Martínez Rodrigo Javier, Farrell James
Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ, 85721, USA.
Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ, 85721, USA.
Chemosphere. 2017 May;175:490-496. doi: 10.1016/j.chemosphere.2017.02.015. Epub 2017 Feb 8.
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) and reaction activation barriers (E) 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 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 ranged from -8.0 to -13.7 kcal/mol, for the bidentate complexes ΔG ranged from -15.3 to -28.9 kcal/mol. Complexation with Ca decreased the energy for physical adsorption but increased the binding energies for mono- and bidentate complexes. Complexation with Ca also allowed formation of a tridentate ternary surface complex, whereby the Ca ion formed a bridge between three FeO and three PO groups. Physical adsorption had E = 0, but mono- and bidentate complex formation had E values ranging from 36 to 53 kcal/mol. Formation of tridentate ternary surface complexes involving Ca 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.
膦酸盐化合物被广泛应用于各种工农业领域，并且在地表水和地下水中普遍存在。吸附到氢氧化铁上会对膦酸盐化合物在环境中的迁移和归宿产生重大影响。本研究使用密度泛函理论模型来研究次氮基三（亚甲基膦酸）（NTMP）在氢氧化铁上的吸附机制。针对不同可能的吸附机制计算了反应的标准吉布斯自由能（ΔG）和反应活化能垒（E）。NTMP通过负电荷辅助氢键促进其在氢氧化铁上的物理吸附，其ΔG范围为-2.7至-7.4千卡/摩尔。发现NTMP形成三种不同类型的内球络合物，即单齿、双齿单核和双齿双核。对于单齿络合物，ΔG范围为-8.0至-13.7千卡/摩尔，对于双齿络合物，ΔG范围为-15.3至-28.9千卡/摩尔。与钙络合降低了物理吸附的能量，但增加了单齿和双齿络合物的结合能。与钙络合还允许形成三齿三元表面络合物，其中钙离子在三个FeO和三个PO基团之间形成桥。物理吸附的E = 0，但单齿和双齿络合物形成的E值范围为36至53千卡/摩尔。涉及钙的三齿三元表面络合物的形成具有最低的活化能垒，分别为8和10千卡/摩尔。不同吸附模式的不同活化能垒可能解释了先前关于NTMP吸附和解吸异常动力学行为的实验观察结果。
