Article Cite This: J. Chem. Eng. Data 2017, 62, 3981-3990
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Sequestering Ability of Oligophosphate Ligands toward Al3+ in Aqueous Solution Donatella Aiello,§ Paola Cardiano,† Rosalia Maria Cigala,† Peter Gans,‡ Fausta Giacobello,† Ottavia Giuffrè,*,† Anna Napoli,§ and Silvio Sammartano† †
Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ed Ambientali, Università di Messina, Viale F. Stagno d’Alcontres 31, 98166 Messina, Italy ‡ Protonic Software, LS15 0HD Leeds, West Yorkshire, England § Dipartimento di Chimica e Tecnologie Chimiche, dell’Università della Calabria, 87036 Arcavacata di Rende (CS), Italy S Supporting Information *
ABSTRACT: The interactions between Al3+ and ortho-phosphate (PO43−), pyro-phosphate (P2O74−, PP), tripolyphosphate (P3O105−, TPP) and hexa-metaphosphate (P6O186−, HMP) in aqueous solution have been studied. Formation constants, speciation models, and possible structures for the complexes formed are discussed on the basis of potentiometric, calorimetric, 31 P−{1H} NMR, laser desorption mass spectrometric (LD MS) and MS/MS results. 31P− 1 { H} NMR experiments for Al3+−TPP system are in agreement with the speciation model drawn from analysis of potentiometric titration data, suggesting that TPP binds to Al3+ with two adjacent phosphate oxygen atoms, forming a six-membered ring. LD MS has provided identification and structural information for Al3+−PP, −TPP, and −HMP complexes which supports the reliability of speciation models proposed on the basis of the potentiometric data. Moreover, MS/MS spectra obtained from [ML]+ precursor ion show that the ion species involve linear HMP isomers and that two adjacent terminal phosphate groups probably bind Al3+. Standard enthalpy change values were obtained by titration calorimetry; all are endothermic, as is typical for hard−hard interactions. The dependence of formation constants on ionic strength over the range I = 0.1−1 mol kg−1 is also reported. The sequestering ability of all the ligands under study toward Al3+ was also evaluated by the empirical parameter pL0.5.
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INTRODUCTION Aluminum is one of the most abundant metals in the earth’s crust and it is widely distributed in the environment. However, it is not an essential element for any organism.1 For this reason, aluminum is toxic toward both plants and animals.2,3 The primary aluminum source for humans is food and drinking water.1 Widespread aluminum distribution in the environment, derived from natural and anthropogenic sources, makes human exposure to this metal unavoidable.4 Aluminum loads can reach 35−40 mg in human tissues. It interferes with many different biological processes,5 causing diseases of the skeletal, central nervous, and hematopoietic systems in humans.6 In recent years, interest in aluminum has grown considerably due to its many applications and industrial uses.1 Phosphates are of great interest in the field of speciation studies since they play a crucial role in many natural fluids. In seawater phosphate concentration has a maximum at about 1000 m depth. The surface concentration is rather low and is due mainly to the decomposition of phytoplankton and other organisms.7 Sodium hexametaphosphate is employed as water softener and in detergents. It can be also used as scale inhibitor.8 It was recognized that the chemical, toxicological, and biological properties of an element significantly depend on its form. Chemical speciation holds the key for understanding the © 2017 American Chemical Society
impact of aluminum, determining the chemical forms that predominate in different natural media, and under different geochemical conditions.9 In environmental chemistry, the assessment of speciation is crucial for understanding the accumulation, transport, bioavailability, and toxicity of elements within and between water, sediments, soil, biota, and air.10−12 In biology, the speciation of aluminum in serum is of crucial importance to assess its toxicity in humans.4 Consequently the identification and quantification of the chemical species formed between Al3+ and serum constituents, which can bind and transport this element to several target organs (such as brain and bone) is essential.4,13 Aluminum can interact with a variety of organic or inorganic ligands. It binds more strongly to hard Lewis bases, such as sulfates, carboxylates, phosphates, phenolate, and catecholate.1,5,14−17 The affinity of phosphate groups toward Al3+ is well-known.5 In biological systems, several phosphate-containing ligands bind to Al3+.18 If on the one hand binding ability of the phosphate group toward Al3+ in aqueous solution has been clearly demonstrated, on the other hand the complexation is rather complicated and not always determined unambiguously.19 In this study, we Received: July 25, 2017 Accepted: October 19, 2017 Published: October 31, 2017 3981
DOI: 10.1021/acs.jced.7b00685 J. Chem. Eng. Data 2017, 62, 3981−3990
Journal of Chemical & Engineering Data
Article
occurred were ignored in the calculations. Pure nitrogen was bubbled through the solutions in order to maintain an inert atmosphere in the titration vessel. pKw and standard electrode potential values were obtained from strong acid/string base titrations, using the same experimental conditions of temperature and ionic strength as for the metal/ligand titration. Calorimetric Equipment and Procedure. Calorimetric measurements were performed using a CSC 4300 Isoperibol titration calorimeter under computer control for data acquisition. Temperature was maintained at 298.150 ± 0.001 K. The titrant was delivered from a 2.5 mL capacity Hamilton syringe (model 1002TLL). Accuracy was checked by titrating a TRIS buffer solution with HCl.21 The precision was ±0.008 J for calorimetric measurements and ±0.001 mL for titrant volumes. For all the measurements with Al3+, a 25 mL solution containing the metal cation (CM = 4 mmol kg−1), HCl (CH = 2.5−5 mmol kg−1) and NaCl as supporting electrolyte (I = 0.15 mol kg−1) was titrated with the ligand sodium salt (CL = 0.15− 0.2 mol kg−1). The heat of dilution was determined before each experiment. 31 P−{1H} NMR Equipment and Procedure. 31P−{1H} NMR spectra on TPP and Al3+−TPP solutions were recorded on a Bruker AMX R-300 spectrometer operating at 121.49 MHz, by accumulating 512−1024 scans per spectrum. Measurements were carried out in a 9:1 H2O/D2O mixture; the chemical shifts were measured with respect to 85% aqueous phosphoric acid solution as external reference. NMR titrations on Al3+−TPP systems were performed by adding sodium hydroxide solution to mixtures of the ligand (10 mmol kg−1) and various concentrations metal ion (5−7 mmol kg−1), at metal/ligand ratios between 0.5 and 0.7 at pH between 2.0 and 7.5, and ionic strength 0.15 mol kg−1. Prior to the study of Al3+−TPP systems, the protonation behavior of TPP was also investigated by 31P−{1H} NMR spectroscopy, on solutions containing only the ligand at CL = 10 mmol kg−1, at pH between 1.7 and 10.2, and ionic strength 0.15 mol kg−1 in NaCl. Mass Spectrometry. LD MS and MS/MS analyses were performed using a 5800 MALDI TOF-TOF Analyzer (AB SCIEX) equipped with a neodymium−yttrium−aluminum− garnet laser (laser wavelength 349 nm) in reflection positiveion mode with a mass accuracy of 5 ppm. At least 4000 laser shots were typically accumulated with a laser pulse rate of 400 Hz in the MS mode, whereas in the MS/MS mode spectra up to 5000 laser shots were acquired and averaged with a pulse rate of 1000 Hz. MS/MS experiments were performed at a collision energy of 1 kV, and ambient air was used as the collision gas with a medium pressure of 10−6 Torr. After acquisition, spectra were handled using Data Explorer version 4.0. A solution of 2 equiv of ligand (PP, TPP, and HMP, pH 3.5 and 6.5) was added dropwise to 1 mmol of AlCl3 dissolved in water with magnetic stirring for 20 min at room temperature. LD MS and MS/MS was performed by direct spotting of the resulting reaction mixture. MALDI MS and MS/MS were performed using α-cyano-4-hydroxycinnamic acid (α-CHA), and sinapinic acid (SA) as matrix. For these experiments, the sample loading was performed by sandwich layer method and by direct spotting of sample/matrix premixed solution (1:5− 1:25: v/v ratio). Calculations. All the parameters relating to the electrode system calibration (standard electrode potential and junction potential coefficient Ja, (Ej = Ja [H+]), formation constants of the complex species and purity of the reagents were calculated
report the speciation results of a thermodynamic and spectroscopic investigation into the interaction between Al3+ and PO43−, PP, TPP, and HMP. These ligands have several oxygen donor atoms which can be used to form complexes with aluminum ions. This allows for a variety of possibilities in the way that they interact with Al3+. Speciation models in aqueous solution have been obtained by processing potentiometric titration data to determine stability constants of the species formed in solution. Thermodynamic formation parameters have been determined from data obtained with calorimetric titrations. The validation of the speciation results, obtained by potentiometry, was performed by different, independent techniques, namely laser desorption mass spectrometry and 31 P−{1H} NMR spectroscopy. The identification and structural information on Al3+−PP, −TPP, and −HMP complexes were performed by laser desorption mass spectrometry. The Al3+− TPP system has been also investigated by 31P−{1H} NMR, validating the speciation model obtained by potentiometry. The dependence of formation constants on ionic strength over the range I = 0.1−1 mol kg−1 is also reported.
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EXPERIMENTAL SECTION Materials and Methods. Solutions of AlCl3·H2O (SigmaAldrich, purity ≥99%) were prepared by weighing the salt. Aluminum concentrations were determined by titration with ethylenediaminetetraacetic acid (EDTA), which had been standardized against copper sulfate. The phosphates were obtained from commercial sources (Sigma-Aldrich) and were used without further purification: sodium phosphate monobasic, purity ≥99%; sodium pyrophosphate tetrabasic decahydrate, purity ≥99.5%; pentasodium tripolyphosphate hexahydrate, purity 98%. HMP was obtained by the Procter & Gamble Ltd.; product: sodium hexametaphosphate, purity 96%. Ligand concentrations were checked by titration with standard alkali solutions. Sodium hydroxide and hydrochloric acid solutions were prepared using Fluka ampules and were standardized against potassium phthalate and sodium carbonate predried at 383 K. Each sodium hydroxide solution was stored in a dark bottle, protected with a soda lime guard tube. Sodium chloride and sodium nitrate solutions were prepared by weighing the salt, dried at 383 K (Fluka, puriss.). Grade A glassware and ultrapure water (conductivity PP > HMP > PO4 3 −
increasing with the number of phosphorus atoms in the polyphosphate ligand. At physiological pH, TPP shows the highest sequestering ability toward Al3+. The strength of oligophosphate binding to Al3+ is much higher than amino acids, thiocarboxylates, and carboxylates.40,41,46 The sequestering ability of EDDS is an exception to this rule, being intermediate between PP and TPP.40 Literature Comparisons. There is a very detailed discussion in the review of Rubini et al.,16 regarding the interaction between Al3+ and PO43−. Some recommended values are log β = 17.6, 13.5 for MLH and ML, as reported in the paper of Atkari et al.18 The data reported here for Al3+− PO43− system, which includes the species MLH (log β = 17.00) 3988
DOI: 10.1021/acs.jced.7b00685 J. Chem. Eng. Data 2017, 62, 3981−3990
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constants values show higher differences respect Al3+−PP system (in some cases more than an order of magnitude).
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CONCLUSIONS The novel elements of this study consist of the reliable speciation model assessment, of the dependence of formation constants on ionic strengths, the determination of standard enthalpy and entropy values of all the chemical equilibria under study, of the sequestering ability, and of the identification and structural information on the Al3+−PP, −TPP, and −HMP species. Ionic strength dependence and enthalpy/entropy values are important for the application to real systems, such as natural waters. pL0.5 values allow to quantify sequestering ability. A quantitative approach is essential for the assessment of the effectiveness of a ligand as detoxifying agent or in the removal of toxic metal cations from natural systems. Knowledge of the structure of the complex species is very important to clarify the interaction mechanism of the different ligands with the metal cation. It has been performed by LD MS, MS/MS spectrometry, and 31P−{1H} NMR spectroscopy. Both identification and structural information on Al3+−PP, −TPP, and −HMP main species was provided by LD MS, validating the speciation models proposed on the basis of the potentiometric data. LD MS spectra obtained from the Al3+− PP system provide structural information on [ML2PP]+ species.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00685. Additional tables (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:ogiuff
[email protected]. ORCID
Rosalia Maria Cigala: 0000-0003-2054-9191 Ottavia Giuffrè: 0000-0002-8486-8733 Funding
The author O.G. thanks MIUR (Ministero dell’Istruzione, dell’Università e della Ricerca) for financial support (cofunded PRIN project with Prot. 2015MP34H3). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors S.S. and R.M.C. thank Procter & Gamble Ltd. for sodium hexametaphosphate product. REFERENCES
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DOI: 10.1021/acs.jced.7b00685 J. Chem. Eng. Data 2017, 62, 3981−3990
