| On the basis of OECD Guideline 106, batch sorption methods were employed to reveal the effect of Ca2+ ion strength and different cations (0.01 mol/L solution of NaCl, KCl, NH4Cl, MgCl2, CaCl2, ZnCl2 and AlCl3) on norfloxacin (NOR) sorption-desorption process in four tested soils (Ustic Cambosols, Udic Argosols, Gleyic-Stagnic Anthrosols and Ali-Perudic Ferrosols). Results indicate that sorption coefficient Kf values of NOR gradually decrease to a steady state with the increased concentration of calcium ion in solution. There exists a significantly negative correlation between Ca2+ ion strength and Kf value (p≤0.05). Compared with higher NOR concentrations, the effect of calcium ion concentration is negligible to the sorption at lower NOR concentrations. At the low pH, cationic form of NOR is predominant in equilibrium solutions, so cation sorption and hydrogen bonding could be the main sorption mechanism. Whilst at the zwitterionic form of NOR (pH 6.3～8.4), electrostatic adsorption or dipole moment may control the sorption process. This causes the different sorption behavior of NOR in the four soils. Thus, the calcium ion in solutions is competitively adsorbed to the active sorption sites so as to decrease the NOR sorption amount. Moreover, the different cations result in difference of NOR sorption. Cations of higher valencies are adsorbed more strongly to surface active sorption sites, with the following order: M+(Na+、K+、NH＋4)2+(Ca2+、Mg2+、Zn2+)3+(Al3+). |
