Estimation of stability constants of Fe(III) with antibiotics and dissolved organic matter using a novel UV-vis spectroscopy method.

Determining conditional stability constant (K) is paramount in assessing complex stability, particularly in Fe(III) complexes that are prevalent in actual surface water and wastewater matrices. In this study, existing methods of K determination were evaluated and a novel UV-Vis spectroscopy method was proposed based on the evaluation of these approaches. Model ligands (ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), and oxalic acid (OA)), as well as common antibiotics (kanamycin (Kana) and tetracycline (TTC)), were employed to determine the K of the Fe(III)-ligand complexes under neutral conditions (pH 6.5). The obtained fitting results revealed that the logK were in the order of Fe(III)-EDTA (7.08) > Fe(III)-NTA (4.67) > Fe(III)-OA (4.32) > Fe(III)-TTC (4.28) > Fe(III)-Kana (3.07). In addition to these single ligands, the methodology was extended to the Fe(III) complexation with humic acid (HA), a complex mixture of organic components, where the fitting result indicated a logK of 5.02 M. The method's application domain was analyzed by numerical analysis and combined with experimental results. The findings demonstrate that the proposed methodology possesses satisfactory measurement capability for K ranging from 10 to 10 M, suggesting its broad applicability to the majority of complexes. This method can provide valuable insights into the impact of Fe(III) complexes within the water matrix.