Lithium manganese oxide, LiMn 2 O 4 , was synthesized in two temperature stages, where the first consisted by an ecofriendly solution combustion method at 300 °C. Finally, the as-burned powders were thermal treated at 500 and 700 °C. The structural and morphological changes were evaluated by the Rietveld method and density functional theory (DFT) calculations. The Rietveld refinement indicates obtaining the spinel cubic phase LiMn 2 O 4 and a small amount of Mn 2 O 3 . The analyses by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show a porous microstructure composed of nano-sized crystallites for the sample treated at 500 °C. In cyclic voltammetry, it was possible to observe that the reduction-oxidation reaction is reversible due to the shape of voltammograms and the anodic and cathodic peaks of Mn ions. The theoretical calculations considered the experimental crystallographic parameters. The unit cell volume change was evaluated according to distinct amounts of lithium ions in the structure. The removal of the Li + cations from the oxides promotes a volume contraction. Therefore, it was possible to evaluate the participation of the Mn 3+ ions in the frontier region between the valence and conduction bands. The density of states (DOS) calculation shows a predominant contribution of the O 2p and Mn 3d orbitals in the frontier orbitals.

The affinity of the pentaaqua Ni 2 + cation for 16 neutral ligands with distinct functional groups was analyzed with the DFT method. The metal-ligand binding strength was calculated by the interaction enthalpy and Gibbs free energy for the exchange of a water molecule from the [Ni(H 2 O) 6 ] 2 + by a neutral ligand. Geometrical, electric and energetic features of the isolated ligands and the complexes were correlated with the intensity of binding. All the substitution enthalpies and Gibbs free energies are negative, showing exothermic and spontaneous processes. The affinity order is P=O > C=N > C=S > C=O > CÀN > CÀS > CÀO ligands. The EDA calculation reveals that the ionic and the covalent terms are both important in determining the interaction order strength. The electrostatic term is the most relevant for modulating the metal cation interaction. The electric nature of each ligand dictates the metal-ligand bonding strength. Ligands containing the phosphate group have the strongest interaction with the Ni 2 + aquacation and are indicated to be the structural anchoring blocks of chelation agents for aquatic resources decontamination.[a] D.