کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
5746485 | 1618796 | 2017 | 7 صفحه PDF | دانلود رایگان |
- Adsorption of NTMP on ferric hydroxide occurs via physical and chemical adsorption.
- Physical adsorption was promoted by negative charge assisted hydrogen bonding.
- Four types of chemical adsorption complexes were identified.
- Inner-sphere complex formation had high activation barriers.
- Ca2+ ions promoted the formation of ternary complexes with lower activation barriers.
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 (ÎGro) 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 ÎGro ranging from â2.7 to â7.4 kcal/mol. NTMP was found to form three different types of inner sphere complexes, monodentate, bidentate mononuclear and bidentate binuclear. For the monodentate complexes, ÎGro ranged from â8.0 to â13.7 kcal/mol, for the bidentate complexes ÎGro ranged from â15.3 to â28.9 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 FeOâ and three POâ groups. Physical adsorption had Ea = 0, but mono- and bidentate complex formation had Ea 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.
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Journal: Chemosphere - Volume 175, May 2017, Pages 490-496