Sulfite as a Green Co-milling Agent for Mechanochemical Destruction of Polychlorinated Aromatics: Working Mechanism and Structural Dependence

Mechanochemical destruction of obsolete halogenated persistent organic chemicals (especially polychlorinated aromatics) has been documented as a safe non-combustion technology, but the smooth implementation of such a system still calls for an appropriate and sustainable co-milling agent. As an example, we show that Na2SO3 as a co-milling agent not only achieves faster degradation rate for hexachlorobenzene (HCB) than calcium oxide (1.4-fold) and reduced iron powders (2.9-fold) with equal mass but also enables high dechlorination ratio (97.4%) after 4 h of milling in a planetary mill. With the input of mechanical energy, the mechanistic study suggests that sulfite salts suffer partial melt to generate SO32- species on fresh surfaces, which attack HCB molecules via the one-electron-transfer mechanism. Detection on intermediates and characterization on milled samples demonstrate that HCB undergoes dechlorination, polymerization, and hydrogenation and finally converts to amorphous and graphitic carbons. Correlation analysis indicates that the degradation reactivity of a specific polychlorinated aromatic compound in such a system is strongly dependent on its molecular structure (e.g., substituent groups and degree of chlorination). This work provides a new insight for utilizing sulfite for green disposal of halogenated wastes.