Effects of dissolved organic matter on the adsorption of norfloxacin on a sandy soil (fraction) from the Yellow River of Northern China

Dissolved organic matter (DOM), which exists widely in the environment, coming from different sources, may greatly affect the adsorption of antibiotics. However, the adsorption mechanisms of antibiotics in a sandy soil and the effects of DOM from different sources on the adsorption remain poorly understood. This study systematically investigated the adsorption characteristics of norfloxacin (NOR) onto a sandy soil obtained from the banks of Xi'an in Yellow River and in the presence of three DOM including HDOM (commercially available humic acids), LDOM (derived from fallen leaves) and MDOM (derived from cattle manure). Elemental analysis, UV-vis spectroscopy, 3D-EEM, XPS, TOC, SEM, and FTIR were used to analyze the adsorption mechanism. It was found that all the DOM sources we used could reduce the adsorption of NOR on sandy soil and prolong the reaction time to reach adsorption equilibrium. The decreasing adsorption capacities of NOR by the three types of DOM (10 mg/L) followed the order as: HDOM < LDOM < MDOM, which was related to their aromaticity, polarity and hydrophobicity. These adsorption processes of NOR on sandy soil in the presence of DOM were well fitted by Double-chamber first-order kinetics, Linear model and Freundlich models. Besides, the adsorption reaction was endothermic and spontaneous. Adsorption competition of DOM molecules with NOR, or formation of DOM-NOR complexes in solution resulted in a decrease of sandy soil adsorption capacity. Correspondingly, co-adsorption and cumulative adsorption were also considered to be the key processes that determined NOR adsorption towards sandy soil after adding DOM. Moreover, the adsorption of NOR onto sandy soil exhibited strong pH-dependent characteristic and NOR might be more easily leached from sandy soil in the aquifer at an alkaline pH. High-ion strength suppressed the adsorption. These results would help to understand the fate and risk of NOR under the action of different DOM.