Sorption Properties of Bentonite for Removing Oxytetracycline from Solutions and Evaluation of Toxicological Characteristics of Sorption Products

The adsorption of oxytetracycline hydrochloride by bentonite was studied in batch sorption experiments. Antibiotic adsorption was described using the Langmuir, Freundlich, and Dubinin–Radushkevich models. At 293 K, the maximum oxytetracycline adsorption was 1.3 mmol g−1 according to the Langmuir equation (R = 0.99) and 0.77 mmol g−1 according to the Dubinin–Radushkevich equation (R = 0.95). It was shown that oxytetracycline adsorption on bentonite is a pH-dependent process, and various cationic forms of the antibiotic are best sorbed by the mineral. Negative values of ΔG0 across the studied temperature range (283–303 K) indicate that the adsorption process is spontaneous. The magnitude of ΔG0 increases with increasing temperature, consistent with an endothermic process. Experimental data on adsorption kinetics are satisfactorily approximated by a pseudo-second-order equation. Using X-ray diffraction, it was shown that the interaction of oxytetracycline with bentonite leads to an increase in the interlayer space in the mineral, which indicates the intercalation of the antibiotic between mineral layers. Clay complexes with oxytetracycline hydrochloride, similar to the antibiotic, continue to exhibit antibacterial activity against Pseudomonas fluorescens cultures. The size of the inhibition zones increases proportionally to the concentration of the antibiotic in the complex. The samples of clay–antibiotic complexes at all analyzed concentrations showed low toxicity for Daphnia magna.