Effests Of Dissolved Humic Acid On Sorption Of Norfloxacin Onto Typical Clay Minerals

Kaolinite and montmorillonite are used as adsorbents in batch sorptionexperiments in this study. By means of Fourier transform infrared spectroscopy andX-ray diffraction analysis, the mechanisms of norfloxacin sorption to kaolinite andmontmorillonite were discussed under the conditions of solution pH and concentrationand adding order of dissolved humic acid. The study on the mechanisms of antibioticssorption to typical clay minerals is helpful to understand the consequence oflong-term agricultural irrigation using of reclaimed water. The main conclusions areas follows.1. The existence of dissolved humic acid prolongs the equilibration time ofisothermal sorption experiments for norfloxacin sorption to kaolinite andmontmorillonite. Addition of humic acids slows down the initial rate of sorption. Theequilibration time of24hours is selected for the solutions without adding humic acid,and equilibration time of48hours is selected for the solution with coexisting humicacid.2. Experimental data show that the overall capacity of sorption of norfloxacinzwitterions to kaolinite is stronger than that of cations and anions of norfloxacin. Thesorption of cation and zwitterionic of norfloxacin to montmorillonite is stronger thanthat of the corresponding anions. The contribution of various species of norfloxacin tothe total sorption under different pH conditions was calculated by fitting theexperimental data. It is found that the contribution proportion is related to the contentsof relevant norfloxacin species and the sorption mechanisms.3. The differences in layer nature and pH value of electrical zero betweenmontmorillonite and kaolinite caused different sorption mechanisms. The data ofX-ray diffraction analysis proved the occurrence of sorption in layer ofmontmorillonite. Cation exchange became a major mechanism under low pHconditions due to the strong cation exchange capacity of montmorillonite. Since pH value of electrical zero of kaolinite is in the range of experiments pH values, kaoliniteis positively charged at low pH conditions. This caused the different electrostaticinteraction of norfloxacin to montmorillonite. Validated by FTIR and XRD methods,hydrogen bonding and surface complexation adsorption are both the sorptionmechanism for kaolinite and montmorillonite.4. The concentration and adding order of dissolved humic acid can affect thesorption mrchanisms of norfloxacin to kaolinite and montmorillonite. The mainreasons of dissolved humic acid promoting the sorption norfloxacin to kaolinite andmontmorillonite are the results of cumulative sorption and co-sorpiton. The reasons ofinhibited sorption include competition of sorption sites, the enhanced solubility andthe blocking of humic acid molecules on the way of norfloxacin to clay mineralsurface or the interlayers of montmorillonite.