title: Understanding Nitrilotris(methylenephosphonic acid) reactions with ferric hydroxide.
abstract: 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 (ΔGro) and reaction activation barriers (Ea) were calculated for different possible adsorption mechanisms. Physical adsorption of NTMP to ferric hydroxide was promoted by negative charge assisted hydrogen bonding, and had ΔGroranging 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, ΔGroranged from -8.0 to -13.7 kcal/mol, for the bidentate complexes ΔGroranged 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 Ea= 0, but mono- and bidentate complex formation had Eavalues 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.
authors.affiliation: Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ, 85721, USA.
authors.forename: Rodrigo Javier
authors.initials: RJ
authors.lastname: Martínez
authors.affiliation: Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ, 85721, USA. Electronic address: farrellj@email.arizona.edu.
authors.forename: James
authors.initials: J
authors.lastname: Farrell
mesh_terms: D000327:Adsorption; D017610:Calcium Compounds / Q000737:chemistry; D005290:Ferric Compounds / Q000737:chemistry*; D006860:Hydrogen Bonding; D007700:Kinetics; D008958:Models, Molecular; D008962:Models, Theoretical; D009943:Organophosphorus Compounds / Q000032:analysis* / Q000737:chemistry
chemical_list: D017610:Calcium Compounds; D005290:Ferric Compounds; D009943:Organophosphorus Compounds; C041182:(nitrilotris(methylene))triphosphonic acid; C021024:ferric hydroxide
keywords: DFT; Ferric hydroxide adsorbents; Phosphonate; nitrilotris(methylenephosphonic acid) (NTMP)