Study on Metronidazole Acid-Base Behavior and Speciation with Ca2+ for Potential Applications in Natural Waters

Metronidazole (MNZ) is an antibiotic widely used for the treatment of various infectious diseases and as an effective pesticide agent for the cultivation of chickens and fish. Its high resistance to purification processes and biological activity has led to the classification of MNZ as an emerging contaminant. A speciation study, aimed to define the acid-base properties of MNZ and its interaction with Ca2+, commonly present in natural waters, is reported. The protonation constants of MNZ, as well as the formation constant value of Ca2+-MNZ species, were obtained by potentiometric titrations in an aqueous solution, using NaCl as background salt at different ionic strengths (0.15, 0.5, 1 mol L-1) and temperature (15, 25 and 37 degrees C) conditions. The acid-base behavior and the complexation with Ca2+ were also investigated by H-1 NMR and UV-Vis titrations, with results in very good agreement with the potentiometric ones. The dependence of the formation constants on the ionic strength and temperature was also determined. The sequestering ability of MNZ towards Ca2+ was defined by the empirical parameter pL(0.5) at different pH and temperature values. The speciation of MNZ simulating sea water conditions was calculated.