The journey of toluene to complete mineralization via heat-activated peroxydisulfate in water: intermediates analyses, CO2 monitoring, and carbon mass balance

Our study has thoroughly investigated the complete mineralization of toluene in water via heat-activated peroxydisulfate (PDS) by: (1) monitoring concentrations/peak areas of various intermediates and CO2 throughout the reaction period and (2) identifying water-soluble and methanol-soluble intermediates, including trimers, dimers, and organo-sulfur compounds, via non-target screening using high-resolution mass spectrometry. Increased temperature and PDS dosage enhanced toluene removal/mineralization kinetics and increased the rate/extent of benzaldehyde formation and its further transformation. Artificial groundwater and phosphate buffer minimally impacted toluene removal but significantly decreased benzaldehyde formation, indicating a shift in transformation pathways. The stoichiometric PDS dose (18 mM at 40 °C) was adequate to completely mineralize toluene (1 mM), with < 10% PDS needed to transform toluene to intermediates. Toluene transformation to intermediates occurred in 47 h (kobs,toluene = 0.594 h-1) whereas 564 h were required for complete mineralization (kobs,CO2 = 0.0038 h-1). O2 accumulated once mineralization neared completion. A carbon mass balance, including quantification of nine intermediates and CO2 throughout the transformation period, showed that unquantified/unknown intermediates (including yellowish-white precipitates) reached as high as 80% of total carbon before transformation to CO2. Possible toluene transformation pathways via hydroxylation, sulfate addition, and oxidative coupling are proposed.