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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 ML 2 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 pL 0.5 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., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Ethylenediamine-N,N'-disuccinic acid is a biodegradable alternative to EDTA, therefore its use for the sequestration of Ca(2+), Sn(2+), Cu(2+), Zn(2+) and Fe(3+) is analyzed. New data on its binding ability towards these cations were obtained with potentiometric, voltammetric and calorimetric measurements at different ionic strengths and at T = 298.15 K. Real multi-component fluids, namely fresh water, urine, sea water, saliva and blood plasma were chosen as case studies to evaluate the sequestering ability of EDDS in comparison with EDTA. Speciation diagrams were drawn in selected conditions, considering all interactions among the "natural" components of the fluid and those studied in this work, EDDS and EDTA (cL = 1 mmol dm(-3)) as sequestering agents and the cited metal cations (cM ∼ 10(-5) mol dm(-3)). The comparison of the sequestering ability of EDDS and EDTA is done using pM and pL0.5. In blood plasma the plasma mobilizing index was adopted. It was found that EDDS is a good alternative to EDTA, which tends to bind Ca(2+) and Mg(2+) more than EDDS. In particular, EDTA cannot be used as a sequestrant for Sn(2+) when cCa > cEDTA. EDDS is more efficient than EDTA at pH < 8, particularly in urine, where carbonate is absent. In sea water, the sequestering ability of EDDS towards Fe(3+) is higher than that of EDTA. In blood plasma, the PMI of EDDS towards Cu(2+) is higher than that of EDTA. Thermodynamic information, in terms of ΔH and ΔS, for the protonation and metal complex formation reactions are reported., (Copyright © 2016 Elsevier Ltd. All rights reserved.)