Experimental determination of solubilities of di-calcium ethylenediaminetetraacetic acid hydrate [Ca2C10H12N2O8·7H2O(s)] in NaCl and MgCl2 solutions to high ionic strengths and its Pitzer model: Applications to geological disposal of nuclear waste and other low temperature environments

Abstract

In this study, solubility measurements on di-calcium ethylenediaminetetraacetic acid hydrate [Ca2C10H12N2O8·7H2O(s), abbreviated as Ca2EDTA·7H2O(s)] as a function of ionic strength are conducted in NaCl solutions up to I=4.4mol·kg−1 and in MgCl2 solutions up to I=7.5mol·kg−1, at room temperature (22.5±0.5°C). The solubility constant (logKsp0) for Ca2EDTA·7H2O(s) and formation constant (logβ10) for CaEDTA2−,Ca2EDTA·7H2O(s)=2Ca2++EDTA4−+7H2O(1)Ca2++EDTA4−=CaEDTA2−(2)are determined as −15.57±0.10 and 11.50±0.05, respectively, based on the Pitzer model with a set of Pitzer parameters describing the specific interactions in NaCl and MgCl2 media.The solubility measurements and thermodynamic modeling indicate that Ca2EDTA·7H2O(s) could become a solubility-controlling phase for EDTA in geological repositories for nuclear waste when the inventories of EDTA reach the saturation concentrations for Ca2EDTA·7H2O(s). The model developed in this work would also enable researchers to calculate the optimal EDTA concentrations to be used for remediation of soils contaminated with heavy metals, and to calculate the maximum EDTA concentrations that could be present in soils after an ETDA washing technology has been applied.