The scope of this thesis was to investigate the uptake of the actinide neptunium-237 (237Np) by solid phases of relevance for the disposal of spent nuclear fuel (SNF) in deep, granitic underground repositories. Investigations were performed with the mineral phases corundum (α-Al2O3) and montmorillonite ((Na)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O) as well as with bentonite colloids and crushed Kuru gray granite, which are constituents of the Engineered Barrier System (EBS), and the host-rock, respectively. The uptake of neptunium in the pentavalent oxidation state (NpO2+) by these solid phases was investigated by batch sorption experiments, which provided information about the quantity of neptunium(V) removal from solution as a function of pH, neptunium concentration, and mineral concentration. By repeated exchange of the background electrolyte it was possible to obtain information about the desorption of neptunium(V) from the mineral surfaces, and to gain an insight into the potential re-mobilization of the actinide under flowing-water conditions. As batch sorption and desorption experiments do not provide information about the exact chemical species of neptunium on the mineral surface, the macroscopic sorption experiments were complemented by spectroscopic investigations using Attenuated Fourier Transform Infra-red (ATR FT-IR) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopies. This enabled the extraction of the complex structure and speciation of neptunium on the solid surfaces including bond lengths, neighboring atoms, coordination numbers and the type of surface complex formed on the solid phases, i.e. outer- vs. inner-sphere bound neptunium. Most of the investigations were performed under stagnant conditions, however, due to the role of potential stable and mobile colloids in the subsurface environment and their role as carriers and mobilizers of colloid-borne neptunium with flowing groundwater, additional granite column experiments were conducted in the absence and presence of bentonite colloids. Here, the migration of neptunium through the column was investigated as a function of time, and the role of colloids was evaluated from the obtained breakthrough curves. Under the chosen experimental conditions, colloids were found to have a negligible influence on the neptunium(V) breakthrough behavior, where most of the neptunium(V) was found to migrate through the column without adsorption onto the granitic column material.