砷和砷酸铁水化结构和红外光谱理论研究

水体中砷的去除与其水化作用密切相关，而不同质子化砷和砷酸铁水化特征相关报道甚少，且缺乏不同质子化砷和砷酸铁水化层红外光谱解析。在B3LYP/6-311G(d, p)计算水平上比较不同质子化砷酸根H_mAsO₄]m-3(m=0～2)和铁-砷酸盐络合物种FeH_mAsO₄]m+(m=0-2)水化能，利用约化密度梯度函数图形化分析其与水分子相互作用的强度、类型和位置，并解析不同质子化砷酸根和砷酸铁水化层红外光谱特征。结果表明，随着氢质子化，砷酸根H_mAsO₄]m-3(m=0～2)水化能力减弱，而铁-砷酸盐络合物种FeH_mAsO₄]m+(m=0～2)水化能力随着氢质子化增强。当水分子中1个氢与H_mAsO₄]m-3(m=0～2)中1个氧相互作用时倾向形成氢键；而水分子中2个氢同时分别与H_mAsO₄]m-3(m=0～2)中两个氧相互作用时，相互作用变弱，以范德华力相互作用；水分子通过其氢与砷酸根中氧形成的氢键强于水分子通过其氧与质子化砷酸根中氢形成的氢键。未质子化O_N倾向与2~4个水分子形成氢键，而质子化O_P最多与2个水分子形成氢键且O_P…H_W氢键弱于O_N…H_W氢键。红外光谱中，2 954，3 114，3 179，3 252和3 297 cm-1是AsO3-₄第一水化层中水分子Ow-Hw伸缩振动峰，3 277，3 324和3 376 cm-1是HAsO2-₄第一水化层中水分子的Ow-Hw伸缩振动峰，3 189，3 277，3 306和3 383 cm-1是H₂AsO-₄第一水化层中水分子Ow-Hw伸缩振动峰；FeH_mAsO₄]m+(m=0～2)第一水化层中水分子Ow-Hw伸缩振动对应区域依次是2 500~3 060，2 660~3 200和2 900~3 360 cm-1。因此，随质子化，H_mAsO₄]m-3(m=0～2)和FeH_mAsO₄]m+(m=0～2)第一水化层中水分子的Ow-Hw伸缩振动峰蓝移；相对于H_mAsO₄]m-3(m=0～2)，FeH_mAsO₄]m+(m=0～2)第一水化层水分子的弯曲振动峰和伸缩振动峰都明显红移。FeH_mAsO₄]m+(m=0～2)第一水壳层形成Fe-Ow-Hw…Ow-Hw…O_N-As氢键桥，该氢键桥中Ow-Hw具有特殊吸收峰，伸缩振动峰依次位于2 195，2 526和2 673 cm-1，质子化导致明显蓝移但峰强度几乎无变化；而其弯曲振动峰随质子化红移且强度明显降低；独立O_P-H伸缩振动峰不受Fe络合影响，而O_P-H…Ow中O_P-H伸缩振动峰位置因Fe络合而发生明显蓝移。该研究有助于更好地解析不同PH下砷和砷酸铁在水中溶解性，可用于红外光谱监测水溶液中砷和砷酸铁水化特征。

Theoretical Study on the Structures and IR Spectra of Hydration of Arsenates and Iron Arsenates

The removal of arsenate in water is closely related to its hydration, but there are few reports on the hydration characteristics of different protonated arsenates and iron arsenates, and there is no correlation analysis on infrared spectra of hydration layers of protonated arsenates and iron arsenates. The hydration energies of different protonated arsenates H_mAsO₄]m-3(m=0～2) and iron arsenates FeH_mAsO₄]m+(m=0～2) were compared at B3LYP/6-311G(d, p) level. Reduced density gradient functions conducted graphical analyses for the intensities, types and locations of the interaction between water molecules with H_mAsO₄]m-3(m=0～2) and FeH_mAsO₄]m+(m=0～2). And, the characteristics of infrared spectra of the hydration layers of different protonated arsenates and iron arsenates were analyzed. The results show that the hydration of H_mAsO₄]m-3(m=0～2) gradually decreases with hydrogen protonation, while the protonation enhances the hydration of FeH_mAsO₄]m+(m=0～2). Hydrogen bonds tend to form when a water molecule hydrogen interacts with an oxygen of H_mAsO₄]m-3(m=0～2). However, when two hydrogens of water molecules simultaneously interact with two oxygens of H_mAsO₄]m-3(m=0～2), the interaction becomes weaker, and the van der Waals force appears. The hydrogen bond formed by water molecules through hydrogen with the oxygen of arsenates is stronger than the hydrogen bond formed by water molecules through oxygen with the hydrogen of protonated arsenates. The unprotonated O_N tends to form hydrogen bonds with 2~4 water molecules, while the protonated O_P forms hydrogen bonds with at most 2 water molecules, and the O_P…H_W hydrogen bond is weaker than the O_N…H_W hydrogen bond. In the infrared spectra, 2 954, 3 114, 3 179, 3 252 and 3 297 cm-1 is the stretching vibration peaks of Ow-Hw in the first hydration shell of AsO3-₄; 3 277, 3 324 and 3 376 cm-1 is the stretching vibration peaks of Ow-Hw in the first hydration shell of HAsO2-₄; 3 189, 3 277, 3 306 and 3 383 cm-1 is the stretching vibration peaks of Ow-Hw in the first hydration shell of H₂AsO-₄. The stretching vibration regions for Ow-Hw in the first hydration shell of FeH_mAsO₄]m+(m=0～2) are 2 500~3 060, 2 660~3 200, 2 900~3 360 cm-1. Therefore, the stretching vibration regions for the first hydration waters of H_mAsO₄]m-3(m=0～2) and FeH_mAsO₄]m+(m=0～2) have blue shifts with protonation. Compared with H_mAsO₄]m-3(m=0～2), the water molecules in the first hydration layers of FeH_mAsO₄]m+(m=0～2) exhibit a significantly red shift of the bending vibration peaks and stretching vibration peaks in the infrared spectra. The hydrogen bond bridge Fe-Ow-Hw…Ow-Hw…O_N-As is formed in the first hydration shell of FeH_mAsO₄]m+(m=0～2). The Ow-Hw in this hydrogen bond bridge has a special absorption peak, such as the stretching vibration peak located in 2 195, 2 526 and 2 673 cm-1, respectively. Its stretching vibration peak is significantly blue-shifted, but the peak strength is almost unchanged, while its bending vibration peak is red-shifted with the protonation and the strength is significantly reduced. The stretching vibration peak of independent O_P-H is not affected by the complexation of Fe, while the position of stretching vibration peak for O_P-H in O_P-H…Ow is significantly blue shifted, due to the complexation of Fe. This study is helpful to understand better the solubility of arsenates and iron arsenates in water at different pH.