Thermochemistry of sulfur during pyrolysis and hydrothermal carbonization of sewage sludges

Sulfur (S) is an abundant and redox-active element in urban wastewater systems and plays a critical role in both the wastewater and sludge treatment processes. This study comparatively characterized the transformation of S and several closely associated metals (Cu, Zn, and Fe) during pyrolysis (250 to 750 °C) and hydrothermal carbonization (HTC, 150 to 275 °C) treatments of sewage sludge. S, Fe, Zn, and Cu K-edge X-ray absorption spectroscopy was applied to quantitatively evaluate the fate of S and contribution of different S species in regulating metal speciation. During pyrolysis, aliphatic-S and sulfonate were preferentially degraded at low temperature (below 350 °C) and sulfate was thermochemically reduced at temperature above 450 °C, while metal sulfides (up to 27%) and thiophenes (up to 70%) were increasingly formed. Similar to the pyrolysis process, metal sulfides (up to 40% at temperature above 200 °C) and thiophenes were formed during HTC. The degradation of thiols and organic sulfide, as well as sulfate reduction, released sulfide and strongly affected metal speciation. For example, almost all Cu and half of Zn were transformed into Cu-Fe- or Zn-Fe-sulfides during HTC, whereas they were partially desulfidized during pyrolysis. High abundance of reduced S species (S⁻¹ and S⁻²) in hydrochars may contribute to their strong reductive adsorption of Cr(VI). Results from this work reveal the thermochemical reactions driving the transformations of S and its associated metals during pyrolysis and HTC. The results provide fundamental knowledge for selecting thermochemical sludge treatment techniques for value-added applications of the products.