Experimental and theoretical study on the transformation of typical organic sulfur during hydrothermal carbonization of sludge

This study reports the migration and transformation of sulfur during the hydrothermal carbonization of sludge and two thermal unstable organic-S model compounds (aliphatic-S and aromatic-S) at 150 to 240 degrees C, focusing on the pathways based on the molecular level. The theoretical calculation indicates that the sulfur atom is the initial attacked position by nucleophile in the organic-S molecule, caused by its maximum contribution (i.e., 73.54 % for aliphatic-S) to the highest occupied molecular orbital (HOMO) and the minimum point of electrostatic po-tential (ESP, i.e.,-103.68 kJ/mol for aliphatic-S). This leads to the nucleophilic addition reaction of aliphatic-S and aromatic-S, especially at 210 degrees C, along with the generation of sulfoxide-S and sulfone-S. Compared with aromatic-S, the aliphatic-S is more inclined to oxidation because of its higher HOMO contribution and lower ESP. Due to the low bond dissociation energy of C-S of formed sulfoxide-S and sulfone-S, large of them (i.e., 47.51 % at 210 degrees C) transform to SO32-and SO42-ions via desulfurization and oxidation reactions, resulting in a low sulfur content in hydrochar. Also, the sulfoxide-S derived from aliphatic-S is more likely to decompose than other intermediates, together with the aliphatic-S, released as sulfur-containing odorants (especially for H2S, CH3SH and SO2). These findings indicate that fixing the aliphatic-S is an effectively method to control the emission of odorants.