Hydrothermal carbonization of sewage sludge for hydrochar valorization: Role of feedwater pH on sulfur removal and transformation

Adjusting the pH value is a useful method to improve the hydrochar properties during hydrothermal carbonization (HTC). The distribution of various sulfur species in hydrochar is considered as an important parameter affecting its further utilization. Nevertheless, the detailed effects of pH on the redistribution of different sulfur forms are still unknown. This study investigated the sulfur transformation pathways during HTC of sewage sludge with different feedwater pH (0-14) at 220 degrees C. Both acidic and alkaline feedwater facilitate the conversion of organic sulfur from the hydrochar into the stable species in aqueous products (i.e., sulfate-S). The latter especially leads to a decreased sulfur retention ratio of 31.97 % at pH=0 and 60.13 % at pH=14. Under acidic conditions, organic sulfur species such as sulfone-S and aliphatic-S undergo enhanced cyclization to form aromatic-S/thiophene-S in aqueous and hydrochar products; additionally, the oxidation of sulfone-S predominates the generation of sulfate-S in aqueous products at pH < 1. Under alkaline conditions, the aromatic-S/thiophene-S can be further oxidized into sulfate-S in aqueous products at pH>13. Furthermore, the interaction between water-soluble sulfate-S and hydrolytic intermediates (i.e., glucose, heterocyclic compounds) is intensified at pH=14, resulting in increased formation of sulfone-S and sulfite-S in aqueous products. The insights gained from this study can offer guidance for controlling sulfur distribution during HTC by adjusting the feedwater pH.