Understanding Nitrilotris(methylenephosphonic acid) reactions with ferric hydroxide

Phosphonate compounds are used in a wide variety of industrial and agricultural applications, and are commonly found in surface and ground waters. Adsorption to ferric hydroxide can have a significant effect on the transport and fate of phosphonate compounds in the environment. This research used density functional theory modeling to investigate the adsorption mechanisms of nitrilotris(methylenephosphonic acid) (NTMP) on ferric hydroxide. Standard Gibbs free energies of reaction (ΔG_r^o) and reaction activation barriers (E_a) were calculated for different possible adsorption mechanisms. Physical adsorption of NTMP to ferric hydroxide was promoted by negative charge assisted hydrogen bonding, and had ΔG_r^o ranging from -2.7 to -7.4 kcal/mol. NTMP was found to form three different types of inner sphere complexes, monodentate, bidentate mononuclear and bidentate binuclear. For the monodentate complexes, ΔG_r^o ranged from -8.0 to -13.7 kcal/mol, for the bidentate complexes ΔG_r^o ranged from -15.3 to -28.9 kcal/mol. Complexation with Ca²⁺ decreased the energy for physical adsorption but increased the binding energies for mono- and bidentate complexes. Complexation with Ca²⁺ also allowed formation of a tridentate ternary surface complex, whereby the Ca²⁺ ion formed a bridge between three FeO^- and three PO^- groups. Physical adsorption had E_a = 0, but mono- and bidentate complex formation had E_a values ranging from 36 to 53 kcal/mol. Formation of tridentate ternary surface complexes involving Ca²⁺ had the lowest activation barriers of 8 and 10 kcal/mol. The different activation barriers for different modes of adsorption may explain previous experimental observations of unusual kinetic behavior for adsorption and desorption of NTMP.