Exploring the Speciation of trans-Aconitic Acid in Aqueous Solution: Interactions with Divalent Metal Cations of Environmental Relevance

Trans-aconitic acid is a naturally occurring, biodegradable, unsaturated tricarboxylic acid that represents a promising sustainable alternative to conventional, poorly degradable chelating agents used in metal ion remediation. In this study, its acid–base behavior and chelating properties toward environmentally relevant divalent metal cations (Ca2+, Mg2+, Cu2+, Co2+, and Zn2+) were systematically investigated in an aqueous solution of potassium chloride (I = 0.10–1.00 mol dm–3) and at T = 298.15 K. A combined experimental and computational approach, including potentiometry, UV–vis and NMR spectroscopy, and Density Functional Theory (DFT) calculations, was employed to achieve a molecular-level thermodynamic characterization. The results indicate that trans-aconitic acid exhibits good sequestration efficiency, with a marked affinity toward Cu2+ and Mg2+ cations, as confirmed by pL0.5 and pM parameters as well as binding energy analyses. DFT calculations elucidated the microscopic acidities of the three carboxylic groups and rationalized the different interaction modes of Ca2+ and Mg2+. Overall, this study provides a comprehensive thermodynamic framework for trans-aconitic acid-metal interactions, supporting its potential application as an ecofriendly ligand for metal ion remediation and scale prevention in natural and industrial waters.