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Plutonium (Pu) mobility through the subsurface is dependent on the chemical composition of the source-term and biogeochemical conditions that impact its speciation. Regardless of future decision making, existing stockpiles of Pu need to be isolated from the biosphere. Therefore, understanding the speciation of trace Pu is essential for assessing and mitigating any risk of Pu transport from the source-term to a human or environmental receptor. After a brief introduction (Chapter One) and statement of research objectives (Chapter Two), this dissertation investigated Pu speciation under saturated conditions and in the presence of natural organic matter (NOM) and pure synthesized mineral phases. Batch sorption experiments, infrared spectroscopy, and diffuse-layer modeling were used to better understand Pu speciation in the presence of NOM and mineral surfaces. In addition, new tools were developed for preparing Pu(V) stock solutions for experimental use (Chapter Five), and for simultaneously quantifying Pu and NOM in ternary batch sorption experiments (Chapter Three). In Chapter Three, the effects of citric acid (CA), Desferrioxamine B (DFOB), fulvic acid (FA), and humic acid (HA) on Pu sorption to goethite were studied as a function of organic carbon concentration and pH. Between pH 5 and 7, all of these organic ligands reduced Pu sorption to goethite relative to a ligand-free experiment. At pH 3 however, CA, FA, and HA all increased Pu sorption relative to a ligand-free experiment. The formation of ternary goethite NOM Pu complexes were attributed to the increase in Pu sorption at pH 3. Possible sorption mechanisms for ternary complexes were investigated using Fourier-transform infrared spectroscopy, and a hybrid empirical/diffuse-layer surface complexation model was used to predict Pu sorption to goethite in the presence of CA, HA, and DFOB. In Chapter Four, that hybrid model was refined by narrowing the scope to CA, and studying Pu and CA sorption to goethite and gibbsite from pH 2 – 10. An entirely diffuse-layer model was used to describe Pu and CA sorption to goethite and gibbsite. Ternary mineral-citrate-Pu(IV) surface complexes were necessary to describe plutonium sorption to both minerals from pH 2 – 4.