ESEEM of industrial silica-bearing powders: reactivity of defects during wet processing in the ceramics production

Abstract

A study is undertaken to ascertain whether changes in the speciation of inorganic radicals are occurring during the ceramic industrial production that involves abundant silica powders as raw material. Industrial dusts were sampled in two ceramic firms, immediately after the wet mixing stage, performed with the aid of a relevant pressure. The dusts were then characterised by means of X-ray diffraction, analysis of the trace elements through chemical methods, granulometry, continuous-wave electron paramagnetic resonance (EPR) and pulsed electron spin echo envelope modulation (ESEEM) spectroscopies. The results of the characterisation point to a relevant change in the speciation of the two samples; namely, a prevailing contribution due to an inorganic radical different from that pertaining to pure quartz is pointed out. The combined interpretation of EPR and ESEEM data suggests the attribution of the main paramagnetic contribution to the A-centre in kaolinite, a constituent that is added to pure quartz at the initial stage of the ceramic production. In one of the two samples, a second weak EPR signal is attributed to the quartz's h(Al) species. By taking into account the relative quantities of quartz and kaolinite mixed in the two samples, and the relative abundances of the two radical species, we propose that the partial or complete suppression of the h(Al) species in favour of the A-centre of kaolinite has occurred. Although this change is apparently fostered by the mixture between quartz and another radical-bearing raw material, kaolinite, the suppression of the h(Al) centre of quartz is ascribed to the role played by the pressure and the wet environment during the industrial mixing procedure. This suppression provides a net change of radical speciation associated with quartz, when this phase is in contact with workers' respiratory system

Reaction with Water Vapor Defines Surface Reconstruction and Membranolytic Activity of Quartz Milled in Different Molecular Environments

“…[ 29 ] In an oxygen‐rich environment, the generation of peroxide species in quartz involves the reaction of the silyl (Si • ) radical with molecular oxygen (Figure 1, Reaction (5)). [ 7b,21b ] The reaction is commonly observed during thermal annealing of SiO 2 at T > 400 °C, and followed by the characteristic increase of peroxyl radical EPR signal intensity. [ 30 ] In our experimental settings, the activation energy for the formation of peroxyl radical could be provided by the mechanochemical process, but the oxygen‐free environment requires that oxygen is provided by a structural rearrangement of the quartz lattice during ball milling.…”

“…The distinct reactivity of electronic defects in fractured quartz has drawn the attention of scientists for years, and these centers were investigated either for their possible role in triggering cytotoxicity and pathogenicity, [ 19 ] for catalysis in polymeric chain reactions, [ 20 ] for mineralogic characterization, [ 18 ] industrial engineering of materials [ 21 ] and in prebiotic chemistry as a catalyst for amide bond formation. [ 22 ] Recently, surface radical species that are generated during quartz fracturing have been proposed as an abiotic source of oxidants in the anaerobic Archean era.…”