ESI-MS, UV-Vis, and Theoretical Investigation of ${\mathrm{Fe}}^{3+}$-Amoxicillin Complexation during Coagulation

In this work, the complexation of the hazardous $\beta$-lactamic antibiotic amoxicillin (AMX) with ${\mathrm{Fe}}^{3+}$ ions during coagulation was investigated. New bands in the ultraviolet-visible (UV-vis) spectra suggested that AMX molecules bind to ${\mathrm{Fe}}_{\mathrm{aq}}^{3+}$ and also displace ligands from a stable Fe thiocyanate complex. Electrospray ionization mass spectrometry (ESI-MS) showed strong signals at $m/z=784$ and 816, corresponding to the ${[\mathrm{Fe}+(2\mathrm{AMX}-2\mathrm{H})]}^{+}$ and ${[\mathrm{Fe}+(2\mathrm{AMX}-2\mathrm{H})+{\mathrm{CH}}_3\mathrm{OH}]}^{+}$ species, respectively, also indicating the complexation of ${\mathrm{Fe}}^{3+}$ with AMX molecules. Density functional theory (DFT) calculations showed that among the different coordination modes of AMX to ${\mathrm{Fe}}^{3+}$, complexation via the carboxylate group is more stable and less sterically hindered. These results suggest that during coagulation, the ${\mathrm{Fe}}^{3+}$-AMX complexation-form stable intermediates are probably bound/trapped in the iron hydroxide precipitate. As a result of this process, significant amounts of AMX (approximately 69%) can be removed from aqueous solution. Experiments with heavily contaminated pharmaceutical industrial wastewaters showed very promising results with a strong decrease in AMX, TOC, and acute toxicity.