Biogeochemistry of Chelating Agents - ACS Publications - American

Chapter 3

Stability Constants Data Sources: Critical Evaluation and Application for Environmental Speciation Downloaded by PENNSYLVANIA STATE UNIV on May 16, 2012 | http://pubs.acs.org Publication Date: July 21, 2005 | doi: 10.1021/bk-2005-0910.ch003

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KonstantinI.Popov and Hans Wanner 1

Physical and Colloid Chemistry Department, Moscow State University of Food Technologies, Volokolamskoye sh., 11, 125080 Moscow, Russia ([email protected]) Swiss Federal Nuclear Safety Inspectorate, CH-5232 Villigen, Switzerland ([email protected])

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Numerous computerized chemical speciation models based on thermodynamic principles are nowadays available to estimate pollutant behavior in the environment. However, these models are sensitive to the reliability of their thermodynamic databases. The quality, accuracy, conflicts and diversity of published stability constants are discussed along with a comparative analysis of critically evaluated thermodynamic data sources. Guidelines that a user would employ in selecting data from the different compilations are proposed. Some applications of chemical speciations for prediction of chelating agent remediation activity in a contaminated soil washing test are considered.

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© 2005 American Chemical Society In Biogeochemistry of Chelating Agents; Nowack, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Downloaded by PENNSYLVANIA STATE UNIV on May 16, 2012 | http://pubs.acs.org Publication Date: July 21, 2005 | doi: 10.1021/bk-2005-0910.ch003

Metal chelation is of great importance in soils because it increases the solubility of metal ions and affects many important chemical and biological processes. It has influence on the availability and mobility of numerous plant nutrients (1-3). Besides, the formation of chelated complexes drastically changes the toxicity and sorption ability of both metal ion and ligand as well as the biodégradation rate of complexant (3-7). At the same time the chelating agents used in soil decontamination technologies could have a negative impact on land (3, 8-20). The toxicities and environmental behavior of pollutants strongly depend on the distribution of metals among the ligands present. The prediction of metal chelate equilibria in soils is based on the use of stability constants in chemical chemical speciation calculations (1, 2, J, 10, 12, 19). Recently, numerous computer chemical speciation programs, e.g., JESS, M I N E Q L , WinSGW, M I N T E Q A 2 , SPECIES, P H R E E Q E , G E O C H E M , GEMS-PSI, EQ3/6 are widely available. Figure 1 demonstrates such a speciation for aluminium(III)E D T A system in a ground water. Such a speciation gives an opportunity to identify the dominating chemical forms of metal in the solution as well as the p H range of complex formation, and to estimate the relative maintenance of these forms. Moreover, the speciation programs give the log[M] values of free metal ion at any pH, and therefore are able to estimate the masking ability of a ligand. Unfortunately, there are still abundant recent examples where changes in chelating agent response in the environment are interpreted without due consideration of the role of metal speciation (5). The corresponding research groups still work on the empirical level (3, 8, 9, 11-18, 20). Partly, this situation arises due to a high sensitivity of computer models to the quality of their thermodynamic databases. Another reason for this is associated with an inadequate description of chemical equilibria in a particular system. It is worthwhile to mention that the chemical speciation diagrams of ligand (cation) forms versus p H in aquatic chemistry have a different meaning being compared with an environmental (geochemical) speciation. The latter indicates the percentage of metal M (ligand L ) bound to different soil phases in order to differentiate soluble phase, the exchangeable cations, the carbonate bound etc. Thus it likely describes the physical speciation of a metal (J). The former demonstrates the distribution of a cation (ligand ) among the particular chemical species, both soluble and solid, e.g. M L , M L , M , M(OH)L, M(OH) etc (electric charges are omitted). In soil chemistry, equilibrium constant should correspond to an ionic strength 0.01 mol/L according to 0.003 mol/L C a C l solution, which approximates that of many well-drained soils (1). Numerous stability constants data compilations in book and software forms are recently available embracing more than 400 000 equilibrium constants; see for example (21-32). Although most of these data correspond to ionic strengths 0.1-1.0 mol/L, the 0.01 mol/L values can be calculated from the former ones using SIT (33) or other 2

soKd

n

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In Biogeochemistry of Chelating Agents; Nowack, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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approximations. The Davies equation may be used for corrections at ionic strength below 0.1 mol/L. For the prediction of relative leaching ability of pollutant by different completing agents in soils, the data obtained at /=0.1 mol/L are also suitable.

Figure 1. Aluminum speciation for 0.005 mol/L Al(N0 )3 aqueous solutions in presence of0.005 mol/L EDTA at 25 °C (Solid curves indicate the speciation done for k ^ A ^ a = 16.7. The dominating species are A l (1), [AlHedta] (2), [Aledta]" (3) and [Al(OH) ]' (4). The dashed lines demonstrate the speciation of the same system if l o g P A k d t a is reduced from 16.7 to 15.3, while all other constants are the same: AlHedta (20, Aledta" (3*); the corresponding curves for A l and Al(OH) " are not listed) 3

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4

3 +

4

The purpose of this review is to characterize and to compare the different thermodynamic data sources and to provide some practical guidelines on how to avoid speciation-relàted errors and subsequent faulty interpretations. Stability Constants Diversity Although true equilibrium in soils is hardly possible due to the long equilibration time, the stability constants based approach appears to be quite adequate and fruitful i f the competing equilibria, which include also numerous



53 nonchelated species, are considered (I). The main problem here is mostly associated with the choice of correct stability constants among those published. Indeed, Table I shows that conflicting data are quite abundant in the open literature, and discrepancies of as many as ten orders of magnitude may arise. This drastic data diversity arises mostly from the inadequacy of the experimental method applied and the lack of thermodynamic rigor in treating the equilibria under the study. A t first glance the data contradiction could leave an impression that reliable measurement of stability constants is hardly possible. However, an experimental benchmarking undertaken within seven experienced research groups for the nickel - glycine system resulted in an excellent agreement of the measured values, cf. Table Π (34). Table I. Comparison of EDTA and Sulfate stability constants, measured by independent research groups at 20-25 °C, in 0.1 Μ K N 0 3

Cation

Fe(III)

EDTA Research group 1 2 3 4 Alog3 max = 0.15 1 2 3 AlogpMLmax = 10.6 1 2 AlogP^max = 4.1 1 2 3 4 5 6 AlogPMLmax = 0.8

lOgPML 25.10 24.95 25.10 25.10

ML

Cr(III)

Th(IV)

Sr(II)

23.40 23.1 12.8 21.17 25.3 8.0 8.53 8.60 8.63 8.70 8.8

Sulfate Research Group 1 Fe(ffl) 2 3 4 AlogPMLmax =1.13 1 Ιη(ΠΙ) 2 3 AlogPMLmax = 0.22 Th(IV) 1 2 AlogPniLmax = 0.04 Cation

Ca(H)

1.53 1.65 2.36 2.66 1.78 1.79 2.0 3.32 3.28

1.34 1 1.40 2 3 1.49 4 1.54 AlogP^max = 0.20

NOTE: Δ ^ β ί ^ π Μ χ indicates the maximal diversity of logarithmic values among the research groups; SOURCE: Data from IUPAC Data base (32).

It can be concluded that reliable experimental procedures and calculation methods exist for the characterization of all type of complexes (37).


54 However, the selection of proper values requires profound knowledge of the chemistry of complex equilibria.


Table II. Nickel - Glycine Project; 25 °C, 1.0 mol/L NaCl Research logfimL Group 1 9.629 12.036 2 9.67 12.14 3 9.654 12.067 4 9.656 12.076 5 9.652 12.109 6 9.659 12.071 7 Mean 9.659 12.083 S.d. 0.011 0.031 0.041 0.104 Δ max SOURCE: DatafromReference 34, Copyright

logftau 5.80 5.53 5.625 5.625 5.60 5.625 5.66 5.638 0.071 0.27 1987IUPAC

10.588 10.26 10.357 10.398 10.325 10.381 10.43 10.391 0.089 0.328

14.308 13.59 13.75 13.911 13.65 13.805 14.08 13.922 0.191 0.558

Critical and Noncritical Stability Constants Compilations This situation stimulated the I U P A C Commission on Equilibrium Data to start publication of a continuous series "Critical Surveys of Stability Constants of Metal Complexes" in 1975 with the aim to evaluate the most reliable equilibrium constants from the available literature data (35). Each survey was prepared by an expert, working actively in the field of thermodynamics of complexes, and commented on by the members of the Commission on Equilibria Data. However, it obviously follows from the nature of the data considered, that even the recommended values cannot be regarded as "final" ones and further research may change its rank, embracing the four categories as defined by I U P A C : recommended, tentative, doubtful and rejected. The authors' responsibility was to state clearly why certain data are regarded as reliable and why other data are rejected. A t the same time, this best up-to-date I U P A C evaluation procedure still remains somewhat arbitrary because it relies largely on "expert opinions". According to the first I U P A C regulations, the data could be recommended (R) if the results of at least two independent research groups are in good agreement; i f the surveyor has no doubt as to the adequacy of the applied experimental and calculation procedure; i f the consideration of the activityconcentration relation is correct, and the standard state is unambiguous. The given error of such a constant must be no less than ±0.05 logarithmic units. Data could be regarded as tentative (T) i f all the conditions mentioned for R category



55 are fulfilled, except the first one; or i f the surveyor observes some deviation from the necessary rigorousness, but this probably caused no serious mistakes. The given error of such a constant must not exceed ±0.2 logarithmic units. Data should be considered as doubtful (D) i f the surveyor suspects some errors in the evaluation of the constants, which are nevertheless of semi quantitative value. The probable error of such a constant should not exceed ±0.2 logarithmic units. Data determined by either an inadequate method, or under undefined conditions should be rejected (Rj). The same catergory is given if any serious objection is found in the evaluation. The reliability of the data is normally revealed by the care with which the reaction conditions and measurements are controlled and described. Papers deficient in specifying the measurement conditions (e.g. temperature, ionic strength, nature of supporting electrolyte), the purity of reagents, the details of the calibration procedure and the consideration of possible side reactions, are eliminated from the evaluation. In addition, the adequacy of the applied method is thoroughly analyzed. The agreement of data from different sources is a valuable verification tool, but is not considered of main priority. Cases are known where several published data showed excellent agreement but were nevertheless rejected (55). Moreover, sometimes one and the same value given by one research group is recommended, and by another one is rejected due to an inadequate data presentation (51). As can be seen from Table II, even the highly experienced laboratories fail to reach the R level of agreement for Ni-complexes. Moreover, the standard deviation value for N i L is beyond the error threshold for Tentative data. Indeed, the increasing complexity of particular species leads normally to higher errors. This could be clearly illustrated by a comparison of nickel N i L , N i L and NLL complexes. Generally, the protonation constants of a ligand H L ( log^m) are more accurate than the formation constants of the corresponding complex M L j because the calculation of the latter one includes inevitably the experimental values of the former species. Thus a report on M L complex stability constant of HiL with an accuracy better than the standard error of logP iL indicates likely the reproducibility of experimental data, but not the real range of confidence. For example, for I D A and M I D A the precision of stability constants should be approximately as high as that found for Ni(II)-glycine (34). For E D T A , D T P A , D O T A , T E T A and T T H A it is significantly lower. For example an evaluation of expected uncertainty of the constants for 2 measured by direct potentiometric titration, gave a value of 0.36 logarithmic units (36). In general, the constants of low stability complexes reveal a better accuracy and agreement than those of high stability complexes. This can be explained by the fact that the stability of "weak" complexes can usually be determined directly in one or two steps, while for the "strong" complexes indirect methods are necessary requiring the use of competing ligand or cation 9

3

2

H

[(U0)2dtpa]\


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56 (37, 38). For example, easily hydrolyzed cations such as Al(III), In(III), Tl(III), Bi(III), Zr(IV), Th(IV), Pd(II) etc. are strongly complexed by complexones. £ M L values for many of these metals range between 10 and 10 . In these cases the direct potentiometric or spectrophotometric titrations cannot be readily used and measurements with competitive ligands, e.g. 2,2 ,2 -triaminotriethylamine (tren), N,N -di-(2-hydroxybenzyl)-diaminoethane-N,N-diacetic acid (HBED), or cations ( H g , G a , C u ) are required. This introduces additional systematic errors, associated with protonation and complex formation of the competing ligands and cations. Similar problems can arise due to fairly slow kinetics of ligand-ligand displacement reactions, e.g. for Fe(III) complexonates equilibrium in most cases is established in a few days (38). In the late 1990-ies the I U P A C Commission adopted "milder" and more flexible requirements for evaluation. The four categories have been replaced by only two: recommended (R) and provisional (/*), which approximately correspond to the R and D levels of the former ones: for IWevel s.d.

Biogeochemistry of Chelating Agents - ACS Publications - American

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