Link between black carbon and resistant desorption of PAHs on soil and sediment

Purpose: The effects of black carbon (BC) on resistant desorption of organic pollutants in soil and sediment were evaluated to further understand the mechanisms for the resistant desorption and to find a more accurate desorption model which can improve risk assessment and management of ubiquitous soil/sediment contamination. Materials and methods: A soil and sediment were used to create "combusted" soil/sediment with only BC and no other organic matter remaining by 375°C chemothermal oxidation (CTO-375) method. Polycyclic aromatic hydrocarbons (PAHs) including naphthalene and phenanthrene were used as sorbates, their sorption and desorption by the original soil/sediment (with organic matter and BC), and combusted soil/sediment (only BC) were examined. Results and discussion: All sorption isotherms were fitted with linear model and Freundlich model. Sorption isotherms of naphthalene and phenanthrene on the original soil/sediment were almost linear (Freundlich coefficient n = 0.86, 1.00, 0.87, and 0.75), whereas the BC remaining after the soil/sediment combustion showed nonlinear (n = 0.70, 0.48, 0.56, and 0.55) and higher sorption (e. g., K_OC = 1.36 × 10⁴ with naphthalene equilibrium solution concentration of 0.001 mg l^-1 for combusted soil, one order higher than that for the original sample accordingly). Desorption resistance was observed for both the original and combusted soil/sediment, and the resistant desorption from combusted soil/sediment was much stronger than that from original soil/sediment based on the quantification of desorption hysteresis with thermodynamic index of irreversibility (TII) values (TII = 0.73-0.88). A biphasic desorption isotherm can well describe the resistant desorption for all PAH-sorbent combinations. Conclusions: The BC could have significant effect on desorption of PAHs from soil and sediment, the BC fraction in soils and sediments could be the predominant component responsible for desorption hysteresis. More accurate biphasic desorption models can be used to improve risk assessment and management of ubiquitous soils/sediments contamination by PAHs.