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Chemical Speciation of Environmentally Significant Metals An IUPAC contribution to reliable and rigorous computer modelling by Kipton J. Powell, Paul L. Brown, Robert H.

1 Chemical Speciation of Environmentally Significant Metals An IUPAC contribution to reliable and rigorous computer modelling by Kipton J. Powell, Paul L. Brown, Robert H. Byrne, Tamas Gajda, Glenn Hefter, Ann-Kathrin Leuz, Staffan Sjöberg, and Hans Wanner The mobility and bioavailability of metal ions in natural waters depend on their chemical speciation, which involves a distribution of the metal ions between different complex (metal-ligand) species, colloid-adsorbed species and insoluble phases, each of which may be kinetically labile or inert. For example, in fresh water the metal ions are distributed among organic complexes (e.g., ph measurement of natural water is a routine procedure but conclusive interpretation can be challenging (photo courtesy of Lars Lovgren, Umeå University) The question to be answered is: how does one select the most reliable values? humates), colloids (e.g., as surface-adsorbed species on colloidal phases such as FeOOH), solid phases (e.g., hydroxide, oxide, carbonate mineral phases), and labile complexes with the simple inorganic anionic ligands commonly present in natural waters (e.g., for Zn II, the aqueous species, Zn 2+, ZnOH +, Zn(OH) 2 (aq), Zn 2 OH 3+, ZnSO 4 (aq), ZnCO 3 (aq) ). In contrast to Na +, K +, Ca 2+ and Mg 2+, which are the common cations present in most natural waters, most metal ions of environmental interest occur at trace levels: at very low concentrations (mg dm 3 (ppb) range) in pristine waters and in low concentrations (mg dm 3 (ppm) range) in polluted waters. With the availability of very sensitive analytical techniques such as ASV, ICP and ICP-MS, measurement of the total concentrations of these trace inorganic metal ions is easily accomplished. However, these techniques provide little, if any, information about the distribution of metal ions amongst their labile complex species. The speciation of metal ions, viz., their distribution between soluble labile complexes and inorganic solid phases, can be calculated conveniently by computer simulation. However, the worth of such calculations is critically dependent on the equilibrium model used to define the system, the rigor of the computer modelling program and the reliability of the equilibrium constants used in the calculations. As there are a number of good, well-tested speciation modelling programs now available, reliable simulations depend on the completeness of the equilibrium model and the reliability of the equilibrium constants. The equilibrium model must include all labile species, both soluble and solid phase, that make a measureable contribution to the distribution of the metal ion. In the case of natural waters, this model will include ions such as Ca 2+ and Mg 2+ that will be in competition with trace metal ions for inorganic ligands such as CO 3 2, SO 4 2 and PO 4 3. The equilibrium constants reported in the literature will often be derived from experiments conducted at different ionic strengths, often significantly higher than those applicable to natural waters. For a speciation simulation to be numerically reliable, the equilibrium constants must be valid for the ionic strength of the natural water of interest. This may require extrapolation or interpolation from a set of experimental values determined at different ionic strengths (typically in a 0.05 to 3.0 mol dm 3 medium of a supposedly nonlabile electrolyte) to the value applicable to pristine fresh water (ca mol dm 3 ) or a saline (seawater) medium (ca mol dm 3 ). Choosing the most reliable constants is a serious challenge even for experienced solution chemists. Constants will have been determined by a range of experimental methods, some less appropriate than others, by laboratories with diverse levels of expertise. Even when the experimental measurements are on a simple binary system, such as Pb 2+ + OH, the choice of different experimental ph ranges, total metal ion concentrations, or ionic strengths, or the nature of the medium (e.g., LiClO 4 vs NaClO 4 or KNO 3 ) [1], will lead to different species models and numerically different results. The question to be answered is: how does one select the most reliable values? 15

2 Critical evaluations of equilibrium data - the role of IUPAC One responsibility of IUPAC is the critical evaluation of published experimental data. For equilibrium constants this includes the identification and recommendation of the most reliable values for each metal-ligand system. This is a very time-consuming process which involves the critical evaluation of experimental methods, numerical treatment of data in each publication and at all ionic strengths reported, applying clearly stated criteria to identify reliable publications and reject others, as well as the test of data reliability by establishing correlations between reliable ( accepted ) data for different ionic strengths and rejecting statistical outliers. This process of critical evaluation of metal + ligand equilibrium constants has been applied to the complexes formed between the natural-water-dominant inorganic ligands OH, Cl, CO 3 2, SO 4 2 and PO 4 3, the proton H +, and the environmentally-significant heavymetal ions Hg 2+, Cu 2+, Pb 2+, Cd 2+ and Zn 2+, which all occur at trace levels in fresh water and sea water. The results are the outcome of IUPAC project and are published in a recently-completed series of IUPAC Technical Reports. [2-6] This review of the IUPAC Technical Reports presents examples of speciation diagrams, viz., plots of metal ion distribution as a function of ph. For brevity the examples and tables are limited to the speciation of Zn II in natural waters. Comprehensive examples for each metal ion for binary systems and natural waters are given in the respective Technical Reports. These diagrams are calculated from the stability constants K n or β p,q,r for each ligand-metal combination. In addition to the experimental equilibrium constants, values of the standard state (zero ionic strength) constants, K n or β p,q,r, are also reported. These standard state constants are obtained by extrapolation of the accepted experimental values at finite ionic strengths using the Brønsted Guggenheim Scatchard Specific Ion-Interaction Theory (SIT), [7] which represents a reasonable trade-off between theoretical rigor and numerical simplicity. For the general reaction of a metal ion M with a ligand L (omitting charges except for H + ): pm + ql + rh 2 O M p L q (OH) r + rh + the SIT relationship between the standard equilibrium constant β p,q,r and that determined in an ionic medium of ionic strength I m (molality scale), β p,q,r is: log 10 β p,q,r Δz 2 D rlog 10 a(h 2 O) = log 10 β p,q,r ΔεI m (1) in which Δz 2 = (pz M + qz L r) 2 + r p(z M ) 2 q(z L ) 2. The value of the constant D is defined by the SIT activity coefficient relationship on the molality scale for a single ion i: log 10 γ m (i) = z i 2A I m (1 + a j B I m ) 1 + Σ k ε(i,k) m k = z i 2D + Σ k ε(i,k) m k (2) in which k represents the electrolyte ions N + or X, ε(i,k) is the aqueous SIT coefficient for short-range interactions between ions i and k, and Δε is the reaction SIT coefficient given by: Δε = ε(complex, N + or X ) + rε (H +,X ) pε (M n+,x ) qε (L m, N + ) A is the Debye Hückel limiting slope (0.509 mol 1/2 kg 1/2 at 25 C) and a j B = 1.5. [7] Equation 1 is used to determine log 10 β p,q,r values as the intercept of the extrapolation of a suite of accepted log 10 β p,q,r values. To view all of these extrapolations and a more rigorous treatment, the reader is referred to the IUPAC Technical Reports. [2-6] These reports include all log 10 β p,q,r or log 10 K n and Δε values determined in this IUPAC project. Figure 1: Speciation diagram for the system Zn 2+ + H + + Cl + CO SO HPO 4 2 in a simulated fresh water medium, 25 C, including carbonato- and sulfato- complexes of Mg 2+ and Ca 2+ (not shown). [8] It was assumed that [Zn II ] T = 1 nmol dm 3 and that the system is in equilibrium with air having a CO 2 fugacity of kpa. Formation constants for the major species are given in Table 1. (I c = mol dm 3). 16 CHEMISTRY International January-February 2015 Unauthenticated

Speciation calculations The availability of equilibrium constants and their ionic strength dependencies allows the modelling of metal ion speciation in natural water systems.

3 Speciation calculations The availability of equilibrium constants and their ionic strength dependencies allows the modelling of metal ion speciation in natural water systems. Figures 1 and 2 present examples for Zn II in natural waters. In fresh water the ionic strength is low, which will cause only small changes in the formation constants from values at I c (or I m ) = 0. However, in a sea water system the changes become more pronounced. This is evident from the data presented in Table 1, which contains calculated log 10 K n or log 10 β p,q,r values for species found to contribute significantly to the speciation of Zn II in fresh and sea water systems. The speciation calculations were achieved using WinSGW ( se). This program incorporates the SIT functions and generates the ionic-strength-corrected values of log 10 β p,q,r for each datum in the calculation. Example: a multicomponent freshwater system Figure 1 illustrates the speciation diagram for Zn II in fresh water (I c = mol dm 3 ) in equilibrium with air having a CO 2 fugacity of kpa. The total concentration of Zn II was set to 1 nmol dm 3 and the total concentrations of the inorganic anions were those typically found in fresh water [8]: [Cl ] T = 0.23 mmol dm 3, [SO 4 2 ] T = 0.42 mmol dm 3 and [HPO 4 2 ] T = 0.7 µmol dm 3. The ph was allowed to vary between 6.0 Illustrating chemical speciation in distribution diagrams can be very informative (courtesy of Anneli Sundman, Umeå University) and 9.0. In this range the ionic strength varies from ca. I c = mol dm 3 at ph = 7, to mol dm 3 at ph = 9 due to the increase in [HCO 3 ] and [CO 3 2 ] at constant f(co 2 ). The impact on the stability constant values caused by this change in I c was accounted for in the calculation. This calculation also includes the competing reactions between Mg 2+ and Ca 2+ and the ligands (not shown). [9] Figure 1 shows that the predominant species are Zn 2+ (ph 8.4) and ZnCO 3 (aq) (ph 8.4), and that hydroxido- complexes (Zn p (OH) q (2p q)+) make only a minor contribution (< 10 %). Table 1. Stability constants for selected species critical in modelling the speciation of Zn II in fresh water (I c = mol dm 3) and seawater (I c = 0.67 mol dm 3) at 25 C.* Reaction log 10 K o (I c = 0) log 10 K (I c = ) log 10 K (I c = 0.67) Zn 2+ + H 2 O ZnOH + + H Zn H 2 O Zn(OH) 2 (aq) + 2H Zn 2+ + H 2 CO 3 ZnCO 3 (aq) + 2H H 2 CO 3 HCO 3 + H H 2 CO 3 CO H Zn 2+ + SO 2 4 ZnSO 4 (aq) Zn 2+ + Cl ZnCl Zn Cl ZnCl 2 (aq) Zn Clˉ ZnCl 3ˉ * The constant log 10 K (CO 2 (g) CO 2 (aq)) = 1.5 applies in all calculations. [8] 17

4 Table 2. Equilibrium (stability) constants for the different Zn 2+ systems at K, p = 10 2 kpa, and I m = 0 mol kg 1. Δε values are for ClO 4 medium. Values presented are either Recommended, Provisional or Indicative; see ref. 2 for full details. Reaction Constant Δε (kg mol 1 ) Zn 2+ + H 2 O ZnOH + + H + log 10 *K 1 = 8.96 ± ± 0.02 Δ r H = (56.8 ± 0.9) kj mol 1 Zn H 2 O Zn(OH) 2 (aq) + 2H + log 10 *β 2 = ± ± 0.04 Δ r H = (109 ± 4) kj mol 1 Zn H 2 O Zn(OH) 3 + 3H + log 10 *β 3 = ± ± 0.06 Δ r H = (151 ± 3) kj mol 1 Zn H 2 O Zn(OH) H + log 10 *β 4 = ± ± 0.04 Δ r H = (188 ± 6) kj mol 1 2Zn 2+ + H 2 O Zn 2 OH 3+ + H + log 10 * β 2,1 = 7.9 ± ± 0.1 Zn 2+ + Cl ZnCl + log 10 K 1 = 0.40 ± ± 0.02 Zn Cl ZnCl 2 (aq) log 10 β 2 = 0.69 ± ± 0.04 Zn Cl ZnCl 3 log 10 β 3 = 0.48 ± ± 0.13 Zn 2+ + CO 3 2 ZnCO 3 (aq) log 10 K 1 = 4.75 ± 0.10 Zn CO 3 2 Zn(CO 3 ) 2 2 log 10 β 2 = 5.4 ± 0.6 (a) Zn 2+ + HCO 3 ZnHCO + 3 log 10 K = 1.62 ± ± 0.06 Zn 2+ + SO 2 4 ZnSO 4 (aq) log 10 K 1 = 2.30 ± ± 0.03 Δ r H = 6.0 ± 0.5 kj mol 1 Zn SO 2 4 Zn(SO 4 ) 2 2 log 10 β 2 = 3.2 ± ± 0.08 Zn 2+ + HPO 4 2 ZnHPO 4 (aq) log 10 K = 2.44 ± 0.20 (b) Note (a) I m = 0.68 mol kg 1 ; (b) I m = 0.10 mol kg 1 Example: a multicomponent seawater system In a sea water system high concentrations of chloride (545 mmol dm 3 ) and sulphate (28 mmol dm 3 ) significantly affect the speciation. [8] This is illustrated in Figure 2, which shows the speciation for Zn II in a sea water medium (I c = 0.67 mol dm 3 ), a calculation including carbonato- and sulfato- complexes of Mg 2+ and Ca 2+. It was assumed that [Zn II ] T = 1 nmol dm 3. Similar to the fresh water system, the species Zn 2+ and ZnCO 3 (aq) predominate. However, due to the competing formation of chlorido- and sulphato- complexes, the concentration of Zn 2+ is now ca. 55% of [Zn] T at sea water ph (ca. 8.2), in contrast to ca. 94 % of [Zn] T at the ph of fresh water (ca. 6.5). Similarly, the formation of ZnCO 3 (aq) is also suppressed and this does 18 CHEMISTRY International January-February 2015 not become the predominant species until ph 8.7. IUPAC Recommended values of critical equilibrium constants For a full set of the critically evaluated Recommended or Provisional equilibrium data the reader is referred to the published Technical Reports. [2-6] To illustrate the scope of the data currently available, Table 2 presents the evaluated stability constants, reaction ion interaction coefficients (Δε) and, where available, the reaction enthalpy change for formation of soluble Zn II complexes with the common environmental inorganic ligands. For the sake of brevity the reactions for nine heterogeneous reactions are omitted from this Table. The reader is referred to the Technical Report. [6] Unauthenticated

5 finite ionic strengths, as too were data for some of the systems M 2+ + SO 4 2. It has been possible to evaluate very few enthalpy values, let alone recommend them, which means that a rigorous thermodynamic modelling of the chemical speciation at temperatures other than 25 C is still not possible. Further, the absence of reliable thermodynamic data for metal complexes with relevant organic ligands (e.g. humic substances) remains the major challenge to the modelling of complexation in natural water systems. In this domain there is an urgent need for additional critical evaluations. Figure 2: Speciation diagram for the Zn II system in a simulated seawater medium, 25 C, including carbonato- and sulfato- complexes of Mg 2+ and Ca 2+ (not shown).[7] It was assumed that [Zn II ] T = 1 nmol dm 3 and that the system is in equilibrium with air having a CO 2 fugacity of kpa. Formation constants for the major species are given in Table 1 (I c = 0.67 mol dm -3 ). Comments As a result of this project, the environmental chemist wishing to model inorganic complexation of the important trace metal ions in natural waters now has easy access to the most reliable stability constant values currently available for the relevant equilibria and applicable ionic strengths. An outcome from the critical evaluations [2-6] is that a lack of detailed high-quality data for many of the metal + ligand systems has been identified. This is particularly true for the M 2+ + PO 4 3 systems for which no values could be recommended at I m = 0 mol dm 3. Reliable data in several of the systems M 2+ + CO 3 2 (e.g. M = Cd, Pb, Zn) were also missing at (Ed): Critical evaluation of stability constants is an ongoing activity of IUPAC Analytical Chemistry Division (Division V) through its Sub-Committee on Solubility and Equilibrium Data (SSED) Kipton J. Powell is an Emeritus Professor at University of Canterbury (Christchurch). Paul L. Brown is Principal Advisor, Mineral Waste Management at Rio Tinto Technology and Innovation (Bundoora). Robert H. Byrne is Distinguished University Professor in Seawater Physical Chemistry at the University of South Florida (St. Petersburg). Tamas Gajda is at University of Szeged (Szeged). Glenn Hefter is Professor of Chemistry at Murdoch University. Ann-Kathrin Leuz is Section Head at the Swiss Federal Nuclear Safety Inspectorate (Brugg). Staffan Sjöberg is Professor Emeritus in Chemistry at Umeå University. Hans Wanner is Director General of the Swiss Federal Nuclear Safety Inspectorate (Brugg). References 1. G. T. Hefter, J. Solution Chem. 13, 179 (1984). 2. K. J. Powell, P. L. Brown, R. H. Byrne, T. Gajda, G. Hefter, S Sjöberg, H. Wanner. Pure Appl. Chem. 77, 739 (2005). [Evaluations for Hg II, and H + + CO 3 2, H + + PO 4 3 ] 3. K. J. Powell, P. L. Brown, R. H. Byrne, T. Gajda, G. Hefter, S Sjöberg, H. Wanner. Pure Appl. Chem. 79, 895 (2007). [Evaluations for Cu II ] 4. K. J. Powell, P. L. Brown, R. H. Byrne, T. Gajda, G. Hefter, A-K. Leuz, S. Sjöberg, H. Wanner. Pure Appl. Chem. 81, 2425 (2009). [Evaluations for Pb II ] 5. Ibid.. Pure Appl. Chem. 83, 1163 (2011). [Evaluations for Cd II ] 6. Ibid.. Pure Appl. Chem. 85, 2249 (2013). [Evaluations for Zn II ] 7. I. Grenthe, A. V. Plyasunov, K. Spahiu. In Modelling in Aquatic Chemistry (I. Grenthe, I. Puigdomenech, eds.), pp Organisation for Economic Co-operation and Development, Paris (France) (1997). 8. F. M. M. Morel and J. G. Hering, Principles and Applications of Aquatic Chemistry, John Wiley, New York (1993). 9. L. D. Pettit and K. J. Powell. SC-Database, IUPAC Stability Constants Database. Release 5.8. IUPAC; Academic Software, Otley, UK (2010); for availability see 19

β p,q,r values at higher ionic strengths using

β p,q,r values at higher ionic strengths using Pure Appl. Chem., Vol. 83, No. 5, pp. 1163 1214, 2011. doi:10.1351/pac-rep-10-08-09 2011 IUPAC, Publication date (Web): 29 March 2011 Chemical speciation of environmentally significant metals with inorganic