Uranium mill tailings (UMTs) are one critical source of environmental U pollution. Leaching test has been extensively used to reveal U release capacity and mechanism from UMTs, while little attention has been paid to the effects of re-adsorption process on U release. In this study, the role of U re-adsorption behaviors during leaching test with UMTs was comprehensively investigated. Through paired data on mineralogical composition and aqueous U speciation, the influence of environmentally relevant factors on U re-absorption capacity and mechanism on UMTs with different particle sizes was revealed. Significant amounts of U re-adsorption were observed and primarily attributed to the adsorption on chlorite, albite and muscovite as well as combined reduction-sequestration by muscovite. Uranium re-adsorption predominantly occurred via inner-sphere complexation and surface precipitation depending on leachant pH. Coexisting sulfate or phosphate could further enhance U re-adsorption. The enhanced re-adsorption from sulfate occurred when inner-sphere complexation governed the re-adsorption process. These findings suggest that the environmental hazards and ecological risks of the U containing (waste) solids might have been underestimated due to the ignorance of the readsorption process, since the re-adsorbed U could be easily re-mobilized. The insights from this study are also helpful in developing effective in-situ remediation strategies.(C)2021 Elsevier B.V. All rights reserved.