亚甲基蓝及NH_4^+,Na^+,K^+与SiO_2的表面配位常数测定

Determination of Surface Complexation Constant of Methylene Blue,NH_4^+,Na^+ and K^+ with SiO_2 Surface

采用动电位、等温吸附方法研究了不同pH条件下SiO2的表面电性和对有机染料-亚甲基蓝的吸附行为,运用WinSGW软件计算得出亚甲基蓝在SiO2颗粒表面配位常数.研究发现亚甲基蓝通过静电和氢键作用吸附,相应的化学反应模型和表面配位常数分别为≡Si2O-+MB+≡Si2OMB(lg K=4.48)和≡SiOH+MB+≡SiOHMB+(lg K=2.30).在此基础上研究了一价阳离子NH+4,Na+,K+对亚甲基蓝在SiO2表面吸附的影响,根据WinSGW软件模拟得到的NH+4,Na+,K+与SiO2表面的配位常数分别为:≡Si2O-+M+≡Si2OM[lg K=3.76,2.73,2.58(M+=NH+4,Na+,K+).在铵根离子浓度高时,还会在表面上发生≡SiOH+NH3+MB+≡SiOHNH3MB+(lg K=6.69)化学反应而生成包含有三种组分的≡SiOHNH3MB+表面三元配合物.研究结果可为了解有机染料和氨氮在被污染土壤和水体中的分布提供一定的理论依据.

Understanding and reducing the environmental contaminations of persistent organic pollutants to soil and surface water is the central topic of environmental chemistry. In this study, the adsorption of Methylene Blue （MB）, a typical model substance of organic dye, at the surface of silica, the main soil containing substance, may give insights for understanding the theoretical distribution of organic pollutants in the system of soil and surface water. The adsorption of MB and surface charge in SiO2 system with varying solution pH were characterized by electrokinetic phenomena and adsorption experiments. Test- ing results confirmed that the adsorption of MB at the silica surface belongs to the Langmuir type i.e. monolayer adsorption. Based on the experimental data, complexation constants of MB at the surface of SiO2 was calculated according to Constant Capacitance Model using computer software WinSGW to be： ≡Si20-＋MB＋≡~Si2OMB （lg K=4.48） and≡SiOH-F MB＋≡SiOHMB＋ （1g K=2.3）. These results indicate that the adsorption of MB at the surface of silica includes two types i.e. electrostatic interaction and hydrogen bonding. In addition, the effect of monovalent cation （ NH＋, Na＋, K＋） on the adsorption of MB at the surface of SiO2 was studied, the complexation constants of NH＋, Na＋ or K＋ with SiO2 surface were established respectively as： ≡Si20＋M＋≡=Si2OM [lg K=3.76, 2.73, 2.58 （M＋= NH＋, Na＋, K＋）]. Furthermore, at higher concentration of NH＋, an additional surface reaction ≡SiOH-k-NH3＋MB＋≡SiOHNH3MB＋ （lg K=6.69） was established, which lead to the formation of a ternary surface complex ≡SiOHNH3MB＋. These results suggest that mutual ion exchange reaction between adsorbed and solution monovalent ions can take place. The affinity of monovalent cations studied to the silica surface is in the order of MB＋〉 NH＋ 〉Na＋ 〉K＋. This study has not only provided basic data for quan- tifying the adsorption of various monovalent cation at the surface of silica but also can hopefully provide theoretical basis for understanding the distribution of organic pollutants and ammonia in the system of contaminated soil and surface water.