Sequestration effect and mechanism of PCB1 by high-temperature black carbon

Black carbon (BC) is a substance that significantly affects the migration and transformation of hydrophobic organic compounds (HOCs) in soil/sediment. High-temperature BC is an important form of BC in the environment, and, currently, there is relatively little research on the influence of high-temperature BC on the sorption and the desorption behavior of HOCs and its mechanism. In this study, the sorption isotherms and TENAX-aided desorption kinetics of PCB1 by three typical high-temperature BCs (fly ash (FC), soot (SC), and high-temperature biochar (BC 900)) and a low-temperature biochar (BC 400) were compared. In addition, the sorption-desorption mechanism was clarified through its correlation with the physicochemical properties of BC. The results indicated that the Freundlich sorption parameters of FC, SC, BC 900, and BC 400 were 9947.90, 5417.57, 77690.16, and 2804.54 (mg kg^-1)/(mg L^-1), respectively, indicating that these high-temperature BCs had stronger sorption capacity. The desorption rate of PCB1 on BC 900 was slow, and the ratio of the difficult desorption fraction (F_r) was as high as 96.2%, while those of FC, SC, and BC 400 were only 35.3%, 19.1%, and 54.7%, respectively. The sorption and desorption mechanisms of the three high-temperature BCs were similar to those of BC 400. They exhibited nonlinear adsorption at low PCB1 concentrations and linear partition at high PCB1 concentrations. Moreover, the results demonstrated that different types of high-temperature BCs in the environment have different sequestration effects on HOCs. F_rap, the part that can be quickly desorbed, was predominantly PCB1 sorbed onto BC through a linear partition mechanism, but the surface acidic functional groups and larger pores would also increase the F_rap. Meanwhile, the slow desorption ratio (F_slow) was mainly affected by the degree of surface aromatization; the difficult-to-desorb PCB1 (F_r) was combined with BC through a nonlinear adsorption mechanism and was mainly related to the micropore volume. Graphical abstract.