Biodegradation and Photodegradation of Mg, Ca, Zn, and Cu Complexes of Biodegradable Chelating Agents (EDDS, HIDS, MGDA, and GLDA)

The environmental fate of biodegradable aminopolycarboxylate chelating agents is strongly influenced by metal complexation; however, the combined effects of central metal ions and ligand structure on degradation behavior remain insufficiently understood. In this study, the biodegradation and photodegradation of the Mg, Ca, Zn, and Cu complexes of four biodegradable chelating agents (EDDS, HIDS, MGDA, and GLDA) were investigated under activated sludge and UV-lamp irradiation conditions. Biodegradation tests revealed metal-dependent behavior: complexes with the alkaline earth metals Mg and Ca were readily degraded, whereas complexes with the transition metals Zn and Cu, except for the EDDS complexes, exhibited high persistence. A qualitative inverse relationship was observed between biodegradability and metal–ligand stability. In contrast, photodegradation exhibited a distinct metal-specific pattern: Cu complexes underwent photodegradation, whereas Mg, Ca, and Zn complexes showed limited photodegradation regardless of ligand identity. These results demonstrate that the degradability of metal–chelate complexes cannot be predicted solely from the behavior of the corresponding free ligands and highlight the importance of considering metal complexation when evaluating the environmental persistence of biodegradable chelating agents in metal-containing aquatic systems.