The dominant effect of black carbon on the chemical degradability of PCB1: Sequestration or/and catalysis.

Black carbon (BC) plays a crucial role in the migration, transformation, and remediation of hydrophobic organics (HOCs) in soil/sediment. Previous studies mainly focus on the sorption characteristic of BC, while the chemical degradability of HOCs, which is affected by sequestration and catalytic effects of BC, has not yet been systematically studied. In this study, the dechlorination process of 2-chlorobiphenyl (PCB1), adsorbed on BC prepared at different pyrolysis temperatures, by bimetal modified nano zero-valent iron (nZVI/Pd) was investigated. The results showed that, on the one hand, adsorption limited the dechlorination process. PCB1 in the resistant desorption state exhibited lower degradation efficiency than that in other adsorption state. On the other hand, the catalysis of high-temperature BC reduced the inhibition of adsorption on dechlorination to some extent. As the pyrolysis temperature rose from 400 °C to 900 °C, the degradation efficiency of adsorbed PCB1 within 48 h improved from 53.5% to 95.3%, and the rate constant (kobs) increased from 0.104 h-1 to 0.197 h-1. High-temperature BC promoted the electrons release of Fe0 and the generation of [H], and its conductivity improved the electron utilization efficiency so that the dechlorination reaction could proceed both on the surface of nZVI/Pd particles and BC, thereby promoting the dechlorination of PCB1. Therefore, adsorption effect dominated degradability of PCB1 sequestrated by low-temperature BC, while for high-temperature BC, synergistic catalytic effect played a dominant role. These findings indicate that reductive efficiency of nZVI should be systematically evaluated according to different types of BC in soil/sediment.