Adsorption behavior of tetracycline on the soil and molecular insight into the effect of dissolved organic matter on the adsorption

Tetracycline (TC) is one of the most used antibiotics and has accumulated in soil. Its adsorption behavior was studied, and the effect of dissolved organic matter (DOM) was evaluated based on the molecular interaction between DOM and TC. This paper is helpful for further understanding the mobility and bioavailability of TC in soil. Physiochemical properties of the soil samples (named as QM-SA soil) were measured. DOM was extracted from QM-SA soil. The adsorption of TC on QM-SA soil and QM-SA soil after DOM extracted was studied based on the batch experiments. Three-dimensional excitation–emission matrix (3D-EEM), synchronous fluorescence, two-dimensional correlation spectroscopy (2D-COS) and FT-IR (Fourier transform infrared spectroscopy) were used to reveal the binding of DOM with TC. The maximum adsorption amount (qm) and the initial adsorption rate (K2q2e,cal) were calculated to be 5.241 mg/g and 2.491 mg/(h·g) by the Langmuir model and the pseudo-second-order kinetic model, respectively. Once DOM was extracted from QM-SA soil, the values of qm and K2q2e,cal for QM-SA soil after DOM extracted reduced to 1.274 mg/g and 1.257 mg/(h·g), decreased by 76% and 49% compared with 5.241 mg/g and 2.491 mg/(h·g), respectively. 3D-EEM demonstrated that DOM contained humic-like, protein-like, soluble microbial by-product-like and fulvic-like substances, which bound with TC and contributed to the formation of substance-TC-complexes. In order to find out which substance in DOM has the strongest affinity for tetracycline, synchronous fluorescence, 2D-COS and the site-binding model were used. Protein-like substance bound with TC more strongly than humic-like substance. As for protein-like substances, tryptophan-like substance exhibited higher affinity to TC than tyrosine-like substance. The order of the binding affinity was tryptophan > tyrosine > humic–like substance. FTIR proved that, besides these fluorescent substances, polysaccharide-like substance and aliphatic compound without fluorescence were also responsible for binding with TC. When the initial concentration of TC ranged from 5 to 40 mg/L, all the removal efficiencies were more than 60% at the equilibrium time. The retention degree of TC in QM-SA soil was strong. The ratio of qm between QM-SA soil after DOM extracted and QM-SA soil was about 25%, that is, the effect of DOM on the adsorption of TC was significant.