Why does sludge-based hydochar activate peroxydisulfate to remove atrazine more efficiently than pyrochar?

Abstract Sludge-derived hydrochar (HC) and pyrochar (PC) by hydrothermal and pyrolytic treatments were compared their peroxydisulfate (PDS) activation capacities toward atrazine (ATZ) removal. The HC alone induced lower ATZ removal than PC alone. However, ATZ removal was significantly enhanced in HC/PDS system rather than in PC/PDS system. ATZ removal via adsorption was very low, and singlet oxygen (1O2) primarily contributed to its removal, independently on the PDS addition. Compared to HC, PC possessed stronger electron transfer capacity, exhibiting a higher ATZ removal. After PDS addition, humic substances (HS) in HC (36.3 ± 2.4 mg/g) played a decisive role in enhanced ATZ degradation: PDS excited HS to a triplet state (3HS*); then, PDS and 3HS* together yielded more 1O2. Meanwhile, 3HS* accelerated electron transfer on HC surface. Thus, more electrons were transferred from AZT to 1O2. However, function of small amount of HS in PC (1.5 ± 0.2 mg/g) could be ignored.