Elsevier

Journal of Molecular Structure

Volume 1180, 15 March 2019, Pages 491-498
Journal of Molecular Structure

Calcium complexing behaviour of lactate in neutral to highly alkaline medium

https://doi.org/10.1016/j.molstruc.2018.12.020Get rights and content

Highlights

  • The Ca complexing properties and the acid-base behaviour of D,L-lactate were investigated.

  • Both neutral and alkaline media were used.

  • Ca-ISE potentiometry, 13C NMR and ESI-MS spectroscopies and solubility measurements were applied.

  • In highly alkaline medium, the formation of the CaLacOH0 and CaLac(OH)2 complexes were detected.

  • The relevant formation constants were also determined.

Abstract

The calcium complexing properties as well as the acid-base behaviour of d,l-lactate have been investigated in both neutral and alkaline media by Ca-ISE potentiometry, 13C NMR spectroscopy, ESI-MS spectroscopy and solubility measurements. In close to neutral solutions the formation of the CaLac+ and CaLac20 complexes were detected with formation constants of logK1,1 = 0.99 ± 0.02 and logβ1,2 = 1.42 ± 0.03 (I = 4 M, T = 25 °C). These data are consistent with previously reported literature data measured at lower ionic strengths. The alkaline deprotonation constant of lactate has been determined to be pKa = 15.8 ± 0.2. In highly alkaline medium, the formation of the CaLacOH0 and CaLac(OH)2 complexes were detected with the corresponding formation constants being logβ1,1,1 = 1.35 ± 0.09 and logβ1,1,2 = 1.43 ± 0.06. The structures of the CaLac+ and CaLacOH0 complexes were optimized by molecular modelling calculations.

Introduction

Hydroxy carboxylates are known to be effective complexing agents. In neutral medium, both the carboxylate and the hydroxy groups are possible binding sites. Their complexing ability, however, is more pronounced in alkaline medium, where the hydroxy groups may be deprotonated yielding a more efficiently sequestering alcoholate function.

Under the highly saline and highly alkaline conditions prevailing in low and intermediate level (L/IL) radioactive waste repositories, the presence of small molecular metal chelators are considered to have significant impact on the mobilization of actinides and lanthanides [[1], [2], [3], [4], [5]]. Such complexing agents are formed during the alkaline degradation of cellulose, which is present in these L/IL wastes in significant quantities [[6], [7], [8], [9]]. The main degradation product of cellulose is α-d-isosaccharinate; however, lactate (2-hydroxypropanoate, Lac) was also shown to be formed in the course of these reactions [6,[10], [11], [12]].

The complex formation of Lac has been studied with a series of metal ions [13]. In neutral solutions containing both Ca2+ and Lac ions, the formation of the plausible CaLac+ complex was detected. The formation constant, log K1,1, was found to be 0.48–1.47 at 25 °C at the ionic strength (I) range of 0–1 M [[13], [14], [15], [16], [17], [18], [19], [20], [21]]. In some of these works, however, the formation of the neutral CaLac20 complex was also reported [20,21]. The corresponding stability products, log K1,1 and log β1,2, were determined to be 0.90 and 1.24 [20] as well as 0.92 and 1.62 [21]. The binding sites of Lac in the CaLac+ complex were also revealed by 13C NMR spectroscopic measurements. Lac was found to act as a bidentate ligand binding Ca2+ through both the COO and the OH groups [22]. For the MgLac+ species, conversely, the ligand was suggested to be bound to the COO moiety through a water molecule forming a solvent-shared ion-pair [22].

In acidic medium, in the presence of lanthanides, the formation of MLac2+, MLac2+, MLac30 and MLac4 has been shown, where M = La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III) and Lu(III) [23]. The coordination environment of the La(III)-lactate complexes was studied by DFT calculations [24]. Barkleit et al. also suggested the formation of AmLac2+, AmLac2+ and AmLac30 as well as EuLac2+, EuLac2+ and EuLac30 in the presence of Am(III) and Eu(III), respectively [25]. In Nd(III)-containing solutions, the presence of NdLac2+, NdLac2+ and NdLac30 has been proposed [26].

To the best of our knowledge, the behaviour of Lac in highly alkaline medium has not been investigated before. The scope of the present work was to describe the composition, stability and structure of complexes forming between Lac and Ca2+ both in neutral and highly alkaline media at high ionic strength (I = 4.0 M) relevant to nuclear waste disposal.

Section snippets

Reagents and solutions

CaCl2 stock solutions were prepared by dissolving CaCl2·2H2O (Sigma Aldrich, ≥ 99% purity) in water. The exact concentrations of the solutions were determined by complexometric titrations using EDTA. Stock solutions of NaOH were made by diluting a carbonate-free concentrated NaOH solution [27], and were standardized against HCl of known concentration. Sodium lactate solutions were prepared by neutralizing racemic d,l-lactic acid (Acros Organics, 85% m/m) with NaOH. The ionic strength was

Complex formation between Ca2+ and Lac ions in neutral medium

In order to examine the Ca2+ complexation of Lac in neutral medium, Ca-ISE potentiometric titrations were performed at I = 4.0 M (NaCl) and T = (25.0 ± 0.1) °C (Fig. 1) The systematic decrease in [Ca2+] with increasing [Lac]T,0 is a sound evidence for complex formation. The three data sets were fitted simultaneously with the PSEQUAD program [56] by minimizing the F fitting parameter (F is the average difference between the observed and calculated data).

Assuming the formation of the 1:1

Conclusions

From Ca-ISE potentiometric measurements conducted in neutral medium, Lac was found to form weak complexes with calcium with 1:1 and 1:2 composition (i.e., CaLac+ and CaLac20). The formation of these complexes was confirmed by 13C NMR spectroscopy and ESI-MS measurements. The structure of the CaLac+ complex was optimized by molecular modelling calculations, and Lac was found to coordinate the Ca2+ ion bidentately, through one carboxylate oxygen and the hydroxy group. The coordination number

Acknowledgements

This work was supported by the NKFIH K 124 265 and the UNKP-17-2 grants. All these supports are highly appreciated.

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