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The adsorption of radionuclides (RNs) onto mineral surfaces is a key retardation mechanism and plays a critical role in the long-term safety assessment of radioactive waste repositories. While numerous studies[1,2,3] have investigated the sorption behavior of trivalent actinides (e.g. Am, Cu) and lanthanides (e.g. Eu, Y) on various minerals, most experiments rely on geochemically simplified systems, typically in binary configurations involving single minerals and single sorbing species. Natural systems, however, are considerably more complex, with competitive sorption, bulk and surface precipitation, incorporation, and co precipitation processes all influencing RN retardation.In this study, we examine the competitive sorption of Eu (a chemical analogue for trivalent actinides) and Al onto quartz using a combination of batch and column experiments supported by mechanistic surface complexation modeling (SCM). Aluminum, an abundant component in natural groundwater and porewater, may compete with RNs for sorption sites. In addition, due to its low solubility at near neutral pH, Al may undergo surface precipitation, altering mineral surface charge and modifying sorption behavior.Batch experiments were performed under varying geochemical conditions, including pH, ionic strength, and initial Al and Eu concentrations. The results show that Al sorption onto quartz arises at lower pH compared to Eu, with sorption edges at approximately pH 4.5 for Al and pH 5.5 for Eu (50% sorbed). In the presence of Al, Eu sorption onto quartz is significantly reduced. Batch sorption data is used for SCM calibration via inverse modeling. These models are then validated using experimental data from column experiments to assess the applicability, advantages, and limitations of derived parameters. In addition, column experiments with K‑feldspar are planned and will be evaluated using SCM to further examine competing sorption effects. Ongoing work focuses on further model development. First modelling outcomes, together with initial assessments of the applicability, strengths and limitations of the emerging parameter set will be presented.These investigations contribute to improving previous modelling approaches in which competitive Al sorption may have influenced radionuclide behavior. More broadly, the results highlight the importance of understanding geochemical surface reactions to enhance the reliability of long-term safety assessments for radioactive waste repositories.[1] J. Neumann, J. Colloid Interface Sci. (2020) (https://doi.org/10.1016/j.jcis.2020.11.041)[2] J. Lessing, Colloids and Surfaces A (2024) (https://doi.org/10.1016/j.colsurfa.2024.133529) [3] S. Britz, PhD Thesis, 2018 (https://doi.org/10.24355/dbbs.084-201806051207-0)