Understanding the bioavailability and sequestration of different metal cations in the presence of a biodegradable chelant MGDA in biological fluids and natural waters. (September 2017)

• Record Type: Journal Article
• Authors: Bretti, Clemente; Cigala, Rosalia Maria; De Stefano, Concetta; Lando, Gabriele; Sammartano, Silvio
• Is Part Of: Chemosphere. Volume 183(2017)
• Page Start: 107
• Page End: 118
• Publication Date: 2017-09
• Subjects: MGDA -- Biodegradability -- Natural fluids -- Modeling -- Case studies: sea water, fresh waters, urine -- Interaction with metal cations; Pollution -- Periodicals; Pollution -- Physiological effect -- Periodicals; Environmental sciences -- Periodicals; Atmospheric chemistry -- Periodicals; 551.511
• DOI: 10.1016/j.chemosphere.2017.05.099
• Languages: English
• ISSNs: 0045-6535

Abstract

Abstract: Thermodynamic information about the metal-ligand interaction between Fe 3+, Zn 2+, Cu 2+ and Sn 2+, and a biodegradable ligand as MGDA is reported. The speciation scheme was obtained by means of potentiometric measurements and isothermal titration calorimetry (to determine enthalpy changes) in NaCl medium. The formation of the ML and MLOH species was evidenced for all the metal cations, and for Fe 3+ also the ML2 and ML(OH)2 were found. The relative stability, for the ML species, follows the order: Sn 2+ > Fe 3+ > Cu 2+ > Zn 2+ . Stability constants and enthalpy changes were obtained at different ionic strengths, and data were modeled using the Debye-Hückel and SIT approaches to obtain data in a standard state. At infinite dilution, the enthalpy changes are largely negative for Cu 2+ (−34.1 kJ mol −1 ) and Sn 2+ (−16.6 kJ mol −1 ), slightly negative for Fe 3+ (−3.3 kJ mol −1 ) and positive for Zn 2+ (8.7 kJ mol −1 ). In all cases, the entropic contribution to the stability is predominant. The sequestering ability of MGDA was evaluated determining the pL0.5 values in different conditions. Comparing the data reported in this work and literature ones, some empirical relationships were obtained with predictive purpose. For example, using 11 data in the test set we have: log K (M/MGDA) ± 0.1 = 1.13 + 0.84·log K (M/NTA) Case studies were built up in the conditions of seawater, fresh water and urine to study the possible use of MGDA towards the metal cations here studied. Some considerations were also done in the light of the ocean acidification. Graphical abstract: Highlights: MGDA is a biodegradable alternative to NTA for chelation of metal cations? Complex formation is mainly exothermic and the reactions are entropic in nature. Empirical relationships were found between M/MGDA, M/NTA and M/EDDS complexes. The possible use of MGDA as chelant in hydrosphere and biosphere is tested. MGDA may be used as chelant in freshwaters its concentration is > 1.5 mmol dm −3 .