Thermodynamic solution properties of a biodegradable chelant (MGDA) and its interaction with the major constituents of natural fluids

Abstract

This work is focused on the determination of the thermodynamic parameters of interaction between a biodegradable complexone, as MGDA (methylglycinediacetic acid), towards the major constituents of natural fluids, such as H+, Na+, K+, Ca2+ and Mg2+. The determination of their thermodynamic parameters of interaction was performed by potentiometric and calorimetric measurements in different experimental conditions, in order to determine the protonation and complex formation constants, together with the relative enthalpy changes of reaction.The protonation data were modeled as a function of ionic strength to obtain data in standard conditions using the EDH (extended Debye-Hückel type equation) and the SIT (Specific ion Interaction Theory) approaches. Ionic strength dependence parameters were obtained, enabling the calculation of the equilibrium constants at any value within the ionic strength range here considered (0 < I/mol dm−3 ≤ 5.0). The sequestering ability of this biodegradable molecule was assessed through the use of pL0.5 parameter in comparison to other most used chelants, such as EDTA, EDDS and NTA.The species determined for the Ca2+ and Mg2+/MGDA (defined below as L) systems, namely the ML, MLH and M2L. For Na+, K+ and (CH3)4N+, the M2L species was not determined, but the neutral MH2L was found for Na+ and K+. The stability trend is: Mg2+ > Ca2+ >> Na+ > K+ > (CH3)4N+. In all cases, the experimental data determined show that the stability of all the complex species decreases with increasing the ionic strength.The studies conducted in this work have shown that the MGDA is an effective chelating agent, with a capacity for mobilization comparable with that of NTA.The values of the enthalpy changes are negative, indicating an exothermic reaction for the first and the second protonation step and for the formation of the CaL− species. On the contrary, the third protonation step and the formation reaction of the MgL− species is endothermic. The entropic contribution to the stability of the species was found to be predominant, except for the first protonation constant.Two “case studies” were conducted to evaluate the possible use of MGDA in real systems, the first representing a water for industrial use and second a urine with presence of precipitated calcium oxalate.