Complexation of Eu, Tb, and Dy with Standard Organic Matter Using Fluorescence Quenching: pH Dependence

The complexation behavior of three types of organic matter from the International Humic Substances Society standards and Reference materials was investigated with Eu3+, Tb3+, and Dy3+ ions under different pH conditions. These elements were selected as representative trivalent lanthanides of environmental concern due to their increasing occurrence in aquatic systems from technological waste. Experiments were conducted at pH values of 4, 5, and 6, with a constant ionic strength of 0.01 M NaClO4. Using fluorescence quenching combined with PARAFAC statistical analysis, the apparent stability constants (KML) and the concentrations of complexing sites ([L]0) were determined within a 1L:1M discrete model framework. Both KML and [L]0 increased significantly with pH, regardless of ligand type or metal ion. Tb3+ exhibited the highest affinity, followed by Eu3+ and Dy3+, with binding behavior modulated by pH and ligand structure. Suwannee River humic acid displayed the strongest complexation (4.03 > Log KM‑SRHA –8 ≤ Log[L]0 ≤ 2.14.10–5 mol L–1 for 6.62 mg L–1 of matter), forming particularly stable complexes. These findings highlight the critical role of pH and DOM in controlling the speciation and mobility of technology-critical elements, providing valuable insights for predictive modeling and environmental management strategies.