Cd^(2+)与环丙沙星共存时在石英砂中的吸附特征及作用机理

Adsorption characteristics and mechanism of co⁃existing Cd^(2+)and ciprofloxacin on quartz sand

为探讨重金属与兽药抗生素共存时在土壤及地下水砂层中的吸附行为及机理,以重金属镉(Cd)和环丙沙星(Ciprfloxacin,CIP)为研究对象,石英砂为介质,通过批平衡实验法,研究不同Cd^(2+)浓度、pH及离子强度等因素下Cd^(2+)与环丙沙星共存时在石英砂上的吸附行为及其交互影响.结果表明:①环丙沙星在石英砂上的吸附可分为两个阶段,6 h内为快速吸附,6~24 h为慢速吸附,24 h后达到吸附平衡;Cd^(2+)在石英砂上的吸附表现为瞬时吸附,且吸附行为不受环丙沙星浓度的影响;环丙沙星在石英砂上的吸附动力学过程用伪二级动力学模型拟合效果最优,其次是伪一级动力学方程,颗粒内扩散方程拟合效果最差.②不同Cd^(2+)浓度对环丙沙星吸附的影响不同,当Cd^(2+)浓度较低(5、20 mg·L^(-1))时,Cd^(2+)的存在对吸附起促进作用;当Cd^(2+)浓度为100 mg·L^(-1)时,Cd^(2+)的存在对吸附起抑制作用;pH越高,离子强度越大,石英砂对环丙沙星的吸附量越低,而石英砂对Cd^(2+)的吸附则表现为pH越高,吸附量越大,离子强度越高,吸附量越小;不同Cd^(2+)浓度、pH及离子强度对CIP吸附的影响程度与CIP浓度有关,CIP浓度越低,影响越显著;除了pH为7的条件外,CIP对Cd^(2+)在石英砂上的吸附基本没有影响.③Freundlich方程能较好地拟合不同条件下CIP在石英砂中的吸附曲线,R2均大于0.949,且大部分n值在0.326~0.651之间,说明石英砂对CIP的吸附是非线性的.④Cd可与CIP中的羧基(—COOH)和羰基(C=O)发生络合作用形成络合物而被石英砂所吸附.

This study focuses on the adsorption behavior and mechanism of co⁃existing heavy metals and the veterinary antibiotics in the soil,groundwater and sand layer.The adsorption of ciprofloxacin(CIP)and cadmium(Cd^(2+))on quartz sand was investigated with batch experiments under different Cd^(2+)concentrations,pH and ionic strength.The adsorption of CIP on quartz sand involved two stages,fast adsorption within the first 6 h,slow adsorption from 6 h to 24 h,and adsorption equilibrium after 24 h.While the adsorption of Cd^(2+)on quartz sand was instantaneous,which was not affected by CIP concentration.The CIP adsorption kinetics process was best fitted by the pseudo⁃second⁃order kinetic model,followed by the pseudo⁃first⁃order kinetic equation.By contrast,the fitting of the intra-particle diffusion equation was the least satisfactory.Secondly,the adsorption of CIP varied with Cd^(2+)concentrations.When Cd^(2+)concentration was low(5,20 mg·L^(-1)),the CIP adsorption was promoted.When Cd^(2+)concentration reached 100 mg·L^(-1),however,the CIP adsorption was inhibited.Further,higher pH and higher ionic strength would lower the absorption of CIP on quartz sand.On the other hand,higher pH would increase,while higher ion strength would lower Cd^(2+)adsorption on quartz sand.The influencing efficiencies of Cd^(2+)concentration,pH and ionic strength on CIP adsorption are related to CIP concentration.The CIP in lower concentration would have more significant influence.Except at pH 7,CIP has little effect on the adsorption of Cd^(2+)on quartz sand.Thirdly,CIP adsorption under different conditions is well fitted by the Freundlich equation with R2>0.949 and most of the n values are in the rage of 0.326~0.651,indicating that the CIP absorption on quartz sand is non⁃linear.Finally,Cd^(2+)could bind CIP through the carboxyl group(—COOH)and carbonyl group(C=O)to form a complex,which was subsequently adsorbed by quartz sand.