Determination of dissolved trace metals in seawater with EDTA: Reassessment and optimization

Trace metal determination in seawater is crucial for understanding marine biogeochemical cycles, but organic complexing agents such as ethylenediaminetetraacetic acid (EDTA) and natural organic ligands pose analytical challenges. EDTA, a widely used ligand in phytoplankton cultures, was selected as a model compound in this study. We systematically evaluated two pretreatment methods—HNO 3 /H 2 O 2 digestion and UV/H 2 O 2 irradiation— for improving trace metal recovery in EDTA-rich seawater using an automated preconcentration system ( seaFAST ). Our results demonstrate that both methods achieve nearly 100 % metal recovery. For HNO 3 /H 2 O 2 digestion, we recommend heating seawater samples to 80–90°C for at least 3 hours with 0.5 M HNO 3 and 26 mM H 2 O 2 , followed by pH adjustment to 1.6–1.8 before seaFAST preconcentration. For UV/H₂O₂ treatment, optimal conditions involve 18 mM H 2 O 2 and 2 hours of UV irradiation using cost-effective UV-C lamps for acidified seawater samples stored in high-transmissibility fluorinated ethylene propylene or perfluoroalkoxy alkane bottles. Metals with low EDTA stability constants (e.g., Mn, Zn, and Cd) require shorter pretreatment time, whereas EDTA concentrations exceeding 200 µM necessitate extended digestion for complete degradation. In summary, the UV/H 2 O 2 method provides a simple, energy-efficient, and scalable solution for trace metal analysis in seawater with high EDTA. These findings support reliable trace metal quantification in complex seawater matrices and improve methodological foundations for studying trace metal cycling and phytoplankton-metal interactions. • Accurate determination of Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb in seawater with EDTA. • Reassessment and optimization of HNO 3 /H 2 O 2 and UV/H 2 O 2 digestion methods. • High metal recovery achieved using the recommended HNO₃/H₂O₂ or UV/H₂O₂ protocols.