Aluminum is a metal without known biological function and a common solution constituent in acidic soils covering approximately 40% of the world's arable land. High concentrations of dissolved monomeric aluminum in these soils represent a risk for plants and thus endanger agricultural food production. Understanding factors that control the speciation, mobility, and bioavailability of aluminum in acidic soils is thus of global economic importance. Aluminum interactions with mineral surfaces play a particularly important role for the geochemical cycling of aluminum in soil environments. Although iron oxides are recognized as ubiquitous aluminum sorbents, sorption reactions of aluminum on iron oxides and their controls have received surprisingly little attention, hampering our ability to accurately predict aluminum sorption to important soil iron oxides. Furthermore, spectroscopy-derived molecular-level information on aluminum species formed on iron oxides is not currently available. The same holds for information on the effect of important inorganic soil solutes such as silicic acid on aluminum sorption reactions. Silicic acid is an omnipresent soil solution constituent exhibiting a high affinity for iron-oxide surfaces. Soil iron oxides are therefore frequently enriched in silicon, with hitherto unknown consequences for aluminum mobilization and immobilization reactions. With the proposed research we seek to better understand the interactions between aluminum and iron oxides in the absence and presence of sorbed silicic acid. Our research focuses on aluminum surface speciation as well as on the extent, kinetics, and predictability of aluminum sorption reactions. To address these aspects, we use a combination of batch and flow-through reactor experiments, equilibrium and kinetic modeling, as well as bulk and surface-sensitive spectroscopies (e.g., X-ray absorption and X-ray photoelectron spectroscopy). The knowledge gained in this project will improve our understanding of processes and mechanisms affecting the mobility and bioavailability of aluminum in soils