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Humic acid is ubiquitous in soils and water. It affects the migration of radionuclides by complexation or redox reactions. The effect of humic acid on the diffusion of ReO in GMZ bentonite was investigated at dry density from 1,300 to 1,800 kg/m. The effective diffusion coefficient D and rock capacity factor α were measured by through-diffusion method. The D decreased from 5.5 to 0.59 × 10 m/s and α decreased from 0.2 to 0.12 with the dry density increasing from 1,300 to 1,800 kg/m. The retardation factor R in GMZ bentonite at three dry densities was obtained, which were less than one. Archie's law was used to describe the relationship between accessible porosity ε and D. An empirical parameter n ( n = 2.2 ± 0.2) was obtained for Re diffusion in GMZ bentonite in the presence of humic acid. [ABSTRACT FROM AUTHOR]