Effect of low-molecular-weight organic acids on the adsorption of norfloxacin in typical variable charge soils of China.

Batch equilibrium experiments were used to study the adsorption of norfloxacin (NOR) onto three kinds of variable charge soils in China, namely Rho-Udic Ferralisols collected from Yunnan, Ali-Perudic Ferrisols collected from Jiangxi and Typ-Hap-Udic Ferralisols collected from Guangdong. Results show that NOR is strongly adsorbed by the soils, with lg Kd-values (linear model) of 4.41+/-0.01, 4.50+/-0.02, 4.44+/-0.01 and lg Kf-values (Freundlich model) of 4.32, 4.45, 3.08 for the three tested soils, respectively. Both curves of the Freundlich equation and Langmuir equation for the three soils were in excellent linear correlation. Low-molecular-weight (LMW) organic acids, including citric acid, malic acid and salicylic acid were added in the presence of acetate buffer (pH 4.5) to test their effects on NOR adsorption. It was observed that the addition of LMW organic acids inhibited the NOR adsorption process. It was assumed that several effects, including soil pH, solid surface charge and competitive adsorption of co-existing cations, dominated the NOR sorption processes onto variable charge soils. Further studies should be performed to reveal the mechanism of the adsorption.