Thermodynamics of Al 3+ -thiocarboxylate interaction in aqueous solution Paola Cardiano, Fausta Giacobello, Ottavia Giuffrè ⁎, Silvio Sammartano Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ed Ambientali, Università di Messina, Viale F. Stagno d'Alcontres 31, 98166 Messina, Italy abstract article info Article history: Received 14 July 2016 Accepted 18 July 2016 Available online 20 July 2016 The equilibria between Al 3+ and three thiocarboxylic acids (2-mercaptopropanoic acid (thiolactic acid (H 2 TLA), 3-mercaptopropanoic acid (H 2 MPA), 2-mercaptosuccinic acid (thiomalic acid (H 3 TMA)) were studied in NaCl aqueous solution at 0.15 ≤ I/mol L −1 ≤ 1 and T = 298.15 K. The results showed the formation of ML and MLH spe- cies for all the investigated systems; in addition, MLOH and ML 2 OH have been observed for TLA, as well as ML 2 and ML 2 OH for MPA, MLH 2 and MLOH for TMA. For the ML species, in NaCl at I = 0.15 mol L −1 and T = 298.15 K, the stability is fairly high, with logβ ML = 8.33, 8.756, 9.87 for TLA, MPA and TMA, respectively. The spe- cies formation at I = 0.15 mol L −1 in NaCl was also investigated by using calorimetric, spectrophotometric and 1 H NMR titrations. ΔH values of the main species, obtained by titration calorimetry, are endothermic, as typical for hard-hard interactions, where the contribution to the Gibbs energy of complexation is mainly entropic in nature. TΔS values referring to the ML species, at I = 0.15 mol L −1 and T = 298.15 K, are 75, 53, 67 kJ mol −1 for TLA, MPA and TMA, respectively. In the same conditions, for ML species, ΔH = 28, 3, 11 kJ mol −1 for TLA, MPA and TMA, re- spectively. The speciation models and the formation constants of the main complex species obtained by the spec- trophotometric and 1 H NMR measurements at I = 0.15 mol L −1 fully confirm those gained by potentiometry in the same conditions. The dependence of the stability on ionic strength was also analysed and the speciation pro- files for all the systems under study were calculated at different ionic strengths. The sequestering ability of the thiocarboxylate ligands towards Al 3+ was also evaluated in the same conditions. For example, at pH = 5, in NaCl at I = 0.15 mol L −1 and T = 298.15 K, the sequestering power of thiocarboxylate towards Al 3+ follows the trend TMA N TLA ≈ MPA. © 2016 Elsevier B.V. All rights reserved. Keywords: Al 3+ complexes Thiocarboxylates Speciation in aqueous solutions Thermodynamic parameters Empirical relationships Potentiometric Calorimetric Spectrophotometric 1 H NMR titrations 1. Introduction Aluminum is the most abundant metal in the earth crust [1–3]. Due to its natural occurrence and to its extensive use (such as water purifi- cation, insulation material, food additive, paper production, glass, ce- ramics, wood preservatives, medicines, etc.), it is extensively distributed in the environment [3]. The absorption of aluminum in humans occurs through the diet and in small part from drinking water, and through the use of antiperspirants [4,5]. Aluminum chronic exposure shows mainly neurological and blood effects as well as bone disease [6–10]. Other effects, such as Alzheimer's disease, have not been fully confirmed [5]. In blood serum basal aluminum concentration is approximately 2 μgL −1 [4]. Its toxicity directly depends on the bio- availability so that it is fundamental the identification and the quantifi- cation of the chemical species by which aluminum is distributed in the body [4,11]. For this reason the study of aluminum speciation is of great interest [7,8,12–14]. In natural waters aluminum mainly results from leaching of rocks and minerals [2]. Its concentration in non-polluted seawater, deriving from clay sediments, is 0.1–20 μgL −1 [15]. In the pH range 3–6 dis- solved organic matter (DOM), mainly humic and fulvic acid, forms with aluminum insoluble and soluble complexes. The solution studies are so complicated by hydrolysis since Al 3+ can be extensively hydro- lyzed to form mononuclear and polynuclear hydroxide species [1,16]. Although the formation of sparingly soluble species greatly limits the pH range to be investigated experimentally, complexation significantly increases aluminum solubility and transport [2]. Due to its hard charac- ter, Al 3+ preferably binds hard Lewis bases, such as hydroxides, phos- phates, sulphates, carboxylates, alcoholates [17–20]. The most stable complexes formed by Al 3+ are those involving multidentate ligands containing O-donor groups [17,18]. Among carboxylates, thiocarboxylates are relevant as thiols are nat- urally occurring ligands that can be formed in many pathways. In sea- water and marine sediment interstitial waters their concentration range is reported from nanomolar to millimolar [21]. Interaction be- tween these ligands and several metal cations, such as (CH 3 ) 2 Sn 2+ , Sn 2+ , Hg 2+ , (CH 3 )Hg + , Pb 2+ , Zn 2+ , has been previously investigated [22–27]. To the best of our knowledge, thermodynamic data on Al 3+ - Journal of Molecular Liquids 222 (2016) 614–621 ⁎ Corresponding author. E-mail address: ogiuffre@unime.it (O. Giuffrè). http://dx.doi.org/10.1016/j.molliq.2016.07.077 0167-7322/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq