Wildfire-derived black carbon alters cesium retention in subsurface environments: insights from synchrotron X-ray imaging

Climate change has increased the frequency and intensity of wildfires, resulting in enhanced accumulation of incompletely combusted organic materials such as black carbon (BC) in soil and subsurface environments. The presence of wildfire-derived BC modifies the physicochemical characteristics of groundwater–soil systems and can alter the retention behavior of various contaminants. Among these, radiocesium (Cs), which may be released into the environment following nuclear power plant accidents, is of particular concern due to its high mobility and long-term persistence. Understanding how BC formation conditions influence Cs retention is therefore essential for predicting radionuclide behavior in wildfire-affected subsurface environments.

In this study, BC was produced from oak and pine biomass under controlled laboratory conditions, with final pyrolysis temperatures ranging from approximately 300–400 °C to ≥500 °C. Previous batch sorption experiments showed that Cs is preferentially sorbed onto low-temperature BC, whereas high-temperature BC exhibits reduced Cs uptake. To investigate the physical mechanisms underlying this temperature-dependent behavior, synchrotron-based X-ray computed tomography (CT) was conducted at the Pohang Accelerator Laboratory (PAL 6C) using 25 keV X-rays with a voxel resolution of 3.25 μm. CT images reveal that BC produced at lower temperatures preserves an interconnected internal pore structure inherited from the original biomass, whereas BC produced at ≥500 °C exhibits pronounced microstructural degradation, including pore collapse and loss of pore connectivity. These structural trends were consistently observed despite inherent heterogeneity associated with different biomass precursors. The results indicate that Cs sorption onto BC is controlled by a coupled effect of surface chemical functionalities and microstructural integrity, which governs the accessibility of reactive sites. High-temperature thermal alteration induces physical damage to the BC structure, thereby limiting effective Cs retention even as aromaticity increases. These findings highlight the importance of considering wildfire-induced changes in BC properties when assessing radionuclide retention in subsurface environments.