The overall diffusion of HCB is lower, reflecting its high accumulation potential in SOM . The lowest diffusion coefficient is observed in the SOM III system. This result aligns with a theoretical study on HCB, which reported relatively low diffusion in Leonardite humic acid (LHA) , corresponding to SOM III in this study. In contrast, in the SOM I system, HCB diffusion closely matches that of water, indicating that HCB is predominantly located in the water phase for this SOM type. Conversely, SOM II exhibits the greatest disparity between HCB and water diffusion, probably due to a strong accumulation of HCB in the HS phase.