Assessing the effect on the generation of environmentally persistent free radicals in hydrothermal carbonization of sewage sludge

Abstract

Environmentally persistent free radicals (EPFRs) have attracted increasing research interest in recent years. Herein, the generation of EPFRs during the hydrothermal carbonization of sewage sludge (SS) was studied. First, the surface morphology, functional groups, constituent elements and free radicals were characterized for a holistic description of the raw SS and the selected hydrochar obtained from hydrothermal carbonization of SS (SHC). Then, the impact of hydrothermal temperature, residence time and initial pH on the formation of EPFRs was explored in detail through the investigation of g-factors and intensities of EPFRs identified in SHC. The results have shown that the formation of EPFRs was affected by the factors mentioned above, in which the impact of temperature is the greatest. Two types of EPFRs were spotted in the hydrochar, oxygen-centered (O-centered) and carbon-centered (C-centered) EPFRs, which were caught in 120–150 °C and 260–280 °C, respectively. Moreover, the intensities of Electron Paramagnetic Resonance (EPR) signals enhanced with increasing hydrothermal temperature. Whereas, residence time and initial pH only affected the amount of EPFRs in a manner. Additionally, the half-life of the O-centered EPFRs and the C-centered EPFRs was determined as long as 160.45 days and 401.10 days, respectively, indicating that EPFRs are stable in a long time.

Formation of persistent free radicals in sludge biochar by hydrothermal carbonization

“…1d). Peaks at 1550-1670 cm −1 related to carboxyl, quinonyl, and aldehyde C=O are weakened with increasing temperature, which suggests the formation of oxygen-and carbon-centered radicals in the presence of transition metals (Zhu et al 2019). Last, the presence of aromatic rings is suggested by the presence of a C=C infrared peak at around 1440 cm −1 (Fig.…”

“…In particular, owing to the presence of organics and metals in sludge, radicals are expected to be formed during hydrothermal conversion, but this fact has not been established (Khachatryan et al 2011). Temperature and other factors were reported to influence radical formation during hydrothermal carbonization of sewage sludge (Zhu et al 2019), but little information on their influencing patterns and mechanisms is available for biochar.…”

Environmentally Persistent Free Radicals as Sources of POPs

“…Partial decomposition of these materials produces phenol, catechol, and hydroquinone, which are established EPFR precursors [22]. Sewage sludge offers abundant biomass containing high concentrations of organic compounds [57].…”

“…Thermal degradation of fossil fuels and biodiesels produces carbon-centered aromatic radicals [63], and so are bituminous coal [13]. During hydrochar processing and thermal carbonization of sewage sludge, EPFR can form even at relatively low temperatures of 120-280 o C [57]. Biochar produces oxygen-and carbon-centered EFPRs [64,68] even in the absence of transitions metals with yields as high as ~10 18 spins/g [21].…”

Process Water Recirculation During Hydrothermal Carbonization as a Promising Process Step Towards the Production of Nitrogen-Doped Carbonaceous Materials

“…Concerning proteins, changes in the regions between 1440 and 1400 as well as 1020-1220 cm −1 are visible, which are attributed to amine and amide structures. In addition, the peak at 559 cm −1 , which is assigned to the bending vibrations of the O = C-N group [57,58], indicates proteins. It is distinctly realised from the FT-IR spectra that the variety of [59].…”