity, nanometric, tailored synthetic fibres, which can be modified by one property at a time, need to be considered. Among the various asbestos forms, chrysotile, by far the most used and widespread, [10] is the closest in morphology to many HARNs, including CNTs, because of its flexibility and its tangled and curled fibrous form.…”

Lichen deterioration of asbestos and asbestiform minerals of serpentinite rocks in Western Alps

“…The incubation with oxalic acid a well known chelator able to leach divalent cations from the octahedral mineral layers (Thomassin et al, 1977;Turci et al, 2007) yields asbestos and asbestiform fibres similarly modified in the chemical composition to those observed in the field below the lichen thalli. Only in the case of chrysotile and carlosturanite in vein B, the chemical modification obtained in the laboratory is higher than that detected below C. vitellina, possibly reflecting the poor hyphal penetration observed within this vein type.…”

“…According to Turci et al (2007), the deterioration by lichen metabolites with acidic and chelating functions was mimicked by incubating the mineral fibres with the commercially available (Merck Pro analysis) oxalic acid 0.5 mM.…”