Study of molecular interactions between humic acid from Brazilian soil and the antibiotic oxytetracycline

• Sorption mechanisms of OTC to the humic acid from a Brazilian soil was examined.
• The reported results are the first scientific attempt related to this issue in Brazil.
• Interaction as hydrogen bonding and proton transfer were found.
• OTC behavior at tropical humic acid was different from temperate humic acids.
• The results indicated that OTC could reach the groundwater.

Tetracyclines (TC) are the major class of antibiotics used in global cattle industry for the treatment of diseases and to promote animal growth. The impact of their residues on tropical soils is not completely understood and Brazil, the major agro-industrial country in the tropical regions, has the commitment to study the environmental fate of these antibiotics due its large consumption. Sorption and interactions at molecular level of the antibiotic oxytetracycline (OTC) with Brazilian humic acid (HA) extracted from soil have been determined in this study. 1313C solid-state nuclear magnetic resonance (1313C NMR), elemental analysis, and electron paramagnetic resonance (EPR) were used for the characterization of HA. Fourier transformed infrared (FTIR) and ultraviolet–visible (UV–Vis) absorption spectroscopies were used to evaluate molecular interactions and mechanisms involved. OTC interacted in acid and alkaline medium (range of pH values from 3.1 to 8.3). Interaction mechanisms were pH dependent, and comprise hydrogen bonding, hydrophobic interactions, and proton transferring - weak interactions. These behaviors are different from HA of temperate regions and suggest that OTC residues could reach the groundwater.

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