Computational chemistry is a flexible tool that can quantitatively describe the reactivity of a wide variety of systems, ranging from simple organic molecules to complex heterogenous surfaces. In this thesis, quantum chemical methods are employed in both a diagnostic and predictive capacity to gain insight into the reactions of a number of environmentally relevant chemical systems. Chapter 2 outlines the use of multiple theoretical models for the characterization of the dominant conformers of nerve agents found in aqueous solution and for the understanding of their subsequent inactivation via triethylamine-catalyzed hydrolysis. This computational analysis informed the development of a procedure used to assess the ability of several organic complexes to more efficiently catalyze the decomposition of toxic organophosphates into non-toxic products. Chapter 3 describes the use of Density Functional Theory (DFT) to reveal a complete mechanistic pathway for the oxidation of water by a homogenous copper- bicarbonate catalyst. The proposed mechanistic steps were evaluated by the calculation of redox potentials, pKa values and free energy barriers, which were validated by comparison to experiment. Chapter 4 presents the use of plane-wave periodic DFT to evaluate the details of nitrogen-rich aromatic groups binding to both an aluminum metal surface and two environmentally relevant alumina surfaces. The role that nitrogen substitution on the aromatic ring plays in adsorption on these surfaces was assessed through the analysis of both optimal geometries and computed binding energies.