Environ. Sci. Technol. 2009, 43, 88–93
Outer-Sphere and Inner-Sphere Ligand Protonation in Metal Complexation Kinetics: The Lability of EDTA Complexes HERMAN P. VAN LEEUWEN† AND R A E W Y N M . T O W N * ,‡ Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands, Institute of Physics and Chemistry, University of Southern Denmark, Campusvej 55, DK-5230 Odense, Denmark
Received August 4, 2008. Revised manuscript received October 15, 2008. Accepted October 20, 2008.
A generic framework, based on the Eigen mechanism, is formulated to describe the formation/dissociation kinetics of innersphere metal complexes that may undergo protonation. In principle, all protonated forms of the ligand contribute to the formation of the precursor outer-sphere complexes, but only the sufficiently stable ones effectively contribute to the overall rate of inner-sphere complex formation. The concepts are illustrated by experimental data for Cd(II)-EDTA complexes. Up to pH 8 the dissociation flux in this system is dominated by the protonated inner-sphere complex, even though it is a very minor component of the equilibrium speciation in bulk solution. The results highlight the importance of distinguishing between the thermodynamically predominant species versus the kinetically relevant ones in considerations of dynamic speciation analysis and bioavailability in natural and engineered systems.
1. Introduction The impact of anthropogenic aminocarboxylate ligands, such as EDTA, on metal speciation and ecotoxicology in natural waters has long been of concern (1-3). EDTA forms very stable chelates with a wide range of metal ions M, and thus may play a role in solubilizing and mobilizing heavy metals from sediments and soils (4-6). There are conflicting reports in the literature as to the bioavailability of M(II)-EDTA complexes (1), and rationalization of these disparate results requires consideration of the dynamic features of M(II)-EDTA complexes as pertaining to the time scale of the involved biological process (7). The 1:1 M(II) complexes with fully deprotonated EDTA, MY, where “Y” denotes the EDTA ligand, typically have very low dissociation rate constants. Enhanced dissociation has been observed in acidic solution, and is referred to in the literature as “proton assisted” or “acid-catalyzed” dissociation. This process is proposed to proceed via: MY + H f M + HY (8-12). Some authors propose involvement of a “nitrogen protonated intermediate” with “proton-migration” from nitrogen to oxygen (13, 14), whereas others consider the involvement of a protonated inner-sphere complex, MHY * Corresponding author fax: +45 66158780; e-mail:
[email protected]. † Wageningen University. ‡ University of Southern Denmark. 88
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 1, 2009
at pH around 3, but without ascertaining the mechanistic nature of the pertaining processes (15). Several studies have measured a dissociation rate constant of ca. 10 s-1 for Cd(II)-EDTA at low pH, and presumed that the dissociation proceeded via some protonated form (9, 15-18). In this context it is relevant to mention the notion of a “conditional” stability constant, Kcond, which describes the decreasing stability with decreasing pH: Kcond ) KRY4- (with RY4- ) cY4-/cY,t). Kcond decreases with pH due to the decrease in the association rate constant via the decreasing cY4 -, however, the dissociation rate constant of the inner-sphere complex does not change. So there must be an additional process that renders the EDTA complexes labile at low pH. Understanding of this phenomenon within the framework of the impact of protonation of both outer-sphere and inner-sphere complexes is crucial for defining equilibration features. This in turn is of basic importance to metal speciation in natural waters as relevant in, e.g., bioavailability and metal exchange processes (1, 19-21). Until recently, the role of ligand protonation on metal complexation kinetics had not been explicitly considered. The first detailed treatment of the topic for 1:1 ML innersphere complexes (22) derived expressions for the lability of metal complexes with protonated and unprotonated ligand species being involved in formation of the precursor outersphere complex. This approach was then extended to the case of MLn complexes for which the dissociation to free M involves a sequence of dissociation steps (23). A proton anywhere on the ligand lowers the stability of the metal complex and affects the dissociation rate, irrespective of whether the pertaining site is involved directly in the metal complexation. Here we formulate a generic approach, based on the Eigen mechanism, for systems containing inner-sphere complexes that undergo protonation. We tackle the case of MHnL complexes, in which the proton competes for one of the inner-sphere binding sites. We show that invoking a reaction layer, defined by the association of metal with all the various protonated ligand species available, together with consideration of the stability of the pertaining precursor outersphere complexes, provides a good description of experimental data for the evolution of lability of Cd(II)-EDTA complexes in the pH range 3-4.
2. Experimental Section 2.1. Apparatus. An Ecochemie µAutolab potentiostat was used in conjunction with a Metrohm 663 VA stand. The electrometer input impedance of this instrument is >100 GΩ. The working electrode was a conventional mercury drop electrode (surface area, A ) 5.2 × 10-7 m2). The auxiliary electrode was glassy carbon, and the reference electrode was Ag|AgCl|KCl(sat) encased in a 0.1 mol dm-3 KNO3 jacket. Measurements were performed at 20 °C. Stripping chronopotentiometry (SCP) measurements were performed with a stripping current of 2 × 10-9 A, corresponding to conditions of complete depletion (24). This mode of SCP provides a quantitative link between the analytical signal and metal species concentrations in the sample (25). 2.2. Reagents. All solutions were prepared with distilled, deionized water from a Milli-Q Gradient system (resistivity >18 MΩ cm). Cd(II) solutions were prepared by dilution of a commercial certified standard from Aldrich. KNO3 solutions were prepared from solid KNO3 (BDH, AnalaR). Stock solutions of EDTA were prepared from the disodium salt (BDH, AnalaR, g 99.5%). KHphthalate (SigmaUltra, g 99.95%)/HCl (Fluka, TraceSelectUltra) solutions (26) were 10.1021/es802185h CCC: $40.75
2009 American Chemical Society
Published on Web 12/03/2008
FIGURE 1. Protolytic speciation of EDTA in bulk solution. Computed for K1H ) 109.93, K2H ) 105.98, K3H ) 102.65, K4H ) 102.02, K5H ) 101.4, K6H ) 100.1 dm3 mol-1
FIGURE 2. Cd(II)-EDTA equilibrium speciation in bulk solution. Computed for cCd,t ) 4 × 10-7 mol dm-3, cEDTA,t) 1.45 × 10-5 mol dm-3, KMY ) 1016.5, KMHY ) 108.9 dm3 mol-1, and protonation constants as given in Figure 1. used to buffer the pH in the range 3.0-4.0. All solutions were prepared in a supporting electrolyte of 0.1 mol dm-3 KNO3 (TraceSelect). Solutions were initially purged with oxygenfree nitrogen (
