Valorization of banana peels to highly effective catalysts for micropollutant degradation: Effects of hydrothermal carbonization conditions.

Abstract

In this study, hydrochars were prepared under various hydrothermal carbonization (HTC) conditions and pyrolyzed to fabricate hydropyrochars under the hypothesis that hydrochars are good precursors of pyrochars and that their reactivity and characteristics are controlled by the HTC conditions. For hydrochars and hydropyrochars, graphitization degree, defects, and graphitic/pyridinic/pyrrolic N increased, whereas the stacking degree and number of acidic O-containing groups decreased as the HTC temperature and time increased. The characteristics of the hydrochars significantly affected those of the hydropyrochars, whereas those of the pyrochar were clearly different. In the presence of peroxydisulfate (PDS), acetaminophen (ACT) removal was negligible for hydrochars + PDS, poor for pyrochar + PDS, and excellent for hydropyrochars + PDS. The best ACT removal was >99 % in 20 min with a 20 mg/L hydropyrochar from a hydrochar prepared via HTC at 250 ℃ for 3 h (HY250/03-PY) (PDS 0.05 mM, ACT 5 mg/L). For the hydropyrochars, possible reactive sites, such as the graphitic structure, CO, and pyridinic/graphitic-N, correlated well with the ACT removal rate. Despite the large differences in the performance and characteristics, the dominant reactive species (¹O₂) and ACT degradation pathways were similar in HY250/03-PY + PDS and pyrochar + PDS. However, HY250/03-PY was significantly superior to pyrochar in ¹O₂ generation and PDS adsorption. HY250/03-PY + PDS showed significant inhibition by coexisting ions and natural organic matter; however, most of these effects were overcome by simply increasing HY250/03-PY. These results strongly suggest that hydropyrochars with excellent reactivity and field application potential can be prepared by combining HTC control and pyrolysis.