环丙沙星在黄土中的吸附机制及影响因素

为了解兽药抗生素在西北黄土中的吸附行为及机制，以环丙沙星（Ciprfloxacin，CIP）为目标污染物，采用批平衡实验法研究其在黄土中的吸附动力学、吸附热力学、pH值和粒径等影响因素.结果表明：环丙沙星在黄土中的吸附可分为两个阶段，6h内表现为快速吸附，6~10h表现为慢速吸附，10h后吸附达到平衡；环丙沙星在黄土中的吸附动力学过程较符合准二级吸附动力学模型，吸附速率受颗粒内部扩散和外部液膜扩散共同影响；吸附热力学过程较符合Freundlich等温吸附模型，且吸附等温线符合L-型，表明黄土对环丙沙星的吸附受溶液中溶质和水分子共同作用影响；焓变ΔH^θ、熵变ΔS^θ和吉布斯自由能ΔG^θ均<0，表明黄土对环丙沙星的吸附是混乱度减少的自发进行的放热反应；pH=5时黄土对环丙沙星的吸附量最大，pH值为2~5时，黄土对环丙沙星的吸附量随pH值升高呈上升趋势，pH>5时，黄土对环丙沙星的吸附量随pH值升高呈下降趋势，表明强酸和碱性环境均不利于吸附；吸附量与黄土粒径呈反比，与初始浓度呈正比.由实验结果推断黄土对环丙沙星的吸附主要与有机质含量有关，主要吸附机制为环丙沙星分子中氨基的阳离子交换作用.

Adsorption mechanism and influencing factors of ciprofloxacin on loess

In order to study the adsorption behavior and mechanism of veterinary antibiotics on the loess of Northwest China, ciprofloxacin was selected as the target pollutant, and batch equilibrium method was conducted to investigate its adsorption kinetics, adsorption thermodynamics, and influencing fators, such as pH values and particle sizes, on the adsorption process. Results showed that the adsorption of ciprofloxacin on loess can be divided into two stages:fast adsorption stage (0~6h) and slow adsorption stage (6~10h), and adsorption equilibrium was reached after 10 hours. The adsorption processes fitted well with the pseudo-second-order adsorption kinetics model, and the adsorption rate was controlled by both the internal diffusion of particles and the external diffusion of liquid membrane. The adsorption thermodynamics process was in accordance with the Freundlich model, and the isotherm adsorption was belonged to L-type, which means the adsorption behaviour was affected by the interaction of solute and water molecules in solution. The changes of Enthalpy (ΔH^θ), Entropy (ΔS^θ) and Gibbs free energy (ΔG^θ) were all less than 0, indicated that the adsorption was a spontaneous exothermic reaction with a decrease of chaotic degree. The maximum adsorption amount of ciprofloxacin on loess was obtained at 5 of pH value, and the adsorption of ciprofloxacin on loess increased with the increase of pH value from 2 to 5. While, the adsorption of ciprofloxacin on loess decreased with the increase of pH value beyond 5, thus, strong acid and alkaline environment were unfavorable to adsorption. The adsorption amount of ciprofloxacin on loess was inversely proportional to the size of loess, and was proportional to the initial concentration as well. According to the experimental results, it was inferred that the adsorption of ciprofloxacin on loess was mainly related to the content of organic matter, and the adsorption mechanism was dominated by the cation exchange of amino groups in ciprofloxacin molecule.