Mechanically induced self-propagating reactions: Analysis of reactive substrates and degradation of aromatic sulfonic pollutants

Abstract

Mechanochemical reactions have been identified as a valuable alternative to conventional methodologies for the degradation of toxic pollutants as well as for their abatement in contaminated matrices. This paper discusses the application of the mechanochemical technique to the degradation of sulfonic acids in a contaminated matrix. The degradation of the pollutant compound was carried out by taking advantage of combustive reactions on a suitable reactive system ignited under mechanical treatment conditions. Two systems have been investigated as possible reactive substrates. The first one was a Mg–SiO 2 powder mixture while the second system was a Ca–SiO 2 powder mixture. Milling trials performed under different mechanical processing conditions allowed one to characterise the reactivity of these chemical systems, which basically undergo a reduction/oxidation reaction involving the formation of MgO and Si phases when the Mg–SiO 2 system is considered and CaO and Si phases when the Ca–SiO 2 system is employed, respectively. The systematic change of the stoichiometric ratios Mg:SiO 2 and Ca:SiO 2 permitted to identify the minimum Mg or Ca content necessary for the ignition of the combustive reactions. The experimental runs performed with such systems have shown a greater effectiveness of the Mg–SiO 2 because of less energy inputs required to ignite a combustion. For this reason the Mg–SiO 2 has been considered as a reactive substrate in the following trials. Since the SiO 2 amount in stoichiometric excess may be regarded as inert phase, it was substituted with a different phase consisting of the matrix contaminated by sulfonic acids. This aspect permitted to ignite a combustive reaction with the minimum possible content of Mg–SiO 2 reacting mixture. The chemical analyses performed after the combustive reaction proved the complete removal of the sulfonic acid from the contaminated matrix.