PII S0016-7037(99)00113-1 The rates of water exchange in AI(III)-salicylate and AI(III)-sulfosalicylate complexes DAVID J. SULLIVAN, 1 JAN P. NORDIN, 1 BRIAN L. PHILLIPS, 2 and WILLIAM H. CASEY 1,3, * 1 Department of Land, Air and Water Resources, University of California, Davis, CA 95616 2 Chemical Engineering and Materials Science, University of California, Davis, CA 95616 3 Department of Geology, University of California, Davis, CA 95616 (Received September 25, 1998; accepted in revised form February 26, 1999) Abstract—Rate parameters are reported for exchange of hydration waters from the inner coordination sphere of Al(III)-sulfosalicylate [Al(sSal) + ] and Al(III)-salicylate [Al(Sal) + ] complexes to bulk solution as deter- mined with 17 O-NMR. The rate parameters for the Al(sSal) + complex are: k ex 298 = (3.0  0.2)  10 3 s -1 , H ‡ = 37(3) kJ mol -1 , S ‡ =-54(9) J mol -1 K -1 ; and for the Al(Sal) + complex are: k ex 298 = 4.9(0.3)  10 3 s -1 , H ‡ = 35(3) kJ mol -1 , S ‡ =-57(11) J mol -1 K -1 . These results, along with previous work, suggest that the lability of water molecules in bidentate carboxylate–phenolic complexes scales with the electron-donating properties of the ligand oxygens. Replacement of a coordinated carboxyl with a phenolic group in the ligand increases both the Lewis basicity and the value of k ex 298 . A correlation between these parameters is proposed that can be used to predict rate coefficients for other bidentate aluminum complexes. Copyright © 1999 Elsevier Science Ltd 1. INTRODUCTION The low molecular weight organic compounds present in nat- ural waters have an enormous effect on metal transport and toxicity. The effects arise both because the organic ligands bind to metals and make them mobile, and because the metal– organic complexes are photoactive and can induce changes in metal redox state. For example, aluminum is strongly com- plexed by humic material and redistributed in soils (e.g., Vance et al., 1996, Pohlmann and McColl 1986). Likewise, natural organic matter dramatically modifies the cycling of redox- sensitive metals such as iron and manganese. The experimental lifetime of Fe(III) complexed with carboxylates in natural, well-lit waters is a few minutes or less and the photolysis reactions release highly reactive free radicals (e.g., Faust, 1994). The organic compounds originate as the degradation prod- ucts of cellular material, through direct synthesis by biota as ligands to bind nutrient metals, and as pollutants. Two of the most common moities in fulvic acids that affect aluminum transport are carboxylate and phenolate functional groups, which commonly exist in ratios of 3:1 to 1:1 in terrestrial fulvic acids (Stevenson, 1982; Sparks, 1995). These functional groups probably bind to aluminum in a bidentate fashion similar to salicylate, which is used as a model compound for understand- ing fulvic acids (e.g., Plankey and Patterson, 1987; Ainsworth et al., 1998). In this paper we report the rates of exchange of inner sphere water molecules with the bulk solution for aluminum–salicylate and aluminum–sulfosalicylate complexes: In previous work (Phillips et al., 1997a,b; Casey et al., 1998; Nordin et al., 1998) we showed that ligands, such as fluoride, hydroxide, and carboxylates in the inner coordination sphere of Al(III) complexes, have an enormous affect on the lability of other water molecules also in the inner coordination sphere. In this paper we report new rate data and establish a means of predicting these rate coefficients (see also Phillips et al., 1998). 2. METHODS 2.1. Experimental Solutions Solutions were prepared by dissolving AlCl 3  6(H 2 O) and 5-sul- fosalicylic acid (Aldrich) or salicylic acid (Aldrich) into 15–37 atom percent H 2 17 O (Isotec Inc., Miamisburg, OH). The concentration of sulfosalicylate was 50 or 100 mM and the L/ Al ratio was 1 or 2, where L refers to the salicylate or sulfosalicylate ligand. Salicylate has a much lower solubility than sulfosalicylate and the concentration was 15 or 30 mM, always with a L/ Al ratio of 1. The pH of the solutions was adjusted with small amounts of 1.0 M NaOH or 1.0 M HCl (Fisher Scientific) and the glass electrode that was used for pH measurement was calibrated on the concentration scale in 0.1 M NaCl solutions. Although the ionic strengths of the calibrating solutions were slightly lower than the experimental solutions (l  0.6 M), the uncertainty in pH that arises from this difference in ionic strength is probably less than 0.1 units and can be neglected for the purposes of this work. Samples prepared for 17 O-NMR were made 0.2 M in Mn(II). Abbreviations of the complex stoichiometries are reported in Table 1. Sample compositions are summarized in Table 2. 2.2. Thermodynamic Calculations Thermodynamic data exist for both Al(III)–sulfosalicylate and Al(II- I)–salicylate complex formation (Table 3). These were adjusted to the appropriate ionic strength using the Davies equation. The previous studies indicate that complexation reactions in the Al(III)–sulfosalicy- late system are described by a series of AlL n 3-2n species for n = 1–3. The Al(III)–salicylate system, on the other hand, is described by AlL n 3-2n complexes for n = 1–2, but the AlL 2 - complex hydrolyzes with increased pH to form ternary complexes with hydroxyl: Al(OH)L 2 2- and Al(OH) 2 L 2 3- . We avoid ternary complexes in these experiments by maintaining pH  3.54 where the only complexes formed in significant concentrations are the bidentate AlL + species shown above (see Fig. 1a– c). Both the calculations and the experimental results (Fig. 1a– c) indi- cate that manganese did not sequester a significant fraction of the salicylate or sulfosalicylate ligands (Fig. 1a– c) in these experiments. *Author to whom correspondence should be addressed. Pergamon Geochimica et Cosmochimica Acta, Vol. 63, No. 10, pp. 1471–1480, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/99 $20.00 + .00 1471